Sputtering in analytical glow discharges
The process by which analyte atoms are sputtered from the sample into the discharge where they may be ionised and excited is absolutely fundamental to the utility of glow discharges in quantitative analysis. Key to the success of the technique is that the sputtered material has the same composition as the bulk sample. This is normally true, even though different elements or regions of the sample may be sputtered at different rates, because as sputtering progresses the available area of more easily sputtered species decreases and for bulk samples a steady state is reached in which the sputtered fluxes of various elements have the same stoichiometric ratios as the bulk material. (N.B. at this point the surface composition is no longer the same as the bulk).
Sputtering occurs when energetic ions or neutral atoms hit a surface. If the sputtering particles have energy significantly in excess of the binding energy of the target, then a target atom can be released with a probability known as the sputtering yield for that material – the number of sputtered particles per incident particle as a function of energy and identity of the incident particle. For incident argon ions, sputtering yields have a lower threshold at about 20 eV incident particle energy and then rise rapidly to the order of unity for energies of a few hundred electron volts for most target materials. Sputtering yields depend on the mass of the incident projectile as well as the energy, with heavy atoms such as xenon being much more effective than light ones such as helium. See the web-site of the Technical University Wien for a sputtering yield calculator based on the data of Matsunami et al (1984). The sputtered particles generally are not charged, exceptions being strongly electropositive (e.g. Na) or electronegative (e.g. F) elements. Initial energies of the sputtered particles under typical glow discharge conditions are of the order of 1 eV, but collisions with the neutral gas atoms necessary for the discharge are expected to cool the sputtered atoms to thermal energies (an 800 K gas temperature corresponds to 0.07 eV mean translational energy of particles) on the scale of a few mean free path lengths. Sputtered particles come from close to the surface – the incident particles may penetrate to a depth of about 2 nm in a glow discharge (~8 atomic monolayers), but the ejected particles must arise from nearer the surface. Monolayer resolution from an analytical glow discharge has been reported (Shimizu et al., 2004).
A side effect of the collisional cooling on short spatial scales by the background gas is that a significant fraction of the sputtered material may end up back on the surface (back-diffusion) where it can be sputtered again. Estimates of this fraction vary between 17% and 90%, but the effect on analytical measurements is just one of minor depth resolution degradation compared to the situation in a vacuum. However, this does mean that calculated sputtering rates in a glow discharge should not be compared directly to literature values obtained in vacuum. Normally almost all the sputtering in an analytical glow discharge is done by the noble gas, but under some conditions the sputtered material can itself be ionized and accelerated onto the cathode thus contributing to the sputtering – this is known as self-sputtering. Because the sputtered target atoms are often heavier than the inert gas used for the discharge (e.g. copper sample in argon discharge) they have high sputter yields and can then contribute significantly to the total sputtering rate.
The phenomenon of sputtering is not unique to analytical glow discharges; it is widely used in industry to prepare coatings on surfaces. Some animations showing the details of the sputtering process can be seen on the web pages of the Penn. State chemistry department. Sputtering can liberate not just atoms but also molecules and so glow discharges may also be used to study molecular materials.
- Matsunami N, Yamamura Y, Itikawa Y, Itoh N, Kazamata Y, Miyagowa S, Morita K, Shimizu R and Tawara H, Atom. Data. Nucl. Data Tables 31. 1984
- Shimizu K, Payling R, Habazaki H, Skeldon P and Thompson G E , J. Anal. At. Spectrom.; 19; 2004; pp 692-695.
“Sputtering: Basic Principles”, B V King, chapter 6.1 in “Glow Discharge Optical Emission Spectrometry” ed. R Payling, D Jones and A Bengtson, (1997) John Wiley & Sons Ltd.
First published on the web: 29.10.2007
Author: James Whitby. The text is based on a lecture given by Zoltan Donko, at the first GLADNET training course in Antwerp Sept: 2007.