Silicon Carbide Microsensors for Demanding Applications

February 1, 2005 By: Jeffrey M. Melzak, Chien-Hung Wu Sensors

As MEMS sensors move into more extreme environments, the search for an alternative to silicon is on. Recent advances in processing make silicon carbide (SiC) a viable and attractive choice for harsh environment microsystems.

Over the past decade, microelectromechanical systems (MEMS) have experienced significant growth and market acceptance, driven largely by their reliability, manufacturability, and potential for low unit cost. In the coming decade, microsystems are expected to continue as a leading driver of innovation, with manufacturers and OEMs looking to incorporate small, relatively inexpensive sensors/actuators into their products to improve their performance, reliability, and longevity and to lower their cost.

For the most part, silicon has been the material of choice for MEMS components. However, for an increasing number of demanding applications, silicon isn't the best construction material from a performance standpoint. For example, developing advanced engine systems requires sensors that can operate in the harsh environments near the ignition source (hot) and in the exhaust stream (hot and corrosive). This has led microsensor manufacturers to investigate advanced materials capable of operating reliably in such demanding environments. The challenge lies in bringing these microsystems based on advanced materials to market at a reasonable cost, i.e., using conventional microfabrication techniques to enable suppliers to deliver large volumes cheaply.

This article provides an overview of the advantages of silicon carbide (SiC) for such demanding microsystem applications and describes recent progress in overcoming the difficulties associated with microfabrication of SiC MEMS components.

Why Silicon Carbide?

Silicon carbide is uniquely suited to use in harsh environment sensors. Figure 1 lists the important physical properties of the leading semiconductors under investigation: silicon (Si), gallium arsenide (GaAs), SiC, and diamond. While silicon (specifically, silicon on insulator, or SOI) and GaAs can be pushed to reasonably high operating temperatures (~300°C) and radiation fluxes (1 Mrad), these two materials are not well suited for harsh mechanical and chemical environments. Neither is sufficiently rugged to form the basis for a harsh-environment MEMS platform technology. SiC and diamond stand out from other MEMS materials based on their ability to survive in harsh conditions; in contrast to other durable materials proposed for harsh environment sensors—including alumina, tantalum oxide (Ta2O5), and titanium carbide (TiC)—SiC and diamond are semiconductors and can provide both mechanical and electronic functionality. Both are wide bandgap materials, making them suitable for high-temperature operation and radiation-hard electronics. Both are extremely chemically inert (although diamond is susceptible to oxidizing environments) and mechanically hard. From a mechanical standpoint, SiC has outstanding material properties, including high elastic modulus, fracture toughness, operational stability, wear resistance, chemical inertness, and thermal conductivity. As an electronic material, SiC has a wide bandgap, high breakdown strength, and high saturation drift velocity. Significantly, the commercial viability of SiC electronic circuits is advancing rapidly as wafer size and quality increase, laying the groundwork for successful integration with SiC transducer elements. SiC's wide bandgap, coupled with its high thermal conductivity, provides for high-temperature/-power operation and radiation hardness. Based on these factors and with recent advances in SiC microfabrication technology [1], SiC is now a realistic and commercially viable platform for harsh-environment MEMS products.

Figure 1. Looking at the various material properties, SiC and diamond are superior to silicon and GaAs if you consider the combination of thermal, mechanical, and electrical properties. In addition, SiCs mechanical and electrical properties maintain their advantage over silicon at high temperatures.