Abstract:
A photolithographically patterned spring contact is formed on a substrate and electrically connects contact pads on two devices. The spring contact also compensates for thermal and mechanical variations and other environmental factors. An inherent stress gradient in the spring contact causes a free portion of the spring contact to bend up and away from the substrate. An anchor portion remains fixed to the substrate and is electrically connected to a first contact pad on the substrate. The spring contact is made of an elastic material and the free portion compliantly contacts a second contact pad, thereby electrically interconnecting the two contact pads. The free portion is initially fixed to the substrate to intentionally form the inherent stress gradient in the elastic member. The free portion is released from the substrate by etching a release layer deposited on the substrate so the inherent stress gradient in the elastic member biases the free portion away from the substrate. A contact tip of the spring contact can be coated with solder.

Description:
This is a continuation of application Ser. No. 09/210,552 filed Dec. 14, 1998, which in turn is a continuation of application Ser. No. 08/770,285 filed Dec. 20, 1996 (now issued as U.S. Pat. No. 5,848,685), which in turn is a continuation of application Ser. No. 08/478,578 filed Jun. 7, 1995 (now issued as U.S. Pat. No. 5,613,861). The entire disclosures of the prior applications are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to photolithographically-patterned spring contacts for use in electrically bonding integrated circuits, circuit boards, electrode arrays, or other devices. 
     2. Description of Related Art 
     Standard bonding techniques for electrically connecting integrated circuits, or chips, to a circuit board or other device include wire bonding, tab bonding, solder-bump and gold-bump flip-chip bonding and other techniques. FIG. 1 shows a contact pad  3  formed on a chip  2  wire bonded to a corresponding contact pad  3  formed on a substrate  1 . The contact pads  3  are electrically connected, or bonded, by a wire  4 . Since the chip  2  typically has tens or even hundreds of the contact pads  3 , wire bonding each contact pad  3  on the chip  2  to the corresponding contact pad  3  on the substrate  1  is labor intensive, expensive and slow. Further, the contact pads  3  must be large enough to accommodate both the wire  4  and the accuracy of the wire bonding device used to create the wire bond. Therefore, the contact pads  3  are made larger than otherwise necessary to compensate for the size limitations of the wire  4  and the wire bonding device. 
     FIG. 2 shows the contact pad  3  formed on the chip  2  tab bonded to the corresponding contact pad  3  on the substrate  1 . A flexible substrate  5  having conductive lines formed on its lower surface is forced against the contact pads  3 . A layer of anisotropic adhesive (not shown) is placed between the contact pads  3  and the flexible substrate  5 . When the flexible substrate  5  is pressed against the contact pads  3 , the anisotropic adhesive and the conductive lines formed on the flexible substrate  5  cooperate to complete the electrical connection between the contact pads  3 . Like wire bonding, tab bonding suffers from yield loss, bond fragility, and high cost. 
     Another conventional method for bonding the contact pads  3  formed on the chip  2  to the contact pads  3  formed on the substrate  1  or to some other device is solder-bump flip-chip bonding. FIG. 3 shows the chip  2  inverted with the contact pads  3  facing toward the substrate  1 . The name “flip-chip” derives from the inversion of the chip  2 , since the chip  2  is “flipped over” with the contacts pads  3  facing the substrate  1 , in contrast to both tab bonding and wire bonding where the contact pads  3  on the chip  2  face away from the substrate  1 . Note, however, that tab bonding can also be done with the chip  2  “flipped over”. In standard flip-chip bonding, solder bumps  6  are formed on the contact pads  3  on the substrate  1  or on the chip  2 . The electrical connection between the corresponding contact pads  3  is completed by pressing the contact pads  3  on the chip  2  against the solder bumps  6  and melting the solder bumps  6 . 
     Flip-chip bonding is an improvement over both wire bonding and tab bonding. The relatively soft solder bumps  6  tend to permanently deform when the chip  2  is pressed down against the solder bumps  6 . This deformation of the solder bumps  6  compensates for some irregularity in the heights of the contact pads  3  and any uneven contacting pressure forcing the chip  2  against the solder bumps  6 . 
     However, flip-chip bonding does suffer from both mechanical and thermal variations in the solder bumps  6 . If the solder bumps  6  are not uniform in height or if the substrate  1  is warped, contact between the contact pads  3  and the solder bumps  6  can be broken. Also, if the contacting pressure forcing the chip  2  down on the solder bumps  6  is uneven, contact between some contact pads  3  and corresponding solder bumps  6  can fail. In addition, stresses from thermal expansion mismatches between the chip  2  and the substrate  1  can break the bonds formed by the solder bumps  6 . 
