Abstract:
Microelectronic dies are thinned according to a variety of approaches, which may include bonding the dies to a substrate under vacuum, disposing a film over the dies and the substrate, and/or changing a center of pressure during thinning.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates, in various embodiments, to microelectronic dies and to die-thinning methods applicable to a plurality of released dies. 
       BACKGROUND 
       [0002]    In order to achieve appropriate economies of scale, microelectronic integrated circuitry is most often fabricated on large semiconductor wafers that each contain an array of dies. To facilitate handling, these wafers are typically hundreds of micrometers (“μm”) thick. Before use, these dies are typically (i) thinned in order to facilitate packaging, and (ii) individuated by cleaving the wafer. Most often, the entire wafer of dies is thinned prior to cleaving in order to efficiently achieve a collection of identical dies with matching thicknesses. 
         [0003]    However, for many applications, it may be desirable to thin processed dies following release (i.e., individuation). For example, specialized multichip modules may desirably include a stack of different dies, each of which contains circuitry optimized for a different application or purpose. It may be desirable not to thin such dies prior to individuation from the wafers on which they were processed, because thinned dies are fragile and difficult to handle. Additionally, many dies may be directed toward low-volume application and may not be available and/or cost-effective in full-wafer quantities. Moreover, cracks or damage occurring during the thinning of entire wafers may propagate and affect more than the single die at which they originated, resulting in catastrophic yield loss. Finally, whole-wafer thinning requires significant material removal, as the majority of an entire wafer is removed. Such drastic material removal may be unnecessary if only one or a few of the die thereon need to be thinned. Thus, there exists a need for a process of simultaneously thinning pluralities of released dies to a final desired thickness with a high degree of accuracy and uniformity. 
       SUMMARY 
       [0004]    Limitations of conventional die thinning methods are herein addressed by controllably thinning individual microelectronic dies. Multiple dies may be thinned simultaneously with a high degree of uniformity across each die, and die to die. In various embodiments, thinned dies are handled so as to prevent damage thereto. 
         [0005]    In one aspect, embodiments of the invention feature a method of thinning a plurality of microelectronic dies. The method includes providing a film over a plurality of microelectronic dies disposed on a substrate and removing from a top side of each microelectronic die at least a portion of the film disposed thereover. The thickness of each microelectronic die is decreased, and the plurality of microelectronic dies is removed from the substrate. The film may be an adhesive film. In one embodiment, the microelectronic dies are first adhered to the film, and the film is then adhered to the substrate. The underside of each microelectronic die may be adhered to the substrate with an adhesive material, and the thickness of the adhesive film may be approximately equal to the desired final thickness of the plurality of microelectronic dies. Decreasing the thickness of each microelectronic die may include lapping followed by chemical-mechanical polishing. The chemical-mechanical polishing may be performed at a rate which substantially diminishes when the thickness of each microelectronic die is approximately equal to the thickness of the film. The thickness of each microelectronic die may be decreased to less than 40 μm, and the thickness of each microelectronic die may be decreased such that the total thickness variation of the microelectronic dies is less than ±1.5 μm. 
         [0006]    In another aspect, embodiments of the invention feature another method of thinning a plurality of microelectronic dies. The method includes disposing the plurality of microelectronic dies on a substrate under vacuum. In addition, the thickness of each microelectronic die is decreased, and the microelectronic dies are removed from the substrate. The thickness of each microelectronic die may be decreased to less than 40 μm, and the total thickness variation of the microelectronic dies is less than ±1.5 μm. 
         [0007]    In yet another aspect, embodiments of the invention feature yet another method of thinning a plurality of microelectronic dies. The method includes disposing the plurality of microelectronic dies on a substrate and decreasing the thickness of each by applying pressure to a thinning fixture along a first center of pressure, thereby giving the microelectronic dies an interim total thickness variation. In addition, the thickness of each microelectronic die may be decreased by applying pressure to the thinning fixture along a second (and different) center of pressure, thereby giving the microelectronic dies a final total thickness variation less than the interim total thickness variation. The microelectronic dies may then be removed from the substrate. The final total thickness variation may be less than 9 μm, and the interim total thickness variation may be measured prior to the application of pressure along the second center of pressure. 
