Abstract:
Silicon electrode assembly decontamination cleaning methods and solutions, which control or eliminate possible chemical attacks of electrode assembly bonding materials, comprise ammonium fluoride, hydrogen peroxide, acetic acid, optionally ammonium acetate, and deionized water.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 11/019,727 entitled CLEANING METHODS FOR SILICON ELECTRODE ASSEMBLY SURFACE CONTAMINATION REMOVAL, filed on Dec. 23, 2004 now U.S. Pat. No. 7,247,579, the entire content of which is hereby incorporated by reference. 
    
    
     SUMMARY 
     In one embodiment, a method of cleaning a used electrode assembly comprising a plasma-exposed silicon surface comprises contacting the silicon surface with a solution of isopropyl alcohol and deionized water. The silicon surface is contacted with an acidic solution comprising 0.01-5% ammonium fluoride, 5-30% hydrogen peroxide, 0.01-10% acetic acid, optionally 0-5% ammonium acetate, and balance deionized water. The silicon surface is contacted with deionized water. Preferably, contaminants are removed from the silicon surface. The electrode assembly can be used for etching a dielectric material in a plasma etching chamber after the cleaning. 
     In another embodiment, an acidic solution for removing contaminants from a plasma-exposed silicon surface of a used electrode assembly comprises 0.01-5% ammonium fluoride, 5-30% hydrogen peroxide, 0.01-10% acetic acid, optionally 0-5% ammonium acetate, and balance deionized water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURE 
         FIG. 1A  shows a fixture for supporting an electrode assembly during cleaning and  FIG. 1B  shows an enlarged area of  FIG. 1A . 
         FIG. 2A  shows silicon surface morphology of a new electrode assembly,  FIGS. 2B-D  show silicon surface morphology of a used electrode assembly before polishing, and  FIGS. 2E-G  show silicon surface morphology of a used electrode assembly after polishing. 
         FIGS. 3 and 4  show exemplary used electrode assemblies that have not been cleaned. 
         FIG. 5  shows an exemplary recovered electrode assembly. 
         FIG. 6A  shows discoloration of the silicon surface of an inner electrode assembly that can result from wiping with an acidic solution and  FIG. 6B  shows discoloration of the silicon surface of an outer electrode assembly member that can result from wiping with an acidic solution. 
         FIGS. 7A-D  shows exemplary electrode assemblies before and after recovery. 
     
    
    
     DETAILED DESCRIPTION 
     Used silicon electrode assemblies exhibit etch rate drop and etch uniformity drift after a large number of RF hours (time in hours during which radio frequency power is used to generate the plasma) are run using the electrode assemblies. The decline of etch performance results from changes in the morphology of the silicon surface of the electrode assemblies as well as contamination of the silicon surface of the electrode assemblies, both of which are a product of the dielectric etch process. 
     Silicon surfaces of used electrode assemblies can be polished to remove black silicon and other metal contamination therefrom. Metallic contaminants can be efficiently removed from silicon surfaces of such electrode assemblies without discoloring the silicon surfaces by wiping with an acidic solution, which reduces the risk of damage to electrode assembly bonding materials. Accordingly, process window etch rate and etch uniformity can be restored to acceptable levels by cleaning the electrode assemblies. 
     Dielectric etch systems (e.g., Lam 2300 Exelan® and Lam Exelan® HPT) may contain silicon showerhead electrode assemblies containing gas outlets. As disclosed in commonly owned U.S. Pat. No. 6,376,385, which is incorporated herein by reference, an electrode assembly for a plasma reaction chamber wherein processing of a semiconductor substrate such as a single wafer can be carried out may include a support member such as a graphite backing ring or member, an electrode such as a silicon showerhead electrode in the form of a circular disk of uniform thickness and an elastomeric joint between the support member and the electrode. The elastomeric joint allows movement between the support member and the electrode to compensate for thermal expansion as a result of temperature cycling of the electrode assembly. The elastomeric joint can include an electrically and/or thermally conductive filler and the elastomer can be a catalyst-cured polymer which is stable at high temperatures. For example, the elastomer bonding material may comprise silicon polymer and aluminum alloy powder filler. In order to avoid contacting the acidic solution with the bonding material of the electrode assembly, which may damage the bonding material, the silicon surface of the used electrode assembly is preferably wiped with the acidic solution. 
