Patent Publication Number: US-2005120553-A1

Title: Method for forming MEMS grid array connector

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is related to concurrently filed and commonly assigned U.S. patent application Ser. No. ______, entitled “A Connector For Making Electrical Contact At Semiconductor Scales,” of Dirk D. Brown et al. (Attorney Docket No. EPC-P107.) The aforementioned patent application is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION  
      The invention relates to reconnectable, remountable electrical connectors, and, in particular, to an electrical connector for connecting to semiconductor scale devices.  
     DESCRIPTION OF THE RELATED ART  
      Electrical interconnects or connectors are used to connect two or more electronic components together or to connect an electronic component to a piece of electrical equipment, such as a computer, router, or tester. For instance, an electrical interconnect is used to connect an electronic component, such as an integrated circuit (an IC or a chip), to a printed circuit broad. An electrical interconnect is also used during integrated circuit manufacturing for connecting an IC device under test to a test system. In some applications, the electrical interconnect or connector provides separable or remountable connection so that the electronic component attached thereto can be removed and reattached. For example, it may be desirable to mount a packaged microprocessor chip to a personal computer mother board using a separable interconnect device so that malfunctioning chips can be readily removed or upgraded chips can be readily installed.  
      There are also applications where an electrical connector is used to make direct electrical connection to metal pads formed on a silicon wafer. Such an electrical connector is often referred to as a “probe” or “probe card” and is typically used during the testing of the wafer during the manufacturing process. The probe card, typically mounted on a tester, provides electrical connection from the tester to the silicon wafer so that individual integrated circuits formed on the wafer can be tested for functionality and compliance with specific parametric limits.  
      Conventional electrical connectors are usually made of stamped metal springs, which are formed and then individually inserted into an insulating carrier to form an array of electrical connection elements. Other approaches to making electrical connectors include using isotropically conductive adhesives, injection molded conductive adhesives, bundled wire conductive elements, springs formed by wirebonding techniques, and small solid pieces of metal.  
      Land grid array (LGA) refers to an array of metal pads (also called lands) that are used as the electrical contact points for an integrated circuit package, a printed circuit board, or other electronic component. The metal pads are usually formed using thin film deposition techniques and coated with gold to provide a non-oxidizing surface. Ball Grid array (BGA) refers to an array of solder balls or solder bumps that are used as the electrical contact points for an integrated circuit package. Both LGA and BGA packages are widely used in the semiconductor industry and each has its associated advantages or disadvantages. For instance, LGA packages are typically cheaper to manufacture than ball grid array (BGA) packages because there is no need to form solder balls or solder bumps. However, LGA packages are typically more difficult to assemble onto a PC board or a multi-chip module. An LGA connector is usually used to provide removable and remountable socketing capability for LGA packages connected to PC boards or to chip modules.  
      Advances in semiconductor technologies has led to shrinking dimensions within semiconductor integrated circuits and particularly, decreasing pitch for the contact points on a silicon die or a semiconductor package. The pitch, that is, the spacing between each electrical contact point (also referred to as a “lead”) on a semiconductor device is decreasing dramatically in certain applications. For example, contact pads on a semiconductor wafer can have a pitch of 250 micron or less. At the 250-micron pitch level, it is prohibitively difficult and very expensive to use conventional techniques to make separable electrical connections to these semiconductor devices. The problem is becoming even more critical as the pitch of contact pads on a semiconductor device decreases below 50 microns and simultaneous connection to multiple contact pads in an array is required.  
      When making electrical connections to contact pads, such as metal pads on a silicon wafer or on a land grid array package, it is important to have a wiping action or a piercing action when the contact elements engage the pads in order to break through any oxide, organic material, or other films that may be present on the surface of the metal pads and that might otherwise inhibit the electrical connection.  FIG. 1  illustrates a contact element being applied to engage a metal pad on a substrate. Referring to  FIG. 1 , a connector  10  includes a contact element  12  for making electrical connection to a metal pad  16  on a substrate  14 . Connector  10  can be a wafer probe card and contact element  12  is then a probe tip for engaging pad  16  on silicon substrate  14 . Under normal processing and storage conditions, a film  18 , which can be an oxide film or an organic film, forms on the surface of metal pad  16 . When contact element  12  engages metal pad  16 , contact element must pierce through film  18  in order to make a reliable electrical connection to metal pad  16 . The piercing of film  18  can be resulted from a wiping action or a piercing action of contact element  12  when the contact element engages the metal pad.  
      While it is necessary to provide a wiping or piercing action, it is important to have a well-controlled wiping or piercing action that is strong enough to penetrate the surface film but soft enough to avoid damaging the metal pad when electrical contact is made. Furthermore, it is important that any wiping action provides a sufficient wiping distance so that enough of the metal surface is exposed for satisfactory electrical connection.  
      Similarly, when making contacts to solder balls, such as solder balls formed on a BGA package, a chip-scale package, or a wafer-level package, it is important to provide a wiping or piercing action to break through the native oxide layer on the solder balls in order to make good electrical contact to the solder balls. However, when conventional approaches are used to make electrical contact to solder balls, the solder balls may be damaged or completely dislodged from the package.  FIG. 2A  illustrates a contact element being applied to contact a solder ball. When contact element  12  contacts solder ball  22  formed on a substrate  20  such as for testing, contact element  12  applies a piercing action which often result in the formation of a crater on the top surface (also called the base surface) of the solder ball. When substrate  20  including solder ball  22  is subsequently attached to another semiconductor device, such as a PC board or a chip-scale package, the crater in solder ball  22  can lead to void formation at the solder ball interface.  FIGS. 2B and 2C  illustrate the result of attaching solder ball  22  to a metal pad  26  of a substrate  24 . After solder reflow ( FIG. 2C ), solder ball  22  is attached to metal pad  26 . However, a void is formed at the solder ball interface due to the presence of the crater on the top surface of solder ball  22  which crater was created by the piercing action of contact element  12 . The presence of such a void can affect the electrical characteristics of the connection and more importantly, degrades the reliability of the connection.  
      Therefore, it is desirable to provide an electrical contact element that can be provide a controlled wiping action on a metal pad, particularly for pads with a pitch of less than 50 microns. It is also desirable that the wiping action provides a wiping distance of up to 50% of the contact pad. Furthermore, when electrical contact to solder balls are made, it is desirable to have an electrical contact element that can provide a controlled wiping action on the solder ball without damaging the contact surface of the solder ball.  
