THERMAL CONDUCTIVITY GAUGE

This invention relates to a thermal conductivity gauge. The invention finds particular use as a pressure gauge for use in measuring a sub-atmospheric pressure.

One well-known type of thermal conductivity 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 filament of a Pirani gauge typically comprises a wire carried on a suitable support to minimise loss of heat from the wire by conduction. For example, 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 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.

In the Pirani gauge, the filament provides one arm of a Wheatstone bridge circuit. The gauge may be operated in either a constant temperature or a constant voltage mode. In the former 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. In the latter mode, the variation with gas pressure of the electrical imbalance of the bridge acts as a measure of the degree of vacuum.

A known Wheatstone bridge circuit of a Pirani gauge operated in a constant temperature mode is illustrated in Figure 1. This bridge circuit 100 has the usual four resistances R ₁ , R ₂ , R ₃ and R ₄ , each provided on a respective arm of the bridge circuit 100, and where R ₁ , R3 and R ₄ are fixed resistances and R ₂ is the

resistance of the filament of the Pirani gauge. The balanced condition of the bridge circuit 100 is given by the equation R ₂ = Ri .R ₄ / R ₃ .

In use, the operational amplifier 102 applies a voltage V ₀ to the top of the bridge circuit 100. Assuming that the bridge circuit 102 is initially balanced at a certain pressure, then a variation in the resistance of the filament, due to a variation in the rate at which the filament loses heat to its surroundings causing a reduction in the temperature, will cause the bridge circuit 100 to become unbalanced. This change in the resistance of the filament will introduce a positive error voltage at the input of the amplifier 102. After amplification of this voltage, the signal from the amplifier 102 adjusts the bridge voltage V ₀ and hence also the current through the filament so that the temperature of the filament is adjusted and the bridge balance is restored. In this mode of operation, calibration of the gauge will allow conversion of the bridge voltage V ₀ to pressure.

Thus, in a constant temperature mode, the filament is heated until its hot resistance achieves a balanced bridge. If the manufacturing tolerances to which the filaments are made are relatively low, say around ±2%, the variable resistance of the filaments can mean that the operating temperature can vary between filaments, typically by as much as 40 ⁰ C. Variation in the operating temperature between filaments can result in a variation in the sensitivity of the filaments to pressure changes, which can result in a variation in the accuracy of the vacuum measurements between gauges.

To achieve a constant operating temperature across all manufactured gauges, the filaments have to be manufactured to a very tight resistance tolerance, typically less than ±0.5%. This can make manufacture of the filaments particularly labour intensive, and can increase the overall cost of the gauge.

In order to address this problem, the present invention provides a thermal conductivity gauge comprising a filament in one arm of a Wheatstone bridge circuit, and means, preferably a potentiometer, for varying the resistance of a

diagonally opposite arm of the bridge circuit to set the bridge voltage to a predetermined value and thereby set the temperature of the filament to a predetermined value.

By controlling the bridge voltage in this manner, the temperature of the filament can be maintained at or around a predetermined value irrespective of the filament resistance. Consequently, the manufacturing tolerances of the filaments can be relaxed, which can reduce the level of labour skills required to manufacture the filaments to the hitherto required tight tolerances, and thereby reduce the overall cost of the gauge.

In one arrangement, the potentiometer is a manually adjustable potentiometer, which may form part of an analogue circuit for providing a variable resistance of the bridge circuit. In another arrangement, the potentiometer is a digital potentiometer, the gauge comprising a controller for adjusting the potentiometer to control the bridge voltage and thereby control the operating temperature of the filament. Therefore, the present invention also provides a thermal conductivity gauge comprising a filament in one arm of a Wheatstone bridge circuit, a digital potentiometer in a diagonally opposite arm of the bridge circuit, and a controller for adjusting the resistance of the potentiometer to set the bridge voltage to a predetermined value and thereby set the temperature of the filament to a predetermined value.

The gauge preferably comprises a comparator, for example an operational amplifier, for balancing the bridge circuit.

