Abstract:
An electrostatic chuck for holding a substrate has a circular dielectric member having a top surface configured to support the substrate, the top surface having a plurality of mesas consisting of n subsets, wherein mesas of each subset are distributed along one of a plurality of concentric bolt circles of increasing radii, and wherein all of the concentric bolt circles center about the center of the circular dielectric member.

Description:
BACKGROUND 
       [0001]    Apparatuses and methods consistent with the present invention relate generally to an electrostatic chuck apparatus for holding a substrate. 
         [0002]    Chucks are devices that can be used to stabilize and hold various objects, such as semiconductor substrates, while processing is performed. There are several different types of chucks, such as mechanical chucks, vacuum chucks, or electrostatic chucks. 
         [0003]    Electrostatic chucks stabilize and hold an object by employing the attractive force, e.g., columbic force, between oppositely charged surfaces to hold the object and the chuck together. Electrostatic chucks can be used to perform a wide variety of functions such as holding silicon wafers in a process chamber for chemical and/or physical deposition apparatuses, as well as etching apparatuses. 
         [0004]    Electrostatic chucks have many advantages over mechanical and vacuum chucks. For instance, electrostatic chucks generally apply a more uniform force than mechanical chucks or vacuum chucks. Electrostatic chucks also reduce stress-induced cracks caused by the clamps utilized by mechanical chucks and allow processing of a larger portion of the substrate. Electrostatic chucks can also be used in processes conducted at low pressures. 
         [0005]    By way of example,  FIGS. 1A and 1B  illustrate a cross section of a typical electrostatic chuck  10  used for holding a substrate  30 . The chuck  10  comprises an electrode  15  embedded in dielectric  17 , a flat receiving surface  20 , and a cooling base  25 . When the electrode  15  is electrically charged, an opposing electrostatic charge accumulates in the substrate  30  and the resultant electrostatic force holds the substrate  30  on the receiving surface  20 . Once the substrate  30  is firmly held on the receiving surface  20 , plasma can be used to process the substrate  30 . 
         [0006]    Generally, during the processing of a semiconductor substrate, the substrate is repeatedly heated and cooled while undergoing various processing steps. Frequently, the processing steps, and particularly plasma processing, are performed in a vacuum chamber. However, because a vacuum does not provide heat conduction or convection, a vacuum environment provides limited heat removal from the substrate. 
         [0007]    Typically, it is important to control the temperature of the substrate while processing is performed. However, the thermal contact between the substrate and the chuck, without more, is generally insufficient to accommodate the heat load imposed by the plasma on the substrate. Without some mechanism of improved heat transfer between the substrate being processed and adjacent surfaces, the temperature of the substrate may exceed acceptable limits. Accordingly, a heat transfer medium, which is typically a gas such as helium, is often introduced between the substrate and the chuck to enhance thermal contact and heat transfer from the substrate to the chuck. However, introducing a heat transfer medium presents several problems in conventional electrostatic chucks. 
         [0008]    A first problem with conventional electrostatic chucks is that the need to introduce a heat transfer gas in the region between substrate and the chuck requires that some discontinuity be introduced in the chuck surface. For example, as shown in  FIG. 1 , some type of conduit  5  is typically formed through the surface of the electrostatic chuck  10  to a gas passage behind the surface of the electrostatic chuck  10 . One drawback of introducing such conduits, however, is that plasma arcs may form from the backside of the substrate down to the metal cooling base  25 . Such plasma arcs are undesirable because they can result in damage to the substrate and to the electrostatic chuck. 
         [0009]    A second problem with conventional electrostatic chucks is that to provide a spatially uniform conductive heat transfer from the substrate to the chuck, any heat transfer medium that is introduced must be uniformly distributed along the surface of the substrate that faces the chuck. 
         [0010]    In an attempt to address the aforementioned first problem of undesirable plasma arcing, conventional methods for reducing the likelihood of plasma arcing, include making the diameter of the conduits smaller, or increasing the thickness of the dielectric member. Additionally, plasma arcing can be reduced by moving the electrode farther away from the center of the conduit. On the other hand, if a conduit connects two metal surfaces, then such a configuration effectively increases the likelihood of plasma arcing due to the emission of free electrons from the metal surfaces. This limits the amount of RF power that may be delivered to the substrate. This limitation on power limits the etch rate and, thus, the throughput of the tool. 
         [0011]    More particularly, one of the functions of an electrostatic chuck is to deliver both DC and RF power to the substrate and to the plasma in the chamber. This power delivery creates electric fields, which permeate the dielectrics and conduits which comprise the majority of the structure of an electrostatic chuck. These electric fields can provide energy to free electrons within the conduits which can then, in turn, impart energy to the backside heat transfer gas. This process can lead to ionization of the backside heat transfer gas which can: (1) undesirably heat the backside heat transfer gas, or (2) create a breakdown or catastrophic arc within a conduit. 
         [0012]      FIG. 2  illustrates a simulation representing the electric fields that exist with a conduit and a dielectric of an electrostatic chuck. In particular,  FIG. 2  shows the electric fields for a RF peak-to-peak voltage of 4,000 V, a chucking voltage of 500 V, and a grounded cooling base. The resulting electric fields change somewhat over time and, therefore,  FIG. 2  shows a point in the RF cycle equivalent to the substrate potential (DC and RF) of zero. 
         [0013]    The breakdown of backside heat transfer gas can occur for many reasons. A primary reason that such breakdown can occur is that free electrons gain sufficient energy from the electric fields permeating the conduits. Such energized electrons can then ionize the backside heat transfer gas. 
         [0014]    There are several possible options for minimizing the likelihood of electrons gaining sufficient energy to ionize the backside heat transfer gas: (1) increase the frequency at which the electrons collide with non-electron emitting surfaces, (2) decrease the electric field permeating the conduits, (3) decrease electron collisions with backside gas molecules (decrease the pressure), (4) increase the collision frequency with backside gas molecules (increase the pressure), or (5) minimize the actual voltage drop that the electrons experience in the direction of the electric field. 
         [0015]    However, because the backside gas pressure is set by processing conditions, the options of controlling electron energy by increasing or decreasing collision frequency with the backside gas within the conduit are problematic. As will be understood by those of ordinary skill in the art, the theoretical relationship for the direct current breakdown voltage of two parallel-plate electrodes immersed in a gas, as a function of the gas pressure and electrode separation, is called the Paschen curve. As illustrated in  FIG. 3 , for example, a typical Paschen Curve for helium indicates a minimum voltage of approximately 150 V at p·d of 40 Torr.mm. 
         [0016]    The processing conditions dictate that tool operation occur near the lowest part of the Paschen Curve. Consequently, the options of (3) decreasing electron collisions with backside gas molecules (decreasing the pressure), or (4) increasing the collision frequency with backside gas molecules (increasing the pressure) are not practical. Thus, only options (1) increasing the frequency at which the electrons collide with non-electron emitting surfaces, (2) decreasing the electric field permeating the conduits, and (5) minimizing the actual voltage drop that the electrons experience in the direction of the electric field, are practical options for minimizing the likelihood of electrons gaining sufficient energy to ionize the backside heat transfer gas. 
         [0017]    One possibility for decreasing the electric field permeating the conduits, i.e., for achieving option (2), involves increasing the length of the conduits, since increasing the length of the conduits typically minimizes the electric field permeating the conduits due to the fixed geometric conditions of the substrate relative to the cathode. That is, for a given voltage between the substrate and the cooling base, the electric field that permeates the conduits may be reduced by simply increasing the length of the conduits (i.e., increasing the thickness of the dielectric between the substrate and the cooling base). 
         [0018]    However, if increasing the length of the conduits does not, in fact, decrease the electric field, then simply increasing the length of the conduits may lead to an actual increase in the likelihood of ionizing the backside gas due to the increased p·d product for various gases, as shown below in Table 1: 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Minimum Breakdown Potentials for Various Gases 
               
