Abstract:
A method for fabricating apparatus for testing semiconductor devices includes forming protective structures for bond wires or other intermediate conductive elements thereof by sequentially fabricating one or more material layers. After a first layer is formed, each subsequent layer is superimposed upon, contiguous with, and mutually adhered to an underlying layer of the protective structure. In addition, a fence member may be assembled with or formed on the test substrate to align and receive a semiconductor device and, thereby, to facilitate assembly of the semiconductor device with the test substrate. The fence member can be formed integrally with the protective structures or secured over the protective structures. Stereolithographic processes may be used to fabricate the fence member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of application Ser. No. 09/994,504, filed Aug. 30, 2002, now U.S. Pat. No. 6,537,842, issued Mar. 25, 2003, which is a continuation of application Ser. No. 09/841,923, filed Aug. 16, 2001, pending, which is a divisional of application Ser. No. 09/590,419, filed Jun. 8, 2000, abandoned. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to stereolithography and, more specifically, to the use of stereolithography to fabricate structures on, or components of, semiconductor testing apparatus and to the resulting structures.  
           [0004]    2. Background of Related Art  
           [0005]    In the past decade, a manufacturing technique termed “stereolithography,” also known as “layered manufacturing,” has evolved to a degree where it is employed in many industries.  
           [0006]    Essentially, stereolithography, as conventionally practiced, involves utilizing a computer to generate a three-dimensional (3-D) mathematical simulation or model of an object to be fabricated, such generation usually effected with 3-D computer-aided design (CAD) software. The model or simulation is mathematically separated or “sliced” into a large number of relatively thin, parallel, usually vertically superimposed layers, each layer having defined boundaries and other features associated with the model (and thus the actual object to be fabricated) at the level of that layer within the exterior boundaries of the object. A complete assembly or stack of all of the layers defines the entire object. Surface resolution of the object is, in part, dependent upon the thickness of the layers.  
           [0007]    The mathematical simulation or model is then employed to generate an actual object by building the object, layer by superimposed layer. A wide variety of approaches to stereolithography by different companies has resulted in techniques for fabrication of objects from both metallic and nonmetallic materials. Regardless of the material employed to fabricate objects, stereolithographic techniques usually involve disposition of a layer of unconsolidated or unfixed material corresponding to each layer within the object boundaries. This is followed by selective consolidation or fixation of the material to at least a semisolid state in those areas of a given layer corresponding to portions of the object, the consolidated or fixed material also at that time being substantially concurrently bonded to a lower layer. The unconsolidated material employed to build an object may be supplied in particulate or liquid form and the material itself may be consolidated, fixed or cured, or a separate binder material may be employed to bond material particles to one another and to those of a previously formed layer. In some instances, thin sheets of material may be superimposed to build an object, each sheet being fixed to a next lower sheet and unwanted portions of each sheet removed, a stack of such sheets defining the completed object. When particulate materials are employed, resolution of object surfaces is highly dependent upon particle size. When a liquid is employed, resolution is highly dependent upon the minimum surface area of the liquid which can be fixed (cured) and the minimum thickness of a layer which can be generated given the viscosity of the liquid and other parameters such as transparency to radiation or particle bombardment (see below) used to effect at least a partial cure of the liquid to a structurally stable state. Of course, in either case, resolution and accuracy of object reproduction from the CAD file is also dependent upon the ability of the apparatus used to fix the material to precisely track the mathematical instructions indicating solid areas and boundaries for each layer of material. Toward that end, and depending upon the layer being fixed, various fixation approaches have been employed, including particle bombardment (electron beams), disposing a binder or other fixative (such as by ink-jet printing techniques), or irradiation using heat or specific wavelength ranges.  
           [0008]    An early application of stereolithography enabled rapid fabrication of molds and prototypes of objects from CAD files. Thus, either male or female forms on which mold material might be disposed could be rapidly generated. Prototypes of objects could be built to verify the accuracy of the CAD file defining the object and to detect any design deficiencies and possible fabrication problems before a design was committed to large-scale production.  
           [0009]    In more recent years, stereolithography has been employed to develop and refine object designs in relatively inexpensive materials, and has also been used to fabricate small quantities of objects where the cost of conventional fabrication techniques is prohibitive, such as in the case of plastic objects conventionally formed by injection molding. It is also known to employ stereolithography in the custom fabrication of products generally built in small quantities or where a product design is rendered only once. Finally, it has been appreciated in some industries that stereolithography provides a capability to fabricate products, such as those including closed interior chambers or convoluted passageways, which cannot be fabricated satisfactorily using conventional manufacturing techniques.  
           [0010]    However, to the inventor&#39;s knowledge, stereolithography has yet to be applied to mass production of articles in volumes of thousands or millions, or employed to produce, augment or enhance products including other pre-existing components in large quantities, where minute component sizes are involved, and where extremely high resolution and a high degree of reproducibility of results are required.  
           [0011]    In the electronics industry, computer chips are typically manufactured by configuring a large number of integrated circuits on a wafer and subdividing the wafer to form singulated devices or dice. Such dice, including so-called “flip-chip” dice, have “solder bumps” or other conductors, or conductive structures, for electrically connecting each die to circuitry external thereto. These conductors are also useful for temporary connection of a die to a test circuit to determine its fitness for the intended use. Tests may be conducted before or after the die has been packaged.  
           [0012]    One type of conventional test apparatus that is used to test the electrical characteristics of semiconductor devices includes a carrier substrate, a test substrate positioned on the carrier substrate, and a fence disposed over the test substrate. The carrier substrate includes terminals and electrical traces that lead from the terminals to communicate with test equipment. Terminals of the carrier substrate are wire bonded to contact pads on the test substrate. The contact pads of the test substrate communicate with test pads thereof. The test pads are arranged to correspond to a pattern of conductors, such as solder balls, conductive pillars, bond pads, or other conductive structures of a semiconductor device to be tested. The fence forms an aperture over the test substrate to facilitate alignment of the semiconductor device to be tested relative to the substrate. As a die to be tested is aligned with a test substrate, test pads of the test substrate temporarily mate or contact the conductors of the semiconductor device. Such test apparatus can be configured to test bare or minimally packaged semiconductor dice or packaged semiconductor devices, such as ball grid array (BGA) packages and chip-scale packages (CSPs).  
