Patent Publication Number: US-2004058560-A1

Title: Fast gas exchange for thermal conductivity modulation

Description:
[0001] The present application relates to semiconductor processing technologies, and particularly to a method of modulating thermal conductivity in a rapid thermal processing chamber.  
       BACKGROUND OF THE INVENTION  
       [0002] One of the major processing steps in manufacturing computer chips from semiconductor substrates is thermal processing, which is used to produce uniform thin films and diffusion regions on the substrates. Thermal processing is traditionally dominated by batch furnace applications, but single wafer rapid thermal procesing (RTP) has become a mature and competitive technology. RTP uses short-time, high-temperature thermal treatments to minimize the effect of dopant diffusion caused by the thermal processing. In addition, RTP has several other advantages such as shorter cycle time, low thermal budget, reduced contamination, and high throughput. Its most common use is for annealing, which activates and controls the movement of atoms in the device after implantation. Another common use is for silicidation, which uses heat to form silicon-containing compounds with metals such as tungsten or titanium. RTP is also commonly used for gate dielectric formation, such as growing oxide on the wafer, and CVD glass reflow.  
       [0003] An RTP system typically includes an RTP chamber with heating and cooling elements. Substrates or wafers processed in RTP systems are thermally isolated from the heating elements, so that heating is performed primarily by thermal radiation. Various heating sources including tungsten halogen lamps, arc lamps and resistively heated susceptors are used as the heating elements. The RTP chamber provides a controlled environment and a means of coupling the thermal energy from the heating elements to the substrate being processed.  
       [0004] Thermal processing in an RTP system can last from a few seconds (e.g. spike anneal for source/drain formation) to a few minutes. As the industry moves to extremely short anneal times in advanced devices, the process of ramping up and cooling down account for a significant fraction of the total RTP time. The formation of ultra-shallow junctions, for example, requires precise, rapid (spike) implant anneals that limit high temperature exposure of the wafer to a few seconds. To enable these new device designs, precise temperature control, high temperature ramp rates, exceptional within-wafer uniformity, and wafer-to-wafer repeatability are required for the rapid thermal processes.  
       SUMMARY OF THE INVENTION  
       [0005] The method of the present invention provides a rapid thermal process with enhanced temperature ramp rates and temperature uniformity across the substrate being processed. The rapid thermal process is performed in a processing chamber and comprises a heat-up phase and a cool-down phase. The enhanced temperature ramp rates are achieved by modulating thermal conduction from the substrate to a thermal reservoir through the use of different purge gases. The temperature ramp rates and temperature uniformity across the substrate are further enhanced by controlling the gas pressure in a thermal processing chamber. In one embodiment of the present invention, a flow of a first purge gas is introduced into a thermal processing chamber, while the gas pressure in the thermal processing chamber is maintained at a first pressure level. The first purge gas fills a reflective cavity between the substrate and the thermal reservoir and conducts heat between the substrate and the thermal reservoir while the temperature of the substrate is being ramped up. At or near the time when the substrate is at a predetermined peak temperature, a flow of a second purge gas is introduced into the processing chamber and the gas pressure in the processing chamber is adjusted to a second pressure level. The thermal conductivity of the first purge gas is lower than the thermal conductivity of the second purge gas. Both the first pressure level and the second pressure level are significantly lower than atmospheric pressure. Also, the first pressure level is preferred to be significantly lower than the second pressure level, so that the second purge gas quickly and uniformly fills the reflective cavity after the flow of the first purge gas is terminated and the flow of the second purge gas is started. The lower conductivity of the first purge gas limits heat transfer from the substrate to the thermal reservoir, allowing the substrate to be heated up quickly by a heating element. The higher conductivity of the second purge gas provides faster heat transfer from the substrate to the thermal reservoir when the substrate is being cooled down, resulting in higher substrate temperature ramp-down rate. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0006] Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings, in which:  
     [0007]FIG. 1A is a diagrammatic side view of a portion of an RTP system for processing a substrate according to one embodiment of the present invention;  
     [0008]FIG. 1B is a diagrammatic side view of a fluid injector which produces a substantially laminar flow of a purge gas across a surface of a portion of a reflector plate assembly;  
     [0009]FIG. 2 is a block diagram of a fluid control system that regulates gas flow rates and pressure in a processing chamber of the RTP system;  
     [0010]FIG. 3A is a graph illustrating a heating schedule during a rapid thermal process;  
     [0011]FIG. 3B is a flow diagram of a method for thermally processing a semiconductor substrate in the processing chamber;  
     [0012]FIGS. 4A and 4B are exploded views of the reflector plate assembly and the fluid injector shown in FIG. 1;  
     [0013]FIG. 4C is a diagrammatic top view of the reflector plate assembly and the fluid injector of FIG. 1 (features of the bottom reflector plate are shown using dashed lines);  
     [0014]FIG. 5 is a diagrammatic top view of an alternative fluid injector;  
     [0015]FIGS. 6A and 6B are diagrammatic side and top views of a portion of an alternative fluid injector, respectively;  
     [0016]FIGS. 7A and 7B are diagrammatic side and top views, respectively, of a portion of an alternative fluid injector;  
     [0017]FIGS. 8A and 8B are diagrammatic side and top views, respectively, of another fluid injector. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0018] The method of the present invention can be performed in an RTP system capable of maintaining gas pressure in a processing chamber at a level that is significantly lower than the atmospheric pressure. An example of such an RTP system is the RADIANCE™ CENTURA® SYSTEM commercially available from Applied Materials, Inc., in Santa Clara, Calif.  
