WO2005036272A1

WO2005036272A1 - Method for radiation treating an optical system

Info

Publication number: WO2005036272A1
Application number: PCT/EP2003/010362
Authority: WO
Inventors: Toralf Gruner; Axel Dochnahl
Original assignee: Carl Zeiss Smt Ag
Priority date: 2003-09-18
Filing date: 2003-09-18
Publication date: 2005-04-21
Also published as: AU2003270216A1

Abstract

The invention relates to a method for radiation treating an optical system with radiation of a lighting system located upstream therefrom. According to the invention, different partial lighting apertures of the lighting system are adjusted in succession and illuminate only a partial area of the total treatment area of the optical system while subjecting this partial area to a radiation treatment. The partial areas being illuminated while undergoing radiation treatment in succession complete on another to form the entire field of the optical system. The invention is used, for example, for the photo-cleaning of projection objectives in lithography exposure systems.

Description

Our mark: P 42407 WO September 17, 2003 EW / SR / ry

Description Method for radiation treatment of an optical system

The invention relates to a method for treating an optical system with the radiation of an upstream lighting system. The quality of optical systems, such as of microlithography projection lenses, is often impaired by effects that only arise during operation. Such effects are e.g. the contamination of optical components and the change in material properties in optical components due to the illumination radiation.

A contamination effect can occur in particular if organic molecules are present in an optical system which have a high absorption coefficient in the wavelength range of the illuminating radiation used. In this case, the Irradiation of the molecules, photochemical reactions are started and the surfaces of optically active components are contaminated by reaction products of these chemical processes. Such organic molecules arise, for example, as outgassing products from construction materials as they form under stray light conditions. A purging gas, which is often used in optical systems, also plays a role as a source of contaminants, since this is usually not completely free of organic contaminants even with thorough filtering. As a result of the contamination of optical components, their transmission or reflectivity may deteriorate, so that the performance of the entire optical system is reduced.

The photochemical process, which leads to the attachment of organic molecules on the surface of optical components, is opposed by a photochemical process, which causes the desorption of such organic molecules from the surface. The critical variable, which determines whether the desorption rate or the rate of addition of organic molecules on the surface of optical components predominates, is the surface energy density of the illuminating radiation used. If this exceeds a certain value, the desorption predominates due to the deposition and the components of an optical system are self-cleaning by the illuminating radiation, which is referred to as photo cleaning. Typically, wavelengths of the illuminating radiation of less than 200 nm are particularly effective for such photo cleaning.

Components in an optical system, in particular in an imaging system, are usually not hit uniformly by the radiation passing through the system in normal operation. As a result, the surface energy density in some areas of the optical system may be larger than that for photo cleaning necessary threshold is, but is below this threshold in other areas. Surface areas which are almost free of impurities can therefore appear on an optical component, whereas strong impurities occur in other surface areas which are hit by radiation with a weaker intensity. If the optical system is an imaging system that images an object present in an object plane, the shape of this object places the beam path through the optical system and thus the distribution of the contaminated and less contaminated surface areas on the optical components of the optical system Systems fixed: In particular if the object is a radiation diffractive object and / or a periodic object structure, for example a diffraction grating or a structure mask for semiconductor exposure, the distribution can be markedly uneven. Areas affected by high-intensity illuminating radiation are subject to noticeable photo cleaning, whereas areas with low illuminance are not.

If the optical system is changed after a longer period of operation with one and the same object, the imaging performance of the optical system can deteriorate in that the radiation passing through the optical system strikes areas with strong impurities in its beam path corresponding to the newly used object. which can result in a deterioration of the imaging performance.

As a further effect of a non-homogeneous distribution of the illuminating radiation when operating an optical system, its optical components are heated to different extents depending on the local illuminating intensity under the influence of the illuminating radiation. This heating leads to local expansion or compression of the material from which the optical components are made consist. Such a local expansion or compression can be irreversible, ie it can continue to exist even after the illuminating radiation has been switched off. The resulting inhomogeneity of the material properties and thus the optical properties of the optical components concerned can lead to a degradation of the imaging properties in imaging optical systems. In order to weaken this effect, a suitable irradiation of the entire field of the optical system with the illuminating light can achieve a homogenization of the material properties, in particular a reduction in the gradient of the material properties.

