Photolithography

From ArticleWorld

Photolithography refers to the process in which a pattern from a reticle is copied onto a photosensitive substrate (like silicon wafer). It is used to create printed circuit boards and integrated circuits. The word literally means "light stone writing" in Latin.

Photolithography employs a method that is similar to lithographic printing. Its steps involve the application of many chemicals successively on the flat surface of a substrate.

History

The history of photolithography can be traced to 1826, when French scientist Joseph Niepce took the first photograph on a pewter plate. About twenty years later, the English scientist William Talbot developed dichromated gelatin. The first negative and positive photoresists were developed in the 1930s.

Steps in the process

The following are the steps involved in a typical procedure of silicon lithography.

1. The surface of the substrate is made flat and even.
2. A thin layer of conductive metal is coated on the substrate.
3. A special chemical called photoresist is then applied on top of the conductive metal. This chemical has the property of hardening upon exposure to light.
4. The photoresist is then selectively hardened through the use of a photomask in combination with an ultraviolet light source. A photoresist can be either positive or negative, depending on whether the exposed area or the non-exposed area is the one that is developed.
5. This hardened photoresist is then etched by the use of another chemical treatment, leaving behind a layer of metal of the desired shape.

Technology

This technology essentially involves the introduction of a wafer onto an automated “wafertrack” setup. There are several robots used to transfer wafers from a particular module to another. Since it is necessary to rid the wafer of any moisture, it is heated to a suitable temperature. The polymeric material’s adhesion is improved by the application of liquid or vapour hexa-methyl-disilizane (HMDS). Any excess solvent that happens to accumulate on the wafer is removed by a soft-bake process where the photoresist-coated wafer is kept on a hot plate.

The simplest exposure system is a printer that puts a photomask in direct contact with the wafer. A small gap is put between the photomask and the wafer. The pattern of the photomask is directly imaged onto the wafer-based photoresist. Ideally, contact printing with gap distance of zero offers the best resolution. Another common method involves the projection of the desired pattern onto the wafer in stepper or a scanner.

It may be desired to include minute features onto wafers, but this is limited by the wavelength of the light being used. The use of deep ultraviolet light (DUV) with wavelengths ranging from 248nm to 193nm allows feature sizes as low as 50nm.