Abstract:
The present invention provides an apparatus and method for use in processing semiconductor workpieces. The new apparatus and method allows for the production of thinner workpieces that at the same time remain strong. Particularly, a chuck is provided that includes a body, a retainer removeably attached to the body and a seal forming member. When a workpiece is placed on the chuck body and the retainer is engaged to the body, a peripheral portion of the back side of the workpiece is covered by the retainer while an interior region of the back side of the workpiece is exposed. The exposed back side of the workpiece is then subjected to a wet chemical etching process to thin the workpiece and form a relatively thick rim comprised of semiconductor material at the periphery of the workpiece. The thick rim or hoop imparts strength to the otherwise fragile, thinned semiconductor workpiece. Semiconductor workpieces made according to the present invention offer an improved structure for handling thinned wafers in conventional automated equipment. This results in improved yields and improved process efficiency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a Divisional Application of U.S. patent application Ser. No. 10/923,363, filed Aug. 20, 2004, now U.S. Pat. No. 7,288,489. 
    
    
     TECHNICAL FIELD 
     The invention relates to a process and apparatus for use with workpieces, such as semiconductor wafers, flat panel displays, rigid disk or optical media, thin film heads or other workpieces formed from a substrate on which microelectronic circuits, data storage elements or layers, or micro-mechanical elements may be formed. These and similar articles are collectively referred to herein as a “wafer” or “workpiece.” Specifically, the present invention relates to a process and apparatus for use in thinning semiconductor workpieces. 
     BACKGROUND OF THE INVENTION 
     State of the art electronics (e.g., cellular phones, personal digital assistants, and smart cards) demand thinner integrated circuit devices (“ICD”). In addition, advanced packaging of semiconductor devices (e.g., stacked dies or “flip-chips”) provide dimensional packaging constraints which also require an ultra-thin die. Moreover, as operating speeds of ICDs continue to increase heat dissipation becomes increasingly important. This is in large part due to the fact that ICDs operated at extremely high speeds tend to generate large amounts of heat. That heat must be removed from the ICD to prevent device failure due to heat stress and to prevent degradation of the frequency response due to a decrease in carrier mobility. One way to enhance thermal transfer away from the ICD, thereby mitigating any deleterious temperature effects, is by thinning the semiconductor wafer from which the ICD is fabricated. Other reasons for thinning the semiconductor wafer include: optimization of signal transmission characteristics; formation of via holes in the die; and minimization of the effects of thermal coefficient of expansion between an individual semiconductor device and a package. 
     Semiconductor wafer thinning techniques have been developed in response to this ever increasing demand for smaller, higher performance ICDs. Typically, semiconductor devices are thinned while the devices are in wafer form. Wafer thicknesses vary depending on the size of the wafer. For example, the thickness of a 150 mm diameter silicon semiconductor wafer is approximately 650 microns, while wafers having a diameter of 200 or 300 mm are approximately 725 microns thick. Mechanical grinding of the back side of a semiconductor is one standard method of thinning wafers. Such thinning is referred to as “back grinding.” Generally, the back grinding process employs methods to protect the front side or device side of the semiconductor wafer. Conventional methods of protection of the device side of the semiconductor wafer include application of a protective tape or a photoresist layer to the device side of the wafer. The back side of the wafer is then ground until the wafer reaches a desired thickness. 
     However, conventional back grinding processes have drawbacks. Mechanical grinding induces stress in the surface and edge of the wafer, including micro-cracks and edge chipping. This induced wafer stress can lead to performance degradation and wafer breakage resulting in low yield. In addition, there is a limit to how much a semiconductor wafer can be thinned using a back grinding process. For example, semiconductor wafers having a standard thickness (as mentioned above) can generally be thinned to a range of approximately 250-150 microns. 
