Patent Publication Number: US-7906402-B2

Title: Compensation techniques for substrate heating processes

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/561,851, filed Nov. 20, 2006, by Ranish, et al., and entitled “Compensation Techniques For Substrate Heating Processes,” which application is incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     Embodiments of the present invention generally relate to substrate processing techniques. More specifically, the present invention relates to processing techniques for creating desired thermal profiles during substrate processing. 
     2. Description of the Related Art 
     Rapid thermal processing (RTP) and rapid thermal chemical vapor deposition (RTCVD) annealing processes, and the like (collectively and generically referred to herein as “conventional heating processes”), traditionally use a furnace with infrared radiation generated by halogen lamps to heat a substrate. The substrate, commonly made of silicon, is disposed in a controlled atmosphere enclosure, and the infrared radiation is directed onto the superficial face of the substrate through a transparent window. 
     The temperatures reached during thermal processing operations may be high, often over 1000° C., with thermal gradients liable to reach several 100° C./second or higher. One important parameter of such substrate processing is the uniformity of the temperature over the entire surface of the processed substrate. The presence of thermal gradients of just a few degrees between the various portions of the substrate can cause defects in the substrate. However, heat loss near the edges of the substrate is much greater than near the center, which leads to lower temperatures at the edge of the substrate. 
     Several solutions have been proposed to compensate for this temperature inequality. Some examples include: a metal reflector positioned at the rear of the lamps, heating both sides of the substrate with two sets of lamps arranged along opposite sides of the reactor, heating by zones in the reactor, the use of heated susceptors, and fitting an edge ring to minimize heat transfer through the sides of the substrate. However, despite any improvements these solutions may have provided, thermal gradients continue to exist sufficient to cause defects in the substrates. 
     Therefore, there is a need in the art for a method and apparatus that generates desired substrate thermal profiles when subjected to these heating processes. 
     SUMMARY 
     Methods for compensating for a thermal profile in a substrate heating process are provided herein. In one embodiment, a method of processing a substrate includes determining an initial thermal profile of a substrate resulting from a process; imposing a compensatory thermal profile on the substrate based on the initial thermal profile; and performing the process to create a desired thermal profile on the substrate. 
     In another embodiment, a method of processing a substrate includes determining an initial thermal profile of a substrate resulting from a process; adjusting a local amount of mass heated per unit area of a component proximate the substrate in response to the initial thermal profile; and performing the process to create a desired thermal profile on the substrate. 
     In another embodiment, a method of processing a substrate includes determining an initial thermal profile of a substrate resulting from a process; adjusting a local heat capacity per unit area of a component proximate the substrate in response to the initial thermal profile; and performing the process to create a desired thermal profile on the substrate. 
     In another embodiment, a method of processing a substrate includes determining an initial thermal profile of a substrate resulting from a process; controlling the heat provided by an edge ring to the substrate in response to the initial thermal profile; and performing the process to create a desired thermal profile on the substrate. 
     In another embodiment, a method of processing a substrate includes determining an initial thermal profile of a substrate resulting from a process; adjusting an absorptivity or a reflectivity of a component proximate the substrate in response to the initial thermal profile; and performing the process to create a desired thermal profile on the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. 
         FIG. 1  depicts a schematic, cross-sectional view of a substrate process chamber in accordance with one embodiment of the present invention; 
         FIG. 2  depicts an illustration of an initial thermal profile of the substrate of  FIG. 1 ; 
         FIG. 3  depicts a graphical representation of the initial thermal profile of the substrate of  FIG. 1 , along axis  3 - 3  of  FIG. 2 ; 
         FIG. 4  depicts a graphical representation of a compensatory thermal profile used to compensate for the initial thermal profile of  FIGS. 2 and 3  in accordance with one embodiment of the invention; 
         FIG. 5  depicts a flowchart of one embodiment of a method for creating a desired thermal profile for a substrate; 
         FIG. 6  depicts a flowchart of one embodiment of a method for creating a desired thermal profile for a substrate; 
         FIG. 7  depicts a schematic, cross-sectional view of a susceptor in accordance with one embodiment of the present invention; 
         FIG. 8  depicts a schematic, cross-sectional view of a susceptor in accordance with one embodiment of the present invention; and 
         FIG. 9  depicts a flowchart of one embodiment of a method for creating a desired thermal profile for a substrate. 
