Method for treating a reflective optical element for the EUV wavelength range, method for producing same, and treating apparatus

Abstract

Treating a reflective optical element (104) for the EUV wavelength range that has a reflective coating on a substrate. The reflective optical element in a holder (106) is irradiated with at least one radiation pulse of a radiation source (102) having a duration of between 1 μs and 1 s. At least one radiation source (102) and the reflective optical element move relative to one another. Preferably, this is carried out directly after applying the reflective coating in a coating chamber (100). Reflective optical elements of this type are suitable in particular for use in EUV lithography or in EUV inspection of masks or wafers, for example.

Claims

1. A method for treating a reflective optical element for an extreme ultraviolet (EUV) wavelength range comprising a reflective coating on a substrate, comprising: irradiating the reflective optical element with at least one radiation pulse having a duration of between 1 μs and 1 s, wherein the irradiating is carried out with at least one radiation source and at a wavelength between 800 nm and 1100 nm; and moving the at least one radiation source and the reflective optical element relative to one another to perform a relative movement, wherein the relative movement is performed with a linear component having a velocity of between 1 cm/s and 600 cm/s adapted at least in part to achieve an artificial ageing of the reflective coating on the substrate with respect to a state of the reflective optical element directly after coating, wherein the reflective optical element includes a substrate and a plurality of stacks, each having an absorber-spacer pair, wherein saturated stress relaxation of the reflective optical element that was irradiated is approximately 250 MPa or wherein stress overcompensation of the reflective optical element that was irradiated is between −50 MPa to −150 MPa, and wherein compaction of at least one layer of the reflective optical element is approximately 0.1 to 1.5% relative to an original layer thickness.

2. The method according to claim 1, wherein the relative movement is further performed as a superimposition of at least one linear movement and at least one rotational movement.

3. The method according to claim 1, wherein the at least one radiation pulse is directed onto the reflective coating.

4. The method according to claim 1, wherein the irradiating is performed with at least one radiation pulse having an energy density of between 0.01 J/cm.sup.2 and 15 J/cm.sup.2.

5. A method for producing a reflective optical element for EUV lithography comprising a reflective coating on a substrate, comprising: coating the substrate; and treating the coated substrate according to the method claimed in claim 1.

6. The method according to claim 5, wherein the coating of the substrate is performed in a coating chamber, and wherein the treating is performed in the coating chamber.

7. The method according to claim 5, wherein both the coating and the treating are performed under vacuum or protective atmosphere.

8. The method according to claim 5, wherein the treating is performed directly after the coating.

9. The method according to claim 1, wherein the radiation source is operated with varying power depending on the relative movement.

10. The method according to claim 1, wherein the irradiating is performed with at least one radiation pulse having an energy density of between 0.01 J/cm.sup.2 and 5 J/cm.sup.2.

11. The method according to claim 1, wherein the irradiating is performed with at least one radiation pulse having a duration of between 250 μs to 2 ms.

12. The method according to claim 1, wherein the at least one radiation source is operated in a pulsed manner with a pulse duration between 100 μs and 500 μs and an energy density between 0.01 J/cm.sup.2 and 15 J/cm.sup.2, wherein the pulse duration and the energy density is set based on a desired stress relaxation of at least one layer of the optical element and wherein the energy density is set to increase homogenous ageing.

13. The method according to claim 1, wherein the velocity is between 10 cm/s to 200 cm/s.

14. The method according to claim 1, wherein the irradiation source has a radiation spot size of 1 mm.sup.2 to 1 cm.sup.2, is operated at a power of 5 W to 500 W, and passes through 1 to 1000 cycles in a pulsed operation at a repetition rate of approximately 0.1 Hz to 1000 Hz.

15. The method according to claim 1, wherein the velocity of the linear component is adapted at least in part to achieve densification of individual layers of the reflective optical element in comparison to the state of the reflective optical element directly after coating and the artificial ageing including density changes and saturated stress relaxation.

