CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation of U.S. Ser. No. 11/298,774 filed on Dec. 12, 2005, which is a continuation of U.S. Ser. No. 10/728,834 filed on Dec. 8, 2003, now issued as U.S. Pat. No. 6,991,322, which is a continuation-in-part of U.S. application Ser. No. 10/302,274 filed on Nov. 23, 2002, now issued as U.S. Pat. No. 6,755,509, the entire contents of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a thermal ink jet printhead, to a printer system incorporating such a printhead, and to a method of ejecting a liquid drop (such as an ink drop) using such a printhead. 
       BACKGROUND TO THE INVENTION 
       [0003]    The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 (Stemme). 
         [0004]    There are various known types of thermal ink jet (bubblejet) printhead devices. Two typical devices of this type, one made by Hewlett Packard and the other by Canon, have ink ejection nozzles and chambers for storing ink adjacent the nozzles. Each chamber is covered by a so-called nozzle plate, which is a separately fabricated item and which is mechanically secured to the walls of the chamber. In certain prior art devices, the top plate is made of Kapton™ which is a Dupont trade name for a polyimide film, which has been laser-drilled to form the nozzles. These devices also include heater elements in thermal contact with ink that is disposed adjacent the nozzles, for heating the ink thereby forming gas bubbles in the ink. The gas bubbles generate pressures in the ink causing ink drops to be ejected through the nozzles. 
         [0005]    It is an object of the present invention to provide a useful alternative to the known printheads, printer systems, or methods of ejecting drops of ink and other related liquids, which have advantages as described herein. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect, the present invention provides an ink jet printhead comprising: 
         [0007]    a plurality of nozzles; and, 
         [0008]    at least one heater element corresponding to each of the nozzles respectively, the heater element configured for thermal contact with a bubble forming liquid; such that, heating the heater element to a temperature above the boiling point of the bubble forming liquid forms a gas bubble that causes the ejection of a drop of an ejectable liquid through the nozzle corresponding to that heater element; wherein, 
         [0009]    the gas bubble displaces less than 4 nanograms of the ejectable liquid to cause the ejection of the drop. 
         [0010]    The invention configures the components of the printhead such that the bubble formed on the heater element need only move a very small mass of liquid in order to eject an ink drop. The displacement of a relatively small mass requires less input energy to the heater element, thereby improving efficiency. 
         [0011]    According to a second aspect, the present invention provides a printer system which incorporates a printhead, the printhead comprising: 
         [0012]    a plurality of nozzles; and, 
         [0013]    at least one heater element corresponding to each of the nozzles respectively, the heater element configured for thermal contact with a bubble forming liquid; such that, heating the heater element to a temperature above the boiling point of the bubble forming liquid forms a gas bubble that causes the ejection of a drop of an ejectable liquid through the nozzle corresponding to that heater element; wherein, the gas bubble displaces less than 4 nanograms of the ejectable liquid to cause the ejection of the drop. 
         [0014]    According to a third aspect, the present invention provides a method of ejecting drops of an ejectable liquid from a printhead, the printhead comprising a plurality of nozzles; and, at least one heater element corresponding to each of the nozzles respectively; 
         [0015]    the method comprising the steps of: 
         [0016]    placing bubble forming liquid into thermal contact with the heater element; 
         [0017]    heating the heater element to a temperature above the boiling point of the bubble forming liquid to form a gas bubble such that a drop of an ejectable liquid is ejected through the nozzle corresponding to that heater element; wherein, the gas bubble displaces less than 4 nanograms of the ejectable liquid to cause the ejection of the drop. 
         [0018]    Preferably, the gas bubble displaces less than 3 nanograms of the ejectable liquid to cause the ejection of the drop. In a further preferred form, the gas bubble displaces less than 2 nanograms of the ejectable liquid to cause the ejection of the drop. In a particularly desirable embodiment, the gas bubble displaces less than 1.5 nanograms of the ejectable liquid to cause the ejection of the drop. 
         [0019]    As will be understood by those skilled in the art, the ejection of a drop of the ejectable liquid as described herein, is caused by the generation of a vapor bubble in a bubble forming liquid, which, in embodiments, is the same body of liquid as the ejectable liquid. The generated bubble causes an increase in pressure in ejectable liquid, which forces the drop through the relevant nozzle. The bubble is generated by Joule heating of a heater element which is in thermal contact with the ink. The electrical pulse applied to the heater is of brief duration, typically less than 2 microseconds. Due to stored heat in the liquid, the bubble expands for a few microseconds after the heater pulse is turned off. As the vapor cools, it recondenses, resulting in bubble collapse. The bubble collapses to a point determined by the dynamic interplay of inertia and surface tension of the ink. In this specification, such a point is referred to as the “point of collapse” of the bubble. 
         [0020]    The printhead according to the invention comprises a plurality of nozzles, as well as a chamber and one or more heater elements corresponding to each nozzle. Each portion of the printhead pertaining to a single nozzle, its chamber and its one or more elements, is referred to herein as a “unit cell”. 
         [0021]    In this specification, where reference is made to parts being in thermal contact with each other, this means that they are positioned relative to each other such that, when one of the parts is heated, it is capable of heating the other part, even though the parts, themselves, might not be in physical contact with each other. 
         [0022]    Also, the term “ink” is used to signify any ejectable liquid, and is not limited to conventional inks containing colored dyes. Examples of non-colored inks include fixatives, infra-red absorber inks, functionalized chemicals, adhesives, biological fluids, water and other solvents, and so on. The ink or ejectable liquid also need not necessarily be a strictly a liquid, and may contain a suspension of solid particles or be solid at room temperature and liquid at the ejection temperature. 
         [0023]    In this specification, the term “periodic element” refers to an element of a type reflected in the periodic table of elements. 
     
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         [0024]    Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying representations. The drawings are described as follows. 
           [0025]      FIG. 1  is a schematic cross-sectional view through an ink chamber of a unit cell of a printhead according to an embodiment of the invention, at a particular stage of operation. 
           [0026]      FIG. 2  is a schematic cross-sectional view through the ink chamber  FIG. 1 , at another stage of operation. 
           [0027]      FIG. 3  is a schematic cross-sectional view through the ink chamber  FIG. 1 , at yet another stage of operation. 
           [0028]      FIG. 4  is a schematic cross-sectional view through the ink chamber  FIG. 1 , at yet a further stage of operation. 
           [0029]      FIG. 5  is a diagrammatic cross-sectional view through a unit cell of a printhead in accordance with the an embodiment of the invention showing the collapse of a vapor bubble. 
           [0030]      FIGS. 6 ,  8 ,  10 ,  11 ,  13 ,  14 ,  16 ,  18 ,  19 ,  21 ,  23 ,  24 ,  26 ,  28  and  30  are schematic perspective views ( FIG. 30  being partly cut away) of a unit cell of a printhead in accordance with an embodiment of the invention, at various successive stages in the production process of the printhead. 
           [0031]      FIGS. 7 ,  9 ,  12 ,  15 ,  17 ,  20 ,  22 ,  25 ,  27 ,  29  and  31  are each schematic plan views of a mask suitable for use in performing the production stage for the printhead, as represented in the respective immediately preceding figures. 
           [0032]      FIG. 32  is a further schematic perspective view of the unit cell of  FIG. 30  shown with the nozzle plate omitted. 
           [0033]      FIG. 33  is a schematic perspective view, partly cut away, of a unit cell of a printhead according to the invention having another particular embodiment of heater element. 
           [0034]      FIG. 34  is a schematic plan view of a mask suitable for use in performing the production stage for the printhead of  FIG. 33  for forming the heater element thereof. 
           [0035]      FIG. 35  is a schematic perspective view, partly cut away, of a unit cell of a printhead according to the invention having a further particular embodiment of heater element. 
           [0036]      FIG. 36  is a schematic plan view of a mask suitable for use in performing the production stage for the printhead of  FIG. 35  for forming the heater element thereof. 
           [0037]      FIG. 37  is a further schematic perspective view of the unit cell of  FIG. 35  shown with the nozzle plate omitted. 
           [0038]      FIG. 38  is a schematic perspective view, partly cut away, of a unit cell of a printhead according to the invention having a further particular embodiment of heater element. 
           [0039]      FIG. 39  is a schematic plan view of a mask suitable for use in performing the production stage for the printhead of  FIG. 38  for forming the heater element thereof. 
           [0040]      FIG. 40  is a further schematic perspective view of the unit cell of  FIG. 38  shown with the nozzle plate omitted. 
           [0041]      FIG. 41  is a schematic section through a nozzle chamber of a printhead according to an embodiment of the invention showing a suspended beam heater element immersed in a bubble forming liquid. 
           [0042]      FIG. 42  is schematic section through a nozzle chamber of a printhead according to an embodiment of the invention showing a suspended beam heater element suspended at the top of a body of a bubble forming liquid. 
           [0043]      FIG. 43  is a diagrammatic plan view of a unit cell of a printhead according to an embodiment of the invention showing a nozzle. 
           [0044]      FIG. 44  is a diagrammatic plan view of a plurality of unit cells of a printhead according to an embodiment of the invention showing a plurality of nozzles. 
           [0045]      FIG. 45  is a diagrammatic section through a nozzle chamber not in accordance with the invention showing a heater element embedded in a substrate. 
           [0046]      FIG. 46  is a diagrammatic section through a nozzle chamber in accordance with an embodiment of the invention showing a heater element in the form of a suspended beam. 
           [0047]      FIG. 47  is a diagrammatic section through a nozzle chamber of a prior art printhead showing a heater element embedded in a substrate. 
           [0048]      FIG. 48  is a diagrammatic section through a nozzle chamber in accordance with an embodiment of the invention showing a heater element defining a gap between parts of the element. 
           [0049]      FIG. 49  is a diagrammatic section through a nozzle chamber not in accordance with the invention, showing a thick nozzle plate. 
           [0050]      FIG. 50  is a diagrammatic section through a nozzle chamber in accordance with an embodiment of the invention showing a thin nozzle plate. 
           [0051]      FIG. 51  is a diagrammatic section through a nozzle chamber in accordance with an embodiment of the invention showing two heater elements. 
           [0052]      FIG. 52  is a diagrammatic section through a nozzle chamber of a prior art printhead showing two heater elements. 
           [0053]      FIG. 53  is a diagrammatic section through a pair of adjacent unit cells of a printhead according to an embodiment of the invention, showing two different nozzles after drops having different volumes have been ejected therethrough. 
           [0054]      FIGS. 54 and 55  are diagrammatic sections through a heater element of a prior art printhead. 
           [0055]      FIG. 56  is a diagrammatic section through a conformally coated heater element according to an embodiment of the invention. 
           [0056]      FIG. 57  is a diagrammatic elevational view of a heater element, connected to electrodes, of a printhead according to an embodiment of the invention. 
           [0057]      FIG. 58  is a schematic exploded perspective view of a printhead module of a printhead according to an embodiment of the invention. 
           [0058]      FIG. 59  is a schematic perspective view the printhead module of  FIG. 58  shown unexploded. 
           [0059]      FIG. 60  is a schematic side view, shown partly in section, of the printhead module of  FIG. 58 . 
