JP2008507708A6 - Pirani pressure gauge - Google Patents

Abstract

The Pirani manometer comprises a sensing element that is coiled and heated, for example 90/10 Pt / Ir, formed from an alloy containing platinum and indium. This allows the pressure gauge to be placed in a corrosive environment and ensures that pressures as low as 10 ⁻⁴ mbar can be measured over time.

Description

  The present invention relates to a pressure gauge.

  One well-known type of pressure gauge is the Pirani gauge. Such an instrument is used to measure gas pressure using a heated filament whose temperature is measured with respect to its electrical resistance. The rate at which the filament loses heat to its surroundings is a function of gas pressure and can therefore be used to allow the instrument to measure vacuum.

  In the Pirani gauge, the filament is on the side of the Wheatstone bridge circuit. The instrument can operate in either constant temperature or constant voltage mode. In the former mode, the power supplied to maintain the filament at a constant temperature varies with changes in gas pressure, and therefore this power operates as a measure of the degree of vacuum. In the latter mode, the change operates as a measure of the degree of vacuum using the gas pressure of the bridge electrical imbalance.

The Pirani gauge gauge typically includes a wire that is attached to a suitable support to minimize heat loss from the wire due to conduction. The BOC Edwards APG-MP Pirani gauge includes a filament formed from a platinum / rhodium alloy. The use of materials such as platinum or Pt / Rh alloys for the wire allows the instrument to measure pressures below 10 ^-3 mbar typically encountered in semiconductor processing applications.

Recently, there has been an increasing demand for measuring pressures as low as 10 ⁻⁴ mbar in such corrosive environments. To increase the range of the Pirani gauge, increase the filament length by coiling, thereby minimizing the increase in instrument size while reducing the effects of heat loss through the filament support It has been known. For example, the BOC Edwards APG-L Pirani gauge includes a coil filament formed from gold-coated tungsten and can measure pressures below 10 ^-4 mbar. However, the extended exposure of the filament to corrosive media such as fluorine dramatically reduces the effective life of the instrument. Replacing the filament with a filament formed from platinum or a platinum / rhodium alloy is tight enough to achieve the required filament length within the acceptable instrument size for wires formed from these materials It is not practical because it cannot be wrapped around. As a result, relatively expensive capacitance pressure gauges are typically used for this purpose.

To address this problem, the present invention is an alloy comprising platinum and iridium, wherein the amount of platinum in the alloy is at least 70%, preferably approximately 90% platinum and 10% iridium. A thermal conductivity pressure gauge with a heated coil filament is provided. In contrast to filaments formed from Pt or Pt / Rn alloys, the mechanical properties of Pt / Ir alloys allow the wire to be relatively tightly wound, while the thermal properties of the alloy are 10 ⁻ Try to measure the pressure below ⁴ mbar. It should be noted that the chemical properties of the alloy provide superior corrosion resistance compared to conventional instruments whose filaments are formed from tungsten or gold-coated tungsten.

As a result, the present invention provides a relatively low cost pressure gauge suitable for use in semiconductor manufacturing applications that require pressures as low as ^10-4 mbar to be measured in corrosive environments. And has superior reliability compared to conventional Pirani gauges typically provided for this purpose.

  Embodiments of the present invention will now be further described below by way of example with reference to the drawings.

  Referring initially to FIG. 1, a first embodiment of the head portion of a Pirani gauge includes a corrosion resistance (corrosive resistance) that electrically insulates a base 10 formed from, for example, glass or ceramic. And forms part of an envelope (not shown) adapted to communicate such that the environment is monitored. Examples of such environments include containment devices being evacuated or maintained at low pressure, or evacuation of vacuum pumps.

  One end of a corrosion-resistant rod 12 that is electrically insulated from the filament is embedded in a base 10 made of an insulating material. An electrically insulating bobbin 14 is attached to the other end of the rod 12. Two electrically conductive metal pins 16, 18 are also embedded in the base 10. The lower ends (shown) of pins 16, 18 provide connection pins for the wheelstone bridge circuit 20 shown in FIG.

