Abstract:
A multi-axis compliance spring includes at least an anchor portion attached to a surface of a substrate and a free portion detached from the substrate. The free portion includes at least one first section having at least one curvature originating from an internal stress in the free portion and a second section having at least one second curvature defined in a plane of the substrate prior to being detached from the free portion of the substrate.

Description:
This non-provisional application claims the benefit of U.S. provisional application No. 60/382,602, filed on May 24, 2002. The entire disclosure of the provisional application is hereby incorporated by reference herein in its entirety. 

   This invention was made with United States Government support under ATP Program Award no. 70NANB8H4008 award by NIST. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   This invention relates to photolithographically-formed microelectronic interconnect structure. 
   2. Description of Related Art 
   Design and fabrication of integrated circuits (ICs) have made rapid advances which require similar advances in the design and fabrication of chip-to-substrate electrical interconnect structures, and of high density substrates. For example, millions of transistors can be fabricated onto a 10×10 mm die with input/output (I/O) pad densities reaching about 1600/cm 2 . This fabrication will eventually reach a billion transistors or more on a 10×10 mm die, and, in response, the die-to-substrate I/O pad density will eventually reach about 6000/cm 2  or more. 
   SUMMARY OF THE INVENTION 
   If the die-to-substrate interconnect technologies do not keep pace with advances in semiconductor technologies, a bottleneck may result that hinders future advances in semiconductor technologies. In addition to fabrication constraints, low cost and reliability also influence die-to-substrate interconnect developments. 
   This invention provides systems and methods for a multi-axis spring compliance interconnect structure that comprises an anchor portion and a released portion. The released portion comprises a release portion, a curved portion and a tip portion. The interconnect structure may be defined by a plurality of geometric parameters which includes a length (L), a width (W), a subtended angle (α), and an inner radius (R), among other parameters. Various geometric parameters of the interconnect structure influence a release height (H) of the interconnect structure as well as, for example, the X, Y and Z-axis components of the spring compliance. 
   In various embodiments, the multi-axis spring compliance interconnect structure has an elastic member that includes an elastic material and that has an inherent stress gradient. In various embodiments, the elastic member has an anchor portion, a release portion, a curved portion and a tip portion. In various embodiments, the multi-axis spring compliance of the interconnect structure is produced by a thickness of the elastic member. In various embodiments, the multi-axis spring compliance of the interconnect structure is produced by a width of the elastic member. In various embodiments, the multi-axis spring compliance of the interconnect structure is produced by a length of the release portion. In various embodiments, the multi-axis spring compliance of the interconnect structure is produced by a subtended angle of the curved portion. In various embodiments, the multi-axis spring compliance is produced by an inner radius of the curved portion. 
   These and other features and advantages of the invention are described in, or are apparent from, the following description of various exemplary embodiments of the systems and methods according to the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of this invention will be described in detail with reference to the following figures, wherein: 
       FIG. 1  shows a conventional solder bump “flip-chip” bonding technique for bonding a chip to a substrate; 
       FIG. 2  shows a conventional technique for establishing a temporary electrical contact between two devices; 
       FIG. 3  is a top view of a first exemplary embodiment of an interconnect structure having multi-axis spring compliance according to this invention; 
       FIGS. 4 and 5  are graphs that show the compliance of the interconnect structure in the X, Y and Z axis dimensions, as a function of the length of the interconnect structure for the exemplary embodiment of the interconnect structure shown in  FIG. 3  along the X, Y and Z axes; 
       FIG. 6  is a schematic diagram that shows the X-axis compliance and the Y-axis compliance of the interconnect structure with respect to the length of the interconnect structure for the exemplary embodiment of the interconnect structure shown in  FIG. 3 ; 
       FIGS. 7 ,  8  and  9  are schematic diagrams that show the influence of a subtended angle (α) on the compliance of the interconnect structure for the exemplary embodiment of the interconnect structure shown in  FIG. 3 ; 
       FIGS. 10 and 11  are graphs that show the compliance of the interconnect structure in the X, Y and Z-axis directions as a function of the subtended angle (α) of the interconnect structure for the exemplary interconnect structure shown in  FIG. 3 ; 
       FIGS. 12 and 13  are graphs that show the compliance of the interconnect structure in the X, Y and Z-axis directions as a function of the inner radius (R) of the interconnect structure for the exemplary embodiment of the interconnect structure shown in  FIG. 3 ; 