           [0013]    Conventionally, the bond wires of a test apparatus have been covered with a silicone gel or a nonconductive epoxy “glob-top” material. As such materials can flow, the use of such materials typically also requires that external fences or walls be used to contain such materials in the desired locations. Internal fences or walls may also be required to prevent such glob top, silicone, and other materials from flowing onto the test pads of a test substrate, which can prevent the electrical connection of tested semiconductor devices to the test substrate. Otherwise, if flowable materials are used to cover wire bonds, these materials may have to be removed from the test pads or from the conductors of the tested semiconductor device to ensure adequate electrical connections between the test substrate and the semiconductor device assembled therewith.  
           [0014]    In other test apparatus, a photoresist material is used to cover the bond wires that connect a test substrate to a carrier substrate. When photoresist materials are used to protect bond wires, the use of a mask and several exposure and developing steps are required.  
           [0015]    Accordingly, there is a need for a method of efficiently and effectively protecting the bond wires of semiconductor device test apparatus, as well as protective structures and test apparatus formed by such a method.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention includes a method of fabricating a protective structure over the bond wires of a semiconductor device assembly, such as the bond wires of the semiconductor device test apparatus that connect test pads of a test substrate to a carrier substrate and, thereby, to the semiconductor device test apparatus. The present invention also includes semiconductor device assemblies so formed.  
           [0017]    A test apparatus embodying teachings of the present invention includes a silicon or other known test substrate with test pads on a surface thereof for receiving complementarily arranged conductors, or conductive structures, of a semiconductor device and electrical traces leading from the test pads to peripheral portions of the test substrate. The test pads may be substantially flush with the surface of the test substrate, recessed relative to the surface, or protrude from the surface, depending upon the types of conductors on the semiconductor devices to be tested with the test substrate or upon the configurations of components of the test apparatus that overlie the test substrate.  
           [0018]    The test substrate is secured to a carrier substrate and electrical connections are formed between terminals of the carrier substrate and the traces and test pads of the test substrate. Preferably, bond wires are used to establish the electrical connections between the electrical traces of the test substrate and their corresponding terminals of the carrier substrate. The terminals of the carrier substrate are configured to communicate with known semiconductor device testing equipment.  
           [0019]    The test apparatus also has protective structures located over the bond wires. The structures formed in accordance with teachings of the present invention may be used to physically protect, seal, and isolate the bond wires of a test apparatus so as to prevent physical damage to and shorting of the bond wires.  
           [0020]    A so-called “fence,” which has a large opening therethrough, is positioned over the test substrate. The fence and the opening therethrough are configured to seat a semiconductor device face down over the test substrate, aligning the conductors on the semiconductor device with their corresponding test pads of the test substrate. The opening through the fence may substantially expose a contact surface of the test substrate. The opening through the fence may have a plurality of vertically extending slots spaced about the periphery thereof, which provide additional tolerances at the periphery of the opening to facilitate the insertion of semiconductor devices into, and their removal from, the fence.  
           [0021]    As another alternative, the fence or the protective structure may include a relatively thin layer that is positionable over the test substrate so as to protect the test substrate from damage during the repeated testing of semiconductor devices. Apertures formed through the thin protective layer of the fence over at least test pads of the test substrate allow for contact between the test pads and corresponding conductors of a die to be tested and may be used to facilitate alignment of the semiconductor device relative to the test substrate.  
           [0022]    The present invention employs computer-controlled, 3-D computer-assisted drafting (CAD) initiated, stereolithographic techniques to rapidly form precision layers of material to specific surfaces of a test substrate and carrier substrate of a test apparatus.  
           [0023]    In the stereolithographic processes that are useful in the present invention, one or more layers of a photo-curable liquid, referred to herein as a photopolymer, are sequentially placed on or laterally adjacent to the item to be covered, and the liquid photopolymer of each layer is cured to at least a semisolid state by a precisely directed beam of laser radiation at substantially ambient temperature. Multiple superimposed, contiguous, mutually adhered layers, each separately cured, form one or more precision three-dimensional structures of desired dimensions.  
           [0024]    For example, a substrate may be covered with a layer of liquid polyimide or other photopolymer which is cured only in particular locations to an at least semisolid state by precisely directed laser radiation at a substantially ambient temperature. As the regions of the layer that are cured by the laser may be selected, photopolymer located over certain regions of the substrate, such as the contact pads thereof, may be left uncured. Thus, apertures may be formed through the protective layer substantially simultaneously with formation of solid regions of a structure. A single layer having a uniform thickness of, for example, about 25 μm (1 mil) may be formed on the surface of the wafer. Single layers having thicknesses of up to about 10 mil or more may be formed, the maximum possible thickness of each layer being limited only by the maximum depth into the liquid photopolymer that the laser beam can penetrate. Multiple superimposed layers, each separately cured, may be formed to create structure layers of even greater thickness while maintaining a thickness accuracy not achievable by conventional techniques.  
           [0025]    In one embodiment of the method, the bond wire protectors and the fence are fabricated on a substrate using precisely focused electromagnetic radiation in the form of an ultraviolet (UV) wavelength laser to fix or cure a liquid material in the form of a photopolymer. However, the invention is not so limited and other stereolithographically applicable materials may be employed in the present invention. The apparatus used in the present invention may also incorporate a machine vision system to locate substrates and features on the substrates, such as bond wires and test pads. The method of the present invention encompasses the use of all stereolithographic apparatus and the application of any and all materials thereby, including both metallic and nonmetallic materials applied in any state and cured or otherwise fixed to at least a semisolid state to define a three-dimensional layer or layers having identifiable boundaries.  
           [0026]    The highly precise stereolithographic process provides accurate alignment of the conductors of a semiconductor device to be tested with the test pads of the test substrate, providing good electrical connection without bump deformation.  
           [0027]    The bond wire protectors and the fence may be fabricated separately by use of individual CAD programs. In another embodiment, the fence is formed stereolithographically to be integral with the bond wire protectors.  
