lithography: one construct, many systems

Lithography is planographic printing: an image is drawn in a greasy medium on a flat stone or plate, the surface is chemically treated so the greasy areas accept oil-based ink and the dampened bare areas repel it, and the ink is then transferred to paper from a surface that is neither raised nor incised. Invented by Alois Senefelder around 1798, it was the first printing process to work from a flat matrix through the mutual repulsion of grease and water. Griffiths's Prints and Printmaking describes the process and its history. The referent is pattern-carrying by differential surface chemistry on a flat plane, the same core idea later borrowed, by analogy, to pattern silicon and to mould micro-structures.[1]

Unit

An ink image defined on a flat matrix by grease-and-water surface chemistry

Recovery means

Not applicable

Measured by

Judged by image fidelity, tonal range, and the registration of successive colour runs

Soft lithography is a family of patterning techniques that use a soft elastomeric stamp or mould, cast from a lithographically made master, to transfer a pattern by printing molecules, moulding, or embossing rather than by exposing a resist. Named explicitly after conventional lithography, it extends flat-surface pattern transfer to chemistry, curved surfaces, and features that optical lithography reaches only with difficulty. Xia and Whitesides's review Soft lithography introduced the approach. It is the same construct (defining and transferring a pattern from a planar master), carried into a system built on conformal elastomer contact.[2]

Unit

A pattern transferred from an elastomeric stamp or mould cast from a master

Recovery means

Not applicable

Measured by

Assessed by replicated feature size, pattern fidelity over area, and conformal contact with the substrate

Photolithography carries the lithographic idea into microfabrication: a light-sensitive resist is coated on a wafer, exposed to patterned light through a mask, and developed so the exposed and unexposed regions differ, leaving a patterned resist that guides etching or deposition. As in stone lithography the pattern is defined on a flat surface by a chemistry that distinguishes two kinds of region, but here the distinction is made with light rather than grease and the features are microscopic. Mack's Fundamental Principles of Optical Lithography sets out the optics and resist chemistry. It is the same flat-surface patterning construct, realised in a semiconductor system and limited by diffraction rather than by the grain of a stone.[3]

Unit

A resist pattern defined on a wafer by masked light exposure

Recovery means

Not applicable

Measured by

Characterised by resolution, critical dimension, overlay accuracy, and depth of focus