Patent Publication Number: US-6215106-B1

Title: Thermally processing a substrate

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 08/884,192, filed Jun. 30, 1997, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to systems and methods of thermally processing a substrate. 
     Substrate processing systems are used to fabricate semiconductor logic and memory devices, flat panel displays, CD ROMs, and other devices. During processing, such substrates may be subjected to chemical vapor deposition (CVD) and rapid thermal processes (TP). RTP processes include, for example, rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal CVD (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). RTP systems usually include a heating element formed from one or more lamps which radiatively heat the substrate through a light-transmissive window; RTP systems may also include one or more other optical elements, such as an optically reflective surface facing the backside of the substrate and one or more optical detectors for measuring the temperature of the substrate during processing. Many rapid thermal processes require precise control of substrate temperature over time. 
     SUMMARY OF THE INVENTION 
     The invention features a thermal processing method in which a temperature response of a substrate may be controlled during a heat-up phase or a cool-down phase, or both. This reduces the thermal budget of the substrate and improves the quality and performance of devices formed on the substrate. In particular, the inventors have realized that by controlling the rate of heat transfer between the substrate and a thermal reservoir (e.g., a water-cooled reflector plate assembly) during the thermal process, the temperature response of the substrate may be controlled. 
     In one aspect of the invention, the substrate is heated in accordance with a heating schedule and, during the heating schedule, the rate of heat transfer between the substrate and a thermal reservoir inside the thermal processing system is changed. 
     The rate of heat transfer may changed by changing the thermal conductivity between the substrate and the thermal reservoir, by changing the emissivity of a surface of the thermal reservoir, or by changing the distance between the substrate and the thermal reservoir. 
     The thermal conductivity may be changed by changing the characteristics of a thermal transport medium (e.g., a purge gas) located between the substrate and the thermal reservoir. For example, the thermal conductivity may be changed by changing the composition of the purge gas or the pressure of the purge gas between the substrate and the thermal reservoir. The thermal reservoir may include a relatively cool surface inside the processing chamber. The thermal conductivity between the substrate and the relatively cool surface may be increased during a cool-down phase of the heating schedule. The thermal conductivity may be increased by supplying a gas with a relatively high thermal conductivity between the substrate and the relatively cool surface. A first purge gas (e.g., nitrogen, argon and xenon) may be supplied between the substrate and the relatively cool surface during a heat-up phase of the heating schedule, and a second purge gas (e.g., helium and hydrogen) may be supplied between the substrate and the relatively cool surface during the cool-down phase of the heating schedule, the second purge gas having a thermal conductivity that is greater than the thermal conductivity of the first purge gas. 
     In another aspect of the invention, a first purge gas is supplied into the thermal processing system, the substrate is heated in accordance with a heating schedule, and a second purge gas that is different from the first purge gas is supplied into the thermal processing system. 
     The second purge gas may be supplied into the thermal processing system during a cool-down phase of the heating schedule. In one embodiment, the second purge gas is supplied into the thermal processing system at or near the time the substrate temperature has been heated to a target peak temperature. The second purge gas may be supplied into the thermal processing system while the substrate temperature is decreasing. The first purge gas may be supplied into the thermal processing system during a heat-up phase of the heating schedule. 
     In one embodiment, the thermal conductivity of the second purge gas is greater than the thermal conductivity of the first purge gas. In this embodiment, the second purge gas includes helium or hydrogen or both, and the first purge gas includes nitrogen and the second purge gas includes helium. The second purge gas may be supplied into the thermal processing system between the substrate surface and a thermal reservoir inside the thermal processing system. During a heat-up phase of the heating schedule, the first purge gas may be supplied into the thermal processing system between the substrate surface and the thermal reservoir. During a cool-down phase of the heating schedule, the second purge gas may be supplied into the thermal processing system between the substrate surface and the thermal reservoir. 
     Among the advantages of the invention are the following. The results of certain thermal processing methods (e.g., methods of forming ultra shallow junctions) are improved if the rates at which substrates are heated or cooled inside the thermal processing system are high. By changing the rate at which heat is transferred between a substrate and a thermal reservoir inside the processing chamber during the thermal process, the heat-up phase or the cool-down phase, or both phases, may be optimized to improve the quality of the devices produced. 