       FIGS. 14 and 15  are graphs that show the compliance of the interconnect structure in the X, Y and Z-axis directions as a function of the width (W) of the interconnect structure for the exemplary embodiment of the interconnect structure shown in  FIG. 3 ; 
       FIG. 16  is a side view of a plurality of the interconnect structures shown in  FIG. 3 ; 
       FIG. 17  shows a metal strip with no stress gradient; 
       FIG. 18  shows a model usable to determine a curvature of an interconnect structure according to this invention due to a stress gradient; 
       FIG. 19  shows a model usable to determine an amount of reaction force exerted at the tip of an interconnect structure according to this invention; 
       FIGS. 20–23  outline one exemplary embodiment of a method for forming an exemplary multi-axis spring compliance interconnect according to the invention; 
       FIG. 24  is a top view of a second exemplary embodiment of an interconnect structure having multi-axis spring compliance according to this invention; 
       FIG. 25  shows a first exemplary embodiment of a chip electrically connected to a substrate using an interconnect structure having multi-axis spring compliance according to this invention; 
       FIG. 26  shows a second exemplary embodiment of the chip electrically connected to the substrate using the interconnect structure having multi-axis spring compliance according to this invention; 
       FIG. 27  shows a third exemplary embodiment of the chip electrically connected to the substrate by using the interconnect structure having multi-axis spring compliance according to this invention; 
       FIG. 28  shows one exemplary embodiment of an immediate wafer interconnect structure having multi-axis spring compliance according to this invention; 
       FIG. 29  shows a probe card having a plurality interconnect structure having multi-axis spring compliance according to the invention that is usable to test an electrical device; and 
       FIG. 30  shows a liquid crystal display and a device for testing the operation of the liquid crystal display. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a conventional solder bump “flip-chip” bonding technique for bonding contact pads  3  formed on a chip  2  to the contact pads  3  formed on a substrate  1 . Solder bumps  6  are formed on the contact pads  3  of the substrate  1  or on those of the chip  2 . The corresponding contact pads  3  are electrically connected by pressing the contact pads  3 , which the solder bumps  6  are not formed on, against the solder bumps  6  and melting the solder bumps  6 . The deformation of the solder bumps  6  compensate for some irregularity in the heights of the contact pads  3  and any uneven contacting pressure forcing those contact pads  3  against the solder bumps  6 . However, the flip-chip bonding technique suffers from mechanical and thermal variations in the solder bumps  6 . For instance, if the solder bumps  6  are not uniform in height, or if the substrate  1  is warped, contact between some contact pads  3  and corresponding solder bumps  6  can be broken. Also, if the contacting pressure forcing those contact pads  3  against 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 between the contact pads  3  formed by the solder bumps  6 . 
     FIG. 2  shows a conventional 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  electrically connects 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, for example, a matrix of display electrodes which are part of an active-matrix liquid crystal array display. 
   The probe card  7  has many more applications other than for testing a liquid crystal display. 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, conventional techniques for producing the probe card  7  are time consuming and labor intensive. Each probe card  7  is 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 card  7  is 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 . 
     FIG. 3  shows a top view of an exemplary interconnect structure  100  having multi-axis spring compliance (C) according to this invention. As shown in  FIG. 3 , in various exemplary embodiments, the interconnect structure  100  is disposed on a substrate  200 . The multi-axis spring compliance interconnect structure  100  includes an anchor portion  120  and a released portion  130 . The released portion  130  comprises a release portion  132 , a curved portion  134  and a tip portion  136 . The interconnect structure  100  is defined by a plurality of geometric parameters which includes a length (L)  140 , a width (W)  150 , a subtended angle (α)  160 , and an inner radius (R)  170 , among other parameters. Various geometric parameters of the interconnect structure  100  influence a release height (H)  180  (see, for example,  FIG. 16 ) of the interconnect structure  100  as well as, for example, the X, Y and Z-axis components C x , C y  and C z  of the spring compliance (C). The X-Y plane may represent a substrate surface  210  containing the interconnect structure  100 . The Z-axis may represent the direction normal to the substrate surface  210 . The influences the various geometric parameters have in shaping the interconnect structure  100  allow for the interconnect structure  100  to have a predetermined compliances C x , C y  and C z  in each of the X, Y, and Z-axis, directions respectively, that, for example, accommodates the differential displacement between a die to be connected to the substrate  200  by the interconnect structure  100  and the substrate  200  due to mechanical displacements and/or mismatches in the coefficients of thermal expansion (CTE) of these elements under thermal excursions. 
