A novel surface micromachining process to fabricate AlN unimorph suspensions and its application for RF resonators

A novel surface micromachining process is reported for aluminum nitride (AlN) thin films to fabricate piezoelectric unimorph suspension devices for micro actuator applications. Wet anisotropic etching of AlN thin film is used with a Cr metal mask layer in the microfabrication process. Tetra methyl ammonium hydroxide (TMAH) of 25 wt.% solution is used as an etching solution for the AlN thin films. Polysilicon is used as a structural layer. Highly c-axis oriented AlN thin films are deposited by RF reactive sputtering. Thin layers of chromium on either side of the AlN are used as top and bottom electrodes and also as a mask to etch the AlN and polysilicon layers. The fabricated suspended unimorph structures are tested for scattering parameters using a vector network analyzer. Results show resonant frequencies of devices above 1.7 GHz with an effective electromechanical coupling factor, $K^2_{\mathrm eff} \approx 1.7\%$