Graduation Semester and Year

2016

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering

Department

Electrical Engineering

First Advisor

Donald Butler

Abstract

Microelectromechanical systems (MEMS) sensors using ultrathin aluminum nitride (AlN) film were developed and fabricated using conventional photolithography techniques in the class 100 clean room with a view to integrate them in flexible substrates along with flexible electronics. The MEMS sensors were designed, analytically modeled, fabricated and characterized. Some of the MEMS sensors were only designed and simulated using finite element method (FEM) for the scope of the dissertation. These MEMS sensors can be applied to many applications such as automobile, robotics, biomedical, biometrics, health condition monitoring, GPS tracking devices, smartphones and aircrafts. MEMS pressure sensors using AlN based piezoelectric film were designed, fabricated and characterized in the form of array of cantilever based structures. A 300 nm thick ultrathin and flexible AlN film with a feature size of ~12 µm which was deposited using DC reactive magnetron sputtering system and sandwiched between two electrodes to induce cantilever shaped structures acted as the sensing element of the cantilever sensors. After fabrication, several cantilevers were chosen for electrical characterization. The pressure sensors were characterized in a probe station system to measure the piezoelectric voltage signals and power spectral densities. With the help of simulation results, numerical modeling was also carried out to find the theoretical output voltage ranges and sensitivity of the cantilevers. The simple and flexible cantilevers form the basis for future piezoelectric energy harvesters, pressure sensors, fingerprint sensors and accelerometers using ultrathin AlN film those can be integrated on a system-on-chip (SoC) circuit. Initially, the ultrathin AlN films were developed by changing the deposition temperature and Ar/N2 gas flow ratio and characterized using SEM, XRD and EDX to analyze the quality of the film. Stress analyses were taken into consideration to check the mechanical strength and reliability of the pressure sensors. In addition, bending performance was also analyzed by calculating the radius of curvature (ROC) of the cantilevers. Finally, noise performance was also analyzed. Ultra-thin AlN based novel flexible MEMS fingerprint sensors were designed using finite element method i.e., CoventorWare® with a view to improve the pixel resolution and, hence, the quality of scanned fingerprint image. Two different sized pixel dimensions were used for the design of three fingerprint sensors; they are: a) FPS725A b) FPS725B, and c) FPS1016. The pixel dimension for FPS725A and FPS725B was 35 µm by 35 µm. The pixel feature was equivalent to an imaging resolution of 725 dot-per-inch (dpi). The other sensor had a pixel size of 25 µm by 25 µm and was equivalent to an imaging resolution of 1016 dpi. In both type of sensors, 200 nm thick, ultrathin AlN film was used as the sensing element. The difference between FPS725A and FPS725B was the location of the sensing element. In FPS725A, AlN film was deposited on top of Al2O3 diaphragm while in FPS725B, AlN was located inside the diaphragm. The fabrications process flow will be discussed in details in the fingerprint sensor chapter. In brief, the fingerprint sensors were comprised of array of pixels and each pixel was made of a cavity like structure which was basically an aluminum oxide (Al2O3) based structure. Underneath the cavity like structure, there was an adjacent piezoelectric plate or film which was sandwiched between two metal electrodes. The total area of the sensors is identical and considered to be 15 mm by 15 mm for practical use. Piezoelectric output voltage with respect to various applied finger pressure were calculated using the stress contour found from the simulation results. Finally, piezoelectric response for each sensor for different finger pressure was found from the slope of the piezoelectric voltage versus applied force plot. The average piezoelectric responses are found to be 225.74 V/N, 115.58 V/N, and 125.52 V/N for FPS725A, FPS725B, and FPS1016, respectively. Stress analysis and noise performance of the sensors were studied. For practical use, the CMOS readouts will be taken from the Silicon substrate through the electrical metallization of pure metal electrodes which will be covered in the chapter. An AlN based piezoelectric z-axis MEMS accelerometer was designed and simulated using CoventorWare®. Modal harmonic analysis was carried out and the simulated resonant frequency was found to be 2.26 kHz. Various loads were applied on top proof mass of the accelerometer ranging from 1g to 10g. Piezoelectric output voltages due to applied loads were calculated. The voltages ranged from 0.00082 V to 0.000082 V. The piezoelectric response or sensitivity was also calculated and found to be 0.000082 V/N. Noise performances was also analyzed and noise equivalent acceleration (NEA) was calculated. Noise equivalent acceleration was found to be 0.253 V/rtHz

Keywords

Piezoelectric, MEMS, Sensors

Disciplines

Electrical and Computer Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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