DE102021200490A1 - Method for forming a protective layer, optical element and optical arrangement - Google Patents

Abstract

The invention relates to a method for forming a protective layer (6) on a reflective optical element (1) for the VUV wavelength range, the reflective optical element (1) having a substrate (2) on which a metal layer (3) is formed , on whose side facing away from the substrate (2) a metal fluoride layer (4) is applied, the method comprising: forming the protective layer (6) on the metal fluoride layer (4) by irradiating the metal fluoride layer (4) with radiation (5) at at least one wavelength (λ) of less than 300 nm, preferably less than 200 nm, the irradiation being carried out in an environment with a water content (CH2O) of less than 0.5 ppmV, preferably less than 0.3 ppmV. The invention also relates to a reflective optical element (1) on which such a protective layer (6) is formed, and to an optical arrangement which has at least one such reflective optical element (1).

Description

Background of the invention

The invention relates to a method for forming a protective layer on a reflective optical element for the VUV wavelength range, the reflective optical element having a substrate on which a metal layer is formed, on whose side facing away from the substrate a metal fluoride layer is applied. The invention also relates to such a reflective optical element and to an optical arrangement which has at least one such reflective optical element.

In the context of this application, the VUV wavelength range is understood to mean a wavelength range between 100 nm and 200 nm (VUV wavelength range according to DIN 5031 Part 7). In this wavelength range it is often not possible to work exclusively with transmissive optical elements; rather, it is usually necessary to use reflective optical elements as well. Reflective optical elements which have a substrate and a metal layer or a metallic surface, which often consists of aluminum or contains aluminum, have proven useful. An overlying metal fluoride layer, which is intended to serve as a protective layer to protect the metal layer from oxidation, can be applied to the metal layer.

Despite the application of protective layers in the form of metal fluorides, it has been shown that with high radiation intensities such as occur in lithography and especially when inspecting masks and wafers, severe degradation of the reflective optical element already occurs within a few days or possibly hours occurs, which is associated with a high loss of reflectivity. When the metal layer is degraded, the material of the metal layer, for example Al, is typically converted into a metal oxide, for example Al ₂ O ₃ , which results in a significant loss of reflection.

In the DE 10 2018 211 499 A1 describes a reflective optical element for the VUV wavelength range, which is designed as described above. At least one oxide layer is applied to the at least one metal fluoride layer in order to protect the underlying layers, in particular the metal layer, and in this way to increase the service life of the reflective optical element, in particular when irradiated with high power density. In the DE 10 2018 211 499 A1 it is described that the metal fluoride layer and the metal oxide layer are preferably deposited on the substrate by means of atomic layer deposition. The application of an oxide layer, for example a layer made of SiO ₂ , by means of plasma-assisted deposition is described there as being advantageous.

When applying an additional protective layer in the form of an oxide layer, however, there is the problem that most oxides have a high absorption at wavelengths of less than 160 nm and can therefore lead to large losses in reflectivity. In the DE 10 2018 211 499 A1 it is proposed to reduce the loss of reflectivity in that a standing wave of the electric field that forms during the reflection has a minimum in the region of the at least one oxide layer.

Object of the invention

The object of the invention is to provide a method for producing a protective layer, a reflective optical element and an optical arrangement with such an optical element, which make it possible to extend the service life of the optical element.

Subject of the invention

According to a first aspect, this object is achieved by a method of the type mentioned at the beginning, comprising: forming the protective layer on the metal fluoride layer by irradiating the metal fluoride layer with radiation at at least one wavelength of less than 300 nm, preferably less than 200 nm, the irradiation in an environment with a water content of less than 0.5 ppmV, preferably less than 0.3 ppmV, particularly preferably less than 0.1 ppmV, extraordinarily preferably less than 0.03 ppmV, in particular less than 0 , 01 ppmV.

According to the invention, it is proposed, by irradiating the metal fluoride layer, to form a protective layer (passivation layer) on the optical element, more precisely on the top of the metal fluoride layer, which increases the service life of the optical element. In this way, it is possible to dispense with the application of a further protective layer in the form of an oxide layer, as shown in FIG DE 10 2018 211 499 A1 is suggested.

The inventors have found that when irradiated with radiation in the UV wavelength range of less than approx. 300 nm, in particular less than approx. 200 nm, and generally at wavelengths of at least approx. 115 nm, in a dry atmosphere, ie with a water content of less than approx. 0.3 ppmV (corresponding to a water partial pressure of less than 3 × 10 ^-4 mbar) possibly less than 0.5 ppmV (corresponding to a Water partial pressure of less than 5 × 10 ^-4 mbar) the metal fluoride layer changes to a comparable final state on a comparatively short time scale, even if the experimental conditions are varied. For this purpose, the inventors varied the duration of the irradiation, the irradiation dose, the irradiation intensity, the initial state of the optical element (different coating batches), the radiation structure and the wavelength of the radiation used for the irradiation. In each case, a comparable final state was observed after the irradiation.

