Patent Publication Number: US-2006016554-A1

Title: Substrate holder having electrostatic chuck and method of fabricating the same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
      This application claims the benefit of Korean Patent Application No. 10-2004-0056857, filed on Jul. 21, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a substrate holder, and more particularly, to a substrate holder having an electrostatic chuck that uses electrostatic forces to hold a semiconductor substrate. The present invention also relates to a substrate holder having an electrostatic chuck and a susceptor flatly attached to each other and a method of fabricating the same.  
      2. Description of the Related Art  
      A plasma device used in manufacturing a semiconductor device typically includes a process chamber.  FIG. 1  is a cross-sectional view of a process chamber  100  including a typical substrate holder. Referring to  FIG. 1 , the pressure of the process chamber  100  can be controlled using a vacuum pump  180  during a plasma process. The chamber  100  includes a substrate holder  130  for holding a substrate, i.e., a wafer  110 . When the wafer  110  is loaded onto the substrate holder  130  by a transport system, a high frequency power supply  170  is electrically connected to a susceptor  150  or an upper electrode (not shown) to generate plasma onto the wafer  110 .  
      To control the temperature of the wafer  110 , a cooling gas is supplied along the rear surface of the wafer  110  during an etching process and a deposition process requiring uniform etching and deposition. To accomplish this, a groove (not shown) is formed in the top surface of the electrostatic chuck  120  and a cooling gas is supplied from a cooling gas source  160  into the groove.  
      There are two types of electrostatic chuck  120  for holding a wafer using electrostatic forces: polyimide electrostatic chuck and anodized coating electrostatic chuck. The polyimide electrostatic chuck is formed by attaching polymer such as polyimide to the top surface of the susceptor  150 . The anodized coating is formed by anodizing the surface of the susceptor  150  made of aluminum and is used as a dielectric.  
      While the polyimide electrostatic chuck exhibits excellent performance in terms of stacking and withstand voltage, it is vulnerable to oxygen plasma and shows a non-uniform wafer temperature distribution due to poor thermal conductivity. Anodized coating suffers variation in adhesion strength between the wafer  110  and the susceptor  150  due to a change in the dielectric constant of a dielectric layer. That is, reaction products that are formed during the process may adhere to the anodized coating that are loosely packed.  
      Recently, electrostatic chucks made of a ceramic material such as alumina or aluminum nitride have been introduced to address these problems.  
      Referring to  FIG. 1 , the ceramic electrostatic chuck  120  typically incorporates an electrode  140 . The ceramic electrostatic chuck  120  is affixed to the top surface of the susceptor  150  by a silicon-based adhesive  190 . The susceptor  150  may also be called a lower electrode for plasma generation. The ceramic electrostatic chuck  120  also has a plurality of cooling gas supply holes  161  and  162  for controlling the temperature of the wafer  110 . The plurality of holes  161  and  162  penetrate the susceptor  150  and connect with the cooling gas supply  160 .  
      The ceramic electrostatic chuck  120  may not be attached flatly to the top surface of the susceptor  150  by the silicon-based adhesive  190  due to the fluidity of the silicon-based adhesive  190 . This causes several problems which will now be described with reference to  FIGS. 2A-2C .  
       FIGS. 2A-2C  are cross-sectional views of a conventional substrate holder. Referring to  FIG. 2A-2C , a silicon-based gel adhesive  290  mixed with a curing agent at a predetermined ratio is applied between a ceramic electrostatic chuck  220  and a susceptor  250  and cured at predetermined pressure and temperature for attachment. The adhesive  290  has viscosity and thermal conductivity that can compensate for a difference between thermal expansion coefficients of the ceramic electrostatic chuck  220  and the susceptor  250 .  
      Cooling gas supply holes  261  and  262  formed in the susceptor  250  must be accurately aligned with their counterparts formed in the electrostatic chuck  220  before adhesion. A slight misalignment error results in a warped cooling gas supply path and a change in the flow of cooling gas. This change causes non-uniform temperature distribution within a wafer, thereby reducing the manufacturing yields for semiconductor chips.  