     Additionally, an electrode assembly may comprise an outer electrode ring or member surrounding an inner electrode and optionally separated therefrom by a ring of dielectric material. The outer electrode member is useful for extending the electrode to process larger wafers, such as 300 mm wafers. The silicon surface of the outer electrode member may comprise a flat surface and a beveled outer edge. Similar to the inner electrode, the outer electrode member is preferably provided with a backing member, e.g., the outer ring may comprise an electrically grounded ring to which the outer electrode member may be elastomer bonded. The backing member of the inner electrode and/or outer electrode member may have mounting holes for mounting in a capacitively coupled plasma processing tool. Both the inner electrode and outer electrode member are preferably comprised of single crystalline silicon, in order to minimize electrode assembly contaminants. The outer electrode member may be comprised of a number of segments (e.g., six segments) of single crystalline silicon, arranged in an annular configuration, each of the segments being bonded (e.g., elastomer bonded) to a backing member. Further, adjacent segments in the annular configuration may be overlapping, with gaps or joints between the adjacent segments. 
     Silicon electrode assemblies used in dielectric etch tools deteriorate after a large number of RF hours are run using the electrode assemblies, in part due to the formation of black silicon. “Black silicon” can form on a plasma-exposed silicon surface as a result of the surface being micro-masked by contaminants deposited on the surface during plasma processing operations. Specific plasma processing conditions affected by the formation of black silicon include high nitrogen and low oxygen and C x F y  concentrations at moderate RF power, as used during etching of low K vias. The micro-masked surface regions can be on the scale of from about 10 nm to about 10 microns. While not wishing to be bound to any particular theory, black silicon formation on the plasma-exposed surface of a silicon electrode (or other silicon part) is believed to occur as a result of non-contiguous polymer deposition on the silicon electrode during plasma processing operations. 
     A non-contiguous polymer deposit can form on the plasma-exposed surface, e.g., the bottom surface of a silicon upper electrode, during a main etching step for etching a dielectric material on a semiconductor substrate, such as silicon oxide or a low-k dielectric material layer. The polymer deposits typically form three-dimensional, island-like formations that selectively protect the underlying surface from etching. Once needle-like formations are formed, polymer deposits then form preferentially on the needle tips, thereby accelerating the micro-masking mechanism and black silicon propagation during the main etching step for successive substrates. The non-uniform, anisotropic etching of the micro-masked surface region(s) results in the formation of closely-spaced, needle-like or rod-like features on the surface. These features can prevent light from reflecting from the modified regions of the silicon surface, which causes those regions to have a black appearance. The needle-like micro features are closely spaced and can typically have a length of from about 10 nm (0.01 μm) to about 50,000 nm (50 μm) (and in some instances can have a length as high as about 1 mm or even greater), and can typically have a width of from about 10 nm to about 50 μm. 
     Silicon surfaces of electrode assemblies affected by black silicon may be recovered by polishing. Prior to polishing, the electrode assembly may be pre-cleaned to remove foreign materials. Such pre-cleaning may include CO 2  snow blasting, which involves directing a stream of small flakes of dry ice (e.g., generated by expanding liquid CO 2  to atmospheric pressure through a nozzle, thereby forming soft flakes of CO 2 ) at the surface being treated, so that the flakes hit small particulate contaminants less than one micron in size on the substrate, then vaporize via sublimation, lifting the contaminants from the surface. The contaminants and the CO 2  gas then typically are passed through a filter, such as a high efficiency particulate air (HEPA) filter, where the contaminants are collected and the gas is released. An example of a suitable snow-generating apparatus is Snow Gun-II™, commercially available from Vatran Systems, Inc. (Chula Vista, Calif.). Prior to polishing, the electrode assembly may be cleaned with acetone and/or isopropyl alcohol. For example, the electrode assembly may be immersed in acetone for 30 minutes and wiped to remove organic stains or deposits. 
