Abstract:
An electrostatic chuck for preventing warpage of a ceramic layer and cooling gas leakage while providing enhanced electrostatic attraction and an improved detachment performance and its manufacturing method is disclosed. The chuck comprises at least one electrode ( 90, 91, 92 ) located in the middle of the ceramic layer ( 80 ) in its thickness direction, a cooling gas channel ( 81 ) is formed on a surface of the ceramic layer within an outer edge of the electrode and above the electrode, wherein the electrode extends beyond the cooling gas channel. Preferably the electrodes are shaped in the form of two interlocked structures comprising multiple interconnected C-shaped ring portion ( 91   c   , 92   c ).

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electrostatic chuck, and associated manufacturing process, for use in an etching apparatus for manufacturing semiconductors. 
   2. Description of the Related Art 
   In a semiconductor manufacturing process, etching processes are repeatedly performed together with insulating film formation, diffusion processes, and photolithographic processes. There are two types of etching processes: wet etching and dry etching. The dry etching process is implemented using a plasma etching apparatus as shown in  FIG. 4 . For example, with a semiconductor wafer W held on a chuck  12  in a processing chamber  11  of the etching apparatus, a reactive gas is introduced from an inlet  13  into the processing chamber  11  while high-frequency electric power  15  is applied between the chuck  12 , which serves as a lower electrode, and an upper electrode  14  to generate a plasma in the processing chamber  11 . Chemical reactions with radicals in the plasma and accelerated ions cause the semiconductor wafer W to be etched. More particularly, either the semiconductor wafer W itself or an insulating film (not shown) thereon is etched. 
   As mentioned above, during the dry etching process, the semiconductor wafer W is held on the chuck  12 . In recent etching apparatuses, the chuck  12  has been specified to be an electrostatic chuck. Electrostatic chucks have demonstrated excellent characteristics in vacuum plasma processors. 
   The electrostatic chuck generates electrostatic forces for attracting material to the chuck. The electrostatic forces include two types: a Coulomb force and a Johnsen-Rahbek force. 
   Additionally, there are two types of electrostatic chucks: a unipolar type and a bipolar type. With respect to  FIG. 5 , the unipolar type of electrostatic chuck includes an anode  22  formed in a dielectric material  21 . Also in the unipolar type, a cathode is defined by the apparatus such that a plasma electric potential is produced as shown in  FIG. 5 . With respect to  FIG. 6 , the bipolar type of electrostatic chuck includes both an anode  32  and a cathode  33  formed in a dielectric material  31 . 
     FIG. 7  is an illustration showing a cross-sectional view of a conventional electrostatic chuck, in accordance with the prior art. The conventional electrostatic chuck includes an anodized aluminum film  42  disposed as a dielectric material on a surface of a disc-shaped aluminum electrode  41 . The anodized aluminum film  42  has a thickness within a range extending from 50 μm to 60 μm. The semiconductor wafer W is placed on the anodized aluminum film  42 . A cooling gas channel  43  is formed on the surface of the aluminum electrode  41  that is covered by the anodized aluminum film  42 . The cooling gas channel  43  extends in a circumferential direction following a periphery of the aluminum electrode  41 . A helium cooling gas is fed from gas feed orifices (not shown), penetrating the anodized aluminum film  42  and the aluminum electrode  41 , to the cooling gas channel  43 . The helium cooling gas flows into the cooling gas channel  43 , fills the cooling gas channel  43 , and then diffuses along the entire interface between the anodized aluminum film  42  and the semiconductor wafer W. The helium cooling gas diffusion occurs through fine gaps present along the interface between the anodized aluminum film  42  and the semiconductor wafer W. The fine gaps are defined by a rough surface of the anodized aluminum film  42 . The helium gas diffusion serves to cool the semiconductor wafer W. In the dry etching process performed using the apparatus of  FIG. 4 , a temperature of the semiconductor wafer W can significantly affect the resulting etching characteristics. Use of the helium cooling gas as previously described serves to cool the semiconductor wafer W by as much as 30° C. to 60° C., thus improving the resulting etching characteristics, especially a uniformity characteristic. 
