Patent Publication Number: US-2007102283-A1

Title: PVD method to condition a substrate surface

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      Embodiments of the present invention generally relate to the deposition of thin films. More particularly, this invention relates to a process and apparatus for depositing a thin film onto a substrate surface to condition the substrate surface.  
      2. Description of the Related Art  
      Developing environmentally friendly energy sources has gained significant interest recently in various industries related to the generation of power and electricity. Various types of fuel cells can be used to directly produce electricity for a number of applications, such as portable electronics, cell phones, wireless devices, PDA&#39;s, cameras, portable players, computer notebooks, vehicles, stationary large size energy equipment, residential electricity and others. Since semiconductor machining technology can be easily utilized in fuel cell manufacturing, efficient production of electricity by fuel cells is feasible.  
      A fuel cell is an electrochemical device in which a gaseous or liquid fuel reacts with an oxidant to produce electricity. Generally, an electrolyte is sandwiched by two electrodes, an anode and a cathode, to form a fuel cell unit. A fuel, such as pure hydrogen or hydrogen reformed from any hydrocarbon fuel, is fed into the anode to be oxidized into a proton and an electron. An oxidant, such as air or oxygen, is flowed into the cathode to react with the proton, which has passed through the electrolyte and, in some cases, through a proton-permeable membrane. The electron forms a separate current that can be utilized to generate electricity before returning to the cathode to be reunited with the proton and the oxidant, resulting in by-products, such as heat and water. Each fuel cell unit is stacked or arranged together to form a fuel cell stack or module. A number of modules or fuel cell stacks are piled, and electrical terminals, electrical insulators and end plates are disposed at opposite ends of the pile of modules to collectively produce electricity.  
      Thus, the essentials of a fuel cell are very simple, leading to highly reliable and long-lasting electricity/energy generating applications. Fuel cells are also highly efficient, converting hydrogen fuel into useful energy at an efficiency rate as high as 55-60 percent, as compared to about 35 percent for combustion gas engines or alkaline batteries. Further, the by-product of the main fuel cell reaction, when hydrogen is the fuel, is pure water, which means the carbon dioxide (CO 2 ) emission of a fuel cell is essentially zero. Also, fuel cells are very quiet, even those with extra equipment (e.g., fuel pump, air pump, thermal control systems, etc.), making them suitable for both portable power applications and for large-scale power generation.  
      In order for a fuel cell to operate properly with high electrical output and reliability, the fuel gas, oxidizing gas and waste fluids, such as liquid water, must move freely through the channels and conduits (discussed in conjunction with  FIGS. 2A and 2B  below) inside the fuel cell without interruption under a wide variety of operating conditions. As such, the surface properties of any fuel cell parts must be conditioned to facilitate and enable this movement. For example, the surface of fuel cell parts should provide a low liquid contact angle to the electrolyte.  
      The liquid contact angle of a surface or material, also known as the wetting angle, is used to quantify how readily a drop of a given liquid will wet or bead up on the surface.  FIG. 1  is a schematic cross-sectional view of a solid-liquid system  100  consisting of a droplet  105  on a solid  102 . The contact angle  101  of solid-liquid system  100  is a measurement of the angle formed between the surface of solid  102  and the line  103  tangent to the radius  104  of droplet  105  on the solid  102  from the point of contact  106  with the solid  102 . For ease of measurement of the contact angle  101 , the solid  102  typically is a flat, horizontal surface, as shown in  FIG. 1 . The contact angle  101  will vary for different systems of liquids and solids, resulting from the interaction of the surface tension of the droplet  105  and the surface chemistry of the solid  102 . When contact angle  101  is less than 90° for a solid-liquid system  100 , good wetting is produced and the droplet  105  tends to spread across the surface of solid  102 . When contact angle  101  is greater than 90°, wetting is poor, i.e., droplet  105  is repelled by the surface of solid  102 , and droplet  105  tends to bead or shrink away from solid  102 . For aqueous solutions, such as the electrolyte solution or liquid waste water in a fuel cell, a surface or material that creates a contact angle smaller than 90° is referred to as hydrophilic and a surface or material that creates a contact angle larger 90° than is referred to as hydrophobic.  FIG. 1A  shows a droplet  105   a  of an aqueous solution in contact with a solid surface  102   a  and forming a contact angle  101   a  that is greater than 90°. Solid surface  102   a  is considered highly hydrophobic.  FIG. 1B  shows a droplet  105   b  of an aqueous solution in contact with a solid surface  102   b  and forming a contact angle  101   b  that is equal to 90°. Droplet  105   b  demonstrates significant beading on solid surface  102   b  and therefore solid surface  102   b  is considered hydrophobic.  FIG. 1C  shows a droplet  105   c  of an aqueous solution in contact with a solid surface  102   c  and forming a contact angle  101   c  that is less than 90°. Droplet  105   c  spreads out across solid surface  102   c , therefore solid surface  102   c  is considered hydrophilic.  
