Characterisation of micromechanical materials and MEMS structures using optically generated ultrasound

Due to the increased commercial demand for miniaturisation, MEMS manufacturers have to characterise their components to improve quality control with ever-higher accuracy and resolution. One method of characterisation, using laser-generated and detected ultrasonic Lamb waves, is described in this thesis. Laser generated ultrasound is an attractive approach for material testing as it provides a non-contact non-destructive characterisation method. In the work reported here, Lamb waves are generated using a broadband laser source, giving a high temporal and spatial bandwidth, and the generated Lamb waves are detected using either a commercially available vibrometer or a custom built large bandwidth Michelson interferometer. An investigation has been conducted on the optical conversion efficiency into Lamb waves. Lamb wave amplitudes were measured on a silicon wafer and a variety of thin films on silicon wafers. This was to determine how parameters such as absorption coefficient and optical reflectivity as functions of excitation wavelength, influence the generation of Lamb waves. As a result, it was concluded that light of wavelength 532 nm is the optimal wavelength for Lamb wave generation. The capabilities of laser-generated Lamb waves, coupled with a large bandwidth Michelson interferometer, have been demonstrated to accurately measure the thickness of a MEMS pressure sensor membrane in the direction of a silicon wafer. Using the reassigned Gabor time-frequency method, to produce group velocity dispersion curves, the thickness was determined to be 35.01 μm ± 0.18 μm, from a single measurement. For comparison, the thickness was measured using an independent technique; obtaining a value of 34.60 μm ± 0.27 μm. Values for Young's modulus and Poisson's ratio were also determined to be 163 GPa ± 11.7 GPa and 0.351 respectively and these are in good agreement (to within 3.6% for Young's modulus and 2.5% for Poisson's ratio) with values found in the literature.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral-pg

Grantor dc:publisher.institution

University of Strathclyde

Year dc:date.issued

2014

Author dc:creator

• McKee, Campbell

DOI

10.48730/v55b-d611

Identifier

T13969

OAI identifier oai:identifier

oai:strathclyde:wm117n995