Abstract:
A method and apparatus for removing a deposited layer on a bottom surface of a substrate, the deposited layer proximate to an edge of the substrate. The method comprises: providing a chuck for supporting the bottom surface of the substrate, an peripheral portion of the bottom surface proximate to the edge extending past a periphery of the chuck; positioning a shield spaced away from and over a top surface of the substrate, a bottom surface of the shield opposite a top surface of the substrate; directing a reactant containing gas to the bottom surface of the substrate proximate to the edge of the substrate; and converting the reactant gas to a reactant species, the reactant species reacting with the deposited layer in order to cause removal of the deposited layer from the substrate.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of semiconductor substrate cleaning; more specifically, it relates to an apparatus for removal of polymer deposits from the backside and edge of semiconductor substrates.  
         BACKGROUND OF THE INVENTION  
         [0002]    Fluorocarbon based plasma etch processes for dielectric materials used to create damascene wiring patterns produce, to various degrees, an unwanted fluorocarbon polymer layer on the underside of semiconductor substrates proximate to the edge of the substrate. While these unwanted polymers are produced using ordinary interlevel dielectric (ILD) materials such as silicon oxides, it has been found that etching more advanced dielectric materials, for example SILK™ (a polyphenylene oligomer) manufactured by Dow Chemical, Midland, Mich. produces even greater quantities of these polymers.  
           [0003]    Subsequently deposited materials, for example silicon oxides, do not adhere well to the fluorocarbon polymer and consequently flake off, contaminating process tools with resultant high maintenance costs and causing defects to the integrated circuit wiring thus degrading yields. Therefore, there is a need in the industry for removal of backside edge polymers.  
         SUMMARY OF THE INVENTION  
         [0004]    A first aspect of the present invention is an apparatus for removing a deposited layer on a bottom surface of a substrate, the deposited layer proximate to an edge of the substrate comprising: a chuck for supporting the bottom surface of the substrate, a peripheral portion of the bottom surface proximate to the edge extending past a periphery of the chuck; a shield spaced away from and positioned over a top surface of the substrate, a bottom surface of the shield opposite a top surface of the substrate; a supply of a reactant containing gas capable of directing the reactant containing gas to the bottom surface of the substrate proximate to the edge of the substrate; and means for converting the reactant gas to a reactant species, the reactant species capable of reacting with the deposited layer in order to cause removal of the deposited layer from the substrate.  
           [0005]    A second aspect of the present invention is a method for removing a deposited layer on a bottom surface of a substrate, the deposited layer proximate to an edge of the substrate comprising: providing a chuck for supporting the bottom surface of the substrate, a peripheral portion of the bottom surface proximate to the edge extending past a periphery of the chuck; positioning a shield spaced away from and over a top surface of the substrate, a bottom surface of the shield opposite a top surface of the substrate; directing a reactant containing gas to the bottom surface of the substrate proximate to the edge of the substrate; and converting the reactant gas to a reactant species, the reactant species reacting with the deposited layer in order to cause removal of the deposited layer from the substrate.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0006]    The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0007]    [0007]FIG. 1 is partial cross-section view of a semiconductor substrate illustrating the location of a layer of polymer that is removed by the present invention;  
         [0008]    [0008]FIG. 2 is schematic diagram of an apparatus for removal of a backside edge polymer according to a first embodiment of the present invention;  
         [0009]    [0009]FIG. 3 is a detailed view of the apparatus of FIG. 2 near the edge of the substrate;  
         [0010]    [0010]FIG. 4 is a detailed view of an alternative configuration of the apparatus of FIG. 2 near the edge of the substrate;  
         [0011]    [0011]FIG. 5 is schematic diagram of an apparatus for removal of a backside edge polymer according to a second embodiment of the present invention;  
         [0012]    [0012]FIG. 6 is a detailed view of the apparatus of FIG. 5 near the edge of the substrate FIG. 7 is schematic diagram of an apparatus for removal of a backside edge polymer according to a third embodiment of the present invention; and  
         [0013]    [0013]FIG. 8 is a detailed view of the apparatus of FIG. 7 near the edge of the substrate. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    In the context of the present invention, the term wafer should be understood to include a variety of semiconductor substrates, such as bulk silicon substrates, silicon on insulator substrates, quartz substrates and sapphire substrates.  
