Abstract:
Apparatus for retaining a workpiece on a workpiece support and method for manufacturing same. The apparatus contains an adhesive layer, an insulating layer made of a base material having a first resistivity, an electrode layer, a hybrid/adhesive layer and a workpiece support layer made of a base material and a dopant, the dopant having a second resistivity wherein a resistivity of the resultant workpiece support layer is lower than the first resistivity. The multi-resistivity layers establish a Johnsen-Rahbek effect for electrostatic chucking while not unduly compromising chuck strength or longevity. The method consists of the steps of disposing an adhesive layer, disposing an insulating layer, disposing an electrode layer, disposing a hybrid/adhesive layer, disposing a workpiece support layer, curing the layers and forming a plurality of grooves in the workpiece support layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to an apparatus for retaining a workpiece within a semiconductor wafer processing system and, more specifically, to an improved composition of a polyimide based electrostatic chuck that maximizes electrostatic clamping ability without loss of material strength or modulus of elasticity. 
     2. Description of the Background Art 
     Electrostatic chucks are used for retaining a workpiece in various applications including retaining a semiconductor wafer within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece. 
     In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a pedestal during processing. The pedestal may form an electrode and a heat sink or heater as used in etching, physical vapor deposition (PVD) or chemical vapor deposition (CVD) applications. For example, FIG. 1 depicts a cross-sectional view of a reaction chamber used in semiconductor wafer processing. For a detailed understanding of the reaction chamber and its operation in processing the wafer, the reader should refer to the drawings and the detailed description contained in U.S. Pat. No. 5,228,501, issued Jul. 20, 1993, incorporated herein by reference. That patent teaches a PVD wafer processing chamber manufactured by Applied Materials, Inc. of Santa Clara, Calif. Additionally, the operation of a conventional electrostatic chuck is disclosed in U.S. Pat. No. 5,350,479 issued Sep. 27, 1994 to the assignee hereof, and its disclosure is incorporated herein by reference. 
     The chamber  100  contains a pedestal  106  supporting an electrostatic chuck  104 . The electrostatic chuck  104  has at least one electrode  116  which is insulated from a wafer  102  placed upon an upper surface  105  of the electrostatic chuck  104 . Specifically, the electrode  116  is either embedded within the body of the electrostatic chuck  104  or encased in layers of dielectric material which comprise the electrostatic chuck. The electrode(s)  116  are coupled to a power supply (not shown) via electrical conductors  118 . The voltage from the power supply creates the electrostatic (or clamping) force which draws the wafer  102  to the chuck  104 . Additionally, a variety of components may circumscribe the pedestal  106  to protect the wafer  102  and chamber  100  from improper or excessive deposition, etching or the like. Specifically, a deposition ring  108  contacts the edges of the wafer  102  and a deposition shield  124  circumscribes the deposition ring  108  to define a reaction zone  126 . Lift pins  110  are mounted on a platform  112 . The platform is coupled to an actuator shaft  114  located below the pedestal  106 . The lift pins  110  engage the wafer and lift it off the pedestal  106  after processing is completed. 
     The mechanism of attraction in the electrostatic chuck used in these types of wafer processing systems is generally Coulombic force. That is, the increase of charges in the insulated electrode  116  induce opposite charges to gather on the backside of the wafer. The resultant force is generally weak per unit area i.e., 15 g/cm 2  at 1500V DC because of the composition of the chuck. For example, a commonly used type of dielectric material for fabricating electrostatic chucks is polyimide. Specifically, electrodes are usually sandwiched between two sheets of polyimide to form an electrostatic chuck. Among the beneficial characteristics of polyimide are its high strength and high modulus of elasticity. This material also has high volume resistivity (on the order of 10 14  ohm-cm) and surface resistivity (on the order of 10 14  ohm/cm 2 ). Since the electrode(s) are insulated and a high resistivity dielectric is used, the charges creating the chucking force are not mobile i.e., the electrode and wafer are separated by the dielectric layer. As such, the wafer must come into contact with a large area of the chuck so that an adequate charge accumulation is established for wafer retention. 
     Additionally, the backside of the wafer  102  and the top surface  105  of the electrostatic chuck  104  are relatively smooth. However, imperfections in each of these surfaces create interstitial spaces when these surfaces come into contact. As such, not all of the wafer is in direct thermal contact with the chuck. Maintaining a uniform temperature across the entire wafer is essential to proper wafer processing. To maintain proper thermal transfer conditions at the wafer during processing, an inert thermal transfer gas is pumped into the interstitial spaces or specially formed grooves in the chuck surface when the clamping force is applied. More specifically, a feed-through pipe  122  in the pedestal  106  provides thermal transfer gas to an aperture  120  in the top surface  105  of the electrostatic chuck  104 . The gas, usually Helium or Argon, acts as a thermal conduction medium between the wafer  102  and the chuck  104  that has better thermal transfer characteristics than the vacuum it replaces. To further enhance thermal transfer conditions (i.e., cooling or heating of the wafer), the pedestal temperature is typically controlled using water-cooled conduits within a cooling plate (not shown) below the chuck  104  and/or with resistive heating elements buried in or clamped to the chuck  104 . This cooling technique is known as backside gas cooling. 