     Accordingly, it is common to apply a wet chemical etch process to a semiconductor wafer after it has been thinned by back grinding. This process is commonly referred to as stress relief etching, chemical thinning, chemical etching, or chemical polishing. The aforementioned process relieves the induced stress in the wafer, removes grind marks from the back side of the wafer and results in a relatively uniform wafer thickness. Additionally, chemical etching after back grinding thins the semiconductor wafer beyond conventional back grinding capabilities. For example, utilizing a wet chemical etch process after back grinding allows standard 200 and 300 mm semiconductor wafers to be thinned to 100 microns or less. Wet chemical etching typically includes exposing the back side of the wafer to an oxidizing/reducing agent (e.g., HF, HNO 3 , H 3 PO 4 , H 2 SO 4 ) or alternatively to a caustic solution (e.g., KOH, NaOH, H 2 O 2 ). Examples of wet chemical etching processes may be found in co-pending U.S. patent application Ser. No. 10/631,376, filed on Jul. 30, 2003, and assigned to the assignee of the present invention. The teachings of application Ser. No. 10/631,376 are incorporated herein by reference. 
     Although methods for thinning semiconductor wafers are known, they are not without limitations. For example, mounting a semiconductor wafer to a submount or “chuck” (as it is commonly known) so that the wafer can be thinned requires expensive coating and bonding equipment and materials, increased processing time, and the potential for introducing contaminates into the process area. Additionally, adhesives for bonding a wafer to a chuck that may be useful in a mechanical grinding process will not withstand the chemical process fluids used in wet chemical etching. Furthermore, the current use of a photoresist or adhesive tape fails to provide mechanical support for very thin wafers either during the back grind process or in subsequent handling and processing. The use of tape also creates obstacles in the removal process. For example, tape removal may subject a wafer to unwanted bending stresses. In the case of a photoresist, the material is washed off the device side of a wafer with a solvent, adding to the processing time and use of chemicals, and increasing the risk of contamination. The use of taping and protective polymers are also costly, since both equipment and materials are necessary to apply and remove the protective media. 
     Further, thinned semiconductor wafers are prone to warping and bowing. And because thinned semiconductor wafers can be extremely brittle, they are also prone to breakage when handled during further processing. Thinned semiconductor wafers (e.g., below 250 microns) also present complications in automated wafer handling because, in general, existing handling equipment has been designed to accommodate standard wafer thicknesses (e.g., 650 microns for 150 mm wafer and 725 microns for 200 and 300 mm wafers). 
     Accordingly there is a need for a process and equipment for producing thinner semiconductor workpieces. At the same time, there is a need to provide thinner workpieces that are strong enough to minimize the risk of breakage, yet remain compatible with conventional automated semiconductor wafer handling equipment. Finally, it would be advantageous to develop a system that reduces the number of processing steps for thinning a semiconductor workpiece. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for use in processing semiconductor wafers. The new system and apparatus allows for the production of thinner wafers that at the same time remain strong and resistant to bowing and warping. As a result, the wafers produced by the present process are less susceptible to breakage. The process and equipment of the present invention also offers an improved product structure for handling thinned wafers, while reducing the number of processing steps. This results in, among other things, improved yields and improved process efficiency. 
     In one aspect, the present invention provides a chuck for receiving and supporting a semiconductor workpiece having a device side, a bevel and a back side. The chuck has a body for supporting the workpiece, a retainer removeably attached to the body and adapted to cover a peripheral portion of the back side of the workpiece, and at least one member for creating a seal between the retainer and the back side of the workpiece. Due to its configuration, the chuck permits an interior region of the back side of the workpiece to be exposed, while protecting the peripheral portion of the back side of the workpiece. The workpiece is then thinned via a wet etching process. The result is a processed semiconductor workpiece that has a thinned main body (e.g., less than approximately 125 microns) and a thick rim (e.g., in a range of approximately 600 to 725 microns). The relatively thicker rim provides strength to the thinned workpiece and permits the workpiece to be handled for additional processing with conventional automated handling equipment. 
     In another aspect, the present invention provides a semiconductor workpiece having a main body and a rim comprised of semiconductor material. The main body is integrally connected to the rim and has a thickness less than approximately 50% of the rim thickness. The relatively thick rim provides strength to the workpiece, preventing the main body from bowing and warping. Meanwhile, the main body of the semiconductor workpiece can be thinned to a thickness less than 300 microns, preferably less than 125 microns, more preferably less than 100 microns, especially less than 50 microns and even less than 25 microns. The structural configuration of thinned semiconductor workpieces of the present invention meet the industry demand for thinned ICDs necessary in today&#39;s state of the art electronics and advanced packaging techniques, while at the same time, reducing the risk of breakage due to the fragile state of the thinned workpiece. 