         [0015]    [0015]FIG. 1 is partial cross-section view of a semiconductor substrate illustrating the location of a layer of polymer that is removed by the present invention. In FIG. 1, wafer  100  has a top surface  105 , a bottom surface  110  and an edge  115 . Wafer  100  has a thickness of “T1.” In the case of a 200 mm diameter bulk silicon substrate, “T1” may be about 750 microns. Because edge  115  of wafer  100  is beveled, the lower portion of the bevel is “shadowed” from the etchant species, which are generally directed normal to top surface  105  and which would otherwise remove any polymeric deposits. Thus a polymer layer  120  is deposited on any portion of bottom surface  110  exposed to the etch chamber environment and on a contiguous lower portion of edge  115 . Polymer layer  120  extends a distance “D1” along bottom surface  110  from edge  115 . “D1” is determined by the distance wafer  100  extended past the edge of the wafer chuck of the plasma etch tool that formed polymer layer  120 . Typically, “D1” is about 2 mm. Polymer layer  120  extends from bottom surface  110  to about midpoint  125  of edge  115 . In a typical dielectric plasma etch process, Polymer layer  120  is a fluorocarbon polymer whose thickness “T2” may be about 0.1 micron or greater.  
         [0016]    [0016]FIG. 2 is schematic diagram of an apparatus for removal of a backside edge polymer according to a first embodiment of the present invention. In FIG. 2, a plasma ash apparatus  130  includes a chamber  135 , a wafer chuck  140  and a shield  145  centered above the wafer chuck. Shield  140  and wafer chuck  145  are electrically conductive. In one example, shield  140  and wafer chuck  145  are formed from anodized aluminum. Wafer  100  is approximately centered on and held to wafer chuck  140  by electrostatic or other means. Top surface  105  of wafer  100  faces shield  145 . A portion of bottom surface  110  (proximate to edge  115 ) of wafer  100  overhangs wafer chuck  140 . Chamber  135  includes an exhaust  150  that is connected to a high vacuum pump (not shown) for producing a relatively high vacuum in the chamber and a reactant gas supply tube  155 . Reactant gas supply tube  155  supplies a reactant gas or gas mixture through shield  145  for distribution throughout a gap  160  between shield  145  and top surface  105  of wafer  100 . Plasma ash apparatus  130  also includes an RF source as illustrated in FIGS. 3 and 4 and described infra.  
         [0017]    [0017]FIG. 3 is a detailed view of the apparatus of FIG. 2 near the edge of the wafer  100 . In FIG. 3, an RF source  165  is coupled between shield  145  and ground. Wafer chuck  140  (as well as wafer  100 ) is coupled to ground. RF source  165  generates a plasma discharge region  170  proximate to bottom surface  110  and edge  115  of wafer  100 . Plasma discharge region  170  forms around the bottom surface  110  and edge  115  of wafer  100 . Plasma discharge region  170  generates, from the reactant gases, oxidizing species such as oxygen ions or oxygen free radicals that react with polymer layer  120  forming a volatile reaction product and thus removing the polymer layer. Shield  145  is spaced a distance “G1” from top surface  105  of wafer  100  forming a gap  160 . The value of “G1” is chosen to be too small to support a discharge region between shield  145  and top surface  105  of wafer  100 . Thus, no reactant species that could etch structures or materials formed on top surface  105  of wafer  100  are generated over the top surface and the top surface is protected by shield  145 . Bottom surface  110  of wafer  100  extends a distance “D2” beyond wafer chuck  140  in the direction  172 . Top surface  105  of wafer  100  extends a distance “D3” beyond shield  145  in the direction  172 .  
