starting with a silicon wafer substrate , fig4 shows a silicon substrate 1 covered with a photoresist mask 2 . the photoresist is then exposed and developed with photolithography technique to produce a patterned mask 3 shown in fig5 . the patterned mask presented here is photoresist but this material could be any patterned material that is chemically non reactive with the processing gases used to create the cavity . the mask needs to show enough strain to hold as a cantilever membrane above the intended cavity . for example , commonly used masks are silicon oxide , silicon nitride , metal and spin on glass . they are usually patterned using photolithography and selective stripping . it then leaves the film as the patterned mask 3 in fig5 . another way to form a mask is to etch throughout the whole depth of a first substrate ( not shown ). this substrate is then bonded to another substrate . the criteria for the masks are always the same : pattern mask that can hold in tension above the cavity , and which the intrinsic nature is etched at a slower rate than the substrate . furthermore , the initial thickness needs to be larger than the thickness involuntary etched during the whole process . after the mask has been created above the substrate , the substrate is then exposed to a spontaneously reacting enchants species that tend to etch isotropically . this is shown in fig6 a . the cavities etched under each opening of the mask 3 will rapidly joint to form a large cavity 4 , shown in fig6 b . to create this cavity , it is preferable to use a dry etching technique . using liquid etching techniques would require special handlings to prevent membrane breaking , membrane stiction or in order to prevent liquid to be trapped in the cavity 4 . furthermore , any possible residues need to be avoided in order to process the next step of the invention . in the present work , the cavity was produced using fluorine - rich plasma . fluorine ions easily react with silicon to form volatile species . it is then pumped away out of the processing chamber and the etching is quasi - isotropic . many other techniques can be used to create a cavity under a mask . to mention a second example of aways to form a cavity under a mask , reference is made to u . s . pat . no . 6 , 712 , 983 , the contents of which are herein incorporated by reference . fig6 c and fig6 d show scanning electron microscopy ( sem ) images of cavities formed under a photoresist mask and a silicon oxide mask respectively . the thick photoresist mask with a square above the oxide in fig6 d is not necessary for the invention . it was used for other purposes . these cavities were produced using the icp system ase hrm from surface technology system ( sts ), with a radio - frequency power ( rf power ) on the coil only ( at 3000 w and 2000 w for fig6 c and 6 d respectively and , no power applied on the substrate holder ), a flow of sulphur hexafluoride ; sf6 , ( 500 sccm and 350 sccm for fig6 c and 6 d respectively ) at a pressure of 60 mtorrs and 40 mtorrs for fig6 c and 6 d respectively . in fig6 c , the parameters were : ( coil power = 3000 w , sf 6 = 500 sccm , pressure = 60 mtorrs , during 36 min ); in fig6 d , the parameters were : ( coil power = 2000 w , sf 6 = 350 sccm , pressure = 40 mtorrs , during 50 min ). finally , as shown in fig7 a , a drie technique is used to create etched structures 5 into the bottom of the cavity that respects the image of the suspended mask . the drie technique needs to be capable of high aspect ratios since the aspect ratios that need to be aimed are the total depth 6 over the mask opening dimension 3 rather than , only the depth of the pattern 5 over the mask opening 3 . many drie techniques can be used as shown in the literature ( u . s . pat . nos . 5 , 501 , 893 , and 6 , 127 , 273 ). this is generally done in an inductively coupled plasma ( icp ) system . fig7 b , 7 c and 7 d show three sem micrographs of patterns in the bottom of a cavity under a photoresist and 2 oxide masks . the cavity and the etched pattern were done sequentially in the icp system ase hrm sts . the anisotropic etch was performed using the technique described in u . s . pat . nos . 5 , 501 , 893 , and 6 , 127 , 273 known as the “ bosch ” technique . in fig7 b , the parameters were : ( isotropic etch during 53 min 30 sec : coil power = 2000 w , sf6 = 350 sccm , pressure = 40 mtorrs , followed by a switched “ bosch ” recipe 30 min of : etch [ 2 . 5 sec coil = 1750 w , platen = 65 w , sf 6 = 450 sccm , o 2 = 45 sccm ]/ passivation [ 2 . 0 sec coil = 1100 w , platen = 0 w , c 4 f 8 = 125 sccm ]); in fig7 c , the parameters were : ( isotropic etch during 39 min : coil power = 2000 w , sf6 = 350 sccm , pressure = 40 mtorrs , followed by a switched “ bosch ” recipe 34 min 30 sec of : etch [ 2 . 5 sec coil = 1750 w , platen = 65 w , sf6 = 450 sccm , o2 = 45 sccm ]/ passivation [ 2 . 0 sec coil = 1100 w , platen = 0 w , c 4 f 8 = 125 sccm ]); in fig7 d , the parameters were : ( isotropic etch during 56 min : coil power = 2000 w , sf6 = 350 sccm , pressure = 40 mtorrs , followed by a switched “ bosch ” recipe 19 min 30 sec of : etch [ 2 . 5 sec coil = 1750 w , platen = 65 w , sf6 = 450 sccm , o2 = 45 sccm ]/ passivation [ 2 . 0 sec coil = 1100 w , platen = 0 w , c4f8 = 125 sccm ]). as shown in fig8 a , the mask can optionally be selectively stripped to show a final cavity with pattern in the bottom without membrane . fig8 b shows a sem micrograph of a real device . when through the wafer vias are needed , fig9 a illustrate the usage of an etch stop 7 . fig9 b shows a sem image of a device made in dalsa ( bromont , canada ) ( silicon oxide is the etch stop in this case ). this etch stop is deposited or grown on the other face of the substrate . this etch stop 7 , is necessary since the back side of the substrate usually needs to be cooled with an inert gas ( generally helium ) that keep the temperature of the substrate stable during drie etching . losing cooling when pinching through would cause loss of dimensions and erosion of the back face . when selecting the proper mask for its selectivity and thickness , one should consider fast erosion of the mask in the region suspended above the cavity . this is due to the loss of the thermal an electrical conduction for this region . chase et al . ( u . s . pat . no . 6 , 500 , 348 b2 ) and rich et al . ( u . s . pat . no . 6 , 685 , 844 b2 ) explain how the pattern etch in a membrane by drie can be rapidly eroded . analogically the mask membrane above a cavity is etched faster than the mask in contact with the substrate . this effect can be minimized by improving thermal and electrical conductance of the mask as explained in u . s . pat . nos . 6 , 500 , 348 b2 and 6 , 685 , 844 . this technique is sometimes not wanted for the design and simply having a thicker mask will bypass this disadvantage . an important feature of the invention is that cavity etching and drie patterning can be done in situ within the same piece of equipment . however , it could be done in two pieces of equipments if desired . using one mask and one etching equipment leads higher throughputs .