           [0028]    Alternatively, a fence can be fabricated on the test and carrier substrates by other known processes or fabricated separately from the test apparatus by known processes and subsequently assembled with the test substrate and carrier substrate assembly. As another alternative, a stereolithographically formed fence can be formed separately from the remainder of the test apparatus and then assembled therewith.  
           [0029]    Other features and advantages of the present invention will become apparent to those of skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0030]    The figures of the application illustrate exemplary embodiments of the invention, wherein the drawings are not necessarily to scale, and wherein like indicia is used for like and similar elements, and wherein:  
         [0031]    [0031]FIG. 1 is a schematic elevation of an exemplary stereolithography apparatus suitable for use in practicing the method of the present invention;  
         [0032]    [0032]FIG. 1A is an enlarged portion of FIG. 1 showing a structure of the invention being formed in a stereolithographic method of the invention;  
         [0033]    [0033]FIG. 2 is a perspective view of an exemplary test substrate useful for forming a test apparatus of the invention for testing a semiconductor flip-chip die;  
         [0034]    [0034]FIG. 3 is a perspective view of an exemplary test substrate joined to a carrier substrate for forming a semiconductor device test apparatus of the invention;  
         [0035]    [0035]FIG. 4 is a side cross-sectional view of a test substrate joined to a carrier substrate for forming a semiconductor device test apparatus of the invention, as taken along line  4 - 4  of FIG. 3;  
         [0036]    [0036]FIG. 5 is a perspective view of a test apparatus of the invention as formed by the method of the invention;  
         [0037]    [0037]FIG. 6 is a side cross-sectional view of a test apparatus of the invention, as taken along line  6 - 6  of FIG. 5;  
         [0038]    [0038]FIG. 7 is a perspective view of one embodiment of a test apparatus of the invention as formed by the method of the invention;  
         [0039]    [0039]FIG. 8 is a side cross-sectional view of one embodiment of a test apparatus of the invention, as taken along line  8 - 8  of FIG. 7;  
         [0040]    [0040]FIG. 9 is a perspective view of another embodiment of a test apparatus of the invention;  
         [0041]    [0041]FIG. 10 is a side cross-sectional view of another embodiment of a test apparatus of the invention, as taken along line  10 - 10  of FIG. 9;  
         [0042]    [0042]FIG. 11 is a side cross-sectional view of another embodiment of a test apparatus of the invention;  
         [0043]    [0043]FIG. 12 is a perspective view of a further embodiment of a test apparatus of the invention;  
         [0044]    [0044]FIG. 13 is a side cross-sectional view of a further embodiment of a test apparatus of the invention, as taken along line  13 - 13  of FIG. 12;  
         [0045]    [0045]FIG. 14 is a perspective view of another embodiment of a test apparatus of the invention;  
         [0046]    [0046]FIG. 15 is a side cross-sectional view of another embodiment of a test apparatus of the invention, as taken along line  15 - 15  of FIG. 14;  
         [0047]    [0047]FIG. 16 is a perspective view of an additional embodiment of a test apparatus of the invention;  
         [0048]    [0048]FIG. 17 is a side cross-sectional view of an additional embodiment of a test apparatus of the invention, as taken along line  17 - 17  of FIG. 16;  
         [0049]    [0049]FIG. 18 is a perspective view of a test apparatus of the invention with a semiconductor device to be tested inserted into the test apparatus;  
         [0050]    [0050]FIG. 19 is a side cross-sectional view of a test apparatus of the invention with a semiconductor device therein, as taken along line  19 - 19  of FIG. 18; and  
         [0051]    [0051]FIG. 20 is a perspective view of another embodiment of a test apparatus of the invention, showing additional features. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0052]    [0052]FIG. 1 schematically depicts various components and operation of an exemplary stereolithography apparatus  10  to facilitate the reader&#39;s understanding of the technology employed in implementation of the present invention, although those of ordinary skill in the art will understand and appreciate that apparatus of other designs and manufacture may be employed in practicing the method of the present invention. The preferred stereolithography apparatus for implementation of the present invention, as well as operation of such apparatus, are described in great detail in United States Patents assigned to 3D Systems, Inc. of Valencia, Calif., such patents including, without limitation, U.S. Pat. Nos. 4,575,330; 4,929,402; 4,996,010; 4,999,143; 5,015,424; 5,058,988; 5,059,021; 5,059,359; 5,071,337; 5,076,974; 5,096,530; 5,104,592; 5,123,734; 5,130,064; 5,133,987; 5,141,680; 5,143,663; 5,164,128; 5,174,931; 5,174,943; 5,182,055; 5,182,056; 5,182,715; 5,184,307; 5,192,469; 5,192,559; 5,209,878; 5,234,636; 5,236,637; 5,238,639; 5,248,456; 5,256,340; 5,258,146; 5,267,013; 5,273,691; 5,321,622; 5,344,298; 5,345,391; 5,358,673; 5,447,822; 5,481,470; 5,495,328; 5,501,824; 5,554,336; 5,556,590; 5,569,349; 5,569,431; 5,571,471; 5,573,722; 5,609,812; 5,609,813; 5,610,824; 5,630,981; 5,637,169; 5,651,934; 5,667,820; 5,672,312; 5,676,904; 5,688,464; 5,693,144; 5,695,707; 5,711,911; 5,776,409; 5,779,967; 5,814,265; 5,850,239; 5,854,748; 5,855,718; 5,855,836; 5,885,511; 5,897,825; 5,902,537; 5,902,538; 5,904,889; 5,943,235; and 5,945,058. The disclosure of each of the foregoing patents is hereby incorporated herein by reference. Improvements in the conventional stereolithographic apparatus, as described in copending application Ser. No. 09/259,143, filed Feb. 26, 1999, and of even assignment, relate to a so-called “machine vision” system in combination with suitable programming of the computer controlling the stereolithographic process. This improvement eliminates the need for accurate positioning or mechanical alignment of workpieces to which material is stereolithographically applied. Alignment of the laser beam or other fixing agent may be item specific (e.g., substrate specific) so that, for example, a plurality of micromachined silicon test substrates  40  may be attached to a carrier substrate  50  and alignment and protective structure  60  (see FIG. 1A) independently formed in selected patterns on each test substrate. Using a machine vision system, accuracy of the process is not dependent on a fiduciary mark  62  (FIG. 2) on a test substrate  40  or on a carrier substrate  50  but on the visual recognition of specific substrate characteristics, such as the locations of test pads  42 , bond wires  56 , or other features of test substrate  40  or carrier substrate  50 .  
