Canon Science Lab

Semiconductor lithography equipment project a reduced image of the photomask, the original circuit pattern, onto the silicon wafer, which is coated in a photosensitive layer called a photoresist. Projecting the circuit pattern onto the photoresist leaves an image of the circuit on the photoresist in much the same way that illuminating light through a film negative leaves a latent image on photo printing paper. After exposure, the wafer is subjected to developing, etching, doping and other processes that leave just the circuit on the wafer.

Developing involves the immersion of the wafers in developer to dissolve and remove unexposed, unnecessary photoresist, while etching removes the surface membrane of silicon dioxide through a chemical reaction, and doping adds conductive impurities through ion irradiation. This series of processes is repeated over 30 times to complete a single IC chip. The diagram below shows the steps involved in manufacturing a simple transistor.

In the completed transistor, n-type silicon has been embedded in p-type silicon by doping, after which the non-conductor (isolator) and conductor are incorporated. If electric current runs through this circuit, electrons gather through electrostatic induction in the area of the p-type silicon opposite the conductor, enabling current to pass between the n-type silicon points. The way in which the p-type silicon is affected by the electric field of the conductor, despite its being isolated by the non-conductor, is referred to as field effect. This transistor works by switching the current on and off.

This kind of transistor, in which a non-conductor (isolator) and metal conductor are placed on top of a semiconductor, is known as an MOS (Metal Oxide Semiconductor), and an MOS that makes use of field effect is further known as an MOS FET.

Because photolithographic optical apparati project very detailed circuit patterns onto wafers, they need to use very short wavelength light sources. Current devices use 248 nm KrF (krypton fluoride) or 193 nm ArF (argon fluoride) excimer lasers as light sources, which are capable of creating circuit patterns of a width of about 100 nanometers. Various other light sources are currently being studied to get this size down to the 50 nanometers that will be required in the future. In addition to fluoride dimer excimer lasers with wavelengths of 157 nm and X-ray lasers, EUV (Extreme Ultra-Violet) light sources are attracting keen attention.

Exposure technology is also used in the manufacture of large liquid crystal panels — the exact opposite of small silicon chips — such as those used to create large LCD televisions. Liquid crystal panels are manufactured by projecting a detailed pixel pattern drawn on an optical mask to etch (expose) and develop a large glass substrate. Mirrors and lenses are used in the projection process, but reflective optical systems using mirrors have a simpler composition than transparent optical systems using lenses, providing such benefits as no chromatic aberration and no deterioration in imaging performance.

In order to expose patterns measuring just several micrometers in size, the mirrors used in mirror projection aligners must be extremely precise. Specifically, large-diameter concave mirrors able to ensure the exposure width and scan distance required for seamlessly exposing large screens with a single pass. Furthermore, due to the size of the glass substrates used, with some measuring as large as 2,200 x 2,500 mm ("8th generation" substrates), an ultra-large stage that moves with precision is also required.