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A Superior Etchant for Alumina Disclosure Number: IPCOM000010199D
Original Publication Date: 2002-Nov-04
Included in the Prior Art Database: 2002-Nov-04
Document File: 2 page(s) / 40K

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A Superior Etchant for Alumina

Thin film alumina etching using ethylene-diamine-tetra-acetic acid (EDTA) and sodium hydroxide to adjust the pH above 9 has a number of distinct disadvantages. (1) It rapidly attacks copper which is commonly used as a conductor in thin film devices using alumina. (2) It is such an outstanding chelate that it may render heavy metal hydroxides in the waste stream soluble. Therefore used etchant must be disposed of in containers as heavy metal waste. (3) The pH is unstable and tends to drift with time, contributing to a variable etch rate. A superior etchant based on a potassium carbonate/bicarbonate solution avoids these disadvantages and has several other advantages as well.

The preferred solution uses a twenty to one ratio of potassium carbonate to potassium bicarbonate in D.I. water with a small concentration, (approximately 0.1m Molar), of a non-ionic surfactant to enhance uniform wetting. These materials are safe, (the sodium salts are widely used as baking powder), free flowing, stable, and easily weighed.

Small increases in the bicarbonate concentration can be used to decrease the pH and to compensate for activity coefficient effects. Once mixed the pH is buffered and will not change with modest absorption of atmospheric carbon dioxide unlike solutions using potassium hydroxide. The pH is also stable against evaporation or dilution because both concentrations of the buffer change at the same rate.

pH = 10.25 + log(f2K2CO3/f1KHC...