     
    
    
     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. Where possible, identical reference numerals are used herein to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     The present invention provides methods for processing a substrate utilizing thermal compensation techniques either prior to or during a heating process for creating a desired thermal profile for a substrate during processing. 
       FIG. 1  is a schematic cross-sectional view of a process chamber  100  in accordance with one embodiment of the present invention. The process chamber  100  is suitable for thermally processing substrates  130  such as semiconductor wafers, glass or sapphire substrates, and the like. As used herein, thermally processing refers to any process performed on a substrate in which the temperature of the substrate is controlled. Accordingly, the process chamber  100  may be adapted for performing at least one of deposition processes, etch processes, plasma-enhanced deposition and/or etch processes, and thermal processes, among other processes performed in the manufacture of integrated semiconductor devices and circuits. Specifically, such processes may include, but are not limited to, rapid thermal processes (RTP), rapid thermal chemical vapor deposition (RTCVD), annealing processes (such as flash annealing), and the like. 
     In the embodiment depicted in  FIG. 1 , the process chamber  100  illustratively comprises a chamber body  102 , support systems  160 , and a controller  150 . The chamber body  102  generally includes an enclosure  104  having an upper portion  106 , a lower portion  108 , and, optionally, a chamber divider  170 . 
     Typically, one or more heat sources  110 , a susceptor  120 , and a susceptor lift  122  may be disposed within the chamber body  102 . The susceptor  120  is configured to support a substrate  130  thereupon. Optionally, an edge ring  140  may be disposed upon the susceptor  120 . The edge ring  140  is generally configured to surround the substrate  130  and may optionally include a heating element, such as a resistive heater  142 . Optionally, the substrate  130  may be held by the edge ring  140  and the susceptor  120  may be absent. 
     The heat sources  110  may be disposed at any location throughout the chamber. Typically, the heat sources  110  are disposed in at least one portion of the chamber, for example, the upper portion  106  and/or the lower portion  108  of the chamber body  102 , and may be separated by the chamber divider  170 . However, some embodiments may provide heat sources  110  on a side  180  of the chamber in addition to or instead of in the upper portion  106  and/or the lower portion  108 . Suitable heat sources  110  include heat lamps, hot plates, bottom-radiant devices, infrared (IR) radiation sources, or any other type of heat source suitable for heating the substrate  130 . 
     The susceptor  120 , which serves as a support surface for the substrate  130 , is disposed on a susceptor lift  122  in the lower portion  108  of the process chamber  100 . The susceptor lift  122  may readily raise and lower the susceptor  120  and substrate  130  as desired. The substrate  130  is placed on the susceptor  120  and during a heating process, a temperature distribution is formed across the surface of the substrate  130  by the heat sources  110  (which may vary from a center  131  to an edge  132  of the substrate  130 ). Depending on the type of process being performed, an edge ring  140  may optionally be used to modify the thermal behavior of the edge (for example, by supplying or removing heat to the substrate edge for higher/lower heating rates, such as by conductively providing/removing heat to or from the substrate edge and/or by reflecting radiation onto the substrate edge or shielding the substrate edge from radiation, or the like). 
     The support systems  160  of the process chamber  100  include components used to execute and monitor pre-determined processes (e.g., growing epitaxial silicon films) in the process chamber  100 . Such components generally include various sub-systems (e.g., gas panel(s), gas distribution conduits, vacuum and exhaust sub-systems, and the like) and devices (e.g., power supplies, process control instruments, and the like) of the process chamber  100 . These components are well known to those skilled in the art and are omitted from the drawings for clarity. 
     The controller  150  generally comprises a central processing unit (CPU)  152 , a memory  154 , and support circuits  156  and is coupled to and controls the process chamber  100  and support systems  160 , directly (as shown in  FIG. 1 ) or, alternatively, via computers associated with the process chamber  100  and/or the support systems  160 . In one embodiment, a software routine  162  is disposed in the memory  154 , which, when executed, implements compensation techniques for an initial thermal profile  158 , discussed below. 
