Tailoring The Microstructure And Properties Of Ti-Al-N Thin Films Deposited By High Power Pulsed Magnetron Sputtering

Ti₂AlN thin films has the advantages of both metal and ceramic.For the fabrication and industrial application of Ti₂AlN thin films,the vacuum annealing Ti/AlN multilayers is a promising method.The crystal structure of Ti₂AlN is graphite-like nano-layere,the（002）plane has higher conductivity and elastic modulus.The weak bonding between（002）plane make it easy to slip.Therefore,the（002）textured Ti₂AlN thin films can be used as ohmic connection layer in the integrated circuits,damping device in MEMS,wear resistance and anti-friction films.In addition,the physical,chemical and mechanical properties of thin films are closely related to its grain size and surface roughness.Therefore,it is worthwhile make the attempts to tailor the microstructure of the Ti/AlN multilayers,in turn tailor the texture,grain size and surface roughness of the Ti₂AlN thin films.High-power pulsed magnetron sputtering（HPPMS）has high ionization rate of sputtering species and the ion energy can be easily regulated by substrate bias.It pave the way to tailor the crystal structure,texture,grain size and surface roughness of Ti/AlN multilayers.In this study,the microstructure of Ti thin film and AlN thin film was tailored by HPPMS firstly,and the microstructures of Ti/AlN multilayers with different modulation ratio and period were tailored by selected appropriate Ti film and AlN film deposition parameters.The as-deposited Ti/AlN multilayers were subsequently annealed at different temperature in a vacuum chamber to synthesise Ti₂AlN thin films.The relationship of microstructures between the Ti/AlN multilayers and the Ti₂AlN thin films was studied,and its influence on the meannical properties of the Ti₂AlN thin films was dicussed.In the last section of this study,the attempts to release the residual stress and enhance the mechanical properties of Ti N thin film deposited by HPPMS by introduction Ti₂AlN in it was made.In the first section of this study,the influences of magnetic field strength of the sputtering target surface and HPPMS pulse width on the Ti ion and atom flux and ion/atom ratio in front of the substrate were studied.The results show that,with the decreasing of the magnetic field strength,the ion/atom ratio in front of the substrate is increased because of the weakened constraint effect to ions by target surface.In addition,combination with increasing the HPPMS pulse width,the Ti ion and atom flux and ion/atom ratio in front of the substrate further increased due to the increased sputtering power.By the way,we changed the ion energy of Ti by applying a negative substrate bias.Sputtering the target which has low magnetic field strength with longer pulse width,and none substrate bias was applied,the plasma characteristic in front of the substrate is high Ti ion and atomic flux and ion/atom ratio,low ion energy.The microstructure characteristic of as deposited Ti thin film is low surface roughness,large grain size,（002）textured.The residual stress of thin film is tensile,the hardness is low,and the adhension is strong.Sputtering the target（low magnetic field strength）with shorter pulse width,and a-100 V substrate bias was applied,the plasma characteristic in front of the substrate is low Ti ion and atomic flux and ion/atom ratio,high ion energy.The microstructure characteristic of as deposited Ti thin film is high surface roughness,small grain size,（100）textured.The residual stress of thin film is compressive,the hardness is high,and the adhension is weak.In the second section of this study,the influences of pulse width and N₂ gas flow ratio on the Al target sputtering mode and the plasma characteristics in front of the substrate,and the microstructure of AlN film were studied.The results show that,with the increase of N₂ gas flow ratio,the Al-N compound layer will gradually formed on the target surface,and the sputtering mode of the Al target will changed from metallic（sputtering）mode to transitional（sputtering）mode,and finally to the compound（sputtering）mode.Since the secondary electron emission coefficient of the Al-N compound is higher than that of Al,the target discharge current is will continuously increased from the metallic mode to the transitional mode,and kept constantly in the compound mode.As the sputtering mode changes from the metallic mode to compound mode,the Al ions and atoms flux in front of the substrate decrease because of the sputtering yield of the Al-N compound is lower than that of Al.and the ion/atom ratio increase due to the increased peak sputtering power.With the increasing of pulse width,both the average and peak sputtering power is increased,and both the Al ions and atoms flux and ion/atom ratio increase,but a increasing N₂ gas flow ratio is required for the sputtering mode change from metallic to transitional mode due to increased sputtering rate of the Al-N compound layer.By sputtering the Al target in transitional mode,the deposition rates of AlN thin films are high,the residual stresses are small and the surface roughnesses are low.In adition,combination with longer pulse width,a（002）textured AlN thin film can be deposited.In the third section of this study,the Ti₂AlN thin films were synthesised by vacuum annealing Ti/AlN multilayers.The results show that,at 400°C,AlN is decomposed,and at600°C,the Ti₂AlN is formed.By increasing the annealing temperature to 700°C,the Ti₂AlN begins to be decomposed,and Ti N_x and Ti_xAl_y is formed in the annealed thin films.For synthesis high crystalline quality Ti₂AlN thin films with higher hardness,vacuum annealing Ti/AlN multilayers which modulation ratio is 3:2 and the modulation period is less than 30 nm is appropriate.In addition,for synthesis（002）textured,small grain size,dense and low surface roughness Ti₂AlN thin film,during Ti/AlN multilayers deposition,sputtering the Ti target which has low magnetic field strength with longer pulse width,and none substrate bias was applied,and sputtering the Al target in transitional mode are appropriate.This textured,fine grain,dense and low surface roughness Ti₂AlN thin film’s hardness is 32 GPa,friction coefficient is 0.15,and adhered well with substrate.In the last section of this study,the influence of substrate bias on the microstructure and residual stress of Ti N thin films deposited by HPPMS were investigated.The attempts to release the residual stress and enhance the mechanical properties of Ti N thin film deposited by HPPMS by introduction Ti₂AlN in it was made.The results show that,with the inlet of N₂,Ti-N compound layer will formed on the target surface,and more secondary electrons are emitted from it,which leads to an increase of the target discharge current.In front of the substrate,the Ti ion and atom flux decrease,and the Ti ion/atom ratio increases.During Ti N thin film deposition,while none substrate bias was applied,the etching and bombardment effects by Ti ions is weak,as a results,the deposition rate of the film is high,and residual compressive stress of the film is small.By appling a-150 V bias on the substrate,the bombardment effects by Ti ions is intensified,as a results,the Ti N thin film’s grain size is small,the hardness is high,the surface roughness is low,and the residual compressive stress is large.By introduction Ti₂AlN in Ti N thin film,the residual compressive stress generated from bombardment effects by Ti ions in HPPMS can be completely released.In addition,the hardness and wear resistance of Ti₂AlN/Ti N multilayers are comparable to those of Ti N film,and the adhension and toughness are enhanced.In summary,the effect of magnetic field strength of the sputtering target surface and HPPMS pulse width and reactive gas（N₂）flow ratio on the ion and atom flux and ion/atom ratio in front of the substrate was revealed in this dissertation.The effect of ion and atomic flux,ion/atom ratio,ion energy on the microstructure of Ti and AlN films deposited by HPPMS was revealed also.The microstructure of relationship between the as-deposited Ti/AlN multilayers and the annealed Ti₂AlN films was found.The（002）textured,grain size,surface roughness,hardness,wear resistance and bonding strength of the Ti₂AlN film was acquired.The results shown in this dissertation are beneficial to the application of Ti₂AlN thin films and open a new perspective toward the microstructure and properties tuning of M_n+1AX_n phase thin film families.