Patent Abstract:
A substrate heater comprising a ceramic substrate support having a substantially flat upper surface for supporting a substrate during substrate processing; a resistive heater embedded within the substrate support; a heater shaft coupled to a back surface of the substrate support, the heater having an interior cavity that extends along its longitudinal axis and ends at a bottom central surface of the substrate support; and a supplemental heater, separate from the ceramic substrate support, positioned within the interior cavity of the heater shaft in thermal contact with a portion of the bottom central surface of the substrate support such that the supplemental heater can alter the temperature of a central area of the upper surface of the substrate support.

Full Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/334,386, filed May 13, 2010, and entitled “HEATER WITH INDEPENDENT CENTER ZONE CONTROL,” which is incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to the field of substrate processing equipment. More specifically, the present invention relates to an apparatus and method for controlling the temperature of substrates, such as semiconductor substrates, used in the manufacture of integrated circuits. 
         [0003]    Modern integrated circuits (ICs) contain millions of individual elements that are formed by patterning the materials, such as silicon, metal and/or dielectric layers, that make up the integrated circuit to sizes that are small fractions of a micrometer. Many of the steps associated with the fabrication of integrated circuits include precisely controlling the temperature of the semiconductor substrate upon which the ICs are formed. 
         [0004]    One challenge semiconductor manufacturers face in such process steps is controlling the temperature of the substrate uniformly across the entire surface of the substrate. Even minor differences in temperature between various locations of the substrate may result in undesirable differences in physical characteristics of one or more of the layers formed at those locations on the substrate. 
         [0005]    One type of heater that is particularly useful in high temperature substrate processing is a pedestal design that employs a ceramic substrate support. A resistive heating element is buried beneath the upper surface of the ceramic substrate support and an electrical feed for the resistive heater is positioned within a pedestal that attaches to the bottom of the heater and raises the substrate above the floor of the substrate processing chamber.  FIG. 1  is an example of a previously known pedestal heater  2  that includes a ceramic substrate support  4  that is attached to a hollow stem or pedestal  6 . Embedded within ceramic support  4  is an RF electrode  8  and a resistive heater  10 . Electrical connector rods  12  and  14  provide power to RF electrode  8  and resistive heater  10 , respectively. Some pedestals heaters also include a vacuum lines (not shown) that allow a substrate to be chucked to the pedestal by vacuum pressure. 
         [0006]    The temperature control challenge discussed above often manifests itself for pedestal heaters, such as heater  2 , in that the center of the substrate heater is slightly cooler than other parts of the heater. This is because electrical connections for the heater and an RF electrode are typically made near the center of the pedestal providing less area for the resistive heating element than is available in other areas of the heater. 
         [0007]    Accordingly, while the substrate heater shown in  FIG. 1  is useful for many substrate processing operations, new and improved substrate heaters and methods for accurately controlling substrate temperature are desired. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Some embodiments of the invention provide a substrate heater that includes two separately controllable heating systems including a first, primary heater embedded within a substantially flat upper surface of a substrate support and a second, supplemental heater positioned within a hollow pedestal coupled to a back surface of the substrate support. The primary heater can be, for example, a resistive heater embedded within the substrate support and laid out in a two-dimensional pattern covering a footprint of the support surface. The supplemental heater can be operatively coupled to the substrate support such that the supplemental heater can alter the temperature in a central area of the upper surface of the substrate support. 
         [0009]    According to one embodiment of the invention, a substrate heater is provided that comprises a ceramic substrate support having a substantially flat upper surface for supporting a substrate during substrate processing. A resistive heater is embedded within the substrate support and a heater shaft is coupled to a back surface of the substrate support. The heater shaft can have an interior cavity that extends along its longitudinal axis and ends at a bottom central surface of the substrate support. The substrate heater may further include a supplemental heater, separate from the ceramic substrate support, positioned within the interior cavity of the heater shaft in thermal contact with a portion of the bottom central surface of the substrate support such that the supplemental heater can alter the temperature of a central area of the upper surface of the substrate support. 
