GB2540654A - Method of variably attenuating a beam of projection light in an optical system of a microlithographic apparatus - Google Patents

Abstract

A pattern of light absorbing regions is created on or within a diffractive or reflective optical element 44 mounted in the optical system of a microlithographic apparatus. Projec­tion light is directed onto the absorption pattern then either the light absorbing regions (Fig. 6: 120) disappear by themselves or they are actively removed. These steps are carried out at least at two consecutive portions (Fig. 4: 56a, 56b) of the optical element, so that at a given time the projection light is directed either to the first or to the second portion. As an example a rotatable disk shaped optical element 44 may be placed at one of the pupil planes 36, 38 of the projection objective 20 as a time variable apodization filter. A print head 50 may be used to form absorbing regions 58 which are removed using an erasing head 52, or a laser printer may provide writing light (Fig. 4: 72) causing illuminated areas on a rotating photosensitive drum (Fig. 4: 78) to attract toner from a toner drum (Fig. 4: 80). Writing light from a laser printer may be directed onto regions of photochromic materials including CaF2 or BaF2 doped with rare earth elements, to cause excited state absorption (ESA). Light absorbing regions may disappear as a result of recombination processes, or using an erasing light, and a rotating filter plate (Fig. 10: 344) including regions of the material may be placed very close to the mask plane. Excited state absorption may alternatively be induced in a replaceable polymer film (Fig. 12a: 445) formed on a substrate.

Description

METHOD OF VARIABLY ATTENUATING A BEAM OF PROJECTION LIGHT IN AN OPTICAL SYSTEM'OF A MICROLITHOGRAPHIC APPARATUS

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention generally relates to the field of microlithography, and is particularly concerned with variably attenuating a beam of projection light in an illumination system or a projection objective of a mi c ro i i t ho graph i c projection exposure apparatus or a mask inspection apparatus. The attenuation may take place in a field plane or a pupil plane of such an apparatus, for example. 2. Description of Related Art

Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other micro-structured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist 'which is a material that is sensitive to radiation, such as deep ultraviolet. (DUV) , vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus, The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed.

Repetition of this process with different masks results in a multi-layered micro-structured component. A projection exposure apparatus typically includes an illumination system, a mask alignment stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example .

In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or simply scanner, each target portion is irradiated by progressively scanning the mask pattern under the beam of projection light in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification β of the projection lens, Ά typical value for the magnification is β = ±1/4.

It is to be understood that the term "mask" (or reticle) is to be interpreted broadly as a patterning means. Commonly used masks contain opaque, transparent or reflective patterns and may be of the binary, alternating phase-shift., attenuated phase-shift or various hybrid mask type, for example.

In a mask inspection apparatus a mask to be inspected is not imaged on a resist, but on an electronic imaging device such as a CCD camera. The aerial image produced by the objective on the imaging device is not reduced as in exposure apparatus, but strongly magnified. The illumination systems of exposure apparatus and mask inspection apparatus are ouite similar, however.

One of the essential aims in the development of projection exposure apparatus is to be able to lithographically produce structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, which generally has a favorable effect on the performance of the microstracturea components produced with the aid of such apparatus . Furthermore, the more devices can be produced on a single wafer, the higher is the throughput of the production process .

The size of the structures that can be generated depends primarily on the resolution of the projection objective being used. Since the resolution of projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths currently used are 24” nm, 193 nm or 157 nm ana thus lie in the deep or vacuum ultraviolet spectral range. Also apparatus using EUV light having a 'wavelength of about. 13 ran are meanwhile commercially available. Future apparatus will probably use EUV light having a wavelength as low as 6,9 nm.

The correction of aberrations (i.e. image errors) is becoming increasingly important for projection objectives having a very high resolution. But there are also other important issues that have an impact on the correct transfer of the mask pattern on the photoresist. Among these issues are undesired irradiance variations in field and pupil planes of the illumination system or the projection objective,

Undesired irradiance variations in field planes such as the mask plane or the wafer plane directly translate into CD variations, i.e. var1atiοns of the critical aimensiοns.

Irradiance variations in the pupil plane are more difficult to understand. The amplitude part of the complex pupil transmission. function describes the angular transmission properties of the optical system, while the phase part of the pupil transmission function defines its aberrations. Further explanations in this respect may be gleaned from WQ 2014/154229 Ά1.

Usually, there is an ideal irradiance distribution in the pupil plane, and a transmission filter (sometimes also referred to as apod.izat.ion filter) may used to correct the real irra-diance distribution so that it approaches, at least to some extent, the ideal irradiance distribution. Sometimes, however, no correction in this sense is required, because a modification of the irradiance distribution suffices. For example, it may be possible to modify the irradiance distribution in the pupil plane in such a manner that adverse effects caused by the residual deviations from the ideal irradiance distribution can be eliminated by other measures. Such measures include, among others, modifications of the angular light distribution produced by the illumination system, or displacements of the wafer or of lenses contained in the cb-j ective.

If the real irradiance distribution in a field or pupil plane does not vary, it usually suffices to use an attenuation filter having a fixed spatial filter function, i.e. an attenuation distribution that, cannot be modified. In microlitho-graphic projection exposure apparatus, however, the real irradiance distribution often varies at least to some extent so that it is desirable to be able to vary the filter function of the attenuation filter. The changes of the real irradiance distributeon may be, for example, a result of lens heating or ageing effects. Sometimes the irradiance distribution in a field or pupil plane shall be changed in order to achieve a corrective effect for shortcomings that are caused by a particular mask pattern or a particular i 11urrdnacion setting. Thus there is a need in the art to variably attenuate a beam of. projection light in an optical system of a microlithograph ic apparatus .

