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Publication numberWO2006054209 A1
Publication typeApplication
Application numberPCT/IB2005/053694
Publication date26 May 2006
Filing date9 Nov 2005
Priority date17 Nov 2004
Also published asCN101057161A, EP1815277A1
Publication numberPCT/2005/53694, PCT/IB/2005/053694, PCT/IB/2005/53694, PCT/IB/5/053694, PCT/IB/5/53694, PCT/IB2005/053694, PCT/IB2005/53694, PCT/IB2005053694, PCT/IB200553694, PCT/IB5/053694, PCT/IB5/53694, PCT/IB5053694, PCT/IB553694, WO 2006/054209 A1, WO 2006054209 A1, WO 2006054209A1, WO-A1-2006054209, WO2006/054209A1, WO2006054209 A1, WO2006054209A1
InventorsAdrianus Sempel, Bernardus H. W. Hendriks, Coen T. H. F. Liedenbaum, Stein Kuiper
ApplicantKoninklijke Philips Electronics N.V.
Export CitationBiBTeX, EndNote, RefMan
External Links: Patentscope, Espacenet
Fluid ultraviolet lens
WO 2006054209 A1
Abstract
An optical element (100) for ultraviolet radiation comprises a fluid chamber (102) containing a first fluid (104) and a second fluid (106) which is non-miscible with the first fluid (104). The first and the second fluid (104, 106) are in contact with each other over a meniscus (114) extending transverse an optical axis of said optical element. At least one of the fluids is substantially transparent for ultraviolet radiation, such as deep ultraviolet radiation. The optical element (100) can be a lens or a mirror and may have adjustable optical properties. The optical elements may be used in optical systems such as mastering tools for data carriers, laser systems or lithography systems. One of the fluids may be a gas.
Claims  (OCR text may contain errors)
CLAIMS:
1. An optical element (100, 200, 250, 300, 350) suitable for ultraviolet radiation, comprising a fluid chamber (102) containing at least a first fluid (104) and a second fluid (106) in contact with each other over a meniscus (114) extending transversely to an optical axis (112) of said optical element (100, 200, 250, 300, 350), said first and second fluids (104, 106) being immiscible, wherein at least one of said fluids is substantially transparent such that said optical element (100, 200, 250, 300, 350) has a transparency of at least 20% for ultraviolet radiation.
2. An optical element (200, 300, 350) according to claim 1, wherein said optical element (200, 300, 350) furthermore comprises an at least partially reflective material (202), located at said meniscus (114), extending transverse the optical axis (112) of said optical element (200).
3. An optical element (100, 200, 250, 300, 350) according to claim 1, wherein one of said first fluid or said second fluid is a gas or vapor.
4. An optical element (100, 200, 250, 300, 350) according to claim 1, wherein said first fluid (104) and said second fluid (106) are substantially transparent such that said optical element (100, 200, 250, 300, 350) has a transparency of at least 20% for ultraviolet radiation.
5. An optical element (100, 200, 250, 300, 350) according to claim 1, wherein said fluid chamber (102) comprises ultraviolet transparent windows (108, 110) along said optical axis (112).
6. An optical element (100, 300, 350) according to claim 1, each of said first and second fluids (104, 106) having a refractive index, wherein the refractive index of said first fluid (104) differs from the refractive index of said second fluid (106) so as to obtain a focusing effect.
7. An optical element (100, 200, 250, 300, 350) according to claim 1, said first fluid (104) being an aqueous fluid and said second fluid being a non-aqueous fluid, wherein said first fluid (104) and said second fluid (106) are positioned with respect to a hydrophobic surface region of said fluid chamber (102).
8. An optical element (300, 350) according to claim 1, wherein said optical element furthermore comprises a means for adjusting the shape of said meniscus (114).
9. An optical element (300, 350) according to claim 8, wherein the means for adjusting the shape of the meniscus (114) comprises at least one first electrode (302) and at least one second electrode (304), and a voltage source for applying a voltage between the at least one first electrode and the at least one second electrode.
10. A projection apparatus for projection of an object in an image plane, the apparatus comprising at least one optical element (100, 200, 250, 300, 350) suitable for UV radiation, the optical element (100, 200, 250, 300, 350) comprising a fluid chamber (102) containing at least a first fluid (104) and a second fluid (106) in contact with each other over a meniscus (114) extending transversely to an optical axis (112) of said optical element (100, 200, 250, 300, 350), said fluids (104, 106) being immiscible, wherein at least one of said fluids is substantially transparent such that said optical element (100, 200, 250, 300, 350) has a transparency of at least 20% for ultraviolet radiation.
11. A projection apparatus according to claim 10, said projection apparatus being a tool for performing a lithography process step.
12. A projection apparatus according to claim 10, said projection apparatus being a data carrier reading or writing tool for reading/writing a data carrier.
13. A projection apparatus according to claim 12, wherein said data carrier is any of a compact disk, a digital versatile disk or a blu-ray disk.
14. A data carrier master tool for creating a master mould, the master tool comprising: a means for providing a substrate covered with a photosensitive layer a focusing means for focusing a laser beam on said photosensitive layer, the focusing means comprising at least one optical element (100, 200, 250, 300, 350) suitable for UV radiation, the optical element (100, 200, 250, 300, 350) comprising a fluid chamber (102) containing at least a first fluid (104) and a second fluid (106) in contact with each other over a meniscus (114) extending transversely to an optical axis (112) of said optical element (100, 200, 250, 300, 350), said fluids (104, 106) being immiscible, wherein at least one of said fluids is substantially transparent such that said optical element (100, 200, 250, 300* 350) has a transparency of at least 20% for ultraviolet radiation.
15. An ultraviolet laser system, said system comprising an optical element (200, 250) suitable for ultraviolet radiation, the optical element (200, 250) comprising a fluid chamber (102) containing at least a first fluid (104) and a second fluid (106), the first fluid (104) and the second fluid (106) being immiscible with each other, the first fluid (104) and the second fluid (106) being in contact with each other over a meniscus (114) and at least one of said fluids being substantially transparent such that said optical element (100, 200, 250, 300, 350) has a transparency of at least 20% for ultraviolet radiation, wherein an at least partially reflective material is located at said meniscus (114), extending transversely to an optical axis (112) of said optical element (200, 250).
16. • . A device manufactured according to a lithographic method, the method comprising: illuminating a substrate covered with a photosensitive layer with an ultraviolet radiation beam, said ultraviolet radiation beam being focused using an optical element (100, 200, 250, 300, 350) suitable for ultraviolet radiation comprising a fluid chamber (102) containing at least a first fluid (104) and a second fluid (106) in contact with each other over a meniscus (114) extending transversely to an optical axis (112) of said optical element, said fluids (104, 106) being immiscible and at least one of said fluids being substantially transparent such that said optical element (100, 200, 250, 300, 350) has a transparency of at least 20% for ultraviolet radiation, developing said photosensitive layer, and removing developed material or undeveloped material.
17. A method of manufacturing a device by means of lithography using ultraviolet radiation, the method comprising: illuminating a substrate covered with a photosensitive layer with an ultraviolet radiation beam, said ultraviolet radiation beam being focused using an optical element (100, 200, 250, 300, 350) suitable for ultraviolet radiation comprising a fluid chamber (102) containing at least a first fluid (104) and a second fluid (106) in contact with each other over a meniscus (114) extending transversely to an optical axis (112) of said optical element, said fluids (104, 106) being immiscible and at least one of said fluids being substantially transparent such that said optical element (100, 200, 250, 300, 350) has a transparency of at least 20% for ultraviolet radiation.
18. A method according to claim 16, the method furthermore comprising, after said illuminating a substrate covered with photosensitive layer with an ultraviolet radiation beam, - developing said photosensitive layer, and removing developed material or undeveloped material.
19. A method of manufacturing an optical data carrier master by means of optical mastering using ultraviolet radiation, the method comprising: - providing a carrier with a photosensitive layer focusing an ultraviolet beam on said photosensitive, using an optical element (100, 200, 250, 300, 350) suitable for ultraviolet radiation comprising a fluid chamber (102) containing at least a first fluid (104) and a second fluid (106) in contact with each other over a meniscus (114) extending transversely to an optical axis (112) of said optical element, said fluids (104, 106) being immiscible and at least one of said fluids being substantially transparent such that said optical element (100, 200, 250, 300, 350) has a transparency of at least 20% for ultraviolet radiation.
Description  (OCR text may contain errors)