     FIGS. 4 and 5  are graphs that show the compliance C x , C y  and C z  of the interconnect structure  100  in each of the X, Y and Z-axis directions and having 90° subtended angle  160 . In particular,  FIGS. 4 and 5  illustrate that the compliance C of the interconnect structure  100  increases as the interconnect length (L)  140  of the interconnect structure  100  increases. Usually, the compliance C z  in the Z-axis direction is larger than the compliances C x  and C y  in either the X-axis direction or the Y-axis direction. This may be explained by the moment of inertia I for a rectangular cross section of the interconnect structure  100 , which can be expressed as: 
             I   =       WT   3     12             (   1   )               
   W is the width of the cross-section of the interconnect structure  100 ; and 
   T is the thickness of the cross-section of the interconnect structure  100 . 
   In various exemplary embodiments, the interconnect structure  100  has a thickness of approximately 1 μm and has a width of approximately 10 μm. Since the Z-axis moment of inertia I z  is smaller than the X-Y plane moment of inertia I xy , the compliance C z  of the interconnect structure  100  in the Z-axis direction is larger than both of the compliances C x  and C y  of the interconnect structure  100  in the X-axis direction and the Y-axis direction. 
   In various exemplary embodiments, the compliance C x  of the interconnect structure  100  in the X-axis direction is smaller than the compliance C y  of the interconnect structure  100  in the Y-axis direction. As shown in  FIG. 6 , the X-axis extends along the longitudinal direction of the interconnect structure  100 . The Y-axis extends perpendicular to the X-axis within an X-Y plane. In addition, because the subtended angle  160  of the interconnect structure  100  is approximately 90°, the subtended angle  160  influences the compliances C x  and C y  of the interconnect structure  100  in the X-axis and the Y-axis directions. The influence of the subtended angle (α)  160  on the X, Y and Z-axis compliances C x , C y  and C z , respectively, of the interconnect structure  100  will be further described with respect to  FIGS. 7–11 . 
   Using the anchor portion  120  of the interconnect structure  100  as a pivot of the X-Y plane rotation anchored on the substrate  200  and the origin of the X-Y plane coordinate system, the normal distance from the interconnect tip portion  136  to the X-axis is less than the distance from the interconnect tip portion  136  to the Y-axis. The displacement of the tip portion  136  with respect to the anchor portion  120  is measured after a force is applied to the interconnect tip portion  136 . The difference between these two distances becomes larger as the length  140  increases. Consequently, the Y-axis compliance C y  of the interconnect structure  100  increases faster than the X-axis compliance C x  of the interconnect structure  100 . As the interconnect length  140 , and thus the release height (H)  180 , plays a role in the X, Y and Z-axis compliances C x , C y  and C z  of the interconnect structure  100 , various geometric parameters that provide for a desired release height (H)  180  at the interconnect tip portion  136  need to be considered. 
     FIGS. 7–9  illustrate the influence of the subtended angle  160  on the compliance C of the interconnect structure  100 . In the exemplary embodiment of the interconnect structure  100  illustrated in  FIGS. 7–9 , the interconnect structure  100  has a thickness of approximately 1 μm, a width (W)  150  of approximately 10 μm, a release height (H)  180  of approximately 30 μm and an inner radius (R)  170  of approximately 50 μm. However, the subtended angle (α)  160  of the interconnect structure  100  is approximately 90°, 135° and 180° in  FIGS. 7 ,  8  and  9 , respectively. As shown in  FIGS. 7–9 , the released height (H)  180 , which is the normal distance from the substrate surface  210  to the interconnect tip portion  136 , increases monotonically until the subtended angle (α)  160  begins to exceed a threshold angle α T . Depending on how the interconnect structure  100  is formed, the threshold angle α T  is somewhere in the range of approximately 120°. Beyond this threshold angle α T , any further increase in the subtended angle (α)  180  causes the interconnect tip portion  136  to fall back towards the substrate surface  210 . 
   In the exemplary embodiment shown in  FIG. 7 , the subtended angle (α)  160  is approximately 90°. In this exemplary embodiment, the release height (H)  180  increases continuously as the length (L)  140  increases. In this exemplary embodiment, the maximum height is located at the interconnect tip portion  136 . However, in the exemplary embodiments shown in  FIGS. 8 and 9 , for the subtended angles (α)  160  of approximately 135° and 180°, respectively, the release height (H)  180  does not increase monotonically. Instead, in the exemplary embodiments shown in  FIGS. 8 and 9 , the release height (H)  180  reaches a maximum height at approximately roughly 120°. The release height then starts to decrease from this maximum height. 