         [0053]    With reference to FIGS.  1 - 19  and as noted above, a 3-D CAD drawing of an object such as a protective structure  60  to be fabricated in the form of a data file is placed in the memory of a computer  12  controlling the operation of apparatus  10  if computer  12  is not a CAD computer in which the original structure design is effected. In other words, an object or structure design may be effected in a first computer in an engineering or research facility and the data files transferred via wide or local area network, tape, disc, CD-ROM or otherwise as known in the art to computer  12  of apparatus  10  to fabricate a protective structure  60  or other object comprising one or more applied layers  64  (see FIG. 1A).  
         [0054]    Each layer  64  is formed or consolidated from a flowable, curable material  16 , which is also referred to herein as liquid material  16 , by a pass of the laser beam  28  thereinto. Test substrate  40  has an active surface  38  having test pads  42  thereon. The completed test apparatus  30  comprises test substrate  40 , carrier substrate  50 , and protective structure  60  formed over bond wires  56  that electrically connect test substrate  40  to carrier substrate  50 . The invention relates specifically to the stereolithographic fabrication of protective structure  60  to shield bond wires  56  of a semiconductor test apparatus.  
         [0055]    The data for protective structure  60  is preferably formatted in an STL file, STL being a standardized format employed by a majority of manufacturers of stereolithography equipment. Fortunately, the format has been adopted for use in many solid-modeling CAD programs, so translation from another internal geometric database format is often unnecessary. In an STL file, the boundary surfaces of protective structure  60  are defined as a mesh of interconnected triangles.  
         [0056]    Apparatus  10  also includes a reservoir  14  (which may comprise a removable reservoir interchangeable with others containing different materials) of liquid material  16  to be employed in applying the intended layer(s)  64  of solidified material to the test substrate  40  and/or carrier substrate  50 . In a currently preferred embodiment, liquid material  16  is a photo-curable polymer (hereinafter “photopolymer”) responsive to light in the UV wavelength range. The surface level  18  of the liquid material  16  is automatically maintained at an extremely precise, constant magnitude by devices known in the art responsive to output of sensors within apparatus  10  and preferably under control of computer  12 . A support platform or elevator  20 , precisely vertically movable in fine, repeatable increments in directions  46  responsive to control of computer  12 , is located for movement downward into and upward out of liquid material  16  in reservoir  14 . A UV wavelength range laser plus associated optics and galvanometers (collectively identified as  22 ) for controlling the scan of laser beam  26  in the X-Y plane across platform  20  has associated therewith mirror  24  to reflect beam  26  downwardly as beam  28  toward surface  32  of platform  20  or, more particularly, toward active surface  38  of test substrate  40  and toward surface  54  of carrier substrate  50  positioned on surface  32 . Beam  28  is traversed in a selected pattern in the X-Y plane, that is to say, in a plane parallel to surface  32 , by initiation of the galvanometers under control of computer  12  to at least partially cure, by impingement thereon, selected portions of liquid material  16  disposed over active surface  38  to at least a semisolid state. The use of mirror  24  lengthens the path of the laser beam  26 , effectively doubling same, and provides a more vertical beam  28  than would be possible if the laser  22  itself were mounted directly above platform surface  32 , thus enhancing resolution.  
         [0057]    Data from the STL files resident in computer  12  is manipulated to build protective structure  60  or another object on active surface  38 , the surface of another substrate, or on surface  32  of platform  20  one layer at a time. Accordingly, the data mathematically representing protective structure  60  is divided into subsets, each subset representing a layer or slice  64  of protective structure  60 . This is effected by mathematically sectioning a 3-D CAD model into a plurality of horizontal layers  64 , a “stack” of such layers representing protective structure  60 . Each slice or layer may be from about 0.0001 to about 0.0300 inches thick. As mentioned previously, a thinner slice promotes higher resolution by enabling better reproduction of fine vertical surface features of protective structure  60 . In some instances, a base support or supports  34  for the object (e.g., test apparatus  30 ) upon which the protective structure  60  is fabricated may also be programmed as a separate STL file. Such base supports  34  may be fabricated before the overlying protective structure  60  and even prior to the disposal of an object, such as test apparatus  30 , on surface  32  of platform  20 . Base supports  34  facilitate fabrication of protective structure  60  with reference to a perfectly horizontal plane. Such supports also facilitate removal of the object (e.g., carrier substrate  50  bearing one or more test substrates  40  and protective structures  60  from surface  32  of platform  20 ). Where a “recoater” blade is employed, as described below, the interposition of base supports  34  precludes inadvertent contact of blade  85  with surface  32 .  
         [0058]    Before fabrication of protective structure  60  is initiated with apparatus  10 , the primary STL file for protective structure  60 , the file for the object upon which protective structure  60  is fabricated, and the file for base support(s)  34  are merged. It should be recognized that, while reference has been made to the formation of a single test apparatus  30 , protective structures  60  may be concurrently fabricated on multiple test apparatus  30  positioned on surface  32  of platform  20 . In such an instance, the STL files for protective structures  60  and base supports  34 , if any, are merged. Operational parameters for apparatus  10  are then set, for example, to adjust the size (diameter, if circular) of the laser light beam  28  used to cure liquid material  16 .  
         [0059]    In the exemplary method described herein, test substrate  40  or carrier substrate  50  may be precisely coated with a structural layer  64  irrespective of substrate size or number of test substrates  40 . Thus, current stereolithographic equipment will accommodate objects up to 12 or more inches in X and Y dimensions, and it is expected that equipment size will increase as the need to produce larger groups of test substrates  40  becomes commonplace. Bond wires  56  and other structures may be totally enclosed without introducing any temperature-induced or flow-induced bending stresses.  