Various approaches have been proposed in this respect; US 5,444,336 discloses a projection objective of a microlith-ographic projectiοn exposure apparatus in wYiich different grey filters can be inserted into the pupil plane of the objective. However, the number of different, filter functions is necessarily restricted, US 2006/0092396 discloses a projection objective of a micro-lithographic projection exposure apparatus in which a variable attenuation filter formed by an array of individual ly programmable elements, for example LCD cells, is arranged in a pupil plane of the objective. By controlling the elements of the array individually, the attenuation distribution of the apodiration filter can be varied. One drawback of this known approach is that it is difficult to finely adjust the attenuation produced by each element. US 2010/0134891 Ά1 discloses another variable attenuation filter for an objective of a microlithographic projection exposure apparatus. Here a reflective coating applied on a curved mirror surface is detuned so as to locally vary the coefficient of ref lectio·', A similar approach is also described in 08 7,791, 7.1.1 "*'. However, the changes in the mir ror surface are permanent, and thus it is not possible to reverse the corrective effect. OS 5,614,990 discloses an illumination system containing a photochromic filter. The filter consists of a photochromic glass plate on which an array of light sources having a variable intensity is imaged. The glass plate is fixedly arranged in the illumination system. DE 10 2013 205 567 Al discloses a projection objective having a variable apodization filter. A glass plate in the projection light path is illuminated by radiation that induces absorption in the glass plate. Tine effect of induced absorption occurs in CaFa and some other materials if the radiation has a wavelength of 157 ran and is sufficiently intense. One drawback of this approach is that the generation of such radiation requires a bulky and expensive exciter laser.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method of variably attenuating a beam of projection light in an optical system of at mi era 1. .11 h og r aphi c apparatus that overcomes the above mentioned shortcomings. In particular it shall be possible to attenuate the beam of projection light very flexibly, quickly and cost effective.

In accordance with the present invention,, this object is achieved by a method of variably attenuating a beam of projection light, in an optical system of a microlithographic apparatus that transfers a pattern to a surface, wherein the method comprises the following steps: a) creating an absorption pattern of light absorbing regions on or within an optical element that is mounted in the optical system; b) directing the projection light on the absorption pattern; c) 'waiting until the light absorbing regions have disappeared by themselves or, if the light absorbing regions do not disappear by themselves, actively removing the light absorbing regions from the optical element; wherein steps a) to c) are carried out at a first portion of the optical element 'while steps a) to c) are carried out, but in a time shifted manner, at a second portion of the optical element so that the projection light is directed, at a given time, either to the first portion or to the second portion.

The invention is based on the conception that a wide variety of different absorption patterns can be created on or in an optical element while it is mounted in the optical system.

The ability to vary the absorption pattern is a result of step c) in which the light absorbing regions are actively removed from the optical element if they do not disappear by themselves. After the light absorbing regions have disappeared or have been actively removed, the same or a different absorption pattern can be created again on or within the optical element. Since these steps are performed while the optical element is mounted In the optical system, the absorption pattern can be changed very quickly, because there is no need to dismount the optical element for processing it outside the optical system.

While the absorbing regions disappear or are actively removed, the optical element cannot fulfill its function as an attenuation filter. But between subsequent exposures, or during wh.mle the wafer is exchanged, there is usually sufficient time to actively remove the light absorbing regions and to create a new absorption pattern on or within the opt.·.cal element. Depending on the process which is used to create and to remove the light absorbing regions, it is even possible to use the short times between subsequent light pulses to perform these steps. This particularly applies if the light absorbing regions disappear by themselves, as this will be explained in further detail below. Then the attenuation brought about by the optical element can be changed during a single scan cycle, i.e. while one die is exposed.

The light absorbing regions are created on and/or within the optical element. If the optical element is a reflective optical element, the regions will usually be created on the element. However, absorption may also be locally increased in a reflective optical element, namely in its reflective coating.

If the light absorbing regions disappear by themselves, it may be necessary to create the same absorption pattern many times again. Otherwise step b) is continued until a different absorption pattern shall be created on or within the optical element. In other words, the steps of removing the light absorbing regions and subsequently creating a different absorption pattern of light absorbing regions are performed only if there is a need for varying the attenuation of the beam of projection light.

According to the invention the steps a) to c) are carried out at a first portion of the optical element and also - but in a time shifted manner - at a second portion of the optical element, The projection light is directed, at a given time, either to the first portion or to the second portion. This makes it possible to attenuate the beam of projection light with the first portion of the optical element, while the same or a different absorption pattern is created in the second portion, or the second portion is simply waiting for being processed. In order to ensure that the projection light is directed either to the first portion or the second portion, the path of the projection light may be changed using one or more moveable mirrors, for example, so that the projection light impinges alternately on the first and second portion.

Often, however, it will be easier to keep the light path of the projection light spatially fixed and to move the optical element so that either the first portion or the second portion is in the light path of the projection light. Such a movement may be a linear displacement of the optical element, for example perpendicularly to the light path direction.

Sines such oscillating movements may cause uncles!red vibrations of the optical system, it may be prudent to - preferably continuously - rotate the optical element at least between steps a) and Pi, Rotation makes it possible to exchange the first and second portions very quickly, for example between two successive exposures. If the optical element is rotated with a rotational frequency of at least 10.000 Rpm, it is even possible to change the absorption pattern between two consecutive pulses of the projection light source.

One way to create the absorption pattern is to apply a light absorbing material on the optical element exclusively at the light absorbing regions during step a), The absorbing material should have, the property that it can be removed quickly and cleanly in step c). Often only a very small amount of projection light shall be attenuated. Therefore it suffices to apply the absorbing material only ac very few locations on the optical element.

If it is desired to attenuate projection light over substantially the entire cross section of the beam of projection light, it maybe quicker to create the absorption pattern in step a) by applying an absorbent layer on the optical element and to locally remove the absorbent layer outside the light absorbing regions, for example by using a scanning laser beam.

In one embodiment the absorption pattern is applied on the optical element by a printing process. A printing process is characterized by the transfer of an entire pattern using a master form or a template.

In principle any printing technology can be used for creating the absorption pattern in step a). However, for this particular purpose the printing technology should be fast and take into account the limited volume that is available for accommodating the printer. It should also be considered how the absorbing material can be easily removed in step c) from the o p t i c a 1 e I erne n t.,

Laser printing is particularly suitable for this purpose. In a laser printer an imager directs writing light to a photosensitive drum. Those portions of the drum which have been, exposed to the writing light attract the absorbing material. The drum then transfers the patterned material to a surface of the optical element while the drum rotates.

Particularly if the material is not fused by the application of heat, the material can be easily removed from the surface of the optical element. If the optical element is rotated daring the printing process, the drum should have a conical shape that is adapted to the diameter of the rotatable optical element.

For removing the absorbing material from the optical element in the step c) , different means are available, In the simplest case a negative pressure is applied so that the material is sucked off. Also mechanical means such as brushes or polishing clothes may be used in this respect. In one embodiment the absorbing material is thermally removed from the surface of the optical element. If the absorbing material, is heated up sufficiently, it will eventually evaporate and can be sucked off as a gas.

