Abstract:
A method for manufacturing an electrostatic chuck is disclosed wherein a sintered ceramic body having a dielectric layer made from Alumina (Al2O2) and Titanium Nitride (TiN) having a specific range of particle size is heat treated in an oxygen-rich environment in order to produce a uniform dielectric layer having no pores or micro-cracks.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrostatic chucks, and more particularly to electrostatic chucks used to support a substrate. More specifically, the present invention relates to a method for manufacturing an electrostatic chuck having an extremely uniform dielectric layer with no pores or microcracks. 
     2. Prior Art 
     Typically, electrostatic chucks are used as a clamping surface for electrostatically securing a semiconductor wafer thereon during a vapor deposition or etching process. An electrostatic chuck may comprise a sintered ceramic structure having an electrode interposed between a dielectric layer and a ceramic insulation material with a conductive path established from the electrode through the ceramic insulation material. In operation, an electrical potential is applied to the conductive path through a terminal lead connected thereto that energizes the electrode of the electrostatic chuck. When energized, an electrostatic force is generated between an external electrode, such as a semiconductor wafer, and the internal electrode embedded inside the electrostatic chuck. 
     One method for manufacturing an electrostatic chuck having an embedded electrode includes forming a first layer of a green ceramic material, screen printing a film electrode onto the first layer, depositing a second layer of the green ceramic material over the screen printed electrode and sintering the resulting ceramic structure. However, electrostatic chucks made with this method of manufacture can display fluctuations or non-uniformities in the thickness of the dielectric layer as well as extremely small cracks and pores which can adversely affect the chuck&#39;s ability to electrostatically secure the substrate to the chucking surface. 
     Therefore, there appears a need in the art for a method of manufacturing an electrostatic chuck having a dielectric layer with no pores or microcracks. There also appears a need in the art for a method of manufacturing an electrostatic chuck having an extremely uniform dielectric layer with no fluctuations. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, the primary object of the present invention is to provide an electrostatic chuck having an extremely uniform dielectric layer. 
     Another object of the present invention is to provide a method for manufacturing an electrostatic chuck that employs a novel heat oxidation process to a sintered ceramic structure in order to produce an extremely uniform dielectric layer. 
     A further object of the present invention is to provide a method of manufacturing an electrostatic chuck that uses a predetermined range of particulate size for the conductive material in order to produce a crack-free dielectric layer having no pores. 
     Yet another object of the present invention is to provide a method of manufacturing an electrostatic chuck that permits direct control of dielectric layer thickness. 
     Another further object of the present invention is to provide a method of manufacturing an electrostatic chuck wherein a ceramic structure comprising either a titanium nitride or carbide or other transition metal nitride or carbide is heat oxidized to form the dielectric layer. 
     These and other objects of the present invention are realized in the preferred embodiment of the present invention, described by way of example and not by way of limitation, which provides for a method of manufacturing an electrostatic chuck. 
     In brief summary, the present invention overcomes and substantially alleviates the deficiencies in the prior art by providing a method for manufacturing an electrostatic chuck comprising an internal electrode sandwiched between a dielectric layer and an underlying ceramic insulation material. The method of manufacturing according to the present invention comprises the steps of providing a fixed amount of alumina (Al 2 O 3 ) powder with an organic binder (i.e. acrylic emulsion) that is poured into a metal die or mold and then flattened to cover the entire mold. The alumina powder is loosely compacted and a hollow tube inserted through the alumina powder to the bottom of the mold at the center of the compact. A predetermined amount of titanium nitride (TiN) and alumina mixture is fed through the inside of the tube to the top of the mold such that a conducting path is established for linking to an electrical charge source. The space of the mold outside the tube and above the alumina powder compact is filled with a titanium nitride and alumina mixture to serve as an electrically conductive electrode material that communicates with the conducting path. 
     Once the rest of the mold is filled with the titanium nitride and alumina mixture, the tube is pulled out of the mold without disturbing the compact so that the continuity of the conducting path is maintained. The entire contents of the mold is then pressed to form a rigid compact structure. Preferably, the compact structure is then sintered to form a ceramic structure which is machined to have flat and parallel surfaces by employing conventional methods known in the art. 
     In accordance with one aspect of the present invention, after the sintering and machining operations are completed, the ceramic structure is heat treated in an oxidative environment to create an extremely uniform dielectric layer along the electrode material. 
