Abstract:
A heating apparatus for isothermally distributing a temperature across a semiconductor device or wafer during processing. The invention includes a chamber configured to receive a single semiconductor wafer. Housed within the chamber is a heating member or heating plate. Disposed on a periphery of the heating member is a heat source. Heat energy radiating from the heat source conducts through the heating member to create an isothermal temperature distribution across the heating member. Wafer supports are included on the heating plate which support the wafer in close proximity to the heating plate, such that the temperature of the heating plate establishes the temperature of the wafer. Advantageously, this configuration permits the temperature to be uniformly and isothermally distributed over the wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to semiconductor manufacturing equipment and, more particularly, to an apparatus and method used for rapid thermal processing of a single semiconductor wafer. 
     2. Description of the Related Art 
     The advances in fabrication processes, especially of semiconductor devices of decreased dimensions, have necessitated the development of new processing and manufacturing techniques. One such processing technique is know as Rapid Thermal Processing (RTP), which reduces the amount of time that a semiconductor device is exposed to high temperatures during processing. The RTP technique, typically includes irradiating the semiconductor device or wafer with sufficient power to quickly raise the temperature of the wafer and hold it at that temperature for a time long enough to successfully perform a fabrication process, but which avoids such problems as unwanted dopant diffusion that would otherwise occur at the high processing temperatures. Generally, RTP uses a light source and reflectors to heat the wafer. In a conventional rapid thermal processor a lamp is used as the light source because of its low thermal mass, which makes it easy to power up and down very quickly. 
     Unfortunately, conventional lamp-based RTP systems have considerable drawbacks with regard to uniform temperature distribution. Any single variation in the power output from the lamps can adversely affect the temperature distribution across the wafer. Because most lamp-based systems use lamps with filaments, the wafer is usually rotated to ensure that the temperature non-uniformity due to the filament array is not transferred to the wafer during exposure. The moving parts required to rotate the wafer, adds to the cost and complexity of the system. Another particularly troublesome area for maintaining uniform temperature distribution is at the outer edges of the wafer. Most conventional RTP systems have no adequate means to adjust for this type of temperature non-uniformity. As a result, transient temperature fluctuations occur which may cause the formation of slip dislocations in the wafer at high temperatures (e.g.˜1000° C.). 
     Conventional lamp-based RTP systems have other drawbacks. For example, there are no adequate means for providing uniform power distribution and temperature uniformity during transient periods, such as when the lamps are powered on and off. Repeatability of performance is also usually a drawback of lamp-based systems, since each lamp tends to perform differently as it ages. Replacing lamps can also be costly and time consuming, especially when one considers that a given lamp system may have upwards of 180 lamps. The power requirement may also be costly, since the lamps may have a peak power consumption of about 250 kWatts. 
     For the above reasons, what is needed is an apparatus and method for isothermally distributing a temperature across the surface of a semiconductor device during rapid thermal processing. 
     SUMMARY OF THE INVENTION 
     The present invention provides a heating apparatus for isothermally distributing a temperature across a semiconductor device or wafer during processing. The invention includes an RTP chamber configured to receive a single semiconductor wafer. Housed within the chamber is a heating member or heating plate. Disposed on a periphery of the heating member is a heat source. Heat energy radiating from the heat source conducts through the heating member to create an isothermal temperature distribution across the heating member. Wafer supports are included on the heating member which support the wafer in close proximity to the heating plate, such that the temperature of the heating plate establishes the temperature of the wafer. Advantageously, this configuration permits the temperature to be uniformly and isothermally distributed over the wafer since the temperature distribution over the heating member is easier to achieve and maintain. The heating member may have a larger dimension than the outer dimension of the wafer, to ensure that the uniform temperature distribution at the outer edge of the wafer is maintained isothermal. Alternatively, a plurality of heating members may be stacked together within the process chamber. In this manner, the heating plates may be positioned above and below the wafer, such that the temperature of both heating plates influences the temperature distribution across the wafer. 
