Abstract:
A plurality of vertically spaced-apart microsprings are provided to increase microspring contact force, contact area, contact reliability, and contact yield. The microspring material is deposited, either as a single layer or as a composite of multiple sub layers, to have a tailored stress differential along its cross-section. A lower microspring may be made to push up against an upper microspring to provide increased contact force, or push down against a substrate to ensure release during manufacture. The microsprings may be provided with similar stress differentials or opposite stress differentials to obtain desired microspring profiles and functionality. Microsprings may also be physically connected at their distal ends for increased contact force. The microsprings may be formed of electrically conductive material or coated with electrically conductive material for probe card and similar applications.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to photolithographically patterned spring contacts, and more particularly to a plurality of such photolithographically patterned spring contacts vertically displaced from one another for use in electrically contacting integrated circuits, arrays, and the like.  
         [0003]     2. Description of the Prior Art  
         [0004]     Photolithographically patterned spring devices (referred to herein as “microsprings”) have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. Such microsprings are disclosed and described, for example, in U.S. Pat. No. 5,914,218, which is incorporated by reference herein. A microspring is generally a micrometer-scale elongated metal structure having a free (cantilevered) portion which bends upward from an anchor portion which is affixed directly or indirectly to a substrate. The microspring is formed from a stress-engineered metal film (i.e., a metal film fabricated to have a stress differential such that its lower portions have a higher internal compressive stress than its upper portions) that is at least partially formed on a release material layer. The free portion of the microspring bends away from the substrate when the release material located under the free portion is removed (e.g., by etching).  
         [0005]     The stress differential is produced in the spring material by one of several techniques. According to one technique, different materials are deposited in layers, each having a desired stress characteristic, for example a tensile layer formed over a compressive layer. According to another technique a single layer is provide with an intrinsic stress differential by altering the fabrication parameters as the layer is deposited. The spring material is typically a metal or metal alloy (e.g., Mo, MoCr, W, Ni, NiZr, Cu), and is typically chosen for its ability to retain large amounts of internal stress. Microsprings are typically produced using known photolithography techniques to permit integration of the microsprings with other devices and interconnections formed on a common substrate. Indeed, such devices may be constructed on a substrate upon which electronic circuitry and/or elements have previously been formed.  
         [0006]     Such microsprings may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, scanning probes, and actuated mirrors. For example, when utilized in a probe card application, the tip of the free portion of a microspring is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the microspring as an electrical contact).  
         [0007]     Microsprings typically terminate at a tip, spaced apart from the substrate. In certain applications, the microspring has a tip profile (e.g., an apical point) capable of physically penetrating an oxide layer that may form on the surface to which electrical contact is to be made. In order to provide a reliable contact with a surface to be contacted, the microspring must provide a relatively high contact force (the force which the spring applies in resisting a force oppositely applied from the surface to be contacted). This is particularly true in applications in which the apical point must penetrate an oxide layer. For example, most probing and packaging applications require a contact force on the order of 50-100 mg between the tip and the structure being contacted.  
         [0008]     One problem faced by typical microsprings is the tradeoff made between contact force and spring geometry. In general the contact force at the tip of the microspring is given by:  
         F   tip     =         wh   2     ⁢   Δσ       12   ⁢   x           
 
 where w is the width of the microspring, h is the thickness of the microspring, Δσ is the total stress difference vertically across the cross-section of the microspring, and x is the distance from the microspring tip to the anchor. Thus, there are several ways to increase the microspring&#39;s contact force, but at the cost of altering the microspring geometry. 
 
         [0009]     First, as one decreases the length (X) of the microspring one increases its contact force. However, it is critical that when released the microspring tip should be at a certain height above the substrate. The spring must also provide a certain amount of compliance in response to a downward force being applied by the surface to be contacted. Too short a microspring produces problematic contact and inadequate compliance. Thus, there is a limit to the extent that one can reduce microspring length to increase the contact force.  
         [0010]     Second, one can increase the microspring thickness (h) to increase the contact force. However, thickness also affects the extent of curvature resulting from a stress differential, thus again affecting tip height and compliance. Above a certain thickness a microspring is incapable of reaching the design requirement for tip height, as well as sufficient compliance.  
