Patent Number: 06198793&
Section: summary

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 .sigma. 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 .ltoreq.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 .ltoreq.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 .ltoreq.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; PA1 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; PA1 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 PA1 an overlapping of the images is achieved in the reticle plane and PA1 the secondary light sources are imaged in the exit pupil by the optical elements. PA1 LC.sub.ill. =.sigma.LC.sub.obj =0.236 mm.sup.2. PA1 LC=x.multidot.y.multidot.NA.sup.2 =A.multidot.NA.sup.2. PA1 diameter of 50 .mu.m-200 .mu.m PA1 range of angle of radiation: 4 .pi. PA1 Bending magnets PA1 Wigglers PA1 Undulators 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 satisfy the telecentry requirements--as defined previously, a second mirror or lens with raster elements is advantageously provided. The raster elements of the first mirror or the honeycombs of the lens will be designated below as field honeycombs, and the raster elements of the second mirror or the honeycombs of the second lens will be designated as pupil honeycombs. In order to take up the previously described "any desired" illumination (circular, rectangular, composite distributions) in surface A, and, on the other hand, to illuminate the pupil uniformly, it is of advantage if the honeycombs are made available on the mirror surface via the degrees relative to freedom of position and tilt. In the case of lens systems, these degrees of freedom can be produced, instead of the tilt and position of the individual honeycombs or raster elements, by orientation, deflection or leading-edge angle and position of the prisms in front of the individual honeycombs. According to the invention, the field honeycombs are to be arranged in such a way that the illuminated entrance plane is optimally covered. The position of the pupil honeycombs is determined in such a way that one images the secondary light sources given in advance in the entrance pupil of the objective opposite the direction of light through the field lens into the diaphragm plane of the illumination system. After the arrangement of the individual honeycombs on the respective mirrors or lenses was determined based on the above boundary conditions, a connection of the light channels to the honeycombs can be produced, for example, by tilting of the field and pupil honeycombs in the mirror systems or prismatic components. It is particularly preferred if the assignment of field and pupil honeycombs is selected in such a way that the tilt angle and the prismatic component are minimized. Alternatively, an ordering can be aimed at, in which the intensity distribution in the pupil is extensively homogeneous. In order to correct the tilt in the pupil illumination that remains due to the zigzag beam path, an adjustment of the reflectivity of the mirror or mirrors can be provided. The preferred further developments of the invention based on mirror systems will be described below as an example, without this being seen as a limitation to reflective systems. The person skilled in the art will transfer the measures named as examples without inventive activity also to refractive systems, without this being explicitly mentioned. In general, in the systems according to the invention, the uniformity of the field illumination and the distribution of the secondary light sources, which is responsible in turn for the filling degrees, will be given by the number and arrangement of honeycombs of the first mirror, thus of the honeycomb condenser. The shape of the illuminated field is given by the shape of the individual honeycombs on the first honeycomb plate. If a high aspect ratio of the individual honeycombs is present, then a uniform distribution of the secondary light sources can be achieved by line-wise displacement of the honeycombs. In the case of systems with two mirrors with raster elements, the shape of the honeycombs of the second mirror is adapted to the shape of the secondary light sources and thus is different from the shape of the first honeycombs. It is particularly preferred that these honeycombs are round, if the light source is also shaped as round. The optical elements arranged subsequently to the mirrors with raster elements, particularly honeycombs, serve for the purpose of imaging the second honeycomb plate, i.e., the pupil plane, in the entrance pupil of the projection objective, shaping the annular field, and producing the illumination distribution according to the requirements of the exposure process. It is particularly preferred if the optical elements comprise grazing-incidence mirrors with incidence angles .ltoreq.15.degree.. In order to minimize the light losses associated with any reflection, it is provided advantageously, if the number of field mirrors is kept small. Particularly preferred are forms of embodiment with two field mirrors at most. A numerical example will be given below, from which the values typical for EUV lithography can be found. If one requires an aperture in the wafer plane NA.sub.wafer =0.1-0.15, then this indicates in the case of 4:1 systems an aperture in the reticle plane of NA.sub.reticle =0.025-0.375. If the illumination system will illuminate this aperture up to a filling degree of .sigma.