         [0008]    In still another aspect, embodiments of the invention feature still another method of thinning a plurality of microelectronic dies. The method includes suspending the dies within an alignment frame and applying an adhesive to a back side of each of the dies without causing them to undergo damage from lateral motion. The back sides of the dies are adhered to a substrate, the thickness of each die is decreased, and the dies are removed from the substrate. Suspending the dies within the alignment frame may include adhering a film to the alignment frame and disposing the dies on the film in a radially symmetric pattern. The thickness of each microelectronic die may be decreased to less than 40 μm, and the total thickness variation of the microelectronic dies may be less than ±1.5 μm. 
         [0009]    In another aspect, embodiments of the invention feature a structure that includes a plurality of microelectronic dies (each having a thickness less than 40 μm) removably bonded with an adhesive material to a substrate, and an adhesive film disposed on the substrate between the microelectronic dies. The total thickness variation of the plurality of microelectronic dies may less than ±1.5 μm. The substrate may consist essentially of glass, and the thickness of the adhesive film may be approximately equal to the thickness of each microelectronic die. 
         [0010]    These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and may exist in various combinations and permutations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0012]      FIG. 1  illustrates a plurality of microelectronic dies disposed on a film in accordance with one embodiment of the invention; 
           [0013]      FIG. 2  illustrates a breakaway view of exemplary elements that may be utilized to mount microelectronic dies to a substrate in accordance with one embodiment of the invention; 
           [0014]      FIG. 3  illustrates an assembled view of the exemplary elements depicted in  FIG. 2 ; 
           [0015]      FIG. 4  illustrates a plurality of microelectronic dies mounted on a substrate and covered by a film in accordance with one embodiment of the invention; 
           [0016]      FIG. 5  illustrates a magnified cross-sectional view of a microelectronic die disposed over a substrate in accordance with one embodiment of the invention; 
           [0017]      FIG. 6  illustrates a side view of a mechanical lapping apparatus utilized to thin microelectronic dies in accordance with various embodiments of the invention; 
           [0018]      FIG. 7  illustrates a cross-sectional view of an exemplary microelectronic die after a lapping process in accordance with one embodiment of the invention; 
           [0019]      FIG. 8  illustrates a cross-sectional view of an exemplary microelectronic die after a chemical-mechanical polishing process in accordance with one embodiment of the invention; 
           [0020]      FIGS. 9 and 10  illustrate schematic views of an apparatus utilized for releasing thinned microelectronic dies from a substrate in accordance with various embodiments of the invention; 
           [0021]      FIG. 11  illustrates thinned microelectronic dies disposed on a screen after release from a substrate in accordance with one embodiment of the invention; 
           [0022]      FIGS. 12A and 12B  illustrate side and bottom views, respectively, of one embodiment of a handling device for microelectronic dies; 
           [0023]      FIGS. 13A and 13B  illustrate side and bottom views, respectively, of another embodiment of a handling device for microelectronic dies; and 
           [0024]      FIGS. 14A and 14B  illustrate side and bottom views, respectively, of the handling device depicted in  FIGS. 13A and 13B  manipulating a thinned microelectronic die in accordance with one embodiment of the invention. 
       
    
    
     DESCRIPTION 
       [0025]      FIG. 1  depicts a plurality of microelectronic dies  110  disposed on a film  120 , although, more generally, as few as a single microelectronic die may be disposed on the film  120 . Each microelectronic die  110  is to be thinned in accordance with the techniques described herein. A first surface of each microelectronic die  110  is in contact with film  120  and an opposing, second surface typically contains circuitry fabricated thereon. Each microelectronic die  110  may include or consist essentially of at least one semiconductor material such as Si, GaAs, or InP. In various embodiments, each microelectronic die  110  is a microcontroller, a central processing unit, or other type of chip utilized in various electronic components such as sensors or computers. In one embodiment, each microelectronic die  100  is a “module” containing a plurality of microelectronic chips packaged together. For example, a plurality of microelectronic chips may be encapsulated, e.g., in a dielectric material, in a lateral configuration to fabricate a single microelectronic die  100 . As illustrated, the microelectronic dies  110  may be disposed on film  120  in a radially symmetric pattern in order to improve uniformity in a die thinning process, as described below. 
