Study on materials and fabrication of functional thin film AlN force sensors

Aluminum nitride (AlN) thin films are widely used in sensors, bulk/surface acoustic wave (B/SAW) filters, and optoelectronic devices, etc. It often requires overlapping of two different thin films to obtain functional devices. Normally a part of the AlN film is deposited on a bottom substrate (such as Si, Glass, etc.) while the remaining part spreads over an electrode material (such as Al, Au, Pt, etc.). This often results in cracks at the step edge between the metal electrode and AlN films, leading to short circuiting when the top metallic electrode is deposited. It is thus of significance to study the thin film materials and their deposition parameters in order to successfully fabricate functional thin film AlN sensors without cracking. Various underlying materials were studied. Al2O3 is identified as a suitable underlying material below the Au electrode. The AlN deposited at the Au/Al2O3 showed no cracks or short circuiting. Thin film AlN piezoelectric force sensors were successfully fabricated on the Al2O3 coated Si wafers. The longitudinal charge-to-force coefficient of the thin film force sensors were also characterized.