Once used for theater posters, lithography is now essential to microchip manufacturing.
Lithography is a printing technique invented in Germany in the 18th Century. The name is derived from the Greek words for stone and writing, a reference to the smooth limestone surface used as a printing medium in original process. In the 20th Century, lithography gave rise to the high speed offset press and today is finding new uses in high technology.
A Brief History of Lithography
The lithographic process was invented in 1798 by German playwright Alois Senefelder. Senefelder discovered that if he drew on a slab of limestone with oil-based ink, then dampened the stone, he could re-ink the stone with a roller and make multiple copies of the image. Ink from the roller would be repelled by the wet stone, and stick only where the surface was already inked. This was a more versatile printing technology than moving type, which handled images and script-based texts poorly.
Lithography and Photographs
Senefelder discovered that an original drawing could be used to transfer an image to the stone for reproduction. In the second half of the 19th Century, photography was adapted to this purpose. The stone was prepared with a coating of photosensitive chemicals. The original image was printed on a transparent surface, or mask, then used to mask part of the surface, as it was "exposed" in the same manner as a photographic plate.
Photolithography and Masks
The process of using light to create an image for reproduction has been transferred from printing to the manufacture of modern semiconductors. In this usage, the desired image, or mask, is still created on coated glass or a similar substance. Visible light, or another form of radiation, is then used to project that pattern onto wafers of silicon coated with photosensitive chemicals. Like swimsuit lines on a sunbather, the pattern is left on the surface. The surface is then etched, alternately removing the pattern or leaving the pattern while removing the surrounding area.
Masks, Photolithography and Microelectromechanical Devices (MEMS)
These same basic principles have been harnessed to a newer technology known as microelectromechanical devices (MEMS). MEMS are miniscule, or even microscopic, devices which replicate the function of larger machines, on a scale sometimes as small as a few molecules. To achieve the desired level of accuracy, masks used in MEMS foundries are usually created larger than the finished product, then focused through a lens mechanism to achieve the desired specifications. At such small sizes, the focal lengths of the optics used and the depth of the silicon wafer become significant design considerations.