     Since the distribution of thermal transfer gas to the interstitial spaces and chuck groove is osmotic and the interstitial spaces may not all be interconnected, some spaces do not receive any gas. This condition can also lead to a non-uniform temperature profile across the backside of the wafer  108  during processing and result in wafer damage. As such, it is advantageous to have as large a gas aperture and groove width as possible to maximize thermal transfer gas flow and pressure beneath the wafer. However, the limited attractive wafer clamping (Coulombic) force establishes a limit on the size of this aperture and the gas pressure therein. Additionally chuck groove width is limited to approximately 1-2 mm. Specifically, if the thermal transfer gas pressure becomes greater than the Coulombic chucking force, the wafer may shift on the pedestal thereby causing a processing anomaly on the wafer. In an extreme situation, the wafer may even pop off the pedestal onto the chamber floor and likely break, rendering the wafer useless. Since effective and uniform heat conduction away from and/or into the wafer is an important aspect of the manufacturing process, different types of chucks are designed in an attempt to maximize clamping force and thermal transfer. 
     One example of an improved electrostatic chuck is one that employs the Johnsen-Rahbek (J-R) effect. In such a chuck, the dielectric material has an intermediate resistivity instead of a high resistivity. As such, there are mobile charges present in the dielectric material. These mobile charges create a small but effective current flow between the backside of the wafer and the top surface of the electrostatic chuck. Specifically, at points where these two surfaces come into contact, a zero potential exists. These contact points are extremely small in comparison to the total area of a wafer being retained on the chuck. As such, not all of the mobile charges are able to pass through the contact points. The resultant movement and accumulation of the mobile charges within the top surface of the electrostatic chuck and the backside of the wafer creates a very high electrostatic force across the interstitial spaces between the surfaces. This electrostatic force clamps the wafer to the chuck. 
     Electrostatic chucks using the J-R effect are usually fabricated from a ceramic having an intermediate or “leaky” dielectric characteristic. Materials such as aluminum and silicon oxides and nitrides are popular and well known for use in electrostatic chucks. However, these types of materials must be carefully machined when creating the gas aperture or similar openings grooves or features; otherwise, they may fracture and become unusable. Additionally, the different coefficients of thermal expansion of the wafer and the ceramic may contribute to the phenomenon of “microgrinding” during processing. Microgrinding causes minute contaminant particles from the surface of the electrostatic chuck to become embedded on the backside of the wafer. Such particles may also be released in the process chamber and contaminate succeeding wafers. Polyimide, exhibits none of the undesirable microgrinding or fracturing characteristics of ceramics. Unfortunately, polyimide exhibits only high resistivity characteristics which is not useful in establishing the J-R effect. 
     Therefore, there is a need in the art for an improved apparatus for retaining a wafer having a strength and modulus of elasticity comparable to polyimide, but have a reduced resistivity level so as to take advantage of the J-R effect for clamping the wafer. Additionally, such an apparatus must be simple and cost-effective in design and construction to allow for optimal thermal transfer gas aperture and groove size in the apparatus and flow of the thermal transfer gas beneath the wafer. 
     SUMMARY OF THE INVENTION 
     The disadvantages heretofore associated with the prior art are overcome by an apparatus for retaining a workpiece on a workpiece support having improved clamping force and a relatively high modulus of elasticity. The inventive apparatus contains an insulating layer disposed on a top surface of the workpiece support, the insulating layer having a first resistivity; an electrode layer disposed on top of the insulating layer; and a workpiece support layer disposed on top of the electrode layer, the workpiece support layer having a second resistivity wherein the second resistivity is lower than the first resistivity. A hybrid/adhesive layer having the second resistivity may also be disposed between the electrode layer and the workpiece support layer. With the altered resistivity of the workpiece support layer, a Johnsen-Rahbek effect is established. As such, a workpiece, e.g., a semiconductor wafer, is retained on the workpiece support with a greater force than previously possible, i.e., when using just the workpiece support layer having the first, higher resistivity. 
