Abstract:
Thermal chucks for processing semiconductor substrates include a cooling passage that provides a laminar flow. The thermal chuck can be fabricated by forming the cooling passage into a selected one of a planar support surface portion and an underside portion; sandwiching a cladding material between the planar support surface portion and the underside portion to form a thermal chuck assembly; and heating the thermal chuck assembly to a temperature and under conditions to fuse the cladding material to the planar support surface portion and the underside portion, wherein the cooling passage is sealed therein. The cooling passage can be milled or cast and has radially curved section connected to adjacent linear sections to form a serpentine-like shape.

Description:
BACKGROUND  
       [0001]     The present disclosure generally relates to the manufacture of thermal chucks employed in semiconductor processing, and more particularly, to thermal chucks having cooling passages formed therein and for methods of manufacturing the thermal chuck with the cooling passages.  
         [0002]     Thermal chucks are generally manufactured from a unitary member having a defined height, width, and thickness. For processing semiconductor substrates, the thermal chucks are circular and have a planar support surface. Cooling passages formed in these chucks generally have a serpentine-like shape so as to provide uniform temperature regulation and control during use. To make the serpentine-like shape of the cooling passage, prior art chucks are drilled at edge locations radially disposed about the chuck. Each linear drilled passage intersects with another drilled passage so as to form the serpentine like-shape. Inlet and outlet openings are formed at the terminal ends of the serpentine-like shape cooling passage, which in the course of manufacture are drilled through the underside of the chuck. The drill hole openings about the radial edge of the chuck are then press fitted with a plug, e.g., a two-piece friction lock plug.  
         [0003]      FIG. 1  illustrates an exemplary thermal chuck  10  that includes a serpentine-like shaped cooling passage. The illustrated thermal chuck  10  is circular in shape having a planar support surface  12  for supporting substrates during processing. Concentric recesses  14  may be formed in the planar support surface  12  to provide vacuum hold down capabilities. In this exemplary chuck, eleven passageways  16  are drilled through the radial edge  18  of the chuck to collectively form the serpentine-like shaped cooling passage. Inlet and outlet openings  20 ,  22  are drilled through the underside of the chuck at terminal end locations for the serpentine-like shape cooling passage. The openings  24  about the radial edge  18  are then press fitted with a  2 -piece plug (not shown).  
         [0004]     One of the problems with manufacturing the cooling passages in this manner is that the press fitted plugs can fail during operation causing fluid to leak into the process chamber during use. The press fitted-plugs are generally rated for temperatures significantly less than the operating temperatures in which the thermal chuck is exposed. However, in practice the chuck is routinely used in excess of these temperatures, which can result in thermal fatigue and failure of the plugs. Moreover, the process of press fitting the plugs into openings can result in distortion of the planar support surface  12  and affect the mechanical integrity of the chuck. Although the distortion can generally be resolved by milling excess material from the distorted surface, to a desired flatness specification, residual stress concentrations coupled with thermal cycling can cause the chuck to fracture.  
         [0005]     Another problem inherent to the method of manufacture of the serpentine-like cooling passage is the transitional/turbulent flow of coolant inherent to the manner in which the cooling passage is formed. Because the serpentine-like cooling passage requires each passage section to be drilled linearly from a radial edge location, the intersecting passage sections are at an angle to one another, e.g., perpendicular as shown. The resulting transitional/turbulent flow of fluid therein affects temperature uniformity and can impart mechanical stresses to the chuck. Moreover, the friction fitted plugs have a finite length, which do not always terminate at the point of intersection, thereby resulting in dead ends within the serpentine -like cooling passage. Additionally, liquid is trapped in the dead end portions when the cooling passage is purged with pressurized air. This liquid vaporizes when the thermal chuck is reheated, causing excess pressure within the cooling passage.  
         [0006]     Accordingly, there remains a need for improved manufacturing methods and thermal chucks to overcome the problems noted in the art.  
       BRIEF SUMMARY  
       [0007]     Disclosed herein are thermal chucks having cooling passages machined therein and for methods of manufacturing the thermal chuck. In one embodiment, a process for fabricating the thermal chuck comprises forming a cooling passage into a selected one of a planar support surface portion and an underside portion; sandwiching a cladding material between the planar support surface portion and the underside portion to form the thermal chuck; and heating the thermal chuck to a temperature and under conditions to fuse the cladding material to the planar support surface portion and the underside portion, wherein the cooling passage is sealed therein.  
