Abstract:
Methods and apparatuses are disclosed for applying a twin wire arc spray composite coating to achieve surface effects on a substrate having predetermined characteristics.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to the field of coating deposition. More specifically, the present invention is directed to methods and apparatuses for depositing coatings onto substrates using modified Twin Wire Spray Arc methodology.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the field of processing chamber deposition, chamber components and surfaces are often modified to facilitate optimum production of work pieces being coated. Deposition processes entail subjecting work piece surfaces to conditions and coatings requiring stringent quality control. Coating thicknesses for many work pieces, such as, for example, semiconductors, are extremely small, often as small as several thousandths of an inch or less. Various plasma coating technologies create byproducts in the deposition chamber that are vented from the chamber atmosphere during or after coating occurs. However, some plasma byproduct remains adhered to the inner surfaces, or walls, of the chamber as well as other chamber components exposed to the inner chamber environment. When byproduct builds up to a predetermined amount, usually based on hours of operation, the production must be brought off line, and the chamber surfaces replaced or cleaned to remove the byproduct build up. If this regular maintenance is not conducted, byproduct may separate from the chamber walls and other exposed components and contaminate the work pieces being coated. This contamination often results in work piece failure or malfunction. As a result, in the coating industry, chamber inner surfaces and chamber components exposed to the inner chamber have modified surfaces designed to increase the adherence of plasma and other coating process byproducts. This enhanced adherence of byproduct to the chamber walls and chamber components increases the processing time achieved between chamber cleanings, which, in turn, increases overall system productivity as the process remains “on line” longer resulting in greater overall product yield. In addition, if the by-product is particularly valuable or otherwise recyclable, by-product adherence to the chamber walls and components is also desired.  
         [0003]     Attempts at modifying chamber component surfaces to achieve the proper surface characteristics to effect the desired byproduct adherence are known. However, it has been difficult creating adequate “roughness” on chamber surfaces to increase adhering coating process byproducts thereto.  
         [0004]     The substrates of many chamber components are often made from aluminum-alloys or stainless steels. The outer surfaces of these components must then be treated to effect the desired surface features and microscopic contours. Known surface “roughing” techniques include grit blasting, or chemically etching metal surfaces. In addition, coatings have been applied to metal substrate surfaces to create irregular surfaces. These known methods have worked to roughen the metal substrate surfaces. However, all known methods have been somewhat unacceptable in terms of the finite degree of adherence with respect to the amount or volume of byproduct able to be “trapped” by the modified chamber wall surface  
         [0005]     In addition, the application of coatings to metal surfaces, introduces additional problems with respect to the nearly permanent adherence of the coating to the metal that must be realized. In other words, coatings applied to chamber wall surfaces that “roughen” the metallic chamber wall surface are not useful if the coatings themselves will eventually delaminate and contribute to the contamination of the work pieces. Known coatings designed to make metallic and non-metallic (e.g. ceramic) chamber walls less smooth must be deposited such that irregularities are presented to the finished surface. However, known methods have not only risked delamination, but have been deposited in a fashion that creates potential gaps that leave the substrate surface coated discontinuously. This results in plasma byproduct being able to react with the substrate surface, or otherwise work to accelerate the delamination of the coating from the chamber wall substrate, further exacerbating the chamber contamination issues and frustrating by-product recycling issues.  
       SUMMARY OF THE INVENTION  
       [0006]     In one embodiment, the present invention is directed to a method for coating deposition chamber component substrate. A deposition chamber component substrate having a substrate surface is provided along with a twin wire arc spray coating apparatus for applying coatings. A composite coating is applied. The composite coating comprises a first twin wire arc spray coating that is applied on the substrate surface followed by a second twin wire arc spray coating applied on the first twin wire arc spray coating. Through regulating coating components, nozzle flow, and substrate composition, the coating surface displays predictable preselected characteristics, in particular a desired roughness for adhering chemical species. Most desirably, the surface roughness of the second coating is greater than the surface roughness of the first coating.  