      In addition to a low liquid contact angle with the electrolyte, the surface of fuel cell parts should withstand by-product heat and cycles of temperature variation between low temperatures (e.g., −40° C.) and high temperatures (e.g., 100° C.). Furthermore, the surfaces of fuel cell parts must be resistant to chemical etching by acid, water (H 2 O), oxygen (O 2 ) and any other chemicals in the electrolyte to ensure long part lifetime and long-term reliable operation. Also, because an acidic environment in the fuel cell may be created from the release of fluorine present in the membrane electrode assembly, a fuel cell part may need to be conditioned to resist etching by hydrofluoric acid. A typical membrane electrode assembly is described below in conjunction with  FIGS. 2A and 2B .  
      Further, various parts of a fuel cell stack or module may need to provide good electrical contact when mated with other parts in the fuel cell, as well as possessing low bulk resistivity. This minimizes current loss between stacks of fuel cell units.  
      Thus, there is still a need for methods and apparatus for conditioning a surface of a substrate, such as a fuel cell part, to provide improved fluid contact angle, low acid etch rate, and low resistivity to the surface of the substrate.  
     SUMMARY OF THE INVENTION  
      Embodiments of the invention generally provide a method for conditioning surfaces of a substrate, particularly a substrate useful in a fuel cell. In one aspect, a method is disclosed for treating a surface of a substrate useful in a fuel cell to increase the substrate&#39;s resistance to acid etching. The method includes depositing a layer of etch-resistant material via a physical vapor deposition (PVD) process onto a surface of the substrate. The substrate may comprise a carbon composite material, a conductive polymer, a metal alloy or graphite. The layer of etch-resistant material is deposited to a thickness to enhance resistance to acid corrosion with a wet etch rate of less than about 0.03 A/mm in the presence of about 10 ppm of hydrofluoric acid in water. In one aspect, the layer of etch-resistant material deposited on the surface of the substrate is about 1000 Å thick or less. In another aspect, the layer of etch-resistant material is a TiN layer.  
      In another embodiment, a method is provided for treating a surface of a substrate useful in a fuel cell to decrease the substrate&#39;s liquid contact angle. The method includes depositing a layer of hydrophilic material via a PVD process onto a surface of the substrate. The substrate may comprise a carbon composite material, a conductive polymer, a metal alloy or graphite. The layer of hydrophilic material is deposited to a thickness to reduce the substrate&#39;s liquid contact angle to less than about 50 degrees in an aqueous solution. In one aspect, the layer of hydrophilic material deposited on the surface of the substrate is about 1000 Å thick or less. In another aspect, the layer of hydrophilic material is a TiN layer.  
      In one aspect of the invention, the layer of material deposited by a PVD process may be a low resistivity material. For example, the deposited material layer may exhibit a resistivity of less than about 100 ohm-cm, such as less than about 10 ohm-cm, on the surface of a carbon composite substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
       FIG. 1  (Prior Art) is a schematic cross-sectional view of a solid-liquid system consisting of a droplet on a solid.  
       FIG. 1A  (Prior Art) illustrates a droplet of an aqueous solution in contact with a solid surface and forming a contact angle that is greater than 90°.  
       FIG. 1B  (Prior Art) illustrates a droplet of an aqueous solution in contact with a solid surface and forming a contact angle that is equal to 90°.  
       FIG. 1C  (Prior Art) illustrates a droplet of an aqueous solution in contact with a solid surface and forming a contact angle that is less than 90°.  
       FIG. 2A  illustrates an exploded perspective view of a fuel cell stack that may benefit from the present invention.  
       FIG. 2B  illustrates a schematic view of an exemplary fuel cell unit that may be used in a fuel stack.  
       FIG. 3  is a schematic cross-sectional view of a PVD system that may be used to perform aspects of the invention.  
       FIG. 4A  illustrates a partial sectional view of the flow field surface of an anode separator plate.  
       FIG. 4B  illustrates a partial sectional view of the flow field surface of an anode separator plate.  
       FIG. 5  illustrates a partial sectional view of the cooling channel surface of an anode separator plate. 
    
    
      For clarity, identical reference numerals have been used, where applicable, to designate identical elements that are common between figures.  
