Lithography comes from two Greek words, “lithos” which means stone and “graphein” which means write. Lithography means literally “writing a pattern in stone’. In microelectronics – used to describe a process in which a pattern is delineated in a layer of material sensitive to photons, electrons or ions. The principle is similar to that of a photo camera in which an object is imaged on a photo-sensitive emulsion film. After development, the exposed regions of the film are left as metallic silver, while theunexposed regions are removed, resulting in a printed image of the object.

Types of lithography

1.Optical lithography

• from g-line (436nm) of high pressure mercury arc lamp to i-line (365nm)

• KrF excimer laser (248nm) – test production

• ArF excimer laser (193nm) – is under development Optical lithography is a technique used by industry but the limitation is due to diffractioni.e the ability of waves to spread around corners of a slit.
2. Electron beam lithography

Electron beam is limited by proximity effect and resist contrast which is a means of enhancing image resolution below 100nm by accelerating electrons instead of photons. Electrons, like photons, have particle and wave properties, when accelerated, acquire a wavelength, W = 1.23/sqrt(V), where W is in nm, V is the accelerating voltage.e.g. electrons accelerate at 10keV, then W = 0.0123nm which is much smaller than wavelength of UV light. This has good overlay tolerance of less than 50nm, has greater depth of focus, has low throughput and proximity effects.This method is used in mask fabrication for UV and X-ray lithography, in high resolution patterning of special devices, in research and development and in mix and match schemes.