WO2003079062A1

WO2003079062A1 - Optical element, method for the production thereof and for determining its optical properties

Info

Publication number: WO2003079062A1
Application number: PCT/DE2003/000810
Authority: WO
Inventors: Jörg HEBER; Norbert Kaiser; Christian MÜHLIG; Wolfgang Triebel
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.; Institut für Physikalische Hochtechnologie e.V.
Priority date: 2002-03-18
Filing date: 2003-03-07
Publication date: 2003-09-25
Also published as: DE10213088B4; WO2003079062A8; AU2003223861A1; DE10213088A1

Abstract

The invention relates to optical elements, methods for the production of these optical elements and for determining the attained optical properties. These optical elements are, in particular, ones used for reflecting, transmitting, guiding and/or filtering electromagnetic radiation in a wavelength range of 100 to 400 nm. At least one layer consisting of a metallic fluoride is formed on a substrate. The inventive solution should improve the optical properties and, in particular, prevent the interfering influence of luminescence to the greatest possible extent. Following the layer formation, the optical elements produced in this manner can be irradiated with electromagnetic radiation of a preset wavelength, and the luminescence intensity can be determined by using optical detectors.

Description

Optical element, process for its production and for the determination of its optical properties

The invention relates to optical elements for reflecting, transmitting, guiding and / or filtering electromagnetic radiation in the wavelength range between 100 and 400 nm, at least one layer of a metallic fluoride being formed on a substrate. The invention further relates to a method for producing such optical elements, a method for the detection of optical properties and / or the layer qualities, and uses for an optical element according to the invention.

Because of their advantageous optical properties in the short-wave spectral range of ultraviolet light, that is from 100 to 400 nm, layers or layer systems made of metallic fluorides are used for optical elements. The formation of such layer ten takes place with known vacuum coating processes.

The known solutions and products, however, have deficits with regard to the desired reflection and transmission parameters as well as disadvantages due to discoloration and stress cracking, which can lead to ablation. As a result, they are problematic in long-term use and in particular in the high-performance range, that is to say at high intensities, so that long-term use is not easily achieved.

Increased absorption due to the presence of hydrocarbon deposits within such layers could be countered by a targeted reduction in the hydrocarbon content.

However, it has been found that further absorption losses and additional deteriorations in the optical properties occur due to the occurrence of actually undesirable luminescence. This results in increased absorption interactions, especially in layer systems made of metallic fluorides. Furthermore, increased layer degradation and reduced layer stability are disadvantageous. It is also obvious that the luminescence emissions represent an undesirable influence of stray light for many applications.

It is therefore an object of the invention to provide optical elements for use in the wavelength range from 100 to 400 nm of electromagnetic radiation, which have improved absorption and reflection properties and in which the interfering

Influence of luminescence is largely avoided that can.

According to the invention, this object is achieved with an optical element which has the features of claim 1. Advantageous uses are the subject of claim 9, the manufacture of the optical elements according to the invention is defined by claim 10 and a method for determining the optical properties and / or the layer qualities of such optical elements is defined by the method according to claim 13.

The optical elements according to the invention, in which at least one layer or a layer system of several such layers of metallic fluorides are formed on a substrate, have a proportion of at least one additional element which is 10 10 ppm by mass, preferably 5 5 ppm by mass, on. This can be one or more elements that are additionally contained in the metallic fluoride, and the elements in question have the property that they luminesce within at least one predefinable wavelength range of the electromagnetic rays.

The undesirable elements in the layers formed from metallic fluorides are generally transition metals and / or rare earth metals, which can be contained in neutral form, as ions or isotopes. They can form mixed crystals with the respective metallic fluoride, so that they are relatively difficult to identify and consequently difficult to remove.

Rare earth metals from the group of lanthanides are particularly problematic. Depending on the respective element, these tend to luminescence, with at least one wavelength with very high luminescence intensity being recorded for such elements in the wavelength range in question. For example, cerium can cause very high lumuninescence intensities at wavelengths in the range between 290 to 305 nm and with praseodymium at approximately 250 nm. Somewhat lower luminescence intensities are also found for cerium at wavelengths of approx. 365 nm and 405 nm.

MgF ₂ , YF ₃ , LaF ₃ , A1F ₃ , NdF ₃ , BaF ₂ , Chiolith, DyF ₃ , GdF ₃ , cryolite, LiF, NaF, LuF ₃ , SmF ₃ , SrF ₂ have been found to be suitable for use in the formation of layers , TbF ₃ , YbF ₃ , ZrF highlighted.

With such metallic fluorides and advantageously selected pairings of fluorides, alternating layer systems, preferably as λ / 4-

Layer systems are formed on optical substrates.

With regard to their optical properties (transmission / reflection) and their design, the optical substrates can be used for a wide variety of optical elements in accordance with the requirements.

For example, non-transparent substrates for the reflective optical elements can generally be used without further ado. For other uses of such optical elements, however, the transparency of the substrates is a basic requirement.

