Partial coherence varier for microlithographic system

DETAILED DESCRIPTION A microlithographic projection imager 10 suitable for the inventive partial coherence varier is schematically illustrated in FIG.
1.
It includes an illuminator 20 and an objective imaging system 50, both preferably controlled by a computer 60.
Light or radiation from a source not indicated is directed through an optical system of illuminator 20 to a reticle plane R where a mask or reticle to be imaged is positioned.
Illumination passing through a mask at reticle plane R proceeds through objective imaging system 50 to wafer plane W where the mask is imaged by the transmitted illumination.
Projection imaging can vary with the masks or reticles involved, and different masks can especially benefit from different partial coherences.
For example, phase shifting masks or reticles generally need lower partial coherence values, and standard masks or reticles generally need higher partial coherence values.
Also, making the partial coherence of the projection imager readily variable allows it to be empirically optimized for any given mask or reticle.
To meet these needs, the invention provides a way of making the numerical aperture of the optical system of illuminator 20 readily adjustable.
Instead of making such adjustments by changing the size of an aperture stop at the pupil of illuminator 20, which can waste some of the illumination, this invention proposes an improved way of varying partial coherence.
A portion of the optical system of illuminator 20 is schematically illustrated in FIG.
2 to show a preferred way of varying the partial coherence by changing the numerical aperture of the illuminator according to the invention.
Member 21 schematically represents a source of radiation and a generally known system of lenses for delivering illumination to the downstream members of illuminator 20.
The elements represented by member 21 are generally known and preferably not involved in varying partial coherence according to the invention.
A series of preferred members of illuminator 20 are arranged downstream of member 21, as shown in FIG