Hot-cathode Gauge Research Articles

A Hot Cathode Magnetron Gauge using a Divergence-Type Magnetic Field for Electric Field Control

A new sensitive type of magnetron gauge has been developed in order to obtain a linear response of ion current, Ii, to pressure, P, with a high sensitivity over a wide range. The gauge is equipped with two ion collectors. The ratio of the ion currents of the two ion collectors is measured and kept constant by controlling the filament voltage, in order to keep the electric field constant within the anode over a wide range of pressure measurement. When a divergent-magnetic field is applied to this hot-cathode magnetron gauge, the slope of the plot of the filament voltage vs the ratio of the ion current of the two separate two ion collectors is improved from -0.28 to +1.1–+3.9 (V-1). When the diameters of the ion collectors were investigated, it was found that the current ratio η=1 was attained when the outer diameters of the ion collectors were identical, and their inner diameters were 2 and 8 mm respectively. With this configuration, ion current was directly proportional to pressure in the range of 1×10-8–1×10-4 Pa.

Comparison of the stability of hot and cold cathode ionization gauges

When ionization gauges are used as reference or secondary standards, their stabilities are the most important features. Generally, only hot cathode gauges are used as reference or secondary standards in calibration services. Some traditional reasons preventing the use of cold cathode gauges are the presence of discontinuities in the current versus pressure characteristic, the ignition delay at very low pressures, and the poor stability especially under contaminating vacuum environments. Though current inverted magnetron designs are believed to avoid some of these problems, at least under clean conditions, no “hard facts” have been published to support this claim. Here, we present the experimental results of calibrations of three hot cathode gauges and two inverted magnetrons against a primary high-vacuum standard over the range 10−7 to 10−3 Pa with gases of N2, Ar, He, and H2, respectively. During a continuous observation period of 72 h at a constant pressure of about 1×10−4 Pa, hot cathode gauges showed better stabilities than inverted magnetrons in N2, Ar, and He, but all gauges behaved similarly in H2. Repeated calibrations over a period of about 6 months showed that all gauges had similar long-term stabilities in N2, Ar, and He. For H2, however, the stability of inverted magnetrons was better than that of hot cathode gauges. For different gases, the discontinuities of inverted magnetrons occurred at different pressures, making their calibrations more elaborate.

The influence of a hot cathode vacuum gauge on the residual gas composition

The residual atmosphere of an UHV chamber is known to be influenced by the hot cathode gauge (HCG) via processes on the hot cathode and by ionisation. In the presence of such a gauge, some gases are pumped while several residuals are generated, depending on the condition of the chamber's inner surface, gauge parameters and gases that are released spontaneously or introduced on purpose. We analysed the influence of the HCG in three different cases of gas accumulation lasting typically for 1 h: (1) in the leak tight vessel, where the background outgassing from the walls was the only inflow Q backg=1.6×10 −8 mbar l H 2 s −1, (2) during the constant nitrogen inflow in the vessel and (3) during the constant deuterium inflow. Both inflow values were in the order of 1×10 −6 mbar l s −1. The absolute total pressure in the 13 l system was monitored by a capacitance manometer. After the accumulation, the composition of the accumulated gases was analysed by a quadrupole mass spectrometer (QMS), calibrated with the gases of interest. Regarding the operation of the HCG, an obvious difference is noted both in the pressure rise curves and QMS spectra. The generation of carbon-contained residuals by the HCG was evident, but the most unexpected was its pumping action: nitrogen content decreased from 99% (HCG off) to 62% (HCG on) and deuterium content decreased from 65% (HCG off) to only 2% (HCG on).

