Patent Publication Number: US-2015083046-A1

Title: Carbon fiber ring susceptor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of United States Provisional Application Ser. No. 61/883,167, filed Sep. 26, 2013 (Attorney Docket No. APPM/020377/USL), of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     Embodiments of the present disclosure generally relate to a carbon fiber susceptor, and more specifically, a carbon fiber ring susceptor. 
     2. Description of the Related Art 
     Semiconductor substrates are processed for a wide variety of applications, including the fabrication of integrated devices and microdevices. One method of processing substrates includes depositing a material on an upper surface of the substrate. For example, epitaxy is a deposition process that grows a thin, ultra-pure layer, usually of silicon or germanium on a surface of a substrate. The material may be deposited in a lateral flow chamber by flowing a process gas parallel to the surface of a substrate positioned on a susceptor, and thermally decomposing the process gas to deposit a material from the gas onto the substrate surface. 
     The most common epitaxial (Epi) film deposition reactors used in modern silicon technology provide similar process conditions. However, the reactor design is essential for film quality as epitaxial growth relies on the precision of gas flow to enhance epitaxial deposition uniformity. Prior susceptor designs restrict process uniformity by causing uneven thermal transfer to the substrate, which negatively influences deposition uniformity over the substrate. 
     Substrate heating during Epi film deposition processes is performed at high temperatures of up to 1300 degrees Celsius. Traditional susceptors are usually made from silicon carbide (SiC) or sintered graphite coated with silicon carbide, and have a high thermal mass. In instances where the susceptor is a ring susceptor, the high thermal mass of the susceptor results in inefficient and uneven thermal transfer to the backside and edge of the substrate, where there is maximum substrate to susceptor contact. The slower transfer of heat from the susceptor to the substrate, in turn, induces non-uniformity in film material properties across the substrate, and particularly at the edge of the substrate. 
     Thus, there is a need for an improved susceptor. 
     SUMMARY 
     Embodiments described herein generally relate to an apparatus for heating substrates. In one embodiment, a susceptor comprises a ring shaped body having a central opening and a lip extending from an edge of the body that circumscribes the central opening. The susceptor comprises carbon fiber or graphene which have lower thermal mass than traditional susceptors. 
     In another embodiment, a method for forming a susceptor comprises molding carbon fiber with an organic binder into a shape of a ring susceptor and firing the organic binder. In yet another embodiment, a method for forming a susceptor comprises layering graphene sheets into a shape of a ring susceptor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure 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. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         FIG. 1  is a schematic view of a process chamber. 
         FIG. 2  illustrates an enlarged cross-sectional view of a susceptor. 
         FIG. 3  illustrates a flow diagram for processing a substrate. 
         FIG. 4  illustrates a cross-section view of another embodiment of a susceptor suitable for use in the process chamber of  FIG. 1 . 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present disclosure. These embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical, and other changes may be made without departing from the scope of the present disclosure. 
       FIG. 1  illustrates a schematic view of a processing chamber  100  according to one embodiment. The processing chamber  100  may be used to process one or more substrates  108 , including the deposition of a material on an upper surface of the substrate  108 . The substrate  108  may include, but is not limited to 200 mm, 300 mm or larger single crystal silicon (Si), multi-crystalline silicon, polycrystalline silicon, germanium (Ge), silicon carbide (SiC), glass, gallium arsenide (GaAs), cadmium telluride (CdTe), cadmium sulfide (CdS), copper indium gallium selenide (GIGS), copper indium selenide (CuInSe 2 ), gallilium indium phosphide (GaInP 2 ), as well as heterojunction substrates, such as GaInP/GaAs/Ge or ZnSe/GaAs/Ge substrates. The processing chamber  100  may include an array of radiant heating lamps  102  for heating, among other components, a back side  104  of a susceptor  120  disposed within walls  101  of the processing chamber  100  and the substrate  108 . In the embodiment shown in  FIGS. 1 and 2 , the susceptor  120  is has a ring shaped body with a central opening  103  and a lip  121  that extends from the edge of the susceptor  120  and circumscribes the central opening  103 . The lip  121  and the front side  102  of the susceptor  120  create a pocket  126  that supports the substrate  108  from the edge of the substrate to facilitate exposure of the substrate  108  to the thermal radiation of the lamps  102 . The susceptor  120  is supported by a support  118 . Details of the susceptor  120  will be discussed further below in reference to  FIG. 2 . The susceptor  120  is located within the processing chamber  100  between an upper dome  110  and a lower dome  112 . The upper dome  110 , the lower dome  112  and a base ring  114  that is disposed between the upper dome  110  and lower dome  112  generally define an internal region of the processing chamber  100 . In some embodiments, the array of radiant heating lamps  102  may be disposed over the upper dome  110 . The substrate  108  can be brought into the processing chamber  100  and positioned onto the susceptor  120  through a loading port (not shown). 