     The present invention also provides several processes for thinning a semiconductor workpiece. In one aspect, the process includes the steps of placing the semiconductor workpiece into a chuck adapted to cover a peripheral portion of the back side of the workpiece, leaving approximately 95% of the back side surface of the workpiece exposed. The semiconductor workpiece is then thinned via a wet chemical etching process wherein the back side of the workpiece is exposed to an oxidizing agent (e.g., HF, HNO 3 , H 3 PO 4 , H 2 SO 4 ) or alternatively to a caustic solution (e.g., KOH, NaOH, H 2 O 2 ). During the wet chemical etching step, the exposed back side of the workpiece is thinned to a thickness less than 50% of the pre-wet chemical etching thickness of the workpiece. As a result, a rim is formed at the periphery of the workpiece, or as it is commonly referred to in the industry, the “exclusion zone.” The rim has a thickness approximately equal to the thickness of the workpiece prior to the wet chemical etch step (e.g., in a range of 600 to 725 microns). The remainder of the workpiece (i.e., the thinned main body) has a thickness less than 50% of the rim thickness (e.g., less than 300 microns, preferably less than 125 microns, more preferably less than 100 microns, especially less than 50 microns and even less than 25 microns). This process eliminates the limitations associated with known methods of thinning semiconductor workpieces mentioned above, while increasing overall manufacturing efficiencies. 
     Any of the described aspects of the invention may be combined and/or repeated one or more times to achieve optimal results. The invention resides as well in sub-combinations of the aspects described. These and other objects, features and advantages of this invention are evident from the following description of preferred embodiments of this invention, with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a chuck according to the present invention with a semiconductor workpiece secured therein prior to thinning. 
         FIG. 1B  is a cross-sectional view of the chuck and workpiece shown in  FIG. 1A . 
         FIG. 1C  is a partial enlarged view of the chuck and workpiece shown in  FIG. 1B , demonstrating the cooperation between the chuck and the workpiece. 
         FIG. 1D  is an exploded cross-sectional view of the chuck and workpiece shown in  FIG. 1A . 
         FIG. 1E  is a partial enlarged view of the chuck and workpiece section identified as X shown in  FIG. 1D . 
         FIG. 2A  is a cross-sectional view of another embodiment of a chuck according to the present invention with a workpiece secured therein prior to thinning. 
         FIG. 2B  is a partial enlarged view of the chuck and workpiece shown in  FIG. 2A , demonstrating the cooperation between the chuck and the workpiece. 
         FIG. 3A  is a cross-sectional view of yet another embodiment of a chuck according to the present invention with a workpiece secured therein prior to thinning. 
         FIG. 3B  is a partial enlarged view of the chuck and workpiece shown in  FIG. 3A , demonstrating the cooperation between the chuck and the workpiece. 
         FIG. 4A  is a cross-sectional view of another embodiment of a chuck according to the present invention with a workpiece secured therein prior to thinning. 
         FIG. 4B  is a partial enlarged view of the chuck and workpiece shown in  FIG. 4B , demonstrating the cooperation between the chuck and the workpiece. 
         FIG. 5A  is a cross-sectional view of another embodiment of a chuck according to the present invention with a workpiece secured therein prior to thinning. 
         FIG. 5B  is a partial enlarged view of the chuck and workpiece shown in  FIG. 5A , demonstrating the cooperation between the chuck and the workpiece. 
         FIG. 6A  is a cross-sectional view of yet another embodiment of a chuck according to the present invention with a workpiece secured therein prior to thinning. 
         FIG. 6B  is a partial enlarged view of the chuck and workpiece shown in  FIG. 6A , demonstrating the cooperation between the chuck and the workpiece. 
         FIG. 7A  is a cross-sectional view of an embodiment of a chuck according to the present invention with a workpiece secured therein prior to thinning. 
         FIG. 7B  is a partial enlarged view of the chuck and workpiece shown in  FIG. 7A , demonstrating the cooperation between the chuck and the workpiece. 
         FIGS. 8 and 9  are flow diagrams depicting aspects of process flows in accordance with the present invention. 