     In contrast to the relatively permanent bonds described above, FIG. 4 shows a standard technique for establishing a temporary electrical contact between two devices. A probe card  7  having a plurality of probe needles  8  contacts the contact pads  3  by physically pressing the probe needles  8  against the contact pads  3 . The physical contact between the probe needles  8  and the contact pads  3  creates an electrical connection between the probe needles  8  and the lines  9  formed on the substrate  1 . 
     The probe cards  7  are generally used to create only temporary contacts between the probe needles  8  and the contact pads  3 , so that the device  10  can be tested, interrogated or otherwise communicated with. The device  10  can be a matrix of display electrodes which are part of an active-matrix liquid crystal display. Testing of the devices  10 , such as liquid crystal display electrode matrices, is more thoroughly described in a U.S. patent application Ser. No. 8/473,912, now abandoned to the same inventor, co-filed and co-pending herewith. 
     The probe cards  7  have many more applications than only for testing liquid crystal displays. Any device  10  having numerous and relatively small contact pads  3 , similar to those found on the chip  2 , can be tested using the probe card  7 . However, standard techniques for producing the probe card  7  are time consuming and labor-intensive. Each probe card  7  must be custom-made for the particular device  10  to be tested. Typically, the probe needles  8  are manually formed on the probe card  7 . Because the probe cards  7  are custom-made and relatively expensive, the probe cards  7  are not typically made to contact all of the contact pads  3  on the device  10  at one time. Therefore, only portions of the device  10  can be communicated with, tested or interrogated at any one time, requiring the probe card  7  be moved to allow communication, testing or interrogation of the entire device  10 . 
     The probe cards  7  are also used to test the chips  2  while the chips  2  are still part of a single-crystal silicon wafer. One such probe card  7  is formed by photolithographic pattern plated processing, as disclosed in  Probing at Die Level , Corwith, Advanced Packaging, February, 1995, pp.  26 - 28 . Photolithographic pattern plated processing produces probe cards  7  which have essentially the same design as the standard probe card  7 . However, this new type of processing appears to automate the method for producing probe needles  8 , thus avoiding manually forming the probe needles  8 . Also, this article discloses a probe card  7  which is bent at the end nearest the probe needles  8 , as shown in FIG.  5 . The bend in the probe card  7  allows the probe needles  8  to contact the contact pad  3  at an angle. As the probe card  7  pushes the probe needles  8  into the contact pads  3 , a mechanical scrubbing action occurs which allows the probe needles  8  to break through the oxide formed on the top surface of the contact pad  3 . All of the standard probe cards  7 , however, are limited to testing contact pads  3  which are arranged in a linear array. 
     SUMMARY OF THE INVENTION 
     Accordingly, this invention provides a spring contact which exhibits the speed and ease of solder-bump flip-chip bonding while eliminating the need to create uniform solder bumps or uniform contacting pressure. Also, the invention provides finer-pitch contact arrays than solder-bump flip-chip bonding. 
     This invention further provides a spring contact which has elastic properties enabling the spring contact to maintain physical contact with a contact pad despite variations in contact pad heights, contacting pressure, thermal variations or mechanical shock. 
     This invention also provides an elastic spring contact having a stress gradient formed in the spring contact, which causes the spring contact to bend away from the substrate and thus provide compliant contact with a contact pad. 
     This invention further provides a probe card and a method for producing the probe card having spring contacts in place of standard probe needles. 