     The susceptor  120  is shown in an elevated processing position, but may be moved vertically by an actuator (not shown) to a loading position below the processing position to allow lift pins  122  to pass through holes in the susceptor support  118 , and raise the substrate  108  from the susceptor  120 . A robot (not shown) may then enter the process chamber  100  to engage and remove the substrate  108  therefrom though the loading port. The susceptor  120  then may be actuated up to the processing position to place the substrate  108 , with a device side  124  facing up, on a front side  102  of the susceptor  120 . 
     The susceptor  120  and the susceptor support  118 , while located in the processing position, divide the internal volume of the processing chamber  100  into a process gas region  128  that is above the substrate  108 , and a purge gas region  130  below the susceptor  120  and the susceptor support  118 . The susceptor  120  and susceptor support  118  are rotated during processing by a supporting cylindrical central shaft  132 , to minimize the effect of thermal and process gas flow spatial anomalies within the processing chamber  100  and thus facilitate uniform processing of the substrate  108 . The central shaft  132  moves the substrate  108  in an up and down direction  134  during loading and unloading, and in some instances, processing of the substrate  108 . 
     In general, the central window portion of the upper dome  110  and the bottom of the lower dome  112  are formed from an optically transparent material such as quartz. One or more lamps, such as an array of the lamps  102 , can be disposed adjacent to and beneath the lower dome  112  in a specified, optimal desired manner around the central shaft  132  to independently control the temperature at various regions of the substrate  108  as the process gas passes over, thereby facilitating the deposition of a material onto the upper surface of the substrate  108 . While not discussed here in detail, in one embodiment, the deposited material may include silicon (Si), germanium (Ge) or dopants to create a single crystalline layer on the substrate. 
     The lamps  102  may be configured to include bulbs  136  and be configured to heat the substrate  108  to a temperature within a range of about 200 degrees Celsius to about 1600 degrees Celsius, for example, about 300 degrees Celsius to about 1200 degrees Celsius or about 500 to about 580 degrees Celsius. Each lamp  102  is coupled to a power distribution board (not shown) through which power is supplied to each lamp  102 . The lamps  102  are positioned within a lamphead  138  which may be cooled during or after processing by, for example, a cooling fluid introduced into channels  152  located between the lamps  102 . The lamphead  138  conductively and radiatively cools the lower dome  112  due in part to the close proximity of the lamphead  138  to the lower dome  112 . The lamphead  138  may also cool the lamp walls and walls of the reflectors (not shown) around the lamps. Alternatively, the lower dome  112  may be cooled by a convective approach known in the industry. Depending upon the application, the lampheads  138  may or may not be in contact with the lower dome  112 . As a result of backside heating of the substrate  108 , the use of an optical pyrometer  142  for temperature measurements/control on the substrate  108  and the susceptor  120  may also be performed. 
     A reflector  144  may be optionally placed outside the upper dome  110  to reflect infrared light that is radiating off the substrate  108  back onto the substrate  108 . The reflector  144  may be fabricated from a metal such as aluminum or stainless steel. The efficiency of the reflection can be improved by coating a reflector area with a highly reflective coating such as with gold. The reflector  144  can have one or more machined channels  146  connected to a cooling source (not shown). The channel  146  connects to a passage (not shown) formed on a side of the reflector  144 . The passage is configured to carry a flow of a fluid such as water and may run horizontally along the side of the reflector  144  in any desired pattern covering a portion or entire surface of the reflector  144  for cooling the reflector  144 . 
