The polyimide can prevent the pinhole formation while the silicon nitride can minimize the effect of undercutting.. The characteristics of a diaphragm-based pressure sensor device are de
Trang 1another 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 (Na∼ 1018cm−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 (VSCE) 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)
Trang 2another 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 (Na∼ 1018cm−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 (VSCE) 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
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)
Trang 3Deep 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 µm to several hundred microns Three methods for bulk Glenn M Beheim
NASA Glenn Research Center
Trang 4Microfabricated 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 NOx and CO
Detection • Electrochemical Cell Oxygen Detection • SiC-Based Hydrogen and Hydrocarbon Detection • NASICON-Based CO2 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.
Trang 5Packaging 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, optchem-ical, 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 nonelectrmechan-ical 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