   The conventional electrostatic chuck as described above can be adversely affected through reaction product deposition. More specifically, reaction products present in the chamber  11  can adhere to a surface of the anodized aluminum film  42  after the semiconductor wafer W is removed from the chuck following the etching process, thus weakening the electrostatic attraction capability of the chuck in subsequent etching processes. Additionally, reaction products adhering to the surface of the anodized aluminum film  42  can increase gaps between the anodized aluminum film  42  and the semiconductor wafer W, causing leakage of the helium cooling gas from an outer peripheral edge of the aluminum electrode  41 . Consequently, leakage of the helium cooling gas can cause the semiconductor wafer W to be insufficiently cooled, thus causing the etching characteristics to be adversely affected. Furthermore, during the etching process, the anodized aluminum film  42  can be deteriorated by reactive gases or ions which pass through end portions of the semiconductor wafer W, thus further weakening the electrostatic attraction capability of the chuck. 
   In response to the aforementioned problems, an electrostatic chuck has been developed that incorporates a ceramic layer as the dielectric material.  FIG. 8  is an illustration showing an electrostatic chuck incorporating a ceramic dielectric layer, in accordance with the prior art. This electrostatic chuck of  FIG. 8  has a ceramic layer  53  bonded onto a disc-shaped metal base plate  51  by means of an adhesive layer  52 . A high-melting point electrode  54  is laid in the ceramic layer  53 . In this arrangement, from the viewpoint of increasing electrostatic attraction, the electrode  54  is positioned near to the surface of the ceramic layer  53 . In one example, the ceramic layer  53  is 1 mm thick and the electrode  54  is positioned 0.3 mm away from a top surface of the ceramic layer  53  and 0.7 mm away from a bottom surface of the ceramic layer  53 . As with the electrostatic chuck of  FIG. 7 , the electrostatic chuck of  FIG. 8  also includes a cooling gas channel  55  formed in the top surface of the ceramic layer  53  and extending in a circumferential direction following a periphery of the ceramic layer  53 . The cooling gas channel  55  needs to have a certain depth considering a flow of the helium cooling gas. Therefore, if the cooling gas channel  55  is formed at a location overlying the electrode  54 , the close proximity of the electrode  54  to the top surface of the ceramic layer  53  causes a distance between the bottom of the channel  55  and the electrode  54  to become short. If the distance between the bottom of the channel  55  and the electrode  54  becomes too short, the ceramic layer  53  spanning the short distance can have an insufficient dielectric strength. To avoid the insufficient dielectric strength issue, the electrostatic chuck of  FIG. 8  has the channel  55  formed 1 mm to 2 mm within an outer periphery of the ceramic layer  53 , and the electrode  54  formed within an outer boundary defined by the channel  55 . 
   The electrostatic chuck of  FIG. 8 , however, is not without problems. More specifically, since the electrode  54  has a coefficient of linear thermal expansion different from that of the ceramic layer  53 , and given that the electrode  54  is located near a top surface of the ceramic layer  53 , the ceramic layer  53  having been formed by firing is susceptible to warpage. In addition to the warpage problem, the electrostatic chuck of  FIG. 8  can also be adversely affected by a cooling gas leakage problem. Leakage of the cooling gas from the channel  55  to the outer periphery of the ceramic layer  53  is intended to be prevented by the sealed interface between the ceramic layer  53  and the semiconductor wafer W extending between the channel  55  and the outer periphery of the ceramic layer  53 . However, if the distance between the channel  55  and the outer periphery of the ceramic layer  53  (sealed distance) is short (e.g., 1 mm to 2 mm) a gas leakage can occur through the sealed distance. 
     FIG. 9  is an illustration showing the electrostatic chuck of  FIG. 8  with a modification to assist in preventing gas leakage through the sealed distance, in accordance with the prior art. In the electrostatic chuck of  FIG. 9 , the channel  55  is formed approximately 3 mm to 10 mm inside the outer periphery of the ceramic layer  53 , thus providing a substantial sealed distance. Placing the channel  55  further from the outer periphery of the ceramic layer  53 , however, requires the electrode  54  to be redefined to remain within the outer boundary represented by the channel  55 , thus effectively decreasing an area of the electrode  54 . Decreasing the area of the electrode  54  can cause insufficient electrostatic attraction in the region between the channel  55  and the outer periphery of the ceramic layer  53 . 
