GB2444280A

GB2444280A - Optical element in a microlithography optical system and method for changing optical properties of optical element

Info

Publication number: GB2444280A
Application number: GB0623982A
Authority: GB
Inventors: Axel Scholz; Uwe Spengler
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2006-11-30
Filing date: 2006-11-30
Publication date: 2008-06-04
Also published as: GB0623982D0

Abstract

An optical element suitable for use in microlithography optical systems includes an electrochromic or photochromic layer (22) to change optical properties of the optical element and the microlithography optical system by voltage control provided by electrodes in the case of electrochromic material or by light supply from a laser (see fig 10) in the case of photochromic material. The optical element may be used to provide a variable aperture.

Description

OPTICAL ELEMENT IN A MICROLITHOGRApHy OPTICAL SYSTEM AND

METHOD FOR CHANGING OPTICAL PROPERTIES OF THE OPTICAL ELEMENT

Technical Field

The present invention relates to optical elements in microlithography optical systems, e.g. to lenses, mirrors, filters and the like. Further, it relates to microlithography opti-cal systems including such optical elements and other optical elements. Moreover, the invention also relates to methods for changing optical properties of the optical element.

Background Art

Optical systems for microlithography apparatus have been known for substantial time, mainly in semiconductor technology and related fields, especially optical sys-tems for producing images of a reticle as well as illumination systems. A number of different materials have been used to produce optical elements for such systems, namely different types of glasses and also other transmissive materials, especially for deep UV applications, metals for reflecting elements and various materials for anti-reflective coatings, etc. Further, aperture stops, shutters and the like have been used to block certain rays within microlithography optical systems. Further, electrical sys-tems have been used to motorize certain movements within microlithography optical systems. Within optical systems of microlithography steppers, it has even been known to use certain electrical systems for fine-adjustments of positions of optical elements, namely lenses and mirrors.

Summary of the invention

It is an object of the present invention to provide improved optical elements for micro-lithography optical systems.

It is a further object of the invention to provide microlithography optical systems in- corporating such improved optical elements. Especially, it is an object of the inven-tion to provide improved optical systems for microlithography apparatus as scanners and steppers and such rnicrolithography apparatus.

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Further, it is an object of the invention to provide a method for changing optical prop-erties of an optical element in a microlithography optical system. It is another object of the invention to provide a microlithography method comprising controlling an opti-cal system. More specifically, it is an object of one aspect of the invention to provide a method for controlling an aperture stop in a microlithography optical system. Ac- cording to another aspect of the invention, it is an object to provide a method of con-trolling illumination homogeneity in a microlithography optical system. Furthermore, it is an object of the invention to provide a method for controlling the angular distribu-tion of exposure light in a microlithography optical system. According to still another aspect of the invention, it is an object to provide a method for correcting wavefront deviations in a microlithography optical system. According to still another aspect of the invention, it is an object to provide a method for measuring an illumination dose in a microlithography illumination system. Further, it is an object of the invention to provide a method for producing an optical element. According to another aspect of the invention, it is an object to provide a method for changing the transmissivity of an op-tical element.

The invention is defined in the appended claims. The following disclosure is to be understood as relating to the optical element, the optical system, the microlithogra-phy apparatus and the various method aspects of the invention as well.

A basic idea of the invention is to use a layer comprising an electrochromic or photo-chromic material in a microlithography optical system. In case of the electrochromic material the term layer is to be understood as a layer made of a solid material and changing its optical properties due to the electrochromic effect. In case of the photo-chromic material the term layer is to be understood as layer made of a solid material changing its optical properties due to the photochromic effect, in particular such a layer can be made of a supporting material, as glass or synthetic material, being doped with a photochromic material.

For simplicity, the layers comprising electrochromic or photochromic material are from now on often referred to as electrochromic layer and photochromic layer respec-tively. (

In both cases, an induced change of the optical properties of the layer is used for the function of microlithography. Both, the electrochromic and the photochromic effect, allow for a reversible change of the optical properties of the corresponding layer.

Thus, a further degree of freedom can be implemented in microlithography optical elements and systems.

The optical element according to the invention can have the sole function to carry the electrochromic layer or the photochromic layer or could consist of the layer as such.

However, it can also be advantageous to combine the layer with an optical element having additional functions, e.g. to provide such a layer on a lens or a mirror in a mi-crolithography system.

Both, the electrochromic and the photochromic effect, allow for a fast change in the optical properties of the layer compared to conventional mechanical optical elements.

Furthermore, both effects allow for a high spatial resolution of these changes. In case of the electrochromic material, these changes depend on the way a voltage is applied (see below). In case of the photochromic material, these changes depend on the way the photochromic effect is induced (see below).

The optical properties to be changed can be the real and/or imaginary part of the complex refractive index, resulting in a change of the coefficient of reflectivity and/or transmissivity and/or absorptance and the like. Thus, the type of controllability the invention provides may be different from case to case. Preferred aspects of the in-vention relating to such controlled properties of optical elements will be explained hereunder.

