Abstract:
A stereolithographic method of applying material to form a protective layer on a preformed semiconductor die with a high degree of precision, either in the wafer stage, when attached to a lead frame, or to a singulated, bare die. The method is computerized and may utilize a machine vision feature to provide precise die-specific alignment. A semiconductor die may be provided with a protective structure in the form of at least one layer or segment of dielectric material having a controlled thickness or depth and a very precise boundary. The layer or segment may include precisely sized, shaped and located apertures through which conductive terminals, such as bond pads, on the surface of the die may be accessed. A plurality of discrete protective structures may be formed on corresponding semiconductor devices that are carried by a large-scale semiconductor substrate. Dielectric material may also be employed as a structure to mechanically reinforce the die-to-lead frame attachment.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a divisional of application Ser. No. 09/589,841, filed Jun. 8, 2000, pending. 
     
    
     
       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 form protective layers on substrates, such as semiconductor dice or wafers, and 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 and 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 an object, stereolithographic techniques usually involve disposition of a layer of unconsolidated or unfixed material corresponding to each layer within the object boundaries. The layer of material is selectively consolidated or fixated 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 was to enable rapid fabrication of molds and prototypes of objects from CAD files. Thus, either male or female forms on which mold material might be disposed might be rapidly generated. Prototypes of objects might 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 for the same, 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 inventors&#39; 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 is 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. A coating of protective dielectric material, such as polyimide, can be formed over the wafer prior to cutting in order to prevent physical damage to each die and the circuitry thereon during handling of the die. Polyimide coatings are formed by applying a liquid polymer in a volatile carrier to the center of a spinning wafer. Thus, the polyimide layer is said to be “spun on” to the wafer. Typically, such polyimide layers have thicknesses of about 6μ. When the volatile carrier evaporates and as the polyimide layer shrinks, it warps, with the polyimide layer typically being thicker at the periphery of the wafer than at the center of the wafer, a phenomenon referred to as “dishing”. Given the required tolerances for semiconductor devices, even slight dishing may drastically reduce the yield of useable dice from the wafer. Moreover, when polyimide layers are spun onto semiconductor wafers in this manner, it is difficult to provide a repeatable thickness between wafer batches.  
           [0012]    In addition, the cure temperature typically required to adequately cross-link the polyimide layer is typically about 300° C. This high cure temperature may be detrimental to the integrated circuits of the dice, as well as to the bond pads exposed at the active surface of the dice.  
           [0013]    As conventional processes, such as spin-on techniques, form polyimide layers that substantially cover an active surface of a wafer, shrinkage or warpage of the polyimide layer during evaporation or volatilization of the carrier material or during curing can stress the wafer and may damage the circuitry of semiconductor devices on the wafer.  
           [0014]    Moreover, the resulting cure of the polyimide layer may not be a full “hard” cure. Consequently, if a die having such a polyimide layer on the active surface thereof is packaged with a polymer filled with particles of silica, such as sand with low a particle emission, the silica particles can impinge, or pass through, the polyimide layer, which is supposed to act as a physical barrier to the silica. Such impingement of silica particles can damage the circuits of the die unless expensive round silica particles are used as a filler in the packaging material.  
           [0015]    After disposal on the active surfaces of one or more dice and curing of the polyimide layer, each of the bond pads covered by the polyimide layer is exposed to facilitate access to the bond pads. Typically, a complex, conventional mask and etch process is employed. For example, a photoresist material may then be used to form a mask and an etchant can be employed to remove portions of the polyimide layer that overlie bond pads through apertures of the mask. As is well known in the art, slight dimensional differences between wafers, including nonflatness, can occur, causing misalignment or distortion of the mask and, thus, of structures formed therethrough, such as the bond pad openings through a protective polyimide layer.  
           [0016]    The use of mask and etch processes to pattern polyimide protective layers is also somewhat undesirable in that these processes introduce additional fabrication steps and, thus, increase fabrication time, as well as increasing the likelihood that the die or dice being fabricated will be damaged. The use of mask and etch processing also consumes expensive dielectric materials and etchants.  
