Chemistry of inorganic compounds – Sulfur or compound thereof – Oxygen containing
Reexamination Certificate
1994-11-08
2001-09-25
Tung, T. (Department: 1743)
Chemistry of inorganic compounds
Sulfur or compound thereof
Oxygen containing
C206S213100, C206S524800, C216S093000, C216S108000
Reexamination Certificate
active
06294145
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of providing a strong oxidizing solution containing sulfuric acid and peroxydisulfuric acid or H
2
S
2
O
8
(PDSA) having relatively long shelf life and to the stored strong oxidizing solution.
2. Brief Description of the Prior Art
Caro's acid (H
2
SO
5
) is a powerful oxidizing agent used in preparation of standard piranha etch solutions for use in semiconductor fabrication technologies. The piranha etch solution (Caro's acid) is generally used to remove photoresist from semiconductor devices being fabricated. Pirhana etch solution generally contains from about 4 to about 5 parts by volume concentrated H
2
SO
4
to one part by volume hydrogen peroxide. Caro's acid can be formed by mixing sulfuric acid (H
2
SO
4
) and hydrogen peroxide (H
2
O
2
) in a reversible reaction which yields the Caro's acid and water. In the required ratios, this reaction is exothermic, raising the temperature of the bath from ambient to about 110° to about 130° C. Caro's acid formulated in this manner is also relatively unstable and has therefore presented storage problems. In general, when Caro's acid has been required, it has been formulated and used immediately after formulation.
Oxidizing solutions have also been produced in the prior art by mixing peroxydisulfuric acid or H
2
S
2
O
8
(PDSA) with sulfuric acid. This formulation has also been found to have poor shelf life because, after about 1 week, there was a 50 percent loss of product where a solution of 10 parts by volume concentrated sulfuric acid to one part by volume PDSA was used. Accordingly, this formulation has only been prepared and applied at the time and point of use.
The economics of piranha solution formulation would be materially improved if the strong oxidizing solution required could be an off-the-shelf item with materially extended shelf life by storage at the point of use rather than having to be formulated separately in conjunction with each use. Not only is the cost of preparation of strong oxidizing solution high but the cost of delivery in a pipeline is also quite high.
SUMMARY OF THE INVENTION
In accordance with the present invention, strong oxidizing solution has been formulated and stored with a shelf life in excess of one week. Briefly, this has been accomplished by providing such strong oxidizing solution, preferably by mixing peroxydisulfuric acid having a concentration of from about 2 to about 5 percent and preferably about 4 percent with concentrated sulfuric acid in the ratio of from about 1:8 to about 1:20 parts by volume and preferably 1:10 parts by volume. The acid is then placed in a container which is inert to the chemicals involved, preferably one having high purity and low contamination and of polytetrafluoroethylene (PTFE) or polyfluoroalkoxy (PFA) having an atmosphere which is non-oxidizing and inert to the oxidizing solution or has an evacuated region above the acid. The container can then be transported to the point of use and retained at that location for extended periods in excess of one week. While concentrated sulfuric acid and PDSA of from 2 to 5 percent is preferred in the ratios stated above, it is readily apparent that other ratios will be used if less concentrated sulfuric acid is used and/or if PDSA which is more or less concentrated than 4 percent is used.
The same result with respect to shelf life should be available when formulating a mixture of hydrogen peroxide having a concentration of from about 40 percent to about 60 percent and preferably 50 percent with concentrated sulfuric acid having a ratio of from about 1:8 to about 1:20 parts by volume and preferably 1:11 parts by volume.
The strong oxidizing solution can be used as a cleaning solution after diluting said solution with water.
DESCRIPTION OF THE PREFERRED EMBODIMENT
REFERENCES:
patent: 2899272 (1959-08-01), Flach et al.
patent: 3801512 (1974-04-01), Solenberger
patent: 3927189 (1975-12-01), Jayawant
patent: 3931396 (1976-01-01), Bardy et al.
patent: 4022703 (1977-05-01), Bakes et al.
patent: 4229544 (1980-10-01), Haynes et al.
patent: 4334610 (1982-06-01), Ota et al.
patent: 4526650 (1985-07-01), Blomquist et al.
patent: 4579725 (1986-04-01), Jayawant
patent: 4917122 (1990-04-01), Lapham et al.
patent: 5304360 (1994-04-01), Lane et al.
Hall Lindsey H.
Schraeder Charles R.
Sees Jennifer A.
Brady III W. James
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
Tung T.
LandOfFree
Piranha etch preparation having long shelf life and method... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Piranha etch preparation having long shelf life and method..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Piranha etch preparation having long shelf life and method... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2508822