         [0007]     In a further embodiment, the present invention is directed to a method for immobilizing tantalum-containing compounds from a tantalum deposition process. A deposition chamber is provided, with the chamber comprising chamber components and a chamber inner surface. The chamber components and chamber inner surface further comprise a plasma composite coating, with the composite coating comprising first and second layers. A tantalum-containing compound is provided to a plasma coating process. The process releases tantalum species from the tantalum-containing compound, and deposits an amount of the tantalum species on a work piece. An amount of tantalum species contacts and adheres to the composite coating. The composite coating is then treated to release the tantalum species from the composite coating. An amount of the tantalum species is then reclaimed from the composite coating.  
         [0008]     In yet another embodiment, the present invention is directed to a composite coating deposited onto a substrate surface, said coating comprising a first coating layer having a first roughness value and a second coating layer having a second roughness value greater than the first roughness value. The first and second coating layers comprise a metal selected from the group consisting of aluminum, aluminum alloy, nickel, and molybdenum.  
         [0009]     In a still further embodiment, the present invention is directed to a processing chamber comprising a chamber having an inner chamber, said chamber having an inner surface and processing chamber components, said components having an outer surface. The chamber inner surface and component outer surfaces comprise a composite coating, having a first layer having a first roughness value and a second layer having a second roughness value greater than the first roughness value. The composite layer is deposited onto the chamber inner surface and component outer surfaces substantially continuously.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is an enlarged cross-sectional photograph of metal substrates coated with known TWAS technologies (Prior Art).  
         [0011]      FIGS. 2   a - 2   d  are photographs showing TWAS-coated quartz samples. (Prior Art).  
         [0012]      FIGS. 3   a - 3   b —are photographs of quartz samples coated according to one embodiment of the processes of the present invention.  
         [0013]      FIG. 4  is a schematic representation of the twin wire arc spray assembly of one embodiment of the present invention.  
         [0014]      FIG. 5  is an enlarged cross-sectional photograph of a metal substrate coated with a TWAS system of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Thermal spraying is a known material processing technique used in various high-tech industries. Twin Wire Arc Spraying (TWAS) processes are particularly useful thermal spraying processes. In the TWAS processes two wires are fed into respective contact tips that pass electrical current into the wires. The tips are oriented toward each other so the wires extend toward an intersection. A high current is applied across the wires causing an electrical arc to form across the tips of the wires. The electric current then melts the feed wire portion in the arc zone. A nozzle device is located proximate to and between the contact tips and is oriented to emit a gas stream toward the arc zone. The gas stream sprays the molten metal onto the work surface forming a coating.  
         [0016]     TWAS processes have been used to treat deposition chamber component surfaces. However, such processes have not yielded optimum results. Specifically, TWAS coatings have provided random coating to surfaces of substrates used in deposition chambers. As shown in  FIG. 1  (prior art), a thin, non-continuous TWAS coating layer  12  was deposited on a substrate  10  used in a deposition chamber for semiconductor processing. The coated substrate was then exposed to work piece processing conditions including the release of tantalum and tantalum nitride vapor within the deposition chamber for deposition onto the work pieces, specifically semiconductor wafers. A coating of tantalum particles  14  is shown adhered to the TWAS coating layer  12 .  FIG. 1  clearly shows the non-continuous nature of the TWAS coating such that potential gaps are created  16 , potentially allowing tantalum species to attack substrate  10 .  
         [0017]      FIGS. 2   a - 2   d  show a plan overhead view of aluminum TWAS-coated quartz substrates.  FIGS. 2   a  and  2   c  show two aluminum TWAS-coated substrates  20 ,  22 , respectively, under ordinary light.  FIG. 2   b  shows the substrate  20  of  FIG. 2   a  under backlit conditions showing light penetration though the substrate, evidencing the incomplete and non-continuous coating effected by the TWAS process. Similarly,  FIG. 2   d  shows the substrate of  FIG. 2   c  under backlit conditions showing light penetration though the substrate, evidencing the incomplete and non-continuous coating effected by the TWAS process.  
