Patent Document

RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional application 60/220,709 filed Jul., 26, 2000, which is herein incorporated by reference. 
     
    
     BACKGROUND OF INVENTION  
       [0002]     The invention relates to the field of semiconductor circuits and devices packaging generally, and in particular, Active Packaging of the type described in U.S. Pat. No. 5,496,743 incorporated herein by reference. Active Packaging relates to the bonding of a flip chip onto a carrier. For this purpose, a carrier is any electronic circuit containing structure, such as a wafer, a plate, a printed circuit board or another chip and a flip chip is a circuit containing structure that undergoes partial processing on one side, is then flipped and further processing is performed on the other side of the chip. In Active Packaging, the partially processed flip chip is bonded onto the carrier before processing of the integrated circuit chip is complete. Thus, in a typical scenario, a semiconductor flip chip is partially processed on one side, bonded onto a carrier such that electrical and mechanical connections between the flip chip and carrier are accomplished, then final processing on the other side of the flip chip occurs. Final processing can include lithography, etching, layer deposition, doping, thinning and other processing steps well known to one of ordinary skill in the art. This technique is often used as a preferred alternative to wire bonding two separate circuit-containing parts.  
         [0003]     Several methods other than wire bonding are known for bonding an integrated circuit chip onto a carrier. One technique previously utilized was Z-axis conductive film adhesives. A typical example of this technique is illustrated in  FIG. 1 . There, flip chip  11 , and carrier  15  are electrically and mechanically connected using a Z-axis conductive film adhesive which consists of conductive particles  13  with diameter of 5-100 μm (microns) contained in an adhesive resin  14 . The resin  14 , mechanically holds the carrier wafer  15  to the flip chip  11  and also insulates the conductive particles  13  from one another. Conductive particles  13  mechanically interface with contact pads  12  on the flip chip  11  and carrier  15 , thereby ensuring electrical connection between respective contact pads  12  of the flip chip  11  and those of the carrier  15 .  
         [0004]     This technique suffers from the disadvantage that the number of conductive particles per contact pad is not large, which dictates that large forces will have to be applied between flip chip  11  and carrier  15  in order to ensure sufficient electrical contact between the respective contact pads  12 . This relatively large force creates substantial stress on the flip chip after bonding, which makes the technique unsuitable for the brittle and/or thin flip chips that are used in Active Packaging. Additionally, the differences in thermal expansion coefficients of a thin flip chip and the adhesive resin or epoxy create further mechanical stresses during thermal cycling. Further, in today&#39;s high density integrated circuits, the electrical contact pads are so closely spaced that the conductive particles may be too large to ensure the contact pads are electrically isolated from each other. Finally, the need for an adhesive such as an epoxy or resin to provide mechanical bonding between the flip chip and the carrier creates problems when the back of the flip chip must remain free from contamination so post-bonding processing may occur.  
         [0005]     Another bonding technique known in the art is solder ball and epoxy encapsulation. This technique is illustrated in  FIG. 2A . In this method solder balls  23  are bonded on electrical contact pads  22  of a carrier  25 , and the contact pads  22  of the flip chip  21  are aligned with and soldered to respective ones of the solder balls  23 . The size of the solder ball is typically between 50 and 150 microns. Epoxy resin  24  is applied after soldering to make a stable bonded structure. This method of epoxy encapsulation suffers from the same disadvantages as the Z-axis adhesive film technique previously described.  
         [0006]     In an effort to overcome the problems of contaminating the flip chip with epoxy or other resin, a revised solder ball epoxy bonding technique has been proposed, as illustrated in  FIG. 2B . In this revised technique, the adhesive layer  33  is pre-formed on the carrier  15 . Moreover, the solder balls  31  are first soldered to the contact pads  14  of the flip chip  11 . The solder balls  31  are formed with a pointed end so that they may penetrate the adhesive layer  33  during the mounting process when pressure is applied between the flip chip  11  and the carrier  15 . The pointed solder balls are pressure bonded to respective electrodes  32  of the chip carrier  15  without the adhesive film coming into contact with the flip chip  11 . Although this technique overcomes the problem of contamination of the flip chip by the adhesive material, it still suffers from the other defects previously mentioned, including the presence of large mechanical stresses on the flip chip after bonding.  
