Abstract:
A method of forming a heater assembly for use in semiconductor processing includes thermally securing a heater substrate to an application substrate; and applying a layered heater having at least one functional layer to the heater substrate after the heater substrate is secured to the application substrate. The heater substrate defines a material having a coefficient of thermal expansion that is matched to a coefficient of thermal expansion of the functional layer. The material of the functional layer is not capable of withstanding the elevated temperature of the thermal securing step.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 13/541,006 filed on Jul. 3, 2012, the entire disclosure of which is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to layered heaters, and more specifically to methods of forming layered heaters bonded to a semiconductor processing apparatus with improved reliability at elevated temperatures. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0004]    A layered heater typically includes a plurality of functional layers applied on a substrate by layered processes. The plurality of functional layers may include a dielectric layer on the substrate, a resistive heating layer on the dielectric layer, and a protective layer on the resistive heating layer. The materials for the different functional layers and the substrate are carefully chosen to have compatible coefficient of thermal expansion (CTE) to reduce shear stress generated at the joining interfaces at elevated temperatures. The shear stress may cause generation of cracks or delamination at the joining interfaces, resulting in heater failure. 
         [0005]    Only a limited number of materials can be used to form the different functional layers by a specific layered process, thereby limiting the selection of materials for the substrate, which should have a CTE matching the CTE of the dielectric layer applied on the substrate or matching the CTE of the heating layer. For example, when alumina ceramic is used to form the dielectric layer, alumina nitride or molybdenum is generally used to form the substrate due to its chemical and CTE compatibility with the alumina ceramic. 
         [0006]    The layered heater may need to be joined to a heating target in some applications. For example, the layered heater may be joined to an electrostatic chuck to form a heated electrostatic chuck. However, the limited selection of materials for the substrate makes joining the layered heater to the electrostatic chuck difficult. When the substrate of the layered heater has a CTE that does not match the CTE of the chuck body, the heated electrostatic chuck is likely to fail due to generation of cracks or delamination at the joining interface at elevated temperatures. 
       SUMMARY 
       [0007]    In one form, a method of forming a heater assembly for use in semiconductor processing includes thermally securing a heater substrate to an application substrate; and applying a layered heater having at least one functional layer to the heater substrate after the heater substrate is secured to the application substrate. The heater substrate defines a material having a coefficient of thermal expansion that is matched to a coefficient of thermal expansion of the functional layer. The material of the functional layer is not capable of withstanding the elevated temperature of the thermal securing step. 
         [0008]    According to another aspect of the disclosure, the functional layer is a bond layer and the application of the layered heater includes applying a first dielectric layer onto the heater substrate, applying a resistive heating layer onto the first dielectric layer, and applying a second dielectric layer onto the resistive heating layer. 
         [0009]    In another form, a method of forming a heated electrostatic chuck includes providing an electrostatic chuck having a chuck top, securing an application substrate to the chuck top, and forming a heater assembly according to the method described above and further defined herein. 
         [0010]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0011]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0012]    In order that the invention may be well understood, there will now be described an embodiment thereof, given by way of example, reference being made to the accompanying drawing, in which: 
           [0013]      FIG. 1  is an exploded view of a layered heater constructed in accordance with the teachings of the present disclosure; 
           [0014]      FIG. 2  is a cross-sectional view of a heater assembly including a layered heater and a heating target and constructed in accordance with the teachings of the present disclosure; 
           [0015]      FIG. 3  is a cross-sectional view of a variant of a heater assembly including a layered heater and a heating target and constructed in accordance with the teachings of the present disclosure; 
           [0016]      FIG. 4  is a flow diagram of a method of forming a heater assembly for use in semiconductor processing; and 
           [0017]      FIG. 5  is a flow diagram of a method of forming another heater assembly for use in semiconductor processing. 
       
    
    
       [0018]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0019]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0020]    Referring to  FIG. 1 , a layered heater  10  constructed in accordance with the teachings of the present disclosure includes an application substrate  12 , a heater substrate  14 , a first dielectric layer  16  formed on the heater substrate  14 , a resistive heating layer  18  formed on the first dielectric layer  16 , and a second dielectric layer  20  formed on the resistive heating layer  18 . The first dielectric layer  16 , the resistive heating layer  18  and the second dielectric layer  20  are formed by a layered process, such as thick film, thin film, thermal spray and sol gel. 
         [0021]    Referring to  FIG. 2 , the layered heater  10  is joined to a heating target  22  to form a heater assembly  25 . For example, the heating target  22  may be a chuck top of a heated electrostatic chuck for semiconductor processing. The application substrate  12  and the heater substrate  14  are made of different materials and are joined by brazing to form a composite chuck. 
         [0022]    A brazing layer  24  is formed between the application substrate  12  and the heater substrate  14 . The brazing material may be a sliver brazing material. Other joining processes, such as welding, soldering, diffusion bonding, epoxying, vulcanizing, may be used to join the application substrate  12  and the heater substrate  14  without departing from the scope of the present disclosure. Similarly, the application substrate  12  may be joined to the heating target  22  by any conventional joining method, such as brazing, welding, soldering, diffusion boding, epoxying, vulcanizing. 
         [0023]    The application substrate  12  includes a material having a coefficient of thermal expansion (CTE) that matches the CTE of the heating target  22 . Alternatively, the application substrate  12  may be an integral part of the heating target to which the heat from the layered heater is transferred. The heater substrate  14  includes a material having a CTE that matches the CTE of the first dielectric layer  16 . In other words, the material of the application substrate  12  depends on the materials of the heating target  22 , whereas the material of the heater substrate  14  depends on the materials of the first dielectric layer  16 . 