Fluid ultraviolet lens

The present invention relates to optical elements suitable for short wavelength, e.g. ultraviolet radiation wavelength, optical systems using these optical elements, such as lithography systems, and mastering tools and methods for using these tools.

In order to fulfill the requirements for lithographic processing in semiconductor processing, to date, more and more radiation with shorter wavelengths is used. Typical wavelengths that are presently used or that are being explored for use in the near feature are ultraviolet wavelengths such as 365 nm, typically obtained from Mercury lamps, and deep ultraviolet wavelengths, such as 248 nm, obtained from a KrF Excimer laser, 193 nm, obtained from an ArF Excimer laser and 157 nm, obtained from a fluorine laser. Introduction of the use of these short wavelengths has lead to a number of additional difficulties.

Short wavelength radiation is used or will be used in the near future in scanning devices for optical data carriers, such as mastering tools or reading/writing devices for compact disk (CD), digital versatile disk (DVD), high definition DVD (HD-DVD) or blu- ray disks (BD). The short wavelength radiation is needed in order to cope with the pit requirements of high capacity storage systems.

One of the major difficulties to overcome when reducing the wavelength used, is the lack of optical elements adapted for use with these short wavelengths. Optical elements typically are made of solid materials, such as optical glass. Nevertheless, the number of solid materials that are transparent in the deep ultraviolet range is very limited. For instance, at 157 nm, one of the only solid materials that is transparent enough for lithography applications is CaF2. Although lenses have been made out of it, the material is expensive and birefringent. Especially the birefringence makes the design of an optical system with good optical performance difficult. Furthermore, having only one type of material severely limits the optical design freedom. As the number of solid materials that can be used is very limited, an often-used alternative for optical systems based on deep ultraviolet wavelengths is the use of optical reflection elements. These are in general difficult to manufacture with good optical performance.

For visible light, alternatives for solid optical elements are known, e.g. from US 6,369,954. This patent describes a variable focus lens for visual wavelengths based on an electro-wetting effect. Two immiscible fluids are confined in a sealed space. The term immiscible indicates that the two fluids do not mix. The first fluid is an electrical insulator, whereas the second fluid is electrically conductive. The fluids are localized in the sealed space by providing areas in the sealed space that are hydrophobic such that they repel the second fluid, and by hydrophilic areas attracting the second fluid. The fluids furthermore have different refractive indices.

The prior art does not provide a lens for ultraviolet radiation with good performance and broad optical design freedom.

It is an object of the present invention to provide optical elements for use with short wavelength, e.g. radiation in the ultraviolet wavelength range with a good performance and a broad optical design freedom. The above objective is accomplished by methods and devices according to the present invention.

The invention relates to an optical element suitable for use with ultraviolet radiation, comprising a fluid chamber containing at least a first fluid and a second fluid in contact with each other over a meniscus extending transversely to an optical axis of the optical element, the first and second fluids being immiscible, wherein at least one of the fluids is substantially transparent such that the optical element has a transparency of at least 20% for ultraviolet radiation. The transparency may be between 20% and 99.9999% for ultraviolet radiation. The optical element furthermore may comprise an at least partially reflective material, located at the meniscus, extending transverse the optical axis of the optical element. One of the first fluid or the second fluid may be a gas. The first fluid and the second fluid may be substantially transparent such that the optical component has a transparency between 20% and 99.9999%. With the transparency of the optical element, the transparency of the optical element on the light path for incident radiation is meant. With substantially transparent it is meant that the first fluid and/or the second fluid is sufficiently transparent such that the transparency of the optical element is in a range having a lower limit of 20%, preferably 50%, more preferably 75%, even more preferably 90%, still more preferably 95% and an upper limit of 99%, preferably 99.9%, more preferably 99.99%, even more preferably 99.999%, still more preferably 99.9999%. The restrictions for the transparency of the optical element may be valid for wavelengths in a range having an upper limit of 380 nm, 370 nm, 350 nm, 320 nm, 300 nm, 280 nm or 250 nm and a lower limit of 240 nm, preferably 220 nm, more preferably 190 nm, even more preferably 170 nm, still more preferably 150 nm, even more preferably 100 nm, still more preferably 7 nm. The restrictions for the transparency of the optical element may be valid in the deep ultraviolet radiation range. The fluid chamber may comprise ultraviolet transparent windows along the optical axis. Each of the first and second fluids may have a refractive index, wherein the refractive index of the first fluid may differ from the refractive index of the second fluid so as to obtain a focusing effect. Obtaining a focusing effect may be creating convergence of incident ultraviolet radiation or creating divergence of incident ultraviolet radiation. The first fluid may be an aqueous fluid and the second fluid may be a non-aqueous fluid, wherein the first fluid and the second fluid may be positioned with respect to a hydrophobic surface region of the fluid chamber. The optical element furthermore may comprise a means for adjusting the shape of the meniscus. The means for adjusting the shape of the meniscus may comprise at least one first electrode and at least one second electrode, and a voltage source for applying a voltage between the at least one first electrode and the at least one second electrode.

< The invention also relates to a projection apparatus for projection of an object in an image plane, the apparatus comprising at least one optical element suitable for use with UV radiation, the optical element comprising a fluid chamber containing at least a first fluid and a second fluid in contact with each other over a meniscus extending transversely to an optical axis of the optical element, the fluids being immiscible, wherein at least one of the fluids is substantially transparent such that the optical element has a transparency of at least 20% for ultraviolet radiation. The transparency of the optical element may be between 20% and 99.9999% for ultraviolet radiation. The projection apparatus may be a tool for performing a lithographic process step. The projection apparatus may be a data carrier reading or writing tool for reading/writing a data carrier. The data carrier may be any of a compact disk, a digital versatile disk or a blu-ray disk. The invention furthermore relates to a data carrier master tool for creating a master mould, the master tool comprising a means for providing a substrate covered with a photosensitive layer and a focusing means for focusing a laser beam on the photosensitive layer, the focusing means comprising at least one optical element suitable for use with UV radiation, the optical element comprising a fluid chamber containing at least a first fluid and a second fluid in contact with each other over a meniscus extending transversely to an optical axis of the optical element, the fluids being immiscible, wherein at least one of the fluids is substantially transparent such that the optical element has a transparency of at least 20% for ultraviolet radiation. The transparency of the optical element may be between 20% and 99.9999% for ultraviolet radiation.