         [0007]     A revised approach has been proposed to overcome some of the limitations associated with prior bonding techniques. This approach, illustrated in  FIG. 3 , utilizes a Z-axis oriented multiple metal fibrils or tubules  16  embedded in a soft porous membrane  17 , such as liquid crystal or a polymer. This technique is described in detail in U.S. Pat. Nos. 5,805,424, 5,805,425, 5,805,426 and 5,818,700, which are incorporated herein by reference. According to this technique, the diameter of the metal fibrils  16  and the distance between adjacent fibrils in the membrane is much smaller than the typical spacing between adjacent contact pads  18  on the flip chip  11  and the carrier  15 , and the typical contact pad size. In this manner, many metal fibrils are in electrical contact with each contact pad  18  on the flip chip  11  and carrier  15  so that the electrical contact resistance between opposing contact pads  18  is much smaller than in the conventional Z-axis conductive film techniques. Bonding between the flip chip  11  and carrier  15  is achieved by applying pressure to the thermo-compressible material.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to provide an improved method and system for electrical and mechanical bonding of two circuit containing elements such as a chip carrier and flip chip. This advantage is achieved by the use of an anisotropically conductive bonding interface (referred to herein as an anisotropically conductive layer) which is composed of a rigid insulating substrate or membrane with top and bottom planar surfaces. A plurality of conductive rigid pins are embedded in the substrate, and each pin extends beyond the top and bottom planar surfaces, forming what might appear to be a “bed of nails”. This arrangement provides for electrical conductivity through the pins in the direction normal to the planar surfaces, but not in other directions, since the pins are electrically isolated from one another. The pins are arranged so that when the anisotropically conductive layer is placed between two circuit containing structures, the electrical contact pads on each structure are contacted by a plurality of the pins. When the two circuit containing structures are bonded to one another, a plurality of pins will connect one contact pad on the first circuit containing structure to an associated contact pad on the other circuit containing structure. Moreover, those pins extending beyond the planar surfaces of the conductive layer&#39;s insulating substrate that are not involved in the electrical connection of respective electrical contact pads of the two circuit containing structures act to provide mechanical support to one or both of the circuit containing structures.  
         [0009]     In an exemplary embodiment, the diameter of the portion of the pins that extends beyond the planar surfaces of the insulating substrate is substantially the same as the diameter of the portion of the pins inside the substrate.  
         [0010]     In another exemplary embodiment, the diameter of the portion of the pins that extends beyond the planar surfaces of the insulating substrate may be enlarged compared to the diameter of the portion of the pins inside the substrate.  
         [0011]     In a further preferred embodiment, the diameter of the pins providing mechanical, but not electrical contact are larger than the diameter of the pins providing electrical connection between contact pads.  
         [0012]     In another preferred embodiment, the nominal diameter of the pins is between 0.01 microns and 0.4 microns.  
         [0013]     In another exemplary embodiment, the pins protrude from the planar surfaces of the insulating substrate by an amount substantially equal to the distance the electrical contact pads protrude from the circuit containing structure (i.e. the pad thickness).  
         [0014]     In another exemplary embodiment, the pins protrude from the planar surfaces of the insulating substrate by an amount that is substantially the same as the distance between pins.  
         [0015]     In yet another exemplary embodiment, the pins are substantially evenly distributed throughout the insulating substrate with an average distance between neighboring pins. This distance may be equal to or less than the thickness of the flip chip (which is bound to a carrier by the anisotropically conductive layer) after final processing.  
         [0016]     In a still further exemplary embodiment, the distance between the two planar surfaces of the insulating substrate is between 5 and 25 microns, and the substrate may be formed from SiC, SiNx, SiO 2 , mica, polycarbonate or alumina (aluminum oxide), which may be formed by anodization of high purity (i.e. over 99.9% pure) aluminum foil.  