     The spring contacts of this invention are formed of a thin metal strip which is in part fixed to a substrate and electrically connected to a contact pad on the substrate. The free portion of the metal strip not fixed to the substrate bends up and away from the substrate. When the contact pad on a device is brought into pressing contact with the free portion of the metal strip, the free portion deforms and provides compliant contact with the contact pad. Since the metal strip is electrically conductive or coated with a conductive material, the contact pad on the substrate is electrically connected to the contact pad on the device via the spring contact. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be described in relation to the following drawings, in which reference numerals refer to like elements and wherein: 
     FIG. 1 shows a chip wire bonded to a substrate; 
     FIG. 2 shows the chip tab bonded to the substrate; 
     FIG. 3 shows the chip solder-bump flip-chip bonded to the substrate; 
     FIG. 4 shows a probe card contacting an electronic device; 
     FIG. 5 shows a probe card having an angled probe needle; 
     FIG. 6 is a spring contact in an undeformed free state and another spring contact deformed when contacting a contact pad; 
     FIG. 7 shows a metal strip with no stress gradient; 
     FIG. 8 shows a model for determining the curvature of a spring contact due to the stress gradient; 
     FIG. 9 shows a model for determining the amount of reaction force exerted at the tip of the spring contact; 
     FIG. 10 shows the first steps in a method of forming a spring contact according to the invention; 
     FIG. 11 shows additional steps following those shown in FIG. 10 in a method of forming a spring contact according to the invention; 
     FIG. 12 shows additional steps following those shown in FIG. 11 in a method of forming a spring contact according to the invention; 
     FIG. 13 shows a final step in a method of forming a spring contact according to the invention; 
     FIG. 14 is a graphic representation of the film stress in a sputter deposited nickel-zirconium alloy as a function of plasma gas pressure; 
     FIG. 15 is a top view of a spring contact; 
     FIG. 16 is a device for testing the contact resistance of a plurality of spring contact pairs; 
     FIG. 17 is a graphical representation of the detected resistance of a plurality of spring contact pairs; 
     FIG. 18 is a graphic representation of the contact resistance of a spring contact as a function of the distance between the contact pad and the substrate; 
     FIG. 19 is a spring contact having a flat end; 
     FIG. 20 is a spring contact having a pointed end; 
     FIG. 21 is a spring contact having two points at the tip end; 
     FIG. 22 is a spring contact having multiple points at the tip end; 
     FIG. 23 is a spring contact having a deformable tab at the tip end; 
     FIG. 24 shows a spring contact having a deformed tab end when forced against a contact pad; 
     FIG. 25 is a chip having a plurality of spring contacts electrically bonded to a substrate; 
     FIG. 26 is a chip bonded to a dust cover and electrically contacted to a substrate having a plurality of spring contacts; 
     FIG. 27 is a chip bonded to a substrate and electrically contacted to a substrate by a plurality of spring contacts on the chip; 
     FIG. 28 is a chip electrically bonded to a substrate by way of an intermediate wafer having a plurality of spring contacts; 
     FIG. 29 is a probe card having a plurality of spring contacts used for testing an electronic device; and 
     FIG. 30 is a liquid crystal display and a device for testing the operation of the display. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 6 shows a side view of a bonding structure  100  having a plurality of spring contacts  15 . Each spring contact  15  comprises a free portion  11  and an anchor portion  12  fixed to an insulating underlayer  13  and electrically connected to a contact pad  3 . Each spring contact  15  is made of an extremely elastic material, such as a chrome-molybdenum alloy or a nickel-zirconium alloy. Preferably, the spring contacts  15  are formed of an elastic conductive material, although they can be formed of a non-conductive or semi-conductive material if they are coated or plated with a conductor material. More preferably, the spring contacts  15  are formed of a nickel-zirconium alloy having 1% zirconium. Zirconium is added to the nickel to improve the elastic properties of the alloy while not greatly reducing the conductivity of the nickel. When the elastic material is not conductive, it is coated on at least one side with a conductive material, such as a metal or metal alloy. 
     The contact pad  3  is the terminal end of a communication line which electrically communicates with an electronic device formed on the substrate  14  or device  101  such as a transistor, a display electrode, or other electrical device. The contact pad  3  is typically made of aluminum, but can be made of any conductive material. If the contact pad  3  on device  101  is made of aluminum, the contact pad  3  is preferably coated with a conductive material, such as gold, indium tin oxide, or nickel. This allows the spring contact  15  to make better electrical contact with the contact pad  3 , since the spring contact  15  cannot “scrub” the uncoated contact pad  3  to break through the aluminum oxide that forms on an uncoated aluminum contact pad  3 . The insulating underlayer  13  is made of silicon nitride or other etchable insulating material. However, the insulating underlayer  13  is not necessary and can be eliminated. The insulating underlayer  13  and the contact pad  3  are formed on or over a substrate  14 , which is also formed of an insulating material, such as oxidized silicon or glass. 
     As shown in FIG. 7, a strip of metal having no stress gradient inherent in the metal will lie flat. However, as shown in FIG. 8, when the strip is bent into an arc, a uniform stress gradient Δσ/h is introduced into the strip. Likewise, if a uniform stress gradient Δσ/h is introduced into the flat metal strip, the metal strip will bend into an arc shape. 