           [0017]    Finally, as is well known to those in the art, alpha particles emitted by solders, leads, sand, and other sources can create electron-hole pairs in a semiconductor device and thereby cause “soft” errors during the operation of the semiconductor device. While not permanently damaging to the semiconductor device, “soft” errors are often a nuisance to the user of a computer or other electronic device. A 6μ layer of polyimide may act to minimize such “soft” errors.  
           [0018]    The art does not teach methods of fabricating protective layers on the active surfaces of semiconductor devices that do not stress a wafer on which the semiconductor devices are contained or that do not require a significant number of additional fabrication steps. Nor does the art teach protective layers so fabricated that reduce the incidence of soft errors in the protected semiconductor devices.  
         SUMMARY OF THE INVENTION  
         [0019]    The present invention provides a method of applying a layer of protective material to precisely defined areas on individual semiconductor devices fabricated on a semiconductor wafer, on individual dice, or on other semiconductor device component substrates. The present invention employs computer-controlled, 3-D computer-assisted design (CAD) initiated, stereolithographic techniques to rapidly form precision layers of material to specific surfaces of a substrate, such as a plurality of dice on a wafer, and to an individual die.  
           [0020]    In a preferred embodiment, the layer and associated structures are fabricated on the 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.  
           [0021]    For example, a silicon wafer comprising a large number of distinct semiconductor devices 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 the contact pads of the substrate can be left uncured. Thus, apertures are formed through the protective layer substantially simultaneously with formation of the protective layer. 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 a protective layer of even greater thickness while maintaining a thickness accuracy not achievable by conventional techniques. Multiple superimposed layers may also be used to form one or more three-dimensional structures on the surface of the protective layer or on the surface of the substrate.  
           [0022]    When protective layers are being formed on a wafer bearing a plurality of semiconductor devices, the protective layer may have several laterally separated regions, each corresponding to the location of one of the semiconductor devices and having apertures formed therethrough to expose bond pads of the semiconductor devices. Further, “streets” traversing the wafer between adjacent semiconductor devices may also be left uncoated, thereby reducing stress to which the wafer would otherwise be exposed if a continuous, protective layer covered the semiconductor devices and the streets therebetween. Thus, for example, each semiconductor device on a wafer may be covered with a discrete, well-defined pattern of protective material of controlled depth, shape, and size, with precisely located apertures therethrough.  
           [0023]    The robust, relatively thick protective layer formed in accordance with teachings of the present invention may be used to physically protect, seal, and isolate circuitry of semiconductor device components from physical damage that may occur during handling, as well as electrical shorting of a semiconductor device component to other devices, damage by chemical agents, and alpha particle-induced “soft” errors. When thermoplastic materials that soften or melt at temperatures that will not damage the semiconductor device are used as the protective layer, the protective layer can also act to adhere the semiconductor device to a higher level substrate. Such protective layers are particularly useful on a die to be packaged in a “leads-over-chip” (LOC)-type arrangement where, when packaged, the leads extend over and are secured to the active surface of the semiconductor die. The protective layer may also be applied following the attachment of leads to the bond pads of a semiconductor die, for example, around the lead fingers.  
           [0024]    The apparatus used in the present invention may also incorporate a machine vision system to locate substrates, individual semiconductor devices on a wafer, and features on substrates.  
           [0025]    The method of the present invention encompasses the use of all known stereolithography 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 layer or layers having identifiable boundaries.  
