Atomic emission spectroscopy of a hollow cathode discharge used for deposition applications

In this work a d.c. discharge with a copper hollow cathode in argon used for deposition applications was investigated using atomic emission spectroscopy. Typical discharge parameters during our investigation were pressures between 80 and 200 Pa and current densities up to 7 mA cm−2. The sputtered copper atoms were deposited onto cylindrical stainless substrates inserted into the cathode. The radial light intensity profiles of some selected copper and argon lines were measured. An analysis of these profiles gives insights into the excitation rates of the filling gas atoms and metal atoms as well as into the radial dimension of the negative glow. The different excitation mechanisms lead to a sharp change of the radial light intensity profiles emitted by many copper spectral lines, which indicates the transition region between the cathode fall and the negative glow. The length of the cathode fall was observed to be very insensitive to all the macroscopic discharge parameters. A nearly constant value was found for the cathode fall length which corresponds to approximately 22% of the cathode radius. Copper films with a thickness of approximately 5 μm showing a good uniformity could be deposited only onto substrates with small diameters in relation to the cathode inner wall diameter, i.e. φs≤0.2 φw. A correlation between the relative light intensities of the Cu I line at 510.7 nm and the deposition rates was found by plotting these quantities as a function of the invariant parameter associated with the current density. Furthermore, from the experimental points, a threshold value was determined for the invariant parameter of approximately 0.15 mA cm−1 Pa−1, above that the deposition starts. These results might be scaled up for other discharges with a cooper hollow cathode in argon, because they were obtained in terms of the invariant parameter.