         [0026]    In order to facilitate accurate placement of a plurality of microelectronic dies  110 , film  120  may be placed over an alignment guide (not shown) containing outlines of various sizes and shapes in a radially symmetric pattern. Film  120  may be at least partially transparent, and, as such, the plurality of microelectronic dies  110  may be placed on film  120  in locations defined on the alignment guide thereunder. Film  120  may also be an adhesive film, e.g., Kapton, and may be supported around its perimeter by an alignment ring  130 . In an embodiment, alignment ring  130  includes or consists essentially of a rigid material such as a metal, and is utilized in the die mounting process described below with reference to  FIGS. 2-4 . In order to facilitate subsequent mounting onto a rigid substrate (as described below), a thin layer of adhesive material  140 , e.g., wax or shellac, may be applied to the exposed surface (i.e., second surface) of microelectronic die  110 . In an embodiment, adhesive material  140  is non-conductive. In addition, adhesive material  140  may be used to form only a temporary bond, i.e., it may be used to form a bond that is reversible upon heating, dissolving, or melting. 
         [0027]      FIG. 2  illustrates exemplary elements that may be utilized to mount one or more microelectronic dies  110  to a substrate  200 , upon which the microelectronic dies  110  may be subsequently thinned to a desired thickness as described below. In one embodiment, a lamination fixture  210  includes a platen  220  and a vacuum port  230 , and includes or consists essentially of a rigid material such as a metal. A pressure plate  240  may include or consist essentially of a rigid material such as a metal, and may be sized and shaped to apply pressure to film  120  and substrate  200  and to seal to lamination fixture  210  such that vacuum may be drawn in the space therebetween. In an embodiment, platen  220  is capable of being heated (e.g., resistively) to an elevated temperature (e.g., 160° C.) in order to apply heat to substrate  200 , film  120 , and/or microelectronic dies  110  during die mounting. Substrate  200  may include or consist essentially of a rigid material, and may be transparent. In an embodiment, substrate  200  includes or consists essentially of glass, e.g., borosilicate glass, and has a thickness of approximately 6.5 mm. 
         [0028]      FIG. 3  illustrates in assembled form the exemplary elements depicted in  FIG. 2 . With reference to both figures, in an embodiment, substrate  200  is placed on platen  220 , and alignment ring  130  (with film  120  and microelectronic dies  110 ) is placed thereover such that the adhesive material  140  of the microelectronic dies  110  directly contacts the substrate  200 . Pressure plate  240  may then placed over and in contact with lamination fixture  210 , sealing the two together. In an embodiment, pressure plate  240  includes a conformal pad  250  which conforms and applies pressure evenly to film  120  as it conforms to the contours of microelectronic dies  110  during bonding. Thereafter, pressure plate  240  is engaged to contact film  120  such that microelectronic dies  110  (and adhesive material  140  placed thereon) firmly contact substrate  200 . Vacuum may be drawn via a vacuum pump attached to vacuum port  230  in order to remove substantially all air around the dies that could be trapped during bonding (as described below), and platen  220  may be heated such that adhesive material  140  softens or at least partially melts. Adhesive material  140  thus bonds microelectronic dies  110  to substrate  200 , and pressure plate  240  (e.g., with conformal pad  250 ) presses portions of film  120  around each microelectronic die  110  (and between each of the plurality of microelectronic dies  110 ) into contact with substrate  200 , thus “sealing” microelectronic dies  110  to substrate  200  (as shown in  FIG. 5 ). The vacuum may additionally remove any trapped air bubbles between microelectronic die  110  and substrate  200  to facilitate a flat, uniform interface therebetween. 
         [0029]    As depicted in  FIG. 4 , substrate  200  (having microelectronic dies  110  mounted thereon and film  120  covering the microelectronic dies  110 ) may then be removed from pressure plate  240  and lamination fixture  210 , and any excess film  120  may be removed from the edges of substrate  200 . 
