The determination of partial pressures with a simple omegatron

VACUUMVol. 13, !ap. 359-366. PergamonPress Ltd. Printed in Great Britain.

The D e t e r m i n a t i o n of Partial Pressures w i t h a Simple O m e g a t r o n P. G. W. ALLEN and B. LANG* Vacuum Deposition Research Division, Edwards High Vacuum Ltd., Crawley, Sussex, England (Received 29 March 1963 ; accepted 15 August 1963) It is shown that a simple form o f omegatron, without guard rings, may be calibrated to determine partial pressures. An omegatron, with a tungsten emitter, was connected Jbr calibration to the vacuum vessel o f an ultra-high vacuum system made o f stainless steel. The calibrations were made using a magnetic field o f 3.5 kG and an r . f fieM o f O.4 V/cm. The controls o f the omegatron head were set at the optimum bias to give maximum sensitivity and resolution Jbr the nitrogen ion N2+. The calibrations were made with commercially available tank gases viz. 1-12, D2, He, N2, 02, A, CH4, CO and C02. Mass spectraJor each gas were taken over a range o f pressures and from these were determined the fragmentation pattern, relative ion abundances and ion current to pressure relationship. There was evidence that the partial pressure calibration depended on the tubulated speeds o f the ion gauge and omegatron head. Mass spectra were then determined in sequence with four Bayard-Alpert-type ion gauges all having similar electrode assemblies but with differing conductances viz. 3.5 ; 16 ; 50 l./sec and a " nude "gauge which had an infinite conductance. The tubulation speed o f the omegatron head was 4 I./sec and from this value the true pressure in the head could be estimated.

I. Introduction

Hippie (1951) 4, McNarry (1956) s, Warnecke (1957) 6, (1959) 7, (1960)8, Stark (1959)9, Klopfer and Schmidt (1960)10, Dummler (1961)11, 12, Lawson (1962)13, to name but a few of the many papers, so the authors will confine themselves to describing their omegatron and its associated electronic equipment.

Omegatron mass spectrometers of the simple type similar to that described by Alpert and Buritz (1954)1 have been in general use in this laboratory for some time to determine the nature of residual gases in ultra-high vacuum systems (Holland and Bateman (1960))2, and the gas atmospheres produced when evaporating metals at low pressures (Holland, Laurenson and Allen (1960))3. Although the simple form of omegatron i.e. without guard rings, lacks high resolution, it is an inexpensive mass spectrometer and affords much more useful information than is obtained merely by measuring the ultimate pressure. To extend the usefulness of our omegatrons it became necessary to calibrate them with known gases so that such gases and their fragments might be identified in complex mass spectra and their corresponding partial pressures determined. All the calibrations were made with the electrical parameters of the omegatron set at optimum conditions for sensitivity and resolution with respect to the nitrogen molecular ion N2 +. These calibrations were to determine the fragmentation patterns and the relationship between the collected ion current and pressure for the known gases. However certain difficulties were encountered in that the collected ion current to pressure relationship changed with different ion gauges. The results although not absolute are presented here to aid other workers using similar omegatrons as mass spectrometers.

Omegatron Head A schematic diagram of the omegatron used in these determinations is shown in Fig. 1. The electrode assembly was made from Pt/Ir alloy having overall dimensions of 2.5 × 2.5 × 1.6 era. The electrodes were mounted on a glass B7G valve base and sealed into a glass envelope with a conductance to the system of 4.2 1./sec. The cathode filament was made from 0.2 mm tungsten wire. The magnetic field across the omegatron was supplied by a permanent magnet giving 3.5 k G in a 4.5 cm gap. The magnet was mounted on a special trolley which permitted exact positioning in all planes. The omegatron was degassed only by the baking cycle of the ultra-high vacuum plant to which it was attached, the bake out temperature was 350°C. The entire magnet assembly was removed during the baking cycle. The electrical parameters of the omegatron were previously adjusted to give maximum sensitivity and resolution for nitrogen. They were as follows : Electron accelerating potential 180 V Potential on box with respect to cathode electrode + 90 V Potential on box with respect to anode potential -- 90 V Amplitude of r.f. voltage 1 V RMS Trapping potential bias VT + 1.5 V