     In one aspect of the present invention, an apparatus is provided for heating a wafer during processing. The apparatus includes a process chamber, which defines a cavity. Disposed within the cavity is a heatable plate, which is configured to receive a wafer thereon. A heat source is disposed at a periphery of the heatable plate for creating a uniform temperature distribution across the heatable plate. 
     In another aspect of the present invention, a reactor is provided for rapidly and uniformly annealing a semiconductor wafer. The reactor includes a chamber, which defines a cavity. Disposed in the cavity is a conductive heating member. The heating member is configured to receive a wafer thereon. The reactor also includes a plurality of resistive heating elements positioned proximate to the chamber. A thermal energy output from each of the resistive heating elements provides a substantially isothermal temperature in the cavity. 
     The present invention overcomes the disadvantages of a lamp-based heating system since the heating member can provide a more uniform temperature distribution, for less power and reduced cost. The heating member can also conduct heat at a faster rate than the wafer, so that the wafer can more quickly arrive at the uniform temperature. No moving parts, such as lift pins or wafer spinners, are required within the cavity to load the wafer, nor are other complex and costly components, such as reflectors, actuators, and complex power transformers and controllers. Since the invention does not require large lamps for heating nor moving parts, the size of the chamber, as well as the volume of the cavity, may be substantially reduced relative to other chambers. The reduced volume and size are of particular advantage for reasons that are made apparent below. 
     These and other features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIGS. 1A and 1B are schematic illustrations of a side view and top view, respectively, of one embodiment of a semiconductor wafer processing system for use with the present invention; 
     FIG. 2 is a simplified cross-sectional view of a reactor chamber in accordance with one embodiment of the present invention; 
     FIGS. 3A and 3B, shown an embodiment of the heating member of the present invention; 
     FIG. 4 is an illustration of an embodiment of the present invention; and 
     FIG. 5 is a simplified schematic illustration of an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A and 1B are schematic illustrations of a side view and top view, respectively, of one embodiment of a semiconductor wafer processing system  10  that establishes a representative environment of the present invention. The representative system is fully disclosed in co-pending U.S. patent application Ser. No. 09/451,677, filed Nov. 30, 1999, which is herein incorporated by reference for all purposes. Processing system  10  includes a loading station  12  which has multiple platforms  14  for supporting and moving a wafer cassette  16  up and into a loadlock  18 . Wafer cassette  16  may be a removable cassette which is loaded into a platform  14 , either manually or with automated guided vehicles (AGV). Wafer cassette  16  may also be a fixed cassette, in which case wafers are loaded onto cassette  16  using conventional atmospheric robots or loaders (not shown). Once wafer cassette  16  is inside loadlock  18 , loadlock  18  and transfer chamber  20  are maintained at atmospheric pressure or else are pumped down to vacuum pressure using a pump  50 , if necessary. A robot  22  within transfer chamber  20  rotates toward loadlock  18  and picks up a wafer  24  from cassette  16 . A reactor or thermal processing chamber  26 , which may also be at atmospheric pressure or under vacuum pressure, accepts wafer  24  from robot  22  through a gate valve  30 . Optionally, additional reactors may be added to the system, for example reactor  28 . Robot  22  then retracts and, subsequently, gate valve  30  closes to begin the processing of wafer  24 . After wafer  24  is processed, gate valve  30  opens to allow robot  22  to pick-up and place wafer  24  into a cooling station  60 . Cooling station  60  cools the newly processed wafers, which may have temperatures upwards of 100° C., before they are placed back into a wafer cassette in loadlock  18 . 
     In accordance with the present invention, reactors  26  and  28  are RTP reactors, such as those used in thermal anneals, dopant diffusion, thermal oxidation, nitridation, chemical vapor deposition, and similar processes. Reactors  26  and  28  are generally horizontally displaced, however in a preferred embodiment, reactors  26  and  28  are vertically displaced (i.e. stacked one over another) to minimize floor space occupied by system  10 . Reactors  26  and  28  are bolted onto transfer chamber  20  and are further supported by a support frame  32 . Process gases, coolant, and electrical connections may be provided through the rear end of the reactors using interfaces  34 . 
