Miniaturized metal (metal alloy)/ PdOx/SiC hydrogen and hydrocarbon gas sensors
Inventors
Hunter, Gary W. • Xu, Jennifer C. • Lukco, Dorothy
Assignees
National Aeronautics and Space Administration NASA
Publication Number
US-8001828-B2
Publication Date
2011-08-23
Expiration Date
2026-05-12
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Abstract
A miniaturized Schottky diode hydrogen and hydrocarbon sensor and the method of making same is disclosed and claimed. The sensor comprises a catalytic metal layer, such as palladium, a silicon carbide substrate layer and a thin barrier layer in between the catalytic and substrate layers made of palladium oxide (PdOx ). This highly stable device provides sensitive gas detection at temperatures ranging from at least 450 to 600° C. The barrier layer prevents reactions between the catalytic metal layer and the substrate layer. Conventional semiconductor fabrication techniques are used to fabricate the small-sized sensors. The use of a thicker palladium oxide barrier layer for other semiconductor structures such as a capacitor and transistor structures is also disclosed.
Core Innovation
The invention is a miniaturized Schottky diode hydrogen and hydrocarbon sensor comprising a catalytic metal layer such as palladium, a silicon carbide substrate layer, and a thin palladium oxide (PdOx) barrier layer between the catalytic metal and substrate. This stable device enables sensitive gas detection at elevated temperatures ranging from at least 450 to 600° C, preventing reactions between the catalytic metal and substrate layers that cause device degradation. The sensor can be fabricated using conventional semiconductor techniques and can also be applied with thicker PdOx layers in capacitor and transistor structures.
The problem addressed is the instability and reduced sensitivity of prior hydrogen sensors operating at high temperatures due to reactions between the catalytic metal sensing layer and the semiconductor substrate. Such reactions lead to the formation of metal silicides and oxidative degradation, which incapacitate sensors by altering surface states and disrupting device layers. Existing sensors also suffer from hysteresis and limited temperature operating ranges. The invention overcomes these issues by introducing a controlled and stable palladium oxide barrier layer that prevents silicide formation and preserves sensor performance at high temperatures.
Claims Coverage
The patent includes seven independent claims covering fabrication processes and methods of using gas sensors in various device forms. The main inventive features concern the formation and use of a palladium oxide barrier interlayer in combination with catalytic metal layers on a silicon carbide substrate.
Formation of a palladium oxide barrier layer on silicon carbide substrate
The invention includes sputtering or evaporating palladium in an oxygen atmosphere on a silicon carbide substrate to form a controlled and stable PdOx barrier layer with specified thickness ranges (approximately 50 to 200 Angstroms for diode structures or greater than 200 Angstroms for capacitors/transistors).
Deposition of catalytic metal or metal alloy over the PdOx barrier
A metal selected from Pt, Pd, Au, Ir, Ag, Ru, Rh, In, Os, Cr, and Ti or alloys thereof is sputter deposited on the PdOx barrier layer, forming the catalytic sensing layer adapted for hydrogen and hydrocarbon detection.
Use of silicon carbide substrate with backside electrode contact
The sensor structure includes an n-type silicon carbide substrate approximately 400 microns thick with electrodes deposited on the backside to complete the device structure and enable operation at high temperatures.
Application of bias voltages and measurement methods in Schottky diode sensors
Methods of usage involve applying bias voltage across the device layers and measuring current through the diode which varies with hydrogen or hydrocarbon content, specifying details such as thickness ranges for metal layers and PdOx interlayer.
Operation and measurement methods for capacitor and transistor device embodiments
The invention includes applying bias across capacitor structures with non-conductive PdOx barrier layers thicker than 200 Angstroms to measure capacitance changes due to hydrogen/hydrocarbons, and transistor devices where hydrogen modulates current flow in a channel under gate voltage influence.
Overall, the claims define a high temperature gas sensor structure and corresponding methods that utilize a controlled PdOx barrier layer between catalytic metals and silicon carbide substrates to enhance stability and sensitivity for hydrogen and hydrocarbon detection across diode, capacitor, and transistor devices.
Stated Advantages
The device provides high sensitivity for hydrogen and hydrocarbon gas detection at temperatures from at least 450 to 600° C.
The palladium oxide barrier layer prevents formation of metal silicides between the catalytic sensing layer and substrate, improving long-term stability and sensor durability.
The sensor exhibits prolonged stability with minimal oxidative degradation even in harsh, high-temperature environments.
The device is miniaturized and suitable for mass fabrication using conventional semiconductor microfabrication techniques, enabling cost-effective production.
The sensor operates with high gain and fast response times without requiring high voltage levels.
Documented Applications
Engine emission monitoring.
Fire detection systems.
Fuel leak detection for launch vehicles and aerospace applications.
Monitoring of fuel leaks and emissions in commercial and space vehicles operating at elevated temperatures.
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