         [0030]      FIG. 5  depicts a magnified cross-sectional view of a single microelectronic die  110  and film  120  disposed over substrate  200 . As described above, microelectronic die  110  is disposed over and in contact with substrate  200 . Film  120  “encapsulates” microelectronic die  110 , i.e., a portion of film  120  is disposed over microelectronic die  110 , and other portions are in contact with substrate  200 . In an embodiment, during the mounting process, the heated adhesive material  140  flows around at least the vertical sides of microelectronic die  110  between it and film  120 . This may prevent edge damage (e.g., chipping) of microelectronic die  110 , which may occur, e.g., due to lateral motion (i.e., motion along the plane of the surface of substrate  200 ) of microelectronic die  110  during subsequent thinning processes (described below). 
         [0031]    As illustrated in  FIG. 5 , the top surface of substrate  200  may also contain protrusions  510 . Protrusions  510  may be, e.g., pillars and may each have a height of approximately 12 μm and a diameter of approximately 20 μm. The top surfaces of protrusions  510  may be substantially co-planar, thereby effectively forming a flat but incomplete surface to support microelectronic die  110 . In another embodiment, substrate  200  has a solid, predominately flat top surface. The presence of protrusions  510  may be preferred, however, because 1) the amount of surface area of substrate  200  contacting microelectronic die  110  is decreased, facilitating release of microelectronic die  110  after thinning, and 2) as shown in  FIG. 5 , adhesive material  140  may freely flow between protrusions  510  and microelectronic die  110  to ensure microelectronic die  110  is seated on substrate  200  with a high degree of flatness. 
         [0032]    With continued reference to  FIG. 5 , in one embodiment, microelectronic die  110  has an initial thickness t 1 , which may be greater than 100 μm, e.g., in the range of 150 μm to 330 μm. In the case where more than one microelectronic die  110  is mounted on substrate  200 , each die  110  may have approximately the same thickness or at least one microelectronic die  110  may have a thickness greater than the other microelectronic dies  110 . After thinning (described below), microelectronic dies  110  will all have a final thickness t 2 , which may be less than 100 μm, e.g., approximately 40 μm or even less. In an embodiment, final thickness t 2  is approximately equal to the thickness t f  of film  120 , e.g., within ±10% of, or even equal to, t f . 
         [0033]    Referring to  FIG. 6 , substrate  200  with microelectronic dies  110  and film  120  mounted thereon may be attached to thinning fixture  600 . In an embodiment, substrate  200  is attached to thinning fixture  600  by means of a retaining ring (not shown). Thinning fixture  600  may include a connector  610 , which may connect thinning fixture  600  to various apparatuses for thinning and/or measuring the thickness of microelectronic dies  110 . Adjustment means  620 , which may be, e.g., at least one thumbscrew, may be utilized to alter the center of pressure transmitted through connector  610  and applied to a microelectronic die  110  during thinning processes. Such adjustments may be used, for example, to alter thinning rates as a function of radial distance from the center of thinning fixture  600  (and/or substrate  200 ), thus altering the thickness uniformity of one or more thinned microelectronic dies  110 . Once substrate  200  is attached to thinning fixture  600 , thinning fixture  600  may be connected to, e.g., a pressure head (not shown). The pressure head may apply pressure to bring film  120  and/or microelectronic dies  110  into contact with lapping plate  630 . Lapping plate  630  may be formed of a rigid material, e.g., a copper composite, which acts as a platform for a polishing slurry, e.g., diamond particles suspended in a liquid such as water. In one embodiment, during the lapping process, lapping plate  630  rotates, and the mechanical action of the polishing slurry against film  120  and/or microelectronic die  110  removes material from surface(s) thereof. 
         [0034]      FIG. 7  depicts a single microelectronic die  110  mounted upon substrate  200  after the lapping process. For clarity, protrusions  510  on substrate  200  are not shown in  FIG. 7  and subsequent figures. In one embodiment, during lapping, the portion of film  120  disposed above microelectronic die  110  is removed, as is a portion of the thickness of microelectronic die  110 . As illustrated in  FIG. 7 , microelectronic die  110  has an intermediate thickness t 3  which is less than its initial thickness t 1  but greater than a desired final thickness t 2 . In an embodiment, intermediate thickness t 3  is approximately 50 μm. As described above, the presence of adhesive material  140  around the vertical edges of microelectronic die  110  may prevent edge damage, e.g., chipping, thereto. 
