Abstract:
A selective deposition system and method for initiating deposition of an unconsolidated material at a defined starting surface are disclosed. The system includes a sensor system for measuring an upper surface of a workpiece and a surface level of the unconsolidated material as deposited on the upper surface of the workpiece. The deposition process and measurement of the upper surface of the material on the workpiece are continuously performed as the material is applied in a layer by layer deposition process resulting in a substantially uniform material of a predetermined thickness beginning at the upper surface of the workpiece.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to systems and associated methods for forming three-dimensional objects on or about a defined starting surface. More particularly, the present invention relates to correlating the beginning surface of a workpiece with a surface level of a selective deposition system and method.  
         [0003]     2. Background of Related Art  
         [0004]     Various selective deposition processes exist and have become well defined in the art. One exemplary selective deposition manufacturing technique termed “stereolithography,” also known as “layered manufacturing,” has evolved to a degree where it is employed in many industries. 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.  
         [0005]     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 non-metallic 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, followed by selective consolidation or fixation of the material to at least a partially consolidated, or 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 of the object to be fabricated. The unconsolidated material employed to build an object may be supplied in particulate or liquid form, and the material itself may be consolidated or fixed, or a separate binder material may be employed to bond material particles to one another and to those of a previously formed layer.  
         [0006]     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, whereas when a liquid is employed, surface resolution is highly dependent upon the minimum surface area of the liquid which can be fixed and the minimum thickness of a layer that can be generated. 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.  
         [0007]     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 may 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.  
         [0008]     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 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.  
         [0009]     Stereolithography has been applied to mass production of articles in volumes where minute component sizes are involved, and where extremely high resolution and a high degree of reproducibility of results is required. By way of example, conventional stereolithography relating to semiconductor processing has been identified by the assignee and various aspects of packaging devices using such techniques are described in various patents, for example, in U.S. Pat. No. 6,524,346, U.S. Pat. No. 6,549,821, U.S. Pat. No. 6,537,482, U.S. Pat. No. 6,544,465, U.S. Pat. No. 6,529,027 and U.S. Pat. No. 6,326,698 and assigned to the assignee of the invention disclosed and claimed herein, which references are further incorporated herein by reference. In particular, while stereolithography has been used to fabricate encapsulating structures or partially encapsulating structures, the formation of a structure on or about a workpiece beginning at a predefined location has not been described.  
         [0010]     One particular technology that, in the future, may particularly benefit from such an advancement includes the art of semiconductor packaging. Semiconductor devices, such as memory devices and processors, are generally fabricated in very large numbers. Typically, several semiconductor devices are fabricated on a wafer or other large-scale substrate that includes a layer of semiconductor material (e.g., silicon, gallium arsenide, or indium phosphide). The semiconductor devices are then singulated, or diced, from the wafer or other large-scale substrate to provide semiconductor “chips” or dice.  
         [0011]     Conventionally, semiconductor dice have been packaged for protection and to facilitate the formation of electrical connections to the small bond pads thereof. Conventional semiconductor device packages typically include an assembly of a semiconductor die and a higher level substrate board (e.g., a circuit board) or leads. Bond pads of the semiconductor die are electrically connected (e.g., by wire bonds or otherwise) to contact pads of a higher level substrate or to leads. The assembly may then be packaged. For example, assemblies that include a semiconductor die with leads connected to the bond pads thereof are typically packaged by use of transfer molding techniques to secure the leads in place and to protect the active surface of the semiconductor die and the wire bonds or other intermediate conductive elements. Assemblies including a semiconductor die and a higher level substrate may be packaged by injection molding techniques or with a glob-top type encapsulant, both of which protect the active surface of the semiconductor die and the wire bonds or other intermediate conductive elements.  
         [0012]     Due to the ever-decreasing sizes of state of the art electronic devices, conventional semiconductor device packages are relatively bulky. As a result, alternative semiconductor device packaging configurations have been developed to reduce the amount of area, or “real estate,” on circuit boards consumed by semiconductor device packages.  
         [0013]     Among these state of the art semiconductor device packages are the so-called chip-scale packages, the areas of which are substantially the same as or only slightly larger than the areas of the semiconductor dice thereof. Chip-scale packages may include a semiconductor die and an interposer superimposed over the semiconductor die. The bond pads of the semiconductor die are electrically connected to contact pads of the interposer, which are in turn electrically connected to a circuit board or other carrier substrate through traces extending to other contact elements that mate with terminals on the circuit board or other carrier substrate.  
