Low voltage bias DC reactive sputtering of cermets.

Abstract

Thin films of cermets are technologically important because their chemical composition and physical properties can be tailored as desired. A few important applications call for the easy and reproducible deposition of cermet films which have their composition graded continuously along their thickness. Graded cermet films are easily deposited using the technique of DC reactive sputtering by varying continuously any of the sputtering parameters during deposition. Use of the conventional sputtering parameters leads to the deposition of step-graded films with compositional step heights of around 50 to 100 Å. It is very difficult to deposit films with smaller compositional step heights using these parameters. The electrical condition of the anode (especially when a low voltage (0–15 V) bias is applied to it) is one sputtering parameter that has not received any attention as a parameter to control the composition of the film being deposited. It is of interest because: (i) Negative ions of the reactive gas (most of which have kinetic energies of only a few electron volts) can be cut off from arrival at the anode, thereby affecting the composition of the film being deposited. (ii) The control parameter being an electrical voltage, it can be varied at rates of a few mV/msec. (iii) The population-energy distribution of negative ions may be continuous. Films of slightly different compositions may then be deposited at slightly different values of the low voltage negative bias. These points lead to the hypothesis that cermet films with a continuous gradation of composition along their thickness can be deposited using a variable low voltage negative bias as the control parameter during DC reactive sputtering. This investigation was aimed at testing the above hypothesis by: (i) Determining the population-energy distribution of the negative ions of oxygen in DC reactive sputtering glow discharges, and (ii) Applying the idea of negative ion control by low voltage biasing to deposit cermet films of different compositions under conditions identical except for the applied bias voltage. The population-energy distribution of the negative ions was obtained from both theoretical considerations and experimental results. It was found from theoretical considerations that the method of formation of negative ions of oxygen in the glow discharges is electron attachment. Considerations of the population-energy distributions of electrons in the glow discharge and the electron attachment cross-section of the formation of negative ions in oxygen lead to the population-energy distribution of negative ions. The I?–V? characteristics were plotted using copper and molybdenum targets in different atmospheres of argon and oxygen and at different sputtering parameters on a DC sputtering unit fabricated for this study. From these results, the shape of the population-energy distribution of negative ions was experimentally determined. The idea of negative ion control to deposit films of different compositions was tested by reactively sputtering copper and molybdenum targets when different bias voltages were applied to the anode under otherwise identical conditions and by examining their physical properties (sheet resistance and lattice parameters) and their chemical composition (ratio of the metal to dielectric constituents by ESCA). The variation of properties with the applied bias voltage was as expected only in a part of the range of the applied bias. A phenomenological model has been developed using the concept of reactive material formation in the virtual thin film to explain the observed results over the full range of applied bias. From the above results it can be seen that a variation of the low voltage negative bias on the anode in DC reactive sputtering systems can be used as a parameter to control the composition of the cermet films deposited. It can therefore be concluded that this method can be used to deposit continuously graded cermet films, which are finding increased technological applications such as in the fabrication of high-efficiency solar selective surfaces.