     Each spring contact  15  is formed such that a stress gradient Δσ/h is introduced into the spring contact  15 . When the spring contact  15  is formed, the metal layer comprising the spring contact  15  is deposited such that compressive stress is present in upper portions of the metal layer and tensile stress is present in lower portions of the metal layer. Compressive stress in upper portions of the metal layer is depicted by arrows directed inwardly. Tensile stress is depicted in lower portions of the metal layer by arrows directed outwardly. The stress gradient Δσ/h causes the spring contact  15  to bend into the shape of an arc having a radius r. Equation 1 gives the radius of curvature r of the spring contact  15 :              r   =       (     Y     1   -   v       )          h   Δσ               (   1   )                                
     where Y is the Young&#39;s modulus of the metal, h is the thickness of the metal layer forming the spring contact  15 , Δσ is the total stress difference, and v is the Poisson&#39;s ratio of the metal. 
     Referring again to FIG. 6, r is the radius of curvature of the free portion  11  of the spring contact  15  as predicted in Equation 1, and Θ is the angle separating the radius line directed toward the junction of the free portion  11  with the anchor portion  12  and the radius line directed toward the tip  30  of the free portion  11 . Equation 2 gives the approximate height b of the spring contact tip  30  from the substrate  14  for angles Θ&lt;50°:              b   ≈       L   2       2      r               (   2   )                                
     where L is the length of the free portion  11  and r is the radius of curvature of the free portion  11 . 
     Since each spring contact  15  is preferably made of a highly elastic material, each spring contact  15  can be pushed down at the tip  30  and deformed as shown in FIG. 6, but will not plastically deform. Typically, a contact pad  3  of a device  101  exerts the downward force placed on the tip  30  and electrically contacts the tip  30 . The spring contact  15  resists the downward force placed on the tip  30  and maintains electrical contact with the contact pad  3 . 
     When the force on the tip  30  is released, the spring contact will return to its undeformed state. Thus, the elasticity of the spring contacts  15  allows the spring contacts  15  to make numerous successive electrical connections with different contact pads  3  while maintaining the integrity of the electrical connection between the spring contact tip  30  and the contact pads  3 . 
     Additionally, the spring contact  15  is preferably made of a creep-resistant material. Therefore, when the spring contact  15  is elastically deformed over an extended period by a contact pad  3  pressing down on the spring contact tip  30 , the spring contact  15  resists the downward force and pushes the spring contact tip  30  against the contact pad  3 , maintaining the electrical connection. 
     FIG. 9 shows a model for determining the amount of force F tip  applied by the spring contact tip  30  to a contact pad  3  in reaction to the force of the contact pad  3  pressing down on the spring contact tip  30 . Equation 3 gives the reaction force F tip  of the spring contact tip  30 :                F   tip     =         wh   2        Δσ       12      x               (   3   )                                
     where w is the width of the spring contact  15 , h is the thickness of the spring contact  15 , Δσ is the total stress difference and x is the horizontal distance from the spring contact tip  30  to the point where the spring contact  15  first touches the substrate  14 . 
     For a given width w, thickness h and stress difference Δσ, the reaction force F tip  of the tip  30  varies inversely with the distance x. Therefore, the reaction force F tip  increases as the spring contact tip  30  gets closer to the substrate  14 , since the distance x decreases as the spring contact  15  collapses and presses against the substrate  14  as shown in FIG.  6 . The increase in the reaction force F tip  as the contact pad  3  presses the spring contact tip  30  closer to the substrate  14  generally improves the electrical connection between the spring contact tip  30  and the contact pad  3 . The increasing reaction force F tip  causes the spring contact tip  30  and/or the contact pad  3  to deform locally at the area of contact, increasing the area of contact between the contact pad  3  and the spring contact tip  30 . 
     FIGS. 10-13 show the basic steps in forming a spring contact  15 . In FIG. 10, a contact pad  3  is formed on or over a substrate  14 . Additionally, an insulating underlayer  13  is formed on or over the substrate  14 . However, as mentioned above, the insulating underlayer  13  is not required and can be eliminated. 