      Another problem encountered by electrical connectors is the variation in coplanarity and positional misalignment of the contact points of a semiconductor device to be connected. For instance, variations in the fabrication process for semiconductor wafers and packages often lead to variations in the final position, in each planar dimension, of the contact points (metal pads or solder balls). In an array of contact points, positional misalignment leads to variations in the relative positions of different contact points. Thus, a connector must be capable of accommodating positional variations due to misalignment in order to be useful in most applications. Hence, it is desirable to have a scalable electrical contact element that can behave elastically so that normal variations in coplanarity and positional misalignment of the contact points can be tolerated.  
      Connectors or interconnect systems for making electrical connection to semiconductor devices are known. For example, U.S. Pat. No. 6,032,356, issued to Eldridge et al. on Mar. 7, 2000, discloses an array of resilient contact structures that are mounted directly on the bonding pads of a semiconductor wafer. The contact structures are formed by attaching gold bond wires to the wafer, shaping the bond wires and then overcoating the bond wires to form composite contact elements. Although Eldridge discloses a approach for providing an array of all-metal contacts at semiconductor scales, the contact elements requires an expensive serial manufacturing process where the contact elements are formed one at a time. Also, the inherent pointy shape of the contact structures results in piercing action which is prone to damaging the contact point such as a solder ball when making contact.  
      U.S. Pat. No. 6,184,065, issued to Smith et al. on Feb. 6, 2001, discloses small metal springs created by the inherent stress gradient in a thin metal film. Smith&#39;s approach provides an array of all-metal contacts at semiconductor scales. However, the metal springs point into the surface of the plane to be contacted and therefore is prone to damaging the solder balls when used to probe solder balls.  
      U.S. Pat. No. 6,250,933, issued to Khoury et al. on Jun. 26, 2001, discloses a contact structure in which the contactors are produced on a semiconductor substrate or other dielectric by microfabrication technology and in which each of the contactors is shaped like a bridge, with one or more angled portions supporting a horizontal contacting portion. Khoury&#39;s approach provides an array of all-metal contacts at semiconductor scales but provides a limited amount of wiping action when interfacing with metal pads because the contacting component is parallel to the metal pad. Khoury addresses the lack of wiping problem by adding asperities and making asymmetric structures to induce a wiping action. However, it will be obvious to one skilled in the art that such approaches can provide a wiping distance of only 10% or less of the overall dimension of the contact which is often not enough for a satisfactory electrical connection. In addition, when contacting solder ball arrays, Khoury&#39;s approach requires the base surface of the solder balls to be physically contacted since the contacting surface is parallel to the solder ball array. Such contact can lead to damage on the base surface of the solder ball which in turn can lead to void formation during subsequent solder reflow as shown in  FIG. 2C .  
      In summary, the conventional connectors are not satisfactory for use with small pitch size semiconductor devices. The conventional connects are also not satisfactory for providing wiping/piercing action without damaging the contact points such as the base surface of a solder ball.  
     SUMMARY OF THE INVENTION  
      According to one embodiment of the present invention, a connector for electrically connecting to pads formed on a semiconductor device includes a substrate and an array of contact elements of conductive material formed on the substrate. Each contact element includes a base portion attached to the top surface of the substrate and a curved spring portion extending from the base portion and having a distal end projecting above the substrate. The curved spring portion is formed to curve away from a plane of contact and has a curvature disposed to provide a controlled wiping action when engaging a respective pad of the semiconductor device.  
      According to another aspect of the present invention, a method for forming a connector including an array of contact elements includes providing a substrate, forming a support layer on the substrate, patterning the support layer to define an array of support elements, isotropically etching the array of support elements to form rounded corners on the top of each support element, forming a metal layer on the substrate and on the array of support elements, and patterning the metal layer to define an array of contact elements where each contact element includes a first metal portion on the substrate and a second metal portion extending from the first metal portion and partially across the top of a respective support element. The method further includes removing the array of support elements. The array of contact elements thus formed each includes a base portion attached to the substrate and a curved spring portion extending from the base portion and having a distal end projecting above the substrate. The curved spring portion is formed to have a concave curvature with respect to the surface of the substrate.  
      According to another aspect of the present invention, a method for forming a connector including an array of contact elements includes providing a substrate, providing a conductive adhesion layer on the substrate, forming a support layer on the conductive adhesion layer, patterning the support layer to define an array support elements, isotropically etching the array of support elements to form rounded corners on the top of each support element, forming a metal layer on the conductive adhesion layer and on the array of support elements, patterning the metal layer and the conductive adhesion layer to define an array of contact elements. Each contact element includes a first metal portion formed on a conductive adhesion portion and a second metal portion extending from the first metal portion and partially across the top of a respective support element. The method further includes removing the array of support elements.  
      The array of contact elements thus formed each includes a base portion attached to the conductive adhesion portion which is attached to the substrate and a curved spring portion extending from the base portion and having a distal end projecting above the substrate. The curved spring portion is formed to have a concave curvature with respect to the surface of the substrate.  
      The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a contact element being applied to engage a metal pad on a substrate.  
       FIG. 2A  illustrates a contact element being applied to contact a solder ball.  
       FIGS. 2B and 2C  illustrate the result of attaching a damaged solder ball to a metal pad of a substrate.  
       FIGS. 3A and 3B  are cross-sectional view of a connector according to one embodiment of the present invention.  
       FIGS. 4A and 4B  are cross-sectional diagrams illustrating the use of the connector of  FIG. 3A  for engaging different semiconductor devices.  
       FIGS. 5A and 5B  illustrate a connector according to an alternate embodiment of the present invention.  
       FIGS. 6A and 6B  illustrate connectors according to alternate embodiments of the present invention.  
       FIGS. 7A  to  7 H illustrate the processing steps for forming the connector of  FIG. 3A  according to one embodiment of the present invention.  
       FIGS. 8A  to  8 H illustrate the processing steps for forming the connector of  FIG. 5A  according to one embodiment of the present invention.  
       FIGS. 9A  to  9 H illustrate the processing steps for forming the connector of  FIG. 5A  according to an alternate embodiment of the present invention.  
       FIGS. 10A and 10B  are cross-sectional views of a connector according to an alternate embodiment of the present invention.  
       FIG. 11  is a cross-sectional view of a connector including a ground plane for improving signal integrity and for controlling contact element impedance according to one embodiment of the present invention.  
       FIG. 12  illustrates another embodiment of the connector of the present invention where a pair of contact elements is used to couple to a pair of differential signals.  
       FIG. 13  illustrates a connector incorporating a thermally conductive plane according to one embodiment of the present invention.  
       FIG. 14  is a cross-sectional view of a connector including a coaxial contact element according to one embodiment of the present invention.  