     [0019]FIG. 1A illustrates an RTP system  10  including a processing chamber  14  for processing a disk-shaped semiconductor substrate  12 , according to one embodiment of the present invention. Processing chamber  14  is radiatively heated through a water-cooled quartz window  18  by a heating element comprising a heating lamp assembly  16 . The peripheral edge of substrate  12  is supported by a rotatable support structure  20 , which can rotate at a rate of up to about 120 rpm (revolutions per minute). Beneath substrate  12  is a cooling element comprising a nickel-plated aluminum reflector plate assembly  22  that has an optically reflective surface facing the backside of substrate  12  to enhance the effective emissivity of substrate  12 . Reflector plate assembly  22  is mounted on a water-cooled base  23 , which is typically maintained at about room temperature, e.g., 23° C. Between the top surface of reflector plate assembly  22  and the backside of substrate  12  is a reflective cavity  15 .  
     [0020] In a system designed for processing eight inch (200 mm) silicon wafers, reflector  22  has a diameter of about 8.9 inches, the separation between substrate  12  and the top surface of reflector  22  is about 5-10 mm, and the separation between substrate  12  and the bottom surface of quartz window assembly  18  is about 25 mm. In a system designed for processing twelve-inch (300 mm) silicon wafers, reflector  22  has a diameter of about 13 inches, the separation between substrate  12  and the top surface of reflector  22  is about 18 mm, and the separation between substrate  12  and the bottom surface of quartz window assembly  18  is about 30 mm.  
     [0021] The temperatures at localized regions of substrate  12  are measured by a plurality of temperature probes  24  that are positioned to measure substrate temperature at different radial locations across the substrate. Temperature probes  24  receive light from inside the processing chamber through optical ports  25 ,  26 , and  27 , which extend through the top surface of reflector plate assembly  22 . While processing system  10  typically may have a total of ten temperature probes, only six probes are shown in FIG. 1. At the reflector plate surface, each optical port may have a diameter of about 0.08 inch. Sapphire light pipes deliver the light received by the optical ports to respective optical detectors (for example, pyrometers), which are used to determine the temperature at the localized regions of substrate  12 . Temperature measurements from the optical detectors are received by a controller  28  that controls the radiative output of heating lamp assembly  16 . The resulting feedback loop improves the ability of the processing system to uniformly heat substrate  12 .  
     [0022] During processing, the top surface of substrate  12  is typically exposed to an ambient gas  37 , which can be a process gas, a purge gas, or a combination of two or more gases. RTP processes for annealing are typically performed in inert ambient such as Ar or N 2 . Oxidizing ambient with O 2  or BO x  is used for Rapid Thermal Oxidation, while reactive species are used for RTCVD. Gases for the processing ambient are introduced into a processing region, which is the region on top of the substrate  12  in processing chamber  14 , through ambient gas input  30 . As shown in FIG. 1B, ambient gas  37  flows across the top surface of substitute  12 . Excess ambient gas, as well as any reaction by-products  35 , is withdrawn from processing chamber  14  through ambient gas output  32  by a pump system  34 .  
