Abstract:
An improved substrate holder comprises an electrode supporting a focus ring and a substrate, an insulating member surrounding the electrode and focus ring, a ground member surrounding the insulating member, and a focus ring surrounding the substrate. The focus ring provides a RF impedance substantially equivalent to a RF impedance of the substrate. A method of processing a substrate utilizing the improved substrate holder reduces arcing between the edge of the substrate and the focus ring. The method comprises the steps of placing the focus ring on the electrode, placing the substrate on the electrode and processing the substrate. Additionally, a method of controlling a focus ring temperature and a substrate temperature utilizing the improved substrate holder comprises the steps of placing the focus ring on the electrode, placing the substrate on the electrode, clamping the focus ring and the substrate to the electrode using an electrostatic clamp, supplying heat transfer gas(es) to the space residing between the focus ring and the electrode, and the space between the substrate and the electrode, and controlling the temperature of the electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority and is related to U.S. application No. 60/363,284, filed on Mar. 12, 2002, the entire contents of which are herein incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Invention  
         [0003]     The present invention relates to substrate holders employed in plasma processing and more particularly to an improved substrate holder for plasma processing.  
         [0004]     2. Description of Related Art  
         [0005]     One area of plasma processing in the semiconductor industry which presents formidable challenges is, for example, the manufacture of integrated circuits (ICs). Demands for increasing the speed of ICs in general, and memory devices in particular, force semiconductor manufacturers to make devices smaller and smaller on the wafer surface. And conversely, while shrinking device sizes on the substrate is incurred, the number of devices fabricated on a single substrate is dramatically increased with further expansion of the substrate diameter (or processing real estate) from 200 mm to 300 mm and greater. Both the reduction in feature size, which places greater emphasis on critical dimensions (CD), and the increase of substrate size lead to even greater requirements on plasma processing uniformity to maximize the yield of superior devices.  
         [0006]     One such consequence of non-uniform plasma processing can be, for example, the unequal charging of the substrate surface in contact with the plasma and the focus ring surrounding the substrate. Using current focus ring design practice, the surface potential of the focus ring can be substantially different than the surface potential of the substrate. Subsequently, the difference in surface potential can lead to a non-uniform plasma sheath thickness and, therefore, result in non-uniform plasma properties proximate the substrate edge. Moreover, the difference in surface potential between the substrate edge and focus ring can be sufficiently great to cause an electrical discharge (arc) arising in a catastrophic process failure and reduced device yield.  
         [0007]     In a known plasma processing system, substrate arcing has been observed and can be attributable to the aforementioned focus ring design. For example,  FIG. 1  presents a known substrate holder  1  comprising a RF biasable electrode  10 , electrode insulator  12 , ground wall  14  with surface anodization  16 , and focus ring  18 . The substrate holder  1  further includes an electrostatic clamp (ESC)  20  in order to facilitate holding a substrate  22 . Although, not shown in detail in  FIG. 1 , the electrostatic clamp  20  typically comprises a clamp electrode encased within a ceramic body. The focus ring  18  is generally fabricated from a silicon-containing material such as, for example, silicon or silicon carbide, when processing silicon substrates. However, the material and size of the focus ring  18  can result in a low capacitance or corresponding high RF impedance leading to a surface potential substantially greater than the surface potential of the substrate  22 . As a consequence, the plasma sheath  30  can be substantially non-uniform, comprising a thin region  32  above the focus ring  18 , a thicker region  34  above the substrate  22  and a transitional region  36  existing therebetween.  
         [0008]     As stated above, the potential difference associated with the non-uniform plasma sheath can manifest as substrate arcing, hence, leading to catastrophic reduction in device yield. It is, therefore, desirable to achieve a uniform plasma sheath thickness across the substrate and the surfaces proximate the edge of the substrate.  