           [0026]    Other features and advantages of the present invention will become apparent to those of skill in the art through a consideration of the ensuing description, the accompanying drawings, and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0027]    The drawings of the application illustrate exemplary embodiments of the invention, wherein the illustrated features are not necessarily to scale, wherein like indicia is used for like and similar elements, and wherein:  
         [0028]    [0028]FIG. 1 is a schematic side elevation of an exemplary stereolithography apparatus suitable for use in practicing the method of the present invention;  
         [0029]    [0029]FIG. 2 is an enlarged, schematic top view of a multiple-die wafer mounted on a platform for formation of a protective layer on the dice in accordance with the method of the invention;  
         [0030]    [0030]FIG. 3 is a further enlarged top view of a portion of the wafer of FIG. 2 illustrating several configurations of a protective layer formed by the method of the invention;  
         [0031]    [0031]FIG. 4 is a side cross-sectional view taken along line  4 - 4  of FIG. 3 and depicting a die of an exemplary wafer prior to formation of a protective layer thereon;  
         [0032]    [0032]FIG. 5 is a side cross-sectional view taken along line  5 - 5  of FIG. 3 and depicting a die of an exemplary wafer having a protective layer thereon;  
         [0033]    [0033]FIG. 6 is a side cross-sectional view taken along line  6 - 6  of FIG. 3 and depicting a die of an exemplary wafer having a protective layer thereon;  
         [0034]    [0034]FIG. 7 is a side cross-sectional view taken along line  7 - 7  of FIG. 3 and depicting a die of an exemplary wafer having a protective layer thereon;  
         [0035]    [0035]FIG. 8 is a side cross-sectional view taken along line  8 - 8  of FIG. 3 and depicting a die of an exemplary wafer having a protective layer thereon;  
         [0036]    [0036]FIG. 9 is a side cross-sectional view taken along line  9 - 9  of FIG. 3 and depicting a die of an exemplary wafer having a protective layer thereon;  
         [0037]    [0037]FIG. 10 is a side cross-sectional view of a lead frame-mounted die having a protective layer formed thereon in accordance with the invention; and  
         [0038]    [0038]FIG. 11 is a side cross-sectional view of a lead frame-mounted die configured for forming a protective layer thereon in accordance with another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]    [0039]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.  
         [0040]    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. Referring to FIG. 2, alignment of the laser beam or other fixing agent may be item specific (i.e., die specific) so that a large number of distinct semiconductor devices, or dice  52 , on a wafer  60 , each die  52  defined by “streets”  44 , may each be accurately coated with a protective layer  50  (see FIG. 10) to the same or differing specifications with regard to numbers of sub-layers  51  (see FIG. 3), layer thickness  40  (FIG. 6), and boundaries  58  (FIG. 3) of areas of dice  52  to be covered by each layer  50  of at least semisolid photopolymer material. Using a machine vision system, accuracy of the process is not dependent on the fiduciary mark  62  (FIG. 3) on a wafer  60  but on the visual recognition of specific physical characteristics of a die  52  or other substrate, whether die  52  is part of a large wafer, a partial wafer, or singulated.  
         [0041]    With reference to FIGS.  1 - 9  and as noted above, a 3-D computer-aided design drawing of an object, such as a protective layer  50  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 object design is effected. In other words, an object 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 layer  50  or another object comprising one or more applied sub-layers  51 . Each sub-layer  51  is formed or consolidated by a pass of the laser beam  28  into a layer of photo-curable material, or photopolymer. The term “active surface” is used herein to denote the actual surface as well as vertical projections thereof.  
         [0042]    The data are preferably formatted in an STL type computer 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, translation from another internal geometric database format is often unnecessary. In the particular case where the apparatus  10  is to be used for applying only a single layer  50  of uniform thickness, the program may be somewhat simplified.  
         [0043]    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  50  of protective material to the dice  52  of a wafer  60  or to singulated dice. In a currently preferred embodiment, the 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  30  of platform  20  or, more particularly, toward surface  54  of a wafer  60  positioned on surface  30 . Beam  28  is traversed in a selected pattern in the X-Y plane, that is to say, in a plane parallel to surface  30 , 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 surface  54  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  30 , thus enhancing resolution. In the exemplary method described herein, a wafer  60  may be precisely coated with a protective layer irrespective of wafer size or number of dice  52 . Thus, current stereolithographic equipment will accommodate wafers  60  up to 12 or more inches in X and Y dimensions, and it is expected that equipment size will increase as the ability to produce even larger wafers becomes commonplace. Base supports, not shown, may be placed on platform  20  prior to introduction of the wafer  60 , to support the wafer  60  in a highly planar, level position. Apparatus  10  may be programmed to stereolithographically form such supports on platform  20 .  
