Abstract:
A heating apparatus and method for isothermally distributing a temperature across the surface of a semiconductor device during processing. Specifically, a chamber is provided defining a cavity, which is configured to receive a single semiconductor wafer. A plurality of resistive heating elements are provided and advantageously arranged in the cavity. The heating elements are disposed across the chamber and are aligned in close proximity to one another so as to provide an even heating temperature distribution. In accordance with the present invention, the cavity is divided into heating zones. The resistive heating elements are each individually assigned to a zone and are independently controllable. By individually varying the amount of energy emanating from each resistive heating element, an isothermal temperature distribution may be generated across each zone.

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 Related Art 
     In the semiconductor industry, to continue to make advancements in the development of semiconductor devices, especially semiconductor devices of decreased dimensions, new processing and manufacturing techniques have been developed. 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 used to rotate the wafer, add 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 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 of the system 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 and method for isothermally distributing a temperature across the surface of a semiconductor device or wafer during processing. The invention provides a potentially slip-free RTP process. In one embodiment, a chamber is provided defining a cavity, which is configured to receive a single semiconductor wafer. As described in greater detail below, a plurality of resistive heating elements are provided and advantageously arranged in the cavity. Preferably, the heating elements are disposed across the chamber and are aligned in close proximity to one another so as to provide an even heating temperature distribution. Advantageously, the resistive heating elements may be positioned above and below the wafer to provide dual-sided heating. 
     Preferably, in accordance with the present invention, the cavity is divided into heating zones. The resistive heating elements are each individually assigned to a zone and are independently controllable. By individually varying the amount of energy emanating from each resistive heating element, a substantially isothermal temperature distribution may be generated across each zone. 
     Unlike lamps in a lamp-based RTP system, the resistive heating elements do not require the use of reflectors to evenly distribute the thermal energy. Additionally, the resistive heating elements may be configured into various patterns (e.g. circular, zigzag, and cross-hatched patterns), which may be configured to provide an optimal temperature distribution and reduce the possibility of temperature fluctuations across the surface of the wafer. Advantageously, the heating elements may be covered with a heat diffusing material, which provides uniform temperature dissipation of the heat energy provided by the resistive heating elements. 
     The present invention overcomes the disadvantages of a lamp-based heating system since the system can provide a more uniform temperature distribution, for less power (e.g. about 5 kWatts) and reduced cost. Fortunately, in the present invention rapid thermal processing can be a potentially slip-free processing technique for a wide range of temperatures and time domains, particularly near the wafer edge. 
     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 required, 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. 
     In one aspect of the present invention, an apparatus is provided for rapidly and uniformly heating a wafer during processing. The apparatus includes a process chamber, which defines a cavity configured to receive a wafer. The cavity further defines a plurality of heating zones. The apparatus also includes a plurality of resistive heating elements positioned proximate to the process chamber. Each of the plurality of resistive heating elements may be apportioned to one of the plurality of heating zones. A thermal energy output from each of the resistive heating elements may be capable of heating each of the zones to create a substantially isothermal environment throughout the cavity. 
     In another aspect of the present invention, a reactor is provided for rapidly and uniformly heating a semiconductor wafer. The reactor includes a chamber, which defines a cavity, which further defines a plurality of heating zones. A plurality of resistive heating elements may be positioned proximate to the chamber, and a portion of the heating elements may be apportioned to each one of the plurality of heating zones. The thermal energy output of each of the resistive heating elements heats the zones to provide a substantially isothermal temperature across each of the zones. 
     In yet another aspect of the invention, a method is provided for rapidly and uniformly heating a semiconductor wafer. The method includes apportioning a plurality of resistive heating to a heating zone; and creating a thermal output from each of the plurality of resistive heating elements to change the temperature of at least one of the heating zones in response to temperature fluctuations to provide a substantially isothermal environment over the surface of the semiconductor wafer. 