       FIGS. 2 and 3  depict illustrative top and side views, respectively, of an initial thermal profile  158  of a substrate  130 . The initial thermal profile  158  typically corresponds to a thermal profile of the substrate  130  immediately or shortly after being subjected to a heating process in a process chamber  100 . The thermal profile may be determined by measuring a process result as a function of position on the substrate  130  and converting the process result into temperature differences from knowledge of the process activation energy. The process does not necessarily have to be the same process as the one actually being used in production but can be any well characterized process. For example, a silicon substrate  130  can be subjected to an atmosphere of pure oxygen during a thermal exposure. Afterwards, the silicon dioxide thickness can be used to infer the spatial temperature variation which will be substantially the activation energy weighted temperature distribution during the process. By choosing a characterization process with a very similar activation energy to the production process, the weightings will be approximately the same. Alternately, the weighting can be corrected with knowledge of the respective activation energies. 
     In one example, the initial thermal profile  158  may generally decrease in temperature concentrically from the center of the substrate  130  due to more rapid heat loss near the edges of the substrate. Accordingly, in the embodiment depicted in  FIG. 2 , the substrate is hottest near the center  131  and has a declining temperature approaching the edge  132 . Although  FIGS. 2 and 3  depict an initial thermal profile  158  wherein the center is hotter than the edges, it is contemplated that some processes may result in different thermal profiles, including those with cooler center portions of the substrate  130 . 
       FIG. 4  depicts a graphical representation of a compensatory thermal profile  159  designed to compensate for the initial thermal profile  158  of  FIG. 3 , in accordance with one embodiment of the present invention. The compensatory thermal profile  159  is a thermal profile that, when added to the initial thermal profile  158 , yields a desired thermal profile  157  for the substrate  130 . For example, where a desired thermal profile  157  is a uniform thermal profile (i.e., a substantially flat profile), the compensatory thermal profile  159  is the mathematical inverse of the initial thermal profile  158 . Thus, if the two profiles were superimposed upon one another, a graphical representation would appear as a straight line (e.g., thermal profile  157  in  FIG. 4 ). So long as an initial thermal profile  158  and a desired thermal profile  157  are known, or can be determined, a compensatory thermal profile  159  can be found by subtracting the initial thermal profile  158  from the desired thermal profile  157 . Consequently, the compensatory thermal profile  159  is sought to be imposed on a substrate  130  according to embodiments of the present invention, as discussed in more detail below. 
       FIG. 5  depicts a flowchart of one embodiment of a method  500  for utilizing compensation techniques for creating a desired thermal profile  157  on a substrate  130  during a heating process. The method  500  is described with reference to  FIGS. 1 through 4 . The method  500  starts at step  502 , where the initial thermal profile  158  of the substrate caused by a heating process is determined. Different methods may be used to determine the initial thermal profile  158  of the substrate  130 , including without limitation empirical or experimental testing, computer-based modeling, mathematical modeling, and the like. During empirical or experimental testing, thermal profilers (such as thermocouples, optical pyrometers, radiation pyrometers or the like) may be used to determine the initial thermal profile  158 . 
     At step  504 , the initial thermal profile  158  obtained in step  502  is compensated for by imposing a compensatory thermal profile  159  on the substrate  130 . In one embodiment, imposing a compensatory thermal profile  159  involves pre-heating the substrate  130  in accordance with the compensatory thermal profile  159 . For example, if the desired thermal profile  157  is uniform, areas of the substrate  130  which would normally maintain lower temperatures after a conventional heating process are pre-heated to a temperature greater than areas which normally maintain higher temperatures. The imposition of a compensatory thermal profile  159  may also involve shielding, or protecting particular areas of the substrate from undesired heat transfer. This embodiment provides for the ability to achieve a desired thermal profile  157  without adjusting the heating process for each substrate  130  processed. The substrate  130  may be pre-heated in the process chamber  100  or prior to being introduced into the process chamber  100 . 
     At step  506 , the substrate heating process is performed. By having implemented the compensatory thermal profile  159  at step  504 , heating the substrate  130  no longer yields the initial thermal profile  158 , but rather the desired thermal profile  157 . For example, during a flash annealing process with a particular support/substrate geometry, it may be determined that an initial thermal profile  158  may be hotter near the center of the substrate  130  and cooler near the edge of the substrate  130 . Accordingly, in embodiments where a substantially uniform thermal profile is desired, the substrate  130  may be pre-heated to impose a compensatory thermal profile  159  on the substrate that corresponds to the inverse of the initial thermal profile  158 . As such, when the flash annealing process is performed on the substrate, a substantially uniform thermal profile results, instead of the non-uniform, initial thermal profile  158 . Although one thermal profile is illustratively described above, it is contemplated that any thermal profile may be compensated for using the techniques described herein. 