     Polishing comprises grinding a surface of the electrode assembly on a lathe using a grinding wheel with appropriate roughness grade number and polishing the electrode assembly surface to a desired finish (e.g., 8μ-inches) using another wheel. Preferably, the silicon surface is polished under constant running water, in order to remove dirt and keep the electrode assembly wet. When water is added, a slurry may be generated during the polishing, which is to be cleaned from the electrode assembly surface. The electrode assembly may be polished first using an ErgoSCRUB™ and ScrubDISK. The polishing procedure (i.e., the selection and sequence of the polishing paper used), depends on the degree of damage of the silicon surface of the electrode assembly. 
     If severe pitting or damage is observed on the silicon electrode assembly, polishing can begin with, for example, a 140 or 160 grit diamond polishing disk until a uniform flat surface is achieved. Subsequent polishing can be with, for example, 220, 280, 360, 800, and/or 1350 grit diamond polishing disks. If minor pitting or damage is observed on the silicon electrode assembly, polishing can begin with, for example, a 280 grit diamond polishing disk until a uniform flat surface is achieved. Subsequent polishing can be with, for example, 360, 800, and/or 1350 grit diamond polishing disks. 
     During polishing, the electrode assembly is attached to a turntable, with a rotation speed of preferably about 40-160 rpm. A uniform, but not strong, force is preferably applied during polishing, as a strong force may cause damage to the silicon surface or bonding area of the electrode assembly. Accordingly, the polishing process may take a significant amount of time, depending on the degree of pitting or damage on the electrode assembly. The shape and angle of an outer electrode ring or member (e.g., the interface between the flat surface and the beveled outer edge) is preferably maintained during polishing. In order to minimize particles trapped inside gas outlets and within joints of electrode assemblies, a deionized water gun may be used to remove particles generated during polishing from the gas outlets and joints whenever changing polishing disks and UltraSOLV® ScrubPADs may be used to remove particles from the polishing disks. 
     Following polishing, the electrode assembly is preferably rinsed with deionized water and blown dry. The surface roughness of the electrode assembly may be measured using, for example, a Surfscan system. The surface roughness of the electrode assembly is preferably approximately 8μ-inches or less. 
     The electrode assembly is preferably immersed in deionized water at 80° C. for 1 hour in order to loosen particles that may be trapped in gas outlets and joints in the electrode assembly. The electrode assembly may be ultrasonically cleaned for 30 minutes in deionized water at about 60° C., to remove particles from the surface of the electrode assembly. The electrode assembly may be moved up and down within the ultrasonic bath during the ultrasonic cleaning in order to help remove trapped particles. 
     The electrode assembly, including gas outlets and joints or mounting holes of the electrode assembly, may be cleaned using a nitrogen/deionized water gun at a pressure of less than or equal to 50 psi. Special handling may be needed to avoid damaging or impacting a graphite backing member of the electrode assembly, as the graphite surface of a used electrode assembly might have a loose surface structure. Cleanroom paper, nylon wire, or white thread may be used to check particle removal quality, for example, from gas outlets and joints of the electrode assembly. The electrode assembly may be dried using a nitrogen gun at a pressure less than or equal to 50 psi. 
     Following polishing, the electrode assembly may be cleaned with a solution of deionized water and isopropyl alcohol, preferably ultrasonic, to remove soluble metal contaminants, such as, for example, sodium salts, potassium salts, and combinations thereof, as well as polymer deposition from electrode assemblies. A weakly acidic or near neutral solution, described in detail below, removes insoluble metal salts, such as, for example, calcium silicate, copper oxide, zinc oxide, titania, and combinations thereof. The acidic solution is removed from the electrode assembly using deionized water, ultrasonic preferred. Finally, the electrode assembly is preferably blown dry using filtered nitrogen gas and oven baked prior to final inspection and packaging. 
     The weakly acidic or near neutral solution for the removal of silicon surface metal contaminants may comprise:
     0.01-5% NH 4 F+5-30% H 2 O 2 +0.01-10% HAc+0-5% NH 4 Ac+balance UPW.
 
In another embodiment, the weakly acidic or near neutral solution may comprise:
   0.01-2% NH 4 F+10-20% H 2 O 2 +0.01-5% HAc+0-5% NH 4 Ac+balance UPW.
 
Additives, such as chelating agents, ethylenediaminetetraacetic acid (EDTA), and surfactants, can also be added to the cleaning solution to enhance the efficiency and chemical reaction rate.