       FIGS. 15A  to  15 H illustrate the processing steps for forming an array of connectors according to an alternate embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      In accordance with the principles of the present invention, a connector for providing separable and remountable connection to a device includes an array of contact elements formed on a substrate where each contact element includes a curved spring portion formed to curve away from a plane of contact and having a curvature disposed to provide a controlled wiping action when engaging a contact point of the device. The connector of the present invention can be used to make electrical connection to devices at semiconductor scales, such as a silicon wafer or a packaged integrated circuit. The contact elements can be formed to make electrical connection to contact points having a pitch of 250 micron or less and in particular, the contact elements of the present invention enable electrical connection to contact points having a pitch of 50 micron or less. By providing a controlled wiping action, the connector of the present invention can be used to connect to a variety of contact surfaces without damaging the contact surface. Finally, the contact elements in the connector of the present invention have a large elastic working range approximately equal to or greater than the electrical path length, thereby allowing the contact elements to operate over a large range of compressions often required in normal operating conditions.  
      The connector of the present invention provides numerous advantages over conventional connector systems. First, the connector of the present invention includes contact elements having a curved spring portion that curved away from the plane of contact, that is, the surface of the contact points to be contacted. Thus, the contact elements can provide a soft controlled wiping action when engaging a metal pad or a solder ball, allowing effective electrical connection to be made without damaging the contact surface. Furthermore, the contact elements in the connector of the present invention can achieve an optimal wiping distance with optimal contact force. Conventional connectors often include curved spring members that curved into the plane of contact. Such curvature results in a piercing action when the spring members are engaged with a contact pad and often results in undesirable damages to the pad. Alternately, in other conventional connectors, the contact element either provides no wiping action or insufficient wiping distance. The connector of the present invention overcomes many of the disadvantages of the conventional connectors.  
      Second, the connector of the present invention provides scalable, low profile, low insertion force, high density, and separable/reconnectable electrical connection and is particularly suited for use in high speed and high performance applications. The connector can be built at relatively low cost while exhibiting highly reliable and compliant operating characteristics. In particular, the connector of the present invention can be scaled to contact metal pads on a wafer or lands of a LGA package where the pads or lands are separated by a pitch of 50 microns or less. The connector of the present invention can also be scaled to contact solder balls of a BGA package or solder balls formed on a wafer where the solder balls are separated by a pitch of 250 micron or less.  
      Third, the connector of the present invention can be used to engage pads of semiconductor device which pads are in vertical alignment with the contact elements of the connection. Thus, only the application of a vertical external biasing force is needed to connect the connector to the device to be connected. This is in contrary to many conventional connector systems which require the application of a lateral force to engage a connector and often result in damage to the connection points.  
      The connector of the present invention can be used to make electrical connection to a wide variety of devices. For example, the connector of the present invention can be used to make electrical connection to metal pads on a silicon wafer, to a ball grid array (BGA) package, to a land grid array package, to a wafer-level package, to a chip scale package and other semiconductor or electrical device. In the present description, the term “device” is used to refer to the class of electronic devices or component to which electrical connection or interconnection is necessary. Thus, a semiconductor device can include but is not limited to a semiconductor wafer, a packaged or unpackaged integrated circuit (IC), a ball grid array formed on a semiconductor wafer or as an IC package, a land grid array formed on a semiconductor wafer, on a chip module or on an IC package.  
       FIGS. 3A and 3B  are cross-sectional view of a connector according to one embodiment of the present invention.  FIGS. 3A and 3B  illustrate a connector  50  of the present invention being connected to a semiconductor device  60  including metal pads  64 , formed on a substrate  62 , as contact points. Semiconductor device  60  can be a silicon wafer where metal pads  64  are the metal bonding pads formed on the wafer. Semiconductor device  60  can also be a LGA package where metal pads  64  represent the “lands” or metal connection pads formed on the LGA package. The coupling of connector  50  to semiconductor device  60  in  FIGS. 3A and 3B  is illustrative only and is not intended to limit the application of connector  50  to connecting with wafers or LGA packages only.  
      Referring to  FIG. 3A , connector  50  includes an array of contact elements  54  formed on a substrate  52 . Substrate  52  can be formed as a dielectric material or a semiconductor material. Because connector  50  can be built be for connecting to semiconductor devices at semiconductor scales, connector  50  is usually formed using material that are commonly used in semiconductor fabrication processes. In one embodiment, substrate  52  is made of quartz, silicon or a ceramic wafer and contact elements  54  are formed on a dielectric layer which dielectric layer could be a SOS, SOG, BPTEOS, or TEOS layer formed on the top surface of the substrate. The array of contact elements is typically formed as a two-dimensional array arranged to mate with corresponding contact points on a semiconductor device to be contacted. In one embodiment, connector  50  is formed to contact metal pads having a pitch of 50 microns or less.  
      Contact elements  54  are formed using a conductive material. Each contact element  54  includes a base portion  55 A attached to the top surface of substrate  52  and a curved spring portion  55 B extending from base portion  55 A. Curved spring portion  55 B has a proximal end contiguous with base portion  55 A and a distal end projecting above substrate  52 . Note that  FIGS. 3A and 3B  illustrate connector  50  being turned upside down to engage semiconductor device  60 . The use of directional terms such as “above” and “top surface” in the present description is intended to describe the positional relationship of the elements of the connector as if the connector is positioned with the contact elements facing upward. One of ordinary skill in the art would appreciate that the directional terms used herein are illustrative only and intended only to describe the relative position of different parts of the contact element.  
      Referring still to  FIG. 3A , contact element  54  includes curved spring portion that is formed to curve away from a plane of contact. In the present description, the “plane of contact” refers to the surface of the contact point to which the contact element is to be contacted. In the present illustration, the plane of contact is the surface of metal pad  64 . As shown in  FIG. 3A , curved spring portion  55 B is formed to have a concave curvature with respect to the surface of substrate  52 . Thus, curved spring portion  55 B curves away from the surface of metal pad  64 . Curved spring portion  55 B of contact element  54  has a curvature that is disposed to provide a controlled wiping action when engaging a respective metal pad  64  of the semiconductor device to be contacted.  
      In operation, an external biasing force, denoted F in  FIG. 3A , is applied to connector  50  causing connector  50  to be compressed against metal pads  64  of semiconductor device  60 . The curved spring portion of a contact element  54  engages the respective metal pad in a controlled wiping action so that each contact element makes effective electrical connection to the respective pad. The curvature of contact elements  54  ensures that the optimal contact force is achieved concurrently with the optimal wiping distance. The wiping distance is the amount of travel the distal end of the contact element makes on the surface of the metal pad when contacting the metal pad. In general, the contact force can be on the order of 5 to 100 grams depending on the application and the wiping distance can be on the order of 5 to 400 microns.  