         [0011]     Third, one can apply a plating material (e.g., Ni) over a microspring after its release. However, the plating process risks damage to the microspring, and deposits material underneath the microspring, potentially interfering with the motion of the spring. During the plating process, microsprings tend to adhere to the substrate surface, affecting device yield. Furthermore, in order to sufficiently increase the contact force, the thickness and width of the plated microspring increases, and may lead to the disadvantage of reducing the number of springs per mm in an array (i.e., decreasing spring pitch) as well as the disadvantages associated with excessive cross-sectional thickness and width discussed previously.  
         [0012]     Furthermore, typical microsprings curve upwards and terminate at the apical tip. This tip, whether patterned into a point or a flat edge perpendicular to the long axis of the microspring, tends to dig into the contact point of the structure being contacted. While this has some benefit, for example when attempting to pierce an oxide layer, it is detrimental when there is some variability in the location of the contact point or need to accommodate small amounts of relative motion between the microspring and the point of contact. In the later case, there is a desire for a microspring with a tip profile capable of accommodating lateral tip movement, for example as it is vertically displaced, in order to maintain continued contact with the contact point.  
       SUMMARY OF THE INVENTION  
       [0013]     Accordingly, the present invention is directed to a novel design for a microspring, an array of microsprings, and a method of making same, which provides an improved contact force, improved control of spring tip height, controllable-profile spring geometry, and improved microspring yield as compared to microsprings heretofore available.  
         [0014]     According to one aspect of the invention, a plurality of vertically displaced microsprings are formed over a substrate. The microsprings include tailored stress differentials such that upon release the microsprings assume a non-planar profile resulting from the stress differential. For example, a lower microspring may be caused to be in physical contact with and push upward on an upper microspring, or positioned such that when the upper microspring is deformed it physically contacts the lower microspring to provide increased contact force. The microsprings may be provided with opposite stress differentials such that the lowermost microspring tends to curve upward (i.e., has a negative stress differential) and the uppermost microspring tends to curve downward (i.e., has a positive stress differential). The stresses for the two microsprings may be individually adjusted by controlling the layer materials and/or deposition parameters used to form the microsprings.  
         [0015]     According to another aspect of the invention, the two microsprings may each have a negative (or positive) stress differentials, with the lower microspring providing support for the upper microspring. The force resisting deflection for each microspring may be the same or may be different, so that the net contact force is higher, with a desired force-displacement profile.  
         [0016]     According to yet another aspect of the invention, the upper and lower microsprings may be patterned such that upon their release they become interlocked with one another. Such interlocked dual microsprings are capable of providing a wide variety of force-displacement profiles.  
         [0017]     According to a still further aspect of the invention, the upper and lower microsprings are bonded together. For example, a lowermost microspring may be bonded at its tip to a point on the spring arm of the uppermost microspring. Alternatively, the microsprings may be bonded together at their distal ends. This bonding may provide increased contact force, electrical interconnection between the two microsprings, and may also provide desired spring configurations such as a loop microspring.  
         [0018]     The plurality of vertically displaced microsprings work together to provide a higher net contact force. With a lower microspring pushing upward on an upper microspring, greater tip height may be provided without need to extend the length of the microspring nor reduce the microspring&#39;s cross-sectional thickness. The ability to use a lower microspring to selectively push upwards on an upper microspring also facilitates controlling the cross-section of the resulting spring structure (e.g., the ability to provide non-circular profiles). Finally, improved spring contact force can be provided without having to plate the microsprings, thus yield may be improved. Furthermore, plating adds material to the width of the microsprings, affecting the pitch (e.g., number of springs per mm) in an array of microsprings. (Regardless, however, plating may be employed in any of the embodiments of the present invention, for example to further increase the contact force, if desired.)  
