Method for compensating for environmental parameters on the imaging characteristics of an optical system

摘要

In a system for imaging an original with high precision into an image plane by utilizing narrow-band radiation, environmental parameters become that much more disturbing the finer the structure which is to be imaged becomes. Various environmental parameters such as air pressure change the refractive index of the air so that the ratio of this refractive index to the refractive index of the material of the imaging system is changed with reference to the ratio forming the basis of the computation. This causes a deterioration of the imaging. The method of the invention makes it possible to maintain the imaging characteristics of the system constant in that at least one system parameter is changed. The dependency of this system parameter on the environmental parameter to be compensated is determined and stored. Pursuant to the method, the environmental parameter is continuously measured during operation of the system and in combination with the stored values, a signal is obtained for providing a compensating adjustment of the system parameter. The invention is especially applicable in the area of microlithography.