Monday, October 16, 2006

Lithography in Semiconductor Manufacturing

Lithography is the process of transferring geometric shapes on a mask to the surface of a silicon wafer. These shapes make up the parts of the circuit, such as gate electrodes, contact windows, metal interconnections, an so on. The final integrated circuit (IC) is made by sequentially transferring the features from each mask level by level, to the surface of the silicon wafer. For example, between each successive image transfer an ion implant, drive-in, oxidation, or metallization operation may take place.
In the IC lithographic process, a photosensitive polymer film is applied to the silicon wafer, dried, and then exposed with the proper geometrical patterns through a photomask to ultraviolet (UV) or other radiation. After exposure, the wafer is brought into contact (e.g. by dipping or spraying) with a solution that develops the images in the photosensitive material. Depending on the type of polymer used, either exposed (in the case of positive resists) or non-exposed (for negative resists) areas of the film are removed in the developing process. After development the resist acts as a mask, for instance for etching patterns into underlying layers.
Resists are made that are sensitive to UV light, electron beams, or ion beams. At present, optical lithography is the prevailing method. In the figure below, an example for the application of resist on the wafer is shown.


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