     Process gas supplied from a process gas supply source  148  is introduced into the process gas region  128  through a process gas inlet  150  formed in the sidewall of the base ring  114 . The process gas inlet  150  is configured to direct the process gas in a generally radially inward direction. During the film formation process, the susceptor  120  may be located in the processing position, which is adjacent to and at about the same elevation as the process gas inlet  150 , allowing the process gas to flow up and round along a flow path across the upper surface of the substrate  108  in a laminar flow fashion. The process gas exits the process gas region  128  through a gas outlet  155  located on the side of the process chamber  100  opposite the process gas inlet  150 . Removal of the process gas through the gas outlet  155  may be facilitated by a vacuum pump  156  coupled thereto. As the process gas inlet  150  and the gas outlet  155  are aligned and disposed approximately at the same elevation, it is believed that such a parallel arrangement, when combined with a flatter upper dome  110  provides generally planar, uniform gas flow across the substrate  108 . Further radial uniformity may be provided by the rotation of the substrate  108  through the susceptor  120 . 
     Purge gas may be supplied from a purge gas source  158  to the purge gas region  130  through an optional purge gas inlet  160  (or through the process gas inlet  150 ) formed in the sidewall of the base ring  114 . The purge gas inlet  160  is disposed at an elevation below the process gas inlet  150 . The purge gas inlet  160  is configured to direct the purge gas in a generally radially inward direction. During the film formation process, the susceptor  120  may be located at a position such that the purge gas flows down and round along a flow path across the back side  104  of the susceptor  120  in a laminar flow fashion. Without being bound by any particular theory, the flowing of the purge gas is believed to prevent or substantially avoid the flow of the process gas from entering into the purge gas region  130 , or to reduce diffusion of the process gas entering the purge gas region  130  (i.e., the region under the susceptor  120 ). The purge gas exits the purge gas region  130  and is exhausted out of the processing chamber  100  through the gas outlet  155 , which is located on the side of the processing chamber  100  opposite the purge gas inlet  160 . 
       FIG. 2  illustrates an enlarged cross-sectional view of the susceptor  120  according to one embodiment. While the susceptor  120  is shown in the processing chamber  100 , it is contemplated that the susceptor  120  is suitable for epitaxy, rapid thermal processing, chemical vapor deposition, atomic layer deposition, or any other vacuum processes that requires uniform gas flow or temperature. Additionally, while the susceptor  120  is a ring-susceptor, it is contemplated that other susceptors (i.e., non-ring susceptors) may benefit from the foregoing disclosure. 
     The susceptor  120  is ring shaped having an inner diameter  252  and outer diameter  124 . The inner diameter  252  defines a central opening  258  of the suscepter  120  and is smaller than the diameter of the substrate  108  such that the substrate  108  may rest on the pocket  126  of the susceptor  120 . The pocket  126 , formed between the central opening  258  and the lip  121 , may have a length  254  of about between about 1 mm and about 7 mm, such as about 4 mm. In one embodiment, the lip  121  may have a thickness  260  between about 2 mm and about 20 mm, such as about 16 mm. The thickness  260  of the lip  121  may be uniform from the pocket  126  to the outer diameter  124 . Alternately, the thickness  260  of the lip  121  may increase over at least a portion of the lip  121  from the pocket  126  towards the outer the outer diameter  124 . (See  FIG. 4 ) The increase in the thickness  260  of the lip  121  near the outer diameter  124  advantageously provides strength and warpage resistance. 
     The susceptor  120  may be configured such that a gap  256  of about 0.5 mm is form between the substrate  108  and the lip  121 . In one embodiment, the central opening  258  is about 1 mm smaller than the substrate  108  for which the susceptor  120  is configured to accept. For example, the central opening  258  of the susceptor  120  may be about 449 mm and configured accept at least a 450 mm diameter substrate. In a second example, the central opening  258  of the susceptor  120  may be about 299 mm and configured accept at least a 300 mm diameter substrate. In yet another example, the central opening  258  of the susceptor  120  may be about 199 mm and configured accept at least a 200 mm diameter substrate. The gap  256  distances the substrate  108  from the thermal mass of material associated with the lip  121  and thus promotes temperature uniformity in the substrate  108 . 
