reference will now be made in details to the preferred embodiments of the present invention , examples of which are illustrated in the accompanying drawings . wherever possible , the same reference numbers are used in the drawings and the description to refer to the same or like parts . the embodiments described below are illustrated to demonstrate the technical contents and characteristics of the present invention and to enable the persons skilled in the art to understand , make , and use the present invention . however , it shall be noticed that , it is not intended to limit the scope of the present invention . therefore , any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention . in order to achieve the purpose of eliminating electromagnetic interferences and having electrostatic protection effect , the proposed structure of the present invention provides an integrated circuit common - mode emi filter incorporating esd protection , in which the two coupled spiral inductor coils can be formed in typical ic back - end processes . considering that a cmf filter serves to suppress the common - mode noises as much as possible and to pass differential signals with as little insertion loss as possible , as illustrated in fig2 , the il for the differential signals , characterized by transmission gain ( s 21 ) for the signals measured in s - parameters ( entitled s dd21 ) should be very low within the pass - band , while the common - mode rejection , characterized by common - mode transmission gain ( entitled s cc21 ) must be as high as possible . an alternative expression of these features are the transmission impedance as depicted in fig3 , where the differential mode impedance ( entitled z dm ) should be as low as possible within the pass - band , while the common - mode impedance ( entitled z cm ) must be as high as possible . refer to fig4 , which shows a layout illustration of a cmf filter circuit structure in accordance with one embodiment of the present invention . in the embodiment , a primary spiral inductor coil 10 is disposed corresponding to a secondary spiral inductor coil 20 , and these two spiral inductor coils are separated by a dielectric layer 40 ( see fig1 a and fig1 b ). the primary spiral inductor coil 10 , the secondary spiral inductor coil 20 and the dielectric layer 40 are disposed on top of a high - resistance substrate 30 . a resistivity ( ρ ) of the substrate 30 is more than 100 ω - cm ( ohm - cm ). according to the embodiment of the present invention , the high - ρ substrate 30 serves to suppress the eddy current that is a main source for electrical energy loss and the high - ρ substrate 30 can be made of silicon in a preferred embodiment . fig5 shows an equivalent circuit model for the cmf filter , wherein l is inductance for the primary and secondary inductor spirals , respectively , r s is series resistance of the inductor spiral , ci is coupling capacitance between the two coupled inductor spirals , cp is other parasitic capacitor associated with each inductor spiral , and k is the magnetic coupling coefficient of the two inductor spirals . fig6 a and 6b show the equivalent circuit model of the cmf operating in a common - mode and in a differential - mode , respectively . according to fig6 a , the common - mode impedance can be expressed as , similarly , according to fig6 b , the differential - mode impedance can be expressed as , for an ideally coupled cmf structure , we have k = 1 , which means no loss of magnetic flux between the two spirals . unlike in discrete ferrite or ceramics based cmf structures where magnetic cores serve to enhance magnetic coupling , it is generally difficult to integrate magnetic materials into ic processes . hence , the cmf structure in the first embodiment does not have any magnetic medium integrated . as illustrated in fig7 , to achieve good magnetic coupling inside an ic cmf structure ( i . e ., a higher k ), careful layout design is required to ensure exact same spiral layout shape for the two inductor coils , which should be overlapping each other closely . in addition , a vertical distance ( d ) between the two inductor spirals in the thickness direction should be as short as possible to ensure excellent magnetic coupling , hence the highest possible k value . however , a smaller d directly results in a larger coupling capacitance ci , which dramatically degrades the signal integrity of the differential - mode signals as shown in fig6 b . therefore , in order to balance the design trade - off between k and ci , the distance d must be carefully designed to achieve overall cmf filter performance . fig8 a - 8c depicts the s 21 behaviors related to d as simulated for several split values for the vertical distance between the two inductor spirals . obviously , the coupling effect decreases as d increases , i . e ., k d = 2 um & gt ; k d = 6 um & gt ; k d = 10 um . it is clearly observed that the d has minor impact on the common - mode s