Abstract:
Fine pitch probe array from bulk material. In accordance with a first method embodiment, an article of manufacture includes an array of probes. Each probe includes a probe tip, suitable for contacting an integrated circuit test point. Each probe tip is mounted on a probe finger structure. All of the probe finger structures of the array have the same material grain structure. The probe fingers may have a non-linear profile and/or be configured to act as a spring.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Patent Application 61/607,893, entitled, “A Method to Fabricate Fine Pitch Probe Arrays Using Silicon,” filed 7 Mar. 2012, to Namburi, which is hereby incorporated herein by reference in its entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    Embodiments of the present invention relate to the field of integrated circuit design, manufacture and test. More specifically, embodiments of the present invention relate to systems and methods for fine pitch probe arrays from bulk material. 
       BACKGROUND 
       [0003]    Integrated circuit testing generally utilizes fine probes to make contact with test points of an integrated circuit in order to inject electrical signals and/or measure electrical parameters of the integrated circuit. Conventional circuit probes are produced singly, and manually assembled into an array corresponding to some or all of the test points on an integrated circuit. 
         [0004]    Unfortunately, due to the constraints of producing the probes individually, and assembling them into an array, conventional integrated circuit probe arrays are generally unable to achieve a pitch, e.g., probe to probe spacing, of less than about 50 μm. In addition, conventional probes often have an undesirable high inductance, which may limit the frequency of test signals. Further, conventional integrated circuit probe arrays are typically unable to achieve necessary alignment accuracies in all three dimensions. Still further, such alignment and co-planarity deficiencies of conventional probes deleteriously limit the number of probes and the total area of a probe array, and hence the total area of an integrated circuit that may be tested at a single time. For example, a single conventional integrated circuit probe array assembled at a fine pitch may not be capable of contacting all test points on a large integrated circuit, e.g., an advanced microprocessor. 
       SUMMARY OF THE INVENTION 
       [0005]    Therefore, what is needed are systems and methods for fine pitch probe arrays from bulk material. What is additionally needed are systems and methods for fine pitch probe arrays from bulk material with fine pitches and high positional accuracy. A further need exists for systems and methods for fine pitch probe arrays from bulk material that are compatible and complementary with existing systems and methods of integrated circuit design, manufacturing and test. Embodiments of the present invention provide these advantages. 
         [0006]    In contrast to the conventional art in which an array of electronic probes is constructed by adding individual probes to form an assembly, embodiments in accordance with the present invention form an array of electronic probes from a bulk material, removing material to render the basis of an array of electronic probes. 
         [0007]    In accordance with a first method embodiment, an article of manufacture includes an array of probes. Each probe includes a probe tip, suitable for contacting an integrated circuit test point. Each probe tip is mounted on a probe finger structure. All of the probe finger structures of the array have the same material grain structure. The probe fingers may have a non-linear profile and/or be configured to act as a spring. 
         [0008]    In accordance with a method embodiment, a bulk material with first and second substantially parallel faces is accessed. A probe base is formed on the first face. A probe tip suitable for contacting an integrated circuit test point is formed on the probe base. The second face is mounted to a carrier wafer. Portions of the bulk material are removed to form a probe finger structure coupled to the probe base and the probe tip. The probe finger structure is coated with a conductive metal electrically coupled to the probe tip. Formation of the probe tip and probe base may include photolithography. 
         [0009]    In accordance with another embodiment of the present invention, an electronic probe array for testing integrated circuits includes a plurality of individual probes, mechanically coupled and electrically isolated. Each individual probe includes a probe tip functionally coupled to a probe finger structure. The probe tip is of a different material from the probe finger structure. The probe tip is configured for contacting an integrated circuit test point. Each probe finger structure is formed from a same piece of bulk material. Each individual probe is coated with conductive metal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. Unless otherwise noted, the drawings are not drawn to scale. 
           [0011]      FIG. 1  illustrates a portion of an exemplary “through-silicon via” (TSV) carrier wafer, in accordance with embodiments of the present invention. 
