Patent Publication Number: US-2006019027-A1

Title: Method for forming microelectronic spring structures on a substrate

Description:
RELATED APPLICATION  
      This application is a continuation-in-part of the co-pending U.S. patent application Ser. No. 09/710,539, filed Nov. 9, 2000, entitled “LITHOGRAPHIC SCALE MICROELECTRONIC SPRING STRUCTURES WITH IMPROVED CONTOURS,” by Eldridge and Wenzel (hereinafter the “FIRST PARENT CASE”), which is a continuation-in-part of co-pending application Ser. No. 09/364,788, filed Jul. 30, 1999, entitled “INTERCONNECT ASSEMBLIES AND METHODS,” by Eldridge and Mathieu (hereinafter, the “SECOND PARENT CASE”), which applications are incorporated herein, in their entirety, by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to electrical contact elements for electrical devices, and more particularly to lithographic-scale, microelectronic spring contacts with improved contours.  
      2. Description of Related Art  
      Recent technological advances, such as described in U.S. Pat. No. 5,917,707 to Khandros et al., have provided small, flexible and resilient microelectronic spring contacts for mounting directly to substrates, such as semiconductor chips. The &#39;707 patent discloses microelectronic spring contacts that are made using a wire bonding process that involves bonding a very fine wire to a substrate, and subsequent electro-plating of the wire to form a resilient element. These microelectronic contacts have provided substantial advantages in applications such as back-end wafer processing, and particularly for use as contact structures for probe cards, where they have replaced fine tungsten wires. It is further recognized, as described, for example, in U.S. Pat. Nos. 6,032,446 and 5,983,493 to Eldridge et al, that such substrate-mounted, microelectronic spring contacts can offer substantial advantages for making electrical connections between semiconductor devices in general, and in particular, for the purpose of performing wafer-level test and burn-in processes. Indeed, fine-pitch spring contacts offer potential advantages for any application where arrays of reliable electronic connectors are required, including for making both temporary and permanent electrical connections in almost every type of electronic device.  
      In practice, however, the cost of fabricating fine-pitch spring contacts has limited their range of applicability to less cost-sensitive applications. Much of the fabrication cost is associated with manufacturing equipment and process time. Contacts as described in the aforementioned patents are fabricated in a serial process (i.e., one at a time) that cannot be readily converted into a parallel, many-at-a-time process. Thus, new types of contact structures, referred to herein as lithographic-scale microelectronic spring (or contact, or spring contact) structures, have been developed, using lithographic manufacturing processes that are well suited for producing multiple spring structures in parallel, thereby greatly reducing the cost associated with each contact. Exemplary lithographic-scale spring contacts, and processes for making them, are described in the commonly owned, co-pending U.S. Patent Applications “LITHOGRAPHICALLY DEFINED MICROELECTRONIC CONTACT STRUCTURES, Ser. No. 09/032,473 filed Feb. 26, 1998 by Pedersen and Khandros, and “MICROELECTRONIC CONTACT STRUCTURES”, Ser. No. 60/073,679, filed Feb. 4, 1998 by Pedersen and Khandros, both of which are incorporated herein, in their entirety, by reference.  
      In general, lithographic processes allow for a great deal of versatility in design of spring contacts, which in turn permits numerous improvements over prior art designs. For example, although prior art lithographically formed structures in general typically have essentially flat rectangular cross-sections, contoured non-rectangular cross-sections are desirable for many spring contact applications. For a given thickness of resilient material, a lithographic type spring contact can be made stiffer and stronger by providing it with a suitably contoured, non-rectangular cross section. Other performance benefits may be realized by utilizing various other more complex shapes. However, prior art manufacturing methods are unsuitable for making lithographic type spring contacts with such suitably contoured, non-rectangular cross-sections, and other types of more complex shapes. Additionally, prior methods, for example, as disclosed in the above-referenced U.S. patent application Ser. Nos. 09/032,473 and 60/073,679, fabricate the spring structures using a series of lithographic steps, thereby building up a z-component extension (i.e., extension of the spring tip away from the substrate surface) with several lithographic layers. However, the use of multiple layers adds undesirable cost and complexity to the manufacturing process. Layered structures are also subject to undesirable stress concentration and stress corrosion cracking, because of the discontinuities (i.e., stepped structures) that result from layering processes.  
      A need therefore exists for method of making microelectronic spring structures more quickly and easily by eliminating process layering steps and the associated costs, while providing springs with improved properties, such as improved strength, stiffness, resistance to stress concentration cracking, and elastic range. Additionally, a need exists for a method of making lithographically formed, microelectronic spring structures with defined contoured surfaces and more complex shapes.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method for forming microelectronic spring structures and methods to address the foregoing needs, while achieving adequate z-extension without requiring the use of multiple stepped lithographic layers.  
      The present invention provides a method for fabricating molded microelectronic spring structures, and methods for making and using such structures, using a molded pre-cursor form. In one embodiment, a method is provided for making resilient contact structures. First, a layer of sacrificial material is formed over a substrate. Then, a contoured surface is developed in the sacrificial material, preferably by molding the sacrificial material using a mold or stamp. The contoured surface provides a mold for at least one spring form, and preferably for an array of spring forms. If necessary, the sacrificial layer is then cured or hardened. A layer of resilient material is deposited over the contoured surface of the sacrificial material, and patterned to define at least one spring form, and preferably an array of spring forms. The sacrificial material is then at least partially removed from beneath the spring form to reveal at least one freestanding spring structure. A separate conducting tip is optionally attached to each resulting spring structure, and each structure is optionally plated or covered with an additional layer or layers of material, as desired.  
      In another embodiment, a method for making a resilient contact structure using the properties of a fluid meniscus is disclosed. First, a layer of material is formed over a substrate. Then, a recess is developed in the material, and fluid is provided in the recess to form a meniscus. The fluid is cured or hardened to stabilize the contoured shape of the meniscus. The stabilized meniscus is then used in the method in the same manner as the molded surface in the sacrificial material.  
      The method according to the present invention is readily adaptable for use with lithographic manufacturing equipment and processes that are currently available to make large quantities of microelectronic spring structures in parallel. The method is particularly capable for making lithographically formed microelectronic spring contact structures that are low-aspect ratio rectangular in cross-section, and have a z-component extension along a linear or curved slope. The method also provides for shaping springs in plan view, for example, by providing springs with tapered triangular shapes. In particular, the method is capable of forming spring structures over a molded substrate formed in essentially a single process step, thereby reducing the number of processing steps required to form springs with desired shapes. The method additionally provides contoured molding substrates for forming springs with numerous performance improvements. For example, the method may be used to readily form structures having a U-shaped cross-section, a V-shaped cross-section, and/or a rib running along a length of the spring.  
      A more complete understanding of the method for forming microelectronic spring structures will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a flow diagram showing exemplary steps of a method according to the invention.  
       FIGS. 2A-2H  are cross-sectional views of a substrate and materials layered thereon during exemplary steps of a method according to the invention.  
       FIG. 3A  is a perspective view of a substrate having molding surfaces impressed thereon as during an exemplary step of a method according to the invention.  
       FIG. 3B  is a perspective view of a portion of an exemplary stamping tool for use in a method according to the invention.  
       FIGS. 3C-3G  are perspective views of exemplary teeth for use on a stamping tool in a method according to the invention.  
       FIG. 4  is a flow diagram showing exemplary steps for forming a molding surface according to an embodiment of the invention, particularly suitable for forming spring structures on uneven substrates.  
       FIGS. 5A-5G  are cross-sectional views of a substrate and materials layered thereon during exemplary steps of an embodiment of the invention, particularly suitable for forming spring structures on uneven substrates.  
       FIG. 6  is a flow diagram showing exemplary steps for forming a molding surface according to an embodiment of the invention, also suitable for forming spring structures on uneven substrates.  
       FIG. 7  is a flow diagram showing exemplary steps for forming a molding surface according to an embodiment of the invention, using a fluid to shape a molding surface in the form of a fluid meniscus.  
       FIG. 8A  is a plan view of a substrate and materials layered thereon during an exemplary one of the steps shown in  FIG. 7 .  
       FIGS. 8B-8F  are cross-sectional views of a substrate and materials layered thereon during the exemplary steps shown in  FIG. 7 .  
       FIG. 8G  is a perspective view of an exemplary spring structure formed using the exemplary steps shown in  FIG. 7 .  
       FIG. 9  is a flow diagram showing exemplary steps for forming a spring structure according to an embodiment of the invention suitable for use with PVD and CVD material deposition techniques.  
       FIGS. 10A-10D  are cross-sectional views of a substrate and materials layered thereon during the exemplary ones of the steps shown in  FIG. 9 .  
       FIG. 11A  is a cross-sectional view of a portion of an exemplary stamping tool having a re-entrant tooth form for creating an impression cavity with an overhanging lip.  
       FIG. 11B  is a cross-sectional view of a typical impression formed by the stamping tool shown in  FIG. 11A .  
