Patent Publication Number: US-6713374-B2

Title: Interconnect assemblies and methods

Description:
This application is a divisional application of U.S. patent application Ser. No. 09/364,855, filed Jul. 30, 1999 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to interconnect assemblies and methods for making and using interconnections and methods for making these interconnect assemblies. 
     BACKGROUND OF THE INVENTION 
     There are numerous interconnect assemblies and methods for making and using these assemblies in the prior art. Interconnect assemblies for making electrical interconnections with semiconductor integrated circuits must support the close spacing between interconnect elements, sometimes referred to as the pitch of the interconnect elements. Certain interconnect assemblies perform their functions through testing and the useful life of the integrated circuit. One type of interconnect assembly in the prior art uses a resilient contact element, such as a spring, to form either a temporary or a permanent connection to the contact pads on a semiconductor integrated circuit. Examples of such resilient contact elements are described in U.S. Pat. No. 5,476,211 and also in co-pending U.S. patent application entitled “Lithographically Defined Microelectronic Contact Structures,” Ser. No. 09/032,473, filed Feb. 26, 1998, and also co-pending U.S. patent application entitled “Interconnect Assemblies and Methods,” Ser. No. 09/114,586, filed Jul. 13, 1998. These interconnect assemblies use resilient contact elements which can resiliently flex from a first position to a second position in which the resilient contact element is applying a force against another contact terminal. The force tends to assure a good electrical contact, and thus the resilient contact element tends to provide good electrical contact. 
     These resilient contact elements are typically elongate metal structures which in one embodiment are formed according to a process described in U.S. Pat. No. 5,476,211. In another embodiment, they are formed lithographically (e.g. in the manner described in the above-noted patent application entitled “Lithographically Defined Microelectronic Contact Structures”). FIG. 1A illustrates an example of resilient contact elements which are formed using a technique described in U.S. Pat. No. 5,476,211. FIG. 1B shows an example of a resilient contact element which is formed using lithographic techniques such as those described in the above-noted U.S. patent application entitled “Lithographically Defined Microelectronic Contact Structures.” In general, resilient contact elements are useful on any number of substrates such as semiconductor integrated circuits, probe cards, interposers, and other electrical assemblies. For example, the base of a resilient contact element may be mounted to a contact terminal on an integrated circuit or it may be mounted onto a contact terminal of an interposer substrate or onto a probe card substrate or other substrates having electrical contact terminals or pads. The free end of each resilient contact element can be positioned against a contact pad on another substrate to make an electrical contact through a pressure connection when the one substrate having the resilient contact element is pressed towards and against the other substrate having a contact element which contacts the free end of the resilient contact element. 
     It will be appreciated that in certain instances, it may be desirable to secure the free end to the corresponding contact element by an operation such as soldering. However, in many instances, it is appropriate to allow the contact to be made by pressure between the two substrates such that the contact end of the resilient contact element remains free. 
     The resilient contact elements are useful for making electrical contacts because their resilience maintains pressure for good electrical contact and because they allow for tolerance in the vertical or Z direction such that all contact elements will be able to make a contact even if their heights vary slightly. However, this pressure sometimes leads to the deformation or degradation of resilient contact elements as they are compressed too much in the vertical direction. One approach to prevent the deformation or degradation of such resilient contact elements is to use a stop structure on one of the two substrates. The stop structure effectively defines the maximum deflection of the resilient contact element such that each of the resilient contact elements is prevented from overflexing (undue deflection) or being destroyed as a result of the two substrates being pressed toward each other. FIG. 1A shows an example of an integrated circuit having contact pads  103  and having, for each of the contact pads, a resilient contact element  110  mounted thereto. A plurality of stop structures  104  and  105  are disposed on the surface of the integrated circuit  102 . These stop structures will prevent undue deflection and may engage another substrate which is pressed towards the surface of the semiconductor integrated circuit  102 . 
     FIG. 1B shows an example of a lithographically defined resilient contact element on a substrate, such as a semiconductor integrated circuit  120 . The integrated circuit includes on its surface a stop structure  150 . 
     The lithographically defined resilient contact structure of FIG. 1B includes an intermediate layer  123  which makes an electrical interconnection with the bonding pad  122  through an opening in the passivation layer  121  on the surface of the substrate of the integrated circuit  120 . A first metal layer  125  and a second metal layer  126  are then formed to create a beam having a step  128  and a beam portion  127 . In this example, the beam portion is substantially parallel to the surface of the substrate  120 . A tip structure including components  181 ,  182 ,  183 ,  184 , and  185  is then subsequently mounted to the end of the beam  127  to create a resilient contact structure. Further details concerning methods for creating and using such lithographically defined resilient contact structures are described in the co-pending U.S. patent application entitled “Lithographically Defined Microelectronic Contact Structures” which is referred to above and which is hereby incorporated herein by reference. While this lithographically defined resilient contact element provides the advantage that it can be formed lithographically using modern photolithographic techniques which are prevalent in the semiconductor industry, there are certain disadvantages with this type of resilient contact element. For example, when a force F as shown in FIG. 1B is applied downwardly against the tip  185 , a torqueing action occurs at the base of the resilient contact element. This torqueing action results from the pressure contact which occurs when a contact element on another substrate is pressed towards the tip  185 . This torque at the base tends to place stress at the base and along the beam portion. If the beam portion  127  is a rectangular shape, this results in the stress being localized at the portion of the beam which is next to the base of the resilient contact element. While the stop structure  150  provides some assurance that certain levels of stress will not be exceeded, there are still certain concentrated areas of stress, as a result of the rectangular shape, which must be accounted for when designing a resilient contact element. Typically, accounting for these concentrated areas of stress requires increasing the use of the amount of materials at certain points in the resilient contact element. This in turn may limit the ability to design resilient contact elements to be smaller. This is particularly undesirable as the sizes of structures on semiconductor integrated circuits are being reduced over time. 
     Thus, it is desirable to provide an improved resilient contact element. 