             
          
           
               
                   
                 Gas 
                 Vs min (V) 
                 p · d at Vs min (Torr mm) 
               
               
                   
                   
               
             
          
           
               
                   
                 Air 
                 327 
                 5.67 
               
               
                   
                 Ar 
                 137 
                 9 
               
               
                   
                 H 2   
                 273 
                 11.5 
               
               
                   
                 He 
                 156 
                 40 
               
               
                   
                 CO 2   
                 420 
                 5.1 
               
               
                   
                 N 2   
                 251 
                 6.7 
               
               
                   
                 N 2 O 
                 418 
                 5 
               
               
                   
                 O 2   
                 450 
                 7 
               
               
                   
                 SO 2   
                 457 
                 3.3 
               
               
                   
                 H 2 S 
                 414 
                 6 
               
               
                   
                   
               
               
                   
                 (data from Naidu, M. S. and Kamaraju, V., High Voltage Engineering, 2nd ed., McGraw Hill, 1995, ISBN 0-07-462286-2). 
               
             
          
         
       
     
         [0019]    In addition, the diameter of the electrode exclusion around the conduit may also dictate the actual maximum electric field in the conduit. 
         [0020]    Thus, there are many limitations with respect to increasing the length of the conduits (i.e., increasing the thickness of the dielectric material containing the conduit) based on RF delivery, cost of the dielectric material, and the physical limitations of manufacturing. Further, there are also limitations with respect to the electrode exclusion based on chucking requirements. That is, the diameter of the electrode exclusion cannot be so large that the chucking force is lost. 
         [0021]    Accordingly, if it is not practical to decrease the electric field, the remaining options for minimizing the likelihood of electrons gaining sufficient energy to ionize the backside heat transfer gas are (1) increasing the collisions of the electrons with non-electron emitting surfaces and (5) minimizing the actual voltage drop that the electrons experience in the direction of the electric field. In particular, if the electric field cannot be practically decreased, then the likelihood of electrons gaining sufficient energy for ionization can be minimized by reducing the distance traveled by the electron as provided by Expression 1: 
         [0000]    
       
         
           
             
               
                 
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                        
                       
                         
                           E 
                           → 
                         
                         · 
                         
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                             s 
                             → 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Expression 
                      
                     
                         
