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Sputtering in rf discharges

Control of the sputtering energy

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One of the interesting features of an RF discharge is that it allows more control of the effective DC bias potential at the sample and thus to some extent separate control of the energy and flux of the sputtering ions. This control allows sputtering to be done with very low-energy ions if desired – conditions that have found use both in the cleaning of samples prior to analysis and in the preparation of damage free surfaces for inspection by high resolution electron microscopy and atomic beam methods (K. Shimizu and T Mitani, 2007).

How is this control possible?
In a DC discharge, most of the applied potential occurs across the cathode sheath – the thin region with a positive space charge in which ions from the negative glow are accelerated towards the surface to cause sputtering of neutral atoms and electrons. It is these secondary electrons that allow the discharge to be self-sustaining, and the bias voltage in a DC glow normally depends on the secondary electron yield, bias voltage increasing as the secondary electron yield decreases (see secondary_electron_yield.htm). The actual energy of the ions arriving at the cathode (i.e. the sample) depends on the number of collisions with neutral atoms that an ion makes in the sheath, but can be comparable to the sheath voltage. The energy of ions incident upon the sample in an RF discharge can be lower than for a similar DC discharge for two reasons which are explained below.
First, in an RF discharge there is an additional mechanism available to accelerate electrons to energies where electron impact ionization can occur. The electrode sheaths oscillate at the applied RF frequency, moving towards and away from the electrodes as each electrode changes between acting as an anode or a cathode; this motion can impart energy to electrons as they are reflected by the sheath potential in a process known as ‘wave-riding’ or stochastic heating (e.g. Belenguer and Boeuf, 1990).  At low discharge powers the plasma electrons are predominately heated by the wave-riding mechanism (the alpha regime) whereas at high discharge powers it is the secondary electrons emitted from the electrodes by ion bombardment that do most of the work, having been accelerated within the sheaths. This transition between wave-riding or secondary emission being the dominant electron production process is also marked by a decrease in the mean electron energy. An RF plasma in the secondary electron regime must have a root mean square voltage comparable to that of a similar DC discharge, but the voltage in the wave-riding regime can be considerably lower.
Second, in an RF discharge with asymmetric electrodes, as is the case for an analytical glow discharge, there will be at steady-state a constant self-bias voltage between the electrodes on which the RF voltage is superimposed (e.g. Lieberman and Lichtenberg, 1994). It is this bias voltage which will determine the energy of ions which strike the sample (acting most of the time as a cathode because it is the electrode with the smaller effective surface area). The DC bias voltage is determined by the electrode geometry and the discharge conditions but will generally be of the order of half the peak-to-peak RF voltage. 
So, the bias voltage in an RF discharge is considerably less than the peak-to-peak applied RF voltage and the voltage necessary to sustain an RF discharge is anyway lower than for a DC discharge. Thus the energies of the ions incident upon the sample may also be lower allowing greatly reduced or no sputtering when the discharge is used at low powers.

References:

  1. P Belenguer and J P Boeuf “Transition between different regimes of rf glow discharges”, Phys. Rev. A 41(8);1990; pp4447-4459.
  2. K Shimizu and T Mitani (2007) “A novel use of rf-GD sputtering for sample surface preparation for SEM: its impact on microscopy and surface analysis” presented at the European Working Group on Glow Discharge Spectroscopy meeting, Brussels, September 2007.
  3. M A Lieberman and A J Lichtenberg “Capacitive Discharges” chapter 11 in “Principles of plasma discharges and materials processing”, Wiley-Interscience 1994 (1st edition) ISBN 0-471-00577-0

Additional background material:

  1. B. Chapman, Glow Discharge Processes; Wiley, New York, 1988
  2. Bogaerts A., Neyts E., Gijbels R., van der Mullen J., Gas discharge plasmas and their applications, Spectrochim. Acta: Part B 57; 2002; 609.

James WhitbyFirst published on the web: 15.02.2008

AuthorJames Whitby. The text is based on a lecture given by Philippe Belenguer (LAPLACE Toulouse), at the first GLADNET training course in Antwerp Sept. 2007.