The experiments have shown that the water content in the vicinity of the reflective optical element during the irradiation is a critical parameter for the occurrence of the passivation effect: If a certain limit value for the water content is exceeded, for example around 0.4 - 0.5 ppmV, the passivation effect does not occur and the metal layer degrades with the formation of a metal oxide.

The metal fluoride layer or its material is passivated by the irradiation; The protective layer formed during the irradiation therefore typically does not extend over the entire thickness of the metal fluoride layer. The protective layer generally has less than 50%, less than 40%, less than 30% or less than 10% of the thickness of the metal fluoride layer. The passivating protective layer has (potentially) a high density, as will be described in more detail below.

It has been experimentally shown that the passivation effect described above takes place on short time scales and the rate for subsequent (rapidly occurring) degradation or for the subsequent reduction in reflectivity when the reflective optical element is irradiated again and / or for longer periods of time with UV radiation significantly reduced.

In the following an attempt is made to give an explanation for the experimental observations. It goes without saying that this explanation does not necessarily have to be correct, i.e. it cannot be ruled out that there is another explanation for the experimental observations.

1. 1. Absorption von Strahlung im UV- bzw. VUV-Wellenlängenbereich mit Energien nahe der Bandkante des Metallfluorids und damit die Anregung von Elektronen ins Leitungsband und/oder Exzitonenzuständen und/oder Defektzuständen nahe der Bandkante,
2. 2. Relaxation der zuvor angeregten Elektronen unter Abgabe der Energiedifferenz an das ionische Gitter des Metallfluorids (Farbzentren),
3. 3. In Konsequenz dieses Mechanismus, Fluorfehlstellengeneration und Bildung von interstitiellem (ungebundenen) Fluor,
4. 4. Diffusion von Fluoratomen und Verlust von Fluor über die der Umgebung zugewandte Oberfläche der Metallfluoridschicht, sowie
5. 5. Oxidation des Metallatoms der Metallfluoridschicht.

The formation of the passivating protective layer presumably involves the following steps:

1. 1. Absorption of radiation in the UV or VUV wavelength range with energies near the band edge of the metal fluoride and thus the excitation of electrons into the conduction band and / or exciton states and / or defect states near the band edge,
2. 2. Relaxation of the previously excited electrons, releasing the energy difference to the ionic lattice of the metal fluoride (color centers),
3. 3. As a consequence of this mechanism, generation of fluorine vacancies and formation of interstitial (unbound) fluorine,
4. 4. Diffusion of fluorine atoms and loss of fluorine over the surface of the metal fluoride layer facing the surroundings, as well
5. 5. Oxidation of the metal atom of the metal fluoride layer.

It can also be assumed that atomic mass transport on the surface makes a significant contribution to this passivation effect. Rearrangements or mass transport on an exposed surface on substrates in the form of metal fluoride single crystals, for example MgF ₂ , CaF ₂ , LiF, by introducing an energy input, for example by UV irradiation, are for example in the PCT / EP2019 / 083632 which is incorporated by reference in its entirety into the content of this application. It is also stated there that a low water content of less than 10 ppmV or less than 1 ppmV in the vicinity of the substrate or the exposed surface contributes to a high rearrangement rate and thus to efficient nanostructuring. Such a morphological surface change was also observed in AFM (“Atomic Force Microscopy”) measurements for the reflective optical elements described here.

The atomic mass transport could help to seal the intrinsic porosity of the metal fluoride layer (grain boundaries, self-shadowing effects during the coating, ...) close to the surface and thus prevent oxygen transport channels to the underlying interface between the metal fluoride layer and the metal layer.

1. 1. Ab einer kritischen Dicke wirkt die Schutzschicht als Diffusionsbarriere für das interstitiell diffundierende Fluor und/oder Sauerstoff.
2. 2. Zur Generation von Farbzentren im Metallfluorid und/oder an der Grenzfläche und/oder an der der Umgebung zugewandten Oberfläche ist Strahlung hinreichend hoher Energie nötig. Oxide haben signifikant kleinere Bandlücken, so dass mit wachsender Oxid-Schichtdicke zunehmend solche Photonen absorbiert werden, die potenziell zur Bildung von Farbzentren notwendig sind.

Assuming the correctness of the physical processes described above for forming the protective layer, this implies that the protective layer is self-limited in its thickness. There are two independent arguments for this:

1. 1. Above a critical thickness, the protective layer acts as a diffusion barrier for the interstitially diffusing fluorine and / or oxygen.
2. 2. To generate color centers in the metal fluoride and / or at the interface and / or Radiation of sufficiently high energy is required on the surface facing the environment. Oxides have significantly smaller band gaps, so that with increasing oxide layer thickness, more and more photons are absorbed that are potentially necessary for the formation of color centers.

In both cases, the protective layer is self-limited (with possibly logarithmic dynamics) and its thickness is only so thick that it counteracts the fundamental cause of the degradation, i.e. the generation of color centers or the fluorine and / or oxygen diffusion length. The mechanism described here offers the possibility of realizing functional protective layers that are below the coalescence thicknesses of vapor-deposited / sputtered protective layers made of metal oxides. The influence of such a protective layer on the optical performance at VUV wavelengths between approx. 115 nm and approx. 190 nm is correspondingly less.