                     
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                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0022]    One way to minimize the energy gain of the electrons is to minimize the diameter of the conduits and to thereby increase the likelihood of the electrons colliding with the walls of the conduit (thus minimizing energy gain). While this technique is helpful to minimize backside gas ionization, the efficacy of this technique is nevertheless restricted by manufacturing limitations based on the aspect ratio of the conduit (aspect ratio=length/diameter). Further, laser drilling techniques may be employed, for example, to create conduits having small diameters, but such laser drilling techniques are quite expensive. 
         [0023]    Thus, in view of these manufacturing limitations, there is a need for an electrostatic chuck for holding a substrate that minimizes the likelihood of plasma arcing and ionization of the backside heat transfer gas. In particular, there is a need for an electrostatic chuck which increases the likelihood of electrons colliding with the walls of the conduit and thereby minimizes energy gain of the electrons. There is also a need for an electrostatic chuck which minimizes the actual voltage drop that the electrons experience in the direction of the electric field. 
         [0024]    Turning next to the second problem discussed above-that of providing a spatially uniform conductive heat transfer from the substrate to the chuck, by introducing a heat transfer medium that is uniformly distributed along the surface of the substrate that faces the chuck—this problem is particularly complicated. The thermal resistances across the interface between the substrate and the electrostatic chuck control both the absolute substrate temperature and substrate temperature uniformity. It is particularly desirable to provide temperature uniformity because features such as etch rate and selectivity are affected by substrate temperature during the plasma etching process. Moreover, non-uniform heat transfer can lead to local temperature non-uniformity on the substrate, thereby lowering yields. 
         [0025]    As such, both the uniform distribution of the heat transfer gas, as well as the surface morphology of the electrostatic chuck, are critical. Uniform heat transfer can be accomplished by balancing the following three heat transfer mechanisms: (1) uniform backside gas pressure distribution (gas conductance, h), (2) uniform solid contact (contact conductance, k) and (3) radiation. 
         [0026]    Hence, the design of the embossment pattern on the surface of the electrostatic chuck is very important to the uniform distribution of backside gas and is important to balancing the relationship between gas-phase heat transfer and solid contact heat transfer. In addition, the ability to adjust mesa contact area relative to the backside gas distribution, without a major redesign, improves the ability to test new electrostatic chuck designs rapidly. 
         [0027]    However, conventional embossment distributions can vary, as discussed below with reference to  FIGS. 4 and 5 . As shown in  FIG. 4 , for instance, conventional electrostatic chucks employing a hexagonal embossment pattern often have a non-uniform embossment distribution toward the edge of the substrate. Indeed, as shown in  FIG. 4 , conventional hexagonal embossment patterns form a series of aligned rows and columns and, therefore, do not provide for uniform embossment distribution at the circular boundaries (such as sealing bands and lift pin holes, for example). Further, as shown in  FIG. 5 , for example, conventional electrostatic chucks employing a linear embossment pattern also exhibit non-uniform embossment distribution. Thus, a drawback of conventional electrostatic chucks, such as those depicted in  FIGS. 4 and 5 , is that the non-uniform embossment distribution may cause non-uniform temperature distribution on the substrate. 
         [0028]    Accordingly, there is a need for an electrostatic chuck having a surface embossment pattern which effectively balances the uniform distribution of backside gas, gas-phase heat transfer and solid contact heat transfer. 
       SUMMARY 
       [0029]    The following summary is provided in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention, and as such it is not intended to particularly identify key or critical elements of the invention, or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
         [0030]    Exemplary embodiments of the present invention relate to an electrostatic chuck for holding a substrate that addresses many of the problems discussed above, and other needs which are not expressly mentioned above. Also, the present invention is not required to overcome the disadvantages described above, and an exemplary embodiment of the present invention may not overcome any of the problems described above. 
         [0031]    According to an aspect of the present invention, there is provided an electrostatic chuck apparatus for holding a substrate, the electrostatic chuck comprising a circular dielectric member having a top surface configured to support a substrate, the top surface comprising a plurality of mesas consisting of n subsets, wherein mesas of each subset are distributed along one of a plurality of concentric bolt circles of increasing radii, and wherein all of the concentric bolt circles center about the center of the circular dielectric member. The radius of each of the concentric bolt circle may be a multiple of the radius of the smallest of the concentric bolt circle. The distance between each of the mesas among a subset arranged at a circle of radius R n  and a closest neighboring mesa from among a subset arranged at a radial distance R n+1  may equal a constant distance l. Each of the plurality of mesas may be circular and have an equal diameter d. The radial distance R 1  of the smallest bolt circle may equal d+l. The radial distance R n  of each subset of mesas may equal n×R 1 . The total number of mesas m of a subset along concentric circle n may equal n×6. The electrostatic chuck may further comprise a single mesa at the center of the top surface. The plurality of mesas in each subset may be arranged such that a distance between each of the