         [0019]     The above is a summary of a number of the unique aspects, features, and advantages of the present invention. However, this summary is not exhaustive. Thus, these and other aspects, features, and advantages of the present invention will become more apparent from the following detailed description and the appended drawings, when considered in light of the claims provided herein.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     In the drawings appended hereto like reference numerals denote like elements between the various drawings. While illustrative, the drawings are not drawn to scale. In the drawings:  
         [0021]      FIG. 1  is a profile view of a vertically spaced plural microspring structure according to an embodiment of the present invention;  
         [0022]      FIGS. 2A through 2I  are profile views of the process of forming a vertically spaced microspring structure according to an embodiment of the present invention;  
         [0023]      FIGS. 2J through 2K  are micrographs of a vertically spaced microspring structure according to an embodiment of the present invention;  
         [0024]      FIG. 2L  is a profile view of a vertically spaced plural microspring structure according to another embodiment of the present invention;  
         [0025]      FIG. 3  is a profile view of a vertically spaced plural microspring structure according to an embodiment of the present invention in which the microspring body has material removed therefrom;  
         [0026]      FIG. 4  is a profile view of a vertically spaced plural microspring structure according to yet another embodiment of the present invention in which the microsprings have oppositely-oriented stress differentials;  
         [0027]      FIG. 5  is a planform view of a microspring structure with contact pad according to still another embodiment of the present invention;  
         [0028]      FIGS. 6A and 6B  are a profile view and top view, respectively, of a vertically spaced plural microspring structure according to an embodiment of the present invention having a shaped apical tip;  
         [0029]      FIGS. 6C and 6D  are micrographs of another embodiment of a vertically spaced plural microspring structure according to the present invention, the microsprings having similarly-oriented stress differentials;  
         [0030]      FIG. 7A  is a top or planform view,  FIG. 7B  of a close-up tip micrograph view, and  FIG. 7C  is a standard micrograph view of an embodiment of a vertically spaced plural microspring structure having an engagement mechanism according to the present invention;  
         [0031]      FIG. 8A  is a top or planform view, and  FIG. 8B  is a close-up tip micrograph view of an embodiment of a vertically spaced plural microspring structure having an engagement and stop mechanism according to the present invention;  
         [0032]      FIG. 8C  and  FIG. 8D  are top or planform views of alternate embodiments of a vertically spaced plural microspring structure having an engagement and stop mechanism according to the present invention;  
         [0033]      FIG. 9A  is a profile view of a first embodiment of a clamped microspring structure according to the present invention;  
         [0034]      FIG. 9B  and  FIG. 9C  are top or planform views of alternate embodiments of a clamped microspring structure according to the present invention; and  
         [0035]      FIG. 10  is a profile view of a vertically spaced plural microspring structure according to another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     Shown in  FIG. 1  is a side view of one embodiment of a microspring structure  100  having a plurality of microsprings such as upper microspring  10  and lower microspring  12 . Each microspring  10 ,  12  comprises a free portion  14  and an anchored portion  16  fixed to a substrate  18  (e.g., glass, silicon, quartz, etc.) at an anchor  20 ,  22 , respectively. Optionally, microsprings  10 ,  12  may be electrically connected to a contact pad  24 ,  26 , respectively. Alternatively, contact pads  24 ,  26  may be formed under anchors  20 ,  22 , with vias or other means for making electrical contact with microsprings  10 ,  12  formed in or on anchors  20 ,  22  (not shown). Furthermore, anchor  20  and contact pad  24  may be a single component, and likewise anchor  22  and contact pad  26  may also be a single component. Anchor  20 , together with contact pad  24  when employed, form a spacing layer, serving among other purposes to space the upper and lower microsprings  10 ,  12  apart from one another. For clarity of explanation, the following discussion assumes that anchor  20  and contact pad  24  are a single component, and likewise anchor  22  and contact pad  26  are also a single component.  
         [0037]     Each microspring  10 ,  12  is made of an elastically deformable material, such as a molybdenum-chrome (MoCr) alloy, a nickel-zirconium (NiZr) alloy, etc. Preferably, the material from which microsprings  10 ,  12  are formed is electrically conductive, although one or both can be formed of a non-conductive or semi-conductive material. If formed of a non-conductive or semi-conductive material, at least upper microspring  10  may be coated or plated with an electrically conductive material. Microsprings  10 ,  12  may be formed of the same material, may be formed of layers of materials, each microspring comprising the same layer order and constituents, or may be formed of different materials or different material layers and/or layer order. In addition to MoCr and NiZr alloys, microsprings  10 ,  12  may be any of a variety of metals or alloys suitable for the creation of microsprings, such as Mo, MoCr, W, Ni, NiZr, or Cu (or a non metal), or other oxides, nitrides, diamond, or organic material.  
