Abstract:
In accordance with an embodiment of the invention, there is provided an electrostatic chuck. The electrostatic chuck comprises a surface layer activated by a voltage in an electrode to form an electric charge to electrostatically clamp a substrate to the electrostatic chuck. The surface layer includes a plurality of polymer protrusions and a charge control layer to which the plurality of polymer protrusions adhere, the plurality of polymer protrusions extending to a height above portions of the charge control layer surrounding the plurality of polymer protrusions to support the substrate upon the plurality of polymer protrusions during electrostatic clamping of the substrate.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/216,305, filed on May 15, 2009. The entire teachings of the above application are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    An electrostatic chuck holds and supports a substrate during a manufacturing process and also removes heat from the substrate without mechanically clamping the substrate. During use of an electrostatic chuck, the back side of a substrate, such as a semiconductor wafer, is held to the face of the electrostatic chuck by an electrostatic force. The substrate is separated from one or more electrodes in the face of the electrostatic chuck by a surface layer of material that covers the electrode. In a Coulombic chuck, the surface layer is electrically insulating, while in a Johnsen-Rahbek electrostatic chuck, the surface layer is weakly conducting. The surface layer of the electrostatic chuck may be flat or may have one or more protrusions, projections or other surface features that further separate the back side of the substrate from the covered electrode. Heat delivered to the substrate during processing can be transferred away from the substrate and to the electrostatic chuck by contact heat conduction with the protrusions and/or by gas heat conduction with a cooling gas. Contact heat conduction is generally more efficient than gas heat conduction in removing heat from the substrate. However, controlling the amount of contact between the substrate and the protrusions can be difficult. 
         [0003]    In microelectronics production, as semiconductor and memory device geometries become progressively smaller and the sizes of wafers, flat screen displays, reticles and other processed substrates become progressively larger, the allowable particulate contamination process specifications become more restrictive. The effect of particles on electrostatic chucks is of particular concern because the wafers physically contact or mount to the chuck clamping surface. If the mounting surface of the electrostatic chuck allows any particulate to become entrapped between the mounting surface and the substrate, the substrate may be deformed by the entrapped particle. For example, if the back side of a wafer is clamped electrostatically against a flat reference surface, the entrapped particle could cause a deformation of the front side of the wafer, which will therefore not lie in a flat plane. According to U.S. Pat. No. 6,835,415, studies have shown that a 10-micron particle on a flat electrostatic chuck can displace the surface of a reticle (i.e., a test wafer) for a radial distance of one inch or more. The actual height and diameter of the particle-induced displacement is dependent on numerous parameters such as the particle size, the particle hardness, the clamping force and the reticle thickness. 
         [0004]    During substrate processing it is important to be able to control the temperature of the substrate, limit the maximum temperature rise of the substrate, maintain temperature uniformity over the substrate surface, or any combination of these. If there are excessive temperature variations across the substrate surface due to poor and/or non-uniform heat transfer, the substrate can become distorted and process chemistry can be affected. The greater the area of direct contact with the electrostatic chuck, the greater the heat transferred by contact heat conduction. The size of the area of direct contact is a function of the roughness, flatness and hardness of the contact surfaces of the substrate and electrostatic chuck, as well as of the applied pressure between the contact surfaces. Since the characteristics of the contact surface vary from substrate to substrate, and since the characteristics of the contact surface can change over time, accurately controlling contact heat conductance between the electrostatic chuck and substrate is difficult. 
         [0005]    Controlling the temperature of a substrate and the number of particles on its back side is important for reducing or eliminating damage to microelectronic devices, reticle masks and other such structures, and for reducing or minimizing manufacturing yield loss. The abrasive properties of the electrostatic chuck protrusions, the high contact area of roughened protrusions, and the effect of lapping and polishing operations during manufacture of electrostatic chucks may all contribute adder particles to the back side of substrates during use with an electrostatic chuck. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with an embodiment of the invention, there is provided an electrostatic chuck. The electrostatic chuck comprises a surface layer activated by a voltage in an electrode to form an electric charge to electrostatically clamp a substrate to the electrostatic chuck. The surface layer includes a plurality of polymer protrusions and a charge control layer to which the plurality of polymer protrusions adhere, the plurality of polymer protrusions extending to a height above portions of the charge control layer surrounding the plurality of polymer protrusions to support the substrate upon the plurality of polymer protrusions during electrostatic clamping of the substrate. 
