X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
1999-05-04
2001-03-06
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S145000, C378S146000, C378S147000
Reexamination Certificate
active
06198793
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns an illumination system according to the preamble of claim I as well as a projection exposure unit with such an illumination system.
In order to be able to further reduce the structural widths of electronic components, particularly in the submicron range, it is necessary to reduce the wavelengths of the light utilized for microlithography. Lithography with soft x-ray radiation, so-called EUV (extreme UV) lithography, is conceivable at wavelengths smaller than 193 nm, for example.
2. Description of the Prior Art
An illumination system suitable for EUV lithography will illuminate with as few reflections as possible the field provided for EUV lithography, particularly the annular field of an objective in a homogeneous manner, i.e., uniformly; further, the aperture diaphragm or pupil of the objective will be illuminated independent of field up to a specific filling ratio &sgr; and the exit pupil of the illumination system will lie in the entrance pupil of the objective.
An illumination system for a lithographic device, which uses EUV radiation, has been made known from U.S. Pat. No. 5,339,346. For uniform illumination in the reticle plane and filling of the pupil, U.S. Pat. No. 5,339,346 proposes a condenser, which is constructed as a collector lens and comprises at least 4 pairs of mirror facets, which are arranged symmetrically. A plasma light source is used as the light source.
In U.S. Pat. No. 5,737,137, an illumination system with a plasma light source comprising a condenser mirror is shown, in which an illumination of a mask or a reticle to be illuminated is achieved by means of spherical mirrors.
U.S. Pat. No. 5,361,292 shows an illumination system, in which a plasma light source is provided, and the point plasma light source is imaged in an annular illuminated surface by means of a condenser, which has five aspherical mirrors arranged off-center. The annularly illuminated surface is then imaged in the entrance pupil by means of a special subordinate sequence of grazing-incidence mirrors.
From U.S. Pat. No. 5,581,605, an illumination system has been made known, in which a photon beam is split into a multiple number of secondary light sources by means of a honeycomb condenser. In this way, a homogeneous or uniform illumination is achieved in the reticle plane. The imaging of the reticle on the wafer to be exposed is produced by means of a conventional reduction optics. A gridded mirror is precisely provided with equally curved elements in the illumination beam path. The contents of the above-mentioned patents are incorporated by reference.
SUMMARY OF THE INVENTION
The task of the invention is to provide an illumination system that is constructed as simply as possible and a process for the design of such, with which the requirements for an exposure system for wavelengths of ≦193 nm, particularly in the EUV region, can be fulfilled, and any desired light sources with any desired type of illumination in a predetermined surface A could be used as a light source. In particular, in addition to a uniform illumination of the reticle, the telecentry requirements of a system will be fulfilled for wavelengths ≦193 nm.
In the present application, telecentry is understood to mean that the entire system is telecentric at the wafer. This requires an adaptation of the exit pupils of the illumination system to the entrance pupils of the objective, which are lying in the finite for a reflective reticle.
In the present application, the telecentry requirement is thus fulfilled, if the aberration of the centroid beam of illumination and objective in the reticle plane does not exceed a certain extent, for example ±14.0 mrad, preferably ±1.0 mrad, and the centroid beams impinge telecentrically on the wafer.
According to the invention, the task is resolved in that in the case of the illumination system of this overall concept, the raster elements or facets of the mirror or lens device are shaped and arranged on the mirror or the lens in such a way that the images of the raster elements or facets cover by means of the optical elements the major portion of the reticle plane, and that the exit pupil of the illumination system, which is defined by the aperture and filling degree, which is the entrance pupil of the reduction optics, is illuminated in an extensively uniform manner. Thus, the entrance pupil can also be varied according to field height, e.g., it can be axially displaced.
Whereas the system is purely reflective for wavelengths in the EUV region, i.e., is designed exclusively with mirror components, refractive components are utilized as lenses in the 193-nm or 157-nm system.
The system also makes available in the 193-nm or 157-nm region an illumination system or a construction principle, which has as few as possible optical elements as possible, such as, for example, lenses or prisms. This is important for 193 nm or 157 nm for that reason, since optical elements have high absorptions.
Further, the invention makes available a process for the design of an illumination system for wavelengths ≦193 nm, particularly for EUV lithography.
The process for the design of an illumination system comprising a light source with any desired illumination A in a predetermined surface, with a mirror or lens device comprising at least two mirrors or lenses, which are organized in raster elements, as well as optical elements, which are arranged between the mirror or lens device and the reticle plane, comprises the following steps according to the invention:
the raster elements of the first mirror or lens are arranged in such a way that the field is covered and have a form, which corresponds to that of the field to be illuminated, whereby a secondary light source is assigned to each raster element;
the raster elements of the second mirror or of the lens are arranged in such a way that they sit at the site of the secondary light source and have a form, which corresponds to that of the secondary light source;
a light path is produced between the mirrors or lenses by rotating and tilting the individual raster elements of the mirror or by orienting and selecting the deflection angle of the prismatic component of the lens(es), whereby a predetermined ordering of the raster elements of the first mirror or of the first lens to the raster elements of the second mirror or the second lens is maintained; so that
an overlapping of the images is achieved in the reticle plane and
the secondary light sources are imaged in the exit pupil by the optical elements.
The field in the reticle or object plane is illuminated homogeneously and with partially filled aperture with the illumination system according to the invention and the process according to the invention. By introducing a field lens into such an illumination system, the exit pupil of the illumination system is put together with the entrance pupil of the objective.
Preferably, in the case of projection systems, this involves annular field systems, so that the field to be illuminated in the reticle plane represents an annular segment.
For forming the annular field, for producing the Etendu or optical flux and for homogenizing the field distribution, one or more of the mirrors is formed with raster elements similar to a honeycomb condenser in a special form of the invention.
A honeycomb condenser with planar carrier surface is represented from U.S. Pat. No. 5,581,605, whose disclosure content is particularly included also to the full extent in the disclosure of this application with respect to the production of such condensers.
In a particularly preferred manner, the honeycombs of the honeycomb condenser are to be configured in their geometry similar to that of the field to be illuminated, i.e., in the case of an annular field system with aspect ratio 1:V, the honeycombs can be configured as rectangles with an aspect ratio of 1:V.
In order to obtain a uniform illumination of the reticle even in the case of asymmetric light sources or those deviating from the point form and to
Dinger Udo
Schultz Jörg
Schuster Karl-Heinz
Wangler Johannes
Bruce David V.
Carl-Zeiss-Stiftung (trading as Carl Zeiss)
Ho Allen C
Ohlandt Greeley Ruggiero & Perle L.L.P.
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