METHOD AND DEVICE FOR THE POST-TREATMENT OF A FLUORIDE LAYER FOR AN OPTICAL SYSTEM FOR THE VUV WAVELENGTH RANGE, AND OPTICAL ELEMENT COMPRISING SAID FLUORIDE LAYER

Abstract

A method for aftertreating a fluoride layer for an optical element for the use in the VUV wavelength range, which method comprises irradiating the fluoride layer with UV/VUV radiation in the presence of an active fluorination agent. An optical element with a fluoride layer is aftertreated using this method. An optical arrangement has at least one such optical element.

Claims

1. A method, comprising: irradiating a fluoride layer of an optical element with UV/VUV radiation in the presence of an active fluorination agent, wherein the active fluorination agent comprises at least one member selected from the group consisting of F.sub.2, HF, XeF.sub.2, NF.sub.3, CF.sub.4 and SF.sub.6.

2. The method of claim 1, wherein the UV/VUV radiation has a first spectral range comprising a wavelength having an energy at least equal to a dissociation energy of the active fluorination agent.

3. The method of claim 2, wherein a highest energy of the first spectral range is at most 100% greater than the dissociation energy of the active fluorination agent.

4. The method of claim 3, wherein the highest energy of the first spectral range is at most a band gap energy of the fluoride layer.

5. The method of claim 2, wherein a highest energy of the first spectral range is at most a band gap energy of the fluoride layer.

6. The method of claim 2, wherein the UV/VUV radiation comprises a second spectral range in an energy range of between 75% and 100% of a band gap energy of the fluoride layer.

7. The method of claim 1, wherein the UV/VUV radiation comprises a spectral range in an energy range of between 75% and 100% of a band gap energy of the fluoride layer.

8. The method of claim 1, wherein the fluoride layer comprises AlF.sub.3.

9. The method of claim 1, wherein the fluoride layer is an AlF.sub.3 layer.

10. The method of claim 1, wherein: the UV/VUV radiation or further electromagnetic radiation additionally used to irradiate the fluoride layer comprises a spectral range to anneal a crystal defect of the fluoride layer; and the spectral range at least partly overlaps with an absorption range of the at least one crystal defect.

11. The method of claim 1, wherein the fluoride layer is irradiated in a protective gas atmosphere.

12. The method of claim 1, wherein, when irradiating the fluoride layer, an oxygen concentration is less than 10 parts per million by volume.

13. The method of claim 1, wherein, when irradiating the fluoride layer, an H.sub.2O concentration is less than 10 parts per million by volume.

14. The method of claim 1, wherein, when irradiating the fluoride layer, a partial pressure of the active fluorination agent is between 0.05 parts per million by volume (ppm V) and 10.sup.6 ppm V.

15. The method of claim 1, wherein, when irradiating the fluoride layer: an oxygen concentration is less than 10 parts per million by volume (ppm V); an H.sub.2O concentration is less than 10 ppm V; and a partial pressure of the active fluorination agent is between 0.05 ppm V and 10.sup.6 ppm V.

16. The method of claim 1, wherein, when irradiating the fluoride layer, a partial pressure of the active fluorination agent is adjusted to a target value.

17. The method of claim 1, wherein, when irradiating the fluoride layer, the fluoride layer is heated.

18. An optical element, comprising: a fluoride layer prepared using the method of claim 1.

19. An optical arrangement, comprising: an optical element prepared using the method of claim 1, wherein the optical arrangement is a VUV lithography apparatus or a VUV wafer inspection system.

20. A device, comprising: an aftertreatment chamber; a supply unit configured to supply inert gas and an active fluorination agent into the aftertreatment chamber, an inner side of the aftertreatment chamber being resistant to the active fluorination agent and its conversion products; and a UV/VUV radiation source configured so that, when an optical element comprising a fluoride layer is present in the aftertreatment chamber, the UV/VUV radiation source irradiates the fluoride layer with UV/VUV radiation in the presence of the active fluorination agent in the aftertreatment chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Exemplary embodiments are shown in the schematic drawings and are explained in the description which follows.

[0069] FIG. 1 shows a schematic illustration of the aftertreatment of a fluoride layer for an optical element for the use in the VUV wavelength range using the aftertreatment method according to the disclosure.