         FIG. 10  is a perspective view of a semiconductor workpiece thinned according to a process of the present invention. 
         FIG. 11  is a cross-sectional view of the thinned semiconductor workpiece shown in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 1A-1E , there is shown a chuck  10  for supporting a semiconductor workpiece  50  during processing in accordance with one embodiment of the present invention. The chuck  10  is comprised of a supporting body  12 , a retainer  14  and a sealing members  16 ,  24 . The retainer  14  has two grooves or recesses  18 . The sealing members  16 ,  24  are housed in the annular grooves  18 , respectively. The retainer  14  is preferably in the form of a ring and is removeably attached to the supporting body  12 . In use, the workpiece  50 , which has a device side  51 , a bevel (i.e., peripheral edge)  52  and a back side  53 , is placed onto a supporting surface  18  of the supporting body  12  of chuck  50 , device side  51  down. The retainer  14  is then attached to the outer periphery of the supporting body  12 . As shown specifically in  FIG. 1C , when the retainer  14  is engaged to the supporting body  12 , the retainer  14  wraps around the outer end of the supporting body  12  and covers a peripheral portion of the back side  53  of the workpiece  50 , securing the workpiece  50  in the chuck  10 . 
     When engaged, preferably the retainer  14  covers only a small peripheral portion of the back side  53  of the workpiece  50 , leaving a majority of the back side  53  of the workpiece  50  exposed. In a preferred embodiment, the back side  53  surface area covered by the retainer  14  extends inwardly from the bevel  52  for about a distance of approximately 1-10 mm, more preferably between about 1-5 mm, and especially between about 2-4 mm. Preferably, at least 95% (or even 97% or 99%) of the back side  53  surface area of the workpiece  50  is left exposed. The exposed portion of the back side  53  of the workpiece  50  is then subjected to a process fluid and thinned to a desired thickness. As a result of covering the peripheral portion of the back side  53  of the workpiece  50 , during thinning, process fluid cannot interact with the periphery of the back side  53  of the workpiece  50 . Accordingly, the periphery of the back side  53  of the workpiece  50  remains in substantially its same pre-thinning form, configuration and thickness. For purposes of this invention, the semiconductor material remaining at the periphery of the workpiece  50  after thinning is referred to as a rim. It is the rim that imparts strength to the thinned workpiece  50  and permits automated handling equipment to handle the thinned semiconductor workpieces  50  processed according to the present invention. 
     Turning to  FIGS. 1D and 1E , in order to facilitate attachment of the retainer  14  to the supporting body  12 , the retainer  14  has an engagement member  20  that cooperates with a recess  22  formed in the supporting body  12 . In this manner, a simple mechanical snap connection between the retainer  14  and the supporting body  12  is achieved. Although not shown in  FIGS. 1A-1D , the present invention includes a configuration where the engagement member  20  extends from the supporting body  12  and cooperates with a recess  22  formed in the retainer  14  to removeably connect the retainer  14  and supporting body  12 . In either configuration, preferably the engagement member  20  and the recess  22  are positioned between the first and second sealing member  16 ,  24 . 
     With reference to  FIG. 1C , the retainer  14  has an outer peripheral end  30  with an angled surface  32 . When the retainer  14  is attached to the supporting body  12 , the angled surface  32  of the outer peripheral end  30  of the retainer  14  mates with an angled surface  34  at an outer peripheral end of the supporting body  12  to form a notch  36 . The notch  36  accepts a tool (not shown) and facilitates removal of the retainer  14  from the supporting body  12 . 
     Turning now to  FIG. 1E , the supporting body  12  has a lip or step  26  formed circumferentially therein. The lip  26  acts to register or guide the workpiece  50  as it is loaded into the chuck  10 . When properly aligned, the workpiece  50  will rest entirely on the supporting surface  28  of the supporting body  12 . While the chuck  10  can be any shape (e.g., square, rectangular, circular, etc), as shown in  FIGS. 1A-1E , in a preferred embodiment the chuck is disk-shaped and will have a diameter slightly larger than the diameter of the workpiece  50  to be processed. 