      Predetermined temperature and pressure must be applied simultaneously when the electrostatic chuck  220  is affixed to the susceptor  250  by a silicon-based adhesive  290 . Thus, when the pressure is not transferred uniformly, the adhesive  290  may concentrate on edge or center of the susceptor  250  as shown in  FIGS. 2B and 2C . This degrades the planarity of the electrostatic chuck  220  or causes the creation of air bubbles through which the cooling gas leaks out when the adhesive  290  solidifies.  
      The conventional substrate holder has the following problems. First, when a ceramic electrostatic chuck is affixed to a susceptor, the planarity of the ceramic electrostatic chuck is difficult to precisely control depending on temperature and pressure applied because of the use of a liquid adhesive. Furthermore, the silicon adhesive tends to flow into cooling gas supply holes formed in the ceramic electrostatic chuck and the susceptor due to pressure exerted thereon, thereby resulting in clogging of the holes.  
      Second, pores created due to non-planarity of the electrostatic chuck and viscosity of the adhesive permits a part of cooling gas to leak from a space between the electrostatic chuck and the susceptor into a process chamber, thereby resulting in creation of an arc inside the chamber.  
      Thus, there is a need for a method capable of attaching the electrostatic chuck flatly to the susceptor while preventing the leakage of cooling gas due to air bubbles or for an adhesive enabling flat adhesion between the electrostatic chuck and the susceptor.  
     SUMMARY OF THE INVENTION  
      The present invention provides a substrate holder designed to accurately maintain the planarity of a ceramic electrostatic chuck that is affixed to a susceptor while preventing clogging of cooling gas supply holes.  
      The present invention also provides a substrate holder designed to prevent creation of an arc due to leakage of cooling gas through pores created due to non-planarity of the electrostatic chuck and viscosity of the adhesive.  
      According to an aspect of the present invention, there is provided a substrate holder including a susceptor having edge protrusion formed on edge thereof and an electrostatic chuck mounted inside the edge protrusion and on the susceptor. The electrostatic chuck is attached to the susceptor by a gel adhesive sheet containing a plurality of wires and a silicon or corrosion-resistant epoxy based material is filled between the electrostatic chuck and the edge protrusion.  
      The shape of the plurality of wires may be meshed, concentrically circular, or radial and the contraction rate may be less than 5%. Meshed wires may be made of an optical fiber. The edge protrusion and the susceptor may be formed in one body and an upper end of the edge protrusion may be lower than or at the same level as the electrostatic chuck affixed to the susceptor.  
      Alternatively, the substrate holder may include: a susceptor having ring-shaped edge protrusion formed on edge thereof; and an electrostatic chuck that incorporates an electrode and is mounted inside the edge protrusion and on the susceptor.  
      According to another aspect of the present invention, there is provided a method of manufacturing a substrate holder including: preparing a susceptor having edge protrusion formed on edge thereof; mounting an adhesive sheet containing a plurality of wires inside the edge protrusion and on the susceptor; mounting an electrostatic chuck on the adhesive sheet; applying a pressure to the top surface of the electrostatic chuck at predetermined temperature and attaching the electrostatic chuck to the susceptor; and filling a silicon or epoxy-based material between the electrostatic chuck and the edge protrusion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  is a cross-sectional view of a chamber including a conventional substrate holder;  
       FIGS. 2A-2C  are cross-sectional views of a conventional substrate holder showing an uneven distribution of an adhesive;  
       FIGS. 3A-3D  are cross-sectional views schematically showing a method of manufacturing a substrate holder according to an embodiment of the present invention; and  
       FIG. 4  is a cross-sectional view of a substrate holder according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity.  
      Referring to  FIG. 3A , a susceptor  350  having edge protrusion  351  is prepared. The edge protrusion  351  is ring-shaped and formed integrally with the susceptor  350 .  
