WO2010015296A2

WO2010015296A2 - Transmitting optical element consisting of magnesium-aluminium spinel

Info

Publication number: WO2010015296A2
Application number: PCT/EP2009/004534
Authority: WO
Inventors: Wilfried Clauss; Yvonne Koehne; Christian Thiele
Original assignee: Carl Zeiss Smt Ag
Priority date: 2008-08-06
Filing date: 2009-06-24
Publication date: 2010-02-11
Also published as: WO2010015296A3; DE102008041055A1

Abstract

The invention relates to an optical element (133) for transmitting radiation in wavelengths of less than 250 nm, especially of 193 nm, consisting of a material containing magnesium-aluminium spinel. In the optical element (133), the ratio (V) of the aluminium part in the magnesium-aluminium spinel to the magnesium part in the magnesium-aluminium spinel varies by less than 200 ppm, preferably less than 100 ppm, especially preferably less than 50 ppm, and/or is between 2.0 and 2.2, preferably between 2.02 and 2.10, especially preferably between 2.04 and 2.08. The invention also relates to a projection objective and a projection illumination system comprising such an optical element, and to a production method for a blank (1) for producing the optical element (133).

Description

Transmitting optical element of magnesium-aluminum spinel

Background of the invention

The invention relates to an optical element, in particular with a diameter of at least 30 mm, for the transmission of radiation at wavelengths of less than 250 nm, in particular at 193 nm, from a magnesium-aluminum spinel-containing material, as well as a projection objective and a projection exposure apparatus such an optical element. The invention also relates to a method for producing a blank for such an optical element.

Microlithography projection exposure equipment is used to fabricate semiconductor devices and other finely-structured components. Such a projection exposure apparatus includes a light source and a lighting system for illuminating a photomask or reticle, often called a reticle, a projection lens which projects the pattern of the reticle onto a photosensitive substrate, such as a silicon wafer coated with a photoresist.

To produce ever smaller structures on the order of less than 100 nm, three approaches have hitherto been pursued. First, an attempt is made to further increase the image-side numerical aperture NA of the projection objectives. Secondly, the wavelength of the illumination light is continually reduced, preferably to UV wavelengths, in particular wavelengths below 250 nm, for example 248 nm or 193 nm. Finally, further measures to improve the resolution are used, such as phase-shifting masks, multipole illumination or oblique illumination.

Another approach to increasing the resolution is immersion lithography. In this case, in the intermediate space, which is also called resist between a last optical element, in particular a lens, of the projection objective and the photoresist, or another photosensitive light to be exposed Layer remains, introduced an immersion liquid. Projection objectives that are designed for immersion operation are also referred to as immersion objectives.

The advantages of immersion lithography lie in the fact that the higher refractive index of the immersion liquid compared to the vacuum reduces the exposure wavelength to an effective exposure wavelength. This is accompanied by an increase in the resolution and the depth of field. When using high refractive index immersion liquids, significant increases in the angle of incidence into the resist are possible compared to systems without immersion. However, in order to make maximum use of the advantage of high-index immersion liquids, it is necessary for the last optical element of the projection objective, which is in contact with the immersion liquid, to have a high refractive index.

As material for optical elements in an objective of a projection exposure apparatus for microlithography, quartz glass is generally used at UV wavelengths, in particular wavelengths below 250 nm, which has a refractive index of about 1.56 at a wavelength of 193 nm. Furthermore, a number of high-index materials are known, which have a refractive index of 1, 6 or more at a wavelength of 193 nm, including magnesium-aluminum spinel (MgAlO ₄ , refractive index about 1, 92), lutetium-aluminum garnet 2.17), yttrium aluminum garnet (Y ₃ Al ₅ O 12, refractive index about 2), magnesium oxide (MgO, refractive index about 1.96), sapphire (Al 2 O 3, refractive index about 1, 92) or mixtures of these materials.

These optical elements made of high refractive index materials can be either monocrystalline or polycrystalline. In monocrystalline materials with cubic crystal structure, such as MgO, CaO, SrO or BaO, as well as mixed crystals such as MgAI ₂ O ₄ or Li _1-X Mg _x Al ₂ O ₄ makes in the wavelength range below 260 nm and even more at shorter operating wavelengths such as 193 nm the effect of intrinsic birefringence is noticeable. The dependence of the refractive index on the polarization state of the incident light caused by the intrinsic birefringence restricts the imaging quality of the projection objectives produced with these materials. It is therefore complex compensation measures, such as special lens designs with Combinations of different lens materials or crystal orientations required to ensure a sufficient image quality of such projection lenses.