     Additionally, a method of manufacturing such an apparatus is disclosed. The method consists of the steps of disposing an insulating layer upon the top surface of a workpiece support; disposing an electrode layer on top of the insulating layer; disposing a doped workpiece support layer on top of the electrode layer and curing the layers. Alternately, an adhesive layer can be disposed between the top surface of the workpiece support and the insulating layer and a hybrid/adhesive layer can be disposed between the electrode layer and the doped workpiece support layer. The described method allows for manufacturing of an electrostatic chuck with a layered polyimide construction that exploits the Johnson-Rahbek effect on its top-most layer to electrostatically retain a workpiece, e.g., a semiconductor wafer, to the workpiece support. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view of a prior art wafer processing chamber; 
     FIG. 2 a  is a cross-sectional view of a first embodiment of the inventive apparatus; 
     FIG. 2 b  is a cross-sectional view of a second embodiment of the inventive apparatus; 
     FIGS. 3 a  through  3   i  are a step-by-step schematic depiction of the fabrication process of the first embodiment of the inventive apparatus; 
     FIGS. 4 a  through  4   g  are a step-by-step schematic depiction of the fabrication process of the second embodiment of the inventive apparatus; 
     FIG. 5 is a graph of volume resistivity of a material vs. the percentage by weight of a dopant added to the material; 
     FIG. 6 is a graph of surface resistance of a material vs. the percentage by weight of a dopant added to the material; and 
     FIG. 7 is a top perspective view of the inventive apparatus. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     FIG. 2 a  depicts a detailed, cross-sectional view of a first embodiment on the inventive electrostatic chuck  202  mounted on a workpiece support  106  (e.g., a pedestal) for supporting a workpiece  102  (e.g., a semiconductor wafer) in a semiconductor wafer processing chamber (e.g., chamber  100  of FIG.  1 ). More specifically, the electrostatic chuck  202  is a multilayer device that covers the top surface  105  of the pedestal  106 . Preferably, the pedestal  106  is fabricated from a durable material (i.e., a metallic substance such as an alloy of aluminum or copper, stainless steel or the like or a ceramic such as aluminum nitride, silicon nitride, silicon dioxide or the like. The electrostatic chuck  202  is affixed to the top surface  105  of the pedestal  106  in stages to form a complete device. 
     In the first embodiment of the invention, the electrostatic chuck  202  has five layers. The first layer  204  is an adhesive layer. Preferably, the adhesive layer  204  is fabricated from an insulating material such as ordinary polyimide. Polyimide is a widely known and used product in the field of electrostatic chuck manufacturing. The polyimide can be in the form of a thin (e.g., 20-50 μm thick) sheet or is preferably a layer of a paste-like material that can be spread across the top surface  105  to a thickness of approximately 30 μm. The next layer  208  is an insulating layer that is preferably a polyimide film approximately 30-300 μm thick. The insulating layer  208  is applied to a top surface  206  of the adhesive layer  204 . The insulating layer  208  forms an electrical barrier between the workpiece support  106  and electrical elements within the electrostatic chuck  202  described below. 
     The next layer of the electrostatic chuck is an electrode layer  212 . The electrode layer  212  contains at least one electrode and, in the depicted bipolar chuck of FIG. 2 a,  contains a pair of coplanar electrodes  116  disposed on a portion  222  of the insulating layer  208 . The electrodes  116  are fabricated from a conductive material (i.e., copper) are approximately 20-300 μm thick and may be in any shape required to maximize chucking force applied to the wafer  102 . For example, the electrodes may be in the form of two half-moons, concentric circles, a plurality of individual pads electrically connected by an interconnection trace or the like. The electrode layer  212  is connected to a power source  210  via electrical conductors  118  and electrical feedthroughs  218 . The electrical feedthroughs  218  are disposed in insulating conduits  220  within the workpiece support  106 . The power source  210  provides the necessary voltage to the electrode layer  212  to clamp the wafer  102  to the pedestal  106 . The electrodes  116  may be connected to the power source in any configuration necessary to form the required chucking force. FIG. 2 depicts a bipolar configuration, but this does not preclude the use of other types of configurations including a monopolar configuration. 
     The next layer of the electrostatic chuck is a hybrid/adhesive layer  214 . The hybrid/adhesive layer  214  is disposed over electrode layer  212  and a portion  224  of insulating layer  208  not covered by the electrode layer  212 . Preferably, hybrid/adhesive layer  214  is a special semiconducting layer. This semiconducting layer differs in composition and function from the other layers  204  and  208 . Specifically, the hybrid/adhesive layer  214  is fabricated from a base material and a dopant. The base material and dopant each have a resistivity that is based on the properties of said material. The resistivity of the base material is greater than the resistivity of the dopant. In a preferred embodiment of the invention, the base material is polyimide and the dopant is carbon. Alternatively, the dopant is a polymer. This doping material increases the conductivity of the polyimide. Thus, it is possible to control the volume and surface resistivity of the chuck by altering the composition of the hybrid/adhesive layer  214  of the chuck. Adding the doping material creates a modified polyimide that is characteristically more conducting than previously possible. That is, more mobile charges are introduced into the polyimide as a result of the doping. As such, the resistivity of the hybrid/adhesive layer is less than the original base material (polyimide). The principle mechanism of attraction in the chuck changes from the weak Coulombic forces to the highly desirable Johnson-Rahbek effect. This hybrid/adhesive layer  214  effectively encapsulates and affixes the electrode layer  212  between layers  208  and  214 . 