         [0044]    While the invention is described in terms of a liquid material polymerizable to a semisolid and/or solid, the process may be varied to use a 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 at least a semisolid state.  
         [0045]    Wafer  60  or another substrate may be secured to the platform  20  so that it will not move thereon during formation of the layer  50 . As seen in FIG. 2, wafer  60  may be attached to the platform  20  by at least two small edge supports  38  which may be formed in situ by apparatus  10  prior to forming layers  50  on the dice  52  of the wafer  60 . Edge supports  38  are formed from liquid photopolymer material  16  which is at least partially cured by a laser beam  28  prior to forming layers  50  on the dice  52 . The locations of edge supports  38  are preferably at the edges  42  of unusable partial dice  52 A (on a round wafer  60 ), and align the wafer edge  42  and surface  30  of the platform  20 . Edge supports  38  may be formed prior to placing a wafer  60  on platform  20  and configured to hold wafer  60  in place upon platform  20 . Edge supports  38  may also be configured to align wafer  60  on platform  20 . Alternatively, edge supports  38  may be formed after a wafer  60  has been positioned on platform  20 . Edge supports  38  may be formed of a partially cured material whose attachment to the platform is readily releasable or a solvent may be used to dissolve the edge supports  38 . Such 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. The data for forming such edge supports  38  may be programmed as a separate STL file.  
         [0046]    A camera  70  is then activated to locate the position and orientation of each die  52  or other substrate on which a layer  50  is to be placed by scanning platform  20  and comparing the features of dice  52  with those in the data file residing in memory, the locational and orientational data for each die then also being stored in memory. It should be noted that the data file representing the design, size, shape, and topography for dice  52  or other substrates may be used at this juncture to detect physically defective or damaged dice  52 . It should also be noted that data files for more than one type (size, thickness, configuration, surface topography) of a die  52  or other substrate may be placed in computer memory and computer  12  programmed to recognize locations and orientations of dice  52  or other substrates, as well as the boundaries  58  which define the layer  50  which is to be formed, and a laser path for forming the layer.  
         [0047]    Referring now to both FIGS. 1 and 2, data from the STL files resident in computer  12  is manipulated to form layer  50  one sub-layer  51  at a time on each die  52  or other substrate. Accordingly, where layer  50  is formed of a plurality of individually formed sub-layers  51 , the data mathematically representing layer  50  are divided into subsets, each subset representing a slice or sub-layer  51  of layer  50 . This is effected by mathematically sectioning the 3-D CAD model into a plurality of horizontal sub-layers  51 , a “stack” of such sub-layers  51  representing layer  50 . Each slice or sub-layer  51  may 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 sub-layer  51 . Where a “recoater” blade  32  is employed as described below, the interposition of base supports precludes inadvertent contact of blade  32  with the surface  54  of wafer  60  or the surface of another substrate.  
         [0048]    Before fabrication of layer  50  is initiated with apparatus  10 , the primary STL file is configured for the particular dice  52  or other substrates upon which layers  50  are to be formed. Thus, by way of example and not by way of limitation, the primary STL file is integrated with the file used for forming the integrated circuits on the dice  52  or the features on other substrates. It should be recognized that, while reference is made herein to sequentially forming a layer  50  on dice  52  of a wafer  60 , the same process may be used to form a layer on a singulated die  52  or other individual substrate. Furthermore, a wafer  60  having dice  52  of differing dimensions and/or layout may be accommodated by merging of the STL files of the different dice  52  into the primary STL file. Operational parameters for apparatus  10  are then set, for example, to adjust the size (e.g., diameter, if circular) of the laser light beam  28  used to alter the state of (i.e., cure) material  16 .  