[0070] FIG. 2 shows a schematic illustration of the absorption and spectral ranges relevant to the irradiation of the fluoride layer using the aftertreatment method.

[0071] FIG. 3 shows a schematic illustration of a device for aftertreating a fluoride layer for an optical element, which is designed for use in the VUV wavelength range.

[0072] FIG. 4 shows a schematic illustration of an optical arrangement for the VUV wavelength range in the form of a VUV lithography apparatus.

[0073] FIG. 5 shows a schematic illustration of an optical arrangement for the VUV wavelength range in the form of a wafer inspection system.

[0074] FIG. 6 shows a schematic illustration of an optical element with a fluoride layer, which was aftertreated using the method according to the disclosure.

DETAILED DESCRIPTION

[0075] FIG. 1 shows a schematic illustration of the aftertreatment of a fluoride layer 1 of an optical element 2 for use in the VUV wavelength range. The fluoride layer 1 of the optical element 2 that is applied to a substrate 3 is depicted at three snapshots M1, M2, M3, before the aftertreatment (snapshot M1), during the aftertreatment (snapshot M2) and after the aftertreatment (snapshot M3), with only a small portion of a surface 4 of the fluoride layer 1 facing the environment being shown in each case.

[0076] Prior to the aftertreatment (first snapshot M1), an oxyfluoride or hydroxyfluoride or a mixture of both is present on the surface 4 of the fluoride layer 1 and along grain boundaries 5 of the fluoride layer 1 on account of the exposure of the fluoride layer 1 e.g. to ambient air. For example, the fluoride layer 1 is superficially oxidized due to the saturation of previously unsaturated bonds and has defect-rich grain boundaries 5 with unsaturated bonds and/or bonds saturated by O or OH from the atmosphere. These at least partially oxidized or defect-rich regions 6 can have an undesirable effect on the optical performance of the optical element 1. By contrast, a stoichiometric fluoride is typically present in the grain volume 7.

[0077] For the aftertreatment, the fluoride layer 1 or the substrate 3 with the fluoride layer 1 applied thereto is initially transferred into an aftertreatment chamber, which is not depicted in FIG. 1. Subsequently, as shown in the second snapshot M2, the fluoride layer 1 is irradiated with UV/VUV radiation 8 in the presence of an active fluorination agent FW. As a consequence of the irradiation, the active fluorination agent FW dissociates and forms fluorine species F, F.sub.2, F*. The fluorine species F, F.sub.2, F* react with the at least partially oxidized or defect-rich regions 6, and a fluoride is formed there. By way of example, the active fluorination agent FW is NF.sub.3, but it may also be a different substance that is able to provide the fluorine species F, F.sub.2, F* by way of photodissociation, for example at least one substance from the group comprising: F.sub.2, HF, XeF.sub.2, CF.sub.4 and SF.sub.6.

[0078] Furthermore, the fluoride layer 1 is irradiated by further electromagnetic radiation 9 during the aftertreatment in the example illustrated, but this is not mandatory. This serves the annealing of crystal defects 10 in the fluoride layer 1.

[0079] Moreover, the fluoride layer 1 may additionally be heated during the irradiation, although this is not depicted in FIG. 1. However, heating is not a mandatory constituent of the method.

[0080] The at least partially oxidized or defect-rich regions 6 of the fluoride layer 1 are refluorinated following the aftertreatment (third snapshot M3). Now, an (at least approximately) stoichiometric fluoride is present there as well. As a consequence, the optical performance of the optical element 2 is significantly improved and comparatively stable in relation to environmental influences.

[0081] FIG. 2 illustrates the absorption and spectral ranges relevant to the irradiation of the fluoride layer 1. Energy is plotted on the abscissa axis, and the absorption cross section is plotted on the ordinate axis. The dissociation energy E.sub.diss of the active fluorination agent FW, the absorption cross section 12 of the fluoride layer 1, including an Urbach tail 12, and the absorption cross section 13 of a crystal defect 10 in the fluoride layer 1 are depicted schematically.