         [0018]     By contrast,  FIGS. 3   a  and  3   b  show quartz substrate  30  coated by the TWAS processes of the present invention, whereby the TWAS-coated quartz substrate  30  is shown under natural light ( FIG. 3   a ) and under backlit conditions ( FIG. 3   b ). In this example, the substrate  30  shown in  FIG. 3   b  shows no light penetration, evidencing a continuous aluminum TWAS coating. According to one preferred embodiment of the present invention, the TWAS coating was deposited as a composite coating, in that, a bond coat and a top coat of aluminum were deposited.  
         [0019]      FIG. 4  shows a schematic representation of one embodiment of the TWAS system of the present invention. The depicted system shows a TWAS gun body  40  comprising wire sleeve contact tips  42 ,  44  housing consumable wire  46 ,  48  respectively. Gun nozzle  50  comprises aperture  52  through which wires  46 ,  48  extend to converge at an area  53  in the arc zone  54 . A gas component nozzle  56  provides control to release of a compressed gas such as, for example, air, nitrogen, argon, etc. The energy provided to the system heats the wires, creates an arc zone and creates molten metal particles  58  that are directed with velocity in the direction of the straight arrows toward a substrate surface  60 .  
         [0020]      FIG. 5  shows an enlarged cross-sectional photograph of the coated substrate of the present invention obtained by one embodiment of the present invention. As shown in  FIG. 5 a  thin continuous TWAS coating layer  72  was deposited on a substrate  70 . Even though the coating was deposited in a two-step process, first depositing a bond coat to the substrate, followed in quick succession by a top coat applied to the bond coat, the coating is deposited more uniformly than conventional TWAS methods, resulting in a highly desirable, continuous coating with no perceptible gaps occurring in the coating that could allow penetration of tantalum to the substrate metal surface. In strong contrast to known TWAS coating methods, the TWAS composite coating methods of the present invention produce a substantially continuous coating on the substrate surface.  
         [0021]     Aluminum is the most common material currently used for coatings on deposition chamber components, regardless of substrate. This is due at least in part to the knowledge that the aluminum metal of the chamber components, in use prior to the introduction of textured applied coatings, does not adversely affect the thin films being deposited due to secondary sputtering, ion mobility, etc. Other materials may be chosen for the coatings to allow selective stripping of the coatings without damage to the substrates. Examples of this can include many species, such as a common nickel/aluminum alloy or molybdenum coatings for aluminum substrates. The coatings mainly adhere to the substrates by mechanical bonds, such that many choices for bond coat/top coat combinations would work mechanically but must be studied for possible effects on the processes involved.  
         [0022]     One embodiment of the present invention is directed to a process by which mechanical bond strength is greatly increased while maintaining desired surface roughness through the application of at least two layers. The first layer or “bond coat” is applied using a nozzle assembly that produces high velocity molten particles that conform very well to the pre-roughened surface of the substrate and to previously deposited metal splats. “Splats” is the technical term, referring to the shape of the solidified metal after contacting the substrate and solidifying). While this coating leads to very good adhesion to the substrate, it possesses a fairly smooth surface roughness (Ra) that is not optimized for collecting the deposition process residues. In one embodiment of the present invention, the bond coat is applied to have a surface roughness (Ra μm) of from about 10 to about 20 microns, more preferably from about 10 to about 18 microns.  
         [0023]     The “top coat” is then sprayed onto the bond coat using a nozzle assembly that produces much lower velocity molten particles. These lower velocity particles do not flatten to the same extent that the high velocity particles do, and so yield a higher porosity coating as well as one with a much higher surface roughness. In one embodiment of the present invention, the surface roughness of the top coat is from about 15 to about 30 microns, more preferably from about 17 to about 23 microns. The same results may be obtained by varying the propellant gas flow rate, but for this embodiment the nozzle diameter was manipulated. According to the present invention, the roughness of the bond coat and top coats may vary, but for purposes of constructing the composite which comprises the two coats, the bond coat will have a roughness value that is less than the roughness of the top coat.  