           [0060]      FIG. 61  is a schematic plan view of the printhead module of  FIG. 58 . 
           [0061]      FIG. 62  is a schematic exploded perspective view of a printhead according to an embodiment of the invention. 
           [0062]      FIG. 63  is a schematic further perspective view of the printhead of  FIG. 62  shown unexploded. 
           [0063]      FIG. 64  is a schematic front view of the printhead of  FIG. 62 . 
           [0064]      FIG. 65  is a schematic rear view of the printhead of  FIG. 62 . 
           [0065]      FIG. 66  is a schematic bottom view of the printhead of  FIG. 62 . 
           [0066]      FIG. 67  is a schematic plan view of the printhead of  FIG. 62 . 
           [0067]      FIG. 68  is a schematic perspective view of the printhead as shown in  FIG. 62 , but shown unexploded. 
           [0068]      FIG. 69  is a schematic longitudinal section through the printhead of  FIG. 62 . 
           [0069]      FIG. 70  is a block diagram of a printer system according to an embodiment of the invention. 
           [0070]      FIG. 71  is a schematic, partially cut away, perspective view of a further embodiment of a unit cell of a printhead. 
           [0071]      FIG. 72  is a schematic, partially cut away, exploded perspective view of the unit cell of  FIG. 71 . 
           [0072]      FIG. 73  is a schematic, partially cut away, perspective view of a further embodiment of a unit cell of a printhead. 
           [0073]      FIG. 74  is a schematic, partially cut away, exploded perspective view of the unit cell of  FIG. 73 . 
           [0074]      FIG. 75  is a schematic, partially cut away, perspective view of a further embodiment of a unit cell of a printhead. 
           [0075]      FIG. 76  is a schematic, partially cut away, exploded perspective view of the unit cell of  FIG. 75 . 
           [0076]      FIG. 77  is a schematic, partially cut away, perspective view of a further embodiment of a unit cell of a printhead. 
           [0077]      FIG. 78  is a schematic, partially cut away, perspective view of a further embodiment of a unit cell of a printhead. 
           [0078]      FIG. 79  is a schematic, partially cut away, exploded perspective view of the unit cell of  FIG. 78 . 
           [0079]      FIGS. 80 to 90  are schematic perspective views of the unit cell shown in  FIGS. 78 and 79 , at various successive stages in the production process of the printhead. 
           [0080]      FIGS. 91 and 92  show schematic, partially cut away, schematic perspective views of two variations of the unit cell of  FIGS. 78 to 90 . 
           [0081]      FIG. 93  is a schematic, partially cut away, perspective view of a further embodiment of a unit cell of a printhead. 
           [0082]      FIG. 94  is a schematic, partially cut away, perspective view of a further embodiment of a unit cell of a printhead. 
       
    
    
     DETAILED DESCRIPTION 
       [0083]    In the description than follows, corresponding reference numerals, or corresponding prefixes of reference numerals (i.e. the parts of the reference numerals appearing before a point mark) which are used in different figures relate to corresponding parts. Where there are corresponding prefixes and differing suffixes to the reference numerals, these indicate different specific embodiments of corresponding parts. 
       Overview of the Invention and General Discussion of Operation 
       [0084]    With reference to  FIGS. 1 to 4 , the unit cell  1  of a printhead according to an embodiment of the invention comprises a nozzle plate  2  with nozzles  3  therein, the nozzles having nozzle rims  4 , and apertures  5  extending through the nozzle plate. The nozzle plate  2  is plasma etched from a silicon nitride structure which is deposited, by way of chemical vapor deposition (CVD), over a sacrificial material which is subsequently etched. 
         [0085]    The printhead also includes, with respect to each nozzle  3 , side walls  6  on which the nozzle plate is supported, a chamber  7  defined by the walls and the nozzle plate  2 , a multi-layer substrate  8  and an inlet passage  9  extending through the multi-layer substrate to the far side (not shown) of the substrate. A looped, elongate heater element  10  is suspended within the chamber  7 , so that the element is in the form of a suspended beam. The printhead as shown is a microelectromechanical system (MEMS) structure, which is formed by a lithographic process which is described in more detail below. 
         [0086]    When the printhead is in use, ink  11  from a reservoir (not shown) enters the chamber  7  via the inlet passage  9 , so that the chamber fills to the level as shown in  FIG. 1 . Thereafter, the heater element  10  is heated for somewhat less than 1 micro second, so that the heating is in the form of a thermal pulse. It will be appreciated that the heater element  10  is in thermal contact with the ink  11  in the chamber  7  so that when the element is heated, this causes the generation of vapor bubbles  12  in the ink. Accordingly, the ink  11  constitutes a bubble forming liquid.  FIG. 1  shows the formation of a bubble  12  approximately 1 microsecond after generation of the thermal pulse, that is, when the bubble has just nucleated on the heater elements  10 . It will be appreciated that, as the heat is applied in the form of a pulse, all the energy necessary to generate the bubble  12  is to be supplied within that short time. 
         [0087]    Turning briefly to  FIG. 34 , there is shown a mask  13  for forming a heater  14  (as shown in  FIG. 33 ) of the printhead (which heater includes the element  10  referred to above), during a lithographic process, as described in more detail below. As the mask  13  is used to form the heater  14 , the shape of various of its parts correspond to the shape of the element  10 . The mask  13  therefore provides a useful reference by which to identify various parts of the heater  14 . The heater  14  has electrodes  15  corresponding to the parts designated  15 . 34  of the mask  13  and a heater element  10  corresponding to the parts designated  10 . 34  of the mask. In operation, voltage is applied across the electrodes  15  to cause current to flow through the element  10 . The electrodes  15  are much thicker than the element  10  so that most of the electrical resistance is provided by the element. Thus, nearly all of the power consumed in operating the heater  14  is dissipated via the element  10 , in creating the thermal pulse referred to above. 
         [0088]    When the element  10  is heated as described above, the bubble  12  forms along the length of the element, this bubble appearing, in the cross-sectional view of  FIG. 1 , as four bubble portions, one for each of the element portions shown in cross section. 
         [0089]    The bubble  12 , once generated, causes an increase in pressure within the chamber  7 , which in turn causes the ejection of a drop  16  of the ink  11  through the nozzle  3 . The rim  4  assists in directing the drop  16  as it is ejected, so as to minimize the chance of a drop misdirection. 
         [0090]    The reason that there is only one nozzle  3  and chamber  7  per inlet passage  9  is so that the pressure wave generated within the chamber, on heating of the element  10  and forming of a bubble  12 , does not effect adjacent chambers and their corresponding nozzles. 
         [0091]    The advantages of the heater element  10  being suspended rather than being embedded in any solid material, is discussed below. 
         [0092]      FIGS. 2 and 3  show the unit cell  1  at two successive later stages of operation of the printhead. It can be seen that the bubble  12  generates further, and hence grows, with the resultant advancement of ink  11  through the nozzle  3 . The shape of the bubble  12  as it grows, as shown in  FIG. 3 , is determined by a combination of the inertial dynamics and the surface tension of the ink  11 . The surface tension tends to minimize the surface area of the bubble  12  so that, by the time a certain amount of liquid has evaporated, the bubble is essentially disk-shaped. 
         [0093]    The increase in pressure within the chamber  7  not only pushes ink  11  out through the nozzle  3 , but also pushes some ink back through the inlet passage  9 . However, the inlet passage  9  is approximately 200 to 300 microns in length, and is only approximately 16 microns in diameter. Hence there is a substantial viscous drag. As a result, the predominant effect of the pressure rise in the chamber  7  is to force ink out through the nozzle  3  as an ejected drop  16 , rather than back through the inlet passage  9 . 
         [0094]    Turning now to  FIG. 4 , the printhead is shown at a still further successive stage of operation, in which the ink drop  16  that is being ejected is shown during its “necking phase” before the drop breaks off. At this stage, the bubble  12  has already reached its maximum size and has then begun to collapse towards the point of collapse  17 , as reflected in more detail in  FIG. 5 . 
         [0095]    The collapsing of the bubble  12  towards the point of collapse  17  causes some ink  11  to be drawn from within the nozzle  3  (from the sides  18  of the drop), and some to be drawn from the inlet passage  9 , towards the point of collapse. Most of the ink  11  drawn in this manner is drawn from the nozzle  3 , forming an annular neck  19  at the base of the drop  16  prior to its breaking off. 
         [0096]    The drop  16  requires a certain amount of momentum to overcome surface tension forces, in order to break off. As ink  11  is drawn from the nozzle  3  by the collapse of the bubble  12 , the diameter of the neck  19  reduces thereby reducing the amount of total surface tension holding the drop, so that the momentum of the drop as it is ejected out of the nozzle is sufficient to allow the drop to break off. 
         [0097]    When the drop  16  breaks off, cavitation forces are caused as reflected by the arrows  20 , as the bubble  12  collapses to the point of collapse  17 . It will be noted that there are no solid surfaces in the vicinity of the point of collapse  17  on which the cavitation can have an effect. 
       Manufacturing Process 
       [0098]    Relevant parts of the manufacturing process of a printhead according to embodiments of the invention are now described with reference to  FIGS. 6 to 29 . 
         [0099]    Referring to  FIG. 6 , there is shown a cross-section through a silicon substrate portion  21 , being a portion of a Memjet printhead, at an intermediate stage in the production process thereof. This figure relates to that portion of the printhead corresponding to a unit cell  1 . The description of the manufacturing process that follows will be in relation to a unit cell  1 , although it will be appreciated that the process will be applied to a multitude of adjacent unit cells of which the whole printhead is composed. 
         [0100]      FIG. 6  represents the next successive step, during the manufacturing process, after the completion of a standard CMOS fabrication process, including the fabrication of CMOS drive transistors (not shown) in the region  22  in the substrate portion  21 , and the completion of standard CMOS interconnect layers  23  and passivation layer  24 . Wiring indicated by the dashed lines  25  electrically interconnects the transistors and other drive circuitry (also not shown) and the heater element corresponding to the nozzle. 
         [0101]    Guard rings  26  are formed in the metallization of the interconnect layers  23  to prevent ink  11  from diffusing from the region, designated  27 , where the nozzle of the unit cell  1  will be formed, through the substrate portion  21  to the region containing the wiring  25 , and corroding the CMOS circuitry disposed in the region designated  22 . 
         [0102]    The first stage after the completion of the CMOS fabrication process consists of etching a portion of the passivation layer  24  to form the passivation recesses  29 . 
         [0103]      FIG. 8  shows the stage of production after the etching of the interconnect layers  23 , to form an opening  30 . The opening  30  is to constitute the ink inlet passage to the chamber that will be formed later in the process. 
         [0104]      FIG. 10  shows the stage of production after the etching of a hole  31  in the substrate portion  21  at a position where the nozzle  3  is to be formed. Later in the production process, a further hole (indicated by the dashed line  32 ) will be etched from the other side (not shown) of the substrate portion  21  to join up with the hole  31 , to complete the inlet passage to the chamber. Thus, the hole  32  will not have to be etched all the way from the other side of the substrate portion  21  to the level of the interconnect layers  23 . 
