J.H. Leck

Performance improvements for a miniature quadrupole with a micromachined mass filter

Conventional quadrupole mass filters are usually constructed from metallic rods of length 50—225 mm and diameters 5—15 mm. In this study the conventional arrangement has been replaced with a micromachined mass filter made from silicon with Au metallised specially drawn glass fibres of length 30 mm and diameter 0.5 mm. This assembly was mounted on a vacuum flange with a VG ANAVAC ion source and a Faraday plate collector. Conventional electronics were adapted to run at 6 MHz and mass spectra in the range 0—50 amu were obtained. The results indicate a linear mass scale with 5—10% valley separation between O 2 /N 2 peaks and a best resolution at 10% peak height of 2.7 amu at mass 40. It is believed that the recent improvements in performance are due to optimisation of the electronics coupled with refinements in the micromachining technique employed. ( 1999 Elsevier Science Ltd. All rights reserved.

An evaluation of the titanium sublimation pump

Abstract Experiments have been carried out to investigate the basic characteristics of the titanium sublimator vacuum pump. For this work an apparatus has been specially designed to enable the molecular ‘sticking coefficient’ to be measured over a range from in excess of 0.99 to below 0.01. Results show that for the chemically active gases such as oxygen and carbon monoxide virtually all molecules striking a clean film, stick on their first impact. This has been observed over the temperature range 77 to 373 K. Nitrogen and hydrogen show a more complex behaviour, their sticking characteristics being more dependent upon impurities in the film and also on the temperature of the substrate, both during film deposition and gas sorption. The use of a mass spectrometer for all pressure measurements has shown that the adsorption of the active gas causes considerable desorption of molecules already held on the film surface. Methane is a good example, this gas being desorbed by all the others investigated. In a comparison of published results it is suggested that some of the considerable disagreements can be explained by replacement. This is a process which cannot fully be taken into account in experiments where the measurements of the partial pressures of the gas components in the system are not made.

Further investigations of the bistable operation of the Bayard-Alpert gauge

Abstract Observations of the influence of the electron emission current, the electron collector potential, gas composition and wall cleanliness upon the bistable operation of the Bayard-Alpert gauge are reported. This bistable behaviour is observed as a duality of manometric sensitivities and ion pumping speeds and the magnitude and variation of these with electron collector potential was measured in both modes of operation for several gases. It is concluded that it is advisable to operate gauges at electron collector potentials less than 250 V, to avoid instability and maintain optimum sensitivity.

An investigation of the factors determining maximum resolution in a quadrupole mass spectrometer

Describes a series of experiments designed to investigate the factors which limit the maximum resolution obtainable with a conventional quadrupole mass spectrometer under normal operating conditions. Included in the investigation were the effects of imperfections in mechanical construction and electrical supplies and also operation with contaminated electrode surfaces. In normal operation and with reasonable care taken in the mechanical construction and assembly, the finite length of the quadrupole rods, in this case 150 mm, was the principle factor setting an upper limit to the resolution. All the results suggested that the ions did not spend a sufficient time in the quadrupole to reach an equilibrium oscillation in the transverse AC field. Under all operating conditions resolution was found to be dependent, to a first approximation, on the number of cycles of AC field N experienced by the ions. The dependence of maximum resolution Rm on N was found to follow the simple empirical relationship, Rm=KN1.7. This formula applied for both low and high energy ions (2-32 eV) and for the mass range H2+ to Xe+.

The pumping of nitrogen in an ionization gauge

Abstract The mechanism of pumping of nitrogen in ionization gauges is discussed with reference to earlier experimental measurements and some recent observations. It is shown that the hitherto inexplicably large pumping speed of this gas, and the onset of pumping before ion production, can be resolved by postulating the chemisorption of nitrogen atoms at the glass walls. The atoms may be formed by electron impact in the gas phase, or by dissociation of excited and ionized molecules at the walls. The sorption process is shown to be somewhat analogous to the case of hydrogen pumping.

Application of a static magnetic field to the mass filter of a quadrupole mass spectrometer

Abstract We consider the case of a generalised quadrupole mass spectrometer in which a static magnetic field is applied to the body of the filter. We have solved numerically the extended Mathieu equation numerically constructing a theoretical transmission mass spectrum. The analysis shows for what values of the parameters an enhancement of the quadrupole resolution is achieved. Initial experimental results for argon confirm the predicted theoretical trends.

Operation of the quadrupole mass-spectrometer in the ac mode

Abstract The useful range of the small quadrupole mass-spectrometer has been extended to 600 amu for possible application as a monitor in high vacuum systems containing high molecular weight hydrocarbons. An important feature of the instrument is its ability to maintain a constant sensitivity over the whole operating range, at the expense, however, of a poor resolution. Because the sensitivity is high, partial pressure can be measured down to 10 −11 mbar. The instrument uses the ac mode of operation with a special collector assembly playing a major part in the filtering action.

A small analytical quadrupole mass spectrometer designed for operation in high and ultra high vacuum systems

Abstract The characteristics of a small quadrupole mass spectrometer are described in detail. These show that the requirements of an analytical instrument for use in high and ultra high vacuum systems have been met. The performance is adequate for the measurement of partial pressures down to and below 10 −11 torr. This is achieved with constant sensitivity over the range 0–200 amu with a resolution adequate to separate completely individual peaks up to 50 amu. The instrument does not have an electron multiplier. The consequent disadvantages of a relatively slow response and a limitation to the minimum detectable pressure are compensated by a higher stability and a smaller, simpler, and reliable instrument. Observations with a number of units over a period of many months indicate a maximum change in sensitivity of any one and a difference between individual instruments to be of the order of ±20%. This is therefore a gauge head of approximately the same size as a modulated BA gauge with the same sensitivity, stability and baking characteristics.

A viscosity gauge for pressure measurement in the range 10−6 to 10−3 torr☆

Abstract A pressure gauge based upon the pressure dependence of molecular damping forces on a vane swinging with a pendulum-like motion in a vacuum system is described. This instrument has a useful operating range from 10 −4 to below 10 −6 torr. The control and monitoring of the vane oscillation is entirely electronic allowing operation as either a decrement or as a direct reading gauge. In the latter case a driving force is applied to the vane so as just to balance the gas damping, thus maintaining a constant amplitude of swing. The level of the driving force is taken as a measure of the pressure. The demountable gauge head has metal gaskets making it bakeable to over 300°C and therefore suitable for incorporation in an ultrahigh-vacuum system. The main advantage of this gauge is its freedom from thermal gradients and ionization phenomena therefore making it completely inert with respect to the gases and vapours in the vacuum system.

An evaluation of the performance of gas flow meters which use a porous membrane as the control element

Results are presented on the suitability of a porous membrane as the basic element for the control and measurement of gas flow into a vacuum system. A porous plug of silicon carbide is shown to have a very low conductance, of the order of 10 −6 litres sec −1 for argon, and to be stable over a long period of time. The Knudsen laws of molecular flow are obeyed up to pressures in excess of 100 torr, ie flow is proportional to pressure difference up to at least 100 torr for most gases, and also inversely proportional to the square root of the gas molecular weight. It is estimated that a high accuracy of measurement, to the order of ±1 per cent can be maintained for flow rates down to 10 −8 litres torr sec −1 .