Abstract:
Embodiments of the present invention generally include an apparatus for uniform heat distribution across the surface of a substrate during processing. The apparatus includes a substrate heater with a heated substrate support surface that is removable attached to a heater shaft via a fastening mechanism. The interface between the heated substrate support and the heater shaft may include a soft metal gasket and a vacuum or purge channel disposed therein. The substrate support surface may include regions for independently varying the back pressure of a substrate disposed thereon.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. provisional patent application Ser. No. 61/052,078, filed May 9, 2008, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the present invention generally relate to an apparatus for providing uniform thin film deposition across a substrate. More particularly, embodiments of the present invention relate to a substrate support heater that provides uniform temperature distribution across a substrate, while minimizing the cost of ownership. 
         [0004]    2. Description of the Related Art 
         [0005]    A primary step in the fabrication of modern integrated circuits involves the formation of an insulating film, such as a silicon dioxide film, on a substrate. One such insulating film is a pre-metal dielectric (PMD) film. The PMD film insulates the substrate from a first metal layer, and provides the base film upon which numerous other layers are deposited. Thus, in order to control the uniformity of features formed in the fabrication of integrated circuits, the uniformity of the PMD film deposited across the substrate is critical. 
         [0006]    One problem associated with sub-atmospheric chemical vapor deposition (SACVD) processes used in manufacturing of integrated circuits is non-uniformity of the thickness of films, such as PMD films, deposited across the substrate. Such non-uniformity may be due, in part, to non-uniform temperature distribution across the substrate. 
         [0007]    In SACVD processes, reactive gases are introduced into a reaction chamber at sub-atmospheric pressures. The reactive gases flow over the heated substrate (e.g. 500-600° C.), where desired chemical reactions occur, and the film is deposited. Unwanted deposition occurs on areas such as a substrate support surface of a substrate heater situated within the reaction chamber. It is common to remove the unwanted deposited material from the surface of the heater with in situ chamber clean operations. Common chamber cleaning techniques include the use of an etchant gas, such as nitrogen trifluoride (NF 3 ), to remove the deposited material from the substrate support surface. 
         [0008]    However, typical substrate heaters are made from aluminum, aluminum oxide, or aluminum nitride. One problem with using NF 3  or other fluorine-containing etchant gases for cleaning unwanted deposits from these aluminum-containing heaters after a high temperature deposition process is that active fluorine species from the etchant gas reacts with the aluminum, resulting in the formation of AlF 3  film on the surface of the substrate heater. This film has relatively high vapor pressures and relatively low sublimation temperatures and may attain a thickness of several hundred micrometers when conditions for self-passivation are not met, such as at the outer annular region of the substrate supporting surface of the substrate heater. 
         [0009]    The build-up of AlF 3  film on the uncovered and/or partially covered edge region of the substrate supporting surface of the substrate heater results in uneven thermal conduction between the heater and the substrate, resulting in uneven temperature distribution across the substrate. In order to deal with this problem, the industry practice is to polish the substrate supporting surface, at specified intervals, to remove the AlF 3  film. However, the substrate supporting surface of the substrate heater is consumed after only a few of the polishing processes, and the entire substrate heater must be replaced at significant cost. 
         [0010]    Therefore, a need exists for a heater pedestal that promotes uniform film deposition by increasing the temperature uniformity across the substrate, while minimizing the cost of ownership. 
       SUMMARY OF THE INVENTION 
       [0011]    In one embodiment of the present invention, a substrate heater comprises a substrate support member comprising a ceramic material and having a heating element disposed therein, a shaft member having an upper flange configured to support the substrate support member, and a fastening member configured to removably attach the substrate support member to the upper flange of the shaft member. 
         [0012]    In another embodiment of the present invention, a processing chamber comprises a chamber wall, a gas distribution showerhead, a vacuum source, a valve member in fluid communication with the vacuum source at an input location, a controller, and a substrate heater. In one embodiment the substrate heater comprises a substrate support member have an upper surface for supporting a substrate thereon and a heating element disposed therein, a hollow shaft member having an upper surface configured to mate to a lower surface of the substrate support member, and a fastening member configured to removably attach the lower surface of the substrate support member to the upper surface of the hollow shaft member. In one embodiment, the substrate support member is comprised of a ceramic material. In one embodiment, the hollow shaft member has an annular groove in the upper surface thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    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. 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. 
