Lithography

Lithography is a method for printing on a smooth surface, as well as a method of manufacturing semiconductor and MEMS devices.

Printing

Lithography as a manual process is based on the repulsion of oil and water. The image is placed on the surface with an oil-based medium; acid is then used to 'burn' the oil into the surface. When printing, the surface is covered in water, which remains on the non-oily surface and avoids the oily parts; a roller can then apply an oil-based ink that adheres only to the oily portion of the surface.

In the early days of lithography, a smooth piece of limestone was used (hence the name "lithography"—"lithos" is the Latin word for stone). After the oil-based image was put on the surface, acid burned the image onto the surface; gum arabic, a water soluble solution, was then applied, sticking only to the non-oily surface and sealing it. During printing, water adhered to the gum arabic surfaces and avoided the oily parts, while the oily ink used for printing did the opposite.

Today, however, aluminum plates are used. The plates already have a brushed, or "roughened" texture, but they are covered with a smooth photosensitive emulsion. A photographic negative of the desired image is laid on top of the plate, and exposed to light, transferring a positive image to the emulsion. The emulsion is then chemically treated to remove the unexposed portions of the emulsion. The plate is affixed to a drum on a printing press, and water is rolled over the plate, which adheres to the rough, or negative portions of the image. A roller coated with ink is then rolled over the plate, which adheres to the smooth, or positive portions of the image. If this image were directly transferred to paper, it would create a positive image, but the paper would be moistened. Instead, a drum covered with a rubber surface is rolled over the plate, which squeezes away the water, and picks up the ink. The drum is then rolled over the paper, transferring the ink. Because the image is first transferred to the rubber drum, the process is called "offset lithography," due to the fact that the image is offset to the drum before being applied to the paper.

Many innovations and technical changes have occurred to this process over the years, including the development of presses that utilize several plates to build up a multi-color image in one pass through the press, and the Dahlgren inking system, which eliminates the separate moistening step (instead combining it in the inking step).

Semiconductor Lithography

Semiconductor lithography was developed for use in manufacturing microchips. It is also used in MEMS applications, as it is one of the best methods currently in use for manufacturing devices on scales much smaller than a micrometer. Although silicon lithographic technology is most advanced, other materials are also used.

Lithography involves some combination of etching, chemical deposition, and chemical treatments in repeated steps on an initially flat substrate. A part of a typical silicon lithography procedure would begin by depositing a layer of conductive metal several nanometers thick on the substrate. A layer of photoresist -- a chemical that hardens when exposed to light -- is applied on top of the metal layer. The photoresist is selectively hardened by illuminating it in specific places. For this purpose a transparent plate with patterns printed on it, called a mask, is used together with an illumination source to shine light on specific parts of the photoresist. Then, the photoresist that was not exposed to light and the metal underneath is etched away with a chemical treatment. Finally, the hardened photoresist is etched using a different chemical treatment, and all that remains is a layer of metal in the same shape as the mask.

Lithography is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously. Its main disadvantages are that it requires a substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.