     SUMMARY OF THE INVENTION 
     The present invention provides an interconnect assembly and methods for making and using the assembly. An exemplary embodiment of an aspect of the invention includes a contact element which includes a base portion adapted to be adhered to a substrate and a beam portion connected to and extending from the base portion. The beam portion has a beam geometry which is chosen to optimize stress. The beam portion is in one embodiment substantially triangular in shape and is adapted to be freestanding. 
     An exemplary embodiment of another aspect of the invention involves a method for forming a contact element. This method includes forming a base portion to adhere to a substrate of an electrical assembly and forming a beam portion connected to the base portion. The beam portion has a beam geometry which is chosen to optimize stress. The beam portion extends from the base portion and is in one embodiment substantially triangular in shape and is adapted to be freestanding. 
     In another example of the present invention, a resilient electrical contact element includes a base portion which is attached to a substrate and a beam portion which is connected to and extends from the base portion. The beam portion is adapted to be free-standing and has a beam geometry which is selected to produce a substantially optimized stress parameter given a desired size constraint of the beam portion and a desired spring constant (e.g. spring rate) for the resilient contact element. In one embodiment of this example, the geometry is selected to improve performance of the resilient contact element over a cantilever beam which has a substantially constant cross-sectional area. The performance may be improved by, for example, substantially evenly distributing stress throughout the beam when it is deflected. 
     It will be appreciated that in certain embodiments of the invention, a plurality of contact elements are used together to create an interconnect assembly. 
     Various other assemblies and methods are also described below in conjunction with the following figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements. 
     FIG. 1A shows an example of a plurality of resilient contact elements disposed on a substrate with stop structures on that substrate. FIG. 1A is a perspective view of the substrate  102  with its resilient contact elements and stop structures. 
     FIG. 1B shows an example of a lithographically formed resilient contact element formed on a substrate with a stop structure. FIG. 1B is a cross-sectional view of the resilient contact element and the stop structure. 
     FIG. 2A is a cross-sectional view of a resilient contact element according to one embodiment of the present invention. 
     FIG. 2B is a cross-sectional view of another embodiment of a resilient contact structure of the present invention with a stop structure disposed over a base of the resilient contact element. 
     FIG. 2C shows a cross-sectional view of another example of an embodiment of a resilient contact element according to the present invention in which a stop structure is disposed over a base of the resilient contact element. 
     FIG. 2D shows a cross-sectional view of another example of a resilient contact element of the present invention. 
     FIGS. 2E,  2 F,  2 G, and  2 H show cross-sectional views which compare the deflection of an initially straight resilient contact element, shown in FIG. 2E, to the deflection of an initially curved resilient contact element shown in FIG.  2 G. 
     FIG. 3A is a top view of one example of a resilient contact element according to the present invention. FIG. 3B is a top view of another example of a resilient contact element according to the present invention. FIG. 3C is a top view of another example of a resilient contact element according to the present invention. FIG. 3D shows, in top view, an array of resilient contact elements disposed on a substrate. 
     FIG. 4 shows, in cross-sectional view, an example of an interconnect assembly of the present invention which includes a resilient contact element having a stop structure disposed over the base of the contact element and FIG. 4 also shows another substrate being brought into contact with the substrate carrying the stop structures and the resilient contact element. 
     FIG. 5 is a flowchart showing an example of a method for forming a resilient contact element and a stop structure according to one embodiment of the present invention. 
     FIG. 6A shows a cross-sectional view of the structure of an interconnect assembly during the formation of a resilient contact element according to one method of the present invention. 
     FIG. 6B shows another cross-sectional view at a later processing stage in the fabrication process which produces a resilient contact element according to one embodiment of the present invention. 
     FIG. 6C shows a top view of a portion of the structure  608  shown in FIG.  6 B. 
     FIG. 6D shows a cross-sectional view of the structure which results after several processing steps shown in FIG.  5 . 
     FIG. 6E shows a cross-sectional view of an alternative substrate onto which a resilient contact element of the present invention may be formed. 
     FIG. 6F shows a cross-sectional view after certain processing steps in the method of FIG.  5 . 
     FIG. 6G shows another cross-sectional view after further processing steps of the method of FIG.  5 . 
     FIG. 6H shows a top view of a portion of the structure shown in FIG.  6 G. 
     FIG. 6I shows another cross-sectional view of the structure being formed according to the method shown in FIG.  5 . 
     FIG. 6J shows another cross-sectional view of a resilient contact element as it is being formed according to the method shown in FIG.  5 . 
     FIG. 6K shows a further cross-sectional view after further processing steps according to the method of FIG.  5 . 
     FIG. 6L shows a top view according to one embodiment of the structure shown in FIG.  6 K. 
     FIG. 7A shows in cross-sectional view an embodiment with multiple resilient contact elements and their corresponding stop structures. 
     FIG. 7B shows a perspective view of one resilient contact element disposed within a well formed by the contact structure according to one exemplary embodiment of the present invention. 
     FIG. 8A shows an alternative embodiment in cross-sectional view of a resilient contact element according to the present invention. 
     FIG. 8B shows in perspective view the resilient contact element of FIG. 8A after the element has been assembled. 
     FIGS. 9A,  9 B,  9 C and  9 D show cross-sectional views of a substrate during another method, according to the invention, for forming a resilient contact element. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to interconnection assemblies and methods and particularly to interconnect assemblies and methods for making mechanical and electrical connection to contact elements on an integrated circuit. The following description and drawings are illustrative of the invention and are not to be construed as limiting the invention. Numerous specific details are described to provide a thorough understanding of the present invention. However, in other instances, well known or conventional details are not described in order to not unnecessarily obscure the present invention in detail. 
