Patent Publication Number: US-2016237570-A1

Title: Gas delivery apparatus for process equipment

Description:
TECHNICAL FIELD 
     The present disclosure is in the field of plasma processing equipment. More specifically, embodiments that reduce contamination from plasma generators that operate at relatively high pressures are disclosed. 
     BACKGROUND 
     In plasma processing, plasmas create ionized and/or energetically excited species for interaction with workpieces that may be, for example, semiconductor wafers. To create and/or maintain a plasma, one or more gases are introduced into a space within a plasma generator, and one or more radio frequency (RF) and/or microwave generators generate electric and/or magnetic fields to ignite a plasma from the gases to create the ionized and/or energetically excited species. The ionized and/or energetically excited species, along with unreacted gases from which they are generated, are collectively referred to herein as “plasma products.” In some wafer processing systems, a plasma is generated in the same location as one or more wafers being processed; in other cases, a plasma is generated in one location and moves to another location where the wafer(s) are processed. Plasma products often include highly energetic and/or corrosive species and/or highly energetic electrons, such that the equipment that produces them sometimes degrades from contact with the energetic species and/or electrons. Plasmas can be generated at a variety of pressures, with typical pressures for generation and/or use of plasma products ranging from milliTorr to thousands of Torr. The effects of plasma products on the items being processed, and the processing equipment, can vary according to the pressure utilized. 
     SUMMARY 
     In an embodiment, a method of preparing an aluminum tube for use as a gas line includes plating a nickel alloy throughout internal surfaces of the aluminum tube, to form the gas line. 
     In an embodiment, a gas line for transport of gases includes an aluminum tube with a nickel alloy coating throughout internal surfaces of the tube. 
     In an embodiment, a plasma processing apparatus includes two process chambers for exposing a workpiece to a plasma, and a gas line that supplies, from one or more inlet ports, one or more gases for generating the plasma to two outlet ports. Each of the two outlet ports interfaces to a respective one of the process chambers, and the gas line includes an aluminum tube with a nickel alloy coated internal surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be understood by reference to the following detailed description taken in conjunction with the drawings briefly described below, wherein like reference numerals are used throughout the several drawings to refer to similar components. It is noted that, for purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale. In instances where multiple instances of an item are shown, only some of the instances may be labeled, for clarity of illustration. 
         FIG. 1  schematically illustrates major elements of a plasma processing system, according to an embodiment. 
         FIG. 2A  schematically illustrates major elements of a plasma processing system, in a cross-sectional view, according to an embodiment. 
         FIG. 2B  shows a perspective view of an exemplary gas line  215  that connects one inlet gas source to two plasma sources, according to an embodiment. 
         FIGS. 3A and 3B  show scanning electron microscope (SEM) photos and elemental analyses of representative particles from SST lines. 
         FIG. 4  is a flowchart of a process for manufacturing and testing a Ni alloy plated Al gas line, according to an embodiment. 
         FIG. 5A  schematically shows, in plan view, an exemplary wet chemical apparatus for cleaning and plating internal surfaces of gas lines, according to an embodiment. 
         FIG. 5B  shows a schematic cross section of apparatus of  FIG. 5A . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates major elements of a plasma processing system  100 , according to an embodiment. System  100  is depicted as a single wafer, semiconductor wafer plasma processing system, but it will be apparent to one skilled in the art that the techniques and principles herein are applicable to processing systems for any type of workpiece (e.g., items that are not necessarily wafers or semiconductors). Processing system  100  includes a housing  110  for a wafer interface  115 , a user interface  120 , a plasma processing unit  130 , a controller  140  and one or more power supplies  150 . Processing system  100  is supported by various utilities that may include gas(es)  155 , external power  170 , vacuum  160  and optionally others. Internal plumbing and electrical connections within processing system  100  are not shown, for clarity of illustration. 
     Processing system  100  is shown as a so-called indirect plasma processing system that generates a plasma in a first location and directs the plasma and/or plasma products (e.g., ions, molecular fragments, energized species and the like) to a second location where processing occurs. Thus, in  FIG. 1 , plasma processing unit  130  includes a plasma source  132  that supplies plasma and/or plasma products for a process chamber  134 . Process chamber  134  includes one or more wafer pedestals  135 , upon which wafer interface  115  places a workpiece  50  (e.g., a semiconductor wafer, but could be a different type of workpiece) for processing. In operation, gas(es)  155  are introduced into plasma source  132  and a radio frequency generator (RF Gen)  165  supplies power to ignite a plasma within plasma source  132 . Plasma and/or plasma products pass from plasma source  132  through a diffuser plate  137  to process chamber  134 , where workpiece  50  is processed. 