     DETAILED DESCRIPTION  
      The invention generally relates to a method of improving the surface properties of a substrate. The invention describes treating or conditioning a substrate surface by depositing a material layer with good adhesion to the substrate, a smaller fluid contact angle with the electrolyte than the untreated substrate surface, low electrical resistivity (high conductivity) and resistance to acid etching.  
      In one embodiment, the invention includes a process for conditioning various parts of a fuel cell by depositing a thin film having a consistent set of surface properties to promote the proper flow of gas and fluids through a fuel cell device. In addition, the deposited thin film resists dissolution or etching in acidic conditions, e.g., a diluted hydrofluoric acid environment, to provide high reliability of the fuel cell and long part lifetime. The thin film may be deposited to condition/treat any parts of a fuel cell, such as a conductive plate, separator plate, bipolar plate or end plate, among others.  
      Exemplary Fuel Cell  
       FIG. 2A  illustrates an exploded perspective view of a fuel cell stack  10  that may benefit from the present invention. Fuel cell stack  10  consists of two fuel cell units: fuel cell unit A and fuel cell unit B. Generally, a fuel cell stack  10  uses a number of conductive plates  201  and membrane electrode assemblies  205 . The conductive plates  201  are placed between adjacent fuel cell units in a fuel cell stack  10 , such as fuel cell unit A and fuel cell unit B, to separate each fuel cell unit. A conductive plate placed between adjacent fuel cell units in a fuel cell stack is typically referred to as a separator plate  201   a , or bipolar plate, and a conductive plate placed at either end of a fuel cell stack is called an end plate  201   b . Each separator plate  201   a  generally includes an anode separator plate  260  and a cathode separator plate  270 . A fuel cell stack typically has one end plate  201   b  that acts as an anode separator plate  260  and one end plate  201   b  that acts as a cathode separator plate  270 , as illustrated in  FIG. 2A . Anode separator plate  260  and cathode separator plate  270  usually include a series of flow channels or grooves, such as anode flow field  262  and cathode flow field  272 , that distribute and route fuel gas or oxidant through a fuel cell unit. Anode flow field  262  and cathode flow field  272  are not shown in  FIG. 2A , but are described in more detail below in conjunction with  FIG. 2B . Referring back to  FIG. 2A , each anode separator plate  260  or cathode separator plate  270  includes one side for flowing fuel gases or oxidant gases. The other side of each anode separator plate or cathode separator plate generally contains cooling channels or conduits. These conduits are mated with the cooling channels from an adjacent fuel cell unit in a fuel cell stack  10  to form a mated conductive plate, such as separator plate  201   a , with internal cooling channels, such as cooling channels  264 . Hence, separator plate  201   a  includes one side to serve as an anode for one fuel cell unit, such as fuel cell unit B, and a second side to act as a cathode for an adjacent fuel cell unit, such as fuel cell unit A.  
       FIG. 2B  illustrates a schematic view of an exemplary fuel cell unit  200  that may be used in fuel stack  10 , similar to fuel cell unit A or fuel cell unit B illustrated in  FIG. 2A . A membrane  210  is generally sandwiched between an anode catalyst  220  and a cathode catalyst  230  to form a membrane electrode assembly  205 . The membrane  210  can be made from ion exchange resins or a polymer material. For example, an ionic perfluorinated sulfonic acid polymer membrane, such as Nafion™, available from the E.I. DuPont de Nemours &amp; Co. Other suitable membrane materials include Gore Select™, sulphonated fluorocarbon polymers, the polybenzimidazole (PBI) membrane (available from Celanese Chemicals, Dallas, Tex.), polyether ether ketone (PEEK) membranes and other materials. Various suitable catalyst formulations for the cathode catalyst  220  and the anode catalyst  230  are known in the art and are generally platinum-based with very finely divided catalytic particles supported on internal and external surfaces of a carbon binder, such as a polytetrafluorethylene (PTFE) binder.  
      The electrolyte plays a key role in a fuel cell to carry protons from one electrode, the anode, to the other electrode, the cathode. The electrolyte may include various types of organic and inorganic chemicals. Different types of fuel cells are formed depending on the types of chemicals used. In addition, the electrolyte may include a membrane, such as a polymer membrane for a direct methanol fuel cell (DMFC) or an immobilized liquid molten carbonate for a molten carbonate fuel cell (MCFC). One type of fuel cell, a proton exchange membrane fuel cell (PEMFC), uses a thin proton exchange membrane, such as membrane  210 , with both sides of the surfaces coated with different catalysts, such as the cathode catalyst  220  and the anode catalyst  230 , which accelerate the different chemical reactions at the anode and the cathode. The membrane is sandwiched by two microporous conductive layers (which function as the gas diffusion layers and current collectors) to separate the hydrogen fuel from the anode and the oxidant from the cathode, forming a membrane electrode assembly  205  as shown in  FIG. 2B . In the exemplary fuel cell unit  200  illustrated in  FIG. 2B , the two microporous conductive layers in membrane electrode assembly  205  are anode conductive gas diffusion layer  240  and a cathode conductive gas diffusion layer  250 . Anode conductive gas diffusion layer  240  and a cathode conductive gas diffusion layer  250  serve as primary current collectors on each side of membrane electrode assembly  205 . Both conductive gas diffusion layers are porous, gas-permeable and are generally made from paper or cloth-based carbon fibers, graphite materials or finely-meshed noble metal screens or foams, among other materials known in the art.  