         [0008]     In another embodiment, the process comprises forming a cooling passage into a selected one of a planar support surface portion and an underside portion; sandwiching a cladding material between the planar support surface portion and the underside portion to form a thermal chuck assembly; and vacuum brazing the planar support surface portion, the cladding material, and the underside portion to form a an integrated structure, wherein the cooling passage is sealed within the integrated structure.  
         [0009]     A thermal chuck for processing semiconductor substrates comprises a planar top surface for supporting a substrate; a cooling passage spanning underneath the planar top surface comprising a plurality of linear sections and at least one radially curved section connecting adjacent ones of the plurality of linear sections, a first end, and a second end; and a bottom surface having therein an inlet opening fluidly connected to the first end and an outlet opening fluidly connected to the second end, wherein the cooling passage is sandwiched between the planar top surface and the bottom surface.  
         [0010]     The above described and other features are exemplified by the following figures and detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Referring now to the figures, which are exemplary embodiments and wherein like elements are numbered alike:  
         [0012]      FIG. 1  is a perspective of a prior art thermal chuck;  
         [0013]      FIG. 2  is an exploded perspective view of a thermal chuck in accordance with one embodiment of the disclosure;  
         [0014]      FIG. 3  is a perspective view of the thermal chuck of  FIG. 2  detailing the planar support surface for supporting a substrate; and  
         [0015]      FIG. 4  is a plan view of a serpentine-like shaped cooling passage in accordance with one embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0016]     Disclosed herein are a thermal chuck that includes cooling passages and a method of manufacturing the thermal chuck, which overcomes the problems noted in the prior art. The method of manufacture generally includes vacuum brazing a planar support surface portion to an underside portion to form the thermal chuck, wherein the cooling passages are milled into a selected one of the planar support surface portion and the underside portion prior to vacuum brazing. Consequently, upon vacuum brazing, the two components are joined together to seal the milled cooling passage. In this manner, the use of plugs is eliminated and the milled pattern can be made to provide a laminar flow through the cooling passages without dead ends.  
         [0017]      FIGS. 2 and 3  illustrate an exploded view of the thermal chuck  100  in accordance with one embodiment. The thermal chuck  100  generally includes a planar support surface portion  102  and an underside portion  104 , wherein one of the selected components ( 102  or  104 ) includes a cooling passage pattern  106  milled therein. The planar support surface portion  102  and an underside portion  104  are preferably made of a metal resistant to erosion by the environment in which the thermal chuck is employed, e.g., heat treatable aluminum alloys, aluminum or aluminum alloys with an anodized aluminum oxide coating, and the like.  
         [0018]     The planar support surface portion  102  includes a planar top surface  108  upon which a substrate is placed during processing. In the illustrated embodiment, the bottom surface  110  of the planar support surface portion  102  is milled with the desired cooling passage pattern  106 . Although a serpentine-like cooling passage pattern is shown, it should be apparent to those skilled in the art that any pattern can be milled therein. As such, the present disclosure is not intended to be limited to the particular pattern shown. Moreover, it should be apparent that more than one passage (i.e., more than one inlet and outlet) could be provided. Additionally, although reference has been made to milling, other methods for defining the cooling passage can be employed, e.g., casting.  
         [0019]     The planar support surface portion  102  can also include those features commonly found in thermal chucks employed for processing semiconductor wafers. For example, as shown, the planar top surface  106  may include a plurality of concentric annular recesses  112  about a central axis  114 . In addition, the planar support surface portion  102  may optionally include openings  116  for attachment of perimeter pins, heating elements, gas transfer holes, thermocouples, and the like. Depending on the desired application, the openings  116  I combination with the concentric annular recesses  112  may also be employed for providing a vacuum to the backside of the substrate for increasing the number of contact points between the bottom surface of the substrate and the planar top surface  106  such as by elastic deformation of the substrate. If a vacuum hold down is utilized, the increased number of contact points between the substrate and the planar top surface  106  resulting from the vacuum can increase the rate at which the substrate comes to process temperature. In this case, the vacuum hold down openings and/or vacuum passages (not shown) are preferably connected to a vacuum line, which is in turn connected downstream of a process chamber isolation valve, a flow control valve, or the like (not shown).  
