Date of Award

2012

Document Type

Doctoral Thesis

Degree Name

Doctor of Philosophy

Department

Electronic Engineering

First Advisor

Dr. Martin Hill

Second Advisor

Dr. Maryna Lishchynska

Abstract

MEMS devices are micromechanical components fabricated using semiconductor fabrication technologies. With advanced functionalities and miniaturization, MEMS devices are being employed in a wide variety of applications, especially in mobile networks. The MEMS ohmic switch is one of the most promising MEMS devices and has the same principle of a mechanical moving switch to manipulate an electrical signal. Due to the existence of mechanical movement, and its compactness, reliability issues have been observed as the main barrier to commercialisation.

This thesis addresses the key issue of reliability in the rapidly expanding area of MEMS. In particular, MEMS ohmic switches are considered. The reliability issues are broad and this thesis does not have the ambition to cover all reliability aspects of this type of device. The thesis focuses on identifying operational reliability issues, proposing models for design optimisation and control methods to reduce factors that affect the ' lifetime of the devices. There are 3 main contributions to the field.

Firstly, this thesis presents a new generalized analytical method for determining the pull-in instability of cantilever beams subjected to partial electrostatic load. It employs a non-linear stiffness analysis. The method is then developed to achieve an analytical design guide to avoid pull-in instability issues, thus reducing the impact force of contact, of cantilever beams. The difference between the developed models and Finite Element Model (FEM) are less than 3%.

Secondly the integration of nonlinear contact mechanics with adhesion into a dynamic 2-D model is outlined. The contact resistance and switch bouncing effects can be captured with high accuracy with experiment results. The developed model is helpful for evaluating the effect of materials and designs on the dynamic behaviour of the switch. The developed model is then employed to evaluate the switch degradation effects to predict the life-time of the switches.

Finally, the thesis introduces a novel energy-based approach to adaptive pulse shaping for control of MEMS ohmic switch closure. The method includes all of the most important practical effects. The method reduces the bouncing effect while maintaining fast switching. Experiment evaluation shows that the highest error is 6.5% at 45V input. Error reduces as the input voltage increases. The analytical method can be easily modified to adapt with the variance of system parameters drift during operation.

In this dissertation, the key failure mechanisms of the devices are identified. This thesis develops system models which allow for quick high accuracy system evaluation. The models are used to improve the switch geometric design, to capture the dynamics of switch bouncing and to optimise the switch actuation voltage to eliminate contact bouncing. Through collaboration with our commercial partner the work can have an immediate benefit in improving switch reliability and therefore over-coming the principle barrier to commercialisation.

Access Level

info:eu-repo/semantics/openAccess

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