cc21 within the concerned distance range as shown in fig8 b and 8c . however , d has significant influence on the differential - mode s dd21 as shown in fig8 a , i . e ., ci with d = 2 μm is too large while k for d = 10 μm is too small , both will significantly affect s dd21 . it is found that a well - balanced inter - spiral distance , for example , d = 6 μm in this embodiment , shall achieve an optimum cmf filter performance . in order to reduce insertion loss to differential mode signals , the metal spiral series resistance must be minimized . therefore , very thick metal line thickness ( up to 10 μm ), as opposed to normal metal interconnect thickness of 1 - 3 μm used in standard ic processes , should be used in cmf designs . meanwhile , metals with low - resistance , such as copper ( cu ) and silver ( ag ), etc ., can also be used to reduce metal wire resistance compared with aluminum ( al ). moreover , please refer to fig9 a and fig9 b for the second and third embodiment of the present invention , wherein fig9 a shows an embodiment of the cmf filter including the primary spiral inductor coil and the secondary spiral inductor coil having the same layout shapes and completely overlapping each other vertically . fig9 b shows an embodiment of the cmf filter including the primary spiral inductor coil and the secondary spiral inductor coil having the same layout shapes , but do not overlap each other completely . as shown in the embodiments of fig9 a , since the primary spiral inductor coil 10 and the secondary spiral inductor coil 20 have the same layout shapes and completely overlap each other vertically , the cmf filter achieves the highest coupling effect of k ≧ 0 . 97 . however , the coupling capacitance c i becomes very large , which results in severe degradation of differential mode signals and very low f c . another major problem associated to the complete - overlap layout design corresponding to fig9 a is shown in fig1 a where since very thick metal wires ( e . g ., 10 μm ) are needed for the primary and secondary spirals , it will cause serious surface uniformity issue in ic back - end processing and requires complicated and expensive process steps to achieve good surface flatness . on the other hand , fig1 b shows an alternative non - overlap spiral layout corresponding to fig9 b , which significantly reduce the surface roughness and improve the surface uniformity . compared fig1 b to fig1 a , it is easily seen that the distance h2 is much shorter than the distance h1 . another major benefit of the non - overlap layout design is that the coupling capacitance c i can be significantly reduced . meanwhile , the inductive coupling coefficient k only slightly decreases , i . e ., k & lt ; 0 . 01 reduction observed in this design . to simultaneously achieve high inductive coupling and low coupling capacitance , the dielectric materials must have very low dielectric constant ( i . e ., permittivity ) and high magnetic permeability . refer to fig1 for the fourth embodiment of the present invention , in which fig1 shows a layout illustration of an exemplar 4 - metal - layer cmf filter circuit structure in accordance with one embodiment of the present invention . in this embodiment , the primary spiral inductor coil is implemented by using multiple metal layers , i . e ., the metal layers 1 and 3 ( from the bottom ), while the secondary spiral inductor coil is implemented by using the metal layer 2 and 4 . in the fourth embodiment , multiple - layer metal spirals ( i . e ., stacked spirals 1 - 4 ), separated by dielectric layers , are used to form both primary and secondary inductor coils that occupy alternative layers . the multiple - layer stacked spiral structures not only deliver the required very high inductance values ( typically more than 100 nh ), but also significantly enhances the inductive coupling effect between the primary and secondary coils due to its inter - digitized layer structure . therefore , the multiple - layer cmf filter as shown in fig1 has much better coupling k factor . further , the two spiral layout designs , i . e ., complete - overlap previously shown in fig9 a and non - overlap in fig9 b , can also be applied to the multiple - layer cmf filter structures . fig1 shows a layout illustration of a same - layer cmf filter circuit structure in accordance with the fifth embodiment of the present invention . referring to fig1 , the primary spiral inductor coil 10 is formed in a first metal layer and the secondary spiral inductor coil 20 is formed in a second metal layer . in the fifth embodiment , the primary 10 and secondary spiral inductor coils 20 have special layout so that the first and second metal coils are formed laterally on the same metal layer . however , in an inter - digitized format , which significantly enhances the inductive coupling effect of the two spiral inductors , the spirals of the two coils shall be insulated by dielectric materials laterally on the same