Silicon wafer etching compositions
View Patent ↗A process for etching silicon wafers using a caustic etchant in the form of an aqueous solution comprising water, a hydroxide ion source, and a chelating agent. The process produces silicon wafers substantially free from diffused metal ions.
1. A caustic etchant for etching silicon from the surface of a silicon wafer, the caustic etchant comprising water, at least about 65% by weight of a hydroxide ion source, and chelates formed between a chelating agent and metal ion impurities in the caustic etchant wherein the concentration of free metal ions in the caustic etchant is less than about 1 ppb and,
wherein the chelating agent is selected from the group consisting of ethylenediaminetetraacetic acid, diethylenetriaminepentacetic acid, glycolic acid, glycine (oxidase), dimethylgloxime, iminodisuccinic acid, nitrilotriacetic acid, and mixtures thereof.
2. The caustic etchant of claim 1 wherein the chelating agent is ethylenediaminetetraacetic acid.
3. The caustic etchant of claim 1 wherein the caustic etchant further comprises at least about 100 ppm by weight of free chelating agent.
4. The caustic etchant of claim 1 wherein the caustic etchant further comprises at least about 200 ppm by weight of free chelating agent.
5. The caustic etchant of claim 1 wherein the caustic etchant further comprises at least about 350 ppm by weight of free chelating agent.
6. The caustic etchant of claim 1 wherein the caustic etchant further comprises at least about 500 ppm by weight of free chelating agent.
7. The caustic etchant of claim 1 wherein substantially all of the metal ion impurities in the caustic etchant are sequestered in chelates.
8. The caustic etchant of claim 1 wherein the molar ratio of chelating agent combined with the hydroxide ion source to metal ion impurities in the hydroxide ion source is at least about 5:1.
9. The caustic etchant of claim 1 wherein the molar ratio of chelating agent combined with the hydroxide ion source to metal ion impurities in the hydroxide ion source is at least about 10:1.
10. The caustic etchant of claim 1 wherein the concentration of free metal ions is less than about 0.5 ppb.
11. The caustic etchant of claim 1 wherein the concentration of free metal ions is less than about 0.05 ppb.
12. The caustic etchant of claim 1 further comprising a salt selected from the group consisting of potassium fluoride, potassium carbonate, and a combination thereof.
13. The caustic etchant of claim 1 wherein the hydroxide ion source is selected from the group consisting of potassium hydroxide, ammonium hydroxide, tetramethylammonium hydroxide, sodium hydroxide, lithium hydroxide, cesium hydroxide, and mixtures thereof.
14. The caustic etchant of claim 13 wherein the hydroxide ion source is potassium hydroxide.