     In one embodiment, the susceptor  120  comprises carbon fiber. The light weight and low thermal mass of carbon fiber yields a thermally agile susceptor  120  which can respond to temperature changes faster than traditional silicon carbide susceptors. In one embodiment, the susceptor  120  is thinner than traditional susceptors and has a uniform thickness less than about 5 mm, for example less than 3 mm. The thinness of the susceptor  120  advantageously minimizes the amount of physical contact between the substrate  108  and the susceptor  120 . 
     In one embodiment, the susceptor  120  is formed by molding carbon fiber with an organic binder. The organic binder may be carbonized or graphitized during a firing process. In one embodiment, the carbon fibers in the susceptor  120  are radially aligned to provide optimal heat transfer to the substrate  108 . In another embodiment, the susceptor  120  comprises graphene, an allotrope of carbon. The susceptor  120  is formed by using layers of graphene sheets such as pyrolytic carbon sheets. The graphene sheets may be about  10  microns to about  100  microns thick. In another embodiment, the susceptor  120  may be formed by layers of the pyrolytic sheets bonded with carbon fiber-carbon composites layers. In yet another embodiment, the graphene or carbon fiber susceptor  120  may be coated with silicon carbide by sintering in a furnace or oven, or any other suitable mechanism for coating. 
     In one example, the susceptor  120  may be formed from polyacrylonitrile (PAN)-based carbon fibers, where the carbon atoms are more randomly folded together. In another example, the carbon fiber susceptor  120  may be more graphitic, such as a heat treated mesophase pitch derived carbon fiber. In yet another example, the carbon fiber susceptor  120  may also be comprised of a composite of PAN or pitch derived carbon fiber along with other suitable materials. The graphitic carbon fiber susceptor  120  may have a higher thermal conductivity than a PAN-based carbon fiber susceptor  120  and thus the heat transfer rate may be tuned accordingly. For example, the graphitic carbon fiber susceptor  120  has quicker heat transfer across the material and heats a substrate  108  thereon more uniformly in a radial direction. Thus, the substrate  108  on the carbon fiber susceptor  120  will have a minimal thermal gradient and advantageously the carbon fiber susceptor  120  promotes uniformly in processing substrates  108  thereon. 
       FIG. 3  illustrates a process sequence  300  which heats a substrate. In one embodiment, the sequence  300  corresponds to a process performed in the processing chamber  100 . However, it is contemplated that the sequence  300  may be performed in any vacuum processing chamber that requires uniform gas flow. The process sequence  300  starts at block  302  by providing a substrate, such as the substrate  108  depicted in  FIGS. 1 and 2 , into a processing chamber, such as the chamber  100  depicted in  FIG. 1 . At block  302 , the substrate  108  advantageously absorbs radiant energy from the lamps  102  at the backside of the substrate  108  through the opening  103  in the ring susceptor  120 . In one embodiment, the sequence  300  is a rapid thermal processing sequence and the substrate  108  is transparent at wavelengths between about 1050 nm to about 1100 nm. The lamps  102  generate radiant energy and heat the substrate  108  to about 500 degrees Celsius or about 580 degrees Celsius, wherein the substrate  108  becomes opaque. At block  306 , process gas flows into the process gas region  128 . Block  306  may be performed before or after heating the substrate  108 . At block  308 , the temperature of the substrate  108 , may be controlled (e.g., increased, decreased or maintained) depending on the process sequence  300 . In one embodiment, the process sequence  300  is a rapid thermal processing sequence and the temperature is ramped up at about 300 degrees Celsius/second to reach about 1200 degrees Celsius. The power to the lamps  102  is then turned off, to allow the temperature of the substrate  108  to cool down. 
       FIG. 4  illustrates a cross-section view of another embodiment for a susceptor  420  suitable for use in the process chamber of  FIG. 1 , among others. The susceptor  420  has a body  410 , a bottom surface  404 , a top surface  426  and an outer perimeter  423 . The body  410  of the susceptor  420  may have a plurality of lift pin holes  422  disposed therethrough from the bottom surface  404  to the top surface  426 . The susceptor  420  may be circular in shape and have a lip  421  extending from the bottom surface  404  to above the top surface  426  along the outer perimeter  423  of the susceptor  420 . 