         [0014]     An exemplary ball grid array type chip-scale package  201  is illustrated in  FIG. 1 . Package  201  includes a semiconductor die  202  and an interposer  206  positioned over an active surface  203  of semiconductor die  202 . Interposer  206  is secured to semiconductor die  202  with a layer  215  of adhesive material. A quantity of underfill material  216  is introduced between semiconductor die  202  and interposer  206  to fill any remaining open areas therebetween.  
         [0015]     Interposer  206  includes a slot  207  formed therethrough. Bond pads  204  on an active surface  203  of semiconductor die  202  are exposed through slot  207 . Bond pads  204  are connected by way of wire bonds  205  or other intermediate conductive elements to corresponding first contact pads  208  on interposer  206 . As illustrated, wire bonds  205  extend through slot  207 . Each first contact pad  208  communicates with a corresponding second contact pad  209  on interposer  206  by way of a conductive trace  210  carried by interposer  206 . Second contact pads  209  may be arranged so as to reroute the output locations of bond pads  204 . Thus, the locations of second contact pads  209  may also impart interposer  206  with a desired footprint, and particularly one which corresponds to the arrangement of terminal pads on a carrier substrate (not shown) to which package  201  is to be connected. Bond pads  204 , wire bonds  205 , and first contact pads  208  have each previously been protected by a quantity of an encapsulant material  211 .  
         [0016]     Package  201  is electrically connected to a carrier substrate by way of conductive structures  213 , such as solder balls, connected to second contact pads  209  and corresponding contact pads of the carrier substrate. Package  201  is configured to be connected to a carrier substrate in an inverted, or flip-chip, fashion, which conserves real estate on the carrier substrate. It is also known in the art to connect a chip-scale package to a carrier substrate by way of wire bonds or other conductive elements. Such assemblies, packages and interposers are disclosed, for example, in U.S. Pat. No. 5,719,440, issued to Walter L. Moden and assigned to the assignee of the invention disclosed and claimed herein.  
         [0017]     In addition to encapsulating the interconnections between bond pads  204  of die  202  and contact pads  208  of interposer  206 , it may also be desirable to further encapsulate other facets of chip scale package  201  including side and bottom surfaces. Suitable encapsulation techniques must be precisely applied and controlled in order to maintain tolerances for subsequent processing steps, an example of which is the application of conductive structures  213 . Accordingly, there is a need for a system and method for forming a deposed layer of a determined thickness on one or more facets of an assembly with the selective deposition beginning at a defined starting surface. Furthermore, there is a need for a method and system for sensing a coating thickness and determining a starting and endpoint of a coating process.  
       SUMMARY OF THE INVENTION  
       [0018]     The present invention is directed to a selective deposition system and method for initiating deposition of a material at an upper surface of a workpiece, an example of which is a semiconductor die integral with a semiconductor wafer. In one embodiment of the present invention, a system for selectively depositing a specific thickness of a material on a previously formed workpiece is disclosed. The system includes a platform for supporting the workpiece during a deposition process, a sensing system for measuring both a level of an upper surface of the workpiece and a surface level of the material deposited on the upper surface of the workpiece until the surface level of the material corresponds to the specific thickness of the material. The system further includes a deposition system for depositing the material on the workpiece to the specific thickness as monitored by the sensing system.  
         [0019]     In another embodiment of the present invention, a selective deposition system for depositing a material at selective locations in the X/Y plane, parallel to a major plane of a previously formed workpiece and on the surface thereof is disclosed. The system includes a controller and a platform for moveably supporting in a Z direction, perpendicular to the X/Y plane, the workpiece during a layer by layer deposition of the material at selected locations on the workpiece surface. The system further includes a reservoir for retaining the material into which the platform may be submerged with the workpiece thereon during the layer by layer deposition process. A scanning laser configured to move a laser beam over the workpiece is responsive to the controller and exposes a portion of the material corresponding to the selective locations for a current deposition layer on the workpiece. In order to monitor the thickness and suspend any further deposition processes, at least one sensing system is responsive to the controller for determining a workpiece surface level when the workpiece is supported by the platform and for determining a surface level of the material deposited on the workpiece. It is contemplated that more than one sensing system may be employed.  
         [0020]     In a further embodiment of the present invention, a method for selectively depositing a material on a workpiece is disclosed. According to the method, a workpiece is secured to a platform and the level of the top surface of the workpiece is measured to determine a starting point for depositing at least a portion of the material thereon. A portion of the material is deposited on the workpiece and an upper surface of the material as deposited on the workpiece is measured to determine a thickness of the material on the workpiece. The depositing and measuring of the upper surface of the material continues until the thickness of the material corresponds to a preselected thickness.  