      As illustrated in  FIG. 2B , a pair of gas impermeable, non-porous, electrically conductive plates, such as an anode separator plate  260  and a cathode separator plate  270 , is sandwiched between the primary current collectors, i.e., membrane electrode assembly  205 , to serve as secondary current collectors for conducting current between adjacent fuel cells and at the ends of a fuel cell stack. The separator plates provide electrical contact between the anodes and cathodes of neighboring fuel cell units while preventing the hydrogen and oxygen reactant gases from mixing. Anode separator plate  260  and cathode separator plate  270  may be made of a metal alloy, such as stainless steel or aluminum protected with a corrosion-resistant conductive coating, or a conductive polymer, such as a carbon-filled composite, or other conductive compound, such as graphite. The carbon composite material provides the adequate electrical and thermal conductivity to be used as a separator. For example, a high-carbon composite with graphite powder in the range of 70% to 90% by volume can be used. However, these composite materials may be inherently brittle and dense, resulting in cracking and breaking. The separator plates in a fuel cell unit are continuously exposed to an acidic and polarized environment and to pressurized air and hydrogen. Further, the electrolyte of the fuel cell is generally in an environment containing up to 10 ppm of hydrofluoric acid, a strong etchant for many materials. The hydrofluoric acid is generated by the combination of protons produced at the anode and a small amount of fluorine ions released into the electrolyte by the fluorinated polymers of the separator plates. Thus, the separator plate must be resistant to such a hostile environment. Because of this, separator plates currently known in the art, such as an anode separator plate  260  and a cathode separator plate  270  may benefit from aspects of the invention, such as the application of a TiN coating.  
      In one aspect of the invention, a PVD process is used to apply a durable conductive coating, such as a TiN coating, to the separator plates of a fuel cell, such as an anode separator plate  260  and a cathode separator plate  270 . A TiN coating provides a more durable surface than the polymers and carbon composites generally used to construct separator plates and still possess very low resistivity. Such a non-friable surface extends fuel cell life and widens the conditions under which the fuel cell may operate. Further, the use of a PVD process (described below in conjunction with  FIG. 3 ) ensures that a coating applied to the irregular surfaces of the separator plates, such as flow fields  262 ,  272 , will have acceptable “step coverage”, i.e. uniform coverage by a coating of all features on a surface.  
      As noted above in conjunction with  FIG. 2A  and illustrated in  FIG. 2B , one side of each separator plate contains a flow field consisting of a plurality of small channels, grooves, or passages. The anode separator plate  260  contains a flow field  262  that distributes and routes gaseous reactants, such as H 2  and other fuel gases, to the surface of the anode conductive gas diffusion layer  240 , facing the anode side of membrane electrode assembly  205 . The cathode separator plate  270  contains a flow field  272  that distributes and routes gaseous oxidants, such as O 2  or air, to the surface of the cathode conductive gas diffusion layer  250 , facing the cathode side of membrane electrode assembly  205 . The flow fields  262 ,  272  provide a means through which the gaseous reactants can flow between gas supplies  280 ,  290  leading into exemplary fuel cell unit  200  and gas exhausts (not shown) leading out of exemplary fuel cell unit  200 .  
      The area between the plurality of flow channels contained in cathode separator plate  270  and anode separator plate  260  are generally called land areas which contact the primary current collector. The other sides of the anode and cathode separator plates  260  and  270 , respectively, generally contain cooling channels  264  and  274  or other conduits for flowing a coolant. Typically, cooling channels  264 ,  274  are formed by mating two conductive plates together, such as cathode separator plate  270  and anode separator plate  260  of separator plate  201   a , illustrated in  FIG. 2A . Cooling channels  264 ,  274  form coolant paths to dissipate heat, water and other reaction by-products away from fuel cell unit, such as exemplary fuel cell unit  200 , illustrated in  FIG. 2B . The mated conductive plates become a separator plate or a bipolar plate to be placed between the anode of one fuel cell and a cathode of an adjacent fuel cell and, thus, the separator plate can be made out of one or more conductive plates. Alternatively, a bipolar plate can be made from one plate having cooling channels therein. In general, a bipolar plate includes one side for flowing a fuel gas through the flow field  262  and other side for flowing an oxidant through the flow field  272 . Reactant gases from each side of the bipolar plate may pass along the flow fields  262 ,  272  and diffuse through the gas diffusion layers  240 ,  250  to reach the membrane electrode assembly  205 .  