         [0060]    As shown in FIG. 1A, base supports  34  may be placed on platform  20  prior to the placement of test apparatus  30  onto platform  20 . In addition, lateral supports  36  may be similarly fabricated to secure test apparatus  30  to platform  20 , preventing lateral movement during fabrication of protective structure  60  over bond wires  56  of test apparatus  30 . The fabrication of lateral supports  36  can be facilitated by one or more individual STL files or an STL file for lateral supports  36  may be merged with the other STL files for the entire STL process. Alternative methods and apparatus for securing test apparatus  30  to platform  20  and immobilizing test apparatus  30  relative to platform  20  may also be used and are within the scope of the present invention.  
         [0061]    Base supports  34  and lateral supports  36  may be formed of an at least partially cured material whose attachment to the platform is readily releasable. Alternatively, a solvent may be used to dissolve the supports  34 ,  36  to release test apparatus  30  from platform  20  and supports  34 ,  36 . Such release and solvent materials are known in the art. See, for example, U.S. Pat. No. 5,447,822 referenced above and previously incorporated herein by reference.  
         [0062]    While the invention is described in terms of a liquid material polymerizable to a semisolid or a solid state, the process may be varied to use a finely divided, powdered material, for example. The term “unconsolidated” will be used herein to denote the unpolymerized material which becomes “altered” or “consolidated” by the laser radiation to an at least semisolid state.  
         [0063]    As shown in FIG. 2, a test substrate  40  includes a layer  41  of silicon upon which conductive test pads  42  are located. Conductive test pads  42  are connected by way of electrical traces  44  to contact pads  48 , which are located at or near the periphery of test substrate  40 . Test pads  42  may be depressed, raised, or level with active surface  38  of test substrate  40  to accommodate the particular type of semiconductor devices to be tested with test apparatus  30 .  
         [0064]    As depicted in FIGS. 3 and 4, test substrate  40  is secured on a higher level carrier substrate  50 , which has contact pads  52  on a surface  54  thereof. Contact pads  52  are connected by way of bond wires  56  to corresponding contact pads  48  (FIG. 3) of test substrate  40 . The test substrate  40  carrier substrate  50  assembly is secured to platform  20  of stereolithographic apparatus  10  as already described and shown in FIG. 1A. In FIG. 4, traces  44  and contact pads  52  are shown to illustrate their general location. In the remaining cross-sectional views of FIGS. 6, 8,  10 ,  11 ,  13 ,  15 ,  17 , and  19 , traces  44  and contact pads  52  are not shown for the sake of clarity.  
         [0065]    [0065]FIGS. 5 and 6 depict test apparatus  30  of FIGS. 3 and 4, upon which a protective structure  60  has been formed, such as by the stereolithographic process disclosed herein.  
         [0066]    The position and orientation of each test apparatus  30  on which protective structure  60  is to be formed is located by scanning platform  20  and comparing the features of that test apparatus  30  with corresponding features stored in the data file residing in memory, the locational and orientational data for each test apparatus  30  then also being stored in memory. It should be noted that the data file representing the design size, shape and topography for one or more test apparatus  30  on platform  20  may be used at this juncture to detect those test apparatus  30  which may be physically defective or damaged. It should also be noted that data files for more than one type (size, thickness, configuration, surface topography) of test apparatus  30  may be placed in computer memory and computer  12  programmed to recognize the locations and orientations of test substrates  40  and carrier substrates  50 , as well as of test pads  42 , contact pads  48 , bond wires  56 , contact pads  52 , and boundaries  58  which define the protective structure  60  which is to be formed, and a laser path for forming protective structure  60 .  
         [0067]    Data from the STL files resident in computer  12  is manipulated to form one layer  64  at a time on the test apparatus  30  disposed on platform  20 . Accordingly, where the final protective structure  60  is formed of a plurality of individually formed layers  64 , the data mathematically representing protective structure  60  is divided into subsets, each subset representing a slice or layer  64 . This is effected by mathematically sectioning the 3-D CAD model into a plurality of horizontal layers  64 , “stacks” of such layers representing protective structures  60 . Slices or layers  64  may each be from about 0.0001 to about 0.0300 inch thick. As mentioned previously, a thinner slice promotes higher resolution by enabling better reproduction of fine vertical surface features of protective structure  60 .  
         [0068]    Before initiation of a first layer  64  for a support  34 ,  36  or for protective structure  60  is commenced, computer  12  automatically checks and, if necessary, adjusts by means known in the art, the surface level  18  of liquid material  16  in reservoir  14  to maintain same at an appropriate focal length for laser beam  28 . U.S. Pat. No. 5,174,931, referenced above and previously incorporated herein by reference, discloses one suitable level control system. Alternatively, the height of mirror  24  may be adjusted responsive to a detected surface level  18  to cause the focal point of laser beam  28  to be located precisely at the surface of liquid material  16  at surface level  18  if level  18  is permitted to vary, although this approach is somewhat more complex. Platform  20  may then be submerged in liquid material  16  in reservoir  14  to a depth equal to the thickness of one layer or slice  64  to be formed on test apparatus  30 . The surface level  18  of liquid material  16  can be readjusted as required, such as to accommodate liquid material  16  displaced by submergence of platform  20 . Laser  22  is then activated so that laser beam  28  will scan liquid material  16  in a defined path over surface  54  of carrier substrate  50  or active surface  38  of each test substrate  40  of each test apparatus  30 , in turn, to at least partially cure (e.g., at least partially polymerize) liquid material  16  at selected locations on each test apparatus  30 , including around and over bond wires  56 .  
         [0069]    Boundaries  58  of protective structure  60  circumscribe test substrate  40  below active surface  38  and circumscribe a central opening  66  above active surface  38  (see FIG. 5). Central opening  66  has precise inner wall surfaces  86  configured to accurately guide packaged semiconductor devices  80  (or alternatively unpackaged semiconductor devices) (see FIGS. 18 and 19) thereinto so that the contact pads  82  of semiconductor device  80  precisely contact the test pads  42  for testing each of the semiconductor devices without the necessity for undue pressure. The placement of the inner wall surface  86  is based on the location of the test pads  42  (in computer memory) rather than the carrier substrate  50 , so that accurate positioning is achieved even when the test substrate  40  is joined to the carrier substrate  50  in a less accurate fashion. The outer boundaries  58 A of the protective structure  60  are shown as being in agreement with the edges  88  of the carrier substrate  50 , but need not be.  