     Other features and advantages will become apparent from the following description, including the drawings and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic side view of a portion of a thermal processing system, including a reflector plate assembly and a fluid injector. 
     FIG. 2A is a flow diagram of a method of processing a substrate. 
     FIG. 2B contains plots of substrate temperature over time during a spike anneal thermal process using a helium purge gas and during a spike anneal thermal process using a nitrogen purge gas. 
     FIGS. 3A and 3B are exploded views of the reflector plate assembly and the fluid injector shown in FIG.  1 . 
     FIG. 3C 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. 
     FIG. 4 is a diagrammatic view of a purge gas control system of the substrate processing system of FIG.  1 . 
     FIG. 5 is a diagrammatic top view of an alternative fluid injector. 
     FIGS. 6A and 6B are diagrammatic side and top views of a portion of an alternative fluid injector, respectively. 
     FIGS. 7A and 7B are diagrammatic side and top views of a portion of an alternative fluid injector, respectively. 
     FIG. 8A and 8B are diagrammatic side and top views of another fluid injector, respectively. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a system  10  for processing a substrate  12  includes a processing chamber  14  that is radiatively heated by a water-cooled heating lamp assembly  16  through a quartz window  18 . The peripheral edge of substrate  12  is supported by a rotatable support structure  20 , which can rotate at a rate of up to about 300 rpm (revolutions per minute). Beneath substrate  12  is a reflector plate assembly  22  that acts as a thermal reservoir and has an optically reflective surface facing the backside of substrate  12  to enhance the effective emissivity of substrate  12 . A reflective cavity  15  is formed between substrate  12  and the top surface of reflector plate assembly  22 . In a system designed for processing eight-inch (200 mm (millimeters)) silicon wafers, reflector plate assembly has a diameter of about 8.9 inches, the separation between substrate  12  and the top surface of reflector plate assembly  22  is about 5-10 mm, and the separation between substrate  12  and quartz window  18  is about 25 mm. Reflector plate assembly  22  is mounted on a water-cooled base  23 , which is typically maintained at a temperature of about 23° C. 
     The temperatures at localized regions of substrate  12  are measured by a plurality of temperature probes  24  which 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  (processing system  10  may have a total often temperature probes, only three 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 . 
     As shown in FIG. 1, in some thermal processes, a process gas  39  may be supplied into processing chamber  14  through a gas input  30 . The process gas flows across the top surface of substrate  12  and reacts with a heated substrate to form, for example, an oxide layer or a nitride layer. Excess process gas, as well as any volatile reaction by-products (such as oxides given off by the substrate), are withdrawn from processing chamber  14  though a gas output  32  by a pump system  34 . In other thermal processes, a purge gas (e.g., nitrogen) may be supplied into thermal processing chamber  14  through gas input  30 . The purge gas flows across the top surface of substrate  12  to entrain volatile contaminants inside processing chamber  14 . 
     In reflective cavity  15 , a purge fluid injector  40  produces a substantially laminar flow of a purge gas  42  across the top surface of reflector plate assembly  22 . Purge gas  42  is removed from reflective cavity  15  though an exhaust port  44 , which may have a diameter of about 0.375 inch and may be located about 2 inches from the central axis of reflector plate assembly  22 . In operation, purge gas is injected into a 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 reflector assembly  22  by a distance, for example, of about 0.01 inch (0.25 mm), to produce the substantially laminar flow of purge gas  42 . 
