Stress relaxation and creep analysis of high temperature thin film materials on CTGS substrates
Synthesis and Properties of Functional Materials
Metallurgical, Thermal and Thermomechanical Treatment of Materials
Final Report Abstract
The aim of the project was directed towards obtaining a thin film system suitable for application as an electrode material in high temperature sensors operating on the SAW principle. The criteria for the electrode system’s applicability were high temperature stability, high electrical conductivity, low residual stresses and microstructure with small grains with low content of defects. In order to achieve long term stability of the thin film, an understanding of the thermomechanical behavior becomes crucial. a) While the aim was to further study the thermomechanical behavior, this could not be performed due to un-repairable technical problems with the measurement tool. As a result, the development of a new tool was initiated which can be used to anneal thin film samples up to 1000 °C under UHV while measuring the stress evolution using a state-of-the-art substrate curvature measurement device kSA MOS. This tool is presently being utilized for the above mentioned studies. b) The main limitations for the use of nanocrystalline metal films are related to stress-induced damaging processes, manifested as voids, hillocks, and mechanical hysteresis that lead to the structural failure and thus consequently lead to the electrical failure of the device. In this work, W/Mo based thin films have been tailored to circumvent the above issues by optimizing the deposition conditions as well as the material combination so as to obtain films that were stable, electrically conducting and with low amount of stresses. High stability of W/Mo films on CTGS substrate was demonstrated up to 800 °C with no cracking or delamination of the films. c) An anomalous result for the electrical resistivity of W/Mo thin films was observed wherein nano-size effects were observed to be non-operational. A systematic study indicated that this anomaly arises due to coherent grain-growth across layers of W and Mo due to their low lattice mismatch. A template-like effect of the W-layer was observed in all the films and hence provides scope to alter the microstructure so as to reduce the influence of grain size in the nanocrystalline thin films. d) A new electrode system, namely Mo-1 wt. % La2O3, was studied as an alternative electrode system which retained a nanocrystalline microstructure even upon annealing up to 800 °C for several days. This was achieved by the pinning effect of uniformly dispersed fine particles of La2O3 in the Mo matrix. e) A first demonstrator of the sensor was designed and produced by structuring the electrodes using a novel reactive ion etching method. A good quality of structuring was achieved with high reproducibility and good edge profiles. Initial studies up to 600 °C show promising results for utilization of W electrodes for SAW sensors.