         [0024]     As stated above, in a physical vapor deposition (PVD) coating process for semiconductors tantalum species are directed to a work piece substrate for precision coating. However, tantalum and tantalum nitride species that are not effectively deposited onto the work piece substrate surface are either vented from the chamber atmosphere or adhere to the inner surface or the chamber and exposed chamber components. The tantalum species that adhere to the chamber inner surfaces assume a dendritic crystal formation. It is this tantalum species formation or “build up” that necessitates an enhanced surface area on the inner surface of the deposition chamber. Therefore, the processes used to “roughen” the inner surface, are, in fact, increasing surface area for the purpose of increasing the volume of attachment sites for entrapping and retaining stray unreacted or unvented particle species in the deposition chamber.  
         [0025]     The known TWAS coating processes, when applied to the stainless steel or aluminum-containing inner surfaces of the processing chamber were applied in single applications. It was believed that too thick of a coating, or the application of multiple coatings, would be disadvantageous and itself lead to chamber contamination due to delamination. However, it has been discovered that the standard TWAS coatings can be microscopically discontinuous (See  FIG. 1 ) to a degree sufficient to allow deposition chamber byproduct species, such as tantalum and tantalum nitride to diffuse past the TWAS coating layer o the chamber components to the base substrate, causing unacceptable delamination.  
         [0026]     According to the present invention, a composite TWAS coating is applied to deposition chamber components that provides high roughness while maintaining coating continuity and effective diffusion barriers. The term “composite” refers to the presence of two separate layers being deposited, with each layer potentially having varying characteristics to obtain the desired overall coating characteristics.  
         [0027]     In one embodiment, the spray distance from gun tip to substrate surface was about three to about 5 inches (from about 76 to about 127 mm), and preferably four inches (about 100 mm). The turn table speed is about 150 rpm with an applied current of about 125 amps. The air pressure supplied was about 60 psi. The spray gun had a nozzle diameter set for the bond coat (first coat on the substrate surface) of about 7.85 mm, and a nozzle diameter set for the top coat (second coat applied in the process, this coat applied to the bond coat) of about 22.5 mm. The metal applied was supplied to the TWAS process as aluminum wire having a diameter of about 1.59 mm. The TWAS process applied a wire feed rate of about 69 mm/second for each of two wires supplied. The coating thicknesses applied were from about 0.10 to about 0.15 mm for the bond coat and from about 0.15 mm to about 0.20 mm for the top coat. The combined preferred coating thickness is from about 254 to about 356 microns.  
         [0028]     Indeed, according to one embodiment of the present invention, the overall thickness of the composite TWAS coatings (bond coat plus top coat) obtained was comparable to the known TWAS coating thicknesses used that fail to offer adequate protection against delamination due to by-product species diffusion. See Table 1.  
                                                         TABLE 1                                               Bond           Ra (μm)   Rz (mm)   Thickness (mm)   Strength (mPa)                                    Current TWAS   27   0.132   0.305   30       Composite   25   0.124   0.305   72       TWAS                  
 
         [0029]     As shown in Table 1, while the coating thickness of the TWAS coatings of the present invention are comparable to the conventional TWAS coatings, the bond strength of the TWAS composite coating to the chamber substrate was improved greatly. In addition, the surface roughness was comparable to conventional coatings. According to the present invention, the desired bond strengths attainable are preferably in the range of from about 40 mPa to about 77 mPa.  