       FIG. 12A  is a perspective view of an exemplary progressive stamping tool for creating an impression cavity with an overhanging lip.  
       FIG. 12B  is a cross-sectional view of a portion of the stamping tool shown in  FIG. 12A .  
       FIGS. 12C-12D  are cross-sectional views of typical impressions formed by the stamping tool shown in  FIGS. 12A-12B , during successive steps of a progressive stamping process.  
       FIGS. 12E-12F  are plan views of exemplary impressions formed by the stamping tool shown in  FIGS. 12A-12B , after completion of a progressive stamping process.  
       FIG. 13  is a flow diagram showing exemplary steps for forming a spring structure according to an embodiment of the invention that avoids a masking step by forming a mold cavity with an overhanging lip.  
       FIGS. 14A-14C  are cross-sectional views of a substrate and materials layered thereon during the exemplary ones of the steps shown in  FIG. 13 .  
       FIG. 14D  is a perspective view of an exemplary spring structure formed by a method as shown in  FIG. 13 .  
       FIG. 15  is a flow diagram showing exemplary steps for forming a spring structure according to an embodiment of the invention that avoids a masking step by using a partially encircling overhanging lip.  
       FIG. 16A  is a plan view of an exemplary mold cavity with materials layered thereon during an exemplary one of the steps shown in  FIG. 15 .  
       FIGS. 16B-16D  are cross-sectional views of a substrate and materials layered thereon during the exemplary ones of the steps shown in  FIG. 15 .  
       FIG. 17  is a flow diagram showing exemplary steps for forming a spring structure according to an embodiment of the invention that uses a radiation-curable substrate.  
       FIGS. 18A-18E  are cross-sectional views of a substrate and materials layered thereon during the exemplary ones of the steps shown in  FIG. 17 .  
       FIG. 18F  is a perspective view of exemplary molding surfaces formed by a method as shown in  FIG. 17 .  
       FIG. 19  is a flow diagram showing exemplary steps for forming a spring structure according to an embodiment of the invention using a line-of-sight deposition method for patterning the resilient material.  
       FIG. 20A  is a perspective view of a substrate and molded material during an exemplary step of the method shown in  FIG. 19 .  
       FIGS. 20B-20E  are cross-sectional views of a substrate and materials layered thereon during exemplary ones of the steps shown in  FIG. 19 .  
       FIGS. 21A-21C  are cross-sectional views of a substrate and materials layered thereon during exemplary steps of the method shown in  FIG. 19 , wherein the electroplating step is omitted; and further shows an embodiment of the invention for forming a spring structure with an integral redistribution trace.  
       FIG. 21D  is a perspective view of an exemplary spring structure with an integral redistribution trace having elevated bridges.  
       FIG. 22  is a perspective view of a plurality of spring structures having integral redistribution traces, showing an exemplary configuration thereof.  
       FIGS. 23A-23C  are perspective views, taken at successively higher levels of magnifications of an exemplary spring structure and stop structure formed by a method according to the invention, wherein the substrate comprises a wafer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      The present invention satisfies the need for a method of forming microelectronic spring structures, that overcomes the limitations of previous forming methods. In the detailed description that follows, like element numerals are used to describe like elements illustrated in one or more figures.  
      Various terms and acronyms are used throughout the detailed description, including the following:  
      “Microelectronic” means pertaining to that branch of electronics dealing with components of miniature size, such as integrated circuits. A “microelectronic spring” is not limited to springs used as electrical contacts, but also includes springs used as electro-mechanical devices and as purely mechanical springs.  
      “Sacrificial layer,” “layer of sacrificial material,” and “sacrificial material layer” means a layer of photoresist or similar material that is deposited on a substrate during formation of a desired component or structure, such as a microelectronic spring component, and later removed from the substrate.  
      “Sacrificial substrate” means a substrate that is used for formation of a desired component or structure, such as a microelectronic spring component, and later removed from the component or structure.  
      “Substrate” means a material having a supporting surface for supporting a desired structure or component. Suitable substrates upon which microelectronic spring contacts may be formed according to the present invention include, but are not limited to, semiconductor materials, such as semiconductor wafers and dice (with or without integrated circuitry), metals, ceramics, and plastics; all of the foregoing materials may be in various geometric configurations and intended for various applications.  
      The foregoing definitions are not intended to limit the scope of the present invention, but rather are intended to clarify terms that are well understood by persons having ordinary skill in the art, and to introduce terms helpful for describing the present invention. It should be appreciated that the defined terms may also have other meanings to such persons having ordinary skill in the art. These and other terms are used in the detailed description below.  
      The present invention provides methods for forming microelectronic springs on a substrate using lithographic techniques that are readily adoptable by manufacturers of semiconductor electronic devices. The microelectronic springs structures are preferably configured as described in the FIRST PARENT CASE referenced above, although various other configurations may also be formed using the methods described herein. Various exemplary methods are disclosed herein, and it should be apparent that the choice of a preferred method will depend on factors such as the type of manufacturing equipment available, the characteristics of the substrate, the desired properties of the springs, and so forth, that will vary depending on the circumstances. In some circumstances, two or more methods may be equally preferable. Additionally, selected steps of the exemplary methods may be combined in various ways, and optional steps may be omitted, depending on these and similar variable factors.  
       FIG. 1  shows exemplary steps of a generally applicable method according to the invention, and  FIGS. 2A-2H  show views of a substrate  32 , and materials layered thereon, during the steps of the method shown in  FIG. 1 . In step  102 , a layer of sacrificial material  30  is deposited on a surface of a substrate  32 , such as an upper surface of a semiconductor device, chip, die, or wafer. Substrate  32  is typically a semiconductor substrate for an integrated circuit having numerous electrical terminals, one of which is shown as the contact pad  46  in  FIG. 2A . Contact pads, such as contact pad  46 , are typically coupled by conductive traces, such as trace  44 , to internal circuitry within the integrated circuit. However, the present invention is not limited for use with a particular type or configuration of substrate. In some embodiments of the invention, contact pad  46  is electrically and mechanically coupled to an intermediate conducting layer (not shown), which is disposed above it. When present, the intermediate layer is typically a manufacturing artifact of a shorting layer used during an electroplating step of a process for forming the microelectronic spring structure. The method  100  may be used to form a spring structure for conduction of electrical signals and/or power between a mating substrate, and contact pad  46 .  
      As known in the semiconductor art, substrate  32  is typically comprised of numerous layers, such as insulating layers interposed with conducting and semiconducting layers, and a passivating layer (not shown) optionally provided on an upper surface of the substrate  32 . The passivating layer may be an insulating layer, a polysilicon layer, or other layers as known in the art, and is commonly present on semiconductor devices. When a passivating layer is present, contact pad  46  is preferably exposed through an opening in the passivating layer. Prior to depositing subsequent layers, a passivating layer (if there is one present) may optionally first be roughened, such as by exposure to an oxygen plasma, to enhance adhesion of the first subsequent layer. Choice of roughening techniques and materials suitable for deposition on passivating layers is as known in the art.  
      Referring to  FIGS. 1 and 2 A, in a preparatory step  102  of a method for making a contoured spring, a substrate  32 , optionally provided with a contact pads  46  for connecting to a integrated circuit, is coated with a moldable sacrificial layer  30 . Sacrificial layer  30  may be any number of materials, such as PMMA (poly methyl methacrylate), which can be coated on a substrate to the desired thickness, which will deform when pressed with a mold or stamp, which will receive the resilient material to be deposited thereon, and which can then readily be removed without damaging the spring structures formed thereon. Additional candidate materials for layer  30  include acrylic polymers, polycarbonate, polyurethane, ABS plastic, various photo-resist resins such as phenol-formaldehyde novolac resins, epoxies and waxes. The sacrificial layer  30  preferably has a uniform thickness slightly greater than the desired height of the contact tips of the finished spring structure above the substrate  32 . For example, if the desired height is 50 microns (about 2 mils), layer  30  may have a thickness of 55 microns (2.2 mils). Various methods known in the art, for example, spin coating, may be used to deposit layer  30  onto substrate  32 .  
      In an embodiment of the invention, layer  30  comprises multiple layers, for example a soft material that is in contact with substrate  32 , covered with a hard or brittle material on the top surface that will cleave or cut cleanly when impressed with molding tool  34 . This type of bi-layer could be formed with successive addition and curing of wet materials by spin coating or casting, by successive lamination of dry film polymers, or by lamination of a dry film that consists of multiple layers. The brittle layer mentioned above could also be a metallic layer, that would cleave and eliminate the metal layer deposition step required to form a conducting surface, such as step  106  shown in  FIG. 1 . In yet another embodiment, layer  30  comprises at least one layer of photo-patternable material and at least one layer of moldable material that is not photo-patternable. This would provide, for example, the ability to photo-pattern some regions, then follow the photo-patterning step with a molding step, or vice versa.  