   Furthermore, there are many examples of conventional electrostatic chucks in which a semiconductor wafer attracted to the chuck cannot always be readily detached, or “dechucked,” therefrom after a completion of etching or other processes. In some cases, it takes a considerable time for detachment of the semiconductor wafer. 
   In view of the foregoing, an apparatus is needed to overcome the problems associated with prior art electrostatic chuck arrangements. More specifically, the apparatus needs to prevent warpage of a ceramic layer and leakage of a cooling gas, while also enhancing electrostatic attraction and requiring only a short time for detachment. 
   SUMMARY OF THE INVENTION 
   The present invention provides an electrostatic chuck in which a disc-shaped ceramic layer having a predetermined thickness is bonded to a metal base plate by means of an adhesive layer. A planar electrode is located in the middle of the ceramic layer, relative to a thickness direction. A cooling gas channel is formed on a surface of the ceramic layer at a location overlying the electrode such that the electrode extends radially beyond the cooling gas channel. 
   Another aspect of the invention provides a method for manufacturing an electrostatic chuck. The method includes preparing a first disc-shaped ceramic material compact having a thickness which is approximately one-half of a completed ceramic layer thickness. As electrode is formed on a surface of the first ceramic material compact The method further includes preparing a second disc-shaped ceramic material compact having a thickness which is approximately one-half of the completed ceramic layer thickness. A cooling gas channel is included on the surface of the second disc-shaped ceramic material compact within an area overlying the electrode. The method also includes placing the second ceramic material compact on the first ceramic material compact to form a laminate followed by firing the entire laminate to complete the ceramic layer. Also, the completed ceramic layer is bonded onto a metal base plate by means of an adhesive layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of an embodiment of an electrostatic chuck according to the present invention; 
       FIG. 2  is a top plan view of an electrode of the electrostatic chuck illustrated in  FIG. 1 ; 
       FIG. 3  is a top plan view of a ceramic layer of the electrostatic chuck illustrated in  FIG. 1 ; 
       FIG. 4  is a schematic view illustrating a dry etching apparatus, in accordance with the prior art; 
       FIG. 5  is a schematic view illustrating a unipolar-type electrostatic chuck, in accordance with the prior art; 
       FIG. 6  is a schematic view illustrating a bipolar-type electrostatic chuck, in accordance with the prior art; 
       FIG. 7  is a cross-sectional view illustrating a conventional first electrostatic chuck, in accordance with the prior art; 
       FIG. 8  is a cross-sectional view illustrating a conventional second electrostatic chuck, in accordance with the prior art; and 
       FIG. 9  is a cross-sectional view illustrating a conventional third electrostatic chuck, in accordance with the prior art. 
   

   DETAILED DESCRIPTION 
   The present invention provides an electrostatic chuck and a method for manufacturing the same. Several exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The electrostatic chuck of the present invention can be used in conjunction with a dry etching apparatus. A general configuration of the dry etching apparatus has been previously discussed with respect to  FIG. 4 . Accordingly, reference should be made to the discussion of  FIG. 4  for the description of the general configuration of the dry etching apparatus. The electrostatic chuck of the present invention is described in detail below. 
     FIG. 1  is an illustration showing a cross-sectional view of an electrostatic chuck, in accordance with one embodiment of the present invention. As shown in  FIG. 1 , the electrostatic chuck includes a disc-shaped ceramic layer  80  having a predetermined thickness bonded onto a disc-shaped metal base plate  60  by means of an adhesive layer  70 . A planar electrode  90  is disposed within the ceramic layer  80  and is oriented to be parallel to a surface of the ceramic layer  80 . In one exemplary embodiment, the planar electrode  90  is made of tungsten. The planar electrode  90  is disposed within the ceramic layer  80  at a depth of one-half of the thickness thereof. In other words, the planar electrode  90  is positioned in the middle of the ceramic layer  80  relative to a thickness direction. 