         [0105]    If, instead, the hole  32  were to be etched all the way to the interconnect layers  23 , then to avoid the hole  32  being etched so as to destroy the transistors in the region  22 , the hole  32  would have to be etched a greater distance away from that region so as to leave a suitable margin (indicated by the arrow  34 ) for etching inaccuracies. But the etching of the hole  31  from the top of the substrate portion  21 , and the resultant shortened depth of the hole  32 , means that a lesser margin  34  need be left, and that a substantially higher packing density of nozzles can thus be achieved. 
         [0106]      FIG. 11  shows the stage of production after a four micron thick layer  35  of a sacrificial resist has been deposited on the layer  24 . This layer  35  fills the hole  31  and now forms part of the structure of the printhead. The resist layer  35  is then exposed with certain patterns (as represented by the mask shown in  FIG. 12 ) to form recesses  36  and a slot  37 . This provides for the formation of contacts for the electrodes  15  of the heater element to be formed later in the production process. The slot  37  will provide, later in the process, for the formation of the nozzle walls  6 , that will define part of the chamber  7 . 
         [0107]      FIG. 13  shows the stage of production after the deposition, on the layer  35 , of a 0.25 micron thick layer  38  of heater material, which, in the present embodiment, is of titanium nitride. 
         [0108]      FIG. 14  shows the stage of production after patterning and etching of the heater layer  38  to form the heater  14 , including the heater element  10  and electrodes  15 . 
         [0109]      FIG. 16  shows the stage of production after another sacrificial resist layer  39 , about 1 micron thick, has been added. 
         [0110]      FIG. 18  shows the stage of production after a second layer  40  of heater material has been deposited. In a preferred embodiment, this layer  40 , like the first heater layer  38 , is of 0.25 micron thick titanium nitride. 
         [0111]      FIG. 19  then shows this second layer  40  of heater material after it has been etched to form the pattern as shown, indicated by reference numeral  41 . In this illustration, this patterned layer does not include a heater layer element  10 , and in this sense has no heater functionality. However, this layer of heater material does assist in reducing the resistance of the electrodes  15  of the heater  14  so that, in operation, less energy is consumed by the electrodes which allows greater energy consumption by, and therefore greater effectiveness of, the heater elements  10 . In the dual heater embodiment illustrated in  FIG. 38 , the corresponding layer  40  does contain a heater  14 . 
         [0112]      FIG. 21  shows the stage of production after a third layer  42 , of sacrificial resist, has been deposited. The uppermost level of this layer will constitute the inner surface of the nozzle plate  2  to be formed later. This is also the inner extent of the ejection aperture  5  of the nozzle. The height of this layer  42  must be sufficient to allow for the formation of a bubble  12  in the region designated  43  during operation of the printhead. However, the height of layer  42  determines the mass of ink that the bubble must move in order to eject a droplet. In light of this, the printhead structure of the present invention is designed such that the heater element is much closer to the ejection aperture than in prior art printheads. The mass of ink moved by the bubble is reduced. The generation of a bubble sufficient for the ejection of the desired droplet will require less energy, thereby improving efficiency. 
         [0113]      FIG. 23  shows the stage of production after the roof layer  44  has been deposited, that is, the layer which will constitute the nozzle plate  2 . Instead of being formed from 100 micron thick polyimide film, the nozzle plate  2  is formed of silicon nitride, just 2 microns thick. 
         [0114]      FIG. 24  shows the stage of production after the chemical vapor deposition (CVD) of silicon nitride forming the layer  44 , has been partly etched at the position designated  45 , so as to form the outside part of the nozzle rim  4 , this outside part being designated  4 . 1   
         [0115]      FIG. 26  shows the stage of production after the CVD of silicon nitride has been etched all the way through at  46 , to complete the formation of the nozzle rim  4  and to form the ejection aperture  5 , and after the CVD silicon nitride has been removed at the position designated  47  where it is not required. 
         [0116]      FIG. 28  shows the stage of production after a protective layer  48  of resist has been applied. After this stage, the substrate portion  21  is then ground from its other side (not shown) to reduce the substrate portion from its nominal thickness of about 800 microns to about 200 microns, and then, as foreshadowed above, to etch the hole  32 . The hole  32  is etched to a depth such that it meets the hole  31 . 
         [0117]    Then, the sacrificial resist of each of the resist layers  35 ,  39 ,  42  and  48 , is removed using oxygen plasma, to form the structure shown in  FIG. 30 , with walls  6  and nozzle plate  2  which together define the chamber  7  (part of the walls and nozzle plate being shown cut-away). It will be noted that this also serves to remove the resist filling the hole  31  so that this hole, together with the hole  32  (not shown in  FIG. 30 ), define a passage extending from the lower side of the substrate portion  21  to the nozzle  3 , this passage serving as the ink inlet passage, generally designated  9 , to the chamber  7 . 
         [0118]      FIG. 32  shows the printhead with the nozzle guard and chamber walls removed to clearly illustrate the vertically stacked arrangement of the heater elements  10  and the electrodes  15 . 
         [0119]    While the above production process is used to produce the embodiment of the printhead shown in  FIG. 30 , further printhead embodiments, having different heater structures, are shown in  FIG. 33 ,  FIGS. 35 and 37 , and  FIGS. 38 and 40 . 
       Control of Ink Drop Ejection 
       [0120]    Referring once again to  FIG. 30 , the unit cell  1  shown, as mentioned above, is shown with part of the walls  6  and nozzle plate  2  cut-away, which reveals the interior of the chamber  7 . The heater  14  is not shown cut away, so that both halves of the heater element  10  can be seen. 
         [0121]    In operation, ink  11  passes through the ink inlet passage  9  (see  FIG. 28 ) to fill the chamber  7 . Then a voltage is applied across the electrodes  15  to establish a flow of electric current through the heater element  10 . This heats the element  10 , as described above in relation to  FIG. 1 , to form a vapor bubble in the ink within the chamber  7 . 
         [0122]    The various possible structures for the heater  14 , some of which are shown in  FIGS. 33 ,  35  and  37 , and  38 , can result in there being many variations in the ratio of length to width of the heater elements  10 . Such variations (even though the surface area of the elements  10  may be the same) may have significant effects on the electrical resistance of the elements, and therefore on the balance between the voltage and current to achieve a certain power of the element. 
         [0123]    Modern drive electronic components tend to require lower drive voltages than earlier versions, with lower resistances of drive transistors in their “on” state. Thus, in such drive transistors, for a given transistor area, there is a tendency to higher current capability and lower voltage tolerance in each process generation. 
         [0124]      FIG. 36 , referred to above, shows the shape, in plan view, of a mask for forming the heater structure of the embodiment of the printhead shown in  FIG. 35 . Accordingly, as  FIG. 36  represents the shape of the heater element  10  of that embodiment, it is now referred to in discussing that heater element. During operation, current flows vertically into the electrodes  15  (represented by the parts designated  15 . 36 ), so that the current flow area of the electrodes is relatively large, which, in turn, results in there being a low electrical resistance. By contrast, the element  10 , represented in  FIG. 36  by the part designated  10 . 36 , is long and thin, with the width of the element in this embodiment being 1 micron and the thickness being 0.25 microns. 
         [0125]    It will be noted that the heater  14  shown in  FIG. 33  has a significantly smaller element  10  than the element  10  shown in  FIG. 35 , and has just a single loop  36 . Accordingly, the element  10  of  FIG. 33  will have a much lower electrical resistance, and will permit a higher current flow, than the element  10  of  FIG. 35 . It therefore requires a lower drive voltage to deliver a given energy to the heater  14  in a given time. 
         [0126]    In  FIG. 38 , on the other hand, the embodiment shown includes a heater  14  having two heater elements  10 . 1  and  10 . 2  corresponding to the same unit cell  1 . One of these elements  10 . 2  is twice the width as the other element  10 . 1 , with a correspondingly larger surface area. The various paths of the lower element  10 . 2  are 2 microns in width, while those of the upper element  10 . 1  are 1 micron in width. Thus the energy applied to ink in the chamber  7  by the lower element  10 . 2  is twice that applied by the upper element  10 . 1  at a given drive voltage and pulse duration. This permits a regulating of the size of vapor bubbles and hence of the size of ink drop ejected due to the bubbles. 
         [0127]    Assuming that the energy applied to the ink by the upper element  10 . 1  is X, it will be appreciated that the energy applied by the lower element  10 . 2  is about 2×, and the energy applied by the two elements together is about 3×. Of course, the energy applied when neither element is operational, is zero. Thus, in effect, two bits of information can be printed with the one nozzle  3 . 
         [0128]    As the above factors of energy output may not be achieved exactly in practice, some “fine tuning” of the exact sizing of the elements  10 . 1  and  10 . 2 , or of the drive voltages that are applied to them, may be required. 
         [0129]    It will also be noted that the upper element  10 . 1  is rotated through 180° about a vertical axis relative to the lower element  10 . 2 . This is so that their electrodes  15  are not coincident, allowing independent connection to separate drive circuits. 
       FEATURES AND ADVANTAGES OF PARTICULAR EMBODIMENTS 
       [0130]    Discussed below, under appropriate headings, are certain specific features of embodiments of the invention, and the advantages of these features. The features are to be considered in relation to all of the drawings pertaining to the present invention unless the context specifically excludes certain drawings, and relates to those drawings specifically referred to. 
       Suspended Beam Heater 
       [0131]    With reference to  FIG. 1 , and as mentioned above, the heater element  10  is in the form of a suspended beam, and this is suspended over at least a portion (designated  11 . 1 ) of the ink  11  (bubble forming liquid). The element  10  is configured in this way rather than forming part of, or being embedded in, a substrate as is the case in existing printhead systems made by various manufacturers such as Hewlett Packard, Canon and Lexmark. This constitutes a significant difference between embodiments of the present invention and the prior ink jet technologies. 
         [0132]    The main advantage of this feature is that a higher efficiency can be achieved by avoiding the unnecessary heating of the solid material that surrounds the heater elements  10  (for example the solid material forming the chamber walls  6 , and surrounding the inlet passage  9 ) which takes place in the prior art devices. The heating of such solid material does not contribute to the formation of vapor bubbles  12 , so that the heating of such material involves the wastage of energy. The only energy which contributes in any significant sense to the generation of the bubbles  12  is that which is applied directly into the liquid which is to be heated, which liquid is typically the ink  11 . 
         [0133]    In one preferred embodiment, as illustrated in  FIG. 1 , the heater element  10  is suspended within the ink  11  (bubble forming liquid), so that this liquid surrounds the element. This is further illustrated in  FIG. 41 . In another possible embodiment, as illustrated in  FIG. 42 , the heater element  10  beam is suspended at the surface of the ink (bubble forming liquid)  11 , so that this liquid is only below the element rather than surrounding it, and there is air on the upper side of the element. The embodiment described in relation to  FIG. 41  is preferred as the bubble  12  will form all around the element  10  unlike in the embodiment described in relation to  FIG. 42  where the bubble will only form below the element. Thus the embodiment of  FIG. 41  is likely to provide a more efficient operation. 
         [0134]    As can be seen in, for example, with reference to  FIGS. 30 and 31 , the heater element  10  beam is supported only on one side and is free at its opposite side, so that it constitutes a cantilever. This minimises any direct contact with, and hence reduces heat transfer to, the solid material of the nozzle. 