         [0009]     An additional shortcoming of current focus ring design practice includes a substantially different temperature between the substrate and the focus ring. In fact, it is not unrealistic to observe focus ring temperatures exceeding the substrate temperature by more than several hundred degrees centigrade. This observation is primarily attributable to the poor thermal contact between the focus ring and the temperature controlled electrode. As a consequence, the “hot” focus ring can heat the substrate edge leading to non-uniform substrate temperatures and, hence, non-uniform substrate processing particularly local to the substrate edge. It is, therefore, desirable to control the focus ring temperature as well as the substrate temperature.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides for an improved substrate holder for a plasma processing system in order to alleviate the aforementioned shortcomings of known substrate holders. The improved substrate holder comprises an electrode supporting a focus ring and a substrate on an upper surface thereof, an insulating member surrounding the electrode and focus ring, a ground member surrounding the insulating member, and a focus ring surrounding the substrate. The focus ring comprises a RF impedance substantially equivalent to a RF impedance of the substrate.  
         [0011]     It is a further object of the present invention to provide an improved substrate holder further comprising an electrostatic clamp, wherein the electrostatic clamp can serve as the upper surface of the electrode.  
         [0012]     It is a further object of the present invention to provide an improved substrate holder further comprising a heating and cooling system for controlling the temperature of the electrode.  
         [0013]     The present invention further describes a method of processing a substrate utilizing the improved substrate holder in order to minimize arcing between the edge of the substrate and the focus ring. The method comprises the steps of placing the focus ring on the electrode, placing the substrate on the electrode and processing the substrate.  
         [0014]     Additionally, the present invention describes a method of controlling a focus ring temperature and a substrate temperature utilizing the improved substrate holder. The method comprises the steps of placing the focus ring on the electrode, placing the substrate on the electrode, clamping the focus ring and the substrate to the electrode using an electrostatic clamp, supplying heat transfer gas(es) to a first space residing between the focus ring and the electrode, and a second space residing between the substrate and the electrode, and controlling a temperature of the electrode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     These and other objects and advantages of the invention will become more apparent and more readily appreciated from the following detailed description of the exemplary embodiments of the invention taken in conjunction with the accompanying drawings, where:  
         [0016]      FIG. 1  presents a schematic cross-section of a known substrate holder indicating a non-uniform plasma sheath;  
         [0017]      FIG. 2A  presents a schematic cross-section of an improved substrate holder according to an embodiment of the present invention;  
         [0018]      FIG. 2B  presents a schematic cross-section of an improved substrate holder according to another embodiment of the present invention;  
         [0019]      FIG. 2C  presents a schematic cross-section of an improved substrate holder according to another embodiment of the present invention;  
         [0020]      FIG. 3  presents a flow diagram for a method of minimizing arcing between a substrate and a focus ring according to a first embodiment of the present invention; and  
         [0021]      FIG. 4  presents a flow diagram for a method of controlling substrate and focus ring temperature according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0022]     The present invention relates to a substrate holder employed in plasma processing and more particularly to an improved substrate holder for plasma processing. According to the illustrated embodiment of the present invention depicted in  FIG. 2A , an improved substrate holder  100  can comprise an electrode  110 , an insulating member  112  and a ground member  114 . A focus ring  118 , comprising an upper surface  150 , a lower surface  152 , an outer surface  154  at an outer diameter and an inner surface  156  at an inner diameter, is coupled to an upper surface  140  of electrode  110 . The inner diameter of inner surface  156  of focus ring  118  is sufficiently large to accommodate substrate  122  and to center substrate  122  about an axis of revolution  111  of electrode  118 . Substrate  122  comprises an upper surface  160 , a bottom surface  162 , and an outer surface  164  at an outer diameter facing inner surface  156  of focus ring  118 . Substrate  122  is coupled to electrode  110  in such a way that bottom surface  162  of substrate  122  opposes upper surface  140  of electrode  110 .  