         [0030]     Ra and Rz values are measured using a profilometer, a device that can measure deflections a stylus makes as it is drawn across a surface. The arithmetic average roughness (Ra) is defined as the arithmetic average height of roughness component irregularities from the mean line measured within the sample length (L). This measurement conforms to ANSI/ASME B46.1 “Surface Texture—Surface Roughness, Waviness and Lay”. Ra (formerly known as AA or Arithmetic Average in the U.S., and CLA Centerline Average in the U.K.) is usually expressed in microinches (μin), and performed by moving a stylus or profilometer in a straight line along the surface. A consistent and measurable surface finish can be specified for a desired roughness of, for example, from about 12 to about 30 microns. Rz is the sum of the height of the highest peak plus the lowest valley depth within a sampling length.  
       EXAMPLES  
       [0031]     Additional features, advantages and details of the present invention are included in the following description of exemplary embodiments of the invention, which description must be taken in conjunction with the accompanying drawings.  
       Example 1  
       [0032]     Pull testing (tensile) was performed under ASTM C 633-01 Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings. The samples were aluminum (Al), stainless steel (SST) or alumina ceramic coupons, as defined in the tables below.  
         [0033]     Final Ra: 500&gt;Ra&gt;700 microinches  
                                                                       Group #1 - Al TWAS on SST Tensile Tests            #   SAMPLE ID   LBS.   PSI   FAILURE MODE               1   1-1   8514   10846   100% Adhesive       2   1-2   8204   10451   100% Adhesive       3   1-3   8628   10991   100% Adhesive       4   1-4   8469   10789   100% Adhesive       5   1-5   8187   10429   100% Adhesive       6   1-6   8239   10496   100% Adhesive       7   1-7   8214   10464   100% Adhesive       8   1-8   8225   10478   100% Adhesive            AVERAGE FAILURE===&gt;   8335   10618                  
 
         [0034]     Final Ra: 700&gt;Ra&gt;800 microinches  
                                                                       Group #2 - TWAS 2-1-2-8 Tensile Tests            #   SAMPLE ID   LBS.   PSI   FAILURE MODE               1   2-1   8648   11017   100% Adhesive       2   2-2   8543   10883   100% Adhesive       3   2-3   8241   10498   100% Adhesive       4   2-4   8259   10521   100% Adhesive       5   2-5   8192   10436   100% Adhesive       6   2-6   8340   10624   100% Adhesive       7   2-7   8740   11134   100% Adhesive       8   2-8   8319   10597   100% Adhesive            AVERAGE FAILURE===&gt;   8410   10714                  
 
       Example 2  
       [0035]     Pull testing (tensile) was performed under ASTM C 633-01 Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings. The samples were aluminum (Al), stainless steel (SST) or alumina ceramic coupons, as defined in the tables below.  
         [0036]     Final Ra: 700&lt;Ra&lt;900 microinches  
                                                                       TWAS/SST - Tensile Tests            #   SAMPLE ID   LBS.   PSI   FAILURE MODE               1   3-1   8619   10980   100% Adhesive       2   3-2   8264   10527   100% Adhesive       3   3-3   8460   10777   100% Adhesive       4   3-4   8262   10525   100% Adhesive       5   3-5   7968   10150   100% Adhesive       6   3-6   8004   10196   100% Adhesive       7   3-7   8221   10473   100% Adhesive       8   3-8   8032   10232   100% Adhesive            AVERAGE FAILURE ===&gt;   8229   10483                  
 
         [0037]     Final Ra: 1000&lt;Ra&lt;1200 microinches  
                                                                       TWAS/SST - Tensile Tests            #   SAMPLE ID   LBS.   PSI   FAILURE MODE               1   SS-1   6555   8350   80% Coating Break -                       Middle/20% Adhesive       2   SS-2   6233   7940   70% Coating Break -                       Middle/30% Adhesive       3   SS-3   6263   7978   60% Coating Break -                       Middle/40% Adhesive       4   SS-4   6810   8675   70% Coating Break -                       Middle/30% Adhesive       5   SS-5   5994   7636   90% Coating Break -                       Middle/10% Adhesive       6   SS-6   5963   7596   75% Coating Break -                       Middle/25% Adhesive       7   SS-7   6227   7932   60% Coating Break -                       Middle/40% Adhesive       8   SS-8   5892   7506   75% Coating Break -                       Middle/25% Adhesive            AVERAGE FAILURE ===&gt;   6242   7952                  
 
       Example 3  
       [0038]     Pull testing (tensile) was performed under ASTM C 633-01 Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings. The samples were aluminum (Al), stainless steel (SST) or alumina ceramic coupons, as defined in the tables below.  