     FIGS. 10 and 11  are graphs that show the compliances of the interconnect structure  100  shown in  FIG. 3  in the X, Y and Z-axis directions C x , C y  and C z , respectively, as a function of the subtended angle (α)  160 . As shown in  FIG. 11 , the Z-axis compliance C z  of the interconnect structure  100  increases and reaches a maximum when the subtended angle (α)  160  is approximately 150°. After the subtended angle (α)  160  reaches approximately 150°, the Z-axis compliance C z  of the interconnect structure  100  decreases with further increases in the subtended angle (α)  160 . For the X-axis compliance C x , the normal distance from the interconnect tip portion  136  to the release portion  132  keeps increasing as long as the subtended angle (α)  160  increases, as shown in  FIG. 10 . However, the rate of increase becomes smaller C x  as the subtended angle (α)  160  approaches to 180°. For the Y-axis compliance of the interconnect structure  100 , the normal distance from the interconnect tip portion  136  to the anchor portion  120  increases with the subtended angle (α)  160  until the subtended angle (α)  160  exceeds approximately 90°. The Y-axis compliance C y  of the interconnect structure  100  decreases as the subtended angle (α)  160  increases when the subtended angle (α)  160  is between approximately 90° and approximately 180°. In particular, for some exemplary embodiments of the interconnect structure  100  according to this invention, the optimum compliance C of the interconnect structure  100  increases in the X-axis, the Y-axis and the Z-axis directions is reached when the subtended angle (α)  160  is approximately 120°. Therefore, in the exemplary embodiments, the optimum release height (H)  180  of the interconnect tip portion  136  is obtained when the subtended angle (α)  160  is approximately 120°. 
     FIGS. 12 and 13  are graphs that show the influence of the inner radius (R)  170  on the X, Y and Z-axis compliances C x , C y  and C z , respectively, of the interconnect structure  100 . As shown  FIGS. 12 and 13 , increasing the inner radius (R)  170  increases all of the X-axis; Y-axis and Z-axis compliances C x , C y  and C z , respectively, of the interconnect structure  100 . Unlike the interconnect length (L)  140 , increasing inner radius (R)  170  simultaneously makes the normal distances from the interconnect tip portion  136  to the X-axis and the Y-axis longer. Therefore, the X-axis compliance C x  and the Y-axis compliance C y  is inner radius compliance curve are approximately parallel to each other. The Z-axis compliance C z  of the interconnect structure  100  also increases as the inner radius (R)  170  increases. 
     FIGS. 14 and 15  are graphs that show the influence of the width (W)  150  on the X, Y and Z-axis compliances C x , C y  and C z , respectively, of the interconnect structure  100 . As shown in  FIGS. 14 and 15 , increasing the width (W)  150  decreases all of the X-axis, Y-axis and Z-axis compliances C x , C y  and C z . For example, the X-axis compliance C x  and the Y-axis compliance C y  significant decrease when the width (W)  150  increases from approximately 5 μm to approximately 10 μm. However, because narrow interconnects structures  100  are susceptible to damage, a wider interconnect structure  100  may be desirable at the expense of its compliance (C). 
     FIG. 16  shows a side view of a number of the interconnect structures  100  having a multi-axis compliance disposed on the substrate  200 . Each of the interconnect structures  100  has an anchor portion  120  that is fixed to an insulating underlayer  230  and electrically connected to a contact pad  240  formed on the substrate  200 . One or more of the interconnect structures  100  may be made of an extremely elastic material, such as a chrome-molybdenum (MoCr) alloy or a nickel-zirconium (NiZr) alloy. In various embodiments, at least some of the interconnect structures  100  are formed using a conductive elastic material, although one or more of the interconnect structures  100  can be formed of a nonconductive or semi-conductive material if such an interconnect structure  100  is coated or plated with a conductive material. In various embodiments, one or more of the interconnect structures  100  are formed using an alloy comprising about 80% molybdenum (Mo) and about 20% chromium (Cr) by weight. The elastic properties of such an interconnect structure  100  is mainly attributed to the elastic properties of MoCr. When the elastic material used to form the interconnect structures  100  is not conductive, such interconnect structures  100  can be coated on at least one side with a conductive material, such as a metal or metal alloy. 