16. The method according to claim 1, wherein the velocity of the linear component is adapted at least in part to achieve one or more structural changes of the reflective optical element on account of interdiffusion at layer boundaries of the reflective optical element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be explained in more detail with reference to preferred exemplary embodiments. In the figures:

(2) FIG. 1 shows a schematic view of a coating chamber with an apparatus for treating a reflective optical element for the EUV wavelength range;

(3) FIG. 2 schematically shows a first embodiment of the apparatus from FIG. 1;

(4) FIG. 3A schematically shows a second embodiment of the apparatus from FIG. 1 in side view;

(5) FIG. 3B schematically shows a second embodiment of the apparatus from FIG. 1 in plan view rotated by 90°;

(6) FIG. 4 schematically shows a third embodiment of the apparatus from FIG. 1;

(7) FIG. 5A schematically shows the structure of a reflective optical element for the EUV wavelength range before the artificial ageing; and

(8) FIG. 5B schematically shows the structure of a reflective optical element for the EUV wavelength range after the artificial ageing.

DETAILED DESCRIPTION

(9) FIG. 1 illustrates a schematic view of a coating chamber 100 with an apparatus for treating a reflective optical element 104 for the EUV wavelength range. The apparatus comprises a holder 106, in which the reflective optical element 104 is mounted, and a radiation source 102, which are arranged in the coating chamber 100. In the example illustrated here, the holder 106 is provided with a linear drive 108, with the aid of which the holder 106 with the reflective optical element 104 can be linearly moved relative to the radiation source 102 in particular during short-pulse irradiation in order to be able to ensure for instance that the reflective coating of the reflective optical element 104 is treated as homogeneously as possible. In further examples not shown, the apparatus comprises at least one rotary drive instead of or in addition to the linear drive. The apparatus may also comprise more than one linear drives, eventually in combination with one or more rotary drives.

(10) Since holder 106 and radiation source 102 are arranged in the coating chamber 100, the short-pulse treatment can be carried out directly after the coating process by the coating device 110. The coating process is preferably carried out under vacuum, particularly if physical deposition methods such as, for instance, sputtering methods or electron beam evaporation are involved. In the method illustrated here, the linear drive 108 can be used to move the holder 106 with coated reflective optical element 104 from a coating position below the coating device 110 into an ageing position below the radiation source 102. In other variants, both positions could also be spatially identical and the radiation source 102 could be positioned by mechanical aids, for example, such that at least one radiation pulse is directed from said radiation source onto the previously applied reflective coating of the reflective optical element, or the coating device 110 is removed or shielded. Specifically, it is possible to use devices for homogenizing the coating process, for instance drives and mask arrangements, in order also to homogenize the radiation treatment over the area of the reflective coating. By virtue of the treatment by very short irradiation pulses also likewise being carried out in vacuum, the risk of contamination of the reflective coating by particles or oxidation can be reduced. In further variants, the irradiation treatment can also be carried out under protective atmosphere, for example on the basis of noble gas or other inert gases such as nitrogen.

(11) FIG. 2 schematically illustrates a first embodiment of an apparatus for treating reflective optical elements for the EUV wavelength range with short-pulse irradiation. It can for example be arranged within a coating chamber or be situated by itself. Preferably, even then it is arranged in a chamber in order to carry out the irradiation under vacuum conditions or protective gas atmosphere. The apparatus comprises a radiation source 202 and a holder 206, in which an EUV mirror 204 is arranged, the reflective coating 214 of which is intended to be treated. In order to carry out the treatment, the EUV mirror 204 is irradiated with at least one radiation pulse having a duration of between 1 μs and 1 s, wherein the at least one radiation pulse is directed onto the reflective coating 214. During the irradiation, the holder 206 performs a rotational movement by a rotary drive (not illustrated), such that radiation source 202 and EUV mirror 204 perform a relative movement with respect to one another, which can contribute in particular to increasing the homogeneity of the density change and/or relaxation over the entire area of the reflective coating 214. By way of example, the rotational movement can be continuous or be synchronized with the repetition rate in the case of more than one radiation pulse, such that the area assumes a different position for each pulse. If e.g. the entire area of the reflective coating 214 is irradiated during each pulse, then it is possible to average out intensity fluctuations over the area.

