Fotoresist

Fotoresist, or photoresist, is a light-sensitive polymer used to form a patterned coating on a substrate through exposure to light followed by development. The term is widely used in microfabrication and printed circuit board manufacturing. Photoresists respond to light by undergoing chemical changes that alter their solubility in a developer solution.

There are two major classes: positive-tone and negative-tone resists. In positive resists, exposure to light cleaves protective groups or causes chemical reactions that increase solubility in aqueous or alkaline developers, so the exposed regions are removed during development. In negative resists, exposure induces crosslinking or polymerization that reduces solubility, leaving the unexposed regions to be removed.

Many modern resists are chemically amplified, using a photoacid generator that, upon exposure, creates many acid molecules that catalyze depolymerization or deprotection during a post-exposure bake, greatly increasing sensitivity. Common materials include novolac resins with diazonaphthoquinone for traditional positive resists, and various epoxy or acrylate systems for negative resists. Deep UV, EUV, and electron-beam resists exist for different lithography techniques.

Processing steps typically include coating by spin, soft bake, exposure, post-exposure bake, development, and hard bake. The choice of developer—aqueous basic solutions such as tetramethylammonium hydroxide for positive resists, organic developers for certain negative resists—determines pattern fidelity. Resolution, contrast, and environmental stability influence performance, as do issues like standing waves, pinholes, and line-edge roughness.

Photoresists are central to semiconductor device fabrication, MEMS, and printed circuit boards. They enable microscopic pattern transfer through etching or deposition steps. Selection of resist type, thickness, and processing conditions is driven by target feature size, exposure wavelength, and the subsequent processing sequence.