mesas and a closest neighboring mesa from among the same subset equals a constant value s. The electrostatic chuck may further comprise fluid conduits extending through the dielectric member, and wherein the conduits are symmetrically arranged about the top surface of the dielectric member. At least a section of the conduit may be arranged along an oblique angle to the top surface of the dielectric member. A section of the conduit may be arranged along an oblique angle comprises a laser-drilled passage. The electrostatic chuck may further comprise a laser drilled passage in fluid communication with the conduit. The electrostatic chuck may further compare at last one fluid conduit extending through the dielectric member and a plug situated inside the conduit, the plug having elongated fluid passages having an axis at an oblique angle to the top surface. The electrostatic chuck may further comprise: at last one fluid conduit extending through the dielectric member; a first plug situated inside a segment of the conduit, the first plug having elongated fluid passages having an axis at a first oblique angle to the top surface; and, a second plug situated inside a segment of the conduit, the second plug having elongated fluid passages having an axis at a second oblique angle to the top surface, the second angle being different from the first angle. The electrostatic chuck may further comprise at last one fluid conduit extending through the dielectric member and a plurality of plugs situated eccentrically inside a segment of the conduit, each plug being offset from a neighboring plug, thereby enabling fluid passage about periphery of the plug. 
         [0032]    According to other aspects of the invention, a method of fabricating an electrostatic chuck is provided, comprising: fabricating a circular dielectric member having a flat top surface; embossing on the top surface a plurality of mesas, the mesas arranged along a plurality of concentric bolt circles of increasing radii, wherein all of the concentric bolt circles center about the center of the circular dielectric member. The radius of each of the concentric bolt circle may be a multiple of the radius of the smallest of the concentric bolt circle. The total number of mesas, m, provided along a concentric circle n may equal n×6, wherein n is the number of circle counting from the inner most circle outwards. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The accompanying drawings, which are incorporated in, and constitute a part of, this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
           [0034]    The aspects of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which: 
           [0035]      FIG. 1  illustrates a straight conduit formed in an electrostatic chuck according to the related art; 
           [0036]      FIG. 2  illustrates a simulation of electric fields permeating an electrostatic chuck; 
           [0037]      FIG. 3  illustrates Paschen curves for different electrode materials in helium; 
           [0038]      FIG. 4  shows a related art electrostatic chuck surface employing a hexagonal embossment pattern; 
           [0039]      FIG. 5  shows a related art electrostatic chuck surface employing a linear embossment pattern; 
           [0040]      FIG. 6  shows an electrostatic chuck apparatus comprising an angled conduit, consistent with an exemplary embodiment of the present invention; 
           [0041]      FIG. 7  shows an electrostatic chuck apparatus comprising an angled laser drilled passage, consistent with an exemplary embodiment of the present invention; 
           [0042]      FIG. 8  shows an electrostatic chuck apparatus comprising an angled laser drilled passage and an angled conduit, consistent with an exemplary embodiment of the present invention; 
           [0043]      FIG. 9A  shows an electrostatic chuck apparatus comprising a plug inserted into a conduit, consistent with an exemplary embodiment of the present invention, while  FIG. 9B  illustrates an embodiment similar to that of  FIG. 9A , except that the laser drilled hole is not oblique; 
           [0044]      FIG. 10  shows a top surface of a plug, which is inserted into a conduit of an electrostatic chuck apparatus, consistent with an exemplary embodiment of the present invention; 
           [0045]      FIG. 11  shows a perspective illustration of the top surface, bottom surface and the exterior surface of a plug, which is inserted into a conduit of an electrostatic chuck apparatus, consistent with an exemplary embodiment of the present invention; 
           [0046]      FIG. 12  shows an electrostatic chuck apparatus comprising a plurality of plugs inserted into a conduit, consistent with an exemplary embodiment of the present invention; 
           [0047]      FIG. 13  shows a top surface of a top plug among the plurality of plugs shown in  FIG. 12 , consistent with an exemplary embodiment of the present invention; 
           [0048]      FIG. 14  shows a bottom surface of a bottom plug among the plurality of plugs shown in  FIG. 12 , consistent with an exemplary embodiment of the present invention; 
           [0049]      FIG. 15A  shows an electrostatic chuck apparatus comprising a plurality of eccentric plugs inserted into a conduit, while  FIG. 15B  is a perspective schematic of the top eccentric plug, consistent with an exemplary embodiment of the present invention; 
           [0050]      FIG. 16  shows a center portion of a receiving surface of an electrostatic chuck comprising a plurality of embossments or mesas consistent with an exemplary embodiment of the present invention; 
           [0051]      FIG. 17  shows an enlarged view of the center portion of  FIG. 16 , illustrating a receiving surface of an electrostatic chuck comprising a plurality of embossments or mesas consistent with an exemplary embodiment of the present invention; 
           [0052]      FIG. 18  shows an edge portion of a receiving surface of an electrostatic chuck comprising a plurality of embossments consistent with an exemplary embodiment of the present invention; 
           [0053]      FIG. 19  shows a receiving surface of an electrostatic chuck comprising a plurality of embossments consistent with an exemplary embodiment of the present invention, and illustrates the relative locations of the center portion shown in  FIG. 16  and the edge portion shown in  FIG. 18 ; 