     The lip  421  is ring shaped having an inner perimeter  425 . Similar to lip  121  discussed above, the lip  421  may have a uniform thickness or have a taper  430 . The taper  430  extends upward from the top surface  426  to at or near the outer perimeter  423 . That is, the tapper  430  may extend to a top lip surface  432  or to the outer perimeter  423  in an embodiment without a defined top lip surface. 
     The inner perimeter  425  is configured to accept the substrate  108  disposed on the top surface  426  of the susceptor  420 . The top surface  426  may have a length  452  corresponding to the inner perimeter  425 . The length  452  may be greater than the diameter of a substrate  108 , such as a 450 mm, or 300 mm or 200 mm substrate, such that a gap  457  is uniformly formed between the substrate  108  and the lip  421 . The gap  457  may be about 0.1 mm to about 1 mm, such as about 0.5 mm. For example, the length  452  may be about 451 mm for a susceptor  420  configured for the 450 mm substrate. 
     The susceptor  420 , excluding the lip  421 , has a substantially uniform thickness  456  between the top surface  426  and the bottom surface  404 . The thickness  456  of the susceptor  420  may be between about 1 mm and about 5 mm, such as about 3 mm. The thickness  456  may be selected to make the susceptor  420  thin yet opaque. Thus, IR thermal energy provided from below the substrate  108  placed on the susceptor  420  may uniformly and quickly change the temperature profile of the substrate  108  with little adverse impact to the pyrometers in the chamber. 
     In one example, the susceptor  420  may be formed from a material having a higher thermal conductivity along the length  452  than along the thickness  456 . The thermal mass of the susceptor  420  may be configured by the material used to form it. The susceptor  420  may be anisotropic being stronger in the fiber direction than across the fibers. The susceptor  420  may be formed from PAN carbon fibers wherein the thermal conductivity along the fiber is high promoting a substantially uniform thermal load with little gradient from center to edge. Aligning the carbon fibers in the plane of the top surface  426  of generates a customizable thermally conductive profile for the susceptor  420 . For example, the susceptor  420  may have a lower thermal conductivity from the bottom surface  404  to the top surface  426  going across the fiber grain then the thermal conductivity along the length  452  going with the fiber grains. Thus, the susceptor  420  has good in plane thermal conductivity to promote a rapid temperature profile that is uniform from center to edge of the substrate  108  place on the susceptor  420 . In one embodiment the thermal conductivity in the plane of the top surface  426  is between about 10 W/(m*K) to about 1000 W/(m*K), such as about between about 60 W/(m*K) an about 600 W/(m*K), such as about 220 W/(m*K). Perpendicular to the plane of the top surface  426 , the thermal conductivity of the suscepter  420  may be about 10 W/(m*K) to about 120 W/(m*K). In some embodiment, such as for composites, the perpendicular to plane thermal conductivity may be about ¼ to about 1/10 of the in plane thermal conductivity for the suscepter  420 . 
     Advantageously, the carbon fiber or graphene susceptors  120 ,  420 , as described above, respond quickly to the increased and decreased temperature change and has a short lag time on transfer of heat from the susceptor  120 ,  420  to the substrate  108 . Additionally, the faster response time of the susceptor  120 ,  420  to temperature change makes it easier to reach a desired processing temperature. Due to the low thermal mass and thinness of the susceptor  120 ,  420 , the susceptor  120 ,  420  will not draw heat from the edge of the substrate  108  and can sustain ramping of high temperatures and quick cooling down without warping or flexing. Therefore, the susceptor  120 ,  420  allows for more uniform heat transfer to the edge of the substrate  108  and, in turn, results in more uniform film deposition on the substrate  108 . 
     In one example, a method for forming a susceptor may be described by molding carbon fiber with an organic binder into a shape of a ring susceptor, and carbonizing or graphitizing the organic binder in a firing process. 
     In another example, a method for forming a susceptor may be described by layering graphene sheets into a shape of a ring susceptor. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.