Abstract:
A thermal processing apparatus is disclosed which separates the load bearing components from the thermal components, improving heating time, cooling time, thermal response, and energy efficiency. The thermal processing apparatus comprises an array of cylindrical heating elements which rest on support plates of high temperature, low density material. The support plates and heating elements are then supported by a rigid skeletal structure for load bearing support.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/381,876, filed on Sep. 10, 2010, and incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    Not Applicable. 
       FIELD OF THE INVENTION 
       [0003]    This invention relates to an improved thermal processing apparatus and process for heat treatment of semiconductor and solar substrates. 
       DISCUSSION OF RELATED ART 
       [0004]    Heating systems are commonly used in many different applications. In the semiconductor and solar substrate processing industry, the heating systems are used to anneal or grow films with a specific gas or vapor onto a semiconductor substrate, or wafer, for achieving desired electrical properties. Furthermore, the annealing process is performed for a predetermined amount of time to cure the already grown films on the substrate to improve the film quality and performance. 
         [0005]    The wafers are typically heat treated in large numbers, one above the other, in a cylindrical array spaced apart by a certain pitch. The batch of wafers is isolated from outside particles and contamination by a process tube, and a process gas is introduced at a precise flow rate into the process tube from an inlet and exhausted from an outlet. 
         [0006]    The temperature range can vary from 200 degrees C. to 1300 degrees C. for different applications and process steps. The heating systems are required to be responsive to temperature changes and maintain a uniform temperature throughout the process in order to achieve a desired film thickness. As such, the temperature accuracy and uniformity across the load of substrate wafers from end to end and within a given substrate is critical in film uniformity and quality. 
         [0007]    In typical heating systems, trays or blocks are used to support the heating elements and to provide load bearing support for the entire heating system. To perform both of these tasks, the trays or blocks are made from high temperature and high density materials having a high thermal mass. As such, extra energy is stored in the trays or blocks. Several attempts have been made to reduce the amount of heat that is obstructed and stored, but these designs are restricted because of the load bearing requirements of the trays or blocks. 
         [0008]    U.S. Pat, No. 2,035,306 to Fannin on Mar. 24, 1936, describes a furnace intended for the metal melting industry, where refractory blocks of porcelain are placed circumferentially end to end and one on the other to form a continuous groove providing a space for insertion of continuous flat ribbon heating element after the blocks are placed in their operating positions. These blocks are stacked in a loose manner to allow them to expand and contract freely. This furnace has a high thermal mass that stores extra energy in its own body, lowers the thermal response time of the heating apparatus during ramp up, increases the temperature stabilization time, and increases the cool down time. These factors result in increased processing time and higher energy costs. 
         [0009]    U.S. Pat. No. 6,005,255 to Kowalski et al. on Dec. 21, 1999, describes a furnace intended for treating semiconductor wafers, where helicoids are placed in high temperature insulation trays that are stacked on top of each other to create the structure of the furnace. The structure works as a load bearing body to build the heat treatment apparatus, as insulation to maintain the precise temperatures that are required for the heat treatment of the substrates, and as a retainer to hold the helicoids in place. The coupling of load bearing and thermal dynamic functions require that the trays have higher density to carry the structural load, which lowers thermal response time, increases temperature stabilization time, and increases cool down time. These factors result in increased processing time and higher energy costs. 
         [0010]    U.S. Pat. No. 6,807,220 to Peck on Oct. 19, 2004, describes a furnace intended for treating semiconductor wafers, where wired helical heating elements are encircled by thermal insulation and ceramic spacers are used for spacing the heating elements apart. As the wires are wound, the separators are placed on top of each other to create a stack of separators, with a guide rod keeping each separator in its intended vertical position. This type of straight wire heating has a higher failure rate due to the expansion and elongation of the wire, which is constrained by the separators, causing buckling between the separator columns. Lastly, the separators come in direct contact with the heating element, requiring a higher density material to be used. 
