Mechanical Engineering ETDs

Publication Date

Spring 5-16-2020

Abstract

AlN (Aluminium Nitride) is a piezoelectric material that is often used in numerous applications as a transducer. C-axis crystal orientation control of AlN is very important as it corresponds to a high piezoelectric coefficient. Crystal orientation of sputtered AlN is strongly affected by its deposition parameters such as sputtering pressure, target to substrate distance, growth temperature, gas pressure and frequency/duty cycle in the case of pulsed DC sputtering. Crystal orientation of AlN depends on the energy of the sputtered atom and the mobility of the adatoms on the surface of the substrate. Varying the deposition parameters can alter the kinetic energy of the atoms and vary the surface mobility of the adatoms thus modifying the crystalline structure of AlN. Along with crystal orientation the polarity of AlN can be critical. Typically, researchers deposit a single layer where polarity is not critical, but for multilayers or bimorphs, polarity control is needed. Since controlling polarity of AlN has not been well established for sputtered films, current devices are limited to single layer. The motivation for creating bimorph structure is demonstrated using Finite Element Modelling of a ultrasound membrane structure. Molybdenum (Mo) is one of the most common electrode materials used as a seed layer for AlN. The properties of AlN are dependent on the quality of Molybdenum thin film. Therefore, the morphological effects of Mo thin film by varying the sputtering parameters was investigated. Molybdenum was deposited using DC and pulsed DC sputtering techniques by varying power and temperature and its properties including stress, electrical resistvities and crystalline properties were studied. The goal of the project was to demonstrate that controlling polarity of a bimorph (stacked) layer of AlN can lead to an increase in device performance, and to develop a method of controlling polarity. Being able to control the polarity of AlN will lead to fabrication of high classes of devices including high power and high-frequency field effect transistors and efficient ultraviolet (UV) light emitting diodes. It also opens possibility of designing and fabricating MEMS (Micro- Electro Mechanical Systems) devices with higher actuation and lower power consumption. In order to accomplish this the following systematic approach was outlined: 1. FEM analysis of the stacked layer (chapter 3) 2. Deposition of bottom Mo (chapter 4) which acts as an electrode 3. Deposition of AlN (chapter 5) 4. Investigate method to control polarity (chapter 6)

Keywords

AlN, sputtering, Molybdenum, thinfilm, XRD

Degree Name

Mechanical Engineering

Level of Degree

Masters

Department Name

Mechanical Engineering

First Committee Member (Chair)

Dr. Nathan Jackson

Second Committee Member

Dr. Matthias W. Pleil

Third Committee Member

Dr. Sakineh Chabi

Document Type

Thesis

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