         [0017]     In another exemplary embodiment, the anisotropic conducting layer is bonded onto the circuit containing structures by soldering some of the pins onto the electrical contact pads of the circuit containing structure. In a further exemplary embodiment, the anisotropic conducting layer is bonded onto the circuit containing structures by physically penetrating some of the pins into the electrical contact pads of the circuit containing structure by respective pins making electrical contact to the pads and without soldering. For this purpose, it may be advantageous to form the contact pads from a material that is softer than the material from which the pins are formed. For example, if the pins were formed of copper, the contact pads may be formed from Sn, Pb, In or alloys thereof.  
         [0018]     In a still further exemplary embodiment, a soft insulating material is applied either to the whole surface of one of the circuit containing structures (including the contact pads), or to those portions of the circuit containing structure that are not electrical contact pads. The anisotropic conducting layer is then bonded onto the circuit containing structure by penetrating respective pins into the electrical contact pads and the soft insulating material.  
         [0019]     In a final exemplary embodiment, a semiconductor device, such as a high-speed heterojunction bipolar transistor is manufactured by performing a series of processing steps on a semiconductor substrate to partially fabricate a semiconductor device, bonding the partially-fabricated semiconductor substrate to a carrier chip in a flip chip fashion using an anisotropic conducting layer of the type previously described, and performing a series of final processing steps on the bonded partially-fabricated semiconductor device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  shows the cross-sectional view of a prior art technique for bonding a chip onto a carrier using conductive particles and an adhesive resin  
         [0021]      FIG. 2A  shows the cross-sectional view of a prior art technique for bonding a chip onto a carrier using solder balls and an epoxy.  
         [0022]      FIG. 2B  shows the cross-sectional view of a modified version of the technique illustrated in  FIG. 2A .  
         [0023]      FIG. 3  shows the cross-sectional view of a prior art technique for bonding a flip chip to a carrier using a soft Z-axis conductive film with embedded conductive tubules.  
         [0024]      FIGS. 4A-4F  show the cross-sectional view of an exemplary technique for forming an anisotropically conductive layer in accordance with the present invention.  
         [0025]      FIGS. 5A-5E  show the cross-sectional view of another exemplary technique for forming an anisotropically conductive layer in accordance with the present invention.  
         [0026]      FIGS. 6A-6F  show the cross-sectional view of another exemplary technique for forming an anisotropically conductive layer in accordance with the present invention.  
         [0027]      FIGS. 7A-7B  show the cross-sectional view of an exemplary technique for bonding a circuit containing structure to an anisotropically conductive layer in accordance with the present invention.  
         [0028]      FIGS. 8A-8B  show the cross-sectional view of another exemplary technique for bonding a circuit containing structure to an anisotropically conductive layer in accordance with the present invention.  
         [0029]      FIG. 9  shows the cross-sectional view of a partially processed semiconductor device suitable for bonding as a flip chip (shown before it is flipped for further processing) in an exemplary embodiment of the present invention.  
         [0030]      FIG. 10  shows the cross-sectional view of a circuit containing structure suitable for use as a carrier in an exemplary embodiment of the present invention.  
         [0031]      FIG. 11  shows the cross-sectional view of an anisotropically conductive layer disposed above a carrier wafer before bonding on a carrier in an exemplary embodiment of the present invention.  
         [0032]      FIG. 12  shows the cross-sectional view of a partially processed semiconductor device chip bonded in a flip chip fashion (shown after flipping) to a carrier using an anisotropically conductive layer in an exemplary embodiment of the present invention.  
         [0033]      FIG. 13  shows the cross-sectional view of a fully processed semiconductor device chip bonded to a carrier using an anisotropically conductive layer in accordance with an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0034]     Referring to  FIGS. 4A-4F , the steps involved in an exemplary method of forming an anisotropically conductive layer  50  of the present invention are illustrated. The illustrated steps involve the use of the nuclear track-etch method, which is well known to one of ordinary skill in the art. The pin diameter  58 , or pin hole diameter  57 , obtained by this method can range from 0.01 microns to a few tens of microns. The distribution of hole diameters is sharp, with deviations from the rated diameter of typically less than 20%. Preferably, the hole diameters range from 0.01 micron to about 0.5 micron. Porous membranes prepared by this method can be purchased commercially, or prepared as described below. The commercial product can be obtained from Corning (Nuclepore membranes) or from Osmonics (Poretics Policarbonate Track-Etch (PCTE) membranes). If a commercial product is employed, pins must be formed in the porous material using, for example, the electroplating technique discussed herein.  