In WO 2006/061225 A1, it has therefore been proposed to use optical elements made of polycrystalline high refractive index material, in particular of polycrystalline magnesium-aluminum spinel (MgAl ₂ O ₄ ), in a microlithographic projection exposure apparatus. Due to the statistical orientation of the crystal axes of the individual crystalline units, also referred to below as crystallites, in a polycrystalline solid, the mean value of the intrinsic birefringence approaches zero in all spatial directions. This eliminates complicated devices for compensation of intrinsic birefringence. Magnesium-aluminum spinel has a high refractive index of more than 1.8 at a wavelength of 193 nm and is therefore particularly suitable for immersion lithography.

It is known from WO 2008/074503 that most of the foreign elements contained in a lens blank, which typically consist of transition metals (eg Fe, Cr, Mn), have a greater absorption in a higher oxidation state than in a low oxidation state because the absorption bands of these foreign element ions in the higher oxidation state are in the range of the useful wavelength below 250 nm. Furthermore, it is known from WO 2008/074503 that the transmission can be reduced if aluminum ions occupy lattice sites on which magnesium ions are arranged in an ideal crystal. This problem can occur when the ratio between the aluminum content in the magnesium-aluminum spinel and the magnesium content in the magnesium-aluminum spinel deviates from the stoichiometric ratio of 2: 1. It states that sufficient transmission at a wavelength of 193 nm can be ensured if the Al / Mg ratio deviates by more than 4% from the stoichiometric ratio of 2: 1, with a maximum deviation of 2% being preferred.

Object of the invention

The object of the invention is to provide an optical element comprising a material containing magnesium-aluminum spinel, a projection objective and a projection exposure. To specify plant with such an optical element, a blank for the production of such an optical element and a manufacturing method for the blank, which are suitable for use in the (immersion) lithography.

Subject of the invention

This object is achieved by an optical element of the type mentioned, in which in the volume of the optical element, the ratio of the aluminum content in the magnesium-aluminum spinel to the magnesium content in the magnesium-aluminum spinel to less than 200 ppm, preferably to less than 100 ppm, more preferably less than 50 ppm.

In addition to the transmission of an optical element, it is very important for the optical quality of the system to adhere as accurately as possible to the optical properties predetermined by the optical design of the optical system, in particular as regards the refractive index. It is known from the literature that the composition of spinel, in particular the ratio of aluminum to magnesium, can be varied within wide limits. Thus, e.g. a large excess of aluminum can be incorporated into the spinel lattice without collapsing the spinel structure. However, it has not been known hitherto whether or how much the compositional variation alters the refractive index of both monocrystalline and polycrystalline spinel material at wavelengths of 250 nm or less, especially at 193 nm.

The following definition is introduced for the quantitative description: Ideal Mg-Al spinel has the chemical composition MgAl ₂ O ₄ . In the following text, this name is still retained as a shorthand for the general name of spinel, even if the stoichiometry deviates from the ideal value. A deviation from the ideal stoichiometry is described in each case by the ratio V between the aluminum content and the magnesium content of the entire element composition. Any variations of the measured total metal contents compared to the oxygen content, which are difficult to detect by measurement, are not relevant for this ratio formation. The inventors have found that even slight variations of the ratio V lead to a large variation in the refractive index n at 193 nm, which is also referred to below as n (193). In addition to an absolute value of the ratio V as precisely as possible, it is also favorable if the ratio V over the volume of the optical element undergoes only slow fluctuations, ie the gradient of the Al / Mg ratio does not become too large in order to achieve high optical homogeneity , The optical homogeneity can also be improved by suitable control of the chemical composition of the optical element. For microlithography, the highest demands are placed on optical homogeneity.

For the evaluation of the optical homogeneity, as shown in FIG. 4, a plane-parallel, polished lens blank 200 ("blank") is measured by an interferometric method in which the deformation ΔL (x, x) caused by refractive index variations within the volume of the lens blank 200 is measured. y) a plane optical or electromagnetic wave 202 is spatially resolved after passing through the material of the lens blank 200 as a function of the location (x, y) transverse to the propagation direction z of the plane wave 202. If one forms the quotient of the wavefront deformation ΔL (x, y) with the thickness D of the optical material of the lens blank 200 traversed by the shaft, the value of the deviation of the refractive index averaged over the thickness D results from the average refractive index of the entire lens blank 202:

Δn = ΔL (x.y) / D.