         [0021]     In a yet additional embodiment of the present invention, a method for fabricating a semiconductor assembly is disclosed. A level of a top surface is measured to determine a deposition starting point of at least one semiconductor die integral with a semiconductor wafer. A layer of an encapsulant material is deposited in a predetermined form beginning at the deposition starting point on the at least one semiconductor die. The level of an upper surface of the layer of the encapsulant material is measured as deposited on the at least one semiconductor die to determined thickness of the material on the at least one semiconductor die. Additional layers of the encapsulant material are deposited until the level of the measured upper surface of the current layer of the encapsulant material substantially equals a predetermined thickness.  
         [0022]     Other features and advantages of the present invention will become apparent to those of ordinary 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  
       [0023]      FIG. 1  is a cross-sectional view of an exemplary chip-scale package with a partial encapsulant placed over the intermediate conductive elements thereof;  
         [0024]      FIG. 2  is a perspective view of a semiconductor wafer including multiple dice having active portions thereon, in accordance with one or more embodiments of the present invention;  
         [0025]      FIG. 3  is a cross-sectional view of a semiconductor device of the semiconductor wafer including an interposer and scribe cuts of the semiconductor wafer of  FIG. 2 , taken along line  3 - 3  thereof, in accordance with an embodiment of the present invention;  
         [0026]      FIG. 4  is a cross-sectional view of a semiconductor device including the assembly of  FIG. 3 , depicting an encapsulant material disposed over the top surface including the scribe cuts surrounding the semiconductor die, in accordance with an embodiment of the present invention;  
         [0027]      FIG. 5  is a cross-sectional view of a semiconductor device including the assembly of  FIG. 4 , depicting removal of a portion of the back or passive side of the semiconductor wafer, in accordance with an embodiment of the present invention;  
         [0028]      FIG. 6  is another cross-sectional view of a semiconductor device package shown in  FIG. 5 , which includes an encapsulant layer formed over the partially removed back or passive side of the semiconductor device, in accordance with an embodiment of the present invention;  
         [0029]      FIG. 7  is another cross-sectional view of the semiconductor device package shown in  FIG. 6 , as singulated from the semiconductor wafer, in accordance with an embodiment of the present invention;  
         [0030]      FIG. 8  is a partial view of a sensing system of an exemplary spin-on system, in accordance with an embodiment of the present invention;  
         [0031]      FIG. 9  is a partial view of a sensing system of an exemplary stereolithography system, in accordance with an embodiment of the present invention; and  
         [0032]      FIG. 10  is a schematic representation of an exemplary stereolithography apparatus that may be employed in the method of the present invention to fabricate the encapsulant of a semiconductor die, in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     While the present exemplary description is drawn to applications for semiconductor wafers, applications to other electrically active or passive configurations are also contemplated wherein formation of a subsequent structure beginning at a specific surface or location of an existing structure is desired.  FIG. 2  illustrates a semiconductor wafer including multiple dice having active portions thereon, in accordance with one or more embodiments of the present invention. A semiconductor wafer  10  includes a plurality of semiconductor dice  12  having corresponding active areas  14  thereon. Typically, semiconductor wafer  10  includes a two-dimensional array-like arrangement of dice  12  across an active surface  16 . Accordingly, separation or “singulation” of individual die from the semiconductor wafer includes one or more initial steps for forming scribe cuts  18 ,  20  in respective X and Y directions along one or more surfaces, such as active surface  16 , of the semiconductor wafer  10 .  
         [0034]      FIG. 3  depicts an assembly  30  including an interposer  22  and a semiconductor die  32  with bond pads  34  positioned on an active surface  36  thereof in, for example, one or more centrally located rows. An interposer  22  is a substantially planar member formed from, for example, semiconductor material (e.g., silicon), or any other known substrate material having a coefficient of expansion (CTE) sufficiently similar to that of the material of the semiconductor die  32  and having an upper surface  23  and a lower surface  24 . As illustrated in  FIG. 3 , interposer  22  includes an elongate slot  26  formed therethrough. Slot  26  is positioned substantially along the center of interposer  22 . Interposer  22  also includes first contact pads  28 , or contacts, located proximate slot  26 . Electrical traces  27  carried by interposer  22  connect each first contact pad  28  to a corresponding second contact pad  29  carried on upper surface  23  of interposer  22 . As depicted, second contact pads  29  are arranged in an array over upper surface  23 . As illustrated and by way of example and not limitation, two parallel strips of adhesive  38  may be placed between active surface  36  of semiconductor die  32  and lower surface  24  of interposer  22  so as to secure interposer  22  to semiconductor die  32 . Intermediate conductive elements  40 , which are illustrated as wire bonds but may also be any other known type of intermediate conductive elements, extend through slot  26  to electrically connect bond pads  34  of semiconductor die  32  to corresponding first contact pads  28  of interposer  22 .  