      Also, a stamping tool  34 , having a molding face provided with different molding regions  36 ,  38 , and  42 , is prepared for molding sacrificial layer  30 . Various methods may be used to prepare tool  34 . For example, the stamping tool  34  may be formed from a relatively hard material using a computer controlled, ultraviolet (“UV”) laser ablation process, using an excimer laser, or a pulsed NdYag laser such as are available from Lambda Physik, Inc., located in Fort Lauderdale, Fla., or from Heidelberg Instruments Mikrotechnik GmbH, located in Heidelberg, Germany. In the alternative, a laser microchemical process, also called laser assisted etching, available from Revise, Inc. located in Burlington, Mass., may be used to form the stamping tool. Yet another alternative is to use a gray-scale photolithography mask, such as available from Canyon Materials, Inc., of San Diego, Calif., to form a pattern with a surface profile in a photopatternable glass or a layer of photoresist (which may be used as a mold for the stamping tool). The latter method—using a gray-scale mask to pattern a layer of photoresist—may also be used to form sacrificial layer  30  directly, but this is less preferred because it is generally slower than using a stamping tool. All of the foregoing methods for forming a stamping tool are capable of defining features with submicron resolution, and thus may be used for forming spring structures with molded features to about 0.1 micron in size. For example, a spring structure with a cantilevered beam portion as narrow as about 0.1 microns may be made using method  100 . Maximally protruding molding regions, or “teeth”  36  of tool  34  are used for deforming the sacrificial layer  30  in the area of the contact pads  46 , where bases of the contact structures will be formed. Contoured molding regions  38  are used for deforming layer  30  in a beam region of the contact structures to be formed. Maximally recessed molding regions  42  are used for receiving excess material, i.e., “flash,” pushed aside by teeth  36 . Molding regions  42  also define spacing between adjacent spring structures on substrate  32 . Depending on the choice of materials for sacrificial layer  30  and stamping tool  34 , a layer of mold release material (not shown) is optionally provided on the molding face of tool  34 . It should be recognized that further layers and materials may be present on substrate  32  without departing from the method described herein. For example, a metallic shorting layer (not shown) is optionally provided between layer  30  and substrate  32 , to protect any integrated circuits embedded in the substrate during processing operations. In an initial phase of forming and curing step  104 , the stamping tool  34  is applied against substrate  32  with sufficient pressure to bring the teeth  36  nearly to the surface of substrate  32 , and to fully mold layer  30  in all contoured molding regions  48 , as shown in  FIG. 2B . To avoid damaging substrate  32 , and particularly because the surface of substrate  32  is typically not perfectly planar, teeth  36  are preferably not brought into contact with substrate  32 . Tool pressures are preferably relatively low, such as less than about 7 mega-Pascals (“MPa”, about 1000 pounds per square inch (“PSI”)), and more preferably, less than about 0.7 MPa (about 100 PSI). In a preferred embodiment, when teeth  36  have sunk into layer  30  to the desired depth, flash substantially fills the maximally recessed regions  42  forming a surface sufficiently uniform to permit later deposition of a layer of masking material between the spring structures after the stamping tool  34  is removed from layer  30 . Stamping tool  34  may be heated to assist deformation of layer  30 , and then cooled to harden layer  30  into place. In an alternative embodiment, layer  30  is selected of a material that is sufficiently deformable to flow under pressure without application of heat, and sufficiently viscous to hold its shape after tool  30  is removed. In yet another alternative embodiment, heat, UV light, or chemical catalysts are used to harden sacrificial layer  30  while under stamping tool  34 , and then tool  34  is removed. In yet another embodiment, ultrasonic energy is applied by tool  34  to soften layer  30  for molding. Whatever molding technique is used, the cycle times are preferably relatively short to permit faster manufacturing throughput.  
       FIG. 2C  shows the shape of the sacrificial layer  30  after removal of the stamping tool  34 , in a subsequent phase of molding and curing step  104 . A thin layer of residue  51  is shown over the area of each contact pad  46 ; however, in some alternative embodiments, the contact pad  46  is essentially free of residue after removal of the stamping tool. Negative mold surfaces  48  are also present, each bearing a negative impression of the desired contour for the contoured beams to be formed thereon. When present, it is necessary to remove the residue  51  to expose the substrate  32  in the areas  50  where the bases of the contact structures are to be formed. To remove the residue  51 , the entire substrate with its molded layer  30  may be isotropically etched by immersion in a bath of wet etchants, by oxygen plasma, or other methods as known in the art. Isotropic etching is suitable for relatively flat substrates for which the residue  51  is of a relatively uniform thickness in all areas  50  on substrate  32 . Preferably, the isotropic etch is performed so as to remove the residue  51  while at the same time reducing the thickness of layer  30  to equal the desired height of the spring structures to be formed. In the alternative, an anisotropic etching method that etches more rapidly in a direction perpendicular to the substrate  32 , such as reactive ion etching, may be used. An anisotropic etch is preferably used in cases where the substrate is relatively uneven, causing non-uniformity in the thickness of residue  51 , or in cases when the lateral dimensions must be held in close tolerance.  
      The appearance of the molded sacrificial layer  30  after etching is shown in  FIG. 2D , at a later time during forming and curing step  104 . The contact pads  46  are preferably exposed, along with a surrounding area of substrate  50  sufficient for providing adhesion of the base of the spring structure to be formed. In typical semiconductor applications, an exposed area of substrate  32  of between about 10,000 and about 40,000 square microns, most preferably in excess of about 30,000 square microns, is provided. After etching, the mold surfaces  48  preferably take on the desired contoured shape, and the ends of all mold surfaces  48  distal from substrate  32  are preferably within substantially the same plane.  
      In step  106 , a seed layer is sputtered over the surface of sacrificial layer  30  and exposed base areas  50 . The seed layer is typically a relatively thin layer of uniform thickness, such as about 4500 Å (Angstroms; or about 0.45 microns) thick, of sputtered metal, used for electroplating the resilient spring material. Suitable metals for seed layer  52  include copper, gold, or palladium; or potentially, titanium-tungsten (Ti—W). Less preferably, surface modifications of layer  30  and base areas  50 , e.g., plasma treatment, may be used to render them conductive, thereby creating a seed layer in a surface layer of the materials.  
      In an alternative embodiment, an electrically conductive mold material, such as a conductive polymer, conductive composite material, or metal alloy having a low melting point, is used to form mold layer  30 , thereby eliminating the need to deposit a seed layer in preparation for plating. In such an embodiment, the resilient spring material can be plated directly onto the conductive mold material. In addition, the substrate is optionally covered with a protective shorting layer, as known in the art, before the layer of conductive mold material is applied the substrate. The shorting layer, if present, protects any integrated circuit elements in the substrate, and carries the plating current.  
      In step  108 , a patterned layer of a masking material, such as a photo-resist layer  54 , is deposited, to cover areas of the seed layer where no resilient material is to be deposited. The photo-resist layer  54  may be selected from various commercially available resist materials, including wet or dry positive or negative resists, or wet positive or negative electrophoretic resist systems. The photo-resist layer may be patterned using any appropriate method, for example by exposing to UV light through a mask, except where the spring structures are to be formed, thus curing it in the exposed areas (in the case of a negative-acting resist).  FIG. 2E  shows substrate  32  after application of a seed layer  52  and a photo-resist layer  54 . In  FIGS. 2E-2H , the relative thickness of seed layer  52  is exaggerated. The uncured portions of photo-resist layer  54  are then dissolved away by a suitable solvent, as known in the art.  
      The masking material  54  is preferably stable in the environment of subsequent deposition methods. For example, a typical positive photoresist masking material contains residual solvent or monomer that can outgas under the high vacuum conditions present during sputtering operations. Similar difficulties may be encountered when sputtering over a layer of sacrificial material, which typically is an organic material that may also contain residual solvent or other low-molecular-weight constituents. In preparation for a subsequent deposition step, the masking or sacrificial material is preferably pre-treated, for example by baking or by exposure to light, to drive off residual solvent or cross-link residual monomer, as the case may be, or to otherwise stabilize the material. A disadvantage of pre-treating is that the masking or sacrificial material may thereby be made more difficult to remove later in the process. A suitable sacrificial material and deposition process can be selected by one skilled in the art.  
      After the uncured portions of resist layer  54  are dissolved away, exposed areas  56  of seed layer  52  are revealed, as shown in  FIG. 2F . Exposed areas  56  have the projected shape, in plan view, of the desired microelectronic spring structure. For example, if a triangular beam is desired, the exposed area has a generally triangular shape, in plan view. In step  110 , one or more layers of resilient material  58  are then deposited onto the seed layer in the exposed areas  56 , using various methods, such as electroplating, as known in the art. Where the seed layer is covered by resist layer  54 , no electroplating will occur. In the alternative, a layer of resilient material may be built up using a process such as CVD or PVD selectively applied to areas  56  through a mask (such as a shadow mask), eliminating the need for the step  106  of depositing a seed layer. Using any of various deposition methods, a spring structure  60  comprising an integrally formed base and beam is formed on the exposed area  56 , as shown in  FIG. 2G . In step  112 , the layers of sacrificial material  30  and masking material  54  are removed using a suitable solvent, such as acetone, that will not attack substrate  32  or the resilient material  58 , as known in the art. Freestanding spring structures  60 , as shown in  FIG. 2H , are the result.  