           [0054]      FIG. 20  shows a receiving surface of an electrostatic chuck comprising a plurality of embossments consistent with an exemplary embodiment of the present invention including an exemplary arrangement conduits, a lift pin hole, and a backside gas channel. 
           [0055]      FIG. 21  illustrates a processing chamber according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0056]    Reference will now be made in detail to exemplary embodiments of the present invention, which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The exemplary embodiments provided below are intended in all respects to be exemplary only, with the true scope and spirit of the invention being defined by the following claims. 
         [0057]    As shown in  FIG. 6 , an exemplary electrostatic chuck apparatus  110  consistent with the present invention is provided in a process chamber (not shown). As shown in  FIG. 6 , the electrostatic chuck apparatus  110  comprises a receiving surface  120  for receiving a substrate  130 , such as a semiconductor wafer. The electrostatic chuck apparatus  110  can be used to securely hold the substrate  130  during various processing steps. The electrostatic chuck apparatus  110  further comprises a DC/RF electrode  115 , which is disposed inside a dielectric member  117 . Moreover, a cooling base  125  and an optional porous pill  127  are disposed below the dielectric member  117 . 
         [0058]    To operate the electrostatic chuck apparatus  110 , a desired voltage is applied to the electrode  115  to electrostatically hold the substrate  130  to the receiving surface  120 . In general, the axis  147  of the electric field resulting from the electrode  115  is roughly perpendicular to the receiving surface  120 , as illustrated in  FIG. 6 . This is because the electrode  115  generates electrostatic forces which operate in a direction to electrostatically hold the substrate  130  to the receiving surface  120 . 
         [0059]    Additionally, the process chamber (not shown) comprises a process gas, which is energized to form a plasma by coupling RF energy to the process gas. As RF energy is applied to the plasma, the plasma is energized and charged particles are accelerated toward the substrate  130 , which is held on the receiving surface  120  by electrostatic forces, to thereby process the substrate  130 . 
         [0060]    A heat transfer gas, such as Helium, is provided to enhance heat transfer rates between the substrate  130  and the electrostatic chuck apparatus  110 . For example, as shown in  FIG. 6 , a laser drilled passage  150  is provided in the dielectric member  117 . The laser drilled passage  150  connects to an angled conduit  160 , which extends along an axis different from the axis of the electric field  147 . In other words, the angled conduit  160  extends at an oblique non-normal or non-orthogonal angle to the receiving surface  120 . A heat transfer gas, is delivered from the cooling base  125 , through the angled conduit  160  and the laser drilled passage  150 , to the interface between the receiving surface  120  and the substrate  130 . In the context of this invention, the term oblique is used in its normal and widely accepted manner, e.g., neither perpendicular nor parallel to a given line or surface. 
         [0061]    According to the exemplary embodiment shown in  FIG. 6 , an optional porous pill  127  is disposed between the dielectric member  117  and the cooling base  125 . The porous material of which the porous pill  127  is composed allows backside gas to flow through to the angled conduit  160  without exposing the angled conduit  160  to the metal cooling base  125 , which delivers the backside gas. If the porous pill  127  is not used, then the backside gas is delivered directly to the conduit  160 . 
         [0062]    Importantly, as shown in  FIG. 6 , the angled conduit  160  is angled off-axis relative to the axis  147  of the electric field. In other words, the angled conduit  160  extends along an axis that is different from the axis  147  of the electric field. Because the angled conduit  160  is angled in this way, the likelihood of free electrons from the energized plasma colliding with a non-electron emitting surface is increased. In addition, such an angled configuration of the angled conduit  160  decreases the distance traveled by such free electrons that are being accelerated by the electric field. Accordingly, angling the angled conduit  160  off-axis relative to the axis  147  of the electric field decreases the likelihood of the free electrons gaining sufficient energy to ionize the backside heat transfer gas and thereby helps to minimize plasma arcing and backside gas ionization. 
         [0063]    According to another exemplary embodiment consistent with the present invention, as shown in  FIG. 7 , an electrostatic chuck  210  comprises a receiving surface  220  for receiving the substrate  230 , an electrode  215  disposed in a dielectric member  217 , a cooling base  225  and an optional porous pill  227 . Further, as shown in  FIG. 7  the angled laser drilled passage  250  provided in the dielectric member  217  is angled off-axis relative to the axis  247  of the electric field, while the conduit  260  is not angled in such a manner. That is, as shown in  FIG. 7 , the angled laser drilled passage  250  extends along an oblique axis that is different from the axis  247  of the electric field. 
         [0064]    Laser drilling of the dielectric member  217  creates an angled laser drilled passage  250  having a smaller diameter than passages formed with other techniques. Such a smaller diameter of the angled laser drilled passage  250  helps to decrease the potential of ionizing backside gas. Additionally, much like the angled conduit  160  described above, since the angled laser drilled passage  250  is angled off-axis relative to the axis  247  of the electric field, the likelihood of free electrons from the energized plasma colliding with a non-electron emitting surface is increased and the distance traveled by such free electrons is decreased. Accordingly, by angling the angled laser drilled passage  250  off-axis relative to the axis  247  of the electric field, the likelihood of free electrons gaining sufficient energy to ionize the backside heat transfer gas is decreased and the likelihood of plasma arcing and backside gas ionization is reduced. 