           [0012]      FIG. 2A  illustrates formation of a probe block, in accordance with embodiments of the present invention. 
           [0013]      FIG. 2B  illustrates a formation of slots between rows of probes along one axis to form a probe block, in accordance with embodiments of the present invention. 
           [0014]      FIG. 2C  illustrates a plan view of a portion of a substrate after the formation of slots, in accordance with embodiments of the present invention. 
           [0015]      FIG. 3  illustrates a die bonding of a probe block to a carrier wafer, in accordance with embodiments of the present invention. 
           [0016]      FIG. 4  illustrates a sectional view of an array of individual probes, in accordance with embodiments of the present invention. 
           [0017]      FIG. 5  illustrates application of a conductive metal coating to an array, in accordance with embodiments of the present invention. 
           [0018]      FIG. 6  illustrates removal of the masking layer, exposing the probe tip, in accordance with embodiments of the present invention. 
           [0019]      FIG. 7  illustrates a typical application of an array of probes, in accordance with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with these embodiments, it is understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the invention, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be recognized by one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the invention. 
       Notation and Nomenclature 
       [0021]    Some portions of the detailed descriptions which follow (e.g.,  FIGS. 1-7 ) are presented in terms of procedures, steps, logic blocks, processing, and other symbolic representations of operations on data bits that may be performed on computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. A procedure, computer executed step, logic block, process, etc., is here, and generally, conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
         [0022]    It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions utilizing terms such as “accessing” or “forming” or “mounting” or “removing” or “coating” or “attaching” or “processing” or “singulating” or “roughening” or “filling” or “performing” or “generating” or “adjusting” or “creating” or “executing” or “continuing” or “indexing” or “computing” or “translating” or “calculating” or “determining” or “measuring” or “gathering” or “running” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
       Fine Pitch Probe Array from Bulk Material 
       [0023]      FIG. 1  illustrates a portion of an exemplary “through-silicon via” (TSV) carrier wafer  100 , in accordance with embodiments of the present invention. Wafer  100  is illustrated as being formed in silicon, although any suitable material may be utilized. Wafer  100  should generally have parallel top and bottom faces. Any suitable plan-view shape may be used. Wafer  100  comprises a silicon substrate  101  with oxide on sidewalls of the silicon via to insulate the metal via from the semiconducting silicon. 
         [0024]    Carrier wafer  100  also comprises a sacrificial ground layer, formed of any suitable material. Sacrificial ground layer  102  will be utilized during wire electrical discharge machining (wire-EDM) processing, further described below, and should be suitable for such purpose. Carrier wafer  100  further comprises a plurality of solder pads  103 . Solder pads  103  may comprise an alloy of gold (Au) and tin (Sn), at an exemplary thickness of 2 μm. Under laying solder pads  103  are a plurality of under-bump-metallurgy (UBM) thin film stacks  105 . UBM thin film stacks  105  may comprise a film of, for example, titanium (Ti), platinum (Pt) and gold (Au). It is appreciated that other suitable materials may also be used. An insulating layer  104 , e.g., silicon dioxide (SiO 2 ), or other suitable material, separates the stacks of solder pads  103  and UBM  105 . 
         [0025]    Carrier wafer  100  further comprises a plurality of through-silicon vias (TSV)  106 . Through silicon vias  106  provide electrical coupling from the solder pads  103  to the other side of the carrier wafer  100 , and to sacrificial ground layer  102 . 
         [0026]      FIG. 2A  illustrates formation of a probe block  200 , in accordance with embodiments of the present invention. Probe block  200  comprises a substrate  201  comprising silicon, although any suitable material may be utilized, for example, beryllium copper. Silicon substrate  201  may be similar to silicon substrate  101 , illustrated in  FIG. 1 . Silicon substrate  201  may comprise highly doped p-type silicon, doped with boron (B) to a concentration of about 10 18  dopants/cm 3 , for example, which may produce an electrical resistivity of 0.001 ohm-cm. The thickness of the substrate  201  determines the overall height of the probe array. 