In another croup of embodiments writing light is directed to the light absorbing regions in step a). The optical element includes a material in which a reversible change of absorption is caused by the writing light. Thus, instead of applying an absorbing material and removing the latter from the surface of the optical element, a material is used that may remain on or in the optical element (or may even completely form the optical element), and in which the absorption is locally increased using the writing light. Suitable materials include fluoride crystals such as CaF or BaF doped with a rare earth element.

The change of absorption may rely on the effect of excited state absorption that is well understood in the art. Compared to the known effect of "induced absorption", excited state absorption has the advantage that the writing light usually has a center wavelength that is much larger than the center wavelength of the projection light. For example, if the center wavelength of the projection light is 193 nm, the writing light may have a center wavelength in the IF. spectral range, for example 808 nm. For such long wavelength small and cheap lasers or other light sources are easily available.

Preferably the light absorbing regions disappear by themselves as a result of a recombination process in the material. Since the time scale for the recombination process is typically in a range between 10 ps to 1 ms, the absorption pattern can be changed extremely quickly. On the other hand, as mentioned above, the "seif erasing" effect makes it necessary to create the absorption pattern hundreds of even thousand times per second.

For creating the absorbing regions in the material, at least the direction of the writing light, the intensity of the writing light or the time during which the 'writing light interacts with the light absorbing region should be controlled in step a).

If the absorbing regions do not disappear by themselves in the material, erasing light should be directed on the light absorbing regions in step c). The erasing light may stimulate a radiation like in a laser material, or it may pump the electrons to higher levels such that the absorption window shifts to a shorter wavelength domain which is outside the operating wavelength of the apparatus.

In principle it. is possible to use writing light and erasing light haring the same wavelengths. Then it is even possible to use the same device for writing and erasing the absorption pattern .

Since suitable materials, in which a reversible change of absorption can be induced by the writing lig.hr, may have properties that are not well suited for use as an optical element, it may be considered to use a substrate on which the material is applied as a layer. The substrate may be made of a glass or. another suitable optical material for which an established technology exists for bringing the substrate into a desired shape. The material which provides the attenuating effect may then be applied on this substrate as a thin layer, for example,

Applying the material as a layer has also the advantage that it may be easily removed from the substrate in case the layer has degraded. Then a new layer is applied on the substrate, and absorption patterns are written in the new layer with the help of writing light.

Generally the optical element may be a refractive optical element through which the projection light propagates. However, the optreal element may also be a mirror.

The optical system may be an illumination system or a projection objective of a projection exposure apparatus or a mask inspection apparatus .

Subject of the invention is also an optical system of a mi-crolithographic apparatus that transfers a mask pattern to a surface using projection light, The optical system has a variable attenuation filter device comprising an optical element that is at least partially arranged in a light path of the projection light. The attenuation filter device further comprises a writing unit that is configured to create light absorbing regions on or within the optical element., and an erasing unit that is configured to remove the light absorbing regions ,

Subject of the invention is further an optical system of a mlcrolithographic apparatus that transfers a mask pattern to a surface using projection light having a projection light center wavelength, The optical system has a variable attenuation filter device comprising an optical element that is at least partially arranged in a light path of the projection light, and a writing; unit that is configured to direct writing light on the optical element. The writing light has a center wavelength that is larger than the projection light center wavelength so that it produces light absorbing regions that are a result of excited state absorption.

The variable attenuation filter device may also comprise an erasing unit that is configured to direct erasing light on the light, absorbing regions.

DEFINITIONS

The term "light” is used herein to denote any electromagnetic radiation, in particular visible light, UV, DUV and V'UV light.

The term "operating wavelength" is used herein to denote the wavelength, or strictly speaking a center wavelength of a narrow range of wavelengths, for which the projection exposure apparatus is designed.

The term "light ray" is used herein to denote light 'whose path of propagation can be described by a line.

The term "light beam" is used herein to denote a plurality of light rays, A light beam usually has an irradiance profile across its diameter that znay vary along the propagation path. Ά single light, beam can usually be associated with a. single point or extended light source.

The term "surface" is used herein to denote any planar or curved surface in the three-dimensional space. The surface may be part of a body or may be completely separated therefrom.

The term "optically conjugate" is used herein to denote an imaging relationship between two points or two surfaces, Imaging relationship means that a light bundle emerging from a point converges at the optically conjugate point.

The term "field plane" is used herein to denote a plane that is optically conjugate to the mask plane.

The term "pupil plane" is used, herein to denote a plane in which ail light rays, which converge or diverge under the same angle in a field plane, pass through, the same point.. As usual in the art, the term "pupil plane" is also used if it. is in fact not a plane in the mathematica1 sense, but is slightly curved so that, in a strict sense, it should be referred to as pupil surface.

The term "projection light path” is used, herein to denote the entire space which may be exposed to projection light under any reasonable operating conditions defined by il.luin.ination setting and mask. The projection light path is therefore primarily defined by optical parameters of the illumination system and the objective, for example the numerical aperture and the size of object and image field.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings in which: FIG, 1 is a schematic perspective view of a projection exposure apparatus in accordance with the present invention ; FIG, 2 is a schematic meridional section through the apparatus shown in FIG, 1 comprising a projection objective having a variable attenuation filter device according to a first embodiment in which an ink jet printer is used to apply a pattern of light, absorbing regions on an optical element; FIGS. 3a to 3d FIGS. 3a to 3d are schematic top views on the optical element shown in FIG, 2 in different rota-t i ο n a 1 p o s i t ί ο n s; FIG. A is a perspective view on an attenuation filter device according to a second embodiment in which a laser printer is used to apply the pattern of light absorbing regions on the optical element; FIG, S is a meridional section through an illumination system of the apparatus shown in FIG. 1 comprising a variable attenuation filter device according to a third embodiment in which light absorbing regions are created in the optical element using the effect of excited state absorption; FIG. 6 is an enlarged cross section through the optical element shown in FIG. 5 during the creation of light absorbing regions; FIG. 7 is a top view on the optical element shown in FIG. 6; FIG. 8 is a graph illustrating the scan integrated absorption profile associated with the optical element shown in FIGS, 6 and 7; FIG. 9 is a graph illustrating the scan integrated irradi-ance pro£ile at mask IsveI; FIG. 10 is a meridional section through an illumination system comprising a variable attenuation filter device according to a fourth embodiment in which the light absorbing regions are created and removed during rotation of the optical element; FIG. 11 is a schematic perspective view of the attenuation filter device shown in FIG. 10; FIGS. 12a to 12c schematically illustrate in cross sections how a material displaying the effect of excited state absorption is applied on and removed from a substrate; FIG. 13 is a flow? diagram illustrating important method steps of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS I.