     FIG. 2  is an illustration showing a top plan view of the planar electrode  90 , in accordance with one embodiment of the present invention. The planar electrode  90  includes a first electrode  91  and a second electrode  92 . Therefore, the electrostatic chuck of this embodiment is of a bipolar type. The first electrode  91  includes a disc portion  91   a  defined about the center of the ceramic layer  80  and a linear portion  91   b  extending linearly from a part of the disc portion  91   a  toward an outer peripheral edge of the ceramic layer  80 . The first electrode  91  further includes a number of “C-shaped” ring portions  91   c  positioned at predetermined intervals so as to surround the disc portion  91   a . More particularly, each of the number of “C-shaped ring” portions  91   c  has a different diameter relative to a center of the disc portion  91   a  and extends in a “C” shape on both sides of the linear portion  91   b . The second electrode  92  includes a linear portion  92   b  positioned opposite to the linear portion  91   b  of the first electrode  91  and beyond the disc portion  91   a  of the first electrode  91 . Also, the second electrode  92  includes a number of “C-shaped” ring portions  92   c  positioned at predetermined intervals and arranged complementary to the number of “C-shaped” ring portions  91   c  of the first electrode  91 . More particularly, each of the number of “C-shaped” ring portions  92   c  has a different diameter relative to the center of the disc portion  91   a  and extends in a “C” shape on both sides of the linear portion  92   b  to be engaged with the number of “C-shaped” ring portions  91   c  of the first electrode  91 . Furthermore, the electrode  91  includes a circular ring portion  92   d  connected to an outer edge of the linear portion  92   b , such that the circular ring portion  92   d  forms the outermost peripheral portion of the planar electrode  90 . 
   With respect to  FIG. 1 , a cooling gas channel  81  is formed on a top surface of the ceramic layer  80 . The cooling gas channel  81  is provided in a ring shape along the outer peripheral edge of the ceramic layer  80 , as shown in a top plan view in  FIG. 3 . In addition, the cooling gas channel  81  is formed within the outer peripheral edge of the planar electrode  90  and over the planar electrode  90 , as shown in  FIG. 1 . Accordingly, the planar electrode  90  passes under the bottom of the cooling gas channel  81  and extends beyond the cooling gas channel  81  near to the outer periphery of the ceramic layer  80 . 
   Gas feed orifices  82  are provided at a bottom of the cooling gas channel  81  in a number of locations along the circumferential direction of the cooling gas channel  81 , as shown in  FIG. 3 . Additionally, a number of gas feed orifices  83  are provided at a number of locations on a top surface of the ceramic layer  80 . The number of gas feed orifices  83  are positioned toward a center of the ceramic layer  80  and at a common radius from the center of the ceramic layer  80 . The gas feed orifices  82  and  83  extend through the ceramic layer  80 , the adhesive layer  70 , and the base plate  60 . A cooling gas such as helium gas, for example, is fed from the bottom of the base plate  60 . In accordance with the foregoing, the helium gas fed through the gas feed orifices  82  and  83  is dispensed from the bottom of the cooling gas channel  81  and from the central locations on the top surface of the ceramic layer  80 . The helium gas diffuses from both the outer peripheral portion and the central portion of the ceramic layer  80  over the entire interface between the ceramic layer  80  and the semiconductor wafer W, as shown in  FIG. 1 , so as to cool the semiconductor wafer W. 
   In one embodiment, the electrode  90  of the electrostatic chuck includes a high-melting point metal having a coefficient of linear thermal expansion that is different from a coefficient of linear thermal expansion of the ceramic layer  80 . However, since the electrode  90  is disposed in the middle of the ceramic layer  80 , relative to the thickness direction of the ceramic layer  80 , differential thermal expansion between the electrode  90  and the ceramic layer  80  will not cause the top surface of the ceramic layer  80  to warp. Furthermore, even if the ceramic layer  80  is formed by firing, placement of the electrode  90  at the middle of the ceramic layer  80  will allow the top surface of the ceramic layer  80  to remain flat. In addition, improvement of the flatness of the ceramic layer  80  leads to improvement of semiconductor wafer W etching characteristics (for example, uniformity) and an increase in the attraction of the semiconductor wafer W to the electrostatic chuck. 