         [0035]     In order to facilitate encapsulation of assembly  30 , an encapsulation material is formed over upper surface  23  of interposer  22  and around the exposed end surfaces of interposer  22  and die  32  as exposed by scribe cuts  18 ,  20 . The encapsulant material may also be formed within slot  26  for the protection and sealing of intermediate conductive elements  40 . By way of example and not limitation, the encapsulant material is applied to the assembly  30  through the use of stereolithographic techniques as will be described herein.  
         [0036]      FIG. 4  is a cross-sectional view of a semiconductor device including the assembly  30  of  FIG. 3 , depicting an encapsulant material disposed over the top surface including the scribe cuts surrounding the semiconductor die, in accordance with an embodiment of the present invention. In  FIG. 4 , an assembly  50  has generally disposed thereon an encapsulant material  52  in an unconsolidated state. By way of example and not limitation, the unconsolidated encapsulant material may be applied to the upper surface  23  as well as the voids formed by the scribe cuts  18 ,  20  using a spin-on process commonly used for application of photoresist during a semiconductor patterning process or through the use of a stereolithographic process of applying liquefied or otherwise unprocessed or unconsolidated material across the upper surface  23  of assembly  50 . Specifics regarding spin-on process are known by those of ordinary skill in the art while the stereolithographic process and an apparatus for performing the process are described in the previously incorporated by reference U.S. patents and further described below.  
         [0037]     The assembly  50  undergoes a curing or hardening process for selectively forming in place encapsulant material  52  while retaining apertures  54  for enabling electrical contact with the second contact pads  29 . The encapsulant material may also flow into the slot  26  for protection of intermediate conductive elements  40 . The curing or hardening process may include a layer-by-layer curing process for all of the layers or may include a two-step process of curing which first fills the various voids with unconsolidated encapsulant material and then caps the uncured encapsulant material with a cured cap. The capped but uncured portion of the encapsulant material is thereafter cured in a follow-up curing step.  
         [0038]     Encapsulant material  52  of assembly  50  exhibits a substantially planar surface  56 . Since surface  56  may be substantially planar, the overall thickness of assembly  50  is reduced relative to packages that employ conventional glob-top type encapsulant materials of greater viscosity and thus having convexly curved surfaces. In addition, when surface  56  is substantially planar, encapsulant material  52  is not as likely as a semiconductor device package with a convexly curved glob-top type encapsulant to interfere with the flip-chip connection of conductive contact pins  76  ( FIG. 7 ) to the terminals of a higher level substrate.  
         [0039]     To ensure that contacts with, for example, the second contact pads  29  may be of a repeatable and reliable nature, the thickness of the encapsulant material  52  located on the upper surface  23  should be consistently formed. For example, in a “flip chip” configuration utilizing a ball grid array contact methodology, where discrete electrically conductive elements in the form of a ball, bump, stud or pillar contacts of solder, other metals or alloys or conductive or conductive filled polymer are applied to the second contact pads  29 , the thickness of the encapsulant material  52  should be controlled in accordance with design specifications to enable proper placement of the electrically conductive ball contacts within apertures  54  and mounting clearance for mounting the final assembly to, for example, a printed circuit board. Prior encapsulation approaches have not measured and monitored the thickness of the encapsulant material with reference to the upper surface  23  of the workpiece such as the semiconductor die  32  of the present example. The encapsulating apparatus disclosed below provides for the measurement location of the upper surface  23  of semiconductor die  32  and the further measurement of the location of the upper surface  56  of the encapsulant material  52 .  
         [0040]      FIG. 5  is a cross-sectional view of a semiconductor device including the assembly of  FIG. 4 , depicting removal of a portion of the back or passive side of the semiconductor wafer for enabling further encapsulation of the back, or passive, side of the semiconductor wafer having one or more semiconductor die thereon, in accordance with an embodiment of the present invention. On a semiconductor die, active circuitry is generally fabricated only on one side and is largely superficial to that side. Therefore, a portion of the back or passive side  58  may be removed to form a thinned surface  62  through mechanical, chemical or otherwise to a depth  60  which at least corresponds to the thickness of the remaining semiconductor wafer located below the scribe cut  18 ,  20 . Abrasive back-grinding and wet chemical etching are suitable techniques to effect such material removal.  