(12) In the case of particularly large reflective optical elements, the relative movement can have at least one linear component, if appropriate in addition to one or more rotational components, in order to be able to irradiate the entire area of the reflective coating as homogeneously as possible. In this case, a different partial area of the reflective coating can be irradiated during each pulse.

(13) In the example illustrated in FIG. 2, a flash tube, preferably on the basis of xenon or krypton is used as the radiation source 202. If they are operated at power densities of approximately 3000 W/cm.sup.2, they emit in particular a line spectrum in the range of approximately 800 nm to 1000 nm for krypton flash lamps or approximately 900 nm to 1100 nm for xenon flash lamps. If they are operated for example at a triple power density, they emit a continuous spectrum of from approximately 160 nm to 200 nm high up to approximately 1000 nm to 1100 nm. In the example illustrated here, they are operated with pulse durations of between 100 μs and 500 μs and energy densities of between approximately 0.01 J/cm.sup.2 and approximately 15 J/cm.sup.2. This corresponds to an energy input per pulse into the reflective coating 214 which corresponds to temperatures of between approximately 200° C. and 1500° C. Depending on the constitution of the reflective coating 214 and the desired compaction or stress relaxation, one, a plurality or a multiplicity of pulses can be employed. In this case, the repetition rates in the example illustrated here are between approximately 0.1 Hz and 1000 Hz, if irradiation is carried out using more than one radiation pulse. The wavelength range and the energy density of the radiation 212 are also advantageously coordinated with the constitution of the reflective coating 214 and the desired density change and/or relaxation. When choosing the wavelength range, it is additionally advantageous also to take account of the material of the holder 206. Since at least the holder surface 216 may likewise be exposed to the radiation 212, wavelength ranges are preferred in which the holder material has a high reflection rate, while the reflective coating 214 has a high absorption rate. If what is involved is for example a reflective coating 214 on the basis of a molybdenum-silicon multilayer system, such as is widely used for reflective optical elements for EUV lithography or EUV inspection, and a metallic holder, preferably composed of high-grade steel and particularly preferably composed of aluminium, a wavelength range of between approximately 800 nm and 1100 nm is well suited.

(14) In the exemplary embodiment illustrated in FIGS. 3A and 3B of an apparatus for artificial ageing comprising radiation source 302 and holder 306 for receiving an EUV mirror 304, the radiation source 302 is configured as a linear arrangement of a plurality of lasers (see plan view in FIG. 3B). This laser arrangement 302 performs a linear movement relative to the EUV mirror 304 by a linear drive (not illustrated). In this way the laser arrangement 302 moves across the reflective coating 314 to be compacted in one direction. Variants that are not illustrated can also involve, inter alia, one or a few lasers, the beam spot of which is correspondingly fanned out, or one laser or a plurality of lasers by which the reflective coating to be irradiated is scanned over the entire area thereof, by two linear drives for respectively different directions, for example. The lasers used can have a wavelength of the emitted radiation 312 of between approximately 160 nm and approximately 2000 nm. Higher wavelengths are rather unsuitable in particular with regard to metallic holders 306, which would absorb too much heat in particular via the holder surface 316 facing the laser arrangement 302 and could thus bring about macroscopic deformations and/or additional stresses in the EUV mirror 304.

(15) In the example illustrated here, lasers are used which have a radiation spot size of between approximately 1 mm.sup.2 and approximately 1 cm.sup.2, are operated at a power of approximately 5 W to approximately 500 W per laser and can pass through 1 to 1000 cycles in pulsed operation, at repetition rates of approximately 0.1 Hz to 1000 Hz. In the present example, the laser arrangement 302 can be moved at a velocity of approximately 5 cm/s to 500 cm/s, preferably 10 cm/s to 200 cm/s. The velocity is preferably adapted to the current beam spot size, the area size of the reflective coating 314, the pulse length, the laser power and to the constitution of the reflective coating 314 and the desired ageing process. At very high repetition rates, for the coordination of the individual parameters it is possible to take account of an effective pulse length as a measure of how long the radiation spot stays on a partial area of comparable size of the reflective coating 314. On account of the curved surface of the EUV mirror 304 it is moreover particularly advantageous, in a manner synchronized with the velocity of the laser arrangement 302 and the relative position of the individual lasers of the arrangement relative to the reflective coating 314, to adapt the power of the individual lasers, for example to set the power to be lower in the case of a relatively small distance with respect to the area of the reflective coating 314 currently being acquired by the beam spots, and higher in the case of a relatively large distance, in order to achieve as homogenous ageing as possible. Particularly preferred types of laser are solid-state lasers and diode lasers, in particular surface emitters such as so-called VCSEL (vertical cavity surface emitting laser) and VECSEL (vertical external cavity surface emitting laser).