      Suitable materials for the resilient material include but are not limited to: nickel, and its alloys; copper, cobalt, iron, and their alloys; gold (especially hard gold) and silver, both of which exhibit excellent current-carrying capabilities and good contact resistivity characteristics; elements of the platinum group; noble metals; semi-noble metals and their alloys, particularly elements of the palladium group and their alloys; and tungsten, molybdenum and other refractory metals and their alloys. Use of nickel and nickel alloys is particularly preferred. In cases where a solder-like finish is desired, tin, lead, bismuth, indium, gallium and their alloys can also be used. The resilient material may further be comprised of more than one layer. For example, the resilient material may be comprised of two metal layers, wherein a first metal layer, such as nickel or an alloy thereof, is selected for its resiliency properties and a second metal layer, such as gold, is selected for its electrical conductivity properties. Additionally, layers of conductive and insulating materials may be deposited to form transmission-line-like structures.  
      After formation of the spring structures  60 , substrate  32  is optionally coated in a patterned layer with an insulating encapsulant material over its surface, as further described in the SECOND PARENT CASE referenced above. The encapsulant layer (not shown) preferably covers the base areas  50  of the contact structures, thereby mechanically reinforcing the attachment of the resilient contact structures to the surface of the substrate. In addition, spring structures  60  are optionally provided with separate tip structures. Separate tip structures may be formed on a sacrificial substrate, and transferred to structure  60 , to be joined adjacent to its free tip, as is further described, for example, in the commonly owned, co-pending application Ser. No. 09/023,859, filed Feb. 13, 1998, which is incorporated herein, in its entirety, by reference.  
      It should be apparent that method  100 , and variations thereof, may be used to readily form numerous contoured spring structures on a substrate during a single production cycle. For example, method  100  may be used to produce tens of thousands of contoured spring structures on a wafer with multiple dice. Additionally, each of the tens of thousands of structures so formed will have a precise size, shape, and location as defined during the embossing and lithographic manufacturing process. In general, dimensional errors are anticipated to be on the order of 10 microns or less. Because so many structures can be formed at the same time, the cost of forming each structure will be relatively low.  
      Furthermore, numerous variations of the above-described sequence of steps will become apparent to one skilled in the art, for producing integrally formed spring structures according to the present invention. For example, a spring contact structure may be fabricated at an area on a substrate that is remote from a contact pad to which it is electrically connected. Generally, the spring contact structure may be mounted to a conductive line (not shown) that extends from a contact pad of the substrate to a remote position. In this manner, a plurality of spring contact structures can be mounted to the substrate so that their tips are disposed in a pattern and at positions that are not limited to the pattern of the contact pads on the substrate. Additionally, in an embodiment of the invention, molds for both the desired spring structures and the redistribution layer are formed simultaneously, by impressing a suitably shaped stamping tool into the moldable substrate. In yet another embodiment, molds for spring contacts are formed on opposite or adjacent surfaces of substrates, which is useful, for example, for forming interposer or space-transformer components. Such molds can be formed either sequentially br simultaneously, with appropriate tooling.  
      For further example, method  100  may further be adapted to permit the resilient material to be permanently deposited in areas of the substrate that are not specifically intended for making interconnections. Generally, any area on the substrate that is not masked will be plated. This may be useful for, e.g., building mechanical elements on the face of the die for standoffs. For example, the edges of the substrate could be plated to provide stand-offs or stop structures for spring structures  60 . Alternatively, the opposite side of the substrate can be plated with a shielding or shorting layer. Variations such as the foregoing may similarly be made in each of the alternative methods disclosed herein.  
      Although various adaptations may be made to the methods disclosed herein, in general, a molding or other forming process using a relatively thick layer of sacrificial material, such as layer  30 , is preferred for providing adequate height of the spring structure without requiring building up of multiple layers of photo-resist. Additionally, use of a deformable (moldable) sacrificial material layer facilitates duplication and mass production of relatively complex, contoured beam shapes.  
      Accordingly, in the preferred embodiments of the method, the entire spring structure (with the exception of optional features such as separate tips) is definable in a layer of material deposited (such as by electroplating, CVD, or PVD) on the surface of a mold form. The resulting spring structures are thus comprised of an integral sheet, which may comprise a single layer, or multiple coextensive layers, of resilient, conductive, and/or resistive material. The integrated sheet may be folded and contoured, and is preferably essentially free of any overlapping portion in the direction that the materials are deposited (typically from above the structure towards a substrate), so it may be more readily formed by depositing a layer or layers of material over a molded layer of sacrificial material, according to the methods described herein. However, substantial overlap may be achieved using some deposition methods, such as electroplating in conjunction with a “robber” to drive electrically charged material under an overhang.  
      It should be apparent that the open molding method  100  according to the present invention may be adapted to form contoured beams for spring structures in a wide variety of shapes and sizes. For the purpose of microelectronic spring contact structures, certain sizes and structural properties are preferred, as further described in the FIRST PARENT CASE referenced above. However, method  100  is capable of forming structures both smaller and larger than in the preferred ranges. Current available techniques for forming stamping tools place a lower limit on feature size at about 0.1 micron. While there is no clearly defined upper limit on feature size, above a certain feature size, for example, features that require forming the sacrificial layer  30  to depths of greater than about 10,000 microns (about 1 cm or 400 mils), prior art fabrication methods, such as sheet metal forming, are likely to be more economically feasible.  
      A perspective view of an exemplary impression formed by a stamping tool is shown in  FIG. 3A . A similar view of an exemplary portion of a stamping tool used for making the impression is shown in  FIG. 3B . It should be appreciated, however, that the impression need not define or correspond to the plan shape of the desired spring structure, because the desired plan shape may be defined using a pattern mask. The impression need only define the desired contour in the z-direction for the spring structure to be formed. In alternative embodiments of the invention, the plan shape of the impression—e.g., a recess formed in the moldable substrate—may be used to define the spring shape. Exemplary ones of these embodiments are described in more detail later in the specification.  
      As shown in  FIG. 3B , a plurality of teeth  36  are arranged on a face  35  of stamping tool  34 , each having an identical contoured surface  38 , corresponding to a molding surface  48  formed in layer  30  on substrate  32 . The teeth  36  may be arranged in a rectangular array, or in any pattern desired on face  35 . Teeth  36  may be made substantially identical to each other, or may comprise various different shapes on the same stamping tool  34 , depending on the desired spring structures to be formed. Exemplary tooth shapes include a tooth  36  with a ribbed surface, for forming a ribbed beam of a spring structure, shown in  FIG. 3C ; a tooth  36  with a corrugated surface  64  shown in  FIG. 3D , for forming a corrugated beam; and a tooth  36  with a V-shaped surface  66  shown in  FIG. 3E , for forming a V-shaped beam. Teeth may additionally be shaped differently to form spring structures having various shapes in plan view. For example,  FIG. 3C  shows a tooth for forming a spring structure with a beam and base that are both rectangular in plan view;  FIG. 3D  shows a tooth for forming a rectangular beam and a semi-elliptical base; and  FIG. 3E  shows a tooth for forming a triangular beam and semi-elliptical base. An exemplary tooth  36  for forming a structure with a beam that is U-shaped in plan view is shown in  FIG. 3F ; and an exemplary tooth  36  for forming a bifurcated beam with parallel arm is shown in  FIG. 3G . Advantages and characteristics of various contoured spring structures are described in the FIRST PARENT CASE referenced above. It will be apparent that the desired shape of tooth  36  will be defined by the molding counterpart of the desired spring structure shape.  
      Furthermore, although a specific configuration of stamping tool  34  is shown in  FIG. 3B , it should be apparent that tool  34  (and therefore, the impression made by it) may be provided in various configurations, without departing from the scope of the invention. For example, tool  34  may comprise as few as a single embossing tooth. In the alternative, tool  34  may comprise a plurality of embossing teeth  36 , disposed in a pattern. In such case, the embossing teeth may be positioned for forming molds on an entire surface of a substrate, or on a selected portion of a substrate surface. In a tool  34  having a plurality of embossing teeth  36 , all of the teeth may have the same size and shape. Alternatively, teeth on the same tool may have various different sizes and shapes, depending on the application requirements. The embossing teeth  26  may be disposed in the same plane, or disposed in different planes, or disposed on a curved surface, such as a cylinder. For example, a cylindrical stamping tool may be used to form molding surfaces by rolling over a substrate, which may be useful, for example, for forming spring structures on continuous webs of material.  