The metal fluoride layer of the optical element is irradiated after the coating has been applied, typically in an irradiation device provided specifically for this purpose. The irradiation can optionally also take place in an optical arrangement in which the reflective optical element is installed in order to fulfill its intended function when the optical arrangement is in operation. The optical arrangement can be, for example, a device for inspecting wafers or masks or a lithography system. In this case, the irradiation typically takes place before the optical arrangement is put into operation.

In a variant of the method, the irradiation is carried out in an environment with an oxygen content of less than 0.1 ppmV. For the generation of the passivation effect, it has proven to be advantageous if not only the water content but also the oxygen content in the vicinity of the reflective optical element is as low as possible during the irradiation.

In a further variant, the metal fluoride layer is irradiated with an irradiance of at least 50 mW / cm ² , preferably of at least 100 mW / cm ² , in particular of at least 200 mW / cm ² . The duration of the irradiation and the irradiance are correlated with one another, with a greater irradiance reducing the time that is required until the passivating (potentially self-limiting) protective layer is formed.

In a further variant, the metal fluoride layer is irradiated for a period of at least one day. As described above, the formation of the passivating protective layer takes place on comparatively short time scales, which are in the order of magnitude of days. With an irradiance of approx. 100 mW / cm ² or more, the time until a protective layer is formed, which effectively prevents the degradation of the metal layer, is typically less than approx. 2 days, with an irradiance of approx. 400 mW / cm ² approx. 1.5 days, etc.

For the irradiation at the radiation intensities specified above, UV lamps (excimer lamps) which emit radiation at a predetermined wavelength are typically used as radiation sources. However, it is also possible to use a broadband UV radiation source for the irradiation, for example in the form of a gas discharge lamp, for example a deuterium (D ₂ ) gas discharge lamp.

In a further variant, the radiation is focused on the surface of the metal fluoride layer during the irradiation. The radiation is usually focused on a spot or a focus with a comparatively small spot size. In order to irradiate the entire surface of the metal fluoride layer, the spot is moved relative to the surface of the metal fluoride layer, typically the surface of the metal fluoride layer is scanned with the spot. By focusing, the above-described irradiance levels can be achieved without the need for an irradiation source with a very high power or a comparatively long irradiation time for this purpose.

In a further variant, an area surrounding the optical element is flushed with a flushing gas that is transparent to the radiation during the irradiation. In this variant, during the irradiation, the reflective optical element is typically introduced into an environment or into an atmosphere in which (essentially) atmospheric pressure (approx. 1 bar) prevails. This is favorable because it has been shown that no passivating protective layer is formed in the case of irradiation in a vacuum at a total pressure of less than approx. 4 × 10 ^{-7 mbar.}

A transparent flushing gas is introduced in the vicinity of the optical element or a flushing gas is supplied to the environment in order to avoid contamination of the surface of the metal fluoride layer during the irradiation. The purge gas is preferably a noble gas, for example from the group He, Ne, Ar, Kr, Xe and mixtures thereof, another type of inert gas, for example nitrogen, or forming gas, ie typically a mixture of nitrogen or a noble gas such as argon and hydrogen. A drying of the supplied Purge gas is beneficial in order not to exceed the value given above for the water content in the environment.

In a further variant, the water content in the vicinity of the reflective optical element is determined and adjusted or controlled during the irradiation. In principle, it is sufficient if the water content in the vicinity of the reflective optical element is below a critical limit value which, according to the experimental data available, is approx. 300 ppbV or 0.3 ppmV. Experiments have shown that the passivation effect described above does not occur at a water content of more than approx. 0.4 or 0.5 ppmV, which results in oxidation of the metal layer. With the help of precise sensors, the water content in the vicinity of the reflective optical element can be determined and, if necessary, adjusted or regulated by appropriately influencing the amount of flushing gas supplied and / or the drying of the flushing gas in order to prevent the Water content rises above the limit value. A control or regulating device can be provided for regulating the water content in the vicinity of the optical element.

Another aspect of the invention relates to a reflective optical element of the type mentioned at the outset, in which a protective layer is formed on the metal fluoride layer, which is formed or produced according to the method described above. As described above in connection with the method, the protective layer prevents or slows down degradation of the metal layer of the optical element and thus increases its service life.

In one embodiment, the metal fluoride layer has at least one material selected from the group comprising: magnesium fluoride, aluminum fluoride, sodium fluoride, lithium fluoride, chiolite, cryolite, erbium fluoride, neodymium fluoride, gadolinium fluoride, dysprosium fluoride, samarium fluoride, holmium fluoride, lutium fluoride, hafnantium fluoride , Cerium fluoride, barium fluoride, yttrium fluoride. In the VUV wavelength range, these fluorides have a low absorption and a comparatively high radiation resistance.

