Method and apparatus for maintaining emission capabilities of hot cathodes in harsh environments

Abstract

A method and apparatus for operating a multi-hot-cathode ionization gauge is provided to increase the operational lifetime of the ionization gauge in gaseous process environments. In example embodiments, the life of a spare cathode is extended by heating the spare cathode to a temperature that is insufficient to emit electrons but that is sufficient to decrease the amount of material that deposits on its surface or is optimized to decrease the chemical interaction between a process gas and a material of the at least one spare cathode. The spare cathode may be constantly or periodically heated. In other embodiments, after a process pressure passes a given pressure threshold, plural cathodes may be heated to a non-emitting temperature, plural cathodes may be heated to a lower emitting temperature, or an emitting cathode may be heated to a temperature that decreases the electron emission current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIG. 1 is a perspective view of an embodiment of a hot-cathode ionization gauge employing two cathodes;

FIG. 2 is a circuit block diagram of an embodiment of a hot-cathode ionization gauge control electronics;

FIG. 3 is a table illustrating different modes of operation of an embodiment of a hot-cathode ionization gauge employing two cathodes; and

FIG. 4 is a cross-sectional view of an embodiment of a triode gauge employing two cathodes.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

FIG. 1 is a perspective view of a hot-cathode ionization gauge 100 employing two cathodes 110, 115 according to one embodiment. The hot-cathode ionization gauge 100 includes a cylindrical wire grid 130 (i.e., anode) defining an ionization volume 135 (i.e., anode volume). Two collector electrodes 120, 125 are disposed within the ionization volume 135 and the two cathodes 110, 115 are disposed external from the cylindrical wire grid 130. The above elements of the hot-cathode ionization gauge 100 are enclosed within a tube or envelope 150 that opens into a process chamber via port 155. The hot-cathode ionization gauge 100 also includes a shield 140, such as a stainless steel shield, to shield various electronics components of the ionization gauge from ionized process gas molecules and atoms and other effects of charged particles.

An ionization gauge controller (not shown) may heat one cathode 110 (e.g., an “emitting” cathode) to a controlled temperature of about 2000 degrees Celsius to produce a specified electron emission current, such as 100 μA or 4 mA. The ionization gauge controller may not heat the other cathode 115 (e.g., a “non-emitting” or “spare” cathode) so that it may be used as a spare when the emitting cathode becomes inoperative. However, as described above, the electron emission characteristics of the spare cathode may degrade and the spare cathode may eventually become inoperative because gaseous products from a process in a vacuum chamber or sputtered material from the gauge may deposit on the spare cathode or process gasses may react with the spare cathode material.

In one embodiment, the spare cathode is instead heated to a temperature above room temperature while the emitting cathode is heated to emit electrons from the cathode surface. The spare cathode is heated to a temperature that is sufficient to evaporate any material that coats or deposits on the spare cathode and to decrease chemical interactions between the spare cathode and process gasses. The spare cathode, for example, may be heated to a temperature between about 200 to 1000 degrees Celsius depending on the process environment to which the spare cathode is exposed while the emitting cathode is operated. As a result, the spare cathode is maintained in a nearly clean condition and is ready to be used as a spare should the emitting cathode become inoperative.

The spare cathode, however, is heated to a temperature that is significantly less than the emitting temperature so that the spare cathode does not wear out for metallurgical reasons, such as embrittlement from grain growth due to long operation at these high temperatures. Also, there are optimum temperatures to decrease or prevent chemical poisoning of the spare cathode depending on the process gases. Thus, by heating the spare cathode to an optimum temperature above room temperature but significantly less than the emitting temperature, the overall operation and life of the ionization gauge is enhanced.