         [0023]     In order to preserve a uniform plasma sheath thickness  130  across both the upper surface  150  of focus ring  118  and the upper surface  160  of substrate  122  and, hence, a spatially homogeneous surface potential, focus ring  118  is designed and implemented as an electrical element comprising an RF impedance substantially similar to that of substrate  122 . In a first embodiment, focus ring  118  comprises, for example, at least one of silicon and silicon carbide when processing a substrate  122  comprising, for example, silicon. The material properties of focus ring  118  are specifically chosen to produce a RF impedance for focus ring  118  that is substantially equivalent to the RF impedance of substrate  122 . Focus ring  118  can comprise material properties such that its inherent capacitance, inductance and resistance are similar to that of substrate  122 . For example, focus ring  118  can comprise heavily doped silicon carbide when processing a substrate  122  comprising silicon. In an alternate embodiment, the upper surface  150  of focus ring  118  can comprise a shape other than flat, such as, for example, an inclined surface as shown in  FIGS. 2B and 2C . In an alternate embodiment (not shown), the upper surface  150  of focus ring  118  comprises at least one of a convex and a concave surface. Furthermore, the thickness of focus ring  118  is designed to be tailored to the thickness of substrate  122 . The thickness of substrate  122  can be, for example, 750 micron. In one embodiment, the focus ring has a thickness of 100 to 2000 microns. In another embodiment, the focus ring has a thickness substantially equivalent to the thickness of the substrate  122 . Exemplary thicknesses of the focus ring include, but are not limited to, (1) a thickness within 20% of the thickness of the substrate, (2) a thickness within 10% of the thickness of the substrate, (3) a thickness within 5% of the thickness of the substrate, and (4) a thickness within 1% of the thickness of the substrate. In an alternate embodiment, the thickness of focus ring  118  is substantially different than the thickness of substrate  122 .  
         [0024]     Electrode  110  can be, for example, generally cylindrical comprising an outer surface  144  at an outer diameter and an axis of rotation  111 . Additionally, electrode  110  can comprise aluminum and, therefore, it can be anodized, hence, comprising an anodization layer  142 , as depicted in  FIG. 2A . Desirably, the outer diameter of outer surface  144  of electrode  110  is substantially equivalent to outer diameter of outer surface  154  of focus ring  118 . In an alternate embodiment, the outer diameter of outer surface  144  of electrode  110  is different than the outer diameter of outer surface  154  of focus ring  118 .  
         [0025]     Insulating member  112 , can also be, for example, generally cylindrical comprising an inner surface  145  at an inner diameter, an outer surface  146  at an outer diameter and an axis of revolution  111 . Desirably, the inner surface  145  corresponds to an inner diameter substantially equivalent to the outer diameter of outer surface  144  of electrode  110 . Moreover, the inner diameter of inner surface  145  of insulating member  112  can be substantially equivalent to the outer diameter of the outer surface  154  of focus ring  118 . Therefore, insulating member  112  can comprise an inner edge  190  substantially flush with the outer surface  154  of focus ring  118  in order to serve as a means of centering focus ring  118  about axis of revolution  111 . In an alternate embodiment, insulating member  112  can comprise an inner surface  145  having an inner diameter different than the outer diameter of outer surface  154  of focus ring  118  and, therefore, allow an edge (or groove)  190  to be machined within the upper surface of insulating member  112  in order to serve the centering function described above. Preferably, insulating member  112  comprises a dielectric material such as, for example, quartz or alumina.  
         [0026]     Ground member  114  can also be, for example, generally cylindrical comprising an inner surface  147  at an inner diameter, an outer surface  148  at an outer diameter and an axis of revolution  111 . Desirably, the inner surface  147  corresponds to an inner diameter substantially equivalent to the outer diameter of outer surface  146  of insulating member  112 . Additionally, ground member  114  can comprise aluminum and, therefore, it can be anodized, hence, comprising an anodization layer  116 , as depicted in  FIG. 2A .  
         [0027]     Alternately, the substrate  122  can be, for example, affixed to the substrate holder  100  via an electrostatic clamp  120 . Electrostatic clamp  120  comprises a clamp electrode  121  connected to a high voltage (HV), direct current (DC) voltage source (not shown). Typically, the clamp electrode is fabricated from copper and embedded within a ceramic element. The electrostatic clamp  120  can be operable in either a monopolar or bipolar mode; each mode is well known to those skilled in the art of electrostatic clamping systems. Desirably, clamp electrode  120  can serve as upper surface  140  of electrode  110  and extends under the lower surface  152  of focus ring  118  and the lower surface  162  of substrate  122 . In one embodiment, electrostatic clamp  120  can be utilized to clamp both the focus ring  118  and the substrate  122 . In another embodiment, electrostatic clamp  120  can comprise two or more independent clamp electrodes with separate HV DC voltage sources for independently clamping the focus ring  118  and the substrate  122 .  