General Construction of Projection Exposure Apparatus FIG, 1 is a perspective and simplified view of a microlitbo-graphie projection exposure apparatus 10 in accordance with the present invention. The apparatus 10 comprises a light source LS, which produces projection light having a central wavelength of 193 ma, and an illumination system 12 that conditions the projection light and directs it on a field 14 on a mask. 16 comprising a pattern 18 of fine features 19. In this embodiment the illuminated field 14 has a rectangular shape. However, other shapes of the illuminated field 14, for example ring segments, and also other central wavelengths, for example 157 nrn or 248 ran, are contemplated as well. Ά projection objective 20 having an optical axis OA and containing a plurality of lenses LI to L4 images the pattern 18 within the illuminated field 14 on a light sensitive layer 22, for example a photoresist, which is supported by a substrate 24. The substrate 24, which may be formed by a silicon wafer, is arranged on a wafer stage (not shown in FIG. 1) such that a top surface of the light sensitive layer 22 is precisely located in an image plane of the projection objective 20. The mask 16 is positioned by means of a mask stage {not shown, in FIG. 1) in an object plane of the projection objective 20. Since the latter has a raagnificatiοη β with ! β j < 1, a minified image 18’ of the pattern 18 within the illuminated field 14 is projected onto the light sensitive lay-e r 2 2 .

During the projection the mask 16 and the substrate 24 move along a scan direction which corresponds to the Y direction indicated in FIG, 1, The illuminated field 14 then scans over the mask 16 so that patterned areas larger than the illuminated field 14 can be continuously imaged. The ratio between the velocities of the substrate 24 and the mask 16 is equal to the magnification B of the projection objective 20. If the projection objective 20 does not invert the image (β >0) , the mask 16 and the substrate 24 move along the same direction, as this is indicated in FIG. 1 by arrows A1 and A2, However, the present invention may also be used with catadioptric projection objectives 20 having off-axis object and image fields, and with apparatus of the stepper type in which the mask 16 and the substrate 24 do not move during the projection . FIG. 2 is a schematic meridional section through the apparatus 10 shown in FIG. 1. In this section also the mask stage denoted by 26, which supports and moves the mask 16 in an object plane 28 of the projection objective 20, and the wafer stage denoted by 32, which supports and moves the substrate 24 in. an image plane 30 of the projection objective 20, are s c h erna t i c all y il 1 a s t r a t e d.

In this embodiment the projection objective 20 has an intermediate image plane 34. A first pupil plane 36 is located between the object plane 28 and the intermediate image plane 34, and a second pupil plane 38 is located between the intermediate image plane 34 and the image plane 30 of the projection objective 20. As illustrated in FIG. 2, all light rays converging or diverging under the same angle from any of the field planes, i. e, the object plane 28, the intermediate image plane 34 and the image plane 30, pass through the same point in the first, and second pupil planes 36, 38,

Hi

Constitution of Filter Device

The projection objective 20 further includes a variable attenuation filter device 42 comprising a rotatable disk-shaped optical element 44 having a portion that is arranged in the first pupil plane 36. The optical element; may be made of glass or another optical material that is transparent for the projection light PL, By absorbing some of the projection light PL, the filter device 42 corrects, or more generally variably modifies, the light irradiance distribution in the first pupil plane 36, as this 'will be explained in more detail in tlie next s ec t ion 111.

The filter device 42 further comprises a drive 46 that is configured to rotate the optical element 44 around a rotational axis 48, The latter is arranged parallel to the optical axis OA. and outside the path of projection light PL.

Part of the filter device 42 is also a printing head 50 and an erasing head 52. As schematically indicated by arrows, the printing head 50 and the erasing head 52 can be displaced with the help of suitable drives along directions perpendicular to the rotational axis 48. In this embodiment the print-ing head 50 is configured as an ink jet printer that applies a plurality of minate ink droplets 58 (their size is grossly exaggerated in FIG. 2) on an upper surface of the optical el-ement 44. The ink droplets 58 adhere to the surface of the optical element 44 and may be dried by the application of heat, if necessary.

The drive 46, the printing head 50 and the erasing head 52 are connected to a control unit 51 which coordinates and controls the function of these components. The control unit 51 is connected to an overall system control 52 which controls the other components of the apparatus 10, for example the mask stage 26 and the wafer stage 32. FIG, 3a is a top view on the optical element 44 of the filter device 42. In this embodiment three filter portions 56a, 56b and 56c are arranged on the upper surface of the optical element 44. Each filter portion 56a, 56b and 56c may have a different density distribution of ink droplets 58 that absorb at least a portion of impinging projection light PL. This will be explained in more detail in the following section. in.

Function ot Filter Device

In the following the function of the variable attenuation filter device 42 will be explained with reference to FIGS. 3a to 3d.

Due to its arrangement in one of the pupil planes 36, 38 of the projection objective 20, the filter device 42 may be used as an apodization filter that absorbs portions of projection light, that would, if allowed to reach the image plane 30, deteriorate the quality of the image of the mask 16.

In a first step the required apodiration (i.e. filter function) in the first pupil plane 36 is determined. This determination may be based on simulations only, on measurements only, or on a combination of measurements and simulations.

For a measurement of the angular light distribution in the image plane 30, a pupil measurement unit 54 may be brought into a measurement position in which an entrance window 54a of the pupil measurement unit 54 is precisely arranged in the image plane 30 of the projection objective 20 (see FIG. 2). The pupil measurement unit 54 may comprise Fourier optics 54b that transfer the angular light distribution in the image plane 30 into a spatial light distribution in a Fourier related plane in which an imaging device 54c such as a CCD sen-s o r 1s a rranged,

If the angular light distribution measured in that mannot deviates from an ideal angular light distribution to an extent that cannot be tolerated, a filter function is computed that, if applied to the projection light PL in the first pupil plane 36, modifies the angular light distribution in the image plane 30 so that it sufficiently approaches the desired angular light distribution. The desired filter function is converted by the system control 52 into a density distribution of the ink droplets 58,

In a next step the control unit. 51 controls the printing head 50 so that it prints a pattern of ink droplets 58 corresponding to the computed density distribution on the first filter portion 56a. In. FIG. 3a it is assumed that the printing process has not yet completed so that a part 60a of the first filter portion 56a is still devoid of any ink droplets 58,

After the desired pattern of ink droplets 58 has been completely printed on the first filter portion 56a, the control unit 51 initiates a rotation of the optical element 44 by 120° along the direction 62 indicated by an arrow in FIG. 3a.