     In FIG. 11, a layer of metal  16  is deposited on or over the substrate  14 . In the preferred embodiment of the invention, the metal is the nickel-zirconium alloy described above. Part of the metal layer  16  is electrically connected to or directly contacts the contact pad  3  and another portion of the metal layer  16  is deposited on or over the insulating underlayer  13 . There are many methods available for depositing a metal layer  16  on or over the substrate  14 , including electron-beam deposition, thermal evaporation, chemical vapor deposition, sputter deposition and other methods. Preferably, the metal layer  16  is sputter deposited. 
     When sputter-depositing a metal, a plate of the metal, called the target, is placed on a cathode, which is set to a high negative potential and immersed in a low-pressure, typically 1 to 100 millitorr, gas. This causes a glow-discharge plasma to ignite, from which positive ions are accelerated into the negatively charged target. This ion bombardment knocks metal atoms off the target, and many of these deposit on nearby surfaces, such as the substrate  14 . 
     The metal layer  16  can be thought of as deposited in several sub-layers  16 - 1  to  16 -n to a final thickness h of approximately 1 μm. The stress gradient Δσ/h is introduced into the metal layer  16  by altering the stress inherent in each of the sub-layers  16 - 1  to  16 -n of the metal layer  16 , as shown in FIG. 11, each sub-layer  16 -x having a different level of inherent stress. 
     Different stress levels can be introduced into each sub-layer  16 -x of the deposited metal layer  16  during sputter deposition in a variety of ways, including adding a reactive gas to the plasma, depositing the metal at an angle, and changing the pressure of the plasma gas. Preferably, the different levels of stress are introduced into the metal layer  16  by varying the pressure of the plasma gas, which is preferably argon. 
     FIG. 14 is a graph showing a typical relationship of the film stress in the sputter deposited nickel-zirconium alloy and the pressure of the plasma gas used in the deposition. For low pressures of the plasma gas, approximately 1 mTorr, the film stress in the deposited metal is compressive. As the pressure of the plasma gas increases, the film stress in the deposited sub-layer changes to a tensile stress and increases with increasing plasma gas pressure. 
     Preferably, the metal layer  16  is deposited in five sub-layers  16 - 1  to  16 - 5 . The first sub-layer  16 - 1  is deposited at a plasma gas pressure of 1 mTorr, as indicated by numeral  1  in FIG.  14 . The first sub-layer  16 - 1  is the bottom-most layer in the metal layer  16  and has an inherent compressive stress. The second sub-layer  16 - 2  is deposited on top of the first sub-layer  16 - 1  at a plasma gas pressure of approximately 6 mTorr. The second sub-layer  16 - 2  has a slight inherent tensile stress, as indicated by numeral  2  in FIG.  14 . Sub-layers  16 - 3 ,  16 - 4  and  16 - 5  are then deposited one on top of the other at the plasma gas pressures indicated by numerals  3 ,  4  and  5  in FIG.  14 . 
     The process of depositing the metal layer  16  in five separate sub-layers  16 - 1  to  16 - 5  results in the metal layer  16  having a stress gradient Δσ/h which is compressive in the lower portion of the metal layer  16  and becomes increasingly tensile toward the top of the metal layer  16 . Although the stress gradient Δσ/h urges the metal layer  16  to bend into an arc, the metal layer  16  adheres to the insulating underlayer  13 , the substrate  14  and the contact pad  3  and thus lies flat. 
     After the metal layer  16  is deposited, the metal layer  16  is photolithographically patterned into the spring contacts  15 . Photolithographic patterning is a well-known technique and is routinely used in the semiconductor chip industry. First, a positive photosensitive resist  17  is spun on top of the metal layer  16  and soft-baked at 90° C. to drive off solvents in the resist  17 . The photosensitive resist  17  is exposed to an appropriate pattern of ultra-violet light and then developed. Exposed areas of the resist  17  are removed during developing and the remaining resist  17  is hard-baked at 120° C. Wet or plasma etching is then used to remove the exposed areas of the metal layer  16 . The remaining areas of the metal layer  16  after etching form the spring contacts  15 . A top-view of one spring contact  15  is shown in FIG.  15 . The area of the metal layer  16  removed by the etching is described by the dashed line  18 . 