     An indirect plasma processing system for semiconductor wafer processing is illustrated in  FIG. 1  and elsewhere in this disclosure. However, it should be clear to one skilled in the art that the techniques, apparatus and methods disclosed herein are equally applicable to direct plasma processing systems (e.g., where a plasma is ignited at the location of the workpiece(s)) and/or to systems that process workpieces other than semiconductor wafers. 
       FIG. 2A  schematically illustrates major elements of a plasma processing system  200 , in a cross-sectional view, according to an embodiment. Plasma processing system  200  is an example of plasma processing unit  130 ,  FIG. 1 . Plasma processing system  200  includes a process chamber  205  and a plasma source  210 . Plasma source  210  introduces one or more source gases (e.g., gases  155 ,  FIG. 1 ) through an inlet gas line  215  and an internal passage  218  that passes through a chamber lid  232 , an insulator  230  and an RF electrode  225 . As shown in  FIG. 2A , internal passage  218  connects with a nozzle  220  formed in RF electrode  225 . Insulator  230  electrically insulates RF electrode  225  from chamber lid  232 , which may be held at electrical ground (or the polarity of ground vs. powered electrode may be reversed). Inlet gas line  215  slopes downwardly as it approaches plasma source  210 , to reduce the possibility of electrical arcing between inlet gas line  215  and RF electrode  225  by keeping gas line  215  and RF electrode  225  as far as possible from one another. Plasma and/or plasma products pass through apertures  237  formed in a diffuser  235 , toward process chamber  205 . 
     Plasma processing system  200  is shown as a single plasma generator and processing chamber in the cross-sectional plane of  FIG. 2A , but certain features shown, particularly inlet gas line  215 , may be shared with other instances of plasma generators and processing chambers in other cross-sectional planes. 
       FIG. 2B  shows a perspective view of an exemplary gas line  215  that connects one or more source gases from a shared gas inlet to two plasma sources (e.g., plasma sources  210 ,  FIG. 2A ). Accordingly, gas line  215  includes one inlet fixture  240  and two outlet fixtures  250 , as shown. 
     In an embodiment, plasma processing system  200  generates plasma products that are suitable for etching dielectric materials used in semiconductor fabrication. Typical source gases that would be introduced into plasma processing system  200  through inlet gas line  215  include, for example, SF 6 , NF 3 , NH 3 , H 2 , He and Ar. Typical plasmas formed in plasma processing system  200  operate within a range of 1 to 30 Torr, and especially within a range of 10 to 12 Torr. 
     Inlet gas line  215  is advantageously formed of aluminum that is coated with a suitable (e.g., durable and pinhole-free) nickel alloy layer inside and/or outside, in embodiments. It is understood that when nickel (Ni) is referred to herein, either nickel or any nickel containing alloy is meant. Although stainless steel (“SST”) is typically utilized for gas lines of at least some process gases in plasma processing equipment, and is sometimes nickel plated for chemical resistance, SST remains vulnerable to attack by free fluorine. It is believed that Ni alloy plating does not adhere well to SST, and may form pinholes, voids and/or other forms of incomplete coverage that allow local attack of the SST by the free fluorine. Free fluorine may be generated in locations such as nozzle  220  and an adjacent region just above diffuser  235 , and can back diffuse through internal passage  218  to gas line  215 . Back diffusion of fluorine to gas line  215  may especially occur in plasma equipment that operates at a relatively high operating pressure (e.g., greater than about 5 Torr, and especially 10 to 12 Torr in plasma source  210 ). Back diffusion may also occur or increase if gas line  215  serves multiple process chambers. That is, when certain events occur within plasma source  210  and/or downstream components such as chamber  205 , momentary surges of gases and/or plasma products may occur as pressure within gas line  215  balances with respect to a second (and/or third, etc.) plasma generator connected to gas line  215 . Events that may cause such surges include but are not limited to plasma ignition, starting or stopping of gas flows, opening and closing of vacuum gates or doors between chambers, and the like. 
     When SST is used for gas line  215 , attack of the SST by free fluorine can lead to gas line  215  shedding particles that may contain, among other elements, Fe and Cr. Such particles are undesirable in semiconductor processing because they can generate defects (e.g., they can short circuit adjacent conductors, or alter patterns printed on various semiconductor layers) and from an atomic contamination standpoint (e.g., Fe and Cr can incorporate into semiconductor materials and affect electronic properties of the materials).  FIGS. 3A and 3B  show scanning electron microscope (SEM) photos and elemental analyses of representative particles from SST lines. Of interest are the breakdowns of elements by weight % and atomic % available in the elemental analyses. These particular analyses indicate significant amounts of Cr and Fe in the analyzed particles. 
     When gas line  215  is formed of suitably processed Ni alloy coated Al instead, particle generation is suppressed. Aluminum is advantageous in that its satisfactory use in plasma wafer processing systems is well established. For example, any of RF electrode  225 , chamber lid  232 , and/or diffuser  235 ,  FIG. 2A , may also be formed of Al. In embodiments, the base Al is of the well known “6061” alloy type, having the following elemental composition: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Element 
                 Minimum percentage 
                 Maximum percentage 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Al 
                 95.85 
                 98.56 
               