FIG. 2 is a circuit block diagram of hot-cathode ionization gauge circuitry 200 that may be used to operate two cathodes 110, 115 according to one embodiment. An output of a first switch 232 connects to a first end of a first cathode 110 and an output of a second switch 234 connects to a first end of a second cathode 115. A power supply 213 connects to and may supply a bias voltage to both a second end of the first cathode 110 and a second end of the second cathode 115. A heating control unit 242 and an emission control unit 244 both connect to respective inputs of the first switch 232 and the second switch 234.

The heating control unit 242 receives a voltage signal V_iH that represents a desired temperature to heat either or both cathodes 110, 115. The voltage signal V_iH may be provided by a pre-programmed processor (not shown) or by an operator via a processor (not shown). The heating control unit 242 then heats either or both cathodes 110, 115 to the desired temperature by providing a heating current i_H to either or both cathodes 110, 115 via the first switch 232 and the second switch 234, respectively.

The emission control unit 244 receives a voltage signal V_iE that represents a desired electron emission current to emit from either or both cathodes 110, 115. The emission control unit 244 then provides an electron emission current i_E to either or both cathodes 110, 115 via the first switch 232 and the second switch 234, respectively. Because the processes described above may degrade As a result, either or both cathodes 110, 115 may heat to a temperature that is significantly greater than the desired temperature regulated by the heating control unit 242.

A first switch logic unit 222 and a second switch logic unit 224 communicate with and control the first switch 232 and the second switch 234, respectively. The first switch logic unit 222 controls the first switch 232 to connect the first cathode 110 to either the heating control unit 242 or the emission control unit 244. Likewise, the second switch logic unit 224 controls the second switch 234 to connect the second cathode 115 to either the heating control unit 242 or the emission control unit 244. The first switch logic unit 222 and the second switch logic unit 224 may be implemented as computer instructions executed in an ionization gauge processor.

FIG. 3 is a table 300 illustrating different modes of operation of a dual-filament hot-cathode ionization gauge according to one embodiment. The column labeled “Cathode” (311) indicates the cathodes being operated. In this embodiment, “Cathode 1” and “Cathode 2” (e.g., the first cathode 110 and the second cathode 115 in FIG. 2) are being operated. The columns labeled I-IV (323-329) indicate example modes of operation of the cathodes or “cathode status options” (311). In mode I (323), Cathode I is heated to a temperature to emit electrons from its surface and is thus labeled an “emitting” cathode. Cathode 2, however, is only heated so that it does not emit electrons and thus is labeled a “heated only” cathode.

In mode 11 (325), the cathodes switch roles: Cathode 2 is the “emitting” cathode and Cathode 1 is the “heated only” cathode. In mode III (327), both Cathode 1 and Cathode 2 are operated as “heated only” cathodes. Finally, in mode IV (329), both Cathode 1 and Cathode 2 are operated as “emitting” cathodes. In all modes, Cathode 1 and/or Cathode 2 can be operated at either low emission to reduce sputtering of ionization gauge components or at standard emission. For example, in mode IV (329), Cathode 1 and Cathode 2 may be heated to a first temperature to provide 4 mA of electron emission current when a process pressure is in the range of ultra high or high vacuum. If the process pressure increases and exceeds a given pressure threshold, such as 1×10⁻⁵ Torr, Cathode 1 and Cathode 2 may be heated to 20 μA to reduce the sputtering of ionization gauge components as described above. If the process pressure then decreases and passes another given pressure threshold, such as 5×10⁻⁶ Torr, Cathode 1 and Cathode 2 may again be heated to 4 mA.

In various embodiments, the ionization gauge controller may heat the spare cathode in several ways. First, the ionization gauge controller may maintain the spare cathode at a constant temperature that is lower than the temperature of the emitting cathode. Second, the ionization gauge controller may power the spare cathode with periodic voltages, i.e., pulsed, duty-cycled, or alternating, to heat the spare cathode to a temperature that is less than the temperature of the emitting cathode. This further increases the lifetime of the spare cathode because it is heated less often than if the spare cathode was maintained at a constant temperature.

