Mo/La2O3 NANO MULTILAYERS AND THEIR THERMAL STABILITY AND ELECTRICAL PROPERTIES

Abstract

Surface acoustic wave based high-temperature sensors are promising devices owing to advantages such as robustness, passive operation and wireless interrogation making them usable in harsh environments as well as mountable on moving equipments. The device’s potential to respond to any kind of surface changes/alterations as an electrical signal is the basic underlying principal behind its functioning and sensing. The susceptibility to surface perturbations vary with the configuration of the electrode, choice of substrate and metallization system and the surrounding environment. One of the key challenges in this field is the choice of the electrode material that is stable with long lifetime and high reliability up to operating temperatures of about 800 °C. The main limitations for the use of such nanocrystalline metal films are related to stress-induced damaging processes such as agglomeration, cracking, and delamination along with stress-driven mechanisms such as high temperature creep which become significantly operative at ~ 0.4 Tm. Thus, structural failure would consequently lead to the electrical failure of the device. In the present dissertation work, the high temperature stability of a new electrode material, Mo-La2O3 composite, has been studied up to 800 °C. Different multilayer thin film metallization was deposited on a Si (100) wafer using DC magnetron sputtering at same operating conditions to study microstructural and electrical properties in as-deposited and heat-treated conditions. Studies revealed that a combination of Mo and La2O3 together has improved the performance of the thin films than pure Mo. Among all the films designed here, Mo-(La2O3-Mo)8-Si is the most interesting. This film shows the uniform and stable microstructure even after 120 h of vacuum annealing with only a slight decrease in the conductivity. In as-deposited condition, all the films showed a typical rice-like grain morphology. Upon annealing for 120 h, the Mo-(La2O3-Mo)8-Si thin film showed the formation of equiaxed grains with smallest in-plane and out-of-plane grain size. The increase in the RMS roughness value was also found to be small for the annealed Mo-(La2O3-Mo)8-Si thin film. XRD analysis showed the presence of diffraction peaks of (110), (220) and low intensity (211) associated with BCC Mo. It was observed that both in as-deposited and annealed condition, the multilayer film with 8 bilayer period showed the lowest microstrain when compared to pure Mo and other multilayer films. XRR examination revealed that the density of the all the individual Mo layers were lower than that of the values in bulk indicating that the formed Mo-(La2O3-Mo)8-Si film is not dense and consists of pores and voids in both as-deposited and heat-treated condition. The curve fitting showed that the thickness of the individual layers matches well with the intended values. TEM ix investigations evidenced the presence of clear and distinct interlayers containing the agglomerates of La2O3 in 120 h annealed Mo-(La2O3-Mo)8-Si multilayer thin film.