         [0065]    As described in the exemplary embodiments provided above, either a conduit or a laser drilled passage connected to a conduit can be angled off-axis relative to the axis of the electric field to help minimize plasma arcing and backside gas ionization. However, the present invention is not limited to these two exemplary configurations. To the contrary, according to the present invention, both components of the conduit and the laser drilled passage may be angled off-axis relative to the axis of the electric field to help minimize plasma arcing and backside gas ionization. 
         [0066]    For instance, according to an exemplary embodiment of the present invention, as shown in  FIG. 8 , an electrostatic chuck  211  comprises a receiving surface  221  for receiving a substrate  231 , an electrode  216  disposed in a dielectric member  218 , a cooling base  226  and an optional porous pill  228 . As shown in  FIG. 8 , the angled laser drilled passage  251  provided in the dielectric member  218  is angled off-axis relative to the axis  248  of the electric field. In addition, the angled conduit  261  provided in the dielectric member  218  is also angled off-axis relative to the axis  248  of the electric field. Thus, as shown in  FIG. 8 , both the angled laser drilled passage  251  and the angled conduit  261 , extend along axes that are different from the axis  248  of the electric field. 
         [0067]    According to another exemplary embodiment of an electrostatic chuck  310  consistent with the present invention, as depicted in  FIGS. 9A and 9B , an electrostatic chuck  310  comprises a receiving surface  320  for receiving the substrate  330 , an electrode  315  disposed in a dielectric member  317 , and a cooling base  325 . Further, as shown in  FIGS. 9A and 9B , a plug  303  may be inserted into a conduit  360 , which is connected to a laser drilled passage  350 . In the embodiment of  FIG. 9A  the laser drilled passage  350  is angled off-axis relative to the axis  347  of the electric field, while in the embodiment of  FIG. 9B  the laser drilled passage  350  is orthogonal to the axis  347  of the electric field. 
         [0068]    As shown in  FIGS. 9A and 9B , the plug  303  comprises a plurality of exterior channels  307  extending along the exterior surface of the plug  303 . As shown in  FIGS. 9A and 9B , the exterior channels  307  extend from a top surface  308  of the plug  303  to a bottom surface  309  of the plug  303 . Moreover, the exterior channels  307  are arranged such that the top of each respective channel  307  does not align with the bottom of each respective channel  307 . In other words, the exterior channels  307  are angled off-axis relative to the axis  347  of the electric field. It will be understood by those of ordinary skill in the art that the exterior channels  307  can be formed in the surface of the plug  303  by a variety of different methods known in the art. 
         [0069]    As shown in  FIGS. 9A and 9B , the plug  303  is substantially cylindrical in shape, however, the present invention is not limited to this particular embodiment and the plug  303  may comprise a variety of different shapes. The shape of the plug  303  may also have substantially the same shape as the conduit  360 , and may correspond to the shape of the conduit  360  such that an exterior surface  372  of the plug  303  abuts the surface of the conduit  360 . 
         [0070]      FIG. 10  shows a top surface  308  of the plug  303 , consistent with an exemplary embodiment of the present invention. As shown in  FIG. 10 , a plurality of top channels  313  are arranged on the top surface  308 . More particularly, the plurality of top channels  313  extend radially from a center  390  of the top surface  308  to a perimeter of the top surface  308 . 
         [0071]      FIG. 11  shows a perspective view of the top surface  308 , a bottom surface  309  and the exterior surface  372  of the plug  303 . As illustrated in  FIG. 11 , each of the top channels  313  communicates with a respective exterior channel  307 . Likewise, a plurality of bottom channels  314  are arranged on the bottom surface  309  and these bottom channels  314  extend radially from a center  395  of the bottom surface  309  to a perimeter of the bottom surface  309 . As shown in  FIG. 11 , each of the bottom channels  314  communicates with a respective exterior channel  307 . 
         [0072]    Thus, according to the exemplary plug  303  illustrated in  FIGS. 8-10 , a heat transfer gas is delivered from the cooling base  325 , to the bottom channels  314 , and the heat transfer gas travels through the bottom channels  314  to the exterior channels  307 . The heat transfer gas then travels through the exterior channels  307  to the top channels  313 . The heat transfer gas travels through the top channels  313  to the center  390 , and then through the laser drilled passage  350 , to the interface between the receiving surface  320  and the substrate  330 . 
         [0073]    According to this exemplary embodiment, the total flow rate of the heat transfer gas can be adjusted by changing the sizes and number of the exterior channels  307 , top channels  313  and bottom channels  314 . Further, while the exemplary embodiment shown in  FIGS. 8-10  depicts a plug  303  having a particular number of exterior channels  307  and top channels  260 , the present invention is not limited thereto and a wide variety of plugs of different sizes and shapes, having different numbers of channels of different sizes may be used consistent with the present invention. 
         [0074]    Moreover, consistent with the present invention, the diameters of the exterior channels  307  can be minimized to increase the likelihood of electrons colliding with the walls of the exterior channels  307 . As a result, the energy gain of such electrons is minimized and the likelihood of backside gas ionization is reduced. 