         [0035]    Referring now to both  FIGS. 6 and 7 , in order to improve thickness uniformity during the lapping process, the remaining thickness of microelectronic die  110  may be measured even before it reaches the desired intermediate thickness t 3 . For example, a portion of the thickness of one or more microelectronic dies  110  may be removed by lapping, and the remnant thickness thereof may be measured, e.g., by use of a drop gauge. Such measurements may be used to calculate either an initial intradie (i.e., across a single microelectronic die  110 ) or interdie (i.e., die-to-die) total thickness variation (“TTV”), or both. Based on the initial TTV, the center of pressure applied through connector  610  may be adjusted via adjustment means  620 . For example, adjustment means  620  may be utilized to alter the angle of attack of thinning fixture  600 , thus varying (e.g., moving off-center) the region of maximal applied pressure (to microelectronic dies  110 ) during lapping. The lapping process may then be recommenced. After the lapping process, intermediate thickness t 3  of one or more microelectronic die  110  may be measured, providing a post-lapping intradie or interdie TTV, or both. The use of adjustment means  620  to alter the center of pressure during lapping enables a post-lapping TTV less than the initial TTV. In an embodiment, the post-lapping interdie TTV is less than 9 μm. In an embodiment, the post-lapping interdie TTV is less than 5 μm, or even less than 2 μm. 
         [0036]    After lapping, thinning fixture  600  may be connected to a polishing drive shaft (not shown) on, e.g., a chemical-mechanical polishing (“CMP”) system for further material removal. Similar to the lapping process described above, the CMP process may further thin one or more microelectronic dies  110  via combined chemical attack (from, e.g., the CMP slurry) and mechanical action against a polishing pad (analogous to lapping plate  630  described above, but normally formed of a softer material). The polishing pad may be a polymer-based material, e.g., a polyurethane. In an embodiment, the CMP process incorporates a slurry that includes a 1:19 mixture of Glanzox HP-20 (available from Fujimi Corporation of Tualatin, Oreg.) and deionized water, as well as a IC1000-k groove polishing pad (available from Rohm and Haas Company of Philadelphia, Pa.). The CMP process may be performed on any number of suitable CMP tools, e.g., the APD-500 (available from Araca, Inc. of Tucson, Ariz.) or the RotoPol-31 (available from Struers Inc. of Cleveland, Ohio). With reference again to  FIG. 7 , in an embodiment, film  120  around and between microelectronic die(s)  110  acts as a polish stop during the CMP process, as the polishing rate of film  120  is preferably much less than the polishing rate of microelectronic die  110 . Therefore, as the thickness of microelectronic die  110  approaches that of film  120 , the rate of material removal of microelectronic die  110  may slow considerably. Thus, the CMP process preferably thins microelectronic die  110  to a final thickness t 2  approximately equal to the film  120  thickness t f . 
         [0037]    The use of film  120  with an arbitrary substrate  200  for thinning microelectronic dies  110  enables a flexible process; an identical substrate  200  (or any other one) may be used in conjunction with a film  120  having a different thickness t f  to facilitate the thinning of microelectronic die  110  to a different preferred final thickness. Thus, embodiments of the present invention are superior to thinning methods utilizing custom substrates with polish stops fixed at particular heights. 
         [0038]      FIG. 8  illustrates an exemplary microelectronic die  110  after the CMP process. The thickness of microelectronic die is the final desired thickness t 2 , e.g., 40 μm or even less. Moreover, the caustic nature of the CMP slurry has removed substantially all of adhesive material  140  disposed between microelectronic die  110  and film  120  and around the edges of microelectronic die  110  (other than between the microelectronic die  110  and substrate  200 ), thereby facilitating the subsequent removal of thinned microelectronic die  110  from substrate  200 . After the CMP process, the interdie TTV of microelectronic dies  110  may be less than ±1.5 μm, and the intradie TTV of each microelectronic die  110  may be less than 1 μm. 