layer . moreover , according to the present invention , special fine - pitch thick - metal re - distribution layer ( rdl ) process techniques are proposed to form very thick ( 10 - 30 μm ) metal lines , including low - resistivity metals such as copper ( cu ), silver ( ag ) or alloys ( e . g ., cu / au / ni ), etc ., to make high - performance cmf filter structures described in the above - mentioned embodiments . the rdl technique makes it easier to fabricate cmf filters with very large inductance ( up to several hundreds of nano henry , nh ) and very low metal resistance , which is difficult to achieve if using normal ic processes . both low - resistivity and very thick metal layer is critical to reduce metal spiral series resistance . therefore , two types of rdl processes may be used according to the present invention . in one embodiment of the present invention , the rdl process is an island - type process , which starts with a dielectric layer deposition followed by metal ( e . g ., cu ) seed sputtering . photo mask is then applied to form the required metal line patterns . then , electrochemical plating ( ecp ) step is used to form the thick cu layer . metal etching follows to form the required thick cu spirals , which looks like an “ island ” on top of the dielectric layer . normally , this island - type rdl technique forms metal lines with relatively wide pitch , e . g ., about 10 μm in line width and spacing . in another embodiment of the present invention , the rdl process is a trench - type process , which starts with dielectric layer deposition . the metal trench photo mask is then applied followed by reactive - ion etching ( rie ) to create fine - pitch trench in the dielectric layer . next , cu seed sputtering is applied and the ecp step is used to fill the trench and form the damascene cu spirals required . this trench - type damascene cu rdl process can achieve both very thick metal line ( 10 - 30 μm ) and very fine metal pitch ( metal line width and spacing less than 5 μm ). the very thick metal spiral helps to reduce the series resistance substantially , while the fine - pitch of metal lines is critical to manufacturing small footprint cmf as demanded by today &# 39 ; s electronics . in addition , various low - resistivity metals and their alloys can also be used to fabricate high - performance cmf filters using the thick - metal rdl techniques . apart from above , the new thick - metal rdl techniques depicted in the present invention may also be used to manufacture multiple - layer cmf filters as previously - described in the fourth embodiment to achieve even better cmf filter performance . thus , the proposed invention is fully described abovementioned , but not limited thereto . refer to fig1 , which shows a layout illustration of a laterally - laid cmf filter circuit structure in accordance with the sixth embodiment of the present invention . according to the sixth embodiment of the present invention , the laterally - laid cmf filter structure is formed in such a way that the primary spiral inductor coil 10 and the secondary spiral inductor coil 20 are two multiple - turn solenoids with the spiral turns of the primary and secondary inductors placed alternatively and next to each other in an inter - digitized format . the primary and secondary inductors 10 , 20 are formed in two metal layers ( i . e ., upper and lower metal layers ) separated by a dielectric layer . the upper metal line of each turn , which is a long line in diagonal direction , is formed with an angel in the top metal layer ; while the lower metal line of each turn , which is also long but not in diagonal direction , is formed in the bottom metal layer . the directions and angles for the upper and lower metal lines can be reversed in layout designs as needed , or modified as necessary . thick through - dielectric metal vias ( vertical metal pillar plugs ) can be used to connect the upper and lower metal lines of the same inductor ( spiral ). the closely coupled multiple - turn transformer with inter - digitized metal turns ensures excellent inductive coupling between the lateral primary and secondary coils , which results in excellent cmf filter performance . as a result , to sum up , the common - mode emi filter ic of the present invention provides high efficiency in removing common - mode noises , while allowing the useful differential signals passing through with minimum loss . also , the common - mode emi filter ic can be integrated with esd protection components and form small footprint cmf ics in single or multiple - chip module formats . the common - mode emi filter ics with integrated esd of the present invention consists of two coupled spiral inductor coils formed in ic back - end processes . it will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the scope or spirit of the invention . in view of the foregoing , it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the invention and its equivalent .