In the older German patent application 10301799.2 from the applicant, it is proposed for a projection exposure system to direct correction light in a correction mode onto a correction area of an optical component which is in a projection mode, i.e. in normal operation, is not or only to a small extent exposed to the projection light and immediately adjoins a projection area of the optical component which is exposed to the projection light in the projection mode. For this purpose, a correction element is introduced into the beam path instead of a reticle, e.g. a double wedge, a lens unit or a phase grating.

From the US Pat. No. 6,268,904 B1, an exposure system is known which contains an illumination system and a downstream projection objective and additionally a photo cleaning unit with which dirty optical components in the projection objective are cleaned by illuminating them with the radiation from the illumination system. Specifically, it addresses the problem that the entire surface of the dirty optical components in the lens cannot be cleaned with the illumination light if the numerical aperture of the projection lens is larger than that of the Lighting system. As a remedy, several options are given to match the numerical aperture of the lighting system to the numerical aperture of the projection lens in such a way that their ratio becomes greater than or equal to one, for example by introducing a variable aperture diaphragm into the lighting system, which is in addition to that in normal exposure mode used illumination apertures still available, which allow a larger pupil opening and are used in the photo cleaning operation. Accompanying the photo cleaning, it is recommended to flush the optical components of the projection lens with a strongly oxidizing gas, for example with oxygen (0 ₂ ) or ozone (O ₃ ), in order to accelerate the photo cleaning process and to increase its effectiveness.

DE 195 20 563 A1 and the literature cited therein describe an illumination device for a microlithography projection exposure system with a device for generating ring-shaped and multipole illumination. This device makes it possible to direct the light supplied by a laser in a concentrated manner into off-axis angular ranges, so that a large part of the intensity of the illuminating light supplied by an associated light source is available for imaging even in the case of annular or multipole illumination.

The invention has for its object to provide a method of the type mentioned that effective radiation treatment of an optical system across its entire field, e.g. for the purpose of photo cleaning optical components of the same, made possible with relatively little effort.

This object is achieved by a method with the features of claim 1. Advantageous further developments are in the dependent indicated claims. The wording of all claims is hereby incorporated by reference into the content of the description.

In the method according to the invention for the radiation treatment of an optical system, various partial lighting apertures of the lighting system are set in succession. With each partial lighting aperture, an associated partial area of an overall treatment area of the optical system is illuminated in a correspondingly radiation-treating manner, e.g. to remove impurities in a photo cleaning process and / or to homogenize the material properties of optical components of the system. The total treatment area here is the total volume of the optical system that is exposed to the illuminating radiation when the aperture diaphragm is opened to the maximum. Optionally, light-distributing elements, such as diffractive structures, can be present, which distributes the illuminating radiation even in areas that are otherwise not reached.

Overall, the sub-area apertures are selected so that the overall treatment area of the optical system is covered by the assigned sub-areas. The term “lighting system” here includes not only special lighting systems of projection exposure systems, but generally any systems that provide a desired optical radiation to a downstream optical system.

For effective photo cleaning according to the method, the entire optically effective surface of the component (s) of the optical system to be cleaned is illuminated with radiation, the intensity of which is so great that the surface energy density lies above the threshold value. In addition or as an alternative to photo cleaning, any radiation treatment can also be carried out by heating according to the process resulting inhomogeneity of the material properties can be compensated.

In a development of the invention, the partial areas are designed as an inner circle and a plurality of ring areas arranged concentrically around the inner circle. In an alternative development of the invention, the partial lighting apertures are adjusted in such a way that the radiation-treating illumination takes place outside the inner circle through a ring area with a continuously adjustable radius. In both cases, such a selection of the partial areas has the advantage that corresponding partial lighting apertures can be set easily by conventional lighting systems, such as those used in front of a projection lens in microlithography systems. There is generally no need to make any significant modifications to the lighting system in order to be able to use it for the radiation treatment.