    
    
     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 block diagram of an embodiment of an RTP reactor system in accordance with the present invention; 
     FIG. 3 is a simplified cross-sectional view of a reactor chamber as in FIG. 2, in accordance the principles of the present invention; 
     FIG. 4 is a simplified top view of an end effector with a wafer (shown in phantom) in accordance with the present invention; 
     FIG. 5 is a simplified schematic illustration of an embodiment of the present invention; 
     FIG. 6 is a simplified illustration of an embodiment of resistive heating elements in accordance with the present invention; 
     FIG. 6A is a simplified illustration of a portion of an embodiment of a resistive heating element of the present invention; 
     FIGS. 6B-6D are simplified illustrations of embodiments of resistive heating elements in accordance with the present invention; 
     FIGS. 7A-7C are various views of an embodiment of the compact gate valve assembly of the present invention; 
     FIG. 7D is a simplified illustration of force vectors acting upon the gate valve assembly of FIGS. 7A-7C in accordance with one embodiment of the present invention; and 
     FIG. 8 is a simplified illustration of the embodiment of FIG. 3 showing temperature zones in accordance with 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 copending 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 a vacuum pressure using a pump  50 . 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 cooling station  60 . Cooling station  60  cools the newly processed wafers 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. In other embodiments, reactors  26  and  28  may also be other types of reactors, such as those used for 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 . 
     A simplified block diagram of an RTP reactor system of the present invention is shown in FIG.  2 . In a preferred embodiment, reactor system  200  may include a reactor chamber  210 , a controller  212 , a process control computer  214 , a gas network  216 , a compact gate valve assembly  218 , and a pump assembly  220 . Each component of the system is described in greater detail below. 
     Referring now to FIG. 3, a simplified cross-sectional view of reactor chamber  210  is shown in accordance with one embodiment of the present invention. Externally, reactor chamber  210  may be a metallic shell  205 , preferably made of aluminum or similar metal, having an opening provided on a face of shell  205 , configured to receive a wafer for processing. Optionally, to protect users and/or equipment near chamber  210 , the chamber may include thermal insulation layers  356 ,  357 ,  358 , and  359 . The layers may be made of any suitable insulation, such as a ceramic fiber material. Alternatively, a detachable water cooled jacket  360  or similar device may be used to surround chamber  210 . The water cooled jacket  360  ensures that the reactor does not become too hot, so as to be a hazard to nearby equipment or personnel. 
     For ease of understanding, the embodiments of reactor chamber  210  shown in FIG. 3 are described as three sections. Each section has components grouped according to the general function performed by the particular section. The sections include: The processing chamber section, the heating section, and the wafer load/unload section. 
     As shown in FIG. 3, the processing chamber section may generally include a closed-end process chamber or tube  230 , which defines an interior cavity  232 . In one embodiment, tube  230  may be constructed with a substantially rectangular cross-section, having a minimal internal volume surrounding wafer  236 . In this embodiment, the volume of tube  230  is usually no greater than 5000 cm 3 , preferably the volume is less than about 3000 cm 3 . One result of the small volume is that uniformity in temperature is more easily maintained. Additionally, the small tube volume allows reactor chamber  210  to be made smaller, and as a result, system  100  may be made smaller, 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  100  by vertically stacking the reactors as shown in FIG.  1 A. Tube  230  is made of quartz, but may be made of silicon carbide, Al 2 O 3 , or other suitable material. 
     To conduct a process, quartz tube  230  should be capable of being pressurized. Typically, tube  230  should be able to withstand internal pressures of about 0.001 Torr to 1000 Torr, preferably between about 0.1 Torr and about 760 Torr. 
     In another embodiment, within tube  230  are wafer support standoffs  234 , which support the single wafer  236 . Standoffs  234  may be any high temperature resistant material, such as quartz. Standoffs  234  may have a height of between about 50 μm and about 20 mm. In order to monitor the temperature of wafer  236  during processing, at least one thermocouple may be embedded into at least one standoff  234 . 