As a result of this rotation, the first filter portion 56a is now completely within the path of the projection light PL. Then the exposure operation of the apparatus 10 coirunences so that projection light PL passes through the first filter portion 56a, as this is shown in the top view of FIG. 3b. The projection light PL is at least partially absorbed by the ink droplets 58, and in this manner the angular light distribution in the image plane 30 of the projection objective 20 is modified. With this optimized angular light distribution the pattern 18 on the mask 16 is perfectly transferred to the light sensitive layer 22 on the 'wafer 24.

The system control 52 sircultaneous1y simulates how the imaging properties of the projection objective 20 change during the exposure operation. Such changes may be caused by reversible short-term effects such as the heating of the mask 16 and the lenses LI to L4, or by irreversible long-term effects such as material degradations of optical elements that are exposed to the projection light PL.

If these changes of the optical properties require a modified attenuation filter function, a new pattern of ink droplets 58 is computed by the system control 52 and printed by the printing head 50 on the second portion 56b. This printing process, which may be performed while projection light PL passes through the first portion 56a, is illustrated in FIG. 3b. Again, a part 60b is shown that is still devoid of any ink droplets 58. At the time when, according to the simulation, it is necessary to modify the filter function, the optical element 44 is rotated again by 120° along the rotational direction 62 so that the second portion 56b with the already completed pattern of the ink droplets 58 gets into the path of the projection light PL, as this is illustrated in FIG. 3c. This rotation may be carried out while the wafer 24 is exchanged, or even between two consecutive scan cycles.

As a result of this second rotation by 120°, the first portion 56a gets into an erasing position in which the ink droplets 58 are removed from the first portion 56a with the help of the erasing head 52, To this end the erasing head 52 may direct erasing light at the entire first portion 56a, or only at those areas that are covered by (dried) droplets 58 on the top surface of the optical element 44 so that, the ink eventually evaporates. Instead of thermally removing the ink, mechanical means such as brushes or polishing clothes may be used to remove the ink. The erasing process is indicated in FIG. 3c by a part 64a which is already devoid of any ink,

While the second filter portion 56b is in the path of the projection light PL, the third filter portion 56c is in the printing position where the printing head 50 prints another pattern of ink droplets 58 on the optical element 44, Similarly as described above, this pattern has been computed by the system control 52, thereby anticipating changes of the optical properties of the optical elements of the projection objective 20.

Then the optical element 44 is rotated again by 120’% as this is shown in FIG. 3d, so that the third filter portion 56c gets into the path of the projection light PL. Another pattern of ink droplets 53 is printed on the first filter portion 56a, while the ink in the second filter portion 56b is removed by the erasing head 52.

The filter device 42 thus makes it possible to quickly change the attenuation filter function so that a variable apodisail on becomes feasible. IV.

Other Embodiments 1. Laser Printer FIG, 4 is a schematic perspective view of an embodiment of a filter device 42 in which a laser printing process is used to print a desired absorption pattern on the optical element 44, The ink jet printer of the embodiment shown in FIG, 2 is replaced by a laser printer 150 comprising an imacer 70 that produces writing light 72, The imager 70 includes a laser light source 74 and a scan mirror 76 that may also be configured as a rotating prism, for example.

Part of the laser printer 150 is also a photosensitive drum 78 which attracts at areas, on which the writing light 72 has been directed by the scan mirror 76, a toner material. The latter absorbs the projection light PL and is supplied to the photosensitive drum 78 by a toner drum 80. The photosensitive drum 78 transfers the absorbing material attached to said areas to the optical element 40 while the latter and also the drums 78, 80 rotate. The photosensitive drum 78 has a conical shape that is adapted to the inner and outer radius of the optical element 44 on which the absorbing pattern shall be printed.

Since a laser printing process is known in the art as such, it will not be described in more detail here, FIG. 4 schematically shows also an erasing unit 152 that is configured to remove the absorbing pattern that has been printed by the laser printer 150 on the optical element 44. The erasing unit 152 may apply heat to the optical element 44, or it may mechanically remove the toner that adheres to the opt ica1 eIement 4 4, 2. Exited State Absorption (ESA.) FIG. 5 is a meridional section through the illumination system 12 in which a variable attenuation filter device 242 according to a third embodiment is arranged.

From the light source LS the projection light PL travels to the illumination system 12 along a beam delivery in which the beam of projection light PL is expanded by a beam expander 82 and folded several times by one or more folding mirrors 86. The projection light PL then enters a housing 90 of the illumination system 12, is reflected at a further folding mirror 88 and impinges on a diffractive optical element 92 which may be configured as a computer generated hologram (CGH), for example . The diffractive optical element 92 spreads the projection light PL into various directions and mainly determines the irradiance distribution in a subsequent pupil plane 94.

In other embodiments the diffractive optical element 92 is replaced by a spatial light modulator, for example a micromirror array.

The projection light emerging from the diffractive optical element 92 passes through zoom optics indicated at 96 and a pair of axieon elements 98, 100 that are spaced apart by a distance that can be varied by displacing one or both axieon elements 98, 100.

The projection light PL is then incident on an optical integrator 102 comprising two honeycomb channel plates 104, 106 that produce in the pupil plane 94 a plurality of secondary light sources. Each secondary light source illuminates, via a subsequent condenser 108, the same field in a field stop plane 110 in which a moveable field stop 112 is arranged, Since the light beams produced by the secondary light sources all. superimpose in the field stop plane 110, the latter is very uniformly illuminated, A field stop objective 114 images the field stop plane 110 on the mask clane 94 in which the mask 16 is arranged. So far the illumination 12 is known in the art as such.