Comparison of some metrological characteristics of hot and cold cathode ionisation gauges

Five commercial vacuum gauges, including three hot cathode gauges and two inverted magnetrons, were tested against a primary high-vacuum standard over the range 10 −7–10 −3 Pa with gases of N 2, Ar, He and H 2. In a continuous observation for 72 h, hot cathode gauges had better stabilities than inverted magnetrons in N 2, Ar and He, but all gauges behaved similarly in H 2. Repeated calibrations over a period of about 6 months showed that all gauges had similar stabilities in N 2, Ar and He. For H 2, however, the stability of the inverted magnetrons was better than that of the hot cathode gauges. For different gases, the discontinuities of the inverted magnetrons occurred at different pressures. For the hot cathode gauges the changes in relative sensitivities with pressure were small in a wide pressure range, but for the inverted magnetron gauges the changes were strongly pressure dependent. The starting times of the inverted magnetrons varied with pressures, gases and ionisation sources in the system. For the inverted magnetrons, the outgassing rates were much lower than that of the hot cathode gauges, but their pumping speeds were similar to that of a hot cathode gauge with an emission current of 10 mA. Some reasons, which influenced the metrological characteristics of the gauges tested, are also discussed.

Investigation and reduction of spurious peaks caused by electron-stimulated desorption and outgassing by means of a grid heating method in a hot-cathode quadrupole residual gas analyzer

In the year 2001, two issues remain for extreme high vacuum (XHV) pressure measurements with hot-cathode ionization gauges: measurement limits caused by electron-stimulated desorption (ESD) and outgassing. The limit caused by ESD is significantly clarified and at least partially solved by means of a grid heating method in a residual gas analyzer. After system bakeout and at 10−9 Pa, when the surface of a platinum alloy grid remains contaminated with F and Cl atoms and CO molecules, the mass spectrum is dominated by spurious peaks (which set the measurement limit for a total pressure gauge) caused by ESD species of F, Cl, and O, with a yield of 10−11 (species/electron, with neutral/ion ≅0.3) and by CO molecule ESD, with a yield of 10−11 (species/electron, with CO neutral/O ion ≅18). In this condition, a spurious H peak caused by hydrogen ESD is still low. However, once the grid surface is thoroughly cleaned at over 900 °C by ohmic heating or electron bombardment, the major ESD species observed for both neutrals and ions dramatically changes to mainly a spurious H peak with a yield of 10−11 (species/electron, with neutral/ion ≅2). When the operational grid temperature is then increased to 500 °C, the spurious H peak is decreased by over 3 orders of magnitude and the spectrum becomes mainly H2 molecules, reflecting the true pressure of the system; i.e., the relative error for the pressure is reduced to 8% from 614%. The lower limit caused by outgassing in these instruments is due to the grid bulk content of hydrogen. Finding a lower-hydrogen solubility material for the grid may become the last work required for lower-limit XHV gauge development. In order to decrease the limits caused by ESD and outgassing in a hot-cathode gauge, employing ion energy filters and cold-cathode field emitters is unnecessary.

Compact wide-range cold-cathode gauges

Cold-cathode ionization gauges offer many advantages for routine high-vacuum measurements. They are simple and robust. Thermal outgassing and electron-stimulated desorption, both significant problems in hot-cathode gauges, are negligible. There are no x-ray effects to cause errors at low pressures. For many applications it would be useful to reduce the physical size of cold-cathode gauges and especially to reduce the volume in which the associated stray magnetic field is significant. We describe experiments to explore the role of the operating voltage and magnetic field strengths on the performance of cold-cathode gauges. The results allowed the design and development of compact wide-range gauges having internal volumes as low as 5–15 cm3, while maintaining a high sensitivity of 10 A Torr−1 at 10−6 Torr. The magnets are little more than one third the volume used in most conventional designs. This, combined with the use of a double inverted magnetron layout, reduces the external magnetic fields by more than an order of magnitude. At 15 cm from the center of the compact gauge, the external field is comparable with the Earth’s magnetic field. For ultrahigh vacuum applications, an internal field emitter provides quick starting. If necessary, an Americium 241 source or a carbon film cold emitter can be used for essentially instant starting. Testing in nitrogen has covered pressures from the mid-10−11 Torr range to above 10−2 Torr.