         [0007]    In further, related embodiments, the polymer of which the plurality of polymer protrusions are formed may comprise polyetherimide (PEI), polyimide or polyether ether ketone (PEEK). The charge control layer may be formed of a polymer, such as polyetherimide (PEI), polyimide or polyether ether ketone (PEEK). An adhesive layer may underlie the charge control layer, and may comprise polyetherimide (PEI). The electrostatic chuck may comprise an adhesion coating layer. The adhesion coating layer may comprise at least one of silicon containing nitrides, oxides, carbides and non-stoichiometric versions of these, for example but not limited to SiO x N y , silicon nitride, silicon oxide or silicon carbide. The adhesion coating layer may also comprise carbon or a nitride compound of carbon; and may comprise diamond-like carbon. The adhesion coating layer may extend to comprise a metals reduction layer surrounding at least a portion of an edge of the electrostatic chuck. The electrostatic chuck may comprise a ceramic to ceramic bonding layer that bonds a dielectric layer of the electrostatic chuck to an insulator layer of the electrostatic chuck, the ceramic to ceramic bonding layer comprising a polymer, such as at least one of polytetrafluoroethylene (PTFE) and modified polytetrafluoroethylene (PTFE), and/or at least one of perfluoroalkoxy (PFA), fluorinated ethylene-propylene (FEP) and polyether ether ketone (PEEK). The modified polytetrafluoroethylene (PTFE) may comprise at least one of perfluoroalkoxy (PFA) and fluorinated ethylene-propylene (FEP). The plurality of polymer protrusions may be substantially equally spaced across the surface layer as measured by center to center distance between pairs of neighboring polymer protrusions. The polymer protrusions may be arranged in a trigonal pattern. The polymer protrusions may comprise a center to center distance of between about 6 mm and about 8 mm; and may comprise a height of between about 3 microns and about 12 microns; and may comprise a diameter of about 900 microns. The charge control layer may comprise a surface resistivity of between about 10 8  ohms per square to about 10 11  ohms per square. The electrostatic chuck may further comprise a gas seal ring comprising a polymer, such as polyetherimide (PEI), polyimide or polyether ether ketone (PEEK). The plurality of polymer protrusions may comprise a surface roughness of between about 0.02 μm and about 0.05 μm. 
         [0008]    In a further embodiment according to the invention, there is provided a method of manufacturing an electrostatic chuck. The method comprises bonding a dielectric layer of the electrostatic chuck to an insulator layer of the electrostatic chuck using a bonding polymer comprising at least one of polytetrafluoroethylene (PTFE), modified polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), fluorinated ethylene-propylene (FEP) and polyether ether ketone (PEEK); coating the dielectric layer of the electrostatic chuck with an adhesion coating layer comprising at least one of silicon containing nitride, silicon containing oxide, silicon containing carbide, non-stoichiometric silicon containing nitride, non-stoichiometric silicon containing oxide, non-stoichiometric silicon containing carbide carbon and a nitride compound of carbon; bonding a charge control layer comprising a charge control layer polymer to the surface of the electrostatic chuck, the charge control layer polymer comprising at least one of polyetherimide (PEI), polyimide and polyether ether ketone (PEEK); depositing a photoresist onto the charge control layer; reactive ion etching the charge control layer to remove portions of the charge control layer that will surround a plurality of polymer protrusions being formed in the charge control layer; and stripping the photoresist off the electrostatic chuck, thereby revealing the plurality of polymer protrusions being formed of the same charge control layer polymer as the charge control layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0010]      FIG. 1  is a cross-sectional diagram of the top layers of an electrostatic chuck in accordance with an embodiment of the invention. 
           [0011]      FIG. 2  is a cross-sectional diagram showing further layers of an electrostatic chuck in accordance with an embodiment of the invention. 
           [0012]      FIG. 3  is an illustration of a pattern of protrusions on the surface of an electrostatic chuck in accordance with an embodiment of the invention. 
           [0013]      FIG. 4  is a diagram of the surface appearance of an electrostatic chuck in accordance with an embodiment of the invention. 
           [0014]      FIG. 5  is a diagram of the profile of a protrusion on an electrostatic chuck in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    A description of example embodiments of the invention follows. 
         [0016]    In accordance with an embodiment of the invention, there is provided an electrostatic chuck that includes protrusions on its surface for mounting a substrate. The protrusions are formed of a polymer substance, such as polyetherimide (PEI), polyimide or polyether ether ketone (PEEK). Further, the electrostatic chuck features a charge control surface layer, to which the polymer protrusions adhere. The charge control surface layer may be formed of the same polymer substance as the protrusions, such as polyetherimide (PEI), polyimide or polyether ether ketone (PEEK). Such protrusions and charge control surface layer may assist with encouraging contact of the electrostatic chuck with the substrate to promote contact cooling, while also reducing production of undesirable particles. 
