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Conference on Fundamental Problems of Low Pressure Measurements impurity content of metals and semiconductors using the vacuum fusion method for extraction. For small samples, weighing less than 1 g and having a total impurity content of < 1 ppm, a mixture of hydrogen, nitrogen and carbon monoxide (for oxygen) amounting to 1.10 -5 ml at NTP may be extracted. With such small quan- tities of gas available an omegatron mass spectrometer was chosen for its apparent simplicity and small volume. An account of the calibration and reproducibility experiments performed on this device is given. A bakeable vacuum system of 3 1. volume containing a liquid nitrogen trap, Bayard-Alpert ionization gauge and omega- tron was used. This was pumped by a trapped mercury diffusion pump backed by an oil rotary pump. It was possible to isolate the pumps from the system by a bakeable liquid indium valve. After baking overnight at 400°C under vacuum the total pressure in the system had fallen to 1.10 -8 tort when the bakeable liquid-nitrogen trap was filled. A gas inlet system incorporating a Toepler pump in which the gas mixtures were made up enabled them to be added to the system in discrete steps. It was concluded that the omegatron could satisfactorily analyse mixtures of inert gases but that a re- action between oxygen and carbon on the hot filaments in the system prevented the quantitative analysis of gas mixtures contain- ing oxygen. Only a qualitative analysis could be obtained when mixtures containing carbon monoxide were examined. M T Lilburne, Metallurgy Division, National Physical Laboratory Evaluation of a low temperature emitter, lanthanum hexaboride on rhenium for use in demountable total and partial pressure measuring gauges This paper described an experimental study of the thermionic emission properties of a rhenium filament coated with lanthanum hexaboride. Particular regard has been paid to the possibility of using it as an emitter for total pressure measuring gauges and for mass spectrometers used in vacuum systems where the filament may frequently be exposed to the atmosphere or operated in high pressures of contaminating gases. Tungsten has generally been used in such applications, but the elevated operating temperature required coupled with the high degree of reactivity exhibited by tungsten in the presence of several of the gases commonly encoun- tered in vacuum systems limit its usefulness. This applies particu- larly in the field of gas analysis where appreciable and variable modification of the sample may occur in the ion source, thereby making the estimation of true partial pressures difficult and un- reliable. The oxide coated cathode, which operates at a substan- tially lower temperature and is much less reactive, is a far more acceptable emitter but, unfortunately, it is in general quite unsuit- able for use in demountable devices since, on exposure to a moist atmosphere, hydration occurs with complete loss of the coating by flaking. In the search for alternative emitters capable of meeting these rather exacting requirements Lafferty has studied the hexa- borides of various alkaline-earth and rare-earth metals. Lafferty found that whilst a satisfactory cathode could be made if the lan- thanum hexaboride were in the form of a self-supporting rod, difficulties occurred when, as is more generally required, a directly- heated emitter was formed by applying the material to a refractory metal. These difficulties resulted from a strong tendency on the part of the boron atoms to diffuse into the base metal, causing collapse of the boron cage structure and thereby allowing rapid evaporation of the lanthanum atoms. However, using rhenium as the base metal, such difficulties appear to have been at least partially over- come, the rate of boron penetration being sufficiently slow that a filament operating at 10-3 to 10-~A/cm 2 could have a life of many hundreds, or even a thousand or so, hours. The method of coating and processing a filament is important since adhesion to the base metal is not very strong, the coating remaining intact largely by virtue of self-adherence. Fine-grain powder, 5 to 8t~, was suspended in acetone and a small quantity of magnesium nitrate added as an activator; the time of coating and current density employed were found to vary appreciably, being dependent on the exact dimensions of the filament, but the correct parameters were not difficult to ascertain. A boride cathode required no activation as generally implied by the term, complete activity being achieved as soon as it was fully outgassed. A slight loss of activity occurred when a fila- ment was held inoperative under good vacuum conditions, but operation at a slightly elevated temperature soon restored full activity. Exposure to the atmosphere produced a somewhat larger loss of activity and although this increased with the number of exposures, complete recovery could generally be achieved; no physical damage occurred after a large number of such exposures. The contamination of well-activated and of poorly-activated cathodes has been investigated for most of the gases commonly encountered in vacuum systems. The bulk of the measurements have been made with oxygen and hydrogen. The emission deterio- rated rapidly once a critical partial pressure of poisoning gas had been exceeded. The critical poisoning pressure was about 10-~ tort for normal cathode temperatures but this decreased with repeated poisoning. The cathode could be reactivated after an appreciable number of poisonings before permanent impairment of the emission occurred. The speed at which reactivation occurred increased with temperature and decreased with the number of poisonings. J D Buckingham, Edwards High Vacuum InternationalLtd, Crawley, Sussex Field emission, a possible tool for low pressure measurements Field electron and field ion emission are low temperature pheno- mena. Therefore using these phenomena gas reactions, as found with the more common hot filaments, can be avoided. So far several applications have been proposed and applied. Field desorp- tion of adsorbed particles from a metal surface occurs at a critical field strength depending on the gas-metal configuration, the tem- perature and the crystal orientation. The possibility to apply field desorption at low temperatures suggests the development of a field desorption spectrometer analogous to the flash filament technique. The method has some advantages as the ad- and desorption from single faces is observed, the tip remains throughout the process at a low temperature, while the number of molecules desorbed is so small that the pressure rise can be neglected. So calibration even at pressures below 10 10 torr can be carried out for a specific gas by applying a high desorption field and measuring the changes in work function and pre-exponential term. At present it will be difficult to measure a mixture of gases as so far not sufficiently is known on the selective adsorption properties of various gases. A van Oostrom, Philips Research Laboratories, Eindhoven The detection of single atoms and molecules Electron optical focusing experiments show that positive ions impinge on the emitter. When this is in the form of a sharp point covered with a thick gas film, some 400 counts per second were recorded at a pressure of 10 5 mmHg for an accelerating potential of 2 kV. The emitter thus creates and collects the ions; each impact of an ion on the gas film causes variations up to (and exceeding) 30 per cent in the "field" current. "Shot" and "Flicker" effects are small in comparison. It is possible to distinguish between "atomic" and "molecular" ion impacts from the reaction time with the sur- face. The technique opens up a new method for measuring pressures in the high and ultra-high region which is direct and absolute. The effect of the bombardment by single ions of N8+, O8+, CO +, N +, O +, C + on the gas film, will depend on the nature and energy of the approaching ion, the type of collision with a surface atom, and the state of the surface in the neighbourhood of the point of impact. "Duration" times from the traces, of the order of 1 ms are explained in terms of the division of momentum of ion as between a multi- layer of gas (106 atoms over an area of 10-1° square centimetre; cohesion 1/20 ev) and the oxide (lattice energy 10 ev). L Jacob, University of Strathclyde, Glasgow (to be published in Y Electronics) Note: Patent application is pending for the apparatus described in this paper. A study of the flash filament technique An experimental study of the flash filament technique of estimating residual gas pressures and measuring surface contamination by gas layers has been made in which a tungsten ribbon filament of known surface area of which the temperature is monitored by an attached thermo-couple has been used in a stainless steel chamber main- tained at residual gas pressures between 10 -9 and 10-8 torr and isolated from the ultra-high vacuum pump set. In previous work glass chambers have been used, direct temperature monitoring has not been practised in the same way and a gas throughput to the continuously operating pumps has been established. The flash filament method of estimating residual gas pressures is considered to be of little value in that the two possible techniques of calculating 24

