The ARUPS System

The ARUPS cham ber was also bakeable, stainless steel and 12 inches in diam eter. It was fitted with a differentially pum ped noble gas discharge lamp, a m oveable electron analyser (VSW HA45), a channeltron electron counting system, an ion gun for surface cleaning and an ion gauge for pressure m easurem ent in the 10'^ to 5 X 1 0 '" torr pressure range (fig. 2.2).

The main cham ber was pum ped by a liquid nitrogen trapped oil diffusion pum p and a titanium sublim ation pump. Base pressures of 2 x 10"'® torr were routinely achieved.

K ev:

A G Argon Ion Gun

D L Discharge Lamp

EG Electron Gun

Q M S Quadrupole Mass Spectrometer

S Sample

SDA Spherical D eflection Analyser

V Viewport AC QMS EG V DL SDA V

Fig. 2.2 Schematic diagram through the experimental level o f the ARUPS system (not to scale).

2.2

THE VACUUM PUMPS

In order to achieve UHV conditions within the experim ental cham ber, gas particles must be rem oved from it. Vacuum pum ps were used for this purpose. A rotary pum p was used first to ‘ro ugh’ pum p the system from atm ospheric pressure to

'y a pressure range within which the diffusion pum p could begin to function (~ 10" torr). Rotary pum ps are mechanical pum ps which operate by creating a periodically increasing and decreasing cham ber volume. The diffusion pum p consists basically of a pum p body with a liquid nitrogen cooled wall and a three or four stage nozzle system. The low vapor density oil is in a boiler and is vaporized there by electrical heating. The oil vapor stream s through the chim neys and em erges with supersonic speed through the different nozzles. This je t of oil vapor widens like an um brella carrying gas particles dow nw ards with it and com pressing them until they can be rem oved by the rotary pump. M eanwhile, the oil vapor condenses when it reaches the pum p w ails and flow s back in liquid form to the boiler. This auxiliary system (rotary and diffusion pum ps) was also used to pum p the gas handling lines and could be isolated from the main cham ber by an isolation valve.

This system was used to bring the cham ber pressure down to approxim ately 10"^ torr at which stage the ion pum p could be used. The diode ion pum p works by ionizing the gas in the system in a Penning gas discharge between a cathode and an anode in the pump. The pump contains two parallel cathodes made of titanium between which are arranged a system of cylindrical anodes m ade o f stainless steel. The cathode is m aintained at high negative electrical potential, of the order o f a few kV, with respect to the anode. The whole electrode assem bly is m aintained in a strong hom ogeneous m agnetic field. Electrons near the anode are trapped in the high m agnetic field and set up a region of high electron density in the anode cylinders. Here the electrons collide with gas particles in the system and ionize them. Due to their greater mass, these ions will be accelerated tow ard and im pinge on the cathode.

A titanium sublim ation pum p was used to supplem ent the ion pump. This is a form of getter pum p in which the getter material (titanium ) is evaporated and deposited on the inner wall o f the cham ber as a getter film. The titanium is evaporated from a titanium /m olybdenum alloy filam ent by resistance heating. Particles from the gas which im pinge on the getter film becom e bound to it by chem isorption and form stable com pounds with the titanium , which have im m easurably low vapor pressures. This pum p w as not used continuously but switched on in short bursts of a few m inutes, controlled by a tim er, every few hours.

W ith the use o f these pum ps pressures of below 2 x 10“’^ torr were m aintained in the main cham ber. A block diagram of the entire pum ping system is shown in fig. 2.3.

Key; T S P OD P P T A

M

o

-m

T ita n iu m S u b ilm a tio n P u m p Oli D iffusion P u m p P ira n i G a u g e L iquid N itro g en T ra p Air Inlet Leai< V alve A — is o la tio n V alve G a s B o ttle Inlet Ion P u m p R o ta ry P u m p ODP C h a m b e r Main T S P