Optical interferometry offers a powerful tool for the study of the mechanical motion of micro- and nano-electromechanical systems (MEMS and NEMS). By examining the modulation of reflected light the displacement can be measured with sub-nanometre precision. Recent work with fibre interferometers carried out by other groups has studied the motion of nanomechanical systems down to temperatures as low as 1 K. Dissipation measurements in the last few years of a number of devices fabricated from high-stress amorphous silicon nitride have shown a marked increase in quality factors when compared to similar low-stress devices. The high quality factors and small masses of these devices have attracted a great deal of interest within the nanomechanical and optomechanical communities. Measurements of dissipation in nanomechanical resonators carried out in Nottingham to date have used the magnetomotive effect to detect nanomechanical motion. This has required that a layer of metal be applied to the high-stress silicon nitride, modifying the mechanical properties. In this thesis we present an overview of the design and construction of an optical detection system designed to study MEMS and NEMS devices from room temperature to liquid helium temperatures. Optical detection is able to measure the displacement of purely dielectric structures and as such is an ideal method with which to measure dissipation in these high-Q silicon nitride resonators, complementing the other nanomechanical measurement techniques available within Nottingham. Using this system, measurements have been made on a number of micro- and nano-electromechanical systems fabricated using processes developed during this work. Confocal images of these devices obtained using the fibre interferometer show a spatial resolution of 0.75 um, a value close to the diffraction limit of the system. Micromechanical quartz tuning forks have been measured to confirm the frequency response of the interferometer, with a value for the piezo-electro-mechanical coupling constant of 2.18 +/- 0.06 uC/m obtained that is in very good agreement with the values published in the literature. Nanomechanical measurements of 200 um square high-stress silicon nitride membranes have revealed thermoelastic damping to be the limiting dissipation mechanism for these resonators at room temperature. Using elastic theory it is possible to quantify the fQ floor predicted by thermoelastic damping seeing good agreement with experimental data. At lower temperatures inter-membrane coupling was observed, with acoustic vibrations from neighbouring membranes coupling into and being amplified by the membrane under observation. Discrepancies in quality factor between the observed and unobserved membranes are most likely due to optomechanical damping of the observed membrane by the laser. This inter-membrane coupling offers a powerful technique for the indirect observation of the flexural modes of nearby membranes without optically damping the response.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:588330 |
| Date | January 2013 |
| Creators | Patton, Mark James |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://eprints.nottingham.ac.uk/13118/ |
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