In a further embodiment, the metal layer has at least one material which is selected from the group comprising: aluminum, rhodium, ruthenium, palladium, osmium, iridium, platinum, magnesium, germanium or a combination thereof. The metal layer can also be formed from an alloy of the materials mentioned. For example, it can be an aluminum-silicon alloy, an aluminum-manganese alloy, an aluminum-silicon-manganese alloy, etc. The materials mentioned have a sufficiently good reflectivity in the VUV wavelength range. The metal layer can form its own layer, which is applied to the substrate, but it is also possible that the metal layer is formed from the surface of a metallic substrate, which in this case is typically polished.

The substrate can be (synthetic) quartz glass, in particular titanium-doped quartz glass, calcium fluoride, magnesium fluoride, a ceramic, a glass ceramic, silicon, silicon carbide, silicon-silicon carbide composite material, aluminum, copper, an aluminum-copper alloy, etc. Act. By polishing their surface, the metals mentioned can be used directly to reflect radiation in the VUV wavelength range without a separate metal layer being required for this purpose.

In addition to the metal layer and the metal fluoride layer, further layers can be applied to the substrate. For example, a functional layer can be formed between the metal layer and the substrate, which serves, for example, as an adhesion promoter layer or as a polishing layer. A dielectric layer system can also be applied to the metal layer, which has at least one material with a low refractive index and at least one material with a higher refractive index in the VUV wavelength range or at the operating wavelength in order to increase the reflectivity of the reflective optical element.

Another aspect of the invention relates to an optical arrangement for the VUV wavelength range, in particular a VUV lithography system or a wafer inspection system, comprising: an interior, with at least one reflective optical element for reflecting radiation in the VUV wavelength range being arranged in the interior , which is designed as described above. The protective layer formed on the reflective optical element during the previous irradiation increases its service life; more precisely, the decrease in the reflectivity of the reflective optical element with increasing duration of the operation of the optical arrangement is significantly less than without the previous irradiation. As has been described above, the irradiation for forming the protective layer can take place in the optical arrangement itself or in an irradiation device provided for this purpose.

In one embodiment, the water content in the interior is less than 0.5 ppmV, preferably less than 0.3 ppmV, in particular preferably less than 0.1 ppmV, extremely preferably less than 0.03 ppmV, in particular less than 0.01 ppmV. A concentration of water that is less than the specified values is usually sufficient to promote passivation and prevent the protective layer from degrading. The oxygen content in the interior should also not be too great and should be less than approx. 1 ppmV, preferably less than 0.3 ppmV, particularly preferably less than 0.05 ppmV.

In a further embodiment, the optical arrangement comprises at least one water sensor for determining the water content in the interior. The determination of the water content with the help of a suitable water sensor makes it possible to set or regulate the water content in the interior: If the water sensor detects an increase in the water content, measures can be taken that counteract any further increase in the water content. For example, in this case the drying of a purge gas (see below) that may be fed into the interior can be improved.

In a further embodiment, the optical arrangement is designed to supply at least one flushing gas into the interior, in particular via at least one gas inlet. As with the irradiation of the reflective optical element to form the protective layer, it is also advantageous when the optical arrangement is in operation to protect the surface of the reflective optical element from contamination with the aid of a flushing gas. The total pressure in the interior is usually around atmospheric pressure, i.e. typically no vacuum is generated in the interior. The flushing gas can in particular be one or more of the flushing gases mentioned above in connection with the irradiation of the optical element.

Further features and advantages of the invention emerge from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can be implemented individually or collectively in any combination in a variant of the invention.

Figure list

• 1a,b schematische Darstellungen der Bestrahlung eines reflektierenden optischen Elements zur Bildung einer passivierenden Schutzschicht,
• 2a-d schematische Darstellungen der Reflektivität des optischen Elements von 1a,b vor bzw. nach der Bestrahlung mit unterschiedlichen Bestrahlungsparametern,
• 3a-d schematische Darstellungen der Reflektivität bzw. des Verlusts an Reflektivität während der Bestrahlung,
• 4a,b schematische Darstellungen der Reflektivität vor und nach der Bestrahlung bei zwei unterschiedlichen Werten des Wasser-Gehalts in der Umgebung des optischen Elements,
• 5 eine schematische Darstellung einer optischen Anordnung für den VUV-Wellenlängenbereich in Form einer VUV-Lithographieanlage, sowie
• 6 eine schematische Darstellung einer optischen Anordnung für den VUV-Wellenlängenbereich in Form eines Wafer-Inspektionssystems.

Exemplary embodiments are shown in the schematic drawing and are explained in the following description. It shows

• 1a, b schematic representations of the irradiation of a reflective optical element to form a passivating protective layer,
• 2a-d schematic representations of the reflectivity of the optical element of FIG 1a, b before and after irradiation with different irradiation parameters,
• 3a-d schematic representations of the reflectivity or the loss of reflectivity during irradiation,
• 4a, b schematic representations of the reflectivity before and after the irradiation with two different values of the water content in the vicinity of the optical element,
• 5 a schematic representation of an optical arrangement for the VUV wavelength range in the form of a VUV lithography system, and
• 6th a schematic representation of an optical arrangement for the VUV wavelength range in the form of a wafer inspection system.

