Coating processes – Optical element produced – Transparent base
Reexamination Certificate
1999-05-19
2001-07-31
Meeks, Timothy (Department: 1762)
Coating processes
Optical element produced
Transparent base
C427S255190, C427S255360, C427S255391, C065S060500
Reexamination Certificate
active
06268019
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns the deposition of fluorine modified, titanium dioxide films (TiO
2
) onto hot glass by atmospheric pressure chemical vapor deposition (APCVD) using TiCl
4
vapor. The invention is also suitable for depositing other metallic oxide films from their metallic halides such as SnCl
4
, GeCl
4
, and VCl
4
.
2. Technical Advance
Beneficial metallic oxide coatings for glass are known for improving one or more properties of the glass. Commercial glass coaters desire metallic oxide coatings that are free of contaminants or surface irregularities that diminish the optical properties of the glass. To be economical, the coatings must be deposited at rates that are commensurate with the operating speeds of a commercial float glass line. An economical process also requires that the chemical components be readily available and inexpensive. Materials that fulfill these needs are often metal chlorides. However, when metal chlorides are deposited at fast rates the resultant films often have rough surfaces which scatter the impinging light, causing haze. This haze reduces optical transmission and gives the article an aesthetically unpleasing look. This invention is a Atmospheric Pressure Chemical Vapor Deposition (APCVD) method for depositing films of metal oxides at very fast rates which have a low degree of haze using readily available, inexpensive, metal halide precursors.
Prior art processes generally have utilized Solution Spray, Low Pressure CVD, or Atmospheric Pressure CVD (APCVD). Solution sprays often require solvents and produce poor quality films while low pressure CVD techniques are batch operations and produce films at low deposition rates. Films that have been deposited by APCVD techniques are often hazy and frequently require thermal post-treatments to obtain the desired properties, and more expensive starting materials are often used. In contrast, the method of the present invention allows one to use inexpensive metal halides in a continuous APCVD process to produce oxide films at rates ≧900 Å/sec. These films have haze values of <1% and are uniform and continuous with reduced surface roughness.
The method of the present invention enables glass coaters to deposit high quality TiO
2
films on the float line at the high line speeds practiced with commercial float lines without any major disruption to the existing coating set-up.
DESCRIPTION OF THE PRIOR ART
In the publication “Rapid Formation of TiO
2
Films by a Conventional CVD Method” by K. Kamata, et. al.,
J. Mat. Sci. Let
., 9 (1990) 316-319, the authors teach the use of Ti (O-i-Pr)
4
in an APCVD process to deposit TiO
2
films. The method uses vapor transport through a concentric tube nozzle with the substrate above the nozzle. This configuration minimizes contamination on the growing film by particulate TiO
2
that may form in the vapor prior to film deposition. The TiO
2
precursor is vaporized by bubbling a carrier gas through a heated reservoir of precursor liquid. In contrast, the present invention uses a different TiO
2
precursor and vaporizes it by direct injection into a hot carrier gas along with a novel combination of reagents that modify and improve the resulting TiO
2
film morphology and increase deposition rate of the film onto hot glass. The following table compares the chemistry and process conditions of the present invention with the chemistry and process as taught in Kamata:
Coating Comparisons
Kamata
Present Invention
Precursor
Ti(O-i-Pr)
4
TiCl
4
Glass temperature
300-500 ° C.
550-675 ° C.
Precursor
0.22-3.6%
0.25-2.0%
concentration
Nozzle distance to
5-25 mm
25 mm
glass
Vaporization temp.
80-130 ° C.
140-180 ° C.