The filter device 242 of the third embodiment comprises an optical element 244 which again has the shape of a transparent disc. Here, however, the optical element. 242 is made of fluorite (CaF2} which is doped with rare-earth metal ions, for example Y, Yh, Eu, Er, Nd, Tm, with sodium group metal ions or with oxygen impurities. The optical element 242 is arranged in the field stop plane 110 which is optically conjugate to the mask plane 116 in which the features 19 are arranged. Thus a light bundle, which is attenuated at a certain point of the optical element 212, will converge at an optically conjugated point on the mask 16 with a reduced irradi-a nee .

The filter device 242 further comprises a scanning unit 250 which includes, as can be seen best in the enlarged cutout of FIG. 6, a writing/erasing light source 120 which is configured to produce IR laser light having a center 'wavelength of approximately 808 nm. The scanning unit 250 further comprises a scanning mirror 122 that can be tilted around two orthogonal tilt axes, or a similar scanning device.

The scanning unit 250 is thus capable of directing a writing light beam 224 on any arbitrary point on the surface of the optical element 244. Regions that are exposed to the writing light beam 224 become light absorbing regions that partially absorb the projection light PL. This change of absorption in the optical element 244 relies on the effect, of excited state absorption (ESA), This effect is characterised in that the writing light beam 224 lifts electrons in the rare earth metal ions (or the other impurities mentioned above) to a first excited state. These ions may absorb photons having a higher energy (and thus a shorter wavelength) than the -writing light beam 224 by lifting the electrons to a second higher excited state, Without the excitation by the writing light beam 224, the ions would not be able to absorb the higher energy photons of the projection light PL.

Depending on the lifetime of the first excited state, the absorption is stable or transient, A stable absorption can be erased by bringing the electrons from the first excited state back to the ground state. This can be accomplished usually by the same writing light beam 224 that has been used to excite the ions. In such material it is therefore possible to remove the absorbing regions by directing the writing light beam 224 again on these regions.

If the excited state absorption is transient, the excited ions relax by a spontaneous recomhination. process, thereby emitting photons.

Since the effect of excited state absorption is well known in the art, it will not be explained in further detail here. Reference is made to the following papers that explain this effect in more detail: "Rare Earth Infrared Quantum Counter”, L. Esterowitz et al., APPLIED OPTICS Vol. 7, No. 10, 2053, 1968 "Infrared Excited Er3+ Ions in CaF Crystals”f M. Schle-singer, J. Phys. Chem. Solids, Vol. 51. No. l,f 85-89, 1990 "Vacuum ultraviolet and ultraviolet fluorescence and absorption studies of Er3*-doped LiluFi single crystals", E. Sarantopoulou et al., Appl. Phys. Lett. 65 {7), 15 August 1991 "Photon avalanche upconversion in rare earth laser materials", M.-F. Joubert, Optical Materials 11, 181-203, 1999 "Fluoride crystals and high lying excited states of rare earth ions'*, M. ~F. Joubert, Journal of Fluorine Chemis-try, 107, 235-240, 2001

From these papers is also becomes clear that apart from fluorite (CaF2), other materials such as BaFs or fluorides may foe used for the optical element 242 *

Referring- back to FIG. 6, it will now foe explained how the effect of the excited state absorption is used to produce absorbing regions on or within the optical element 244. The scanning unit 250 is controlled by a control unit 251, which is connected to the overall system control 52, such that, the time, during which the writing light beam 224 interacts with the regions where the absorption shall be increased, is sufficient to stimulate the excited state absorption. In FIG. 6 these absorbing regions are indicated with reference numeral 120. FIG. 7 is a top view on the optical element 244 in which the absorbing regions 120 are indicated as areas having a rectangular geometry with different aspect ratios. The longer the rectangles are along the scan direction Y, the higher is the attenuating effect on the projection light after scan integration ,

This is illustrated in FIG. 8 which represents the scan integrated absorption profile A(x) along the cross-scan direction X for the filter device 242. By comparing FIG, 8 with FIG, 7, it can be seen that, as a result of the scan integration, different absorption levels can be achieved although the absorption in the absorbing regions 120 is uniform, The scan integrated absorption profile A(x) is determined in this embodiment such that the irradiance distribution I(x) along the cross-8can direction X in the light path behind the opt.'.cal element 244 is substantially uniform, as this is shown in FIG. 9,

If the excited state absorption in the material of the optical element 244 is a transient effect., the scanning unit 250 is controlled such that the absorbing regions 120 are created repeatedly, for example every few milliseconds. Then the writing light beam 224 scans over the optical element 244 repeatedly and preferably along the same scan path. If the absorption profile A (x) shall be modified, this can simply be accomplished by directing the writing light beam 224 over different portions of the optical element 244. It is also possible to modify the energy of the writing light beam. 224, and/or the duration over which the writing light beam 224 interacts with the material of the optical element 244,

If the absorbing' regions 120 are stable, it is possible to erase the absorbing regions 120 by simply directing the light beam 224 again on the absorbing' regions 120- The light beam 224, which now forms an erasing light beam, stimulates in the rare earth icons contained in the fluorite crystal an emission of radiation having a wavelength of 1064 ran. This corresponds to the energy difference between the second excitation state and the ground state. For changing the pattern of absorbing regions 120, it suffices to erase only the undesired portions of the absorbing regions 120 and to create additional portions. In principle, however, it. is also possible to erase all absorbing regions 120 completely in one go 'with a strong erasing light source, and to produce a modified pattern of absorbing regions by directing the writing light beam 224 on all regions on the optical element 244 where the absorption shall increase. 3. Rotating Optical Element FIG. 10 is a meridional section through the illumination system 12 comprising an attenuation filter device 342 according to a fourth, embodiment. This embodiment combines features of the first embodiment, namely a rotatable optical element 344, and of the third embodiment, namely the creation of absorbing regions based on the effect of excited state absorption.

As it can be seen best, in the schematic perspective view of FIG, 11, the optrcal element 344 has again the shape of a circular disc that can be rotated around a rotational axis 348 with the help of a drive 346. In this embodiment the optical element 344 is arranged very close to the mask plane 116 so that a similar effect on the uniformity of the irradiance distribution is achieved as in the second embodiment.

The optical, element. 34 4 consists of the same material as the optical element. 242 shown in FIG. 5.