         [0028]     Alternately, electrode  110  can further include a cooling/heating system including a re-circulating fluid that receives heat from substrate  122  and focus ring  118  and transfers heat to a heat exchanger system (not shown) when cooling, or when heating, transfers heat from the heat exchanger system to the above elements. In other embodiments, heating elements, such as resistive heating elements, or thermoelectric heaters/coolers can be included as part of the heating/cooling system. The heating/cooling system further comprises a device (not shown) for monitoring the electrode  110  temperature. The device can be, for example, a thermocouple (e.g., K-type thermocouple).  
         [0029]     Moreover, heat transfer gas can be delivered to at least one of a first space  170  between upper surface  140  of electrode  110  and lower surface  152  of focus ring  118  using a first gas supply line  172 , and a second space  180  between upper surface  140  of electrode  110  and lower surface  162  of substrate  122  using a second gas supply line  182  (see  FIG. 2A ). Gas supply lines  172  and  182  can distribute heat transfer gas to one or more orifices or a groove formed in the upper surface  140  of electrode  110 . The implementation of heat transfer gas distribution is well known to those skilled in the art of substrate processing. The supply of heat transfer gas to the first space  170  can improve the gas-gap thermal conductance between the lower surface  152  of focus ring  118  and the upper surface  140  of electrode  110 , while the supply of heat transfer gas to the second space  180  can improve the gas-gap thermal conductance between the lower surface  162  of substrate  122  and the upper surface  140  of electrode  120 . The heat transfer gas can be, for example, at least one of a Noble gas such as helium, argon, neon, xenon, krypton, a process gas such as C 4 F 8 , CF 4 , C 5 F 8 , C 4 F 6  and C 2 F 6 , or a mixture thereof. Therefore, controlling the temperature of electrode  110  via the aforementioned heating/cooling system can lead to control of both the temperature of the focus ring  118  and the temperature of the substrate  122 . In one embodiment, the supply of heat transfer gas to the first space  170  is independent of the supply of heat transfer gas to the second space  180  using independent gas supplies  174  and  184  as shown in  FIG. 2A . Using independent heat transfer gas supplies, the pressure in first space  170  can be adjusted to be different than the pressure in second space  180 . In an alternate embodiment, gas supply lines  172  and  182  are supplied heat transfer gas from a single heat transfer gas supply. In an alternate embodiment, the second space  180  is divided into one or more spaces to which heat transfer gas is supplied independently.  
         [0030]     Substrate  122  can be, for example, transferred into and out of a process chamber (not shown) through a slot valve (not shown) and chamber feed-through (not shown) via robotic substrate transfer system where it is received by substrate lift pins (not shown) housed within substrate holder  100  and mechanically translated by devices housed therein. Therefore, lift pin holes (not shown) in electrode  110  and electrostatic clamp  120  accommodate the passage of lift pins to and from the lower surface  162  of substrate  122 . Once substrate  122  is received from the substrate transfer system, it is lowered to an upper surface  140  of substrate holder  100 .  
         [0031]     In the illustrated embodiment, shown in  FIG. 2A , electrode  110  can, for example, further serve as a RF electrode through which RF power is coupled to plasma in a processing region adjacent substrate  122 . For example, electrode  110  is electrically biased at a RF voltage via the transmission of RF power from a RF generator (not shown) through an impedance match network (not shown) to electrode  110 . The RF bias can serve to heat electrons and, thereby, form and maintain plasma or to provide a RF bias in order to enable control of ion energy at the upper surface  160  of substrate  122 . In this configuration, the system can operate as a reactive ion etch (RIE) reactor, wherein the chamber serves as ground surfaces. A typical frequency for the RF bias can range from 1 MHz to 100 MHz and is preferably 13.56 MHz. RF systems for plasma processing are well known to those skilled in the art. Impedance match network topologies (e.g. L-type, π-type, T-type, etc.) and automatic control methods are also well known to those skilled in the art.  