The inventors have found that the main cause of the measured distribution Δn (x, y) is the variations ΔV in the volume of the lens blank 202 of the ratio V (x, y, z), where at a fixed location (x1, y1 ) gives the deviation from the mean refractive index Δn (x1, y1) as summation (integral) of the variations of the ratio V along the z-direction which occur at the location (x1, y1) or is proportional to this integral:

Δn (x1, y1) ~J (dV (x1, y1) / dz) dz According to the invention, it is therefore proposed to keep the ratio V as constant as possible over the entire volume of the optical element, ie at two arbitrary points of the volume of the optical element this ratio may at most vary within the limits specified above.

In a particularly advantageous embodiment, in the magnesium-aluminum spinel, the spatial variation of the ratio V at any two points of the optical element with a distance of less than 2 mm is less than 50 ppm, preferably less than 20 ppm. In this way, a particularly high optical homogeneity of the optical element can be achieved.

As is known to the pertinent experts, the long-wave, slowly varying components can be separated off from the distribution Δn (x, y) by suitable mathematical methods (Zernike development, Fourier filtering), wherein the lower limit frequency can typically be selected to be about 1 / cm , The standard deviation of the remaining fast varying component of the function is hereafter used, as is common practice, as a measure to assess the quality of the optical homogeneity:

, 2

RestRMS = 1 / N ^* JΣ ( _{W /} -Λ)

where n denotes the refractive index averaged over the total number N of measured values ni.

In a particularly advantageous embodiment, the variation of the ratio V dependent refractive index of the magnesium-aluminum spinel of the optical element, defined by the residual RMS value at a wavelength of less than 250 nm, in particular at 193 nm, less than 2 ppm , preferably less than 0.5 ppm. If the Al / Mg ratio is determined as precisely as possible in the spinel and this has a low spatial variation, then a high optical homogeneity can be achieved, which is expressed in a low residual RMS (root-mean-square) value in the refractive index , In a particularly advantageous embodiment, the ratio of the aluminum content to the magnesium content is in the range between 2.0 and 2.2, preferably in the range between 2.02 and 2.10, particularly preferably in the range between 2.04 and 2, 08th

Experiments have shown that the choice of the ratio V also influences the transmission of the magnesium-aluminum spinel. The inventors have found that better transmission samples have an Al / Mg ratio greater than the ideal spinel ratio of 2: 1. Samples with an excess of magnesium, i. with a ratio of less than 2: 1, however, had a lower transmission. Contrary to the prejudice prevailing in the prior art that the Al / Mg ratio should be set as accurately as possible to 2: 1, it is more favorable for transmission at wavelengths of less than 250 nm, in particular at 193 nm, if this ratio is greater than 2 , 0 and is in particular in the above ranges.

In an advantageous embodiment, the material containing magnesium-aluminum spinel is polycrystalline and is prepared by well-known sintering processes far below the melting temperature. In the case of polycrystalline spinel, the reason for the reduced transmittance at a value of the ratio V <2 is that the phase diagram shows a high Al excess but at most a slight or no Mg excess in the spinel lattice at low Temperatures (<1400 °) is stable, so that in a spinel with Mg excess, the magnesium tends to segregate as MgO (periclase), which can have a negative effect on the transmission behavior by greatly increased scattering. However, the phase diagrams known from the literature do not allow to predict such segregations with sufficient accuracy. The choice of an Al / Mg ratio of more than 2: 1 to produce a high transmission in magnesium-aluminum spinel is thus a completely new finding.

In an alternative embodiment, the material containing magnesium-aluminum spinel is monocrystalline. With monocrystalline spinel, too, with Mg excess, increased scattering can occur, which can have a negative effect on the transmission. The desired Al / Mg ratio can be set here with the required accuracy in the growth of the single crystal. Both in polycrystalline In both the case of monocrystalline and spinel crystals, care must be taken that the Al / Mg ratio varies as little as possible over the entire volume of the optical element.

Another advantage of a spinel material with a ratio V> 2 is the lower density of oxygen vacancies resulting therefrom. In contrast, with Mg excess (V <2), a higher density of oxygen vacancies must prevail in order to adjust the charge neutrality. This has a direct effect on the transmission at 193 nm, which is due to the known absorption band at about 235 nm.

Particularly preferably, the optical element has an absorption coefficient k of at most 0.2 cm ^-1 , preferably of at most 0.1 cm ^-1 , in particular of at most 0.01, at at least one wavelength between 190 nm and 250 nm, in particular at 193 nm cm ^'1. By choosing a suitable Al / Mg ratio, the transmission of the optical element can be improved and, for a good transmission behavior, it must be ensured that the concentration of impurity impurities in the optical element is as low as possible Absorption coefficient k is given by:

k = - log (T / F) / d,

where T denotes the transmission, F the Fresnel correction factor and d the sample thickness. For spinel as a high refractive index material, the refractive index at 193 nm is 1.92 and the Fresnel correction factor is 0.819, at 248 nm the refractive index is 1.884 and the Fresnel correction factor is 0.846.