         [0038]     With reference now to  FIG. 2A through 2I , there is illustrated therein a first embodiment of a process for the production of a plurality (e.g., two) of vertically spaced microsprings according to the present invention. With reference initially to  FIG. 2A , a first contact and anchor layer  32  is formed of Ti on or over a silicon, glass, quartz or a polymer such as polyimide substrate  30 . Optionally, substrate  30  may have previously formed thereon electronic components and/or circuitry (not shown), or alternatively substrate  30  may be a printed circuit board or printed wiring board. First contact and anchor layer  32 , as well as subsequent layers of the plural microspring structure, may be formed by sputtering, plating, evaporation, chemical vapor deposition (CVD), plasma-enhanced CVD (PECVD) or material transfer through bonding and release. An optional insulating underlayer (not shown) may then be deposited and patterned in applications in which electrical insulation is desired between the microspring and other elements of the final plural microspring structure.  
         [0039]     As shown in  FIG. 2B , first contact and anchor layer  32  is next patterned by photolithographic techniques well known in the art in order to form first contact and anchor pad  34 . First release layer  36 , formed of materials known and used in stressed metal applications, such as Ti, Ni, SiO 2 , SiN, SiO x N y  or Si is then deposited over the structure, which is then photolithographically patterned to form first release island  38 , shown in  FIG. 2C . A first material system  40  is next deposited over the structure, then photolithographically patterned. Ultimately, first material system  40  becomes the lowermost of the plural microsprings  42  when released. Accordingly, first material system  40  may be any of a variety of metals or alloys suitable for the creation of microsprings, such as Mo, MoCr, W, Ni, NiZr, or Cu (or a non metal).  
         [0040]     First material system  40  is deposited in such a way as to develop within the layer a stress differential in a vertical direction across the layer&#39;s cross-section. That is, the stress in the system varies from bottom to top. According to one embodiment, first material system  40  comprises a single layer of material, and the gas pressure or power is varied during the deposition process to create a stress-engineered single layer (i.e., a layer comprised of a single material) having a desired cross-sectional stress differential. According to another embodiment, first material system  40  is itself comprised of a number of sub-layers, each sub-layer having a desired intrinsic stress. When properly selected, the assembly of sub-layers mechanically and electrically functions as a single system, but the bulk stress differential across the system of layers is a composition of the individual stresses of the sub-layers. These techniques are further described in U.S. Pat. No. 5,613,861, which is incorporated by reference herein, as well as in previously discussed U.S. Pat. No. 5,914,218. While formed in a plane, the result of the stress in system  40  is that when the layer is patterned into lowermost microspring  42 , then released from the substrate by removal of first release island  38 , the microspring bends into a non-planar profile to form a desired spring shape, such as a circular curvature upward. The released structure is discussed further below.  
         [0041]     With reference now to  FIG. 2D , second contact and anchor layer  44 , formed for example of Ti (it may be but is not necessarily formed of the same material as that of layer  32 ) is deposited over the structure. As shown in  FIG. 2E , second contact and anchor layer  44  is then patterned to become second contact and anchor pad  46 . Second release layer  48 , formed of material similar to that comprising first release layer  36 , is then deposited over the structure. As shown in  FIG. 2F , release layer  48  is then photolithographically patterned to form second release island  50 . Finally, a second material system  52  is formed over the structure. As shown in  FIG. 2G , second material system  52  is then photolithographically patterned, and becomes the uppermost of the plural microsprings  54  when released. Accordingly, second material system  52  may also be any of a variety of metals or alloys suitable for the creation of microsprings, such as Mo, MoCr, W, Ni, NiZr, or Cu, (or a non metal) and may be the same material or a different material than that forming lowermost microspring  42 . Furthermore, similar to first material system  40 , second material system  52  may also be formed with an intrinsic stress differential from bottom to top along its cross-section or as a system of sub-layers, each sub-layer having a desired intrinsic stress such that the system acting as a whole, when released, provides the desired non-planar profile to microspring  54 .  
         [0042]     As illustrated in  FIG. 2H , the structure is next etched to remove first release island  38  and second release island  50 . With these release islands removed, a portion of each of lowermost microspring  42  and uppermost microspring  54  are released from surface contact with layers above and below. Thereafter, the stress in each of lowermost microspring  42  and uppermost microspring  54  cause each of the microsprings to curve out of the plane in which they were formed. The nature of the stress in each microspring determines the direction, profile, and extent of the curve induced in the microspring. For example, microspring  42  may be fabricated with a negative stress differential, meaning that prior to its release the stress goes from compressive at a lower edge to tensile at an upper edge, when viewed in cross-section. This stress differential will produce an upward curvature to microspring  42  when release layer  38  is removed by etching. Furthermore, microspring  54  may be fabricated to have a positive stress differential prior to release, meaning that prior to its release the stress varies from tensile at a lower edge to compressive at an upper edge, when viewed in cross-section. This stress differential will produce a downward curvature to microspring  54  when release layer  50  is removed by etching. These two stress differentials are said to be opposite to one another, since they result in curvature of the microsprings in opposite directions.  