     The next and final layer  216  is a workpiece support layer that completes the construction of the inventive electrostatic chuck  202 . Preferably, the workpiece support layer  216  is also a semiconducting layer of the same doped polyimide as the hybrid/adhesive layer  214 . However, the workpiece support layer  216  is preferably in the form of a doped polymide film approximately 20-300 μm thick and not necessarily a paste. Nonetheless, it contains the same resistivity properties of the hybrid/adhesive layer. 
     FIG. 2 b  depicts an alternate embodiment of the invention wherein the electrostatic chuck  202  is formed of only three layers. In accordance with the previous embodiment, the chuck  202  is affixed to the top surface  105  of a pedestal  106  similar to that shown in FIG. 2 a.  However, in this embodiment, there are no film layers of either doped or ordinary polyimide. Specifically, a coating of ordinary polyimide paste  204  is applied to the top surface  105  of the pedestal  106  preferably to a thickness of approximately 30-300 μm. In this embodiment, the layer  204  is both an adhesive and insulating layer. Next, an electrode layer  212  identical to the one depicted in FIG. 2 a  is disposed upon a portion  226  of the polyimide paste layer  204 . The electrical connections and feedthroughs are likewise identical to that of FIG. 2 a.  The third and final layer is the workpiece support layer  216 . As discussed in the first embodiment, the workpiece support layer  216  for this embodiment is a coating of doped polyimide paste, preferably applied to a thickness of approximately 20-400 μm over the electrode layer  212  and a portion  228  of the ordinary polyimide paste layer  204  not covered by the electrode layer  212 . This workpiece support layer has the same resistivity properties as the hybrid/adhesive layer of the previous embodiment. 
     FIG. 7 depicts a perspective view of the electrostatic chuck  202  without a wafer retained thereon. Specifically, the workpiece support layer  216  is shown in detail. A plurality of grooves  702  are provided in the workpiece support layer  216 . The grooves  702  extend radially outward from a center aperture  704  provided in the chuck  202 . That is, the center aperture extends through the pedestal  106  (though not specifically shown in FIGS. 2A or  2 B) into the chuck  202  to provide a path for a backside gas similar to the port  120  in the prior art apparatus of FIG.  1  and described previously. The backside gas then travels along the grooves  702  to act as a heat transfer medium between the wafer  102  and the chuck  202 . In a preferred embodiment of the invention, eight (8) grooves are provided in the workpiece support layer  216 . 
     The gas grooves  702  are formed into layer  216  by any known method for providing surface features in a polyimide-based structure. Such methods include but are not limited to etching or laser cutting after curing (explained in greater detail below). The improved polyimide of the layers above the electrode layer create a chucking force that is approximately 2 times stronger than the chucking force established in conventional electrostatic chucks. As such, wider gas grooves are provided in the subject invention. The width of the gas grooves are approximately two times greater than width of prior art chuck grooves and preferably in the range of approximately 2-4 mm. The wider gas grooves result in a greater volume of backside gas being provided to the wafer. Hence, temperature control of the wafer is also improved. 
     Additionally, a method of manufacture of an improved electrostatic chuck is disclosed. FIGS. 3 a-i  depict the manufacturing process of the first embodiment of the chuck seen in FIG. 2 a  in a step-by-step manner as follows. FIG. 3 a  depicts the first step of the process wherein a bare pedestal  106  is provided. In the next step, depicted in FIG. 3 b,  one or more insulating conduits  220  are formed in the bare pedestal  106  for providing an insulated path between the electrode layer  212  and the bottom of the pedestal  106 . In FIG. 3 c,  the bare pedestal  106  is coated with an adhesive layer  204 . Preferably, the adhesive layer  204  is an ordinary polyimide paste approximately 30 μm thick. The coating process can be performed by any means known in the art of electrostatic chuck manufacturing including screen printing and the like. In Step  3   d,  an insulating layer  208  is applied over the adhesive layer  204 . The insulating layer  208  is preferably an ordinary polyimide film. A preferred thickness of the first insulating layer  208  is approximately 30-300 μm. 
     As seen in FIG. 3 e,  an electrode layer  212  is disposed over a portion  222  of the insulating layer  208  in the next step of electrostatic chuck fabrication method. The electrode layer  212  as described earlier is preferably at least one copper sheet that is applied over the insulating layer  208 . One or more electrical feedthroughs  218  extend from the electrodes  116  through the pedestal  106 . The feedthroughs are connected to one or more conductors  118  that conduct power from a chucking power supply (not shown) to the electrode layer  212 . In the next step, a hybrid/adhesive layer  214  is applied over the electrode layer  212  and a portion  224  of the insulating layer  208  not covered by the electrode layer  212  as seen in FIG. 3 f.  Preferably, this hybrid/adhesive layer  214  is the doped polyimide paste described earlier and is applied to a thickness of 20-300 μm. This hybrid/adhesive layer effectively affixes and encapsulates the electrode layer  212  thereby preventing the electrode from contacting any other layers. 