     FIG. 2A shows a cross-sectional view of a resilient contact element which is one example of an embodiment of the present invention. The resilient contact element  201  of FIG. 2A includes a first metal layer  209  and a second metal layer  210  which have been formed at an oblique angle relative to the surface of the substrate  202 . The first metal layer  209  may be selected for its resiliency properties in order to impart resiliency to the resilient contact element and the second metal layer  210  may be selected for its electrical conductivity properties in order to provide good electrical conductivity for the resilient contact element. The substrate  202  is typically a semiconductor substrate which includes an integrated circuit which includes various terminals, one of which is shown as the contact pad  207  in FIG.  2 A. Representative terminals carry input/output signals, power or ground. This contact pad  207  is coupled through a wiring layer  206  to internal circuitry within the integrated circuit. The wiring layer  206  is disposed within an insulating layer  204 , and layers  206  and/or  204  may be covered by a passivating layer  205  on the top surface of the substrate  202 . The layer  203  may be an insulating layer or a polysilicon layer or other layers which are known and used in integrated circuits. The contact pad  207  is electrically and mechanically coupled to a shorting layer  208  which is disposed above the contact pad  207 . The metal layers  209  and  210  are formed above the shorting layer  208  with a base portion of each of the metal layers  209  and  210  above and electrically connected to contact pad  207 . Electrical conductivity between the contact pad  207  occurs through the shorting layer  208  and the metal layer  209  and finally to the metal layer  210 . It will be appreciated that the resilient contact element  201  of FIG. 2A may also be used for other types of interconnect assemblies such as probe card assemblies or interposers or other connection systems where a resilient contact element may provide electrical conductivity from a conductor in the substrate  202  to the tip, which is free-standing, of the resilient contact element  201 . As can be seen from FIG. 2A, this resilient contact element is both free-standing and elongate. Also, the resilient contact element, in the embodiment shown in FIG. 2A, is sloped relative to the surface of the substrate  202 . In a preferred embodiment, this slope typically forms an oblique angle relative to the surface of the substrate  202 . 
     In one particular embodiment of the present invention, the resilient contact element  201  may be formed such that the beam portion which extends from the base has a substantially triangular shape. This is shown in the top view of the resilient contact structure  301  shown in FIG.  3 A. That is, the beam portion  215  of the resilient contact element  201  may be formed in such a way as to have a substantially triangular shape such as the beam portion  303  of the resilient contact element  301  shown in FIG.  3 A. The resilient contact element  301  includes a base  302  which is attached to the beam portion and a tip  304  which makes electrical contact with another interconnect terminal/pad. FIG. 3B shows, in a top view, another example of a resilient contact element  301 A which has a beam portion  303 A which has a substantially triangular shape. The beam portion  303 A is attached to the base  302 A and includes a tip  304 A. 
     FIG. 2B shows another embodiment of the present invention in which a stop structure  211  is disposed on the top surface of the substrate  202  and is also disposed on top of a portion of the base  214  of the resilient contact element  212 . In this embodiment shown in FIG. 2B, the substrate  202  includes the similar components  203 ,  204 ,  205 ,  206 ,  207 ,  208 ,  209  and  210  as in the case of the interconnect assembly shown in FIG. 2A. A resilient contact element  212  includes two metal layers  209  and  210  which have been formed such that a base portion  214  of both metal layers rests on the upper surface of the shorting layer  208  while beam portion  215  extends from an end of the base portion  214  at an oblique angle relative to the surface of the substrate  202 . The beam portion of the resilient contact element  212  may have a triangular shape or other shapes such as rectangular shapes or other shapes which are known to be used for resilient contact elements. In one embodiment, however, it is preferred that the beam  215  have a triangular shape such as the beam  303  of FIG. 3A or beam  303 A of FIG.  3 B. The stop structure  211  is formed over the base portion  214  of the resilient contact element  212 . The stop structure  211  acts to prevent overflexing (e.g. undue deflection) of the resilient contact element by determining the maximum amount of deflection of the resilient contact element  212  as it is pressed by a force F towards the surface of the substrate  202 . When another contact element, such as a contact pad on another substrate, is pressed toward and against the tip  216  of the resilient contact element  212 , the upper surface of the stop structure  211  will engage a corresponding surface of the other substrate to prevent the substrate from being pressed further towards the surface of the substrate  202 . That is, the travel between the two substrates is limited by the stop structure  211  such that the stop structure  211  will define or determine a minimum distance between the substrates and hence a maximum flexing of the resilient contact element  212 . Further details in connection with advantages of and uses of stop structures are described in co-pending U.S. patent application Ser. No. 09/032,473, which was filed Jul. 13, 1998 and is entitled “Interconnect Assemblies and Methods;” this co-pending patent application entitled “Interconnect Assemblies and Methods” is hereby incorporated herein by reference. 
     FIG. 2C shows a simplified example of a resilient contact structure with a stop structure according to one exemplary embodiment of the invention. The interconnect assembly  231  includes a substrate  232  which has a wiring layer  237  embedded therein. The wiring layer  237  is electrically coupled to a base portion  235  of a resilient contact element  234 . The base portion  235  is coupled to the beam portion  236 . Typically, as described below, the beam portion  236  and the base portion  235  are formed integrally in one operation. Both the base portion  235  and the beam portion  236  may contain multiple conductive layers such as the layers  208 ,  209 , and  210  as shown in FIGS. 2A and 2B. As in the case of the embodiments shown in FIGS. 2A and 2B, the beam portion  236  of the resilient contact element  234  extends from an end of the base portion  235  at an oblique angle relative to the surface of the substrate  232 . The stop structure  233  is disposed above and adhered to the surface of the substrate  232  and it is also disposed above and adhered to the top surface of the base portion  235 . As shown in FIG. 2C, an end of the wiring layer  237  is mechanically adhered to and electrically coupled to the base portion  235 . Since the stop structure  233  is mechanically adhered to the top surface of the substrate  232  and to the top surface of the base portion  235 , any torque created at the base portion  235  (e.g. as a result of a force exerted against the tip of the beam portion  236  when a contact pad, such as the contact pad  411  shown in FIG. 4, is pressed towards the surface of the substrate  232 ) will be balanced through various parts of the entire structure, including the stop structure  233 , rather than merely the base portion  235  and the beam portion  236 . It will be appreciated that, in an alternative embodiment, a stop structure may be disposed on only the base portion  235  and not the substrate  232  and that this stop structure will tend to balance any torque created at the base portion. 