         [0024]    For example, when the first dielectric layer  16  includes alumina ceramic, the heater substrate  14  may be made of alumina nitride or molybdenum. The application substrate  12  may include a material having a CTE that can easily match the CTE of most suitable materials for the heating target  22 , regardless of the materials of the first dielectric layer  16  and the heater substrate  14 . The application substrate  12  may include austenitic stainless steel, which has a CTE matching a wider selection of materials. Therefore, the layered heater  10  can be relatively easily joined to the heating target  22 . 
         [0025]    The layered heater  10  may be a layered heater formed by thick film, thin film, thermal-spray, and sol-gel process. The resistive heating layer  18  may be formed by applying a resistive layer throughout the first dielectric layer  14 , followed by a laser removal process to form a circuit pattern. 
         [0026]    In still another form, the resistive heating layer  18  is formed of a material having sufficient temperature coefficient of resistance such that the heating layer  18  functions as both a heater and a temperature sensor, commonly referred to as “two-wire control.” Such heaters and their materials are disclosed, for example, in U.S. Pat. No. 7,196,295 and pending U.S. patent application Ser. No. 11/475,534, which are commonly assigned with the present application and the disclosures of which are incorporated herein by reference in their entirety. 
         [0027]    Referring to  FIG. 3 , a heater assembly  30  has a structure similar to that of the heater assembly  10  of  FIG. 2  except that the layered heater further includes a bond coat layer  32  and a topcoat  34 . The bond coat layer  32  is applied on the heater substrate  14 . The topcoat  34  is applied on the second dielectric layer  20 . 
         [0028]    While two substrates are described in the present disclosure to form a composite substrate, more than two substrates may be used to form a composite substrate, which provides a gradual transition over multiple substrates in terms of coefficient of thermal expansion. 
         [0029]    Referring to  FIG. 4 , a method  40  of forming a heater assembly  25  for use in semiconductor processing includes bonding an application substrate  12  to a heating target  22  in step  42 . When the application substrate  12  is an integral part of the heating target  22 , this step is eliminated. The heater substrate  14  is then thermally secured to the application substrate  12  in step  44 . Thermally securing may include brazing, welding, soldering, diffusion bonding, epoxying, vulcanizing at a first temperature. A layered heater is then applied to the heater substrate  14  after the application substrate  12  is joined to the heating target  22 . 
         [0030]    The application of the layered heater on the heater substrate  14  includes applying a first dielectric layer  16  on the heater substrate  14  in step  46 . A resistive heating layer  18  is then applied on the first dielectric layer  16  in step  48 . The resistive heating layer  18  may be applied to form a circuit pattern when applied on the first dielectric layer  16 . Alternatively, the resistive heating layer  18  may be applied by forming a continuous layer on the entire surface of the first dielectric layer  16 , followed by a laser removal process to form the desired circuit pattern. Finally, a second dielectric layer  20  is applied on the resistive heating layer  18  in step  50 . The method  40  ends in step  52 . 
         [0031]    Referring to  FIG. 5 , a method  60  of forming a heater assembly  30  for use in semiconductor processing is similar to the method  40  of  FIG. 4  except for the steps of applying a bond coat layer and a topcoat. More specifically, the method  60  includes bonding the application substrate  12  to the heating target  22  in step  62 . The heater substrate  14  is then thermally secured to the application substrate  12  in step  64 . Thermally securing may include brazing, welding, soldering, diffusion bonding, epoxying, vulcanizing at a first temperature. Thereafter, a layered heater is applied to the heater substrate  14  after the heater substrate  14  is joined to the application substrate  12 . 
         [0032]    The application of the layered heater to the heater substrate  14  includes applying a bond coat layer  32  on the heater substrate  14  in step  66 . A first dielectric layer  16  is then applied on the bond coat layer  32  in step  68 . A resistive heating layer  18  is applied on the first dielectric layer  16  in step  70 . A second dielectric layer  20  is applied on the resistive heating layer  18  in step  72 . A topcoat  34  is applied over the second dielectric layer  20  in step  74 . Finally, portions of the topcoat  34  are removed to achieve a predetermined surface flatness in step  76 . The method  60  ends in step  78 . 
         [0033]    In the methods  40  and  60  of the present disclosure, brazing is used to effectively and reliably join the heater substrate  14  to the application substrate  12 . The application substrate  12  is made of a material having a CTE that matches the CTE of the material of the heating target  22 . The heater substrate  14  is made of a material having a CTE that matches the CTE of the material of the first dielectric layer  16 . The composite substrate enables the layered heater to be applied to a heating target  22  having a CTE that does not match (i.e., is significantly different from) the CTE of the heater substrate  14 . 
         [0034]    Moreover, the brazing process, which requires a relatively high temperature, is performed before the various functional layers of the layered heater are applied to the heater substrate  14 . Therefore, the layered heater is not subjected to the undesirable high temperature during the brazing process and thus can maintain its integrity. 
         [0035]    The present method enables better matching of the CTE of the layered heater to any heating target  22  by using an application substrate. The present method also ensures the integrity of the layered heater by brazing the heater substrate to the application substrate before the different functional layers of the layered heater are formed on the heater substrate. Therefore, the present method can improve physical/material characteristics of the heater assembly such as machinability, surface roughness, surface hardness, chemical compatibility, thermal conductivity, electrical conductivity, emissivity, appearance, cost, etc. 
         [0036]    The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.