Field emission, a possible tool for low pressure measurements

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Conference on Fundamental Problems of Low Pressure Measurements

impurity content of metals and semiconductors using the vacuum fusion method for extraction. For small samples, weighing less than 1 g and having a total impurity content of < 1 ppm, a mixture of hydrogen, nitrogen and carbon monoxide (for oxygen) amounting to 1.10 -5 ml at NTP may be extracted. With such small quan- tities of gas available an omegatron mass spectrometer was chosen for its apparent simplicity and small volume. An account of the calibration and reproducibility experiments performed on this device is given. A bakeable vacuum system of 3 1. volume containing a liquid nitrogen trap, Bayard-Alpert ionization gauge and omega- tron was used. This was pumped by a trapped mercury diffusion pump backed by an oil rotary pump. It was possible to isolate the pumps from the system by a bakeable liquid indium valve. After baking overnight at 400°C under vacuum the total pressure in the system had fallen to 1.10 -8 tort when the bakeable liquid-nitrogen trap was filled. A gas inlet system incorporating a Toepler pump in which the gas mixtures were made up enabled them to be added to the system in discrete steps. It was concluded that the omegatron could satisfactorily analyse mixtures of inert gases but that a re- action between oxygen and carbon on the hot filaments in the system prevented the quantitative analysis of gas mixtures contain- ing oxygen. Only a qualitative analysis could be obtained when mixtures containing carbon monoxide were examined. M T Lilburne, Metallurgy Division, National Physical Laboratory