         [0041]      FIG. 6  is another cross-sectional view of a semiconductor device package shown in  FIG. 5 , which includes an encapsulant layer formed over the partially removed back or passive side of the semiconductor wafer, in accordance with an embodiment of the present invention. In order to further encapsulate an assembly  70 , an encapsulant layer  72  is formed on the thinned surface  62  and further couples to the encapsulant material  52  within scribe cuts  18 ,  20  which surrounds the semiconductor die  32  and the interposer  22 , if present within assembly  70 . The encapsulant layer  72  may be formed, after inversion of the workpiece, by encapsulation techniques similar to that described above or by more rudimentary techniques such as spin-on techniques, since the thickness dimension of the encapsulation layer  72  generally does not demand as precise tolerances as those structures formed on the active side.  
         [0042]      FIG. 7  is a cross-sectional view of the semiconductor device package shown in  FIG. 6 , of the semiconductor wafer, in accordance with an embodiment of the present invention. Electrically conductive contacts or pins  76  (e.g., balls, bumps, studs, pillars, or other structures formed from metal, conductive polymer, conductor-filled polymer, or other conductive material), are illustrated as electrically conductive ball contacts according to an exemplary ball grid array contact methodology. The pins  76  are applied to the second contact pads  29  followed by a further processing step which singulates an assembly  80  from the semiconductor wafer by performing a narrower scribe cut  82  within the previous scribe cut  18 ,  20  ( FIGS. 2-6 ) which was subsequently filled with encapsulant material  52 .  
         [0043]      FIG. 8  is a partial view of a sensing system of an exemplary flowable material spin-on system, in accordance with an embodiment of the present invention. A spin-on thickness characterization system  90  generally in includes (i) a surface level sensory portion  86  for measuring the beginning surface level and monitoring the material surface level and (ii) a material application portion  88  for dispensing and distributing the material. The material application portion  88  includes a dispenser  112  and a support and spinning system  96  for providing support and controlled rotational motion for a workpiece such as semiconductor wafer  98 . The surface level sensory portion  86  includes one or more sensors  92 ,  94  for measuring a level of surface  100  and monitoring changes in the level of surface  106  of a dispensed material  99 . The level of an upper surface  100  of semiconductor wafer  98  is measured by, for example, sensor  94  comprised of a transmitter  102  and a receiver  104 . The measurement of the upper surface  100  defines a reference point upon which a defined thickness of material  99 , such as an encapsulant material, may be dispensed and formed.  
         [0044]     The thickness or surface level of material  99  at surface  106  may be measured using the same sensor  94  when placed within a dispensed material region about the semiconductor wafer  98 , or, alternatively a dedicated sensor  92  may be employed to measure and monitor the level at surface  106  of material  99 . In  FIG. 8 , the sensor  92  includes a separate transmitter  108  and receiver  110 , however, combinations of sensor componentry is also contemplated within the scope of the present invention. The sensors  92 ,  94  are configured to transmit a signal toward the surfaces  100 ,  106  and to receive a reflected signal from the respective surfaces. The transmitted signal may be an energy beam selected from the group comprising a visible light beam, an ultra-violet light beam, an infrared light beam, a radio frequency (“RF”) beam, a microwave beam and an ultra-sound beam. To determine the location of the surface defining a starting point such as the surface of the semiconductor wafer or to determine the surface of the upper surface of the material which facilitates the calculation of the thickness of the material, the reflected signal may be analyzed to determine a relative distance between the sensor and the surface.  
         [0045]     In one embodiment of the present invention, the relative distance between the sensor and the surface of the semiconductor wafer is derived by measuring the time delay between the emission of the transmitted signal and detection of the reflected signal, multiplying the measured time delay by the speed of the transmitted signal and dividing by two. In another embodiment of the present invention, the distance between the sensor and the surface may be determined by indirectly establishing the time delay by measuring a phase difference between the transmitted signal and the reflected signal. In a phase measurement sensor embodiment, the transmitted signal may comprise a modulated signal. In yet another embodiment of the present invention, the transmitted signal may be a pulsed signal and the reflected pulse signal may be detected only during a predetermined time window such that increased time delay between transmission and detection causes less of the pulse to be detected. Thus, the detected power level of the reflected pulse signal is inversely proportional to the distance traveled. Other embodiments for measuring the distance between the sensor and the surface, as presently known in the art, may also be employed.  