     With reference now to  FIGS. 2A-2B , there is shown an alternative embodiment of a chuck  10  according to the present invention. Like the chuck  10  shown in  FIGS. 1A-1E , the chuck  10  includes a supporting body  12  and a retainer  14 . The retainer  14  has first and second sealing members  16 ,  24  disposed within annular grooves  18 ,  38 . The mechanical attaching mechanism in the embodiment illustrated in  FIGS. 2A-2B , however, is slightly different than the mechanism shown in  FIGS. 1A-1E . An engagement member  20  extends from the outer periphery of the supporting body  12 . The retainer  14 , in turn, has a recess  22  that cooperates with the engagement member  20  of the supporting body  12  to provide a simple snap engagement that attaches the retainer  14  to the supporting body  12 . An upper portion of the retainer  14 , including sealing member  16 , covers the exclusion zone of the back side  53  of the workpiece  50  in the engaged position. In this preferred embodiment, the retainer  14  has a plurality of rinse holes  40  for allowing processing fluid to escape from cavities formed in the chuck  10 . A lower portion  42  of the retainer  14  which creates the mechanical snap connection with the engaging member  20  forms an annular recess  44  with a mating lower portion  46  of the supporting body  12 . A tool (not shown) can be inserted into the annular recess  44  so that the retainer  14  can be simply popped off the chuck  10  supporting body  12  after processing is completed. 
     In the embodiments having two sealing members  16 ,  24  (as disclosed in  FIGS. 1A-1E  and  2 A- 2 B), sealing member  16  creates a flexible interface and seal between the workpiece  50  and the retainer  14  to prevent process fluid from accessing the device side  51  and bevel  52  of the workpiece  50 . This flexible interface also relieves some of the stress that is exerted on the workpiece  50  during assembly and disassembly of the chuck  10 . Sealing member  24  creates a flexible interface between the retainer  14  and the supporting body  12  and also helps relieve some of the stress that is exerted on the workpiece  50  during assembly and disassembly of the chuck  10 . 
     With reference now to  FIGS. 3A-3B  through  7 A- 7 B, there is shown various chuck  10  designs having only a single sealing member  16 . Specifically,  FIGS. 3A-3B  illustrate a chuck  10  having a retainer  14 , supporting body  12  and a engagement mechanism similar to the engagement mechanism shown in  FIGS. 2A-2B  and described above. The retainer  14 , however, has only a single annular groove  18  which is adapted to house sealing member  16 . In this embodiment, the annular groove  18  is V-shaped and receives a square-shaped compressible sealing member  16 . Preferably the square-shaped sealing member  16  has semi-circular extensions projecting from each corner to ensure an adequate fit in groove  18 . 
       FIGS. 4A-4B  and  5 A- 5 B show chucks  10  having an engagement ring  48  attached circumferentially to the bottom outer periphery of the supporting body  12 . The engagement ring  48  extends radially outwardly from the supporting body  12 , creating a stepped-relationship between the supporting body  12  and engagement ring  48 , and forming engagement member  20 . The retainer  14  has a lower portion  42  with a U-shaped recess  22  formed therein. The U-shaped recess  22  receives the engagement member  20 . The lower portion  42  of the retainer  14  has an extension  49  that wraps around the engagement member  20  to form a mechanical snap connection between the retainer  14  and the engagement ring  46  of the supporting body  12 . In  FIGS. 4A-4B , the retainer  14  has a two-step annular groove  18  which receives a sealing member  16  having a top part with one width for insertion into one-step of the annular groove  18 , and a bottom part with a second width for insertion into the second step of the annular groove  18 . In  FIGS. 5A-5B , the retainer  14  has a single V-shaped annular groove  18  for housing the sealing member  16 , which in this embodiment is a compressible O-ring. 
       FIGS. 6A-6B  illustrate another preferred embodiment of a chuck  10  according to the present invention. In this embodiment, the lower portion  42  of the retainer  14  has an inner side wall  60  with a convex protrusion  62  extending outwardly therefrom. The supporting body  12  has an end wall  64  with a concave recess  66  for accepting the convex protrusion  62  of the inner side wall  60  of the lower portion  42  of the retainer  14 . In this manner, the retainer  14  engages the supporting body  12  and secures the workpiece  50  on the supporting surface  28  of the chuck  10 . 