      Another feature of the contact element of the present invention is that the curved spring portion of the contact element enables a very large elastic working range. Specifically, because the curved spring portion can move in both the vertical and the horizontal directions, an elastic working range on the order of the electrical path length of the contact element can be achieved. In the present description, the “electrical path length” of the contact element is defined as the distance the electrical current has to travel from the distal end of the curved spring portion to the base portion of the contact element. Basically, the contact elements of the connector of the present invention have an elastic working range that spans the entire length of the contact elements.  
      Contact elements  54  are formed using a conductive metal that can also provide the desired elasticity. In one embodiment, contact elements  54  are formed using titanium (Ti) as a support structure that can later be plated to obtain desired elastic behavior. In other embodiments, contact elements  54  are formed using a copper-alloy (Cu-alloy) or a multilayer metal sheet such as stainless steel coated with Copper-Nickel-Gold (Cu/Ni/Au) multilayer metal sheet. In a preferred embodiment, the contact elements are formed using a small-grained copper-beryllium (CuBe) alloy and then plated with electroless Nickel-Gold (Ni/Au) to provide a non-oxidizing surface. Furthermore, in an alternate embodiment, contact elements  54  are formed using different metals for the base portions and the curved spring portions.  
      In the embodiment shown in  FIG. 3A , contact element  54  is shown as formed by a rectangular shaped based portion with one curved spring portion. This configuration is illustrative only and is not intended to be limiting. The contact element of the present invention can be formed in a variety of configurations and each contact element only needs to have a base portion sufficient for attaching the curved spring portion to the substrate. The base portion can assume any shape and can be formed as a circle or other useful shape for attaching the contact element to the substrate. Furthermore, a contact element can include multiple curved spring portions extended from the base portion as will be discussed in more detail below.  
      The large elastic working range of the connector of the present invention enables the connector to accommodate normal coplanarity variations and positional misalignments in the semiconductor devices to be connected. The connector is thus capable of providing reliable electrical connection despite coplanarity and positional irregularities that may exist in semiconductor devices to be connected.  FIGS. 4A and 4B  are cross-sectional diagrams illustrating the use of connector  50  for engaging different semiconductor devices. In  FIG. 4A , positional variations of the metal pads to be contacted require contact elements at one end of connector  50  to be more compressed than contact elements at the opposite end. In  FIG. 4B , coplanarity variations of the metal pads to be contacted require contact elements in the middle portion of connector  50  to be more compressed than contact elements at the two ends of connector  50 . Because the contact elements of the present invention have a large elastic working range, different contact elements can be compressed at different levels while providing effective electrical connection over all contact elements.  
       FIGS. 5A and 5B  illustrate a connector according to an alternate embodiment of the present invention. Referring to  FIG. 5A , a connector  70  includes an array of contact elements  74  formed on substrate  72 . In the present embodiment, each contact element  74  includes a base portion  75 A and two curved spring portions  75 B and  75 C extending from base portion  75 A. Curved spring portion  75 B and  75 C have distal ends projecting above substrate  72  and facing towards each other. Other characteristics of curved spring portions  75 B and  75 C are the same as curved spring portion  55 B. That is, curved spring portions  75 B and  75 C are formed curved away from a plane of contact and each has a curvature disposed to provide a controlled wiping action when engaging a contact point of a semiconductor device to be contacted. Furthermore, curved spring portions  75 B and  75 C have an elastic working range approximately equal to the electrical path length of the contact element, thus enabling a large range of compression to be applied.  
      In the present illustration, connector  70  is used to contact a semiconductor device  80 , such as a BGA package, including an array of solder balls  84  as contact points.  FIG. 5B  illustrates connector  70  being fully engaged with semiconductor device  80 . Connector  70  can be used to contact metal pads such as pads on a land grid array package. However, using of connector  70  to contact solder balls  84  provides particular advantages.  
      First, contact elements  74  contact the respective solder balls along the side of the solder balls. No contact to the base surface of the solder ball is made. Thus, contact elements  74  do not damage the base surface of the solder balls during contact and effectively elimination the possibility of void formation when the solder balls are subsequently reflowed for permanently attachment.  
      Second, because each curved spring portion of contact elements  74  is formed to curved away from the plane of contact which in the present case is a plane tangent to the side surface of the solder ball being contacted, the contact elements  74  provides a controlled wiping action when making contact with the respective solder balls. In this manner, effective electrical connection can be made without damaging the contact surface, that is, the surface of the solder balls.  
      Third, connector  70  is scalable and can be used to contact solder balls having a pitch of 250 microns or less.  
      Lastly, because each contact element has a large elastic working range on the order of the electrical path length, the contact elements can accommodate a large range of compression. Therefore, the connector of the present invention can be used effectively to contact conventional devices having normal coplanarity variations or positional misalignments.  
      Connectors  50  and  70  in  FIGS. 3A and 5A  are shown as including a curved spring portion that projects linearly from the base portion. The embodiments shown in  FIGS. 3A and 5A  are illustrative only and are not intended to be limited. The connector of the present invention can be configured in a variety manner depending on the types of contact points to be contacted and depending on the desired contact force.  FIGS. 6A and 6B  illustrate connectors according to alternate embodiments of the present invention. Referring to  FIG. 6A , a connector  90  includes a contact element  93  formed on a substrate  92 . Contact element  93  includes a base portion  94 A and a first curved spring portion  94 B and a second curved spring portion  94 C. First curved spring portion  94 B and second curved spring portion  94 C have distal ends that point away from each other. Contact element  93  can be used to engage a contact point including a metal pad or a solder ball. When used to engage a solder ball, contact element  93  cradles the solder ball between the first and second curved spring portions. Thus, first and second curved spring portions  94 B and  94 C contact the side surface of the solder ball in a controlled wiping motion in a direction that curved away from the plane of contact of the solder ball.  
       FIG. 6B  illustrates a contact element  95  formed on a substrate  96 . Contact element  95  includes a base portion  97 A and a first curved spring portion  97 B and a second curved spring portion  97 C extended from the base portion. In the present embodiment, first curved spring portion  97 B and the second curved spring portion  97 C project above substrate  96  in a spiral configuration. Contact element  95  can be used to contact a metal pad or a solder ball. In both cases, first and second curved spring portion  97 B and  97 C curve away from the plane of contact and provide a controlled wiping action.  