The attenuation filter device 342 includes a 'writ.eng light source 320 and a writing light scanning mirror 322 which is configured to direct a 'writing light beam 324 on a first portion 56a of the optical element 344. The attenuation filter device 342 further comprises an erasing light source 325 and an erasing light scanning mirror 327 which is configured to direct an erasing light beam 329 on a second portion 56b of the optical element 344. A special feature of the filter device 342 is that both the generation of the absorbing areas 120 by the 'writing light beam 324 as well as the removal of the absorbing regions 120 by the erasing light beam 329 is performed while the optical element 344 rotates with constant angular velocity, The latter is chosen such that the projection light PL impinges either on the first filter portion 56a, the second filter portion 56b or the third filter portion 56c that are formed on the optical element 344. This is possible because the projection light PL is emitted by the light source LS only during short intervals between which no projection light PL propagates through, the illumination system 12. As a result of this pulsed operation of the light source LS, there is sufficient time between consecutive projection light pulses to move the next filter portion into the path of the projection light PL. FIG. 11 illustrates an angular position of the optical element 34Ί in which the projection light PL impinges only on the third filter portion 56c, while the other two filter portions 56a and 56b are arranged outside the projection light path. More specifically, the first filter portion 56a is in a writing position in which the writing light beam 324 scans over the first filter portion 56a and thereby produces a pattern of absorbing; regions 120 on the first filter portion 56a. The second filter portion 56b is in an erasing position in which the erasing light beam 329 scans over all absorbing regions 120 of the second filter portion 56b, initiates a recombination process and thereby removes the absorbing regions 120.

The scanning mirrors 322 and 327 are controlled such that the writing light beam 324 and the erasing light beam 329 take into account the rotation of the filter portions 56a, 56b during the 'writing and erasing processes. The projection light pulses are so short that the simultaneous rotation of the third filter portion 56c during the light pulse can be neglected, However, it is also possible to take into account the rotation during the computation of the pattern of absorbing regions 120. 4. Decrra da t i on

In the embodiments shown in FIGS, 5 to 11 it has been assumed that the optical element 244 or 341 consists of a material in which excited state absorption occurs. However, the ability to create excited state absorptions may degrade after some t irne , FIGS. 12a to 12c illustrate an alternative embodiment of a filter device 442 in which a polymer film 445 is applied on a conventional glass substrate 443. Excited state absorption only occurs in the polymer film 445, In FIG, 12a the polymer film 445 is applied on the substrate 443 using a rotating application drum 432 which distributes the polymer dispensed from a reservoir 483 over the substrate 443.

The optical element 444 obtained in this manner is then used in the same 'way as this has been described above with reference to FIGS. 5 to 11, This implies that a pattern of absorbing regions 120 is produced by directing a writing light beam 424 produced by a scanning unit 450 on the polymer film 445, as this is shown in FIG. 12b,

If the ability of the polymer film 445 to locally change its coefficient of absorption upon interaction with the writing light beam 424 degrades, the polymer film 445 is removed from, the substrate 443, as this is schematically shown in FIG. 12c. Here the removal is accomplished by a rotating scraping drum 437 which mechanically scrapes the polymer film 445 off the substrate 443, After that a new polymer film 445 can be applied on the substrate 443 in the manner shown in FIG. 12a. These steps are preferably performed within the apparatus 10 so that its operation does not have to be interrupted for the replacement of the polymer film 425. V.

Important Method Steps FIG. 13 is a flow diagram that summarizes important aspects of a method of variably attenuating a beam of projection light in an optical system of a microlithographic apparatus.

In a first step 31 an absorption pattern of light absorbing regions is created on or within an optical element that is mounted in the optical system.

In a second step S2 the projection light is directed on the absorption pattern.

In a third step S3 one waits until the light absorbing regions have disappeared by themselves or, if the light absorbing regions do not disappear by themselves, the light absorbing regions are actively removed from the optical element. VI.