           [0014]      FIG. 1A  is a schematic, cross-sectional view of an exemplary SACVD system that may incorporate embodiments of the present invention. 
           [0015]      FIG. 1B  is an enlarged, schematic, cross-sectional view of the gas delivery system in  FIG. 1A . 
           [0016]      FIG. 2  is a schematic, cross-sectional view of a substrate heater assembly according to one embodiment of the present invention. 
           [0017]      FIG. 3  is a top view of the heater plate in  FIG. 2 . 
           [0018]      FIG. 4  is a top layout view of a heater element according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1A  is a schematic, cross-sectional view of an exemplary SACVD system  100  that may incorporate embodiments of the present invention. The system  100  includes a chamber  120 , a gas delivery system  150 , a substrate heater assembly  160 , and a vacuum system  130 . Reactive gases are introduced into the reaction chamber  120  through an inlet  125  of the gas delivery system  150 . The substrate heater assembly  160  supports and heats a substrate  140 . In order to promote a uniform distribution, the reactive gases are introduced into the chamber  120  from a source positioned opposite the substrate  140 . The gas delivery system  150  may include a heating and cooling means (not shown) for maintaining a constant gas and chamber temperature. The substrate  140  is transferred into and out of the chamber  120  by a transfer robot (not shown) through an opening (not shown) in the side of the chamber  120 . 
         [0020]      FIG. 1B  is an enlarged, schematic, cross-sectional view of the SACVD system  100  illustrating the gas delivery system  150 . Reactive gasses are introduced through the inlet  125  into a heated showerhead  175 . The shower head  175  has a plurality of outlets  180  disposed at specified intervals. The reactive gasses flow over the heated substrate  140  and deposit a thin film thereon. 
         [0021]      FIG. 2  is a schematic, cross-sectional view of a substrate heater assembly  200  according to one embodiment of the present invention. The heater assembly  200  includes a heater plate  210  for supporting and heating the substrate  140 . In one embodiment, the heater plate  210  is substantially disk shaped having an appropriately sized upper surface for supporting the substrate  140 . The heater plate  210  may comprise a ceramic material with good thermal conducting characteristics. In one embodiment, the heater plate  210  may comprise aluminum nitride. The heater plate  210  may be formed by sintering multiple layered sheets of a “green” phase ceramic material, such as aluminum nitride as known in the art. 
         [0022]    The heater plate  210  is detachably coupled to a heater shaft  230 . The heater shaft  230  is substantially cylindrical having a hollow interior volume  232 . The heater shaft  230  includes an upper flange  234  for mounting to the heater plate  210 . In one embodiment, the upper flange  234  extends into the interior volume  232 . The heater shaft  230  may comprise a ceramic material having a thermal conductance less than that of the heater plate  210 . In one embodiment, the heater shaft  230  may comprise aluminum oxide and the heater plate  210  may comprise aluminum nitride. 
         [0023]    In one embodiment, the heater plate  210  is detachably coupled to the heater shaft  230  via two or more fasteners  250 . Each fastener  250  may comprise a threaded stud  252  permanently attached to the heater plate  210 . In one embodiment, each threaded stud  252  is a metal (e.g., Kovar, SST) that is brazed to the bottom surface of the heater plate  210 . When the heater plate  210  is mated to the heater shaft  230 , each stud  252  extends through a corresponding aperture in the upper flange  234  of the heater shaft  230 . A nut  254  is then threaded onto each stud  252 , and the appropriate torque is applied to achieve a seal between the heater plate  210  and the heater shaft  230 . Alternatively, the each fastener  250  may comprise a screw (not shown) inserted through the upper flange  234  and threaded into threaded holes (not shown) in the bottom surface of the heater plate  210 . 