The invention also relates to an ultraviolet laser system, the system comprising an optical element suitable for use with ultraviolet radiation, the optical element comprising a fluid chamber containing at least a first fluid and a second fluid, the first fluid and the second fluid being immiscible with each other, the first fluid and the second fluid being in contact with each other over a meniscus and at least one of the fluids being substantially transparent such that the optical element has a transparency of at least 20% for ultraviolet radiation, wherein an at least partially reflective material is located at the meniscus, extending transversely to an optical axis of the optical element. The transparency of the optical element may be between 20% and 99.9999% for ultraviolet radiation. The invention also relates to a device manufactured according to a lithographic method, the method comprising illuminating a substrate covered with a photosensitive layer with an ultraviolet radiation beam, the ultraviolet radiation beam being focused using an optical element suitable for ultraviolet radiation comprising a fluid chamber containing at least a first fluid and a second fluid in contact with each other over a meniscus extending transversely to an optical axis of the optical element, the fluids being immiscible and at least one of the fluids being substantially transparent such that the optical element has a transparency of at least 20% for ultraviolet radiation, developing the photosensitive layer and removing the developed material or removing the material which is not developed. The transparency of the optical element may be between 20% and 99.9999% for ultraviolet radiation.

The invention furthermore relates to a method of manufacturing a device by means of lithography using ultraviolet radiation, the method comprising illuminating a substrate covered with a photosensitive layer with an ultraviolet radiation beam, the. ultraviolet radiation beam being focused using an optical element suitable for ultraviolet radiation comprising a fluid chamber containing at least a first fluid and a second fluid in contact with each other over a meniscus extending transversely to an optical axis of the optical element, the fluids being immiscible and at least one of the fluids being substantially transparent such that the optical element has a transparency of at least 20% for ultraviolet radiation. The transparency of the optical element may be between 20% and 99.9999% for ultraviolet radiation. The method furthermore may comprise, after illuminating a substrate covered with photosensitive layer with an ultraviolet radiation beam, developing the photosensitive layer, and removing the developed material or removing the material which is not developed. The invention also relates to a method of manufacturing an optical data carrier master by means of optical mastering using ultraviolet radiation, the method comprising providing a carrier with a photosensitive layer and focusing an ultraviolet beam on the carrier, using an optical element suitable for ultraviolet radiation comprising a fluid chamber containing at least a first fluid and a second fluid in contact with each other over a meniscus extending transversely to an optical axis of the optical element, the fluids being immiscible and at least one of the fluids being substantially transparent such that the optical element has a transparency of at least 20% for ultraviolet radiation. The transparency of the optical element may be between 20% and 99.9999% for ultraviolet radiation.

It is an advantage of the present invention that the optical elements suffer less from birefringence than prior art optical elements suitable for ultraviolet radiation.

It is also an advantage of the present invention that optical elements suitable for ultraviolet radiation can be obtained with reduced costs and having a relatively easy construction.

It is furthermore an advantage of specific embodiments of the present invention that optical elements suitable for ultraviolet radiation can be obtained with adjustable optical properties, such as e.g. a switchable focal distance, without the need for mechanical moveable elements. In this way, wearand tear can be reduced. It is also an advantage of the optical elements of embodiments of the present invention that they result in not just a material but rather a new class of materials, i.e. UV transparent fluids, to be used, leading to more optical design freedom for UV optics. It is also an advantage of specific embodiments of the present invention that optical elements suitable for ultraviolet radiation can be obtained which allow optical aberration correction.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

The teachings of the present invention permit the design of improved apparatus for guiding radiation in the ultraviolet wavelength range. These and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

Fig. Ia is a vertical cross-section of an optical element suitable for ultraviolet radiation being a convex lens according to a first embodiment of the present invention. Fig. Ib is a vertical cross-section of an optical element suitable for ultraviolet radiation being a concave lens according to a first embodiment of the present invention.

Fig. 2a is a vertical cross-section of an optical element with a hydrophilically pinned meniscus suitable for ultraviolet radiation being a convex lens according to the first embodiment of the present invention. Fig. 2b is a vertical cross-section of an optical element with a hydrophilically pinned meniscus suitable for ultraviolet radiation being a concave lens according to the first embodiment of the present invention.

Fig. 3 shows the structural formula for cyclo-octane, as can be used in a fluid ultraviolet lens according to embodiments of the present invention. Fig. 4 shows the absorption characteristics in the ultraviolet wavelength range for water and modified water as used in embodiments of the present invention.

Fig. 5 shows an example of a mirror suitable for ultraviolet radiation according to a second embodiment of the present invention.

Fig. 6 shows an alternative example of a mirror suitable for ultraviolet radiation according to the second embodiment of the present invention.

Fig. 7 shows an example of an optical element for ultraviolet radiation having an adjustable focus distance based on the electro-wetting effect according to a third embodiment of the present invention.

Fig. 8 shows an example of an optical element for ultraviolet radiation as can be used for aberration correction according to the third embodiment of the present invention. Fig. 9 shows an example of an optical element for ultraviolet radiation having an adjustable focus distance based on hydrostatic pressure difference according to the third embodiment of the present invention. Fig. 10 shows an ultraviolet reading/writing or mastering device for optical data carriers according to a fourth embodiment of the present invention.

Fig. 11 shows a lithography system using ultraviolet radiation for performing optical lithography according to the fourth embodiment of the present invention. Fig. 12 shows an ultraviolet laser system using a partial mirror suitable for ultraviolet radiation according to the second embodiment of the present invention.

In the different figures, the same reference signs refer to the same or analogous elements.

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. Any reference signs in the claims shall not be construed as limiting the scope. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative ■ purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated. Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

The terms lyophilic (liquid-attractive) and lyophobic (liquid-repelling) describe the tendency of a surface to become wetted by a liquid. The terms hydrophilic and hydrophobic refer to the particular case when the liquid is aqueous, and refer to, respectively, an attractive or a repellent force for aqueous solutions or water. In the following description, e.g. a water-based fluid and a non-water based fluid will be used as polar and non-polar fluids respectively. Consequently, sometimes the terms hydrophobic and hydrophilic are used. However, it should be understood that any combination of fluids and surfaces which provides the necessary combination of polarity and non-polarity and lyophobic/lyophilic effect, respectively, can be used instead.

A fluid is a substance that alters its shape in response to any force, that tends to flow or to conform to the outline of its chamber, and that includes gases, liquids, vapors and mixtures of solids and liquids capable of flow.