       Efficiency of the Printhead 
       [0135]    The printhead of the present invention has a design that configures the nozzle structure for enhancing efficiency by reducing the mass of ink that needs to be displaced in order to eject a drop from the nozzle. As a result, the gas bubble displaces less than 4 nanograms of the ejectable liquid to cause the ejection of the drop. Preferably, the gas bubble displaces less than 3 nanograms. In a further preferred form, less than 2 nanograms of the ejectable liquid is displaced by the gas bubble. In a particularly preferred embodiment, the gas bubble displaces less than 1.5 nanograms of the ejectable liquid to cause the ejection of the drop. 
         [0136]    It will be appreciated by those skilled in the art that prior art devices generally require over 5 microjoules to heat the element sufficiently to generate a vapor bubble  12  to eject an ink drop  16 . Thus, the energy requirements of the present invention are an order of magnitude lower than that of known thermal ink jet systems. This lower energy consumption allows lower operating costs, smaller power supplies, and so on, but also dramatically simplifies printhead cooling, allows higher densities of nozzles  3 , and permits printing at higher resolutions. 
         [0137]    These advantages of the present invention are especially significant in embodiments where the individual ejected ink drops  16 , themselves, constitute the major cooling mechanism of the printhead, as described further below. 
       Self-Cooling of the Printhead 
       [0138]    This feature of the invention provides that the energy applied to a heater element  10  to form a vapor bubble  12  so as to eject a drop  16  of ink  11  is removed from the printhead by a combination of the heat removed by the ejected drop itself, and the ink that is taken into the printhead from the ink reservoir (not shown). The result of this is that the net “movement” of heat will be outwards from the printhead, to provide for automatic cooling. Under these circumstances, the printhead does not require any other cooling systems. 
         [0139]    As the ink drop  16  ejected and the amount of ink  11  drawn into the printhead to replace the ejected drop are constituted by the same type of liquid, and will essentially be of the same mass, it is convenient to express the net movement of energy as, on the one hand, the energy added by the heating of the element  10 , and on the other hand, the net removal of heat energy that results from ejecting the ink drop  16  and the intake of the replacement quantity of ink  11 . Assuming that the replacement quantity of ink  11  is at ambient temperature, the change in energy due to net movement of the ejected and replacement quantities of ink can conveniently be expressed as the heat that would be required to raise the temperature of the ejected drop  16 , if it were at ambient temperature, to the actual temperature of the drop as it is ejected. 
         [0140]    It will be appreciated that a determination of whether the above criteria are met depends on what constitutes the ambient temperature. In the present case, the temperature that is taken to be the ambient temperature is the temperature at which ink  11  enters the printhead from the ink storage reservoir (not shown) which is connected, in fluid flow communication, to the inlet passages  9  of the printhead. Typically the ambient temperature will be the room ambient temperature, which is usually roughly 20 degrees C. (Celsius). 
         [0141]    However, the ambient temperature may be less, if for example, the room temperature is lower, or if the ink  11  entering the printhead is refrigerated. 
         [0142]    In one preferred embodiment, the printhead is designed to achieve complete self-cooling (i.e. where the outgoing heat energy due to the net effect of the ejected and replacement quantities of ink  11  is equal to the heat energy added by the heater element  10 ). 
         [0143]    By way of example, assuming that the ink  11  is the bubble forming liquid and is water based, thus having a boiling point of approximately 100 degrees C., and if the ambient temperature is 40 degrees C., then there is a maximum of 60 degrees C. from the ambient temperature to the ink boiling temperature and that is the maximum temperature rise that the printhead could undergo. 
         [0144]    It is desirable to avoid having ink temperatures within the printhead (other than at time of ink drop  16  ejection) which are very close to the boiling point of the ink  11 . If the ink  11  were at such a temperature, then temperature variations between parts of the printhead could result in some regions being above boiling point, with the unintended, and therefore undesirable, formation of vapor bubbles  12 . Accordingly, a preferred embodiment of the invention is configured such that complete self-cooling, as described above, can be achieved when the maximum temperature of the ink  11  (bubble forming liquid) in a particular nozzle chamber  7  is 10 degrees C. below its boiling point when the heating element  10  is not active. 
         [0145]    The main advantage of the feature presently under discussion, and its various embodiments, is that it allows for a high nozzle density and for a high speed of printhead operation without requiring elaborate cooling methods for preventing undesired boiling in nozzles  3  adjacent to nozzles from which ink drops  16  are being ejected. This can allow as much as a hundred-fold increase in nozzle packing density than would be the case if such a feature, and the temperature criteria mentioned, were not present. 
       Areal Density of Nozzles 
       [0146]    This feature of the invention relates to the density, by area, of the nozzles  3  on the printhead. With reference to  FIG. 1 , the nozzle plate  2  has an upper surface  50 , and the present aspect of the invention relates to the packing density of nozzles  3  on that surface. More specifically, the areal density of the nozzles  3  on that surface  50  is over 10,000 nozzles per square cm of surface area. 
         [0147]    In one preferred embodiment, the areal density exceeds 20,000 nozzles per square cm of surface area, while in another preferred embodiment, the areal density exceeds 40,000 nozzles per square cm. In a preferred embodiment, the areal density is 48 828 nozzles per square cm. 
         [0148]    When referring to the areal density, each nozzle  3  is taken to include the drive-circuitry corresponding to the nozzle, which consists, typically, of a drive transistor, a shift register, an enable gate and clock regeneration circuitry (this circuitry not being specifically identified). 
         [0149]    With reference to  FIG. 43  in which a single unit cell  1  is shown, the dimensions of the unit cell are shown as being 32 microns in width by 64 microns in length. The nozzle  3  of the next successive row of nozzles (not shown) immediately juxtaposes this nozzle, so that, as a result of the dimension of the outer periphery of the printhead chip, there are 48,828 nozzles  3  per square cm. This is about 85 times the nozzle areal density of a typical thermal ink jet printhead, and roughly 400 times the nozzle areal density of a piezoelectric printhead. 
         [0150]    The main advantage of a high areal density is low manufacturing cost, as the devices are batch fabricated on silicon wafers of a particular size. 
         [0151]    The more nozzles  3  that can be accommodated in a square cm of substrate, the more nozzles can be fabricated in a single batch, which typically consists of one wafer. The cost of manufacturing a CMOS plus MEMS wafer of the type used in the printhead of the present invention is, to some extent, independent of the nature of patterns that are formed on it. Therefore if the patterns are relatively small, a relatively large number of nozzles  3  can be included. This allows more nozzles  3  and more printheads to be manufactured for the same cost than in a cases where the nozzles had a lower areal density. The cost is directly proportional to the area taken by the nozzles  3 . 
       Bubble Formation on Opposite Sides of Heater Element 
       [0152]    According to the present feature, the heater  14  is configured so that when a bubble  12  forms in the ink  11  (bubble forming liquid), it forms on both sides of the heater element  10 . Preferably, it forms so as to surround the heater element  10  where the element is in the form of a suspended beam. 
         [0153]    The formation of a bubble  12  on both sides of the heater element  10  as opposed to on one side only, can be understood with reference to  FIGS. 45 and 46 . In the first of these figures, the heater element  10  is adapted for the bubble  12  to be formed only on one side as, while in the second of these figures, the element is adapted for the bubble  12  to be formed on both sides, as shown. 
         [0154]    In a configuration such as that of  FIG. 45 , the reason that the bubble  12  forms on only one side of the heater element  10  is because the element is embedded in a substrate  51 , so that the bubble cannot be formed on the particular side corresponding to the substrate. By contrast, the bubble  12  can form on both sides in the configuration of  FIG. 46  as the heater element  10  here is suspended. 
         [0155]    Of course where the heater element  10  is in the form of a suspended beam as described above in relation to  FIG. 1 , the bubble  12  is allowed to form so as to surround the suspended beam element. 
         [0156]    The advantage of the bubble  12  forming on both sides is the higher efficiency that is achievable. This is due to a reduction in heat that is wasted in heating solid materials in the vicinity of the heater element  10 , which do not contribute to formation of a bubble  12 . This is illustrated in  FIG. 45 , where the arrows  52  indicate the movements of heat into the solid substrate  51 . The amount of heat lost to the substrate  51  depends on the thermal conductivity of the solid materials of the substrate relative to that of the ink  11 , which may be water based. As the thermal conductivity of water is relatively low, more than half of the heat can be expected to be absorbed by the substrate  51  rather than by the ink  11 . 
       Prevention of Cavitation 
       [0157]    As described above, after a bubble  12  has been formed in a printhead according to an embodiment of the present invention, the bubble collapses towards a point of collapse  17 . According to the feature presently being addressed, the heater elements  10  are configured to form the bubbles  12  so that the points of collapse  17  towards which the bubbles collapse, are at positions spaced from the heater elements. Preferably, the printhead is configured so that there is no solid material at such points of collapse  17 . In this way cavitation, being a major problem in prior art thermal ink jet devices, is largely eliminated. 
         [0158]    Referring to  FIG. 48 , in a preferred embodiment, the heater elements  10  are configured to have parts  53  which define gaps (represented by the arrow  54 ), and to form the bubbles  12  so that the points of collapse  17  to which the bubbles collapse are located at such gaps. The advantage of this feature is that it substantially avoids cavitation damage to the heater elements  10  and other solid material. 
         [0159]    In a standard prior art system as shown schematically in  FIG. 47 , the heater element  10  is embedded in a substrate  55 , with an insulating layer  56  over the element, and a protective layer  57  over the insulating layer. When a bubble  12  is formed by the element  10 , it is formed on top of the element. When the bubble  12  collapses, as shown by the arrows  58 , all of the energy of the bubble collapse is focussed onto a very small point of collapse  17 . If the protective layer  57  were absent, then the mechanical forces due to the cavitation that would result from the focussing of this energy to the point of collapse  17 , could chip away or erode the heater element  10 . However, this is prevented by the protective layer  57 . 
         [0160]    Typically, such a protective layer  57  is of tantalum, which oxidizes to form a very hard layer of tantalum pentoxide (Ta 2 O 5 ). Although no known materials can fully resist the effects of cavitation, if the tantalum pentoxide should be chipped away due to the cavitation, then oxidation will again occur at the underlying tantalum metal, so as to effectively repair the tantalum pentoxide layer. 
         [0161]    Although the tantalum pentoxide functions relatively well in this regard in known thermal ink jet systems, it has certain disadvantages. One significant disadvantage is that, in effect, virtually the whole protective layer  57  (having a thickness indicated by the reference numeral  59 ) must be heated in order to transfer the required energy into the ink  11 , to heat it so as to form a bubble  12 . This layer  57  has a high thermal mass due to the very high atomic weight of the tantalum, and this reduces the efficiency of the heat transfer. Not only does this increase the amount of heat which is required at the level designated  59  to raise the temperature at the level designated  60  sufficiently to heat the ink  11 , but it also results in a substantial thermal loss to take place in the directions indicated by the arrows  61 . This disadvantage would not be present if the heater element  10  was merely supported on a surface and was not covered by the protective layer  57 . 