     Several dimensions are labeled in FIG.  2 C and also FIG. 3A in order to provide examples of the present invention. It will be appreciated that the following examples are merely illustrative of the invention and are not intended to limit the invention and are not intended to provide an exhaustive set of examples for possible structures which are within the scope of the present invention. The distance c represents in one example (e.g. see FIG. 4) the maximum displacement or deflection of the resilient contact element. In one example, this is approximately 75 microns (about 3 mils). This distance c is normally less than the element&#39;s compliance which is the total amount of deflection or flexing which is available before “failure” of the resilient contact element. This distance in one example (e.g. see FIG. 4) represents the travel of the tip from a point where there is no force pressing against the tip to the point where the tip has been pressed flush with the top surface of the stop structure  233 . Examples of these two states are shown in FIG. 2C where no force is being applied to the tip and FIG. 4 where a force has been applied to the tip such that one substrate  410  has its surface virtually in contact with the top surface of the stop structures  406  and  407 . 
     The thickness t represents the total thickness of the resilient contact element in a perpendicular cross-section of the resilient contact element as shown in FIG.  2 C. Typically, this thickness will be approximately the same throughout any particular cross-section, although it will be appreciated that in alternative embodiments, this thickness may be different at certain cross-sections. It will also be appreciated that FIG. 2C represents a simplified example of a resilient contact element  234  and that the base and the beam portions may be fabricated from one or more conductive layers. In a typical embodiment, the thickness is approximately 25 microns, and the total height h of the resilient contact element  234  is approximately 150 microns (about 6 mils). 
     For certain embodiments, the spring rate of the resilient contact element may be approximately 1 to 2 grams per mil, where the spring rate “k” represents the force of the spring divided by the displacement of the spring (k=F/x). It will be appreciated that once the geometry and the materials have been selected for a particular resilient contact element, the thickness of the spring generally determines the spring rate. Thickness is a major factor in determining the stress at a maximum deflection, which is also dependent upon the material in the resilient contact element. It will be appreciated that certain design points or goals will be established for a resilient contact element such that the maximum possible stress for the resilient contact element is less than a desired amount. The use of the stop structure  233  will tend to control the maximum amount of stress and also tend to relieve the stress at maximum deflection by counteracting the torque at the base portion of the resilient contact element. This can be seen from FIG. 4 which will be further described below. 
     The resilient contact element may have a curved shape which provides certain advantages that are described here in conjunction with FIGS. 2D,  2 E,  2 F,  2 G and  2 H. FIG. 2D shows a cross-sectional view of a curved resilient contact element  234 A which includes a base portion  235 A, a tip  239 A and a curved beam portion  236 A. The base portion  235 A is coupled electrically to the wiring layer  237 . The curvature of the beam portion may be used to compensate for bending of the beam portion during deflection of the resilient contact element. This compensation is shown in the comparison shown by FIGS. 2E,  2 F,  2 G and  2 H. 
     A stop structure  233  is disposed on the base portion  235 A and (optionally) on the substrate  232 . 
     FIGS. 2E and 2F respectively show an initially straight resilient contact element before and after deflection of the resilient contact element. When a force F is applied to the tip of the beam portion  236 , the beam portion curves as shown in FIG. 2F, and this causes the tip to be tilted away from the surface of the contact point on the other substrate and causes a shallow angle θ 1  to be formed between a line representing the surface of the contact point and a line through the end of the beam portion. These lines are shown in FIG. 2F along with the angle θ. The shallowness of this angle tends to be undesirable for making a good electrical connection. 
     FIGS. 2G and 2H respectively show an initially curved resilient contact element before and after deflection of the resilient contact element. When the force F is applied to the tip of the beam portion  236 A, this beam portion tends to be straightened out as shown in FIG.  2 H. As a result of this straightening out, the angle θ 2  (the angle between the line representing the surface of the contact point and a line through the end of the beam portion) is not as shallow as angle θ 1  and thus better electrical contact may be accomplished using the curved resilient contact element. 
     FIG.  3 A and FIG. 3B show another aspect of the present invention in which the beam portion of the resilient contact element has a substantially triangular shape. The resilient contact element  301  includes a base  302  which corresponds to the base  235  in FIG.  2 C. The beam portion  303  shown in the top view of FIG. 3A corresponds to the beam portion  236  of FIG.  2 C. The beam portion  303  includes a tip  304  which corresponds to the tip  239  of the resilient contact element  234  of FIG.  2 C. The triangular shape of the beam portion  303  tends to more evenly distribute across (e.g. tip to base) the entire beam the stress created when the tip  304  is pressed towards the surface on which the base  302  is resting. This allows the use of less material in the beam  303  while still obtaining good performance for the resilient contact element, such as the ability to withstand continued stress throughout the useful life of the resilient contact element. FIG. 3B shows an alternative triangular shaped resilient contact element. 
     FIG. 3C shows, in top view, another example of a resilient contact element  310  which has a substantially triangular shape. In this case, the triangle is modified at the electrical contact end  312  to include a portion of a rectangle. The beam portion of the resilient contact element  310  is normally free-standing and capable of deflections when depressed and includes a triangular part  314  and a rectangular part  312 . The triangular part is attached to the base portion  316 . The beam portion may be attached to the base portion  316  at an oblique angle and may be lithographically formed together with the base portion  316 . It can be seen that the cross-sectional areas taken at different cross-sections  318  and  320  will be different, and this is also true of the contact elements  301  and  301 A. 
     The geometry of the beam portions of these substantially triangular shaped resilient contact elements provides improved performance over a cantilever beam spring which has a substantially constant cross-sectional area. Other shapes may also be selected to provide improved performance relative to cantilever beam springs which have substantially constant cross-sectional areas. The substantially triangular shape gives better behavior of the spring under stress and allows for tighter packing (small pitches) for the springs. FIG. 3D shows an array of tightly packed (e.g. pitch of 10 microns) resilient contact elements  310 A,  310 B,  310 C and  310 D of the type shown in FIG. 3C on a substrate  322  (which may be an integrated circuit or an electrical connection substrate which may be used to test or make permanent contact to an integrated circuit). The geometry of the beam portion is selected to give optimum stress behavior for a given size constraint (e.g. the size as determined by the packing of the contact elements) and a given spring rate and a given/desired amount of compliance for the contact element. A smaller size, as determined by a size constraint, for a given spring rate and given compliance tends to place greater stress at certain points of a cantilever beam having a constant cross-sectional area. A substantially triangular shaped resilient contact element, on the other hand, allows for tighter packing (see, e.g. FIG. 3D) and smaller sizes while, for the same spring rate and given compliance, allowing better behavior under stress. 
