Stress Evolution During Crystallization and Isothermal Annealing of Titanium-Nickel on (100) Si

Abstract

ABSTRACTThin films of near-equiatomic titanium-nickel are capable of thermally induced shape-strains and have recently been applied to silicon-based micromachined reversible actuators in force-production devices requiring energy-density levels substantially exceeding those available from piezoelectric, electromagnetic, electrostatic, or bimetal systems. Reversible actuation requires the presence of a resetting agent, or ‘bias’ force, capable of deforming the martensite phase during the exothermic transformation on cooling. Here, we show that both the initial martensite ‘programming’ force, and the bias force for subsequent reversible actuation, can readily be provided by manipulating intrinsic and extrinsic stresses developed during low-pressure sputter deposition of TiNi, and subsequent cooldown from either the deposition process temperature or from the crystallization annealing temperature. The stability of both intrinsic and extrinsic stresses at temperatures below the deposition temperature allows considerable flexibility in the design of the force-producing system, and reversible stress-production to levels beyond 0.7 GPa are shown to be readily achieved, corresponding to energy-density levels approaching 10 MJ-m−2. The present paper will review recent results from our group on the development of residual stresses in TiNi sputtered on (100) Si, using the wafer-curvature method applied during isochronal and isothermal vacuum annealing of both amorphous and crystalline thin films, with an emphasis on the combined influence of deposition temperature and annealing temperature on stress-relaxation rates, and on the response of the system to temperature cycling in the displacive transformation range.