Secondary electron emission, surface composition and modes of activation of metal alloy cathodes

surface s c i e n c e ELSEVIER

Applied Surface Science 111 (1997) 285-287

Secondary electron emission, surface composition and modes of activation of metal alloy cathodes B.Ch. Djubua *, E.M. Zemchikhin, A.P. Makarov, O.V. Polivnikova SRPC " Istok", Fryazino, Moscow region 141120, Russia

Received 4 June 1996; revised 13 August 1996; accepted 2 September 1996

Abstract

The interrelation between the secondary electron emission, the elemental composition of the surface, and the temperature heating history has been studied for cathodes made from alloys based on platinum or palladium containing 1.5-2% barium. These cathodes, which are used at low operating temperatures in magnetron amplifiers and oscillators, were tested in the form of foil disks diffusion bonded to molybdenum or copper. The following modes have been shown to result in optimum secondary electron emission: 1.5 to 2 h at 900 to 950°C for Pd-Ba and 1.5 to 2 h at 1100 to 1150°C for Pt-Ba. To activate the cathode by electron bombardment the following modes may also be used: bombardment power density of 85 to 100 W/cm 2, temperature = 450 to 400°C, for 5 h, or 200 to 250 W / c m 2, 650 to 600°C, for 1 h. Keywords: Metal alloy cathode; Secondary electron emission; Secondary electron emission coefficient; Auger electron spectroscopy; Atomic concentration; Barium alloy; Pt-Ba; Pd-Ba

1. Introduction

Metal alloy cathodes (MAC) based on P t - B a and P d - B a have found wide application in 'cold' cathodes for magnetron amplifiers. These cathodes operate at temperatures up to a maximum of 700°C. For cathodes operating at such low temperatures there is a severe problem in removing surface contamination when processing the cathode after exhaust or subsequently during operation of the device. This paper reports a study of the interrelation between the secondary electron emission (SEE) performance, the elemental composition of the cathode surface and the temperature to which the alloy cathode has been heated.

* Corresponding author.

2. Experimental The cathodes used were made of either palladium or platinum alloys containing from 1.5 to 2% barium. Ingots of the alloys were prepared under an atmosphere of argon and then rolled into foil of thickness 0.2 mm. Disks (diameter 6.5 mm) were cut from the foil and then diffusion bonded to a carrier of copper or molybdenum. The SEE coefficient (SEEC) of the foils was measured in pulsed mode as a function of incident primary electron energy (Ep) with automatic recording of the results. The surface composition was determined by Auger electron spectroscopy (AES). The brightness temperatures (T) of the cathodes are given in the text and figures. Fig. 1 shows the SEEC as a function of Ep for P d - B a and P t - B a MACs over a range of tempera-

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At T greater than 600-650°C the SEEC increases. The SEEC growth rate is higher under conditions of simultaneous EB (curve 5c). The same changes of SEEC occur for P d - B a cathodes on heating, but activation by EB is observed to begin at lower T than for P t - B a cathodes. However, pulsed EB at T above 600°C for P d - B a and 750°C for P t - B a does not influence the SEE (curve 6c). Stabilization of the SEEC first cross-over voltage (E~) and the work function takes place after heating for 1.5-2 hours at the highest T. When the cathodes are heated for a longer time (20 h) at operating T, there is an increase in SEEC. Conversely, extended exposure (20 h) to residual air at room T results in an SEEC decrease of 15 to 20%. Low power EB reduces the SEEC at low T, but at T above 4 0 0 500°C higher power EB can be used to activate the cathodes. The stability of the SEE of activated cathodes to EB has been studied at bombardment current densities of up to 0.5 A / c m 2. The most resistant T-intervals to EB are from 450 to 750°C for P d - B a and from 650 to 1000°C for Pt-Ba. Oxidized samples of P d - B a cathodes have a maximum SEEC in the range J i

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B. Ch. Djubua et al. / Applied SurJace Science 111 (1997) 285-287

4.5 to 5.5 and EpJ equal to 25 eV. In this case the cathode characteristics were measured after long term storage in air or after annealing at 500 to 600°C in vacuum worse than 10 -3 Pa. (Preliminary annealing of the cathodes in vacuum prior to welding them into the device reduces the time and T required for activation.) The change in the MAC surface composition when heated is shown in Fig. 2. The initial Auger spectra show a significant number of impurities. The impurity elements present and their relative concentrations are determined by the storage conditions and preliminary processing of the cathodes. The total content of Cu, Ni, Co, and Fe at the surface (not shown in the figure) at T up to 800°C does not exceed a few at%. Above 800°C these impurities vanish from the surface. The P and Si atomic percentages are also low. As shown in Fig. 2, as T increases progressive cleanup of the impurities from the cathode surface takes place, succeeded by the formation of an active layer. Long term electron bombardment of the cathode at low T results in the accumulation of carbon (Fig. 2a). This is derived from the decomposition of carboniferous compounds (CO, CO 2, and others). As shown in Fig. 2, T = 800 to 850°C for P d - B a and 950 to 1000°C for P t - B a is evidentially inadequate for the complete cleaning of the cathode surface. The effect of the emergence of sulphur is more marked for cathodes which have diameters larger than 4 ram. (Special experiments have shown that accumulation of sulphur is related to its re-evaporation from the closely spaced electrode of the electron energy analyzer.)

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At the activation T no oxygen was detected on the surface and within a range of temperatures about the activation T its surface concentration is only fractions of an at%. This accounts for the high resistance of the MAC to EB since changes in surface oxygen content would affect the stability of the SEE. At low T the accumulation of oxygen and the diffusive limitations result in a decrease in SEE. When the cathodes are held at room temperature O, N, and C accumulate on the surface from residual gases in the vacuum system.

3. Conclusions Both secondary electron emission and surface composition have been investigated in MACs and the relationship between them determined. In the activated state the surface of the MAC is free from impurities and enriched in the activating component. After activation the atomic concentration of the activator element in the near-surface layer of the cathode is from 20 to 40 at% depending upon the active element content in the bulk and the manufacturing process. The following activation procedures have been found to facilitate the attainment of optimal SEE: heating for 1.5 to 2 h at 900 to 950°C for P d - B a and 1.5 to 2 h at 1100 to l l50°C for Pt-Ba. To EB activate the MAC the following modes can be used: bombardment power density of 85 to 100W/cm 2, T = 450 to 400°C, for 5 h, or 200 to 250 W / c m 2, 650 to 600°C, for 1 h.