   

     Hydrolysis of ammonium fluoride (NH 4 F) in the acidic solution generates hydrofluoric acid and ammonium hydroxide. Hydrofluoric acid helps etch the silicon surface. However, excess hydrofluoric acid is undesirable in cleaning elastomer bonded silicon electrode assemblies, as hydrofluoric acid may cause decomposition of silicon polymer. Ammonia, provided through solution balance with ammonium ions, is an excellent complexing agent that forms stable complex metal ions with many transition metals, such as, for example, copper and iron. Thus, the presence of ammonium helps improve metal removal efficiency. 
     Hydrogen peroxide (H 2 O 2 ) is a strong oxidizer, which helps to remove not only organic contaminants, but also metal contaminants. As an oxidant, hydrogen peroxide can oxidize transition metals to higher chemical states to form soluble complexes with ammonia, as described above. Further, hydrogen peroxide can form chelating complexes with many metal ions to improve cleaning efficiency. Acetic acid (HAc) and ammonium acetate (NH 4 Ac) serve as buffer solutions to maintain the pH of the solution as weakly acidic or near neutral. The ultra-pure deionized water (UPW) preferably has a resistivity of greater than 10e18 ohm/cm. 
     To further reduce the risk that the bonding material of the electrode assembly is chemically attacked by the acidic solution, metal contaminants are removed by contacting the silicon surface of the electrode assembly with the acidic solution, preferably by wiping, as opposed to soaking the electrode assembly in the acidic solution. Accidental contact of the acidic solution with the backing member or bonding area is thus avoided by contacting only the silicon surface of the electrode assembly with the acidic solution and by means of a fixture that allows the silicon surface of the electrode assembly to be supported facing downward while the silicon surface is cleaned. With the silicon surface of the electrode assembly supported facing downward, excess acidic solution applied to the silicon surface can be collected after dripping off of the silicon surface, as opposed to flowing to the backing member or bonding area. The backing member and bonding area are preferably immediately cleaned with deionized water if contacted with the acidic solution. Additionally, exposed electrode assembly bonding material is preferably protected by covering with masking material and/or chemical resistant tape prior to cleaning with the acidic solution. 
     Additional measures to avoid accidental contact of the acidic solution with the backing member or bonding area include drying the electrode assembly after wiping using compressed nitrogen gas, blown from the backing member down to the silicon surface, and blowing any residual solution from the silicon surface. After wiping, the solution is removed from the electrode assembly by rinsing the electrode assembly with deionized water. Similarly, potential attack of the bonding material by residual acidic solution during rinsing with deionized water may be further reduced by rinsing the backing member with deionized water followed by rinsing the silicon surface with deionized water. With the electrode assembly supported in a fixture with the silicon surface facing downward, the electrode assembly will be rinsed from the backing member down to the silicon surface, and through gas holes, if present. 
     The fixture, sized to the electrode assembly to be cleaned, has a sturdy base and three or more supporting members that raise the electrode assembly above the working bench surface, allowing the surface of the electrode assembly facing downward to be cleaned. As illustrated in  FIG. 1A , showing a fixture for supporting an electrode assembly during cleaning, and  FIG. 1B , showing an enlarged area of  FIG. 1A , the top of each supporting member preferably has a step on which the electrode assembly rests and which prevents the electrode assembly from slipping off the supporting members. The supporting members, and base, are preferably coated with and/or made from a chemically resistant material, such as Teflon® (polytetrafluoroethylene), which is chemically resistant to acids. 
     The electrode assembly is preferably inspected prior to recovery and after recovery to ensure that the recovered electrode assembly conforms to product specifications. Inspection may include measuring, for example, dimensions (e.g., thickness), surface roughness (Ra, e.g., 16μ-inches or less, preferably 8μ-inches or less), surface cleanliness (Inductively Coupled Plasma Mass Spectrometry analysis), surface particle count as measured by, for example, a QIII®+Surface Particle Detector (Pentagon Technologies, Livermore, Calif.), surface morphology (e.g., by scanning electron microscopy (SEM)), and measurement of black silicon pits and etch depths. Further, plasma etch chamber performance of the recovered electrode assemblies are preferably tested to ensure that the recovered electrode assembly exhibits acceptable etch rate and etch uniformity. 
