This is a continuation of U.S. patent application Ser. No. 09/714,023, filed Nov. 15, 2000, now U.S. Pat. No. 6,500,265, which is a divisional of U.S. patent application Ser. No. 08/890,743, filed Jul. 11, 1997, now U.S. Pat. No. 6,177,023. 


   FIELD OF THE INVENTION 
   The present invention relates generally to a substrate support for maintaining an essentially flat substrate, and more particularly to a hot substrate support that electrostatically maintains the flatness of a substrate. 
   BACKGROUND OF THE INVENTION 
   A susceptor is a mechanical part that functions as a ground electrode and holds a substrate in a processing chamber during fabrication, such as plasma-enhanced chemical vapor deposition (PECVD). The susceptor includes a substrate support plate mounted on a stem, along with a lift assembly for raising and lowering the substrate within the processing chamber. The substrate is held essentially flat to facilitate the deposition process. 
   The extent to which the substrate is held flat generally leads to more uniform structural parameters across the substrate surface. For example, it is easier to grow a film of uniform thickness on a flat substrate than on one that may have a degree of curvature due to, for example, thermal stress. Thus, if uniform structural parameters are required by the process, the substrate must be held essentially flat. 
   In the absence of mechanisms which physically hold the substrate flat, substrates tend to become slightly curved during processing for a number of reasons. For example, a nonuniform temperature across the substrate tends to induce a curvature due to different amounts of thermal expansion at different areas within the substrate. In a large substrate, for example 550×650 square millimeters (mm 2 ), a significant difference in thermal expansion may occur because the substrate heater may not be able to provide a uniform temperature across the large dimensions of the substrate. Further, the perimeter of the substrate has more surface area than the central areas and thus radiates heat faster than the central areas, again leading to temperature nonuniformity, thermal stress and consequent curvature. In smaller substrates, for example, substrates around 360×450 mm 2 , the problem is less pronounced but nevertheless evident. 
   All of the above difficulties become worse as the processing temperature rises. At a typical processing temperature of 320 degrees Celsius (° C.), which is common for a class substrate, the glass substrate will lose its flatness due to the kinds of thermal stress mentioned above. 
   It is thus important to hold substrates essentially flat to prevent such curvature. Previous methods and apparatuses for holding substrates flat employ a frame which physically contacts the substrate around the substrate&#39;s perimeter and holds the substrate against the substrate support by the support&#39;s weight. Several difficulties have been noticed with such systems. 
   First, the substrate area covered by the frame is sacrificed. Thus, there is less surface area of the substrate which may be used for devices or deposition. If the entirety of the perimeter of the substrate is under the frame, substantial loss of surface area may result. 
   Second, the thickness of the deposited materials is not constant near the frame. This is primarily a geometric effect and occurs because of the thickness of the frame. In regions near the center of the substrate, impinging deposition gas molecules or atoms strike the surface of the substrate over a solid angle of 2π steradians corresponding to a hemisphere. Near the perimeter of the substrate, the frame partially blocks gas molecules over a significant fraction of the 2π angle. Near a corner of the frame, blockage is even worse. Thus, it is expected that less gas molecules strike the substrate near the substrate&#39;s perimeter. As a result, the thickness of deposition is usually not uniform near the perimeter of the substrate. 
   Third, deposited material may seep under the frame. Such material cannot be used in films because its thickness is uncontrollable. This problem arises because the frame typically does not contact the substrate in an abrupt manner. In other words, the effective deposition “shadow” of the frame (the point at which edge of the frame starts to inhibit deposition) is not at the same point where the same physically touches the substrate. One reason for this is that the frame may not be completely parallel to the substrate when intimate contact is made. As a result, some deposition may occur on the substrate under the frame. Of course, the amount of such deposition is less than on the unframed central reunion of the substrate. This deposition may be problematic in the sense that it is uncontrollable. 
   Fourth, a physical frame for holding the substrate flat constitutes a large structure to be placed in a processing chamber. As such, it is a potential source for contaminant particles in the chamber which may degrade the quality of the deposited film. This may be particularly true as the contact between the frame and the substrate often causes particle release due to friction. Such particles can also adversely affect the quality of the chamber vacuum. 