         [0027]    Probe block  200  additionally comprises a plurality of solder pads  203 . Solder pads  203  may be similar to solder pads  103 , illustrated in  FIG. 1 . Solder pads  203  may comprise an alloy of gold (Au) and tin (Sn), at an exemplary thickness of 2 μm. Under laying solder pads  203  are a plurality of under-bump-metallurgy (UBM) thin film stacks  205 . UBM films  205  may be similar to UBM films  105 , illustrated in  FIG. 1 . UBM films  205  may comprise a film of, for example, titanium (Ti), platinum (Pt) and gold (Au). It is appreciated that other suitable materials may be used. 
         [0028]    Probe block  200  further comprises a plurality of probes  210 . Probes  210  comprise a probe base  211  and a probe tip  212 . Probe tip  212  may comprise any material suitable for the probing application, e.g., suitable to contact an integrated circuit test point, for example, a noble metal, e.g., ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir) and/or, platinum (Pt). (It is appreciated that gold (Au) is often included in the noble metals, but is generally considered too soft for probing.) The probe tip  212  and the upper face of the probe base  211  are masked with a masking layer  213 , e.g., a non-conductive polymer. The probe base  211  may be fabricated by sputtering a seed layer on one side of the wafer, and lithographically patterned and plated. Probe tips  212  may be fabricated on top of the probe base by lithographically patterning photoresist, plating the tip material and etching the seed layer between the tip bases. The probe tips  212  may be planarized if necessary for a smooth finish. The probe tips  212  should then coated to protect them from the rest of the process. 
         [0029]      FIG. 2B  illustrates a formation of slots  251  between rows of probes  210  along one axis to form probe block  250 , in accordance with embodiments of the present invention. It is appreciated that slots  251  represent the absence of substrate material. In some embodiments, slots  251  may remove an entire thickness of substrate  201 . It is appreciated that substrate  201  is not completely singulated; portions of substrate  201  remain coupled outside of the plane of  FIG. 2B . Slots  251  may be formed by any suitable process, including, for example, deep reactive ion etching (DRIE). 
         [0030]      FIG. 2C  illustrates a plan view of a portion of substrate  201  after the formation of slots  251 , in accordance with embodiments of the present invention. Slots  251  are substantially parallel, and separate “rows” of probes  210  from one another. Mask  213  is not illustrated in  FIG. 2C  for clarity. 
         [0031]      FIG. 3  illustrates a die bonding  300  of probe block  250  to carrier wafer  100 , in accordance with embodiments of the present invention. Bond pads  103  ( FIG. 1 ) are bonded to bond pads  203  ( FIGS. 2A ,  2 B) by any suitable process. 
         [0032]      FIG. 4  illustrates a sectional view of an array  400  of individual probes  401 , in accordance with embodiments of the present invention. It is to be appreciated that the plane of  FIG. 4  is perpendicular to the plane of  FIG. 3 . For example, the plane of  FIG. 4  is parallel to, but not coincident with, the slots  251 , as illustrated in  FIG. 2C . Individual proves  401  comprise a probe tip  212 , a probe base  211  and a probe finger structure  402 . It is appreciated that all probe fingers  402  will have the same material grain structure, as they are formed from the same block of material, e.g., single crystal silicon. 
         [0033]    It is to be appreciated that individual probes  401  may have a complex shape in at least one dimension, in accordance with embodiments of the present invention. For example, as illustrated in  FIG. 4 , probe fingers  401  are not linear, e.g., they are “bent” to the right. Such a profile, in one or more dimensions, may allow each individual probe to function as a spring, allowing for compliance to slight irregularities in a surface of an integrated circuit, and providing a restorative force to keep the probe tip, e.g.,  212 , in contact with an integrated circuit test point. 
