Introduction
This thesis is the description of the work carried out during my PhD program at the Department of Information Engineering of the University of Pisa.
The thesis is organized as follows.
In Chapter 1 a technique for fast release of large dielectric membranes for MEMS applications is presented. A brief state of the art on freestanding large MEMS structure is given. The proposed method exploits the anisotropic etch- ing properties of tetramethylammonium hydroxide (TMAH)-based solutions.
The release takes place through underetch starting at convex corners present in the membrane geometry. By pre-patterning the membrane with periodic convex-corner patterns the release times can be radically reduced.
Preliminary laboratory tests confirmed the functionality of the proposed method. Consequently, the design implementation on a standard CMOS pro- cess was discussed. Different periodic patterns are proposed and analysed, and actual release times for fabricated dielectric membranes are presented. Fur- thermore, a mathematical model of the etch evolution and release times which is in good agreement with experimental data has been extracted.
Chapter 2 presents the development of resonant MEMS biosensors. The sensors are fabricated with a MEMS (Micro-electro-mechanical System) post- processing method applied on a standard, CMOS-based VLSI technology, re- taining maximum compatibility with the CMOS process flow. Preliminarily, an overview on resonant MEMS sensors is given. The presented mechanical resonator is based on inductive actuation and detection, and the sensing is
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Introduction
based on the microbalance principle. Design improvements are discussed, fo- cusing on the methodology to increase the sensitivity but also to reduce the direct feedthrough and to increase the output voltage amplitude. A protocol for covalent bonding of organo-functional silanes (to be used as link sites for biomolecular probes), probe and target on the resonator surface is presented.
The fabricated MEMS resonators underwent a complete electro-mechanical characterization, aimed at the verification of its gravimetric sensing perfor- mances. Finally, after a discussion on circuit approaches for microbalance reading, a single chip oscillator based on the MEMS resonator is presented.
The simulated behavior of the oscillator is presented and discussed, and the temperature stability of the oscillator output frequency is analyzed. A first im- plementation of the oscillator circuit is explained and its frequency spectrum and frequency/temperature characteristics are presented.
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