Design and Fabrication of Multilayer Piezoelectric NEMS Resonators
We have successfully demonstrated the fabrication of self-actuating multilayer NEMS resonators including cantilevers and double clamped beams. Resonant sensors are widely used as precision mass sensors for the detection and quantification of small concentration of analyte molecules in a gaseous solution or bacteria from biological sample which requires extremely high mass sensitivity and responsivity of the resonant sensor. Nanofabrication techniques have enabled the use of ultrathin layers for the fabrication of resonant sensors to significantly enhance sensitivity by reducing the mass of the sensor. Aluminum nitride (AIN) as an ultra-thin PZE layer has significant advantages over other transduction techniques due to its high PZE efficiency, good mechanical and thermal stability and compatibility with CMOS processing. 50nm ultra-thin layer of AIN was deposited over 25nm thin layer of platinum (Pt) in order to get well textured columnar growth of AIN crystals. AIN deposition parameters were carefully optimized to enhance piezoelectric response. The residual stress and piezoelectric properties of AIN film were studied as a function of deposition conditions. Short circuiting of electrodes was the most critical problem which was solved after implementing two different process flows. Oxide bridges were fabricated in order to avoid unwanted electrical contact between different metallic layers. At the end, suspended cantilevers and beams were suffering from bending due to high compressive stress in the active layer. Residual stress of -169.80 MPa was calculated using Stoney formula by measuring the curvature of wafer before and after the deposition of individual layers. Numerous studies have already been done on shifting of the resonance frequency due to the variations in attached mass on the surface of a resonator but there is still room for the investigation of surface stress changes on the resonance frequency. Application of surface stress induces a change in beam stiffness and dimensions which alter the resonance frequency of the cantilevers and doubly clamped beams. The motivation of this project was to carried out different experiments to study the effect of surface stress on the resonance frequency by using an active piezoelectric layer for the actuation of resonant structures. Application of voltage across PZE layer generates a longitudinal stress across the beam which leads to a shift in resonance frequency. Vibrometer was used to find the resonance frequency of the released devices.