1. Field of the Invention
The invention relates generally to an illumination system for a microlithographic projection exposure apparatus. Such apparatuses are used in the manufacture of integrated circuits and other microstructured devices. More particularly, the invention relates to a condenser for such an illumination system that transforms a pupil plane into a field plane in which a field stop is arranged. The invention also relates to a field stop objective that images the field stop onto a mask plane.
2. Description of Related Art
Microlithography (also referred to as photolithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. More particularly, the process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After the exposure, the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed.
A projection exposure apparatus typically includes an illumination system, a mask alignment stage, a projection objective and a wafer alignment stage. The illumination system illuminates a region of the mask that is to be projected onto the photoresist.
Usually the illumination system contains a pupil plane in which an optical raster element is positioned. The optical raster element influences the geometry of the region that is illuminated on the mask. The light intensity distribution in the pupil plane determines the angular distribution of the projection light impinging on the mask. For modifying the intensity distribution in the pupil plane, various optical elements, for example axicon elements or diffractive optical elements, may be used in the illumination system.
A condenser, which usually comprises a plurality of lenses, transforms the pupil plane into a field plane. This means that the condenser images an object positioned at infinity on the field plane. Often a field stop comprising a plurality of adjustable blades is positioned in the field plane. The field stop ensures sharp edges of the region that is illuminated on the mask. To this end, a field stop objective images the field stop onto the mask plane in which the mask is positioned.
The illumination system has to ensure a very uniform irradiance in the mask plane. The uniformity of the irradiance is often expressed in terms of the relative change of the irradiance over 1 mm in an arbitrary direction. This gradient of the irradiance in the mask plane should not exceed a certain value that may be as low as 0.1%/mm or even 0.015%/mm.
Furthermore, the illumination system should produce a chief ray distribution in its exit pupil that matches the chief ray distribution of the subsequent projection objective. Usually it is desired that the chief rays are collimated, i.e. the exit pupil is positioned at infinity. In this case the illumination system is referred to as being telecentric on the image side.
Another property of highly advanced illumination systems is a good pole balance. The pole balance denotes the ability of an illumination system to correctly transform an intensity distribution in the pupil plane into an angular distribution in the mask plane. For example, if only two poles are illuminated in the pupil plane with perfect symmetry, a perfect pole balance (PB=0) means that the irradiance at an arbitrary point in the mask plane results from equal contributions from both poles. If PB≠0 in the case of a dipole illumination, light rays impinging from one side on a field point are more intense than light rays impinging from the other side.
Another property, which has to be fulfilled by the condenser of the illumination system and which is closely related to the pole balance, is the extent to which the sine condition is fulfilled. According to the sine condition the distance from the optical axis in the pupil plane is proportional to the sine of the angle of incidence in the field plane. Ideally, the sine condition is perfectly fulfilled for all angles of incidence, and also for all field points.
These properties should be achieved with an illumination system having a short overall length, containing lenses with a small diameter and maintaining a certain minimum distance between the last lens and the mask plane.
Meeting these tight specifications has become more difficult in illumination systems that do not comprise a light homogenization rod. Such a rod, which is known, for example, from U.S. Pat. No. 6,285,443, is used to homogenize the illumination light bundle. Since the rod does not maintain the polarization state of the illumination light bundle, its use is restricted to illumination systems without polarization control.
From U.S. Pat. No. 6,583,937 B1 a condenser of a rod-less illumination system is known that comprises five lenses. The first surface of the condenser is aspherical.
US 2002/0171944 A1 discloses a condenser of a rod-less illumination system that comprises four lenses, namely a negative meniscus lens having an aspherical concave front surface, two bi-convex lenses and a flat convex lens having an aspherical convex rear surface.
U.S. Pat. No. 6,680,803 B2 discloses a field stop-objective for a rod-less illumination system comprising a totality of 9 lenses.
From DE 196 53 983 A1 another field stop objective is known comprising only 7 lenses with at least three aspherical surfaces. In one embodiment, this objective ensures a telecentricity error of less than 0.3 mrad.