Silicon Carbide Microelectromechanical Systems for Harsh Environments

The following sections are included:

• PREFACE

• CONTENTS

This chapter serves as a brief introduction to the basic properties of silicon carbide (SiC) and the advantages of using SiC over other semiconductor materials for microelectromechanical systems (MEMS). Given the excellent and extensive review chapters that follow this one, I have confined this chapter to recent research performed at the University of Edinburgh in the area of SiC microelectromechanical systems (MEMS). Some of the processes involved in the fabrication of microelectromechanical systems in SiC are discussed, together with the problems to be overcome in order for SiC's potential as a MEMS material be exploited in applications for harsh environments.

SiC is a highly attractive material for microelectromechanical systems (MEMS) in applications where the material requirements call for a mechanically durable, chemically inert, and thermally stable semiconductor. Of the collection of materials that meet this requirement, SiC has risen to the forefront in large part because appropriate thin film deposition techniques are currently available and capable of producing MEMS-grade material in amorphous, polycrystalline and single crystal phases. This chapter reviews the major deposition techniques used to produce SiC for MEMS applications. These techniques include APCVD, LPCVD, PECVD, PVD and ion beam synthesis.

The following sections are included:

• Introduction

• Thermal Stability
  • Thermal Stability of Ohmic Contacts
  • Thermal stability of Rectifying contacts

• Ohmic Contacts to p-type SiC
  • Introduction
  • Al-based Contacts
  • Al-Free Contacts

• Ohmic Contacts using Nickel
  • Introduction
  • Thermodynamics
  • Phase Formation Sequence
  • Issues
  • Mechanisms
  • Alloying

• Effects of Defects on Schottky Contacts

• Summary

• Acknowledgments

• References

In this chapter we discuss wet and dry patterning techniques for SiC and the relative merits of these methods for MEMS processing. We describe the basic principles involved in etching SiC and problems that can arise because of the binary nature of the lattice and its relatively high bond strength. Recent developments in the use of high density plasma sources to achieve fast etching rates (in some cases over 1 μm min⁻¹ for bulk 4H-SiC) are discussed. These sources are likely to play a dominant role for processing of SiC MEMS devices since they are capable of producing etch depths from 0.1 to 100 μm with minimal disruption of the SiC surface.

Micro-electro-mechanical systems (MEMS) based on silicon carbide (SiC) have been emerging over the past fifteen years. In the first part of this chapter the material properties of SiC, applications of SiC MEMS, SiC-based material systems and recent achievements in SiC processing technologies are briefly reviewed. The main part categorizes SiC MEMS devices presented so far, such as pressure sensors, accelerometers, motors, resonators and gas sensors. Their characteristics and performances are summarized. Finally, an evaluation of the state-of-the-art of SiC MEMS is given and further developments are discussed that will be necessary to let SiC MEMS devices reach market maturity.