The optical elements can be the electromagnetic Influence radiation in various forms. So there is the possibility of beam guidance, which is certainly easily possible with the reflectors already mentioned. Furthermore, beam shaping can also be carried out, which can be achieved with reflectors as well as with optical lenses.

However, the optical elements can also be designed as a polarizer, polarization splitter, as a phase delay plate, optical window, beam splitter or also as an optical grating.

In any case, however, at least one layer of a metallic fluoride is used. In the case of optical gratings, for example, this can also act as a protective layer against atmospheric and other environmental influences for the actual grating.

Advantageous areas of use for optical elements according to the invention are lithographic applications, as are used in particular in semiconductor technology. However, the optical elements according to the invention can also be used advantageously for monochromatization, in spectral analysis technology for the spectral decomposition of electromagnetic radiation.

With a certain design, for example a layer system, a reduction in reflection at the respective optical element can also advantageously be achieved.

The production of the optical elements according to the invention is based on known methods. The one or more layers of the at least one metallic fluoride are thus formed by loading layering process in vacuum.

For this purpose, the respective metallic fluorides can be thermally evaporated, deposited on the respective substrate by electron beam evaporation, ion-assisted deposition or plasma-assisted evaporation.

For this purpose, the metallic fluorides are usually used in powder or granule form. For the optical elements according to the invention, the powdery or granular metallic fluoride used has in each case proportions of at least one additional element which are 10 10 ppm by mass, preferably ≤ 5 ppm by mass.

However, there is also the possibility of carrying out the layer formation of the various metallic fluorides using metallic targets using known PVD / CVD methods. The metallic fluorides can be reactively formed during the process within an atmosphere containing fluorine. The fluorine does not necessarily have to be elementary in such an atmosphere which is required for the reactive formation of the metallic fluorides, but suitable fluorine compounds which can be split up relatively easily can also be used. The latter can even have an advantageous effect, since some fission products, for example hydrogen, can react with other undesirable components which are not embedded in the respective layers.

Irrespective of the fact that metallic targets or already metallic fluorides in powder or granule form are used for the formation of the layers in a vacuum. are set, the additional elements that luminesce in a specifiable wavelength range should have a proportion of ≤ 10 mass ppm.

In particular if the layer formation is to be carried out in a vacuum using powdered or granular metallic fluorides, the proportion of undesirable impurities which are present in the form of the elements which tend to luminescence can be obtained by appropriate cleaning carried out before the coating of the preliminary product can be reduced or even completely removed.

Such cleaning can be carried out, for example, by zone melting which is known per se, but also vacuum sublimation. In certain cases, however, both different cleaning processes can also be carried out, for example to specifically reduce or eliminate certain elements. With this method, the proportion of undesirable elements can be reduced to ≤ 1 ppm by mass. The reduction in the proportion of these elements can also be reduced by targeted ion exchange in a vacuum.

Taking up the inventive idea, the optical properties achieved and / or the layer qualities can also be determined in optical elements according to the invention. The layer or the layer system is irradiated with electromagnetic radiation with a predefinable wavelength and the intensity of the luminescent light caused by the irradiation is determined with at least one optical detector. For this purpose, electromagnetic radiation with preferably shorter wavelengths than the luminescence radiation to be examined in each case, preferably between 100 to 500 nm, can be used. However, there is also the possibility of using electromagnetic radiation with a significantly smaller wavelength bandwidth in a targeted manner within this wavelength range.

Since, as already indicated, the various undesired elements have luminescent light peak values at very specific wavelengths, the luminescence intensity can also be determined in a wavelength-selective manner and the luminescence intensity measured for such a wavelength both as a measure of the proportion and the detection of the actual presence of such an element within a layer or Layer system can be used. In addition to the luminescence intensity, the temporal decay behavior of the luminescence light can also be evaluated.

For the luminescence excitation, the most varied of beam sources can be used for correspondingly suitable intensive electromagnetic radiation, such as high-pressure mercury lamps, but also various laser light sources suitable for the corresponding wavelength range, e.g. Excimer lasers can be used.

Accordingly, the solution according to the invention can be used to achieve optical elements for the wavelength range in question with significantly improved optical properties, which essentially relates to long-term stability, laser strength, reflectivity and transmissivity. Absorption and luminescence losses in particular lent to be reduced. In addition, a reduction in the interaction of the electromagnetic radiation used, a reduced layer degradation and an increase in the layer stability can be achieved with a significantly reduced interference from luminescence.

The invention will be explained in more detail below by way of example.

Show:

1 shows a diagram of the reflectivity of an optical element with an alternating layer system, consisting of MgF ₂ and LaF ₃ layers, in the wavelength range between 150 and 230 nm

FIG. 2 shows a diagram with which the luminescence influence of cerium within an alternating

Layer systems of LaF ₃ and MgF ₂ layers should be clarified.

An MgF ₂ / LaF ₃ alternating layer system with the layer design (1H 1L) ²⁰ 1H was formed on a substrate. Electromagnetic radiation with a wavelength of 193 nm was directed onto the surface of this alternating layer system by an ArF laser at an angle of 45 °.