     In the embodiments having only a single sealing member  16  (as disclosed in  FIGS. 3A-3B  through  6 A- 6 B), sealing member  16  creates a flexible interface between the workpiece  50  and the supporting body  12  to prevent process fluid from interacting with the device side  51  and bevel  52  of the workpiece  50 , and to relieve stress exerted on the workpiece during the assembly/disassembly process. 
     Turning now to  FIGS. 7A-7B , there is shown a preferred embodiment of a chuck  10 , which combines the retainer  14  and sealing member  16  of the prior embodiments. In this embodiment, retainer  14  is a single-component, compressible annular ring with an annular groove  18  running circumferentially through the middle of the retainer  14 . The supporting body  12  has an outer end  13 , which is inserted into the annular groove  18  in the retainer  14 . The retainer  14  remains engaged to the supporting body  12  as a result of a compression force exerted by the retainer  14  onto the supporting body  12  and the workpiece  50 . In the attached position, an outer peripheral portion of the workpiece  50  (e.g., the exclusion zone) is also positioned within the annular groove  18 . In this preferred embodiment, retainer  14  creates a seal with the back side  53  of the workpiece  50 , preventing process fluid from accessing the bevel  52  and device side  51  of the workpiece  50  during processing. 
     Suitable materials for use in the chuck  10  embodiments according to the present invention will now be discussed. Generally, the chuck  10  can be made from a number of different polymer materials that are stable and highly chemically resistant. Preferably the supporting body  12  comprises polytetrafluoroethylene and the retainer  14  preferably comprises a fluoropolymer such as polyvinylidene fluoride sold by Atofina Chemicals under the KYNAR tradename. In the embodiment illustrated in  FIGS. 7A-7B , the retainer  14  is preferably formed from a material having a Durometer hardness less than that of a fluoropolymer, but greater than the elastomeric materials discussed below with respect to the sealing member. That is, a material compressible enough to form a seal with the workpiece  50 , but stiff enough to provide structure to the retainer  14  for receiving the supporting body  12 . In any embodiment of the present invention, in order to enhance the attachability of the retainer  14  to the supporting body  12 , it is preferred that the supporting body  12  is comprised of a material having a Durometer hardness greater than the Durometer hardness of the material from which the retainer  14  is formed. 
     As illustrated in  FIGS. 1A-1E ,  2 A- 2 B,  5 A- 5 B and  6 A- 6 B, the sealing members  16 ,  24  are preferably shaped like an “O-ring,” but it is contemplated that other shapes can be used as well (e.g., as shown in  FIGS. 3A-3B  and  4 A- 4 B). The sealing members  16 ,  24  are preferably formed from a compressible material having a Durometer hardness equal to or greater than 50. Specific examples of suitable elastomeric materials include: a perfluoroelastomer sold by DuPont under the tradename Kalrez; a perfluoroelastomer sold by Greene, Tweed &amp; Co. under the tradename Chemraz; fluoruelastomers sold by DuPont under the tradename Viton; and hydrocarbon elastomers sold under the tradename EPDM. 
     Turning now to the workpiece thinning processes according to the present invention,  FIG. 8  illustrates one embodiment of a process that may be implemented when the chuck  10  and workpiece  50 , described above, are used to thin the back side  53  of the workpiece  50 . At step  200 , a workpiece  50  is provided having a device side  51 , a bevel  52  and a back side  53 . The back side  53  of the workpiece  50  will have a given surface area depending on its dimensions. Also, the workpiece  50  has a given thickness. 
     At step  210 , the workpiece  50  is placed onto the supporting surface  28  of chuck  10  with the device side  51  immediately adjacent to the supporting body  12  of the chuck  10 . The retainer  14  is attached to the supporting body  12  so that a peripheral portion (e.g., the exclusion zone of the workpiece  50 ) of the back side  53  of the workpiece  50  is covered. In step  210 , the workpiece  50  is secured to the chuck  10 . As a result of the chuck  10  configuration, upon attaching the retainer  14  to the supporting body  12 , in step  220  a majority (and preferably at least 95%, more preferably at least 97% and especially at least 99%) of the back side  53  surface area is exposed, while a small peripheral portion of the back side  53  of the workpiece  50  is covered. 
