Coating processes – Coating by vapor – gas – or smoke
Patent
1997-06-30
1998-11-03
King, Roy V.
Coating processes
Coating by vapor, gas, or smoke
4272551, 4272552, 4271263, 427314, C23C 1618, C23C 1640
Patent
active
058305300
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention concerns chemical vapour deposition of tin oxide.
BACKGROUND OF THE INVENTION
The metal oxide semi-conductor tin oxide (SnO.sub.2) has a range of important applications, such as in transparent and conducting coatings on glass and in gas-sensing devices, especially for carbon monoxide, used in environmental and automotive applications.
Tin oxide gas sensors have been prepared from bulk sintered powder or from deposited thick or thin films. Thin films of tin oxide are especially suitable for gas-sensor applications, due to their better speed of response compared to thick films. Also, they can be fabricated with small dimensions on a large scale using low-cost production techniques. Also, thin films are widely compatible with existing micro-electronics technology and integrated circuits.
Thin films of tin oxide have been grown by a variety of techniques including reactive sputtering, spray pyrolysis and chemical vapour deposition (CVD). Of these techniques CVD has considerable potential due to its capability for large area growth, control of doping and film thickness and superior conformal step coverage.
A variety of precursors have been used for the deposition of tin oxide by CVD, including stannic chloride (SnCl.sub.4) and organotin compounds, such as dibutyltin diacetate (Bu.sub.2 Sn(OAc).sub.2), dimethyltin dichloride (Me.sub.2 SnCl.sub.2) and tetramethyltin (Me.sub.4 Sn). These precursors are generally highly toxic volatile materials, which are hazardous to use, and metal oxide growth is only possible in the presence of a large excess of a separate added oxidant, such as oxygen or water.
For example, U.S. Pat. No. 4,865,883 (Saitoh et al) relates to a method for forming a deposited film by introducing a gaseous starting material containing tin atoms, such as SnH.sub.4 or Sn(CH.sub.3).sub.4, a gaseous oxidizing agent to oxidize the starting material and an oxygen-containing gaseous compound for formation of a precursor for deposition of the film. U.S. Pat. No. 5,279,851 (Minosou et al) relates to a method of depositing a coating of stannic oxide containing potassium onto glass using the precursor, dioctyltin diacetate.
SUMMARY OF THE INVENTION
An object of this invention is, therefore, to provide a technique for depositing tin oxide which involves use of less hazardous source materials.
It has now been surprisingly found that tetraalkoxy tin compounds can be used to deposit tin oxide by chemical vapour deposition in the absence of an added oxygen source.
Accordingly in a first embodiment the invention provides a method of depositing tin oxide by chemical vapour deposition onto a heated substrate using a tetraalkyoxy tin compound.
The addition of dopants, such as platinum and palladium, to tin oxide films has been shown to improve the sensitivity and selectivity of gas sensor devices. It has further been surprisingly found that by including a .beta.-diketonate of platinum or palladium in the chemical vapour deposition method using a tetraalkoxy tin compound can result in a platinum or palladium doped tin oxide film.
Accordingly in a second embodiment the invention provides a method of depositing a platinum or palladium doped tin oxide film on a heated substrate by chemical vapour deposition using a mixture of a tetraalkoxy tin compound and a platinum or palladium .beta.-diketonate.
The invention further provides a substrate having a thin film of tin oxide doped with platinum deposited thereon by chemical vapour deposition from tetraalkoxy tin and a platinum .beta.-diketonate as precursors, wherein the deposited film has reduced levels of carbon contamination.
The invention also provides a gas-sensor comprising a substrate having a film of platinum-doped tin oxide having low levels of carbon contamination deposited thereon produced by the method of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred tin compounds for use in the invention included branched chain tetraalkoxy tin compounds, examples of which are tetratertiarybutoxy tin and tetraisop
REFERENCES:
patent: 4504522 (1985-03-01), Kaiser et al.
patent: 4857095 (1989-08-01), Brown
patent: 4865883 (1989-09-01), Saitoh et al.
patent: 5004490 (1991-04-01), Brown
patent: 5279851 (1994-01-01), Minosou et al.
patent: 5389401 (1995-02-01), Gordon
Pierson, "Handbook of Chemical Vapor deposition (CVD), Principles, Technology and Applications", Noyes Publications, 1992, pp. 235-236.
Houlton et al, Chem. Vap. Deposition (1995), 1(1), 26-8.
Epichem Limited
King Roy V.
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