     In a typical embodiment, the dimensions l 1  and l 2  are about 9 mils while the dimension l 3  is about 27 mils. An array of these resilient contact elements may be formed on a substrate, such as an integrated circuit or a contactor for making contact to an integrated circuit, to create a densely packed array of such resilient contact elements. FIG. 8B shows one example of a layout of an array on a substrate  803 . In one embodiment, these resilient contact elements may be packed so densely that the distance between corresponding points (e.g. bases) on adjacent contact elements is less than 30 mils and even as small as 0.1 mils (approximately 2.5 microns). FIG. 3D shows another example of an array of densely packed (tight pitch) resilient contact elements. 
     The actual use of an interconnect assembly according to one embodiment of the present invention will now be described while referring to FIG.  4 . The substrate  402  includes a resilient contact element  403  having a beam portion  405  and a base portion  404 . In this particular embodiment, the beam portion  405  may have a triangular shape or may have another shape such as a rectangular shape. Two stop structures  406  and  407  are adhered to the upper surface of the substrate  402  and the stop structure  406  is also adhered to the top portion of the base  404 . The base  404  makes mechanical and electrical contact with the wiring layer  415  in the substrate  402 . It will be appreciated that the substrate  402  may be a semiconductor integrated circuit or may be a passive wiring layer such as a probe card or interposer structure. Another interconnection assembly  410  having a contact pad  411  is brought into electrical and mechanical contact with the tip of the beam portion  405  as shown in FIG.  4 . The entire assembly  401 , which includes substrates  410  and  402 , is pressed together to assure a good pressure connection between the tip of the beam portion  405  and the surface of the contact pad  411 . The contact pad  411  makes electrical contact with other components through the wiring layer  412  of the substrate  410 . As the substrate  410  is pressed towards the substrate  402 , the tip of the beam portion  405  makes a wiping action which occurs laterally along the direction of the arrow  413  as shown in FIG.  4 . This improves the electrical contact between the beam portion  405  and the pad  411 , thus improving the electrical contact between the wiring layers  415  and  412 . It will be appreciated that FIG. 4 illustrates the circumstance where the substrate  410  is pressed completely against the substrate  402  and thus no further “travel” is possible (they cannot get closer). This represents the maximum deflection of the resilient contact element  403 . However, in use of the assembly  401 , the two substrates may not be abutting such that the resilient contact element  403  is deflected less than the maximum deflection while still achieving acceptable electrical conductivity between the beam  405  and the pad  411 . The final assembly of substrates  402  and  410  may be held in place by various techniques (with either maximum deflection of the resilient contact elements or less than maximum deflection) including by mechanical forces (e.g. F m  shown in FIG. 4) and/or by an adhesive applied between the surface of substrates  402  and  410  (not shown). 
     A method of fabricating an interconnect assembly will now be described; this method represents one particular example of the present invention and it will be appreciated that various other methods may be employed using alternative techniques and procedures. The method  500  shown in FIG. 5 begins with the operation  502 . This particular method assumes that a redistribution layer will be applied in order to redistribute contact pads having a certain arrangement and geometry to a set of another contact pads having a different arrangement and/or geometry. It will be appreciated that in other circumstances, a redistribution layer may not be necessary, and the contact elements of the invention may terminate directly on the wiring layers of the substrate without a redistribution layer; see FIG.  6 E. In operation  502 , a passivation layer, such as a polyimide layer, is applied to the upper surface of the substrate having a contact pad disposed on the upper surface. In one embodiment, a polyimide layer may be spun onto the upper surface to uniformly cover the surface. Then the passivation layer is patterned using conventional photolithography to create openings in the passivation layer over the contact pads at the surface of the substrate. These contact pads may be input/output interconnections in a semiconductor integrated circuit or they may be contact terminals on a passive or active board or substrate such as a probe card or an interposer or other interconnect assemblies. 
     FIG. 6A shows an example of an interconnect assembly  601  having a contact  604  at the upper surface of the substrate  602 . A wiring layer  603  is disposed in the substrate  602  in order to provide electrical conductivity between the pad  604  and another component, such as a circuit within the substrate  602  or a contact terminal at another location on the substrate  602 . It will be appreciated that typically the wiring layer  603  is disposed within an insulating layer in the substrate  602 . It will also be appreciated that the portion shown in FIG. 6A is the upper portion of the substrate  602  and that other wiring layers and/or circuitry is disposed below this portion shown in FIG.  6 A. The passivation layer applied in operation  502  is shown as passivation layer  605  in FIG.  6 A. This passivation layer has been patterned to create an opening  606  over the contact pad  604 . 
     In operation  504  of FIG. 5, a shorting layer is applied over the surface of the passivating layer  605  and over the surface of the contact pad  604 . Then, a conventional photoresist is applied over the entire surface of the shorting layer and this photoresist is patterned using conventional photolithography to create an opening over the shorting layer. The shorting layer may be formed from copper, titanium, or titanium/tungsten or other appropriate metallization and may be sputtered onto the surface of layer  605 . An example of a resulting interconnect assembly after operation  504  is shown in FIG.  6 B. The interconnect assembly  608  of FIG. 6B includes a shorting layer  609  which is disposed above the passivation layer  605  and the contact pad  604 . The contact pad and the shorting layer  609  are in electrical conductivity. The patterned photoresist layer  610  includes an opening  611  over a portion of the shorting layer. This opening will be used to create a redistribution layer wherein the layout of the contact pad  604  will be redistributed to another location. This may be done to relax the interconnection pitch achieved at the end of the process when the resilient contact elements have been formed on the substrate  602 . FIG. 6C shows a top view of the redistribution wiring layer. In particular, FIG. 6C shows the pattern in the photoresist layer  610  which has created a hole in the patterned photoresist  610 . This hole exposes a portion of the shorting layer  609  as shown in FIG.  6 C. 