In the case of the step-by-step ring areas with successive adjustment of the partial lighting apertures of the lighting system, a safety overlap, which is provided for each adjacent ring area in the radial direction, ensures that there are no areas in the radiation-treating illumination of the overall treatment area, in which there are no areas due to insufficient lighting intensity desired radiation treatment effect occurs.

It is particularly advantageous if the radiation of the lighting system is concentrated on a partial area of the overall treatment area when the respective partial lighting aperture is set, since this increases the available surface energy density, which significantly improves the efficiency of the radiation treatment. This is particularly advantageous if the light intensity of the light source assigned to the lighting system is not high enough to without generating such a sufficient area energy density. By concentrating the lighting intensity on partial areas, the radiation power of the light source required for effective radiation treatment can be kept relatively low.

In a further embodiment of the invention, the surface energy density of the treating radiation of the lighting system is set to more than 1 μJ / cm ² , so that the cleaning of the optical surfaces is particularly efficient since, from this threshold, the organic impurities have a strong tendency to desorb from those affected have optical components, whereas attachment processes are suppressed.

If the system to be cleaned is the projection lens of a lithography exposure system, this system already has an upstream lighting system. This can be used for photo cleaning, whereby the method presented here can be carried out with existing lighting systems without the need for significant modifications to these systems. If the lighting system and the projection lens are coordinated so that the maximum possible aperture ratio when using the lighting system to illuminate a reticle to be projected is one, the entire treatment area can be successively completely covered by the illuminated partial areas without additional changes, e.g. by inserting the Components that change the light path and are required on the lithography exposure system. In addition, the wavelengths used in microlithography are often below 200 nm, so that this condition for effective photo cleaning is also automatically met. A further embodiment of the invention provides for the selection of partial lighting apertures for the radiation treatment by an illumination system, which is also used in a normal operating mode to provide illuminating radiation. In particular, the concentration of the illuminating radiation on off-axis, for example annular partial areas in many conventional illuminating systems, such as, for example, the illuminating systems described in the above-mentioned DE 195 20 563 A1, is already provided for normal operation, so that these systems are used without modifications for radiation treatment can be.

Advantageous exemplary embodiments of the invention are shown in the drawings and are described below. Show it:

1 shows a schematic longitudinal sectional view of an illumination system of a microlithography projection exposure system,

Fig. 2 is a schematic representation of annular partial lighting saperturen of the lighting system of Fig. 1, which are set in a photo cleaning operating mode, and

3 shows a flow diagram of a method for radiation treatment according to the invention.

1 shows an illumination system for a microlithography system, as is also shown and described, for example, in DE 195 20 563 A1 cited above, to which reference can be made for further details. This illumination system comprises a laser 1, a beam expander 14, a first diffractive optical raster element 9 and a zoom lens 2, in the pupil plane of which a second diffractive optical raster element 8 is provided. In the beam path behind this raster element 8 there is a coupling optic 4 for coupling of light onto the entry surface 5e of a glass rod 5. Immediately after the exit surface of the glass rod 5a, an adjustable field diaphragm 51 is attached in an intermediate field plane. This is followed by an objective 6 with a plurality of lens groups 61, 63, 65, a deflection mirror 64 and a pupil plane 62, which images the intermediate field plane on a reticle plane 7.

An axicon pair 21 is arranged in the zoom lens 2. This consists of two lenses, each of which has a conical surface, the conical part of the first lens being convex and the conical part of the second lens being concave. Both lenses can be pushed together so that there is no gap between the conical surfaces. In this position, the conical surfaces have no optical effect, so that the axicon pair acts like an ordinary lens and the lens 2 can be used as a pure zoom lens, in which the imaging scale by changing the position of the lens 22 along the optical Axis is set. In this case, the lighting system provides conventional circular lighting.

