A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It is well-known in the art of lithography that the image of a mask pattern can be improved, and process windows enlarged, by appropriate choice of the angles at which the mask pattern is illuminated. In an apparatus having a Koehler illumination arrangement, the angular distribution of light illuminating the mask is determined by the intensity distribution in a pupil plane of the illumination system, which can be regarded as a secondary source. Illumination modes are commonly described by reference to the shape of the intensity distribution in the pupil plane. Conventional illumination, i.e. even illumination from all angles from 0 to a certain maximum angle, requires a uniform disk-shaped intensity distribution in the pupil plane. Other commonly-used intensity distributions are: annular, in which the intensity distribution in the pupil plane is in the shape of an annulus; dipole illumination, in which there are two poles in the pupil plane; and quadrupole illumination, in which there are four poles in the pupil plane. To create these illumination schemes, various methods have been proposed. For example, a zoom-axicon, that is a combination of a zoom lens and an axicon, can be used to create annular illumination with controllable inner and outer radii (σinner and σouter) of the annulus. To create dipole and quadrupole type illumination modes, it has been proposed to use spatial filters, that is opaque plates with apertures where the poles are desired as well as arrangements using moveable bundles of optical fibers. Using spatial filters may be undesirable because the resulting loss of light reduces the throughput of the apparatus and hence increases its cost of ownership. Arrangements with bundles of optical fibers may be complex and inflexible. It has therefore been proposed to use a diffractive optical element (DOE) to form the desired intensity distribution in the pupil plane. The diffractive optical elements are made by etching different patterns into different parts of the surface of a quartz or CaF2 substrate.
The choice of materials from which lenses useable with deep ultraviolet radiation (DUV), e.g. at 248 nm, 193 nm, 157 nm or 126 nm, can be made is quite limited and even the best materials have significant coefficients of absorption of this radiation. This means that the lenses in the projection system absorb energy during exposures and heat up, leading to changes in their shape, separation and refractive index which introduce aberrations into the projected image. Therefore, many lens systems are provided with one or more actuated lens elements whose shape, position and/or orientation in one or more degrees of freedom can be adjusted during or between exposures to compensate for lens heating effects.
If an illumination mode, such as dipole, in which the energy of the beam is strongly localized in a pupil plane of the illumination system is used, then the energy of the beam will also be strongly localized in and near the pupil plane(s) of the projection system. Lens heating effects are more severe when such localized illumination modes are used because the temperature gradients in the lens elements affected are greater, leading to localized changes in shape and/or refractive index which cause large phase gradients in the beam. These effects are often not correctable by existing actuated lens elements, which generally effect corrections described by only lower order Zernike polynomials e.g. up to Z5 or Z6. Similar effects can be caused by the use of a slit-shaped illumination field, as is common in a scanning lithographic apparatus, but these effects are generally of lower order, and more easily correctable.
Past attempts to deal with the problem of non-uniform lens heating include the provision of additional light sources, e.g. infra-red, to heat the “cold” part, i.e. the part not traversed by the intense part of the beam, of elements of the projection system. See, e.g., Japanese patent application publication JP-A-08-221261, which addresses non-uniform heating caused by zonal or modified illumination. The provision of such additional light sources and guides to conduct the additional heat radiation to the correct place increases the complexity of the apparatus and the increased heat load in the projection system necessitates the provision of a cooling system of higher capacity.
Another proposal to deal with non-uniform heating caused by a slit-shaped illumination field is disclosed in U.S. Pat. No. 6,603,530, which describes a special “lens illumination mark” provided in the reticle stage outside of the reticle area which diverges radiation so that the illumination of the lens elements in the projection system is rotationally symmetric. The lens elements are thermally saturated by illumination through the special mark before production exposures so that the non-rotationally symmetric heating caused by a slit-shaped illumination system does not cause non-rotationally symmetric aberrations.
The problem of non-uniform lens heating caused by localized illumination modes is addressed in WO 2004/051716. In one proposal described in this document, “dummy irradiation” is performed during wafer exchange to heat the cold parts of the lens elements affected by non-uniform heating in production exposures. During the dummy irradiation, the illumination mode is set, using a diffractive optical element or an adjustable diaphragm, to be the inverse of the illumination mode used for production exposures so that the heating effects of the dummy irradiation are the inverse of the heating effects of production exposures and the net heating is more uniform. Another proposal of this document is to use additional infra-red radiation to locally heat selected lens elements.