Abstract:
A system for measuring gas density in a vacuum includes a gauge, a housing for containing the gauge, and a magnet secured to an exterior surface of the housing. The magnet is a flexible magnetic strips, and positioned around the exterior surface of the housing. The gauge includes grid insulator posts extending longitudinally along a tubular section of the housing, and the magnet is secured to the exterior surface of the housing adjacent to the grid insulator posts, and oriented transversely to the grid insulator posts. The magnet is a flexible magnetic strip, and a clamp secures the magnet to the exterior surface of the housing.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates, in general, to ionization vacuum gauges. More specifically, the present invention relates to redirecting contaminants away from ionization vacuum gauges. 
       BACKGROUND OF THE INVENTION 
       [0002]    Ionization vacuum gauges may be used to measure low pressures of gases. Ionization vacuum gauges sense pressure indirectly by measuring the electrical ions produced when a gas is bombarded with electrons. 
         [0003]    Ionization vacuum gauges may include the following types: (1) thermionic emission vacuum gauges, also called hot cathode vacuum gauges; and (2) field emission vacuum gauges, also called cold cathode vacuum gauges. In hot cathode vacuum gauges, an electrically-heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current flowing through the electrode depends on the number of ions, which depends on the gas pressure in the gauge. The principle behind cold cathode vacuum gauges is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. 
         [0004]    A typical example of a hot cathode vacuum gauge is the Bayard-Alpert vacuum gauge disclosed in U.S. Pat. No. 2,605,431, issued to Bayard on Jul. 29, 1952, which is incorporated herein by reference in its entirety. 
         [0005]    The Bayard-Alpert type ionization vacuum gauge is the most common non-magnetic gauge for measuring very low pressures and has been widely used worldwide. The Bayard-Alpert vacuum gauge ionizes the gas molecules within the gauge volume, collects those ions on a thin ion collector wire and measures the resulting current in the ion collector wire. The number of gas molecules present in the volume, or the pressure in the volume, may be determined based on the current measurement. 
         [0006]    The Bayard-Alpert gauge may include at least one heated filament that emits electrons toward an anode, for example a cylindrical wire grid. At least one ion collector electrode is placed within and along the axis of the cylindrical wire grid. The negative electrons, which are emitted from the heated filament (a cathode), are accelerated toward the positively-charged cylindrical wire grid (an anode). Electrons pass into the volume of space enclosed by the cylindrical wire grid. In this volume, the electrons collide with any gas molecules in the vacuum chamber and, thereby, produce positive ions. These positive ions are collected by the ion collector electrode disposed within the cylindrical wire grid. 
         [0007]    The ion collector electrode may be at nearly ground potential and may be negative compared to the cylindrical wire grid. At constant filament-to-grid voltage and electron emission current, the rate that the positive ions are formed is directly proportional to the density of molecules (pressure) in the vacuum gauge for pressures below about 1×10 −3  Torr. The strength of the current may be indicated on a meter, which may be calibrated in units of pressure. 
         [0008]    Vacuum chambers requiring very low pressures are used in many different processes. For example, some processes performed in a vacuum chamber produce metallic evaporates. These processes, however, contaminate the ion gauge which is used to measure the pressure in the chamber. 
         [0009]    The present invention provides a method and apparatus for reducing the contaminants reaching the ion gauge collector. By directing the contaminants away from the ion gauge collector, as will be described, the ion gauge has less of a likelihood for being damaged, is more accurate, and has a longer maintenance-free life cycle, and is more accurate. 
       SUMMARY OF THE INVENTION 
       [0010]    To meet this and other needs, and in view of its purposes, the present invention provides a system for measuring gas density in a vacuum including a gauge, a housing for containing the gauge, and a magnet secured to an exterior surface of the housing. The magnet is a flexible magnetic strip, and positioned around the exterior surface of the housing. 
         [0011]    The gauge includes grid insulator posts extending longitudinally along a tubular section of the housing, and the magnet is secured to the exterior surface of the housing, adjacent to the grid insulator posts, and oriented transversely to the grid insulator posts. The magnet is a flexible magnetic strip, and a clamp secures the magnet to the exterior surface of the housing. The magnet is formed from a high temperature material, and the clamp is formed from stainless steel and is adjustable to press fit the magnet around the exterior surface of the tubular section of the housing. 
         [0012]    Another embodiment of the present invention is a vacuum gauge including an ion collector wire extending longitudinally in a tubular section of a housing; a filament extending longitudinally along and spaced from the ion collector; a grid surrounding the ion collector wire; grid insulators extending longitudinally in the tubular section of the housing including electrical conductors for providing a voltage potential to the grid; and a magnet disposed externally to the tubular section of the housing. The magnet is a flexible magnetic strip, and positioned around the exterior of the tubular section of the housing. The magnet is secured transversely to the grid insulators on the exterior of the tubular section of the housing. A clamp provides a press fit for the flexible magnetic strip around the exterior of the tubular section of the housing. The flexible magnetic strip is oriented transversely and adjacent to the grid insulators. The flexible magnetic strip may include a plurality of magnetized strips oriented circumferentially around the exterior of the tubular section of the housing. 