   The cooling gas channel  81  is formed near an outer peripheral edge of the electrode  90  and over the electrode  90 . Thus, the electrode  90  extends beyond the cooling gas channel  81  and into the gas-sealed region extending from the cooling gas channel  81  to the outer periphery of the ceramic layer  80 . Therefore, with respect to  FIG. 1 , the electrode  90  is positioned to apply electrostatic attraction over the gas-sealed region as indicated by arrows. Consequently, the electrostatic attraction over the gas-sealed region prevents gas leakage from the gas-sealed region. Prevention of gas leakage leads to improved cooling of the semiconductor wafer W, thus improving the etching characteristics such as center-to-edge uniformity as measured across the semiconductor wafer W. 
   Positioning of the electrode  90  in the middle of the ceramic layer  80  relative to the thickness direction of the ceramic layer  80 , allows the cooling gas channel  81  to be formed sufficiently deep, even in locations overlying the electrode  90 . Forming the cooling gas channel  81  in a sufficiently deep manner allows the helium gas to smoothly flow, thus allowing the semiconductor wafer W to be cooled more favorably. In addition, forming the cooling gas channel  81  over the electrode  90  allows the gas-sealed region between the cooling gas channel  81  and the periphery of the ceramic layer  80  to be of sufficient distance so as to more reliably prevent gas leakage. Also, forming the cooling gas channel  81  over the electrode  90  avoids a need to decrease an area of the electrode  90  to accommodate a position of the cooling gas channel  81 . Additionally, even if the electrode  90  is located at a deep position in the ceramic layer  80 , a sufficient electrostatic attraction can be secured because electrostatic attraction is also applied to the gas-sealed region between the cooling gas channel  81  and the periphery of the ceramic layer  80  as described above. 
   Furthermore, the first electrode  91  and the second electrode  92  each occupy an identical area and are each uniformly distributed over the ceramic layer  80 . Therefore, the first and second electrodes,  91  and  92 , allow uniform electrostatic attraction to be achieved over the entire ceramic layer  80 . Still further, the ratio of the area of the first electrode  91  to the area of the ceramic layer  80  can be increased, thus allowing the electrostatic attraction to be enhanced. Additionally, with the electrostatic chuck of the present invention, detaching or “dechucking” of a wafer after processing is improved in comparison with a conventional electrostatic chuck having a conventional doughnut or threading pattern, whereby the conventional doughnut or threading pattern causes an uneven ratio of an area of the first electrode to that of the second electrode. The uneven ratio of the area of the first electrode to the second electrode in the conventional electrostatic chuck can lead to a maldistribution of electric charges and cause poor dechucking performance. 
   In one exemplary embodiment, the base plate  60 , with respect to  FIG. 1 , is preferably formed by using an aluminum  6061  material. A high radio frequency (RF) power is fed to the base plate  60  in order to generate a plasma in the chamber  11  of the etching apparatus as shown in  FIG. 4 . The high RF power to be applied may have a frequency in the range extending from 1 MHz to 40 MHz and a power in the range extending from 15 W to 3000 W. 
   In one exemplary embodiment, the adhesive layer  70  is defined by a suitable, flexible, preferably organic, adhesive. With the ceramic layer  80  bonded to the base plate  60  by means of the flexible organic adhesive layer  70 , the ceramic layer  80  is prevented from cracking as a result of differential stress induced by differential thermal expansion. 
   In one exemplary embodiment, the ceramic layer  80  is formed by adding conductive additive to aluminum oxide, for example, aluminum nitride or magnesium oxide. In this embodiment, the ceramic layer  80  is formed by using aluminum oxide (Al 2 O 3 ) as a predominant component and titanium oxide (TiO 2 ) and glass firing auxiliary material as additives. The additive TiO 2  is added to cause the ceramic layer  80  to be slightly conductive, thus allowing electric charges for generating electrostatic attraction rise from the electrode  90  up to the surface of the ceramic layer  80 . A resistivity of the ceramic layer  80  is selected to be within a range extending from 10 11  Ω/cm to 10 12  Ω/cm, and more preferably within a range extending from 1.0×10 11  Ω/cm to 2.0×10 11  Ω/cm. 
   In one exemplary embodiment, a diameter of the ceramic layer  80  is slightly smaller than that of the semiconductor wafer W. Additionally, a thickness of the ceramic layer  80  is approximately 1 mm taking into consideration dielectric breakdown and an energy loss of the high-frequency power. Also in this exemplary embodiment, a surface roughness of the ceramic layer  80  is Ra=0.8 μm, and a flatness of the ceramic layer  80  is 5 μm or lower. The outer peripheral edge on the top surface of the ceramic layer  80  is chamfered by 0.2 mm to 0.25 mm. 