               
                   
                 Si 
                 0.4 
                 0.8 
               
               
                   
                 Fe 
                 0 
                 0.7 
               
               
                   
                 Cu 
                 0.15 
                 0.40 
               
               
                   
                 Mn 
                 0 
                 0.15 
               
               
                   
                 Mg 
                 0.8 
                 1.2 
               
               
                   
                 Cr 
                 0.04 
                 0.35 
               
               
                   
                 Zn 
                 0 
                 0.25 
               
               
                   
                 Ti 
                 0 
                 0.15 
               
               
                   
                 Others 
                 0 
                 0.05 each, 0.15 total 
               
               
                   
                   
               
            
           
         
       
     
     Advantageously, to increase corrosion resistance of Al, the Ni alloy plating forms a thickness in the range of 0.0008 to 0.0015 inches, especially the range of 0.0010 to 0.0012 inches. Ni alloy plating also advantageously includes a phosphorous content in the range of 8% to 15%, especially the range of 10% to 12%, according to the test methods described in ASTM Practice E 60 or Test Methods E 352. 
     Embodiments that make and use gas line  215  formed of Ni alloy plated Al are now disclosed. 
     Using electroplating to generate a suitable Ni alloy coating on the interior of an Al tube or gas line can be problematic because ions in an electroplating solution are guided by electric fields therein, and such fields will not extend to internal surfaces deep within a tube. Embodiments herein utilize electroless Ni alloy plating and a heat treatment to generate a Ni alloy coated tube that has been found in tests to be suitable for use in equipment that may expose the tube to free fluorine. The methods now described are advantageously capable of producing gas lines that are internally Ni alloy coated or plated throughout; that is, all of the internal surfaces of such gas lines are Ni plated, not just parts of the surfaces. Coating internal surfaces throughout a gas line provides the significant advantage that no parts of the internal surfaces are unprotected from the highly corrosive environment that they may be subjected to. 
       FIG. 4  is a flowchart of a process  300  for manufacturing and testing a Ni alloy plated Al gas line, such as gas line  215 ,  FIG. 2A . It will be evident to those skilled in the art that individual subprocesses or all subprocesses of process  300  may be performed on individual Al components and/or fabricated gas lines, or multiples of such components and/or gas lines in batch processes. Subprocesses of process  300  need not be performed by a single entity or at a single location; components and/or fabricated gas lines may be sent from one location to another, or to different business entities, to perform various ones of the subprocesses. It will also be evident to those skilled in the art that certain subprocesses may be omitted, or their order rearranged, within process  300 . 
     As process  300  begins, Al components that will be joined to form the gas line are chemically cleaned,  310 , which may be considered optional if the Al components are believed to be clean enough as-fabricated, and in view of subsequent cleaning. Cleaning may include use of surfactants and/or chemicals and may optionally be followed by rinsing and/or drying. The Al components are coupled,  320 , to form the gas line. Coupling is typically done by welding, but other forms of coupling are possible; it may be advantageous to utilize coupling methods that result in inner surfaces that are clean and free of residue with minimal crevices, steps or discontinuities. Also, advantageously, all machining and coupling operations are performed before Ni plating, so that all machining induced scratches and the like are covered by the Ni plating. The gas line is chemically cleaned,  330 , again optionally followed by rinsing and/or drying. Chemical cleaning of the gas line may include, for example, cleaning exterior and/or interior surfaces of the gas line with dilute HF and/or HNO 3 , again optionally followed by rinsing and/or drying. 
     Internal surfaces of the gas line are plated with electroless Ni alloy,  340 . In preparation for the internal surface Ni alloy plating, external surfaces that may have a critical flatness or other dimensional requirement may be masked, to avoid incidental electroless Ni buildup on such surfaces. Advantageously, to promote uniform Ni alloy plating on the internal surfaces of the gas line(s), electroless Ni alloy plating solution is pumped through the fabricated gas line. Cleaning  330  and plating  340  may be done on individual fabricated gas lines, or fixtures may be utilized to circulate cleaning or Ni alloy plating solutions through several fabricated gas lines at once, in serial or parallel arrangements (see, e.g.,  FIGS. 5A, 5B ). Electroless Ni plating may use nickel sulfate, NiSO 4  (or its hydrated form, NiSO 4 (H 2 O 6 )) as a Ni source, and sodium hypophosphite, NaPO 2 H 2  as a reducing agent. Other possible Ni sources include nickel chloride, NiCl 2 , and nickel acetate, Ni(CH 3 CO 2 ) 2  or their hydrated forms. Other possible reducing agents are sodium borohydride, NaBH 4 , hydrazine, N 2 H 4 , and dimethylamine borane, (CH 3 ) 2 )NH.BH 3 . 
     External surfaces of the gas line are plated with Ni alloy,  350 . In embodiments, the external surface Ni plating  350  also performs electroless Ni plating, like  340 , but in other embodiments Ni plating  350  is electrolytic Ni plating, since outer surfaces of the gas line would be accessible to ions guided by electric fields in an electrolytic plating bath. Plating  340  and  350  may be performed in either order, optionally with rinses and/or drying in between or following the last of the plating. During the outer surface Ni alloy plating  350 , ends of the gas line are optionally plugged. 