         [0070]    If a recoater blade  85  is employed, the process sequence is somewhat different. In this instance, the surface  32  of platform  20  is lowered into liquid material  16  below surface level  18 , then raised thereabove until it is precisely a thickness  96  (see FIG. 1A) of layer  64  below recoater blade  85 . Recoater blade  85  then sweeps horizontally over the uppermost surface of protective structure  60  on which the next layer is to be formed to remove excess liquid material  16  and leave a film thereof of the precise, desired thickness on the uppermost surface. Platform  20  is then lowered so that the surface of the film and material level  18  are coplanar and the surface of the liquid material  16  is still. Laser  22  is then initiated to scan with laser beam  28  and define the first layer  64  on surface  54  of carrier substrate  50 . The process is repeated, layer by layer, to define each succeeding layer  64  and simultaneously bond same to the next lower layer  64  until protective structure  60  is completed. A more detailed discussion of this sequence and apparatus for performing same is disclosed in U.S. Pat. No. 5,174,931, previously incorporated herein by reference. In general, the recoater blade method cannot be used where any portion of test substrate  40 , carrier substrate  50 , bond wires  56 , or another feature of test apparatus  30  protrudes upwardly above the sweeping portion of recoater blade  85 . Recoater blade  85  may generally be used for forming only an upper portion of protective structure  60 .  
         [0071]    As an alternative to the above approach to preparing a layer of liquid material  16  for scanning with laser beam  28 , a layer of liquid material  16  may be formed on the test apparatus  30  by lowering platform  20  to flood material over surface  54  or over the highest completed layer  64  of protective structure  60 , then raising platform  20  and horizontally traversing a so-called “meniscus” blade across platform  20  (or just across the formed portion of protective structure  60 ) to form a layer  64  of desired thickness thereabove, followed by initiation of laser  22  and scanning of beam  28  to define the next higher layer of protective structure  60 .  
         [0072]    As yet another alternative to layer preparation of liquid material  16 , platform  20  can be lowered to a depth equal to that of a layer  64  of liquid material  16  to be scanned and a combination flood bar and meniscus bar assembly can be horizontally traversed over platform  20  to substantially concurrently flood liquid material  16  over surface  54  and define a layer  64  of precisely a desired thickness of liquid material  16  for scanning.  
         [0073]    All of the foregoing approaches to flooding and layer definition and apparatus of initiation thereof are known in the art, so no further details relating thereto will be provided.  
         [0074]    Each layer of structure  60  is preferably built by first defining any internal and external object boundaries  58 ,  58 A of that layer with laser beam  28 , then hatching solid areas of protective structure  60  with laser beam  28 . If a particular part of a particular layer  64  is to form a boundary  58  of a void in the object above or below that layer, then the laser beam  28  is scanned in a series of closely spaced, parallel vectors so as to develop a continuous surface, or skin, with improved strength and resolution. For example, laser  22  first defines the boundaries  58  of protective structure  60  in first layer  64  and fills in solid portions of layer  64  within boundaries  58  to complete a layer of protective structure  60 . Platform  20  is then lowered by a distance substantially equal to a desired thickness of the next, second layer  64 , and laser beam  28  scanned over the next, second layer  64  to define boundaries of protective structure  60  therein and to fill in the areas of second layer  64  within boundaries  58  while simultaneously bonding the second layer to the first. Additional layers  64  are then added at least partially atop the previously formed layer as needed to complete protective structure  60 . The time it takes to form each layer  64  depends upon its geometry, surface tension and viscosity of material  16 , and thickness of the layer.  
         [0075]    Once protective structure  60  is completed on test apparatus  30  or another substrate, platform  20  is elevated above surface level  18  of liquid material  16 , and the test apparatus  30  may be removed from apparatus  10 . Excess, uncured liquid material  16  on the surface of the apparatus  30  may be removed, for example, by a manual removal step and solvent cleaning. Protective structure  60  on each test apparatus  30  may then require postcuring, as material  16  may be only partially polymerized and exhibit only a portion (typically 40% to 60%) of its fully cured strength. Partially consolidated material or unconsolidated material in contact with at least partially consolidated material will eventually cure due to the cross-linking initiated in the outwardly adjacent photopolymer. Postcuring to completely harden protective structure  60  or portions thereof may be accelerated in another apparatus projecting UV radiation in a continuous manner over protective structure  60  and/or by thermal completion of the initial, UV-initiated partial cure.  
         [0076]    In the embodiment of FIGS. 5 and 6, protective structure  60  is shown as formed to encapsulate and protect bond wires  56  and to provide a top surface  68  to which a preformed fence member  90  may be bonded. In FIGS. 7 and 8, a preformed fence member  90  is shown bonded to top surface  68  with a thin layer  92  of adhesive. Fence member  90  has a central opening  67  that is generally co-aligned with central opening  66  of protective structure  60 , although central opening  66  may be larger than central opening  67 . Fence member  90  is positioned to provide accurate mating of contact pads on a type of semiconductor device to be tested with corresponding test pads  42 .  
         [0077]    Fence member  90  may, by way of example and not limitation, be formed of plastic, ceramic, semiconductor material such as silicon, or glass (e.g., borophosphosilicate glass (BPSG), borosilicate glass (BSG), or phosphosilicate glass (PSG)). Alternatively, the stereolithography processes disclosed herein may be used to form a fence member  90  of the desired configuration. When stereolithography is used, fence member  90  can be fabricated separately from test apparatus  30  or protective structure  60 , directly on protective structure  60 , or integrally with protective structure  60 .  
         [0078]    The external terminals used with the test apparatus  30  may be of any type which enables reliable electrical connection with test circuitry. Thus, a wide variety of external terminals may be used, including wire-contact pads, solder bumps, tabs, pins, and the like, and are not shown in the drawings with the exception of FIGS. 7 and 8. In FIGS. 7 and 8, external terminals are illustrated as exemplary down-formed tab conductors  94 .  