         [0017]      FIG. 1  is a cross-sectional diagram of the top layers of an electrostatic chuck in accordance with an embodiment of the invention. The electrostatic chuck features protrusions  101  that are formed of a polymer, such as polyetherimide (PEI), polyimide or polyether ether ketone (PEEK). The gas seal rings (not shown) of the electrostatic chuck may be formed of a polymer, such as the same polymer as the protrusions  101 . The protrusions  101  adhere to a charge control layer  102 , which may also be formed of a polymer. The purpose of the charge control layer  102  is to provide a conductive layer to bleed away surface charge. The charge control layer  102  reduces the likelihood of “wafer sticking,” which occurs when a wafer or other substrate electrostatically adheres to the chuck surface after the chuck power is removed. A charge control layer  102  having a surface resistivity in an appropriate range, such as, for example, a range of from about 1×10 8  ohms/square to about 1×10 11  ohms/square, has been shown to reduce surface charge retention that can lead to undesirable electrostatic force and ultimately to wafer sticking. The slightly conductive surface layer bleeds charge to ground (not shown) while not interfering with the electrostatic attraction between the electrostatic chuck and the substrate. In one embodiment, both the protrusions  101  and the charge control layer  102  are formed of a single polymer, such as polyetherimide (PEI), polyimide or polyether ether ketone (PEEK). An adhesive layer  103  may be underneath the charge control layer  102 , and may comprise a different polymer from the charge control layer. In particular, where the charge control layer is formed of polyether ether ketone (PEEK), the adhesive layer  103  may comprise polyetherimide (PEI). Alternatively, the adhesive layer  103  need not be present. Underneath the adhesive layer  103  (or directly underneath the charge control layer  102 ), the electrostatic chuck includes an adhesion coating  104  that encourages the polymer layers above it to adhere to the dielectric layer  105 . The adhesion coating  104  stays buried under the polymer layers above it, and hides cosmetic defects in the polymers. The adhesion coating  104  may, for example, include silicon containing nitrides, oxides, carbides and non-stoichiometric versions of these, for example but not limited to SiO x N y , silicon nitride, silicon oxide or silicon carbide. The adhesion coating layer may also comprise carbon or a nitride compound of carbon; and may comprise diamond-like carbon; and/or a combination of any of the foregoing. Underneath the adhesion coating  104  is a dielectric layer  105 , such as an alumina dielectric. 
         [0018]      FIG. 2  is a cross-sectional diagram showing further layers of an electrostatic chuck in accordance with an embodiment of the invention. In addition to protrusions  201 , charge control layer  202 , adhesive layer  203 , adhesion coating  204  and dielectric layer  205 , the electrostatic chuck includes metal electrodes  206 . The metal electrodes  206  are bonded to electrode pins  207  by electrically conductive epoxy bonds  208 . The dielectric layer  205  is bonded to a insulator layer  209 , such as an alumina insulator, by a ceramic to ceramic bond  210 . The ceramic to ceramic bond  210  may be formed of a polymer, such as polytetrafluoroethylene (PTFE) or modified PTFE (which includes PFA and/or FEP in addition to PTFE). Further, the ceramic to ceramic bond  210  may be formed of polymers such as perfluoroalkoxy (PFA), fluorinated ethylene-propylene (FEP) and polyether ether ketone (PEEK). Underneath the insulator  209  there is a thermally conductive bond  211  (which may be formed, for example, using TRA-CON thermally conductive epoxy, sold by TRA-CON, Inc. of Bedford, Mass., U.S.A.) and a water cooled base  212 . The adhesion coating  204  may extend down an edge of the electrostatic chuck (including down the edges of the gas seal rings) to form a metals reduction layer  213 , which prevents beam strikes on the edges of the electrostatic chuck from causing aluminum particles to strike the substrate. 
         [0019]    In accordance with an embodiment of the invention, the polyetherimide (PEI) used for the protrusions  201 , charge control layer  202  or other components of the electrostatic chuck may be formed of unfilled amorphous polyether imide (PEI), in a thickness of between about 12 microns and about 25 microns. For example, PEI sold under the tradename ULTEM 1000 may be used, sold by Sabic Innovative Plastics Holdings BV. Where the protrusions  201  and/or charge control layer  202  or other components are formed of polyether ether ketone (PEEK), they may be made from unfilled PEEK, in a thickness of between about 12 microns and about 25 microns. For example, PEEK sold under the trade name Victrex® APTIV PEEK™ FILM, 2000-006 (unfilled amorphous grade) may be used, sold by Victrex U.S.A., Inc. of West Conshohocken, Pa., U.S.A. 
         [0020]    An electrostatic chuck featuring polymer protrusions and a polymer charge control layer in accordance with an embodiment of the invention may include features of the electrostatic chuck of U.S. patent application Ser. No. 12/454,336, filed on May 15, 2009, published as U.S. Patent Application Publication No. 2009/0284894, the teachings of which are hereby incorporated by reference in their entirety. In particular, features relating to equally spaced protrusions, trigonal pattern protrusions and low particle production may be included, and other features may also be included. 