         [0075]      FIG. 12  illustrates yet another exemplary embodiment of the present invention. As shown in  FIG. 12 , an electrostatic chuck  410  comprises a receiving surface  420  for receiving the substrate  430 , an electrode  415  disposed in a dielectric member  417 , and a cooling base  425 . According to the exemplary embodiment shown in  FIG. 12 , a plurality of plugs  400  are inserted into the conduit  430 , and the plugs  400  are stacked on top of each other to form a stack of plugs. Each of the plugs  400  further comprises a plurality of exterior channels  407  through which a heat transfer gas is provided. As shown in  FIG. 12 , the bottoms of the exterior channels  407  of a top plug, are aligned with the tops of the exterior channels  407  of the plug immediately below, such that the heat transfer gas can be delivered from the exterior channels  407  of the bottommost plug in the stack to the exterior channels  407  of the uppermost plug in the stack. 
         [0076]      FIG. 13  shows a top surface  408  of the top plug  401  from among the plurality of plugs  400  shown in  FIG. 12 . As shown in  FIG. 13 , the top surface  408  comprises a plurality of top channels  413 . More particularly, the plurality of top channels  413  extend radially from a center  490  of the top surface  408  to a perimeter of the top surface  408 . 
         [0077]      FIG. 14  shows a bottom surface  409  of the bottom plug  403  from among the plurality of plugs  400  shown in  FIG. 12 . As illustrated in  FIG. 14 , a plurality of bottom channels  414  are arranged on the bottom surface  409  and these bottom channels  414  extend radially from a center  495  of the bottom surface  409  to a perimeter of the bottom surface  409 . Each of the bottom channels  414  communicates with a respective exterior channel  407  of the bottom plug  403 . 
         [0078]    Thus, according to the exemplary embodiment illustrated in  FIGS. 11-13 , a heat transfer gas is delivered from the cooling base  425 , to the bottom channels  414 , and the heat transfer gas travels through the bottom channels  414  to the exterior channels  407  of the plurality of plugs  400 . Beginning with the exterior channels  407  of the bottom plug  403 , the heat transfer gas then travels through the exterior channels  407  of the plurality of plugs  400  and finally through the exterior channels  407  of the top plug  401 , to the top channels  413 . The heat transfer gas travels through the top channels  413  to the center  490 , and then through the angled laser drilled passage  450 , to the interface between the receiving surface  420  and the substrate  430 . 
         [0079]    Although  FIG. 12  illustrates that the plugs  400  are substantially circular in shape, a wide variety of different shaped plugs  400  may be employed consistent with the present invention. Further, as shown in  FIG. 12 , the laser drilled passage  450  is angled off-axis relative to the axis  437  of the electric field so as to reduce the likelihood of backside gas ionization. The exemplary embodiment shown in  FIG. 12 , among other advantages, provides for long path lengths and addresses the aspect ratio limitations of laser drilling. 
         [0080]      FIGS. 15A and 15B  show an electrostatic chuck apparatus  510  according to another exemplary embodiment of the present invention. As shown in  FIGS. 15A and 15B , an electrostatic chuck  510  comprises a receiving surface  520  for receiving the substrate  530 , an electrode  515  disposed in a dielectric member  517 , and a cooling base  525 . Moreover, as shown in  FIG. 15A , the angled laser drilled passage  550  is angled off-axis relative to the axis of the electric field. 
         [0081]    As shown in  FIG. 15A , a plurality of eccentric plugs  500  are inserted into a ceramic sleeve  565 . The top eccentric plug  501  is centrally positioned to provide annular passage for backside gas flow. Further, this passage can be minimized so as to increase the likelihood of the electrons colliding with non-electron emitting surfaces. The upper part of the top eccentric plug  501  may be provided with spacers  503  to enable fluid flow to the fluid passage  550 . The design of the exemplary electrostatic chuck apparatus  510  shown in  FIGS. 15A and 15B  also enables continuous backside gas flow from the cooling base  525  to the backside of the substrate  530  by having eccentric plugs minimizing the distance traveled by free electrons and thereby reducing the risk of backside gas breakdown. 
         [0082]      FIG. 16  illustrates a receiving surface  620  of an electrostatic chuck  610 , consistent with another exemplary embodiment of the present invention. As shown in  FIG. 16 , a center portion of the receiving surface  620  comprises a plurality of embossments or mesas  600 .  FIG. 18  shows an edge portion of the receiving surface  620  comprising a plurality of embossments or mesas  600 . Further,  FIG. 19  shows the relative location of the center portion depicted in  FIG. 16  on the receiving surface  620  and the relative location of the edge portion depicted in  FIG. 18  on the receiving surface  620 . 
         [0083]    As shown in  FIG. 16 , the plurality of embossments or mesas  600  are arranged in a symmetrical geometric layout of concentric circles about the center C of the receiving surface  620 , so as to form a plurality of bolt circles BC 1 , BC 2 , BC 3 . . . BC n . More particularly, as illustrated in  FIG. 17 , which shows an enlarged section of the center portion shown in  FIG. 16 , a first subset of the plurality of mesas  600  are arranged at a radial distance R 1  from the center C of the receiving surface  620 , so as to form a first bolt circle BC 1 . Similarly, a second subset of the plurality of mesas  600  are arranged at a radial distance R 2  from the center C of the receiving surface  620 , so as to form a second bolt circle BC 2 . Thus, both the first bolt circle BC 1  and the second bolt circle BC 2  are concentric circles formed about the center C, wherein the first bolt circle BC 1  has a radius R 1  and the second bolt circle BC 2  has a radius R 2 . 
         [0084]    According to the exemplary embodiment shown in  FIGS. 16-18 , each of the embossments  600  has the same diameter d. Since there is a relationship between total embossment area and the contact area of the electrostatic chuck  610 , adjusting the embossment diameter d provides for lower or higher contact area of the electrostatic chuck apparatus  610 . 
