Development and implementation of a deflection amplification mechanism for capacitive accelerometers

Micro-Electro-Mechanical-Systems (MEMS) and especially physical sensors are part of a flourishing market ranging from consumer electronics to space applications. They have seen a great evolution throughout the last decades, and there is still considerable research effort for further improving their performance. This is reflected by the plethora of commercial applications using them but also by the demand from industry for better specifications. This demand together with the needs of novel applications fuels the research for better physical sensors.Applications such as inertial, seismic, and precision tilt sensing demand very high sensitivity and low noise. Bulk micromachined capacitive inertial sensors seem to be the most viable solution as they offer a large inertial mass, high sensitivity, good noise performance, they are easy to interface with, and of low cost. The aim of this thesis is to improve the performance of bulk micromachined capacitive sensors by enhancing their sensitivity and noise floor.MEMS physical sensors, most commonly, rely on force coupling and a resulting deflection of a proof mass or membrane to produce an output proportional to a stimulus of the physical quantity to be measured. Therefore, the sensitivity to a physical quantity may be improved by increasing the resulting deflection of a sensor. The work presented in this thesis introduces an approach based on a mechanical motion amplifier with the potential to improve the performance of mechanical MEMS sensors that rely on deflection to produce an output signal.The mechanical amplifier is integrated with the suspension system of a sensor. It comprises a system of micromachined levers (microlevers) to enhance the deflection of a proof mass caused by an inertial force. The mechanism can be used in capacitive accelerometers and gyroscopes to improve their performance by increasing their output signal. As the noise contribution of the electronic read-out circuit of a MEMS sensor is, to first order, independent of the amplitude of its input signal, the overall signal-to-noise ratio (SNR) of the sensor is improved.There is a rather limited number of reports in the literature for mechanical amplification in MEMS devices, especially when applied to amplify the deflection of inertial sensors. In this study, after a literature review, mathematical and computational methods to analyse the behaviour of microlevers were considered. By using these methods the mechanical and geometrical characteristics of microlevers components were evaluated. In order to prove the concept, a system of microlevers was implemented as a mechanical amplifier in capacitive accelerometers.All the mechanical structures were simulated using Finite Element Analysis (FEA) and system level simulations. This led to first order optimised devices that were used to design appropriate masks for fabrication. Two main fabrication processes were used; a Silicon on Insulator (SOI) process and a Silicon on Glass (SoG) process. The SOI process carried out at the University of Southampton evolved from a one mask to a two mask dicing free process with a yield of over 95%, in its third generation. The SoG is a well-established process at the University of Peking that uses three masks.The sensors were evaluated using both optical and electrical means. The results from the first prototype sensor design (1HAN) revealed an amplification factor of 40 and a mechanically amplified sensitivity of 2.39V/g. The measured natural frequency of the first mode of the sensor was at 734Hz and the full-scale measurement range was up to 7g with a maximum nonlinearity of 2%. The measurements for all the prototype sensor designs were very close to the predicted values with the highest discrepancy being 22%. The results of this research show that mechanical amplification is a very promising concept that can offer increased sensitivity in inertial sensors without increasing the noise. Experimental results show that there is plenty of room for improvement and that viable solutions may be produced by using the presented approach. The applications of this scheme are not restricted only to inertial sensors but as the results show it can be used in a broader range of micromachined devices

A dicing free SOI process for MEMS devices

This paper presents a full wafer, dicing free, dry release process for MEMS silicon-on-insulator (SOI) sensors and actuators. The developed process is particularly useful for inertial sensors that benefit from a large proof mass, for example accelerometers and gyroscopes. It involves consecutive front and backside deep reactive ion etching (DRIE) of the substrate to define the device features, release holes, and trenches. This is followed by hydrofluoric acid vapor phase etching (HF VPE) to release the proof mass and the handle wafer underneath to allow vertical displacements of the proof mass. The release process also allows the devices to be detached from each other and the substrate without the need of an extra dicing step that may damage the delicate device features or create debris. In the work described here, the process is demonstrated for the full wafer release of a high performance accelerometer with a large proof mass measuring 4 × 7 mm2. The sensor was successfully fabricated with a yield of over 95

Chemical vapor deposition and Van der Waals epitaxy for wafer-scale emerging 2D transition metal di-chalcogenides