         [0162]    According to the feature presently under discussion, the need for a protective layer  57 , as described above, is avoided by generating the bubble  12  so that it collapses, as illustrated in  FIG. 48 , towards a point of collapse  17  at which there is no solid material, and more particularly where there is the gap  54  between parts  53  of the heater element  10 . As there is merely the ink  11  itself in this location (prior to bubble generation), there is no material that can be eroded here by the effects of cavitation. The temperature at the point of collapse  17  may reach many thousands of degrees C., as is demonstrated by the phenomenon of sonoluminesence. This will break down the ink components at that point. However, the volume of extreme temperature at the point of collapse  17  is so small that the destruction of ink components in this volume is not significant. 
         [0163]    The generation of the bubble  12  so that it collapses towards a point of collapse  17  where there is no solid material can be achieved using heater elements  10  corresponding to that represented by the part  10 . 34  of the mask shown in  FIG. 34 . The element represented is symmetrical, and has a hole represented by the reference numeral  63  at its center. When the element is heated, the bubble forms around the element (as indicated by the dashed line  64 ) and then grows so that, instead of being of annular (doughnut) shape as illustrated by the dashed lines  64  and  65 ) it spans the element including the hole  63 , the hole then being filled with the vapor that forms the bubble. The bubble  12  is thus substantially disc-shaped. When it collapses, the collapse is directed so as to minimize the surface tension surrounding the bubble  12 . This involves the bubble shape moving towards a spherical shape as far as is permitted by the dynamics that are involved. This, in turn, results in the point of collapse being in the region of the hole  63  at the center of the heater element  10 , where there is no solid material. 
         [0164]    The heater element  10  represented by the part  10 . 31  of the mask shown in  FIG. 31  is configured to achieve a similar result, with the bubble generating as indicated by the dashed line  66 , and the point of collapse to which the bubble collapses being in the hole  67  at the center of the element. 
         [0165]    The heater element  10  represented as the part  10 . 36  of the mask shown in  FIG. 36  is also configured to achieve a similar result. Where the element  10 . 36  is dimensioned such that the hole  68  is small, manufacturing inaccuracies of the heater element may affect the extent to which a bubble can be formed such that its point of collapse is in the region defined by the hole. For example, the hole may be as little as a few microns across. Where high levels of accuracy in the element  10 . 36  cannot be achieved, this may result in bubbles represented as  12 . 36  that are somewhat lopsided, so that they cannot be directed towards a point of collapse within such a small region. In such a case, with regard to the heater element represented in  FIG. 36 , the central loop  49  of the element can simply be omitted, thereby increasing the size of the region in which the point of collapse of the bubble is to fall. 
       Chemical Vapor Deposited Nozzle Plate, and Thin Nozzle Plates 
       [0166]    The nozzle ejection aperture  5  of each unit cell  1  extends through the nozzle plate  2 , the nozzle plate thus constituting a structure which is formed by chemical vapor deposition (CVD). In various preferred embodiments, the CVD is of silicon nitride, silicon dioxide or oxi-nitride. 
         [0167]    The advantage of the nozzle plate  2  being formed by CVD is that it is formed in place without the requirement for assembling the nozzle plate to other components such as the walls  6  of the unit cell  1 . This is an important advantage because the assembly of the nozzle plate  2  that would otherwise be required can be difficult to effect and can involve potentially complex issues. Such issues include the potential mismatch of thermal expansion between the nozzle plate  2  and the parts to which it would be assembled, the difficulty of successfully keeping components aligned to each other, keeping them planar, and so on, during the curing process of the adhesive which bonds the nozzle plate  2  to the other parts. 
         [0168]    The issue of thermal expansion is a significant factor in the prior art, which limits the size of ink jets that can be manufactured. This is because the difference in the coefficient of thermal expansion between, for example, a nickel nozzle plate and a substrate to which the nozzle plate is connected, where this substrate is of silicon, is quite substantial. Consequently, over as small a distance as that occupied by, say, 1000 nozzles, the relative thermal expansion that occurs between the respective parts, in being heated from the ambient temperature to the curing temperature required for bonding the parts together, can cause a dimension mismatch of significantly greater than a whole nozzle length. This would be significantly detrimental for such devices. 
         [0169]    Another problem addressed by the features of the invention presently under discussion, at least in embodiments thereof, is that, in prior art devices, nozzle plates that need to be assembled are generally laminated onto the remainder of the printhead under conditions of relatively high stress. This can result in breakages or undesirable deformations of the devices. The depositing of the nozzle plate  2  by CVD in embodiments of the present invention avoids this. 
         [0170]    A further advantage of the present features of the invention, at least in embodiments thereof, is their compatibility with existing semiconductor manufacturing processes. Depositing a nozzle plate  2  by CVD allows the nozzle plate to be included in the printhead at the scale of normal silicon wafer production, using processes normally used for semi-conductor manufacture. 
         [0171]    Existing thermal ink jet or bubble jet systems experience pressure transients, during the bubble generation phase, of up to 100 atmospheres. If the nozzle plates  2  in such devices were applied by CVD, then to withstand such pressure transients, a substantial thickness of CVD nozzle plate would be required. As would be understood by those skilled in the art, such thicknesses of deposited nozzle plates would give rise certain problems as discussed below. 
         [0172]    For example, the thickness of nitride sufficient to withstand a 100 atmosphere pressure in the nozzle chamber  7  may be, say, 10 microns. With reference to  FIG. 49 , which shows a unit cell  1  that is not in accordance with the present invention, and which has such a thick nozzle plate  2 , it will be appreciated that such a thickness can result in problems relating to drop ejection. In this case, due to the thickness of nozzle plate  2 , the fluidic drag exerted by the nozzle  3  as the ink  11  is ejected therethrough results in significant losses in the efficiency of the device. 
         [0173]    Another problem that would exist in the case of such a thick nozzle plate  2 , relates to the actual etching process. This is assuming that the nozzle  3  is etched, as shown, perpendicular to the wafer  8  of the substrate portion, for example using a standard plasma etching. This would typically require more than 10 microns of resist  69  to be applied. To expose that thickness of resist  69 , the required level of resolution becomes difficult to achieve, as the focal depth of the stepper that is used to expose the resist is relatively small. Although it would be possible to expose this relevant depth of resist  69  using x-rays, this would be a relatively costly process. 
         [0174]    A further problem that would exist with such a thick nozzle plate  2  in a case where a 10 micron thick layer of nitride were CVD deposited on a silicon substrate wafer, is that, because of the difference in thermal expansion between the CVD layer and the substrate, as well as the inherent stress of within thick deposited layer, the wafer could be caused to bow to such a degree that further steps in the lithographic process would become impractical. Thus, a 10 micron thick nozzle plate  2  is possible but (unlike in the present invention), disadvantageous. 
         [0175]    With reference to  FIG. 50 , in a Memjet thermal ink ejection device according to an embodiment of the present invention, the CVD nitride nozzle plate layer  2  is only 2 microns thick. Therefore the fluidic drag through the nozzle  3  is not particularly significant and is therefore not a major cause of loss. 
         [0176]    Furthermore, the etch time, and the resist thickness required to etch nozzles  3  in such a nozzle plate  2 , and the stress on the substrate wafer  8 , will not be excessive. 
         [0177]    The relatively thin nozzle plate  2  in this invention is enabled as the pressure generated in the chamber  7  is only approximately 1 atmosphere and not 100 atmospheres as in prior art devices, as mentioned above. 
         [0178]    There are many factors which contribute to the significant reduction in pressure transient required to eject drops  16  in this system. These include: 
         [0000]    1. small size of chamber  7 ;
 
2. accurate fabrication of nozzle  3  and chamber  7 ;
 
3. stability of drop ejection at low drop velocities;
 
4. very low fluidic and thermal crosstalk between nozzles  3 ;
 
5. optimum nozzle size to bubble area;
 
6. low fluidic drag through thin (2 micron) nozzle  3 ;
 
7. low pressure loss due to ink ejection through the inlet  9 ;
 
8. self-cooling operation.
 
         [0179]    As mentioned above in relation the process described in terms of  FIGS. 6 to 31 , the etching of the 2-micron thick nozzle plate layer  2  involves two relevant stages. One such stage involves the etching of the region designated  45  in  FIGS. 24 and 50 , to form a recess outside of what will become the nozzle rim  4 . The other such stage involves a further etch, in the region designated  46  in  FIGS. 26 and 50 , which actually forms the ejection aperture  5  and finishes the rim  4 . 
       Nozzle Plate Thicknesses 
       [0180]    As addressed above in relation to the formation of the nozzle plate  2  by CVD, and with the advantages described in that regard, the nozzle plates in the present invention are thinner than in the prior art. More particularly, the nozzle plates  2  are less than 10 microns thick. In one preferred embodiment, the nozzle plate  2  of each unit cell  1  is less than 5 microns thick, while in another preferred embodiment, it is less than 2.5 microns thick. Indeed, a preferred thickness for the nozzle plate  2  is 2 microns thick. 
       Heater Elements Formed in Different Layers 
       [0181]    According to the present feature, there is a plurality of heater elements  10  disposed within the chamber  7  of each unit cell  1 . The elements  10 , which are formed by the lithographic process as described above in relation to  FIG. 6 to 31 , are formed in respective layers. 
         [0182]    In preferred embodiments, as shown in  FIGS. 38 ,  40  and  51 , the heater elements  10 . 1  and  10 . 2  in the chamber  7 , are of different sizes relative to each other. 
         [0183]    Also as will be appreciated with reference to the above description of the lithographic process, each heater element  10 . 1 ,  10 . 2  is formed by at least one step of that process, the lithographic steps relating to each one of the elements  10 . 1  being distinct from those relating to the other element  10 . 2 . 
         [0184]    The elements  10 . 1 ,  10 . 2  are preferably sized relative to each other, as reflected schematically in the diagram of  FIG. 51 , such that they can achieve binary weighted ink drop volumes, that is, so that they can cause ink drops  16  having different, binary weighted volumes to be ejected through the nozzle  3  of the particular unit cell  1 . The achievement of the binary weighting of the volumes of the ink drops  16  is determined by the relative sizes of the elements  10 . 1  and  10 . 2 . In  FIG. 51 , the area of the bottom heater element  10 . 2  in contact with the ink  11  is twice that of top heater element  10 . 1 . 
         [0185]    One known prior art device, patented by Canon, and illustrated schematically in  FIG. 52 , also has two heater elements  10 . 1  and  10 . 2  for each nozzle, and these are also sized on a binary basis (i.e. to produce drops  16  with binary weighted volumes). These elements  10 . 1 ,  10 . 2  are formed in a single layer, adjacent to each other in the nozzle chamber  7 . It will be appreciated that the bubble  12 . 1  formed by the small element  10 . 1 , only, is relatively small, while that  12 . 2  formed by the large element  10 . 2 , only, is relatively large. The bubble generated by the combined effects of the two elements, when they are actuated simultaneously, is designated  12 . 3 . Three differently sized ink drops  16  will be caused to be ejected by the three respective bubbles  12 . 1 ,  12 . 2  and  12 . 3 . 
