© 2002 by CRC Press LLC another appropriate mask, such as polyimide or polyimide on silicon nitride, anodization reaction will only occur at the appropriate areas. The polyimide can prevent the pinhole formation while the silicon nitride can minimize the effect of undercutting. Polyimide is highly conformal and therefore will plug the pinholes. Silicon nitride is not conductive and is therefore electrically and chemical inactive during PECE. One effective method used to neutralize pinholes in platinum is by double-layer deposition. After the deposition of the first platinum layer, the film is sputter-etched, and subsequently a second platinum layer is deposited. This significantly reduces pinhole formation and pitting associated with the platinum etch mask. In many cases, the p-type SiC layer is not a fully effective etch-stop. This effect was observed in p-type SiC with low doping levels (N a ∼ 10 18 cm − 3 ). Apparently, in lightly doped material, the electric field in the space-charge region is not high enough to prevent all the photogenerated carriers from reaching the surface to cause etching. In addition, the UV light incident on the n–p junction causes higher leakage currents across the junction than higher doped p-type SiC. Although the anodic voltage is applied only through the ohmic contact on the top n-type SiC epilayer, the light-induced current through the junction leads to etching of the p-type SiC. To avoid etching of the p-type SiC epilayer, the reference voltage (V SCE ) must be reduced to a level that curtails the drifting of photocarriers assisted by electric field when the p-epilayer is eventually exposed to the electrolyte. This fabrication procedure described above can be adopted to produce resistors in n-type epilayers with any doping level. The characteristics of a diaphragm-based pressure sensor device are determined by the piezoresistors and by the dimensions of the diaphragm. Two key dimensions that characterize any circular diaphragm are thickness and radius. Because the radius is generally a fixed value determined by the pressure range and FIGURE 20.3 (a) Cross-section view of 6H-SiC after PECS of the top n + -epilayer and ECE of the backside cavity. Notice the curvature in the cavity, which is characteristic of the ECE process. (b) Top view of patterned piezoresistors in n-type 6H-SiC. p-type 6H-SiC etch-stop epilayer n + -type 6H-SiC piezoresistors n-type 6H-SiC substrate High-temperature metallization Oxide (a) © 2002 by CRC Press LLC another appropriate mask, such as polyimide or polyimide on silicon nitride, anodization reaction will only occur at the appropriate areas. The polyimide can prevent the pinhole formation while the silicon nitride can minimize the effect of undercutting. Polyimide is highly conformal and therefore will plug the pinholes. Silicon nitride is not conductive and is therefore electrically and chemical inactive during PECE. One effective method used to neutralize pinholes in platinum is by double-layer deposition. After the deposition of the first platinum layer, the film is sputter-etched, and subsequently a second platinum layer is deposited. This significantly reduces pinhole formation and pitting associated with the platinum etch mask. In many cases, the p-type SiC layer is not a fully effective etch-stop. This effect was observed in p-type SiC with low doping levels (N a ∼ 10 18 cm − 3 ). Apparently, in lightly doped material, the electric field in the space-charge region is not high enough to prevent all the photogenerated carriers from reaching the surface to cause etching. In addition, the UV light incident on the n–p junction causes higher leakage currents across the junction than higher doped p-type SiC. Although the anodic voltage is applied only through the ohmic contact on the top n-type SiC epilayer, the light-induced current through the junction leads to etching of the p-type SiC. To avoid etching of the p-type SiC epilayer, the reference voltage (V SCE ) must be reduced to a level that curtails the drifting of photocarriers assisted by electric field when the p-epilayer is eventually exposed to the electrolyte. This fabrication procedure described above can be adopted to produce resistors in n-type epilayers with any doping level. The characteristics of a diaphragm-based pressure sensor device are determined by the piezoresistors and by the dimensions of the diaphragm. Two key dimensions that characterize any circular diaphragm are thickness and radius. Because the radius is generally a fixed value determined by the pressure range and FIGURE 20.3 (a) Cross-section view of 6H-SiC after PECS of the top n + -epilayer and ECE of the backside cavity. Notice the curvature in the cavity, which is characteristic of the ECE process. (b) Top view of patterned piezoresistors in n-type 6H-SiC. p-type 6H-SiC etch-stop epilayer n + -type 6H-SiC piezoresistors n-type 6H-SiC substrate High-temperature metallization Oxide (a) © 2002 by CRC Press LLC 21 Deep Reactive Ion Etching for Bulk Micromachining of Silicon Carbide 21.1 Introduction 21.2 Fundamentals of High-Density Plasma Etching 21.3 Fundamentals of SiC Etching Using Fluorine Plasmas 21.4 Applications of SiC DRIE: Review 21.5 Applications of SiC DRIE: Experimental Results 21.6 Applications of SiC DRIE: Fabrication of a Bulk Micromachined SiC Pressure Sensor 21.7 Summary 21.1 Introduction It is often desired to insert microsensors and other microelectromechanical systems (MEMS) into harsh (e.g., hot or corrosive) environments. Silicon carbide (SiC) offers considerable promise for such appli- cations, because SiC can be used to fabricate both high-temperature electronics and extremely durable microstructures. One of the attractive characteristics of SiC is the compatibility of its process technologies with those of silicon, which allows for the co-fabrication of SiC and silicon MEMS. However, a very important difference in the processing of these semiconductors arises from the chemical inertness of SiC, a characteristic that makes it attractive for use in corrosive environments but also makes it very difficult to micromachine. Realization