         [0020]     The underside portion  104  includes a top surface  120  and a bottom surface  122 . The top surface is co-planar to and is mated with the bottom surface  122  of the planar support surface portion  102 . An inlet  124  and an outlet  126  for the cooling passage  106  may be drilled through the underside portion  104 . The underside portion  104  may further include one or more plug openings  128  coaxially aligned with an opening or recess  132  in the planar support surface portion  102 . Prior to vacuum brazing, a plug  132  is reamed into the opening  128 ,  130  to provide the proper alignment of the planar support surface portion  102  with the underside portion  104 . The underside portion  104  may further include openings  134  coaxial and complementary to openings  116  in the planar support surface portion  102 . In this manner, the thermal chuck  100  can be fitted with thermocouples, perimeter pins and the like, as may be desired for the intended application for the thermal chuck. Resistance heating elements may also be cast into the underside portion  104  enabling elevated processing temperatures that may be utilized for increased tool throughput such as when performing a bulk photoresist strip or etching process. An annular flange  136  circumscribes the bottom surface  122  to provide a means for securing the thermal chuck to a process chamber. The openings can be drilled before or after the vacuum brazing process is complete.  
         [0021]     In a preferred embodiment, the operating temperature of thermal chuck can be varied preferably via a feedback or a closed loop control system using a proportional integral derivative (PID) controller having a heating and cooling capability. The controller would alternately supply a current to heating elements or cooling fluid (air or water) to passages  106 , as needed. Feedback to the PID controller would be provided by measuring the temperature of substrate during the process using a temperature measuring device such as a spring activated thermocouple mounted within the planar support surface portion  102 . For example, a spring is in operable communication with the thermocouple such that the thermocouple maintains contact with the backside surface of substrate. Alternatively, the temperature of the thermal chuck  100  can be controlled with an open loop process (i.e., without a feedback device) by adjusting the current supplied to heating elements and allowing fluid flow (air or water) through passages  106  at the appropriate point in the process. The support  22  is preferably made of a metal resistant to erosion by the process gases, e.g., aluminum with an anodized aluminum oxide coating.  
         [0022]      FIG. 4  illustrates a plan view of the serpentine-like cooling passage pattern  110 . Noteworthy in the illustrated pattern are the radial curvatures that connect the passage sections that are substantially linear. In this manner, laminar flow through the cooling passage  110  can be obtained.  
         [0023]     The vacuum brazing process is a well characterized joining process, whereby a non-ferrous filler metal and an alloy is heated under vacuum to melting temperature (above 450° C.) and distributed between two or more close-fitting parts by capillary action. At its liquid temperature, the molten filler metal interacts with a thin layer of the base metal, cooling to form an exceptionally strong, sealed joint due to grain structure interaction. The brazed joint becomes a sandwich of layers, each metallurgically linked to each other. In order to work properly, the components  102 ,  104  must be closely fitted and the base metals should be exceptionally clean and free of oxides for achieving the highest strengths for brazed joints. For capillary action to be effective, joint clearances of 0.002 to 0.006 inch (50 to 150 μm) are typically recommended. The vacuum brazing process generally includes a pre-heating step, a series of brazing heating steps, and then a cool down step. The vacuum chamber is generally kept at a vacuum level of 1×10 −3  pascals (Pa) or less.  
         [0024]     When joining aluminum-based materials such as the planar support surface portion  102  and the underside portion  104 , the cladding layer typically includes aluminum as the primary component. Other materials are added to the cladding material to lower its melting point below that of the pieces to be joined. Thus, during the vacuum brazing process the cladding material is melted, flows between the pieces and then forms a solid joint when it is cooled. For example, silicon can be included in the cladding material in order to lower the melting point. In addition, the cladding material typically includes added magnesium. The magnesium diffuses during the brazing process thereby breaking up the external aluminum oxide layer, acting as a surface wetting agent. The diffusion or out-gassing of magnesium permits the cladding material to flow between the aluminum pieces and results in braze joint formation. Thus, magnesium is typically added to the cladding material for this function. The cladding material often comprises other components, the selection of which is well within the skill of those in the art.  
         [0025]     While the disclosure has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.