In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

1a, b show highly schematically a reflective optical element 1 that is a substrate 2 has on which a metal layer 3 is formed. On the metal layer 3 is on one of the substrate 2 facing away from a metal fluoride layer 4th upset. The application of the metal fluoride layer 4th is done in a conventional coating process (by evaporation). The metal fluoride layer 4th serves to protect the underlying metal layer 3 from oxidation. In the example shown, it is the material of the metal layer 3 around aluminum, in the case of the material of the metal fluoride layer 4th around magnesium fluoride (MgF ₂ ).

This in 1a shown optical element 1 has a substrate 2 made of aluminum. With the metal layer 3 it is the polished surface of the aluminum substrate 2 . The in 1b shown optical element 1 is the substrate 2 Formed from quartz glass on which the metal layer 3 made of aluminum is applied in a conventional coating process. It goes without saying that the substrate 2 can also be formed from another metallic or non-metallic material, for example from a glass ceramic, a ceramic, ...

It has been shown that the presence of the metal fluoride layer 4th alone is not sufficient to the optical element 1 protect from degradation when the reflective optical element 1 is irradiated during operation with high powers or irradiation strengths: Despite the presence of the metal fluoride layer 4th becomes the Al material of the metal layer 3 in a comparatively _{oxidizes to Al 2} O _{3 for a} short period of time of several hours or days, resulting in a significant reduction in the reflectivity of the reflective optical element 1 has the consequence. If the loss of reflectivity is too great, the reflective optical element must be replaced.

To the life of the reflective optical element 1 a self-passivating protective layer is used in a method described below 6th on the metal fluoride layer 4th educated. For this purpose, the reflective optical element 1 with radiation 5 irradiated at at least one wavelength λ of less than 300 nm, typically less than 200 nm. The irradiation can take place in particular at one or more wavelengths λ which are between 115 nm and 200 nm.

As in 1b can be seen, the irradiation takes place in an irradiation device specially provided for this purpose 7th . The irradiation facility 7th has an irradiation chamber 8th on, in their interior 11th the reflective optical element 1 is introduced for the irradiation. In the interior 11th can with the help of a gas inlet 9 one for radiation 5 transparent purging gas in the form of nitrogen N _{2 can be} introduced. It goes without saying that, as an alternative or in addition to nitrogen, other purge gases can also enter the interior 11th can be introduced, for example noble gases, for example from the group He, Ne, Ar, Kr, Xe and mixtures thereof, other types of inert gas or a forming gas, typically a mixture of nitrogen or a noble gas, for example argon, and hydrogen. Drying the supplied purge gas N ₂ is beneficial in order to achieve a predetermined water content C _H2O in the interior 11th which is the vicinity of the reflective optical element 1 forms not to exceed.

Experiments have shown that the formation of the passivating protective layer 6th on the metal fluoride layer 4th only occurs if the water content is C _H2O , which during the irradiation in the vicinity of the reflective optical element 1 prevails, is below a limit value. The experiments carried out showed that this critical limit value for the water content C _{H2O is} between approx. 10 ppbV and approx. 300 ppbV, possibly up to 500 ppbV. The in 1b The example shown is the irradiation of the reflective optical element 1 therefore carried out at a value of the water content C _H2O of less than approx. 0.1 ppmV.

In principle, it is beneficial if the water content in the interior is _{C H2O} 11th using a suitable water sensor 10 is monitored, which has sufficient sensitivity for such a low water content C _H2O . Setting or regulating the water content C _H2O is basically possible, but not absolutely necessary, provided that it is ensured that the above-mentioned critical limit value is not exceeded. In order not to exceed the limit value, the flushing gas N ₂ can be dried, for example, or the amount of the flushing gas N ₂ supplied per unit of time can be suitably selected or, if necessary, changed.

For the formation of the protective layer 6th it has proven to be beneficial, albeit the oxygen content _CO2 in the interior 11th has a comparatively low value of less than about 0.1 ppmV.

Experimentally it has been shown that under the specified conditions for the water content C _H2O and for the oxygen content C _O2 in the interior 11th the reflective optical element 1 or the passivating protective layer 6th have a stable end state even with the variation of several parameters used in the irradiation after the irradiation. The final stable state is reached on a short time scale of the duration of irradiation, on the order of typically one or more days.

For irradiation, the in 1b The example shown is an irradiation source 12th used in the form of a UV irradiation lamp. When the optical element is irradiated 1 , more precisely the surface 4a the metal fluoride layer 4th , becomes the radiation 5 the radiation source 12th focused on a focus spot. To on the entire metal fluoride layer 4th the fluoridic protective layer 6th to generate becomes the irradiation source 12th relative to the surface 4a the metal fluoride layer 4th moved as in 1b is indicated in which already part of the protective layer 6th to the right of the focus spot or of the irradiation source 12th while this is not the case to the left of the focus spot.