     Optionally but advantageously, the gas line is heat treated,  360 , to promote grain growth of the electroless Ni alloy plating, to harden the Ni plating and improve its adhesion to Al. For example, in embodiments the gas line is heat treated at 120 C to 130 C for at least one hour; in other embodiments the gas line is heat treated at 140 C to 150 C for at least one hour. The gas line goes through a final clean,  370 , to remove chemicals and contamination from plating  340 ,  350 . Optionally, the gas line (and/or a coupon processed in parallel with the gas line) is tested,  380 . Testing may include for example running an acidic solution through the gas line and/or swabbing inner or outer surfaces of the gas line to obtain a sample of material that remains on the surface(s) and/or is loosened or chemically removed by the acidic solution. Testing may also include visual inspection, plating thickness testing of cross-sectioned coupons as per ASTM B 487, plating thickness testing of gas lines and/or coupons before and after plating using a micrometer, plating thickness testing using Beta backscatter analysis as per ASTM B 567, plating thickness testing using X-ray spectrometry as per ASTM B 568, surface finish testing as per ANSI/ASME B46.1, adhesion testing as per ASTM B 571, porosity testing as per section C2 of Annex C of ISO 4527, phosphorous content testing as per ASTM Practice E 60 or Test Method E 352, corrosion resistance testing as per ASTM G 31, long term HCl exposure testing, microhardness testing as per ASTM B 578, outgassing testing as per ASTM E 1559, ionic contamination testing as per US EPA methods 300.0, 300.7, black light inspection and/or metallography inspection as per ASTM E 3. 
     Process  300  can, in embodiments, be performed on multiple gas lines in parallel to improve manufacturing volumes and consistency of the gas lines so produced. For example, fixtures may be built to flow a chemical or chemical mixture through one or more gas lines, in serial or parallel combinations. Certain gas lines that include branches (e.g., that form T-shaped or Y-shaped, or more complex topographies) may be connected to a chemical source in one branch, such that a chemical stream that is introduced splits internally and drains from the gas line through two or more branches. The chemical or chemical mixture may be an electroless nickel alloy plating solution, a cleaning solution, a rinsing solution, and/or combinations or sequences of such solutions. The chemical or chemical mixture may flow through the gas lines from a source reservoir to a waste reservoir, or may be recycled by being pumped from a single reservoir through the gas lines back to the single reservoir. In embodiments, the chemical or chemical mixture is strained and/or filtered to promote adhesion, cleanliness and uniformity of the plating. Also, Al coupons can be processed at the same time as gas lines, and can be analyzed for thickness of the electroless Ni plating, concentrations of Ni, P and contaminants, hardness of the plating, and the like. The fixtures used for plating of gas lines can have features attached for coupon processing. 
       FIG. 5A  schematically shows, in plan view, an exemplary wet chemical apparatus  400  for cleaning and plating internal surfaces of gas lines  215 .  FIG. 5B  shows a schematic cross section of apparatus  400 . For clarity of illustration,  FIG. 5B  shows certain features of apparatus  400  as if cross-sectioned at line  5 B- 5 B′ in  FIG. 5A , while the remaining features are shown as would be seen in an elevational view with wall  411  of tank  410  removed. It will be appreciated by one skilled in the art upon reading and understanding the disclosure below, that the features of apparatus  400  are exemplary only and may be modified in many ways for cleaning and plating internal surfaces of differing numbers and/or types of devices that include or are formed of tubes, such as gas line  215 . 
     Apparatus  400  is configured to pump one or more chemicals, chemical mixtures and/or rinsing solutions (any of which may be called “a chemical” herein) through gas lines  215  to provide electroless Ni alloy plating, other chemical activity, and/or rinsing, on internal surfaces of the gas lines. As shown, apparatus  400  includes a generally cuboid tank  410  including side walls  411 ,  412 ,  413  and  414  and a bottom surface  415 ; in other embodiments, tank  410  may assume different shapes. Bottom surface  415  includes a sump portion  420  in which a chemical  470  may pool for access by a pump  440 . Racks  430  are configured to hold gas lines  215 .  FIG. 5B  shows two gas lines  215  being held by racks  430 , but tank  410  and racks  430  may configured to accommodate any number and configuration of gas lines for processing. Pump  440  pumps chemical  470  into feed tubes  450 . Feed tubes  450  terminate in fittings  460  that are configured to fit inlet fixtures  240  of gas lines  215 . Chemical  470  thus flows into gas lines  215 , contacting internal surfaces thereof until it exits at outlet fixtures  250 , whereupon chemical  470  drips back into tank  410  for recycling through pump  440 . 
     Apparatus  400  can thus be utilized to implement several subprocesses of process  300 ,  FIG. 4 . For example, a cleaning solution can be utilized as chemical  470  to clean internal surfaces of gas lines  215 , optionally followed by use of water as chemical  470  to rinse the cleaning solution out of gas lines  215 . The same apparatus  400  can then be utilized to pump electroless Ni alloy plating solution as chemical  470  to Ni plate gas lines  215 , which again can optionally be followed by a water rinse. Alternatively, multiple instances of apparatus  400  can be utilized for different subprocesses, to avoid cross-contamination. 
     Many optional features and variations will be apparent to those skilled in the art. For example,  FIGS. 5A and 5B  show sump portion  420  with an optional strainer  480  which may be omitted in embodiments, or replaced with one or more filters, either upstream or downstream of pump  440 . Other optional features include:
         provisions for temperature control of chemical  470  and/or gas lines  215 ;   features for adding, mixing, and/or removing chemical  470  to or from tank  410 ;   manifolds or valves to distribute chemical  470  among gas lines  215 , including valves that allow flow of chemical  470  to individual gas lines  215  to be halted for addition or removal of ones of gas lines  215 , while others of gas lines  215  continue to flow chemical  470 ;   drain tubes fitted to outlet fixtures  250  of gas lines  215  to carry chemical  470  therefrom to a waste tank or to a reservoir used in place of sump portion  420 ; and/or   drying gases (e.g., clean dry air or N2) can be provided through feed tubes  450  and/or fittings  460 , or separate tubes and/or fittings can be provided with the drying gases, to dry internal surfaces of gas lines  215 .       

