Petros Gkotsis

Dynamic Operational Stress Measurement of MEMS Using Time-Resolved Raman Spectroscopy

A dynamic-stress analysis method, based on time-resolved micro Raman spectroscopy, has been developed for reliability studies of microelectromechanical systems. This novel technique is illustrated by measuring temporally and spatially resolved stress maps of a piezoelectrically actuated silicon microcantilever when driven at its first- (6.094 kHz) and second-order (37.89 kHz) resonant frequencies. Stress amplitudes of up to 180 plusmn 10 MPa were measured at the maximum stress locations. The time-resolved Raman stress measurements are compared to the results of finite-element analysis and laser Doppler vibrometry.[2008-0003].

Effects of Fast Neutrons on the Electromechanical Properties of Materials Used in Microsystems

Microelectromechanical systems (MEMS) technology is expected to play a significant role in avionics and in future space missions provided that the effect of radiation on the reliability of MEMS devices is well understood. Most tests, to date, have been performed on MEMS that operate based on electrostatic principles, and radiation-induced dielectric charging has been identified as a mechanism which can potentially limit the reliability of such devices. Experiments with piezoresistive and piezoelectrically actuated MEMS have also revealed changes in the operation of devices after exposure to different types of ionizing radiation. However, not many reports on the damage effects in MEMS due to fast neutrons exist. In this paper, we present results from mechanical and electrical tests on different materials which are typically used in microsystems, before and after exposure to fast neutrons. Changes were detected in the residual stress and in the resonant frequencies of silicon nitride microbeams. There was also evidence of radiation-induced charge trapping in metal-insulator-semiconductor capacitors fabricated from thermal oxide, silicon nitride, and aluminum oxide deposited by atomic layer deposition. Finally, we observed that the electrical properties of ferroelectric capacitors fabricated from lead zirconate titanate and lead strontium titanate thin films do not degrade after exposure to fast neutrons.

A MEMS resonant magnetometer based on capacitive detection

In this paper, a MEMS resonant magnetometer based on Lorentz force is presented. The Lorentz force is stimulated by the interaction of an AC current which circulates in a suspended clamped-clamped conducting beam, obtained through a bulk micro-machining, and a 150 mT external static magnetic field. The off-plane (in the z-direction) structure displacement is converted into a capacitance change by a series of interdigitated combs. The frequency of the AC supply current is chosen to be equal to the resonant frequency of the suspended beam, leading to the increase of its deflection magnitude and the overall sensitivity by a factor equal to Q (Quality-factor). Throughout this paper, the first-order resonance frequency and the capacity are simulated with Comsol® and thereafter compared to the measured values. On the other hand, the quality-factor and the electric model of the sensor are determined from the characterization of the fabricated sensor prototype.

Impact of radiations on the electromechanical properties of materials and on the piezoresistive and capacitive transduction mechanisms used in microsystems

The impact of different types of radiation on the electromechanical properties of materials used in microfabrication and on the capacitive and piezoresistive transduction mechanisms of MEMS is investigated. MEMS technologies could revolutionize avionics, satellite and space applications provided that the stress conditions which can compromise the reliability of microsystems in these environments are well understood. Initial tests with MEMS revealed a vulnerability of some types of devices to radiation induced dielectric charging, a physical mechanism which also affects microelectronics, however integration of novel functional materials in microfabrication and the current trend to substitute SiO2 with high-k dielectrics in ICs pose new questions regarding reliability in radiation environments. The performance of MEMS devices with moving parts could also degrade due to radiation induced changes in the mechanical properties of the materials. It is thus necessary to investigate the effects of radiation on the properties of thin films used in microfabrication and here we report on tests with γ, high energy protons and fast neutrons radiation. Prototype SOI based MEMS magnetometers which were developed in UCL are also used as test vehicles to investigate radiation effects on the reliability of magnetically actuated and capacitively coupled MEMS.