The protective layer 6th is formed on the observed time scales essentially only in the area of the respective focus spot, since due to the focusing the dose (and also the irradiance) at the focus spot is significantly greater than in the vicinity of the focus spot. Provided that the irradiation time is not too long, a sufficient irradiation dose to form the protective layer is provided even without focusing 6th can be generated can be due to the focusing of the radiation 5 may be waived. The formation of the protective layer 6th also depends, among other things, on whether a monochromatic or a polychromatic radiation source 12th is used: In the case of a polychromatic radiation source 12th can be different Wavelength bands potentially have a different effect on the metal fluoride layer 4th to have.

2a-d show the reflectivity R of the reflective optical element 1 from 1 b Depending on the wavelength λ before and after the irradiation, in each case with a water content C _H2O of less than 0.1 ppmV and an oxygen content C _O2 of less than 0.1 ppmV in the surrounding area 11th of the optical element 1 . In the 2a-d The reflectivity R shown relates in each case to an angle of incidence on the optical element 1 reflected radiation of approx. 10 ° to the surface normal. In 2a-d the reflectivity R is shown in each case before the irradiation and after the irradiation in or outside the focus spot that of the irradiation source 12th is produced.

The in 2a In the example shown, the irradiation was carried out over a period of six days with an irradiation source 12th in the form of a single UV irradiation lamp. The in 2 B In the example shown, the duration of irradiation was varied and is two days or four days. The in 2c In the example shown, the irradiation was carried out with an irradiation time of 1.5 days and with an irradiation source 12th , which includes four UV radiation lamps, ie the irradiance has been quadrupled. The in 2d In the example shown, the irradiation time was five days and it became an irradiation source 12th used with a UV irradiation lamp.

As can be seen from a comparison of the reflectivity profiles obtained with different irradiation parameters in the spot after the irradiation, the final state of the reflectivity R is in all in 2a-d Examples shown are comparable, although in some cases different irradiation structures and / or irradiation parameters were used: In all of the examples shown, the reflectivity R of the optical element 1 decreased after irradiation at wavelengths in the VUV wavelength range between approx. 100 nm and approx. 200 nm. As below based on 3a, b is described, this decrease in reflectivity R results in the formation of the protective layer 6th but also that the reduction in reflectivity R when the optical element continues to be irradiated 1 after the formation of the protective layer 6th , for example during operation of the optical element 1 in an optical arrangement, turns out to be significantly lower than without such irradiation.

3a shows the reflectivity R of the optical element 1 from 1b when irradiated with a wavelength λ of 160 nm with an irradiation source 12th in the form of a UV irradiation lamp with an irradiance of 100 W / cm ² on the surface 4a the metal fluoride layer 4th generated, with different durations of irradiation ( 2 Days, 4 days, 6 days, 13.5 days, 19.5 days). 3b shows the reflectivity loss ΔR as a function of the duration t of the irradiation for a wavelength of 134 nm and 150 nm.

4a, b show analogous representations for irradiation with a radiation source 12th with four UV radiation lamps, the radiation 5 at a wavelength of 120 nm and generate an irradiance of 400 mW / cm ² in the focus spot.

As based on the chronological sequence in 3b and 3d can be seen is the reflectivity loss ΔR when the optical element is irradiated 1 initially comparatively large, but decreases significantly after a period of a few days when the passivating protective layer 6th has formed. Until the protective layer has been completely formed 6th is the rate of reflectivity loss ΔR / t at the in 3b Example shown 11.1% / d, after the formation of the protective layer 6th the rate of reflectivity loss ΔR / t is less than about 0.3% / d. The same applies to the in 3d Example shown, in which also the rate of reflectivity loss ΔR / t after the formation of the protective layer 6th significantly decreases.

The amount of time it takes for the protective layer to develop 6th is typically on the order of a day or more and depends on the irradiance of the surface 4a the metal fluoride layer 4th during irradiation by means of the irradiation source 12th . The irradiation is typically carried out with an irradiance of at least 50 mW / cm ² , so that the time until the protective layer is formed 6th is not too big.

4a, b again illustrate the relevance of the water content C _H2O in the vicinity of the reflective optical element 1 for the formation of the passivating protective layer 6th : At the in 4a The example shown is irradiation with a water content C _H2O of less than 0.1 ppmV in the environment 11th of the optical element 1 accomplished. The reflectivity R shows the optical element 1 shows a similar dependence on the wavelength λ as in 2a-d .

The in 4b The example shown is the irradiation with a water content of C _H2O in the vicinity of the reflective optical element 1 of approx. 0.4 ppmV under otherwise identical irradiation parameters. As in 4b can be seen, both with a water content C _H2O between approx. 0.3 and 0.5 ppmV and with a water content C _H2O between approx. 0.5 and 1.0 ppmV, the reflectivity R in the VUV- Wavelength range significantly. A reflective optical element 1 with the in 4b The reflectivity R shown in the VUV wavelength range is unusable for almost all applications in this wavelength range.

However, if the irradiation is carried out in connection with 4a described way, reduces the self-limiting protective layer 6th the degradation or the rate of the loss of reflectivity ΔR / t of the optical element 1 significant during further irradiation, as demonstrated by 3b and from 3d has been described. The lifetime of the reflective optical element 1 with such a protective layer 6th when irradiated in an optical arrangement can therefore be increased significantly. Due to the comparatively small thickness of the protective layer 6th is the one on the protective layer 6th attributable reflectivity loss ΔR of the optical element 1 comparatively low.