         [0085]    Further, the distance between each of the embossments comprising the first bolt circle BC 1  and a closest neighboring embossment from among the embossments comprising the second bolt circle BC 2 , equals a distance l. As shown in  FIG. 17 , the embossments are arranged such that the radial distance R 1  of the first bolt circle BC 1  equals the diameter of each embossment d+the distance l. On the other hand, the embossments are also arranged such that the radial distance R 2  of the second bolt circle BC 2  equals 2 multiplied by the radial distance R 1 . 
         [0086]    Although the above description has set forth one exemplary embodiment comprising two bolt circles BC 1  and BC 2 , the embossments may be arranged to include any number of additional bolt circles consistent with the present invention. For example, as shown in  FIG. 17 , the embossments may be arranged such that the radial distance R 3  of a third bolt circle BC 3  equals 3 multiplied by the radial distance R 1 . That is, according to various exemplary embodiments of the present invention, the embossments may be arranged on the receiving surface  620  such that the radial distance R n  each bolt circle BC n  equals n multiplied by the radial distance R 1 . 
         [0087]    According to an exemplary embodiment of the present invention, the embossments may be arranged on the receiving surface  620  such that a total number of embossments m on the receiving surface  620  equals n×6. Moreover, as shown in  FIG. 17 , the embossments within each bolt circle may be arranged equidistant from each other. For example, each of the embossments comprising the first bolt circle BC 1  may be arranged such that the distance between each of the embossments in the first bolt circle BC 1  and a closest neighboring embossment, from among the embossments in the first bolt circle BC 1 , is a distance f Further, each of the embossments comprising the second bolt circle BC 2  may be arranged such that the distance between each of the embossments in the second bolt circle BC 2  and a closest neighboring embossment, from among the embossments in the second bolt circle BC 2 , is a distance s. According to an exemplary embodiment of the present invention, the embossments can also be arranged such that the distance f equals the distance s. 
         [0088]    The various arrangements of embossments described above provide for a uniform geometric layout as particularly illustrated in  FIG. 19 . That is, as shown in  FIG. 19 , the embossments  600  are uniformly distributed. Accordingly, when maximum contact is desirable, the present invention provides for “close packing” or the densest possible arrangement of mesas. On the other hand, when lesser contact is desirable, the present invention provides that the locations of the mesa centers will not change. 
         [0089]    In addition to the advantages of the symmetrical arrangement of embossments, as described above, the above concepts may also be applied to the arrangement of conduits with respect to the receiving surface  620 . That is, as shown in  FIG. 20 , the conduits  660  on the receiving surface  620  may be located such that the sources of heat transfer gas on the receiving surface  620  provide for a uniform pattern within the symmetry of the embossments  600 . For instance, the conduits  660  may be arranged such that no point on the receiving surface  620  is any farther away from a heat transfer gas source than any other point. 
         [0090]    As shown in  FIG. 20 , the receiving surface  620  further comprises lift pin holes  623  for receiving lift pins (not shown) that are raised and lowered by a pneumatic lift mechanism, for instance, so as to raise or lower the substrate  630  from/to the receiving surface  620 . As shown in  FIG. 20 , the receiving surface  620  also comprises a seal band  624 . 
         [0091]      FIG. 21  illustrates a processing chamber  200  according to an embodiment of the invention. The processing chamber may be utilized to process semiconductor wafers for the fabrication of microchips. In this context, the chamber  200  may be, for example, a plasma chamber for etching of semiconductor wafers. The chamber  200  includes a base  225  upon which the electrostatic chuck  211  is positioned. The electrostatic chuck includes an electrode  215  which is biased by power supplier V. Backside cooling gas from source H is provided to cooling fluid conduits  260 . The fluid conduits may be implemented according to any of the inventive conduits described above. The top surface  220  of the chuck  211  may includes mesas arranged according to any of the embodiments described above. It should be noted, however, that the use of the inventive fluid conduits and inventive mesa arrangement is independent of each other. That is, chuck  211  may include fluid conduits according to the subject invention while having a conventional embossments. On the other hand, the chuck  211  may include conventional fluid conduit, while utilizing embossments according to the subject invention. Of course, the chuck  211  may include both the inventive fluid conduits and embossments. 
         [0092]    Exemplary embodiments of the present invention employing an embossment pattern, as explained above, provide a mechanism for morphing a plurality of embossments to fit into a circular geometry. Such a configuration reduces the non-uniformity around any circular feature on the surface of an electrostatic chuck. In addition to the mesa topology, the groove length and spacing may be aligned for optimal backside gas uniformity to improve heat transfer. The grooves may also follow a symmetry that provides for overall symmetry of the grooves, the conduits and the mesas. 
         [0093]    Exemplary embodiments employing an embossment pattern, as discussed above, also reduce the number of backside gas holes, thereby minimizing cost since it enables uniform gas distribution for substrate cooling by uniformly and minimally locating backside conduits across the electrostatic chuck surface. 
         [0094]    The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Various other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.