      The connectors of the present invention can be manufactured in a variety of processes using different processing sequence. For example, the curved spring portion of each contact element can be formed by stamping. In one embodiment, the connectors of the present invention are formed using semiconductor processing techniques. When formed using semiconductor processing techniques, the connectors of the present invention can be referred to as being built as MicroElectroMechanical Systems (MEMS). Thus, in one embodiment of the present invention, the connector of the present invention is also referred to as a MEMS grid array connector.  
       FIGS. 7A  to  7 H illustrate the processing steps for forming connector  50  of  FIG. 3A  according to one embodiment of the present invention. Referring to  FIG. 7A , a substrate  102  on which the contact elements are to be formed is provided. Substrate  102  can be a silicon wafer or ceramic wafer for example and may include a dielectric layer formed thereon (not shown in  FIG. 7A ). As described above, a dielectric layer of SOS, SOG, BPTEOS, or TEOS layer can be formed on substrate  102  for isolating the contact elements from substrate  102 . Then, a support layer  104  is formed on substrate  102 . Support layer  104  can be a deposited dielectric layer, such as an oxide or nitride layer, a spin-on dielectric, a polymer, or any other suitable etchable material. In one embodiment, support layer  104  is deposited by a chemical vapor deposition (CVD) process. In another embodiment, support layer  104  is deposited by a plasma vapor deposition (PVD) process. In yet another embodiment, support layer  104  is deposited by a spin-on process. In yet another embodiment, when substrate  102  is not covered by a dielectric layer or a conductive adhesive layer, the support layer can be grown using an oxidation process commonly used in semiconductor manufacturing.  
      After the support layer  104  is deposited, a mask layer  106  is formed on the top surface of support layer  104 . Mask layer  106  is used in conjunction with a conventional lithography process to define a pattern on support layer  104  using mask layer  106 . After the mask layer is printed and developed ( FIG. 7B ), a mask pattern, including regions  106 A to  106 C, is formed on the surface of support layer  104  defining areas of support layer  104  to be protected from subsequent etching.  
      Referring to  FIG. 7C , an anisotropic etching process is performed using regions  106 A to  106 C as a mask. As a result of the anisotropic etching process, support layer  104  not covered by a patterned mask layer is removed. Accordingly, support regions  104 A to  104 C are formed. The mask pattern including regions  106 A to  106 C is subsequently removed to expose the support regions ( FIG. 7D ).  
      Referring to  7 E, support regions  104 A to  104 C are then subjected to an isotropic etching process. An isotropic etching process remove material under etch in the vertical and horizontal directions at substantially the same etch rate. Thus, as a result of the isotropic etching, the top corners of support regions  104 A to  104 C are rounded off as shown in  FIG. 7E . In one embodiment, the isotropic etching process is a plasma etching process using SF6, CHF 3 , CF 4  or other well known chemistries commonly used for etching dielectric materials. In an alternate embodiment, the isotropic etching process is a wet etch process, such as a wet etch process using a buffered oxide etch (BOE).  
      Then, referring to  FIG. 7F , a metal layer  108  is formed on the surface of substrate  102  and the surface of support regions  104 A to  104 C. Metal layer  108  can be a copper layer or a copper-alloy (Cu-alloy) layer or a multilayer metal deposition such as Tungsten coated with Copper-Nickel-Gold (Cu/Ni/Au). In a preferred embodiment, the contact elements are formed using a small-grained copper-beryllium (CuBe) alloy and then plated with electroless Nickel-Gold (Ni/Au) to provide a non-oxidizing surface. Metal layer  108  can be deposited by a CVD process, by electro plating, by sputtering, by physical vapor deposition (PVD) or using other conventional metal film deposition techniques. A mask layer is deposited and patterned into mask regions  110 A to  110 C using a conventional lithography process. Mask regions  110 A to  110 C define areas of metal layer  108  to be protected from subsequent etching.  
      Then, the structure in  FIG. 7F  is subjected to an etching process for removing metal layer not covered by mask regions  110 A to  110 C. As a result, metal portions  108 A to  108 C are formed as shown in  FIG. 7G . Each of metal portions  108 A to  108 C includes a base portion formed on substrate  102  and a curved spring portion formed on a respective support region ( 104 A to  104 C). Accordingly, the curved spring portion of each metal portion assumes the shape of the underlying support region, projecting above the substrate surface and having a curvature that provides a wiping action when applied to contact a contact point.  
      To complete the connector, support regions  104 A to  104 C are removed ( FIG. 7H ), such as by using a wet etch or an anisotropic plasma etch or other etch process. If the support layer is formed using an oxide layer, a buffered oxide etchant can be used to remove the support regions. As a result, free standing contact elements  112 A to  112 C are formed on substrate  102 .  
      One of ordinary skill in the art, upon being apprised of the present invention, would appreciate that many variations in the above processing steps are possible to fabricate the connector of the present invention. For example, the chemistry and etch condition of the isotropic etching process can be tailored to provide a desired shape in the support regions so that the contact elements thus formed have a desired curvature. Furthermore, one of ordinary skill in the art would appreciate that through the use of semiconductor processing techniques, a connector can be fabrication with contact elements having a variety of properties. For example, a first group of contact elements can be formed with a first pitch while a second group of contact elements can be formed with a second pitch greater or smaller than the first pitch. Other variations in the electrical and mechanical properties of the contact element are possible, as will be described in more detail below.  
       FIGS. 8A  to  8 H illustrate the processing steps for forming connector  70  of  FIG. 5A  according to one embodiment of the present invention. The processing steps shown in  FIGS. 8A  to  8 H are substantially the same as the processing steps shown in  FIGS. 7A  to  7 H. However,  FIGS. 8A  to  8 H illustrate that different configuration of contact elements can be fabricated by using suitably designed mask patterns.  
      Referring to  FIG. 8A , a support layer  124  is formed on a substrate  122 . A mask layer  126  is formed on the support layer for defining mask regions for forming the connector of  FIG. 5A . In the present embodiment, mask regions  126 A and  126 B ( FIG. 8B ) are positioned closed together to allow a contact element including two curved spring portion to be formed.  
      After an isotropic etching process is performed using mask regions  126 A and  126 B as mask, support regions  124 A and  124 B are formed ( FIG. 8C ). The mask regions are removed to expose the support regions ( FIG. 8D ). Then, support regions  124 A and  124 B are subjected to an isotropic etching process to shape the structures so that the top surface of the support regions includes rounded corners ( FIG. 8E ).  