(16) In the embodiment illustrated in FIG. 4 of an apparatus for treating a reflective optical element 404 with holder 406 and radiation source 402 the relative movement between radiation source 402 and reflective optical element 404 is comprised of a superimposition of linear and rotational movements which can be performed by linear and rotary drives (not illustrated) optionally at the holder 406 and/or at the radiation source 402. Thus, even reflective optical elements 404 having asymmetrical surface profiles such as, for example, a chamfer as in the example illustrated in FIG. 4 or other arbitrary freeform profiles can be impinged on by radiation 412 as homogeneously as possible. It should incidentally be pointed out that the geometric arrangement of the apparatus in space is arbitrary. In this regard, in the present example said arrangement is such that the holder 406 is arranged above the radiation source 402, while in the examples explained above the radiation sources are arranged above the respective holders. Other arrangements are also possible, if appropriate with optical elements that deflect the radiation, as long as in particular the reflective coating is impinged on by the radiation.

(17) A reflective optical element for the EUV wavelength range and in particular for EUV lithography or EUV inspection will be explained schematically and by way of example with reference to FIGS. 5A and 5B. FIG. 5A shows the reflective optical element 504 before irradiation, and FIG. 5B after irradiation.

(18) The reflective optical element 504 comprises a multilayer system 524 on a substrate 540. Typical substrate materials for reflective optical elements for EUV lithography or EUV inspection are for example silicon, silicon carbide, silicon-infiltrated silicon carbide, quartz glass, titanium-doped quartz glass and glass ceramic. Furthermore, the substrate can also be composed of metal, for instance copper, aluminium, a copper alloy, an aluminium alloy or a copper-aluminium alloy.

(19) The multilayer system 504 comprises alternatingly applied layers of a material having a higher real part of the refractive index at the operating wavelength at which for example the lithographic exposure is carried out (also called spacer 541) and of a material having a lower real part of the refractive index at the operating wavelength (also called absorber 542), wherein an absorber-spacer pair forms a stack 543. In certain respects a crystal is thereby simulated whose lattice planes correspond to the absorber layers at which Bragg reflection takes place. Reflective optical elements for instance for optical systems for example for an EUV lithography apparatus, an EUV inspection system for wafers or masks or some other optical application are usually designed in such a way that the respective wavelength of maximum peak reflectivity substantially corresponds to the operating wavelength of the lithography or inspection process or other applications of the optical system.

(20) The thicknesses of the individual layers 541, 542 and also of the repeating stacks 543 can be constant over the entire multilayer system 504 or else vary over the area or the total thickness of the multilayer system 504 depending on what spectral or angle-dependent reflection profile or what maximum reflectivity at the operating wavelength is intended to be achieved. The reflection profile can also be influenced in a targeted manner by the basic structure composed of absorber 542 and spacer 541 being supplemented by further more and less absorbent materials in order to increase the possible maximum reflectivity at the respective operating wavelength. To that end, in some stacks absorber and/or spacer materials can be mutually interchanged or the stacks can be constructed from more than one absorber and/or spacer material. Furthermore, it is also possible to provide additional layers as diffusion barriers at the transition from spacer to absorber layers 541, 542 and/or at the transition from absorber to spacer layer 542, 541. With the aid of diffusion barriers, as is known, the reflectivity, even over relatively long periods of time or under the influence of heat, of real multilayer systems can be increased by the reduction of the effect of the density change on account of structural change. Moreover, optionally it is possible to provide a protective layer 544, which can also be of multilayer design, on that side of the multilayer system 524 which faces away from the substrate 540, and also a substrate protective layer 545 between the multilayer system 524 and the substrate 540, which substrate protective layer inter alia can protect the substrate 540 against radiation damage and/or can compensate for layer stresses exerted by the multilayer system 524. These layers 544 and 545 can also be influenced by the treatment.