   Fifth, a physical frame affects the reliability of transfer when a substrate is processed in one chamber and then moved to another for further processing. In particular, as a substrate is transferred from one chamber to another, a new frame is usually used. Each frame must be aligned in each processing chamber to the same position to avoid a loss of substrate processing area due to misalignment. When misalignment occurs, some of the substrate processing area used in one chamber is shadowed by the frame in the next chamber. Further, some of the substrate previously shadowed by a frame in the one chamber is not covered in the next chamber. In both cases, these areas must be sacrificed as not having been fully processed. To combat this problem, complicated realignment mechanisms must be used to ensure the same area is covered by each frame. Such mechanisms again lead to more particle-releasing surface area in the chamber and ensuing particle contamination and breakdown. Such mechanisms are also expensive and complex, increasing markedly the manufacturing cost of the processing chamber. 
   The inventors have discovered a need to provide a method and apparatus for keeping substrates essentially flat to increase the usable substrate area and to enhance film uniformity across this area, particularly near the edges of the substrate. The method and apparatus should not require complex mechanisms, and should not lead to contamination of the processing chamber. The present invention fulfills these needs. 
   SUMMARY 
   In one embodiment, the invention is directed to a method for holding a substrate on a support layer in a processing chamber. The method includes steps of locating the substrate a predetermined distance from the support layer, starting a plasma in the processing chamber, lowering the substrate to a point where the substrate engages the support layer, and maintaining the plasma for a predetermined time. 
   Implementations of the invention may include one or more of the following. The method may further comprise steps of stopping the plasma and depositing a film on the substrate. The plasma may constitute a gas that is inert to the substrate, for example one selected from the group consisting of nitrogen, hydrogen, arson, helium, krypton, xenon, neon, radon, mixtures thereof, or other similar gases, molecular or otherwise that can form a plasma. The pressure of the gas may be in a range of from about 200 mTorr to about 1 Torr. The power of the plasma may be in a range of from about 100 watts to about 1000 watts. The power density of the plasma may be in a range of from about 0.02 watts per square centimeter of substrate area to about 0.5 watts per square centimeter of substrate area, or about 0.4 watts per cubic centimeter of chamber volume to about 4 watts per cubic centimeter of chamber volume. The substrate may be made of glass. The support layer preferably is a dielectric material, such as anodized aluminum or alumina (Al 2 O 3 ). The method may further comprise the step of depositing a coating on top of the support layer. The preferred coating may be selected from the group consisting of silicon nitrides, silicon oxides, silicon carbides and mixtures thereof. 
   In another embodiment, the invention is directed to a susceptor system for a processing chamber in which a substrate is electrostatically held essentially flat. The system includes a substrate support and a support layer composed of a dielectric material disposed on the substrate support. At least one lift pin is used for supporting the substrate relative to the support layer. Means are provided for moving each lift pin relative to the support layer. Means are also provided for by igniting a plasma within the processing chamber. A gas supply supplies a gas to the interior of the processing chamber. 
   In another embodiment, the invention is directed to a method for processing a substrate in a processing chamber. The method includes steps of locating the substrate a predetermined distance from a support laser, starting a plasma in the processing chamber, lowering the substrate to a point where the substrate engages the support layer, maintaining the plasma for a predetermined time, and depositing or growing a thin film on the substrate. 
   Among the advantages of the invention are the following. A method and apparatus are provided for maintaining a substrate in an essentially flat position. An increase in usable substrate area is achieved, and film uniformity grown thereon is enhanced. The method and apparatus do not require complicated frame mechanisms, and thus have the advantage of not locating potentially contaminant-producing structures into the processing chamber. The method and apparatus may be used in processing substrates in semiconductor processing chambers. 


   
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Like reference numbers and designations in the various drawings indicate like elements. 
       FIG. 1  is a cross-sectional view of a CVD processing chamber. 
       FIG. 2  shows the electrical connections to a heating element used in the CVD processing chamber of FIG.  1 . 
       FIG. 3  is a top view of a substrate support plate used in the CVD processing chamber of FIG.  1 . 
       FIG. 4  is a cross-sectional view of a processing chamber used according to the present invention showing details of the substrate support and the plasma used in the CVD processing chamber of FIG  1 . 
   


   DETAILED DESCRIPTION 
   The present invention is directed to a method and apparatus for maintaining a substrate essentially flat. In the implementation described below, the invention is described with respect to a CVD chamber. However, the invention is also applicable to other sorts of processing chambers. For example, the invention may be used in chambers which carry out the following fabrication processes: CVD, PECVD, etching processes, physical vapor deposition (PVD), and rapid thermal processes such as rapid thermal annealing (RTA). Certain of the details described are specific to this implementation and may be changed as required by the processing conditions and parameters. 