      A metal layer  128  is deposited over the surface of substrate  122  and over the top surface of support regions  124 A and  124 B ( FIG. 8F ). A mask pattern, including regions  130 A and  130 B, is defined on metal layer  128 . After metal layer  128  is etched using mask regions  130 A and  130 B as mask, metal portions  128 A and  128 B are formed ( FIG. 8G ). Each of metal portions  128 A and  128 B includes a base portion formed on substrate  122  and a curved spring portion formed on the respective support region ( 124 A or  124 B). The curved spring portion of each metal portion assumes the shape of the underlying support region, projecting above the substrate surface and having a curvature that provides a wiping action when applied to contact a contact point. In the present embodiment, the distal ends of metal portions  128 A and  128 B are formed facing each other. To complete the connector, support regions  124 A to  124 B are removed ( FIG. 8H ). As a result, a free standing contact element  132  is formed on substrate  102 . In the cross-sectional view of  FIG. 8H , the two metal portions of contact element  132  appears to unconnected. However, in actual implementation, the base portions of the metal portions are connected such as by forming a ring around the contact element or the base portions can be connected through conductive layers formed in substrate  122 .  
       FIGS. 9A  to  9 H illustrate the processing steps for forming connector  70  of  FIG. 5A  according to an alternate embodiment of the present invention. Referring to  FIG. 9A , a substrate  142  including predefined circuitry  145  is provided. Predefined circuitry  145  can include interconnected metal layers or other electrical devices, such as capacitors or inductors, which are typically formed in substrate  142 . In the present embodiment, a top metal portion  147  is formed on the top surface of substrate  142  to be connected to the contact element to be formed. To form the desired contact element, a support layer  144  and a mask layer  146  are formed on the top surface of substrate  142 .  
      The processing steps proceed in a similar manner as described above with reference to  FIGS. 8A  to  8 H. Mask layer  146  is patterned ( FIG. 9B ) and support layer  144  is etched accordingly to formed support regions  144 A and  144 B ( FIG. 9C ). The mask regions are removed to expose the support regions ( FIG. 9D ). Then, an isotropic etching process is carried out the round out the top corners of support regions  144 A and  144 B ( FIG. 9E ). A metal layer  148  is deposited on the surface of substrate  142  and over the support regions ( FIG. 9F ). Metal layer  148  is formed over top metal portion  147 . As a result, metal layer  148  is eclectically connected to circuit  145 .  
      Metal layer  148  is patterned by a mask layer  150  ( FIG. 9F ) and subjected to an etching process. Metal portions  148 A and  148 B are thus formed ( FIG. 9G ) having distal ends pointing towards each other. Support portions  144 A and  144 B are removed to complete the fabrication of contact element  152  ( FIG. 9H ).  
      As thus formed, contact element  152  is electrically connected to circuit  145 . In the manner, additional functionality can be provided by the connector of the present invention. For example, circuit  145  can be formed to electrically connect certain contact elements together. Circuit  145  can also be used to connect certain contact elements to electrical devices such as a capacitor or an inductor formed in or on substrate  142 .  
      Fabricating contact element  152  as part of an integrated circuit manufacturing process provides further advantage. Specifically, a continuous electrical path is formed between contact element  152  and the underlying circuit  145 . There is no metal discontinuity or impedance mismatch between the contact element and the associated circuit. In some prior art connectors, a gold bond wire is used to form the contact element. However, such a structure results in gross material and cross-sectional discontinuities and impedance mismatch at the interface between the contact element and the underlying metal connections, resulting in undesirable electrical characteristics and poor high frequency operations. The contact element of the present invention does not suffer from the limitations of the conventional connector systems and a connector built using the contact elements of the present invention can be used in demanding high frequency and high performance applications.  
      As described above, when the contact elements of the connector of the present invention are formed using semiconductor fabrication processes, contact elements having a variety of mechanical and electrical properties can be formed. In particular, the use of semiconductor fabrication processing steps allows a connector to be built to include contact elements having different mechanical and/or electrical properties.  
      Thus, according to another aspect of the present invention, a connector of the present invention is provided with contact elements having different operating properties. That is, the connector includes heterogeneous contact elements where the operating properties of the contact elements can be selected to meet requirements in the desired application. In the present description, the operating properties of a contact element refer to the electrical, mechanical and reliability properties of the contact element. By incorporating contact elements with different electrical and/or mechanical properties, the connector of the present invention can be made to meet all of the stringent electrical, mechanical and reliability requirements for high-performance interconnect applications.  
      According to one embodiment of the present invention, the following mechanical properties can be specifically engineered for a contact element or a set of contact elements to achieve certain desired operational characteristics. First, the contact force for each contact element can be selected to ensure either a low resistance connection for some contact elements or a low overall contact force for the connector. Second, the elastic working range of each contact element over which the contact element operates as required electrically can be varied between contact elements. Third, the vertical height of each contact element can be varied. Fourth, the pitch or horizontal dimensions of the contact element can be varied.  
      According to alternate embodiments of the present invention, the electrical properties can be specifically engineered for a contact element or a set of contact elements to achieve certain desired operational characteristics. For instance, the DC resistance, the impedance, the inductance and the current carrying capacity of each contact element can be varied between contact elements. Thus, a group of contact elements can be engineered to have lower resistance or a group of contact elements can be engineered to have low inductance.  
      In most applications, the contact elements can be engineered to obtain the desired reliability properties for a contact element or a set of contact elements to achieve certain desired operational characteristics. For instance, the contact elements can be engineered to display no or minimal performance degradation after environmental stresses such as thermal cycling, thermal shock and vibration, corrosion testing, and humidity testing. The contact elements can also be engineering to meet other reliability requirements defined by industry standards, such as those defined by the Electronics Industry Alliance (EIA).  
      When the contact elements in the connectors of the present invention are fabricated as a MEMS grid array, the mechanical and electrical properties of the contact elements can be modified by changing the following design parameters. First, the thickness of the curved spring portion of the contact element can be selected to give a desired contact force. For example, a thickness of about 30 microns typically gives low contact force on the order of 10 grams or less while a flange thickness of 40 microns gives a higher contact force of 20 grams for the same displacement. The width, length and shape of the curved sprint portion can also be selected to give the desired contact force.  
      Second, the number of curved spring portions to include in a contact element can be selected to achieve the desired contact force, the desired current carrying capacity and the desired contact resistance. For example, doubling the number of curved spring portions roughly doubles the contact force and current carrying capacity while roughly decreasing the contact resistance by a factor of two.  