      The individual gas channels and passages contained in flow fields  262 ,  272  are generally small in relation to the overall size of the separator plates, i.e., on the order of 1 mm in width or diameter. Because the waste product of a fuel cell is commonly water, it is known in the art that the small gas channels inside flow fields  262 ,  272  are easily blocked by any water droplets that form inside them (see  Visualization of Liquid Water Transport in a Polymer Electrolyte Fuel Cell , Electrochemical and Solid-State Letters, 7 (11) A408-A411, 2004). This type of blockage is referred to as electrode flooding. When electrode flooding blocks a gas channel at any point inside flow fields  262 ,  272 , the fuel cell reaction for the entire gas channel is shut down, significantly reducing current produced by the fuel cell. It has been shown that the gravitation force on a water droplet approximately 1 mm in diameter is less by one order of magnitude than the surface tension of such a droplet. Because of this, different orientations (i.e. vertical instead of horizontal) of a fuel cell, or of the gas channels therein, will not significantly affect water droplet formation and, hence, electrode flooding. This is particularly true for more hydrophobic surfaces, such as the carbon-containing polymers typically used for separator plates in fuel cells. Instead, electrode flooding may be avoided by decreasing the liquid contact angle between the waste water produced in a fuel cell and the surfaces of the gas passages of flow fields  262 ,  272 .  
      In one aspect of the invention, a PVD coating is applied to the flow field side of cathode separator plate  270  and anode separator plate  260 . In this way, the liquid contact angle of the surfaces of the gas passages of flow fields  262 ,  272  may be reduced by the application of the hydrophilic PVD coating to some or all surfaces of the separator plates. An exemplary PVD process is described below in conjunction with  FIG. 3 . In one example, a PVD coating is only applied to the gas channel surfaces of cathode separator plate  270  and anode separator plate  260  to reduce the possibility of electrode flooding. In this case, the land areas, i.e. the current-transmitting surfaces located between the gas channels, are masked during the PVD process to prevent deposition thereon. When the hydrophilic coating has relatively high resistivity, masking the land areas minimizes contact resistance between adjacent separator plates, such as cathode separator plate  270  and anode separator plate  260  of separator plate  201   a , illustrated in  FIG. 2A . In another example, a low resistivity PVD coating, such as a TiN coating, is applied to all flow field surfaces of cathode separator plate  270  and anode separator plate  260 . In this case, no masking of the land areas of the separator plates is required because no significant contact resistance between adjacent separator plates is created by the low resistivity PVD coating. In yet another example, a low resistivity PVD coating, such as a TiN coating, is applied to all cooling channel surfaces of cathode separator plate  270  and anode separator plate  260 . In this case, the formation of a hydrophilic surface on the cooling channel surfaces reduces the possibility of any cooling channels being blocked by a “vapor lock” scenario, i.e. when a heated liquid begins turning to vapor in a narrow conduit and blocks the flow of liquid. This is because vapor bubbles are less likely to adhere to hydrophilic surfaces. Vapor bubbles adhering to the walls of a narrow conduit restrict liquid flow through the conduit, such as through cooling channels  264 ,  274 . As the liquid flow is reduced, vaporization of the heated liquid increases, until a vapor lock is created.  
      Exemplary PVD Apparatus and Process  
      As noted above, a number of components of a fuel cell may benefit by being coated with a durable, conductive (i.e., low-resistivity) material that reduces the liquid contact angle of a component&#39;s surface and is resistant to etching by dilute hydrofluoric and other acids. A PVD TiN film is such a coating. A method and apparatus for the application of a PVD TiN film to improve the surface properties of fuel cell components is described below in conjunction with  FIG. 3 .  
      One embodiment is illustratively described below in reference to a physical vapor deposition (PVD) system configured to process different types of substrates, such as various PVD systems available from AKT, a division of Applied Materials, Inc., Santa Clara, Calif. However, it should be understood that the invention has utility in other system configurations, such as other PVD systems and any other film deposition systems, including those systems configured to process round substrates.  
