Measuring and testing – Gas analysis – Detector detail
Patent
1997-06-19
2000-08-29
Williams, Hezron
Measuring and testing
Gas analysis
Detector detail
257414, 422 94, G01N 700, G01N 3112, H01L 2714
Patent
active
061090948
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method and a device for gas sensing and particularly for gas sensing at high ambient temperatures.
PRIOR ART
It is known that catalytic metals can be used as gates for gas sensitive field effect devices (transistors, capacitors, diodes, etc). Thus they comprise metal-insulator-semiconductor- or metal-semiconductor structures. Such devices may be used to measure small concentrations of molecules like hydrogen, hydrogen sulfide, alcohols, hydrocarbons, ammonia, amines, etc. The highest operation temperature is determined by the semiconductors used, which e.g. for silicon is about 250.degree. C. but for silicon carbide about 1000.degree. C.
The gas sensitivity occurs because reaction intermediaries (like hydrogen atoms) give rise to electrical phenomena at the metal-insulator or metal-semiconductor interface, which changes the electric field outside the semiconductor. In FIG. 1 is demonstrated, in a simple cross section, a structure of prior art semiconductor sensors. A problem with this type of device is that slow phenomena occurs (structural changes in the metal and/or slow adsorption sites for the reaction intermediaries at the interface) which give rise to stability problems and slow responses (hysteresis).
In U.S. Pat. No. 5,273,779 is described a method of fabricating a gas sensor and the product fabricated thereby. The device comprises a substrate, a buffer layer coated on the substrate, a pair of electrodes disposed on the gas sensing layer, at least one gas sensing layer arranged on the buffer layer and a catalytic layer coated on the gas sensing layer. However the device is not a field effect device but based on conductivity changes in the sensing layers. Additionally the buffer layer is used to prevent reaction between the gas sensing layer and the substrate, i.e. the buffer layer has no function in the gas detection.
In U.S. Pat. No. 4,337,476 is described a method using silicon-rich suicides. Silicon-rich silicides of titanium and tantalum formed by sintering a cosputtered alloy with silicon to metal ratio of three are replacing polysilicon as the gate metal in semiconductor integrated circuits. The technique is used as a normal contact forming procedure in for example standard field effect devices.
In U.S. Pat. No. 4,816,888 is disclosed a way to produce a normal known contact by means of a titanium-gold film. This has nothing to do with the gas sensitivity of the sensor which is based on a moisture sensitive layer.
There is still a demand for a gas sensor array having improved stability and speed of response for use also at high ambient temperatures for instance in a combustion system.
The present invention discloses a gas sensitive semiconductor device suitable for forming arrays having one or more elements containing at least two layers disposed onto a semiconductor substrate, the layers offering better long term stability and faster response compared to elements having only one layer. The layer in contact with the gas to be detected is catalytically active while underlying layers primarily do not have to be catalytic, but at least providing changes in the electric field outside the semiconductor in the presence of the gas to be detected. The sensing electrode may be operated up to about 1000.degree. C. and is therefore of interest for gas sensitive devices based on e.g. siliconcarbide or diamond capable of operation at higher temperatures than that for silicon based devices.
According to a first object of the present invention is provided a gas sensing array comprising at least one sensing device on a semiconductor substrate, wherein changes in the electric field outside the semiconductor occur due to a catalytic layer in contact with the gas to be measured and a mono or multi layer between the catalytic layer and the semiconductor substrate constitutes an intermediate layer in the process of detecting gas, whereby the intermediate layer is different from the catalytically active layer, and has an electrical conductivity making it suitab
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Baranzahi Amir
Lundstrom Ingemar
Spetz Anita Lloyd
Forskarpatent i Linkoping AB
Politzer Jay L.
Williams Hezron
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