By changing the distance 21a of the two conical axicon lenses 21 along the optical axis, they can be used to concentrate the radiation on the entrance side in off-axis ring areas, the size of the ring area, ie its inside and outside diameter, being set by specifying the distance 21a. In other words, when the distance between the two axicon surfaces is different from zero, the radiation is mainly concentrated in the off-axis ring regions of the light distribution, while an inner circle of the illuminating radiation with only low light intensity remains at most in the center of the light distribution. The method for radiation treatment according to the invention can consequently be implemented in a simple manner with the aid of the above illumination system, since this enables the illumination light to be concentrated on annular partial areas. By adjusting the partial area apertures with the Axicon lenses 21 and the zoom lens 22, the entire field of a downstream optical system can be illuminated successively with radiation treatment, the annular partial area apertures being formed either by an infinitely adjustable ring area or by several concentric ring areas, which are arranged one after the other with a safety overlap. In particular, in the application of a lithography exposure system, a projection lens following this illumination system can be effectively illuminated in a radiation-treating manner.

FIG. 2 schematically shows several partial lighting apertures as are used in one embodiment of the method according to the invention. Specifically, these are an inner circular area Ai with radius Rt and four concentric ring areas A ₂ , A ₃ , A ₄ , A ₅ with inner radius Rj-dRj and outer radius Rj ₊ i; i = 1, ..., 4, where dRj denotes a predeterminable safety overlap, which is preferably very small compared to Rj, so that the overlap area is very much smaller than the respective ring area. The safety overlap dRj can be the same or alternatively different for the successive ring areas. The inner circle A1 and the four ring areas A ₂ to A ₅ together cover the overall illumination field of the associated optical system, i.e. the entire volume of the optical system exposed to the maximum radiation aperture opening is successively treated in the respective partial areas or partial volumes The method for radiation treatment is explained on the basis of subareas A ₁ to A ₅ according to FIG. 2, it being understood that a larger or smaller number of subareas is also possible. 3 shows a flowchart to illustrate the method according to the invention. At the start of the method, the lithography exposure system is switched from a normal operating mode to a radiation treatment mode (step 10). For this purpose, any reticle that may be present can be removed from the exposure system. Then the inner circle Ai is first selected as the partial lighting aperture by appropriate adjustment of the lighting system objective 2 (step 11). The radius R _{Ϊ of} the inner circle is the minimally adjustable radius with the objective 2. The optical system following the illumination system 6 is exposed to the inner circle illumination radiation until the radiation treatment has led to the removal of contaminations on the optical components and / or a homogenization of their material properties in the inner circle part region Ai illuminated by the illumination light.

In a first embodiment of the method, the ring region A ₂ with the inner radius R ₁ - dRi and the outer radius R ₂ is then adjusted by correspondingly adjusting the lens 22 and the axicon pair 21 of the illumination system objective ₂ , ie the inner radius is reduced by the safety overlap dRi to ensure effective radiation treatment also at the transition from the inner circle Ai to the inner ring area A ₂ . After setting the ring area A ₂ , the optical system in this area A2 is illuminated with radiation treatment. Then the next ring area A _{3 is} selected. This procedure is successively repeated for the ring regions A3 to A ₅ which follow concentrically radially outwards, that is, until the entire surface of the optical system has been illuminated with radiation treatment. The ring areas A2 to A5 are chosen such that they each cover only a relatively narrow annular area in which the radiation emitted by the laser 1 is concentrated. ie a high intensity of the illuminating light can be achieved in each ring area A ₂ to A5.

In a second embodiment of the method according to the invention, a ring region with an inner radius Ri-dRi and an outer radius R ₂ is then set to the inner circle Ai, and then, when the laser 1 is switched on, the ring region 22 and the axicon pair 21 are shifted radially outward in a stepless manner shifted until its outer edge has reached the radius R5, ie the edge of the entire aperture field. The speed of the displacement of the ring area is controlled in such a way that effective radiation treatment of the subsequent optical system is guaranteed at every point in the entire aperture field. In this embodiment of the method, too, it is advantageous to select an annular region which is as small as possible and on which the illuminating radiation is concentrated, since this increases the light intensity and thus also the surface energy density on the optical components which are subjected to the radiation treatment.