If the electrochrornic effect is used for changing the optical properties of an optical element, the optical element comprises electrodes for applying a control voltage to the electrochromic layer. In many cases, transparent electrodes or at least one transparent electrode will be advantageous. In this respect for the visible spectral range, electrodes comprising indium oxide or tin oxide are preferred, especially so-called ITO materials, mixtures of indium oxide and tin oxide, are contemplated.

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Electrochromic materials include e.g. gadolinium oxide, molybdenum oxide, nickel oxide, niobium oxide, tantalum oxide, tungsten oxide, vanadium oxide and polyani- line. However, the above list is not complete and also other materials could be ad-vantageous.

Depending on the electric conductivity of the materials used as an electrochromic material, it may be advantageous to use at least one insulating layer between at least one of the electrodes and the electrochromic material. Such insulating layers could comprise oxides like silicone oxide or metal fluorides like calcium fluoride or other known insulators.

Further, the electrodes mentioned above may be comprised of two or more partial electrodes being electrically separated. Such electrodes could be used for applying different voltages to different parts of the electrochromic layer. Normally, it will be suf-ticient that one electrode is a system of multiple partial electrodes. However, also electrodes of different polarity might be multiple part electrodes.

If the photochromic effect is used for changing the optical properties of an optical element, no electrodes are necessary for inducing the photochromic effect, Instead, the photochromic effect is induced by light. The more light the photochromic material absorbs the less transparent the photochromic layer becomes.

Thus, the absorption of the photochromic layer can be quantified by 1post lpre e0X; with ai (l) <02 (12) for t < 2, where 1pre is the light intensity in front of the photochromic layer, post is the light inten-sity behind the photochromic layer, a is the coefficient of absorption and Lx is the thickness of the photochromic layer.

Examples for appropriate light sources are exposure light of a microlithography appa-ratus and light of an additional laser.

Since the transmissivity of a photochromic layer decreases with the applied light in- tensity such a photochromic layer can be a self-regulating means for laterally ho-mogenizing the exposure light (see below). The light of an additional laser can also

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be used as external control means to control and to structure the transmissivity within the photochromic layer spatially.

In case the optical element is provided with a photochromic material, it is advanta- geous to provide a layer comprising an amorphous glass or a synthetic material be-ing resistant to ultraviolet light, e.g. a polymer comprising fluorinated hydrocarbon.

Especially preferred is a perfluor polymer. Such a layer can be coated with the photochromic material, or the photochromic material can be embedded within the amorphous glass or the synthetic material, i.e. the layer can be doped with the photochromic material.

If the layer is made of a synthetic material, it is advantageous that the layer is a foil. A foil is especially cheap to produce and easy to apply to lenses and the like to build an optical element according to the invention.

For the photochromic material halogenides of silver are preferred. In particular silver chloride is preferred.

The change of the optical properties due to the photochromic effect as a function of the supplied light can be influenced by additional copper within the photochromic layer. With increasing temperature the photochromic effect decreases, i.e. the coeffi- cient of absorption becomes smaller. This temperature dependence of the photo-chromic effect scales with the amount of copper within the layer. Thus, the amount of copper within the photochromic layer enables adjusting the photochromic effect.

The electrochromic and photochromic layers, layers constituting electrodes, insulat- ing layers, and other layers could all be coated on optical elements for microlithogra-phy systems known as such according to known procedures of thin-film technology.

Thus, conventional microlithography optical elements, even elements with curved optical surfaces, could be substrates therefor.

Application fields of the invention are both in optical systems illuminating the reticle and in optical systems projecting the reticle onto the chip or other workpieces. Both r systems could be named "objectives". Thus, a microlithography apparatus can com-prise more than one such optical system.

In case of a photochromic layer, it is advantageous to provide a corresponding micro-lithography apparatus with a laser for inducing a change of the optical properties of the optical system via the photochromic effect. Since a laser can be focused to a very small spot, high resolution spatial changes of the optical properties are facilitated.

The wavelength of the laser can differ from the wavelength of the exposure light such that the laser only affects the photochromic layer and not the resist.

Since the photochromic effect depends on temperature, it is advantageous to provide the nhicrolithography apparatus with a source for emitting infrared light for heating the photochromic layer. This gives another degree of freedom for employing the photo-chromic effect. The source for emitting infrared light can be a laser as well.

It is known to cover the reticle with a pellicle to avoid contamination of the reticle by e.g. dust. It is preferred to employ a pellicle which corresponds to an optical element according to the invention. It is especially preferred to employ a pellicle which is a foil of a synthetic material doped with a photochromic material.

A method according to the invention thus includes the step to provide a layer com-prising one of an electrochromic material and a photochromic material and 10 induce a change of the optical properties of said layer to change the optical properties of said element in a microlithography optical system.

