First an oxide layer is created on the substrate with thermal oxidation of the silicon surface. This oxide surface is then covered with a layer of photoresist. Photoresist is a light-sensitive, acid-resistant organic polymer which is initially insoluble in the developing solution. On exposure to ultraviolet (UV) light, the exposed areas become soluble which can be etched away by etching solvents. Some areas on the surface are covered with a mask during exposure to selectively expose the photoresist. On exposure to UV light, the masked areas are shielded whereas those areas which are not shielded become soluble.

There are two types of photoresists, positive and negative photoresist. Positive photoresist is initially insoluble, but becomes soluble after exposure to UV light, where as negative photoresist is initially soluble but becomes insoluble (hardened) after exposure to UV light. The process sequence described uses positive photoresist. Negative photoresists are more sensitive to light, but their photolithographic resolution is not as high as that of the positive photoresists. Hence, the use of negative photoresists is less common in manufacturing high-density integrated circuits.

The unexposed portions of the photoresist can be removed by a solvent after the UV exposure step. The silicon dioxide regions not covered by the hardened photoresist is etched away by using a chemical solvent (HF acid) or dry etch (plasma etch) process. On completion of this step, we are left with an oxide window which reaches down to the silicon surface. Another solvent is used to strip away the remaining photoresist from the silicon dioxide surface. The patterned silicon dioxide feature is shown in Figure 12.43

The sequence of process steps illustrated in detail actually accomplishes a single pattern transfer onto the silicon dioxide surface. The fabrication of semiconductor devices requires several such pattern transfers to be performed on silicon dioxide, polysilicon, and metal. The basic patterning process used in all fabrication steps, however, is quite similar to the one described earlier. Also note that for accurate generation of high-density patterns required in submicron devices, electron beam (E-beam) lithography is used instead of optical lithography.