High contrast high thermal stability positive photoresists having novolak resins of lowered hydroxyl content

A method of forming a resist pattern on a substrate comprising:

• I. coating said substrate with a layer of a positive photoresist composition comprising an admixture of a bindingly effective amount of an esterified novolak resin and a photoimaging amount of a photosensitizer;
• II. exposing said layer patternwise to actinic radiation;
• and
• III. removing the exposed portion of said layer with an aqueous alkaline developer for the exposed resist compositon to uncover the areas of the substrate beneath the exposed portions;
• said an esterified novalak resin comprising, prior to exposure:
• (a) a mixture of m- and p-cresol and
• (b) a mixture of aldehydes comprising formaldehyde or formaldehyde precursor and
• a monohydroxy aromatic aldehyde selected from the group consisting of 2-hydroxybenzaldehyde, 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde, wherein said an esterified novalak resin has hydroxyl groups thereof esterified so that an aliphatic or aromatic ester group exists in place of the hydrogen atom in at least a portion of the hydroxyl groups,
• whereby said an esterified novolak resin having a hydroxyl number of about 120 to about 180 grams of resins per equivalent of hydroxyl.