[0082] The VUV radiation 8 used to irradiate the fluoride layer 1 has a first spectral range 14 for the photodissociation of the active fluorination agent FW. By way of example, the first spectral range 14 comprises at least one wavelength whose energy E.sub.ph is at least equal to the dissociation energy E.sub.diss of the active fluorination agent FW. Further, the highest energy E.sub.UP of the first spectral range 14 is here by way of example, but not necessarily, less than 50% greater than the dissociation energy E.sub.diss of the active fluorination agent FW. This suppresses potentially negative and/or competing effects. The highest energy E.sub.UP of the first spectral range 14 may also be no more than the band gap energy E.sub.G of the fluoride layer 1, such as no more than 75% of the band gap energy E.sub.G of the fluoride layer 1.

[0083] Moreover, the fluoride layer 1 is irradiated with further electromagnetic radiation 9 by way of example for annealing at least one crystal defect 10 in the fluoride layer 1. To this end, the further electromagnetic radiation 9 includes a spectral range 16 that overlaps with the absorption range 17 of the at least one crystal defect 10. In the illustrated example, the spectral range 16 of the further electromagnetic radiation 9 lies within the absorption range 17 of the crystal defect 10, which is an F center; however, this is not necessarily required. In an alternative to that, the UV/VUV radiation 8 may include a corresponding spectral range.

[0084] In the illustrated example, the spectral range 16 of the further electromagnetic radiation 9 comprises the absorption energy EA of the crystal defect 10 at which the absorption cross section is maximal. However, this is not necessarily the case. The absorption range 17 of the crystal defect 10 is defined by a drop to one hundredth of the maximum value of the absorption cross section (FWHM) at the absorption energy E.sub.A of the crystal defect 10. Further, it is desirable if a mean energy E.sub.m of the spectral range 16 deviates from the absorption energy E.sub.A of the crystal defect 10 by no more than 0.5 eV, for example by no more than 0.25 eV.

[0085] Moreover, the UV/VUV radiation 8 includes a second spectral range 18 for mobilizing atoms at the surface 4, at the grain boundaries 5 and/or in the grain volume 7 of the fluoride layer 1. In the illustrated example, this second spectral range 18 lies in an energy range of between 75% and 100% of the band gap energy E.sub.G of the fluoride layer 1. The second spectral range 18 may also lie between 80% and 95% of the band gap energy E.sub.G of the fluoride layer 1.

[0086] FIG. 3 shows a device 60 for aftertreating the fluoride layer 1 for the optical element 2 of FIG. 1 using the above-described aftertreatment method. The device 60 comprises an aftertreatment chamber 61, a supply unit 62 and a first UV/VUV radiation source 63.

[0087] The optical element 2 that comprises the fluoride layer 1, which is applied to a substrate 3 in exemplary fashion in this case, is attached to a substrate holder 64, which is rotatable about a rotation axis 65, within the aftertreatment chamber 61. However, deviating from the example illustrated here, the device 1 need not comprise a rotatable substrate holder 64.

[0088] The supply unit 62 serves to supply protective gas in the form of inert gas IG and the active fluorination agent FW into the aftertreatment chamber 61, the supply unit 62 comprising a first valve 66 for controlled supply of the inert gas IG and a second valve 67 for controlled supply of the active fluorination agent FW. The second valve 67 is a controllable metering valve. As a consequence, the fluoride layer 1 may be irradiated in the presence of the active fluorination agent FW in a protective gas atmosphere within the aftertreatment chamber 61. The device 60 moreover comprises a gas outlet 68 for letting out the inert gas IR and reaction products formed during the aftertreatment. In the example illustrated, the inert gas IR is argon, but it is also possible to use other inert gases IR, for example other light noble gases such as helium or neon. Mixtures of noble gases, for example of the noble gases mentioned, can also be used as inert gas IR.

[0089] The first UV/VUV radiation source 63 serves to irradiate the fluoride layer 1 with UV/VUV radiation 8 in the presence of the active fluorination agent FW in the aftertreatment chamber 61. By way of example, in the illustrated example, the UV/VUV radiation 8 enters the aftertreatment chamber 61 through an MgF.sub.2 window 69. The first UV/VUV radiation source 63 serves to generate UV/VUV radiation 8 in the first spectral range 14 described further above.

