GAAs High-Speed Devices: Physics, Technology, and Circuit Applications

5

LITHOGRAPHY

5.1 INTRODUCTION TO LITHOGRAPHY

In semiconductor processing area-patterning techniques are very important. Lithography is the process of transferring patterns of geometric shapes on a mask to a thin layer of radiation-sensitive material called resist covering the surface of a semiconductor wafer [Wil84]. These patterns define the various regions in an integrated circuit such as the implantation regions, the contact windows, and the bonding pad areas. The resist film is then subjected to a development process that selectively removes either the exposed or unexposed resist. The remaining pattern will be replicated in other materials by evaporation or etching. The exposure may be accomplished using light, ultraviolet light, electron beams, X rays, or ion beams. Photoresists can be classified as positive or negative, depending on how they respond to radiation. The positive type removes the exposed resist. The net result is that the patterns formed in the positive resist are the same as those on the mask. The negative type forms patterns that are the reverse of the mask patterns because the exposed regions are made less soluble. The image reversal techniques, which give better resolution, contrast, and process latitude will be discussed in this chapter. Photolithography is the dominant technique using in GaAs processing.

GaAs processing requires different lithography techniques than silicon processing [Wil90]. The basic silicon process uses resist as an etch mask. Consider ...