         [0035]     The process begins with a three membrane layers  51 ,  52 ,  53 . In the exemplary embodiment, the layers consist of 10 microns of mica  52 , sandwiched between two layers of 2.5 micron thick polycarbonate  51 ,  53 . The thickness of the outer layers  51 ,  53  are preferably proportional to the diameter of the pin holes formed using this method, with smaller diameter pins corresponding to thinner outer layers  51 ,  53 , so as to maintain mechanical rigidity of the anisotropically conductive layer. Outer layers  51 ,  53  should be capable of being removed, such as by a selective etch method, without damaging the integrity of the middle substrate layer  52  or the pins  58 .  
         [0036]     Vertically oriented tracks  56  are then formed in the three layer membrane by bombarding the membrane with charged accelerated nuclear fission particles from a radioactive source, such as radioactive Californium or by placing the membrane into a nuclear reactor. The number of tracks per unit area depends primarily on the exposure time and the flux of energetic nuclear particles. By varying these parameters, it is possible to control the average density of resulting tracks. Once the energized particles have created randomly distributed tracks  56  with sufficient density (although the tracks are shown with regular spacing in  FIGS. 4B-4C , in practice the spacing between tracks  56  will vary), which constitute normally-oriented damaged regions in the membrane layers  51 ,  52 ,  53 , the membrane layers are exposed to an anisotropic etchant such that the etch rate for damaged regions is faster than the etch rate for undamaged regions. In the exemplary embodiment, two or three etch and rinse steps are employed using two different etchants, the different steps corresponding to the outer and inner layers of the three membrane layers  51 ,  52 ,  53 .  
         [0037]     In this manner, holes are formed along each of the tracks extending completely through the membrane layers  51 ,  52 ,  53 . For example, the tracks in outer polycarbonate layers  51 ,  53  may be etched with NaOH, which does not substantially etch the mica layer  52 . The nuclear tracks in the mica layer  52  (which is mainly composed of SiO 2 ) may be etched with a solution of hydrofluoric acid HF.  
         [0038]     Once holes have been formed in the three membrane layers  51 ,  52 ,  53  one of the outside layers  53  is coated with metal layer  54 . The metal layer, in turn, is mounted on conductive substrate  55 . Any metal with good conductance, such as copper or gold, will suffice for the conductive substrate  55 . Alternatively, the conductive substrate  55  can be made of a soft metal, such as Indium, and the three layer membrane can be pressed against that metal. In this case it may be unnecessary to cover the outer membrane layer  53  with metal layer  54 .  
         [0039]     In  FIG. 4D , metal pins  58  are formed in the previously formed holes  57  using traditional electroplating of the metal layer  54  in each of the holes  57 . The metal chosen should have a melting temperature higher than the relevant soldering temperature, such as 500° C. so the pins  58  will maintain their shape during any subsequent soldering. Moreover, the metal should be sufficiently hard so as to not deform during the bonding of a flip chip onto a carrier. In the preferred embodiment, copper is chosen for the metal pins  58 .  
         [0040]     After pin formation, the membrane layers  51 ,  52 ,  53  and pins  58  are demounted from backside metal  54  and substrate  55 . This can be achieved by mechanically separating the membrane from the substrate  55 , using, for example, a sharp blade. The connection between the substrate  55  and the membrane  51 ,  52 ,  53  is typically not strong, as the mechanical connection consists primarily of the pin cross-sections that were grown from the metal substrate  55  up into the hole  57 .  
         [0041]     After the deinounting step, the top and bottom membrane layers  51 ,  53  are removed by an etchant that selectively removes polycarbonate layers  51  and  53 , while leaving the middle layer  52  and metal pins  58  intact, resulting in the anisotropically conductive layer shown in  FIG. 4E . An alkaline solution of NaOH may be used for this purpose. As shown in  FIG. 4F , an optional further step of electroplating (such as electroless plating), can be conducted to make the diameter of the exposed portion  59  of each of the metal pins  58  larger than the unexposed portion, thus preventing the metal pins from becoming movable in the vertical direction in the rigid insulating membrane, also referred to as substrate  52 . The resulting anisotropically conductive layer  50 , may then be used to electrically and mechanically bond two circuit containing structures.  