   The contact pad  240  on the substrate  200  may be a terminal end of a communication line that electrically communicates with an electronic element or device, such as a transistor, a display electrode, or any other known or future-developed electronic element or device. The contact pad  240  is often made of aluminum, but can be made of any conductive material. If the contact pad  240  is made of aluminum, the contact pad  240  may be coated with gold, indium tin oxide, nickel or the like. The insulating underlayer  230  is often made of silicon nitride or other etchable insulating material. In various embodiments, the insulating underlayer  230  may not be necessary and can be omitted. The insulating underlayer  230  and the contact pad  240  on the substrate  200  are formed on or over the substrate  200 . The substrate  200  is also often formed of an insulating material, such as oxidized silicon or glass. 
   As shown in  FIG. 17 , a strip of metal having no stress gradient inherent in the metal will lie flat. However, as shown in  FIG. 18 , when the strip is bent into an arc, a uniform stress gradient Δσ/h is introduced into the strip. In contrast, if a uniform stress gradient Δσ/h is introduced into the flat metal strip, that metal strip will bend into an arc shape. 
   In  FIG. 18 , each interconnect structure  100  is formed such that a stress gradient Δσ/h is introduced into that interconnect structure  100 . When such an interconnect  100  is formed, the metal layer  110  comprising the interconnect structure  100  is deposited such that compressive stress is present in the lower portions, as indicated by arrows  112  in the metal layer  110  and tensile stress is present in the upper portions of the metal layer  110 , as indicated by upper arrows  114 . The stress gradient Δσ/h causes the interconnect structure  100  to bend into the shape of an arc having a radius r. The radius of curvature r of the interconnect structure  100 , as a function of the stress gradient Δσ/h, is: 
               r   =       (     Y     1   -   v       )     ⁢     h     Δ   ⁢           ⁢   σ           ,           (   2   )             
 
   where: 
   Y is the Young&#39;s modulus of the material; 
   h is the thickness of the material layer  110  forming the interconnect structure  100 ; 
   Δσ is the total stress difference in the metal layer  110 ; and 
   v is the value of Poisson&#39;s ratio for the material forming the metal layer  110 . 
   As shown in  FIG. 16 , since, in various exemplary embodiments, the interconnect structure  100  is made of a highly elastic material, while the interconnect structure  100  may be pushed down at the interconnect tip portion  136  and deformed, the interconnect structure  100  will not plastically deform. Typically, a contact pad  240  of a device exerts the downward force placed on the interconnect tip portion  136  and electrically contacts the interconnect tip portion  136 . The interconnect structure  100  resists the downward force placed on the interconnect tip portion  136  and maintains electrical contact with the contact pad  240 . 
   When the force on the interconnect tip portion  136  is released, the interconnect structure  100  will return to its original state. Thus, the elasticity of the interconnect structure  100  allows the interconnect structure  100  to make numerous successive electrical connections with different contact pads  240  while maintaining the integrity of the electrical connection between the interconnect tip portion  136  and the contact pad  240 . 
   Additionally, in various exemplary embodiments, the interconnect structure  100  is made of a creep-resistant material. Therefore, when the interconnect structure  100  is elastically deformed over an extended period by being pressed down by a contact pad  240  placed against on the interconnect tip portion  136 , the interconnect structure  100  resists the downward force and pushes the interconnect tip portion  136  against the contact pad  240 , maintaining the electrical connection. 
     FIG. 19  shows a model for determining the amount of force F tip  applied by the interconnect tip portion  136  to a contact pad  240  in reaction to the force of the contact pad  240  pressing down on the interconnect tip portion  136 . The reaction force F tip  of the interconnect tip  136  portion is: 
                 F   tip     =       wh2   ⁢           ⁢   Δσ       12   ⁢   x         ,           (   3   )               
   where 
   w is the width of the interconnect structure  100 ; 
   h is the thickness of the interconnect structure  100 ; 
   Δσ is the total stress difference; and 
   x is the horizontal distance from the interconnect tip  136  to the point where the interconnect structure  100  first touches the substrate  200 . 