         [0046]     Specifically, transmitters  102 ,  108  operate as source elements configured to generate, for example, collimated light beams. By way of example and not limitation, the transmitter may comprise a laser diode. Alternatively, the transmitter may comprise a collimator, such as a lens, configured to collimate or focus light exiting an optical fiber to a desired beam diameter or spot size. The collimated light emitted from the transmitter minimizes extraneous reflections and enhances signal detection. Use of a collimated light beam as an energy beam is currently preferred, although the invention is not so limited.  
         [0047]     The receivers  104 ,  1   10  comprise a plurality of detectors disposed in an array, for example a linear array, wherein the detectors are spatially distributed according to the resolution and thickness of material  99 . Each detector in the receivers  104 ,  110  is configured to produce an electronic sensory signal related to the magnitude of the radiation received thereon. By way of example and not limitation, each detector may comprise a photodiode or a charge coupled device (“CCD”). Alternatively, each detector in the receiver may comprise a collimator, such as a lens, configured to collect light into an optical fiber.  
         [0048]      FIGS. 9 and 10  depict various components, and operation, of an exemplary stereolithography apparatus, in accordance with various embodiments of the present invention.  FIG. 9  is a partial view of a sensing system  120  of an exemplary stereolithography system in which a workpiece, such as a semiconductor wafer  114 , may undergo application of a measurable thickness of an unconsolidated material, such as an encapsulant material. The sensing system  120  measures both the starting position or level of the upper surface  122  of the semiconductor wafer  114  and the surface level  124  of material  126 . The sensory system  120  includes one or more sensors  128 ,  130  comprised of respective transmitters  132 ,  134  and receivers  136 ,  138 . The measurements from the various levels are used to form a relatively precise layer of, for example, an encapsulant material on the upper surface  122  of the semiconductor wafer  114 . Changes in the relative levels of surfaces  122  and  124  are coordinated through movements in the height of a platform  140  and/or through the movement of a material displacement piston  142 .  
         [0049]      FIG. 10  schematically depicts various components, and operation, of an exemplary stereolithography system  150  to facilitate the reader&#39;s understanding of the technology employed in implementation of the method 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. Various aspects of stereolithography apparatus for implementation of the method 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 this reference.  
         [0050]     With continued reference to  FIG. 10  and as noted above, a 3-D CAD drawing of a subsequent object to be fabricated in the form of a data file is placed in the memory of a controller  152  controlling the operation of stereolithography system  150  if controller  152  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 controller  152  of stereolithography system  150  for object fabrication.  
         [0051]     The data is preferably formatted in an STL (for STereoLithography) 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 an object are defined as a mesh of interconnected triangles.  
         [0052]     Stereolithography system  150  also includes a reservoir  154  (which may comprise a removable reservoir interchangeable with others containing different materials) of an unconsolidated material  156  to be employed in fabricating the intended subsequent object. In the currently preferred embodiment, the unconsolidated material  156  is a liquid, photo-curable polymer, or “photopolymer,” that cures in response to light in the UV wavelength range. The surface level  158  of unconsolidated material  156  is automatically maintained at an extremely precise, constant magnitude by devices, such as the piston  142  ( FIG. 9 ) known in the art and responsive to output of sensors within system  150  and preferably under control of controller  152 . A support platform or elevator  160 , precisely vertically movable in fine, repeatable increments responsive to control of controller  152 , is located for movement downward into and upward out of material  156  in reservoir  154 .  
         [0053]     A structure may be fabricated on a substrate, such as a semiconductor wafer  144  disposed on platform  160 . The semiconductor wafer  144  may be secured to the platform  160  through vacuum pressure, adhesive, other otherwise and may be further secured thereto by way of one or more base supports (not shown) to prevent lateral movement of, the substrate relative to the platform  160  particularly when a so-called “recoater” blade  172  is employed to form a layer of material on a substrate, such as semiconductor wafer  144  disposed thereon.  