[0090] Moreover, by way of example, the device 60 in this case comprises, but not necessarily, a second UV/VUV radiation source 70 for irradiating the fluoride layer 1 with UV/VUV radiation 8 in the second spectral range 18, described further above, for mobilizing atoms at the surface 4, at the grain boundaries 5 and/or in the grain volume 7 of the fluoride layer 1. In the illustrated example, the UV/VUV radiation from the second UV/VUV radiation source 8 enters the aftertreatment chamber 61 through an MgF.sub.2 window 69. The device 60 also comprises a further radiation source 71 for irradiating the fluoride layer 1 with further electromagnetic radiation 9 in the spectral range 17, described further above in the context of FIG. 2, for annealing at least one crystal defect 10 in the fluoride layer 1. The further electromagnetic radiation 9 from the further radiation source 71 enters the aftertreatment chamber 71 through a further MgF.sub.2 window 69.

[0091] In place of at least one of the MgF.sub.2 windows 69, 69, 69, it is possible in principle to also use windows made of other materials, for example from CaF.sub.2, SrF.sub.2 and/or BaF.sub.2, with sufficient transparency at the utilized wavelengths being decisive in this respect.

[0092] The aftertreatment chamber 61 may be sealed in a gas-tight manner. Furthermore, the inner side 72 of the aftertreatment chamber 61 is resistant to the active fluorination agent FW and its conversion products. For this purpose, in the example illustrated, the aftertreatment chamber 61, at least on its inner side 72, is formed from a metal in the form of Monel steel, which forms a passivating layer in order to prevent corrosion. In principle, the aftertreatment chamber 61 can also be formed from other corrosion-resistant metals if the latter are free of Cr and Ti.

[0093] Alternatively, a corrosion-resistant coating, e.g. composed of NiP, Pt or Ru/Rh mixtures, can be applied to the inner side 72 of the aftertreatment chamber 61. The corrosion-resistant coating can be applied to the inner side 72 of the aftertreatment chamber 61 via a galvanic process, for example. The components which are arranged in the aftertreatment chamber 61 and which come into contact with the active fluorination agent FW are likewise resistant to the active fluorination agent FW and the conversion products thereof.

[0094] Further the device 60 depicted here comprises, by way of example, but not necessarily, a sensor 73 for measuring the oxygen concentration c.sub.O2 in the aftertreatment chamber 61 and a further sensor 74 for measuring the H.sub.2O concentration c.sub.H2O in the aftertreatment chamber 61.

[0095] By way of example, the oxygen concentration coz in the aftertreatment chamber 61 is less than 50 ppb V during the irradiation of the fluoride layer 1. The oxygen concentration c.sub.O2 should be as low as possible, although it may also be greater than 50 ppbV. However, it is desirable for the oxygen concentration coz to be less than 10 ppmV, such as less than 1 ppm V, for example less than 100 ppb V.

[0096] Further, in the illustrated example, the H.sub.2O concentration c.sub.H2O in the aftertreatment chamber 61 is less than during the irradiation of the fluoride layer 1 In principle, the H.sub.2O concentration c.sub.H2O in the aftertreatment chamber 61 during the irradiation of the fluoride layer 1 should be as low as possible, although the H.sub.2O concentration c.sub.H2O may also be greater than 100 ppbV. However, it is desirable for the H2O concentration c.sub.H2O to be less than 10 ppm V, such as less than 1 ppm V, for example less than 500 ppb V.

[0097] The device 60, in the illustrated example, but not necessarily, moreover comprises a sensor 75 for measuring the partial pressure c.sub.FW of the active fluorination agent FW in the aftertreatment chamber 61 and a closed-loop controller 76 for controlling the partial pressure c.sub.FW of the active fluorination agent FW in the aftertreatment chamber 61 to a target value, the control being implemented using the actual measured value M from the sensor 75 for measuring the partial pressure c.sub.FW of the active fluorination agent FW in the aftertreatment chamber 61 and via the control of the second valve 67. The sensor 75 may be designed only to measure the partial pressure c.sub.FW of the active fluorination agent FW; however, it may also be a residual gas analyzer that is able to also determine the partial pressures of other gases contained in the aftertreatment chamber 21. It is possible that such a residual gas analyzer assumes the function of the three sensors 73, 74, 75 depicted in FIG. 3. If the second valve 67 is a metering valve, for example a mass flow regulator, then it is possible to dispense with the use of the sensor 75 for measuring the partial pressure c.sub.FW of the active fluorination agent FW in the aftertreatment chamber 61.