         [0032]     Referring now to  FIG. 3 , a flowchart  300  describes a method of processing a substrate using the improved substrate holder depicted in  FIG. 2  in order to minimize the possibility of arcing between the substrate edge and the focus ring. The method begins with step  310  wherein a focus ring  118  as described above is placed upon substrate holder  100  and coupled to the upper surface  140  of electrode  110 . The focus ring  118  can, for example, be set atop the electrode  110  by an operator during chamber maintenance. Furthermore, the focus ring  118  can be centered about an axis of revolution  111  by aligning the outer surface  154  of focus ring  118  flush with the inner edge  190  of insulating member  112 . Alternately, focus ring  118  can be received and lowered to the upper surface  140  of electrode  110  by a set of lift pins (not shown), wherein the focus ring  118  is transferred into and out of the chamber via the robotic substrate transfer system described above.  
         [0033]     In step  320 , substrate  122  is placed upon substrate holder  100  and coupled to the upper surface  140  of electrode  110 . The substrate  122  can, for example, be received and lowered to the electrode  110  by a set of lift pins (not shown), as described above, wherein substrate  122  is transferred into and out of the chamber via the robotic substrate transfer system. Furthermore, substrate  122  can be centered about an axis of revolution  111  by aligning the outer surface  164  of substrate  122  flush with the inner edge  156  of focus ring  118 .  
         [0034]     In step  330 , substrate  122  is processed in the plasma processing system according to a process recipe. The process recipe can, for example, include setting the electrostatic clamping voltage (force), backside gas pressure (e.g. gas pressure in spaces  170  and  180 ), RF power to electrode  110 , chamber gas pressure, process gas partial pressure(s) and flow rate(s), etc.  
         [0035]     Referring now to  FIG. 4 , a flowchart  400  describes a method of processing a substrate using the improved substrate holder depicted in  FIG. 2  in order to control the temperatures of focus ring  118  and substrate  122 . The method begins with step  410  wherein, as before, a focus ring  118  as described above is placed upon substrate holder  100  and coupled to the upper surface  140  of electrode  110 . The focus ring  118  can, for example, be set atop the electrode  110  by an operator during chamber maintenance. Furthermore, the focus ring  118  can be centered about an axis of revolution  111  by aligning the outer surface  154  of focus ring  118  flush with the inner edge  190  of insulating member  112 . Alternately, focus ring  118  can be received and lowered to the upper surface  140  of electrode  110  by a set of lift pins (not shown), wherein the focus ring  118  is transferred into and out of the chamber via the robotic substrate transfer system described above.  
         [0036]     In step  420 , substrate  122  is placed upon substrate holder  100  and coupled to the upper surface  140  of electrode  110 . The substrate  122  can, for example, be received and lowered to the electrode  110  by a set of lift pins (not shown), as described above, wherein substrate  122  is transferred into and out of the chamber via the robotic substrate transfer system. Furthermore, substrate  122  can be centered about an axis of revolution  111  by aligning the outer surface  164  of substrate  122  flush with the inner edge  156  of focus ring  118 .  
         [0037]     In step  430 , a voltage supplied from a HV, DC voltage source is applied to electrostatic clamp  120  in order to provide a clamping force between the focus ring  118  and electrode  110  as well as the substrate  122  and electrode  110 . In step  440 , once the focus ring  118  and substrate  122  are clamped, a heat transfer gas can be supplied to the first and second spaces  170 ,  180  described above in order to improve the gas-gap thermal conductance between the focus ring  118  and electrode  110 , and the substrate  122  and the electrode  110 . In an embodiment of the present invention, the gas pressure in first space  170  is substantially equivalent to the gas pressure in second space  180 . In an alternate embodiment, the gas pressure in first space  170  is substantially different than the gas pressure in second space  180 .  
         [0038]     In step  450 , the temperature of electrode  110  is controlled via the heating/cooling system described above, thereby providing temperature control for the focus ring  118  and the substrate  122 .  
         [0039]     Although only certain exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.