     In accordance with another aspect of the present invention, the thickness of the dielectric layer produced in the heat oxidation process may be precisely controlled by the particle size of the titanium nitride used in the electrode as well as heat oxidation time and temperature. 
     Additional objects, advantages and novel features of the invention will be set forth in the description which follows, and will become apparent to those skilled in the art upon examination of the following more detailed description and drawing in which like elements of the invention are similarly numbered throughout. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of the electrostatic chuck and mold according to the present invention; 
         FIG. 2  is a cross sectional view of an electrostatic chuck taken along line  2 — 2  of  FIG. 1  produced using the method according to the present invention; 
         FIG. 3  is a table showing the thickness of the dielectric layer created by the heat treatment method according to the present invention; 
         FIG. 4  is a graphic representation illustrating the increase and decrease in the electrical conductivity of an electrode based on the particle size of the titanium nitride used in the method according to the present invention; 
         FIG. 5  is a graphic representation illustrating the effect of oxidation temperature and titanium nitride particle size on the thickness of the dielectric layer formed along the electrode according to the present invention; 
         FIG. 6  is a graphic representation illustrating the clamping force as a function of applied voltage according to the present invention; and 
         FIG. 7  is a table showing the presence or absence of microcracks as a function of titanium nitride particle size according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, the electrostatic chuck manufactured using the method of the present invention is illustrated and generally indicated as  10  in FIG.  1 . The electrostatic chuck  10  provides a surface and means for clamping a semiconductor wafer (not shown) or other substrate during a vapor deposition or etching process. Referring to  FIGS. 1 and 2 , the electrostatic chuck  10  comprises a disc-like body  11  having an electrode  14  sandwiched between a dielectric layer  12  formed on top of electrode  14  and a ceramic layer  18  formed on the bottom thereof. A conductive path  16  comprising an electrically conductive material is established through ceramic layer  18  for linking electrode  14  to a terminal lead  24  when energizing electrostatic chuck  10 . 
     The method for manufacturing electrostatic chuck  10  according to the present invention comprises the steps of providing a predetermined amount of alumina (AlO 3 ) powder mixed with an organic binder (i.e. acylic emulsion) that is poured into a generally circular-shaped mold. The alumina powder is loosely compacted to the level shown by ceramic layer  18  ( FIG. 2 ) and a hollow tube (not shown) inserted through the alumina powder to the bottom of mold at the center of the compact. An electrically conductive material comprising a predetermined amount of titanium nitride (TiN) and alumina mixture (i.e., 25% by weight of titanium nitride) is fed inside the hollow tube until the tube is completely filled from the bottom of mold to a level that is substantially even with the top of the compact in order to form conducting path  16 . The conductive path  16  permits the electrode  14  of the electrostatic chuck  10  to be linked to an electrical charge source (not shown) through terminal lead  24 . 
     To provide electrode  14 , the area outside the tube and above the compact in mold is filled with a predetermined amount of titanium nitride and alumina mixture similar to the electrically conductive mixture contained in the conducting path  16 . The tube is then gently pulled out of the compact without disturbing the arrangement of mixtures inside mold such that the continuity of the conducting path  16  is maintained. 
     The compact is then pressed to form a rigid compact structure, preferably using pressures ranging between 30-300 MPa, utilizing a uni-axial press and/or followed by isostatic pressing using methods known in the art. Once the compact structure is made, it is then preferably hot pressed using temperatures ranging from 1,400° C. to 1,700° C. with pressures between 10-40 MPa for a period of time of 1 to 8 hours which produces a ceramic structure. In the alternative, the compact structure can be sintered in a non-oxidative atomosphere using temperatures of 1,500° C. to 1,800° C., in pressures ranging between 10-40 Mpa for a period of time from 1 to 8 hours. 
     The resultant ceramic structure may then be machined to have flat and parallel surfaces by conventional grinding methods known in the art. Preferably, a diamond grinding procedure using 75 or 125 um is employed during the grinding process. 
     In accordance with one aspect of the present invention, the ceramic structure is then heat treated in an oxidative environment using a temperature range of 1,000° C. to 1,400° C. for a period of time between 0.1 to 10 hours. This novel heat oxidation process produces an extremely uniform dielectric layer  12  along the top portion of electrode  14  by virtue of the reaction which transforms the electrically conductive titanium nitride of electrode  14  into a nonconductive dielectric titanium oxide (TiO2) as set forth in the formula below:
 
