Heat treatments of NiTi shape memory alloy (SMA) thin films and its application as a micro-array for detection of infrared radiation

Abstract

Uncooled imaging sensors that utilize the thermo-mechanical properties of thin film NiTi shape memory alloys (SMAs) are fabricated for detection of infrared (IR) radiation. The part of the spectrum relevant to this project is the far-infrared range. This is different from the near-infrared range that can be detected by either a CCD image sensor or a CMOS active-pixel sensor available in many domestic digital cameras and video-cameras. Five micrometer thick NiTi SMA thin films are prepared on glass substrates by direct current (DC) magnetron sputter deposition method. The composition of the SMA films can be varied from 49.2 to 51.5 at. % Ti by changing the target-substrate distance. The films deposited at longer target-substrate distance contain higher Ti content. Since the as-deposited films are amorphous, they are heattreated above the crystallization temperature. The thermal heat treatment is followed by an aging process to induce two-way shape memory (TWSM) in the films. The polycrystalline phases and microstructures of the films are analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The transformation characteristics of the films are also studied by differential scanning calorimetry (DSC). It is found that the films developed a multiple-stage transformation (MST) behaviour depending on the annealing temperature and aging time. For the sample, annealing at a higher temperature for a relatively short time has led to one stage R-phase with reverse martensitic transformations above 0 ℃. This one-step transformation is smoother and favours faster response as compared with the multiple-stage transformation. The Ni- 49.8 at. % Ti SMA thin films are patterned to two-dimensional cantilever micro-arrays by photolithographic technique. To ease the lift-off of the NiTi thin film for subsequent heat treatments, a layer of photoresist is spin-coated on a glass substrate before the SMA film deposition. Different baking time and temperatures are applied to optimize the one-step baking process for the bottom photoresist. The higher one-step baking temperature (120 ℃) promotes good interfacial adhesion, and the film has good surface finish for the subsequent photolithographic and wet etching processes. The width and length of each micro-cantilever are 50 and 100 μm, respectively. The phase transformation temperatures are evaluated by DSC as well as displacement measurements. The displacement and the sensitivity of the sample are recorded. After the crystallization and constraint-aging heat treatments, the NiTi SMA thin film cantilever micro-array exhibits a two-way shape memory behaviour in association with the B2 « R transformation at near room temperatures. This two-way shape memory effect has eliminated the need for an external biasing force. Upon the absorption of IR radiation, the NiTi SMA thin film micro-cantilevers are heated up leading to the reverse R-phase transformation. Illumination of the micro-array increases the temperature locally and has caused curving up of the two-way shape memory (TWSM) thin film. In order to improve the resolution of the optical images, an optical collimator is designed and used. In addition, another configuration (50 μm × 500 μm) of the NiTi TWSM thin film cantilever micro-arrays are fabricated and examined. Based on the dimensions of the micro-cantilever, a bending model which simulated the ideal response of a straight micro-cantilever upon the absorption of an incident infrared radiation is developed. It is noticed that a short aperture length will lead to a larger maximum change in the area between the original and the spontaneous reflecting surface (Amax.) of the thin film micro-cantilever, but a longer aperture length would be necessary for allowing sufficiently large deformation of the sample. Accompanying with the long aperture length, the relatively smaller Amax. of the thin film micro-cantilever can be raised by increasing the separation between the collimator and the micro-array. Manipulating the collimator can lead to higher resolution of the optical images by filtering the unwanted light rays off the screen. The change in the light intensity is correlated with the deflection angle of the thin film micro-cantilever, and the deflection angle of the micro-cantilevers is estimated as well.