Method for making photomasks having regions of different light transmissivities

摘要

An improved method for preparing multiple-exposure photomasks having at least three levels of transmissivity is disclosed. A conventional bi-exposure photomask is first formed which contains a macroscopically exposed area, a microscopically exposed area, and a masked area on a transparent substrate. Then a diffractive-refractive light-scattering optical element is formed above the microscopically exposed area so as to provide an exposure area with an intermediate light transmissivity. In a preferred embodiment, the diffractive-refractive light-scattering optical element above the microscopically exposed area is formed by first forming a transparent positive photoresist above the bi-exposure photomask. The transparent positive photoresist is exposed to an irradiation from the bottom of the transparent substrate, and is developed. Finally, the transparent positive photoresist is heated to cause a melt-flow thereof which forms a concave optical element above the microscopically exposed area. The transmissivity of the intermediate exposure area can be adjusted by adjusting the dimensions of the microscopical openings provided therein and the distance between the microscopic openings. More than one microscopic pattern can be provided on the same substrate so as to allow two or more intermediate transmissitivities to be obtained.