Development of Nano-Structured Thin Film Shape Memory Alloys for MEMS Applications

Abstract

The production of thin film TiPdNi shape memory alloys (SMA) using ion beam assisted deposition (IBAD) is being studied as a way to increase the actuation frequencies and transformation temperatures of thin film SMA for micro-actuator applications. The capability to transmit extremely high forces along with a large stroke, large strain memory, and high corrosion resistance makes shape memory alloys prime candidates for use in micro-actuator applications. However, low actuation frequency (~1Hz at macro-scale), and low transition temperature (below 100°C) makes commercially available NiTi incompatible with applications in extreme environments.

The transformation temperature and actuation frequency of shape memory alloys can be improved through the production of thin film TiPdNi. Through the substitution of Pd for Ni in equiatomic NiTi, the transformation temperature can be varied from approximately room temperature to 527°C. The composition that has received the most attention is Ti50Pd30Ni20 because of its transformation temperature of over 200°C. However, the shape memory effect of Ti50Pd30Ni20 is adversely affected by the low critical stress needed for slip at high temperatures, which results in unrecoverable strain. Age hardening or thermo-mechanical treatments such as cold rolling have been found to improve the critical stress for slip in bulk form SMA due to an increased density of dislocations. Precipitation hardening, as well as, ion bombardment, is expected to increase the high temperature properties in IBAD deposited Ti50Pd30Ni20 film SMA. Additionally, ion bombardment during deposition can be used to improve film properties such as morphology, density, stress level, crystallinity, as well as, limit defects. Due to the refined grain size, increased density, and reduced defects, IBAD is able to produce films of 1 micron or less, which will greatly reduces the SMA actuation time due to the increased surface area —to — volume ratio.

In this study, we have deposited thin film TiPdNi using IBAD with thicknesses of less than 2 microns. It had been suggested in a previous study that ion bombardment could produce films with shape memory properties without the need for additional heat treatment. As-deposited films on unheated substrates were found to be highly amorphous without the martensitic crystalline structure needed for shape memory effect. As a result, post deposition annealing of amorphous films was evaluated and found to cause severe cracking and delamination. When films were annealed with a slow heating and cooling rate, severe cracking was present throughout the surface as a result of decohesion. In contrast, delamination from the film/substrate interface occurred when the heating and cooling rates were increased. The SEM cross sectional analysis after annealing showed the transformation from the flawless cross section before annealing to a porous cross section afterwards. Independent of heating and cooling rates, all attempts at annealing films that had been deposited using IBAD on unheated substrates resulted in film failure from extensive tensile stresses.

In this study, moderate compressive stress was found in IBAD deposited films on unheated substrates. Subsequent annealing of the films resulted in extensive tensile stresses. The magnitude of the stress, and the conversion from compressive to tensile stress, lead to the film failure as described above. Deposition, and in-situ crystallization of films deposited on heated substrates, produced a moderate tensile stress. Neither cracking nor delamination was found in these films. As the result, we concluded that only deposition on heated substrates produced both the flawless cross section and crystalline structure needed for high transition temperature thin films SMAs to be used for MEMS actuators.

In depth analysis of films deposited on heated substrates showed a highly crystallized twinned B19 martensitic structure through the bulk of the film without the need for post deposition heat treatment. In a 1.5 micron thick film, a 70nm thick transition layer was identified between the bulk film and silicon substrate. The remaining film showed a twinned martensite structure. The transition layer between the substrate and the fully martensite layer consists of a 50 nm crystalline austenite layer and two amorphous layers of 10 nm thicknesses each, at the interface with the substrate. A very thin layer of silicon oxide was observed between the 10nm amorphous layers and the Si substrate. No precipitates were found within the film, although a slight compositional gradient was identified.

This study shows that the deposition of thin film shape memory alloys using IBAD on unheated substrates will produce amorphous films that require post annealing to produce shape memory effect. It has also been shown that this post deposition heat treatment has undesirable effects such as formation of a porous microstructure, as well as delamination and cracking as a result of high tensile stresses. On the other hand, deposition of films on heated substrate can produce the desired microstructure needed to achieve shape memory properties while reducing film stresses and decreasing processing time by allowing deposition and annealing occur simultaneously.