         [0079]    In practicing the present invention, a commercially available stereolithography apparatus operating generally in the manner as that described with respect to apparatus  10  of FIG. 1 is preferably employed. For example and not by way of limitation, the SLA-250/50HR, SLA-5000 and SLA-7000 stereolithography systems, each offered by 3D Systems, Inc., of Valencia, Calif., are suitable for practice of the present invention. Photopolymers believed to be suitable for use in practicing the present invention include Cibatool SL 5170 and SL 5210 resins for the SLA-250/50HR system, Cibatool SL 5530 resin for the SLA-5000 system and Cibatool SL 7510 resin for the SLA-7000 system. All of these resins are available from Ciba Specialty Chemicals Corporation. Materials are selected for dielectric constant, sufficient purity (semiconductor grade), adherence to other semiconductor device materials, desirable hardness for physical protection, low shrinkage upon cure, and a coefficient of thermal expansion (CTE) sufficiently similar to that of the test substrate  40  and carrier substrate  50  of test apparatus  30 , to which the material is applied. By selecting a photopolymer with a CTE similar to those of substrates  40  and  50 , substrates  40  and  50  and the at least partially cured material thereon will not be unduly stressed during thermal cycling in initial testing at elevated temperature and subsequent normal operation as a semiconductor device testing apparatus  30 . One area of particular concern in determining resin suitability is the substantial absence of mobile ions and, specifically, fluorides. The layer thickness  96  of material  16  to be formed, for purposes of the invention, may vary widely depending upon the required apparatus height for holding a semiconductor device  80  to be tested, but will enclose bond wires  56  and may be configured to apply a dielectric coating over electrical traces  44  on active surface  38  of test substrate  40  or other protective coating on active surface  38 .  
         [0080]    The size of the laser beam “spot”  78  impinging on the surface of liquid material  16  to cure same may be on the order of 0.002 inch to 0.008 inch. Resolution is preferably ±0.0003 inch in the X-Y plane (parallel to platform surface  31 ) over at least a 0.5 inch×0.25 inch field from a center point, permitting a high resolution scan effectively across a 1.0 inch×0.5 inch area. Of course, it is desirable to have substantially this high a resolution across the entirety of surface  54  of a large structure  54  to be scanned by laser beam  28 , such area being termed the “field of exposure.” The longer and more effectively vertical the path of laser beam  26 / 28 , the greater the achievable resolution.  
         [0081]    Referring again to FIG. 1 of the drawings, improved performance of this process is achieved by certain additions to apparatus  10 . As depicted, apparatus  10  includes a camera  70  which is in communication with computer  12  and preferably located, as shown, in close proximity to mirror  24  located above test apparatus  30 . Camera  70  may be any one of a number of commercially available cameras, such as capacitative-coupled discharge (CCD) cameras available from a number of vendors. Suitable circuitry as required for adapting the output of camera  70  for use by computer  12  may be incorporated in a board  72  installed in computer  12 , which is programmed, as known in the art, to respond to images generated by camera  70  and processed by board  72 . Camera  70  and board  72  may together comprise a so-called “machine vision system,” and specifically a “pattern recognition system” (PRS), the operation of which will be described briefly below for a better understanding of the present invention. Alternatively, a self-contained machine vision system available from a commercial vendor of such equipment may be employed. For example, and without limitation, such systems are available from Cognex Corporation of Natick, Mass. The apparatus of the exemplary Cognex BGA Inspection Package™ or SMD Placement Guidance Package™ may be adapted to the present invention, although it is believed that the MVS-8000™ product family and the Checkpoint® product line, the latter employed in combination with Cognex PatMax™ software, may be especially suitable for use in the present invention.  
         [0082]    It is noted that a variety of machine vision systems are in existence, examples of which and their various structures and uses are described, without limitation, in U.S. Pat. Nos. 4,526,646; 4,543,659; 4,736,437; 4,899,921; 5,059,559; 5,113,565; 5,145,099; 5,238,174; 5,463,227; 5,288,698; 5,471,310; 5,506,684; 5,516,023; 5,516,026; and 5,644,245. The disclosure of each of the immediately foregoing patents is hereby incorporated herein by this reference.  
         [0083]    In order to facilitate practice of the method of the present invention with improved apparatus  10 , a data file representative of the substrate surfaces  54  on which a protective structure  60  is to be formed is placed in the memory of computer  12 . The data file will contain information, such as surface dimensions (in three dimensions) and visual features, as well as spacing and layout of features (e.g., test pads  42 , contact pads  48 , bond wires  56 , and contact pads  52 ) on test substrate  40  and carrier substrate  50 . The data file will also contain information defining boundaries  58 ,  58 A of protective structure  60  to be formed and, in addition, a defined path of laser beam  28  as controlled by mirror  24  to achieve the coverage.  
         [0084]    Continuing with reference to FIGS. 1 and 1A of the drawings, a test apparatus  30  on platform  20  may be submerged partially below the surface level  18  of liquid material  16  to a depth the same as, or greater than, the desired thickness  96  of a first layer  64  of material  16  to be at least partially cured to a semisolid state. Then platform  20  is raised to a depth equal to the layer thickness  96  (if previously lowered to a greater depth than a layer thickness) and the surface level  18  of liquid material  16  is allowed to stabilize. The material  16  selected for use in applying a layer  64  to the test apparatus  30  may be one of the above-referenced resins from Ciba Specialty Chemicals Company. Inasmuch as the stereolithography process is conducted without appreciable temperature rise, the need to compensate boundary location (as constructed) for subsequent temperature drop to match semiconductor device dimensions is generally insignificant.  
         [0085]    Camera  70  is initiated to locate the position and orientation of each test apparatus  30  on which one or more protective structures  60  are to be formed by scanning platform  20  and comparing the features of test apparatus  30  with those in the data file residing in memory, the locational and orientational data for each test apparatus  30  then also being stored in memory.  
         [0086]    Laser  22  is then activated and scanned to direct beam  28 , under control of computer  12 , across the desired portion of substrate  50  to effect the partial cure of material  16  to form a first layer  64 . For forming a second and subsequent layers  64 , the platform  20  is lowered into reservoir  14  and raised as before, and the laser activated to form the next layer atop layer  64 , for example. It should be noted that the layer thickness  96  of material  16  in a selected portion of a given protective structure  60  may be altered layer by layer, again responsive to output of camera  70  or one or more additional cameras  74  and  76  shown in broken lines, which detect particular features of certain test apparatus  30 .  