   In one exemplary embodiment, the cooling gas channel  81  is formed so as to extend 5 mm within the outer peripheral edge of the ceramic layer  80  and to be 1 mm wide and 0.25 mm deep. On the bottom surface of the cooling gas channel  81 , eight gas feed orifices  82  are located at 45° intervals in a circumferential direction formed by the cooling gas channel  81 . On the top surface and toward the center of the ceramic layer  80 , four gas feed orifices  83  are located at 90° intervals on the circumference of 0.35 mm diameter. Each gas feed orifice  82  is 0.32 mm in diameter, and each gas feed orifice  83  is 0.35 mm in diameter. The gas feed orifices  83  on the top surface and toward the center of the ceramic layer  80  also serve as insertion orifices for pins for lifting the semiconductor wafer W, and are therefore formed to be slightly larger than the gas feed orifices  82  on the bottom surface of the cooling gas channel  81 . 
   In one exemplary embodiment, the electrode  90  (consisting of the first and second electrodes  91  and  92 ) is made of tungsten having a thickness of 10 μm to 20 μm. The electrode  90  is approximately 1 mm smaller in radius than the ceramic layer  80  and is laid in a position spaced 0.5 mm from each of the top surface and bottom surface of the ceramic layer  80 , wherein the ceramic layer  80  has a thickness of 1 mm. In other words, the electrode  90  is positioned in the middle of the ceramic layer  80 , relative to the thickness direction of the ceramic layer  80 . 
   In one exemplary embodiment, the disc portion  91   a  of the first electrode  91 , as shown in  FIG. 2 , is approximately 30 mm in diameter. The linear portion  91   b  of the first electrode  91  and the linear portion  92   b  of the second electrode  92  are each approximately 6.0 mm wide. There are seven C-shaped ring portions  91   c  of the first electrode  91  having consecutively increasing diameters. There are seven C-shaped ring portions  92   c  of the second electrode  92  having consecutively increasing diameters. A single circular ring portion  92   d  of the second electrode  92  is provided on the outermost periphery of the electrode  90 . The C-shaped ring portions  91   c  of the first electrode  91 , the C-shaped ring portions  92   c  of the second electrode  92 , and the circular ring portion  92   d  have slightly different widths, with an average width of the ring portions  91   c ,  92   c , and  92   d  being approximately 5.0 mm. Each portion of the first electrode  91  is spaced 1.0 mm away from adjacent portions of the second electrode  92 . The total area of the first electrode  91  is 128.3 cm 2 , and the total area of the second electrode  92  is nearly the same size at 128.4 cm 2 . 
   In one embodiment, a manufacturing method is provided. The manufacturing method includes preparing a first disc-shaped ceramic material compact having one-half of a thickness of a ceramic layer. An electrode is formed on a surface of the first ceramic material compact in a screen printing process. The method also includes preparing a second disc-shaped ceramic material compact having one-half of the thickness of the ceramic layer and including a cooling gas channel on its surface at a location overlying an area to be occupied by the electrode. The second ceramic material compact is placed on the first ceramic material compact for contact bonding. In following, the entire material is fired to complete the ceramic layer. In the presently described manufacturing method, no warpage occurs due to the location of the electrode in the middle of the ceramic layer, relative to the thickness direction of the ceramic layer. Subsequently, the ceramic layer is bonded to an aluminum base plate by means of an adhesive layer. 
   While the present invention has been described in terms of an exemplary electrostatic chuck to be used in a dry etching apparatus, it should be appreciated that the electrostatic chuck of the present invention may be also be implemented within a CVD apparatus or the like, for example a target. In addition, the electrostatic chuck of the present invention can be adapted for use in electrostatically attracting materials other than semiconductor wafers, for example, ceramic substrates. In accordance with the disclosure above, the present invention provides an electrostatic chuck, and a method for manufacturing the same, that prevents warpage of a ceramic layer included therein. Furthermore, the electrostatic chuck of the present invention prevents a cooling gas leakage from occurring at a periphery thereof and increases an electrostatic attraction.