In a first embodiment, the invention relates to an optical element 100 which can be used for ultraviolet light, e.g. for deep ultraviolet (UV) light or for radiation in a wavelength range thereof. With ultraviolet radiation, typically electromagnetic radiation in the wavelength range between 380 nm and 7 nm is meant, whereas with deep ultraviolet radiation, typically electromagnetic radiation in the wavelength range between 250 nm and 7 nm is meant. The present invention typically will be applied for ultraviolet wavelengths often used in optical tools, such as e.g., but not limited thereto, 248 nm, 193 nm and 157 nm. An example of an optical element 100 suitable for ultraviolet radiation, e.g. deep ultraviolet radiation or radiation in a wavelength range thereof, is shown in Fig. Ia, to Fig. 2b. The optical element 100 comprises a fluid chamber 102 containing at least two fluids 104, 106. The fluid chamber 102 is defined by walls, whereby at least portions of a front wall 108 and a back wall 110, crossing the optical axis 112 are transparent for ultraviolet light, e.g. for deep ultraviolet light. Depending on the exact wavelength used, the transparent material may be e.g. quartz or calcium fluoride (CaF2). For 157 nm wavelength light, the only known solid material which is sufficiently transparent is CaF2. Although CaF2 suffers from birefringence, the effect is significantly less disturbing than for solid CaF2 lenses, as the front wall 108 is perpendicular to the optical axis of the system, thereby avoiding a birefringence effect and as the back wall 110 only has a limited thickness such that for light rays which are not incident perpendicular to the back wall 110, the disturbance is limited.

In the present embodiment, the two fluids 104, 106 used are substantially transparent for the ultraviolet radiation used, e.g. for deep ultraviolet radiation or radiation in a wavelength range thereof. The transparency of the fluids 104, 106 is such that the optical element has a transparency for the incident radiation in a range having a lower limit of 20%, preferably 50%, more preferably 75%, even more preferably 90%, still more preferably 95% and an upper limit of 99%, preferably 99.9%, more preferably 99.99%, even more preferably 99.999%, still more preferably 99.9999%. With the transparency of the optical element, there is meant the transparency along the optical path of the incident radiation. In other words, the intensity of the radiation after passing the optical element may be maximal reduced to 20%, preferably to 50%, more preferably to 75%, even more preferably to 90%, still more preferably to 95% referred to the intensity of the radiation prior to its incidence on the optical element. The first fluid 104 that is used in an optical element for UV radiation, e.g. deep UV radiation or radiation in a wavelength range thereof, may e.g. be - but is not limited to - water or an aqueous based fluid, such as de-ionized (DI) water, or more preferably DI water with additional surfactants to lower the absorption, which has at least a sufficient transparency for UV radiation with a wavelength down to 180 nm. An overview of the absorption characteristics of DI water, indicated with squares, and DI water with additional surfactants, indicated with discs, which can be used as first fluid 104 is shown in Fig. 4. The second fluid 106 that is used also has a sufficient transparency for UV, e.g. deep ultraviolet (DUV) or a specific range thereof. An example of a second fluid 106 is cyclo- octane, for which the structural formula is shown in Fig. 3. Cyclo-octane is a colorless liquid that can be formed by catalytic hydrogenation of 1,5-cyclooctadiene, which is easily obtained by the dimerization of butadiene. Cyclo-octane has an absorption coefficient of 2.44 cm"1 at 257 nm, which increases for shorter wavelengths The fluids 104, 106 used in the present invention may have an absorption coefficient in a range having an upper limit of 10 cm"1, preferably 5 cm"1, more preferably 2 cm"1, even more preferably 1 cm"1, still more preferably 0.7 cm"1 and a lower limit of 0.5 cm"1, preferably of 0.2 cm"1, more preferably of 0.1 cm"1, even more preferably of 0.05 cm"1, still more preferably of 10"6 cm"1. The latter limits for the absorption coefficients are valid in a wavelength range with an upper limit of 250 nm, 280 nm, 300 nm, 320 nm, 350 nm, 370 nm or 380 nm and a lower limit of 240nm, preferably 220nm, more preferably 190 nm, even more preferably 170 nm, still more preferably 150 nm, even more preferably 100 nm most preferably 7 nm.

The two fluids 104, 106 in the fluid chamber 102 are non-miscible or immiscible, i.e. the two fluids do not mix. Positioning of the fluids can e.g. be done by selecting a front or back wall of the fluid chamber that is hydrophilic in combination with one aqueous fluid, such that the aqueous fluid sticks against the selected wall. The two fluids 104, 106 are preferably arranged to have a substantially equal density. The maximum allowed difference in density depends strongly on the lens diameter. The densities should be so equal that the optical aberrations due to gravity are negligible. In this way, the optical element suitable for UV radiation functions independently of orientation, i.e. without dependence on gravitational effects between the two fluids 104, 106. This typically can be done by an appropriate selection of fluids 104, 106. The density can be changed by adding molecular constituents to increase or decrease the density of one of the fluids to match to the density of the other fluid.

The contact area between the two fluids 104, 106 is called the meniscus 114. The meniscus 114 extends transversely to the optical axis 112 of the optical element 100. The term transverse indicates that the meniscus 114 crosses, i.e. it extends across, the optical axis 112, and that it is not parallel to the optical axis 112. In the present invention, different ways of changing the curvature of the meniscus 114 are included within the scope of the present invention. Two examples will be discussed, although the invention is not limited thereto. A first way of changing the curvature of the meniscus 114, and consequently the corresponding focus distance is by changing the interfacial tensions of the fluids 104, 106. In principle, the meniscus is always spherical, provided that the densities of the fluids are equal. Changing the contact angle of the meniscus 114 and thus the shape of the meniscus 114 can be done by changing the interfacial tensions, e.g. by introducing surfactants in the fluids, or by influencing one of the interfacial tensions with a voltage, if a polar fluid is used. A lowering of the interfacial tensions by a factor of 2 can easily be obtained. Two examples of fluid lenses having a different meniscus shape, based on different interfacial tensions are shown in Fig. Ia and Fig. Ib.

Alternatively, the meniscus can be pinned at the edge of a hydrophilic area 118 provided on certain areas of the walls of the fluid chamber, regardless of the interfacial tensions. In this alternative, one of the fluids is a aqueous fluid, whereas the other fluid is a non-aqueous fluid. The perimeter 116 of the meniscus 114, i.e. that part of the meniscus 114 touching the walls of the fluid chamber 102, then contacts the wall surfaces on fixedly located places, i.e. at locations where an abrupt change in wettability of the surface is present. The interaction between the different areas on the wall and the different fluids is determined by the wettability. Wettability is the extent by which a side is wetted, i.e. covered by a fluid. For instance, if the first fluid 104 is an aqueous polar fluid, and the second fluid 106 is a nonČ aqueous fluid, a hydrophilic part will attract an aqueous fluid and not attract the non-aqueous fluid. The curvature of the meniscus 114 in this case is determined by the amount or volume of each of the fluids that is provided. Two examples of UV fluid lenses having a different meniscus shape are shown in Fig. 2a and 2b. The meniscus perimeter 116, the hydrophilic areas 118 and the hydrophobic areas 120 are indicated.