         [0011]    Therefore, a need exists for a heat treatment apparatus that can provide more uniform and efficient heating across a large batch of substrates by separating the load bearing support from the heating element support, resulting in better device production yield and lower energy costs. The present invention accomplishes these objectives. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention comprises a thermal processing apparatus used for heating substrates such as semiconductor devices, solar cells, LEDs, MEMs, and other substrates. The thermal processing apparatus uniquely decouples the thermal mass from the structural strength of the apparatus. Therefore, the thermal processing apparatus is divided into a skeletal frame structure and an array of heating element support plates. 
         [0013]    The skeletal structure has a minimum mass and provides structural support to carry the load of the heating elements and their circular support plates. As such, the support plates do not have any load bearing functions. This is true for the insulation material on the outside of heating element as well. Therefore, the choice of the material for the skeletal structure and insulation can be of a low mass to increase the efficiency of heating and cooling of the substrates. 
         [0014]    In one embodiment, the support plates have the role of retaining the heating elements in their radial position. The skeletal structure maintains the support plates on their inner and outer circumferences, while the support plates retain the heating elements with edges or lips on their inner circumference. The skeletal structure is stacked, and each support module contains one or several pieces. The shape of the skeletal structure can be circular or rectangular. 
         [0015]    In an alternative embodiment, there is a single heating element which travels to adjacent support plates by gaps within the support plates. The skeletal structure has the role of retaining the heating element in its radial position. The skeletal structure maintains the support plates on their inner and outer circumferences and prevents the heating element from coming in contact with the substrate. The support plates are flat and have no lips or edges. The skeletal structure is stacked, and each support module contains one or several pieces. The shape of the skeletal structure can be circular or rectangular. 
         [0016]    These and other objectives of the present invention will become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiments. It is to be understood that the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a front perspective view of the thermal processing apparatus according to one embodiment of the invention; 
           [0018]      FIG. 2  is a front cross-sectional diagram, taken generally along lines  2 - 2  of  FIG. 1 , illustrating the skeletal structure and support plates in greater detail according to one embodiment of the invention; 
           [0019]      FIG. 3  is a front cross-sectional diagram of the thermal processing structure according to one embodiment of the invention; 
           [0020]      FIG. 4  is a front perspective view of the thermal processing apparatus according to one embodiment of the invention; 
           [0021]      FIG. 5  is a rear cross-sectional diagram, taken generally along lines  5 - 5  of  FIG. 4 , illustrating the skeletal structure and support plates in greater detail according to one embodiment of the invention; 
           [0022]      FIG. 6  is a front cross-sectional diagram of the thermal processing structure according to one embodiment of the invention; 
           [0023]      FIG. 7  is a front partial diagram illustrating the support plate gap and heating element of the thermal processing structure according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    Illustrative embodiments of the invention are described below. The following explanation provides specific details for a thorough understanding of and enabling description for these embodiments. One skilled in the art will understand that the invention may be practiced without such details. In other instances, well-known structures and functions have not been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments. 
         [0025]    Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” Words using the singular or plural number also include the plural or singular number respectively. Additionally, the words “herein,” “above,” “below” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. When the claims use the word “or” in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list and any combination of the items in the list. 
         [0026]    The present invention describes a new method to support the resistance wire  11  in a heating system  10  by uniquely decoupling the load bearing components from the thermal element retention components, thereby achieving maximum thermal efficiency. The invention utilizes a unique skeletal frame structure  13  made of a plurality of axially spaced tubes  22 ,  28  of individually stacked support modules  21 . 
         [0027]    Heater coil support plates  12  are placed between the support modules  21  to form a rigid structure defining parallel spaces for heating elements  11  and defining an enclosed volume of space for substrates  14 . Heating elements  11  are positioned in a cylindrical array of concentric rings, each heating element  11  being spaced from the hot wall process tube according to the specifications of the heating system  10 . The number of support plates  12  and heating elements  11  can increase or decrease to provide more or less heat to the system  10 . 
         [0028]      FIG. 1  is a diagram illustrating a system  10  in which one embodiment of the invention may be practiced. The system  10  represents a plurality of heating elements  11  which rest on a plurality of circular support plates  12  of high temperature material. The position of the support plates  12  is held in place by a rigid skeletal structure  13  of a high strength, low profile material. Each support plate  12  has an inner circumference and an outer circumference. The spacing of the support plates  12  can vary according to the specific requirements of the heating system  10  and the substrate  14 . 