                                                                       TWAS/Al - Tensile Tests            #   SAMPLE ID   LBS.   PSI   FAILURE MODE               1   Al-1   5845   7446   70% Coating Break -                       Middle/30% Adhesive       2   Al-2   5531   7046   70% Coating Break -                       Middle/30% Adhesive       3   Al-3   6037   7690   60% Coating Break -                       Middle/40% Adhesive       4   Al-4   6793   8654   70% Coating Break -                       Middle/30% Adhesive       5   Al-5   6356   8097   70% Coating Break -                       Middle/30% Adhesive       6   Al-6   6866   8746   80% Coating Break -                       Middle/20% Adhesive       7   Al-7   6729   8572   60% Coating Break -                       Middle/40% Adhesive       8   Al-8   6327   8060   70% Coating Break -                       Middle/30% Adhesive            AVERAGE FAILURE ===&gt;   6311   8039                  
 
         [0039]     All aluminum coupons used in the testing, the results of which are set forth in Examples 1-3 were made from 6061 aluminum alloy. The stainless steel coupons used were made from 304L stainless steel.  
       Example 4  
       [0040]     Tests were conducted to determine the adhesion of aluminum TWAS to alumina ceramic. This was achieved using a bond coat applied by a plasma spray process with the substrate pre-heated to between a temperature ranging from about 700° F. to about 900° F.  
         [0041]     It is believed that the pre-heating is needed because the key sites on the blasted ceramic surface are much smaller than those created when you grit blast metal substrates. The increased substrate temperature allows the molten aluminum to cool more slowly on contact, and so allows it to flow into the smaller key sites before it solidifies.  
                                                                           Plasma/TWAS on Ceramic Tests            #   SAMPLE ID   LBS.   PSI   FAILURE MODE                    1   Specimen ID #1   5029   6406   100% Interface       2   Specimen ID #2   3971   5059   100% Substrate                       ceramic substrate                       broke       3   Specimen ID #3   6480   8255   100% Interface       4   Specimen ID #4   5804   7394   100% Interface       5   Specimen ID #5   6182   7875   100% Interface       6   Specimen ID #6   4936   6288   100% Substrate                       ceramic substrate                       broke       7   Specimen ID #7   5856   7460   100% Interface       8   Specimen ID #8   5771   7352   100% Interface       9   Specimen ID #9   5836   7434   100% Interface       10   Specimen ID #10   4867   6200   100% Interface           AVERAGE FAILURE===&gt;   5473   6972                  
 
         [0042]     The present invention facilitates enhanced recovery of species adhered to the improved substrate coating of the present invention. Since the coatings of the present invention have enhanced “roughness”, or have a greater volume of binding sites on the surface, an increased volume of species being coated, such as, for example, tantalum may adhere and grow in crystalline formations. This facilitates an increase in tantalum recovery and recycling during chamber cleaning. This recovery and recycling enhances the overall efficiency of the system compared to known processes. More specifically, with respect to tantalum, aluminum occurring on the chamber walls-between the substrate and the deposited tantalum is dissolved using a solution of potassium hydroxide. The tantalum and stainless steel are insoluble in this solution such that the substrate is not destroyed during the recovery and recycling phases of the process. The tantalum is then recovered from the chamber and reclaimed once this stripping cycle is complete. The recovered tantalum is desirably in a nearly pure form.  
         [0043]     Although the various aspects of the present invention have been described with respect to specific examples and embodiments thereof, it will be understood that the invention is entitled to protection within the full scope of the appended claims.