One of the fluids may be a gas, which may be any gas that is transparent in ultraviolet, e.g. in deep ultraviolet. The combination of the gas, gas conditions and fluid used are preferably selected such that the amount of aberration in the optical element is as small as possible. Furthermore, the amount of optical aberration due to gravity will depend on the size and shape of the optical element. The gas may for example be a gas that is transparent for extreme ultraviolet radiation, such as e.g. purified argon, nitrogen, helium or a mixture thereof. These gasses typically have a significantly high transparency for UV radiation at wavelengths less than 200nm, less than 150nm, less than 50nm, less than 20nm and even less than IOnm. Allowable pressures are such that the transparency of the gas is still significantly high. The pressure may e.g. be between 1200mbar and 5mbar, between 1200mbar and 50mbar or between 1200mbar and 500mbar, although the invention is not limited thereto. Oxygen contamination may reduce the UV transparency, such that, preferably, the gasses are purified and contain only a very low amount of oxygen, e.g. a few ppm, or even no oxygen. By way of example, when dry nitrogen is used with an oxygen content lower than lppm, an absorption coefficient of lower than 2.10"4 cm"1 can be obtained at 157nm.

In the present embodiment, the two fluids 104, 106 furthermore have a different refractive index. For the given example, whereby water and cyclo-octane are used, the refractive indices are 1.38 and 1.51 respectively, as known from e.g. J. Vac. Sci. Techno 1. B 17 (1999) p3306-3309. Because of the different refractive indices between the two fluids 104, 106, and the curvature of the meniscus 114, the meniscus 114 will act as a lens surface. If one of the fluids is a gas, a large difference in refractive index maybe obtained. Fig. Ia and Fig. 2a illustrate an optical element 100 having a meniscus 114 with a convex shape. The meniscus 114 between the first fluid 104 and the second fluid 106 is called convex if the surface of the meniscus 114 is hollow seen from the part upstream the meniscus 114 on the optical axis. The optical element 100 thus acts as a convex lens if nio4 > nioβ- Light rays incident on the optical element 100, pass the transparent portion of the front wall 108 and are incident on the meniscus 114. The meniscus 114 allows to focus the light rays in a focus point 122. Fig. Ib and 2b illustrate an optical element 100 having a concave lens surface, such that light rays incident on the meniscus 114 of the optical element 100 are divergent. The focal distance "f" corresponding with a meniscus 114 of an optical element suitable for UV radiation as described in the present embodiment, is determined by equation [l], i.e.

f U104 I R I whereby nio4 is the refractive index of the first fluid 104, nioδ is the refractive index of the second fluid 106 and R is the radius of curvature of the meniscus 114. It can be seen that the focus distance f of the optical element 100 is determined both by the refractive indices Ti1(M, nioδ of the materials used and by the radius of curvature R of the meniscus 114. Depending on the system used, the radius of curvature R can be changed by changing the surface tension properties of one of the fluids 104, 106 or the surface tension of the wall, if the meniscus shape is determined by the interfacial tensions of the fluids, or it can be changed by changing the relative volumes of the fluids 104,106, if the meniscus is pinned by a hydrophylic coating. In this way a fluid lens suitable for UV radiation, e.g. deep UV radiation or radiation in a wavelength range thereof, can be obtained that can be tuned during production to have the suitable optical properties. Selecting the refractive indices niO4, niO6 and selecting - depending on the system used - the surface tension properties of the fluids 104, 106 or the volumes of the fluids 104, 106 allows to obtain convex and concave lenses, which allows to converge or diverge one or more wavelengths of ultraviolet radiation. It is to be noted that the focus distance as given in formula [1] only expresses the effect of the curved meniscus surface and needs to be further adapted for the absolute values of the refractive indices and the thickness of the fluid volumes, as known by a person skilled in the art.

A second embodiment describes a mirror 200 suitable for ultraviolet radiation, e.g. deep ultraviolet radiation or radiation in a wavelength range thereof, based on an optical element as described in the first embodiment, i.e. comprising a fluid chamber 102 comprising at least two fluids 104, 106 having the same properties as described in the first embodiment, whereby reflective material 202 is provided at the interface between the two fluids 104, 106. This embodiment is illustrated in Fig. 5. If the optical element only functions as mirror based on the additional reflective material 202, it is not a requirement that the two fluids 104, 106 have a different or significantly different refractive index. By providing reflective material 202 at the interface between the two fluids 104, 106, a reflective portion of a mirror is formed. The reflective material 202 may be arranged to be only partially reflective or to be highly reflective, e.g. with a reflectivity larger than 90% or even larger than 98%. If only a reflected portion of impinging UV radiation is of interest, it is sufficient that only the fluid 104 crossed by the UV radiation before reaching the reflective material 202 is substantially UV transparent. Thereby is meant that the transparency of the fluid 104 then is such that the optical element has a transparency for incident radiation in a range having a lower limit of 20%, preferably 50%, more preferably 75%, even more preferably 90%, still more preferably 95% and an upper limit of 99%, preferably 99.9%, more preferably 99.99%, even more preferably 99.999%, still more preferably 99.9999%. In this case the back wall of the fluid chamber does not need to be transparent. The radiation then enters the optical element through a transparent wall portion of the fluid chamber 102, crosses he first fluid 104, is reflected at the reflective material 202 and again crosses the first fluid 104 to leave the fluid chamber 102 through a transparent wall portion. Furthermore, if the non-crossed fluid 106 is well localized by e.g. the electro wetting effect or by e.g. gravitational effects for an optical element used in a specific orientation, a gas can be used as UV radiation transparent fluid 104. The gas may for example be a gas that is transparent for extreme ultraviolet radiation, such as e.g. purified argon, nitrogen, helium or a mixture thereof. These gasses typically may have a significantly high transparency for UV radiation at wavelengths less than 200nm, less than 150nm, less than 50nm, less than 20nm and even less than 10nm. Allowable pressures are such that on the one hand the transparency of the gas is still significantly high, whereas on the other hand the other fluid does not evaporate easily. As described in the first embodiment, oxygen contamination of the gas preferably is reduced. The use of a gas as UV radiation transparent fluid in this embodiment allows to obtain low absorption in the optical element, even for very low wavelengths of the EUV range. Nevertheless, if both fluids 104, 106 used are transparent for ultraviolet radiation, e.g. deep ultraviolet radiation or radiation in a wavelength range thereof, material selections for both convex and concave mirrors can ' easily be made. Furthermore, for a partially reflective mirror, i.e. a mirror that reflects part of the incident UV radiation but transmits the remaining part, there is a need for the two fluids 104, 106 to be UV transparent, e.g. deep UV transparent. The reflective material 202 at the interface between the two fluids 104, 106, i.e. the meniscus 114, may take a number of forms, e.g. it may comprise metal nano-particles, a metal liquid-like film (MELLF) or a thin metal layer on an organic polymer, as known by a person skilled in the art. A more detailed description of how metallic nano-particles coated with organic ligands may be spread over the meniscus 114 is described by Yockell-Lelievre et al. in Applied Optics vol. 42 (2003) pi 882. The particles self assemble at the meniscus 114. Hence the present invention includes the use of self-assembling reflecting particles to form a reflective surface. The application of a metal liquid-like film (MELLFs) consisting of coated silver nano-particles is described in more detail by e.g. Laird et al. in Proceedings SPIE vol. 4839 (2003) p733. The silver nano- particles are partially coated with an organic ligand such that the particles are no longer stable in the aqueous phase and spontaneously assembly at the water-organic interface. The MELLF thus forms an extremely thin layer that follows the surface very closely, allowing precise control of the reflective surface. This may e.g. be a monolayer, whereby, after the layer has been formed, other particles do not have the tendency to settle near the layer. The reflectivity of the nano-particles for UV and deep UV should be large, such that a mirror in the UV and deep UV region is created. Therefore, the nano-particles may be coated with an additional UV reflecting, e.g. deep UV reflecting, layer such as a multilayer dielectric coating.