     [0023] Most of the excess ambient gas and reaction products can be pumped out of processing chamber  14 , but some volatile contaminants  36 , especially those with relatively high vapor pressures such as BO x  and PO x , may leak into reflective cavity  15  and deposit onto the optical components situated around the reflective cavity. The rate at which volatile contaminants are deposited onto these optical components can be substantially reduced by a flow of a purge gas  42  across the top surface of reflective plate assembly  22 . As described in commonly assigned U.S. Pat. No. 6,280,790 B1, which is incorporated herein by reference, a purge fluid injector  40  can be used to produce a substantially laminar flow of a purge gas across the top surface of reflector plate assembly  22 .  
     [0024] As shown in FIG. 1B, purge gas  42  forms a contaminant-entraining bather between substrate  12  and the optical components. Volatile contaminants  36  are entrained in the purge gas flow  42 , and removed through an exhaust port  44  or through ambient gas output  32  by pump system  34 , rather than condense on the optically reflective surface of reflector plate assembly  22 . In one embodiment of the present invention, when exhaust port  44  is provided for purge gas output, exhaust port  42  has a diameter of about 0.375 inch and is located about 2 inches from the central axis of reflector plate assembly  22 . In operation, purge gas is injected into purge gas input  46  and is distributed through a plurality of channels  48  in reflector plate assembly  22 . The purge gas is then directed against a deflector  50 , which is spaced above the top surface of the reflector assembly by a distance, for example, of about 0.01 inch (0.25 mm), to produce the substantially laminar flow of purge gas  42 .  
     [0025] The flow rates of the ambient gas and purge gas, and the gas pressure in processing chamber  14 , are controlled by a fluid control system shown in FIG. 2. Mass flow controller (MFC)  80  is used to regulate the flow of ambient gas into processing chamber  14 . Purge gas is introduced into processing chamber  14  through input  46  which is connected to a filter  86 . A MFC  88  is used to regulate the flow of purge gas into processing chamber  14 . An adjustable flow restrictor  90  and a mass flow meter (MFM)  92  are used to regulate the rate at which purge gas is removed from processing chamber  14 . To reduce the migration of purge gas into the processing region of the processing chamber  14 , which is above substrate  12 , flow restrictor  90  is adjusted such that the rate at which purge gas is introduced into processing chamber  14  is substantially the same as the rate at which purge gas is removed from processing chamber  14 . Solenoid shut-off valves  94  and  96  provide additional control over the flow of purge gas through processing chamber  14 .  
     [0026] A closed-loop pressure control system is used to regulate the gas pressure in processing chamber  14  by controlling the rate at which gases are removed from processing chamber  14 . In one embodiment of the present invention, the pressure control system comprises a pressure control valve  84  at ambient gas output  32 , a pressure gauge  98  coupled to processing chamber  14 , and a programmable logic controller (PLC)  82  coupled between pressure gauge  98  and pressure control valve  84 . During the operation of the processing chamber  14 , the pressure gauge  98  measures the pressure in processing chamber  14  periodically and sends the measured pressure value to PLC  82 . The PLC  82  subtracts the measured pressure value from a pressure set point, which is the intended gas pressure in chamber  14 , and uses an algorithm, such as proportional integral derivative (PID) control, to produce a control signal based on a set of tuning parameters. The control signal is sent to the pressure control valve  84 , and the amount of flow through the valve  84  is adjusted accordingly.  
     [0027] In one embodiment of the present invention, processing chamber  14  is coupled to one or more transfer chambers (not shown), each through a load lock (not shown). The transfer chamber(s) and the associated load lock system facilitate transfers of substrates in and out of processing chamber  14  without substantially changing the gas pressure in processing chamber  14 .  
     [0028] A substrate  12 , after going through a dopant implant process, can be annealed in processing chamber  14  according to a heating schedule, such as the one shown in FIG. 3A. As illustrated in FIG. 3A, substrate  12  is heated to an initial temperature of about 700° C. by heating lamp assembly  16 . At time to, heating lamp assembly  16  begins to heat the substrate to a target peak temperature (e.g. 1000° C. or 1100° C.). After the substrate has been heated to a temperature that substantially corresponds to the target peak temperature (at time t 1 ), the radiant energy supplied by heating lamp assembly  16  is reduced, and the substrate cools down until its temperature is below a threshold temperature (e.g. below 800° C.) and the substrate is removed from thermal processing chamber  14 . Sometimes, when the substrate reaches the target peak temperature, the heating lamp assembly is adjusted to let the substrate soak near the peak temperature for a few seconds of soak time before it is turned off to allow the substrate to cool down.  