     In one embodiment, step  506  may be performed immediately after imposing the compensatory thermal profile  159  on the substrate  130  in step  504 . Alternatively, step  506  may be performed after a period of time elapses. Optionally, the compensatory thermal profile may further compensate for cooling of the substrate during any period of time between the imposition of the compensatory thermal profile and the performance of the thermal process. For example, if the compensatory thermal profile is imposed in a chamber remote from the processing chamber where the thermal process occurs, the compensatory thermal profile may compensate for the cooling that occurs during the time to transport the substrate to the process chamber where the thermal process is to be performed. 
       FIG. 6  depicts a flowchart of another embodiment of a method  600  for utilizing compensation techniques for creating a desired thermal profile for a substrate  130 . The method  600  is described with reference to  FIGS. 1 through 4 . The method  600  begins at step  602  where the initial thermal profile  158  of the substrate due to a process is determined. This step is similar to step  502  described above with respect to  FIG. 5 . 
     Next, at step  604 , a local substrate heating rate adjusted to compensate for the initial thermal profile and result in a desired thermal profile. The local substrate heating rate may be adjusted in a number of ways. In one embodiment, useful for processes which have not reached steady state, the amount of mass heated per unit area may be adjusted, or locally controlled. For example, in the embodiment depicted in  FIG. 7 , the thickness of a susceptor  702  is varied to emulate the compensatory thermal profile  159 , resulting in a change of mass at particular areas of the susceptor  702  (i.e., creating a susceptor having a compensatory heat transfer profile). By varying the thickness of the susceptor  120 , thermal properties such as heat flux, heat transfer rates, and the like, which are dependant upon the mass of the susceptor at a particular location, may be controlled. For example, if the susceptor  702  is thicker at a particular location, the heat transfer through that location is decreased by virtue of having more mass to heat, and conversely, if the susceptor  702  is thinner at a particular location, the heat transfer through that location is increased. In the illustrative embodiment depicted in  FIG. 7 , the susceptor  702  has a thinner section  710  disposed proximate the periphery of a substrate  130 , and thicker section  720  proximate the center of the substrate  130  to compensate for an initial heat profile determined to have a hotter center and a cooler edge. It is contemplated that other thickness profiles of susceptors resulting in varying profiles of mass heated per unit area may be utilized to compensate for particular initial thermal profiles determined for particular substrates undergoing particular thermal processes. Alternatively or in combination, the local mass heated per unit area may be controlled via control of the thermal conductivity in desired locations of the susceptor. For example, different regions of the susceptor may have different thermal conductivity to emulate regions of differing mass, as discussed above. Although described as being useful for processes which have not reached steady state, the above techniques may have effects that persist into the steady state regions of a process. 
     Alternatively or in combination, the local heat capacity per unit per unit area may be adjusted, or locally controlled. For example, in the embodiment depicted in  FIG. 8 , a multi-material susceptor  802  is utilized, in which material variations in the susceptor  802  change the local heat capacity per unit area to emulate the compensatory thermal profile  159 , resulting in a susceptor  802  having a compensatory heat transfer profile. By varying the material selection of the susceptor  802 , the local substrate heating rate may be controlled. The material selection may be based on the heat transfer rate of the material. For example, materials with high heat capacity have lower heat transfer rates. Conversely, materials having low heat capacity have higher heat transfer rates. In embodiments where an edge ring is utilized (as depicted in  FIG. 1 ), the material of the edge ring may similarly be selected to have a desired heat rate to compensate for the initial thermal profile of the substrate. In the illustrative embodiment depicted in  FIG. 8 , the susceptor  802  may comprise a first material  810  with deposits of a second material  820  and a third material  830  where changes in the local heat capacity per unit area are desired to control the local heating rates of the substrate  130  in those areas as a result of a particular thermal process. The heat transfer rates of the first, second, and third materials may be selected to control the local heat capacity per unit area as desired to compensate for the initial thermal profile and result in a desired thermal profile. It is contemplated that the location, geometry, numbers of regions, and/or selection of materials may be varied as desired for particular heating applications. 
