Device Photolithography: Lenses for the Photolithographic System

There are two classes of photographic mask-making systems. In the first class, the pattern is generated through a lens as in a cathoderay-tube plotter or primary pattern generator (PPG), or a lens is used to reduce the size of the pattern to that of the circuit being made. The maximum complexity of pattern in this type of system is limited by the resolution that can be obtained over the field of a lens. A second class of systems uses a lens imaging a single small spot of light that is moved over an area and modulated to write a pattern. In this type of pattern generator the complexity of pattern is limited only by the minimum spot size and the area covered. This system must be used to draw the mask at the same scale as the final circuit or the lens in a reduction camera would limit the resolution. Systems in the first class have been chosen for the mask laboratory in spite of the resolution limitations because of the speed and flexibility of the lens type systems for making a wide variety of masks. As a result the lenses in the system are the principal limitation on the maximum complexity of patterns that can be produced and on the quality of the images. 2105