Residues from Wet Processing of Positive Resists

An ellipsometric study has been made of the residues left from the development and stripping of several positive resists as a function of surface treatment, development, and stripping variables. Residues of up to nearly 5 nm have been ellipsometrically observed in the development of AZ‐1350J and AZ‐111, using commonly employed developer solutions under conditions which yield positive print biases. The use of adhesion promoters HMDS, BSA, and BSTFA generally appeared to increase the amount of residue. The stripping effectiveness of organic solvents such as acetone was found to be extremely sensitive to post‐baking, with temperatures as low as 130°C causing detectable residues after stripping. The use of a postdevelopment exposure helps prevent this problem. High temperature inorganic strippers, such as sulfuric/nitric acid mixtures, are highly effective, but unexplained "negative residues" are observed. Also, for resist that is cross‐linked by ion bombardment, considerable residue is observed at patterned edges and can be removed only by a 400°C bake before stripping. Caro's acid, evaluated as a room temperature inorganic stripper, is fully equivalent to the high temperature acids. In addition, it shows superior stripping properties for cross‐linked resists and minimal attack on Cu‐Al‐Si metallization. A persulfate‐based stripper, similar in composition and properties to Caro's acid but whose makeup does not involve the handling of concentrated hydrogen peroxide, was also evaluated. This type of stripper appears best suited to commonly used process technology, from considerations of safety, effectiveness, and pollution control.