  The filament 22 is spot welded to the upper ends of the pins 16 and 18. Filament 22 is formed from a single length of coiled wire made from an alloy of platinum and iridium. In this embodiment, the filament is made from a 90/10 Pt / Ir alloy containing 90% (by weight) Pt and 10% Ir. The filament 22 preferably moves the bobbin 14 within a circumferential groove formed in the bobbin so that the filament 22 is held under slight tension between the pins 16, 18 in the “V” configuration. Pass by.

  In another embodiment shown in FIG. 2, the Pirani gauge head comprises an electrically insulating base 10 similar to the first embodiment. The corrosion-resistant electrically conducting rod 24 is embedded in the base 10. The lower end (shown) of rod 24 provides a first connection pin to Wheatstone bridge circuit 20. The upper end of the rod 24 has an “L” shaped portion with respect to the main portion of the rod 24. Similar to the first embodiment, the metal pin 26 is also embedded in the base 10 and the lower end (shown) of the pin 26 provides a second connection pin to the Wheatstone bridge circuit 20. The 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 from an alloy of platinum and iridium. Also in this example, the filament is made from a 90/10 PI / Ir alloy.

  Referring to FIG. 3, the Wheatstone bridge circuit 20 has four conventional resistors Ri, R2, R3 and R4, each provided on a respective side of the bridge circuit 20, where Ri, R3 and R4 are It is a fixed resistance, and R2 is the resistance of the filament of the Pirani gauge. The equilibrium condition of the bridge circuit 20 is given by R2 = Ri · R4 / R3.

  In use, the operational amplifier supplies a voltage to the bridge that ensures that the bridge remains balanced at all times. In this case, the hot resistance of the filament remains constant. This is governed by a certain resistance on the other side of the bridge. As the pressure in the instrument head increases, for example, heat loss from the filament increases. The operational amplifier increases the voltage on the bridge to ensure its balance. In this mode of operation, the supply voltage is used as a pressure indicator. Instrument calibration allows conversion of supply voltage to pressure.

In both embodiments described above, the instrument filament is formed from an alloy of platinum and iridium, and a preferred composition is 90/10 Pt / Ir, although different proportions of Pt and Ir weights may be provided. . The mechanical properties of this alloy allow the wire to be substantially replaced so that the filament can replace the tungsten or gold painted tungsten filament typically used to measure pressures below 10 ^-4 mbar. While being tightly wound, the chemical properties of the alloy provide improved corrosion resistance compared to conventional filaments formed from W or W with Au coating.

  For example, FIG. 4 is a graph showing the results of tests performed using five Pirani gauges that were placed for 19 days in an atmosphere containing 20% fluorine at atmospheric pressure. Three of these instruments are instruments according to the second embodiment illustrated in FIG. 2, each having a 90/10 Pt / Ir filament. The four instruments are the same as the first three instruments, except that the filaments were provided with Au-coated W filaments, and the five instruments are now provided with W filaments. Is the same as the first three instruments again.

  As indicated by each of arrows 35 and 38 in FIG. 4, the readout from a meter containing a conventional W or Au coated W filament is at least 2 after 5 days exposure due to filament degradation due to fluorine atmosphere. Bolt fell. In contrast, readouts from three instruments containing Pt / Ir filaments are indicated by arrows 40 and remain substantially constant over all test times, indicating excellent corrosion resistance of Pt / Ir alloys.

Fig. 6 illustrates the display of an embodiment of the head portion of a Pirani gauge. Fig. 6 illustrates the display of the second embodiment of the head part of the Pirani gauge. Fig. 4 illustrates a control circuit incorporating an instrument head filament. FIG. 6 illustrates the time variation of readout from a number of different instruments located in a fluorine environment.

Claims (6)

A thermal conductivity pressure gauge with a coiled heated filament formed from an alloy comprising platinum and indium,
A thermal conductivity pressure gauge wherein the amount of platinum in the alloy is at least 70%.   The pressure gauge of claim 1, wherein the alloy comprises approximately 90% platinum and 10% iridium.   3. The pressure gauge according to claim 1, further comprising means for supplying an electric current to the filament to heat the filament.   The pressure gauge according to claim 1, wherein the filament is formed in a non-linear arrangement.   The pressure gauge according to claim 4, wherein the filament has a substantially V shape.   A pressure gauge according to any one of the preceding claims, wherein the filament forms one side of an electrical bridge arranged to generate an electrical output signal representative of the gas pressure in the pressure gauge.

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