Ionization gauge errors at low pressures

Factors affecting the accuracy of ionization gauge measurements at low pressures are reviewed. In hot-cathode gauges these include electron-stimulated desorption at the electron collector, forward and reverse x-ray effects, the potential of the gauge envelope, outgassing, and various controller-related errors. In cold-cathode gauges they include nonlinearities below the “magnetron knee,” plasma instabilities, and background currents. Case studies are given to illustrate many of these sources of error and their elimination. The case studies were gathered in the course of long-term stability measurements on over 30 ionization gauges at pressures ranging from the 10−8 to 10−11 Torr ranges. The investigation included Bayard–Alpert (both conventional and modulated), extractor, inverted magnetron, double inverted magnetron, and magnetron gauges. Errors caused by outgassing from virtual leaks in all-metal seals are also considered. By far the largest and most unpredictable source of error proved to be electron-stimulated desorption in hot-cathode gauges, avoidable by operating the grid at a sufficiently high temperature. It is concluded that, with proper precautions, better then 10% reproducibility in the 10−10 Torr range is easily achievable over periods of many months with either hot-cathode or cold-cathode gauges. Over shorter periods, reproducibility of better than 1% is obtainable with well-designed hot-cathode gauges.

Cold cathode gauges for ultrahigh vacuum measurements

Nine cold cathode gauges have been evaluated on ion-pumped ultrahigh vacuum systems operating at pressures down to the 10−11 Torr range. The test gauges included magnetrons, inverted magnetrons, and double inverted magnetrons from four different manufacturers as well as experimental variable-geometry gauges built especially for this project. Spinning-rotor and extractor gauges were used for calibration. The investigation covered repeated calibration over the 10−10 to 10−4 Torr range, stability over periods of up to 35 000 h of low pressure operation, tests for discontinuities in the current-pressure characteristics, stray magnetic field measurements, susceptibility to external magnetic fields, outgassing effects, and starting behavior at very low pressures. Our conclusion is that modern cold-cathode gauges are capable of giving far more accurate results than were possible with earlier Penning-type designs. Because of their extremely low outgassing rates, and their relative freedom from x-ray and electron stimulated desorption errors, they may in practice give results at low pressures which are more accurate than those obtained with typical hot-cathode gauges.

History of ultrahigh vacuum pressure measurements

The measurement of pressure in the UHV range started in 1950 with the invention of the Bayard–Alpert gauge which reduced the lowest pressure measurable with hot-cathode gauges of the time by a factor of 200 or more. The processes causing limits to the lowest measurable pressure include the x-ray effect, electron stimulated desorption from electron bombarded electrodes, chemical effects at thermionic cathodes, and thermal desorption. This historical review traces from 1950 to the present day the development of gauges, both hot and cold cathode, designed to reduce these lower limits, and emphasizes the research directed to understanding the physical and chemical processes governing these limitations. The development of, and the limiting processes in, residual gas analysers suitable for UHV is also followed over the same period starting with the omegatron, then the magnetic sector instrument, and finally the quadrupole.

Total pressure measurement down to 10−12 Pa without electron stimulated desorption ion errors

In order to measure a total pressure as low as 10−12 Pa, the avoidance of electron stimulated desorption (ESD) ion errors is more important than the x-ray limit, because even if we can contrive to reduce ESD ions in a gauge, we do not know what fraction the ESD ion error makes in the measured pressure value. This is the largest defect in the reliability of the measurements. Therefore, the development of the ion spectroscopy gauge which avoids ESD ion errors, was recalled with a historical review of the limiting processes in a hot-cathode gauge. The most advanced ion spectroscopy gauge was constructed with a fine platinum alloy spherical grid ion source, immersed in a copper block with stainless steel flanges. Gas phase ions were effectively separated from ESD ions by the actions of electron space charge in the grid and a hemispherical 180° ion energy analyzer. The x-ray limit of the advanced gauge was approximately 2.5×10−13 Pa, and the outgassing rate of 2×10−9 Pa ℓ/s (about 1/100 that of an ordinary nude Bayard–Alpert gauge) was achieved.

Characteristics of cold cathode gauges for outgassing measurements in uhv range

Outgassing rates have been evaluated by measuring pressure with hot cathode gauges as a function of time. However, the large gas release from hot cathode gauges and the limited degassing time cause serious error in outgassing rate measurements in uhv. Although some investigations were reported on the pumping effects of the gauges with glass tubes, there are few data on the commercially available gauges in this application. In this paper, the gas release and pumping speeds of various commercially available gauges are measured quantitatively, including the characteristics of cold cathode/inverted magnetron gauges (IMGs). The maximum pumping speed of the IMGs is 0.1 ls −1, and the pumping speed at pressures in uhv is negligible. It is shown that the IMGs can be utilized in applications for low outgassing rate measurements in uhv.