         [0042]     In a similar exemplary embodiment (not shown), a single material membrane may be used, for example a polycarbonate layer a few microns thick. The process proceeds much like the three-layer technique described, i.e. formation of nuclear track-generated holes and filling of these holes with metal by electroplating. At the end of this process, the outer portions of each pin are exposed by partial etching of the membrane in a chemical that removes the membrane material but does not damage the pin material. An alkaline solution of NaOH is suitable for this purpose.  
         [0043]     The pins  58  of the anisotropically conductive layer  50  must, on average, be spaced sufficiently close so as to keep the circuit containing structure, such as the flip chip, that they support from deforming caused by mechanical forces exerted on the chip by the pins  58 . For example, a flip chip may encounter mechanical forces during bonding with another circuit containing substrate such as a carrier wafer or chip, or during subsequent processing steps such as thinning. The forces encountered by the flip chip may be approximated by traditional stress and strain equations well known to one of ordinary skill in the art and are more easily understood with reference to a thin square plate with side lengths L (corresponding to the average distance between pins) and thickness t. If the plate is supported only at its four corners and a pressure P is applied to the plate, the maximum deformation (i.e. bending of the plate) is:  
                 B   max     =       -   α     ⁢       PL   4       Et   3           ,           (     Eq   .           ⁢   1     )             
 
 where α is a geometrical factor (0.0444 for a square plate) and E is Young&#39;s modulus of elasticity (131 GPa in the &lt;100&gt; direction for silicon). 
 
         [0044]     Using the formula in Eq. 1, it can be shown that for a pressures P of approximately 10 atm (or 10 6  Pa, which is larger than the pressures encountered during most processing operations such as bonding and thinning), a plate thickness t of five microns, and plate length L of one micron, the maximum deformation of the plate is approximately 3×10 −9  microns, which is negligible and should not result in fracturing of the plate. Similarly, assuming the material to be a polyamide (a material typically used to form an insulation layer in semiconductor devices) rather than silicon, the maximum deformation increases to the order of 10 4  microns, which again is negligible and should not result in fracturing of the plate. However, if the length L is increased to ten microns, the maximum deformation increases to approximately 0.35 microns and 3.5 microns for Silicon and polyamide respectively, which is sufficient to fracture the plate. Consequently, to ensure the flip chip does not fracture, the distance L between unsupported portions of the flip chip should remain at approximately 1 micron or less. This implies that the distance between adjacent pins  58  in the anisotropically conductive layer  50  should be approximately 1 micron or less.  
         [0045]     Notably, if the pin distance is 1 micron and the total chip size is approximately 1×1 mm , there will be approximately one million pins  58  supporting the chip. Accordingly, the 10 atm pressure P applied to the flip chip during processing will be spread over all the pins  58 , resulting in a force of about 10 −6  N on each pin, which is sufficiently small to prevent pin deformation, assuming a pin diameter of approximately 0.5 microns.  
         [0046]     Referring now to  FIGS. 5A-5E , an alternative approach to constructing an anisotropically conductive layer using an anodic alumina membrane is illustrated. The porous membranes can be either prepared as described below, or purchased from, for example, Fisher Scientific (Whatman Anodisc filter membranes with pore diameters from 0.02 micron to 0.2 micron). Porous oxide growth on very pure aluminum under anodic bias in various electrolytes has been well known in the art for a number of years. A description of the process appears in U.S. Pat. No. 6,045,677, incorporated herein by reference. Porous membranes formed in this matter are characterized by narrow vertical pores with sharp pore diameter distribution and overall uniformity of pores throughout the membrane surface. More recently it was found that for a narrow range of growth parameters, it is possible to obtain a self-organized densely packed hexagonal pore structure. See H. Masuda et al., Appl. Phys. Lett. vol. 71, p. 2270-72 (1997), incorporated herein by reference.  