      Referring to  FIG. 3B , an adhesive sheet  390  containing a plurality of wires  395  is mounted inside the edge protrusion  352  of the susceptor  350  and on the susceptor  350 .  
      Referring to  FIG. 3C , a prepared ceramic electrostatic chuck  320  is affixed to the top surface of the adhesive sheet  390  by a predetermined pressure at a predetermined temperature. An electrode  340  is embedded in the electrostatic chuck  320 .  
      The temperature and pressure is applied to attach and solidify the adhesive sheet  390 . The adhesive sheet  390  may be a silicon or gel adhesive. More specifically, an available temperature range is 80 to 220° C., preferably, 100 to 180° C. An available pressure range is 2 to 20 kgf/cm 2 .  
      A cooling gas path in the adhesive sheet  390  or a lift pin path may be prepatterned using hydraulic or pneumatic blanking to form holes therein. The electrostatic chuck  320  and the susceptor  350  have preformed cooling gas path and lift pin path. Because a method for forming a cooling gas path and a lift pin path is well known in the art, description thereof will not be given.  
      Referring to  FIG. 3D , an epoxy-based material having corrosion resistance to Freon plasma or silicon is inserted into a gap  351  between the electrostatic chuck  320  and the edge protrusion  351  and cured to thereby complete a substrate holder according to an embodiment of the present invention.  
      The height of the ring-shaped edge protrusion  351  is less than or equal to that of the ceramic electrostatic chuck  320  that has been mounted on the susceptor  350  because a wafer cannot be attracted and held onto the electrostatic chuck  320  if the diameter of the wafer is greater than that of the electrostatic chuck  320 .  
       FIG. 4  is a cross-sectional view of a substrate holder according to an embodiment of the present invention. Referring to  FIG. 4 , an electrostatic chuck  420  of a completed substrate holder can be divided into an upper plate positioned above an electrode  440  and a lower plate positioned below the electrode  440 . The upper and lower plates of the electrostatic chuck may be made of a ceramic material such as alumina, nitrided aluminum, or monocrystalline sapphire.  
      The electrode  440  embedded in the electrostatic chuck  420  may be formed on the lower plate of the electrostatic chuck using screen printing. The electrode  440  may be made of molybdenum (Mo) or tungsten compound containing 5 to 20% nitrided aluminum.  
      A plurality of wires  495  has a meshed or rounded shape and consists of a single or a plurality of layers. The wires  495  may be made of an insulating layer or optical fiber. The optical fiber may have a circular, elliptical or other-shaped cross-section. The present embodiment uses an elliptical optical fiber.  
      The ratio of a minor axis diameter to a major axis diameter in the elliptical optical fiber may be 1:3 to 1:7 and the contraction rate may be less than 5%.  
      The adhesive sheet  490  may be manufactured by inserting a gel adhesive material such as silicon or epoxy material into a gap between the meshed wires  495 . The thickness of the adhesive sheet  490  that varies depending on the diameter of an optical fiber is typically 150 to 350 μm.  
      Furthermore, the adhesive sheet  490  may be selected over various ranges considering factors such as the thermal expansion coefficients of the electrostatic chuck  420  and the susceptor  450 , viscosity, dielectric constant, volume resistivity, and thermal conductivity. The susceptor  450  is typically formed of metal such as aluminum and an anodized coating may be formed on the surface of the susceptor  450  for insulation. Edge protrusion  451  may be formed using mechanical machining.  
      Based on the above-mentioned configuration and manufacturing method, the electrostatic chuck is affixed to a susceptor by an adhesive sheet containing a plurality of meshed wires, thereby preventing the adhesive from flowing into a cooling gas hole or lift pin hole. Furthermore, the planarity of the electrostatic chuck can be maintained due to little variation in thickness of the adhesive sheet.  
      An epoxy filled between ring-shaped edge protrusion formed on the side of the susceptor and the ceramic electrostatic chuck basically prevents leakage of cooling gas from a space between the electrostatic chuck and the susceptor.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.