     Referring to FIGS. 2A and 2B, in one embodiment, an ultra shallow junction may be formed in an impurity-doped semiconductor substrate as follows. The substrate is loaded into thermal processing chamber  14  (step  200 ). A first purge gas (e.g., nitrogen) is supplied into thermal processing chamber  14  through gas input  30 , or into reflective cavity  15  through the output of purge fluid injector  40 , or both (step  202 ). The substrate is heated to an initial temperature of about 700° C. by heating lamp assembly  16  (step  204 ). At time t 0 , heating lamp assembly  16  begins to heat the substrate to a target peak temperature of about 1100° C. (step  206 ). 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 a second purge gas (e.g., helium) is supplied into reflective cavity  15  by purge fluid injector  40  (step  208 ). In practice, the helium purge gas may be initiated just before the target temperature is reached so that reflective cavity  15 , defined between the substrate and reflector assembly  22 , is filled with the second purge gas by the time the substrate has been heated to the target temperature. If the first purge gas is being supplied by purge fluid injector  40  during the heat-up phase, the purge gas supply is switched from the first purge gas to the second purge gas at or near time t 1 . After the substrate has cooled below a threshold temperature (e.g., below 800° C.), the substrate is removed from thermal processing chamber  14  (step  210 ). 
     The second purge gas may be introduced into reflective cavity  15  during any cool-down phase of a thermal process. For example, in another embodiment, the second purge gas may be supplied into reflective cavity  15  during the cool-down phase following a thermal soak period of a thermal process. 
     The inventors have realized that by changing the rate at which heat is transferred between a substrate and a thermal reservoir inside the processing chamber during the thermal process, the heat-up phase or the cool-down phase, or both phases, may be optimized to improve the quality of the devices produced. 
     For example, 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 one aspect, the inventors have realized that a purge gas with a relatively high thermal conductivity (e.g., helium, hydrogen, or a combination of these gases) may increase the cool-down rate of the substrate and, thereby, improve the operating characteristics or processing yield of certain devices (e.g., ultra shallow junction transistors). For example, the rate at which the substrate cools is substantially greater when a helium purge gas is supplied into reflective cavity  15  than when a purge gas (e.g., nitrogen) with a lower thermal conductivity is used. As shown in FIG. 2B, between times t 1  and t 2  (which may be on the order of about 6 seconds) the substrate temperature has cooled down from about 1100° C. to about 650° C. with a helium purge gas, whereas the substrate temperature has cooled down to only about 800° C. in the same amount of time with a nitrogen purge gas. In another aspect, the inventors have realized that a purge gas with a relatively low thermal conductivity (e.g., nitrogen, argon, xenon or a combination of two or more of these gases) may be supplied into reflective cavity  15  to increase the rate at which the substrate temperature increases during the heat-up phase of the thermal process (e.g., between times t 0  and t 1 ; FIG. 2B) by reducing the thermally coupling between substrate  12  and reflector plate assembly  22 . 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., the integral of substrate temperature T(t) over a fixed period of time: ∫T(t)·dt—may be reduced. This improves the quality of certain devices produced by such a thermal process. 
     Referring to FIGS. 3A and 3B, in one embodiment of a purge reflector  40 , 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 the 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 . 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. 
     Referring to FIG. 3C, 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°. 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° sector  70 , which includes nine of the ten optical ports in top reflector plate  54  (including optical ports  25 ,  26 , and  27 ). In the embodiment described above, a high thermal conductivity purge gas  42  (e.g., helium or hydrogen) increases the thermal conductivity between substrate  12  and reflector assembly  22  during the cool-down phase of a rapid thermal process (e.g., between times t 1  and t 2 ; FIG.  2 B). 
     The flow rates of purge gas and process gas are controlled by the fluid control system shown in FIG. 4. A mass flow controller  80  is used to regulate the flow of gas into processing chamber  14  through gas input  30 , and a pressure transducer  82  and a pressure control valve  84  are used to regulate the rate at which gas is removed from processing chamber  14  through gas output  32 . Purge gas is introduced into reflective cavity  15  through input  46  which is connected to a filter  86 . A mass flow controller  88  is used to regulate the flow of purge gas into reflective cavity  15  through purge gas injector  40 . An adjustable flow restrictor  90  and a mass flow meter  92  are used to regulate the rate at which purge gas is removed from reflective cavity  15 . To reduce the migration of purge gas into the processing region of reflective cavity  15 , above substrate  12 , flow restrictor  90  is adjusted until the rate at which purge gas is introduced into reflective cavity  15  is substantially the same as the rate at which purge gas is removed from reflective cavity  15 . Solenoid shut-off valves  94  and  96  provide additional control over the flow of purge gas through reflective cavity  15 . In a system designed for processing eight-inch (200 mm) silicon wafers, purge gas may be flowed through reflective cavity  15  at a rate of about 9-20 slm (standard liters per minute), although the purge gas flow rate may vary depending upon the pressure inside reflective cavity  15  and the pumping capacity of pump system  34 . The pressure inside reflective cavity  15  and processing chamber  14  is typically about 850 torr. 