The experiments described above were carried out on a metal fluoride layer 4th performed, which was formed from magnesium fluoride (MgF ₂ ), which was applied to a metal layer 3 was applied from aluminum. Instead of magnesium fluoride, the metal fluoride layer 4th but also be formed from other metal fluorides, for example from aluminum fluoride, sodium fluoride, lithium fluoride, chiolite, cryolite, erbium fluoride, neodymium fluoride, gadolinium fluoride, dysprosium fluoride, samarium fluoride, holmium fluoride, hafnium fluoride, ytetium fluoride or lutetium fluoride, europium fluoride, lanthanium fluoride, europium fluoride. Also the metal layer 3 does not necessarily have to be formed from aluminum, but can for example have a material which is selected from the group comprising: rhodium, ruthenium, palladium, osmium, iridium, platinum, magnesium, germanium or a combination thereof.

In the following, using 5 and 6th two optical arrangements are described in which the reflective optical element described above 1 which is the protective layer 6th has, can be operated.

In 5 Figure 3 is a schematic of an optical arrangement 21 in the form of a VUV lithography system, in particular for wavelengths in the range between 100 nm and 200 nm or 190 nm. The VUV lithography system 21 has as essential components two optical systems in the form of a lighting system 22nd and a projection system 23 on. To carry out an exposure process, the VUV lithography system 21 a radiation source 24 on, which can be, for example, an excimer laser, the radiation 25th emitted at a wavelength in the VUV wavelength range of, for example, 193 nm, 157 nm or 126 nm and is an integral part of the VUV lithography system 21 can be.

The one from the radiation source 24 emitted radiation 25th is made with the help of the lighting system 22nd prepared in such a way that it becomes a mask 26th , also called reticle, can be illuminated. The in 5 shown example has the lighting system 22nd both transmitting and reflecting optical elements. Representative are in 5 a transmitting optical element 27 the radiation 25th bundles, as well as a reflective optical element 28 shown which the radiation 25th for example deflects. In a known manner, in the lighting system 22nd A wide variety of transmitting, reflecting or other optical elements can be combined with one another in any desired, even more complex manner.

The mask 26th has a structure on its surface that points to an optical element to be exposed 29 , for example a wafer, as part of the production of semiconductor components, using the projection system 23 is transmitted. In the example shown is the mask 26th designed as a transmitting optical element. In alternative designs, the mask 26th can also be designed as a reflective optical element. The projection system 22nd has at least one transmitting optical element in the example shown. In the example shown, two transmitting optical elements are representative 30th , 31 shown, which are used, for example, the structures on the mask 26th on the one for the exposure of the wafer 29 to reduce the desired size. Even with the projection system 23 reflective optical elements can be provided and any optical elements can be combined with one another in a known manner. It should be pointed out that optical arrangements without transmissive optical elements can also be used for VUV lithography.

In 6th is a schematic of an exemplary embodiment of an optical arrangement in the form of a wafer inspection system 41 shown. The following explanations also apply analogously to inspection systems for inspecting masks.

The wafer inspection system 41 exhibits an optical system 42 with a radiation source 54 on whose radiation 55 by means of the optical system 42 on a wafer 49 is steered. To this end, radiation is used 55 from a concave mirror 46 on the wafer 49 reflected. With a mask inspection system, instead of the wafer, 49 place a mask to be examined. The one from the wafer 49 reflected, diffracted and / or refracted radiation also becomes the optical system 42 associated concave mirror 48 via a transmitting optical element 47 on a detector 50 forwarded for further evaluation. At the radiation source 54 it can be, for example, precisely one radiation source or a combination of several individual radiation sources in order to provide an essentially continuous radiation spectrum. In modifications, one or more narrow-band radiation sources can also be used 54 can be used. The wavelength or the wavelength band is preferably that of the radiation source 54 generated radiation 55 in the range between 100 nm and 200 nm, particularly preferably between 110 nm and 190 nm.

The in 5 The example shown has the lighting system 22nd a housing 32 on in which an interior space 33 is formed in which the transmissive optical element 27 as well as the reflective optical element 28 are arranged in the form of the mirror. The optical system also exhibits accordingly 42 of the wafer inspection system 41 from 6th a housing 52 on in which an interior space 53 is formed in which the two mirrors 46 , 48 as well as the transmissive optical element 47 are arranged.

When the respective reflective optical elements are irradiated 28 , 46 , 48 with the radiation 25th , 55 the radiation source 24 , 54 , which typically has a high intensity, the degradation described above and an associated loss of reflectivity R can occur, even if the reflective optical elements 28 , 46 , 48 a passivating protective layer as described above 6th exhibit. To avoid this, the respective interior 33 , 53 a water content C _H2O of less than approx. 0.5 ppmV, usually less than 0.3 ppmV. To determine the water content C _H2O is in the respective interior 33 , 53 a water sensor 10 arranged. In order to counteract oxidation, the oxygen content C _O2 in the respective interior should also be 33 , 53 be as low as possible and not exceed approx. 1 ppmV. The oxygen content C _O2 in the respective interior 33 , 53 can be determined by means of an oxygen sensor (not shown).