   As indicated by Eq. (3), for a given width w, thickness h and stress difference Δσ, the reaction force F tip  of the interconnect tip  136  varies inversely with the distance x. Therefore, as shown in  FIG. 16 , the reaction force F tip  increases as the interconnect tip portion  136  gets closer to the substrate  200 , since the distance x decreases as the interconnect structure  100  is pressed against the substrate  200 . The increase in the reaction force F tip  as the contact pad  240  presses the interconnect tip portion  139  closer to the substrate  200  generally improves the electrical connection between the interconnect tip portion  136  and the contact pad  240 . The increasing reaction force F tip  causes the interconnect tip portion  136  and/or the contact pad  240  to deform locally at the area of contact, increasing the area of contact between the contact pad  240  and the interconnect tip portion  136 . 
   Referring back to  FIG. 3 , it should be appreciated that because of the design and processing of the interconnect structure, the curved portion  134  has its axis of curvature substantially perpendicular to the top surface of the interconnect structure. Also, referring to  FIG. 16 , it should be appreciated that because of the stress gradient present in the interconnect structure, the released portion  130  has its axis of curvature substantially parallel to a top surface of the interconnect structure. 
     FIGS. 20–23  show one exemplary embodiment of a method for forming the interconnect structure  100 . As shown in  FIG. 20 , a contact pad  240  is formed on or over the substrate  200 . Additionally, the insulating underlayer  230  is formed on or over the substrate  200 . However, as mentioned above, in various exemplary embodiments, the insulating underlayer  230  is not necessary and may be omitted. 
   As shown in  FIG. 21 , the layer of metal  110  is deposited on or over the substrate  200 . In various exemplary embodiments, the metal is the MoCr alloy described above. Part of the metal layer  110  is electrically connected to or directly contacts the contact pad  240 . Another portion of the metal layer  110  is deposited on or over the insulating underlayer  230 . There are many techniques available for depositing the metal layer  110  on or over the substrate  200 , including electron-beam deposition, thermal evaporation, chemical vapor deposition, sputter deposition, electroplating and other appropriate known or future-developed techniques. In various exemplary embodiments, the metal layer  110  is sputter deposited. 
   In various exemplary embodiments, the metal layer  110  is deposited in several sub-layers  110   1  to  110   n  to a final thickness h of approximately 1 μm. The desired stress gradient Δσ/h is introduced into the metal layer by altering the stress inherent in each of the sub-layers  110   1  to  110   n  of the metal layer. That is, as formed, each sub-layer  110   x  has a different level of inherent stress. 
   If sputter deposition is used to form the metal layer, different stress levels can be introduced into each sub-layer  110   x  of the deposited metal layer  110  in a variety of ways, including adding a reactive gas to a plasma used during sputter deposition, depositing the metal at an angle, and changing the pressure of the plasma. In various embodiments, the different levels of stress are introduced into the metal layer  110  by varying the pressure of the plasma gas, which is preferably argon. 
   In various exemplary embodiments, the metal layer  110  is deposited in several sub-layers with different intrinsic stress, which results in the metal layer  110  having a stress gradient Δσ/h which is compressive in the lower portion of the metal layer  110  and becomes increasingly tensile toward the top of the metal layer  110 . Although the stress gradient Δσ/h urges the metal layer  110  to bend into an arc, the metal layer  110  initially adheres to the insulating underlayer  230 , the substrate  200  and/or the contact pad  240  and thus lies flat. 
   After the metal layer  110  is deposited, the metal layer  110  is photolithographically patterned to form the individual interconnect structures  100 . Photolithographic patterning is a well-known technique and is routinely used in the semiconductor chip industry. As shown in  FIG. 21 , according to one technique, a positive photosensitive resist layer  250  is spun on top of the metal layer  110  and soft-baked at 90° C. to drive off solvents in the resist layer  250 . The photosensitive resist layer  250  is exposed to an appropriate dose of ultra-violet light and then developed. Exposed areas of the resist layer  250  are removed during developing and the remaining resist layer  250  is hard-baked at 120° C. Wet or plasma etching is then used to remove the exposed areas of the metal layer  110 . The areas of the metal layer  110  under the remaining portions of the resist layer  250  that remain after etching form the interconnect structure  100 .  FIG. 24  shows a top view of one exemplary embodiment of such an interconnect structure  100 . The area of the metal layer  110  removed by etching is described by the dashed line  102 . 