     An opening or aperture  238  at the left end of tube  230  provides access for the loading and unloading of wafer  236  before and after processing. Aperture  238  may be a relatively small opening, but with a height and width large enough to accommodate a wafer of between about 0.5 to 2 mm thick and up to about 300 mm (˜12 in.) in diameter, and robot arm  22  passing therethrough. The height of aperture  238  is no greater than between about 18 mm and 50 mm, and preferably, no greater than 20 mm. The relatively small aperture size helps to reduce radiation heat loss from tube  230 . Also, the small aperture size keeps down the number of particles entering cavity  230  and allows for easier maintenance of the isothermal temperature environment. In one embodiment, during a processing procedure an edge of wafer  236  may be no less than 50 mm from aperture  238  when the wafer is placed on standoffs  234 . 
     FIG. 4 is a simplified schematic illustration of robot arm  22  loading wafer  236  onto standoffs  234 . As shown in FIG. 4, at the end of robot arm  22  is an end effector  62 . In one embodiment, robot arm  22  urges end effector  62  through aperture  238  and subsequently lowers wafer  236  onto standoffs  234 . To hold and transport wafer  236 , end effector  62  may have any number of prongs  64 , usually one or more, preferably two. Prongs  64  are sized and spaced apart from each other so that as end effector  62  enters tube  230 , prongs  64  avoid contacting standoffs  234 . Each prong  64  of end effector  62  has at least one wafer contact point  66 . Preferably, end effector  62  has a total of at least three contact points. Contact points  66  are designed with a minimal surface area to provide a minimum of contact area between wafer  236  and contact points  66 . In a preferred embodiment, the total contact area between contact points  66  and wafer  236  may be less than about 350 mm 2 , preferably less than 300 mm 2 . 
     FIG. 5 shows an alternative embodiment of reactor chamber  210 , which may help to maintain the structural integrity of quartz tube  230  during high temperature processing. In this embodiment, an external cavity  240  may be formed around tube  230  and filled with air, or preferably, N 2 , O 2 , or other process gases. Using pure gases to fill the external cavity may help to extend the usage life of other components, such as heating elements, which may be housed in cavity  240 . In a preferred embodiment, external cavity  240  may be maintained having at least an equal or lower pressure than interior cavity  232  (P 1 ≦P 2 ). In one embodiment, tube  230  may be in communication with loadlock  18 , typically through gate valve  218 , such that the pressure in tube  230  may be equal to the pressure in loadlock  18  (P 2 =P 3 ). In this embodiment, the pressure differential between external cavity  240  and tube  230  creates a force on the internal walls of tube  230 . To create the pressure differential external cavity  240  is evacuated directly at orifice  234  and through pump pipe  237 . Tube  230  is evacuated through loadlock  18  at orifice  235  and through loadlock pipe  236 . Pump pipe  237  and loadlock pipe  236  meet at tube intersection  238  and proceed as one pipe  239  to pump assembly  220 . Since the combined volume of loadlock  18  and tube  230  is greater than the volume of external cavity  240 , it follows that the pressure in external cavity  240  can be less than that in the combined loadlock  18  and tube  230  configuration. In this manner, the internal pressure in tube  230  can be used to fortify tube  230  against failure, and ensures that the structural integrity of tube  230  is maintained. 
     Pump assembly  220  may include any suitable pump for creating the required process pressures within chamber  210 . Pump assembly  220  may also serve other purposes, as are generally required of a pump in a processing system. For example, pump assembly  220  may be used to pump down or create a vacuum in process chamber  230 , such that the cool down rate within the chamber can be controlled. An exemplary pump assembly may include mechanical pump model HC-60B available from Kashiyama Industries Ltd. 
     Referring again to FIG. 3, the heating section of the present invention is configured to surround the process chamber section. The heating section includes heating elements, preferably resistive heating elements  246 . In a preferred embodiment, a plurality of heating elements  246  are used to surround a top and a bottom portion of tube  230 . In one embodiment, shown in FIG. 6, resistive heating elements  246  may be disposed in parallel across chamber  210 . Each element  246  is in relative close proximity to each other element. For example, each resistive heating element  246  may be spaced a distance β from the next closest heating element, which may be between about 5 mm and 50 mm, preferably between about 10 mm and 20 mm. Accordingly, the close spacing of heating elements  246  provides for an even heating temperature distribution across the wafer positioned in cavity  232 . 