         [0043]     By properly selecting the lengths and thicknesses of the free portions of microsprings  42 ,  54 , and the stress differentials in each, it is possible to produce a structure in which the released free portion of lowermost microspring  42  mechanically and electrically engages the released free portion of uppermost microspring  54 , the former providing elastically deformable support for the later, thereby increasing the contact force which may be provided by microspring  54 .  FIG. 2J  is a micrograph of such a structure.  
         [0044]     One consequent advantage of the embodiment described above is that the lowermost microspring  42  can exert sufficient force upon uppermost microspring  54  that tip height, h, may be significantly increased over prior designs. For example, the lowermost microspring may provide sufficient force to cause the uppermost microspring to become oriented virtually normal to the plane of the substrate, as illustrated in the micrograph of  FIG. 2K . Tip heights on the order of 50-1000 μm may be achieved by this technique.  
         [0045]     Optionally, as shown in  FIG. 2I , the structure may then be plated to form a conductive metal layer  57  on the exposed surfaces of microsprings  42 ,  54 . This overplating technique is known for single microspring applications to obtain desirable spring characteristics, such as increased spring stiffness, hardness, wear resistance, and improved electrical conductivity. Such overplating may also rigidly affix lowermost microspring  42  to uppermost microspring  54 , creating a clamped structure with still higher contact force. Any of a wide variety of conductive plating materials may be employed, and as known, a gold or similar seed layer (not shown) may be used to improve the plating adhesion and reduce oxidation prior to plating. Furthermore, the plating process may proceed to such a degree that the open region  55  between lowermost microspring  42  and uppermost microspring  54  becomes filled with plating material. This arrangement produces an exceptionally “thick” spring, particularly when compared to its width, and provides a relatively high resistance to deflection upon contact.  
         [0046]     In addition, while the vertically displaced microsprings disclosed above have anchor pads formed directly atop one another, in certain applications it may be desirable to both vertically and laterally dispose the anchor pads apart from one another. In this embodiment, while the microsprings vertically overlap one another, the microsprings “point” towards one another. Such an arrangement  102  is shown in  FIG. 2L , in which two microsprings  104 ,  106  have their respective anchor pads  108 ,  110 , formed opposite one another on substrate  112 .  
         [0047]     Another consequent advantage of the embodiment described above is that there is a reduced probability of microsprings sticking to the substrate during release. This problem, called stiction, which regularly occurs in prior art structures, is often caused by surface wetting and surface tension during the release step and post etch cleaning. According to the present invention, the two microsprings  42 ,  54  work together, due to their respective stress differentials, to overcome stiction. The gap that will typically form between the plural vertically displaced microsprings further assists in reducing surface area and hence stiction. In addition, the planar surfaces of one or both of microsprings  42 ,  54  may be perforated or provided with openings  58  of varying shape, as shown in  FIG. 3 , to still further reduce surface area and hence reduce stiction.  
         [0048]      FIG. 4  illustrates another embodiment  114  which beneficially overcomes stiction. According to this embodiment, lowermost microspring  116  may be formed to have a positive stress differential, which causes that microspring to curve downward toward substrate  118 , while the uppermost microspring  120  is formed with a negative stress differential which causes it to curve upward for contact. The bow produced by lowermost microspring  116  both overcomes stiction and provides additional contact force to the uppermost microspring  120 .  
         [0049]     A still further advantage of the embodiment described above is that the lowermost microspring  42  and uppermost microspring  54  may each have different thicknesses and planform geometries. Thus, in addition to selecting stress differentials for each microspring, these attributes may be selected to tailor the contact force, contact height, contact area, electrical conductivity, etc. provided by the microsprings. For example, lowermost microspring  42  may have a relatively short anchor-to-tip length to provide high contact force, and uppermost microspring  54  may have a relatively long anchor-to-tip length to provide a large contact surface or increase tip height.  
