The pumping of nitrogen in an ionization gauge

The Pumping of Nitrogen in an Ionization Gauge G. CARTER, L. H. JAMES and J. H. LECK Department of Electrical Engineering, The University of Liverpool (Received 13 March 1962; accepted I June 1962) The mechanism o f pumping o f 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 o f this gas, and the onset o f pumping before ion production, can be resolved by postulating the chemisorption o f nitrogen atoms at the glass walls. The atoms may be formed by electron impact in the gas phase, or by dissociation o f excited and ionized molecules at the walls. The sorption process is shown to be somewhat analogous to the case o f hydrogen pumping.

Introduction

following ion pumping, a second gas species could be introduced to the ion pump, and the ion bombardment of the walls by the second gas, resulted in re-emission of the gas pumped initially14. If this latter method of gas release was used it was always necessary to heat the pump after the second gas bombardment to completely recover all the primary sorbed gas, i.e. ion bombardment induced re-emission was incapable of recovering all sorbed gas. When nitrogen was the primary sorbed gas and was

For some years the problem of the pumping action of nitrogen in an ionization gauge has occupied the attention of a number of workers l, 2.5, 13. The basic difficulties of the problem are that (a) nitrogen pumps at an extremely high rate compared with inert gases of comparable atomic weight and dimensions, (b) pumping apparently commences before N2 + ions are formed. Thus Alpertl, Young19, Bills2 and Carleton, and Cobic6 et al. have observed a nitrogen pumping speed at least an order of magnitude larger than that of argon, whilst Jaeckel and Teloy13 have noted that observable pumping of nitrogen occurs when the nitrogen is subject to electron bombardment of an energy as low as 8 eV. In this communication it is intended to describe some recent experiments, to reappraise the earlier data and, on the basis of these studies, to offer an explanation of the pumping mechanism in nitrogen.

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Experimental "6

A technique has been employed recently in this laboratory to study the re-emission of ionically pumped gases by bombardment with ions of a second gas. The glass envelope of a diode valve (a Comsa-Musa ion pump7) contained in a bakeable ultra-high vacuum system which was evacuated to pressures below 10-8 Torr, was used as the test surface. A full description of the apparatus and experimental technique is given in an earlier paper14. By operation of the ion pump in a constantly flowing stream of the experimental gas at a pressure of about 10-4 Torr, ions of this latter were pumped into the containing glass envelope. Using spectroscopically pure gas and baking the diode to 400°C before starting any experiment, ensured a clean system and allowed reproducible results to be obtained. The pumped gas was subsequently released by heating the diode and recording the pressure transient on a mass spectrometer. Alternatively,

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300

450

temperature,

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FiG. I. Thermal release of sorbed krypton, nitrogen and hydrogen Heating was discontinued above about 450°C. © nitrogen • krypton hydrogen 213

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CARTER,

L . H . JAMES AND J . H .

recovered by the first method, i.e. thermal desorption, a decisive maximum desorption rate was noted in the region of 180"C (as was also observed when krypton or any other inert gas (He, Ne, A and Xe) and hydrogen were th.e primary I©0

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gas, it is natural to seek a sorption process for nitrogen similar to that for hydrogen. This analysis is undertaken in the following section.

Discussion

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Fro. 2. Thermal release of sorbed krypton, nitrogen and hydrogen following bombardment induced emission by argon O nitrogen • krypton :~ hydrogen sorbed gases) and all pumped gas was recovered by heating to 350°C. If however the second method of gas release was employed, and nitrogen was recovered thermally, only after prior bombardment by argon ions, the maximum desorption rate was recorded at temperatures in excess of 300°C. If an inert gas, such as krypton, the behaviour of which is typical of all the inert gases, had been the initially sorbed gas, very little change in the position of the temperature at peak release rate would have occurred, as is shown in Fig. 1 and 2. Since nitrogen would be expected to behave in a like manner to an inert gas (such as He, A and Kr to which it is physically similar), when exposed to argon bombardment on the surface, then one must conclude that the nitrogen is largely bound initially by a different mechanism, and a large number of adentities are relatively loosely bound and are readily released by ion bombardment. When hydrogen was employed as the sorbing gas, the characteristic thermal release with and without bombardment by argon, was found to follow closely the behaviour for nitrogen although shifted to lower temperature generally, i.e. the large temperature at peak rate shift after bombardment, as seen in Fig. 1 and 2. The initial room temperature recovery of H2 is attributed to more loosely bound gas than is found for Kr or N2. Thus since the results for hydrogen and nitrogen exhibit similar bebaviour, which is quite different from that for an inert