   Next, as shown in  FIG. 22 , the released portion  130  of the interconnect structure  100  is released from the insulating underlayer  230  by under-cut etching. Until the released portion  130  is released from the insulating underlayer  230 , the released portion  130  adheres to the insulating underlayer  230  and the interconnect structure  100  lies flat on the substrate  200 . There are two methods for releasing the released portion of the interconnect structure  100  from the substrate  200  or insulating underlayer  230 . In a first method, the insulating underlayer  230 , typically silicon nitride, is deposited by plasma chemical vapor deposition (PECVD) at a temperature of 60–250° C. This gives the insulating underlayer  230  a fast etch rate. The insulating underlayer  230  is then pre-patterned, before the metal layer  110  is deposited, into islands on which the interconnect structure  100  will be formed. 
   After the interconnect structures  100  are formed on or over the islands of the insulating underlayer  230 , the released portion  130  of the interconnect structures  100  are released from the islands of the insulating underlayer  230  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  230  faster than the selective etchant removes metal from the metal layer  110 . This means that the released portions  130  of the interconnect structures  100  are released from the insulating underlayer  230  and are allowed to bend up and away from the insulating underlayer  230  due to the stress gradient Δσ/h in the interconnect structure  100 . The islands of the insulating underlayer  230  can also be formed of a low-melting-temperature material, such as solder or plastic. In this technique, after the interconnect structure  100  is formed, the low-melting-temperature material is heated to release the released portion  130  of the interconnect structure  100 . 
   In a second method for releasing the released portion  130  of the interconnect structure  100 , the insulating underlayer  230 , if used, is not pre-patterned into islands. Instead, after the interconnect structure  100  is formed, a passivation layer, such as silicon oxynitride, is deposited on the interconnect structure  100  and the surrounding areas by PECVD. The passivation layer is patterned into windows, such as the shaded area shown in  FIG. 24 , to expose the released portion  130  of the interconnect structure  100  and surrounding areas of the insulating underlayer  230 . The same selective etchant, the HF solution, can be used to etch the insulating underlayer  230  and release the released portion  130  of the interconnect structure  100 . This method avoids a step discontinuity in the material of the interconnect structure  100  at an edge of the anchor portion  120  and leaves an insulating cover on or over the anchor portion  120 . The insulating cover protects the anchor portion  120  from short-circuiting and also helps hold the anchor portion  120  down on to the substrate  200 . 
   Only those areas of the insulating underlayer  230  under the released portion  130  of the interconnect structure  100  are under-cut etched. The areas of insulating underlayer  230  under-cut etched for each interconnect structure  100  is described by the shaded portion in  FIG. 24 . This means that the anchor portions  120  of the interconnect structure  100  remain fixed to the insulating underlayer  230  and do not pull away from the insulating underlayer  230 . It should be appreciated that in various exemplary embodiments the method for patterning the metal layer  110  into the interconnect structure  100  does not result in any annealing of the metal layer  110 . 
   Additional steps can be added to the under-cut etching process to improve the process, if necessary. For example, etchant vias, or small windows, can be etched into the released portion  130  of the interconnect structure  100 . The etchant vias provide faster access to the selective etchant to the insulating underlayer  230 , speeding up the process of releasing the released portions  130  from the insulating underlayer  230 . Also, a hard mask, made of, for example, silicon, can be applied to the top surface of the interconnect structure  100  to ensure that the etchant does not remove material from the top surface of the interconnect structure  100  in case the photosensitive material  250  protecting the top of the interconnect structure  100  fails during patterning of the interconnect structure  100 . 
   Once the released portion  130  is freed from the insulating underlayer  230 , the stress gradient Δσ/h causes the released portion  130  to bend up and away from the substrate  200 . The stress gradient Δσ/h is still inherent in the anchor portion  120  and urges the anchor portion  120  to pull away from the substrate  200 . 
   To decrease the chance that the anchor portion  120  will pull away from the substrate  200 , the interconnect structure  100  can be annealed to relieve the stress in the anchor portion  120 . This annealing process does not affect the released portion  130  because, once the released portion  130  is released and allowed to bend up, no stress remains on the released portion  130  to be relieved by annealing. Thus, the released portion  130  remains curved up and away from the substrate  200  after annealing. 
     FIG. 23  shows a layer of gold  190  plated over the outer surface of the interconnect structure  100 . The layer of gold  190  may be used to reduce the resistance in the interconnect structure  100 , but any other conductive material can be used in place of gold. In various exemplary embodiments, the gold layer  190  is plated on the interconnect structures  100  using an electroless plating process. 