In an advantageous embodiment, the total concentration of foreign element impurities in the magnesium-aluminum spinel, in particular caused by transition metals, is less than 50 ppm, preferably less than 20 ppm. As described in the Applicant's WO 2008/074503, which is incorporated herein by reference in its entirety, experiments in which the spinel material has been doped with foreign elements have shown concentrations of the individual foreign elements in the spinel material can be tolerated, yet to ensure a high transmission. θ

In a preferred embodiment, the foreign element impurities by iron and chromium in the magnesium-aluminum spinel each have a concentration of less than 0.5 ppm. In the doping experiments it has been shown that, in particular, iron and chromium must be present only in very low concentrations in the magnesium-aluminum spinel material in order to ensure a high transmission.

A further aspect of the invention is embodied in an optical element for transmitting radiation at wavelengths of less than 250 nm, in particular at 193 nm, of a magnesium-aluminum spinel-containing, in particular monocrystalline material in which the ratio V in magnesium-aluminum Spinel is in the range between 2.0 and 2.2, preferably in the range between 2.02 and 2.10, more preferably in the range between 2.04 and 2.08. As indicated above, it is beneficial for the transmission behavior of the magnesium-aluminum spinel when the Al / Mg ratio is greater than 2: 1. It goes without saying that it is favorable to comply at the same time with the ranges of values described above for the variation of this ratio, in order to ensure the most constant refractive index or the best possible optical homogeneity.

Another aspect is realized in a projection objective for microlithography with at least one optical element as described above, wherein the optical element is arranged in the projection objective and preferably forms the image-side termination element of the projection objective, and wherein the design of the projection objective for a target refractive index of the optical element is designed, of which the refractive index of the optical element by less than 5 x 10 ^-5, preferably less than 1 x 10 ^"5 deviates. by adjusting a constant in the above-specified limits Al / Mg ratio in the blank or the optical element, the refractive index can be set very precisely, so that this corresponds almost exactly to the predetermined value by the design.

A further aspect of the invention is realized in a projection exposure apparatus for microlithography, in particular for immersion lithography, with at least one optical element as described above and / or a projection objective as described above. The optical element of magnesium-aluminum spinel with The properties described above have sufficient transmission and sufficient homogeneity to be used in a microlithographic projection exposure apparatus. Due to the high refractive index of the magnesium-aluminum spinel, the optical element is particularly suitable as a termination element of the projection lens, which is brought into contact with an immersion liquid.

The invention is also realized in a method for producing a blank of a magnesium-aluminum spinel-containing polycrystalline material which transmits at wavelengths of less than 250 nm, in particular at 193 nm, comprising: providing a starting powder containing magnesium oxide and alumina, compressing the starting powder a green compact, and sintering of the green compact at temperatures of 1300 ⁰ C or more for the production of the blank, wherein the magnesium content in the starting powder and the aluminum content in the starting powder and the process control during sintering are coordinated such that a ratio of the aluminum content to the magnesium content in the magnesium-aluminum spinel of the blank, which is in the range between 2.0 and 2.2, preferably in the range between 2.02 and 2.10, particularly preferably in the range between 2.04 and 2.08. It is known from the literature that the reduction rates of Mg and Al are strongly temperature-dependent. Depending on the selected temperature control in the sintering process, therefore, the ratio of Al / Mg may change relative to the powder value, so that the choice of the starting concentrations of Al and Mg in the starting powder must be coordinated with the sintering process in order to set the desired ratio in the blank. In order to ensure a high transmission, it is advantageous to align the starting ratio of the production process carried out with the starting powder as a whole so that an Al excess in the finished ceramic of the blank compared to the 1: 1 state of Al ₂ O ₃ : MgO im ideal, ie stoichiometric spinel sets. From such a blank, an optical element having the above-described properties can be produced. For this purpose, the optical element can be cut from the blank, wherein the shape of the blank can already be adapted to the shape of the optical element. As a rule, a further treatment by polishing, coating, etc. is carried out in order to be able to operate the optical element in an optical arrangement. In a particularly advantageous variant, the starting powder is compacted into a green compact, wherein in the green compact the ratio of the aluminum fraction to the magnesium fraction varies by less than 200 ppm, preferably by less than 100 ppm, more preferably by less than 50 ppm. For this purpose, the Al / Mg ratio in the starting powder is adjusted with the required accuracy.

In a preferred variant, a spinel powder is provided as the starting powder. In this case, the stoichiometry of the spinel powder is already adjusted with the required accuracy, with possibly possibly process-dependent losses must be maintained.

