A technology for the low cost production of continuous tone gray scale photomasks for deep UV photolithography applications has been demonstrated. This technology is based on the use of a photosensitive spin-on-glass (SOG) thin film deposited onto a UV transparent substrate such as quartz. Different light exposure energies, from either a lithography setup or a laser pattern generator, onto the photosensitive SOG film changes the UV absorption spectrum at both H and I mercury emission lines. The amount of photo induced attenuation on the film is directly proportional to light exposure energy, hence allowing the formation of fully continuous tone patterns. Once the image pattern is photo-generated with a resolution of 0.1 to 1 micrometer, it is permanently fixed by a thermal treatment step without the need of an etching step. This new continuous tone deep UV photomask technology offers new cost effective opportunities for the production of micro-electro-mechanical systems (MEMS) structures, diffractive optical elements (DOEs), computer generated holograms (CGHs), and kinoform optics.
In previous work we combined fast aerial image simulation with a closed-loop Optical Proximity Correction (OPC) control system to generate pre-compensated mask geometries which account for pattern transfer distortion effects at small feature sizes. We also presented the variable- threshold resist (VTR) model in which an image-dependent threshold is used to calculate linewidths directly from the image intensity. The model parameters can be determined by `tuning' the model with linewidth measurements from chosen sample sites on the wafer. In this paper, we present verify our OPC approach experimentally by showing after etch SEM wafers of corrected and uncorrected designs. In doing so, we show that (1) OPC can eliminate bridging effects in uncorrected designs, (2) VTR model is fairly insensitive to process variations and (3) mask writing effects are important and cannot be ignored.