     [0029] As described in commonly assigned U.S. Pat. No. 6,215,206, which is incorporated herein by reference, the heat-up phase (e.g. between times t 0  and t 1  in FIG. 3A) or the cool-down phase (e.g. between times t 1  and t 2  in FIG. 3A), or both phases, may be optimized to improve the quality of the devices produced, by modulating the rate at which heat is transferred between a substrate and a thermal reservoir inside a processing chamber during the thermal process.  
     [0030] In one aspect, the rate at which the substrate is cooled may be substantially increased by proper selection of the purge gas supplied between substrate  12  and a thermal reservoir, e.g. water-cooled reflector plate assembly  22 , inside processing system  10 . In particular, a purge gas with relatively high thermal conductivity, e.g., helium, hydrogen, or a combination of these gases, provides better heat transfers between the substrate and the thermal reservoir, and therefore increases the cool-down rate of the substrate. The thermal conductivity of helium is about 5 times that of nitrogen. Thus, as illustrated in FIG. 3A, the rate at which the substrate cools is substantially greater when a purge gas such as helium with relatively high thermal conductivity is supplied into reflective cavity  15  than when a purge gas such as nitrogen with relatively low thermal conductivity is used. For certain devices (e.g. ultra-shallow junction transistors), a higher cool-down rate results in improved the operating characteristics or processing yield of the devices.  
     [0031] In another aspect, a purge gas, such as nitrogen, argon, krypton, xenon, or a combination these gases with a relatively low thermal conductivity may be supplied into reflective cavity  15  to limit heat transfer from the substrate to the thermal reservoir during the heat-up phase of the thermal process, so that the rate at which the substrate temperature increases is not significantly reduced by the purge gas. Using a low thermal conductivity purge gas during substrate heat-up is especially important in spike anneal processes, in which the substrate to reflector plate spacing is reduced in order to improve the ramp-down rate.  
     [0032] Thus, by proper selection of the purge gases supplied between the substrate and a thermal reservoir during the heat-up and cool-down phases of the thermal process, the overall thermal budget—i.e., ∫T(t)dt), the integral of substrate temperature T(t) over a fixed period of time, such as from time t 0  to t 2 —may be reduced. This improves the quality of certain devices produced by such a thermal process.  
     [0033] In order to take full advantage of the different thermal conductivity values of the purge gases used in different phases of the thermal process, the higher thermal conductivity purge gas used in the cool down phase must quickly replace the lower thermal conductivity purge gas used in the heat-up phase. Fast gas exchange is especially important for spike anneal processes, where very short (&lt;1 sec) or no soak time is used and cooling of the substrate starts right after the substrate reaches the target peak temperature. Fast exchange of different purge gases is achieved in the present invention by controlling the chamber pressure during the thermal process. FIG. 3B is a flow diagram of a process  300  for annealing semiconductor substrate  12  in processing chamber  14 , according to one embodiment of the present invention. At step  310 , substrate  12  is loaded onto support structure  20  in processing chamber  14 . The gas pressure in processing chamber  14  is adjusted at step  320  by adjusting the pressure set point in the pressure control system to a first pressure value. In one embodiment of the present invention, the first pressure value is in the range of about 0.1-100 Torr, and more typically in the range of 1-20 Torr. In step  340 , ambient gas  37  is supplied into the processing region in processing chamber  14 . Also in step  340 , if purging of the reflective cavity  15  is desired, a first purge gas may be supplied into reflective cavity  15  in processing chamber  14 . The first purge gas is selected from a group of gases with relatively low thermal conductivity, such as nitrogen, argon, krypton, xenon, or a combination of these gases. In one embodiment of the present invention, the first purge gas is argon, which has a thermal conductivity that is 0.65 times that of nitrogen. Once the gas pressure in processing chamber is stabilized, the heating lamp assembly  16  is turned on at step  350  to heat up substrate  12  according to a heating schedule, such as the one shown in FIG. 3A.  