     Next, as shown in FIG. 12, the free portion  11  of the spring contact  15  is released from the insulating underlayer  13  by a process of under-cut etching. Until the free portion  11  is released from the insulating underlayer  13 , the free portion  11  adheres to the insulating underlayer  13  and the spring contact  15  lies flat on the substrate  14 . There are two methods for releasing the spring contacts  15  from the substrate  14  or insulating underlayer  13 . In the first method, the insulating underlayer  13 , typically silicon nitride, is deposited by plasma chemical vapor deposition (PECVD) at a temperature of 200-250° C. This gives the insulating underlayer  13  a fast etch rate. The insulating underlayer  13  is then pre-patterned, before the metal layer  16  is deposited, into islands on which the spring contacts  15  will be formed. After the spring contacts  15  are formed on or over the islands of the insulating underlayer  13 , the spring contacts  15  are released from the insulating underlayer  13  islands by etching the islands with a selective etchant. The selective etchant is typically a HF solution. The etchant is called a selective etchant because it etches the insulating underlayer  13  faster than the selective etchant removes metal from the spring contacts  15 . This means that the spring contacts  15  are released from the insulating underlayer  13  and are allowed to bend up and away from the insulating underlayer  13  due to the stress gradient Δσ/h in the spring contacts  15 . The islands can also be formed of a low melting temperature material, such as solder or plastic. After the spring contacts  15  are formed, the low melting temperature material is heated to release the spring contacts  15 . 
     In the second method for releasing the spring contacts  15 , the insulating underlayer  13 , if used, is not pre-patterned into islands. Instead, after the spring contacts  15  are formed, a passivating layer, such as silicon oxynitride, is deposited on the spring contacts  15  and the surrounding areas by PECVD. The passivation layer is patterned into windows, such as the shaded area shown in FIG. 15, to expose the free portion  11  of the spring contacts  15  and surrounding areas of the insulating underlayer  13 . The same selective etchant, the HF solution, is used to etch the insulating underlayer  13  and release the spring contacts  15 . This method avoids a step discontinuity in the metal of the spring contact  15  at the anchor portion  12  edge and leaves an insulating cover (not shown) on the anchor portion  12 . The insulating cover protects the anchor portion  12  from short-circuiting and also helps hold the anchor portion  12  down on the substrate  14 . 
     Only those areas of the insulating underlayer  13  under the free portion  11  of the spring contact  15  are under-cut etched. The area of insulating underlayer  13  under-cut etched for each spring contact  15  is described by the shaded portion in FIG.  15 . This means that the anchor portion  12  of the spring contact  15  remains fixed to the insulating underlayer  13  and does not pull away from the insulating underlayer  13 . It should be appreciated that the method for patterning the metal layer  16  into the spring contacts  15  should not result in any annealing of the metal layer  16 . 
     Additional steps can be added to the under-cut etching processes to improve the processes if necessary. For example, etchant vias, or small windows, can be etched into the free portions  11  of the spring contacts  15 . The etchant vias operate to provide the selective etchant faster access to the insulating underlayer  13 , thereby speeding the process of releasing the free portions  11  from the insulating underlayer  13 . Also, a hard mask, made of, for example, silicon, can be applied to the top surface of the spring contacts  15  to ensure that the etchant does not remove material from the top surface of the spring contacts  15  in case the photosensitive material  17  protecting the top of the spring contacts  15  fails during patterning of the spring contact  15 . 
     Once the free portion  11  is freed from the insulating underlayer  13 , the stress gradient Δσ/h causes the free portion  11  to bend up and away from the substrate  14 . The stress gradient Δσ/h is still inherent in the anchor portion  12  and urges the anchor portion  12  to pull away from the substrate  14 . 
     To decrease the chance of the anchor portion  12  pulling away from the substrate  14 , the spring contact  15  can be annealed to relieve the stress in the anchor portion  12 . This annealing process does not affect the free portion  11  because, once the free portion  11  is released and allowed to bend up, no stress remains on the free portion  11  to be relieved by annealing. Thus, the free portion  11  remains curved up and away from the substrate  14  after annealing. 
     Finally, FIG. 13 shows a layer of gold  19  plated over the outer surface of each spring contact  15 . The layer of gold  19  is preferably used to reduce the resistance in the spring contacts  15 , but can be replaced with any other conductive material. Preferably, the gold layer  19  is plated on the spring contacts  15  using an electroless plating process. 
     Since the process for forming the spring contacts  15  is limited only by the design rules of photolithographic patterning, many hundreds or thousands of spring contacts  15  can be formed closely together in a relatively small area on the substrate  14 . The typical width w of the spring contact  15  is 10-100 μm. Therefore, the spring contacts  15  can be formed close together, at a spacing of approximately 10-20 μm. This makes the center-to-center distance between adjacent spring contacts  15  approximately 20-120 μm, which is within or less than the typical center-to-center distance between adjacent contact pads  3  on a standard semiconductor chip  2 . 