         [0010]    A substrate heater according to another embodiment comprises a ceramic substrate support having a substantially flat upper surface for supporting a substrate during substrate processing. A resistive heater is embedded within the substrate support and laid out in a two dimensional pattern that is adapted to heat the upper surface of the substrate support in a generally uniform manner, and a heater shaft is coupled to a back surface of the substrate support. The heater shaft includes an interior cavity that extends along its longitudinal axis and ends at a bottom central surface of the substrate support. A detachable supplemental heater is positioned within the cavity and an air gap surrounds the supplemental heater between an interior surface of the heater shaft that defines the cavity and an outer peripheral surface of the supplemental heater. A biasing mechanism is operatively coupled to force the supplemental heater in thermal contact with a portion of the bottom central surface of the substrate support such that the supplemental heater can alter the temperature of a central area of the upper surface of the substrate support. 
         [0011]    Various benefits and advantages that can be achieved by these and other embodiments of the present invention are described in detail below in conjunction with the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a simplified cross-sectional view of a substrate heater according to the prior art; 
           [0013]      FIG. 2  is a simplified cross-sectional view of a substrate heater according to an embodiment of the invention; 
           [0014]      FIG. 3  is a simplified perspective view of a substrate heater according to another embodiment of the present invention; 
           [0015]      FIG. 4  is a simplified perspective view of a substrate heater according to still another embodiment of the present invention; 
           [0016]      FIG. 5  is a simplified cross-sectional view of a substrate heater according to another embodiment of the present invention; 
           [0017]      FIG. 6  is a simplified cross-sectional view of a substrate heater according to yet another embodiment of the present invention; 
           [0018]      FIGS. 7A and 7B  are simplified cross-sectional views of a heater shaft according to different embodiments of the invention; and 
           [0019]      FIGS. 8 and 9  depict test results demonstrating the effectiveness of the invention as compared to previously known substrate heaters. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 2  is a simplified cross-sectional view of a substrate heater  20  according to an embodiment of the invention. Heater  20  includes a ceramic (e.g., AlN, BN, SiC, SiN) substrate support  22  with an RF electrode  24  and resistive heating element  26  embedded therein along with a shaft or pedestal  28 . Heating element  26  is the primary heat source for the substrate support and can be a resistive heating coil laid out in a two dimensional pattern slightly underneath the substrate support surface that is designed to provide generally uniform heating at the substrate support surface across the entire footprint of the substrate support. Pedestal  28 , which can be made from the same ceramic material as substrate support  22 , includes an interior cavity  30  that allows metal rods (not shown) to be extended within cavity  30  and coupled to electrode  24  and heating element  26  to provide power to each of the electrode and the heating element. As shown in  FIG. 2 , substrate heater  20  also includes a supplemental heater  40  that fits within cavity  30  of pedestal  28  to provide an additional source of heat in a central region of the substrate support. The supplemental heater can be any appropriate compact heat source that fits within cavity  30 . In one particular embodiment, heater  40  is a ceramic block (e.g., aluminum nitride) having a second resistive heating element embedded therein that is independently controlled from heating element  26 . In another embodiment, heater  40  includes a cartridge heater that slides into a cavity machined into the heater block. 
         [0021]    Supplemental heater  40  fits within cavity  30  and abuts a bottom surface  32  of substrate support  22  in a position that provides good thermal contact between heater  40  and the substrate support. According to embodiments of the invention, heater  40  is a separate component from and not integrated with or bonded to substrate support  22 . This allows the supplemental heater to be attached, detached and replaced as may be needed over the life of the substrate processing tool. Additionally, not bonding the two components together in a fixed manner reduces or eliminates the chances of cracking at the interface between the surface  32  and supplemental heater  40  due to stresses associated with a difference in coefficients of thermal expansion between heater  40  and substrate support  22 . Some embodiments of the invention include a biasing mechanism (not shown in  FIG. 2 ) that forces heater  40  in thermal contact with the substrate support as described below and that also allows the supplemental heater to be readily disengaged when needed. 