For employing the electrochromjc effect, the method includes a step to provide elec-trodes on the optical element for applying a voltage to these electrodes and thereby create an electrical field in the electrochrom,c layer in order to change its optical properlies and thus the optical properties of an optical element in a microlithography optical system.

For employing the photochromic effect the photochromic layer is illuminated by a light to change the optical properties of the optical element and thus also the optical prop-erties in a microlilhography optical system. g

The above also relates to controlling a microlithography scanner and/or stepper.

According to one aspect of the invention, by applying individual voltages to the above-mentioned partial electrodes, a transmissivity of parts of the electrochromic layer can be switched between a high transmissivity state and a low transmissivity state. According to another aspect of the invention, by applying laser light to the above-mentioned photochromic layer, a transmissivity of parts of the layer can also be switched between a high transmissivity state and a low transmissivity state. In an aperture stop, a part of an optical element is of low or zero transmissivity in order to block certain rays within the optical system. Controlling an aperture means to enable or disable such a blocking function and/or to change the form or size of the blocking area, which might be curved. Thus, by applying a voltage or individual voltages to an electrode or several partial electrodes connected to a layer subsequently, or by ap-plying laser light to parts of a photochromic layer, the existence and/or the size and/or the form of an aperture can be controlled. Therein, the aperture could also be formed on a curved surface of an optical element, i.e. the aperture stop function and another optical function could be integrated in providing a "thin-layer technology" ap-erture stop on a lens or a mirror. In case of a multiplicity of desired aperture settings, the microlithography optical system is designed such that the curved optical surface is an envelope of aperture borders for such settings.

According to another aspect of the invention, an illumination homogeneity in a micro- lithography illumination system can be controlled. Here, grey values of an electro-chromic or photochromic layer are changed. The grey values have a certain structure which can be the result of material or thickness variations of the electrochromic or photochromic layer. The structure can also be due to voltage variations in case of an electrochromic layer with partial electrodes or, in case of a photochromic layer, due to an inhomogeneous supply of light to the layer. In other words, the optical extinction of the layer comprising the electrochromic or photochromic material can have a local variation or structure, i.e. can be different at different locations of the layer.

Thus, a single voltage or light source could be used to switch on or off a grey filter having a certain grey value distribution that could be enhanced or reduced by the

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voltage value. However, multiple and electrically separate partial electrodes or a source of spatially distributed light, e.g. a laser, could be used to change the spatial distribution of the grey values depending on the individual requirements, not only af- ter production but also during application. Especially, for different illumination set- tings, e.g. in the illumination system of a microlithography stepper, different require-ments could apply so that one and the same filter or also a system of filters according to the invention could be adapted thereto. Further, these filters could, as explained above for the aperture stop control, also be integrated on a surface of an optical ele-ment.

Another aspect of the invention refers to controlling the angular distribution of the exposure light. If an optical element according to the invention is positioned in a pupil plane of a microlithography optical system, it can be used to control the angular dis-tribution of the exposure light leaving the objective. A spatial change of the grey value distribution of a filter within a pupil plane effects the angular distribution of the exposure light in the subsequent image planes. E.g. blocking the outer rays of the exposure light within a pupil plane will eliminate rays with large angles in the image planes. In particular, the method can be used to provide a flat angular distribution by providing a homogeneous illumination within the pupil plane comprising the optical element. If the optical element comprises a photochromic layer, lateral intensity in- homogeneities can also be leveled out by the photochromic effect without an addi- tional control. This is due to the above-mentioned fact that a locally higher light inten-sity applied to the photochromic layer corresponds to a locally increased darkening of the layer.

A further aspect of the invention refers to correcting wavefront deviations of optical rays in a microlithography optical system. Therein, phase shifts are created by means of a layer according to the invention. In case of an electrochromic layer, this is done by setting the applied voltage such that the desired phase shifts of rays passing through the layer occur. In case of a photochromic layer, this is done by supplying light such that a corresponding phase shift distribution occurs. E.g. an astigmatism of an objective could be corrected In one embodiment, the electrochromic or photo-chromic material display a corresponding thickness profile, i.e. an astigmatic profile, and is arranged in the pupil of the objective. Then, the voltage or a light supply can be used to switch on or off the correction and possibly to control the degree of the correction.

On the other hand, as in the above explained aspects, a multiplicity of partial elec- trodes with individually applicable voltages can be used in order to create the correc-lion profile within an electrochromic layer electrically, as desired. The same result can be achieved by employing laser light for a photochromic layer, as explained above.

Also the wavefront deviation correction can be done not only during or after produc-tion but also during application. It can thus be used to correct image defects arising or changing during long-term application. The so-called lens heating, i.e. image de-fects arising from locally different heating of regions of the optical element, as well as the so-called compaction or rarefaction, i.e. UV-induced density variations in quartz glass, can be mentioned. UV-induced density variations can e.g. arise from the rec-tangular form of the illuminated held and the thus inhomogeneous UV-dose resulting in image defects varying during long-term application.