(21) Most EUV lithography apparatuses or EUV inspection systems are designed for an EUV wavelength of approximately 13.5 nm. A material combination that is customary for example for an operating wavelength of approximately 13.5 nm is molybdenum as absorber material and silicon as spacer material. In this case, in conventional reflective optical elements a stack 55 often has a thickness of ideally approximately 6.7 nm, that is to say half the operating wavelength, wherein the spacer layer 56 is usually thicker than the absorber layer 57. They typically have around 50 stacks.

(22) The reference numerals in FIG. 5B have the same meaning as in FIG. 5A, whereby the elements provided with an apostrophe have a different thickness due to irradiation. The thickness of a spacer layer 541, 541′ is designated by d.sub.B or, after irradiation, by d′.sub.B, and that of an absorber layer 542, 542′ by d.sub.A or d′.sub.A. In the majority of chemical and/or physical vapour deposition methods, the applied layers have a structure which has a lower density than that of the corresponding solid composed of the respective material, wherein the actual density can depend on the respective coating method. Under the thermal load during the operation of the reflective optical elements in an EUV lithography apparatus or an EUV inspection system, a densification takes place that can lead to a reduction of the layer thicknesses. The wavelengths of maximum reflectivity with a constant angle of incidence also shift with a change in the layer thickness and hence also stack thickness. Furthermore, wave front aberrations can occur as a result of figure changes. At least partially anticipating the ageing process in the form of a density change, it is possible to reduce drifting of the wavelength of maximum reflectivity and occurrence of wave front aberrations as a result of density change. Since the thermal load is higher for mirrors of an EUV lithography apparatus or of an EUV inspection system that are arranged further at the front in the beam path compared with mirrors arranged further at the back, the correspondingly different drifting can lead to losses of reflectivity.

(23) It should be pointed out that possible structural changes on account of interdiffusion at the layer boundaries are not illustrated in FIG. 5A or 5B, for the sake of better clarity. The interdiffusion is to a first approximation proportional to the root of the time since production of the coating. In the case of molybdenum-silicon multilayer systems, molybdenum silicide, which has a higher density than molybdenum or silicon, would form at the layer boundaries. Consequently, densifications in the individual layers and densifications as a result of the formation of molybdenum silicide can combine to form the phenomenon of compaction.

(24) Moreover, as a result of a short-pulse radiation treatment that operates at the lower end of the energy input into the reflective coating per pulse, it is possible primarily to bring about a relaxation of the layer stress that forms when a multiplicity of layers are applied. To that end, the radiation power can be set to be so low that hardly any densification takes place, but layer stresses resulting from the coating process and the different layer materials relax. It is thus possible to achieve a targeted tensile layer stress change and to dispense with other stress-reducing measures such as additional stress-compensating coatings, for instance, which for their part lead to higher total thicknesses of the coating and aberration faults associated therewith.

(25) Depending on the setting of the radiation power over the pulse duration, if appropriate repetition rate, radiated area, energy density of the radiation source and wavelength or wavelength range, during the artificial ageing it is possible to achieve a compaction for which the thickness of a compacted layer is 0.05% to 0.5% less than the thickness of the still uncompacted layer. When applying the coating, it is possible to provide a corresponding margin for the individual layers.

(26) Particularly in the case of reflective optical elements for the EUV wavelength range, the short-pulse treatment described here has made it possible to achieve the effect that the drift of the wavelength of maximum reflectivity after treatment and during operation over a number of months in an EUV lithography apparatus or an EUV inspection system can be limited to a deviation from the thickness set during the treatment at less than 0.2%.

(27) It was observed that the change in the layer stress after a number of radiation pulses, said number being dependent on the individual case, reaches saturation, such that appreciable changes in the layer stress were no longer measured even in later long-term operation of the reflective optical element for example in an EUV lithography apparatus or an EUV inspection system. The stress at which saturation is reached depends in particular on the energy dose that is input. In the case of compaction, by contrast, given a plurality of constant pulses, a dependence on the root of the number of pulses was observed, such that the irradiation makes it possible to achieve the effect that a subsequent compaction during long-term operation is below a defined tolerance limit.