Evaluation of a low temperature emitter, lanthanum hexaboride on rhenium for use in demountable total and partial pressure measuring gauges This paper described an experimental study of the thermionic emission properties of a rhenium filament coated with lanthanum hexaboride. Particular regard has been paid to the possibility of using it as an emitter for total pressure measuring gauges and for mass spectrometers used in vacuum systems where the filament may frequently be exposed to the atmosphere or operated in high pressures of contaminating gases. Tungsten has generally been used in such applications, but the elevated operating temperature required coupled with the high degree of reactivity exhibited by tungsten in the presence of several of the gases commonly encoun- tered in vacuum systems limit its usefulness. This applies particu- larly in the field of gas analysis where appreciable and variable modification of the sample may occur in the ion source, thereby making the estimation of true partial pressures difficult and un- reliable. The oxide coated cathode, which operates at a substan- tially lower temperature and is much less reactive, is a far more acceptable emitter but, unfortunately, it is in general quite unsuit- able for use in demountable devices since, on exposure to a moist atmosphere, hydration occurs with complete loss of the coating by flaking. In the search for alternative emitters capable of meeting these rather exacting requirements Lafferty has studied the hexa- borides of various alkaline-earth and rare-earth metals. Lafferty found that whilst a satisfactory cathode could be made if the lan- thanum hexaboride were in the form of a self-supporting rod, difficulties occurred when, as is more generally required, a directly- heated emitter was formed by applying the material to a refractory metal. These difficulties resulted from a strong tendency on the part of the boron atoms to diffuse into the base metal, causing collapse of the boron cage structure and thereby allowing rapid evaporation of the lanthanum atoms. However, using rhenium as the base metal, such difficulties appear to have been at least partially over- come, the rate of boron penetration being sufficiently slow that a filament operating at 10 -3 to 10-~A/cm 2 could have a life of many hundreds, or even a thousand or so, hours. The method of coating and processing a filament is important since adhesion to the base metal is not very strong, the coating remaining intact largely by virtue of self-adherence. Fine-grain powder, 5 to 8t~, was suspended in acetone and a small quantity of magnesium nitrate added as an activator; the time of coating and current density employed were found to vary appreciably, being dependent on the exact dimensions of the filament, but the correct parameters were not difficult to ascertain. A boride cathode required no activation as generally implied by the term, complete activity being achieved as soon as it was fully outgassed. A slight loss of activity occurred when a fila- ment was held inoperative under good vacuum conditions, but operation at a slightly elevated temperature soon restored full activity. Exposure to the atmosphere produced a somewhat larger

loss of activity and although this increased with the number of exposures, complete recovery could generally be achieved; no physical damage occurred after a large number of such exposures. The contamination of well-activated and of poorly-activated cathodes has been investigated for most of the gases commonly encountered in vacuum systems. The bulk of the measurements have been made with oxygen and hydrogen. The emission deterio- rated rapidly once a critical partial pressure of poisoning gas had been exceeded. The critical poisoning pressure was about 10 -~ tort for normal cathode temperatures but this decreased with repeated poisoning. The cathode could be reactivated after an appreciable number of poisonings before permanent impairment of the emission occurred. The speed at which reactivation occurred increased with temperature and decreased with the number of poisonings. J D Buckingham, Edwards High Vacuum InternationalLtd, Crawley, Sussex

Field emission, a possible tool for low pressure measurements Field electron and field ion emission are low temperature pheno- mena. Therefore using these phenomena gas reactions, as found with the more common hot filaments, can be avoided. So far several applications have been proposed and applied. Field desorp- tion of adsorbed particles from a metal surface occurs at a critical field strength depending on the gas-metal configuration, the tem- perature and the crystal orientation. The possibility to apply field desorption at low temperatures suggests the development of a field desorption spectrometer analogous to the flash filament technique. The method has some advantages as the ad- and desorption from single faces is observed, the tip remains throughout the process at a low temperature, while the number of molecules desorbed is so small that the pressure rise can be neglected. So calibration even at pressures below 10 10 torr can be carried out for a specific gas by applying a high desorption field and measuring the changes in work function and pre-exponential term. At present it will be difficult to measure a mixture of gases as so far not sufficiently is known on the selective adsorption properties of various gases. A van Oostrom, Philips Research Laboratories, Eindhoven

The detection of single atoms and molecules Electron optical focusing experiments show that positive ions impinge on the emitter. When this is in the form of a sharp point covered with a thick gas film, some 400 counts per second were recorded at a pressure of 10 5 m m H g for an accelerating potential of 2 kV. The emitter thus creates and collects the ions; each impact of an ion on the gas film causes variations up to (and exceeding) 30 per cent in the "field" current. "Shot" and "Flicker" effects are small in comparison. It is possible to distinguish between "atomic" and "molecular" ion impacts from the reaction time with the sur- face. The technique opens up a new method for measuring pressures in the high and ultra-high region which is direct and absolute. The effect of the bombardment by single ions of N8 +, O8 +, CO +, N +, O +, C + on the gas film, will depend on the nature and energy of the approaching ion, the type of collision with a surface atom, and the state of the surface in the neighbourhood of the point of impact. "Durat ion" times from the traces, of the order of 1 ms are explained in terms of the division of momentum of ion as between a multi- layer of gas (106 atoms over an area of 10 -1° square centimetre; cohesion 1/20 ev) and the oxide (lattice energy 10 ev). L Jacob, University of Strathclyde, Glasgow (to be published in Y Electronics) Note: Patent application is pending for the apparatus described in this paper.

A study of the flash filament technique An experimental study of the flash filament technique of estimating residual gas pressures and measuring surface contamination by gas layers has been made in which a tungsten ribbon filament of known surface area of which the temperature is monitored by an attached thermo-couple has been used in a stainless steel chamber main- tained at residual gas pressures between 10 -9 and 10 -8 torr and isolated from the ultra-high vacuum pump set. In previous work glass chambers have been used, direct temperature monitoring has not been practised in the same way and a gas throughput to the continuously operating pumps has been established. The flash filament method of estimating residual gas pressures is considered to be of little value in that the two possible techniques of calculating

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