     FIG. 6A shows an exemplary heating element  246 , in accordance with the principles of the present invention. Resistive heating element  246  includes a resistive heating element core  250  and a filament wire  252 . Core  250  is usually made of a ceramic material, but may be made of any high temperature rated, non-conductive material. Filament wire  252  is conventionally wrapped around core  250  to allow for an optimal amount of radiated heat energy to emanate from the element. Filament wire  252  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. 
     Optionally, as shown in FIGS. 6B-6D, resistive heating elements  246  may be positioned in various configurations which may include, for example, circular, zigzag, cross-hatched patterns and the like. The variable patterns may be able to provide more optimal temperature distribution and further reduce the possibility of temperature fluctuations across the surface of the wafer. Advantageously, a direct line voltage of between about 100 volts and about 500 volts may be used to power the resistive elements. Thus, no complex power transformer is needed in the present invention for controlling the output of resistive heating elements  246 . 
     Referring to FIGS. 7A-7D and once again to FIG. 3, the loading/unloading section of chamber  210  includes gate valve assembly  218 , which is generally coupled to external shell  205  at aperture  238 . Housed within gate valve main body  300  and aligned along the valve central axis  301  are gate  304 , bellows  306 , plumbing interface  308 , linear drive shaft  310 , and actuator  326 , all assembled together to provide gate valve assembly  218 . In a preferred embodiment, valve main body  300  defines a port  317 . Port  317  has a first end  321 , which provides initial access to the reactor through gate valve  218 , and a second end  323 , which has an aperture or opening  302  configured to mate with aperture  238  of quartz tube  230 . The geometry and dimensions of valve aperture  302  generally correspond to those of reactor aperture  238 , so that valve aperture  302  and reactor aperture  238  can be used together to provide a seal, which maintains a selected vacuum or pressurized environment within tube  230  or isolates cavity  232  during wafer processing operations. 
     Gate  304  is an elongated plate mounted at an upper end of drive shaft  310 . The elongated plate is well suited for sealing slot-type openings, such as valve aperture  302 . It should be understood that the geometry of gate  304  may be changed to accommodate differently shaped openings. Preferably, as shown in FIGS. 7A and 7B, gate  304  may be sloped relative to central axis  301  to form inclined surface  313 . Inclined surface  313  may be sloped at any angle, such as between about 5° and about 85°, which is adequately suited to allow for the proper performance of the present invention. In a preferred embodiment, inclined surface  313  is angled at between about 30° and about 60°, more preferably about 45° to axis  301 . On a top and bottom portion of inclined surface  313  are contact portions  312  and  314 , which extend along the elongated length of gate  304 . In a preferred embodiment, inclined surface  313  may have a highly polished surface or may be coated with a heat/radiation reflective coating, such as gold, silver, Ni, Molybdenum, or other metal with a high melting point relative to the process temperatures. The reflective surface may reflect radiation energy, which may leak through valve aperture  302 , back into tube  230 . 
     By way of example, when drive shaft  310  is moved up into main body  300 , gate  304  is moved upward, into port  317  (FIG.  7 B). Drive shaft  310  is moved up and/or down through linear guide  319  by a linear action created using actuator  326 . In the preferred embodiment, to move drive shaft  310 , actuator  326  is supplied at plumbing interface  308  with a conventional incompressible fluid, such as water or alcohol. The supply of fluid causes drive shaft  310  to move linearly through linear guide  319  into main body  300 . 
     By moving linear shaft  310  vertically upward, as described in FIG. 7A, linear shaft  310  drives the expansion of bellows  306 . Bellows  306  surrounds shaft  310  along axis  301 . In this embodiment, bellows  306  establishes a vacuum seal between actuator  326  and main body  300  to ensure that tube  230  is not contaminated from the outside environment during opening and closing of the valve. 