Third, the ionization gauge controller may alternate between maintaining the spare cathode at a constant temperature and periodically heating the spare cathode to a constant temperature. For example, at high pressures, where the emitting function of the spare cathode is more prone to being degraded by process gases, the ionization gauge controller could heat the spare cathode to the constant temperature, and at low pressures, where the spare cathode is less prone to being degraded by process gases, the ionization gauge controller could periodically heat the spare cathode.

In some applications, a process may continue up to 100 mTorr or 1 Torr, after the ionization gauge turns off. When the ionization gauge is turned off, there is no longer any sputtering of the tungsten or stainless steel because there are no ions being generated which bombard surfaces and sputter the metal off. However, both cathodes continue to be exposed to contaminating process gases that can deposit on the cathodes or chemically react with the cathode. Thus, in another embodiment, if the ionization gauge turns off and the process pressure passes or exceeds a given pressure threshold, both cathodes may be heated to a temperature that is not sufficient to emit electrons from both cathodes. In this way, the cathodes are maintained free of contaminating process gases that may deposit on the cathodes. For example, after the ionization gauge turns off at 10 or 20 mTorr, the ionization gauge controller may heat both the spare and emitting cathodes to the non-emitting temperature until the process environment reaches a higher pressure level, such as 100 mTorr or 1 Torr.

In another embodiment, an emission control unit (e.g., the emission control unit 244 in FIG. 2) may reduce the power provided to heat the emitting cathode in order to decrease the electron emission current from the emitting cathode at higher pressures. Reducing the electron emission current at higher pressures reduces the quantity of ions produced and, as a result, reduces sputtering and its effects on the surfaces of the ionization gauge. In an example embodiment, the electron emission current may be reduced from 100 μA to 20 μA at high pressures. The emission control unit may also reduce the power provided to heat two or more cathodes, such as the emitting cathode 110 and the spare cathode 115.

FIG. 4 is a cross-sectional view of an embodiment of a non-nude triode gauge 400 which also employs two cathodes 110, 115. The non-nude triode gauge 400 includes two cathodes 110, 115, an anode 130 which may be configured as a cylindrical grid, a collector electrode 120 which may also be configured as a cylindrical grid, feedthrough pins 470, feedthrough pin insulators 475, an enclosure 150, and a flange 460 to attach the gauge to a vacuum system. The anode 130 defines an anode volume 135. Thus, the triode gauge 400 includes similar components and operates in a similar way as the standard B-A gauge described above with reference to FIG. 1, but the triode gauge's cathodes 110, 115 are located within the anode volume 135 and the triode gauge's collector 120 is located outside of the anode volume 135. The methods and control circuitry described above with reference to FIG. 2 and FIG. 3 may be applied to the two cathodes 110, 115 of the triode gauge 400 in order to extend its operational lifetime.

Alternating between turning on one cathode and turning off the other may increase the life of the cathodes by about 1.1-1.2 times in certain applications. However, embodiments of the ionization gauge presented herein may increase the life of the cathodes in certain applications by a significant factor up to nearly double.

An additional advantage of the above embodiments is that the existing components of the multi-cathode ionization gauge tube do not have to be changed. The control algorithm for operating the cathodes may simply be changed such that the spare cathode is heated to a temperature less than the temperature of the emitting cathode.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

It should be understood that all or a portion of the methods or elements disclosed above may be implemented in hardware, software, firmware, or any combination thereof.

It should also be understood that more than two cathodes, more than one collector, and more than one anode of varying sizes and shapes may be employed in example ionization gauges according to other embodiments.

Claims

1. An ionization gauge, comprising: at least two cathodes;an anode defining an anode volume;an ion collector electrode; andcontrol circuitry coupled to the at least two cathodes and configured to heat at least one cathode to a first temperature and configured to heat at least one other cathode to a second temperature that is insufficient to emit electrons from the at least one other cathode.