Further, an optical element according to the invention could also be used to measure an illumination dose in a microlithography illumination system by changing the reflec-tivity and transmissivity of an electrochromic or photochromic layer, i.e. by switching between a high reflectivity state and low transmissivity and a low reflectivity and high transmissivity state in order to switch between a dose-measuring mode and a non- dose measuring mode. In one mode, the light impinging on the optical element ac-cording to the invention is reflected or transmitted for illumination. In another mode, the light or a part of the light could be transmitted or reflected in order to direct it to a dose-measuring system. Therein, only a part of the light might be sufficient so that a high-reflectivity state is a relative term and does not necessarily mean a reflectivity value close to 100%. Especially, the dose-measuring mode could be used for only short periods of time and thus without disturbing the operation of the system in a long-term sense.

Still further, one aspect 01 the invention relates to producing an optical element for a microlithography optical system comprising an electrochromic layer including the step of dividing at least one electrode into a multiplicity of partial electrodes. Here, a shad-ing lithography can be advantageous for defining the pattern of the partial electrodes.

Finally, another aspect of the invention refers to changing the transmissivity of an optical element in a microlithography optical system. The method comprises the steps to provide the optical element with a layer comprising a material causing a re-versible change in the transmissivity of said optical element due to a supply of energy to this material and to adjust the supply of energy to the material for changing the transniissivity of the optical element. Examples for such materials are electrochromic materials and photochromic materials.

Description of the Preferred Embodiments

Hereunder preferred embodiments of the invention will be described in more detail.

These embodiments are merely illustrative and not meant to limit the scope of the invention as defined in the claims. The features disclosed could also be relevant in other combinations.

Figure 1 shows a schematic diagram of an electrochromic layer on a trans-parent substrate together with electrodes and contacts as a top view.

Figure 2 shows the embodiment of figure 1 wherein parts of the electrochro-mic layer are darkened.

Figure 3 shows a schematic top view of a lens comprising a photochromic layer wherein parts of the photochromic layer are darkened.

Figure 4 shows a schematic side view of an optical element according to the invention in an optical system.

Figure 5 shows a schematic side view of a part of the element of figure 4 in detail.

Figures 6 and 7 are diagrams for the explanation of electrode and contact layouts for further embodiments of the invention.

Figure 8 shows a schematic perspective view of a microlithography appara-tus while performing microlithography Figure 9 shows a schematic side view of an objective being part of the micro-lithography apparatus of figure 8.

Figure 10 shows a variation of figure 9.

The following embodiments refer to the field of optical systems in microlithography steppers and scanners.

The first embodiment shown in figures 1 and 2 is a lens in a projection objective for projecting a reticle in a microlithography stepper onto a wafer. The objective is de-signed such that the aperture borders of different desired apertures are all on a lens surface. Thus, a thin layer aperture stop comprising an electrochromic layer accord-ing to the invention on said lens element surface can be used.

Figure 1 shows a top view onto this thin layer aperture. The lens element itself is not shown in detail because it is known as such. In the top view of figure 1, it has a circu-lar shape roughly corresponding to the outer circle 1 in figure 1. The same applies for an electrochromic layer not shown because it is transparent here. This outer circle 1 is the outer border of an annular electrode 2 further defined by an inner circle 3 as an inner border. Electrode 2 is contacted via a radial contact lead 4.

Correspondingly, a second annular electrode 5 is defined between circle 3 and an inner circle 6 and contacted via a radial contact lead 7. Further, an annular electrode 8 is defined between circle 6 and an inner circle 9 and contacted via a radial contact lead 10. Finally, there is an inner circular partial electrode 11 within circle 9 contacted via a radial contact lead 12.

Partial electrodes 2, 5, 8, and 11 form one electrode whereas one further electrode of roughly identical circular shape, but not segmented into partial electrodes, is provided above and not shown. Namely, on a lens surface, a 120 nm ITO layer (mixture of tin oxide and indium oxide) is provided and structured as shown in figure 1. This is done by a shading lithography using a mask so that the curvature of the lens surface is not problematic. Since the structures shown in figure 1 are not very small, the litho-graphic demands are not very high. Separations between the partial electrodes 2, 5, 8, and 11 and between the partial electrodes and the radial contact leads 4, 7, 10, 12 can be etched. The contact pads to be used later are covered during all following process steps.

Then, a 50 nm silicon oxide insulating layer is applied onto which two electrochromic layers of tantalum oxide and tungsten oxide of 350 nm each are applied. A further ITO electrode layer of 120 nm completes the layer sandwich. The second upper elec-trode is not segmented and is a mere counter electrode.