         [0022]    When heater  20  is positioned within a substrate processing chamber, cavity  30  is isolated from the substrate processing region (not shown) of the chamber. Generally cavity  30  is under atmospheric pressure while the substrate processing region is evacuated to a subatmospheric or near vacuum pressure. Thus, heater  40  is not exposed to the environment within the processing chamber which is often corrosive. While not shown in  FIG. 2 , in one particular embodiment heater  40  includes four terminals that run through shaft  30  to the substrate support including two heater terminals, an RF terminal and a thermocouple terminal. 
         [0023]    Embodiments of the invention allow for an additional degree of temperature control at the center of the substrate heater  20  so that a more uniform temperature can be seen by a substrate positioned on surface  21  across the entirety of the surface. As previously mentioned, without such an additional temperature control, the center region of the substrate may sometimes be cooler than the periphery which in turn may result in non-uniform processing of the substrate. For example, a center cold heater temperature profile will result in the deposition of a film having a higher center region during deposition of various SACVD silicon oxide thick or thin films among others. The inventors have determined that this issue is partly caused by a lack of heater coils in the center due to area taken up by required terminal connections to the heater and RF electrode. Even if, however, the heater coil design of heating element  26  is optimized so that a particular heater delivers a uniform temperature profile across the entire substrate surface at a particular temperature, for example, 480° C. or 540° C., the conductivity of AlN varies with temperature. Thus, as the heater set point decreases, AlN thermal conductivity of substrate support  22  and pedestal  28  increases thus increasing heat loss through the pedestal. Temperature difference between the center and periphery of even 0.5% (e.g., 500° C. at the periphery and 497.5° C. at the center) may result in unacceptable performance regarding film uniformity. 
         [0024]    Embodiments of the invention compensate for the temperature drop in the center of the heater with supplemental heater  40  that is operatively coupled to the lower surface of the substrate support  22  within cavity  30  of the shaft  28  at interface  32 . In one embodiment, shown in  FIG. 3 , the supplemental heater is a metal block  50  (e.g., aluminum, copper, nickel, or some combination or alloy thereof, etc.) that is in contact with the back surface of substrate support  22 . An air gap  58  surrounds the periphery of heater block  50  so that the heater block is not in direct contact with the sidewalls of the pedestal shaft  28 . Block  50  can be heated by any appropriate mechanism such as a resistive heater, a cartridge heater or the like. A thermocouple  52  monitors the temperature of the supplemental heater and the desired set point of heater block  50  can be set based on the whether or not temperature sensors (not shown) at various radii of substrate support  22  indicate there is a temperature difference at the substrate center versus the periphery. Terminal rods (e.g., nickel rods) run through ceramic tubes  54  and  56  to provide power to the embedded RF electrode and resistive heating element embedded within the substrate support  22  (neither of which is shown in  FIG. 3 ). As mentioned above, substrate support  22  includes one or more of its own temperature sensors or thermocouples (not shown), different from thermocouple  52 , that measures the temperature of the substrate support at different locations and are operatively coupled to a control element for the resistive heater (e.g., heater  26  shown in  FIG. 2 ) that is the primary heat source to support  22 . 
         [0025]      FIG. 4  is a simplified perspective view of another embodiment of the invention in which a supplemental heater  60  is fitted within the cavity  30  and operatively coupled to the lower surface of substrate support  22  within the cavity. As shown in  FIG. 4 , heater  60  is separated from an interior surface of shaft  28  by airgap  58 . Heater  60  can be made from metal or a ceramic block, such as aluminum nitride, and includes a heater cartridge  61  that fits within a matching sized cavity. A thermocouple  62  monitors the temperature of the substrates support in a manner similar to that described above for thermocouple  52 . Wires  64   a,    64   b  provide power/signals to heater cartridge  61  and thermocouple  62 . Heater cartridge  61  may include, for example, a standard resistive tungsten heater element. 
         [0026]    A ceramic cap  63  can be secured to the end of heater  60  to hold heater cartridge  61  in place. Ceramic cap  63  can be made from an insulating ceramic material, such as aluminum oxide, that has less thermal conductivity than aluminum nitride to isolate components within shaft  30  and below heater  60  from its heat. Spaced apart from ceramic cap  63  is a high temperature ceramic (e.g., Al 2 O 3 ) plate  65  that is operatively attached to a spring  66  near a center point of plate  65 . In other embodiments, plate  65  may be made from a high temperature plastic or similar material. 