of the full potential of SiC MEMS will require the development of a set of micromachining tools for SiC comparable to the tool set available for silicon. Micromachining methods are generally classified as bulk, in which the wafer is etched, or surface, in which deposited surface layers are patterned. Surface micromachining methods for deposited SiC layers have been developed to a high level [Mehregany et al., 1998]. Silicon carbide can be readily etched to the required depths of just several microns using reactive ion etching (RIE) processes [Yih et al., 1997]. Further work remains to be done, however, in developing RIE processes with greater selectivity for SiC. Current RIE processes lack the selectivity needed to etch a SiC layer entirely through while minimally modifying an underlying silicon or silicon dioxide layer. This limitation has motivated the development of a micromolding method in which SiC is deposited into molds formed by RIE of silicon or silicon dioxide [Yasseen et al., 1999]. The emphasis here is bulk micromachining of SiC, for the fabrication of SiC microstructures with vertical dimensions from approximately 10 µ m to several hundred microns. Three methods for bulk Glenn M. Beheim NASA Glenn Research Center © 2002 by CRC Press LLC 22 Microfabricated Chemical Sensors for Aerospace Applications 22.1 Introduction 22.2 Aerospace Applications Leak Detection • Fire Safety Monitoring • Engine Emission Monitoring 22.3 Sensor FabricationTechnologies Microfabrication and Micromachining Technology • Nanomaterials • SiC-Based High-Temperature Electronics 22.4 Chemical Sensor Development Si-Based Hydrogen Sensor Technology • Nanocrystalline Tin Oxide Thin Films for NO x and CO Detection • Electrochemical Cell Oxygen Detection • SiC- Based Hydrogen and Hydrocarbon Detection • NASICON- Based CO 2 Detection 22.5 Future Directions, Sensor Arrays and Commercialization High-Selectivity Gas Sensors Based on Ceramic Membranes • Leak-Detection Array • High-Temperature Electronic Nose 22.6 Commercial Applications 22.7 Summary Acknowledgments 22.1 Introduction The advent of microelectromechanical systems (MEMS) technology is important in the development and use of chemical sensor technology for a range of applications, especially those that include operation in harsh environments or effect safety. As will be discussed in this chapter, chemical microsensors can provide unique information that can significantly improve safety and reliability while decreasing costs of a system or process. Such information can also be used to improve a system’s performance and reduce its effect on the environment. Chemical sensor data also can complement data derived from physical measurements such as temperature, pressure, heat flux etc., further improving overall knowledge of a system and expanding its capabilities. However, the application of even traditional macrosized chemical sensor technology can be problematic. Chemical sensors often need to be specifically designed (or tailored) to operate in a given environment. It is often the case that a chemical sensor that meets the needs of one application will not function Gary W. Hunter NASA Glenn Research Center Chung-Chiun Liu Case Western Reserve University Darby B. Makel Makel Engineering, Inc. © 2002 by CRC Press LLC 23 Packaging of Harsh-Environment MEMS Devices 23.1 Introduction 23.2 Material Requirements for Packaging Harsh-Environment MEMS Substrates • Metallization/Electrical Interconnection System • Die-Attach • Hermetic Sealing 23.3 High-Temperature Electrical Interconnection System Thick-Film Metallization • Thick-Film-Based Wirebond • Conductive Die-Attach 23.4 Thermomechanical Properties of Die-Attach Governing Equations and Material Properties • Thermomechanical Simulation of Die-Attach 23.5 Discussion Innovative Materials • Innovative Structures • Innovative Processes Acknowledgments 23.1 Introduction Microelectromechanical system (MEMS) devices, as they are defined, are both electrical and mechanical devices. Via microlevel mechanical operation, MEMS devices, as sensors, transform mechanical, chem- ical, optical, magnetic and other nonelectrical parameters to electrical/electronic signals; as actuators, MEMS devices transform electrical/electronic signal to nonelectrical/electronic operations. Therefore, MEMS devices very often interact with the environment electrically, magnetically, optically, chemically and mechanically. In order to support these nonconventional device operations (i.e., the device mechan- ical operation and the nonelectrical interactions between the device and their environments), new packaging capabilities beyond those provided by conventional integrated circuit (IC) packaging technol- ogy are required [Madou, 1997]. A chemically inert, optically dark and electromagnetically “quiet” environment for packaging conventional ICs, provided by hermetic sealing and electromagnetic screen- ing, is no longer suitable for packaging most MEMS devices. Because MEMS devices have very specific requirements for their immediate packaging environment, it is expected that the design of MEMS packaging will be very device dependent. This is in contradiction to the conventional IC packaging practice in which a universal package design can accommodate many different ICs. Compared to con- ventional IC packaging, the most distinct issue of MEMS packaging is to meet the requirements imposed by the mechanical operability and reliability of MEMS devices. Liang-Yu Chen NASA Glenn Research Center Jih-Fen Lei NASA Glenn Research Center . Electronic Nose 22.6 Commercial Applications 22.7 Summary Acknowledgments 22.1 Introduction The advent of microelectromechanical systems (MEMS) technology is important in the development and. mechanical devices. Via microlevel mechanical operation, MEMS devices, as sensors, transform mechanical, chem- ical, optical, magnetic and other nonelectrical parameters to electrical/electronic signals;. actuators, MEMS devices transform electrical/electronic signal to nonelectrical/electronic operations. Therefore, MEMS devices very often interact with the environment electrically, magnetically,