Abstract
The sputtering, in low pressure discharge tubes, of carbon by Hg+ and A+ ions at energies from 10 eV to 20 × 103 eV was investigated by direct weight loss measurements. The commonly quoted linear variation of sputtering rate in grammes per ampere-hour held up to 2 to 3 kv, where it was replaced by a variation proportional to (voltage)1/2. Sputtering occurred at 30 volts, a value well below the usual `critical voltages' in the literature. X-rays were generated above 8kv by the action of secondary electrons emitted from the targets. The emission from spherical targets was determined from the dimensions of the positive ion sheaths and was initially proportional to voltage, after which the rate of increase diminished rapidly.
A brief survey of the sputtering of beryllium, aluminium, silicon, iron, nickel, copper, molybdenum, tungsten, lead and uranium indicated similar variations with voltage, and showed that the sputtering rate was roughly proportional to the (atomic weight of the target)1.3 and that it increased with the heavier ion.
These empirical relations are useful for practical assessments, but not for theoretical interpretation, unless the secondary electron emission is distinguished. This was done for carbon and iron and the sputtering rate in atoms per ion determined. The empirical relation of the form s=λ ln (V/V0) was found, where V0 is thought to be related to the `penetration' of the ion into the target, and has a value of a few hundred volts. The ratio of the binding energy of the atom to the energy required to sputter it, for carbon and iron atoms released by Hg+ and A- ions, had a maximum of 0.5% at 2000 volts for carbon and of 2.5% at 1000 volts for iron.