A positive photoresist with uniform reactivity for use in a thick film lithography process, includes thermal curing during soft-baking and photo dissociation through UV exposure. The positive photoresist comprises a phenolic resin, a resin with acid labile groups, a photoacid generator (PAG), and a reactive monomer with vinyl ether or epoxy group. First, the resins react with the reactive monomer to perform a thermal curing step by soft-baking to form network polymers. In the UV lithography process, the exposed network polymers perform both deprotection and depolymerization simultaneously and are rendered alkali-soluble. The resulting photoresist patterns have a high aspect ratio and resolution profile, due to the good alkali dissolution contrast and uniform reactivity.