Figure 2 shows the embodiment of figure 1 wherein partial electrodes 2 and 5 have a voltage relative to the counter electrode (not shown) which creates an electrical field that darkens the corresponding part of the electrochromic material. Thus, a circular aperture 13 within an aperture stop 14 is formed, wherein the circular aperture 13 corresponds to partial electrodes 8 and 11 and aperture stop 14 corresponds to par- tial electrodes 2 and 5. It is clear from figure 2, that by controlling the voltages ap-plied to partial electrodes 2, 5, 8, and 11, the aperture can be maximized up to circle 1 in figure 1 and narrowed stepwise to circle 3, circ'e 6 and circle 9 as well as closed completely.

Figure 3 shows a top view of a lens as another embodiment. This lens is also a part of an objective in a microlithography stepper. The lens is curved (not shown) and consists of amorphous quartz glass which is doped with silver chloride and copper.

Thus, the lens itself corresponds to a photochromic layer. In contrast to the first em-bodiment shown in figures 1 and 2, the lens shown in figure 3 does not comprise an electrochromic layer. As a consequence, it does not comprise any electrodes for ap-plying a voltage. A laser beam 16 is used to darken selected parts of the lens. The circular aperture-stop 15 is shaped by applying laser light 16 to the photochromic layer. However, it is clear from the nature of the photochromic effect and the possi- bilities to manipulate laser light, e.g. by means of a mirror (see figure 10), that arbi-trary subsections of the photochromic layer can be darkened that way.

Thus, with reference to the first two embodiments of the invention, the invention pro-vides a very flexible tool for controlling an aperture stop or, to speak more general, a definition of a bundle of rays within an optical system that is fast, free of wear, has an ideal reproducibility and allows almost arbitrary forms of apertures or ray bundle defi-nitions.

Conventional aperture stop constructions having motorized drives are known. There, lamellas constituting the stop are moved. These lamellas are supported in ball bear- ings. Due to these bearings and the friction of the lamellas, problems may occur. Fur- ther, mechanically moved constructions have a limited reproducibility, limited flexibil-ity and Hmited spatial resolution.

A further embodiment of the invention is shown in figures 4 and 5. Figure 4 sche-matically shows a part of an optical illumination system for illuminating a reticle 20.

This illumination system is represented by an objective lens element 21 and com- prises a grey filter 22. Said grey filter 22 is shown in more detail in figure 5 and com-prises an upper electrode 23 and a lower electrode 24 as well as an electrochromic layer 25 there between (figure 5 does not show a substrate under electrode 24). A voltage U symbolically shown in figure 5 by means of two leads connected to elec-trodes 23 and 24 creates an electric field between electrodes 23 and 24 through electrochromic layer 25 in order to control a grey value of electrochromic layer 25.

Electrochromic layer 25 could have a varying thickness such that by means of volt- age U grey filter 22 can be switched on or off and has a grey value distribution de-termined by said thickness variation and by said voltage. In other words: Voltage U would be used to control the presence and possibly the intensity of the grey filter whereas thethickness variation would determine the spatial distribution of the grey values.

Alternatively or additionally, one of electrodes 23 and 24 could be segmented into multiple partial electrodes as in the embodiment of figures 1 and 2 or along the ex-planations hereunder referring to figures 6 and 7 or in any other manner. Then, the control voltage used for certain parts of electrochromic layer 25 could determine the grey value of this part so that the spatial distribution of grey values could also be con-trolled electrically.

Both embodiments could be combined, i.e. multiple partial electrodes instead of elec-trode 23 and/or electrode 24 could be used together with an electrochromic layer 25 of varying thickness. Electrochrornic layer 25 and thus grey filter 22 could also con-sist of multiple parts in order to implement a certain thickness variation pattern. In other words: Grey filter 22 could be a combination or a 11mosaic" of various small filter elements of varying thickness or material. This mosaic could be switched on or off by common electrodes or could be individually controlled by partial electrodes, e.g. at least one individual electrode for each filter element.

An exemplary layer structure is as follows: Calcium fluoride is used as a substrate of 1 mm thickness. Thereon, a 150 nm zinc oxide layer is used as an electrode 24. Fur-ther, a 100 nm insulating layer of calcium fluoride is applied and a molybdenum oxide layer of varying thickness in the range of between 20 nm and 200 nm is used as an electrochromic material thereon. This corresponds to some percent of extinction, e.g. 0% -5%. Electrode 23 is implemented by a 300 nm layer of zinc oxide.

Instead of employing an electrochromic layer 25 with additional electrodes 23 and 24 one could employ a lens comprising a photochromic layer, as shown in figure 3, and generate the desired grey value distribution by means of a laser beam as mentioned above along figure 3 and hereunder along figure 10.

Presently, grey filters adapted to improve the homogeneity of the illumination in mi- crolithography are already used. However, these filters have a fixed grey value distri-bution so that they cannot be adapted to individual demands of different illumination settings.