Ion spectroscopy gauge: Total pressure measurements down to 10−12 Pa with discrimination against electron-stimulated-desorption ions

The hot-cathode magnetron gauge by Lafferty (which presently holds the possibility of making the lowest-pressure measurements, 10−15 Pa) and the improved Helmer gauge by Benvenuti and Hauer (which carried out the lowest-pressure measurements to date, 2.5×10−12 Pa) have been examined regarding the measurement limit due to electron-stimulated-desorption (ESD) ions. On the basis of this examination, a new hot-cathode total pressure gauge, called an ion spectroscopy gauge, which substantially avoids errors caused by ESD ions down to 10−12 Pa, has been developed. Discrimination against ESD ions is accomplished by combining three technical elements: (i) a spherical grid ion source; (ii) a spherical 180° ion energy analyzer; and (iii) burying the ion source in an aluminum alloy flange. The ion source plays an important role in the generation of a large kinetic energy difference between gas-phase ions and ESD ions, by using electron space charge. The 180° analyzer, which has a high energy resolution, promotes ion focusing and tends to reduce the x-ray limit. The burying technique reduces the outgassing rate of the hot-cathode gauge. Performance measurements of the new gauge have been carried out in a new aluminum extreme high vacuum system. The effective separation of gas-phase ions from ESD ions for two variants of the gauge, one with a molybdenum grid and one with a platinum grid, was demonstrated using oxygen dosing. The performance of the platinum-grid gauge was confirmed down to 2×10−11 Pa, essentially free from errors caused by ESD ions. The x-ray limits and sensitivities of the new gauges were: below 2×10−12 Pa with a sensitivity of 4.5×10−4 A/Pa for the molybdenum grid and 5.6×10−12 Pa with a sensitivity of 1.8×10−4 A/Pa for the platinum grid.

Measurement of x-ray currents in Bayard–Alpert type gauges

The collector current in a hot cathode gauge at low pressure is the sum of the pressure dependent ion current and a nonpressure dependent residual current. A portion of this residual term is photocurrent caused by soft x-ray production within the gauge. At ultrahigh vacuum, the photocurrent can be comparable to the ion current. For accurate measurements, it is necessary to determine the pressure equivalent of the photocurrent and subtract it from the indicated pressure. Several methods have been used to determine the x-ray limit of a gauge. Using the variation of electron energy method, also known as the Alpert method, and the comparison method, the x-ray limits were found for a commercial Bayard–Alpert (BA) gauge and several experimental nude BA gauges. X-ray limits from both methods were in good agreement for gauges with tungsten filaments. In gauges with oxide coated cathodes, agreement varied and the residual current was noted to depend upon operating history. The x-ray limit for the commercial BA gauge was found to be 3.7×10−10 Torr at 1.0 mA grid current and 2.8×10−10 Torr at 10.0 mA. For the experimental gauges, x-ray limits were about 1.5×10−10 Torr.

ＥＳＤイオン誤差を除去したＸ線限界１０｀−１２´Ｐａのイオン分光ゲージ

A new hot-cathode ionization pressure gauge, called an ion spectroscopy gauge, which avoids errors due to electron-stimulated desorption (ESD) ions down to 10-12 Pa has been developed. The elimination of ESD ion errors is accomplished by combining three technical elements, a spherical grid ion-source, a spherical 180° energy analyzer, and reduction of desorption from a hot-cathode gauge. The X-ray limit of the new gauge is below 1.3×10-12 Pa with a sensitivity of 3×10-4 (A/Pa). Performance measurements of the new gauge have been carried out in a new aluminum XHV system and confirmed down to 2×10-11 Pa.