      Third, specific metal composition and treatment can be selected to obtain the desired elastic and conductivity characteristics. For example, Cu-alloys, such as copper-beryllium, can be used to provide a good tradeoff between mechanical elasticity and electrical conductivity. Alternately, metal multi-layers can be used to provide both excellent mechanical and electrical properties. In one embodiment, a contact element is formed using titanium (Ti) coated with copper (Cu) and then with nickel (Ni) and finally with gold (Au) to form a Ti/Cu/Ni/Au multilayer. The Ti will provide excellent elasticity and high mechanical durability while the Cu provides excellent conductivity and the Ni and Au layers provide excellent corrosion resistance. Finally, different metal deposition techniques, such as plating or sputtering, and different metal treatment techniques, such as alloying, annealing, and other metallurgical techniques can be used to engineer specific desired properties for the contact elements.  
      Fourth, the curvature of the curved spring portion can be designed to give certain electrical and mechanical properties. The height of the curved spring portion, or the amount of projection from the base portion, can also be varied to give the desired electrical and mechanical properties.  
       FIGS. 10A and 10B  are cross-sectional views of a connector according to an alternate embodiment of the present invention. Referring to  FIG. 10A , a connector  220  includes a first set of contact elements  224 ,  226  and  228  and a second set of contact elements  225  and  227 , all formed on a substrate  222 . The first set of contact elements  224 ,  226  and  228  has a curved spring portion longer than the curved spring portion of the second set of contact elements  225  and  227 . In other words, the height of the curved spring portion of contact elements  224 ,  226  and  228  is greater than the height of the curved spring portion of contact elements  225  and  227 .  
      By providing contact elements having different height, connector  220  of the present invention can be advantageously applied in “hot-swapping” applications. Hot-swapping refers to mounting or demounting a semiconductor device while the system to which the device is to be connected is electrically active without damaging to the semiconductor device or the system. In a hot-swapping operation, various power and ground pins and signal pins must be connected and disconnected in sequence and not at the same time in order to avoid damages to the device or the system. By using a connector including contact elements with different heights, taller contact elements can be use to make electrical connection before shorter contact elements. In this manner, a desired sequence of electrical connection can be made to enable hot-swapping operation.  
      As shown in  FIG. 10A , connector  220  is to be connected to a semiconductor device  230  including metal pads  232  formed thereon. When an external biasing force F is applied to engage connector  220  with semiconductor device  230 , the tall contact elements  224 ,  226  and  228  make contact with respective metal pads  232  first while shorter contact elements  225  and  227  remain unconnected. Contact elements  224 ,  226  and  228  can be used to make electrical connection to power and ground pins of semiconductor device  230 . With further application of the external biasing force F ( FIG. 10B ), shorter contact elements  225  and  227 , making connection to signal pins, can then make connection with respective metal pads  232  on device  230 . Because the contact elements of the present invention have a large elastic working range, the first set of contact elements can be further compressed than the second set of contact elements without compromising the integrity of the contact elements. In this manner, connector  220  enables hot-swapping operation with semiconductor device  230 .  
      According to another aspect of the present invention, a connector is provided with ground planes and the impedance of the contact elements can be controlled by varying the distance between the contact element for a signal pin and the ground plane or between the contact element for a signal pin and the contact element for a ground pin.  FIG. 11  is a cross-sectional view of a connector including a ground plane for improving signal integrity and for controlling contact element impedance according to one embodiment of the present invention. Referring to FIG.  11 , a connector  250  includes a contact element  254 B which is to be connected to a signal pin on a semiconductor device. Connector  250  further includes contact elements  254 C which is to be connected to the ground potential of the semiconductor device. Connector  250  includes a ground plane  255  which is formed in substrate  252 . Ground plane  255  can be formed on the top surface of substrate  252  or embedded in substrate  252 . In  FIG. 11 , the connection between contact elements  254 A and  254 C and ground plane  255  is shown. In actual implementation, contact elements  254 A and  254 C can be connected to ground plane  255  through metal connection on the surface of substrate  252  or through metal connection embedded in substrate  252 .  
      The inclusion of ground plane  255  in connector  250  has the effect of improving the signal integrity of the AC electrical signals that are connected through connector  250 . Specifically, as integrated circuits are being operated at higher and higher frequencies while the package lead count increases with decreasing lead pitches, the ability to improve signal integrity in a connector used to interconnect such integrated circuits becomes more important. In accordance with the present invention, connector  250  includes ground plane  255  which functions to reduce noise and improve signal integrity of the connector. Furthermore, in the configuration shown in  FIG. 11 , the distance G between contact element  254 B for a signal pin and contact elements  254 A and  254 C for the ground potential can be varied to obtain a desired impedance for contact element  254 B. Elements  257 A,  257 B and  257 C can be included to further control the Electromagnetic emissions and rejection characteristic of the connector.  
       FIG. 12  illustrates another embodiment of the connector of the present invention where a pair of contact elements  262  and  264  is used to couple to a pair of differential signals. In the present embodiment, contact elements  262  and  264  are each formed as including separate base portions  261  and  263 . In this manner, a connector including contact elements  262  and  264  can be used to contact a semiconductor device including a pair of differential signals.  
      According to another aspect of the present invention, a connector incorporates embedded thermal dissipation structures to provide enhanced heat dissipation capability at specific contact elements. For instance, when a contact element engaging a lead of an electronic package carries more than 1 A of current, significant Joule heating can result creating a temperature rise of 20 degrees or more at the contact element. In accordance with the present invention, a connector includes embedded thermal dissipation structures so as to effectively limit the temperature rise at specific contact elements. For example, the amount of temperature rise can be reduced to 10 degrees or less by the use of the embedded thermal dissipation structures in the connector of the present invention.  
       FIG. 13  illustrates a connector incorporating a thermally conductive plane according to one embodiment of the present invention. Referring to  FIG. 13 , connector  270  includes contact elements  274 A to  274 D formed on the top surface of a substrate  272 . A thermally conductive plane  277  is formed in substrate  272  during the manufacturing process of substrate  272 . Thermally conductive plane  277  provides heat dissipation function for contact elements  274 A to  274 D. In one embodiment, the thermally conductive plane is formed using Cu. In another embodiment, the thermally conductive plane is formed using a filled epoxy which is not electrically conductive and thus can be in intimate contact with any circuitry that may be present in substrate  272  and connected to contact elements  274 A to  274 D. In operation, thermally conductive plane  288  dissipates heat generated at the contact elements when the contact elements are coupled to a semiconductor device and are subjected to Joule heating.  