A further embodiment relates to the correction of wavefront deviations in microlitho-graphy, e.g. the correction of astigmatism. This embodiment can also be explained by means of figures 4 and 5.

Assuming that figure 4 shows a wavefront correction element 22 shown in more de-tail in figure 5, and that reference numeral 21 in figure 4 relates to a lens element of an imaging objective in a microlithography scanner whereas a wafer on which an im-age of the reticle is to be projected is referenced as 20 Electrochromic layer 25 of wavefront correction element 22 could be a niobium oxide layer varying in thickness according to an astigmatic profile. An astigmatic profile can be defined as follows by means of the height h depending on radius r and angle 0 in cylinder coordinates: h(r, 0) = 20 nm + 15 nm (r/R)2 cos(2 0); therein R is the free optical radius, in other words: the maximum radius occurring.

This astigmatic electrochromic layer could be sandwiched between a 150 nm elec- trode layer 23 of indium oxide on the one side and a similar electrode layer 24 to- gether with a 200 nm insulating layer of silicone oxide on the other side. The sub-strate could be made from quartz.

Further to figure 5, a silicone oxide layer could be added on top of electrode 23 being varied in thickness such that in case of zero voltage between electrodes 23 and 24, a light ray of the design wave length impinging orthogonally would make the same op-tical path length independent from the place of impinging on wavefront correction element 22. If, however, a voltage is applied between electrodes 23 and 24, the opti-cal path length in electrochromic layer 25 would be altered in order to produce an astigmatic wavefront correction.

A second embodiment for wavefront correction is a quartz lens element arranged near the aperture plane in the imaging objective on which a segmented electrode is used as explained along figures 1 and 2. Thereon, a 200 nm insulating layer of sili- cone oxide and a very thin electrochromic layer of tungsten oxide in a range of be-tween 2 nm and 25 nm are applied. The rest of the layer package is as explained along figures 1 and 2.

The very thin electrochromic layer results in a very low transmission loss so that phase changing properties dominate. These phase changing properties could, alter-natively or additionally to a thickness variation as explained above, be controlled by a fine segmenting, i.e. a fine partial electrode structure (see figures 6 and 7).

A third embodiment for a wavefront correction is a lens element as the one shown in figure 3. It is also arranged near the aperture plane in the imaging objective. The photochromic layer within the lens element is very thin. Darkening selected areas of this photochromic layer by means of a laser gives basically the same phase-shifting effect as explained just before.

Thus, by a controlled voltage distribution for an electrochromic layer or a correspond- ingly adjusted light supply for a photochromic layer, a multiplicity of different wave- front deformation can be produced. Thus, certain wavefront deviations could be cor- rected or an adaptation to an existing population of objectives having certain charac- teristics could be achieved. The latter might be advantageous in view of reproducibil-ity when using different objectives in wafer production.

Examples for segmented electrode structures are shown in figures 6 and 7. In figure 6, the right half circle shown is segmented into four partial electrodes each contacted as shown. Thus, the complete circle would consist of eight partial electrodes. In the lower left corner, a structure leading to 16 individually contacted partial electrodes of a full circle is shown. In the upper left eighth, a structure of three partial electrodes corresponding to 24 partial electrodes for a full circle is shown.

Further, figure 7 shows a finer segmenting with regard to the angle, in the upper third corresponding to 12 partial electrodes for a full circle, in the lower right third corre- sponding to 24 partial electrodes for a full circle and in the lower left sixth corre-sponding to 36 partial electrodes for a full circle. One further partial electrode is shown in the centre of figure 7.

By using for example 16 segments with regard to the angle, i.e. halts of the structure according to figure 6, and by a radial segmenting into sixths and by a further central partial electrode as in figure 7, e.g. also 97 partial electrodes could be implemented.

Wavefront correction could also be implemented by a set of structures as in figure 5 having different thickness profes. Individual phase filters of this set could be switched on or off depending on demand in the completed objective.

Furthermore, an embodiment relating to dosimetry shall be explained. Again, refer-ence can be made to figure 5. By providing a quartz substrate, a 100 nm layer 24 of tin oxide as a transparent electrode, a further 80 nm insulating layer of silicone oxide thereon, a 200 nm gadolinium hydride layer 25 as the electrochromic layer, and a further 100 nm tin oxide electrode layer 23, and possibly proton donating and proton conducting layers between the insulating layer and the electrochromic layer 25, an optical layer 22 having a voltage controllable reflectivity is implemented. The electro-chromic layer 25 can be a more ionic or a more metallic substance depending on the voltage and thus changes its transmissivity and reflectivity. Thus, parts of certain rays within an objective could be reflected to a dosimetry sensor responsive to a control voltage. Thus, a dosimetry system not disturbing the optical system when not used is feasible.

