Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2000-07-08
2003-01-14
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C359S627000, C378S034000, C362S326000, C362S341000
Reexamination Certificate
active
06507440
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns an illumination system, particularly such a system for lithography, thus, for example, VUV and EUV-lithography with wavelengths equal to or less than 193 nm, which illuminates a field with an aspect ratio that is not equal to 1:1, wherein the illumination system comprises at least one light source, one field mirror or one field lens as well as optical components for transforming the light source into secondary light sources.
In order to be able to still further reduce the structural width of microelectronic devices, particularly to the submicron range, it is necessary to reduce the wavelengths of the light utilized for microlithography.
For example, lithography with weak x-ray radiation at wavelengths smaller than 193 nm, is conceivable, as has become known from U.S. Pat. No. 5,339,346.
In addition to illumination according to U.S. Pat. No. 5,339,346, which requires at least four mirror facets arranged symmetrically in pairs in respect to the source, illumination systems can be provided, which operate, for example, by means of plates with reflective raster elements for the homogeneous illumination of the annular field of an exposure objective. Such systems have the advantage that the field of an objective is illuminated homogeneously with as few reflections as possible, and also that an illumination of the pupil up to a specific filling ratio independent of the field is assured.
Reflective raster element plates for EUV-illumination systems have become known from U.S. Pat. No. 5,581,605.
The disclosures content of both of the above-named documents, U.S. Pat. No. 5,339,346 as well as U.S. Pat. No. 5,581,605 are incorporated in their entirety in the present application.
The following light sources are discussed at present as light sources for EUV-illumination systems:
laser plasma sources
pinch plasma sources
synchrotron radiation sources
In the case of laser plasma sources, an intensive laser beam is focussed onto a target (solid, gas jet, droplet). The target is heated so strongly by the excitation that a plasma is formed. This emits EUV-radiation.
Typical laser plasma sources have an angular characteristic like a sphere, that is the source radiates in each direction with nearly the same intensity, i.e., a radiation angle range of 4 &pgr; as well as a diameter of 50 &mgr;m to 200 &mgr;m.
For pinch plasma sources, the plasma is produced by means of electrical excitation.
Pinch plasma sources can be described as volume radiators (=1.00 mm), which radiate in 4 &pgr;, whereby the beam characteristic is given by the mechanical source geometry.
In the case of synchrotron radiation sources, one can currently distinguish among three types of sources:
bending magnets
wigglers
undulators
In the case of bending magnet sources, the electrons are deflected by a bending magnet and photon radiation is emitted.
Wiggler sources comprise a so-called wiggler for deflecting an electron beam, and this wiggler comprises a multiple number of alternating polarized pairs of magnets arranged in rows. If an electron passes through a wiggler, then the electron is subjected to a periodic, vertical magnetic field; the electron correspondingly oscillates in the horizontal plane. Wigglers are also characterized by the fact that no coherency effects occur. The synchrotron radiation produced by means of a wiggler is similar to that of a bending magnet and radiates in a horizontal solid angle. In contrast to the bending magnet, it has a flux which is intensified by the number of poles of the wiggler.
There is no clear dividing line between wiggler sources and undulator sources.
In the case of undulator sources, the electrons in the undulator are subjected to a magnetic field with shorter periods and smaller magnetic field of the deflection poles than in the case of the wiggler, so that interference effects occur in the synchrotron radiation. The synchrotron radiation has a discontinuous spectrum based on the interference effects and emits both horizontally as well as vertically in a small solid-angle element; i.e., the radiation is highly directional.
Since the extension and angular spectrum of the currently discussed EUV-light sources are insufficient for filling or for illuminating field and aperture in the reticle plane of a lithography projection exposure system, the illumination systems presently discussed comprise at least one mirror or one lens with raster elements for producing a multiple number of secondary light sources, which are distributed uniformly in the diaphragm plane. Since the geometric form of the raster elements of the first mirror or of the first lens corresponds to the form of the illuminated field in the reticle plane, raster elements of the first mirror or of the first lens are preferably formed as rectangles, if the field to be illuminated is rectangular or represents a segment of an annular field. The optical effect of the raster elements of the first mirror or of the first lens, which are also designated below as field raster elements or field facets, is designed in such a way that images of the light source are formed in the diaphragm plane. Such light sources are so-called secondary light sources.
An EUV-illumination system, in which a number of secondary light sources are formed in a plane by means of two one-dimensional arrays, which are arranged perpendicular to each other, has been made known, for example, from U.S. Pat. No. 5,677,939. It is a disadvantage with this arrangement that two arrays of cylinder mirrors are necessary in order to illuminate a field with large aspect ratio and the exit pupil of the illumination system simultaneously.
In a second example, a system is shown in U.S. Pat. No. 5,677,939, in which critical Köhler illumination is produced by means of a one-dimensional array of cylinder mirrors. It is disadvantageous in this case that the exit pupil of the illumination system is illuminated by single lines and thus is illuminated nonuniformly.
If the extent of the light source is small, for example, approximately like a point source, as in the case of an undulator source, then the extent of the secondary light sources formed by the field raster elements is also small. All light rays coming from the field raster elements are focused to the point-like secondary light sources. In this case an image of the field raster element is formed in each plane after the corresponding secondary light source wherein the imaging scale is given by the ratio of the secondary light source/reticle distance to the field raster element/secondary light source distance. The field raster elements are tilted such that the images of the field raster elements overlap in the reticle plane, at least in part.
The edge sharpness, i.e., the distance between the 0% point and the 100% point of the intensity distribution of the image of the field raster elements, is almost zero in the case of point-like light sources, i.e., the intensity decreases from 100% directly to 0% for ideal imaging.
In the case of extended light sources, the secondary light sources are also extended, so that the image of the field raster elements in the reticle plane is not sharp. The edge sharpness of the image of the field raster elements increases.
If one wishes to prevent an over-illumination of the pregiven width of the field to be illuminated, then this can be achieved by reducing the height of the field raster elements. Field raster elements designed in such a way have a high aspect ratio.
Field raster elements with high aspect ratio are also caused by the fact that the field to be illuminated has a large aspect ratio, for example, an x-y aspect ratio of 17.5:1. An aspect ratio is also known as a lateral magnification.
Field raster elements with a high aspect ratio, however, can be distributed only unfavorably on a field raster element plate and are expensive to manufacture.
SUMMARY OF THE INVENTION
The object of the invention is thus to create an illumination system, particularly for EUV-lithography, which has a simple structure and in which the disadvantage
Carl-Zeiss-Stiftung
Ohlandt Greeley Ruggiero & Perle L.L.P.
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