Another, alternative configuration for a UV mirror 250 is shown in Fig. 6, combining a UV reflective surface 252 with a fluid UV lens 100 as described in the first embodiment of the present invention. The UV reflective surface 252 may be a planar reflective surface, as in the embodiment illustrated in Fig. 6, although the invention is not limited thereto. The UV reflective surface 252 may e.g. be a UV reflective metal layer. The combination of the shaped meniscus 114 of the fluid lens 100 with the planar reflective surface 252 provides the same optical effect as a curved reflective surface. By selecting the shape of the meniscus 114 of the fluid UV lens 100, the radius of curvature of the resulting UV mirror 250 can be selected during production. It is an advantage of the second embodiment that the shape of the UV mirror 200, 250 can be selected without severe restrictions on the materials used.

In a third embodiment of the present invention, an optical element, such as e.g. a mirror or a lens, is described based on the optical elements described in the previous embodiments, whereby additional means for adjusting the optical properties - i.e. after production - are provided. In this way, a variable or adjustable optical element 300, 350 suitable for UV radiation, e.g. deep UV radiation or radiation in a wavelength range thereof, is obtained. Some examples are shown in Fig. 7, Fig. 8 and Fig. 9. With variable optical element is meant an optical element, e.g. a lens, in which one or more properties can be controllably adjusted e.g., in which either the focal length or the position of the refractive/reflective surface of the optical element can be altered. Several types of means for adjusting the properties of the optical element may be used such as a means for adjusting based on application of an electrical voltage using the electro wetting effect in a fluid optical element, a means for adjusting the properties based on changing the hydrostatic pressure in the fluids of a fluid optical element, etc. Fig. 7 shows an adjustable optical element with a means for adjusting the optical properties, such as focussing distance, based on application of the electro wetting effect of a fluid optical element suitable for UV radiation, such that the configuration of the meniscus 114 is changed. Typically, one of the fluids 104, 106, e.g. the first fluid 104, must be a conductive fluid to experience the electro wetting effect. The means for adjusting in the example shown in Fig. 7 comprises a first electrode 302 for influencing the meniscus shape, which first electrode 302 is not in conductive contact with the conductive fluid 104, e.g. located outside the inner surface of the fluid chamber 102, and for example at a position corresponding to the point at which the meniscus 114 contacts the surface of the fluid chamber 102, as illustrated in Fig. 7. The means for adjusting furthermore comprises a second electrode 304 in direct electrical contact with or capacitively coupled to the polar fluid 104. The first electrode 302 may extend around the perimeter of the meniscus 114. Alternatively, not illustrated in Fig. 7, the first electrode 302 may be an electrode which is located in or near the fluid chamber 102, at the side of the second, non-conductive fluid 106. An electrical voltage is applied from a variable voltage source 306 across the first, polar fluid 104 via the electrodes 302, 304. By applying this voltage, the interaction between the first, polar fluid 104 and the walls of the fluid chamber is changed, leading to a change in the contact angle between the two fluids 104, 106, and thus to a change in the shape of the meniscus 114, e.g. indicated in Fig. 7 by changed meniscus 114' shown by the dashed line. In other words, applying a voltage between the first and the second electrodes 302, 304 allows to attract the polar fluid 104 more or less towards the first electrode 302, thus influencing the interaction of the polar fluid with the walls of the fluid chamber and consequently the position and shape of the meniscus. By adjusting the shape of the meniscus 114, the lens or mirror function provided by the variable optical element 300 suitable for UV radiation can be changed. This can be seen from equation [I]: the electro wetting effect allows to change the curvature of the meniscus 114 of the optical element such that the focal distance f can be changed. If the wettability of a surface is initially small, which is usually referred to as the surface being lyophobic such as e.g. for a Teflon-like surface, a voltage can be used to make it larger in case the fluid is susceptible to a voltage. If the wettability is initially large, usually referred to as a lyophilic surface such as e.g. for a silicon dioxide surface, then applying a voltage will have relatively little effect. It is therefore preferable that in such electro wetting devices, the meniscus 114 is initially in contact with a hydrophobic layer. Furthermore, by changing the position of the first electrode 302, other alternative positions and shapes of the meniscus 114 can be obtained. Similarly, by adding additional electrodes, the shape of the meniscus 114 can be changed from spherical to any suitable shape. The latter is described in more detail for visible electro-wetting fluid lenses in patent application EP04101341.8. It is to be noted that the difference in lens power created can be achieved with different amounts of movement in the meniscus 114, depending on the difference in refractive index between the two fluids. In Fig. 8, a more complex system for adjusting the meniscus 114 of the optical element is illustrated in accordance with a further embodiment of the present invention. The system comprises a plurality of first electrodes 312a to 312j, each being UV transparent, e.g. made of indium tin oxide. The plurality of first electrodes 312a to 312j furthermore are each connected to a voltage supply (not shown in Fig. 8) such that possibly different voltages can be applied to the different first electrodes 312a to 312j. By selecting appropriate voltages for the plurality of first electrodes 312a to 312j, any shape suitable for introducing the desired optical aberration into the UV radiation beam of the system can be obtained. In a specific design (not shown in Fig. 8), the optical element may have a substantially cylindrical fluid chamber and the plurality of first electrodes 312a to 312j may be configured as rings arranged concentrically with respect to the optical axis 112 of the system, the first electrodes 312a to 312j being thin plates having their plane arranged perpendicular to the optical axis 112. The system of the present embodiment typically may be used for aberration correction in the DUV range. Such an aberration correction system can be used for example - but not limited to - in systems handling optical data carriers such as mastering tools or reading/writing devices. The optical element introduces optical aberrations like spherical aberration and/or coma aberration into the UV light beam of the optical system in order to compensate for corresponding aberrations caused by the transparent layer on the optical data carrier, in particular in case of a tilt error or a centring error of the optical data carrier. Fig. 9 shows an alternative means for adjusting the optical properties of an alternative optical element 350 suitable for UV radiation, based on hydrostatic pressure on the meniscus 114 between the two fluids 104, 106. The meniscus is pinned to the wall, e.g. by introducing a hydrophilic area on the walls of the fluid chamber or by a specific shape of the fluid chamber 102. Given that the perimeter 116 of the meniscus 114 is fixed, as described above, the shape of the meniscus 114 is determined by the amount of fluid that is present in the fluid chamber 102 for each fluid 104, 106. In the present alternative, the means for adjusting the optical properties therefore comprises at least one pump 352, for changing the hydrostatic pressure on the meniscus 114. The at least one pump 352 is connected to the fluid chamber 102 and arranged to pump quantities of one or more of the fluids 104, 106 to and from the fluid chamber 102. Pump 352 may be any type of pump suitable for providing a different pressure for the fluids 104, 106 in the fluid chamber 102. This may be - but is not limited to - a mechanical pump. By way of example, in Fig. 9, a pump 352 is shown that is arranged to simultaneously increase the volume of the fluid 104 and to decrease the volume of the fluid 106 and vice versa, so as to maintain the same total volume of the two fluids within the chamber 102. The result will be that the shape of the meniscus 114 will be changed, as the perimeter 116 of the meniscus is pinned to the chamber surface. For instance, if extra fluid 104 is added to the chamber 102, then the meniscus 114 shape may change to be more convex as viewed in a direction from fluid 104 towards fluid 106, i.e. to form the changed meniscus 114". Alternatively, if extra fluid 106 is added, then the meniscus 114 may change shape to changed meniscus 114' i.e. the meniscus becomes concave as viewed in the direction from fluid 104 towards fluid 106. It will be appreciated that by altering the volumes of the fluids 104, 106 within the chamber 102, the meniscus shape can be changed from being convex, to planar, to concave. A more detailed discussion of changing the optical properties of a variable optical element based on changing the hydrostatic pressure and applications thereof is provided in patent application EP 03101328.7. It is expected that the maximum curvature of the meniscus shape would occur when the meniscus 114 forms a half- sphere. However, it will be appreciated that there is likely to be a threshold pressure at which the meniscus 114 moves, when the pressure becomes so great that the pinning action of the meniscus 114 is overcome, with the result that the meniscus will subsequently move position. Such a threshold pressure is dependent on the way of pinning of the meniscus perimeter 116, e.g. of the magnitude of the change in wettability near the meniscus perimeter 116, the interfacial tension between the fluids, the chamber diameter and the chamber shape. In the above-described embodiments, the meniscus 114 is by way of example fixedly located by a change in the wettability of the surface. However, it will be appreciated that other techniques may be used to fix or pin the position of the meniscus perimeter 116. It is to be noted that, if no hydrophilic coating is present, the meniscus will move easily along the wall, without changing the shape. This can be used as a way of focusing.