         [0054]     Stereolithography system  150  has a UV wavelength range laser plus associated optics and galvanometers (collectively identified as laser  162 ) for controlling the scan of laser beam  166  in the X-Y plane across the semiconductor wafer  144  fixed about the platform  160 . Laser  162  has associated therewith a mirror  164  to reflect beam  166  downwardly as beam  168  toward surface  170  of semiconductor wafer  144 . Beam  168  is traversed in a selected pattern in the X-Y plane, that is to say, in a plane parallel to surface  170 , by initiation of the galvanometers under control of controller  152  to at least partially cure, by impingement thereon, selected portions of material  156  disposed over surface  170  to at least a partially consolidated (e.g., semisolid) state. The use of mirror  164  lengthens the path of the laser beam, effectively doubling same, and provides a more vertical beam  168  than would be possible if the laser  162  itself were mounted directly above surface  170 , thus enhancing resolution.  
         [0055]     Data from the STL files resident in controller  152  is manipulated to build a subsequent object such as an encapsulating layer as shown in various configurations which are illustrated in  FIGS. 2-7  one layer at a time. Accordingly, the data mathematically representing one or more of the objects to be fabricated are divided into subsets, each subset representing a slice or layer of the object. The division of data is effected by mathematically sectioning the 3-D CAD model into at least one layer, a single layer or a “stack” of such layers representing the object. Each slice 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 the object or objects to be fabricated.  
         [0056]     Because the various embodiments of the present invention build a subsequent structure, such as encapsulant layer, beginning at a defined surface of an existing underlying structure, an example of which is a semiconductor wafer. Therefore, the underlying structure provides the support or base structure for the underlying or subsequent structure.  
         [0057]     By way of disclosure of one operational stereolithographic process configuration, the operational parameters for stereolithography system  150  are set to adjust the size (diameter if circular) of the laser light beam used to cure material  156 . In addition, controller  152  automatically checks and, if necessary, adjusts by means known in the art the surface level  158  of material  156  in reservoir  154  to maintain same at an appropriate focal length for laser beam  168 . 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  164  may be adjusted responsive to a detected surface level to cause the focal point of laser beam  168  to be located precisely at surface level  158  of material  156  if surface level  158  is permitted to vary, although this approach is more complex. Platform  160  with semiconductor wafer  144  attached thereto may then be submerged in material  156  in reservoir  154  to a depth equal to the thickness of one layer or slice of the object to be formed, and the liquid surface level  158  is readjusted as required to accommodate material  156  displaced by submergence of platform  160  and semiconductor wafer  144 . Laser  162  is then activated so laser beam  168  will scan unconsolidated (e.g., liquid or powdered) material  156  disposed over the surface  170  of semiconductor wafer  144  to at least partially consolidate (e.g., polymerize to at least a semisolid state) material  156  at selected locations, defining the boundaries of a first layer. Platform  160  is then lowered by a distance equal to a thickness of a second layer and laser beam  168  scanned over selected regions of the surface of material  156  to define and fill in the second layer while simultaneously bonding the second layer to the first. The process may then be repeated, as often as necessary, layer by layer, until the subsequent structure, such as an encapsulant layer, is completed. The number of layers required to erect the structure or object to be formed depends upon the height of the object or objects to be formed and the desired layer thickness. The layers of a stereolithographically fabricated structure with a plurality of layers may have different thicknesses.  
         [0058]     If a recoater blade  172  is employed, the process sequence is somewhat different. In this instance, surface  170  of the semiconductor wafer  144  on platform  160  is lowered into unconsolidated (e.g., liquid) material  166  below surface level  158  a distance greater than a thickness of a single layer of material  156  to be cured, then raised above surface level  158  until a substrate disposed thereon, or a structure being formed on platform  160  is precisely one layer&#39;s thickness below blade  172 . Blade  172  then sweeps horizontally over the semiconductor wafer  144  on platform  160  or (to save time) at least over a portion thereof on which one or more objects are to be fabricated to remove excess material  156  and leave a film of precisely the desired thickness. Platform  160  is then lowered so that the surface of the film and surface level  158  are coplanar and the surface of the unconsolidated material  156  is still. Laser  162  is then initiated to scan with laser beam  168  and define the first layer. The process is repeated, layer by layer, to define each succeeding layer and simultaneously bond same to the next lower layer until all of the layers of the object or objects to be fabricated are 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.  
         [0059]     As an alternative to the above approach to preparing a layer of material  156  for scanning with laser beam  168 , a layer of unconsolidated (e.g., liquid) material  156  may be formed on surface  170  of the semiconductor wafer  144  disposed on platform  160 , or on one or more objects being fabricated by lowering platform  160  to flood material  156  over surface  170 , over a substrate disposed thereon, or over the highest completed layer of the object or objects being formed, then raising platform  160  and horizontally traversing a so-called “meniscus” blade horizontally over the substrate, such as semiconductor wafer  144  or each of the objects being formed. Laser  162  is then initiated and a laser beam  168  scanned over the layer of unconsolidated material to define at least the boundaries of the solid regions of the next higher layer of the object or objects being fabricated.  