Figure 8 shows a microlithography projection apparatus (a scanner) 30 according to the invention. The apparatus 30 comprises an illumination system 31, i.e. an illumina-tion objective, generating the exposure light. The exposure light illuminates a reticle 32 which comprises structures 34 to be projected. A rectangular sub-area 33 of the reticle 32 is illuminated. By means of a projection objective 35 the structures 34 of the illuminated area 33 of the reticle 32 are projected 37 onto a wafer 36. For cover-ing the surface of the wafer 36, the projection apparatus scans the wafer 36.

In one embodiment, the reticle 32 is covered with a foil (not shown) as a pellicle based on perfluor polymers and doped with silver chloride. Thus, the foil itself corre- sponds to a photochromic layer. Since the exposure light itself triggers the photo-chromic effect, illumination inhomogeneities are leveled out by means of this foil.

Figure 9 shows a schematic side view of the illumination system 31 of the microlitho-graphy apparatus 30 of figure 8. The illumination system 31 comprises a light source 38 which generates deep ultraviolet light with a wavelength of 193 nm as exposure light. The exposure light then passes through a zoom objective 39 which widens the beam of exposure light such that all rays leave the zoom objective 39 parallel to the optical axis. After the zoom objective 39 the exposure light passes through an optical element 40 in a plane conjugate to the image plane. This optical element 40 is one of the optical elements shown in figures 1 to 7. Subsequent to passing the optical ele-ment 40 the exposure light beam is narrowed again by a zoom objective 41. All rays of the exposure light beam leave the zoom objective 41 parallel to the optical axis.

For widening or narrowing the exposure light beam the zoom objectives 39 and 41 can comprise a plurality of conventional optical elements.

In one embodiment, the optical element 40 is the optical element shown in figure 3.

The exposure light passes through the optical element 40 and thus triggers the photochromic effect. The silver chloride doping is adjusted such that lateral intensity inhomogeneities of the exposure light are leveled out during performing microlitho-graphy. In another embodiment, the optical element 40 corresponds to the optical element shown in figures 1 and 2. A voltage is applied to this optical element to ad-just the desired aperture, as explained above.

Figure 10 shows a variation of figure 9. The optical element 40 corresponds to the optical element of figure 3. The microlithography apparatus comprises an additional laser 42 and a drivable mirror for moving the laser light (spot) along the surface of the optical element 40. Thus, arbitrary grey value distribution within the photochromic layer of the optical element 40 can be generated, as explained above.

In an alternative embodiment, the optical element 40 shown in figures 9 and 10 lies within a pupil plane of the illumination system 31. By selectively darkening parts of the photochromic layer the angular distribution of the exposure light is altered.

In still another embodiment, the illumination system 31 comprises a source for infra-red light, i.e. a laser (not shown). This source is used to warm-up the optical element 40. In this embodiment the optical element 40 corresponds to the one shown in figure 3. Since the photochromic effect depends on temperature, heating the optical ele- ment 40 by the infrared light gives a degree of freedom for adjusting the optical prop-erties of the optical element 40.

Although the above description of the embodiments is explicit with regard to certain details, the embodiments are merely illustrative examples and not intended to limit the scope of the invention as defined in the claims.

