microlithography

Microlithography denotes optical lithography techniques used to print micron-scale patterns onto substrates, primarily for semiconductors and MEMS. Historically focused on features from a few micrometers to sub-micron scales, the field has advanced with shorter light wavelengths to improve resolution. Wavelengths used have included 365 nm (I-line), 248 nm (KrF), and 193 nm (ArF); more recently, extreme ultraviolet at 13.5 nm enables the smallest devices, though this is often treated as a separate class. The aim is high pattern fidelity and precise multi-layer alignment.

Typical process sequence: clean the substrate; apply photoresist by spin coating; soft bake; align and expose through a photomask; develop the resist to reveal the pattern; etch the underlying layer or perform a lift-off; hard bake; and resist stripping. Positive resists dissolve where exposed; negative resists cross-link and remain. The chemistry of resists, masks, and etchants governs resolution, contrast, and process latitude.

Resolution is governed by wavelength, numerical aperture, and process factor k1. The Rayleigh criterion R = k1·λ/NA provides an approximate feature size for a given system. Mask-based projection systems use chromium on quartz masks; alternatives include contact and proximity lithography. Precise mask alignment and overlay accuracy are essential for multi-layer devices.

Microlithography remains central to the fabrication of integrated circuits, MEMS, and microfluidic devices. Ongoing challenges include defect control, mask costs, resist performance, and limits on deep ultraviolet and visible light as feature sizes shrink. Research continues to extend resolution, throughput, and process windows through improved materials, optics, and exposure methods.