      In many cases, the upper surface of the silicon substrate will have substantial irregularities (non-planarities) that will transfer to the upper surface of a uniform layer of sacrificial material, such as a spin-coated layer. The tips of the spring structures formed by the foregoing molding method will accordingly not be in substantially the same plane. If the irregularities are larger than about 10% of the tip height of the spring structures above the substrate, an array of spring structures on the substrate will be unsuitable for making contact with another planar substrate. Also, because mating substrates will also have non-planar surfaces, it is desirable to reduce non-planarities in the tips of the spring structures to avoid errors from tolerance build-up. Accordingly, the present invention provides a method  400  for making spring structures with tips in substantially the same plane, relative to surface irregularities in a substrate.  
      Exemplary steps of the method  400  are shown in  FIG. 4 , and cross-sectional views of a substrate and layered materials during steps of the method are provided in  FIGS. 5A-5G . In an initial step  402 , a substrate  42 , having an irregular upper surface  33 , is mounted in a mold  71 , comprising a cover plate  68 , spacers  70 , a mounting surface  74 , and an injection port  72 . An inner surface  77  of cover plate  68  is planarized to the desired tolerance and polished to the desired surface finish. Substrate  32  is mounted to mounting surface  74 , for example, a wafer chuck, so that the upper surface  33  of substrate  32  is substantially parallel to the inner surface  77 . The depth of the sacrificial layer  30  to be formed in mold  71  is controlled by the thickness of spacers  70 .  
      In step  404 , a moldable material (for forming sacrificial layer  30 ) is injected through port  72  to fill the interior of mold  71 . The moldable material may be any suitable moldable material, including the materials previously described for forming a coated sacrificial layer. In step  406 , the material is cooled or cured to the desired hardness. In step  408 , the cover plate  68  is removed from the substrate  32  with the adhered layer  30 , as shown in  FIG. 5C . After the molding process, the upper surface  78  of layer  30  is substantially planar relative to the irregular upper surface  33  of substrate  32 . In step  410 , contoured molding surfaces are formed in sacrificial layer  30 , using a stamping tool  34 , as shown in  FIG. 5D . Details of step  410  are essentially the same as step  104  of method  100 , described above. In the alternative, features for forming the contoured molding surfaces  48  can be machined directly into inner surface  77  of cover plate  68 , and step  410  may be omitted. The appearance of the molded sacrificial layer after forming the molding surfaces  48  is shown in  FIG. 5E . The upper surface of residue  51  over base areas  50  are located at a uniform depth h from the reference plane defined by the inner surface  77  of cover plate  68 . The reference plane is itself located a distance d 1  from the mounting plane  82  of substrate  32 , where d, is longer than h.  
      In step  412 , the substrate is exposed at the base areas  50 , preferably by etching the sacrificial layer  30  using an anisotropic etch  76 , as previously described. The etch  76  is continued until all substrate areas are exposed, as shown by the dotted lines in  FIG. 5E . Conventional end-point detection techniques can be used to determine the ending point for the etching process. After etching, the base areas  50  are disposed on the irregular upper surface  33 , and thus are no longer disposed at an equal depth from the reference surface. However, the upper surface of layer  30  is still within essentially the same plane, disposed at a distance d 2  from the mounting plane  82  of substrate  32 , where d 2  is less than d 1 . A layer of resilient material is then deposited over the sacrificial layer and patterned, and the sacrificial layer is removed from the substrate  32 , as previously described in connection with method  100 . The resulting spring structures  60  have their tips  80  disposed in substantially the same plane, located a distance d 2  from the mounting plane  82  of substrate  32 . The distance d 2  is preferably constant, but may vary in a regular fashion across any straight section of the substrate (that is, the plane of the spring structure tips need not be exactly parallel to the mounting plane of substrate  32 ), within limits of about 20% of the average tip height of the spring structures  60  above substrate  32 .  
      Similar results may be obtained using an alternative method  600 , exemplary steps of which are shown in  FIG. 6 . In step  602 , a sacrificial layer is deposited on the substrate as previously described in connection with step  102  of method  100 . In step  604 , a molding surface is formed in the sacrificial material layer, as previously described in connection with step  104 . Then, at step  606 , the upper surface of the sacrificial material layer is made planar, using a process known in the art, such as chemical-mechanical polishing. As previously described in connection with method  400 , the upper surface of the sacrificial layer is thus made to be disposed within a plane that is substantially parallel to, or slightly inclined to, the mounting plane of substrate  32 . The remaining steps of method  600  are essentially the same as previously described in connection with steps  106 - 112  of method  100 .  
      In some circumstances it may be advantageous to avoid forming the molding surfaces on a sacrificial layer by a method that requires a stamping tool and ancillary equipment. The present invention provides a method  700  for forming contoured molding surfaces in a sacrificial layer without the need for a stamping tool. Exemplary steps of method  700  are shown in  FIG. 7 . Related views of a substrate with layered materials during steps of the method  700 , and a view of an exemplary resulting spring structure, are shown in  FIGS. 8A-8G . In an initial step  702 , a layer of sacrificial material  30  is deposited on a substrate  32 . Sacrificial layer  30  is preferably deposited in a layer of uniform thickness, using any of the methods previously described. In step  704 , the layer of sacrificial material is patterned to form one or more recesses  86 , as shown in  FIG. 8B , extending to the surface of substrate  32  in at least a portion of the recess  86 . Various methods as known in the art, such as photo patterning, may be used to form recess  86 . In plan view, such as shown in  FIG. 8A , recess  86  has the shape of the spring structure to be formed, which may be any of the shapes previously described, or any other suitable shape. For example, in an embodiment of the invention, the beam shape is triangular in plan view, and the base area is rectangular, as shown in  FIG. 8A .  
      In step  706 , the surfaces of the recess  86 , and in particular, the sidewalls, are preferably treated to alter their wetting properties as desired. The wetting properties can be modified by various techniques as known in the art, such as silanization. For further example, exposure to plasmas of oxygen, nitrogen/hydrogen, and other gases can change surface wetting properties. Further, increasing the surface roughness will generally increase the wetability of the surface. The sidewalls of the recess  86  are treated to alter the surface energy, which determines wetability, relative to the chosen wetting fluid. If a concave meniscus is desired, the surface energy of the sidewalls is decreased (if necessary) such that the chosen wetting fluid will cling to the sidewalls and form a concave meniscus in the recess  86 . Conversely, if a convex meniscus is desired, the sidewalls are treated to repel the wetting fluid, thereby causing the fluid to form a bead having a convex meniscus. In the preferred embodiment of the invention, the selection of the sacrificial material, wetting fluid, and recess shape are such that no surface treatment of the recess  86  is needed to achieve the desired meniscus shape. In general, it is preferred that the surface of the recesses  86  be easily wetted, to avoid difficulties with filling multiple recesses with a uniform quantity of fluid.  
      In step  708 , recess  86  is partially filled with a suitable wetting fluid  84 . A suitable fluid is one with a low enough viscosity and surface tension to wet recess  86 , which may be solidified without significant shrinkage or otherwise distorting the desired meniscus shape, and which may be later removed from the substrate  32  dissolution along with layer  30 . In an embodiment of the present invention, fluid  84  is a photo patternable material such as photoresist (e.g., SU8-25 or SU8-2). Several methods may be used to get a specified volume of fluid  84  into the recess  86 . Generally the recesses  86  are small, for example about 250 microns wide, 250 microns deep, and 1000 microns long. The volume of a “manhattan” (rectangular) cavity with these dimensions is 62.5 nanoliters, and special techniques must be used to accurately deposit a specified volume of liquid, which is preferably less than the recess volume. In one embodiment, a substrate having recesses  86  with a volume of less than about 100 nanoliters is spin coated with a fluid  84 . The spin-coating process leaves a small amount of fluid  84  in each cavity, the volume of which depends on the fluid viscosity, surface wetting properties of the fluid  84  and recess  86 , the shape of recess  86 , and spin process parameters such as rotational velocity and acceleration, and radial distance from the axis of rotation. Fluid  84  may be applied by directing (such as by spraying) a fluid mist onto a rotating substrate, or by immersion. A portion of the fluid  84  is also removed by the spin coating process from recess  86 , so that the fluid  84  only partially fills the recess  86 , as shown in cross-section in  FIG. 8C .  
      The relative surface energies of fluid  84  and the sidewalls of recess  86  are such that the fluid  84  has a meniscus having a first contoured shape  88  in the length direction of the recess  86 , and a second contoured shape  89  in the width direction, as shown in  FIGS. 8C and 8D , respectively. Where recess  86  is narrower, such as toward the point of the triangle shown in  FIG. 8A , the surface tension of fluid  84  preferably causes surface  88  to rise, as shown in  FIG. 8C . Across the width of recess  86 , surface tension pulls the surface  89  into a concave U-shape.  
