Date of Award

8-2009

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Electrical and Computer Engineering

Advisor

John Vetelino

Second Committee Member

David Frankel

Third Committee Member

George Bernhardt

Abstract

The Lateral Field Excited (LFE) sensor is a bulk acoustic wave device that excites the transverse shear mode (TSM) in an AT-cut quartz substrate. The standard quartz crystal microbalance (QCM) excites the same TSM mode in AT-cut quartz. However, the LFE uses a lateral field to excite the TSM while the QCM uses thickness field excitation. This allows the LFE to have electrodes on one surface of the quartz while the QCM requires electrodes on both surfaces of the quartz. The LFE electrode geometry results in a bare sensing surface allowing the device to be sensitive to mechanical and electrical property changes, while the QCM is only sensitive to mechanical properties. The LFE is a potential replacement for the QCM as a rate monitor in thin film deposition systems. In these systems, the rate monitor is used to measure the rate and thickness of material being deposited onto a substrate. The material being deposited onto the substrate is also deposited onto the rate monitor, and as material accumulates on the rate monitor its frequency of oscillation decreases. By measuring this decrease in frequency, the rate and thickness of the deposition can be determined. Since the LFE device had not been previously used as a rate monitor in thin film deposition systems the device had to be designed for existing thin film deposition systems. This included the design of the LFE device on 0.55" diameter AT-cut quartz wafers by testing different LFE electrode configurations and different quartz crystal curvatures to determine the effect of these two parameters on the response of the LFE device. It was found that the LFE electrode configuration had a minimal effect on the impedance response of the LFE device. The quartz crystal curvature, plano-plano quartz and plano-convex quartz, had a large effect on the impedance response of the LFE device. Furthermore, an LFE rate monitor holder needed to be designed to use the LFE device in thin film deposition systems. It was determined that this holder had a large effect on the impedance response of the LFE device as it introduced stress that lead to a damping of the impedance response. To test the viability of the LFE rate monitor and compare it to the QCM rate monitor, the LFE and QCM were simultaneously put into a thin film sputter deposition system and material was deposited onto both sensors. The frequency shift of each sensor was recorded throughout the deposition to build a mass versus frequency curve. Using this data the sensitivity of the LFE was compared to the sensitivity of the QCM. Results showed that the LFE can successfully be used as a rate monitor in thin film deposition systems, and the sensitivity of the LFE rate monitor was similar to the sensitivity of the QCM rate monitor. As technology increases and the thickness of material being deposited decreases, a more sensitive rate monitor is going to be needed in thin film deposition systems.

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