The pumping action in argon and the other inert gases has been established4, 6, t7 to be a result of the propulsion of high speed positive ions into the glass walls of a gauge, and the probability of capture of the incident ion is of the order 0.1. Nitrogen, on the other hand, under the same electron bombardment conditions exhibits a pumping speed of at least an order of magnitude larger than that of argon, indicating a capture probability per ion at least equal to and probably greater than unity. The sorption mechanism in nitrogen, on this evidence and on that presented in the previous section, is thus patently different from that in the inert gases and it is further likely that other active species, apart from ions, participate in the clean up. It has been proposed 2, 3 that the activated particles are N2 molecules and ions excited to a metastable state, and the measurements of Jaeckel and Teloy to some extent support this, since pumping onset occurs at about the electron energy required to form Nz* metastable molecules. This proposal has been criticised by Cobic et al. on the grounds that on collision with the wall the metastability would be rapidly lost, the molecule would relax to the ground state and would not sorb at the walls. (Molecular nitrogen does not show any marked tendency to chemisorb on glass at room temperature.) Another proposal, by Bills and Carleton, that the sorbing entities are nitrogen atoms produced in the gas phase, was also rebutted by these investigators in that such a process, together with the process of recombination of an atom from the gas phase and a sorbed atom, could not account for the rapid decline in pumping speed as the quantity of sorbed gas increased. The present proposal in fact postulates the sorption of nitrogen atoms as is active in the pumping of hydrogen but does not require that they are formed in the gas phase. The production of an active modification of nitrogen in glow discharges was observed as long ago as 1911 by Lord RayleigMS, who then ascribed the activity to the presence of nitrogen atoms in the discharge. In 1953 work by Mitra 16 supported Rayleigh's view and showed that production of atomic nitrogen could arise in two manners. Firstly by direct dissociation of molecular nitrogen (at a bombarding electron energy of 9.76 eV) into atoms in the 4s ground state, and secondly by preformation of an N2 + ion (at 15.58 eV) with dissociative recombination of the N2+ ion in collision with an electron into atoms in the 2D and 2p metastable states. Thus in the problem of ion gauge pumping of nitrogen it appears possible that N2 + ions, on striking the walls, are neutralized, possibly by an auger de-excitation process11 and dissociate into atoms which are sorbed on the walls. It is well known that atomic nitrogen is highly reactive and sorbs strongly on, for example, tungstenlO and behaves in a similar manner to atomic hydrogen. In an ion gauge hydrogen

The Pumping of Nitrogen in an Ionization Gauge readily dissociates at the heated filament and the H atoms sorb strongly on the walls with a binding energy up to about 35 kcal per mole a2. It is thus likely that nitrogen, once atomized, could strongly sorb upon the walls, and the above mechanism appears to offer a partial explanation at least. The fact that pumping occurs at a faster rate than explicable by surface ion bombardment and commences at an electron bombardment energy of about 8 eV (about 7.5 eV below the first ionization potential) can be explained in two ways. Firstly, direct dissociation at 9.76 eV will give rise to pumpable N atoms, whilst metastable N2* molecules formed at 6.3 eV, 7.4 eV and 11.0 eV may dissociate at the walls. Although cross section magnitudes for the first process are unknown they may conceivably be sufficient to account for the larger than ion pumping rates. To understand the second process one must consider the thermodynamic aspect of chemisorption. Ehrlich has shownl0 that in problems of the dissociative chemisorption of molecules (e.g. N2 and H2 on tungsten), spontaneous dissociation and adatom chemisorption is likely to occur if the energy to be supplied for the process is negative, i.e. A E = D --2V' + Vm =

--Ve

and the process is exothermic. Here A E is the energy supplied, D is the molecular dissociation energy, ~pis the heat of adatom adsorption and Vm is the energy difference between the adatom equilibrium site and the transition complex (effectively the energy of activation for surface migration). It is assumed that in the adsorption step, one atom sorbs in the equilibrium position and one in the transition complex. In the experiments in this laboratory, where nitrogen was pumped in a diode ion pump and subsequently released by heating the gauge walls, it was found that the energy of activation for desorption, 2~p--D, possessed a spectrum of values in the range 22 kcal/mole to about 45 kcal/mole. Ehrlich has shown that Vm is generally about 1/59 and adopting the value of 225 kcal/mole for the dissociation energy of nitrogen, D, one arrives at values for A E , for direct dissociation and atom chemisorption, between -+-3 kcal/mole and --18 kcal/mole. The dissociative absorption of molecular nitrogen is, on this basis therefore, an unlikely process. A similar state of affairs exists for hydrogen, where molecular hydrogen does not react strongly with glass, but once dissociated sorbs with an energy between 20 and 35 kcal/mole 12. In the case of hydrogen predissociation occurs at a hot surface prior to adatom sorption on the glass, thus the dissociation energy is added to the system before the sorption step and adatom sorption proceeds by a thermodynamically favourable process since A E is negative. The dissociation of nitrogen by electron bombardment, or merely its activation to a higher energy metastable level, thus provides the precursor of a higher energetic state from which relaxation to the adsorbed phases is a thermodynamically favourable step (i.e. for the N2* 6.1 eV state, the energy loss in adatom sorption is about 140 kcal/mole). It thus appears possible to explain the onset of pumping at electron energies typical of either dissociation or metastable N2 production, and in fact pumping probably proceeds as a