The gauge preferably comprises a temperature compensator in an arm of the circuit located adjacent the arm of the filament for adjusting the current passing through the filament with variation in ambient temperature.

The gauge is arranged to produce an electrical output signal representative of gas pressure adjacent the filament.

By way of example, preferred embodiments of the invention will now be further described with reference to the following figures in which:

Figure 1 illustrates a known circuit of a thermal conductivity gauge;

Figure 2 illustrates a first embodiment of a circuit of a thermal conductivity gauge;

Figure 3 illustrates a second embodiment of a circuit of a thermal conductivity gauge; and

Figure 4 illustrates the variation with manufacturing tolerance of the operating temperature of a filament of a thermal conductivity gauge at a constant pressure.

With reference first to Figure 2, a first embodiment of a thermal conductivity gauge comprises a Wheatstone bridge circuit 10 comprising fixed resistors R1 , R2 and R3, and a filament 12 each disposed in a respective arm 14, 16, 18, 20 of the bridge circuit 10. As is known, the filament 12 can be formed in a number of different configurations, for example a single or double length of a straight wire, a single or a double length of coiled wire and can be made from various materials such as tungsten, platinum and platinum alloys, nickel and nickel alloys. The bridge circuit 10 further includes a variable potentiometer 22 in arm 16 of the bridge circuit 10, where arm 16 is diagonally opposite the arm 20 in which the filament 12 is located. In this embodiment, the potentiometer 22 is a manually adjustable potentiometer, which provides with resistor R2 an analogue circuit for providing a variable resistance of the bridge circuit. A temperature compensator 24 is located in arm 18 of the bridge circuit 10, arm 18 being located adjacent arms 16 and 20 of the bridge circuit 10.

A comparator or operational amplifier 26 receives a supply voltage VSUPPLY and serves to keep the bridge balanced by adjusting the bridge voltage V ₀ to maintain the filament 12 at a constant resistance.

The bridge circuit 10 is calibrated by exposing the filament 12 to a known pressure. The potentiometer 22 is adjusted to set the bridge voltage V ₀ to a predetermined value. Consequently, irrespective of the resistance of the filament 12, the temperature of the filament 12 is set to a predetermined operating temperature T _op . As the pressure to which the filament 12 is exposed varies, the operational amplifier 26 adjusts the bridge voltage V ₀ from the predetermined value in order to maintain the filament 12 at or around T _op . The variation in V ₀ can enable the pressure of the atmosphere to which the filament 12 is exposed to be monitored. The temperature compensator 24 serves to vary the resistance in the arm 18 of the bridge circuit 10 with ambient temperature, so that the operating temperature of the filament can be maintained at a fixed temperature above the ambient temperature.

A second embodiment of a thermal conductivity gauge comprises a Wheatstone bridge circuit 10' illustrated in Figure 3. The second embodiment varies from the first embodiment in that the manually adjustable potentiometer 22 of the first embodiment has been replaced by a digital potentiometer 28 that is controlled by a controller 30. During calibration, the controller 30 monitors the bridge voltage V ₀ , and adjusts the digital potentiometer 28 to set the bridge voltage to the predetermined value. As is known, a digital potentiometer generally comprises an array of switches that can each engage a respective resistor. In response to a signal received from the controller 30, the digital potentiometer activates selected ones of the switches so that the digital potentiometer has the desired resistance.

Figure 4 is a graph illustrating the variation with manufacturing deviation of the operating temperature of a filament of a thermal conductivity gauge at a constant pressure. Trace 32 illustrates the variation of the operating temperature of the filament with manufacturing deviation in the prior thermal conductivity gauge of Figure 1 , whilst trace 34 illustrates the variation of the operating temperature of a filament in the gauge of Figure 2 or Figure 3. As illustrated by the graph, the first and second embodiments of the gauge can set the operating temperature of the

filament to a predetermined value, in this example just below 100 ⁰ C, irrespective of variation in the manufacture of the filament 12.