Preparation of highly textured Mo and AlN films using a Ti seed layer for integrated high-Q film bulk acoustic resonators
Effects of seed layers on the properties of Mo and AlN/Mo (AlN on Mo) films were extensively investigated for developing high-Q film bulk acoustic resonator (FBAR) devices. The Mo films deposited onto a Ti seed layer were more textured and smoother than those deposited with no seed layer, whereas the use of Cr and Ta seed layers degraded the quality of Mo films. The influence of Mo sputtering conditions on the quality of Mo films was also checked, but the effect was negligible in the case of using a Ti seed layer. The AlN films deposited on top of Mo films with a Ti seed layer showed better c-axis orientation and smoother surface morphology than the AlN films without any seed layers. The AlN/Mo films with a Ti seed layer had full width at half-maximum (FWHM) values ranging from 1.6° to 1.8° and surface roughness ranging from 6.0 to 7.4 Å, while the AlN/Mo films without any seed layers had a FWHM value of 3.6° and surface roughness of 12.6 Å. It is considered that properties of AlN films are very dependent on the quality of the bottom electrodes. Film bulk acoustic resonators in the form of a membrane type were fabricated by using a bulk micromachining method and their resonance characteristics were measured and compared. The resonator comprised of Mo/AlN/Mo/Ti/membrane films had an effective mechanical coupling coefficient of 7.3% and a quality factor of 1514, while the resonator without a Ti seed layer had an effective mechanical coupling coefficient of 6.9% and a quality factor of 630, respectively.