         [0029]    The support plates  12  do not have any load bearing functions. As such, the material for the support plates  12  can be of minimum density and low thickness, creating a low mass heating system for faster heating and cooling of the substrate  14 . Any number of support plates  12  can be used according to the specific requirements of the heating system  10  and substrate  14 . The support plates  12  can be made of two pieces, with one piece to support the heating element  11  and the other to retain it. The support plates  12  can be made of Al 2 O 3 —SiO 2 , or any other suitable material. 
         [0030]    The heating elements  11  comprise coils of helically wound resistance heating wire made of Kanthal®, Nikrothal®, Super-Kanthal®, Molybdenum Discilicide, Iron Chromium Aluminum alloys, or other similar materials. The heating element  11  will expand as the temperature increases, and must be restricted properly to protect the substrate  14 . A power source will energize the heating elements  11 . 
         [0031]    A shell of insulating material  15  surrounds the structure to create the desired thermal insulation for the system  10 . The insulating material  15  does not have any load bearing function. As such, the density and thickness of the insulation  15  can vary independent of the load bearing structure for faster heating and cooling of the substrate  14 . The insulating material  15  can be made of Al 2 O 3 —SiO 2 , or any other suitable material. 
         [0032]      FIG. 2  is a diagram illustrating the skeletal structure  13  and support plates  12  in greater detail. The skeletal structure  13  comprises a series of individually linked support modules  21 . Each support module  21  consists of an inner tube  22 , an outer tube  28 , an inner support rod  23 , a flat stub  24 , and an outer support rod  25 . The inner and outer tubes  22 ,  28  are circular in shape and are machined to allow the inner support rod  23  and outer support rods  25 , respectfully, to fit inside of them with minimal clearance. The distance between the support plates  12  can be modified by increasing or decreasing the height of the inner and outer tubes  22 ,  28  and inner and outer support rods  23 ,  250 , respectively. Furthermore, there can be a single inner support rod  23  or outer support rod  25  that travels through all inner and outer tubes  22 ,  28  and flat stubs  24 . 
         [0033]    The flat stub  24  is also machined with an inner through hole  26  that the inner support rod  23  can fit through with minimal clearance, and an outer through hole  27  that the outer support rod  25  can fit through with minimal clearance. The inner and outer support rods  23 ,  25  vertically align all support modules. The flat stub  24  is placed in between the interlocking inner and outer tubes  22 ,  28  and inner and outer support rods  23 ,  25 . There can be one or multiple flat stubs  24  that extend different directions and different distances under the support plates  12 . The support modules  21  can be made of Al 2 O 3 —SiO 2 , or any other suitable material. 
         [0034]    The support modules  21  can be one or several pieces. In one embodiment, the inner tube  22  and the flat stub  24  may be molded as one piece  29 . In another embodiment, the inner and outer tubes  22 ,  28  may have a rectangular shape. 
         [0035]      FIG. 3  is a diagram illustrating the support plates  12  in greater detail. The skeletal structure  13  maintains the support plates  12  between the inner  23  and outer  25  support rods. The support plates  12  retain the heating elements  11  from coming into contact with the substrate  14  with retaining lips  31  on their inner circumference. The retaining lips  31  can be short lips to allow for more radiation of the heating elements  11 . Eyelets (not pictured) can be used liberally to maximize the amount of heat that radiates to the substrates  14 . The inner tubes  22  and inner support rods  23  may provide additional support if the heating elements  11  are not retained by the support plates  12 . 
         [0036]    The relationship between the heating elements  11  and the structure that holds them is optimized by the low profile, high strength support modules  21 , as well as the low density, low thickness support plates  12  and outside insulation  15 . This combination allows for maximum thermal efficiency while maintaining rigid structural support for the heating system  10 . 
         [0037]      FIG. 4  is a diagram illustrating a system  40  in which one embodiment of the invention may be practiced. In this embodiment, there is one interconnected heating element  41  which rests upon a plurality of semi-circular support plates  42 . Each support plate  42  comprises a gap  43 , permitting the heating element  41  to go continue to the adjacent support plate  42 . Each support plate  42  has an inner circumference and an outer circumference. The skeletal structure  13  maintains the support plates  42  on their inner circumference and outer circumference, and the skeletal structure  13  also retains the heating element  41  in their radial position to avoid contact with the substrate  14 . 