         [0024]    In one embodiment, the sealing surfaces of the heater plate  210  and the heater shaft  230  are polished to promote a good seal. In one embodiment, the roughness of each of the sealing surfaces is between about 0.40 microns (μm) and about 0.01 microns. In one embodiment, an annular gasket  236  is disposed between the mating surfaces of the heater plate  210  and the heater shaft  230  to promote a better seal. The gasket  236  may be a soft metal, such as aluminum. It is believed that the aluminum gasket  236  may plastically deform at operating temperatures (such as 500° C.-600° C.), resulting in good conformance to the mating surfaces of the heater plate  210  and the heater shaft  230 . In one embodiment, the annular gasket is disposed within a groove  230 A formed in the heater shaft  230  to support and retain the annular gasket  236 . In one embodiment, the groove  230 A and the back surface  210 A of the heater plate  210  each have at least one raised area (not shown) that is in contact with a portion of the annular gasket  236  to improve the seal formed between the annular gasket  236 , the heater plate  210 , and the heater shaft  230 . The raised areas are used to increase the contact stress between the annular gasket  236 , the heater plate  210 , and the heater shaft  230  when the heater shaft  230  is attached to the heater plate  210  to improve the formed seal. 
         [0025]    Several advantages may be achieved over the prior art by the preceding configuration. In one embodiment, the height of the heater shaft  230  may be significantly shorter than that of prior art unitary substrate heaters. Typical prior art substrate heaters comprise unitary or permanently bonded structures. As such the material properties, such as thermal conductance, of the heater plate portion and the heater shaft portion are substantially identical. Therefore, the height of the shaft must be significant to adequately choke the heat transferred from the heater plate through the shaft. Conversely, embodiments of the present invention provide the heater shaft  230  made of a material having lower thermal conductance than that of the heater plate  210 . Thus, the height of the heater shaft  230  may be significantly shorter than that of the prior art to achieve the same or better heat choking effect. Therefore, embodiments of the present invention provide a heater shaft  230  comprising less material than that of the prior art, and consequently, less costly than that of the prior art. 
         [0026]    Additionally, embodiments of the present invention provide a lower cost of ownership than that of the prior art. For instance, as previously described, the surface of heater plates must be polished periodically to remove AlF 3  film deposits. In prior art substrate heaters, after a few polishing procedures, the entire heater assembly (including the heater shaft) must be removed and discarded. Therefore, the entire heater assembly is a consumable part in prior art configurations. In contrast, in embodiments of the present invention, only the heater plate  210  need be removed and replaced. Thus, in the present invention, the heater plate  210  is a consumable part, and the heater shaft  230  is a reusable part. 
         [0027]    In one embodiment, the heater shaft  230  is a hollow shaft. Heater terminals (not shown), an RF terminal (not shown), and a thermocouple (not shown) may be located within the inner volume  232  of the heater shaft  230 . In one embodiment, the heater shaft  230  includes shaft channels  236 ,  238  disposed therethrough. In one embodiment, the shaft channel  236  is coupled to a vacuum source  260  for vacuum chucking a substrate to the heater plate, as subsequently described. In one embodiment, the shaft channel  238  is coupled to the vacuum source  260  as well. In one embodiment, a valve  265  is positioned between the vacuum source  260  and the shaft channels  236 ,  238 . The valve may be controlled by controller  270 , which may be programmed to vary the gas conductance through the shaft channels  236 ,  238  to the vacuum source  260  to achieve a different vacuum pressure in each of the shaft channels and components connected to the respective shaft channels. In another embodiment, the shaft channel  238  is coupled to a purge gas source  275  that is adapted to deliver a gas to the shaft channel  238 . 
         [0028]    In one embodiment, the upper surface of the heater shaft  230  includes an annular groove  240  disposed therein. The annular groove  240  may be coupled to the shaft channel  238  via shaft channel  239 . In one embodiment, vacuum pressure is applied to the groove  240  to remove atmospheric gas that may leak from the inner volume  232  of the heater shaft or to remove reactive gasses that may leak past the seal prior to reaching the inner volume  232 . In another embodiment, a purge gas is supplied through the groove  240  to prevent leakage of gases past the sealing surfaces of the heater plate  210  and the heater shaft  230 . 
         [0029]    In one embodiment (not shown), the annular groove  240  is disposed within the back surface  210 A of the heater plate  210  and coupled to the shaft channel  238 . In one embodiment (not shown), both the back surface  210 A and the upper surface of the heater shaft  230  have a groove  240  disposed therein and coupled to the shaft channel  238 . 