         [0013]    Yet another embodiment of the present invention is a method of reducing metallic contamination of a Bayard-Alpert vacuum gauge including grid insulator posts. The method includes the steps of: (a) exposing the Bayard-Alpert vacuum gauge to metallic vapor; and (b) providing a magnetic field adjacent to the grid insulator posts. The magnetic field is effective in reducing metallic deposits on the grid insulator posts. 
         [0014]    The step of providing the magnetic field includes attaching a flexible magnetic strip on an exterior surface of a housing surrounding the Bayard-Alpert vacuum gauge. The flexible magnetic strip is attached transversely to and adjacent to the grid insulator posts. 
         [0015]    The attaching step includes clamping the flexible magnetic strip using an adjustable stainless-steel hose clamp. Providing the magnetic field includes providing a magnetic field having a strength of at least 160 gauss. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0016]    The invention may be best understood from the following detailed description when read in conjunction with the accompanying figures: 
           [0017]      FIG. 1  is a cut-away, top view of a right angle ionization vacuum gauge with a molecular shield installed in accordance with an embodiment of the present invention; 
           [0018]      FIG. 2  is a side view of a housing for the right angle ionization vacuum gauge shown in  FIG. 1 , with the molecular shield installed in accordance with an embodiment of the present invention; 
           [0019]      FIG. 3  shows a clamp for securing the molecular shield around the housing of the right angle ionization vacuum gauge of  FIG. 2 , according to one exemplary embodiment of the invention; and 
           [0020]      FIGS. 4A and 4B  show cross-sections of the ionization vacuum gauge shown in  FIG. 1  with different magnet configurations around the grid insulator posts, according to two exemplary embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Many processes require a vacuum chamber for depositing a metallic substance onto a surface of a substrate. During the process, metallic vapor is combined with oxygen in a vacuum forming a metallic oxide. The metallic oxide, however, contaminates the ionization vacuum gauge used for measuring the vacuum pressure of the chamber, in which the substrates are processed. The inner components of the ionization gauge are exposed to the vacuum and, consequently, become coated with metallic evaporates. 
         [0022]    As described below, the present invention provides a method and apparatus for directing contaminants, such as a metallic vapor, away from the inner components of the ionization gauge. 
         [0023]    Referring to  FIGS. 1 ,  2  and  3 , there is shown a right angle ionization vacuum gauge, generally designated as  10 . The ionization vacuum gauge may be a Bayard-Alpert gauge, as shown in a top cut-away view of  FIG. 1 . The Bayard-Alpert gauge includes tungsten filaments (cathode)  24 , helical coil grid (anode)  26 , and collector wire (ion collector)  28 . 
         [0024]    There are two types of materials commonly used for filaments: tungsten and iridium. There are two types of coatings used on the filaments: thoria and yttria. 
         [0025]    Generally, filaments are yttria-coated iridium, thoria-coated iridium, or uncoated tungsten. The most common style is coated iridium, because they operate at a lower temperature than tungsten, and therefore, are less reactive. Coated iridium filaments are also more burnout resistant, when exposed to atmospheric pressure while power is on. Tungsten filaments will burn out immediately if exposed to pressures of 1×10 −2  Torr or higher, while they are on. However, tungsten filaments are the best type to use, when the chemistry (such as halogen compounds) of the vacuum process causes premature failure of coated iridium filaments. 
         [0026]    The amount of emission current that a Bayard-Alpert gauge requires for proper operation depends on many factors such as: the type of filament, the size or style of the gauge, the process in which the gauge is used, the pressure range of operation, and the desired sensitivity of the indicated pressure. Emission currents are typically in the range of 25 μA to 10 mA. 
         [0027]    Also shown in the cut-away view of  FIG. 1  are the grid insulator posts, designated  12   a  and  12   b , which support and insulate the grid conductors, while they are positively charged with respect to filaments  24 . Although not shown, input/output (I/O) pins are connected to the grid conductors for providing a voltage potential to helical coil grid (anode)  28 . The input/output pins are shown in cross-section in  FIGS. 4A and 4B , designated as  40   a  and  40   b.    
         [0028]    It will be understood that  FIG. 1  shows one example of an ionization gauge. There are many other types of ionization gauges, in which the anode, cathode and ion collector have configurations different than the configuration shown in  FIG. 1 . For example, an ultrahigh vacuum (UHV) nude Bayard-Alpert gauge may be used to measure pressure below 2×10 −11  Torr. This lower limit is achieved by modifying two elements of a standard Bayard-Alpert gauge. First, the diameter of the collector is reduced. The smaller cross-sectional area reduces the probability that x-rays created at the grid will strike the collector. Second, the helical grid structure is replaced with a fine-wire mesh grid structure, and there is also a fine-wire structure across both ends of the grid. The fine grid wires provide a more transparent grid for longer electron path lengths, and the grid ends confine the positive ions for better ion collection. Together, these two modifications cause a higher gauge sensitivity. 
         [0029]    Returning to  FIG. 1 , at an end distal from grid insulator posts  12   a ,  12   b , tubular housing  18  includes a right angle bend that continues the tubular section of housing  18  and terminates in a connecting flange, designated as  16 . The connecting flange may be coupled to a pressure chamber (not shown), in which metallic evaporation may be produced during processing of substrates(for example). 
         [0030]    The connecting flange  16  includes a tubular opening (not shown) in tubular housing  18 . This tubular opening permits gas from the pressure chamber to be introduced into the interior of housing  18  for pressure measurement by ionization gauge  10 . 
         [0031]    Since the tubular opening of gauge housing  18  permits metallic vapor to enter the ionization chamber, the present invention provides a molecular shield for a Bayard-Alpert type ionization gauge. The molecular shield reduces contamination of the ionization gauge elements (namely, the filament (cathode), the coil grid (anode), the collector wire and the grid insulators, due to metallic evaporates. 
         [0032]    One exemplary embodiment of a molecular shield is shown in  FIGS. 1 ,  2 ,  3  and  4 A, including magnet  14 . As shown, magnet  14  is a thin, flexible high-temperature magnet having dimensions of approximately 2.25 inches long by 1.0 inches wide, and 0.5 inches high. The magnet  14  is wrapped around a portion of the outer surface circumference of tubular housing  18 . 
         [0033]    The magnet  14  is positioned directly over internal grid insulator posts  12   a ,  12   b  in a direction transverse to the longitudinal direction of the grid insulator posts. The magnet  14  may be fastened around the outer surface of tubular housing  18  using any fastener. As shown in  FIG. 3 , the flexible magnetic strip is fastened to housing  18  using a 2.25 inch stainless hose clamp, designated as  20 . The magnet  14  is fastened to housing  18 , as shown, over a portion of the circumference that is closest to the grid insulator posts. 
         [0034]    The inventor has discovered that the resulting magnetic field created by magnetic strip  14  acts as a shield surrounding the collector and grid insulator posts. When molecules of a metallic vapor come near the field, they are redirected away from the grid insulators, before the metallic vapor adheres to the grid insulators and causes premature gauge failure due to current leakage to ground. 
         [0035]    It will be appreciated that the magnet may be made of any material, so long as the effective magnetic gauss intensity and the pole orientation are similar. The magnet may be made from a rigid magnetic material, if the curvature of the magnetic strip is manufactured to fit the circumferential curvature of the tubular section of housing  18 . 
         [0036]    Furthermore, the strength of the magnetic strip is dependent upon the thickness of housing  18  which typically is made from stainless steel. The strength of the magnetic field produced by magnet  14  must be sufficient to enter the interior of housing  18 . The minimum strength of the magnet is 160 gauss. 
         [0037]    The magnet may include a dry mixture of ferrite powder and rubber polymer resin, which is mixed, calendered and ground. The ground mixture may then form a strip. It is then magnetized. 
         [0038]    The magnet must be of a high temperature component, so that the magnet may withstand temperatures of 160° F. In addition, clamp  20  must be able to withstand the same temperatures without being subject to corrosion or melting. Stainless steel is a good choice for clamp  20 . 
         [0039]    In another embodiment of the present invention, magnet  14  may wrap around the entire circumferential surface of tubular housing  18 . As another embodiment, magnet  14  may be comprised of multiple small strips of magnets, oriented side-by-side, along the circumferential outer tubular surface of housing  18 . Such an embodiment is shown in  FIG. 4B , where four small magnetic strips  34   a ,  34   b ,  34   c  and  34   d  are oriented to fit around the outer tubular surface of housing  18 . 
         [0040]    It will be understood that the polar orientation of magnetic strips  34   a - 34   d  is north, south, north, south, etc., in sequence. Fewer or more numbers of magnetic strips  34  may be used by the present invention. Moreover, the magnetic strip may be made of any dimension in length and width, and of any thickness dimension. 
         [0041]    The present invention may be used in any high vacuum system where metallic evaporates inside the vacuum system accumulate on the ion gauge collector, or grid insulators. 
         [0042]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.