Detection of neutral particles

In document Energy distributions of sputtered particles (Page 167-174)

Some of the experiments in which the sputtered neutral beam has been ionised and detected by mass spectrometry have been

described. However the conditions for the experiments reported

here differed in several major ways from those performed previously. It was desired to retain the same angular and energy resolution

for the neutrals as for the ions. The high angular resolution places

severe limits on the number of particles available for ionisation without having a very significant effect on the background gas

contribution to the measured current. High energy resolution has

a similar effect on the number of particles available to the mass analyser with an added restriction that the time a particle stays within the ioniser and hence the probability of ionisation depends

-h

on E . A particle with energy 1 keV has only 0.005 the chance of

being ionised under given conditions as a similar one at room

temperature. Further the yield of neutral particles falls quite

rapidly as the energy increases so that the particle density at

the energy distribution of the particles must suffer the least perturbation by the ionisation process, (or alternatively the relationship between the energy spectrum of the emerging ions and the incident neutrals must be known) to permit meaningful

conclusions to be drawn about the energy distribution of the neutrals. An order of magnitude assessment of the number of ions available to

the mass filter can be made from published figures. A copper atom yield of 5 atoms per 10 keV argon ions (Rol et al, 1960) would result in 4 x 109 atoms/sec in a cone of half angle one degree for

an incident beam intensity of 10 yA. If the sputtered particles

had an energy distribution similar to that measured by Thompson (1968), that is rising linearly from zero to a peak at say, 5 eV then falling as E 2 there would be 2 x 104 atoms/sec within an

energy window at 3 eV at 500 eV. A Weiss (1961) ioniser with an

efficiency of 0.4% for room temperature gases {the value 1/40 quoted by Weiss is in error due to an incorrectly defined solid angle (Bickes and Bernstein, 1970)} would result in count rates of approximately 6, 25, 150, 600 per second at 500, 250, 100 and

50 eV respectively. These would be detectable by pulse counting

techniques but not so readily with an electrometer method. However the contribution of the background gas would more than swamp the

wanted signal. Several stages of differential pumping would be

necessary to reduce the argon level sufficiently and adequate

foreline trapping would be essential (Holland, 1970). The now

conventional approach of pulsing the incident argon ion beam and the ioniser electron current would also aid substantially in reducing unwanted components.

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The matter of the change in energy of an atom as it

becomes ionised could pose a much more serious problem in using the ions to determine the energy of the neutrals. Once ionised the particle would be able to interact with the charges and electrically biassed electrodes in the vicinity and in the process undergo energy changes. If an electric field were used to extract the ions once formed then the energy of the ion on emerging would depend upon the position within the ioniser the charge state change occurred. A novel approach to this difficulty was devised. The particles of

initial interest had energy greater than one electron volt. Their kinetic energy could be used to render them largely self extracting from the ionisation region. A Weiss (1961) ioniser was used because in addition to its inherently large electron current leading to high probability of ionisation the region occupied by the electron space charge produced potential minimum was quite extensive and essentially uniform within the confines of the beam of these experiments.

The arrangement of the ioniser in relation to the system used to measure the energy of the sputtered ions is shown in

figure 4.15. A screen (S.S.) to suppress the ions produced in the sputtering process was inserted between the target and the ioniser which was mounted on the deceleration grid Hi. The ioniser was modified by inserting in the electron stream another mesh which was used as a control grid to stabilise the current collected by

the anode sections. Further a pair of inlet and exit apertures (each covered by a mesh) was added to the ionisation chamber and maintained at near the potential minimum to ensure that the ionisation

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volume was as near unipotential as possible. The inclination

between the two anode sections could be adjusted ± 10° to produce an internal forward (in the beam direction) or reverse extraction field. This was aimed at reducing the number of ambient gas ions fed into the energy analyser by expelling them in the opposite

direction to the self-extracting ions. However it did not prove

satisfactory and a reversible extraction field was produced by keeping the anode sections parallel and replacing the indirectly heated unipotential cathode by a single strip electron emitter. The D.C. heating potential of 2.5 V became the effective extraction potential.

By recording the energy spectrum of the ambient argon gas after it had been extracted from the ioniser it was possible to

determine the energy spread produced by the ionisation process. The

way in which this was used to deconvolute the measured energy spectrum of the sputtered copper neutrals by a Fourier transform method is explained in chapter 5.

A further significant advantage of using a self-extracting ioniser which produces a very small spread in the ion energy is the ability to discriminate against the background gas (Schmidt-

Bleek et al, 1969). In the higher energy region (that is when the

sputtered neutral has initial energy > W, the spherical analyser pass energy) there is a nett retarding potential between the

grids Hi and II2. If the ambient gas ions acquire less energy than

this retarding value they will not penetrate to the spherical

analyser. The skirts of the spherical analyser response curve

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completely the relatively large currents produced. The retarding

field discriminates against such ions much more effectively. When the ions from the wanted beam have energy less than W and

therefore are accelerated into the analyser the ambient gas ions

are also. Under these conditions a retarding potential placed upon

an electrode after the spherical analyser and before the mass filter (such as shown in the dotted line of figure 4.14b) will prevent most of the unwanted low energy ions from being measured. The suppression of the argon ions by a factor of more than 104 is shown in the series of mass spectrometer scans of figure 4.16. The argon component in which is found by multiplying the peak

height by the electrometer setting given. With the mass spectro­

meter adjusted for say, mass number 60, the suppression is of course even greater.

Neutral beam detectors of similar characteristics and configurations but without the energy analysis equipment have been described recently by Bickes and Bernstein (1970) and Lee

In document Energy distributions of sputtered particles (Page 167-174)