         [0034]    In accordance with embodiments of the present invention, such “non-straight” or non-linear probe profiles may be accomplished by wire electrical discharge machining (wire-EDM). For example, a wire of about 12 μm in diameter may be used to machine probes at fine pitch geometries less than 40 μm. It is appreciated that a probe pitch may be different in X and Y dimensions, and it is not necessarily the same, even in the same dimension. Although the probe fingers  401  are illustrated as being “straight” in the plane of  FIG. 2B , wire electrical discharge machining could be applied to that stage as well, e.g., replacing deep reactive ion etching, to produce a more complex shape in that dimension, as well, in accordance with embodiments of the present invention. It is further appreciated that embodiments in accordance with the present invention may form probes with a pitch greater than about 40 μm. For example, a wire of greater than about 12 μm in diameter may be used to machine probes at larger pitches. Probes formed in accordance with embodiments of the present invention at such larger pitches continue to enjoy significant advantages over the conventional art, including, for example, lower cost, less complexity and exceptional precision in probe tip positional accuracy in all three dimensions. 
         [0035]      FIG. 5  illustrates application of a conductive metal coating  501  to array  400 , in accordance with embodiments of the present invention. The conductive metal coating  501  may comprise gold (Au) and/or copper (Cu), or other suitable materials, and may be applied by any suitable process, including, for example, immersion plating or electro-less plating processes. The thickness of the conductive metal coating  501  may be determined by the required current carrying capability of the probes. Conductive metal coating  501  may not be required in the case that material  201  is a metal such as berellium-copper (BeCu) since it is sufficiently conductive, unlike doped silicon. 
         [0036]    In  FIG. 6 , the masking layer  213  ( FIG. 2 ) is removed, exposing the probe tip  212 , via any suitable process, for example, using a dry reactive ion etch process or by using suitable wet chemistry. In addition, the sacrificial ground layer  102  ( FIG. 1 ) is removed. In this manner, an array of electrical probes  600  is formed from a bulk material, in accordance with embodiments of the present invention. 
         [0037]      FIG. 7  illustrates a typical application of array of probes  600  ( FIG. 6 ), in accordance with embodiments of the present invention. As illustrated in  FIG. 7 , the array of electrical probes  600  is bonded to a space transforming substrate  701 . Space transforming substrate  701  serves to transform the spacing of the probe heads  712 , which may be on a pitch suitable for probing integrated circuits, e.g., less than or equal to about 40 μm, to a pitch more suitable for printed circuit boards, e.g., about 1 mm. 
         [0038]    Substrate  701  may be similar to substrate  101  ( FIG. 1 ), although that is not required. Space transforming substrate  701  is electrically and mechanically bonded to array of probes  600  via any suitable processes and materials, for example via solder bonding pads  703 . Bottom bond pads  704  serve to couple space transforming substrate  701  to a higher level assembly, for example, a printed circuit board. 
         [0039]    In accordance with embodiments of the present invention, the individual probes of the array  600  are formed from a bulk material, e.g., from single crystal silicon with a high modulus. Such material functions as a spring without any appreciable plastic deformation. The complex shape increases the spring characteristic of the probes, allowing for compliance to slight irregularities in a surface of an integrated circuit, and providing a restorative force to keep the probe tip, e.g.,  212 , in contact with an integrated circuit test point. The probe tips exhibit a fine pitch, e.g., less than 40 μm, with excellent planarity and tip positional accuracy, as the probe tips are lithographically defined. The probe array has a high current carrying capability due to the conductive metal coating. Further, probe arrays in accordance with the present invention may be produced with shorter lead times and at reduced cost in comparison with the conventional art, as there is no manually assembly, and the processes leverage the economics of integrated circuit manufacturing. 
         [0040]    Embodiments in accordance with the present invention provide systems and methods for fine pitch probe arrays from bulk material. In addition, embodiments in accordance with the present invention provide systems and methods for fine pitch probe arrays from bulk material with fine pitches and high positional accuracy. Further, embodiments in accordance with the present invention provide systems and methods for fine pitch probe arrays from bulk material that are compatible and complementary with existing systems and methods of integrated circuit design, manufacturing and test. 
         [0041]    Various embodiments of the invention are thus described. While the present invention has been described in particular embodiments, it should be appreciated that the invention should not be construed as limited by such embodiments, but rather construed according to the below claims.