Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than...
Reexamination Certificate
1999-11-15
2001-09-18
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
C257S077000, C257S252000, C257S253000, C257S414000, C257S473000, C438S048000, C438S049000
Reexamination Certificate
active
06291838
ABSTRACT:
BACKGROUND OF THE INVENTION
U.S. Pat. No. 5,270,252 to Papanicolaou is a Schottky diode on beta silicone carbide. The metals taught in the Papanicolaou disclosure are platinum, tungsten, titanium tungsten, gold and aluminum. The Schottky diode of the Papanicolaou disclosure is used for high temperature semiconductor applications and, in particular, it is used as a rectifying diode.
U.S. Pat. No. 5,612,232 to Thero, et al. is a Schottky diode for use in high temperature applications but it cannot be used as a gas sensor. Thero, et al. teaches the use of a silicon carbide semiconductor with nickel and tungsten as metals.
U.S. Pat. No. 4,622,736 to Drobny discloses Schottky diodes for use in connection with a silicon semiconductor. Tungsten, titanium-tungsten, and vanadium are the metals used in the invention The Drobny Schotky diode is not a gas sensor.
The development of a Schotky diode structure has been reported by the researchers at Linkoping University. The structure of the Linkoping sensor is Pt on TaSix on SiO2 on SiC. The Pt/TaSix/SiO2 thicknesses are 100 nm/10 nm/5 nm respectively. The sensor responses are stable and fast but they are not highly sensitive.
SUMMARY OF THE INVENTION
The instant invention discloses a Schottky diode which includes an alpha silicon carbide substrate, an alpha silicon carbide epilayer, a backside contact, and a palladium chrome contact. The silicon carbide epilayer is an n-type carrier as is the silicon carbide substrate. The epilayer is grown on a commercially available n-type 3.5° off-axis polished c-FACE 6H-SiC substrate. The epilayer surface was etched by a dilute hydrofloric solution, rinsed with deionized water and blown dry with nitrogen prior to the deposition of the palladium chrome film thereon. Approximately 400 Angstroms of the palladium chrome alloy are magnetron sputter deposited onto the C-face of the epilayer to form a palladium chrome/silicon carbide diode. The ratio of the palladium to chrome is controlled during the deposition thereof.
In the preferred embodiment the palladium chrome deposition is 90 atomic percent palladium and 10 atomic percent chrome. A backside substrate contact is formed by sputtering aluminum thereon.
The palladium chrome contact surface is a catalytic material in the presence of hydrogen. The presence of hydrogen results in an increased current flow through the diode with a given bias voltage applied to the diode. Hydrogen dissociates when it reacts with the palladium chrome.
It is an object of the invention to provide a sensor which is stable and sensitive at high temperatures of 425° C. and above.
It is a further object of the present invention to provide a palladium chrome contact on an alpha 6H silicon carbide substrate with a metal backside contact.
It is an object of the present invention to provide a gas sensor using an alloy on the C-face of a silicon carbide epilayer on a silicon carbide substrate.
It is an object of the present invention to provide a sensor which is stable and sensitive following exposure to high temperatures for long periods of time.
It is an object to provide a Schottky diode employing silicon carbide as the semiconductor to detect hydrogen and hydrocarbons at low concentrations.
It is an object to provide a hydrogen and/or hydrocarbon sensor which can be used at elevated temperatures for prolonged periods of time for use in catalytic combustion control systems or other applications which depend on the presence of hydrogen or hydrocarbons.
REFERENCES:
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patent: 3660154 (1972-05-01), Scott-Monck et al.
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patent: 5442200 (1995-08-01), Tischler
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Tobias et al. “Fast Chemical Sensing with Metal Insulator Silicon Carbide Structures” IEE Elector Device Letters, vol. 18; p. 287-289 (Jun. 6, 1997).
Karlstein, et al. “Electrical Properties of MIS Structures on 6N-Sic”. Linkoping University, Conference. Pg. X-17 to X-22. (Jun. 5-10, 1994).
Mintel William
Stone Kent N.
The United States of America as represented by the Administrator
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