For setting or regulating the water content C _H2O or the oxygen content C _O2 in the respective interior 33 , 53 a control or regulating device can be used. To set the water content C _H2O , the control or regulating device can, for example, influence the degree of dryness of the purge gas supplied (see below). A determination or setting of the water content C _H2O in the interior 33 , 53 but is not absolutely necessary, provided it is ensured that the values described above for the water content C _{H2O are} not exceeded.

Around the reflective optical elements 28 , 46 , 48 To protect against contaminating substances, show the optical arrangements 21 , 41 one gas inlet each 34 , 54 on, through which a purge gas, which is argon in the example shown, into the interior 33 , 53 is admitted. In order not to exceed the values described above for the water content C _H2O , it may be necessary to remove the flushing gas before it is fed into the respective interior 33 , 53 to dry (see above). The total pressure in the respective interior 33 , 53 is typically on the order of atmospheric pressure.

In summary, a self-passivation of the metal fluoride layer can be carried out in the manner described above 4th can be achieved which significantly extends the service life of the respective reflective optical elements 1 , 26th , 46 , 48 has the consequence.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102018211499 A1 [0004, 0005, 0008]
• EP 2019/083632 PCT [0015]

Claims (15)

Method for forming a protective layer (6) on a reflective optical element (1, 28, 46, 48) for the VUV wavelength range, the reflective optical element (1) having a substrate (2) on which a metal layer (3) is formed, on the side facing away from the substrate (2) a metal fluoride layer (4) is applied, the method comprising: forming the protective layer (6) on the metal fluoride layer (4) by irradiating the metal fluoride layer (4) with radiation (5) at least a wavelength (λ) of less than 300 nm, preferably of less than 200 nm, the irradiation being carried out in an environment with a water content (C _H2O ) of less than 0.5 ppmV, preferably less than 0.3 ppmV will. Procedure according to Claim 1 , in which the irradiation is carried out in an environment with an oxygen content (C _O2 ) of less than 0.1 ppmV. Procedure according to Claim 1 or 2 , in which the irradiation of the metal fluoride layer (4) is carried out with an irradiance of at least 50 mW / cm ² . Method according to one of the preceding claims, in which the irradiation of the metal fluoride layer (4) is carried out over a period of at least one day. Method according to one of the preceding claims, in which the radiation (5) during irradiation is focused on a surface (4a) of the metal fluoride layer (4). Method according to one of the preceding claims, in which, during the irradiation, an area (11) of the optical element (1) is flushed with a flushing gas that is transparent to the radiation (5). Procedure according to Claim 6 in which the gas transparent to radiation (5) is a noble gas from the group He, Ne, Ar, Kr, Xe and mixtures thereof, another type of inert gas, in particular nitrogen, or a forming gas, ie a mixture of nitrogen or a noble gas and hydrogen. Method according to one of the preceding claims, in which the water content (C _H2O ) in the vicinity of the reflective optical element (1) is set during the irradiation. Optical element (1) for reflecting radiation (5) in the VUV wavelength range, comprising: a substrate (2) on which a metal layer (3) is formed, wherein on a side of the metal layer (3) facing away from the substrate (2) a metal fluoride layer (4) is applied, characterized in that a protective layer (6) is formed on the metal fluoride layer (4), which protective layer is formed according to a method according to one of the preceding claims. Optical element according to Claim 9 wherein the metal fluoride layer (4) has at least one material selected from the group comprising: magnesium fluoride, aluminum fluoride, sodium fluoride, lithium fluoride, chiolite, cryolite, erbium fluoride, neodymium fluoride, gadolinium fluoride, dysprosium fluoride, samarium fluoride, holmium fluoride, haafnium fluoride , Lutetium fluoride, cerium fluoride, barium fluoride, yttrium fluoride. Optical element according to Claim 9 or 10 in which the metal layer (3) has at least one material selected from the group comprising: aluminum, rhodium, ruthenium, palladium, osmium, iridium, platinum, magnesium, germanium or a combination thereof. Optical arrangement for the VUV wavelength range, in particular VUV lithography system (21) or wafer inspection system (41), comprising: an interior (33, 53), wherein in the interior (33, 53) at least one reflective optical element (28, 46, 48) is arranged for reflecting radiation (25, 55) in the VUV wavelength range, which according to one of the Claims 9 until 11th is trained. Optical arrangement according to Claim 12 , in which the interior (33, 53) has a water content (C _H2O ) of less than 0.5 ppmV, preferably less than 0.3 ppmV. Optical arrangement according to one of the Claims 12 or 13th , further comprising: a water sensor (10) for measuring the water content (C _H2O ) in the interior space (33, 53). Optical arrangement according to one of the Claims 12 until 14th which is designed to supply at least one flushing gas into the interior space (33, 53), in particular via a gas inlet (34, 54).

Images