      Physical vapor deposition using a magnetron is one of the principal methods of depositing a metal layer of uniform thickness onto a substrate. During a PVD process a target is electrically biased so that ions generated in a process region can bombard the target surface with sufficient energy to dislodge atoms from the target. The process of biasing a target to cause the generation of a plasma that in turn causes ions to bombard and remove atoms from the target surface is commonly called sputtering. The sputtered atoms travel generally ballistically toward the substrate being sputter-coated and the sputtered atoms are deposited on the substrate. Alternatively, the atoms may react with another gas in the plasma, for example, nitrogen, to reactively deposit a compound on the wafer. Reactive sputtering is often used to form layers of titanium nitride or tantalum nitride.  
      DC magnetron sputtering is the most usually practiced commercial form of sputtering. The metallic target is biased to a negative DC bias in the range of about −400 to −600 VDC to attract positive ions of the working gas (e.g., argon or an argon-nitrogen mixture) toward the target to sputter the metal atoms. Usually, the sides of the sputter reactor are covered with a shield to protect the chamber walls from sputter deposition. The shield is typically electrically grounded and thus provides an anode in opposition to the target cathode to capacitively couple the DC target power into the chamber and its plasma.  
      A magnetron having at least a pair of opposed magnetic poles is typically disposed near the back of the target to generate a magnetic field close to and parallel to the front face of the target. The induced magnetic field from the pair of opposing magnets trap electrons and extend the electron lifetime before they are lost to an anodic surface or recombine with gas atoms in the plasma. Due to the extended lifetime, and the need to maintain charge neutrality in the plasma, additional argon ions are attracted into the region adjacent to the magnetron to form there a high-density plasma. Thereby, the sputtering rate is increased.  
       FIG. 3  illustrates a vertical cross-sectional view of one embodiment of a processing chamber  300  that may be used to perform aspects of the invention described herein. In general, the processing chamber  300  contains a lid assembly  20  and a lower chamber assembly  35 . The lower chamber assembly  35  generally contains a substrate support assembly  60 , chamber body assembly  40 , a shield  50 , a process gas delivery system  45  and a shadow frame  52 . The shadow frame  52  is generally used to shadow the edge of a substrate  12  to prevent or minimize the amount of deposition on the edge of the substrate  12  and substrate support  61  during processing. The chamber body assembly  40  generally contains one or more chamber walls  41  and a chamber base  42 . The one or more chamber walls  41 , the chamber base  42  and a surface of the target assembly  124  generally form a vacuum processing area  17  that has a lower vacuum region  16  and a processing region  15 . The process gas delivery system  45  generally contains one or more gas sources  45 A that are in fluid communication with one or more inlet ports  45 B that are in direct communication with the lower vacuum region  16  and/or the processing region  15 , to deliver a process gas that can be used during the plasma process.  FIG. 3  illustrates a substrate  12  that is positioned in a processing position on substrate support  61  in the processing region  15 .  
      The lower chamber assembly  35  will also generally contain a substrate lift assembly  70 , slit valve  46  and vacuum pumping system  44 . The lift assembly  70  is adapted to remove and replace a substrate positioned on a robot blade (not shown) that has been extended into the lower chamber assembly  35  from a central transfer chamber (not shown). The extended robot blade enters the lower chamber assembly  35  through the access port  32  in the chamber wall  41  and is positioned above the substrate support  61  that is positioned in a transfer position (not shown).  
      The vacuum pumping system  44  evacuates the lower vacuum region  16  and processing region  15  to a desired base and/or processing pressure. A slit valve actuator (not shown) which is adapted to position the slit valve  46  against or away from the one or more chamber walls  41  may be a conventional pneumatic actuator which is well known in the art.  
      To control the various processing chamber  300  components and process variables during a deposition process, a controller  3101  is used. The processing chamber&#39;s processing variables may be controlled by use of the controller  3101 , which is typically a microprocessor-based controller. The controller  3101  is configured to receive inputs from a user and/or various sensors in the plasma processing chamber and appropriately control the plasma processing chamber components in accordance with the various inputs and software instructions retained in the controller&#39;s memory.  
      The lid assembly  20  generally contains a target assembly  124 , a lid enclosure  22 , a ceramic insulator  26 , one or more o-ring seals  29  and a magnetron assembly  23  that is positioned in a target backside region  21 . Generally, each magnetron assembly  23  will have at least one magnet  27  that has a pair of opposing magnetic poles (i.e., north (N) and south (S)) that create a magnetic field (B-field) that passes through the target assembly  124  and the processing region  15 . An exemplary magnetron assembly that may be adapted to benefit the invention described herein is further described in the commonly assigned U.S. patent application Ser. No. 10/863,152[APPM 8841], filed Jun. 7, 2004, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/534,952, filed Jan. 7, 2004, and is hereby incorporated by reference in its entirety to the extent not inconsistent with the claimed invention.  