     The workpiece  50  is then thinned to a desired thickness at step  230  by applying a process fluid to the exposed back side  53  of the workpiece  50 . Due to the overlapping configuration of the retainer  14 , by thinning the exposed back side  53  of the workpiece, at step  240 , a rim and a main body is formed in the workpiece  50 . The rim is formed at the outer periphery of the workpiece  50  and has a thickness, RT and the main body of the workpiece  50  has a thickness, MBT. In the preferred embodiment of  FIG. 8 , the MBT is less than approximately 50% of the RT. A desired MBT is preferably less than approximately 40% of the RT; more preferably less than approximately 30% of the RT; especially less than approximately 20% of the RT; and even less than approximately 10% of the RT. It should be understood that after thinning the workpiece  50 , the RT should be substantially the same as the workpiece  50  thickness prior to the thinning process. Thus, for conventional 200 mm and 300 mm workpieces, the RT after thinning will be about 725 microns. And the RT of a conventional 150 mm workpiece after thinning will be about 650 microns. 
     It is within the scope of the present invention, however, to process a workpiece  50 , which has previously been thinned by some other method, e.g., mechanical grinding. Thus, a workpiece  50  having a thickness of anywhere from 150-725 microns can be thinned according to the present invention to create a workpiece  50  with a rim having a RT in a range of substantially the same thickness as the workpiece  50  (i.e., about 150-725 microns, even about 600-725, or even about 300-725) and a main body having a MBT in a range of about 25-300 microns, preferably in a range of about 100-125 microns, more preferably in a range of about 50-100 microns, especially in a range of about 25-50 microns. 
     Turning now to  FIG. 9 , there is shown another embodiment of a process that may be implemented when the chuck  10 , described above, is used to thin a workpiece  50 . At step  300 , a workpiece  50  having a thickness, WPT, is provided. The workpiece  50  has a device side  51 , a bevel  52  and a back side  53 . The workpiece  50  is placed onto the chuck  10  with the device side  51  immediately adjacent to the supporting body  12  of the chuck  10  at step  310 . At step  320 , the retainer  14  is attached to the supporting body  12  so that a peripheral portion of the back side  53  of the workpiece  50  is covered. In this step, the workpiece  50  is secured to the chuck  10 . As a result of the chuck  10  configuration, when the retainer  14  is attached to the supporting body  12 , with the exception of the covered exclusion zone, substantially all of the back side  53  of the workpiece  50  is exposed. 
     Still referring to  FIG. 9 , at step  330  the chuck  10  and workpiece  50  are placed into a process chamber. The process chamber may be manual or automated and is preferably within a spray acid tool platform like those available from Semitool, Inc., of Kalispell, Mont. Once inside the process chamber, a process fluid is applied to the exposed back side  53  of the workpiece  50  at step  340 . The thinning process of step  340  preferably comprises a conventional wet chemical etch process or a polishing process. In either process, the process fluid preferably consists of one, or a combination of: deionized water, hydrogen peroxide, ozone, potassium hydroxide, sodium hydroxide, hydrofluoric acid, nitric acid, sulfuric acid, acidic acid and phosphoric acid. A number of other acidic and basic solutions may also be used, depending on the particular surface to be treated and the material that is to be removed. 
     The process fluid can be applied to the workpiece  50  in any conventional manner. In one preferred embodiment, however, the process fluid is sprayed through a nozzle or a plurality of nozzles onto the back side  53  of the workpiece  50 . In another preferred embodiment, the chuck  10  and workpiece  50  are immersed into a volume of process fluid, or sequentially into a plurality of volumes of the same process fluid (at different concentrations or temperatures) or different process fluids. 
     Depending on the composition of the material to be removed and the amount of material to be removed (i.e., the desired end thickness of the workpiece), the process fluid will have a desired concentration, a temperature and a flow rate. By monitoring and maintaining these process fluid variables, the process fluid can be applied to the exposed back side  53  of the workpiece  50  at a first etch rate, and then subsequently at a second etch rate. Preferably, the first etch rate is greater than the second etch rate. That is, semiconductor material is etched away quickly at first, and then more slowly as the thickness of the workpiece  50  approaches the desired thickness. 
