A Research Of Highly C-axis Oriented AlN Thin Films Deposited On Nickel Alloy For High Temperature Applications

In recent years, the flexible surface acoustic wave(SAW) sensor has been widely applied in many fields due to its various advantages and potential application value.Highly c-axis oriented aluminum nitride(AlN) film is one of the most promising piezoelectric materials for these flexible SAW sensors due to its excellent physical and chemical properties. So far, AlN films were prepared on flexible substrates and various flexible SAW devices had been reported. However, these flexible SAW devices were deposited on organic polymer substrates and cannot work at high temperature. To our best knowledge, there was few report of depositing AlN films on flexible metal substrate, which can operate at high temperature. Hastelloy tapes are widely used as flexible metal substrates which can work at high temperature. Then it is interesting and important to deposited AlN piezoelectric films onto Hastelloy tapes for developing flexible SAW sensors.Firstly, we deposited AlN thin films on Hastelloy substrate and Si(100), and it is found that the RMS roughness of the substrate affect the AlN thin films c-axis orientation greatly. The FWHM of the AlN/Hastelloy(002) peak is as high as 16.3°, as a control group, the FWHM of the AlN/Si(100) peak is only 2.3°. A Y2O3 buffer layer was deposited on Hastelloy tape by solution deposition planarization, and the surface of the Y2O3/Hastelloy substrate is very smooth with RMS roughness less than 1 nm. We deposited AlN thin film on Y2O3/Hastelloy substrate using the same deposition parameters and the FWHM of the AlN/Y2O3/Hastelloy(002) peak decreased to 4.4°.Secondly, The effects of the substrate temperature, power, N2 concentration,pressure, target-substrate distance on the c-axis orientation of the AlN films were systematicly studied. It is found that the substrate temperature and power affect the AlN thin films c-axis orientation greatly, and the target-substrate distance have the smallest influence on the c-axis orientation of the AlN films. With optimal process conditions,the FWHM of the AlN(002) peak decreased to 3.7°, and the RMS roughness of the prepared AlN films was 5.46 nm, the piezoelectric coefficient d33 is about 5.02 pm/V.The flexible AlN/Y2O3/Hastelloy samples can be greatly bent and the AlN films don’t delaminate from the Hastelloy tape.Then, we analyzed the SEM cross-section microstructure of the AlN films and gotthe growth mechanism of c-axis oriented AlN films which prepared with the two-step deposition process. The growth of the c-axis oriented AlN films can be mainly divided into three stages: seed layer growth, alignment layer growth, column layer growth, and the first two stages of growth for c-axis preferred orientation of AlN films is very important.Finally, a so-called three-step deposition process have been raised. We gradually increased N2 concentration from 30 % to 50 % during the AlN films sputtering, and improved the abnormal phenomenon of the grain size on the surface of the AlN films.The prepared AlN film is highly c-axis oriented, the FWHM of the AlN(002) peak decreased to 4.2° and the surface RMS roughness is 8.42 nm, which suggested that the prepared AlN films are good enough for the fabrication of the high temperature flexible SAW sensors.