         [0050]     An arrangement in which the vertically displaced microsprings each have a different geometry generally dictates that each layer be patterned after its deposition and before deposition of the layer above it. However, in those cases in which lowermost microspring  42  and uppermost microspring  54  have the same geometry, the various layers described above may be deposited one after the other, and the final multi-layered structure photolithographically patterned, then etched to release the microsprings.  
         [0051]     While it will be common for there to be electrical contact between lowermost microspring  42  and uppermost microspring  54 , according to other embodiments it may be desirable to electrically isolate lowermost microspring  42  and uppermost microspring  54 . In such a case, an insulative layer (not shown) may be deposited between the first material system  40  and second material system  52 , for example, below second release layer  50 , during fabrication. This insulative layer may be patterned to provide electrical isolation between the lowermost microspring  42  and uppermost microspring  54  in the region nearest contact and anchor pads  34 ,  46  or elsewhere. In addition, multiple insulative layers may be applied to isolate additional numbers of microsprings and other structures formed on the substrate.  
         [0052]     According to the foregoing, each layer of the device was deposited in bulk, photolithographically patterned, then etched prior to the deposition of the layer above it. While such a method provides precise control over the geometries of the elements forming the microspring structure, it also requires a large number of different masks and masking and etching steps. Therefore, an alternative to the foregoing is to deposit the bulk layers one after another, pattern the uppermost layer, then etch downward through the various layers. In this embodiment, the materials for the various layers described above are selected in part for their etch rates, such that the release layers etch more quickly than the metal layers. Furthermore, first contact and anchor pad  34  ( FIG. 2B ) and second contact and anchor pad  46  ( FIG. 2E ) are typically patterned to have a width, w 1 , and length l 1 , which are greater than the width of a microspring w 2 , as shown in  FIG. 5  in order to preserve the bond provided by the anchor pads post etch. For a properly timed etch, the release material underlying the microsprings is completely removed while sufficient anchor material remains for a mechanical bond to the substrate or layers formed thereover. Openings formed in the microsprings, such as openings  58  shown in  FIG. 3 , also assist in selectively etching the release material more rapidly than the microspring material.  
         [0053]     According to the embodiment shown in  FIG. 2H , the structure provides a rounded surface, or “skid”  56  for contact with a surface to be contacted. Skid  56  permits lateral translation of the microspring under compression and ensures contact between the microspring and the surface to be contacted for different angles for improved contact reliability and quality without spring buckling. However, a wide variety of tip geometries and functionalities may be provided by the plural microspring structure disclosed and taught herein. For example, with reference to  FIG. 6A , there is shown therein a dual microspring structure  60  formed on a substrate  62 . Structure  60  includes a first contact and anchor pad  64 , a lowermost microspring  66 , a second contact and anchor pad  68 , and an uppermost microspring  70 , similar to the embodiment shown in  FIG. 2H . The stress differentials for each of the microsprings in this embodiment are negative. Accordingly, the curvature of both microsprings  66 ,  70  is the same. However, the degree of stress in the two microspring layers was selected such that lowermost microspring  66  achieves a greater curvature than uppermost microspring  70 , resulting in the former being in mechanical and electrical contact with the later despite the same direction of curvature. In this embodiment, the tip  72  of microspring  60  is oriented for presentation to a region to be contacted. Accordingly, any number of tip designs, such as the apical, chisel point tip shown in  FIG. 6B , designs such as disclosed in the aforementioned U.S. Pat. No. 5,613,861, etc. may be employed. Alternatively, such tip designs may also be provided on microsprings having nearly identical stress differentials, such as illustrated in the micrographs of  FIGS. 6C and 6D .  
         [0054]     According to another embodiment of the present invention, each microspring is provided with a feature which allows mechanical interconnection with another such microspring. The interconnection features may allow for stopped or unstopped slidable interconnection or fixed interconnection. For example, with reference to  FIG. 7A , there is  
         [0055]     shown therein microspring  80  in its unreleased state. According to this embodiment, microspring  80  is provided with an anchor region  82 , and a spring arm  84  having at one end thereof an engagement tip  86  which includes a spring arm receiving slot  88 . When two such microsprings are vertically disposed proximate one another, the lowermost microspring having a negative stress differential and the uppermost microspring have a positive stress differential, and released from the substrate as disclosed above, the spring arm  84  of the lowermost microspring engages into the receiving slot of the uppermost microspring. The tips of two engaged microsprings are shown in close-up micrograph in  FIG. 7B .  FIG. 7C  shows a complete spring structure using this interlocking mechanism. This embodiment permits relative vertical and one-directional lateral motion between the two microsprings, and limits a second lateral directional deflection of the one microspring relative to the other. More controlled and uniform spring action is thereby provided. A noteworthy advantage provided by this embodiment is that only a single masking step, with etching of the top and bottom microsprings, is required to produce the interlocking structure.  