     FIG. 2 is a simplified cross-sectional view of reactor chamber  100  in accordance with one embodiment of the present invention. Externally, reactor chamber  100  may be a metallic shell  102 , preferably made of aluminum or similar metal, having an opening  104  provided on a face of shell  102 , configured to receive a wafer for processing. Optionally, to protect users and/or equipment near reactor chamber  100 , the chamber may include thermal insulation layers  106  and  108 . The insulation layers may be made of any suitable insulation, such as a ceramic fiber material. Alternatively, a detachable water cooled jacket  110  or similar device may be used to surround reactor chamber  100 . The water cooled jacket  110  ensures that the reactor does not become too hot, so as to be a hazard to nearby equipment or personnel. 
     As shown in FIG. 2, reactor chamber  100  includes a process chamber  112 , which defines an interior cavity  114 . In a preferred embodiment, the volume of process chamber  112  may be kept small, for example, usually no greater than 3000 cm 3 . Preferably, the volume of process chamber  112  is 1000 cm 3 . Accordingly, the small process chamber volume allows reactor chamber  100  to be made smaller, and as a result, system  10  may be made more compact, requiring less clean room floor space. The smaller reactor size, in conjunction with the use of the robot loader, allows multiple reactors to be used in system  10  by vertically stacking the reactors as shown in FIG.  1 A. 
     Opening or aperture  104  of reactor chamber  100  allows for the loading and unloading of wafer  116  before and after processing. Aperture  104  may be a relatively small opening, but with a width large enough to accommodate a wafer of between about 0.5 to 2 mm thick and up to 300 mm (˜12 in.) in diameter, and robot arm  22  passing therethrough. In one embodiment, during a processing procedure an edge of wafer  116  may be no less than 50 mm from aperture  104  when the wafer is positioned in the process chamber. Preferably, the height of aperture  104  is no greater than between about 15 mm and 40 mm, preferably, no greater than 20 mm. The relatively small aperture size helps to reduce radiation heat loss from process chamber  112 . Also, the small aperture size keeps down the number of particles entering process chamber  112  and allows for easier maintenance of the isothermal temperature environment. 
     FIGS. 3A and 3B, show an embodiment of the heating assembly of the present invention. Heating assembly  118  includes heating member or plate  120 , at least one heat source  122 , and a coupling mechanism  124 . In a preferred embodiment, heating assembly  118  may be positioned suspended within process chamber  112 , in a cantilevered relationship on a wall of process chamber  112 . Alternatively, heating assembly  118  may rest on mounts emanating up from a floor of process chamber  112 . 
     Heating plate  120  may have a large mass relative to wafer  116 , and may be fabricated from a material, such as silicon carbide, quartz, inconel, aluminum, steel, or any other material that will not react at high processing temperatures with any ambient gases or with wafer  116 . Arranged on a top surface of heating plate  120  may be wafer supports  126 . In a preferred embodiment, wafer supports  126  extend outward from the surface of heating member  120  to support the single wafer  116 . Wafer supports  126  are sized to ensure that wafer  116  is held in close proximity to heating member  120 . For example, wafer supports  126  may each have a height of between about 50 μm and about 20 mm, preferably about 2 mm to about 8 mm. The present invention includes at least three wafer supports  116  to ensure stability. However, the total contact area between wafer supports  126  and wafer  116  is less than about 350 mm 2 , preferably less than about 300 mm 2 . In one embodiment, thermocouples may be strategically placed embedded within at least one wafer support  126  to provide feedback as to the temperature of wafer  116 . The thermocouples may be conventional R-type or K-type thermocouples available from Omega Corporation of Stamford, Conn. 
     Heating plate  120  may be formed into any geometric shape, preferably a shape which resembles that of the wafer. In a preferred embodiment, heating plate  120  is a circular plate. The dimensions of heating plate  120  may be larger than the dimensions of wafer  116 , such that the surface area of the wafer is completely overlaid by the surface area of heating plate  120 . As illustrated in FIG. 3A, the diameter of heating plate  120  may be no less than the diameter of wafer  236 , preferably, the diameter of heating plate  120  is greater than the diameter of wafer  116 . For example, the radius of heating plate  120  is greater than the radius of wafer  116  by about a length γ, which is between about 1 mm and 100 mm, preferably 25 mm. 
