title: Polycrystalline diamond micro-electromechanical systems (MEMS) for passive micro-rheology and sensor applications
Creators: name: McGlone, Andrew William
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: MEMS, diamond, polycrystalline, micro-rheology, silicon, dry etch, wet etch, etching, resonators, viscosity, cantilever, micro-cantilever, boron doped diamond, piezo, piezoresistive.

description: Owing to its unique mechanical and electrical properties, diamond is an attractive candidate for use in micro-electro-mechanical systems (MEMS) devices. This thesis pertains to the development, fabrication and characterisation of polycrystalline diamond (PCD) micro-electromechanical systems (MEMS) devices for passive micro-rheology and sensor applications. Intrinsic PCD and boron doped PCD (BDD) materials are investigated. Micro-rheology is the study of soft matter rheological properties, often performed by observing interactions with mechanical devices, such as micro-cantilevers, at the micro scale. In order to overcome significant fluid dampening, these devices are actuated at or around their resonant frequency, and several measurements are taken at different frequencies to build a data set. We present an intrinsic diamond-based micro-cantilever micro-rheometer device, the passively actuated thermal fluctuations of which can be characterised in a fluid at least up to the viscosity of water (8.90 × 10−4 Pa.s ). A possible data analysis method to extract a fluid’s viscoelastic properties from the power spectrum of the thermal fluctuations of a device submerged in the fluid is also presented. This method negates the requirement for measurements at multiple actuation frequencies and provides useable data up to the sample rate of the data acquisition system. Intrinsic PCD cantilevers for passive micro-rheology were fabricated from polished (~3 nm Ra) 500 nm thick PCD on Silicon &lt;100&gt; substrate films. Cantilever dimensions range from 5 μm to 150 μm in length and 1 μm to 4 μm in width, the highest height/width/length ratio cantilevers yet reported. PCD samples were patterned using electron beam lithography and highly anisotropic diamond etching was achieved using an RIE Ar/O2 plasma etching method. A new fabrication process to minimize cantilever undercut is presented. The thermal fluctuations of the free-standing cantilever structures in air and water at room temperature were successfully captured by a laser Doppler vibrometer system. Resonant frequencies of devices are presented, ranging from 38 – 554 kHz in air and 42 – 148 kHz in water, comparable to that of similar single crystal diamond devices. Polycrystalline Diamond MEMS for Passive Micro-rheology and Sensor Applications ii PCD micro-cantilevers have been investigated extensively in different sensor applications. Recently, boron-doped diamond micro cantilevers exhibiting piezoresistive behavior have been fabricated from multi-layer PCD material. We present a boron-doped PCD micro-cantilever piezoresistive sensor fabricated from a single layer of BDD thin film on silicon. BDD material was electrically characterised and found to be electrically stable for up to at least 60 seconds within the I/V ranges investigated. BDD micro-cantilevers were fabricated from polished (~3 nm Ra) 480 nm thick BDD on Silicon &lt;100&gt; substrate films. The Ushaped cantilever’s dimensions ranged from 60 μm to 110 μm in length with legs 4 μm in width. The deflection sensitivity of the fabricated cantilever devices is reported, ranging from 0.029 mΩ/Ω-μm to 0.063 mΩ/Ω-μm. An analysis of the nature of the piezoresistive mechanism in the BDD devices is presented.