         [0049]    Before initiation of a first sub-layer  51  of layer  50  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 the 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. The platform  20  may then be submerged in liquid material  16  in reservoir  14  to a depth equal to the thickness of one sub-layer  51  or slice of the layer  50  or another object to be fabricated and the liquid surface level  18  readjusted as required 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 each substrate, such as the illustrated die  52 , in turn to at least partially cure (e.g., at least partially polymerize) liquid material  16  at selective locations on each die  52 . For example, laser  22  first defines the boundaries  58  of a first sub-layer  51  on die  52  and fills in solid portions thereof to complete the layer. The boundaries  58  may circumscribe bond pads  34  (FIG. 3) or other features on die surface  56  of die  52  which are intended to be left exposed for connection to higher-level packaging as by wire bonding, tape-automated bonding (“TAB”) using flex circuits, use of projecting conductive connectors in a “flip-chip” configuration, or otherwise, as known in the art. The circumscription of such features is specified in the STL files in computer  12 . In the event that complete layer  50  comprises more than one single sub-layer  51 , platform  20  is then lowered by a distance equal to the thickness of a sub-layer  51 , and the laser beam  28  scanned to define and fill in a second sub-layer  51  while simultaneously bonding the second sub-layer  51  to the first sub-layer  51 . The process is then repeated, sub-layer by sub-layer, until layer  50  is completed.  
         [0050]    If a recoater blade  32  is employed, the process sequence is somewhat different. In this instance, the surface  30  of platform  20  is lowered into liquid material  16  below surface level  18 , then raised thereabove until it is precisely a thickness of sub-layer  51  below blade  32 . Blade  32  then sweeps horizontally over surface  56  of die  52 , or (to save time) at least over a portion thereof on which layer  50  is to be fabricated, to remove excess liquid material  16  and leave a film thereof of the precise, desired thickness on surface  56 . Platform  20  is then lowered so that the surface of the film and material level  18  are coplanar and the surface of the material  16  is still. Laser  22  is then initiated to scan with laser beam  28  and define the first sub-layer  51 . The process is repeated, sub-layer by sub-layer, to define each succeeding sub-layer  51  and simultaneously bond the same to the next lower sub-layer  51  until layer  50  is completed. The process is then repeated for each die  52  of wafer  60  or on each of another type of semiconductor device component substrate on which a layer  50  is to be formed. A more detailed discussion of this sequence and apparatus for performing the same is disclosed in U.S. Pat. No. 5,174,931, previously incorporated herein by reference.  
         [0051]    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 surface  56  of a substrate, such as the illustrated die  52 , by lowering platform  20  to flood material  16  over surface  56  or over the highest completed sub-layer  51  of layer  50 , then raising platform  20  and horizontally traversing a so-called “meniscus” blade across the platform (or just the formed portion of layer  50 ) of a thickness equal to one sub-layer  51  thereabove, followed by initiation of laser  22  and scanning of beam  28  to define the next higher sub-layer  51 .  
         [0052]    Yet another alternative to layer preparation of liquid material  16  is to merely lower platform  20  to a depth equal to that of a layer of liquid material  16  to be scanned and then traverse a combination flood bar and meniscus bar assembly horizontally over platform  20  to substantially concurrently flood liquid material  16  over surface  54  of wafer  60  and define a precise layer thickness of liquid material  16  for scanning.  
         [0053]    All of the foregoing approaches to flooding, layer definition and apparatus of initiation thereof are known in the art, therefore, no further details relating thereto will be provided.  
         [0054]    Each sub-layer  51  of complete layer  50  is preferably built by first defining any internal and external object boundaries  58  of that sub-layer  51  with laser beam  28 , then hatching solid areas of complete layer  50  with laser beam  28 . If a particular part of a particular sub-layer  51  is to form a boundary  58  of a void in the object above or below that sub-layer  51 , 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. The time it takes to form each sub-layer  51  depends upon its geometry, surface tension and viscosity of material  16 , and thickness of the layer.  