2. Apparatus The theory and construction of the omegatron mass spectrometer are well documented, Sommer, Thomas and

*Now at Strasbourg University. 359

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Electronic Apparatus A schematic diagram of the omegatron circuit is shown in Fig. 2. The r.f. field was supplied from a signal generator (Airmec 828) having a frequency range of 30 kc/s to 30 mc/s. The proton resonance frequency for the omegatron with a 3.5 k G magnetic field was 5.3 Mc/s. The r.f. power was fed to the omegatron via a wide band amplifier which had a continuously variable output up to a maximum of 1.5 V. The collected ion current from the omegatron was measured with

a d.c. amplifier having a maximum sensivity of 10-13 A.f.s.d., having a response time of 0.3 sec. The amplifier output was taken to a chart recorder (Kent Multilec III) with a full scale response time of one second. As the filament emission was not stabilized, the electron beam current Ie varied with pressure. This effect necessitated the knowledge of the influence of the electron beam current Ie upon the collected ion current Ii. To determine this relationship Ii was plotted against le for a known mass over a range of pressures : a typical result is shown in Fig. 3. In this case it can be seen that Ii reaches a maximum value when le = 7 #A.

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As there is no apparent reason why the number of ions formed should depart from a linear relationslfip with the ionizing current Ie, the presence of a peak in the experimental curve must be due to a decrease in the ion collection efficiency. Edwards (1955)14 has attributed the foregoing effect to spac

T h e D e t e r m i n a t i o n o f Partial Pressures w i t h a S i m p l e O m e g a t r o n

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charge produced b y the electron beam when the ionization reaches a certain level and to non-resonant ions. However within a certain limited range Ii may be taken as proportional to Ie, the range varying both with the total pressure and the nature of the ion. The authors found experimentally that for the gases and pressure range described later on that the ratio li/le was constant in the range of/re from 1 to 4 #A.

Vacuum System The omegatron was attached to the baseplate of an ultrahigh vacuum plant (Holland (1960))1s, as shown in Fig. 1. The system was made of stainless steel and was degassed by baking from room temperature to 350 °C in 3 h. The vacuum chamber was of 12 in. dia. and 14 in. in height having a volume of 26 1. The vessel was exhausted by a fractionating oil diffusion pump, charged with Silicone 704, through a water-cooled baffle and a liquid nitrogen trap. After baking and charging the trap the ultimate pressure of the plant measured with a Bayard-Alpert ionization gauge was 2 × 10-9 tort. The Bayard-Alpert ionization gauge was mounted on top of the plant chamber, see Fig. 1. All pressure values given in this paper are nitrogen equivalents.

extent the omegatron is the cause of dissociation. Dissociation of particles within the omegatron is due to at least two factors, firstly the ionization process, Cobine (1941) 16, Engel (1955) ]7, Reed (1962) 18, and secondly, the cathode filament temperature. Some knowledge of the effect of filament temperature may be gained by comparing the authors' fragmentation patterns for a " hot " filament with those obtained by Klopfer and Schmidt 10 who used a " c o l d " oxide filament which caused less dissociation.

Results : Relationship o f Collected Ion Current to Pressure The collected ion current to pressure relationship is best represented by the ratio of collected ion current to electron beam current Ii/Ie as a function of pressure. Representative curves for this relationship are shown in Fig. 4-12 ; pressures were measured with a Bayard-Alpert ion gauge having a tubulation of 50 1./sec. Conduction of ion gauge tubulacion 50 I./sec.

Admission o f Gases The calibration gases were admitted to the chamber of the ultra-high vacuum plant via a metal needle valve in which a metal bellows sealed the needle from atmosphere. The valve could only be baked to 150°C for degassing but this was adequate to prevent it from being a source of contamination.

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3. Calibration procedure and results Procedure When a calibration gas was admitted to the system and the pressure reached an equilibrium, the ion gauge was outgassed and the pressure allowed to return to equilibrium before the mass spectrum was taken. This procedure was repeated at each calibration point. The mass spectra recorded at each pressure included the primary peak of the calibration gas, e.g. Mass 28 for the nitrogen molecular ion, and the secondary peaks due to doubly ionized or fragmentary ions due to the parent gas. The entire spectrum was scanned at frequent intervals to ensure that contamination was not occurring. Repeatedly raising the pressure to 2 × 10-s torr with different calibration gases over a considerable period of time (weeks) did not prevent re-attaining the ultimate pressure of 2 × 10-9 torr without further baking. The calibrations were determined in the pressure range 2 × 10-8 to 2 × 10-5 torr. In this range the possibility of errors due to residual gas components was negligible.

