Multi-level resist image reversal lithography process

Claim 1

A single development step, dual layer photoresist photolithographic process for forming hourglass shaped photoresist structures, comprising the steps of: depositing a first layer of positive photoresist having a base composition of a phenol-formaldehyde resin with a diazo-naphthoquinone sensitizer with a first solubility characteristic of becoming soluble in an aqueous alkaline developer solution after exposure to light and post baking; depositing a second layer of negative photoresist having said base composition plus the addition of a cross linking agent 1-hydroxyethyl-2-alkyl imidazoline on top of said first photoresist layer, having a second solubility characteristic of becoming insoluble to said developer solution after said exposure to light and said post baking; said positive photoresist and said negative photoresist mutually intermixing at said top of said first layer forming a gradation region where the solubility in said developer gradually changes from said first characteristic in the upper portion thereof to said second characteristic in the lower portion thereof; exposing said first and said second photoresist layers to a patterned light source; post baking said first and said second photoresist layers; flood exposing said first and said second photoresist layers without a pattern to the flooding light; developing said first and said second photoresist layers in said developer solution; whereby a composite photoresist structure consisting of said first and said second layers of photoresist will remain in those regions exposed to said patterned light source, said structure having well-defined upper edges and a recessed contour near the interface of said first and said second photoresist layers forming an hourglass shaped photoresist structure.