     In operation  506  of FIG. 5, a redistribution layer is applied over the shorting layer. This redistribution layer may be applied by electrolytically plating (or electroless plating) a metal layer (e.g. copper or gold) onto the exposed portion of the shorting layer, where the shorting layer is used as the cathode in an electrolytic plating operation. It will be appreciated that on a typical substrate there will be many such redistribution traces created on the surface of the substrate in each of the patterned openings in the patterned photoresist  610 . 
     After the redistribution layer has been applied, the patterned photoresist layer  610  is removed and the shorting layer which is exposed after removing the patterned photoresist layer is also removed. The redistribution layer may be used as a mask to remove the shorting layer. Thus, in this case, typically one metal (e.g. Ti/W) will be used to create a shorting layer and another metal (e.g. Cu) will be used to create the redistribution layer such that the redistribution layer will remain intact while the shorting layer is etched away or removed under the action of a solvent or etching agent which is resisted by the redistribution layer metal. The result of operation  506  is shown in FIG.  6 D. It can be seen that the patterned photoresist layer  610  has been removed and that the unprotected portion of the shorting layer  609  has also been removed, leaving the structure of the interconnect assembly  614  shown in FIG.  6 D. In an alternative embodiment in which the shorting layer  609  is required for subsequent electrolytic plating (e.g. because layer  642  has electrically isolated regions due to openings in the layer), then the shorting layer  609  is not removed in operation  506  but is removed in operation  514 . 
     It will be appreciated that operations  502 ,  504 , and  506  have been used to create a plurality of redistribution traces, such as a redistribution layer, on the surface of the substrate  602 . This may be required in certain cases where the contact pads were designed to be connected to an interconnect mechanism other than a resilient contact element or it may be desirable for other reasons. It will also be appreciated that in certain instances, no such redistribution layer is necessary and thus a resilient contact element may be fabricated on a via or other contact element on the surface of a substrate. An example of a via is shown in FIG. 6E in which a post  623  of conductive material is exposed at the upper surface  622  of the substrate  621 . This conductive post  623  is electrically coupled to the wiring layer  624  which is also in the substrate  621 . Typically, the material encasing the wiring layer  624  and the post  623  is an insulating layer. The substrate  621  will typically be a part of a semiconductor integrated circuit although other interconnect assemblies may also be used, such as printed wiring substrates, interposers, etc. The processing from the structure  620  shown in FIG. 6E is similar to the processing of the structure  614  shown in FIG. 6D from steps  508  through  516  of FIG.  5 . That is, while the structure  620  does not need to be processed in operations  502 ,  504 , and  506 , it may be processed with operations  508 ,  510 ,  512 ,  514 , and  516  as will structure  614  shown in FIG.  6 D. 
     The next operation in the method  500  is operation  508  in which a photoresist is applied and patterned to include an opening which has a slope on the photoresist. The resulting structure of the interconnect assembly is shown as structure  631  in FIG.  6 F. The photoresist  633  has been applied and patterned to create an opening  632  which has a slope  634  on a portion of the opening. The opening is disposed at least partially over a portion of the conductive layer  615 . It will be appreciated that typically, the flat portion of this opening will be utilized to construct a base for the resilient contact element while the sloped portion  634  will be used to construct the beam portion of the resilient contact element. 
     There are numerous techniques known in the art for creating openings in photoresist which include a slope on the photoresist. For example, a gray-scale mask having a relatively continuous gradient of opacity from clear to black may be used to create a slope on the photoresist. Other methods may be used to provide the tapered side wall, which include: gently reflowing the masking material to taper a side of the opening; controlling the light exposure intensity or time to the masking material; during exposure, varying the distance of the mask from the masking layer; exposing the masking layer two or more times, one through the mask having a small transparent area and separately with a mask having a larger transparent area; or combinations of these methods. Methods for forming an opening having a tapered side wall are further described in co-pending U.S. patent application entitled “Lithographically Defined Microelectronic Contact Structures” which has been referred to above and which is hereby incorporated herein by reference. Also described below is a method in which a mold is used to stamp a slope in a deformable material which is then used to form a resilient contact element. 
     After operation  508 , the operation  510  involves applying a seed layer and then applying and patterning a photoresist layer to create a triangular opening (in one exemplary embodiment) for a resilient contact element. The seed layer  642  shown in FIG. 6G may be applied by conventionally sputtering an appropriate metallic layer (e.g. Cu or Ti or Ti/W) onto the surface of the photoresist  633 . If the shorting layer  609  was removed in operation  506 , then the seed layer  642  should provide a continuous conductive surface except for the side wall  645 , but if the shorting layer  609  was retained in operation  506 , then the seed layer  642  may be electrically discontinuous over its entire surface. 
     In one preferred implementation, care is taken in the sputtering operation to avoid or prevent the sputtered material from remaining on the vertical side wall  645  so the opening in the photoresist  633  does not receive a continuous layer of the seed material. In another implementation, sputtering does cover some or all of the vertical side wall  645 . In this implementation, it is generally preferable that the subsequent masking and patterning step cover some or all of the sputtered side wall so as to minimize or prevent subsequent deposition of additional conductive materials on the vertical side wall. 
     After the seed layer  642  is applied, a photoresist layer is applied over the surface of the seed layer  642  as shown in FIG.  6 G. This photoresist layer is then patterned to create an opening  643  in the patterned photoresist layer  646  as shown in FIG.  6 G. This opening will be used to deposit at least one conductive layer, such as a metal layer, on the top of the exposed portion of the seed layer  642 . This will include the deposition of a beam portion of the resilient element on the sloping portion  644  of the exposed portion of the seed layer  642 . It is generally desirable for the subsequent plating or other deposition operation to fill the final shape contours in a regular manner. In one preferred embodiment, a seed layer is deposited so as to be primarily at the bottom of an opening in photoresist, particularly where the photoresist is from the masking and patterning phase of operation  510 . 
