Analysis and optimization of sputter deposited AlN-layers for flexural plate wave devices

Aluminum nitride (AlN) thin films deposited by reactive radio frequency magnetron sputtering in an Ar/N-2 discharge on Si(001) substrates were studied with respect to structure, stress, and piezoelectric properties. In order to optimize the AlN layers for flexural plate wave (FPW) devices, the influence of process pressure and N-2 concentration has been evaluated by means of spectroscopic ellipsometry, residual stress measurements, x-ray diffraction, atomic and piezoresponse force microscopy, along with analysis of the piezoelectric charge coefficient d(33,f). FPW devices with low compressively stressed (-200 to -300 MPa) AlN layers were prepared and characterized by white light interferometry and Raman measurements. With increasing pressure from 3 x 10 (3) to 8 x 10 (3) mbar, a transition from -840 MPa compressive stress to +300 MPa tensile stress was measured. Increasing the nitrogen concentration from 3.3% to 50% resulted in a change in stress from +150 to -1170 MPa. All films exhibited a high degree of c-axis orientation. A piezoelectric charge coefficient up to d(33,f) approximate to -6.8 pC/N was obtained. Furthermore, it is shown that the film surface morphology is also very much dependent on the growth conditions. A model regarding the mean free path of the sputtered particles and the film surface morphology is proposed. The authors show that the optimization of the film stress by means of the nitrogen concentration in the sputter gas mixture is beneficial as the process window is larger. (C) 2016 American Vacuum Society.