Preparation And Characterization Of ScAlN Thin Films With High Performance

Scandium doped aluminum nitride(ScAlN)piezoelectric thin film has many advantages such as high velocity,good thermal stability,board bandgap,especially the fabrication process is compatible with CMOS.Meanwhile,ScAlN overcomes the shortages of AlN such as small piezoelectric coefficient and low electromechanical coupling factor.Having important applications in bulk acoustic waves,surface acoustic waves,energy harvesting,ultrasonic detection,and HEMTs,ScAlN has been a hotspot in various countries in recent years.Focusing on key scientific and technology issues in preparation and characterization of ScAlN functional film in high-tech MEMS/NEMS piezoelectric devices,this thesis has important needs background.Based on the research and analysis of crystal structure,material properties and available preparation methods,the method of pulsed-DC magnetron sputtering was chosen in this thesis.The ScAlN thin film with high scandium concentration,good crystal quality and strong piezoelectric response was prepared.The working principle of magnetron sputtering was analyzed after a investigation on research actuality;the mechanism of ScAlN film deposition in magnetron sputtering was studied;the influence of key parameters such as power,gas ratio,total gas flow and seed layer on film properties were studied and the process parameters were optimized;the doping mechanism of scandium in ScAlN was discussed and the reason why the piezoelectric property was enhanced after doping was explained.The Sc_0.35Al_0.65N piezoelectric thin film prepared at room temperature has a FWHM of 2.1°and a high vertical piezoelectric constant d₃₃ of 27.5 pC/N.The main research contents and relevant conclusions of this article are as follows:(1)Four sets of experiments were designed through the principle of control variables and the controlled variable method was used to optimize the process parameters.Combined with the characterization results of XRD and SEM,the effects of sputtering power,nitrogen-argon ratio,total gas flow rate and seed layer on the crystal quality and micro-morphology of aluminum scandium thin film were studied one by one.The affection of process parameters on thin film growth were studied,a reference curve was fitted for optimization of process parameter.The Sc_0.35Al_0.65N thin film was successfully prepared under room temperature.The half-width of the rocking curve of the film is 2.1°,and the longitudinal piezoelectric constant d₃₃ is 27.5 pC/N,which is 4 times as much that of pure aluminum nitride(2)Designed an exclusive structure for d₃₃ test,the testing was simplified to improve the test efficiency.(3)The piezoelectric constant d₃₃,Young's modulus and dielectric constant of the film were tested using the dielectric analysis modules in the piezoelectric tester quasi-static,and nano-indenter.The electromechanical coupling coefficient of the film was calculated,(k₃₃^l)² is 42.2%and thickness-extensional coupling factor(k₃₃^t)² is 29.5%,which is 4 times as much that of pure AlN.(4)Combined with XRD,SEM,EDS,XPS,Raman spectrum and nano indenter,the crystal structure and doping mechanism of aluminum scandium nitride film were analyzed,the reason why the piezoelectric properties was enhanced was explained.It is believed that the film is not simply a stack of hexagonal aluminum nitride and cubic scandium nitride cell.Aluminum atoms and scandium atoms in the crystal will compete for nitrogen,resulting in softening of the elastic coefficient in the c-axis direction,which greatly improves the piezoelectric response sensitivity in this direction.At the same time,scandium nitride may exist in the hexagonal wurtzite transition state rather than the cubic rock salt in the film,the ScAlN film within the doping range prepared by the experiment still shows the wurtzite structure as a whole.