     FIG. 6G shows an example of a structure  641  after completion of the operation  510 . FIG. 6H shows a top view of a portion of the structure  641 . In particular, the portion of the structure  641  over the opening  643  is shown in the top view of FIG.  6 H. It can be seen that the patterned photoresist layer  646  exposes only a portion of the seed layer  642 . In the particular example shown in FIG. 6H, the base portion of the resilient contact element will be formed in the rectangular portion of the exposed seed layer  642  and the beam portion of the resilient contact element will be formed in the triangular portion of the exposed seed layer  642 . This will result in a resilient contact structure having a beam portion which is substantially triangular in shape, such as the resilient contact structure shown in FIG.  3 A. It will be appreciated that in other embodiments, other shapes, such as rectangular shapes, may be used for the beam portion and thus top views of such structures would look differently than FIG.  6 H. 
     After structure  641  has been created by completing the operation  510 , operation  512  is performed on the structure  641  to achieve the structure  651  shown in FIG.  61 . Operation  512  typically involves in one exemplary embodiment the electrolytic plating of a first metal layer and then a second metal layer into the triangular opening. In alternative embodiments, the opening may be a different shape (e.g. a rectangular shape to create a rectangular beam portion) and alternative methods may be employed to deposit one or more conductive layers into the opening. The seed layer  642  (or a retained underlying shorting layer if the seed layer  642  is electrically discontinuous over its surface) will be used as the cathode in the electrolytic plating operation to plate the metal layers  652  and  653  onto the exposed portion of the shorting layer in the opening  643  as shown in FIG.  6 I. In one embodiment, the first metal layer  652  is selected to provide sufficient mechanical resiliency so that the final resilient contact element has sufficient resiliency for its intended operation. In one particular embodiment, a nickel cobalt alloy may be used. This alloy may be 70% nickel and 30% cobalt. This alloy may be heat treated as described in co-pending application Ser. No. 08/931,923 filed Sep. 17, 1997. The second metal layer  653  is typically selected to provide good electrical conductivity; for example gold, or rhodium or a palladium cobalt alloy may be used. The composition of the various other layers and materials will be described further below. It will also be appreciated that numerous other types of materials may be selected for these metal layers, and these materials have been described in U.S. Pat. No. 5,476,211. 
     After one or more conductive layers have been deposited into the opening  643  and the structure  651  has been completed, then operation  514  is performed to remove the photoresist layers and the sputtered shorting layers to achieve the structure  661  shown in FIG.  6 J. Conventional solvents or dry etching methods for removing or stripping the photoresist layers are utilized and solvents or etchants which will selectively remove the sputtered seed layer, such as seed layer  642 , are used to remove the shorting layer. Thus, the patterned photoresist layer  646  and the patterned photoresist layer  636  is removed. Also, almost all of the seed layer  642  is removed except for the portion of the layer under the base of the resilient contact element  662  as shown in FIG.  6 J. This results in the structure shown in FIG. 6J which then may be used as an interconnect assembly (without a travel stop structure) or may be used with a travel stop structure after such structure has been created in operation  516 . 
     If it is desired to create a stop structure for the interconnect assembly, then step  516  is performed on the structure  661 . This operation involves the application of a photo-imageable material (PIM), such as a negative photoresist such as SU8 which is commercially available, over the entire surface of the structure  661 . It is desirable that this PIM be applied uniformly to a uniform thickness and relatively flat, preferably as flat as possible. Thus it is preferred to spin the PIM, such as a photoresist, onto the surface of the structure  661 . Care should be taken to apply enough photoresist material such that after it is spun on, it covers the base of the resilient contact element  662 . That is, the height of the final structure created by this spun-on photoresist should be more than the height h of the base of the resilient contact element. Typically, the height of the final structure should also be less than the height of the freestanding contact point of the resilient contact element. Typically, the difference between the height of the freestanding contact point and the height of the final stop structure created by this photoresist should be fabricated to be the maximum deflection amount c described above. 
     After the proper amount of photoresist has been applied to achieve the desired height of the photoresist relative to the height h of the base of the resilient contact element (elements), then the photoresist layer is exposed and developed. The photoresist layer is exposed through a mask  690  such that an area around and below the beam portion of the resilient contact element remains unexposed while adjacent regions become exposed. Because the PIM in one embodiment is a negative photoresist, this will mean that the area which did not receive an exposure due to the mask  690  will be developed to remove the photoresist (and the exposed portions of the photoresist will remain), creating the opening  674  as shown in FIG.  6 K. In one embodiment, this mask  690  may be a rectangular mask which provides sufficient clearance for the beam portion to move up and down in the opening  674 . In an alternative embodiment, the mask  690  may be a triangular mask which is designed to fit around the triangular shape of the beam portion of the resilient contact element when the resilient contact element has a triangular beam shape. Thus, the structure  671  shown in FIG. 6K results from operation  516  and produces a stop structure  672  which is adhered to the passivation layer  605  and to the conductive layer  615  and to the base portions  652 A and  653 A of the resilient contact structure. Note that a portion  673  of the stop structure  672  is adhered to and is over the base portions  653 A and  652 A. 
     FIG. 6L shows a top view of the structure shown in FIG.  6 K. FIG. 6L assumes that the mask  690  had a triangular shape rather than a rectangular shape. Thus, the opening in the patterned photoresist  672  which forms the stop structure is a triangular opening which matches the shape of the triangular beam portion while providing sufficient clearance for the beam portion to move up and down within the opening  674 . The base  653   a  is shown underlying the travel stop structure  672  in FIG.  6 L. In an alternative embodiment, a rectangular mask  690   a  may be used over the beam portion in the exposure operation  516  to create a rectangular opening  674   a  shown in FIG.  7 B. This rectangular opening in the stop structure  672  may be used either for a triangular beam portion as shown in FIG. 7B or may be used for a beam portion which is rectangular. It will be appreciated that whichever mask geometry is selected, it should provide sufficient clearance for the beam portion to move up and down within the opening in the stop structure  672 . 
     FIG. 7A shows a cross-sectional view of a plurality of resilient contact elements which include stop structures surrounding and disposed on the bases of the resilient contact elements. 