     Alternatively or in combination, the absorptivity or reflectivity of an edge ring or susceptor edge may be adjusted to compensate for the initial thermal profile  158 . For example, as discussed above with respect to  FIG. 1 , the material composition, surface properties (i.e., finish, angle, or the like), or thickness of the edge ring  140  or susceptor edge  124 , may be adjusted to control the level of absorptivity or reflectivity as desired. Optical coatings or films, including dielectric film stacks, can also be used to alter the surface properties. As the local absorptivity is increased, more irradiation is retained by the edge ring  140  or susceptor edge  124 , and the temperature increases at the substrate edge  132 . Conversely, increasing the local reflectivity causes more irradiation to reflect from the edge ring  140  or susceptor edge  124 , resulting in a temperature decrease at the substrate edge  132 . In another embodiment, an edge ring  140  may be provided with an optional feature (not shown) to reflect additional energy to the substrate edge  132  to heat the substrate edge  132 . 
     Returning to  FIG. 6 , at step  606 , the thermal process is performed resulting in a desired thermal profile formed on the substrate. Thus, by instituting one or more of the above techniques (i.e., varying the mass heated per unit area, varying the heat capacity per unit area, or controlling the absorptivity or reflectivity of the edge ring or susceptor edge), the heating rate of the substrate may be locally controlled, and thereby compensate for an initial thermal profile to yield a desired thermal profile. The effectiveness of the adjustments of mass heated per unit area generally decreases as the heat rate increases (i.e., as the heat rate increases, the amount of mass heated-per-unit area becomes less of a factor in determining the resulting thermal profile of a substrate). For Example, at exceedingly high heating rates like laser surface heating, where the irradiance on the heated piece is on the order of ˜1×10 9  W/m 2 , only the layers exposed to the radiation (those nearest the surface for visible radiation on bare silicon) are effectively heated. Layers of substrate a couple hundred microns below the surface remain at the starting temperature. In this case, the thermal properties of the substrate support (mass, heat capacity, and the like) are immaterial. The heated thickness depends on the balance of radiation applied and heat dissipated conductively in the substrate. Therefore, although the method described above with respect to  FIG. 6  may be utilized in any thermal process, it is particularly useful for low-heat processes (i.e., processes with heat rates on the order of hundreds of degrees Celsius per second). 
       FIG. 9  depicts a flowchart of yet another embodiment of a method  900  for utilizing compensation techniques for creating a uniform thermal profile on a substrate. The method  900  is discussed with reference to  FIG. 1 . The method  900  begins at step  902  where an initial thermal profile of a substrate due to a process is determined, similar to steps  502  and  602 , discussed above with respect to  FIGS. 5 and 6 , respectively. 
     Next, at step  904 , a heater disposed within the edge ring  140  (such as resistive heater  142 ) is controlled to heat the substrate in a manner that compensates for the initial thermal profile of the substrate and yields a desired thermal profile. The heater may be controlled manually or via the controller  150 . 
     The average temperature of the edge ring  140  may be inferred by monitoring the electrical resistance of the resistive heater  142 . As such, the current supplied to the heater  142  may be controlled to produce a temperature of the edge ring  142  that compensates for the initial thermal profile. For example, in embodiments where the initial thermal profile of the substrate has a cooler edge (such as depicted in  FIGS. 2 and 3 ) the temperature of the edge ring  140  may be increased to reduce the more rapid heat loss near the edge of the substrate and provide the desired thermal profile. In other embodiments, the temperature of the heater  142  may be kept at a lower temperature to prevent excessive heating of the edge of the substrate. Moreover, in embodiments where the edge ring has a negative temperature coefficient, the resistive heating will tend to heat the cooler regions of the edge ring more and even out any non-uniformities in the edge ring temperature. 
     At step  906 , which may be performed subsequent to or simultaneously with step  904 , the process is performed, resulting in a substrate with a substantially desired thermal profile. 
     Thus, embodiments of methods for processing a substrate utilizing thermal compensation techniques for creating a desired thermal profile on a substrate have been provided. The disclosed techniques advantageously compensate for non-desired initial thermal profiles caused by a process and provide for the creation of a desired thermal profile on a substrate. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.