In both variants, the aperture adjustment and optionally the additional adjustment of the radiation emission power of the light source 1 ensure that the surface energy density is higher than 1μJ / cm ² in the inner circle Ai and in the ring areas A ₂ to A5, which are gradually adjusted, or in the continuously adjustable ring area. This ensures effective photo cleaning of contaminated surfaces of the components of the optical system to which the illuminating radiation is supplied by the illuminating system. Wavelengths below 200 nm, as are typically used in microlithography projection exposure systems for semiconductor wafer structuring, which work with UV radiation, are particularly suitable for photo cleaning. After the radiation treatment is completed, the person concerned Operating mode deactivated (step 13) so that the system is available again for normal operation.

It goes without saying that various further process variants for the successive, radiation-concentrating ring aperture illumination described above are possible for the purpose of radiation treatment. In this way, the different partial lighting apertures can be set in any other order instead of the one described. Furthermore, depending on the need and the lighting system used, other partial lighting aperture forms can be used instead of the inner circle and the concentric ring areas, which together cover the entire treatment area of the optical system to be treated or a predeterminable part of the overall treatment area.

Instead of the transmission described above, the invention can also be used for reflection-working systems. Particularly in the case of lithography systems which work in the EUV wavelength range with reflective masks instead of the transmitting masks described above, the illuminating light is also guided into the objective for photo cleaning via a reflective reticle. This reticle can be designed to be uniformly reflective. In this case, the light is immediately effective in the distribution provided by the lighting system. As an alternative to this, diffractive structures can be applied to the reticle, which specifically redistribute the incident light in the pupil and in this way lead to a further increase in intensity in selected areas at the expense of other areas.

As can be seen from the embodiments described above, the method according to the invention enables systematic, effective radiation treatment of an optical system. It goes without saying that the method according to the invention is not limited to the treatment of projection objectives in lithography exposure systems, but that optical systems of various types can be subjected to an effective radiation treatment with it. The radiation treatment according to the invention in partial areas of the overall treatment area that are illuminated successively in any order that can be predetermined can include, in particular, photo cleaning and / or complete or at least partial elimination of lighting-related material inhomogeneities of the optical system components, but also any other conventional radiation treatment purposes. For this purpose, the treatment parameters are selected in a suitable manner, in particular the number and shape of the partial lighting apertures, the radiation duration, the radiation wavelength and the radiation energy density.

Claims

claims

1. A method for treating an optical system with radiation from an upstream lighting system, characterized in that different partial lighting apertures (A 1, A ₂ , A ₃ , A ₄ , A ₅ ) of the lighting system are set in succession, each of which only covers a partial area of an overall treatment area of the optical Illuminate the system in a radiation-treating manner, with the successively illuminated partial areas complementing one another to form the overall treatment area of the optical system.

2. The method according to claim 1, characterized in that as successively radiation-illuminated partial areas, an inner circle (A and several concentrically arranged around the inner circle ring areas (A ₂ , A ₃ , A ₄ , A ₅ ) or an inner circle and an adjoining these Ring area with infinitely adjustable radius can be selected.

3. The method according to claim 2, characterized in that the partial lighting apertures are set when using the plurality of concentric ring areas so that adjacent concentric ring areas each have a radial overlap.

4. The method according to any one of claims 1 to 3, characterized in that the radiation supplied by a radiation source of the lighting system is concentrated on the respective partial lighting aperture.

5. The method according to any one of claims 1 to 4, characterized in that the energy density of the radiation of the lighting system is set to more than 1 μJ / cm ² in each partial area illuminated by radiation treatment.

6. The method according to any one of claims 1 to 5, characterized in that the optical system to be treated is the projection lens of a lithography exposure system.

7. The method according to any one of claims 1 to 6, characterized in that those selected as partial lighting apertures for the radiation treatment are those which can be set by an illumination system which is also used in a normal operating mode to provide illuminating radiation for the optical system.