ABSTRACT
Photolithography is widely used in the semiconductor industry to delineate ultra large scale integration (ULSI) patterns on a Si wafer. Advanced photolithography employs an excimer laser as a light source in the exposure tools. The principle of photolithography is similar to that of photography: the exposure of mask images through an optical system produces latent images in a photosensitive resist material (photoresist), which is then developed in a developer solution to form resist patterns. Photolithography is a system composed of various sub-systems such as light sources, exposure tools (optics and exposure systems), masks, pellicles and photoresist materials. There are two types of photoresist: positive and negative tone. As shown in figure D1.5.1, in a positive-tone photoresist material, the area exposed to light becomes soluble in the developer. This produces positive-tone patterns in the photoresist layer. In contrast, in a negative-tone photoresist, the area exposed to light becomes insoluble in the developer, producing negative-tone patterns in the photoresist layer. In the industrial world, positive-tone photoresists are widely used because they provide better resolution. However, due to the development of high-resolution negative-tone photoresist materials and the fact that various resolution-enhancement techniques require a negative-tone resist, these resists are starting to see greater use in specific fields.