In an alternative variant, to provide the starting powder, a magnesium oxide powder and an alumina powder are preferably mixed in a mixed grinding process. The stoichiometry can also be set with the required accuracy. The required spatial homogeneity of the mixture hereby may be e.g. be ensured by a Mischmahlungsprozess.

Further features and advantages of the invention will become apparent from the following description of 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 realized individually for themselves or for several in any combination in a variant of the invention.

drawing

Embodiments are illustrated in the schematic drawing and will be explained in the following description. Show it:

FIGS. 1a-c show schematic diagrams of process steps in the production of a magnesium-aluminum spinel blank; 2 is a schematic representation of the relationship between refractive index and the ratio of the aluminum content to the magnesium content in magnesium-aluminum spinel,

Fig. 3 is a schematic representation of a projection exposure system for

Immersion lithography with an optical element produced from the blank of Fig. 1c, and

4 shows an illustration of a lens blank when carrying out an interferometric measurement for determining the variation of the refractive index in the volume of the lens blank.

In Fign. 1a-c schematically show a variant of a method for producing a blank 1 made of polycrystalline magnesium-aluminum spinel, which transmits radiation at wavelengths of less than 250 nm, in particular at 193 nm. In the method, first, an alumina (Al ₂ O ₃ ) powder 2 and a magnesium oxide (MgO) powder 3 are mixed in a mixing grinder 4, and thus a starting powder 5 for producing the blank 1 is produced. The mixing-milling process can ensure that the starting powder 5 has the desired homogeneity and the desired structure size of the individual crystallites. Furthermore, the ratio V _A between aluminum and magnesium in the starting powder 5 can be adjusted in a particularly simple way by supplying the desired proportions of aluminum powder 2 and magnesium powder 3 to the mixing grinder 4. It is understood that, alternatively, a starting powder 5 made of magnesium-aluminum spinel can be used, ie a powder which consists of crystallites of magnesium-aluminum spinel, in which the desired Al / Mg ratio V _A by suitable, known Production process has been discontinued.

To produce a polycrystalline optical element or blank 1 with good transmission properties, the starting powder 5 must have the highest possible purity, ie its proportion of foreign element impurities should be as low as possible and their total concentration should not be 50 (wt) ppm exceed. In particular, the proportion of foreign elements, which should have a particularly negative influence on the transmission, such as iron or chromium, to be as low as possible and are each less than 0.5 (ppm by weight). An initial powder 5 having such a low content of impurity impurities can be obtained by repeatedly performing known purification processes until the desired purity is obtained.

In a subsequent process step shown in FIG. 1 b, the starting powder 5 is compacted dry or in liquid at room temperature to form a so-called green body 6 by means of a compression device indicated by an arrow. In a further, shown in Fig. 1c, the green compact 6, which still has relatively large pores and even channels, at temperatures Ti of, for example, about 1400 ⁰ C or 1700 ⁰ C under ambient pressure p _u in ambient atmosphere 7a in an oven. 7 sintered. This process step leads to a further densification of the green body 6, so that there are no more channels and the remaining isolated pores have a smaller size than before sintering. The sintered green compact 6 is also referred to below as blank 1. One or more annealing steps can subsequently be carried out on the blank 1, in particular the so-called hot isostatic pressing (HIP) _1, in which the blank 1 is exposed to comparable temperatures under sintering for several hours at a pressure of typically more than 100 MPa To achieve compaction.

In the context of FIGS. 1a-c described process, the magnesium content in the starting powder 5 and the aluminum content in the starting powder 5 and the process control during sintering and possibly further subsequent to the sintering process steps are coordinated so that a desired Al / Mg Ratio V in the blank 1 sets. In particular, the Al / Mg ratio V in the blank 1 due to the process may differ from the Al / Mg ratio V _A in the starting powder 5 since it is known from the literature that the reduction rates of magnesium and aluminum are highly temperature dependent. Depending on the selected temperature control in the sintering process, the Al / Mg ratio may therefore change compared to the powder value. The Al / Mg ratio V _A in the starting powder 5 is therefore to be matched to the process control such that a change in this ratio caused by the process control leads precisely to the desired Al / Mg ratio V in the blank 6. The desired Al / Mg ratio V in the blank 6 is chosen so that the absorption coefficient k is as small as possible. In the experimental investigation of the chemical composition of spinel samples, it was found that samples with better transmission at a wavelength of 193 nm systematically differ in the stoichiometric Al / Mg ratio V of lower transmission samples. Samples with better transmission generally have a higher Al concentration. Conversely, samples without Al excess or Al deficiency (Mg excess) show inferior transmission values. Associated experimental results on a plurality of samples are shown in the following Table 1:

Table 1

One explanation for this behavior could be that in the phase diagram of magnesium-aluminum spinel, although a high Al excess, but at most a slight or no excess of magnesium in the spinel lattice at low temperatures (<1400 °) is stable , so that in a spinel with Mg excess, the magnesium tends to segregate as MgO (periclase), and has a negative effect on the transmission behavior by greatly increased scattering. Therefore, it is advantageous to align the output ratio V _A in the starting powder 5 and the manufacturing process carried out with the starting powder 5 in such a way that an excess of aluminum in the finished ceramic of the blank 1 compared to the 1: 1 state of Al 2 O 3: MgO in the ideal spinel setting. The Al / Mg ratio V should in this case be in the range between 2.0 and 2.2, with an aluminum excess in the range of approximately 2 to 3% already causing a marked increase in the transmission. It is understood that also in the case of monocrystalline spinel above considerations are relevant, since there may also be increased scattering in the case of excess magnesium, so that even there the choice of the Al / Mg ratio should be within the range specified above.

Furthermore, it is favorable if the Al / Mg ratio V in the volume of the blank 6 is as constant as possible, i. if this varies by less than 200 ppm, preferably by less than 100 ppm, in particular by less than 50 ppm, since not only the transmission of the magnesium-aluminum spinel blank 6, but also other optical properties, e.g. whose refractive index, depend on this ratio.

For the determination of the absolute value of the refractive index of magnesium-aluminum spinel, it is possible to extrapolate from the literature known Sellmeier curves for magnesium oxide and aluminum oxide, which have been determined for visible wavelengths, to wavelengths below 250 nm. This results in a value for n (193) of about 1, 92 or 1, 96 for the pure oxides of aluminum and magnesium. For spinel with a ratio of Al / Mg of about 2: 1, n (193) is also in the range of about 1.92, so the maximum refractive index difference to the extreme case of pure oxides is on the order of about 0.04. This rough (linear) estimate thus gives an order of magnitude of the sensitivity of max. 0.04 with variations of the Al / Mg ratio in the range of 100%. However, as shown by exact determinations of the Al / Mg ratio V in different samples, even small variations in the ratio V lead to a large change in the refractive index at 193 nm. For the quantitative determination of the dependence of the refractive index n (193) on the ratio V Therefore, an ellipsometric refractive index measurement at a wavelength of 193 nm was performed on three samples which were far apart in the Al / Mg ratio, the results of which are shown in FIG. For the three measurement points in FIG. 2, a regression line was determined using the method of least squares, for which the following applies:

n (V) = -0.07V + 2.06.

It is understood that this is also an approximation, but in the same order of magnitude as the above estimate. Also, the relative refractive index difference in a sample allows the ratio Δn /

Estimate ΔV (Al / Mg) according to the slope of the straight line of FIG. Interferograms were used to read a maximum stroke (without deduction of long-wave zernike components) for the stoichiometry-induced variations of the refractive index Δn (x, y) of about 50-100 ppm over the entire sample volume. These are to be compared with the variations found in energy-dispersive X-ray spectroscopy (EDX) and ellipsometry within a sample, which are approximately 0.01, resulting in a slope of

Δn / ΔV (Al / Mg) = 50 E-6 / 0.01 = 0.005.

Although this value is smaller by a factor of about 10 than the slope determined above from the absolute values. However, since the interferogram provides a value for the refractive index variation averaged over the sample thickness, this is not a contradiction, cf. also the considerations made above in connection with FIG. 4.

In addition to the lowest possible absolute variation of the Al / Mg ratio in the volume of the blank 6, it is therefore advantageous if the spatial variation (the gradient) of this ratio in the volume of the blank is as low as possible. Thus, the spatial variation of the ratio V of the aluminum content to the magnesium content at any two points P1, P2 of the optical element, shown by way of example in FIG. 1c, should be less than 2 mm at less than 50 ppm, preferably less than 20 ppm. In this way it can be ensured that the e.g. by interferometry determinable residual RMS value, which indicates the spatial variation of the refractive index is in the volume of the blank 6 at less than 2 ppm, preferably less than 0.5 ppm.

From the treated in the manner described above, usually cylindrically symmetrical and / or already oriented on the final geometry blank 6 of the polycrystalline magnesium spinel is a subsequent transmissive optical element 133 produced by subsequent, not described in detail here grinding and polishing processes, which the same optical Material properties as the blank 1 and used in a projection exposure apparatus 101 for microlithography can be, which will be described below in connection with FIG. 3.