2. The ionization gauge of claim 1, wherein the ion collector electrode is disposed inside of the anode volume and the at least two cathodes are disposed outside of the anode volume.

3. The ionization gauge of claim 1, wherein the ion collector electrode is disposed outside of the anode volume and the at least two cathodes are disposed inside of the anode volume.

4. The ionization gauge of claim 1, wherein the first temperature is sufficient to emit electrons from the at least one cathode and the ion collector electrode is configured to collect ions formed by impact between the electrons and gas atoms and molecules.

5. The ionization gauge of claim 4, wherein the second temperature is between about 200° C. and 1000° C.

6. The ionization gauge of claim 4, wherein the control circuitry alternates between: (i) heating the at least one cathode to the first temperature and the at least one other cathode to the second temperature and (ii) heating the at least one other cathode to the first temperature and the at least one cathode to the second temperature.

7. The ionization gauge of claim 4, wherein the second temperature is a variable temperature.

8. The ionization gauge of claim 4, wherein the control circuitry constantly heats the at least one other cathode to the second temperature.

9. The ionization gauge of claim 4, wherein the control circuitry periodically heats the at least one other cathode to the second temperature.

10. The ionization gauge of claim 4, wherein the control circuitry alternates between: (i) constantly heating the at least one other cathode to the second temperature and (ii) periodically heating the at least one other cathode to the second temperature.

11. The ionization gauge of claim 4, wherein the second temperature is sufficient to decrease the amount of material that deposits on the at least one other cathode or decreases the chemical interaction between a process gas and a material of the at least one other cathode.

12. The ionization gauge of claim 4, wherein the control circuitry is further configured to heat the at least one cathode to a temperature that decreases the electron emission current emitted from the at least one cathode in response to a process pressure passing a given pressure threshold.

13. The ionization gauge of claim 1, wherein the first temperature is equal to about the second temperature, the control circuitry further configured to heat the at least two cathodes to the second temperature in response to a process pressure passing a given pressure threshold or the ionization gauge turning off.

14. A method of measuring a gas pressure from gas molecules and atoms, comprising: heating at least one cathode to a first temperature to generate electrons;heating at least one other cathode to a second temperature less than the first temperature;collecting ions formed by impact between the electrons and the gas atoms and molecules in an anode volume defined by an anode.

15. The method of claim 14, wherein the second temperature is between about 200° C. and 1000° C.

16. The method of claim 14, further comprising alternating between: (i) heating the at least one cathode to the first temperature and the at least one other cathode to the second temperature and (ii) heating the at least one other cathode to the first temperature and the at least one cathode to the second temperature.

17. The method of claim 14, wherein the second temperature is a variable temperature.

18. The method of claim 14, wherein heating the at least one other cathode to the second temperature includes constantly heating the at least one other cathode to the second temperature.

19. The method of claim 14, wherein heating the at least one other cathode to the second temperature includes periodically heating the at least one other cathode to the second temperature.

20. The method of claim 14, wherein heating the at least one other cathode to the second temperature includes alternating between: (i) constantly heating the at least one other cathode to the second temperature and (ii) periodically heating the at least one other cathode to the second temperature.

21. The method of claim 14, wherein the second temperature is sufficient to decrease the amount of material that deposits on the at least one other cathode or decreases the chemical interaction between a process gas and a material of the at least one other cathode.

22. The method of claim 14, further comprising heating the at least one cathode to a temperature that decreases the electron emission current emitted from the at least one cathode in response to a process pressure passing a given pressure threshold.

23. A method of measuring a gas pressure from gas molecules and atoms, comprising: heating plural cathodes to a first temperature to generate electrons;heating the plural cathodes to a second temperature less than the first temperature in response to a process pressure passing a given pressure threshold;collecting ions formed by impact between the electrons and the gas atoms and molecules.

24. The method of claim 23, wherein heating the plural cathodes to the second temperature reduces sputtering of ionization gauge components.