(28) Reflective optical elements for EUV lithography or EUV inspection comprising reflective coatings comprising a multilayer system as described in association with FIGS. 5A and 5B were subjected to various short-pulse radiation treatments.

(29) A first reflective optical element was irradiated by a xenon flash lamp having a continuous spectrum of between 900 nm and 1100 nm with 10 pulses each having a duration of 300 μs and each having an energy density of 1.5 J/cm.sup.2. This achieved a compaction of approximately 0.1% relative to the original layer thicknesses and a saturated stress relaxation at a layer stress of approximately 250 MPa.

(30) A second reflective optical element was irradiated by a krypton flash lamp having a continuous spectrum of between 800 nm and 1000 nm with 20 pulses each having a duration of 250 μs and each having an energy density of 1 J/cm.sup.2. Since the area of the reflective coating of the second reflective optical element was somewhat larger than the area illuminated by the krypton flash lamp, said reflective optical element was moved relative to the krypton flash lamp in order each time to irradiate a different area section and overall to achieve a homogenous irradiation over the entire area of the reflective coating. This likewise lead to a compaction of approximately 0.1% relative to the original layer thicknesses and a saturated stress relaxation at a layer stress of approximately 250 MPa.

(31) A third reflective optical element was irradiated by a xenon flash lamp having a continuous spectrum of between 900 nm and 1100 nm with 10 pulses each having a duration of 350 μs and each having an energy density of 5 J/cm.sup.2. This achieved a compaction of approximately 1.5% relative to the original layer thicknesses and a slight stress overcompensation of approximately −50 MPa.

(32) A fourth reflective optical element was irradiated by an NdYAG-laser at a wavelength of 106 nm, with a laser power of 20 W and a beam spot of 1 mm.sup.2. The beam spot swept over the reflective coating at a velocity of 100 cm/s and needed 100 passes, which were laterally offset in each case, in order to scan the entire area of the reflective coating. This corresponds to a power density of 2 kW/cm.sup.2 or an energy density of 2 J/cm.sup.2 for a respective duration of 1 ms. This achieved a compaction of approximately 0.2% relative to the original layer thicknesses and a slight stress overcompensation of approximately −100 MPa.

(33) A fifth reflective optical element was irradiated by an NdYAG-laser at a wavelength of 1064 nm, with a laser power of 50 W and a beam spot of 1 mm.sup.2. The beam spot swept over the reflective coating at a velocity of 50 cm/s and needed 100 passes, which were laterally offset in each case, in order to scan the entire area of the reflective coating. This corresponds to a power density of 6 kW/cm.sup.2 or an energy density of 3 J/cm.sup.2 for a respective duration of 2 ms. This achieved a compaction of approximately 1.2% relative to the original layer thicknesses and a slight stress overcompensation of approximately −150 MPa.

REFERENCE SIGNS

(34) 100 Coating chamber

(35) 102 Radiation source

(36) 104 Reflective optical element

(37) 106 Holder

(38) 108 Linear drive

(39) 110 Coating device

(40) 202 Halogen flash lamp

(41) 204 EUV mirror

(42) 206 Holder

(43) 212 Radiation

(44) 214 Reflective coating

(45) 216 Holder surface

(46) 302 Short-pulse laser

(47) 304 EUV mirror

(48) 306 Holder

(49) 312 Radiation

(50) 314 Reflective coating

(51) 316 Holder surface

(52) 402 Radiation source

(53) 404 EUV mirror

(54) 406 Holder

(55) 412 Radiation

(56) 414 Reflective coating

(57) 504, 504′ Reflective optical element

(58) 524, 524′ Reflective coating

(59) 540 Substrate

(60) 541, 541′ Spacer

(61) 542, 542′ Absorber

(62) 543, 543′ Stack

(63) 544 Protective layer

(64) 545 Substrate protective layer

(65) d.sub.A, d′.sub.A Thickness

(66) d.sub.B, d′.sub.B Thickness