         [0186]    It will be appreciated that the size of the elements  10 . 1  and  10 . 2  themselves are not required to be binary weighted to cause the ejection of drops  16  having different sizes or the ejection of useful combinations of drops. Indeed, the binary weighting may well not be represented precisely by the area of the elements  10 . 1 ,  10 . 2  themselves. In sizing the elements  10 . 1 ,  10 . 2  to achieve binary weighted drop volumes, the fluidic characteristics surrounding the generation of bubbles  12 , the drop dynamics characteristics, the quantity of liquid that is drawing back into the chamber  7  from the nozzle  3  once a drop  16  has broken off, and so forth, must be considered. Accordingly, the actual ratio of the surface areas of the elements  10 . 1 ,  10 . 2 , or the performance of the two heaters, needs to be adjusted in practice to achieve the desired binary weighted drop volumes. 
         [0187]    Where the size of the heater elements  10 . 1 ,  10 . 2  is fixed and where the ratio of their surface areas is therefore fixed, the relative sizes of ejected drops  16  may be adjusted by adjusting the supply voltages to the two elements. This can also be achieved by adjusting the duration of the operation pulses of the elements  10 . 1 ,  10 . 2 —i.e. their pulse widths. However, the pulse widths cannot exceed a certain amount of time, because once a bubble  12  has nucleated on the surface of an element  10 . 1 ,  10 . 2 , then any duration of pulse width after that time will be of little or no effect. 
         [0188]    On the other hand, the low thermal mass of the heater elements  10 . 1 ,  10 . 2  allows them to be heated to reach, very quickly, the temperature at which bubbles  12  are formed and at which drops  16  are ejected. While the maximum effective pulse width is limited, by the onset of bubble nucleation, typically to around 0.5 microseconds, the minimum pulse width is limited only by the available current drive and the current density that can be tolerated by the heater elements  10 . 1 ,  10 . 2 . 
         [0189]    As shown in  FIG. 51 , the two heaters elements  10 . 1 ,  10 . 2  are connected to two respective drive circuits  70 . Although these circuits  70  may be identical to each other, a further adjustment can be effected by way of these circuits, for example by sizing the drive transistor (not shown) connected to the lower element  10 . 2 , which is the high current element, larger than that connected to the upper element  10 . 1 . If, for example, the relative currents provided to the respective elements  10 . 1 ,  10 . 2  are in the ratio 2:1, the drive transistor of the circuit  70  connected to the lower element  10 . 2  would typically be twice the width of the drive transistor (also not shown) of the circuit  70  connected to the other element  10 . 1 . 
         [0190]    In the prior art described in relation to  FIG. 52 , the heater elements  10 . 1 ,  10 . 2 , which are in the same layer, are produced simultaneously in the same step of the lithographic manufacturing process. In the embodiment of the present invention illustrated in  FIG. 51 , the two heaters elements  10 . 1 ,  10 . 2 , as mentioned above, are formed one after the other. Indeed, as described in the process illustrated with reference to  FIGS. 6 to 31 , the material to form the element  10 . 2  is deposited and is then etched in the lithographic process, whereafter a sacrificial layer  39  is deposited on top of that element, and then the material for the other element  10 . 1  is deposited so that the sacrificial layer is between the two heater element layers. The layer of the second element  10 . 1  is etched by a second lithographic step, and the sacrificial layer  39  is removed. 
         [0191]    Referring once again to the different sizes of the heater elements  10 . 1  and  10 . 2 , as mentioned above, this has the advantage that it enables the elements to be sized so as to achieve multiple, binary weighted drop volumes from one nozzle  3 . 
         [0192]    It will be appreciated that, where multiple drop volumes can be achieved, and especially if they are binary weighted, then photographic quality can be obtained while using fewer printed dots, and at a lower print resolution. 
         [0193]    Furthermore, under the same circumstances, higher speed printing can be achieved. That is, instead of just ejecting one drop  14  and then waiting for the nozzle  3  to refill, the equivalent of one, two, or three drops might be ejected. Assuming that the available refill speed of the nozzle  3  is not a limiting factor, ink ejection, and hence printing, up to three times faster, may be achieved. In practice, however, the nozzle refill time will typically be a limiting factor. In this case, the nozzle  3  will take slightly longer to refill when a triple volume of drop  16  (relative to the minimum size drop) has been ejected than when only a minimum volume drop has been ejected. However, in practice it will not take as much as three times as long to refill. This is due to the inertial dynamics and the surface tension of the ink  11 . 
         [0194]    Referring to  FIG. 53 , there is shown, schematically, a pair of adjacent unit cells  1 . 1  and  1 . 2 , the cell on the left  1 . 1  representing the nozzle  3  after a larger volume of drop  16  has been ejected, and that on the right  1 . 2 , after a drop of smaller volume has been ejected. In the case of the larger drop  16 , the curvature of the air bubble  71  that has formed inside the partially emptied nozzle  3 . 1  is larger than in the case of air bubble  72  that has formed after the smaller volume drop has been ejected from the nozzle  3 . 2  of the other unit cell  1 . 2 . 
         [0195]    The higher curvature of the air bubble  71  in the unit cell  1 . 1  results in a greater surface tension force which tends to draw the ink  11 , from the refill passage  9  towards the nozzle  3  and into the chamber  7 . 1 , as indicated by the arrow  73 . This gives rise to a shorter refilling time. As the chamber  7 . 1  refills, it reaches a stage, designated  74 , where the condition is similar to that in the adjacent unit cell  1 . 2 . In this condition, the chamber  7 . 1  of the unit cell  1 . 1  is partially refilled and the surface tension force has therefore reduced. This results in the refill speed slowing down even though, at this stage, when this condition is reached in that unit cell  1 . 1 , a flow of liquid into the chamber  7 . 1 , with its associated momentum, has been established. The overall effect of this is that, although it takes longer to completely fill the chamber  7 . 1  and nozzle  3 . 1  from a time when the air bubble  71  is present than from when the condition  74  is present, even if the volume to be refilled is three times larger, it does not take as much as three times longer to refill the chamber  7 . 1  and nozzle  3 . 1 . 
         [0000]    Heater Elements Formed from Materials Constituted by Elements with Low Atomic-Numbers 
         [0196]    This feature involves the heater elements  10  being formed of solid material, at least 90% of which, by weight, is constituted by one or more periodic elements having an atomic number below 50. In a preferred embodiment the atomic weight is below 30, while in another embodiment the atomic weight is below 23. 
         [0197]    The advantage of a low atomic number is that the atoms of that material have a lower mass, and therefore less energy is required to raise the temperature of the heater elements  10 . This is because, as will be understood by those skilled in the art, the temperature of an article is essentially related to the state of movement of the nuclei of the atoms. Accordingly, it will require more energy to raise the temperature, and thereby induce such a nucleus movement, in a material with atoms having heavier nuclei that in a material having atoms with lighter nuclei. 
         [0198]    Materials currently used for the heater elements of thermal ink jet systems include tantalum aluminum alloy (for example used by Hewlett Packard), and hafnium boride (for example used by Canon). Tantalum and hafnium have atomic numbers 73 and 72, respectively, while the material used in the Memjet heater elements  10  of the present invention is titanium nitride. Titanium has an atomic number of 22 and nitrogen has an atomic number of 7, these materials therefore being significantly lighter than those of the relevant prior art device materials. 
         [0199]    Boron and aluminum, which form part of hafnium boride and tantalum aluminum, respectively, like nitrogen, are relatively light materials. However, the density of tantalum nitride is 16.3 g/cm 3 , while that of titanium nitride (which includes titanium in place of tantalum) is 5.22 g/cm 3 . Thus, because tantalum nitride has a density of approximately three times that of the titanium nitride, titanium nitride will require approximately three time less energy to heat than tantalum nitride. As will be understood by a person skilled in the art, the difference in energy in a material at two different temperatures is represented by the following equation: 
         [0000]        E=ΔT×C   p   ×V   OL ×ρ, 
         [0000]    where ΔT represents the temperature difference, C p  is the specific heat capacity, V OL  is the volume, and ρ is the density of the material. Although the density is not determined only by the atomic numbers as it is also a function of the lattice constants, the density is strongly influenced by the atomic numbers of the materials involved, and hence is a key aspect of the feature under discussion. 
       Low Heater Mass 
       [0200]    This feature involves the heater elements  10  being configured such that the mass of solid material of each heater element that is heated above the boiling point of the bubble forming liquid (i.e. the ink  11  in this embodiment) to heat the ink so as to generate bubbles  12  therein to cause an ink drop  16  to be ejected, is less than 10 nanograms. 
         [0201]    In one preferred embodiment, the mass is less that 2 nanograms, in another embodiment the mass is less than 500 picograms, and in yet another embodiment the mass is less than 250 picograms. 
         [0202]    The above feature constitutes a significant advantage over prior art inkjet systems, as it results in an increased efficiency as a result of the reduction in energy lost in heating the solid materials of the heater elements  10 . This feature is enabled due to the use of heater element materials having low densities, due to the relatively small size of the elements  10 , and due to the heater elements being in the form of suspended beams which are not embedded in other materials, as illustrated, for example, in  FIG. 1 . 
         [0203]      FIG. 34  shows the shape, in plan view, of a mask for forming the heater structure of the embodiment of the printhead shown in  FIG. 33 . Accordingly, as  FIG. 34  represents the shape of the heater element  10  of that embodiment, it is now referred to in discussing that heater element. The heater element as represented by reference numeral  10 . 34  in  FIG. 34  has just a single loop  49  which is 2 microns wide and 0.25 microns thick. It has a 6 micron outer radius and a 4 micron inner radius. The total heater mass is 82 picograms. The corresponding element  10 . 2  similarly represented by reference numeral  10 . 39  in  FIG. 39  has a mass of 229.6 picograms and that heater element represented by reference numeral  10 . 36  in  FIG. 36  has a mass of 225.5 picograms. 
         [0204]    When the elements  10 . 1 ,  10 . 2  represented in  FIGS. 38 and 39 , for example, are used in practice, the total mass of material of each such element which is in thermal contact with the ink  11  (being the bubble forming liquid in this embodiment) that is raised to a temperature above that of the boiling point of the ink, will be slightly higher than the above discussed masses as the elements will be coated with an electrically insulating, chemically inert, thermally conductive material. This coating increases, to some extent, the total mass of material raised to the higher temperature. 
       Conformally Coated Heater Element 
       [0205]    This feature involves each element  10  being covered by a conformal protective coating, this coating having been applied to all sides of the element simultaneously so that the coating is seamless. The coating  10 , preferably, is electrically non-conductive, is chemically inert and has a high thermal conductivity. In one preferred embodiment, the coating is of aluminum nitride, in another embodiment it is of diamond-like carbon (DLC), and in yet another embodiment it is of boron nitride. 
         [0206]    Referring to  FIGS. 54 and 55 , there are shown schematic representations of a prior art heater element  10  that is not conformally coated as discussed above, but which has been deposited on a substrate  78  and which, in the typical manner, has then been conformally coated on one side with a CVD material, designated  76 . In contrast, the coating referred to above in the present instance, as reflected schematically in  FIG. 56 , this coating being designated  77 , involves conformally coating the element on all sides simultaneously. However, this conformal coating  77  on all sides can only be achieved if the element  10 , when being so coated, is a structure isolated from other structures—i.e. in the form of a suspended beam, so that there is access to all of the sides of the element. 