         [0021]      FIG. 3  is an illustration of a pattern of protrusions  314  on the surface of an electrostatic chuck, in accordance with an embodiment of the invention, in which the protrusion pattern is used to reduce the forces between a substrate and the protrusions  314 . Protrusion patterns that equally distribute such forces may be used, for example trigonal or generally hexagonal patterns of protrusions. It should be appreciated that, as used herein, a “trigonal” pattern is intended to mean a regularly repeating pattern of equilateral triangles of protrusions, such that the protrusions are substantially equally spaced apart. (Such a pattern may also be viewed as being generally hexagonal in shape, with a central protrusion in the center of an array of six protrusions that form the vertices of a regular hexagon). Forces may also be reduced by increasing the diameter  315  of the protrusions, or by decreasing the center-to-center spacing  316  of the protrusions  314 . As shown in the embodiment of  FIG. 3 , the protrusions may be disposed in an equally spaced arrangement, in which each protrusion is substantially equally spaced apart from the adjacent protrusions by a center to center spacing dimension  316 . By virtue of such spacing, a substantial portion of the back side of the substrate contacts the top portion of the protrusions, leaving a gap between the protrusions for helium or other gas for back side cooling. By contrast, without such protrusion spacing, only a small portion, 10% or less, of the protrusions may contact the substrate. In accordance with an embodiment of the invention the substrate may contact greater than 25% of the protrusion&#39;s top surface area. 
         [0022]    In one example, the electrostatic chuck may be a 300 mm configuration, including an aluminum base, an alumina insulator  209  of about 0.120 inches in thickness, an alumina dielectric  205  of about 0.004 inches thickness, and having a rotary platen design to allow rotating and tilting of the substrate that is mounted to the electrostatic chuck. The diameter of the electrostatic chuck may, for example, be 300 mm, 200 mm or 450 mm. The protrusions  314  may be in a trigonal pattern, with a center to center spacing dimension  316  of from about 6 mm to about 8 mm, for example. The diameter  315  of the protrusions may, for example, be about 900 microns. The height of the protrusions  314  may, for example, be from about 3 microns to about 12 microns, such as about 6 microns. The protrusions  314  may be formed entirely of polymer, as may be the charge control layer  202  (see  FIG. 2 ). 
         [0023]      FIG. 4  is a diagram of the surface appearance of an electrostatic chuck in accordance with an embodiment of the invention. The electrostatic chuck surface includes gas inlets  417 , a ground pin passage  418 , a gas seal ring  419 , a lift pin passage  420  that includes its own gas seal ring (outer light-colored structure of lift pin passage  420  in  FIG. 4 ), and a small gas inlet at  421  in the center of the chuck (inlet not visible in  FIG. 4 ). The ground pin passage  418  may include its own gas seal ring (outer ring of ground pin passage  419  in  FIG. 4 ). A detail view (inset  422  in  FIG. 4 ) shows the protrusions  414 . The gas seal ring  419  (and the gas seal rings of the lift pin passages  420  and ground pin passages  418 ) may be about 0.1 inches in width and may have an equal height to that of the protrusions  414 , such as from about 3 microns to about 12 microns, for example about 6 microns, although other widths and heights are possible. 
         [0024]    In accordance with an embodiment of the invention, an electrostatic chuck may be made by the process of, first, preparing the ceramic assembly using a ceramic to ceramic bond. For example, the dielectric layer  205  may be bonded to the insulator layer  209  using the bonding substances described above in connection with the embodiment of  FIG. 2 . Next, the ceramic assembly is coated with the adhesion coating  204 , such as the substances discussed above in connection with the embodiment of  FIG. 1 , to a thickness of about 1 or 2 microns. Next, the polymer substance that will make up the charge control layer  202  and protrusions  201  is bonded to the surface of the adhesion coating  204 . The top of the polymer substance may then be plasma treated to help photoresist (applied next) to stick. Next, photoresist is deposited on the polymer substance, and is exposed and developed. Next, a reactive ion etch process is used to remove a thickness of the polymer substance (such as between about 3 microns and about 12 microns, in particular about 6 microns) to create the areas between the protrusions  201 . The amount etched away (resulting in the height of the protrusions) may be optimized for the back side gas pressure that will be used with the electrostatic chuck. The height of the protrusions is preferably approximately the same as, or substantially equal to, the mean free path of the gas used in back side cooling. After etching, the photoresist is then stripped off, and the process proceeds to final assembly of the electrostatic chuck. 
         [0025]      FIG. 5  is a diagram of the profile of a protrusion on an electrostatic chuck in accordance with an embodiment of the invention. The width and height are shown in micrometers. The protrusion is about 6 microns in height, and has a very smooth wafer contact surface  523 . For example, the protrusion may have a surface roughness on the wafer contact surface  523  of about 0.02 to about 0.05 μm. Likewise, the gas seal rings may have a similarly smooth surface, which results in a good seal with the substrate. Table 1, below, shows the results of a gas leak rate experiment in accordance with an embodiment of the invention. The left column shows the back side gas pressure applied, the right column shows the back side gas flow, which occurs as a result of gas leaks out from under the edges of the electrostatic chuck, and the middle column shows the chamber pressure, which will rise as more gas leaks out the edge of the electrostatic chuck. Results of less than 1 sccm back side gas flow rate (as here) are considered desirable. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Gas Leak Rate Test 
               