     Gas lines with internal nickel alloy plating can be tested to assure that the nickel alloy plating is functioning as designed. For example, gas line  215  can be tested by using a swab to rub one or more internal surfaces with a mildly acidic solution, and performing elemental analysis on particles found on the swab (e.g., with inductively coupled plasma mass spectroscopy, or ICP-MS). Because the swabbing method is technique sensitive, a total number of particles obtained is not a reliable indicator of suitability. However, elemental analysis can be performed on the particles that are found. This analysis can serve as a monitor for efficacy of the gas line base material, nickel alloy plating and/or other process variables in suppressing elements that will be harmful in workpiece processing. Particles obtained by swabbing will generally contain Ni, but other elements found on the particles can provide information relevant to suitability. For example, when SST gas lines are analyzed in this manner, high ratios of Fe and/or Cr to Ni are found, whereas when Al gas lines are analyzed in the same way, much lower ratios of Fe and/or Cr to Ni are found. Also, a ratio of Ni to P can be determined in order to monitor P concentration of the electroless Ni plating. The same technique can be utilized to evaluate variables such as gas line surface finish, thickness of nickel alloy plating, cleaning techniques, heat treatment variables and the like. This technique has repeatedly validated that aluminum gas lines with electroless nickel plating as described herein reduce Fe and Cr, in particles obtained on swabs, to nearly undetectable levels (e.g., reduction of Fe and Cr by factors of at least 10, often by factors of 100 or greater). 
     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention. 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included. 
     As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” or “a recipe” includes a plurality of such processes and recipes, reference to “the electrode” includes reference to one or more electrodes and equivalents thereof known to those skilled in the art, and so forth. Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups.