Lithography Explained!

Lithography

• Lithography is a process used to create integrated circuits (ICs). ICs are commonly made using optical lithography (also called photolithography), which uses ultraviolet light to expose the desired IC patterns onto a semiconductor wafer. Though other types of IC lithography exist, such as ion beam, electron beam, and x‐ray lithography, their basic principles and processes are similar.
• The physical laws of light and optics determine the precision and accuracy of a lithographic device, and the device’s precision and accuracy determine the minimum feature size of the IC. Therefore, the effects of diffraction and interference of light waves, and the numerical aperture of lenses are critical factors in creating more dense integrated circuits.

Process:

• The basic components of an optical lithography device are a prepared silicon wafer, a light source, an illuminator, a mask, and a lens. First, the silicon wafer is cleaned and covered with a barrier layer, usually of silicon dioxide (SiO2), and a photoresist layer. The silicon wafer is the substrate on which the IC is built, and the photoresist is a photosensitive polymer that becomes soluble when exposed to light. The prepared wafer is then soft baked to set the layers and remove solvents.
• The lithographic tool’s light source provides wavelength‐limited light to the illuminator. The illuminator is composed of mirrors and light pipes which shape the light to the necessary spatial uniformity and intensity across the surface of the mask. The mask is a plate, usually, quartz, with the pattern of a desired layer of IC sketched in chrome, like a stencil. Several masks are used to create the different layers and features of the IC. The light is changed into patterns of lighter and darker areas as it passes through to mask to the lens. The mask must be carefully aligned with the silicon wafer for proper transfer of the pattern to the silicon wafer.
• The lens focuses and resolves the patterned light image into a smaller image, usually one‐quarter the size of the mask. This smaller image is reflected on the photoresist surface of the silicon wafer. The parts of the photoresist exposed to light are washed away, leaving exposed and unexposed regions that match the pattern on the mask. The wafer is then hard baked to set the remaining photoresist. The unexposed regions are protected from the next step, in which the SiO2 layer is removed by etching from the exposed regions. Then the remaining photoresist is removed. This entire process, from preparing the silicon wafer to etching and photoresist removal, is repeated with several masks to create the different layers of an IC. 

 ~Jay Mehta

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