Study On Prepration Of Niti Based Shape Memory Alloy Thin Films By Magnetron Sputtering And Its Properties

NiTi shape memory alloy thin film is a promising material in MEMS applications.The moisture absorbed can have negative effect on device reliability.Hydrophobic property is an important factor in the utilization of NiTi thin films.Surface morphology and hydrophobic property of NiTi thin films are investigated under various in situ and post annealing temperatures.The XRD and AFM results reveal that in situ annealed NiTi thin films crystallize at lower temperature and have larger surface roughness than post annealed samples.Heating during sputtering provides energy for atoms to move to position of equilibrium,thus more regular crystal structure is observed in in situ annealed NiTi thin films.Another reason for the smoother surface observed in post annealed thin films is that large amount of Ni₄Ti₃ precipitation in hinders the formation of B19'phase.The contact angle becomes larger with the increase of surface roughness and the maximum contact angle of 123.5°is achieved in 600°C in situ annealed thin film with 41.7 nm surface roughness.The addition of a third element into NiTi thin films can improve the performance.Ti rich NiTiAl shape memory alloy thin films of different target current are fabricated using DC magnetron sputtering.The effect of aluminum on the phase content,surface morphology,mechanical property and phase transformation behavior is studied.NiTiAl thin films have B2phase and Ti₂Ni phase in room temperature and the content of Ti₂Ni phase increases with Ti content increases.Within a limited range of Ti target current?1.3A to 1.5A?,addition of aluminum can decrease the size of grain and improve hardness,elastic modulus,recovery rate of thin films.The particles on surface of NiTiAl thin films have two different sizes.Spherical particles have smaller size of 10-40nm and rod-like particles have bigger size of 100nm.Phase transformation behavior is studied by resistivity temperature curve and synchrotron radiation XRD over wide temperature range.The temperature coefficient of resistance?TCR?of all the NiTiAl thin films are negative from room temperature to 10K,indicating that R phase is stable in NiTiAl shape memory alloy thin films at large temperature range?from 10K to above room temperature?.Martensite phase transformation is completely prohibited,which corresponds with synchrotron radiation XRD results at 173K.B2-R transformation temperatures in NiTiAl thin films range from 370K to 380K with different elemental composition,which is an advantage for MEMS applications.