      In step  710 , after the fluid  84  partially fills recess  86 , the fluid is solidified, for example, by curing with a chemical catalyst or UV light, by heating to drive out solvents, or by cooling below its melting point. The solidified fluid  92  may then be further patterned to define a mold for the spring structure. For example, as shown in  FIG. 8E , a portion of the solidified fluid  92  may be removed in a base area  50 , by exposing the solidified fluid  92  to an anisotropic etch  76  through mask  90 . The remaining solidified fluid  92  defines a contoured molding surface  48  and exposed base area  50  as shown in  FIG. 8F , upon which a suitable resilient material may be deposited according to the previously described method  100 , or other suitable method. The resulting spring structure has a beam with a U-shaped contour across its width, as shown in  FIG. 8G .  
      Each of the foregoing fabrication methods may be used to define a spring structure having a defined contoured shape. In general, one of the advantages of contouring the beam of a spring structure is that a contour can be used to reduce the thickness of material that is needed to achieve a beam of adequate stiffness for use as a microelectronic spring contact. Accordingly, alternative deposition techniques, such as physical vapor deposition (“PVD”) or chemical vapor deposition (“CVD”), can be used to deposit the resilient spring material over the molding surface. For example, PVD and CVD are generally less suitable than electroplating for depositing layers more than 5 microns thick, which is a suitable range of thickness for contoured springs. The present invention accordingly provides a method  900  for forming a microelectronic spring structure using an alternative material deposition technique, as shown in  FIG. 9 . Views of a substrate and materials layered thereon during exemplary steps of method  900  are shown in  FIGS. 10A-10D .  
      Steps  902  and  904  of method  900 , for depositing a sacrificial layer  30  on substrate  32 , and forming the molding surfaces, are substantially the same as corresponding steps  102  and  104  of method  100 , as previously described. Other methods, such as method  400  also described herein, may also be used to form a molding surface in the sacrificial material. At step  906 , the surface of the sacrificial layer  30  is coated with a layer of resilient material  58  using a process such as CVD or PVD, to a uniform thickness of at least about one micron, and preferably about five microns. To achieve a thickness greater than about 5 microns, it is preferred to deposit the resilient material  58  by electroplating, after first depositing a seed layer, as described in connection with method  100 . A cross-section of the substrate after the deposition process is shown in  FIG. 10A . In step  908 , a patterned layer of masking material, such as a photo-resist layer  54 , is applied to cover the resilient material in areas where spring structures are to be formed, as shown in  FIG. 10B . At step  910 , the excess (unmasked) resilient material is removed using a etching process as previously described, resulting in the layered materials as shown in  FIG. 10C . At step  912 , the sacrificial layer  30  and masking layer  54  are removed in a suitable solvent, leaving the spring structure  60 , comprised of resilient material  58 , adhered to substrate  32 . Spring structure  60  is then typically post-processed, for example, by plating with gold and/or by adhering a separate tip structure (not shown), as further described herein and in the co-pending applications referenced herein.  
      The steps required to pattern layers of resilient materials and/or seed layers may be reduced or eliminated by providing at least a portion of the molding surfaces and base areas with an overhanging lip. Such techniques may be generally applied to the methods previously described to reduce manufacturing costs. An overhanging lip may be provided using a mold tooth of suitable form, such as the re-entrant tooth  98  provided on tool  34  and shown in  FIG. 11A . When re-entrant tooth  98  is pressed into a layer of sacrificial material, the recess formed thereby is provided with an overhanging lip  96 . It should be apparent that, to remove tooth  98  from layer  30  after being fully impressed therein without damaging lip  96 , it is helpful for the layer of sacrificial material  30  to be a visco-elastic material. A visco-elastic material will deform sufficiently to permit removal of tooth  98  without damaging lip  96 , but will recover its shape after the tooth is removed. Similar benefits may be realized if layer  30  is formed from a soft, elastic material that does not adhere to tool  34 . Generally, layer  30  should comprise a solid material with a low shear modulus, i.e., a gel. The gel may have a viscous component, making it visco-elastic, or it may be more purely elastic, e.g., a soft elastic material.  
      As an alternative to using a re-entrant tooth, progressive stamping tools may be used to form an overhanging lip.  FIG. 12A  shows an exemplary progressive stamping tool, having a primary tooth  36  and a secondary tooth  37 . Primary tooth  36  is shaped as previously described. Secondary tooth  37  is shaped as a relatively shallow ring that partially or fully encloses the perimeter of the recess formed by tooth  36 . A cross-sectional view of primary tooth  36  and a representative portion of secondary tooth  37  are shown in  FIG. 12B . The primary and secondary teeth are designed to be sequentially impressed on substrate  30  by first impressing the primary tooth  36 , lifting the tool  34  from the sacrificial material  30 , relocating the stamping tool  34  so that secondary tooth  37  is positioned over the recess formed by the primary tooth, and impressing the tool a second time. In the alternative, the primary and secondary teeth may be provided on separate stamping tools (not shown) which are then applied in sequence to the sacrificial layer  30 . It should be apparent that progressive stamping is not limited to use with two progressive tools, and any number of sequential impression tools may be used without departing from the scope of the invention.  
      The resulting impressions formed by sequential impression of the primary and secondary teeth are shown in  FIGS. 12C-12F .  FIG. 12C  shows a cross-section of an exemplary layer of sacrificial material  30  after being impressed with primary tooth  36 .  FIG. 12D  shows the same exemplary material layer  30 , after the progressive stamping tool  34  is shifted a distance and re-impressed upon the material, forming an overhanging lip  96  around the perimeter of the molding surface  48  and base area  50 . The sequence may be repeated to provide the next recess formed by the primary tooth  36  with an overhanging lip, and so forth, as the tool  34  progresses across the surface of the material layer  30 . A plan view of an exemplary triangular/rectangular recess  86  with an overhanging lip is shown in  FIG. 12E , and a similar rectangular recess  86  is shown in  FIG. 12F .  
      A fully enclosing overhanging lip, as shown in  FIGS. 12E and 12F , may be used to pattern a layer of resilient material according to the method  1300 , shown in  FIG. 13 . Cross-sectional views of the substrate and layered materials during steps of the method  1300  are shown in  FIGS. 14A-14C . In an initial step  1302 , a layer of conductive material  53  is deposited on substrate  32 , to serve as a shorting layer, according to methods known in the art. The conductive layer  53  may be titanium-tungsten (Ti—W) alloy, a chrome-gold (Cr—Au) bi-layer, or any other appropriate conductive precursor layer, typically deposited by sputtering to a thickness between about 300 and 10,000 Å. The shorting layer  53  substantially conforms to and contiguously covers the surface of the substrate  32 , and any contact pads or other features that may be present on the substrate. Alternatively (but less preferably for the purposes of method  1300 ), shorting layer  53  can be deposited in a pattern of multiple, non-contiguous regions. Patterning the shorting layer  53  is generally for the purpose of defining a redistribution layer between contact pads on the substrate  32  and the spring structures to be formed.  
      At step  1304 , the sacrificial material layer  30  is deposited according to a method previously described. At step  1306 , a molding surface  48  with an overhanging lip  96  is formed in the layer of sacrificial material, preferably using a re-entrant tooth or progressive stamping tool, as previously described. At step  1308 , a seed layer  52  and  55  is deposited on the surface of the sacrificial layer, using a process such as sputtering (especially ionized physical-vapor deposition (I-PVD)), or similar line-of-sight deposition process. It will be apparent that the overhanging lip  96  shields the perimeter of the molding surface from deposition of the seed layer, resulting in a first portion  52  of the seed layer disposed over the molding surface  48  and base area in recess  86 , and a second portion  55  of the seed layer over the surrounding area of the sacrificial material layer, as shown in  FIG. 14A . If will further be apparent that, so long as the overhanging lip  96  fully encloses the recess  86 , the first portion  52  of the seed layer will be connected to the shorting layer  53 , and the second portion  55  will be isolated from the shorting layer  53  and from the first portion  52 .  
      Then, in step  1310 , the substrate is electroplated with a resilient material using shorting layer  53  to apply a plating potential to the first portion  52 . The resilient material  58  thus is selectively plated on the first portion  52  of the seed layer, and does not cover the second portion  55 . Then, at step  1312 , the sacrificial material layer and second portion  55  of the seed layer are removed by dissolving the sacrificial material in a suitable solvent, as previously described. It should be noted, however, that even if the resilient material  58  is incidentally plated over the second portion  55 , this unwanted plated material can later readily be removed without harming the desired spring structures, so long as it is not continuous with the resilient material  58  plated over the first portion  52 . In either case, a separate, free-standing spring structure results from application of the method  1300 , an exemplary one of which is shown in  FIG. 14D , without the need for any separate patterning step.  
