Hollow Cathode Sputtering

Hollow cathode gas flow sputtering operates in the pressure range of 0.1-1 mbar. The particles are thermalized at the substrate. Plasma activation of growth processes can be achieved either by applying a substrate bias or by pulsed mode operation of the discharge. 

---

Another study (200) presented IR data for a number of hydride and deuteride species. Using matrix-isolation spectroscopy in conjunction with a hollow-cathode, sputtering source (the apparatus for which is shown in Fig. 36), the IR-active vibrations of the diatomic hydrides and deuterides of aluminum, copper, and nickel were observed. The vibra-... 

FIGURE 10-19. Hollow cathode sputtering chamber. [From B. M. Gatehouse and A. Walsh, Analysis of Metal Samples by Atomic Absorption Spectroscopy, Spectrochim. 

Use of glow-discharge and the related, but geometrically distinct, hollow-cathode sources involves plasma-induced sputtering and excitation (93). Such sources are commonly employed as sources of resonance-line emission in atomic absorption spectroscopy. The analyte is vaporized in a flame at 2000—3400 K. Absorption of the plasma source light in the flame indicates the presence and amount of specific elements (86). 

In general, the speed at which transfer and deposition take place is low, but it may be improved by magnetron sputtering, of which the types available include planar, closed field, hollow cathode, and post cathode—all giving coatings with good geometrical array. 

Fig. 5.33. Schematic diagram of hollow cathode gas flow sputtering. Total pressure pt.ot, 0.1 — lmbar. Gas flow q( Ar) fsl — 5slm for 75 cm target length...

Brief mention should also be made here of high intensity (also known as boosted output ) hollow cathode lamps.7 In these lamps an auxiliary current of around 200-400 mA is applied to the dilute cloud of atoms sputtered outside the central zone of the normal hollow cathode. The atoms are thus excited and emit intense radiation which may be used in AAS or AFS. Once again an auxiliary power supply is required, and the lamps themselves are more complex and correspondingly more expensive. Such lamps have had a rather chequered history, finding great favour in some environmental analytical laboratories but never being widely used on any routine basis. 

Source operating power may have a substantial influence upon both absorbance and upon signal-to-noise ratio. For hollow cathode lamps, the maximum safe routine operating current is often printed on the lamp label. If this current is exceeded for more than a very short period, the lamp may be permanently damaged. Normally a much lower current of around 4-1 mA is used in routine work. Consider what happens to atoms in the immediate vicinity of the hollow cathode if the lamp current is excessive (Figure 1). Sputtered atoms will escape from the immediate vicinity of the hollow cathode. 

Cases have been observed where the isotopic line absorption profiles completely overlap, e.g. boron-10 and -11 in a krypton-filled lamp at 249.7 nm [244]. Hannaford and Lowe [245] later showed that this was caused by an unusually large Doppler half-width induced by the fill-gas, and, if neon is used, the 208.9 and 209.0 nm lines can allow the determination of boron-10 and boron-11 isotope ratios. The 208.89/208.96 nm doublet was found to be more useful than the 249.68/249.77 nm doublet. Enriched isotope hollow-cathode lamps were used as sources. A sputtering cell was preferred to a nitrous oxide/acetylene flame as the atom reservoir, as it could be water-cooled to reduce broadening and solid samples could be used, thus avoiding the slow dissolution in nitric acid of samples of boron-10 used as a neutron absorber in reactor technology. 

The reaction of metals with energetic hydrogen or deuterium ions is important in nuclear reactors. Ion beams may be generated thermally and allowed to interact with the metal, and the reaction products then may be examined by matrix-isolation techniques. Alternatively, metal atoms are sputtered from a cathodic surface by a low-energy plasma. If or at low P is added to the discharge, then molecular species are formed by the interaction with the sputtered metal atoms. Applied to Cu this technique leads to the identification of CuH and CuD in Ar matrices by their IR spectra. The reacting species are believed to be atomic Cu and H or D formed in the hollow-cathode discharge . 

While the first hollow cathode tubes were constructed in such a way that they could be repeatedly flushed with the purified noble gas, the inconvenience connected with such equipment led to the development of permanently sealed tubes. In order to insure a reasonable lifetime of such tubes, they have to be of a certain minimum volume. One of the reasons for the lifetime limits is leakage of air into the tube, but more important seems to be the loss of the filler gas which is slowly absorbed by the metal and the glass surface. Since the lamp operates by the sputtering off of the cathode lining, gradual loss of the latter leads to eventual deterioration of the lamp. Lamps for metals that sputter abundantly, like the alkali metals, or zinc and cadmium, have short lifetimes, mostly well below a hundred hours. 

The principle of the hollow cathode tube, production of a vapor of atoms by cathodic sputtering, has been employed by Gatehouse and Walsh (Gl) for sample vaporization. The sample is introduced into a vacuum chamber and is made the cathode which produces a cloud of activated atoms. The light of a separate hollow cathode tube is passed through this vapor and absorption is measured in a spectrophotometer. 

The hollow-cathode plume (HCP) [137] makes rather different use of the hollow-cathode effect. The hollow-cathode base is a sample disc with a 1-2 mm orifice. By choosing appropriate gas pressures and discharge currents, the energetic plume is ejected through the orifice. A constricted hollow-cathode discharge localized in the narrow orifice creates intense sputter action on the surface of the disc and causes direct transport of the atoms into the plume for excitation — and ionization, if required. 

Sputtering The process whereby an atomic vapor is produced by collisions with excited ions on a surface such as the cathode in a hollow-cathode lamp. 

---