Projection printing systems for forming an image on a light sensitive substrate or wafer are particularly suitable for effecting the exposure of photoresist-coated semiconductor wafers in the manufacture of integrated circuits. These systems normally include a photomask or reticle (hereinafter "mask") having a transparent substrate with a pattern of opaque and transparent areas formed on one surface thereof, an illumination system for directing a beam of radiant energy such as light or ultraviolet rays through the mask to the wafer, optical means for forming a focused image of the mask pattern on the wafer and a pellicle for maintaining the images of any dust particles out of focus on the surface of the mask.
The pellicle is a free-standing thin optical film mounted on a frame. Normally, the frame is attached to the mask and the optical film stands away from the mask surface by a given distance. Dust particles which might otherwise settle on the mask and be projected onto the wafer, will settle on the pellicle instead and be defocused. Consequently, when pellicles are used in a conventional projection printing system, one or more dust particles will not affect the yield of a given wafer and yields can be significantly improved.
Ideally, pellicles should be invisible to the radiant energy of the projection printing system. That is, in order to produce clear, well-defined patterns, the optical film of the pellicle should transmit 100% (i.e. reflect 0%) of the radiant energy used during the projection printing process.
In the past, pellicle films have typically been made of nitrocellulose. With increasing miniaturization, the pattern elements have become smaller and smaller and the optical transmission of the pellicle films, especially over a broad range of energy wavelengths, has become increasingly insufficient. Accordingly, different materials have been proposed for pellicle films.
In U.S. Pat. No. 4,131,363, Shea et al. discloses polyoxyethylene terephthalate and parylene as suitable materials for pellicle films. In U.S. Pat. Nos. 4,378,953 and 4,536,240, Winn teaches acrylics as pellicle films. And Japanese patent publication Kokai Patent No. 58-219023 discloses pellicle films made of cellulose esters.
Ward uses the reaction product of a polyvinyl butyral resin and a silane as a pellicle film in U.S. Pat. No. 4,476,172. And in U.S. Pat. No. 4,482,591 to Ward and U.S. Pat. No. 4,499,231 to Ward et al., a composition comprising a mixture of a polyvinyl butyral resin and a dispersion of colloidal silica and a silicone resin is disclosed as a suitable material for a pellicle film.
Japanese patent publication Kokai Patent No. 60-083032 discloses a fluorine polymer of refractive index below 1.42 in a pellicle film.
U.S. Pat. Nos. 4,465,759 and 4,523,974 disclose a polymethylmethacrylate pellicle film. And U.S. Pat. No. 4,796,973 to Gordon discloses cellulose acetate butyrate pellicle films.
Although these various materials form pellicle films which transmit a substantial portion of incident radiant energy, there is still room for improvement. For example, many of the materials deteriorate rapidly upon exposure to deep ultraviolet radiant energy and some transmit a high percentage of incident energy only within a narrow range of energy wavelengths. Thus, there is still a need in the art for a pellicle film which transmits a high percentage of incident light and is useful and durable over a broader wavelength of incident radiant energy.