       FIG. 2A  (Ra=16μ-inches) shows silicon surface morphology of a new electrode assembly,  FIGS. 2B-D  (Ra=240, 170, and 290μ-inches, respectively) show silicon surface morphology of a used electrode assembly before polishing, and  FIGS. 2E-G  (Ra=9, 9, and 10μ-inches, respectively) show silicon surface morphology of a used electrode assembly after polishing.  FIGS. 2A-G  show SEM images of a silicon surface at a magnification of 100 times. The electrode assembly of  FIG. 2  has an inner electrode and an outer electrode member, as discussed above.  FIGS. 2B and 2E  are images taken from the center of the inner electrode,  FIGS. 2C and 2F  are images taken from the edge of the inner electrode, and  FIGS. 2D and 2G  are images taken from the outer electrode member.  FIG. 2  shows that polishing recovers the silicon surface morphology and roughness of a used electrode assembly to the state of a new electrode assembly. 
       FIGS. 3 and 4  show exemplary used electrode assemblies that have not been cleaned and  FIG. 5  shows an exemplary recovered electrode assembly.  FIG. 6A  shows discoloration of the silicon surface of an inner electrode assembly that can result from wiping with an acidic solution and  FIG. 6B  shows discoloration of the silicon surface of an outer electrode assembly member that can result from wiping with an acidic solution.  FIGS. 7A  (Ra&gt;150μ-inches) and  7 B (Ra&gt;300μ-inches) shows exemplary used electrode assemblies before recovery, while  FIGS. 7C and 7D  (both having Ra&lt;8μ-inches) show exemplary electrode assemblies after recovery.  FIGS. 7A and 7C  show outer electrode members, while  FIGS. 7B and 7D  show inner electrodes. 
     EXAMPLE 
     The following method for cleaning silicon electrode assembly surfaces is provided to be illustrative, but not limiting. 
     Soak (immerse) the electrode assembly in an ultrasonic tank filled with a 50/50 solution of deionized water and isopropyl alcohol. Ultrasonically clean the electrode assembly for 30 minutes at room temperature. If necessary, lightly scrub the silicon surfaces of the electrode assembly with a lint-free wipe to remove any residue. Remove the electrode assembly from the solution of deionized water and isopropyl alcohol. Rinse the electrode assembly for at least five minutes using ultra-pure deionized water. Flow water through the gas holes from both sides, beginning with the backing side and followed by the silicon side. If necessary, repeat the above to remove any remaining visible residue. 
     Place the electrode assembly on a fixture with the silicon surface facing downward. Air dry the electrode assembly using filtered nitrogen gas. Wet a polyester cleanroom wipe with a solution of 0.01-2% NH 4 F, 10-20% H 2 O 2 , 0.01-5% HAc, optionally 0-5% NH 4 A c , and balance UPW and wipe the electrode assembly silicon surface. Replace the wetted polyester cleanroom wipe as necessary, until there is no visible residue on the wipe. If there is any visible residue on the wipe, repeat the wiping until there is no visible residue on the wipe. Use a clean, dry cleanroom wipe to gently wipe off residual solution from the silicon surface. 
     Rinse the electrode assembly, including the gas holes, with deionized water for at least five minutes. Soak (immerse) the electrode assembly in ultra-pure deionized water and ultrasonic clean in ultra-pure deionized water for 30 minutes. Rinse the electrode assembly, including the gas holes, with deionized water for at least five minutes. Air dry the electrode assembly, including gas holes, using filtered nitrogen gas. Place the electrode assembly in an unheated oven and heat the oven a rate less than 10° C./minute to 120° C. Heat the electrode assembly at 120° C. for two hours. Turn off the oven and allow it to cool. After the oven has cooled to below 60° C., place the electrode assembly in a clean, dry area to cool to room temperature. 
     Inspect the electrode assembly for any surface residue, water marks, gas holes blockage, and/or bonding material damage. Clean the electrode assembly again if any surface residue, water marks, and/or gas holes blockage is found. Particles may be removed from surfaces and/or gas holes using filtered nitrogen. 
     While various embodiments have been described, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto.