Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L21/00—Vacuum gauges
  • G01L21/10—Vacuum gauges by measuring variations in the heat conductivity of the medium, the pressure of which is to be measured
  • G01L21/12—Vacuum gauges by measuring variations in the heat conductivity of the medium, the pressure of which is to be measured measuring changes in electric resistance of measuring members, e.g. of filaments; Vacuum gauges of the Pirani type

Definitions

• This invention relates to a pressure gauge.
• Pirani gauge One well-known type of pressure gauge is a Pirani gauge. Such gauges are used for measuring the pressure of a gas by means of a heated filament of which the temperature is measured in terms of its electrical resistance. The rate at which the filament loses heat to its surroundings, is a function of the gas pressure, and hence may be used to permit the gauge to measure vacuum.
• the filament is in one arm of a Wheatstone bridge circuit.
• the gauge may be operated in either a constant temperature or a constant voltage mode.
• the power supplied to keep the filament at a constant temperature varies with changes in gas pressure, and hence this power acts as a measure of the degree of vacuum.
• the variation with gas pressure of the electrical imbalance of the bridge acts as a measure of the degree of vacuum.
• the filament of a Pirani gauge typically comprises a wire carried on a suitable support to minimise loss of heat from the wire by conduction.
• the BOC Edwards APG-MP Pirani gauge contains a filament formed from a platinum/rhodium alloy.
• the use of materials such as platinum or Pt/Rh alloy for the wire enables the gauge to measure pressures down to 10 '3 mbar in corrosive environments typically encountered in semiconductor processing applications.
• the BOC Edwards APG-L Pirani gauge contains a coiled filament formed from gold- coated tungsten, which is able to measure pressures down to 10 "4 mbar.
• a corrosive medium such as fluorine
• the present invention provides a thermal conductivity pressure gauge comprising a heated, coiled filament formed from an alloy comprising platinum and iridium, the amount of platinum in the alloy being at least 70%, preferably approximately 90% platinum and 10% iridium.
• the mechanical properties of the Pt/lr alloy enable the wire to be coiled relatively tightly, whilst the thermal properties of the alloy enable the gauge to measure pressure down to 10 "4 mbar.
• the chemical properties of the alloy provide superior corrosion resistance in comparison to conventional gauges in which the filament is formed from tungsten or gold-coated tungsten.
• the present invention can provide a relatively low cost pressure gauge that is suitable for use in semiconductor manufacturing applications where pressures as low as 10 '4 mbar need to be measured in a corrosive environment, and which has superior reliability in comparison to conventional Pirani gauges typically provided for this purpose.
• Figure 1 illustrates a view of one embodiment of a head of a Pirani gauge
• Figure 2 illustrates a view of a second embodiment of a head of a Pirani gauge
• Figure 3 illustrates a control circuit incorporating the filament of the gauge head
• Figure 4 illustrates the variation with time of the reading from a number of different gauges located in a fluorine environment.
• a first embodiment of a head of a Pirani gauge comprises a corrosion resistant, electrically insulating base 10, formed for example, from glass or ceramic, which forms part of an envelope (not shown) adapted to communicate with the environment to be monitored.
• a corrosion resistant, electrically insulating base 10 formed for example, from glass or ceramic, which forms part of an envelope (not shown) adapted to communicate with the environment to be monitored. Examples of such an environment include an enclosure under evacuation or maintained at a low pressure, or the exhaust of a vacuum pump.
• One end of a corrosion resistant, rod 12 electrically insulated from the filament is embedded in the base 10 made from an insulating material.
• An electrically insulating bobbin 14 is attached to the other end of the rod 12.
• Two electrically conducting metal pins 16, 18 are also embedded in the base 10. The lower (as shown) ends of pins 16, 18 provide connecting pins to a Wheatstone bridge circuit 20, shown in Figure 3.
• a filament 22 is spot welded to the upper ends of the pins 16, 18.
• the filament 22 is formed from a single length of coiled wire made of an alloy of platinum and iridium. In this embodiment, the filament is made of 90/10 Pt/lr alloy, containing 90% Pt and 10%lr (by weight).
• the filament 22 passes over the bobbin 14, preferably within a circumferential groove formed in the bobbin, such that the filament 22 is retained under slight tension between the pins 16, 18 in a "V" configuration.
• the Pirani gauge head comprises a similar electrically insulating base 10 to the first embodiment.
• a corrosion resistant, electrically conducting rod 24 is embedded in the base 10.
• the lower (as shown) end of the rod 24 provides a first connecting pin for the Wheatstone bridge circuit 20.
• the upper end of the rod 24 has an "L" shaped portion relative to the main portion of the rod 24.
• a metal pin 26 is also embedded in the base 10, the lower (as shown) end of the pin 26 providing a second connecting pin to the Wheatstone bridge circuit 20.
• a filament 28 is spot welded to the upper end of the pin 26 and the upper end of the rod 24. Similar to the first embodiment, the filament 28 is formed from a single length of coiled wire made of an alloy of platinum and iridium. In this embodiment also, the filament is made of 90/10 PI/I r alloy.
• the Wheatstone bridge circuit 20 has the usual four resistances Ri, R2, R3 and R 4 , each provided on a respective arm of the bridge circuit 20, and where Ri, R 3 and R 4 are fixed resistances and R 2 is the resistance of the filament of the Pirani gauge.
• the operational amplifier supplies the bridge with voltage ensuring that the bridge remains balanced at all time.
• the hot resistance of the filament remains constant. This is governed by the constant resistance of the other arms of the bridge.
• the operational amplifier will increase the voltage to the bridge to ensure its balance. In this mode of operation, the supplied voltage will be used as a pressure indicator. Calibration of the gauge will allow conversion of the supply voltage to pressure.
• the filament of the gauge is formed from an alloy of platinum and iridium, the preferred composition being 90/10 Pt/lr, although different percentages, by weight, of Pt and Ir may be provided.
• this alloy enable a wire to be coiled sufficiently tightly to enable the filament to replace tungsten or gold-coated tungsten filaments typically used to measure pressures down to 10 '4 mbar, whilst the chemical properties of the alloy provide improved corrosion resistance in comparison to conventional filaments formed from W or Au- coated W.
• Figure 4 is a graph showing the results of tests performed using five Pirani gauges located for around 19 days within an atmosphere containing 20% fluorine at atmospheric pressure. Three of these gauges were gauges according to the second embodiment, as illustrated in Figure 2, each having a 90/10 Pt/lr filament. The fourth gauge was similar to the first three gauges, except that the filament comprised a Au-coated W filament, and the fifth gauge was again similar to the first three gauges, but with the exception here that the filament comprised a VV filament.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measuring Fluid Pressure (AREA)

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• 2004
  • 2004-07-26 GB GBGB0416633.6A patent/GB0416633D0/en not_active Ceased
• 2005
  • 2005-07-12 WO PCT/GB2005/002722 patent/WO2006010884A1/en active Application Filing
  • 2005-07-12 KR KR1020077001730A patent/KR20070045201A/ko not_active Application Discontinuation
  • 2005-07-12 EP EP05759334A patent/EP1771710A1/de not_active Withdrawn
  • 2005-07-12 JP JP2007523139A patent/JP2008507708A/ja not_active Abandoned

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