Comparison of hot cathode and cold cathode ionization gauges

Ionization gauges are commonly used for pressure measurement from the lowest achievable pressures to 10−2 Torr. Two types exist: In the hot cathode gauge (HCG) the source of the ionizing electrons is a thermionic cathode. In the cold cathode gauge (CCG) it is a circulating space charge current of electrons trapped in crossed electric and magnetic fields. Indications of both types of gauge are gas dependent. Relative sensitivities for different gases are not the same for different HCGs or CCGs, or for HCGs compared to CCGs. Both types of gauge are susceptible to contamination. The filament heating power of the HCG is sufficient to cause local outgassing of adsorbed material, and possibly large pressure errors. Each has certain spurious effects. The lower limit of the HCG is set by x‐ray photocurrent and electron impact desorption. The CCG is subject to discontinuities in the current versus pressure characteristic. Starting of the discharge in a CCG may be delayed at ultrahigh vacuum (UHV). Pumping speeds o...

7549. Structure and morphology of thin SnO 2 films : L I Popova et al,Thin Solid Films,186, 1990, 107–112

An experimental study is described in which residual gases were analysed by means of a cyclotron resonance mass spectrometer of the omegatron type. Analysis was continuous before, during and after both (a) ion pumping with a Bayard-Alpert pattern hot-cathode gauge and (b) the dispersal of a barium getter. Results are more extensive than any published hitherto and are presented both as mass spectra and the variation of gas constitution with time throughout each stage of the above operations. The study illustrates the potentialities of the omegatron as a tool for solving many problems existing in vacuum technology. In particular, by regarding the Bayard-Alpert pattern gauge as an electron tube, it is shown that the omegatron is well suited to the analysis of residual gases during the manufacture and life of tubes.

6090. Pumping of methane by a low power hot-cathode ion pump/gage

An experimental study is described in which residual gases were analysed by means of a cyclotron resonance mass spectrometer of the omegatron type. Analysis was continuous before, during and after both (a) ion pumping with a Bayard-Alpert pattern hot-cathode gauge and (b) the dispersal of a barium getter. Results are more extensive than any published hitherto and are presented both as mass spectra and the variation of gas constitution with time throughout each stage of the above operations. The study illustrates the potentialities of the omegatron as a tool for solving many problems existing in vacuum technology. In particular, by regarding the Bayard-Alpert pattern gauge as an electron tube, it is shown that the omegatron is well suited to the analysis of residual gases during the manufacture and life of tubes.

Calibration stability of some hot-cathode ion gauges

Data are presented on the stability of calibration of some hot-cathode ion gauges: a conventional triode used in the range of 0.0002–0.2 Pa, and Bayard-Alpert-type gauges used in the range of 0.0002–0.02 Pa. Numerical magnitudes are given of the systematic corrections that must be applied to the gauge indication in order to achieve the highest accuracy of pressure indication. The relative sensitivities of these gauges to Ar, N2, Ne, and He are also given.

A report from the AVS Standards Committee: Comparison of ion gauge calibrations by several standards laboratories

Calibrations by four U.S. laboratories of four hot-cathode ion gauges, in the range 0.07–13 mPa, showed systematic differences among laboratories that were much larger than the expected error of any one calibration. They also suggested that any of the four gauges tested, if properly packaged and shipped, was able to serve as a transfer standard with probable error of 2%. A second comparison was made of the calibrations by two U.S. laboratories of some other gauges that had also been calibrated by the National Physical Laboratory, England. Results did not permit conclusive determination of whether differences were due to the laboratories or to changes in the gauges.

Ion Desorption by Electron Bombardment; Relation to Total and Partial Pressure Measurement

When electrons strike certain adsorbed gases, positive ions, neutrals, and/or excited neutrals are released. This electron impact desorption (E.I.D.) effect can cause significant errors in total pressure measurement with hot-cathode gauges and can change the striking characteristics of magnetic, cold-cathode gauges. In certain mass spectrometers, “surface ion” peaks appear in the mass spectrum resulting from E.I.D. of gases sorbed at the ion source electrodes. The general characteristics of E.I.D. and the results of measurements of E.I.D. from metal surfaces are briefly reviewed. The effects of E.I.D. on the characteristics of hot- and cold-cathode gauges and mass spectrometers are considered in some detail. Methods of minimizing errors caused by E.I.D. are discussed.