In a fourth embodiment, the invention relates to the use of an optical element as described in any of the previous embodiments in an optical system using UV radiation, e.g. deep UV radiation or radiation in a wavelength range thereof. Such an optical system may be e.g. a UV-based mastering tool or reading/writing device for optical data carriers or a UV-based lithographical system. Fig. 10 shows an optical scanning device 500, i.e. a mastering or reading/writing tool suitable for UV radiation, incorporating an optical element as described in any of the previous embodiments. Optical scanning devices 500 are devices that scan an optical data carrier 502, for reading and/or writing information from/to the carrier 502. Such a UV-based optical scanning device 500 may be compatible with a variety of optical data carrier formats e.g. CD format, DVD format and BD (Blu-ray Disc format). The use of UV radiation, e.g. deep UV radiation, allows to cope with the high pit requirements of high capacity storage systems. Typically, each optical data carrier 502 will comprise a transparent layer 504, one side of which is provided with an information layer 506. The side of the information layer 506 facing away from the transparent layer 504 is protected from ambient influences by a protection layer 508. The side of the transparent 504 layer facing the device 500 is referred to as the entrance face. Information may be stored in the information layer 506 of the data carrier 502 in the form of optically detectable marks arranged in substantially parallel, concentric or spiral tracks, not indicated in Fig. 10. These marks may have any optically readable form. By way of example, a system is shown that comprises a separate radiation source 520a, 520b, 520c for each type of optical data carrier or for different types of data marks to be written/read. Each radiation source 520a, 520b, 520c is suitable for providing the correct wavelength of electromagnetic radiation for scanning the relevant optical data carrier, whereby at least one radiation source is an ultraviolet radiation source, e.g. a deep ultraviolet radiation source. However, it will be appreciated in other embodiments, a single tuneable optical source, tuneable into UV radiation, could replace the three illustrated sources. Light from each optical source 520a, 520b, 520c passes through a respective pre-collimator lens 522, and through a grating 524, and into the radiation beam path via a respective beam splitter 540, 542, 544, which reflects light towards the optical data carrier 502. The light then passes through collimator lens 530, is reflected off folding mirror 532, through a quarter-wave plate 534 and into the objective lens 536. Light incident on the objective lens 536 should be in the form of a collimated beam, such that the objective lens 536 transforms the collimated radiation beam into a converging beam incident on the information layer 506 of the optical data carrier 502. Light from the information layer 506 of the optical data carrier 502 then passes back through the system, including being transmitted through each of the relevant beam splitters 540, 542, 544 (without reflection), through a servo lens 546, to be detected by detector 548. Typically, in order to correct for the different wavelengths of electromagnetic radiation used to scan each respective data carrier, the collimator lens 530 is moved (as indicated by double headed arrow 550). In this particular embodiment, the collimator lens 530 is therefore a variable fluid lens suitable for UV radiation, according to an embodiment of the present invention. Alternatively, accurate collimation of the radiation beam incident upon the objective lens 536 from the quarter- wave plate 534 instead may be achieved by utilising a variable mirror suitable for UV radiation in the position of the folding mirror 532. Consequently, a device used to alter the position of the collimator lens 530 (which may have been susceptible to mechanical fatigue), can be replaced. Furthermore the other optical elements used in the reading/writing tool also may be fluid lenses according to the present invention.

A mastering tool allows to create a master mould that can be used to manufacture information carriers. Such a mastering tool typically is equipped for irradiating a photosensitive layer applied to a substrate, which typically is made from glass. Irradiation typically is performed by spinning the substrate and displacing the light beam such that the light spot creates a spiral-shaped track on the photosensitive layer, although other relative motions also could be applied. The radiation beam, which in the present example is a UV radiation beam focussed with an optical element as described in any of embodiments 1 to 3, is modulated such that a series of irradiated and non- irradiated elements is formed on the spiral-shaped track, which correspond to the desired data content to be provided on the information carrier. After developing the photosensitive layer and removing the developed material or removing the undeveloped material, typically a metal layer structure is provided on the substrate, e.g. in an electro-deposition process. The metal layer, which thus comprises a structure corresponding with the desired data content, subsequently is detached from the < substrate and can be used as a master mould for information carriers to be manufactured

Variable optical elements suitable for UV radiation according to embodiments of the present invention may also be used to correct for the difference in thickness of the cover layer between CDs, DVDs and BDs such that high quality data reproduction signals • can be obtained. Furthermore, switchable optical elements as described in the previous embodiments, also can be advantageously used for reading/writing on multi- layer data carriers. In a multi- layer data carrier, multiple information layers are situated at different depths in the data carrier. When refocusing from one layer to the other, due to the difference in information layer depth, unwanted spherical wavefront aberration arises, which needs to be compensated. One way to achieve this is to change the convergence/divergence of the incoming beam using a mechanical actuator, for example moving a collimator lens in the device, which is relatively expensive. The problem can be solved by applying a switchable optical element as described in the present invention, such that focussing on the different information layer depths is possible. As a result, the root mean square value of the wavefront aberration can be reduced. It is to be noted that a similar effect can be obtained using different combinations of meniscus curvatures, since only a variation in lens power is required.

