Effect Of Sputtering Pressure On Properties Of TiNi Alloy Thin Films

In recent years,TiNi thin films by the sputter-deposition method are expected tobe applicable in the field of micro-engineering. Some unique properties like shapememory effect, superelasticity, excellent corrosion resistance, and goodbiocompatibility makes this material acompetitive candidate for the micro-electricalmechanical system （MEMS）（e.g. microvalves, micro-cantilevers and micropumps）.To utilize these properties, some factors need to be considered in view of therelationship between the processing, annealing conditions,phase transformationbehaviors and microstructure of TiNi films. There are some basic requirements forthe successful application of TiNi films,such as the thickness, compositionuniformity across a large area,and a good adhesion with substrate and free ofmacroscopic defects.The establishment of sputtering pressure and annealingconditions for precise control of films quality is the primary challenge. However,there are few reports the important effects of gas pressure and annealing conditionson properties of TiNi thin films in the literature. The aim of this study is toinvestigate the properties of sputter-deposited TiNi thin films under different Ar gaspressures and annealing conditions.The Ni-rich TiNi films were deposited onto glass substrates and Si substrates bymagnetron sputtering in different argon atmosphere using a sputtering target of anequiatomic TiNi alloy. Ar gas pressure varied from2.0×10²Pa to12.0×10²Pa,films were deposited at room temperature and the as-deposited films were annealedunder different conditions. The films composition is determined to be Ti–51.17at.%Ni under Ar gas pressures of6.0×10²Pa by energy dispersive X-rayspectroscopy（EDX）.The microstructures and the phase constituent of thin films wasdetermined by X-ray diffraction. The growth behavior and the mechanical behaviourof rough surfaces of TiNi films has been investigated by Atomic Force Microscopy（AFM），Scanning Electron Microscopy （SEM） has been used to characterize thecross-section and surface morphology of the TiNi films. Themechanical propertiesare studied by Nano-Hardness Tester. In addition, The properties of TiNi films were measured by some other instruments in this work,such as TEM, step Profiler and soon. Through the analysis, these reseach results in this article as following:1.The phenomenology was found for nucleation-coalescence-growth in theforepart of TiNi films’growth,Surface morphology of growing film is determinedfrom several surface processes: adsorption of precursor atoms or molecules,desorption of atoms, and migration of atoms.Migration enhances nucleation ofclusters, growth of islands, and coalescence of islands.2.At a high Ar gas pressure，the film presents a rough surface texture thatconsisted of coalescent islets，building up high mountains and deep valleys on thesurface. With gas pressure increased to10.0×10²Pa, more small-sized islets growand fill up the deep valleys, which increases the surface evenness and the density ofthe islets with the increasing of TiNi thin film. The film thickness、deposition rate andsurface roughness of film decreased with the increment of sputtering pressure. Inaddition,AFM studies on the TiNi tin films were performed and found that the averagegrain size and roughness decrease by increasing the annealing temperaturetemperature.3.TiNi films prepared under Ar gas pressures of4.0×10²Pa at room temperatureand the as-deposited films were annealed in a vacuum under the525℃, this time theTiNi thin film was amorphous, and the thin film crystallized at about600℃. Withsputtering pressure increased to6.0×102Pa、8.0×10²Pa and10.0×102Pa, thesesamples were complete crystallizing under525℃annealing temperature for1hour, and this time only B2and Ti3Ni4phase existed in TiNi films. When theannealing temperature and the annealing time increases, the Ti3Ni4precipitateshave to partially dissolve into the TiNi matrix, the martensite （B19’） and theTiNi3precipitates appeared according to the XRD profiles.4.With a lower sputtering pressure,the residual stress in the depositedfilms is compressive stress, At a low gas pressure, the sputtered atoms pass tosubstrate with little loss of kinetic energy, causing much more atomic peeningeffect and enhancing the mobility of adatoms by transferring kinetic energy tothem.These could promote the generation of compressive stress. As the increasing of sputtering pressure and the residual stress decreased. With an increasein temperature，the tensile components increase due to densification of TiNifilm, When the crystallization event completes,cooling from high temperatureresults in the development of tensile stresses caused by the difference inthermal expansion coefficient between TiNi film and glass substrate. Duringcooling, martensitic transformation occurs and the tensile stress relaxessignificantly due to the formation and alignment of twining structures andshear-variant boundary motion. Large residual stress could lead to either filmcracking and decohesion in tension, or film delamination and buckling incompression.5.The elastic modulus and hardness of the TiNi films were found to bedifferent under different sputtering pressure and annealing conditions, Bothhardness and elastic modulus showed an increase with increasing annealing time oncethe films crystallized,However, with increasing temperature, the elastic modulus andhardness fluctuates but remains increase.Which is attributed to the precipitationharden induced by Ti₃Ni₄and TiNi₃. with the annealing temperature increased to650℃, a layer of TiO₂forms on the surface of film and the oxidation layer becomesdenser, which induce the increase of the elastic modulus and hardness.