FIG. 3 schematically shows a microlithographic projection exposure apparatus 101, which is provided for the production of highly integrated semiconductor components by means of immersion lithography. The projection exposure apparatus 101 comprises as the light source an excimer laser 103 with a working wavelength of 193 nm. Alternatively, light sources of other working wavelengths, for example 248 nm or 157 nm, could also be used. A downstream illumination system 105 generates in its exit plane or object plane 107 a large, sharply delimited illumination field which is very homogeneously illuminated and adapted to the telecentricity requirements of the downstream projection objective 111. The illumination system 105 has means for controlling the pupil illumination and setting a predetermined polarization state of the illumination light. In the beam path behind the illumination system 105, a device (reticle stage) for holding and moving a mask 113 is arranged such that it lies in the object plane 107 of the projection lens 111 and can be moved in a departure direction 115 in this plane for scanning operation.

The object plane 111, which is also referred to as the mask plane, is followed by the projection objective 111, which images an image of the reduced-scale mask onto a substrate 119, for example a silicon wafer, which is provided with a photoresist, also called resist 121. The substrate 119 is arranged such that the planar substrate surface coincides with the resist 121 substantially with the image plane 123 of the projection lens 111. The substrate is held by a device 117 which includes a drive to move the substrate 119 in synchronism with the mask 113. The device 117 also includes manipulators for moving the substrate 119 both in the z direction parallel to the optical axis 125 of the projection lens 111 and in the x and y directions perpendicular to this axis.

The device 117 (wafer stage) provided for holding the substrate 119 is constructed for use in immersion lithography. It comprises a recording device 127, which can be moved by a scanner drive, for an immersion liquid 131, the bottom of which has a flat recess for receiving the substrate 119. The projection objective 111 has an image-side numerical aperture NA of at least NA = 1, 2, but preferably of more than 1, 35, particularly preferably of more than 1.5, and is thus particularly adapted to the use of immersion liquids of high refractive index. The projection objective 111 has, as the last, the image plane 123 next optical element, a hemispherical plano-convex lens 133 whose exit surface 135 is the last optical surface of the projection objective 111. The exit side of the last optical element is completely immersed in the immersion liquid 131 during operation of the projection exposure apparatus 101 and is wetted by the latter. The hemispherical plano-convex lens 133 is made of polycrystalline magnesium-aluminum spinel. By contrast, the further lens 137 in the projection objective 111 consists of monocrystalline magnesium-aluminum spinel. In the growth of the magnesium-aluminum spinel single crystal, the Al / Mg ratio was adjusted so that the further lens 137 has the same optical material properties (transmission, refractive index and, in particular, spatial variation of the refractive index) as the plano-convex lens 133. It will be understood itself that, alternatively, the plano-convex lens 133 may consist of monocrystalline magnesium-aluminum spinel and the further lens 137 may consist of polycrystalline material.

Optionally, the plano-convex lens 133 and the further lens 137 are provided with an anti-reflection coating. This anti-reflection coating has a sequence of alternating low-refractive and high-refractive materials. Suitable low-index materials are, in particular at a working wavelength of the projection exposure apparatus 101 of 193 nm, MgF.sub.2, AIF.sub.3, Na.sub.5 Al.sub.3Fi.sub.4, Na.sub.3AIF.sub.6, SiO.sub.2, LiF and NaF. Suitable high-index materials include LaF3, GdF3, NdF3, Al2O3 and DyF3.

The optical design of the projection objective 111 is optimized here for an optical element with a desired refractive index n _{s of} the plano-convex lens 133, which may be, for example, 1.9201. The plano-convex lens 133 in this case has a refractive index n _e in which from this target refractive index n _s in absolute value by less than 5 x 10 ^"5, preferably less than 1 x 10 ^'5 is different. This requirement can be met if the Al / Mg ratio in the volume of plano-convex lens 133 varied by less than 200 ppm. In summary, in the manner described above, it is possible to use a polycrystalline or monocrystalline magnesium-aluminum spinel material for (immersion) lithography at wavelengths of less than 250 nm, especially at 193 nm. It is understood that optical elements of the type described above can also be used in areas other than microlithography, in which a high transmission for UV radiation and a refractive index, which can be met with high accuracy, are required.

Claims

claims

1. Optical element (133, 137), in particular with a diameter of at least 30 mm, for the transmission of radiation at wavelengths of less than 250 nm, in particular at 193 nm, from a magnesium-aluminum spinel (MgAI ₂ O ₄ ) containing Material, wherein in the volume of the optical element (133, 137) the ratio (V) of the aluminum content in the magnesium-aluminum spinel (MgAl ₂ O ₄ ) to the magnesium content in the magnesium-aluminum spinel (MgAl ₂ O ₄ ) by less than 200 ppm, preferably by less than 100 ppm, more preferably by less than 50 ppm.

