The key to achieving high density during the hot press operation is to press the material at high temperatures. However, oxides, sulfides, selenides and tellurides can out-gas at elevated temperatures due to the volatile nature of O, S, Se, and Te. Each compound has an optimal temperature for sintering, where high density can be achieved without triggering the evaporation of the material. This optimal temperature is lowered also due to the fact that pressing occurs in a vacuum.
One key aspect to lowering the propensity of the material to out-gas is to work with a stable compound. Stable compounds can be determined in binary, ternary, quaternary and five-element systems, depending on the line compounds present in phase diagrams.
Physical vapor deposition can stress the target surface immensely. Any differences in electrical and thermal characteristics in the target material can be exacerbated by poor thermal conductivity for many ceramic and semiconductor targets. For this reason, many targets exhibit poor performance at thicknesses much greater than 6 mm. Metallic targets can be sputtered at thicknesses greater than 20 mm with no issues due to the metals’ excellent thermal conductivity.
The poor thermal conductivity can affect how much power can be applied during sputtering. While some metallic targets can be sputtered at high powers of 10-30 kW, many semiconductor targets sputtered by pulsed-DC can only be sputtered at < 5 kW. Insulating ceramic targets sputtered by RF are at even lower powers of < 1 kW. The lower power levels and smaller duty cycles result in reduced deposition rates. The target surface can still be exposed to temperatures of several hundred degrees C despite the circulation of cooling water on the backing plate. This temperature can be even higher with materials with poor thermal conductivity. Target cracking can occur when the temperature of the target surface gets too high. Depending on the desired film thickness, the deposition rate can be a determining factor in the viability of the material for use in commercial production.
Another determinant of the target microstructure and the quality of the sintering process is the powder size. Finer powders can lead to higher density targets. However, oxidation can be increased as well since finer powders are associated with larger surface area. Figures 5 and 6 show the difference between an (In,Ga)2Se3 target cross-section for an average powder size of 150 µm and a target with an average powder size of 75 µm.
The strength of the materials is also directly related to the quality of the sintering and the material microstructure. Various techniques can be employed in the powder processing steps to minimize the oxidation of the materials. One of the drawbacks of finer powder sizes is the loss in materials, leading to higher costs.