enormous lamps and long exposure times. This was solved through the introduction of the steppers, which used a mask at much larger sizes and used lenses to reduce the image. These systems continued to improve in a fashion similar to the aligners, but by the late 1990s were also facing the same issues. At the time, there was considerable debate about how to continue the move to smaller features. Systems using excimer lasers in the soft-X-ray region were one solution, but these were incredibly expensive and difficult to work with. It was at this time that resolution enhancement began to be used. Basic concept The basic concept underlying the various resolution enhancement systems is the creative use of diffraction in certain locations to offset the diffraction in others. For instance, when light diffracts around a line on the mask it will produce a series of bright and dark lines, or "bands". that will spread out the desired sharp pattern. To offset this, a second pattern is deposited whose diffraction pattern overlaps with the desired features, and whose bands are positioned to overlap the original pattern's to produce the opposite effect - dark on light or vice versa. Multiple features of this sort are added, and the combined pattern produces the original feature. Typically, on the mask these additional features look like additional lines lying parallel to the desired feature. Adding these enhancement features has been an area of continual improvement since the early 2000s. In addition to using additional patterning, modern systems add phase-shifting materials, multiple-patterning and other techniques. Together, they have allowed feature size to continue to shrink to orders of magnitude below the diffraction limit of the optics. Using resolution enhancement Traditionally, after an IC design has been converted into a physical layout, the timing verified, and the polygons certified to be DRC-clean, the IC was ready for fabrication. The data files representing the various layers were shipped to a mask shop, which used mask-writing equipment to convert each data layer into a corresponding mask, and the masks were shipped to the fab where they were used to repeatedly manufacture the designs in silicon. In the past, the creation of the IC layout was the end of the involvement of electronic design automation. However, as Moore's law has driven features to ever-smaller dimensions, new physical effects that could be effectively ignored in the past are now affecting the