     FIG. 3 g  depicts the next step of the electrostatic chuck fabrication process in which a workpiece support layer  216  is disposed over the hybrid/adhesive layer  214 . Specifically, a premade sheet of doped polyimide is applied to the hybrid/adhesive layer  214  to form the complete electrostatic chuck  202 . In a preferred embodiment of the invention, the workpiece support layer  216  is approximately 20-300 μm thick. In step  3   h,  the entire pedestal/chuck assembly  106 / 202  is placed in a furnace for curing. For example, the assembly is cured at a temperature of approximately 100-350° C. at a pressure of approximately 1 atm for approximately 30 min.-10 hrs. This curing step cures the adhesive and hybrid/adhesive polyimide paste layers  204  and  214  respectively. In step  3   i,  a plurality of grooves (see in FIG.  7  and described above) are formed in the workpiece support layer  216 . The final product is therefore a pedestal  106  with a layered electrostatic chuck  202  on the top surface  105  of the pedestal  106  wherein the top (workpiece support) layer  216  of the chuck  202  is a doped polyimide. The doped layer exhibits a lower resistivity than ordinary polyimide. As such, a greater number of mobile charges exist in the polyimide and which leads to establishment of the Johnson-Rahbek effect. Further, the increased chucking force established by this chuck  202  allows for wider grooves having greater backside gas flow. 
     A method of manufacture of the alternate embodiment of the improved electrostatic chuck is also disclosed. FIGS. 4 a-g  depict the manufacturing process of the alternate embodiment of the chuck seen in FIG. 2 b  in a step-by-step manner as follows. FIG. 4 a  depicts the first step of the process wherein a bare pedestal  106  is provided. In the next step, depicted in FIG. 4 b,  one or more insulating conduits  220  are formed in the bare pedestal  106  for providing an insulated path between the electrode layer  212  and the bottom of the pedestal  106 . In FIG. 4 c,  the bare pedestal  106  is coated with an insulating and adhesive layer  204 . Preferably, the insulating and adhesive layer  204  is an ordinary polyimide paste approximately 30-300 μm thick. The coating process can be performed by any means known in the art of electrostatic chuck manufacturing including screen printing and the like. In Step  4   d,  an electrode layer  212  is disposed over a portion  226  of the insulating and adhesive layer  204 . In step  4   e,  a workpiece support layer  216  is applied over the electrode layer  212  and a portion  228  of insulating and adhesive layer  204  not covered by the electrode layer  212 . Preferably, this workpiece support layer  216  is the doped polyimide paste described earlier and is applied to a thickness of 20-400 μm. This workpiece support layer  216  effectively affixes and encapsulates the electrode layer  212  thereby preventing the electrode from contacting any other layers and forms a support surface upon which a workpiece ( 102  of FIG. 2 b ) is retained. 
     In step  4   f,  the entire pedestal/chuck assembly  106 / 202  is placed in a furnace for curing. For example, the assembly is cured at a temperature of approximately 100-350° C. at a pressure of approximately 1 atm for approximately 30 min.-10 hrs. This final curing step cures the insulation and workpiece support paste layers  204  and  216  respectively. In step  4   g,  a plurality of grooves (see in FIG.  7  and described above) are formed in the workpiece support layer  216 . 
     Controlling the amount of dopant material is critical to the functionality of the inventive electrostatic chuck. Essentially, the quantity of dopant material must be controlled during the manufacturing of the doped polyimide sheet so as to yield the desired results. Specifically, the ratio of the weight of the dopant material versus the total weight of the improved polyimide is adjusted until the desired volume and surface resistivity are attained. FIG. 5 depicts a graph of volume resistivity vs. percentage by weight of dopant material in the improved polyimide. As a greater amount of dopant material is added to the improved polyimide, the resistivity drops. By increasing the dopant level to 50% by weight of the total improved polyimide, it is possible to reduce the resistivity level by a factor of 10 4 . FIG. 6 depicts a graph of surface resistivity vs. percentage by weight of dopant material in the improved polyimide. Similar reductions in this characteristic are also realized as the dopant level approaches 50% by weight of the total improved polyimide. The dark points on each graph represent post-cured resistivity and the white points represent pre-cured resistivity. 
     Table 1 displays important dielectric characteristics of ordinary and improved polyimide as the level of dopant material is increased. As seen from this table, there is approximately a 30% drop in the modulus of elasticity, extension and strength of ordinary polyimide as dopant levels are increased through 50% by weight. The reduced levels of these characteristics are not considered to be detrimental to the chuck&#39;s longevity. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Mechanical Characteristics of Improved Polyimide Film 
               