         [0056]     According to another embodiment of the present invention, a microspring  90 , similar to that described with reference to  FIG. 7A , may be provided with a slide stop  92 , shown in  FIG. 8A , to limit the relative motion of the two microsprings. The result is to provide a “clamped” contact force in response to vertical displacement. That is, the microsprings slide relative to one another up to a point, providing a first level of contact force, then, when engagement tip  86  buts up against slide stop  92 , the microspring provides a second level of contact force which is greater than the first level of contact force.  FIG. 8B  is a micrograph of the engaged spring arm  84  and engagement tip  86  of two such microsprings.  FIGS. 8C and 8D  illustrate two further variations  94 ,  96  of the above embodiment, in that the engagement tip  86  and slide stop  92  do not protrude from the spring arm  84 , but rather are notched into spring arm  84 .  
         [0057]     Finally, according to another embodiment of the present invention, the plurality of microsprings may be clamped together at their free ends to produce desirable structures. For example,  FIG. 9A  illustrates a loop contact  124 , comprised of first microspring  126 , second microspring  128 , and third microspring  130 , formed on substrate  132 . First microspring  126  is physically and electrically connected to third microspring  130  by first junction  134 . Likewise, second microspring  128  is physically and electrically connected to third microspring  130  by second junction  136 . Loop contact  124  is formed by forming first microspring  126  and second microspring  128 , having a common anchor pad  138 , but oriented to face away from one another, over a release layer (not shown). First microspring  126  and second microspring  128  are formed to have a negative stress differential. A release layer is formed thereover, then third microspring  130  is formed above first microspring  126  and second microspring  128 . Third microspring  130  may optionally be formed to have a positive (or negative) stress differential. Vias may be opened and material introduced into those vias in order to form first and second junctions  134 ,  136 . The release layers are thereafter removed, causing first microspring  126  and second microspring  128  to bow upwards off the substrate, and third microspring  130  to bow downward toward the substrate. When vertically deflected by contact with another structure, third microspring  130  deflects downward, resisted by the contact force provided by first microspring  126 , second microspring  128 , and third microspring  130 , acting as a single unit connected by junctions  134 ,  136 .  
         [0058]      FIG. 9B  illustrates a top view of a structure  140  similar to that described above with reference to  FIG. 9A , but including two loop contacts  142 ,  144 , oriented roughly perpendicularly and clamped at their ends by junctions  146   a ,  146   b ,  146   c , and  146   d .  FIG. 9C  also illustrates a top view of a structure  148  similar to that described above with reference to  FIG. 9A , but including four loop contacts  150 ,  152 ,  154 ,  156  (indeed, larger numbers of such contacts may also be formed) clamped at their ends by junctions  158   a ,  158   b ,  158   c ,  158   d ,  158   e ,  158   f ,  158   g , and  158   h . Alternating layers of microspring material and release material are required to construct such structures, as will be apparent to one of ordinary skill in the art given the present disclosure.  
         [0059]     While a plurality of preferred exemplary embodiments have been presented in the foregoing detailed description, it should be understood that a vast number of variations exist, and that these preferred exemplary embodiments are merely representative examples, and are not intended to limit the scope, applicability or configuration of the invention in any way. For example, the above description has described embodiments well suited to sputter deposition techniques. However, selective deposition through patterned resist mask techniques may also be used to construct plural microspring structures of the types discussed herein by way of plating processes. In addition, each of the embodiments described above have included two microsprings. However, the disclosure herein is equally applicable to structures including more than two microsprings, each such microspring having similar or unique geometry, composition, and stress differential, such as shown in  FIG. 10 . Accordingly, the foregoing detailed description provides those of ordinary skill in the art with a convenient guide for implementation of the invention, and contemplates that various changes in the functions and arrangements of the described embodiments may be made without departing from the spirit and scope of the invention defined by the following claims thereto.