TiN+O2−→&gt;TiO2+½N2
 
     The applicants have discovered that the dielectric layer  12  resulting from this heat oxidation process of the ceramic structure produces a layer  12  that is extremely flat and uniform in thickness with standard deviation as shown in FIG.  3 . The table in  FIG. 3  illustrates that the dielectric layer  12  produced by the heat oxidation of the titanium nitride in the TiN/Al 2 O 3  mixture of the electrode  14  ranges in thickness between 240.5 to 260 um having an average thickness of 249.8 um with a standard deviation of only 3.7 um. 
     If desired, the surface of dielectric layer  12 , which normally has a roughness (Ra) of −0.5 um, can be further machined to achieve a flatness have a roughness of less than 0.5 um using conventional diamond polishing. Preferably, the polishing process employs 0.5 um diamond particles and/or 0.05 um colloidal silica to properly polish the surface of dielectric layer  12 . The inventors have discovered that this polishing step eliminates the need for costly additional grinding &amp; lapping steps which can significantly lower the cost of making the electrostatic chuck  10 . 
     Once the electrostatic chuck  10  is manufactured according to the present invention, a terminal lead wire  24 , made from nickel or silver plated copper wire can be brazed or soldered onto the center of the conducting path  16  for connection to an electrical source. For low temperature applications, a soldering process using a silver-tin-titanium alloy can be applied to bind the terminal lead wire  24  at temperatures between 260-420° C. For high temperature applications, an active brazing alloy, such as TICUSIL, may be used to bind the terminal lead wire  24  at a processing temperature of about 900° C. under vacuum or inert atmosphere. 
     Referring to  FIG. 6 , the electrostatic chuck  10  manufactured using the method of the present invention has a clamping force that is a function of the applied voltage shown in the equation set forth below:
 
 F ( V )=∈ V   2 /2( Hd/k+Hgap )2
 
wherein V is applied voltage; ∈ is the permittivity of vacuum; Hd and Hgap are the thickness of the dielectric layer  12  and airgap (not shown), respectively; and K is the relative dielectric constant of the dielectric layer  12 . The inventors have discovered that the pressure exerted on the wafer or substrate by the electrostatic chuck  10  was 7000 Pa when a bias of 900V was applied at a room temperature of 23° C.
 
     In accordance with another aspect of the present invention, the inventors have found that the thickness of dielectric layer  12  can be precisely controlled by manipulating the ratio of titanium nitride to alumina used in the electrically conductive material of electrode  14 , the particle size of the titanium nitride, and the heat oxidation time and temperature used in the heat oxidation process. For example, a dielectric layer  12  with a thickness of 50-300 um can be obtained without any surface cracking or delamination occurring by using the method of the present invention. 
     Referring to  FIGS. 5 and 7 , the thickness of the dielectric layer  12  may also be increased as a function of heat oxidation time during the heat oxidation process. For example, although the particle size of the titanium nitride in electrode  14  does not appear to significantly affect the thickness of dielectric layer  12  when the temperature of the heat treatment process is below 1,300° C., the thickness of the dielectric layer  12  appears to increase when formed from electrode  14  material comprising titanium nitride having a particle size of 6 um oxidized at a temperature of 1,300° C. It was found that the thickness of the oxidized dielectric layer  12  formed from electrode  14  using larger titanium nitride particles size, e.g. 6 um, achieved around a 10% increase in the thickness of the dielectric layer  12  when compared with electrode  14  material containing smaller particle sizes of titanium nitride, e.g. 4.75-0.75 um. This relationship is illustrated in  FIG. 5  which shows the effect of oxidation temperature and titanium nitride particle size on the thickness of the dielectric layer  12  for a period of one hour. 
     The resultant dielectric layer  12  manufactured using the method of the present invention is relatively dense and has only an extremely small amount of porosity, for example, less than 1% porosity for the entire dielectric surface  12 . Further, the dielectric layer  12  shows a uniform grain structure having an average grain size of 7 um. 
     In accordance with yet another aspect of the present invention, the inventors have also found that the conductivity of electrode  14  can be controlled by the varying the concentration of titanium nitride used in the TiN/Al 2 O 3  mixture, the particle size of the titanium nitride in the TiN/Al 2 O 3  mixture, and controlling certain processing parameters, such as sintering time and temperature. Referring to  FIG. 4 , the inventors have discovered, for example, that with a concentration of titanium nitride of about 25 percent by weight in the TiN/Al 2 O 3  mixture, the conductivity of electrode  14  increases when the particle size of the titanium nitride is between 2-6 um, while the conductivity decreases significantly when the particle size is 0.75 um or below. 
     The present invention contemplates that the particle size of the titanium nitride is also critical in producing a crack-free dielectric layer being formed during oxidation process of the inventive method. Referring to  FIG. 7 , it has been discovered that a titanium nitride particle size of 3.5 um or smaller produced no surface micocracks in the dielectric layer. 
     Preferably, the material for the electrode  14  and ceramic layer  18  used to manufacture the electrostatic chuck  10  are made from a powder; however, a conventional green tape made from the same materials can be used in the present inventive method in lieu of powder without departing from the spirit of the invention. 
     Preferably, the acrylic emulsion used as an organic binder is a Rohm &amp; Hass B1002, although any suitable organic binder such PVA, PVB, etc., is felt to fall within the scope of the present invention. 
     It should be understood from the foregoing that, while particular embodiments of the invention have been illustrated and described, various modifications can be made thereto without departing from the spirit and scope of the invention. Therefore, it is not intended that the invention be limited by the specification; instead, the scope of the present invention is intended to be limited only by the appended claims.