The MgF _{2 formed} the low refractive index and the LaF the higher refractive index component of such a layer system and the MgF layers had a thickness of 34 nm and the LaF ₃ a thickness of 29 nm.

As is clear from the diagram shown in FIG. 1 a reflectivity above 97% could be achieved in the range of the wavelength used.

The entire layer system, consisting of the 41 individual layers, had a proportion of the undesired elements ≤ 5 mass ppm.

The diagram shown in FIG. 2 is intended to illustrate the undesired influence of cerium contained in such a layer system, as an example of such an element.

An alternating layer system was again used, in which LaF ₃ and MgF _{2 were} used as layer materials. The system was for 182 nm

Design wavelength optimized at 0 ° angle of incidence. The layer thicknesses of the alternating layer system were adjusted accordingly to this wavelength. Intensive pulsed electromagnetic radiation with a wavelength of 193 nm was used to excite the luminescence. The diagram in FIG. 2 clearly shows that the cerium concentration within such an alternating layer system means that the luminescence intensity is very high at a wavelength of

290 nm occurs, which is clear from the relative information on the abscissa of the diagram, especially when compared to a hydrocarbon-containing system. This high luminescence intensity accordingly has a reduced laser resistance of the

Component due to the shown absorption change effect and at the same time leads to an increased influence of stray light, which for certain applications the use of appropriate optical filter elements, which in turn reduces the actually desired reflected electromagnetic see radiation causes, requires.

Claims

claims

1. Optical element for reflection, transmission, guidance and / or filtering of electromagnetic radiation in the wavelength range between 100 and 400 nm, in which there is at least one layer formed from a metallic fluoride on a substrate;

the proportion of at least one element additionally contained in the layer which luminesces within at least one predefinable wavelength range of the electromagnetic radiation is less than or equal to mass 10 ppm.

2. Optical element according to claim 1, characterized in that an alternating layer system formed from at least two different metallic fluorides is formed on the substrate.

3. Optical element according to claim 1 or 2, characterized in that the alternating layer system is designed as a λ / 4 layer system.

4. Optical element according to at least one of the preceding claims, characterized in that the proportion of transition metals and / or rare earth metals in the layers formed from metallic fluorides is kept ≤ 10 mass ppm.

5. Optical element according to at least one of the preceding claims, characterized in that the proportion of one or more additional is contained in one or more layers of elements ≤ 5 mass ppm.

6. Optical element according to at least one of the preceding claims, characterized in that the rare earth metals from the group of

Lanthanides are selected.

7. Optical element according to at least one of the preceding claims, characterized in that the metallic fluorides selected from MgF ₂ , YF ₃ , LaF ₃ , AlF ₃ , NdF ₃ , BaF ₂ , Chiolith, DyF ₃ ,

GdF _{3 /} cryolite, LiF, NaF, LuF ₃ , SmF ₃ , SrF ₂ , TbF ₃ , YbF ₃ , ZrF ₄ .

8. Optical element according to one of claims 1 to 7, characterized in that it is designed as an optical reflector, antireflection, optical lens, polarizer, polarization splitter, phase delay plate, optical window, beam splitter or the optical grating.

9. Use of an optical element according to one of claims 1 to 8, for lithographic applications, laser technology, for monochromatization, for spectral analysis, spectral decomposition, transmission increase or reflection reduction.

10. A method for producing an optical element according to any one of claims 1 to 8, characterized in that one or more layers of at least one metallic fluoride on a substrate in a vacuum by thermal evaporation or electron beam evaporation, ion-assisted deposition or plasma-assisted evaporation is / are being trained, powdery or granular metallic fluorides, the proportion of which is kept at least one additional element, which is ingested within at least one predefinable wavelength range of the electromagnetic radiation, Masse mass 10 ppm, or

by a PVD / CVD process reactively using metallic targets, in which the proportion of at least one additional element which luminesces within at least a predefinable wavelength range of the electromagnetic radiation, ≤ mass 10 ppm, is used.

11. The method according to claim 10, characterized in that powdered or granular metallic fluorides are subjected to a cleaning process before evaporation in vacuo.

12. The method according to claim 11, characterized in that the cleaning by

Zone melting and / or vacuum sublimation is carried out.

13. A method for determining the optical properties and / or layer qualities in optical elements according to one of claims 1 to 8, characterized in that the one or more layer (s) formed from metallic fluorides is irradiated with electromagnetic radiation of a predeterminable wavelength and the luminescence intensity is determined with an optical detector.

14. The method according to claim 13, characterized in that a laser light source is used for the irradiation.

15. The method according to claim 13 or 14, characterized in that for the irradiation electromagnetic radiation with a shorter wavelength than that of the respective luminescent radiation is used.

16. The method according to any one of claims 13 to 15, characterized in that with certain wavelengths of electromagnetic radiation, the presence or absence of certain elements in layers or alternating layer systems formed from metallic fluorides is determined.