2.6 Photoresist

Once the aerial image, i.e., the incident light intensity on top of the wafer, is defined, it has to be transferred into the photoresist to generate a replica of the mask directly on the wafer surface. The basic categories into which photoresists can be divided is their polarity. A positive photoresist responds to the light in such a way as to make the exposed regions more soluble. A positive image of the mask is thereby stored. A negative photoresist acts in the opposite way. As most negative photoresists rely on photoinduced cross polymerization, a process in which large resin molecules attach to each other to become less soluble, they suffer of ``swelling.'' Swelling limits the possible resolution due to linewidth broadening during the development stage. For this reason negative resists are not suited to define features less than 2.0 $\mu$m. Hence, primarily positive photoresists are used in IC fabrication.