Development of high quality AlN epitaxial film for 2.4 GHz front-end SAW matched filter

We have developed aluminum nitride (AlN) epitaxial growth technology using Knudsen pressure MOCVD method. The thickness uniformity was ±1%. However groove-like cracks were formed on the surface of the AlN epitaxial film. AlN deposition on off-angle substrates and the AlN deposition at high temperature have been investigated for eliminating the cracks on the surface. AlN deposition on an Al2O3 surface which is -4 degree off-angle from c'-axis has resulted in elimination of the cracks from the SEM (Scanning Electron Microscope) observations. The cracks in an AlN film deposited at a higher temperature of 1140°C/40mTorr are found to be completely eliminated on the whole 2"-φ wafer from SEM and laser-scan microscope observations. The propagation loss has been evaluated from the characteristics of time domain impulse response of 2.4 GHz matched filters fabricated on 2"-φ wafers. The propagation loss of crackless AlN films is drastically improved by one order compared with that of cracked AlN films.