         [0207]    It is to be understood that when reference is made to conformally coating the element  10  on all sides, this excludes the ends of the element (suspended beam) which are joined to the electrodes  15  as indicated diagrammatically in  FIG. 57 . In other words, what is meant by conformally coating the element  10  on all sides is, essentially, that the element is fully surrounded by the conformal coating along the length of the element. 
         [0208]    The primary advantage of conformally coating the heater element  10  may be understood with reference, once again, to  FIGS. 54 and 55 . As can be seen, when the conformal coating  76  is applied, the substrate  78  on which the heater element  10  was deposited (i.e. formed) effectively constitutes the coating for the element on the side opposite the conformally applied coating. The depositing of the conformal coating  76  on the heater element  10  which is, in turn, supported on the substrate  78 , results in a seam  79  being formed. This seam  79  may constitute a weak point, where oxides and other undesirable products might form, or where delamination may occur. Indeed, in the case of the heater element  10  of  FIGS. 54 and 55 , where etching is conducted to separate the heater element and its coating  76  from the substrate  78  below, so as to render the element in the form of a suspended beam, ingress of liquid or hydroxyl ions may result, even though such materials could not penetrate the actual material of the coating  76 , or of the substrate  78 . 
         [0209]    The materials mentioned above (i.e. aluminum nitride or diamond-like carbon (DLC)) are suitable for use in the conformal coating  77  of the present invention as illustrated in  FIG. 56  due to their desirably high thermal conductivities, their high level of chemical inertness, and the fact that they are electrically non-conductive. Another suitable material, for these purposes, is boron nitride, also referred to above. Although the choice of material used for the coating  77  is important in relation to achieving the desired performance characteristics, materials other than those mentioned, where they have suitable characteristics, may be used instead. 
         [0000]    Example Printer in which the Printhead is Used 
         [0210]    The components described above form part of a printhead assembly shown in  FIG. 62 to 69 . The printhead assembly  19  is used in a printer system  140  shown in  FIG. 70 . The printhead assembly  19  includes a number of printhead modules  80  shown in detail in  FIGS. 58 to 61 . These aspects are described below. 
         [0211]    Referring briefly to  FIG. 44 , the array of nozzles  3  shown is disposed on the printhead chip (not shown), with drive transistors, drive shift registers, and so on (not shown), included on the same chip, which reduces the number of connections required on the chip. 
         [0212]      FIGS. 58 and 59  show an exploded view and a non-exploded view, respectively, a printhead module assembly  80  which includes a MEMS printhead chip assembly  81  (also referred to below as a chip). On a typical chip assembly  81  such as that shown, there are 7680 nozzles, which are spaced so as to be capable of printing with a resolution of 1600 dots per inch. The chip  81  is also configured to eject 6 different colors or types of ink  11 . 
         [0213]    A flexible printed circuit board (PCB)  82  is electrically connected to the chip  81 , for supplying both power and data to the chip. The chip  81  is bonded onto a stainless-steel upper layer sheet  83 , so as to overlie an array of holes  84  etched in this sheet. The chip  81  itself is a multi-layer stack of silicon which has ink channels (not shown) in the bottom layer of silicon  85 , these channels being aligned with the holes  84 . 
         [0214]    The chip  81  is approximately 1 mm in width and 21 mm in length. This length is determined by the width of the field of the stepper that is used to fabricate the chip  81 . The sheet  83  has channels  86  (only some of which are shown as hidden detail) which are etched on the underside of the sheet as shown in  FIG. 58 . The channels  86  extend as shown so that their ends align with holes  87  in a mid-layer  88 . The channels  86  align with respective holes  87 . The holes  87 , in turn, align with channels  89  in a lower layer  90 . Each channel  89  carries a different respective color of ink, except for the last channel, designated  91 . This last channel  91  is an air channel and is aligned with further holes  92  in the mid-layer  88 , which in turn are aligned with further holes  93  in the upper layer sheet  83 . These holes  93  are aligned with the inner parts  94  of slots  95  in a top channel layer  96 , so that these inner parts are aligned with, and therefore in fluid-flow communication with, the air channel  91 , as indicated by the dashed line  97 . 
         [0215]    The lower layer  90  has holes  98  opening into the channels  89  and channel  91 . Compressed filtered air from an air source (not shown) enters the channel  91  through the relevant hole  98 , and then passes through the holes  92  and  93  and slots  95 , in the mid layer  88 , the sheet  83  and the top channel layer  96 , respectively, and is then blown into the side  99  of the chip assembly  81 , from where it is forced out, at  100 , through a nozzle guard  101  which covers the nozzles, to keep the nozzles clear of paper dust. Differently colored inks  11  (not shown) pass through the holes  98  of the lower layer  90 , into the channels  89 , and then through respective holes  87 , then along respective channels  86  in the underside of the upper layer sheet  83 , through respective holes  84  of that sheet, and then through the slots  95 , to the chip  81 . It will be noted that there are just seven of the holes  98  in the lower layer  90  (one for each color of ink and one for the compressed air) via which the ink and air is passed to the chip  81 , the ink being directed to the 7680 nozzles on the chip. 
         [0216]      FIG. 60 , in which a side view of the printhead module assembly  80  of  FIGS. 58 and 59  is schematically shown, is now referred to. The center layer  102  of the chip assembly is the layer where the 7680 nozzles and their associated drive circuitry is disposed. The top layer of the chip assembly, which constitutes the nozzle guard  101 , enables the filtered compressed air to be directed so as to keep the nozzle guard holes  104  (which are represented schematically by dashed lines) clear of paper dust. 
         [0217]    The lower layer  105  is of silicon and has ink channels etched in it. These ink channels are aligned with the holes  84  in the stainless steel upper layer sheet  83 . The sheet  83  receives ink and compressed air from the lower layer  90  as described above, and then directs the ink and air to the chip  81 . The need to funnel the ink and air from where it is received by the lower layer  90 , via the mid-layer  88  and upper layer  83  to the chip assembly  81 , is because it would otherwise be impractical to align the large number (7680) of very small nozzles  3  with the larger, less accurate holes  98  in the lower layer  90 . 
         [0218]    The flex PCB  82  is connected to the shift registers and other circuitry (not shown) located on the layer  102  of chip assembly  81 . The chip assembly  81  is bonded by wires  106  onto the PCB flex and these wires are then encapsulated in an epoxy  107 . To effect this encapsulating, a dam  108  is provided. This allows the epoxy  107  to be applied to fill the space between the dam  108  and the chip assembly  81  so that the wires  106  are embedded in the epoxy. Once the epoxy  107  has hardened, it protects the wire bonding structure from contamination by paper and dust, and from mechanical contact. 
         [0219]    Referring to  FIG. 62 , there is shown schematically, in an exploded view, a printhead assembly  19 , which includes, among other components, printhead module assemblies  80  as described above. The printhead assembly  19  is configured for a page-width printer, suitable for A4 or US letter type paper. 
         [0220]    The printhead assembly  19  includes eleven of the printhead modules assemblies  80 , which are glued onto a substrate channel  110  in the form of a bent metal plate. A series of groups of seven holes each, designated by the reference numerals  111 , are provided to supply the 6 different colors of ink and the compressed air to the chip assemblies  81 . An extruded flexible ink hose  112  is glued into place in the channel  110 . It will be noted that the hose  112  includes holes  113  therein. These holes  113  are not present when the hose  112  is first connected to the channel  110 , but are formed thereafter by way of melting, by forcing a hot wire structure (not shown) through the holes  111 , which holes then serve as guides to fix the positions at which the holes  113  are melted. When the printhead assembly  19  is assembled, the holes  113  are in fluid-flow communication with the holes  98  in the lower layer  90  of each printhead module assembly  80 , via holes  114  (which make up the groups  111  in the channel  110 ). 
         [0221]    The hose  112  defines parallel channels  115  which extend the length of the hose. At one end  116 , the hose  112  is connected to ink containers (not shown), and at the opposite end  117 , there is provided a channel extrusion cap  118 , which serves to plug, and thereby close, that end of the hose. 
         [0222]    A metal top support plate  119  supports and locates the channel  110  and hose  112 , and serves as a back plate for these. The channel  110  and hose  112 , in turn, exert pressure onto an assembly  120  which includes flex printed circuits. The plate  119  has tabs  121  which extend through notches  122  in the downwardly extending wall  123  of the channel  110 , to locate the channel and plate with respect to each other. 
         [0223]    An extrusion  124  is provided to locate copper bus bars  125 . Although the energy required to operate a printhead according to the present invention is an order of magnitude lower than that of known thermal ink jet printers, there are a total of about 88,000 nozzles in the printhead array, and this is approximately 160 times the number of nozzles that are typically found in typical printheads. As the nozzles in the present invention may be operational (i.e. may fire) on a continuous basis during operation, the total power consumption will be an order of magnitude higher than that in such known printheads, and the current requirements will, accordingly, be high, even though the power consumption per nozzle will be an order of magnitude lower than that in the known printheads. The busbars  125  are suitable for providing for such power requirements, and have power leads  126  soldered to them. 
         [0224]    Compressible conductive strips  127  are provided to abut with contacts  128  on the upperside, as shown, of the lower parts of the flex PCBs  82  of the printhead module assemblies  80 . The PCBs  82  extend from the chip assemblies  81 , around the channel  110 , the support plate  119 , the extrusion  124  and busbars  126 , to a position below the strips  127  so that the contacts  128  are positioned below, and in contact with, the strips  127 . 
         [0225]    Each PCB  82  is double-sided and plated-through. Data connections  129  (indicated schematically by dashed lines), which are located on the outer surface of the PCB  82  abut with contact spots  130  (only some of which are shown schematically) on a flex PCB  131  which, in turn, includes a data bus and edge connectors  132  which are formed as part of the flex itself. Data is fed to the PCBs  131  via the edge connectors  132 . 
         [0226]    A metal plate  133  is provided so that it, together with the channel  110 , can keep all of the components of the printhead assembly  19  together. In this regard, the channel  110  includes twist tabs  134  which extend through slots  135  in the plate  133  when the assembly  19  is put together, and are then twisted through approximately 45 degrees to prevent them from being withdrawn through the slots. 
         [0227]    By way of summary, with reference to  FIG. 68 , the printhead assembly  19  is shown in an assembled state. Ink and compressed air are supplied via the hose  112  at  136 , power is supplied via the leads  126 , and data is provided to the printhead chip assemblies  81  via the edge connectors  132 . The printhead chip assemblies  81  are located on the eleven printhead module assemblies  80 , which include the PCBs  82 . 
         [0228]    Mounting holes  137  are provided for mounting the printhead assembly  19  in place in a printer (not shown). The effective length of the printhead assembly  19 , represented by the distance  138 , is just over the width of an A4 page (that is, about 8.5 inches). 