      A similar process may be used, utilizing a partially enclosing overhanging lip, according to the method  1500  shown in  FIG. 15 . No shorting layer is needed for method  1500 , however, an additional step is needed to separate the resilient material of the spring structures from the surrounding material. A plan view of the substrate during a step of the method is shown in  FIG. 16A , and cross-sectional views of the substrate and materials layered thereon during steps of the method are shown in  FIGS. 16B-16D . In step  1502 , a layer of sacrificial material is deposited according to one of the previously described methods. In step  1504 , a molding surface is formed as described above, except that the overhanging lip  96  does not completely enclose the molding surfaces within recess  86 . As shown in  FIG. 16A , the overhanging lip  96  is formed to enclose the recess  86  on three sides, and no lip is formed on the side adjacent to the top of the sacrificial layer, where the tip of the spring structure will be formed. In step  1506 , a seed layer  52  is deposited over the surface of the sacrificial layer  30 , using a line-of-sight method, as previously described. Because recess  86  is not completely enclosed by the overhanging lip, the seed layer  52  is electrically connected to the deposited seed layer everywhere else on the surface of layer  30 , as shown in  FIG. 16A . Seed layer  52  can thus be used for electroplating the resilient material  58 , and no shorting layer is needed for this purpose (although one may optionally be present, for other reasons).  
      The appearance of the substrate after deposition of the resilient material layer is shown in  FIG. 16B . As is also clear from  FIG. 16A , the layer of resilient material  58  will be parted on all sides of the recess  86 , where no seed layer was deposited, except for near the surface of layer  30 , where it is connected to a more generally extending layer. It is necessary, therefore, to remove the excess resilient material, which is done is step  1510 , by any suitable precision machining method, such as chemical/mechanical polishing. At the same time, the surface of layer  30  is preferably planarized, so the tips of the spring structures will reside in the same plane, for the reasons discussed previously. A cross-section of the substrate after step  1510  is shown in  FIG. 16C . The next step is to remove the remaining portion of sacrificial layer  30 , using any of the methods described herein, to leave the free standing spring structure  60 , as shown in  FIG. 16D .  
      In some cases it may be advantageous to form a plurality of microelectronic spring contacts by replicating a single mold tooth (or a relatively small group of teeth), instead, of by using a stamping or molding tool having a plurality of teeth covering a relatively large area, such as the area of an die or wafer. The present invention provides a “one-up” method  1700 , exemplary steps of which are shown in  FIG. 17 , for such cases. For example, method  1700  may be advantageous for small production runs, or runs involving atypical “custom” positioning of the spring structure, because it avoids the need for an intricate stamping tool having many teeth.  FIGS. 18A-18E  show cross-sectional views of a substrate  32  and layered materials during steps of the method  1700 .  FIG. 18F  shows a perspective view of exemplary molding surfaces  48  which may be formed using method  1700 , for molding spring structures, or for use as a stamping tool with many teeth. In an initial step  1702 , a layer of sacrificial material  30  is deposited on substrate  32 . In an embodiment of the invention, the layer  30  is a material that may be cured (hardened) by exposure to a radiation, such as by exposure to UV light, or to an electron beam.  FIG. 18A  shows the sacrificial layer after deposition during step  1702 . Also shown is an exemplary single-tooth stamping tool  34 , having a tooth  36 . Tooth  36  is as previously described; however, in an embodiment of the invention, tooth  36  is additionally provided with a radiation-transparent portion  39  and an opaque portion  41 .  
      A process loop, comprising steps  1704  through  1708 , is then performed. In a first cycle of the loop, a single contoured molding surface is formed using tooth  36 , at step  1704 .  FIG. 18B  shows the substrate  32 , layer  30 , and tooth  36  during step  1704 , with tooth  36  fully impressed into substrate  30 . Flash  49  is evident on either side of tooth  36 . In step  1706 , while tooth  36  is in position, molding surface  48 , which is under transparent portion  39  of tooth  36 , is preferably selectively cured. In an embodiment of the invention, UV light is beamed through the tooth  36  to cure portion  31 . The opaque portion  41  preferably prevents the sacrificial layer  30  from being cured in the area of the base, so that the substrate may be more readily exposed to a layer of resilient material there. Steps  1704  and  1706  are repeated until the desired number of molding surfaces  48  have been defined, as indicated by decision step  1708 . The appearance of the substrate during a second cycle of the process loop is shown in  FIG. 18C , and the appearance of the substrate after the second cycle is shown in  FIG. 18D . Two cured portions  31  are shown, surrounded by uncured areas of flash  49 . These uncured portions are readily removed in step  1710 , by dissolving in a suitable solvent, leaving only the molding surfaces  48  comprised of cured portions  31 . The molding surfaces may be used for forming spring structures as previously described. In the alternative, the molding surfaces  48  may be used as the teeth of a stamping tool. It should be apparent that stamping methods using a transparent tooth, such as method  1700 , are readily adaptable to methods using tools with a plurality of transparent teeth separated by opaque regions, which may be used, for example, to form a plurality of spring structures in parallel at single-die, multiple-die and wafer scales.  
      A similar “one-up” method may be used to form molding surfaces for spring contacts, using plunge EDM. According to a plunge EDM method, a suitable plunge EDM tool is shaped like, and replaces, the transparent stamping tooth  36  discussed above with respect to method  1700 . Instead of embossing a deformable substrate, the plunge EDM tool is used to form molding surfaces in a substantially non-deformable, electrically conductive substrate. Candidates for molding surfaces include metals and polymers filled with conductive particles or fibers. The surface so formed may be used as a mold form for spring contacts, or as a multi-toothed forming tool, depending on the characteristics of the conductive substrate and the desired objective.  
      In yet another embodiment of the invention, a spring structure is formed on a molded substrate utilizing the properties of a line-of-sight material deposition technique, such as sputtering or evaporation, so as to eliminate certain process steps. Exemplary steps of a method  1900  using a line-of-sight deposition technique are shown in  FIG. 19 . Exemplary views of a substrate and layered materials during method  1900  are shown in  FIGS. 20A-20E . At step  1902 , a substrate  32  is provided, typically having at least one exposed contact pad  46 . At step  1904 , dielectric layer  43  is optionally deposited and patterned as known in the art. At optional step  1906 , a shorting or adhesion layer  53 , such as a layer of titanium, titanium-tungsten, or chromium, is deposited over layer  43  and contact pad  46  as known in the art. The purpose of layer  53  is to facilitate the subsequent optional plating step  1916 . If step  1916  is to be omitted, step  1906  is preferably omitted also. At step  1908 , a sacrificial layer of moldable material  30  is deposited on substrate  32 , and formed, such as by embossing with a stamping tool, to provide a mold for a microelectronic spring. Any suitable moldable material, such as described herein, may be used. At step  1910 , any residual moldable material  30  covering contact pad  46  is removed, such as by using a suitable anisotropic etch process. A layer of metallic material  52  is then deposited over the moldable layer  32  using a line-of-sight process, such as sputtering or evaporation, at step  1912 .  
      Exemplary views of a substrate and layered materials after completion of step  1912  are shown in  FIGS. 20A and 20B . A recess  86  having vertical or relatively steep sidewalls  87  has been provided in layer  30 , such as by a stamping tool having a suitably shaped embossing tooth. For the purposes of method  1900 , “steep” means inclined (positively or negatively) less than about 45° from vertical, and preferably, less than about 30° from vertical. Still more preferably, sidewalls  87  are inclined between about 0°-5° from vertical. A bottom surface of recess  86  comprises a molding surface  48  for defining the shape of a microelectronic spring structure. Molding surface  48  is isolated from the top surface  57  of the moldable layer  30  by the sidewalls  87  which preferably surround the entire periphery of recess  86 , thereby separating molding surface  48  from the upper surface  57  of layer  30 . As shown in  FIG. 20C , because of the properties of line-of-sight deposition, the thickness “t 1 ” of the layer  52  is substantially greater on the molding surface  48  of layer  30  than the thickness “t 2 ” on the sidewalls  87 . In particular, if sidewalls  87  are substantially vertical, or overhang the molding surface  48  (that is, are inclined with respect to the line of deposition of the line-of-sight deposition method so as to not present a face for deposition of material thereon), no material will be deposited on the sidewalls. Although the upper surface  57  of layer  30  is shown as being substantially horizontal and planar, the shape and inclination of surface  57  is not critical, and may have a variety of different shapes, so long as sidewalls  87  are present and inclined so as to isolate surface  57  from molding surface  48 .  
      At step  1914 , if present on sidewalls  87 , layer  52  is etched isotropically to remove all of layer  52  adhering to sidewalls  87  while leaving it substantially intact on molding surface  48  and top surface  57 . That is, the etching step  1914  preferably is halted as soon as sidewalls  87  are free of deposited metallic material, at which point layer  52  on molding surface  48  will preferably be of a desired thickness. After step  1914 , layer  52  on molding surface  48  will remain electrically connected to shorting layer  53 . An isolated portion  55  of layer  52 , on the upper surface  57  of layer  30 , will be physically isolated from layer  52  on molding surface  48 , and preferably, also electrically isolated from shorting layer  53 . Layer  52  is thus patterned to define a spring structure, by separation of the molding surface  48  from upper surface  57  by sidewalls  87 , and elimination of metallic (or resilient) material from the sidewalls. It should be apparent that if there is no metallic layer  52  on sidewalls  87  after deposition step  1912  (such as if sidewalls  87  are vertical or overhanging) step  1914  will be unnecessary and may be omitted.  