2. An optical element according to claim 1, wherein in the magnesium-aluminum spinel (MgAl ₂ O ₄ ), the spatial variation of the ratio (V) at any two points (P1, P2) of the optical element (133, 137) with a Distance less than 2 mm at less than 50 ppm, preferably less than 20 ppm.

The optical element according to claim 1 or 2, wherein the variation of the ratio (V) dependent refractive index (n (V)) of the magnesium-aluminum spinel (MgAl ₂ O ₄ ) of the optical element (133, 137) defines by the residual RMS value at a wavelength of less than 250 nm, in particular at 193 nm, less than 2 ppm, preferably less than 0.5 ppm.

4. Optical element according to one of the preceding claims, wherein the ratio (V) in the range between 2.0 and 2.2, preferably in the range between 2.02 and 2.10, particularly preferably in the range between 2.04 and 2 , 08 lies.

5. An optical element according to any one of the preceding claims, wherein the material containing magnesium-aluminum spinel (MgAl ₂ O ₄ ) is polycrystalline.

6. An optical element according to any one of claims 1 to 4, wherein the magnesium-aluminum spinel (MgAI ₂ O ₄ ) containing material is monocrystalline.

7. Optical element according to one of the preceding claims, which at least one wavelength between 190 nm and 250 nm, in particular at 193 nm, an absorption coefficient k of at most 0.2 cm ^-1 , preferably of at most 0.1 cm ^-1 , in particular of at most 0.01 cm ^-1 .

An optical element according to any one of the preceding claims, wherein the total concentration of impurity foreign elements in the magnesium-aluminum spinel (MgAl ₂ O ₄ ) is less than 50 ppm, preferably less than 20 ppm.

9. An optical element according to any one of the preceding claims, wherein the impurity elements by iron (Fe) and chromium (Cr) in the magnesium-aluminum spinel (MgAI ₂ O ₄ ) each have a concentration of less than 0.5 ppm.

10. Optical element (133, 137) for the transmission of radiation at wavelengths of less than 250 nm, in particular at 193 nm, from a magnesium-aluminum spinel (MgAI ₂ O ₄ ) containing, in particular monocrystalline material in which the ratio ( V) of the aluminum content in the magnesium-aluminum spinel (MgAl ₂ O ₄ ) to the magnesium content in the magnesium-aluminum spinel (MgAl ₂ O ₄ ) in the range between 2.0 and 2.2, preferably in the range between 2 , 02 and 2.10, more preferably in the range between 2.04 and 2.08.

11. Microlithography projection objective (111) with at least one optical element (133, 137) according to one of claims 1 to 10, wherein the optical element (133, 137) is arranged in the projection objective (111) and preferably the image-side terminating element (11). 133) of the projection objective (111), and wherein the design of the projection objective (111) is designed for a desired refractive index (n _s ) of the optical element (133) of which the refractive index (n _e ) of the optical element (133) by less than 5 x 10 ^"5, preferably less than 1 x ^10" ⁵ deviates.

12 projection exposure apparatus (101) for microlithography, in particular for immersion lithography, with at least one optical element (133, 137) according to any one of claims 1 to 9 and / or a projection lens (111) according to claim 11.

13. A method of producing a polycrystalline material containing magnesium aluminum spinel (MgAl ₂ O ₄ ) at wavelengths less than 250 nm, in particular at 193 nm, comprising: providing a magnesium oxide (MgO) and alumina (AI2O3) containing starting powder (5),

Compacting the starting powder (5) to a green compact (6), and sintering the green compact (6) at temperatures of 1300 ⁰ C or more for the production of the blank (1), wherein the magnesium content in the starting powder (5) and the aluminum Part in the starting powder (5) and the process control during sintering are coordinated so that a ratio (V) of the aluminum content to the magnesium content in the magnesium-aluminum spinel (MgAI ₂ O ₄ ) of the blank (1) which is in the range between 2.0 and 2.2, preferably in the range between 2.02 and 2.10, particularly preferably in the range between 2.04 and 2.08.

14. The method according to claim 13, wherein as the starting powder (5) a spinel powder is provided.

A method according to claim 13, wherein for providing the starting powder (5), an alumina powder (2) and a magnesia powder (2) are preferably mixed in a compound grinding process.

16. The method according to any one of claims 13 to 15, wherein the starting powder (5) to a green compact (6) is compressed, wherein in the green compact (6) the ratio (V) of the aluminum content to the magnesium content by less than 200 ppm, preferably less than 100 ppm, more preferably less than 50 ppm.