   The present invention may be used in a model AKT-3500 PECVD System, manufactured by Applied Komatsu Technology of Santa Clara, Calif. The AKT-3500 PECVD is designed for use in the production of substrates for large liquid crystal flat panel displays. It is a modular system with multiple process chambers which can be used for depositing amorphous silicon, silicon nitrides, silicon oxides, and oxynitride films. More details regarding the system may be found in U.S. patent application Ser. No. 08/707,491, entitled “A Deposition Chamber Cleaning Technique Using a High Power Remote Excitation Source”, filed Sep. 16, 1996, assigned to the assignee of the present invention and incorporated herein by reference. The present invention, however, may be used with any commercially-available deposition system. 
   PECVD or CVD are processes used to deposit a thin film layer onto a substrate. We intend the term “substrate” to broadly cover any object that is being processed in a process chamber. The term “substrate” includes, for example, flat panel displays, and glass or ceramic plates or disks. The present invention is particularly applicable to large substrates such as glass plates having areas of 360×450 mm 2 , 550×650 mm 2 , and larger. The remainder of this detailed description describes an embodiment in which a glass substrate is used. However, as noted above, other substrates may also be used. 
   In general, the substrate is supported in a vacuum deposition process chamber and is heated to several hundred ° C. Deposition gases are injected into the chamber, and a chemical reaction occurs to deposit a thin film layer onto the substrate. The thin film deposited layer may be a dielectric layer (such as silicon nitride or silicon oxide), a semiconductor layer (such as amorphous silicon), or a metal layer (such as tungsten). The deposition process may be PECVD or thermally-enhanced chemical vapor deposition. In the chamber shown in  FIG. 1 , a plasma is used. Thus, appropriate plasma ignition means, such as the radio frequency (RF) voltage described below, are generally required. 
   As shown in  FIG. 1 , a CVD apparatus  130  includes a susceptor  135  having a substrate support plate  20  mounted on a stem  137 . Support plate  20  may be fabricated, for example, of high purity unanodized cast aluminum or of aluminum alloys. Susceptor  135  is shown centered within a vacuum deposition process chamber  133 . A support layer  22  is located on a plate surface  176  of support plate  20  to support a substrate such as a glass panel (shown in  FIG. 4 ) in a substrate processing or reaction region  141 . As discussed in more detail below, and in accordance with the present invention, support layer  22  constitutes a dielectric material. A lift mechanism (not shown) is provided to raise and lower the susceptor  135 . Commands to the lift mechanism are provided by a controller in known fashion. Substrates are transferred into and out of chamber  133  through an opening  142  in a sidewall  134  of the chamber  133  by a robot blade (not shown). 
   The deposition process gases (indicated by arrow  123 ) flow into chamber  133  through an inlet manifold  126 . The gases then flow through a perforated blocker plate  124  and holes  121  in a process gas distribution faceplate  122  (indicated by small arrows in the substrate processing region  141  of FIG.  1 ). Support layer  22  of support plate  20  is parallel and spaced-closely to faceplate  122 . An RF power supply  172  (shown in  FIG. 4 ) may be used to apply electrical power between gas distribution faceplate  122  and susceptor  135  so as to excite the process gas mixture to form a plasma. The constituents of the plasma react to deposit a desired film on the surface of the substrate on support plate  20 . 
   The deposition process gases may be exhausted from the chamber through a slot-shaped orifice  131  surrounding the reaction region  141  into an exhaust plenum  150 . From exhaust plenum  150 , the gases flow by a vacuum shut-off valve  152  and into an exhaust outlet  154  which connects to an external vacuum pump (not shown). 
   Referring to  FIG. 2 , support plate  20 , as noted, is attached to stem  137  of susceptor  135 . Support plate  20  may include a top plate  40 , a base plate  42 , and a braised region  44  therebetween. Disposed in support plate  20  between top plate  40  and base plate  42  are heaters  24  and  26 . In the illustrated embodiment, one or more heating element tubes  50  (only one is visible in  FIG. 2 ) are disposed within the hollow core of stem  137 . Each tube includes a conductive lead wire  52  for attachment to an end of a filament of a heating element. Tubes  50  are terminated at the end of stem  137 , and lead wires  52  are connected to a heater controller which powers the heating elements and monitors the temperature of support plate  20 . The heaters may be located about 0.25 in. beneath the uppermost surface of support layer  22  on support plate  20 . In this embodiment, one heater is run at approximately 0.66 in. from the outer edge of the plate, while another is run at approximately 7.75 in. from the outer edge. This configuration provides for uniform heating of a substrate  165  placed on support plate  20 . 
   Stem  137  includes a hollow core and is configured to mate with base plate  42  of support plate  20 . A vacuum tight joint  85  is made such that the inside of the hollow core is at ambient (atmospheric) pressure. 