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Results : Fragmentation Patterns Fragmentation patterns for the calibration gases are shown in in Table 1. The height of the primary peak corresponding to the parent gas is represented at a nominal value of 100 and all secondary peaks attributed to the parent gas are shown in proportion to the primary peak. It would be of considerable interest to determine to what

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The Determination of Partial Pressures with a Simple Omegatron However, the Ii/Ie pressure relationship was found to change according to the tubulation of the ion gauge used. Comparative calibrations were taken in sequence with four ion gauges, all having similar electrode assemblies but with differing conductances viz. 3.5, 16 ; 10 1./sec and a " n u d e " gauge which theoretically had an infinite conductance. Comparative calibration curves in Fig. 13 show that, with exception of the " n u d e " gauge, as the conductance of the gauge tubulation increases, the ratio It/Ie is obtained at a lower pressure. To ensure that the discrepancy in the pressure readings of the " n u d e " gauge did not arise either from desorption effects in the vessel or the omegatron head, the calibration curve for A + was measured on five separate occasions in several cycles during which the vacuum apparatus was baked twice. The combined results, Fig. 14, show that the relationship between the pressure given by the " n u d e " gauge and the Ii/Ie rate was constawt. The results for the " n u d e " gauge cannot however be correlated with those for the tubulated gauges, but a possible explanation for this is that the electrical characteristics of the gauge head have been disturbed by removing the glass envelope which is known to become electrically charged in normal use. The omegatron head has a tubulation of 4 l./sec and its internal area for desorption is somewhat comparable to that of an ion gauge. Thus true calibration of the li/le in terms of pressure probably corresponds to the readings obtained with the ion gauge having a speed of 3.5 1./see. However disregarding the discrepancies due to the ion gauges used, the following can be stated.

Primary Peaks The peaks corresponding to the parent gases are characterized by exhibiting li/Ie ratios which are directly proportional to pressure to an upper limit imposed by mean free path considerations.

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Secondary Peaks From the results obtained for the primary ions, the secondary ions might have been expected to exhibit the same relationship with respect to pressure. However, this is not always so, as can be seen from the calibration curves. In the cases of nitrogen, Fig. 4, and oxygen, Fig. 5, their respective ~/le pressure relationship for ions having mass to charge ratios of half that of the parent gas ions, had slopes significantly less than one. These half mass ions occur in two ways either being due to the doubly ionized parent gas molecules i.e. N22+, or due to singly charged atoms i.e. N +. In what proportions these ions are present is as yet undetermined. It is believed that the proportion of charged atoms present influenced the result causing the deviation from direct proportionality. Hydrogen, Fig. 6, and deuterium, Fig. 7, gave interesting results in that both gases yielded large quantities of their triatomic ions, respectively I-I3+ and D3 +. In both cases the singly charged diatomic and triatomic ions were directly proportional to pressure whilst their atomic ions H + and D + followed a square root law with respect to pressure. The doubly charged deuterium ion D32+ however followed a

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curve with a power of 0.25. No such observation was made for H32+ as its abundance was negligible. The inert gases argon, Fig. 8, and helium, Fig. 9, gave somewhat conflicting results in that both the argon ions A + and A2+ were directly proportional to pressure, whilst this was only true of the singly charged helium ion He +. The doubly charged helium ion He 2+ followed a square root law with respect to pressure. Now consider the more complex gas molecules which exhibit considerable fragmentation. In the case of methane, Fig. 10, the CH4 + and CH3 + ions, the Ii/Ie to pressure relations were found to be proportional to pressure. The other fragment ions from methane followed curves having a range of powers from 0.6 to 0.9.

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The Determination of Partial Pressures with a Simple Omegatron

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singlycharged argon ion A+. particles which undoubtedly effect the " c u t - o f f " pressure e.g. in practice the " c u t - o f f " for H2+ was found to be approximately 10-6 torr. The lower pressure limit of the omegatron is imposed by various factors, such as errors due to background ion current, the amplification limit imposed by the electronic equipment associated with the omegatron and the magnetic field strength ; the latter also influences the high pressure limit.