     FIG. 8A shows an alternative embodiment for creating a resilient contact element which includes three portions, a base portion, a beam portion, and a contact portion. The contact portion is useful for contacting a second electronic component as described above in detail. In addition, the contact portion may be used to attach a tip structure onto the end of the beam. Examples of various tip structures and methods for mounting tip structures are described in the co-pending U.S. patent application entitled “Lithographically Defined Microelectronic Contact Structures” and also in co-pending U.S. patent application Ser. No. 08/819,464, which was filed Mar. 17, 1997. In particular, FIG. 8A shows the structure after a stage in the fabrication process of an interconnection assembly after operation  512  (without operations  502 ,  504 , and  506  because this assembly does not utilize a redistribution layer). The structure of FIG. 8A includes a conductive post  604   a  disposed in an insulating layer of the substrate  602   a . The conductive post is electrically coupled to a seed layer  642   a  which corresponds to the seed layer  642  of FIG.  6 I. The seed layer  642   a  has been sputtered over a patterned photoresist layer  633   a  which includes an opening having a sloped side wall. The sputtered seed layer  642   a  is used to electrolytically plate two metal layers to create the structure shown in FIG.  8 A. The electroplating operation occurs through the patterned photoresist mask  646   a  which corresponds to the patterned mask  646  of FIG.  6 I. After the plating operation has occurred, each metal layer includes a base portion, such as the base portion  653   a , a beam portion, such as the beam portion  653   b , and a contact portion, such as the contact portion  653   c . After the electroplating operation is completed, operation  514  may be performed to remove the photoresist layers  646   a  and  633   a  and then the sputtered seed layer  642   a  may be removed except for the portion under the base  653   a . The resulting structure may be used as a resilient contact element in an interconnect assembly without a stop structure or the stop structure may be formed by performing operation  516  as described above. 
     FIG. 8B illustrates an array  800  of two resilient contact elements  801  and  802  on the surface of the substrate  803 . FIG. 8B is a perspective view and it will be appreciated that typically a large number of resilient contact elements may be disposed on the surface of the substrate, such as a semiconductor integrated circuit or other interconnect assemblies. The resilient contact elements shown in FIG. 8B are similar to the type shown in FIG. 8A in that they include a contact portion at the end of the beam structure. The beam structure, such as beam portion  653 B, may be a substantially triangular shape which is attached to the base portion  653 A. The contact portion  653 C may serve itself as the contact tip for making contact to another contact terminal (e.g. contact terminal  411  as shown in FIG. 4) or a tip structure as described above may be mounted onto the foot portion  653 C to provide a tip structure. This structure may also be adapted for permanent connection to the contact pad (e.g. by use of solder or conductive epoxy). 
     FIGS. 9A,  9 B,  9 C and  9 D illustrate another method for forming a resilient contact element according to the present invention. A mold  901 , which may be photolithographically formed, includes a “negative” image of at least a portion of a resilient contact element. The mold  901  is shown in FIG. 9A prior to being used on the deformable material  903  and is positioned above the deformable material  903 , which is disposed on a substrate  905  which includes wiring layer  906 . The substrate  905  and wiring layer  906  is similar to the structure shown in FIG.  6 E. The deformable material  903  may be any number of materials, such as PMMA (poly methyl methacrylate), which are deformable when pressed with a mold or stamp and which can be used to receive the deposition of spring metals to form a resilient contact element and can be subsequently removed. The mold  901  includes, in the embodiment shown in FIG. 9A, a base portion  901 B and a sloped portion  901 A. It will be appreciated that other geometries may be used, including a rotated “L” shape (e.g. ) or a shape that will generate the curved beam portion of FIG.  2 D. 
     The mold  901  is pressed into the deformable material as shown in FIG.  9 B. Several pounds of pressure may be necessary (depending on the type of deformable material) in order to deform the deformable material to achieve the desired shape. This in one exemplary embodiment causes the base portion  901 B to be nearly touching the surface of substrate  905 , leaving a thin region of the deformable material separating this surface from the base portion  901 B. The mold  901  forces the deformable material to take the positive of its negative shape as shown in FIG.  9 B. The mold  901  is then separated from the substrate  905  and deformable material  903 , leaving the structure shown in FIG.  9 C. This structure is then “cleaned” to remove the thin region  903 A of the deformable material which was under the base portion  901 B. The structure may be cleaned with an isotopic etch which removes all exposed deformable material but does not effect the substrate  905 . This etch is performed for a sufficient duration to remove all of the thin region  903 A while leaving most of the remainder of the deformable material  903 , including the sloped portion  903 B. The structure may be cleaned to remove the thin region  903 A with a plasma etch or a reactive ion etch or with a laser ablation etch. After removal of the thin region  903 A, the structure is shown in FIG. 9D, and it is ready for further processing (e.g. operations  510 ,  512 ,  514 , and  516  of FIG. 5) to create a resilient contact element using the molded deformable material. 
     There are numerous ways to create the mold. The mold may be formed from a silicon wafer by laser etching the surface of the wafer. A glass backing substrate with a photo-imageable material (which can be hardened) may be used with a mask to photolithographically define the mold. A silicon carbide wafer may be machined to define the mold by using electro-discharge machining techniques on the silicon carbide wafer. A negative of the mold may be formed in wax (e.g. paraffin), then sputtering a shorting layer on the surface of the negative, after which the mold is formed by electrolytically plating a metal onto the shorting layer on the wax. 
     The foregoing discussion has provided certain details with respect to materials and process steps. It will be appreciated that the present invention may be practiced with other types of materials and process variations. For example, in just some of many preferred implementations, the sputtered shorting layers may use copper, gold, aluminum, titanium, titanium/tungsten or other appropriate metallization. Further, the redistribution trace  615  may use copper or gold material in creating the trace. Other materials may be employed to achieve similar results as will be appreciated by those skilled in the art. As another alternative embodiment, it will be appreciated that other methods for forming various layers may be employed. For example, processes based on electroless plating, chemical vapor deposition (CVD), or phase vapor deposition (PVD) may be utilized rather than electrolytic plating. 
     In the foregoing specification, the present invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader scope and spirit of the invention as set forth in the appended claims.