Silicon carbide as a new MEMS technology

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Abstract

Silicon carbide (SiC) is a material with very attractive properties for microsystems applications. Its mechanical strength, high thermal conductivity, ability to operate at high temperatures and extreme chemical inertness in several liquid electrolytes, make SiC an attractive candidate for MEMS applications, both as structural material and as coating layer. The recently reported progress in material growth and processing techniques has strengthened the potential of this material for MEMS, especially for applications requiring operation at high temperature or in severe environments. Examples of SiC microsensors and microstructures are given and interesting development in both material characteristics and micromachining processes are discussed.

Introduction

An increasing demand for sensors that can operate at temperatures well above 300°C and often in severe environments (automotive and aerospace applications: combustion processes or gas turbine control; oil industry) has stimulated the search for alternatives to silicon. In order to benefit from the enormous know how on silicon integrated circuit (IC) technology attention is first directed to silicon derivatives. Silicon carbide (SiC) is a material that has attracted much attention for a long time, particularly due to its wide bandgap, its ability to operate at high temperatures, its mechanical strength and its inertness to exposure in corrosive environments. However, the difficulty in growing crystalline material and the limited knowledge in the areas of oxidation, doping, etching and metallization have limited its use to very specific applications area, such as light emitting devices and specific high-temperature, high-power or high-frequency applications that are not suitable for Si- or GaAs-based devices. For other applications and particularly for SiC-MEMS devices, large area substrates are essential. In the past decade great progress has been made with respect to the growth of single crystal wafers now commercially available and what is much more relevant to the MEMS community, in epitaxial growth of single and poly-crystalline SiC layers on silicon [1]. These developments have stimulated research on deposition, characterization and further processing of SiC for MEMS.

In this paper we review some of the most interesting results both in terms of material characteristics and micromachining processes and give a few examples of MEMS devices which indicate the great progress made in SiC MEMS technology and underline the potential of this material for MEMS.

Section snippets

Single crystal SiC

Several techniques are used to grow single crystal SiC. Next to seed-sublimation techniques used for growing SiC bulk material, significant progresses have been made in the growth of mono (and poly) crystalline layers on a silicon substrate by using Metalorganic Chemical Vapor Deposition (MOCVD), gas-source Molecular Beam Epitaxy (MBE), electron cyclotron resonance (ECR) plasma and liquid phase epitaxy 1, 2. Breakthroughs in crystal growth reported in the past decade resulted in the

Devices and microstructures

The progress made so far is quite encouraging and the number of projects and groups working on SiC MEMS is steadily increasing. In order to illustrate the state of the art of SiC devices and microstructures, a few examples are given. β-SiC films grown on silicon have been used to realize bulk micromachined membranes for pressure sensors or for gas sensing [42]. Epitaxial layers of single crystal 3C-SiC have been used to realize pressure sensors designed for operations at 400°C [43]. High

Conclusions

Silicon carbide appears to be a good candidate for MEMS applications, particularly when high operation temperatures or harsh environments are involved. The recent developments on SiC deposition techniques, the improved control on its properties and the progress on SiC micromachining are quite promising and have resulted in a number of SiC MEMS devices and structures. Although quite some work still needs to be done, SiC MEMS technology has definitely demonstrated its potentials and entered new

Acknowledgements

The author would like to thank all DIMES colleagues and graduate students who have contributed to the material presented in this paper. Particularly the contributions of C.R. de Boer, who is responsible for the SiC PECVD depositions, Axel Berthold for his work on wafer bonding and Andrea Irace for the thermal conductivity measurements, are greatly appreciated.

Pasqualina M. Sarro received the Laurea degree in solid-states physics from the University of Naples, Italy, in 1980. From 1981 to 1983, she was a post-doctoral fellow in the Photovoltaic Research Group of the Division of Engineering, Brown University, Rhode Island, U.S.A., where she worked on thin-film photovoltaic cell fabrication by chemical spray pyrolysis. In 1987, she received the Ph.D. degree in Electrical Engineering at the Delft University of Technology, the Netherlands, her thesis

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    Pasqualina M. Sarro received the Laurea degree in solid-states physics from the University of Naples, Italy, in 1980. From 1981 to 1983, she was a post-doctoral fellow in the Photovoltaic Research Group of the Division of Engineering, Brown University, Rhode Island, U.S.A., where she worked on thin-film photovoltaic cell fabrication by chemical spray pyrolysis. In 1987, she received the Ph.D. degree in Electrical Engineering at the Delft University of Technology, the Netherlands, her thesis dealing with infrared sensors based on integrated silicon thermopiles. Since then, she has been with the Delft Institute of Microelectronics and Submicron Technology (DIMES), at the Delft University, where she is responsible for research on integrated silicon sensors and microsystems technology. Since April 1996 she is also Associate Professor in the Electronic Components, Materials and Technology Laboratory of the Delft University. She has been an IEEE member since 1984 and a Senior Member since 1997. She acts as reviewer for numerous technical journals and has served as technical program committee member of the ESSDERC Conferences (since '95), the SPIE 5th Annual Symposium on SMART STRUCTURES and MATERIALS '98 and EUROSENSORS '99.

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