     To test the effectiveness of the spring contacts  15  in applications similar to those found in solder-bump flip-chip bonding, a test array of the spring contacts  15  at a center-to-center spacing of 80 μm was developed as shown in FIG.  16 . Four sets of arrays  20  of the spring contacts  15  were formed on a bottom substrate  21 . Four corresponding arrays of linked contact pads  22  were formed on an upper substrate  23 . The upper substrate  23  and the lower substrate  21  were brought together such that the spring contacts  15  contacted a corresponding contact pad  3 . The resistance R was then measured across pairs of the spring contact  15  leads. 
     FIG. 17 graphically depicts the measured resistance R for each spring contact pair in the test apparatus. The measured resistance R within each array generally trends upward from left to right because of the increased conductor length of the spring contacts  15  positioned to the right compared to the spring contacts  15  positioned to the left in each array. Most of the approximately 25-30 ohms of resistance measured for each spring contact  15  pair is due to the length and geometry of the conductors extending between the spring contacts  15  and the resistance R probing points. 
     FIG. 18 shows the total resistance of the connection between a spring contact  15  and corresponding contact pad  3  with most of the resistance R shown in FIG. 17 removed by using a 4-point probing geometry. As shown in FIG. 18, approximately 1.3 ohms of resistance is due to the conductors leading to the contact pad  3  and the spring contact  15 . Approximately 0.2 ohms of resistance is due to the shape of the spring contact tip  30 . The remaining resistance, approximately 0.1 ohms for b&lt;80 μm, is the resistance at the interface between the contact pad  3  and the spring contact tip  30 . 
     In general, the resistance at the interface between the contact pad  3  and the spring contact tip  30  decreases as the height b decreases. As mentioned above, the reaction force F tip  that the spring contact tip  30  exerts against the contact pad  3  increases as the contact pad  3  pushes the spring contact tip  30  closer to the substrate  14 . The increased reaction force F tip  causes the spring contact tip  30  to locally deform at the contact pad  3 , thereby increasing the contact area and decreasing the resistance at the interface. 
     The shape of the spring contact tip  30  can take different forms, depending on the application. Since the spring contacts  15  are photolithographically patterned, the spring contact tips  30  are easily formed in a variety of shapes. FIG. 19 shows a spring contact tip  30  having a flat end. The spring contact tip  30  shown in FIG. 20 has a pointed end which concentrates the force F tip  exerted by the spring contact  15  at a single point on the contact pad  3 . This pointed shape aids the spring contact tip  30  when breaking through some oxides which may be present on the contact pads  3 . FIGS. 21 and 22 show spring contact tips  30  having multiple points for applications where contact redundancy is required. FIG. 23 shows a spring contact tip  30  having a deformable tab. The deformable tab increases the contact area with the contact pad  3 , by deforming as shown in FIG. 24 when the spring contact  15  forces the tip  30  against the contact pad  3 . 
     Other methods are used to lower the contact resistance between the spring contact tip  30  and the contact pad  3 . The spring contact tips  30  can be ultrasonically scrubbed into the contact pads  3  to increase the area of contact. Also, the spring contact tips  30  and the contact pads  3  can be coated with solder  52  which is melted after the tips  30  and the contact pads  3  are brought into contact. Melting the solder bonds the spring contacts  15  to the contact pads  3 . 
     As mentioned above, since the production of the spring contacts  15  is limited only by the design rules of photolithographic patterning, the spring contacts  15  can be used to interconnect numerous different types of devices. For example, FIG. 25 shows one preferred embodiment of the invention. The spring contacts  15  are formed on the lower surface of the chip  2 . The spring contacts  15  contact corresponding contact pads  3  on the substrate  14 . The adhesive  24  holds the chip  2  stationary with respect to substrate  14 . FIG. 26 shows the substrate  14  having a plurality of spring contacts  15  formed on the top surface of the substrate  14 . The contact pads  3  formed on the lower surface of the chip  2  are electrically connected to corresponding spring contacts  15  on the substrate  14 . An adhesive  24  holds the chip  2  stationary relative to a dust cover, or can,  25  covering the chip  2  and hermetically seals the dust cover  25  to the substrate  14 . The dust cover  25  assures that moisture and other foreign substances do not corrode the spring contacts  15  or the contact pads  3 , or otherwise interfere with the electrical connections between the individual spring contacts  15  and the corresponding contact pads  3 . Optional cooling fins  50  and the dust cover  25  provide a heat sink to cool the chip  2 . FIG. 27 shows an alternate form of the embodiment shown in FIG.  26 . The adhesive  24  holds the chip  2  stationary to the substrate  14 . No heat sink is provided by the dust cover  25 . 