     As shown in FIG. 7D, when linear shaft  310  reaches the end of its effective travel length (See FIG.  7 B), portions  312  and  314  of gate  304  contact sealing surfaces  316  and  318 , respectively, with a force F to create the positive seal, which isolates tube  230 . Inclined surface  313  causes force F to create reaction forces Rx and Ry at contact surfaces  316  and  318 . The reaction force Rx acts equally on contact surfaces  316  and  318 , as shown in FIG. 7D, normal to inclined surface  313 . It should be understood that the horizontal force component Rx, which provides the sealing force that causes gate  304  to form a positive seal with contact surfaces  316  and  318  at opening  302 . Beneficially, since the sealing force can be provided without the need for a horizontal action or horizontal movement of shaft  310  or gate  304 , gate valve  218  may be designed with a reduced profile and size. It is another advantage of the present invention that when a vacuum is drawn in tube  230 , outside pressure forces the inclined surface  313  against contact surfaces  316  and  318  and O-ring  320  to create the seal. Because of the O-ring, sliding contact between the inclined surface  313  and the contact surfaces  316  and  318  is substantially avoided. The portions of inclined surface  313  which may contact the gate main body, may also be coated with a soft buffer material to avoid metal-to-metal sliding contact, which helps to avoid the creation of contaminating particles. 
     To re-open the valve from the closed configuration, gate  304  is removed from bore  317  when drive shaft  310  is moved down and out from main body  300  through linear guide  319  (FIG. 7A) by actuator  326 , thus removing the sealing force from gate  304  to open bore  317  for loading/un-loading of the wafer. 
     Additionally, gate valve  218  also has an exhaust port  322 , which allows for controlling of the process chamber pressure. Also, cooling fluid ports  324  are provided, which allow a coolant to flow so as to reduce the external temperature of the gate valve main body during RTP. 
     As better understood with reference to FIG. 8, in conjunction with FIG. 2, in a preferred embodiment, reactor chamber  210  may include up to any number of heating zones  400 . In the embodiment shown in FIG. 8, reactor chamber  210  has three parrallel heating zones, which includes a central zone  402  and two adjacent outer zones  404  and  406 . In an alternative embodiment, shown in FIG. 6B, the heating elements may be in a circular configuration and therfore may include at least two heating zones, a central inner zone and an outer zone. Referring again to FIG. 8, each heating element  246  can be apportioned to a specific heating zone  402 ,  404 , and  406 . As described in more detail below, each heating zone  402 ,  404 , and  406  has at least one temperature sensor  354 , which provides feedback to controller  212 . Accordingly, as fluctuations in temperature within a heating zone  402 ,  404 , and/or  406  are sensed by temperature sensors  354 , real-time controller  212  (FIG. 2) can cause the power from power supply  221  (FIG. 2) to increase or decrease, as necessary, to increase or decrease the energy output (heat) from each of resistive elements  246 . For example, if a drop in temperature is sensed in zone  404 , the thermal energy output from resistive heating elements  246  apportioned to zone  404 , increases until the temperature in zone  404  is returned to the desired level. In this manner, the temperature from zone-to-zone across the surface of wafer  236  may be kept substantially isothermal. 
     The number of resistive elements  246  apportioned to each zone may vary based on the energy output desired per zone. The size of each zone (i.e. the heating volume) is also variable. In a preferred embodiment, central zone  402  can encompass at least one wafer of up to about 300 mm in diameter. For example, as shown in FIG. 8, when placed on standoffs  234 , wafer  236  is fully within the boundary of zone  402  as indicated by the dashed lines. Advantageously, the size of each zone can be scaled up or down as desired. For example, zone  402  can be scaled up for processing of larger wafers, by re-apportioning heating elements  246  from zone  404  to zone  402 . This means that the number of heating elements  246  assigned to zone  402  is increased, while the number of heating elements assigned to zone  404  is decreased. The heating elements added to zone  402  are controlled by controller  212  (FIG. 2) to responed in the same manner as the heating elements already assigned to zone  402 . 