      To perform a PVD deposition process on a substrate  12 , such as a conductive plate  201  used in fuel cell stack  10 , illustrated in  FIG. 2A , the controller  3101  commands the vacuum pumping system  44  to evacuate the processing chamber  300  to a predetermined pressure/vacuum. Chamber evacuation allows the processing chamber  300  to receive a substrate  12  from a system robot (not shown) mounted to a central transfer chamber (not shown) which is also under vacuum. To transfer a substrate  12  to the processing chamber  300 , the slit valve  46 , which seals off the processing chamber  300  from the central transfer chamber, opens to allow the system robot to extend through the access port  32  in the chamber wall  41 . The lift pins  74  of substrate lift assembly  70  then remove the substrate  12  from the extended system robot, by lifting the substrate from the extended robot blade (not shown) using a lift actuator  71 . The system robot then retracts from the processing chamber  300  and the slit valve  46  closes to isolate the processing chamber  300  from the central transfer chamber. The substrate support  61  then lifts the substrate  12  from the lift pins  74  and moves the substrate  12  to a desired processing position below the target assembly  124 . After achieving a desired base pressure, (e.g., 10 −6  to 10 −9  Torr) by use of the vacuum pumping system  44 , a desired flow of a processing gas is injected into the processing region  15  and a bias voltage is applied the target region  127  of the target assembly  124  by use of a power supply  128 A attached to the target region. The application of a bias voltage by the power supply causes the gas in the processing region  15  to become ionized and bombard the surface of the cathodically biased target region  127  and thus “sputter” metal atoms that land on the surface of the substrate  12 . For the formation of a TiN film on the substrate surface, the process gas is typically a nitrogen or nitrogen-argon mixture. The flow rate of process gas varies as a function of chamber size, ranging from about 100 sccm for a chamber with a processing region  15  that is 200 mm in diameter to 20,000 sccm for chamber with a processing region  15  that is 1300 mm×1500 mm in area. One skilled in the art can readily determine an appropriate process gas mixture and flow rate, depending on the size of PVD chamber and deposition rate desired.  
      PREFERRED EMBODIMENT  
      In one aspect of the invention, surfaces inside a fuel cell that may benefit from a conditioning or protective coating, such as a TiN coating, include the flow field side of a separator plate, such as anode separator plate  260  and a cathode separator plate  270  contained in fuel cell unit  200  and illustrated in  FIG. 2B . Because the conductive plates in fuel cell unit  200  are generally composed of brittle and/or non-durable materials, such as graphite, carbon-composites, or polymers, for example, a durable conductive coating, such as TiN may greatly improve their operational lifetime. Further, TiN is resistant to etching by hydrofluoric acid, which is commonly present inside fuel cells in concentrations up to about 10 ppm.  
       FIG. 4A  illustrates a partial sectional view of the flow field surface  400  of an anode separator plate  260 , which is also illustrated in  FIGS. 2A and 2B . Gas channels  401  contained in flow field  262  are shown in cross-section. A PVD coating  402 a is depicted on anode separator plate  260  as covering the entire flow field surface  400  of anode separator plate  260 . This includes not only the sides  420  and bottom  421  of gas channels  401 , but also the land areas  422 . As illustrated in  FIG. 2B and 4B , the land areas  422  of anode separator plate  260  are designed to contact anode conductive gas diffusion layer  240  when the fuel cell is assembled. In this aspect, no masking is used to prevent deposition of PVD coating  402   a  on the land areas  422 . When PVD coating  402   a  consists of a TiN coating, very little additional electrical resistance is introduced into the fuel cell when the land areas are also coated. This is due to the low resistivity of a PVD TiN coating; TiN possesses high bulk conductivity (less than about 10 ohm-cm) and a PVD TiN coating on a separator plate is also very thin. It is important to note that the TiN coating, i.e. PVD coating  402   a , is illustrated in  FIG. 4A  with a greatly exaggerated thickness relative to the size of the gas channels. Typically, the width  403  and depth  404  of gas channels  401  is on the order of about 1 mm. In contrast, the thickness required for a TiN coating to provide adequate protection of flow field surface  400  is on the order of about 1000 Å or less—four orders of magnitude smaller in size than gas channels  401 . Due to the high resistance to etching of TiN, a thicker coating is not required.  