     Referring to step  350  of  FIG. 9 , the thinning process forms a rim  70  and a main body  72  in the workpiece  50 . The thinning process is carried out until the main body  72  reaches a desired thickness, MBT. Preferably, the MBT is less than 50% of the WPT, more preferably less than 40% of the WPT, even more preferred less than 30% of the WPT, especially less than 20% of the WPT and especially preferred less than 10% of the WPT. It is preferable to measure the thickness of the main body  72  of the semiconductor workpiece  50  throughout the thinning process. This can be accomplished by employing conventional infrared monitoring technology in the process chamber, or by any other known measuring technique such as a capacitive measurement technique. If need be, the process fluid variables described above can be adjusted based on the continued monitoring of the workpiece thickness. 
     At step  360 , the thinned workpiece  50  is rinsed and dried. For example, the workpiece may be sprayed with a flow of deionized water, nitrogen or phosphoric acid during the rinsing step and may then be subject to any one or more known drying techniques thereafter. Finally, the workpiece  50  is then removed from the chuck (step  370 ) and the thinned workpiece  50  is diced into a plurality of dies (step  380 ). 
     Thinning of semiconductor workpieces  50  can be carried out on a single workpiece  50 , or on a plurality of workpieces  50  simultaneously, according to the present invention. When thinning a plurality of workpieces  50 , it is desirable to place each workpiece  50  into a corresponding chuck  10  and then place the plurality of chucks  10  and workpieces  50  into a carrier such as the carriers disclosed in co-pending U.S. patent applications Ser. Nos. 10/200,074 and 10/200,075, the disclosures of which are incorporated herein by reference. Once the plurality of workpieces  50  (and associated chucks  10 ) are placed in the carrier, the carrier is loaded into a process vessel and a process fluid is applied to the exposed back sides  53  of the plurality of workpieces  50 . In order to ensure an adequate application of the process fluid to the workpieces  50 , it is preferable to rotate the chucks  10  or the carrier, or both, within the process vessel during processing. The process vessel can be a stand alone tool, or one of a plurality of workstations making up a larger, workpiece  50  processing system. 
     With reference now to  FIGS. 10-11 , the resulting thinned semiconductor workpiece  50  processed according to the process of the present invention will be described. As described above, the thinned workpiece  50  is comprised of a rim  70  and a main body  72 . The rim  70  is formed at the periphery of the workpiece  50  and is integral with the main body  72 . Generally, when processing standard semiconductor workpieces  50 , the processed workpiece  50  will have a main body  72  with a thickness less than 125 microns and a rim  70  with a thickness in a range of approximately 600 to 725 microns. In a preferred embodiment, however, the main body  72  thickness will be less than 100 microns, mre preferably less than 50 microns, and especially less than 25 microns. As mentioned, the rim  70  is formed at the exclusion zone of the workpiece  50  and will have a width (shown as w in  FIG. 10 ) in a range of 1-10 mm, preferably a range of 1-5 mm and especially in a range of 1-2 mm. The main body  72  and rim  70  are formed from substantially the same material as the pre-thinned workpiece  50 . Most preferably the main body  72  and rim  70  are comprised of silicon. 
     As also mentioned above, it is contemplated that workpieces  50  that have previously been thinned by another process can be thinned according to the present invention. In these instances, the initial thickness of a workpiece  50  to be thinned according to the present invention may be 200 microns or less. In such case, a workpiece  50  thinned according to the present invention will have a main body  72  thickness less than about 50% of the rim  70  thickness, preferably less than about 40% of the rim  70  thickness, more preferably less than 30% of the rim  70  thickness, preferentially less than 20% of the rim  70  thickness, even less than 10% of the rim  70  thickness and especially less than 5% of the rim  70  thickness. It is also contemplated that the present invention can be used to thin workpieces  50  of varying sizes. Accordingly, the rim  70  will preferably comprise less than approximately 5% of the back side  53  surface area (BSSA) of the workpiece  50 , more preferably less than 3% of the BSSA, and even less than 1% of the BSSA. 
     Numerous modifications may be made to the foregoing invention without departing from the basic teachings thereof. Although the present invention has been described in substantial detail with reference to one or more specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the scope and spirit of the invention.