      At optional step  1916 , a layer of resilient material  58  is electroplated onto the portion of layer  52  on molding surface  48 . Preferably, no additional material will be plated onto the isolated portion  55 , because it is preferably not connected to shorting layer  53 , through which the plating current flows. A view of the substrate and layered materials after completion of step  1916  is shown in  FIG. 20D . Note that resilient layer  58  does not contact the isolated portion  55 . Hence, isolated portion  55  and sacrificial moldable layer  30  are readily removed at step  1918 , such as by dissolution in an etchant, without damaging the resilient material  58  deposited on molding surface  48 . It should further be apparent that if metallic layer  52  is sufficiently thick to provide the desired strength and stiffness, plating step  1916  may be omitted. In particular, where the spring structure is to be provided with stiffening features, such as a contoured or ribbed cantilevered portion, plated layer  58  (which may be used to provide strength and stiffness) is less likely to be necessary.  FIG. 20E  shows a cross-sectional view of the resulting spring structure  60  after removal of the sacrificial moldable layer  30  at step  1918 . Isolated portion  55  and exposed portions of shorting layer  53  are also removed at step  1918 . A plurality of microelectronic spring structures, such as structure  60 , may thus be formed in parallel using method  1900 , without the need for any pattern-masking step.  
      Other structures may be formed on the surface of a substrate at the same time as, and using the same processes as used for forming a microelectronic spring. In particular, redistribution traces, bridges, and bumps may be formed with a spring structure according to the present invention.  FIGS. 21A-21D  show a substrate and layered materials during exemplary steps of a method for forming a redistribution trace  45  and bridges  59  with a spring structure  60 . Although method  1900  is adapted for this purpose to illustrate an application thereof that omits the plating steps, any other suitable method described herein may also be used to form features in parallel with a spring structure.  FIG. 21A  shows a substrate having a contact pad, dielectric layer, and moldable layer  30 , as described above in connection with method  1900 . After preparation of moldable layer  30 , a stamping tool  34  is used to define a molding surface  48 , a trace-defining portion  63  for molding for a redistribution trace, and bumps  61 .  
       FIG. 21B  shows the substrate with stamping tool  34  fully impressed into moldable layer  30 . Bumps  61  may be any suitable shape, and have a height less than the tip height of the spring structure to be formed. In an embodiment of the invention, bumps  61  have a height and shape suitable for acting as stop structures, i.e., structures capable of preventing over-compression, for their companion spring structure. For example, suitable shapes include those with arched, semi-circular, triangular, or rectangular cross sections, having a height above the substrate sufficient to prevent over-compression of the spring structure. Bumps  61  may be configured as connected to trace-defining portion  63 , or as isolated from it.  
      Residue  51  is typically present on the substrate  32  after the tool  34  is removed. Such residue is removed to reveal the contact pad  46  and dielectric layer  43  at the bottom of recess  86  in the area of the redistribution race and base for the spring structure. By suitable design of tool  34 , recess  86  is surrounded by steep sidewalls  87  which separate molding surface  48  and the bottom of recess  86  from the upper surface  57  of moldable layer  30 , as previously described herein. A layer of resilient material is deposited generally on the substrate, including over the bottom of recess  86  and over molding surface  48 , using a line-of-sight deposition technique.  FIG. 21C  shows the molded resilient material  52  after deposition of resilient layer  58 . In this example, layer  58  is sufficiently thick so that no additional resilient layer is needed.  
      Moldable layer  30  is then removed, revealing a spring structure  60  with an integral redistribution trace, as shown in  FIG. 21D . In this example, spring structure  60  has a contoured beam for enhanced stiffness. Bridges  59  correspond to bumps  61  formed by tool  34 . Bridges  59  may serve to provide stress relief to trace  45 , particularly if trace  45  is relatively lengthy. Bridges  59  may also serve as stop structures for spring structure  60 . Additional bridges (not shown) may additionally be provided, that are electrically isolated from any contact element, and therefore perform a purely mechanical function, such as a mechanical stop. Thus, a complete contact system, including a plurality of spring contacts, associated redistribution traces, and stop structures, can be made using relatively few process steps. To further illustrate an application of the method,  FIG. 22  shows an exemplary two of many contact structures with integral redistribution traces, for performing a pitch spreading function from a relatively fine pitch “p 1 ” at the contact pads, to a coarser pitch “p 2 ” at the tips of the spring elements. A wide variety of geometric configurations for pitch spreading and other redistribution purposes are possible, without departing from the scope of the invention.  
      In an alternative embodiment, a separately formed stop structure, as further described in the co-pending application Ser. No. 09/364,855, filed Jul. 30, 1999, entitled “INTERCONNECT ASSEMBLIES AND METHODS,” by Eldridge and Mathieu, which is hereby incorporated herein by reference, is provided to prevent over-compression of the microelectronic spring structures under application of a contact force, according to methods described therein. Perspective views of a substrate  32  with an array of contoured, microelectronic spring contacts  60 , and provided with stop structures  47 , are shown in  FIGS. 23A-23C . The substrate is shown at a wafer level in  FIG. 23C . A view of a single die  97  on the wafer, showing an array of spring structures  60  on the die, is shown in  FIG. 23B . A detailed view of a single contoured spring structure  60  and surrounding stop structure  47  is shown in  FIG. 23C . It should be apparent that the spring structures may be disposed in any desired pattern on the substrate. In particular, spring structures may be disposed at locations on the substrate remote from underlying contact pads and vias, by creating an intermediate redistribution layer between the contact pads or vias and the spring structures, as further described in the co-pending application Ser. No. 09/364,855 referenced above.  
      It should be appreciated that the contoured microelectronic spring structures  60  described herein, such as shown in  FIGS. 23A-23C  may also be used for other types of interconnect assemblies, such as probe card assemblies, interposers, and other connection systems where electrical contact to or through a substrate is desired. In particular, such spring structures may be used both for making high-temperature, temporary connection during a wafer or chip level burn-in process, and subsequently, for making a more permanent, ambient temperature connection between the substrate and an electronic component such as a printed circuit board. It is anticipated that the low cost and versatility of the spring structures will greatly reduce the costs associated with high-temperature testing by permitting testing at higher temperatures, and thus achieving higher throughput than is possible using methods according to the prior art.  
      The methods of the present invention are further illustrated by the following example:  
     EXAMPLE  
      A silicon wafer with a 0.5 micron surface oxide layer was selected for a prototype substrate. A layer of chrome was sputtered on a surface of the substrate, followed by a layer of gold, to provide a shorting layer. A 4.0 mil (100 micron) thick layer of negative dry-film photoresist was applied to the sputtered gold layer using a vacuum laminator. A second 3.0 mil (75 micron) thick layer of the same type of photoresist was applied over the first layer. The substrate was placed on a hot plate and heated until the photoresist was soft. An embossing tool with protruding triangular teeth contoured to produce the desired spring shape was pressed into the photoresist laminate while the laminate was soft. The substrate was cooled and the embossing tool was removed. A photolithography mask and UV light were used to expose (and thus cross link) the photoresist laminate everywhere except in the area of the spring base contact. The photoresist was developed using a spray developer with standard sodium carbonate developer solution, which removed the unexposed photoresist from the spring-base contact. The spring-base contacts were then cleaned using an oxygen plasma descum for ten minutes. A seed layer of metal (palladium/gold) for a subsequent electroplating step was sputtered over the entire surface of the photoresist laminate and exposed base area. A 4.0 mil layer of dry-film photoresist was applied over the sputtered layer using a vacuum laminator at 80° C. The photoresist was exposed with UV light using a photolithography mask to shield the resist over the molding surface, where the spring was to be formed. The photoresist was then developed to remove in the area of the molding surface, and then a plasma descum was used to clean the molding surface as before. A resilient spring metal (nickel) was deposited in the mold form by electroplating for 20 minutes at about 50 ASF current density. The substrate was removed from the electroplating solution and immersed in a solution of RD87 negative resist stripper to remove all layers of photoresist. A free-standing spring structure remained on the substrate having a thickness of 12 microns (about 0.5 mil), a cantilevered beam that was triangular in plan view and that extended about 180 microns (7 mils) from the surface of the substrate.  
      Having thus described a preferred embodiment of a method for forming microelectronic spring structures, it should be apparent to those skilled in the art that certain advantages of the invention have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. For example, a method for forming microelectronic spring contact structures has been illustrated, but it should be apparent that the inventive concepts described above would be equally applicable to form similar structures for other purposes. For example, electromechanical spring contacts, such as relays, or purely mechanical springs could be formed on a variety of substrates for various applications using the methods described herein. Additionally, other lithographic type structures comprising open contoured sheets of materials, such as channels, funnels and blades, may be made at microscopic scales by suitably adapting the methods herein. The invention is further defined by the following claims.