Thin Film Substrate
New applications in the photovoltaic, thermoelectric, storage, and semiconductor markets are spurring innovation in ceramic and semiconductor sputtering targets. Magnetron sputtering was initially developed using metal or alloy targets with materials having high electrical conductivity (e.g., Al, Ag, Au, Cu, Ti, Mo, etc.). In order to achieve acceptable deposition rates, the target material needed to be electrically and thermally conductive. Ceramic sputtering targets were developed for transparent conductive oxides (TCOs) and usually consisted of films made from compositions of ZnO:Al2O3 (2% wt) or In2O3:SnO2 (10% wt). However, as the name implies, the materials were fairly conductive and were suited for DC magnetron sputtering.Pulsed-DC and RF magnetron sputtering allows for the deposition of materials with poor electrical conductivity. Semiconductor materials with better electrical conductivity can be sputtered with pulsed-DC power supplies, while insulating materials (mainly ceramics) require RF sputtering. The deposition rates for RF sputtering are generally much lower than with pulsed-DC. Also, pulsed-DC sputtering has a lower deposition rate than DC sputtering. New applications in photovoltaic, thermoelectric, storage, and semiconductor markets are spurring innovation in ceramic and semiconductor sputtering targets.
DC sputtering with metallic targets has fewer process problems since the metals are ductile and the materials feature high conductivity. Conversely, semiconductor and ceramic sputtering targets are more prone to process difficulties due to the brittle nature of the materials and the poor electrical and thermal conductivities. In order to achieve consistent sputtering over the life of the target, it is essential to have a well-sintered target material with high density. Voids and cracks in the material can propagate and lead to sputtering problems such as arcing, target cracking, and particle generation.
One of the key benefits of RF and pulsed-DC magnetron sputtering is the wider array of materials available to sputtering, including fully insulating ceramics and semiconductors. Challenges must be overcome due to these materials’ brittle nature and poor electrical and thermal properties. Achieving a well-sintered, fully dense target results in uniform performance throughout the target life.
Key requirements for being able to achieve this goal include understanding the materials’ characteristics and combining the optimal compound formation parameters, powder size, sintering temperature, and sintering pressure while maintaining the desired purity and composition. Finally, the target assembly must be well constructed to ensure adequate heat flow between the target surface and the backing plate.