         [0055]    Once the layers  50  or other objects are completed on the substrate, such as the illustrated dice  52  of wafer  60 , platform  20  is elevated above surface level  18  of liquid material  16 , and wafer  60  with protective layers  50  on dice  52  thereof may be removed from apparatus  10 . Excess, uncured liquid material  16  on the surface of wafer  60  may be removed, for example, by a manual removal step and solvent-cleaning. Layer  50  on each die  52  of wafer  60  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. Postcuring to completely harden layers  50  over the entire wafer  60  or portions thereof, on singulated dice  52 , or on other semiconductor device component substrates may be effected in another apparatus projecting UV radiation in a continuous manner over wafer  60  or dice  52  and/or by thermal completion of the initial, UV-initiated partial cure. Singulation of the individual dice  52  of a wafer  60  is preferably done following the completed cure but may also be effected prior to a completed cure.  
         [0056]    A small portion of wafer  60  is shown in FIG. 3, having a plurality of rectangular dice  52 A,  52 B,  52 C,  52 D,  52 E and  52 F, etc., with die edges  48  separated by streets  44  in the X and Y directions. FIGS. 3 and 4 illustrate a die  52  prior to the formation of a protective layer  50  thereon. For the sake of simplicity, other details of the surface  56  of die  52  are not shown.  
         [0057]    [0057]FIGS. 3 and 5 illustrate the same type of die  52 B with a sub-layer  51  of protective material formed thereon within boundary  58 . Boundary  58  excludes the entire end of die  52 B having bond pads  34  thereon.  
         [0058]    In FIGS. 3 and 6, die  52 C has two rows of centrally located bond pads  34 . Coverage by sub-layer  51  is complete except for the bond pads  34  and adjacent areas.  
         [0059]    As shown in FIGS. 3 and 8, die  52 E has a first sub-layer  51  on the surface thereof. A second sub-layer  51 A is then formed over particular areas of layer  51  to increase the thickness of layer  50  in those areas. Again, the bond pads  34  and adjacent areas are exempted from coverage by both sub-layers  51 ,  51 A. It is noted that in this method, the walls of apertures formed through layer  50  above a bond pad  34  may be substantially vertical, angled or otherwise shaped. When layer  50  has a plurality of sub-layers, the orientation or configuration of the aperture walls can be programmed into the STL file.  
         [0060]    The ability to precisely direct the laser beam  28  to uniformly cover a small target area enables the method to provide very accurate and precise inner or outer boundaries  58 . As illustrated in FIGS. 3 and 9, die  52 F is shown with a sub-layer  51  precisely surrounding each bond pad  34 . Similarly, in FIGS. 3 and 7, die  52 D is illustrated as representing a die surface  56  covered by protective sub-layer  51 . Sub-layer  51  is formed so as to leave bond pads  34 , which are located in centrally extending rows, exposed therethrough.  
         [0061]    The various types of layer coverage depicted in FIGS.  3 - 9  are exemplary only and not intended to be limiting. Any portion of any die  52  of any configuration may have a layer  50  formed thereon by this invention, using one material layer or two or more superimposed, contiguous, mutually-adhered sub-layers  51 .  
         [0062]    When layer  50  or regions thereof are fabricated from a thermoplastic material having a softening or melting temperature that will not damage die  52 , the thermoplastic regions of layer  50  may also be used as an adhesive, or glue, to secure die  52  to a substrate in face-down orientation. The thermoplastic material of layer  50  is disposed in a configuration and thickness that, when heated to at least the softening temperature thereof, will adhere die  52  to a substrate or other component of a semiconductor device assembly in a desired manner. When the material of layer  50  softens at a temperature that also wets or melts conductive structures secured to bond pads  34  while continuing to laterally support the conductive structures, die  52  may be secured to a substrate or other component of a semiconductor device assembly substantially simultaneously with the electrical connection of a conductive structure to a contact of the other component.  