Transition metal di-chalcogenides (TMDCs) such as MoS2, MoSe2, WS2 and WSe2 have become promising complimentary materials to graphene sharing many of its attributes. They may however offer properties that are unattainable in graphene, in particular TMDCs offer a bandgap tunable through both composition and number of layers. This has led to use of TMDCs in applications such as transistors, photodetectors, electroluminescent and bio-sensing devices. The current challenge in this emerging research field is to provide a reliable process to fabricate large area of atomically thin 2D TMDCs on the desired substrate. Chemical vapor deposition (CVD) technology has the advantage of offering conformal, scalable, and controllable thin film growth on a variety of different substrates. In addition, Van der Waals epitaxy could provide the vapor phase epitaxy of these TMDCs on the substrates with mismatched lattice constants. In this talk we describe our recent development in TMDCs materials using CVD technology and Van der Waals epitaxy and discuss their properties and potential applications

Emerging CVD technology for functional chalcogenide materials

Chalcogenide materials, formed from metallic alloys of S, Se, and Te, have received considerable attention for applications in optoelectronic devices over the past two decades in part due to their unique properties such as high infrared transparency, strong photosensitivity, large nonlinearity, capability of high rare-earth doping, and ability to readily change phase. Thin amorphous chalcogenide films are of particular interest because their diverse active properties are easily exploited in integrated planar optical circuits, as well as for memory and other optoelectronic applications. More recently, transition metal dichalcogenides (TMDCs), two-dimensional (2D) layered materials, such as MoS2, MoSe2, WS2, and WSe2 have become a noteworthy complimentary material to field. Sharing many of the properties of graphene they also offer properties that are unattainable in 2D graphene including a tunable bandgap; easily modified through both composition and the number of layers. This has led to use of TMDCs in applications such as transistors, photodetectors, electroluminescent and bio-sensing devices. In this talk we describe our development of functional chalcogenide materials by the chemical vapour deposition technology and discuss their potential applications

Observation of Complete Photonic Bandgap in Low Refractive Index Contrast Inverse Rod-Connected Diamond Structured Chalcogenides

Three-dimensional complete photonic bandgap materials or photonic crystals block light propagation in all directions. The rod-connected diamond structure exhibits the largest photonic bandgap known to date and supports a complete bandgap for the lowest refractive index contrast ratio down to nhigh/nlow ∼ 1.9. We confirm this threshold by measuring a complete photonic bandgap in the infrared region in Sn–S–O (n ∼ 1.9) and Ge–Sb–S–O (n ∼ 2) inverse rod-connected diamond structures. The structures were fabricated using a low-temperature chemical vapor deposition process via a single-inversion technique. This provides a reliable fabrication technique of complete photonic bandgap materials and expands the library of backfilling materials, leading to a wide range of future photonic applications

A Sub-30 mpH Resolution Thin Film Transistor-Based Nanoribbon Biosensing Platform

We present a complete biosensing system that comprises a Thin Film Transistor (TFT)-based nanoribbon biosensor and a low noise, high-performance bioinstrumentation platform, capable of detecting sub-30 mpH unit changes, validated by an enzymatic biochemical reaction. The nanoribbon biosensor was fabricated top-down with an ultra-thin (15 nm) polysilicon semiconducting channel that offers excellent sensitivity to surface potential changes. The sensor is coupled to an integrated circuit (IC), which combines dual switched-capacitor integrators with high precision analog-to-digital converters (ADCs). Throughout this work, we employed both conventional pH buffer measurements as well as urea-urease enzymatic reactions for benchmarking the overall performance of the system. The measured results from the urea-urease reaction demonstrate that the system can detect urea in concentrations as low as 25 μM, which translates to a change of 27 mpH, according to our initial pH characterisation measurements. The attained accuracy and resolution of our system as well as its low-cost manufacturability, high processing speed and portability make it a competitive solution for applications requiring rapid and accurate results at remote locations; a necessity for Point-of-Care (POC) diagnostic platforms

Single stage deflection amplification mechanism in a SOG capacitive accelerometer

This work discusses on a novel mechanical amplification concept for micro-electro-mechanical systems (MEMS). A single stage deflection amplifying mechanism, comprising microlevers, is implemented for a single axis in-plane capacitive accelerometer. Such a mechanism can provide a higher signal to noise ratio in a wide bandwidth with improved performance. Results from the analysis and evaluation of the fabricated prototype sensor are presented in this paper