             
          
           
               
                 BSG Pressure 
                 Chamber Pressure 
                 BSG Flow 
               
               
                 (Torr) 
                 (Torr) 
                 (sccm) 
               
               
                   
               
             
          
           
               
                 0 
                 2.44E−06 
                 na 
               
               
                 4 
                 5.17E−06 
                 0.09 
               
               
                 10 
                 9.04E−06 
                 0.34 
               
               
                 15 
                 1.24E−05 
                 0.56 
               
               
                 25 
                 2.02E−065 
                 1.1  
               
               
                   
               
             
          
         
       
     
         [0026]    In accordance with an embodiment of the invention, the gas seal rings of the electrostatic chuck may comprise a surface roughness of less than about 8 microinches, or less than about 4 microinches, or less than about 2 microinches, or less than about 1 microinches. 
         [0027]    In accordance with an embodiment of the invention, the electrostatic chuck is a Coulombic chuck. The dielectric can include aluminum, for example alumina or aluminum nitride. In a further embodiment according to the invention, the electrostatic chuck is a Johnsen-Rahbek electrostatic chuck. Alternatively, the electrostatic chuck may not be a Johnsen-Rahbek electrostatic chuck, and the dielectric may be chosen so that a Johnsen-Rahbek (JR) force or partial hybrid Johnsen-Rahbek force does not act on the wafer or substrate. 
         [0028]    The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety. 
         [0029]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.