Claims

Claims 1. An optical element for a microlithography optical system,

said optical element comprising a layer comprising one of an electrochromic material and a photochromic ma-terial, said element being adapted to change its optical properties by means of an induced change of the optical properties of said layer.
2. The optical element of claim 1 comprising electrodes for applying a control voltage to said material and in which element said material is an electrochromic material.
3. The optical element of claim 2 in which said electrodes comprise at least one of the group of indium oxide, tin oxide, and ITO.
4. The optical element of claim 2 in which said layer comprises at least one of the group of gadolinium oxide, molybdenum oxide, nickel oxide, niobium ox-ide, tantalum oxide, tungsten oxide, vanadium oxide, and polyaniline.
5. The optical element of claim 2 in which at least one of said electrodes is sepa- rated from said electrochromic material by an insulating layer, preferably com- prising at least one of an oxide and a fluoride, preferably at least one of sili-cone oxide and calcium fluoride.
6. The optical element of claim 2 in which at least one of said electrodes com-prises multiple electrically separate partial electrodes for applying different voltages to different parts of said layer.
7. The optical element of claim 1 in which said material is a photochromic mate-rial.
8. The optical element of claim 7 in which said layer comprises at least one of amorphous glass and synthetic material being resistant to ultraviolet light.
9. The optical element of claim 8 in which said synthetic material is a foil.
10. The optical element of claim 7 in which said photochromic material is a halo-genide of silver.
11. The optical element of claim 10 in which said layer comprises copper.
12. The optical element of claim 1 in which said optical element has a curved opti-cal surface.
13. A microlithography optical system, preferably an objective, comprising the op-tical element of claim 1 and further optical elements having a focal length, such as lens elements or mirror elements.
14. A microlithography apparatus comprising the optical system of claim 13.
15. The microlithography apparatus of claim 14 comprising a laser for inducing a change of the optical properties of the optical element via the photochromic ef-fect.
16. The microlithography apparatus of claim 14 comprising a source for emitting infrared light for warming said layer.
17. The microlithography apparatus of claim 14 comprising a pellicle, said peUicle comprising said optical element.
18. A method for changing optical properties of an optical element in a microlitho-graphy optical system, comprising the steps: providing a layer comprising one of an electrochromic material and a photo-chromic material and inducing a change of the optical properties of said layer to change the optical properties of said element.
19. The method of claim 18 in which said layer comprises an electrochromic mate-rial, comprising the steps: providing electrodes on said optical element for applying an electric field to said layer and applying a voltage to said electrodes for changing the optical properties of said optical element.
20. The method of claim 18 in which said layer comprises a photochromic mate-rial, comprising the step: supplying light to said photochromic layer for changing the optical properties of said optical element.
21. The method of claim 20 in which said light is the exposure light of said micro-lithography optical system.
22. The method of claim 20 in which said light is emitted by a laser, comprising the steps: illuminating a spot of said layer with light emitted by said laser and moving said spot along said layer.
23. The method of claim 20 comprising the step: changing the temperature of said layer for changing its sensitivity to said light.
24. The method of claim 23 comprising the steps: providing a source for infrared light and warming said layer with said infrared light.
25. A microlithography method comprising controlling an optical system in a mi-crolithography apparatus, said controlling including the method of claim 18.
26. A method for controlling an aperture stop including the method of claim 18, wherein by inducing a change of the optical properties of said layer at least a part of said layer is switched between a high transmissivity state and a low transmis-sivity state.
27. The method of claim 26 including the method of claim 19, wherein at least one electrode consists of at least two electrically separate partial elec-trodes, and by applying voltages individually to said partial electrodes the transmissiv-ity of parts of said layer can be switched.
28. The method of claim 26 including the method of claim 22, wherein by applying laser light as a moving spot to said layer the transmissivity of parts of said layer can be switched.
29. The method of claim 26, wherein said optical element has a curved optical surface and said layer is provided on said surface, said surface being an envelope of aperture borders for different aperture settings.
30. A method of controlling illumination homogeneity in a microlithography optical system including the method of claim 18, wherein said optical element is arranged in a plane that is conjugate to a plane to be illuminated and said induced change of the optical propertied of said element is controlled such that different parts of said layer have different grey values.
31. The method of claim 30 including the method of claim 19, wherein at least one of said electrodes consists of multiple electrically separale partial electrodes and by applying voltages individually to said partial electrodes the grey value of parts of said layer can be varied.
32. The method of claim 30 including the method of claim 22, wherein by moving said spot along said layer the grey value of parts of said layer can be varied.
33. The method of claim 30, wherein said homogeneity is set by setting said in-duced change responsive to changes of illumination settings.
34. A method of controlling the angular distribution of exposure light in a micro-lithography optical system including the method of claim 18, wherein said optical element is arranged in a pupil plane of said system and said induced change of the optical properties of said layer is controlled such that different parts of said layer have different grey values.
35. A method of correcting waveform deviations of optical rays in a microlithogra-phy optical system including the method of claim 18, wherein said layer varies a phase of optical rays passing therethrough depending on the induced change of the optical properties of said layer and said induced change is set such that said layer corrects a wave front de-viation of said optical rays.
36. The method of claim 35, wherein an astigmatism of an objective is corrected.
37. The method of claim 36, wherein a layer thickness of said layer has an astig-matic profile and is arranged in the pupil plane.
38. The method of claim 35, wherein a microlithography objective is adapted to an existing population of other microlithography objectives.
39. The method of claim 35 including the method of claim 19, wherein at least one of said electrodes consists of multiple electrically separate partial electrodes and by applying voltages individually to said partial electrodes the phase of optical rays passing through parts of said layer can be varied.
The method of claim 35 including the method of claim 22, wherein by applying laser light as a moving spot to said layer the phase of op-tical rays passing through parts of said layer can be varied.
41. A method of measuring an illumination dose in a microlithography illumination system including the method of claim 18, wherein said layer varies its reflectivity depending on said induced change of said opti-cal properties of said layer and said induced change is set to switch between a high reflectivity state to reflect light and a low reflectivity state to transmit light in order to switch between a dose measuring mode and a non-dose measur-ing mode.
42. A method of producing the optical element of claim 2, wherein at least one of said electrodes is divided into multiple partial electrodes by means of shading lithography.
43. A method for changing the transmissivity of an optical element in a microlitho-graphy optical system, comprising the steps: providing said optical element with a layer comprising a material causing a reversible change in the transmissivity of said optical element due to a supply of energy to said material and adjusting said supply of energy to said material for changing the transmissivity of said optical element.