Photolithography

Photolithography is a process used in microfabrication to transfer geometric patterns from a photomask to a light-sensitive film, or resist, coated on a substrate such as silicon. The procedure typically begins with substrate cleaning and film deposition, followed by spin-coating a thin layer of photoresist. After a soft bake to drive off solvent, the wafer is aligned with a photomask and exposed to a patterned light source. Depending on the resist type, exposure alters the solubility of the resist in a developer solution: in positive resists, exposed regions become more soluble; in negative resists, exposed regions become insoluble. Development removes the targeted resist, revealing the substrate beneath.

The remaining resist serves as a mask for subsequent processing, most commonly etching or ion implantation. After etching or implantation, the remaining resist is stripped, and additional layers may be deposited or patterned in subsequent lithography steps. Modern semiconductor fabrication relies on optical lithography with wavelengths in the near-ultraviolet to deep-ultraviolet range, including 365 nm (i-line), 248 nm (KrF), 193 nm (ArF), and increasingly extreme ultraviolet (EUV) at 13.5 nm. Resolution is influenced by wavelength, numerical aperture, and resist chemistry; techniques such as immersion lithography, phase-shift masks, and multiple-patterning extend pattern fidelity.

Applications include the production of integrated circuits and MEMS devices. The process favors high throughput and repeatability but requires sophisticated, costly equipment and strict process control. Ongoing research aims to push shorter wavelengths and new resist and mask technologies to sustain continued scaling of device features.