         [0047]     High purity aluminum foil  61  (more than 99.9% pure) with thickness of a few microns, is mounted on a conductive metal plate  62 , such as a copper plate. Regularly distributed holes  63  are formed in the membrane  61  by slow anodization in 0.3 M oxalic acid solution, at 15-17 degrees C., under a constant voltage of 40 V, as described in H. Masuda and K. Fukuda, Science vol. 268, p. 1466-68 (1995), incorporated herein by reference. The diameters of holes  63  can be further adjusted by dipping of the anodized porous membrane into various acidic solutions. Preferred acids include sulfuric, phosphoric and oxalic acids. Hole size using this technique can vary from 0.01 microns to 0.4 microns. Typically, a lower layer of the aluminum foil  61  remains in the metallic state, and the holes in the anodized alumina terminate without going all the way through, as shown in  FIG. 5C . After anodization, the aluminum substrate and the bottom part of the porous layer can be etched away with saturated HgCl 2 .  
         [0048]     Holes  63  are then ready for filling with metal. In an exemplary embodiment, the holes  63  are filled with metal to make pins  64  using AC electroplating of the metal plate  62  in each of the holes  64 , or using electroless electroplating, which are well known to one of ordinary skill in the art. See R. M. Metzger et. al., IEEE Trans. on Magnetics, vol. 36, p. 30 (2000). Both methods allow for filling the incomplete hole  63  with metal despite the fact that there is a thin partition of insulating alumina between the interior of the hole  63  and the metal substrate  62 . Regular electroplating can be used if the bottom part of the membrane has been etched with HgCl 2  and the holes have been opened on both ends.  
         [0049]     Once the pins  64  are formed, the membrane and pins are mechanically demounted from the conductive substrate  62 . Etching in an appropriate acid or other chemical, such as saturated HgCl 2  which removes alumina but not the metal pins, is then performed to remove residual Aluminum from the membrane and to thin the alumina membrane so as to expose the top and bottom portions of the pins, resulting in the structure shown in  FIG. 5D . Again, it is possible to use electroless or other standard plating techniques to increase the diameter of the exposed portions of the pins  64  as a final processing step. The resulting anisotropically conductive layer  60  is shown in  FIG. 5E .  
         [0050]     Referring now to  FIGS. 6A-6F , a third approach to constructing an anisotropically conductive layer is presented using a traditional photolithography and etching technique. An approximately 40 to 100 micron thick insulating membrane  72 , such as Silicon is formed on a conductive substrate  73 . A photoresist layer  71  is applied to coat the top surface of the membrane  72  as illustrated in  FIG. 6A . The insulating membrane  72  may be a carbon doped silicon material, mica, or quartz platelet. The photoresist may be any general negative photo resist such as AZ-5200, manufactured by AZ Electronic Materials of Somerville, N.J. The closely spaced hole array pattern  74  is then exposed in the photoresist using a suitable mask. To ensure the holes are spaced closely enough, it is preferred to employ optical lithography using UV light or electron beam lithography. After the exposed photoresist  71  is developed to form the photoresist pattern shown in  FIG. 6B , the membrane  72  masked by the photoresist pattern is subjected to a highly anisotropic etchant, such as induction coupled plasma (ICP) etching. If membrane  72  is composed of SiO 2 , the plasma may consist mainly of CF 4 ; if the membrane  72  is composed of SiC, it may consist mainly of SF 6 . The ICP technique is known to produce high aspect ratio vertical holes  75  through the membrane, as shown in  FIG. 6C . The holes  75  are filled with metal by electroplating the metal substrate  73  through the holes  75 , as shown in  FIG. 6D . The membrane with metal filled holes  76  is then demounted from substrate  73 . The photoresist layer  71  is then removed and further etching takes place to expose the top and bottom portion of the pins  76 , forming the structure shown in  FIG. 6E . Electroless or regular electroplating may then be employed to increase the diameter of the exposed portion  77  of the pins  76 , forming the anisotropically conductive layer  70 , as shown in  FIG. 6F .  
