Week 8 : Spin Coating & Nanofabrication through Optical Lithography

Optical lithography: Optical lithography or photolithography is the process of forming a micro or nano pattern in a layer of light sensitive polymer called photoresist, that can be transferred by selective etching, into an underlying film. The most widely used lithography technique today is optical lithography, which uses UV light, however there are other technologies such as x-ray and electron beam lithographies that promise better fresolution. These newer technologies have many drawbacks to date, make them unattractive for use in integrated circuits or microprocessors production. There is a minimum limit of the pattern size that can be achieved by UV lithography of about (~500 nm), due to diffraction limit of the light wavelength. The photo exposure is typically performed using the 356 nm UV light from a mercury lamp, but to achieve the line widths of sub 100 nm an extreme UV source or even an X-ray source is needed.

Spin coating: To do optical lithography first silicon wafer is covered with a thin layer of light sensitive material called photoresist by spin coating. In this technique, a viscous, liquid solution of photoresist is dispensed onto the wafer, and the wafer is spun rapidly to produce a uniformly thin layer. Later, this photoresist layer is exposed to light and developed to make required nano pattern or nanotemplate into photoresist layer. 

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