         [0038]    The support plates  42  do not have any load bearing functions. As such, the material for the support plates  42  can be of minimum density and low thickness, creating a low mass heating system for faster heating and cooling of the substrate  14 . Any number of support plates  42  can be used according to the specific requirements of the heating system  40  and substrate  14 . The support plates  42  can be made of Al 2 O 3 —SiO 2 , or any other suitable material. 
         [0039]    The heating element  41  comprises a solid, continuous, resistance heating wire made of Kanthal®, Nikrothal®, Super-Kanthal®, Molybdenum Discilicide, Iron Chromium Aluminum alloys, or other similar materials. The heating element  41  will expand as the temperature increases, and must be restricted properly to protect the substrate  14 . A power source will energize the heating elements  41 . 
         [0040]    A shell of insulating material  15  surrounds the structure to create the desired thermal insulation for the system  40 . The insulating material  15  does not have any load bearing function. As such, the density and thickness of the insulation  15  can vary independent of the load bearing structure for faster heating and cooling of the substrate  14 . The insulating material  15  can be made of Al 2 O 3 —SiO 2 , or any other suitable material. 
         [0041]      FIG. 5  is a diagram illustrating the skeletal structure  13  of this embodiment in greater detail. Each support module  21  consists of an inner tube  22 , outer tube  28 , an inner support rod  23 , a flat stub  24 , and an outer support rods  25 . The inner and outer tubes  22 ,  28  are circular in shape and are machined to allow the inner support rod  23  and outer support rod  25 , respectfully, to fit inside of them with minimal clearance. The distance between the support plates  42  can be modified by increasing or decreasing the height of the inner and outer tubes  22 ,  28  and inner and outer support rods  23 ,  25 , respectively. Furthermore, there can be a single inner support rod  23  or outer support rod  25  that travels through all inner and outer tubes  22 ,  28  and flat stubs  24 . 
         [0042]    The flat stub  24  is also machined with an inner through hole  26  that the inner support rod  23  can fit through with minimal clearance, and an outer through hole  27  that the outer support rod  25  can fit through with minimal clearance. The inner and outer support rods  23 ,  25  vertically align all support modules. The flat stub  24  is placed in between the interlocking inner and outer tubes  22 ,  28  and inner and outer support rods  23 ,  25 . There can be one or multiple flat stubs  24  that extend different directions and different distances under the support plates  42 . The support modules  21  can be made of Al 2 O 3 —SiO 2 , or any other suitable material. 
         [0043]    The support modules  21  can be one or several pieces. In one embodiment, the inner tube  22  and the flat stub  24  may be molded as one piece  29 . In another embodiment, the inner and outer tubes  22 ,  28  may have a rectangular shape. 
         [0044]      FIG. 6  is a diagram illustrating the support plates  42  in greater detail. The skeletal structure  13  maintains the support plates  42  on their inner and outer circumferences with the flat stubs  24  while also retaining the heating element  41  from damaging the substrate  14 . 
         [0045]    The relationship between the heating element  41  and the structure that holds them is optimized by the low profile, high strength support modules  21 , as well as the low density, low thickness support plates  42  and outside insulation  15 . This combination allows for maximum thermal efficiency while maintaining rigid structural support for the heating system  40 . 
         [0046]      FIG. 7  is a diagram illustrating the support plate  42  gaps  43  according to one embodiment of the invention. Here, the solid and continuous resistance heating element  41  rests on each support plate  42  as it travels through the gaps  43  of the support plates  42 . The overall shape of the heating element  41  inside of the system  40  can generally be described as a spiral. 
         [0047]    The above detailed description of the embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above or to the particular field of usage mentioned in this disclosure. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. Also, the teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various embodiments described above can be combined to provide further embodiments. 
         [0048]    While certain aspects of the invention are presented below in certain claim forms, the inventor contemplates the various aspects of the invention in any number of claim forms. Accordingly, the inventor reserves the right to add additional claims after filing the application to pursue such additional claim forms for other aspects of the invention.