     On a periphery of heating plate  120  is at least one heat source  122 . Heat source  122  may be a resistive heating element or other conductive/radiant heat source, which can be made to contact a peripheral portion of heating plate  120  or may be embedded within heating plate  120 . The resistive heating element may be made of any high temperature rated material, such as a suitable resistively heatable wire, which is made from a high mass material for increased thermal response and high temperature stability, such as SiC, SiC coated graphite, graphite, AlCr, AlNi and other alloys. Resistive heating elements of this type are available from Omega Corp. of Stamford, Conn. 
     The temperature of heating plate  120  may be controllable to provide a variable temperature depending on the application. However, once heating plate  120  is heated to a preferred temperature, the temperature of the plate remains uniform and consistent. Preferably, the temperature of heating plate  120  may be varied between about 50° C. and about 1500° C., preferably between about 100° C. and about 1200° C. 
     Coupling mechanism  124  includes a mounting bracket  128  and electrical leads  130  to provide an electrical power connection to heat source  122 . Mounting bracket  128  is coupled to an internal wall of process chamber  112  using conventional mounting techniques. Once mounted, electrical leads  130  can extend outside of process chamber  112  to be connectable to an appropriate power source. The power source may be a direct line voltage of between about 100 volts and about 500 volts. 
     FIG. 4 is an illustration of yet another embodiment of the present invention. As shown in the figure, a plurality of heating members  120  may be stacked together within process chamber  112 . In a preferred embodiment, mounting holes  132  (FIG. 3A) are provided on a periphery of heating member  120  and extend therethrough. Any appropriate number of mounting holes may be used to ensure that heating plates  120  are well supported. However, each mounting hole is positioned, such that the loading/unloading of wafer  116  is not hampered. Preferably, as illustrated in FIG. 3A, each mounting hole  132  is positioned on a half of heating member  120  away from aperture  104  and near coupling mechanism  124 . This arrangement ensures that the loading/unloading of wafer  116  onto heating member  120  is not impeded. In one embodiment, a rod  134  or similar member is threaded through mounting holes  132  and spacers  136 . Spacers  136  keep heating members  120  an appropriate distance away from one another, which ensures that wafer supports  126  and wafer  116  can be fit in-between the stacked heating members by, for example, robot arm  22 . Preferably, the distance between the stacked heating plates may be between about 10 mm and 50 mm, more preferably about 20 mm. The top most stacked heating plate  138  may be the same in structure and performance as the other heating members, except that the top most heating member  138  may not have a wafer placed upon it. As can be best understood from FIG. 4, in the stacked arrangement, wafer  116  can be heated from top and bottom, which may provide a more uniform and consistent heating environment across the wafer. 
     FIG. 5 shows an alternative embodiment of the present invention. In the alternative embodiment, a plurality of heating elements  140 , surround a top and a bottom portion of process chamber  112 . Resistive heating elements  140  may be disposed in parallel across process chamber  112 . Resistive heating element  140  includes a resistive heating element core and a filament wire. The element core is typically made of a ceramic material, but may be made of any high temperature rated, non-conductive material. The filament wire is conventionally wrapped around the core to allow for an optimal amount of radiated heat energy to emanate from the element. Filament wire may be any suitable resistively heatable wire, which is made from a high mass material for increased thermal response and high temperature stability, such as SiC, SiC coated graphite, graphite, NiCr, AlNi, and other alloys. Preferably, resistive heating filament wire  252  is made of a combination Al—Ni—Fe material, known commonly as Kantal A- 1  or AF, available from Omega Corp. of Stamford, Conn. A direct line voltage of between about 100 volts and about 480 volts may be used to power the resistive elements. Thus, no complex power transformer are needed in the present invention for controlling the output of resistive heating elements  140 . 
     It should be understood that the wafer described above may be made of conventional materials commonly used in the industry, such as silicon, gallium arsenide, or other similar compound or the wafer may be a semiconductor wafer, made from quartz or glass. 
     Having thus described the preferred embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. Thus the invention is limited only by the following claims.