         [0018]    In one example, “G1” is about 0.5 to 1 mm, “D2” is about 3 to 5 mm, “D3” is about 0.5 to 1 mm and the reactant gas comprises oxygen, an oxygen/tetraflouromethane mixture, an oxygen/fluorine mixture, oxygen diluted with argon or nitrogen, an oxygen/tetraflouromethane mixture diluted with argon or nitrogen or an oxygen/fluorine mixture diluted with argon or nitrogen.  
         [0019]    An exemplary backside edge ash process for the apparatus illustrated in FIGS. 2 and 3 may be run at a pressure of about 2 to 2 torr, an oxygen flow rate of about 1000 to 3000 sccm sccm/sec and about 500 to 1500 watts forward bias for about 30 to 60 seconds seconds.  
         [0020]    [0020]FIG. 4 is a detailed view of an alternative configuration of the apparatus of FIG. 2 near the edge of the substrate. FIG. 4 is identical to FIG. 3 except that an auxiliary ring electrode  175  has been added. Ring electrode  175  is electrically conductive. In one example, ring electrode  175  is formed from anodized aluminum. Ring electrode  175  is coupled to RF source  165 . Shield  145  and chuck  140  are coupled to ground. Ring electrode  175  is positioned a distance “D4” beyond edge  115  of wafer  100  in direction  172 . In one example, “D4” is about 10 to 15 mm. Reactant gases are the same as those discussed supra in reference to FIG. 3.  
         [0021]    An exemplary backside edge ash process for the apparatus illustrated in FIGS. 2 and 4 may run at a pressure of about 2 to 3 torr, an oxygen flow rate of about 1000 to 3000 sccm/sec and about 500 to 1500 watts forward bias for about 30 to 60 seconds.  
         [0022]    [0022]FIG. 5 is schematic diagram of an apparatus for removal of a backside edge polymer according to a second embodiment of the present invention. In FIG. 3, an ozone clean apparatus  180  includes a chamber  185 , a wafer chuck  190  and a shield  195  centered above the wafer chuck. Wafer  100  is approximately centered on and suspended above wafer chuck  190  by lift pins  200 . Top surface  105  of wafer  100  faces shield  195 . A lip  205  of wafer chuck  190  surrounds edge  115  of wafer  100 . Wafer chuck  195  includes optional channels  210  that may contain electrical heating coils or through which a hot fluid may be circulated in order to heat the wafer chuck. Electrical heating is preferred. Chamber  185  includes an exhaust  215  that is connected to a vacuum pump (not shown) for producing a medium to high vacuum in the chamber, a reactant gas supply tube  220  and a purge gas supply tube  225 . Reactant gas supply tube  220  supplies ozone or an ozone mixture (generated by an ozone generator, not shown) through wafer chuck  190  for distribution throughout a gap  230  between wafer chuck  190  and bottom surface  110  of wafer  100 . Purge gas supply tube  225  supplies an inert gas or gas mixture through shield  195  for distribution throughout a gap  235  between shield  195  and top surface  105  of wafer  100 .  
         [0023]    [0023]FIG. 6 is a detailed view of the apparatus of FIG. 5 near the edge of the wafer  100 . In FIG. 6, edge  115  of wafer  100  is posited a distance “D5” from an inside surface  240  of lip  205  of wafer chuck  190 . Ozone flowing past bottom surface  110  and edge  115  of wafer  100  reacts with polymer layer  120  forming a volatile reaction product and thus removing the polymer layer. A top edge  245  of lip  205  is positioned a distance “D6” below a plane defined by a lower surface  250  of shield  195 . Distance “D6” is selected to reduce back diffusion of ozone onto top surface  105  of wafer  100 . The purge gas also helps to keep ozone away from top surface  105  of wafer  100 .  
         [0024]    In one example, “D5” is about 1 to 2 mm, “D6” is about 1 to 2 mm, the reactant gas is ozone, an ozone/argon mixture, an ozone/nitrogen mixture or an ozone/oxygen mixture and the purge gas is nitrogen or argon and the wafer chuck is heated to between about room temperature (i.e. 20° C.) and 300° C. Heating will increase the reaction rate and hence the removal rate of polymer layer  120 .  