         [0060]     Yet another alternative to layer preparation of unconsolidated (e.g., liquid) material  156  is to merely lower the semiconductor wafer  144  on platform  160  to a depth equal to that of a layer of material  156  to be scanned, and to then traverse a combination flood bar and meniscus bar assembly horizontally over a substrate disposed on platform  160 , or one or more objects being formed to substantially concurrently flood material  156  thereover and to define a precise layer thickness of material  156  for scanning.  
         [0061]     All of the foregoing approaches to liquid material flooding and layer definition and apparatus for initiation thereof are known in the art and are not material to practice of the present invention, so no further details relating thereto will be provided herein.  
         [0062]     In practicing the present invention, a commercially available stereolithography apparatus operating generally in the manner as that described above with respect to stereolithography system  150  of  FIG. 10  is preferably employed, but with further additions and modifications as hereinafter described for practicing the method of the present invention. For example and not by way of limitation, the SLA-250/50HR, SLA-5000, SLA-7000 and SLA-3500 stereolithography systems, each offered by 3D Systems, Inc, of Valencia, Calif. and stereolithography systems available from Sony Precision Technology America, Inc. of Tokyo, Japan, are suitable for modification. Photopolymers believed to be suitable for use in practicing the present invention include Cibatool SL 5170, SL 5210, SL 5530 and SL 7510 resins available from Ciba Specialty Chemicals Company as well as SI-40 resin available from RPC, a wholly owned subsidiary of 3D Systems, Inc., of Switzerland.  
         [0063]     By way of example and not limitation, the layer thickness of material  156  to be formed, for purposes of the invention, may be on the order of about 0.0001 to 0.0300 inch, with a high degree of uniformity. It should be noted that different material layers may have different heights, so as to form a structure of a precise, intended total height or to provide different material thicknesses for different portions of the structure. The size of the laser beam “spot” impinging on surface level  158  of material  156  to cure same may be on the order of 0.001 inch to 0.008 inch. Resolution is preferably ±0.0003 inch in the X-Y plane (parallel to surface  170 ) 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  170  of semiconductor  144  on platform  90  to be scanned by laser beam  168 , such area being termed the “field of exposure,” such area being substantially coextensive with the vision field of a machine vision system employed in the apparatus of the invention as explained in more detail below. The longer and more effectively vertical the path of laser beam  166 ,  168 , the greater the achievable resolution.  
         [0064]     Referring again to  FIG. 10 , it should be noted that stereolithography system  150  useful in the method of the present invention includes the sensing system  120  of  FIG. 9  which measures both the starting position or level of the upper surface  170  of the semiconductor wafer  144  and the surface level  158  of the material  156 . The sensing system  120  includes one or more sensors  128 ,  130  comprised of respective transmitters  132 ,  134  and receivers  136 ,  138 . The measurements from the various levels are used to form a relatively precise layer of, for example, an encapsulant material on the upper surface  170  of the semiconductor wafer  144 .  
         [0065]     Processing in accordance with the one or more methods of the present invention for selectively depositing a material on a workpiece, such as a semiconductor wafer  144 , includes securing the semiconductor wafer  144  to the platform  160  or other support structure. The location of the top or upper surface  170  of the semiconductor wafer  144  is measured and identified as a starting or reference point upon which the formation of a structures, such as an encapsulant, is formed. A portion of unconsolidated material  200  is deposited upon the upper surface  170  of either the semiconductor wafer  144  or a previously consolidated layer of material  156 . A thickness of unconsolidated material may be determined by measuring the surface level  158  of unconsolidated material  156  and with reference to the previously identified upper surface  170  of the semiconductor wafer. The thickness of the unconsolidated material  200  is also relative to the distance of movement of the platform  160  when submerged into the material  156 . At least a portion of the unconsolidated material is then consolidated according to a defined pattern on the semiconductor wafer  144  or upon a previously consolidated layer on the semiconductor wafer  144 . The steps of depositing, measuring and consolidating are then repeated until the thickness of the material corresponds to a desired or preselected thickness.  
         [0066]     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 invention claimed herein. Similarly, features from one embodiment may be combined with those of another while remaining within the scope of the invention.