     FIG. 3  shows a top view of substrate support plate  20 . As noted, support plate  20  includes a support layer  22  for supporting a substrate  165  (shown in phantom) in the vacuum deposition process chamber. Heaters  24  and  26  (both shown in phantom) are disposed beneath support layer  22  on support plate  20 . Support plate  20  is rectangular in shape, and has a width “w” of about 26.26 inches and a length “l” of about 32.26 inches. 
     FIG. 4  shows the depth “d” and length of support plate  20 . A typical depth of support plate  20  may be about one inch. This allows for the processing of a glass substrate for that panel displays of up to about 1 square meter (m 2 ). The size of support plate  20  is scalable to accommodate either larger or smaller substrates. 
   Support layer  22  is disposed on top of support plate  20 . Support layer  22  may be a separate plate placed in intimate contact with a surface  176  of support plate  20  or a thick layer of a material coated on surface  176  of support plate  20 . The material constituting support layer  22  is anodized or otherwise treated so that support layer  22  has the properties of a dielectric. For example, anodized aluminum or alumina (Al 2 O 3 ) may be used. In this way, any charge induced on a surface  23  of support layer  22  is stationary. 
   A coating (not shown) may be deposited on surface  23  of support layer  22  to enhance the dielectric properties of support layer  22 . In other words, if support layer  22  by itself is not a superior dielectric, the coating may improve its dielectric properties. Such coatings may constitute dielectrics, and may be, for example, silicon nitrides (Si x N y ), silicon oxides (Si w O z ), silicon carbides (Si r C s ) or other such dielectrics. One type of SiN deposition is described in U.S. Pat. No. 5,399,387, assigned to the assignee of the present invention and incorporated herein by reference. This coating enhances the effect of the invention but is not inherently required for its practice. 
   As mentioned above, a robot blade facilitates the transfer of substrates into and out of chamber  133  through an opening  142  in sidewall  134  of chamber  133 . Once the robot blade moves substrate  165  into position, lift pins  171  move upwards to support substrate  165  prior to lowering into a processing position. In particular, lift pins  171  move through lift pin holes  162  to contact and support substrate  165 . Lift pins  171  may move through lift pin holes  162  by the action of a lift means  180  such as known translation mechanisms or linear feedthroughs. 
   After substrate  165  has been contacted and supported by lift pins  171 , the robot blade is withdrawn and substrate  165  may be lowered into position for processing. In the method of the invention, substrate  165  is not lowered into intimate contact with support layer  22  (or a coating) until after a plasma  169  is ignited in the chamber. 
   In particular, lift pins  171  retract and lower substrate  165  until a bottom surface  173  of substrate  165  is at a predetermined separation distance, in a range of about 20 to 50 mils above surface  23  of support layer  22 . This position is termed here an “intermediate height”. Lift pins  171  may be attached to a moving means which is remotely and/or computer-controlled by a controller  177 . At this point, a plate-charge inducing plasma  169  is started or ignited in chamber  133 . The separation distance is chosen to be wide enough for this plasma to enter the volume between substrate  165  and support layer  22 . If the distance is too small, no plasma will enter this volume and no charge effects will occur. If the distance is too large, the plasma may become unstable near the edges of substrate  165 . 
   Plate-charge inducing plasma  169  is formed from a gas that is relatively inert to the substrate, is such as nitrogen (N 2 ), hydrogen (H 2 ), argon (Ar), helium (He), krypton (Kr), xenon (Xe), radon (Rn), or mixtures thereof. Other gases with similar plasma properties may also be used. A requirement of the gas used is that it not itself deposit on substrate  165 . The power of plasma  169  may be relatively low, such as in a range of about 100 watts to 1000 watts for a chamber having a volume of 250 cubic centimeters and a substrate processing area of 550×650 mm 2 ; the power would scale up or down for chambers of larger or smaller volumes, or larger or smaller substrate processing areas, respectively. For example, as the power scales with the size of substrate  165 , a usable range of power densities may be 0.02 watts per square centimeter to 0.5 watts per square centimeter of substrate area. As the power also scales with the volume of chamber  133 , a usable range of power densities may be 0.4 watts per cubic centimeter to 4 watts per cubic centimeter of chamber volume. 
   The pressure of plate-charge inducing plasma  169  may be between about 200 milliTorr (mTorr) and 3 Torr. Generally lower pressures are preferred because they allow a larger bias to be induced between substrate  165  and support layer  22  (in a manner to be explained below). This bias refers to the amount of charge induced on the bottom surface  173  of substrate  165  and the amount of charged induced on support layer  22 . These charges are responsible for an electric field having features and advantages discussed below. 