         [0051]     The anisotropically conductive layer of the present invention  50 ,  60 ,  70  may be used to electrically and mechanically bond two circuit containing structures, such as a flip chip and chip carrier.  FIGS. 7A-7B  illustrate an exemplary method of bonding an anisotropically conductive layer  50 ,  60 ,  70  to a circuit containing structure  81 . The structure  81  has electrical contact pads  82 , which protrude from its surface. The bonding of the anisotropically conductive layer  50 ,  60 ,  70  to the structure  81  is achieved by soldering respective pluralities of the metal pins  94  with the contact pads  82 , or by mechanically penetrating the contact pads  82  with respective pluralities of the metal pins  94  of anisotropically conductive layer  50 ,  60 ,  70 , as shown in  FIG. 7A . The pins not soldered to or penetrating the contact pads  82 , contact the surface of structure  81 , providing mechanical support to the structure  81 . Consequently, the length of the exposed portion of the pins in the anisotropically conductive layer  50 ,  60 ,  70  is comparable to or greater than the distance the electrical contact pads  82  extend from the surface of structure  81  (i.e. the pad thickness).  
         [0052]     Referring now to  FIGS. 8A-8B , another exemplary method of bonding the anisotropically conductive layer  50 ,  60 ,  70  onto circuit containing structure  91  is illustrated. In this embodiment, a soft insulating material is applied to coat the surface of structure  91 , but not to coat the surface of electrical contact pads  93 . The thickness of the soft insulating layer  92  should be approximately the same as the distance the electrical contact pads  93  protrude from the surface of structure  91 . The bonding of the anisotropically conductive layer  50 ,  60 ,  70  to the structure  91  is achieved by soldering respective pluralities of the metal pins  94  to the contact pads  93 , or by mechanically penetrating the contact pads  93  with the metal pins of anisotropically conductive layer  50 ,  60 ,  70 . The pins not bonded to the electrical contact pads  93  penetrate the soft insulating material  92 , forming mechanical bonds that help to distribute forces applied to the anisotropically conductive layer over the bonded surface of the structure  91 . Alternatively, (although not shown in  FIG. 8A ), the soft insulating material may cover the entire surface of structure  91 . In that case, the pins would penetrate the soft insulator material both to make electrical contact to the pads and to make mechanical contact to the rest of the surface of the structure.  
         [0053]     Referring now to  FIGS. 9-13 , an exemplary embodiment of a method for manufacturing a semiconductor device, such as a heterojuntion bipolar transistor (HBT)  130  using the anisotropically conductive layer  50 ,  60 ,  70  of the present invention is illustrated.  FIG. 9  illustrates a partially processed HBT chip  100 . The partially processed HBT chip  100  consists of InP substrate  109  and five epitaxially grown layers including a 100 nm thick n+ type InGaAs emitter contact layer  103 , an n type InP emitter layer  104 , a p+ type InGaAs base layer  106 , an n− type InGaAs subcollector layer  107  and an n+ type InGaAs collector layer  108 . The emitter layer  104  and the emitter contact layer  103 , are etched to form a mesa shape. A Si 3 N 4  passivation layer  1010  is then formed on the base layer  106  and the mesa layers  103  and  104  using PECVD. The Si 3 N 4  layer  1010  is then etched to make openings for ohmic contact electrode layers  105  and  1011  for the base and emitter, respectively, which are formed by conventional metallization, photolithography and etching. The surface of the chip  100  is then covered with a layer of insulator material  101 , such as a polyamide. Holes are then formed in the layer of insulating material  101  from the top surface to each of the base and emitter ohmic contact electrodes  105  and  1011 , using photolithography and anisotropic plasma etching. Electrical contact terminals or pads  102  are formed on the base and emitter ohmic contact electrodes  105  and  1011  using electroplating.  
         [0054]      FIG. 10  illustrates an electronic circuit containing structure or carrier,  110 , having conventional electronic circuits (e.g., integrated circuits) contained in semiconductor layer  116 . The semiconductor layer  116  is covered by a layer of Si 3 N 4 , which has openings for contact electrodes  113  to the electronic circuits in the layer  116 . A further layer  112 , preferably of a polyamide, is formed over the Si 3 N 4  layer  115  and the contact electrodes  113 . Holes are formed in the layer  112  extending from its top surface to respective contact electrodes  113  by conventional photolithography and anisotropic etching, the holes are filled with metal and a contact pad  111  is formed over each hole by electroplating of the contact electrode through each of the holes. In this manner, electrical contact terminals or pads  111  are provided at the surface of layer  112  for electrical connection to the electronic circuits contained in the semiconductor layer  116 .  