     FIG. 28 shows an alternate embodiment of a connecting device for electrically connecting two devices. A wafer  26  is shown having a plurality of spring contacts  15  formed on opposite sides of the wafer. Pairs of the spring contacts  15  on opposite sides of the wafer  26  communicate with each other by way of vias etched in the wafer  26  and electrically connect the contact pads  3  on both the chip  2  and the substrate  14 . This embodiment of the invention allows processing of the chip  2  and the substrate  14  without risking damage to the spring contacts  15 . The wafer  26  is used to interconnect the chip  2  and the substrate  14  only after all processing is completed on the chip  2  and the substrate  14 . 
     The spring contacts  15  are not limited to interconnecting a chip  2  to a substrate  14  or circuit board. The spring contacts  15  are used equally well to interconnect two chips  2 , two circuit boards, or other electronic devices to each other. Two exemplary applications are mounting driver chips to visual displays and assembling multi-chip modules (MCM&#39;s) for computers. Another alternative use for the spring contacts  15  is in probe cards. As discussed above, probe cards  7  are used to temporarily connect two devices, typically when one of the devices is tested. Such testing is common in the semiconductor industry, where the probe cards  7  are used to test semiconductor chips while the chips are still part of a single-crystal silicon wafer. 
     FIG. 29 shows an embodiment of the invention where the probe card  27  has an array of spring contacts  15  used in place of the standard probe needles  8 . The probe card  27  operates identically to the standard probe card  7  except for having spring contacts  15 . The probe card  27  is aligned with the device  10  such that the spring contacts  15  compliantly contact the corresponding contact pads  3  on the device  10 . The device  10  is then tested or communicated with by a testing device electrically connected to the probe card  27 . 
     An example testing device is shown in FIG. 30 which is more thoroughly described in the application U.S. patent application Ser. No. 08/473,912, now abandoned, filed concurrently herewith. A display pattern generator  40  communicates with driver chips  42  mounted on the two full-width probe cards  27 . The probe cards  27  have the spring contacts  15  which contact associated addressing lines  43  formed on the display plate  44 . The addressing lines  43  communicate with display electrodes (not shown). Therefore, the display pattern generator  40  can drive the display electrodes to produce a matrix of electric potentials corresponding to a test image. Sensors (not shown) on the sensor plate  45  detect the matrix of electric potentials on the display electrodes and generate signals each corresponding to the electric potential. The signals are read out by scanner chips  46  mounted on the sensor plate  45 . The test signal analyzer  41  receives the signals from the scanner chips  46  and forms a sensed image corresponding to the signals. The test signal analyzer  41  then compares the sensed image with the test image output by the display pattern generator  40  to determine if the display plate  44  and display electrodes are working properly. 
     Since producing a standard probe card  7  having probe needles  8  is labor intensive and time-consuming, standard probe cards  7  are not generally made to contact all of the addressing lines  43  on the display plate  44 . Therefore, testing of the display plate  44  must be done in sections since the probe cards  7  cannot accommodate the full width of the addressing lines  43 . In contrast, the probe card  27  made with spring contacts  15  can be made easily and inexpensively. Also, the probe cards  27  having the spring contacts  15  can be made to any width and therefore can test all of the data or address lines of an apparatus, such as the display shown in FIG. 30, at one time. 
     In another example, wafer-scale testing and burning-in of chips  2  can be performed by a single probe card  27  contacting all contact pads  3  of all chips  2  while the chips  2  are still part of a single semiconductor wafer. The probe card  27  can be a silicon wafer containing microcircuitry to distribute test signals to and from each chip  2  on the wafer under test. The test signals can be distributed either all at once or sequentially to the chips  2 . 
     While the invention has been described with reference to specific embodiments, the description of the specific embodiments is illustrative only and is not to be construed as limiting the scope of the invention. Various other modifications and changes may occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.