     FIG. 25  shows a first exemplary embodiment of a chip  300  electrically connected to a substrate  320  using an interconnect structure  100  having multi-axis spring compliance according to this invention. As shown in  FIG. 25 , the interconnect structures  100  are formed on the lower surface of the chip  300 . The interconnect structures  100  contact corresponding contact pads  322  on the substrate  320 . The adhesive  310  holds the chip  300  stationary with respect to the substrate  320 . As shown in  FIG. 26 , a substrate  400  has a plurality of interconnect structures  100  formed on a top surface of the substrate  400 . The contact pads  422  formed on the lower surface of a chip  420  are electrically connected to the corresponding interconnect structures  100  on the substrate  400 . The contact pads  422  formed on the lower surface of the chip  420  are thus electrically connected to the corresponding interconnect structures  100  on the substrate  400 . A first adhesive  410  holds the chip  420  stationary relative to a dust cover  430  covering the chip  420  and a second adhesive  450  hermetically seals the dust cover  430  to the substrate  400 . The dust cover  430  assures that moisture and other foreign substances do not corrode the interconnect structures  100  or the contact pads  422 , or otherwise interfere with the electrical connections between the individual interconnect structures  100  and the corresponding contact pads  422 . Optional cooling fins  440  and the dust cover  430  provide a heat sink to cool the chip  420 . As shown in  FIG. 27 , in various other exemplary embodiments, the adhesive  420  holds the chip  420  stationary to the substrate  400 . In the exemplary embodiment shown in  FIG. 27 , no heat sink function is provided by the dust cover  430 . 
   As shown in  FIG. 28 , in various other exemplary embodiments, a connecting device for electrically connecting two devices includes an intermediate wafer  500 . The intermediate wafer  500  has a plurality of interconnect structures  100  formed on opposite sides of the wafer  500 . Pairs of the interconnect structures  100  on opposite sides of the wafer  500  communicate with each other by way of vias etched in the intermediate wafer  520  and electrically connect the contact pads  522  and  532  on both a chip  520  and a second substrate  530 . In these exemplary embodiments, the chip  520  and the second substrate  530  without risking damage to the interconnect structures  100 . The wafer  500  is used to interconnect the chip  520  and the substrate  530  only after all processing is completed on the chip  520  and/or the second substrate  530 . 
   The interconnect structures  100  are not limited to interconnecting the chip  520  to the second substrate  530 , such as a circuit board. The interconnect structures  100  are equally well usable to interconnect two chips, two circuit boards, or any other two 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. Various other exemplary uses for the interconnect structures  100  are in probe cards. As discussed above, probe cards  7  (see  FIG. 2 ) are used to temporarily connect two devices, typically when one of the devices is to be 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 a probe card  600  that has an array of the interconnect structures  100  used in place of the standard probe needles  8  used in the probe card  7 . The probe card  600  operates identically to the standard probe card  7 , except for having the interconnect structures  100 . The probe card  600  is aligned with the device  10  such that the interconnect structures  100  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  600 . 
     FIG. 30  shows an examplary testing device. A display pattern generator  610  communicates with one or more driver chips  620  mounted on the two full-width probe cards  600 . The probe cards  600  have the interconnect structures  100 , which are used to contact associated lines  710  formed on a display plate  700 . The addressing lines  720  communicate with display electrodes (not shown). Therefore, the display pattern generator  610  can drive the display electrodes to produce a matrix of electric potentials corresponding to a test image. Sensors (not shown) on the sensor plate  730  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  740  mounted on the sensor plate  730 . The test signal analyzer  750  receives the signals from the scanner chips  740  and forms a sensed image corresponding to the signals. The test signal analyzer  750  then compares the sensed image with the test image output by the display pattern generator  610  to determine if the display plate  700  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  720  on the display plate  700 . Therefore, testing the display plate  700  must be done in sections, since the probe cards  7  cannot accommodate the full width of the addressing lines  720 . In contrast, the probe card  600  that uses the interconnect structures  100  can be made easily and inexpensively. Also, the probe cards  600  that uses the interconnect structures  100  may 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 various exemplary embodiments, wafer-scale testing and burning-in of the chips may be performed by a single probe card contacting all of the contact pads of all chips while the chips are still part of a single semiconductor wafer. The probe card may be a silicon wafer containing microcircuitry to distribute test signals to and from each chip on the wafer under test. The test signals can be distributed either all at once or sequentially to the chips. 
   While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of this invention.