Other Important Features of the Invention

Important system features of the present invention are summarized in the following sentences, 1, An optical system of a microlithographic apparatus that transfers a mask pattern (18} to a surface (22) using projection light (PL} , 'wherein the optical system (12; 20) has a variable attenuation filter device (42; 24 2; 342; 442) comprising; a) an optical element (44; 244; 344; 444) that is at least partially atrranged in a light path of the projection light, b) a writing unit (50; 150; 250; 320, 322; 450) that is configured to create light absorbing regions (1210) on or within the optical element, e) an erasing unit (52; 152; 325, 327) that is configured to remove the light absorbing regions. 2, The optical system of sentence 1, wherein the variable attenuation filter device comprises a moving mechanism (46; 346) that is configured to move the optical element between a first position and a second position, wherein in the first position a first portion (56a) of the optical element is arranged in the light path and a second portion (56b) is arranged outside the light path such that at least one of the writing unit and the erasing unit is capable to interact with the second portion, and wherein in the second position the first portion is arranged outside the light path and the second portion is arranged in the light path. 3» The optical system of sentence 2, wherein in the second position the first portion is arranged outside the light path such that at least one of the writing unit and the erasing unit is capable to interact with the of t hi e o p t .1 c a 1 e l ernen t. 4. The optical system of any sentences 2 or 3, wherein the moving mechanism comprises a drive (46; 346} configured to rotate the optical element around a rotational axis (48; 348). 5. The optical system of sentence 4, wherein the drive is configured to rotate the optical element with a rotational frequency of at least 10,000 Rpru. 6. : The optical system of any of the preceding sentences, wherein the writing unit (50; 150) is configured to apply a light absorbing material on the optical element exclusively at the light absorbing regions (120). 7. The optical system of sentence 6f wherein the writing unit is a printer (50; 150) that is configured to print a pattern of light absorbing regions on the optical element . 8. The optical system of sentence 1, 'wherein the printer is a laser printer (ISO) comprising a} an imager (70) configured to produce writing light (72), and b) a photosensitive drum (78) configured such that portions of the drum, which have been exposed to the writing light, have a modified property to attract a .material that is absorbent for the pr o jection 11ght, aπd such that ~ the drum transfers the material to a sur face of the optical element while the drum rotates , 9». The optical system of sentence 8, wherein the erasing unit comprises a cleaning device configured to remove the absorbing material from the surface of the optical element. 10. The optical system of any of sentences 1 to 5, wherein the writing unit (250; 320, 322; 450) is configured to direct writing light (224; 324; A 24) to the region of the optical element, wherein the optical element includes a material in which a reversible change of absorption is inducible by the writing light, 11. The optical system of sentence 10, wherein the change of absorption relies on the effect of excited state absorption. 12. The optical system of sentence 10 or 11, wherein the 'writing unit is configured to control at least one of the group consisting of: direction of the writinc: light, intensity of the writing light, and time during 'which the writing' light interacts with the region. 13. The optical system of any of sentences 10 to 12, wherein the material is a fluoride crystal doped with a rare e a r t h element. 14, The optical system of any of sentences 10 to 13, wherein the erasing unit (250; 320, 322; 450) is configured to direct erasing light to the region. 15. The optical system of sentence 14, wherein the writing unit (250; 320, 322; 450) forms also the erasing unit. 16.. The optical system of any of sentences 10 to 15, where-in the optical element comprises a substrate (443} on which the material (445) is applied as a layer, 17. The optical system of sentence 16, wherein the filter device comprises an application apparatus (482, 483, 487} that is configured to apply the layer to the substrate and to remove the layer after it has degraded, 18. The optical system of sentences 1 to 5, wherein the optical element comprises a substrate, and wherein the writing unit comprises an application apparatus that is configured to apply an absorbent layer on the substrate and to locally remove the absorbent layer outside the regions . 19. The optical system of sentence 18, wherein the erasing unit is configured to remove the absorbent layer from the substrate. 20. The optical system of sentences 18 or 19, wherein the writing unit is configured to direct a scanning light beam over the optical element. 21. The optical system of any of the preceding sentences, wherein the optical element (44; 244; 344; 444} is a refractive optical element through which the projection light propagates, 22. The optical system of any of the preceding sentences, 'wherein the optical system is an illumination system (12) or a projection objective (20) that is coni loured to image the pattern on the surface. 23:, An optical system of a rrdcrolithographic apparatus that transfers a mask pattern to a surface using projection .light having a projection light center wavelength, wherein the optical system has a variable attenuation fi11er deviee comprising: a) an optical element (244; 344; 444) that is at least partially arranged in a light path of the projection light, b) a writing unit (50; 150; 250; 320, 322; 450) that, is configured to direct writing light on the optical element, wherein the writing light has a center -wavelength that is larger than the projection light center wavelength so that it produces in a material (244; 344; 445) contained in the optical element light absorbing regions (120) t h a t are a r e s u .1. t o f e x c i t e d s t a t e a b sorption, 24. The system of sentence 23, comprising an erasing unit that is configured to direct erasing light on the light absorbing regions , 25. The method sentence 23 or 24, wherein the light absorbing regions disappear by themselves as a result of a recombination process in the material. 26. The method of any of sentences 23 to 25, 'wherein the writing light has a center wavelength that is greater than a center wavelength of the projection light. 27. The method of sentence 26, wherein the center wavelength of the 'writing light is between 600 run and 1000 nm. 28. The method of any of sentences 23 to 27, wherein the attenuation filter device is configured to control at least one of the following parameters of the writing light: direction of the writing light, intensity of the writing light, and time during which the writing light interacts with the light absorbing regions, 29, The method of any of sentences 23 to 28, wherein the attenuation filter device is configured to direct erasing light on the light absorbing regions in. step. 30. The method of sentence 29, wherein the writing light and the erasing light have the same center wavelengths, 31, The method of any of sentences 23 to 30, wherein the optical element comprises a substrate (443) on which the material (445) is applied as a layer. 33. The method of sentence 31, wherein the layer is removed from the substrate after the layer has degraded.

Claims (10)

1, A method of variably attenuating a beam of projection light (PL) in an optical system of a microlithographic apparatus that transfers a pattern to a surface, wherein said method comprises the following steps; a) creating an absorption pattern of light absorbing regions (120) on or within an optical element {44; 244; 344; 444) that is mounted in the optical system; b) directing the projection light on the absorption pa 1.1 e rn; c) waiting until the light absorbing regions (120) have disappeared by themselves or, if the light absorbing regions do not disappear by themselves, actively removing the light absorbing regions from the optical element; wherein steps a) to c) are carried out at a first portion (56a) of the optical element while steps a) to o) are carried out, but in a time shifted manner, at a second portion (56b) of the optical element so that the projection light is directed, at a given time, either to the first portion or to the second pertion.

2. The method of claim 1, wherein in step a) a different absorption pattern is created at the first portion (56a) and at the second portion (56b). c. The method of claim 1 or 2, wherein a light path of the projection light (PL) is spatially fixed, and wherein the optical element moves so that either the first portion or the second portion is in the light path of the p r o j e o t i ο n .1. i ght. 4* The method of claim 3, wherein the optical element is rotated at least between steps a) and b)»

5, The method of any of the preceding claims, wherein a light absorbing material (58) is applied on the optical element (44) exclusively at the light absorbing regions (120 ) du ri ng s t ep a} , 6> The method of claim 5, wherein the absorption pattern is printed on the optical element. 7,· The method of claim 6, wherein in step a) a) an imager (70) directs writing light (72} to a photosensitive drum (78), b} those portions of the drum which have been exposed to the writing light, attract the absorbing material, c) the drum transfers the material to a surface of the optical element while the drum rotates. 8» The method of any of claims 5 to 7, wherein during step c) the 'absorbing material is thermally removed from the surface of the optical element. 9* The method of any of claims 1 to 4, wherein writing light (224; 324; 424) is directed to the light absorbing regions in step a), and wherein the optical element (244; 344; 444} includes a material in which a reversible change of absorption is caused by the writing light.

10. The method of claim 9, wherein the change of absorption relies on the effect of excited state absorption. 1.1, The method claim 9 or 10, wherein the light absorbing regions {120} disappear by themselves as a result of a recombination process in the material.

12. The method of claims 10 or 11, wherein the writing light (224; 324; 424) has a center wavelength that is greater than a center wavelength of the projection light.

13. The method of any of claims 10 to 12, wherein at least one of the following parameters of the writing light (224; 324; 424) is controlled in step a); direction of the writing light, intensity of the writing light, and time during which the writing light interacts with the light absorbing regions.

14. The method of any of claims 10 to 13, wherein erasing light (224; 329; 424) is directed on the light absorbing regions in step c).

15. The method of claim 14, wherein the writing light and the erasing light have the same wavelengths,

16. The method of any of claims 10 to 15, wherein the optical element comprises a substrate (443) on which the material (445) is applied as a layer.

17. The method of claim 16, wherein the layer is removed from the substrate after the layer has degraded.