     In yet another embodiment, shown in FIGS. 3 and 8, chamber  210  includes heat diffusing members  350 , which are positioned proximate to and typically overlay heating elements  246 . Heat diffusing members  350  absorb the thermal energy output from heating elements  246  and dissipate the heat evenly within tube  230 . It should be appreciated that by heating wafer  236  from above and below, and further by keeping the distance Δ between heat diffusing members  350  small, the temperature gradient within tube  230  is more easily isothermally maintained. For example, if top heat diffusing member  351  is maintained at 1000° C. and bottom heat diffusing member  352  is also maintained at 1000° C., the temperature gradient in the small space between them should also be substantially maintained at 1000° C. with very little fluctuation. Heat diffusing members  350  may be any suitable heat diffusing material that has a sufficiently high thermal conductivity, preferably Silicon Carbide, Al 2 O 3 , or graphite. In a preferred embodiment, temperature sensors  354 , preferably thermocouples, are embedded within heat diffusing members  350 . Thermocouples  354  are strategically placed such that they can provide feedback as to the temperature conditions of the heat diffusing members. For example, at least one thermocouple  354  is placed at each end and at the center of heat diffusing members  351  and  352 . In this configuration, the temperature of each zone  402 ,  404 , and  406  can be monitored with feedback provided to controller  212  (FIG.  2 ). By positioning the thermocouples  354  at known positions on the heat diffusing members  350 , the temperture gradient can be determined with reference to a position within tube  230 . This data is used by controller  212  to control the temperature within each zone  402 ,  404 , and  406  more precisely. Thermocouples  354  are conventional R-type or K-type thermocouples available from Omega Corporation of Stamford, Conn. 
     A microprocessor or process control computer  214 , generally controls the processing of a semiconductor wafer placed in the RTP reactor and may be used to monitor the status of the system for diagnostic purposes. In one embodiment, process computer  214  provides control signals to controller  212  in response to temperature data received from temperature sensors  354  in chamber  210 . Process computer  214  may also direct pressure setpoints to pump assembly  220  as well as gas and plasma inlet flow signals to mass-flow controllers in gas network  216 . In one embodiment, controller  212  is a real-time Proprtional Integral Derivitive (PID), multi-zone controller, available from Omega Corporation. Controller  212  provides control signals to a SCR-based phase controlled power supply  221 , which provides power to the resistive heating elements provided in chamber  210 . In operation, the multi-zone controller receives temperature sensor outputs via sensing line  222  from chamber  210 , as well as the desired wafer temperature setpoint from computer  214  via line  224  and delivers controlled power setpoints to the heating element power supply  221 . As described in greater detail below, the heating elements increase or decrease their energy output in response to the increase or decrease in power supplied from power supply  221 . 
     A primary advantage of the present invention is the ability to conduct substantially slip free RTP of a silicon wafer. The advantages of embodiments of the present invention will be further understood by reference to the following parameters, which are provided below to illustrate the present invention and not to limit or restrict it. 
     In one exemplary embodiment, in order to conduct substantially slip-free RTP in chamber  210  in accordance with the present invention, the following parameters should be used. A wafer having a diameter of 150 mm or greater is loaded into the processing chamber of the present invention. The wafer is loaded using an end effector that has one or more wafer contact points with a cumulative contact point area no greater than 300 mm 2 . The loading/unloading speed of the wafer is done between about 50 mm/s and about 600 mm/s in the horizontal plane and between about 5 mm/s and about 100 mm/s in the vertical plane. The wafer is placed on one or more standoffs positioned within the chamber. The standoffs will generally have a height of between about 50 μm and about 20 mm. The total contact area between the standoffs and the wafer is less than about 350 mm 2 , preferably less than about 300 mm 2 . The processing may occur at temperatures between about 900° C. and 1200° C., preferably between 1000 ° C. and 1200° C., at a chamber pressure of between 0.1 Torr and about 1000 Torr, preferably at atmospheric pressure. 
     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.