     [0034] At a predetermined point during the heating schedule, a second purge gas is introduced into reflective cavity  15  at step  360 . The first purge gas flow into reflective cavity  15  may be terminated before or near the time when the second purge gas flow is started. In one embodiment of the present invention, the predetermined point is selected to be at or near the time when substrate  12  reaches the target peak temperature, such as within a second or so before or after substrate  12  reaches the target peak temperature, or within a second or so before or after the end of the soak time. The second purge gas is selected from a group of gases with relatively high thermal conductivity, such as helium, hydrogen, or a combination of these gases. In one embodiment of the present invention, the second purge gas is helium.  
     [0035] Because the gas pressure in processing chamber  14  is significantly lower than atmospheric pressure, the time it takes for the second purge gas to replace the first purge gas in reflective cavity  15  is also much shorter than in conventional annealing processes performed at a pressure level that is comparable to atmospheric pressure. For example, at 760 Torr, the time it takes for the second purge gas to replace the first purge gas is in the order of a few seconds, but at 10 Torr, this time is reduced to the order of milliseconds.  
     [0036] To further reduce the time for purge gas exchange, when the second purge gas is supplied into reflective cavity  15 , the gas pressure in processing chamber  14  is adjusted at step  370  by adjusting the pressure set point in the pressure control system to a second pressure value. In one embodiment of the present invention, the second pressure value is about 1-500 Torr, and more typically about 10-100 Torr. The second pressure value is also about 5-10 times the first pressure value. The sudden pressure increase helps to achieve a much quicker increase in the mole fraction of the second purge gas in reflective cavity  15  after switching between the two different purge gases. Moreover, since thermal conductivity of a gas increases gradually with pressure in the above pressure range, the higher chamber pressure when the second purge gas is supplied further increases the conductivity of the purge gas between the substrate and the reflective plate assembly.  
     [0037] The rate at which the first purge gas is injected into reflective cavity  15  is not critical, as long as it is sufficiently high to achieve the purpose of entraining volatile contaminants and to assure a certain degree of temperature uniformity across the wafer. In one embodiment of the present invention, the first purge gas flow is in the range of about 0.1-5 standard liters per minute (slm), and more typically in the range of 1-3 slm. The rate at which the second purge gas is injected into or exhausted from reflective cavity  15  is relatively high during the time when the second purge gas is replacing the first purge gas. It has been found that when the rate at which the second purge gas is injected into the reflective cavity is approximately equal to the rate at which that gas is exhausted from the reflective cavity, the temperature uniformity across the substrate is optimized during the cool down phase. The rate at which the second purge gas is exhausted from the reflective cavity may also need to be optimized to achieve maximum temperature uniformity. For example, if the exhaust rate is too high, the second purge gas (e.g. helium) will flow out of the chamber too fast, causing cold spots to form on the substrate. On the other hand, if the exhaust rate is too low, the second purge gas flow will take too long to reach the entire region of the substrate, resulting in a temperature gradient across the substrate. In one embodiment of the present invention, the rate at which the second purge gas is injected into or exhausted from reflective cavity  15  is about 10-30 slm, and more typically about 20 slm, during a short period of time, such as a second or so, after the second purge gas flow is started. Afterwards, the rate at which the second purge gas is injected into reflective cavity  15  may be maintained at the same flow rate, or reduced to a lower flow rate, or the flow of the second purge gas may be stopped if purging in the reflective cavity is not desired.  
     [0038] The temperature uniformity across substrate  12  during thermal processing may also be improved by optimizing the manner in which purge gases are introduced into reflective cavity  15 . The purge gases may be supplied into reflective cavity  15  in a variety of different ways. Referring to FIGS. 4A and 4B, in one embodiment of a purge reflector  40 , the reflector plate assembly  22  includes a deflector ring  52 , a top reflector plate  54 , and a bottom reflector plate  56 . Bottom reflector plate  56  has a horizontal channel  58  for receiving purge gas from input  46  and for delivering the purge gas to a vertical channel  60 , which communicates with a plurality of horizontal channels  48  in top reflector plate  54 . Horizontal channels  48  distribute the purge gas to different locations at the periphery of top reflector plate  54 . Deflector ring  52  includes a peripheral wall  62  which rests on a lower peripheral edge  64  of bottom reflector plate  56  and, together with the peripheral wall of top reflector plate  54 , defines a 0.0275 inch wide vertical channel which directs the-purge gas flow against deflector  50  to produce a substantially laminar flow of purge gas across the top surface of reflector plate  54 . The purge gas and any entrained volatile contaminants are removed from the processing chamber through exhaust port  44 , or through ambient gas output  32 . When exhaust port  44  is provided, a horizontal channel  66  in bottom reflector plate  56  receives the exhausted gas from exhaust port  44  and directs the exhausted gas to a line  68  that is connected to a pump system. Each of the channels  48 ,  58 , and  60  may have a cross-sectional flow area of about 0.25 inch by about 0.1 inch.  