      In one example of this aspect, a PVD coating  402   b  is only deposited on the internal gas channel surfaces of a flow field  262  on a separator plate, such as an anode separator plate. This is illustrated in  FIG. 4B . The internal gas channel surfaces that are treated with PVD coating  402   b  include the sides  420  and bottom  421  of gas channels  401 . Masking material placed on the land areas  422  is used to prevent deposition of the PVD coating thereon. In this example, a PVD coating that is not as conductive as TiN may be used to protect and reduce the liquid contact angle of the sides  420  and bottom  421  of gas channels  401  without introducing additional electrical resistance between the conductive plates of a fuel cell. For example, the resistivity of PVD coating  402   b  in this example may be as high as about 100 ohm-cm.  
      In another aspect of the invention, the cooling channel side of a separator plate, such as anode separator plate  260  or cathode separator plate  270  (illustrated in  FIG. 2B ) is coated with a layer of material deposited via a PVD process. In one example, the PVD coating is a TiN coating. The increased wettability, i.e., smaller liquid contact angle, created thereby may improve the performance of the cooling channels by reducing the possibility of vapor lock occurring in the cooling channels  264  formed into the separator plate. Further, the TiN coating may significantly increase the durability and acid resistivity of the separator plate surface and, hence, increase the operational lifetime of the separator plate as well.  
       FIG. 5  illustrates a partial sectional view of the cooling channel surface  500  of an anode separator plate  260 , which is also illustrated in  FIGS. 2A and 2B . Cooling channels  264  are shown in cross-section. A PVD coating  502  is depicted on anode separator plate  260  as covering the cooling channel surface  500  of anode separator plate  260 . This includes the bottom  520  and the land areas  522  of the cooling channels  264 . Alternatively, masking may be applied to the land areas  522  to prevent deposition of PVD coating  502  thereon. In order to condition the cooling channel surface  500  against acid etching and to provide a durable coating, the thickness of PVD coating  502 , when a TiN coating, may be relatively thin, for example no thicker than about 1000 Å. This is adequate to protect the material of an anode separator plate  260  or cathode separator plate  270  from the hostile environment of the fuel cell interior.  
      Other surfaces inside a fuel cell, particularly surfaces that are preferably highly conductive yet highly acid resistant, may also benefit from a TiN coating. A thin film, i.e. less than about 1000 Å, may be deposited via a PVD process to condition any parts of a fuel cell that are routinely exposed to the acidic environment, such as the end plates, the fuel cell housing and the fuel gas and oxidant gas supply and exhaust conduits, among others.  
      In another aspect, a surface undergoes a pre-clean process prior to deposition of the PVD coating. One example of a beneficial pre-clean process is a plasma-clean process, also known as a sputter clean process. Another example is a wet clean process. Either process may be performed on the desired fuel cell surface in order to remove unwanted contamination present on a fuel cell surface that may subsequently result in the delamination or peeling of the PVD coating from the treated fuel cell surface.  
      A plasma-clean process may be performed in a plasma-processing chamber immediately prior to deposition of the PVD film. Preferably, the plasma-clean process is performed on the fuel cell surface without removing the treated surface from vacuum and exposing it to atmospheric conditions. Exposure to atmosphere may be avoided between a plasma-clean process and a PVD process when the PVD processing chamber and the plasma-processing chamber share access to a common central transfer chamber that is also maintained at vacuum. By keeping the fuel cell surface at vacuum between the surface cleaning process and the deposition process, contamination of the fuel cell surface is minimized and adhesion of the PVD coating is improved.  
      A plasma-clean process may be performed in a chamber that is inductively coupled, capacitively coupled or utilizes a remote plasma source. The application of reactive gases, such as ammonium or nitrogen trifluoride, for example, may also be used as part of the plasma-clean process to enhance the cleaning of the fuel cell surface. As part of the plasma-clean process, the selection of chemical species, process gas flow rates and plasma power are strongly dependent on the composition of the substrate being processed. The plasma-clean process is well known in the art and anyone skilled in the art may select an appropriate cleaning method for a given fuel cell surface.  
      A wet-clean process may also be performed in a plasma-processing chamber immediately prior to deposition of the PVD film. The wet-clean process may be performed on fuel cell components singly or in batches. Chemical selection, cleaning time, cleaning bath temperature, rinse times and other process details are dependent on the composition of the surface being cleaned. The wet-clean process is well known in the art and anyone skilled in the art may select the appropriate cleaning method for a given fuel cell surface.  
      Fuel cell components that may benefit from the invention may be of any shape, e.g., circular, square, rectangle, polygonal, etc., and size. Also, the composition of fuel cell components is not limiting and can be any substrate comprised of a material of carbon-containing polymer, composite, metal, plastic, or glass, particularly where the substrate&#39;s surface fluid contact angle and resistance to wet etch conditions need to be improved while maintaining good conductivity, i.e., low resistivity.  
      Although several preferred embodiments which incorporate the teachings of the present invention have been shown and described in detail, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.  
      While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.