         [0027]    One or more ceramic tubes  67  are positioned between plate  65  and cap  63  that allow the heater and RF terminals to be run through it to substrate support  22 . Spring  66  biases the assembly of plate  65 , tube(s)  67  and cap  63  so that, in operation, an upper surface of heater  60  is in thermal contact with the lower surface of substrate support  22 . Spring  66  is positioned against an aluminum heater base plate  68  that is fixedly attached to pedestal  28 . 
         [0028]    In another embodiment shown in  FIG. 5 , a supplemental heater  70  is positioned within the cavity  30  of a pedestal  28  and coupled to a spring-loaded mechanism  72  that allows heater  70  to be moved between a first position in which the heater is operatively engaged with the substrate support when additional heat control is desired or moved into a second position in which heater  70  is not in physical contact with the substrate support. Heater  70  includes one or more holes  71  through which terminal rods  73  (e.g., terminal rods for electrode  24  and RF heater  26 ) extend. Holes  71  allow heater  70  to slide up and down within the pedestal cavity. If desired, corrugated foil (e.g., Al, Cu, BeCu, etc.) or a ceramic foil or a similar component can be positioned between the interface of heater  70  (as well as heaters  40 ,  50 ,  60  or  80  shown in other embodiments) and the substrate support to effect heat transfer between the two bodies. 
         [0029]    In still another embodiment shown in  FIG. 6 , a supplemental heater  80  is operatively coupled to a spring so that the heater can be engaged with an inner surface  81  of pedestal shaft  28 , which is coupled to the bottom of the substrate support  22 . To engage surface  81 , heater  80  can be expanded radially by separating along a line  85 . Once engaged, heat from heater  80  is transferred through shaft  28  to an annular area at the bottom of support  22 . As shown in  FIG. 7A , which is a simplified cross-sectional view of shaft  28 , in some embodiments the cross section of shaft  28  is circular at an outer surface  83 , but has a rectangular, rounded rectangular or oval shape at the inner surface  81 . Such a shape, which is non-symmetric with respect to a circular substrate, is particularly useful when substrate support  22  includes a vacuum chuck (not shown) that is operatively coupled to vacuum lines  84 . Heater  80  can be designed to compensate for the non-symmetric shape by providing a higher temperature at the portions of shaft  28  that have less surface area contact with the bottom of substrate support  22  than other areas of shaft  28 . In other embodiments, both the outer and inner surfaces of shaft  28  have a circular cross section as shown in  FIG. 7B  and are thus symmetric with respect to a circular substrate being processed on support  22 . 
         [0030]      FIGS. 8 and 9  depict the results of tests that demonstrate the effectiveness of one particular embodiment of the present invention. Specifically,  FIG. 8  shows that embodiments of the invention can be used to improve temperature uniformity over the surface of a wafer being processed on a substrate heater according to the present invention as compared to a previously known heater (the “baseline” test). Note, at 0% power, the temperature uniformity for the particular 550° C. process is worse than the previously known heater because the heater acts as a heat sink transferring heat away from the center of the wafer which is already cooler than the periphery at 550° C. in this heater design. As the supplemental heater is powered at 50% though, the average temperature difference drops and temperature is more uniform across the substrate with the techniques of the present invention than without.  FIG. 9  shows the actual temperature for the tested power levels depicted in  FIG. 8  measured at different radii of the substrate. 
         [0031]    In some instances a substrate support designs may have a center temperature that is actually hotter than the periphery at some or all temperature ranges. Embodiments of the invention can improve uniformity for these substrate supports as well by not powering the heater within the supplemental heater or by driving the supplemental heater at a lower set point than the substrate temperature. In such situations, the supplemental heater, which has a relatively large mass, acts as a heat sink drawing heat away from the center of the substrate thus cooling the center of the substrate relative to the periphery.

Technology Classification (CPC): 7