[0098] The active fluorination agent FW is added to the inert gas IR in the supply unit 62. The partial pressure c.sub.FW of the active fluorination agent FW in the aftertreatment chamber 61 lies typically between 0.05 and 10.sup.6 ppmV, such as between 0.075 ppm V and 50 ppmV, for example between 0.1 ppmV and 10 ppmV, during the irradiation of the fluoride layer 1.

[0099] FIG. 4 shows an optical arrangement for the VUV wavelength range in the form of a VUV lithography apparatus 21. The VUV lithography apparatus 21 comprises two optical systems, namely an illumination system 22 and a projection system 23. The VUV lithography apparatus 21 additionally comprises a radiation source 24, which may be an excimer laser, for example.

[0100] The radiation 25 emitted by the radiation source 24 is conditioned with the aid of the illumination system 22 such that a mask 26, also called a reticle, is illuminated thereby. In the example shown, the illumination system 22 comprises a housing 32, in which both transmissive and reflective optical elements are arranged. In a representative manner, the illustration shows a transmissive optical element 27, which focuses the radiation 25, and a reflective optical element 28, which deflects the radiation.

[0101] The mask 26 comprises, on its surface, a structure which is transferred to an optical element 29 to be exposed, for example a wafer, with the aid of the projection system 23 for the purpose of producing semiconductor components. In the example shown, the mask 26 is designed as transmissive optical element. In alternative embodiments, the mask 26 may also be designed as reflective optical element.

[0102] The projection system 22 comprises at least one transmissive optical element in the example illustrated. The example shown illustrates, in a representative manner, two transmitting optical elements 30, 31, which serve, for example, to reduce the structures on the mask 26 to the size desired for the exposure of the wafer 29.

[0103] Both in the illumination system 22 and in the projection system 23, a wide variety of transmissive, reflective or other optical elements can be combined with one another as desired, including in a more complex manner. Optical arrangements without transmissive optical elements can also be used for VUV lithography.

[0104] FIG. 5 shows an optical arrangement for the VUV wavelength range in the form of a wafer inspection system 41, but this may also be a mask inspection system. The wafer inspection system 41 comprises an optical system 42 with a radiation source 54, from which the radiation 55 is directed onto a wafer 49 via the optical system 42. For this purpose, the radiation 55 is reflected off a concave mirror 46 to the wafer 49. In the case of a mask inspection system, it would be possible to replace the wafer 49 with a mask to be examined. The radiation reflected, diffracted and/or refracted by the wafer 49 is directed onto a detector 50 for further evaluation by a further concave mirror 48, which is likewise associated with the optical system 42, via a transmissive optical element 47. The wafer inspection system 41 additionally comprises a housing 52, in which the two mirrors 46, 48 and the transmissive optical element 47 are arranged. The radiation source 54 may, for example, be exactly one radiation source or a combination of a plurality of individual radiation sources in order to provide a substantially continuous radiation spectrum. In modifications, it is also possible to use one or more narrowband radiation sources 54.

[0105] At least one of the optical elements 27, 28, 30, 31 of the VUV lithography apparatus 21 shown in FIG. 5 and at least one of the optical elements 46, 47, 48 of the wafer inspection system 41 shown in FIG. 6 are designed here as described further above. The at least one of the optical elements 27, 28, 30, 31 thus comprises at least one fluoride layer that was aftertreated using the above-described method.

[0106] FIG. 6 shows an optical element 2 for the VUV wavelength range in the form of an Al mirror which comprises an Al layer 90 applied to a substrate 3 and which is protected by a fluoride layer 1 in the form of an AlF.sub.3 layer. The fluoride layer 1 was aftertreated using the above-described method. As a consequence, the reflectance of the optical element 2 is increased, and the degradation during the operation of the optical element 2 is reduced.