      According to yet another aspect of the present invention, a connector includes one or more coaxial contact elements.  FIG. 14  illustrates a connector  300  including a coaxial contact element according to one embodiment of the present invention. Referring to  FIG. 14 , connector  300  includes a first contact element  320  and a second contact element  340  formed on the top surface of a substrate. Contact elements  320  and  340  are formed in proximity to but electrical isolated from each other. In the present embodiment, contact element  320  includes a base portion  322  formed as an outer ring including an aperture while contact element  340  includes a base portion  342  formed inside the aperture. Each of contact elements  320  and  340  includes multiple curves spring portions. Specifically, contact element  320  includes eight curved spring portions  324  dispersed along the circular base portion  322 . Curved spring portions  324  are formed linear projection from the base portion. On the other hand, contact element  340  includes two curved spring portions  344 A and  344 B, each curved spring portion projecting in a spiral configuration from the base portion.  
      The curved spring portions of contact element  320  do not overlap with the curved spring portions of contact element  340 . Thus, contact element  320  is electrically isolated from contact element  340 . As thus constructed, connector  300  can be used to interconnect a coaxial connection on a semiconductor device. Typically, the outer contact element is coupled to a ground potential connection while the inner contact element is coupled to a signal connection, such as a high frequency signal. A particular advantage of the connector of the present invention is that the coaxial contact elements can be scaled to dimensions of 250 microns or less. Thus, the connector of the present invention can be used to provide coaxial connection even for small geometry electronic components.  
      According to another aspect of the present invention, each of the contact elements of the connector further includes a conductive adhesion layer in the base portion of the contact element for improving the adhesion of the contact element to the substrate.  FIGS. 15A  to  15 H illustrate the processing steps for forming an array of connectors according to an alternate embodiment of the present invention. Like elements in  FIGS. 7A  to  7 H and  15 A to  15 H are given like reference numerals to simplify the discussion.  
      Referring to  FIG. 15A , a substrate  102  on which the contact elements are to be formed is provided. Substrate  102  can be a silicon wafer or ceramic wafer and may include a dielectric layer formed thereon (not shown in  FIG. 15A ). A conductive adhesion layer  103  is deposited on substrate  102  or on top of the dielectric layer if present. Conductive adhesion layer  103  can be a metal layer, such as copper-beryllium (CuBe) or titanium (Ti), or a conductive polymer-based adhesive, or other conductive adhesive. Then, a support layer  104  is formed on the adhesion layer  103 . Support layer  104  can be a deposited dielectric layer, such as an oxide or nitride layer, a spin-on dielectric, a polymer, or any other suitable etchable material.  
      After the support layer  104  is deposited, a mask layer  106  is formed on the top surface of support layer  104 . Mask layer  106  is used in conjunction with a conventional lithography process to define a pattern on support layer  104  using mask layer  106 . After the mask layer is printed and developed ( FIG. 15B ), a mask pattern, including regions  106 A to  106 C, is formed on the surface of support layer  104  defining areas of support layer  104  to be protected from subsequent etching.  
      Referring to  FIG. 15C , an anisotropic etching process is performed using regions  106 A to  106 C as a mask. As a result of the anisotropic etching process, support layer  104  not covered by a patterned mask layer is removed. The anisotropic etching process stops on conductive adhesion layer  103  or partially in conductive adhesion layer  103 . Thus, conductive adhesion layer  103  remains after the anisotropic etch process. Accordingly, support regions  104 A to  104 C are formed on the conductive adhesion layer. The mask pattern including regions  106 A to  106 C is subsequently removed to expose the support regions ( FIG. 15D ).  
      Referring to  15 E, support regions  104 A to  104 C are then subjected to an isotropic etching process. An isotropic etching process remove material under etch in the vertical and horizontal directions at substantially the same etch rate. Thus, as a result of the isotropic etching, the top corners of support regions  104 A to  104 C are rounded off as shown in  FIG. 15E .  
      Then, referring to  FIG. 15F , a metal layer  108  is formed on the surface of conductive adhesion layer  103  and the surface of support regions  104 A to  104 C. Metal layer  108  can be a copper layer or a copper-alloy (Cu-alloy) layer or a multilayer metal deposition such as Tungsten coated with Copper-Nickel-Gold (Cu/Ni/Au). In a preferred embodiment, the contact elements are formed using a small-grained copper-beryllium (CuBe) alloy and then plated with electroless Nickel-Gold (Ni/Au) to provide a non-oxidizing surface. Metal layer  108  can be deposited by a CVD process, by electro plating, by sputtering, by physical vapor deposition (PVD) or using other conventional metal film deposition techniques. A mask layer is deposited and patterned into mask regions  110 A to  110 C using a conventional lithography process. Mask regions  110 A to  110 C define areas of metal layer  108  to be protected from subsequent etching.  
      Then, the structure in  FIG. 15F  is subjected to an etching process for removing metal layer and conductive adhesion layer not covered by mask regions  110 A to  110 C. As a result, metal portions  108 A to  108 C and conductive adhesion portions  103 A to  103 C are formed as shown in  FIG. 15G . Each of metal portions  108 A to  108 C includes a base portion formed on a respective conductive adhesion portion and a curved spring portion formed on a respective support region ( 104 A to  104 C). Accordingly, the curved spring portion of each metal portion assumes the shape of the underlying support region, projecting above the substrate surface and having a curvature that provides a wiping action when applied to contact a contact point. The base portion of each metal portion is attached to a respective conductive adhesion portion which functions to enhance the adhesion of each base portion to substrate  102 .  
      To complete the connector, support regions  104 A to  104 C are removed ( FIG. 15H ), such as by using a wet etch or an anisotropic plasma etch or other etch process. If the support layer is formed using an oxide layer, a buffered oxide etchant can be used to remove the support regions. As a result, free standing contact elements  112 A to  112 C are formed on substrate  102 . As thus formed, each of contact elements  112 A to  112 C effectively includes an extended base portion. As shown in  FIG. 15H , each conductive adhesion portion serves to extend the surface area of the base portion to provide more surface area for attaching the contact element to substrate  102 . In this manner, the reliability of the contact elements can be improved.  
      The above detailed descriptions are provided to illustrate specific embodiments of the present invention and are not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. For example, one of ordinary skill in the art would appreciate that references to the “top” and “bottom” surfaces of a structure are illustrative only and the “top” and “bottom” references are merely used to refer to the two opposing major surfaces of the structure. Furthermore, while the above description refers to the use of the connector of the present invention for connecting to wafers, to LGA packages and to BGA packages, one of ordinary skill in the art would appreciate that the connector of the present invention can be used as an interconnect for any types of area array formed using pads or land oar solder balls as the eletrical connections or the contact points. The references to specific types of semiconductor device to be connected are illustrative only. The present invention is defined by the appended claims.