         [0087]    It should be noted that the laser treatment may be carried out to form a boundary  58  which adheres to the substrate surface  54  or the surface of previous layer  64  and the layer within the boundary is lightly cured to form a semisolid “skin” which encloses liquid material  16 . The final cure of protective structure  60  may be effected subsequently by broad-source UV radiation in a chamber or by thermal cure in an oven. In this manner, an extremely precise protective structure  60  may be formed in minimal time within apparatus  10 .  
         [0088]    As illustrated in FIGS. 9, 10 and  11 , fence member  90  may be configured with portions  100  having reduced elevation. These portions may have any shape, including sloped portions  100 A (FIGS. 9 and 10) and slotted portions  100 B (FIG. 11). Sloped portions  100 A and slotted portions  100 B may be useful for manipulation of a semiconductor device (not shown) inserted into central opening  66  of protective structure  60 . Use of such portions also reduces the quantity of material used to construct fence member  90 .  
         [0089]    In another embodiment of the invention, a test apparatus  30  is formed without the use of a preformed fence member  90 . Thus, as illustrated in FIGS. 12 through 15, the formation of structure  60  previously shown in FIGS. 5 and 6 is continued to a desirable higher elevation to provide a guide for semiconductor devices  80  inserted into the central opening  66 . In this embodiment, use of a separately formed fence member  90  is unnecessary.  
         [0090]    In FIGS. 14 and 15, a test apparatus  30  is shown with cut-out wall portions  100  ( 100 A,  100 B) as previously described.  
         [0091]    As depicted in FIGS. 16 and 17, protective structure  60  may include a thin layer  104  of dielectric material formed over an inner portion of active surface  38  of test substrate  40  to protect active surface  38 , including electrical traces  44  (not shown) from damage or shorts under repeated use. Layer  104  may also be useful for protecting a semiconductor device during assembly thereof with test apparatus  30 . While layer  104  may be formed by conventional methods, this invention encompasses the incorporation of its construction as a part of the stereolithography process. Layer  104  can have one or two sublayers of material that are at least partially cured to give layer  104  a thickness of about 10 to about 50 μm, but layer  104  may have any thickness that will permit the formation of electrical connections between test substrate  40  and conductive elements of a semiconductor device to be assembled therewith. As shown, the test pads  42  are left uncovered, eliminating any additional step to remove cured material therefrom. The methodology is incorporated as a STL file into the total stereolithography program.  
         [0092]    [0092]FIGS. 18 and 19 show a completed test apparatus (exterior terminals not shown) of the type illustrated in FIG. 12, with a semiconductor device  80  inserted therein for testing. In addition, the gap  106  between the central opening  66  and the semiconductor device  80  is precisely configured to facilitate insertion of semiconductor device  80  into central opening  66  and to align contact pads  82  of semiconductor device  80  or other conductors communicating therewith and the corresponding test pads  42 . In the various embodiments of this invention, a minimum of downward force  108  is required to maintain electrical contact between all contact pads  82  of semiconductor device  80  and the corresponding test pads  42  of test substrate  40 . If conductors, such as the illustrated solder balls  84 , protrude from contact pads  82  of semiconductor device  80 , the conductors  84  need not be deformed to provide a sufficient electrical connection.  
         [0093]    It should be noted that in any of the embodiments described thus far, the inner wall surfaces  86  of central opening  66  may be vertical, sloped slightly inward, sloped slightly outward, or undercut (e.g. see FIG. 7). In addition, as shown in FIG. 20, wall surfaces  86  may have vertically extending slots  98 , or notches. Such slots  98  reduce the frictional forces in inserting or removing a semiconductor device  80  to be tested and also result in material savings, weight reduction, and reduced manufacturing time.  
         [0094]    Also shown in FIG. 20 are various optional open spaces  102  in protective structure  60 , which result in weight, material and time savings. The open spaces  102  may be located anywhere in protective structure  60 , so long as their location does not hinder the testing of semiconductor devices  80  or reduce the useful life of test apparatus  30 . Each of these features is incorporated into the STL data file.  
         [0095]    It is notable that the present invention provides a rapid method for forming structures of protective material precisely on specified areas of a test apparatus  30 . The method is frugal of material  16 , since all such material in which cure is not initiated by laser beam  28  remains in a liquid state in reservoir  14  for continued use.  
         [0096]    The method of the present invention is conducted at substantially ambient temperature, the small laser beam spot  78  size and rapid traverse of laser beam  28  on test substrate  40 , carrier substrate  50 , bond wires  56 , and other features of test apparatus  30  resulting in negligible thermal stress thereon.  
         [0097]    Furthermore, forming a protective structure  60  on a test apparatus  30  by stereolithographic processes is advantageous in that such processes enhance the precision of material placement and the precision with which structures of desired dimensions can be fabricated, reduces fabrication time, reduces subsequent packaging costs, and enables computer control of the protective structure fabrication process using commercially available equipment.  
         [0098]    Referring to FIGS. 1 through 20 of the drawings, it will be apparent to the reader that the present invention involves a substantial departure from prior applications of stereolithography, in that the structures of preformed electrical components are modified by forming multilayered structures thereon using computer-controlled stereolithography. Moreover, the use of stereolithography facilitates the fabrication of protective structures  60  that have different configurations and are made from different materials than existing bond wire protective structures.  
         [0099]    It should be re-emphasized that the stereolithographic technique of the present invention is suitable for covering, or leaving uncovered, any desired portion of a substrate, so that electrical connections for connection to semiconductor devices and other devices may be left bare, eliminating a material removal step.  
         [0100]    While the present invention has been disclosed in terms of certain preferred embodiments, those of ordinary skill in the art will recognize and appreciate that the invention is not so limited. Additions, deletions and modifications to the disclosed embodiments may be effected without departing from the scope of the invention as claimed herein. Similarly, features from one embodiment may be combined with those of another while remaining within the scope of the invention.