         [0055]      FIG. 11  illustrates the anisotropically conductive layer  50 ,  60 ,  70  positioned above the carrier  110  just before bonding thereon. The bonding technique may be any of the methods previously described with reference to  FIGS. 7A-7B ,  8 A- 8 B. As can be seen, proper alignment between the structure  110  and the anisotropically conductive layer is not necessary as long as electrical contact terminals  111  are in contact with an adequate number of pins of the anisotropically conductive layer  50 ,  60 ,  70 ; however, to ensure maximum mechanical support of the carrier and/or the flip chip, the anisotropically conductive layer  50 ,  60 ,  70  should include a sufficient number of pins to cover as much of the interface surface of carrier  110  as possible.  
         [0056]     Referring now to  FIGS. 9 and 12 , the electrical and mechanical bonding of the carrier  110  and flip chip  100  is shown. Anisotropically conductive layer  50 ,  60 ,  70  (previously bonded onto carrier  110 ) is bonded on its other side to the partially processed HBT chip  100 , flipped upside down, using the methods previously described with reference to  FIGS. 7A-7B ,  8 A- 8 B, forming the structure shown in FIG.  12 . At this stage, each contact electrode  113  of the carrier  110 , is electrically connected to a respective one of the contact electrodes  105 ,  1011  of the flip chip  100 . As shown in  FIG. 12 , each contact electrode  113  of the carrier  110  is electrically connected to the contact electrode  105 ,  1011  of the partially processed HBT flip chip positioned directly above the respective carrier contact electrode  111 . Again, it will be appreciated that precise alignment of the carrier  110  and flip chip is not necessary because the surface of the electrical contact terminals or pads  102 ,  111 , will usually be contacted by more than 10,000 metal pins, thus ensuring electrical connection between the carrier  110  and flip chip  100  despite some misalignment between the two.  
         [0057]     Once the carrier and flip chip have been electrically and mechanically bonded using the anisotropically conductive layer, further processing of the backside of flip chip  100  may take place to form a completed HBT  130  as shown in  FIG. 13 . In the exemplary embodiment, the further processing includes selective etching away the entire InP substrate  109 , etching the collector layer  104  and the collector contact layer  103  to form a collector mesa structure that rises above the base layer  106 , forming a Si 3 N 4  passivation layer  134  covering the base layer  106  and the collector mesa, forming a contact window in the Si 3 N 4  layer above the collector mesa, forming a collector contact electrode  131  in the contact window by making ohmic contact between the collector contact layer  108  and a deposited metal layer patterned by conventional photolithography and etching, covering the back side of the HBT with a layer of polyamide  132 , forming a hole extending from the top surface of the polyamide layer  132  to the collector contact electrode  131  using photolithography and etching, and filling the hole with metal forming a contact pad over the hole by electroplating of the collector contact electrode through the hole. These steps are all familiar to one of ordinary skill in the art.  
         [0058]     The foregoing merely illustrates the principles of the invention in exemplary embodiments. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. For example, the pin density of the anisotropically conductive layer might be varied to produce regions with few pins so as to not interfere with any surface elements of a circuit having non-planar structure. Additionally, the surface of a circuit having non-planar structure may be coated with a hard protective layer to protect it from the possibility of damage caused by the pins of the anisotropic conducive layer. As another example, it may be desirable to introduce structural elements similar to the described electrical contact terminals or pads on a circuit containing structure where the number of electrical contact terminals or pads is low. These introduced structural elements would enhance and more fully distribute the mechanical bonding and support provided by the pins but would not provide electrical connection between the circuit containing structures. It will thus be fully appreciated that those skilled in the art will be able to devise numerous systems and methods which, although not explicitly shown or described, embody the principles of the invention and thus are within the spirit and scope of the invention as defined in the appended claims.

Technology Category: 5