Design and Fabrication of Self-Packaged, Flexible MEMS Accelerometer and Aluminum Nitride Tactile Sensor

The work presented in this dissertation describes the design, fabrication and characterization of a Micro Electro Mechanical System (MEMS) capacitive accelerometer on a flexible substrate. To facilitate the bending of the accelerometers and make them mountable on a curved surface, polyimide was used as a flexible substrate. Considering its high glass transition temperature and low thermal expansion coefficient, PI5878G was chosen as the underlying flexible substrate. Three different sizes of accelerometers were designed in CoventorWare RTM software which utilizes Finite Element Method (FEM) to numerically perform various analyses. Capacitance simulation under acceleration, modal analysis, stress and pull-in study were performed in CoventorWareRTM . A double layer UV-LIGA technique was deployed to electroplate the proof mass for increased sensitivity. The proof mass of the accelerometers was perforated to lower the damping force as well as to facilitate the ashing process of the underlying sacrificial layer. Three different sizes of accelerometers were fabricated and subsequently characterized. The largest accelerometer demonstrated a sensitivity of 187 fF/g at its resonant frequency of 800 Hz. It also showed excellent noise performance with a signal to noise ratio (SNR) of 100:1. The accelerometers were also placed on curved surfaces having radii of 3.8 cm, 2.5 cm and 2.0 cm for flexibility analysis. The sensitivity of the largest device was obtained to be 168 fF/g on a curved surface of 2.0 cm radius. The radii of robotic index and thumb fingertips are 1.0 cm and 3.5 cm, respectively. Therefore, these accelerometers are fully compatible with robotics as well as prosthetics.;The accelerometers were later encapsulated by KaptonRTM superstrate in vacuum environment. KaptonRTM is a polyimide film which possesses similar glass transition temperature and thermal expansion coefficient to that of the underlying substrate PI5878G. The thickness of the superstrate was optimized to place the intermediate accelerometer on a plane of zero stress. The KaptonRTM films were pre-etched before bonding to the device wafer, thus avoiding spin-coating a photoresist layer at high rpm and possibly damaging the already released micro-accelerometers in the device wafer. The packaged accelerometers were characterized in the same way the open accelerometers were characterized on both flat and curved surfaces. After encapsulation, the sensitivity of the largest accelerometer on a flat and a curved surface with 2.0 cm radius were obtained to be 195 fF/g and 174 fF/f, respectively. All three accelerometers demonstrated outstanding noise performance after vacuum packaging with an SNR of 100:1. Further analysis showed that the contribution from the readout circuitry is the most dominant noise component followed by the Brownian noise of the accelerometers. The developed stresses in different layers of the accelerometers upon bending the substrates were analyzed. The stresses in all cases were below the yield strength of the respective layer materials.;AlN cantilevers as tactile sensors were also fabricated and characterized on a flexible substrate. Ti was utilized as the bottom and the top electrode for its smaller lattice mismatch to AlN compared to Pt and Al. The piezoelectric layer of AlN was annealed after sputtering which resulted in excellent crystalline orientation. The XRD peak corresponding to AlN (002) plane was obtained at 36.54º. The fabricated AlN cantilevers were capable of sensing pressures from 100 kPa to 850 kPa which includes soft touching of human index finger and grasping of an object. The sensitivities of the cantilevers were between 1.90 x 10-4 V/kPa and 2.04 x 10 -4 V/kPa. The stresses inside the AlN and Ti layer, developed upon full bending, were below the yield strength of the respective layer materials.