   After plasma  169  is ignited, substrate  165  is lowered onto support layer  22 , to a “processing height”, by retracting lift pins  171 . The rate at which the substrate is dropped may-be on the order of an inch every few seconds, such as one inch every three seconds. After placement of substrate  165  on support layer  22 , plasma  169  is maintained for a predetermined time which may be measured in seconds. For example, ten seconds has been found to be appropriate for a large glass substrate. However, it should be noted that there is a wide range of variation in the length of this time period. Longer or shorter time periods may also be used depending on the substrate material, the plasma power, the support layer material, the coating material, and the type of gas used in the plasma. 
   After engaging substrate  165  on support layer  22  and maintaining plasma  169  for several seconds, substrate  165  is held in an essentially flat manner against support layer  22  and is highly resistant to curvature. 
   One reason why substrate  165  is held to support layer  22  is believed to be because of electrostatic attraction. In particular, it is noted that plate-charge inducing plasma  169  is composed of electrons (denoted in  FIG. 4  as “−e”) and positive ions (denoted in  FIG. 4  as “+i”). The voltage applied via voltage source  172  to ignite plasma  169  is an RF voltage which alternates between a positive value and a negative value. The potential of susceptor  135  is held to ground. When the applied voltage is positive, electrons are attracted to faceplate  122 . When the applied voltage is negative, positive ions are attracted to faceplate  122 . As even the smallest positive ion is about 2000 times more massive than an electron, the electrons move much faster due to lower inertia. When the electrons are attracted to the faceplate  122 , a net positive charge is left in plasma  169  because most of the remaining species within plasma  169  are positive ions. Even when electrons are repelled by a negative voltage swing of source  172 , they are not believed to be repelled so far as to render the net charge of plasma  169  neutral. This is indicated in  FIG. 4  by the placement of several electrons adjacent faceplate  122 . The net positive charge of plasma  169  (indicated in  FIG. 4  by a preponderance of positive ions in plasma  169 ) then induces a negative charge on a top surface  175  of substrate  165  (indicated by a series of negative signs near surface  175 ) By conservation of charge, this induced negative charge in turn induces a positive charge on bottom surface  173  of substrate  165  (indicated by a series of positive signs on surface  173 ). This positive charge then induces a negative charge on top surface  23  of support layer  22  (indicated by a series of negative signs on surface  23 ). 
   Another factor enhances the induced negative charge on top surface  23 . Because surface  23  is not completely electrically screened from plasma  169  (such partial screening occurring because of the placement of substrate  165 ), the net positive charge of plasma  169  also tends to induce a negative charge on surface  23  of support layer  22 . Thus, an electrostatic attraction is formed between surface  23  of support layer  22  and bottom surface  173  of substrate  165 . 
   To summarize, bottom surface  173  of substrate  165  is left with an induced positive charge, and top surface  23  of support layer  22  is left with an induced negative charge. By Coulomb electrostatic attraction, substrate  165  is held substantially flat against support layer  22 . 
   Following several seconds of plasma  169 , plasma  169  may be stopped or extinguished and further processing of substrate  165  may begin. Further processing may also begin without stopping plasma  169 . In this method, the inert gas or gases forming plasma  169  are simply replaced by appropriate process gases while source  172  continually causes the gases to enter the plasma state. 
   Further processing may include film deposition. Such processing may involve reinstatement of a different plasma for use in deposition or other processes, and this plasma may be of an inert gas or otherwise. 
   Once further processing is completed, substrate  165  is removed from support layer  22 . This may be accomplished by using lift pins  171  to force the substrate off of support layer  22 . Another way to remove substrate  165  from support layer  22  is described in U.S. Pat. No. 5,380,566, issued Jan. 10, 1995, assigned to the assignee of the present invention and incorporated herein by reference. 
   In summary, a substrate support including an electrostatic substrate attachment feature has been disclosed. This feature allows a substrate to be held essentially flat against a support layer without loss of usable substrate area. 
   A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.
An apparatus and method for holding a substrate on a support layer in a processing chamber. The method includes the steps of positioning the substrate a predetermined distance from the support layer, introducing a plasma in the processing chamber, lowering the substrate to a point where the substrate engages the support layer, and maintaining the plasma for a predetermined time. The apparatus is directed to a susceptor system for a processing chamber in which a substrate is electrostatically held essentially flat. The apparatus includes a substrate support and a support layer composed of a dielectric material disposed on the substrate support. At least one lift pin is used for supporting the substrate relative to the support layer. Means are provided for moving each lift pin relative to the support layer. Means are also provided for producing a plasma within the processing chamber.