         [0063]    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 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, purity (semiconductor grade), good adherence to other semiconductor device materials, and a coefficient of thermal expansion (CTE) sufficiently similar to that of the dice  52  to which the material is applied so that the dice  52  and cured material thereon are not unduly stressed during thermal cycling in testing and subsequent normal operation. One area of particular concern in determining resin suitability is the substantial absence of mobile ions and, specifically, fluorides. By way of example and not limitation, the layer thickness  40  of material  16  to be formed, for purposes of the invention, may be on the order of about 0.001 inch (1 mil) to about 0.020 inch (20 mils), with a high degree of uniformity over a field on a surface  56  of a die  52  or other substrate. In order to achieve a desired physical, environmental, and alpha particle protection of a semiconductor die  52 , it is presently believed that a preferred total layer thickness  40  is about 0.004 to about 0.015 inches (4 to 15 mils). As noted previously herein, such thicknesses may be achieved with a single layer. It should be noted that different sub-layers  51  may be of different thicknesses so as to form a completed layer  50  of a precise, intended total height or to provide different material thicknesses for different portions of layer  50 . The size of the laser beam “spot” impinging on the surface of liquid material  16  to cure the 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  30 ) 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 whole wafer  60  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.  
         [0064]    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 surface  54  of wafer  60  or another substrate on which a layer  50  according to the invention is to be formed. 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), 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.  
         [0065]    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 by this reference.  
         [0066]    In order to facilitate practice of the present invention with apparatus  10 , a data file representative of the surface of a substrate, such as a die  52  or wafer  60 , on which a layer  50  is to be formed, is placed in the memory of computer  12 . The data file will contain information, such as die surface dimensions and visual features of the substrate. When the substrate is a wafer, the data file can also include information about the spacing and layout of dice  52  on wafer  60 . The data file will also contain information defining the specific area(s) of the surface of the substrate which are to be covered with layer  50  and the desired depth of coverage and, in addition, a defined path of laser beam  28  as controlled by mirror  24  to achieve the desired coverage.  
         [0067]    Continuing with reference to FIG. 1 of the drawings, a substrate, such as the illustrated wafer  60 , 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  40  of a first sub-layer  51  of material  16  to be at least partially cured to a semisolid state. If platform  20  was lowered to a greater depth than a layer thickness, platform  20  is then raised to such a depth that the thickness of material  16  over an upper surface of wafer  60  is equal to the layer thickness  40 , the surface level  18  of liquid material  16  being allowed to stabilize. The material  16  selected for use in applying a protective layer  50  to dice  52  may be one of the above-referenced resins from Ciba Specialty Chemical Company which exhibits a desirable dielectric constant, is of sufficient (semiconductor grade) purity, of a desirable hardness for physical protection, and which is similar in coefficient of thermal expansion (CTE) to that of the substrate, such as die  52 , on which layer  50  is to be formed so that neither the substrate nor layer  50  are unduly stressed during thermal cycling in subsequent packaging, testing and normal operation.  
         [0068]    Laser  22  is then activated and scanned to direct beam  28 , under control of computer  12 , across the desired portion of the surface of the substrate (e.g., surface  56 ) to effect the partial cure of material  16  to form a first sub-layer  51  on the substrate. If the substrate is a die  52  or a wafer  60 , the laser  22  is then refocused to another die  52  and the process repeated. If any portion of die surface  56  is to have a second (or more) sub-layer(s)  51 , the platform  20  is lowered further into reservoir  14 , and can also be raised as before, to submerge the substrate and formed sub-layers  51  into liquid material  16 . The laser is then activated to form another sub-layer  51 A atop sub-layer  51 , for example. It should be noted that thickness  40  of material  16  over a selected portion of a given substrate (e.g., die  52 ) may be altered die by die, again responsive to output of camera  70  or one or more additional cameras  74 ,  76 , and  78 , shown in broken lines, which detect particular features of the substrate.  