     Purge gas may be supplied into reflective cavity  15  in a variety of different ways. 
     Referring to FIG. 5, in one 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 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  102 , and exhaust port  106  may be located about 2 inches from the center of reflector plate  102 . This embodiment may be used when optical ports  108  are distributed over the entire surface of reflector plate  102 . 
     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 reflector plate assembly  110  includes a deflector plate  112  and a top reflector plate  114  that together define flow channels for producing a substantially laminar flow of purge gas in circumferential regions  116 - 122  surrounding optical ports  124  and  126 . The purge gas flows through vertical annular channels  128 ,  129  in top reflector plate  114 . The purge gas may be exhausted through an 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). 
     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). 
     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  also are 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 of support structure  20 . Purge gas is removed from the reflector cavity by a pump system  162  through an exhaust port  164 . 
     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. 
     Other embodiments are within the scope of the claims. 
     For example, although the embodiments disclosed above have been described with reference to a single, relatively cool thermal reservoir (e.g., reflector plate assembly  22 ), other thermal reservoir configurations are possible. The thermal reservoir may be positioned at a different location inside thermal processing system  10 . Two or more independent thermal reservoirs may be provided. The thermal reservoir may include a relatively hot surface, and different purge gases may be supplied into reflective cavity  15 , which is defined between the thermal reservoir and the substrate, to control the temperature response of the substrate. In some embodiments, the temperature of the thermal reservoir may be changed during the thermal process to improve the temperature response of the substrate. 
     In another embodiment, the rate of heat transfer between a substrate and a thermal reservoir inside processing system  10  may be optimized by changing the emissivity of the thermal reservoir during the thermal process. For example, the top surface of reflector plate assembly  22  may include an electro-chromic coating with a reflectivity that may be selectively varied by changing the voltage applied across the coating. In operation, the reflectivity of reflection plate assembly  22  may be maximized during the heat-up phase of a thermal process, and the reflectivity may be minimized during the cool-down phase. In this way, the rate of heat transfer between the substrate and reflector plate assembly  22  may be decreased during the heat-up phase and increased during the cool-down phase. 
     In yet another embodiment, the rate of heat transfer between a substrate and a thermal reservoir inside processing system  10  may be optimized by changing the distance separating the substrate from the thermal reservoir. For example, support structure  20  may be configured to move up and down relative to the top surface of reflector plate assembly  22 . In operation, in one embodiment, support structure  20  may position the substrate a relatively far distance from reflector plate assembly  22  during the heat-up phase of a thermal process, and support structure  20  may position the substrate a relatively close distance from reflector plate assembly  22  during the cool-down phase of the thermal process. In this way, the thermal conductivity between the substrate and reflector plate assembly  22  may be reduced during the heat-up phase of the thermal process and may be increased during the cool-down phase to improve the quality of devices produced on the substrate. 
     In another embodiment, the rate of heat transfer between a substrate and a thermal reservoir inside processing system  10  may be optimized by changing the pressure of a purge gas between the substrate and the thermal reservoir during a thermal process. For example, during a heat-up phase of the thermal process the pressure of the purge gas may be reduced to a sub-atmospheric pressure (e.g., 1-5 Torr), and during a cool-down phase of the thermal process the pressure may be increased to atmospheric pressure (760 Torr). The composition of the purge gas also may be changed during the thermal process. For example, during the heat-up phase the purge gas may consist of nitrogen, and during the cool-down phase the purge gas may consist of helium. 
     Systems and methods have been disclosed for controlling the temperature response of a substrate during rapid thermal processing. The invention may enable certain devices (e.g., ultra shallow junction transistors) to be formed with improved physical features and improved operating characteristics.