         [0039]    At least one thinned microelectronic die  110  may be removed from substrate  200  for further handling, inspection, and/or processing.  FIGS. 9 and 10  depict schematic top and side views, respectively, of an apparatus utilized for releasing thinned microelectronic dies  110  from a substrate  200  in accordance with various embodiments of the invention. As illustrated, substrate  200  may be placed within a release holder  900  with microelectronic die  110  (still adhered to substrate  200  with adhesive material  140 ) facing downward. Substrate  200  and release holder  900  may be placed above a screen  910 . In one embodiment, release holder  900  is sized and shaped to suspend substrate  200  above screen  910  with a small gap  920  (see  FIG. 10 ) therebetween to facilitate the flow of a liquid release agent to remove the remnant adhesive material  140  that holds microelectronic die  110  on substrate  200 . Screen  910  may include openings large enough to allow the free flow of the release agent, but small enough to catch and support released microelectronic die  110 . 
         [0040]    As illustrated in  FIG. 10 , a flow chamber  1000  may be placed above screen  910  and sealed to prevent the leakage of release agent  1010  once flow chamber  1000  is filled therewith. Release agent  1010  is preferably a liquid capable of dissolving adhesive material  140  without etching or damaging microelectronic die  110 , substrate  200 , release holder  900 , or screen  910 . In an embodiment, release agent  1010  is a solvent such as ethanol. A pump (not shown) may be attached to inlet  1020  and outlet  1030  to circulate release agent  1010  in the general direction toward screen  910  (i.e., in the “release direction”). Such circulation may speed the release of microelectronic die  110 . The entire release apparatus  1040  (including flow chamber  1000 , screen  910 , release holder  900 , substrate  200 , and release agent  1010 ) may be placed in a heated environment (not shown) in order to enhance the removal of adhesive material  140  (e.g., by softening or melting). 
         [0041]    As illustrated in  FIG. 11 , after release from substrate  200 , at least one thinned microelectronic die  110  is disposed on screen  910 , ready for further handling. After release from substrate  200 , thinned microelectronic dies  110  may be extremely fragile and are thus, in accordance with one embodiment, handled with a great degree of care. 
         [0042]    Typical handling means for handling microelectronic die  110 , e.g., an end effector, utilize vacuum to pick up the die  110 .  FIGS. 12A and 12B  depict the side view and bottom view, respectively, of a typical end effector  1200  that includes a single vacuum inlet  1210 . However, such handling means  1200  typically draw vacuum from a single point, thus concentrating the force upon the die  110  at that point. Such a concentration of force may result in damage, e.g., cracking, of the die  110  originating at that point. Accordingly, end effector  1200  is typically not feasible for use with thinned microelectronic die  110 . 
         [0043]      FIGS. 13A and 13B  depict the side view and bottom view, respectively, of a handling means  1300  that is optimized for the handling of a thinned microelectronic die  110  having a thickness of less than 100 μm, e.g., 40 μm or less. Handling means  1300  is, e.g.,an end effector, and includes diffuser  1310 . Diffuser  1310  has a plurality of holes extending therethrough such that vacuum drawn through diffuser  1310  is not concentrated at a single point (as with handling means  1200  depicted in  FIGS. 12A and 12B ). In an embodiment, diffuser  1310  includes or consists essentially of a porous material, e.g., a metal or a polymer such as polyethylene, or a polymer or other mesh. Diffuser  1310  may be an integrated portion of handling means  1300  or may be removable (in which case handling means  1300  with diffuser  1310  removed will resemble handling means  1200 ). 
         [0044]      FIGS. 14A and 14B  depict side and bottom views, respectively, of the handling means  1300  manipulating a thinned microelectronic die  110  in accordance with one embodiment of the invention. In one embodiment, diffuser  1310  facilitates the handling of thinned microelectronic die  110  without causing damage thereto. Handling means  1300  may also be attached to an automated pick-and-place machine (not shown) for automated handling of thinned microelectronic die  110 . 
         [0045]    As described, embodiments of the present invention enable the thinning and subsequent handling of microelectronic dies without causing damage thereto. Several released dies may be thinned to a thickness of less than 40 μm with a high degree of uniformity and without permanently mounting them to a handling substrate. Embodiments of the present method enable flexibility in the thinning process, as the final thickness of the microelectronic dies may be selected via selection of the thickness of a tape that may act as a polishing stop. Damage from, e.g., lateral motion, may be minimized or prevented by “encapsulating” the die edges with the adhesive material adhering them to the substrate. Finally, post-thinning handling may advantageously utilize handling means applying a diffuse vacuum rather than vacuum force concentrated at a single point. 
         [0046]    The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.