Device Photolithography: The Primary Pattern Generator - Part I - OpticalDesign

The basic design concept of the primary pattern generator (PPG) is the production of a linearly scanning, small, constant-size light spot. The scanning system consists of a regular polygonal-prism mirror which rotates about its axis of highest symmetry. The mirror faces are used sequentially to reflect a collimated light beam into a lens (for example, the scanning lens of Fig. 1). The collimated light is focused to a spot which scans a line in the focal plane of the lens as the polygonal mirror rotates. Located in the focal plane of the lens is a flat, glass photographic plate. The glass plate is moved by the desired scan line separation during the time required to bring the succeeding mirror facet into proper position. The collimated beam incident onto the rotating mirror is formed by the scanning lens from a diverging beam obtained from a laser. The location of the reflecting mirror facet must be close to the aperture plane of the scanning lens in order to insure that the mode is not truncated by the physical lens apertures after the light is reflected from the mirror facet, Translation of the reflecting facet will not affect the position of the focused spot; the spot position is uniquely determined by the directions of the incident collimated beam and of the reflecting mirror facet relative to the optic axis of the lens. A barrel distortion is designed into the scanning lens such that the linear velocity of the focused spot is proportional to the angular velocity of the rotating mirror.