4. Sensitivity of the omegatron

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Influence of Pressure upon the Results At certain defined pressures, dependent upon the individual ion, the li/Ie ratio passes through a maximum value with respect to pressure. This phenomenon is due to a decrease in collection efficiency caused by a reduction in mean free paths within the omegatron box. Thus the upper pressure limit at which an omegatron may be used is determined by the mean free path of the gas or gases under investgation, with reference to the path travelled by an ion orbiting to collection. The theoretical path length travelled by an 1-12+ ion to collection in the authors' omegatron is 1.8 × 103 cms. Now the mean free path of the H2 molecule at 10-6 torr is 9 × 103 cms, therefore " c u t - o f f " pressure, that is the pressure at which the li/Ie reaches a maximum is theoretically 5 × 10-6 torr for H2+. This however ignores the presence in the system of other

(1)

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5. Conclusions It has been shown that the simple form of omegatron may be calibrated to determine partial pressures of residual and reaction gases. Difficulties can be experienced in relating

366

P. G. W. ALLEN and B. LANG

the Ii/Ie ratio to the true pressure in the system, because the c a l i b r a t i o n curve o b t a i n e d is influenced by the t u b u l a t i o n of b o t h the ion gauge a n d the o m e g a t r o n head. T h e concent r a t i o n o f the ionic species (mass/charge) in the f r a g m e n t a t i o n p a t t e r n s o f the calibration gases are d e p e n d e n t o f pressure. T h u s if the pressure indication is effected by the gauge or o m e g a t r o n t u b u l a t i o n t h e n this will influence the relation o b t a i n e d between pressure a n d f r a g m e n t a t i o n pattern.

Acknowledgements A c k n o w l e d g e m e n t s are m a d e to Mr. L. H o l l a n d of the V a c u u m D e p o s i t i o n R e s e a r c h Division for suggesting this work, a n d to h i m a n d Mr. L. L a u r e n s o n , o f the same laboratory, for m a n y useful discussions. W e t h a n k Mr. W. Steckelmacher o f the I n s t r u m e n t R e s e a r c h Division for the provision o f the o m e g a t r o n h e a d a n d electronic e q u i p m e n t w h i c h m a d e the w o r k possible. Finally we t h a n k the Directors o f E d w a r d s H i g h V a c u u m Ltd. for permission to publish this work.

References 1 D. Alpert and R. S. Burtiz, .L .4ppl. Phys., 25, (1954), 202-209. 2 L. Holland and S. K. Bateman, Trans, 8th Nat. Symp. Vac. Techn., (1961), 1201-1210, Pergamon Press, (1962). 3 L. Holland, L. Laurenson and P. G. W. Allen, Trans. 8th Nat. Symp. Vac. Techn. (1961), 208-219, Pergamon Press, (1962). 4 a . Sommer, H. A. Thomas and J. A. Hippie, Phys. Rev., 82, (1951), 697-702. 5 L. R. McNarry, National Research Council of Canada, NRC No. 4259., (1956). 6 R. J. Warnecke, Ann. Radioelectricit~, 12, (49), (1957), 258-281. 7 R. J. Warnecke, Ann. Radioelectricitd, 14, (58), (1959), 339-365. 8 R. J. Warnecke, Ann. Radioelectricitd, 15, (60), (1960), 169-199. 9 D. S. Stark, Vacuum, 9, (1959), 288-294. 10 A. Klopfer and W. Schmidt, Vacuum, 10, (1960), 370. 1l S. Dummler, Vakuumtechnik, 10, (1961), 131-138. 12 S. Dummler, Vakuumtechnik, 10, (1961), 184-190. 13 R. W. Lawson, J. Sci. lnstrum., 39, (6), (1962), 281-286. 14 A. G. Edwards, Brit. J. AppL Phys., 6, (1955), "1.4 48. 15 L. Holland, Trans. 7th Nat. Symp. Vac. Techn., (1960), 168, Pergamon Press (1961). 16 j. D. Cobine, Gaseous Conductors, McGraw Hill, (1941). 17 A. von Engel, lonized Gases, Clarendon Press, Oxford, (1955). 18 R. I. Reed, Ion Production by Electric Impact, Academic Press, (1962).