     [0039] Referring to FIG. 4C, a purge gas may be introduced into reflective cavity  15  at the top surface of top reflector plate  54  along a peripheral arc of about 75 degrees. The resulting substantially laminar flow of purge gas  42  extends over a region of the top surface of top reflector plate  54  corresponding to the 75-degree sector  70 , which includes nine of the ten optical ports in top reflector plate  54  (including optical ports  25 ,  26 , and  27 ).  
     [0040] Referring to FIG. 5, in another embodiment, a reflector plate assembly  100  is similar in construction to reflector plate assembly  22 , except reflector plate assembly  100  is designed to introduce a purge gas  102  from different locations around the entire periphery of a top reflector plate  104 . Purge gas  102  is removed through ambient gas output  32 , or through an exhaust port  106  that extends through top reflector plate  104 . Purge gas  102  may be introduced at locations about 4.33 inches from the center of reflector plate  104  for 200 mm substrates. When exhaust port  106  is provided, it may be located about 2 inches from the center of reflector plate  104 . This embodiment may be used when optical ports  108  are distributed over the entire surface of reflector plate  104 .  
     [0041] Referring to FIGS. 6A and 6B, in another embodiment, a reflector plate assembly  110  is also similar in construction to reflector plate assembly  22 , except that reflector plate assembly  110  includes a deflector plate  112  and a reflector plate  114  that together define flow channels for producing a substantially laminar flow of purge gas in circumferential regions  116  and  122  surrounding optical ports  124  and  126 , respectively. The purge gas flows through vertical annular channels  128 ,  129  in top reflector plate  114 . The purge gas may be exhausted through ambient gas output  32 , or through a separate exhaust port (not shown) that extends through top reflector plate  114 ; the purge gas may alternatively be exhausted over the circumferential edge of reflector plate assembly  110 . In this embodiment, the top surface of deflector plate  112  acts as the primary optically reflective surface that faces the backside of the substrate. Deflector plate  112  may be spaced above top reflector plate  114  by a distance of 0.01 inch (0.25 mm).  
     [0042] Referring to FIGS. 7A and 7B, in another embodiment, a reflector plate assembly  130  includes a vertical channel  132  for receiving a flow of a purge gas, and a slot-shaped deflector  134  for deflecting the flow of purge gas  136  as a rectangular curtain across an optical port  138  that extends through a reflector plate  140 . A slot-shaped exhaust port  142  is used to remove purge gas  136 . Deflector  134  may be spaced above the top surface of reflector plate  140  by a distance of about 0.01 inch (0.25 mm).  
     [0043] As shown in FIGS. 8A and 8B, in another embodiment, a reflector plate assembly  150  may include a plurality of orifices  152 ,  154 ,  156  which are coupled to a common gas plenum  158  which, in turn, is coupled to a purge gas input  160 . Orifices  152 - 156  are arranged to uniformly introduce purge gas into the reflector cavity defined between substrate  12  and reflector plate assembly  150 . Orifices  152 - 156  are also arranged to accommodate the locations of optical ports  25 - 27  through which temperature probes  24  receive light emitted by substrate  12 . In operation, the purge gas flows into the reflector cavity at a flow rate of about 9-20 slm; in general, the flow rate should be less than the rate required to lift substrate  12  off support structure  20 . Purge gas is removed from the reflector cavity by a pump system  162  through an exhaust port  164 , or through ambient gas output  32 .  
     [0044] Still other purge gas delivery systems are possible. For example, purge gas may be supplied by the rotating gas delivery system described in U.S. application Ser. No. 09/287,947, filed Apr. 7, 1999, and entitled “Apparatus and Methods for Thermally Processing a Substrate,” which is incorporated herein by reference.  
     [0045] The exact order of some of the steps in the process  300  and/or the operation of the processing chamber  14  as described above can be altered. In addition, steps may be added or omitted and process parameters varied depending upon the requirements of a particular processing application and the particular RTP system in which the annealing process takes place. The above operations and the order in which they are presented are chosen for illustrative purposes and to provide a picture of a complete run sequence.