         [0030]      FIG. 3  is a top view of the heater plate  210  from the heater assembly  200  in  FIG. 2 . In one embodiment, the heater plate  210  may have a substrate support surface  212  surrounded by a raised annular flange  214 . In one embodiment, the heater plate  210  has an inner groove  216  disposed in the substrate support surface  212 . In one embodiment, the heater plate  210  has an outer groove  218  disposed in the substrate support surface  212 . 
         [0031]    In one embodiment, the inner groove  216  is coupled to the shaft channel  236  via a heater plate channel  226 . The shaft channel  236  supplies vacuum pressure to the inner groove  216  for vacuum chucking a substrate to the substrate support surface  212 . In one embodiment, the outer groove  218  is coupled to the shaft channel  238  via a heater plate channel  228 . The shaft channel  238  may supply vacuum pressure to the outer groove  218  for chucking the substrate to the substrate support surface  212  as well. 
         [0032]    In one embodiment, the vacuum pressure may be varied such that the vacuum applied to the substrate through the outer groove  218  is less or greater than the vacuum applied to the substrate through the inner groove  216 . This configuration allows the contact pressure between the back surface of the substrate  140  and the heater plate  210  to be varied and controlled across different regions of the substrate  140 , thus resulting in greater control of the heat transfer and temperature uniformity across the substrate during processing. As such, the heat distribution across the substrate may be substantially uniform, resulting in a more uniform film deposition across the substrate. 
         [0033]    In another embodiment, purge gas may be provided to an outer peripheral region of the substrate through the outer groove  218 . The flow of purge gas may reduce deposition in unwanted areas of the substrate and the heater plate  210 . Additionally, the flow of purge gas may increase the heat transfer between the heater plate  210  and the outer peripheral region of the substrate, resulting in a more uniform heat distribution across the substrate. Thus, a more uniform film deposition across the substrate may be achieved. 
         [0034]    Referring to  FIG. 2 , fabrication of the heater plate channel  228  may be accomplished via various techniques. In one embodiment, the heater plate  210  may comprise sheets of “green” phase material sintered to form a unitary body. In one embodiment, slots may be formed in one or more layer(s) within the stacked layers of “green” phase material prior to sintering to form the heater plate channel  228 . In another embodiment, the channel  228  may be drilled from the side  210 B of the heater plate  210 . A plug  229  is inserted into the drilled hole and bonded to the side of the heater plate  210 . Bonding may be performed by use of a high temperature adhesive material or by bonding of the plug  229  to the heater plate  210  by use of a sintering process. In one embodiment, it is desirable to assure that the seal formed between the plug  229  and the heater plate  210  is formed between a surface  229 A of the plug  229  and the side  210 B of the heater plate  210  to reduce the need for maintaining tight tolerances between the channel  228  and plug shank  229 B. 
         [0035]    One or more heating elements  220  are embedded within the heater plate  210 .  FIG. 4  is a schematic view of a heating element  220  layout according to one embodiment of the present invention. In one embodiment, the heating element  220  comprises a wire of a material having good stability at high operating temperatures, such as tungsten or molybdenum. In one embodiment, the heating element  220  may be a screen printed layer. In one embodiment, the heating element  220  may be a tungsten or molybdenum wire mesh. In one embodiment, the heating element  220  is comprised of a substantially planar strip of perforated foil. The substantially planar shape provides positional stability in the fabrication process as opposed to a cylindrical wire. In addition, the perforated surface allows displacement of the heater plate  210  material during fabrication through the perforations in the heating element  220 , again resulting in greater positional stability than prior art wires. The positional stability can improve the temperature uniformity across the substrate supporting surface  212  by reducing the variation in distance between the heating element  220  and the substrate supporting surface  212 . In one embodiment, the perforated foil may comprise tungsten or molybdenum. 
         [0036]    In one embodiment, shown in  FIG. 4 , the heating element  220  comprises two or more heating elements  220 ,  222  electrically connected in parallel and routed throughout the heating plate  210 . The parallel heating elements  220 ,  222  allow greater distribution and control of the heat density across the surface of the substrate, resulting in more uniform heat distribution across the substrate. Therefore, a more uniform film distribution across the substrate may be achieved. 
         [0037]    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.