Another illustration of an optical system using optical elements suitable for UV radiation may be an UV-based lithography system, as shown schematically by way of example in Fig. 11. By way of example, a transmission lithography system is shown, although a reflection lithography system also may be used. The use of the optical elements of the present invention allows to use a transmission lithography system for radiation of the UV range, e.g. in the deep UV region, whereas previously typically reflection lithography systems needed to be used for deep UV or extreme UV. The latter therefore significantly broadens the design possibilities for the lithographic set-up, especially for deep UV and extreme UV. The lithography set-up 600 comprises a projection column accommodating a projection lens system 602, a mask holder 604 for accommodating a mask 605 and a substrate table 606 supporting a substrate holder 608 for accommodating a substrate 610. This may be any suitable substrate, such as a semiconductor substrate like e.g. Si, Ge InP or GaAs wafers. This substrate 610 is provided with a UV radiation sensitive layer, for example a photosensitive layer 612, on which a lithographic pattern must be imaged, e.g. by performing lithography on a number of adjacent areas on the substrate 610. The apparatus further includes an illumination system which is provided with an illumination source 614, a lens system 616, a reflector 618 and a condenser lens 620. The optical elements such as the projection lens system 602, the lens system 616 the reflector 618 and the condenser lens 620 all need to be UV transparent or UV reflective, e.g. deep UV transparent or reflective. The latter is obtained by using at least one optical element as described in any of the previous embodiments. Different types of illumination sources 614 can be used for UV lithography. Well known illumination sources 614 are the deep UV lines at 248 nm of a KrF laser, at

193 nm of an ArF laser and 157 nm of a fluorine laser, having a typical energy delivered at the wafer surface of 20 mJ/cm2. KrF excimer lasers are commercially available from e.g. Cymer Inc., Lambda Physik or Komatsu. Examples of other illumination sources 614 that can be used are a frequency-quadrupled neodymium yttrium-aluminium-garnet (YAG) laser or a frequency-doubled copper vapour laser. In operation, the projection beam supplied by the illumination system 614 illuminates the pattern of the mask. This pattern then is imaged on the substrate 610 by the projection lens system 616. Other typical features of lithography systems such as - but not limited to - e.g. controlling features for optimising outlining, also may be present. The use of UV radiation, especially deep UV radiation in optical lithography allows to cope with the high resolution demands to date in semiconductor processing.

During lithographic processing, typically a substrate covered with a photosensitive layer is subjected to an ultraviolet light beam. The ultraviolet light beam thereby comprises information on the pattern to be obtained on the substrate. The ultraviolet light beam interacts with the photosensitive and thereby changes the properties of certain parts of the photosensitive layer, in agreement with the pattern to be obtained. After illuminating, the photosensitive layer is developed whereafter, typically by etching, the developed material or the undeveloped material is selectively removed. Using UV radiation, especially deep UV radiation, allows to obtain high-resolution lithography. Optical systems as described above, such as e.g. lithographic systems, mastering tools or reading/writing systems, may e.g. use a fluid mirror as described in the second and third embodiment. The optical element may e.g. be used in combination with a polarizing beam splitter. Radiation with a first polarization direction then is typically directed through the beam splitter, changed by a quarter wavelength retarder, reflected by a fluid mirror thus influencing the convergence of the radiation, changed again by the quarter wavelength retarder and reflected by the polarizing beam splitter. In this way radiation with a specific polarization and with good convergence properties is obtained for further use in the optical system.

Optical elements, especially optical mirrors, as described in the previous embodiments, can be advantageously used in UV laser applications. Part of a typical UV laser is shown in Fig. 12. A laser cavity 700 is shown comprising a first mirror 702 and a second mirror 704, whereby at least mirror 704 is a fluid mirror as described in any of embodiments 2 or 3 or the present invention. The partially transmissive mirror 704 allows the partial light outcoupling from the gain medium 706 to outside the laser cavity 700. As both fluids 104, 106 of the partially transmissive mirror 704 are UV transparent, e.g.' deep ultraviolet transparent, the part of the light that is coupled out through the mirror 704 is not reduced substantially in intensity anymore after passing the mirror meniscus 114. Furthermore, if mirror 704 is an adjustable UV mirror, adjusting the shape or position of the meniscus 114 may be used to provide the desired optical resonance mode. The effect of the curvature upon the resonance mode has been described extensively by Kogelnik and Li in

Applied Optics 5 (1966) pi 550-1567 and in "Lasers" by Siegman, University Science Books, Mill Valley, California, Chapter 19. Typical UV lasers are e.g. - but not limited to - gas lasers based on N2, Ar, Kr, diode pumped solid state lasers, etc.

The optical elements described in embodiments 1 to 3 of the present invention can also be used in image capturing devices including a lens e.g. for microscopy, for telescopes and for optics in cameras used for e.g. photoluminescence studies based on UV- radiation, UV-emission studies, etc. If variable optical elements such as variable lenses or variable mirrors are provided, in which the shape of the lens can easily be adjusted by controllably altering the shape of the meniscus between the two fluids, no mechanical elements are required within the optical path, such that the optical systems do not suffer from mechanical wear and tear. Further, the lens may be adjusted between having a positive power and a negative power.

Whereas the use of a gas as one of the fluids has been described for UV transparent optical elements such as fluid lenses, fluid mirrors and adjustable fluid lenses and mirrors and their application in different systems, the use of a gas in fluid optical elements such as fluid lenses, fluid mirrors and adjustable fluid lenses and mirrors for other regions of the electromagnetic (E.M.) spectrum such as e.g. but not limited to visible light and infrared light, and their application is different systems is also disclosed by the present invention. If transparency is needed, e.g. in lenses, the gas used then typically is transparent in the regions of the E.M. spectrum wherein the optical elements are used.

It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention. For example, although the meniscus 114 of the optical elements suitable for UV radiation has been indicated as being curved, generally symmetrical with respect to the optical axis and generally perpendicular to the optical axis at the point at which it crosses the optical axis, it will be appreciated that, depending upon the desired optical function to be performed by the i meniscus, any or all of these conditions can be changed. Although the lens chambers shown in the present examples typically are cylindrical, other shapes such as e.g. - but not limited to — conical shapes also can be used. Furthermore, although in the above embodiments and examples optical elements are described using two fluids, the number of fluids can be higher. By way of example, the optical elements could use three fluids.

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Classifications
International ClassificationG02B3/14, G02B13/14, G02B26/02, G03F7/20
Cooperative ClassificationG02B26/0816, G02B13/143, G02B26/0825, G02B26/005, G03F7/70958, G02B3/14
European ClassificationG03F7/70P10B, G02B26/00L1, G02B13/14B, G02B26/08M, G02B3/14, G02B26/08M2
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