         [0069]    It should be noted that the laser treatment may be carried out to form a boundary  58  which adheres to the surface (e.g., die surface  56 ) of the substrate and the sub-layer  51  within the boundary is lightly cured to form a semi-solid “skin” which encloses liquid material  16 . Trapped, unconsolidated material will eventually cure due to the cross-linking initiated in the outwardly adjacent photopolymer. The cure of sub-layer  51  may be subsequently accelerated by broad-source UV radiation in a chamber, or by thermal cure in an oven. In this manner, an extremely thick protective layer  50  may be formed in minimal time within apparatus  10 .  
         [0070]    As illustrated in FIG. 10, the method of the invention may be adapted to form layers  50  on dice  52  (e.g., LOC dice) already mounted on lead frames  66 . In the example of FIG. 10, a series of dice  52  have active surfaces  64  secured to lead frames  66  of strip  80  and electrically connected thereto, such as by wire bonds  68 , thermocompression bonding, TAB bonding, or otherwise as known in the art. A layer  50  of semi-solid material formed from material  16  may be formed on any particular portion of the active surface  64  or back side  82  (including lead frame  66 ) of each die  52 , for protection, insulation or other purpose. In the example of FIG. 10, a layer  50  of semi-solid material is to be formed on portions of the active surface  64  of a die  52  suspended from lead frame strip  80  and supported on platform  20 . Layer  50  surrounds lead fingers of the lead frame  66  and provides attachment thereof to die  52 . As already described, a film of liquid material  16  is formed atop the active surface  64  and lead frame  66 . A narrow beam  28  of UV laser radiation is precisely scanned by stereolithographic means over particular areas to partially cure the material  16  to form a semi-solid layer  50 . The lead frame strip  80  is then repositioned to place the next sequential die  52  in place for formation of layer  50 . It should be noted that the process may be conducted without an underlying platform  20  provided that the die  52  and lead frame strip  80  are securely joined and a vertical position of the combination may be precisely attained and retained without underlying support.  
         [0071]    In another variation, shown in the example of FIG. 11, a narrow sub-layer  51  defining an attachment  84  of semi-solid material may be first formed with the dice  52  positioned atop the lead frame strip  80 , this layer formed adjacent the periphery of the dice  52  to join outer portions of the lead fingers to the dice. Attachment  84  may be formed by submerging the lead frame strip and die to a level providing the desired reinforcement member and partially curing by laser radiation. Following this step, the lead frame strip  80  may then be inverted and a layer  50  (not shown) applied to the active surface  64  of lead frame  66  side of the die  52 , electrically connected thereto, such as by wire bonds  66 , as indicated above.  
         [0072]    It is notable that the method depicted and described with respect to FIGS. 10 and 11 have utility with a variety of lead frame configurations, including conventional lead frames having a die mounting paddle, or paddle-less leads-over-chip (LOC) lead frames or leads-under-chip (LUC) lead frames. The method is adaptable to dice  52  configured for packaging as zig-zag in-line (ZIP) packages, single in-line (SIP) packages, dual in-line packages (DIP), thin small outline packages (TSOP), quad flat packs (QFP), direct die connect packages (DDC), or otherwise.  
         [0073]    It is also notable that the present invention provides a rapid method for forming layers of protective material precisely on specified areas of singulated dice  52 , many dice of a wafer  60 , or dice joined to a lead frame strip  80 . The method conserves material  16 , since all such material in which cure is not initiated by laser  28  remains in a liquid state in reservoir  14  for continued use.  
         [0074]    As the packaging method of the present invention is conducted at substantially ambient temperature, the small beam spot size and rapid traverse of laser beam  28  on the semiconductor dice  52  or other substrate results in negligible thermal stress thereon.  
         [0075]    Referring to FIGS. 1 through 10 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 layered structures thereon using computer-controlled stereolithography.  
         [0076]    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 semiconductor device component substrate, such as semiconductor die  52 , so that electrical connections for connection to lead frames, test equipment or higher-level packaging may be left bare.  
         [0077]    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.