         [0229]    Referring to  FIG. 69 , there is shown, schematically, a cross-section through the assembled printhead  19 . From this, the position of a silicon stack forming a chip assembly  81  can clearly be seen, as can a longitudinal section through the ink and air supply hose  112 . Also clear to see is the abutment of the compressible strip  127  which makes contact above with the busbars  125 , and below with the lower part of a flex PCB  82  extending from a the chip assembly  81 . The twist tabs  134  which extend through the slots  135  in the metal plate  133  can also be seen, including their twisted configuration, represented by the dashed line  139 . 
       Printer System 
       [0230]    Referring to  FIG. 70 , there is shown a block diagram illustrating a printhead system  140  according to an embodiment of the invention. 
         [0231]    Shown in the block diagram is the printhead  141 , a power supply  142  to the printhead, an ink supply  143 , and print data  144  (represented by the arrow) which is fed to the printhead as it ejects ink, at  145 , onto print media in the form, for example, of paper  146 . 
         [0232]    Media transport rollers  147  are provided to transport the paper  146  past the printhead  141 . A media pick up mechanism  148  is configured to withdraw a sheet of paper  146  from a media tray  149 . 
         [0233]    The power supply  142  is for providing DC voltage which is a standard type of supply in printer devices. 
         [0234]    The ink supply  143  is from ink cartridges (not shown) and, typically various types of information will be provided, at  150 , about the ink supply, such as the amount of ink remaining. This information is provided via a system controller  151  which is connected to a user interface  152 . The interface  152  typically consists of a number of buttons (not shown), such as a “print” button, “page advance” button, an so on. The system controller  151  also controls a motor  153  that is provided for driving the media pick up mechanism  148  and a motor  154  for driving the media transport rollers  147 . 
         [0235]    It is necessary for the system controller  151  to identify when a sheet of paper  146  is moving past the printhead  141 , so that printing can be effected at the correct time. This time can be related to a specific time that has elapsed after the media pick up mechanism  148  has picked up the sheet of paper  146 . Preferably, however, a paper sensor (not shown) is provided, which is connected to the system controller  151  so that when the sheet of paper  146  reaches a certain position relative to the printhead  141 , the system controller can effect printing. Printing is effected by triggering a print data formatter  155  which provides the print data  144  to the printhead  141 . It will therefore be appreciated that the system controller  151  must also interact with the print data formatter  155 . 
         [0236]    The print data  144  emanates from an external computer (not shown) connected at  156 , and may be transmitted via any of a number of different connection means, such as a USB connection, an ETHERNET connection, a IEEE1394 connection otherwise known as firewire, or a parallel connection. A data communications module  157  provides this data to the print data formatter  155  and provides control information to the system controller  151 . 
       FEATURES AND ADVANTAGES OF FURTHER EMBODIMENTS 
       [0237]      FIGS. 71 to 94  show further embodiments of unit cells  1  for thermal inkjet printheads, each embodiment having its own particular functional advantages. These advantages will be discussed in detail below, with reference to each individual embodiment. However, the basic construction of each embodiment is best shown in  FIGS. 72 ,  74 ,  76  and  79 . The manufacturing process is substantially the same as that described above in relation to  FIGS. 6 to 31  and for consistency, the same reference numerals are used in  FIGS. 71 to 94  to indicate corresponding components. In the interests of brevity, the fabrication stages have been shown for the unit cell of  FIG. 78  only (see  FIGS. 80 to 90 ). It will be appreciated that the other unit cells will use the same fabrication stages with different masking. Again, the deposition of successive layers shown in  FIGS. 80 to 90  need not be described in detail below given that the lithographic process largely corresponds to that shown in  FIGS. 6 to 31 . 
         [0238]    Referring to  FIGS. 71 and 72 , the unit cell  1  shown has the chamber  7 , ink supply passage  32  and the nozzle rim  4  positioned mid way along the length of the unit cell  1 . As best seen in  FIG. 72 , the drive circuitry is partially on one side of the chamber  7  with the remainder on the opposing side of the chamber. The drive circuitry  22  controls the operation of the heater  14  through vias in the integrated circuit metallisation layers of the interconnect  23 . The interconnect  23  has a raised metal layer on its top surface. Passivation layer  24  is formed in top of the interconnect  23  but leaves areas of the raised metal layer exposed. Electrodes  15  of the heater  14  contact the exposed metal areas to supply power to the element  10 . 
         [0239]    Alternatively, the drive circuitry  22  for one unit cell is not on opposing sides of the heater element that it controls. All the drive circuitry  22  for the heater  14  of one unit cell is in a single, undivided area that is offset from the heater. That is, the drive circuitry  22  is partially overlaid by one of the electrodes  15  of the heater  14  that it is controlling, and partially overlaid by one or more of the heater electrodes  15  from adjacent unit cells. In this situation, the center of the drive circuitry  22  is less than 200 microns from the center of the associate nozzle aperture  5 . In most Memjet printheads of this type, the offset is less than 100 microns and in many cases less than 50 microns, preferably less than 30 microns. 
         [0240]    Configuring the nozzle components so that there is significant overlap between the electrodes and the drive circuitry provides a compact design with high nozzle density (nozzles per unit area of the nozzle plate  2 ). This also improves the efficiency of the printhead by shortening the length of the conductors from the circuitry to the electrodes. The shorter conductors have less resistance and therefore dissipate less energy. 
         [0241]    The high degree of overlap between the electrodes  15  and the drive circuitry  22  also allows more vias between the heater material and the CMOS metalization layers of the interconnect  23 . As best shown in  FIGS. 79 and 80 , the passivation layer  24  has an array of vias to establish an electrical connection with the heater  14 . More vias lowers the resistance between the heater electrodes  15  and the interconnect layer  23  which reduces power losses. 
         [0242]    In  FIGS. 73 and 74 , the unit cell  1  is the same as that of  FIGS. 71 and 72  apart from the heater element  10 . The heater element  10  has a bubble nucleation section  158  with a smaller cross section than the remainder of the element. The bubble nucleation section  158  has a greater resistance and heats to a temperature above the boiling point of the ink before the remainder of the element  10 . The gas bubble nucleates at this region and subsequently grows to surround the rest of the element  10 . By controlling the bubble nucleation and growth, the trajectory of the ejected drop is more predictable. 
         [0243]    The heater element  10  is configured to accommodate thermal expansion in a specific manner. As heater elements expand, they will deform to relieve the strain. Elements such as that shown in  FIGS. 71 and 72  will bow out of the plane of lamination because its thickness is the thinnest cross sectional dimension and therefore has the least bending resistance. Repeated bending of the element can lead to the formation of cracks, especially at sharp corners, which can ultimately lead to failure. The heater element  10  shown in  FIGS. 73 and 74  is configured so that the thermal expansion is relieved by rotation of the bubble nucleation section  158 , and slightly splaying the sections leading to the electrodes  15 , in preference to bowing out of the plane of lamination. The geometry of the element is such that miniscule bending within the plane of lamination is sufficient to relieve the strain of thermal expansion, and such bending occurs in preference to bowing. This gives the heater element greater longevity and reliability by minimizing bend regions, which are prone to oxidation and cracking. 
         [0244]    Referring to  FIGS. 75 and 76 , the heater element  10  used in this unit cell  1  has a serpentine or ‘double omega’ shape. This configuration keeps the gas bubble centered on the axis of the nozzle. A single omega is a simple geometric shape which is beneficial from a fabrication perspective. However the gap  159  between the ends of the heater element means that the heating of the ink in the chamber is slightly asymmetrical. As a result, the gas bubble is slightly skewed to the side opposite the gap  159 . This can in turn affect the trajectory of the ejected drop. The double omega shape provides the heater element with the gap  160  to compensate for the gap  159  so that the symmetry and position of the bubble within the chamber is better controlled and the ejected drop trajectory is more reliable. 
         [0245]      FIG. 77  shows a heater element  10  with a single omega shape. As discussed above, the simplicity of this shape has significant advantages during lithographic fabrication. It can be a single current path that is relatively wide and therefore less affected by any inherent inaccuracies in the deposition of the heater material. The inherent inaccuracies of the equipment used to deposit the heater material result in variations in the dimensions of the element. However, these tolerances are fixed values so the resulting variations in the dimensions of a relatively wide component are proportionally less than the variations for a thinner component. It will be appreciated that proportionally large changes of components dimensions will have a greater effect on their intended function. Therefore the performance characteristics of a relatively wide heater element are more reliable than a thinner one. 
         [0246]    The omega shape directs current flow around the axis of the nozzle aperture  5 . This gives good bubble alignment with the aperture for better ejection of drops while ensuring that the bubble collapse point is not on the heater element  10 . As discussed above, this avoids problems caused by cavitation. 
         [0247]    Referring to  FIGS. 78 to 91 , another embodiment of the unit cell  1  is shown together with several stages of the etching and deposition fabrication process. In this embodiment, the heater element  10  is suspended from opposing sides of the chamber. This allows it to be symmetrical about two planes that intersect along the axis of the nozzle aperture  5 . This configuration provides a drop trajectory along the axis of the nozzle aperture  5  while avoiding the cavitation problems discussed above.  FIGS. 92 and 93  show other variations of this type of heater element  10 . 
         [0248]      FIG. 93  shows a unit cell  1  that has the nozzle aperture  5  and the heater element  10  offset from the center of the nozzle chamber  7 . Consequently, the nozzle chamber  7  is larger than the previous embodiments. The heater  14  has two different electrodes  15  with the right hand electrode  15  extending well into the nozzle chamber  7  to support one side of the heater element  10 . This reduces the area of the vias contacting the electrodes which can increase the electrode resistance and therefore the power losses. However, laterally offsetting the heater element from the ink inlet  31  increases the fluidic drag retarding flow back through the inlet  31  and ink supply passage  32 . The fluidic drag through the nozzle aperture  5  comparatively much smaller so little energy is lost to a reverse flow of ink through the inlet when a gas bubble form on the element  10 . 
         [0249]    The unit cell  1  shown in  FIG. 94  also has a relatively large chamber  7  which again reduces the surface area of the electrodes in contact with the vias leading to the interconnect layer  23 . However, the larger chamber  7  allows several heater elements  10  offset from the nozzle aperture  5 . The arrangement shown uses two heater elements  10 ; one on either side of the chamber  7 . Other designs use three or more elements in the chamber. Gas bubbles nucleate from opposing sides of the nozzle aperture and converge to form a single bubble. The bubble formed is symmetrical about at least one plane extending along the nozzle axis. This enhances the control of the symmetry and position of the bubble within the chamber  7  and therefore the ejected drop trajectory is more reliable. 
         [0250]    Although the invention is described above with reference to specific embodiments, it will be understood by those skilled in the art that the invention may be embodied in many other forms. For example, although the above embodiments refer to the heater elements being electrically actuated, non-electrically actuated elements may also be used in embodiments, where appropriate.
Provided is a pagewidth printhead assembly having a plurality of printhead modules. Each of these modules includes a printhead integrated circuit (IC) having a plurality of ink ejection nozzles, and an ink distribution assembly formed from a stack of ink distribution layers. Each layer defines apertures therein at predetermined locations, with the layers laminated together so that the apertures complementarily define ink distribution channels to the printhead IC. Each module also includes a tape automated bonding (TAB) film connected to the printhead IC for arranging said IC in signal communication with a controller of the printhead assembly.