         [0025]    An exemplary backside edge ozone clean process for the apparatus illustrated in FIGS. 5 and 6 may be run at a pressure of about 100 to 200 torr and an ozone flow rate of about 3000 to 5000 sccm/sec at temperature of about 200 to 300° C. for about 60 to 120 seconds.  
         [0026]    [0026]FIG. 7 is schematic diagram of an apparatus for removal of a backside edge polymer according to a third embodiment of the present invention. In FIG. 2, a plasma torch clean apparatus  260  includes a chamber  265 , a rotatable wafer chuck  270  and a shield  275  centered above the wafer chuck. Wafer chuck  270  is rotated by rotating shaft  280 . Wafer  100  is approximately centered on and held to wafer chuck  270  by electrostatic or other means. Top surface  105  of wafer  100  faces shield  275 . A portion of bottom surface  110  (proximate to edge  115 ) of wafer  100  overhangs wafer chuck  270 . Chamber  265  includes an exhaust  285  that is connected to an exhaust fan (not shown) for removing waste gas process gas and reaction products. Apparatus  260  is run at essentially room pressure. A purge gas supply tube  290  supplies an inert gas or gas mixture through shield  275  for distribution throughout a gap  295  between shield  275  and top surface  105  of wafer  100 . A reactant gas supply tube  300  supplies a reactant gas or gas mixture to a plasma torch  305 . Plasma torch  305  produces a plasma region  310  that contacts an exposed portion of bottom surface  110  and a contiguous portion of edge  115  of wafer  310 . Use of shield  275  and purge gas  295  ensures that plasma region  310  does not damage any structures formed on top surface  105  of wafer  100  or that any reaction products formed by removal of polymer layer  120  do not re-deposit on top surface  105 .  
         [0027]    [0027]FIG. 8 is a detailed view of the apparatus of FIG. 7 near the edge of the substrate. In FIG. 8, wafer  100  extends a distance “D7” from wafer chuck  270 . If optional shield  275  is used, the shield is spaced a distance “D8” from top surface  105  of wafer  100 . Plasma torch  305  includes a RF source  315 . RF source  315  generates plasma region  310  that contacts bottom surface  110  and edge  115  of wafer  100 . Plasma torch  305  is positioned a distance “D9” from bottom surface  110  of wafer  100 . Plasma region  310  includes oxidizing species such as oxygen ions or oxygen free radicals that react with polymer layer  120  forming a volatile reaction product and thus removing the polymer layer as wafer  100  is rotated past plasma torch  305 .  
         [0028]    There are two types of plasma torches available, an inductively coupled device and a capacitively coupled device. An example of an inductively coupled is a RAP (reactive atom plasma) device manufactured by RAPT Inc. of Livermore, Calif. and is described in United State Patent Publication 2002/0100751A1, which is hereby incorporated by reference. An example of a capacitively coupled device is manufactured by Apjet Inc. of Los Alamos, N. Mex.  
         [0029]    Plasma torch  305  has a length “L1” and a diameter of “W1.” In one example “L1 is about 3 inches and “W1” is about 1 inch.  
         [0030]    In one example, “D7” is about 50 mm, “D8” is about 1 to 2 mm, “D9” is about 1 to 5 mm, the reactant gas is oxygen, an oxygen/tetraflouromethane mixture, an oxygen/fluorine mixture, oxygen diluted with argon or nitrogen, an oxygen/tetraflouromethane mixture diluted with argon or nitrogen or an oxygen/fluorine mixture diluted with argon or nitrogen and the purge gas is nitrogen or argon. It is possible for the reactant gas and the purge gas to be the same.  
         [0031]    An exemplary backside edge plasma torch clean process for the apparatus illustrated in FIGS. 7 and 8 may be run at an oxygen flow rate of about 500 to 1000 sccm and about 500 to 1000 watts forward bias (torch) for about 30 to 60 seconds while the wafer is rotated at about 5 to 10 RPM.  
         [0032]    The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. For example, the wafer chuck may be heated in the first and third embodiments of the present invention as well as the first embodiment as illustrated in FIG. 5 and described supra. Therefore it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.