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result of both processes. Cross-sections for metastable atom and ion production are probably of the same energy functional dependence as ionization cross-sections, although the magnitude of the former are unknown, and thus the increase in pumping speed with electron bombardment energy as observed by Cobic et al., and the pumping speeds, too high by a factor of three, to be accounted for by ion pumping, measured by Jaeckel and Teloy, can be explained. Bills and Carleton have criticized the atomic sorption hypothesis on the grounds that surface saturation is too rapid to be explained by recombination by collision of gas phase atoms with adatoms. However, it has been shown 14 that energetic ions have a high gas sputtering rate on collision with a glass sorbing surface, thus considerably higher release rates than envisaged by Bills and Carleton on atom recombination rates will occur and lead to rapid apparent surface saturation. Further the mechanism of trapping as envisaged by these workers was different from that postulated here and conclusions based on their arithmetic would probably be invalid. Finally the thermodynamic considerations, as outlined above, lead one to suspect that the energy barrier to surface diffusion is high (approximately equal to the activation energy for desorption), and so the nitrogen layer is immobile. This is as observed in hydrogen absorption on glass 12 and thus, in addition to ionically induced gas sputtering, some release of nitrogen atoms by recombination with bombarding gas will occur. It is also noted that no appreciable desorption of nitrogen occurs at room temperature, whereas some hydrogen is readily released at this temperature, as shown by the initial release rate of hydrogen in Fig. 1. Thus nitrogen appears to be more tightly bound than hydrogen, as is generally the case for materials upon which both gases sorb in the atomic statO0. Final evidence in support of these hypotheses are the observations of White et al. 18 and Daly9, who observed that N2+ ions are completely dissociated into their atomic components by collision with organic or metallic thin films; it is supposed that glass may be an equally good catalyst.

Conclusions One thus concludes that the enhanced pumping of nitrogen in ion gauges can be adequately explained by the chemisorption of atoms at the glass surface, where the atoms are formed by either molecular dissociation in the gas phase, or by dissociation of N2 + ions and N2* metastable molecules at the glass surface. The process is thus a similar one to that for hydrogen atom absorption, except that in the latter case dissociation is effected thermally rather than by electron excitation.

Acknowledgments The authors are grateful to Professor J. D. Craggs for his advice and encouragement. L . H . J . also acknowledges the receipt of a D.S.I.R. maintenance grant.

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References D. Alpert; J. Appl. Phys., 24, 1953, 860. 9 D. G. Bills and N. P. Carleton; J. Appl. Phys., 29, 1958, 692. 3 R. N. Bloomer and M. E. Haine; Vacuum, 3, 1953, 128. 4 G. Carter and J. H. Leek; Proc. Roy. Soc., 1961, A261, 303. 5 B. Cobic, G. Carter and J. H. Leek; Brit. J. Appl. Phys., 12, 1961, 384. 6 B. Cobic, G. Carter and J. H. Leck; Brit. J. Appl. Phys., 12, 1961, 288. 7 G. Comsa and G. Musa; J. Sci. Instrum., 34, 1957, 291. 8 J. D. Craggs and H. S. W. Massey; Handbueh der Physik, 1959. Springer-Verlag, Berlin, Vol. 37/!.

9 N. R. Daly; Rev. Sci. lnstrum., 31, 1960, 720. 10 G. Ehrlich; Proc. Second International Vacuum Conj,, 14/ashington, 1961. (To be published.) ll H. D. Hagstrum; Phys. Rev., 96, 1954, 336. 12 T. W. Hickmott; J. Appl. Phys., 31, 1960, 128. 13 R. Jaeckel and E. Teloy; Zeits. f. Naturforsch, 15A, 1960, 1009. 14 L. H. James and G. Carter; Brit. J. Appl. Phys., 13, 1962, 3. is Lord Rayleigh; Proc. Roy. Soc., A85, 1911, 219. 16 S. K. Mitra; Phys. Rev., 90, 1953, 516. ~7 H. J. Schwarz; Zeits. f. Phys., 117, 1940, 23. 18 F. A. White, F. M. Rourke and J. C. Sheffield; Rev. Sci. btstrum., 29, 1958, 182. 19 J. R. Young; J. Appl. Phys., 27, 1956, 926.