=0.6 in a homogeneous manner, independent of the field, then the EUV source must make available the following 2-dim Etendu or Lagrange optical invariant (light conductance (LC)): The Etendu or Lagrange optical invariant LC is generally defined as follows: In order to produce this Etendu for light sources, particularly EUV sources, the latter must be appropriately adapted. In the case of a too large Etendu of the source, the Etendu is reduced to the required dimension at a suitable place by vignetting. In the case of the Etendu of the source that is too small, the Etendu can be effectively increased. The use of scattering disks or a partially filled pupil is possible, as it is proposed by segmenting according to the invention. A laser-plasma source can be provided in a first form of embodiment as the light source for the EUV illumination system according to the invention. The laser plasma source has the following parameters: Since the radiation of a laser-plasma source is spherical, in order to take up the radiated light power, advantageously an ellipsoid mirror is used, which remodels the light of the spherical source into a convergent pencil beam. In expanded laser-plasma sources, for example with a diameter of 200 .mu.m, in order to avoid blurrings and for correct superimposition of the beam pencils of the field honeycombs, it is advantageous to provide a second mirror with raster elements. In the case of a point light source, for example, with a diameter of 50 .mu.m, the blurrings are so small that such a measure can be dispensed with. Another light source for the illumination system according to the invention is a pinch-plasma source. A pinch-plasma source can be described as a surface radiator (.O slashed.=1.00 mm), which irradiates in the solid angle element .OMEGA.=0.3 sr. For the correct superimposition of the individual honeycomb images, as in the case of expanded laser-plasma sources, a honeycomb plate is necessary at the position of the secondary light source. A system according to the invention with a pinch-plasma source preferably comprises at least the following elements: a collector mirror, a plane facet mirror or first honeycomb condenser for producing the secondary light sources and for field shaping and a pupil facet mirror or second honeycomb condenser for the correct field overlay as well as subsequent optical elements, for example, two field mirrors. The honeycombs of the first honeycomb condenser are shaped in a planar manner in a first form of embodiment and tilted individually, while, on the other hand, the honeycombs of the second honeycomb condenser are curved spherically and are regularly arranged on the mirror without tilting. In order to avoid vignetting, the entire system is advantageously arranged in a zig-zag beam path. In order to be able to keep to a short structural length, particularly in systems with laser-plasma sources, a tele-system is provided in a special configuration of the invention. This tele-system can be coupled with the honeycomb condenser, so that no other mirrors are necessary. The tele-system has the further advantage that secondary light sources are arranged packed densely on the second honeycomb plate. The tele-effect can be produced, for example, in such a way that the honeycombs are introduced onto a curved carrier surface, or that a prismatic component is produced for each honeycomb, which is produced in lens systems, for example, by prisms pre-assigned to the honeycombs, or in mirror systems by tilting the honeycombs. Of course, in addition to the special EUV light sources described, for example above, such as laser-plasma, plasma or pinch-plasma sources, also still other EUV light sources are conceivable without deviating from the invention. Particularly preferred additional EUV light sources are synchrotron radiation sources. Synchrotron radiation is emitted if relativistic electrons are deflected in a magnetic field. The synchrotron radiation is emitted tangentially to the electron path. In the case of synchrotron radiation sources, currently one distinguishes three types of sources: 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 the deflection of the electron or of an electron beam, and this wiggler comprises a multiple number of alternating poled pairs of magnets arranged in a series. If an electron passes through a wiggler, then the electron is subjected to a periodic, vertical magnetic field; the electron oscillates correspondingly in the horizontal plane. Wigglers are further 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 steradian. In contrast to the bending magnet, it has a flow reinforced by the number of poles of the wiggler. The transition from wiggler source to undulator source is indistinct. 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 pole than in the case of the wiggler, so that interference effects of synchrotron radiation occur. Due to the interference effects, the synchrotron radiation has a discontinuous spectrum and radiates both horizontally and vertically in a small steradian element; i.e., the radiation is strongly directed. All of the above-described synchrotron EUV radiation sources with suitable dimensioning make available EUV radiation, for example, from 13 or 11 nm with sufficient power for EUV lithography. Concerning synchrotron radiation, reference is made to Ernst Eckhart Koch, "Handbook of Synchrotron Radiation", 1983, Elseiver Science, New York, whereby the disclosure content of this publication is included to its full extent in the present application.