Carrier-N
2
/O
2
2/1-1/2
2/1-1/2
Carrier flow
1.5-3.5 m/s
1.9-3.1 m/s
H
2
O concentration
0-21%
0.25-6%
Deposition rate
800-1000 Å/s
900-1200 Å/s
Under certain conditions, Kamata obtains smooth, crystalline, anatase TiO
2
films at rates of
~
1000 Å/sec, contrary to others who get rough films at high deposition rates. However, Kamata's maximum deposition rate occurs at
~
500° C. and at these high temperatures, the process often produces significant amounts of powder in the vapor phase and films have a rough surface. Using Kamata's precursor and deposition conditions at temperatures typically encountered in a glass float bath,
~
575-600° C., the high deposition rates were not achieved. In contrast, at the higher temperatures encountered in a float bath, the rates obtained using the present invention are faster. This represent a significant commercial advantage over the Kamata process. Although Kamata makes no specific mention of film haze, we have found that the films deposited with the isopropoxide in accordance with the teaching of Kamata and under the same conditions as the fluorine modified TiCl
4
process of the present invention had haze values ≧1% as opposed to films of the present invention, which consistently measure <1% haze. Accordingly, the present invention produces fluorine modified TiO
2
film having less haze at faster rates and at higher substrate temperatures then Kamata, thereby making the present process and product more suitable for production on a commercial float line.
“Laser CVD-applications in materials processing”,
Proc. Soc. Photo-Opt. Instrum. Eng
., 198 (1980) 49-56. CAN 92:204194, S. D. Allen describes the deposition of TiO
2
films at rates of
~
333 Å/sec by use of a focused CO
2
laser beam to locally heat the substrate and a mixture of TiCl
4
, H
2
, and CO
2
gases. The method does not produce a fluorine modified TiO
2
film, is not practical on a large scale, and operates at a significantly slower rate.
S. Hayashi and T. Hirai, in “Chemical vapor deposition of rutile films”,
J. Crystal Growth
, 36(1), 157-64 (1976). CAN 6:63641 describe the deposition of rutile titania films on metallic substrates between 400-900° C. from a TiCl
4
-H
2
O system using argon as a carrier gas. The carrier was bubbled through the reagents and the saturated vapors were transported to the heated substrate. Maximum deposition rates of
~
70 Å/sec were obtained at 900° C. The method does not produce a fluorine modified TiO
2
and although high temperatures are used, film deposition rates are low, and the bubbler feed method is not efficient.
In “Photoconducting TiO
2
prepared by spray pyrolysis using TiO
2
” by S. Zhang, et. Al.,
Thin Solid Films
. 213,265-270 (1992), nitrogen is bubbled through a reservoir of TiCl
4
at room temperature and the saturated vapors are directed to a heated chamber where the substrate is mounted in the inverted position about 15 cm above the nozzle. Water is excluded from the system by design. Above a substrate temperature of 362° C., hazy films composed of both rutile and anatase phases are deposited at a deposition rate of <2 Å/sec. Below 322° C., clear anatase films were obtained. This process produces hazy films at very slow rates by an inefficient method.
In “Method of depositing titanium dioxide films by chemical vapor deposition”, U.S. Pat. No. 3,916,041, Oct. 28, 1975, T.Chu, et. al. describes the preparation of titania films on various substrates in a tube reactor. A gaseous mixture of an inert carrier gas, TiCl
4
, H
2
in molar excess, and O
2
are passed over the substrate heated to a temperature between 227-927° C. At a preferred temperature of 600° C., oxygen deficient, rutile phase titania films are deposited at rates of
~
1 Å/sec. The TiCl
4
vapors are generated by bubbling a carrier gas through a liquid reservoir. The method does not produce a fluorine modified TiO
2
film, has low film deposition rates, uses an inefficient bubbler system, and a tube reactor not practical for large scale work.
There are a number of other references which mention rapid deposition rates, but the precursor used in all cases was Ti (O-i-Pr)
4
and haze is not discussed. For example:
In “Particle precipitation aided CVD for rapid growth of ceramic films” by H. Komiyama, et. al.,
Proc. Electroche
ATOFINA Chemicals, Inc.
DeBenedictis Nicholas J.
Marcus Stanley A.
Meeks Timothy
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