             
          
           
               
                   
                 Quantity of 
                 Modulus of 
                   
                   
               
               
                   
                 doped material 
                 elasticity 
                   
                 Strength 
               
               
                   
                 (wt %) 
                 (kgf/mm2) 
                 Extension (%) 
                 (kgf/mm2) 
               
               
                   
                   
               
             
          
           
               
                 Ordinary 
                 0 
                 750 
                 18 
                 35 
               
               
                 polyimide 
               
               
                 Improved 
                 25 
                 530 
                 16 
                 27 
               
               
                 polyimide 1 
               
               
                 Improved 
                 40 
                 515 
                 13 
                 24 
               
               
                 polyimide 2 
               
               
                 Improved 
                 50 
                 520 
                 10 
                 24 
               
               
                 polyimide 3 
               
               
                   
               
             
          
         
       
     
     In sum, the above described method and apparatus provide an electrostatic chuck with altered physical properties of a top layer (i.e., the layer that supports the wafer) of polyimide to establish an improved chucking force to retain a substrate material (i.e., a semiconductor wafer). Specifically, ordinary polyimide is doped to alter the mechanism of attraction from Coulombic force to the Johnsen-Rahbek effect. The resultant device is capable of retaining a wafer with a greater attractive force. As such, a greater thermal transfer gas flow and pressure can be attained under the wafer without the wafer popping off of the chuck due to an inadequate chucking force. The increased thermal transfer gas conditions promote greater temperature control of the wafer and more accurate temperature uniformity during wafer processing. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.