DE102014223411A1 - Arrangement and method for measuring the intensity of electromagnetic radiation in an optical system - Google Patents

Abstract

The invention relates to an arrangement and a method for measuring the intensity of electromagnetic radiation in an optical system, in particular an optical system of a microlithographic projection exposure apparatus or a mask inspection system. An inventive arrangement for measuring the intensity of electromagnetic radiation in an optical system, wherein the optical system is designed for a working wavelength less than 100nm, comprises a detector material, wherein the measurement of the intensity is based on an interaction of the electromagnetic radiation with this detector material, and wherein the detector material comprises a group III nitride.

Description

The invention relates to an arrangement and a method for measuring the intensity of electromagnetic radiation in an optical system, in particular an optical system of a microlithographic projection exposure apparatus or a mask inspection system.

Microlithography is used to fabricate microstructured devices such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus which has an illumination device and a projection objective. The image of a mask (= reticle) illuminated by means of the illumination device is hereby projected onto a substrate (eg a silicon wafer) coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection objective in order to apply the mask structure to the photosensitive coating of the Transfer substrate.

Mask inspection equipment is used to inspect reticles for microlithographic projection exposure equipment.

In EUV-designed optical systems, e.g. EUV projection lenses, i. at wavelengths of e.g. about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process, due to the lack of availability of suitable translucent refractive materials. The use of faceted mirror arrangements with a multiplicity of mirror elements (for example in the form of a field facet mirror or a pupil facet mirror) is also known in particular in the illumination device.

Basically, there is a need in such a mirror or mirror arrangements in a microlithographic projection exposure apparatus to monitor the proper functioning, in particular with regard to a possible decrease in the reflectivity. For this purpose, the use of sensor arrangements for measuring the illuminance or intensity in the relevant optical system is known, wherein e.g. From the ratio of the intensity measured values, which are obtained with respect to the light propagation direction before or after a mirror or a mirror element, the functionality or reflectivity of the relevant optical element is concluded.

Such sensor arrays are typically based on an interaction of the electromagnetic radiation with a detector material, this interaction being e.g. in a diode-based embodiment of the sensor arrangement in the generation of an intensity-dependent photocurrent, in the case of also possible realization of the sensor array in the form of so-called quantum converter in the intensity-dependent generation of electromagnetic secondary radiation may exist.

A problem that arises in practice, however, is that the sensor or detector materials used can in turn prove to be unstable over the lifetime, which is reflected in a corresponding change in the photosensitivity of the sensor arrangement over the lifetime. This is due, in particular, to radiation damage caused by short-wave electromagnetic radiation in a projection exposure apparatus designed for operation in the EUV, which radiation is e.g. by the breaking of chemical bonds or the generation of crystal defects (which in turn may possibly have an undesirable absorption effect).

The above-described problem basically exists in all solid-state based detector systems and is common to the so-called standard materials such as e.g. Silicon (Si) is pronounced, e.g. in the case of diode-based configuration of the sensor arrangement based on the presence of a space charge zone (for example in a Schottky diode) or else in the case of realization of the sensor arrangement in the form of so-called quantum converters. In the latter case, the effect is exploited that the electromagnetic radiation to be detected ejects electrons from the valence band (or an inner electron shell), with the result that an electron transfer from the conduction band into the valence band (or between two electron shells) causes secondary radiation leads. While in the first-mentioned, diode-based or based on the generation of an electric photocurrent configuration of the sensor arrangement, the defects generated by the short-wave EUV radiation act as impurities that affect the electrical current flow, in the "quantum converter design" the corresponding secondary radiation the said defects are recaptured or absorbed with the result that a subsequent detection of this secondary radiation is no longer possible.

As a result, the problems described above lead in practice to the fact that the results provided by the respective measuring arrangement can not conclusively state a functional failure of the optical component to be monitored (eg a mirror element of a field or pupil facet mirror), since, for example, one in the Measurement detected change can also be based on a radiation-induced damage to the detector material in the measurement or detector arrangement itself.

The prior art is merely an example DE 10 2010 006 326 A1 . US 2013/0032724 A1 . US 2013/0099249 A1 as well as the publications A. Ionascut-Nedelcescu et al .: "Radiation Hardness of Gallium Nitride", IEEE Transactions on Nuclear Science Vol. 49 (2002), pp. 2733-2738 ; Xueping Xu et al .: "Fabrication of GaN wafers for electronic and optoelectronic devices", Optical Materials 23 (2003), pp. 1-5 and PJ Sellin et al .: "New materials for radiation hard semiconductor detectors", CERN-OPEN-2005-005, pp. 1-24 directed.

It is an object of the present invention to provide an arrangement and method for measuring the intensity of electromagnetic radiation in an optical system while avoiding unwanted radiation-induced changes in measurement characteristics over the lifetime or compared to standard materials such as e.g. Silicon (Si) can be significantly reduced.

This object is achieved by the arrangement according to the features of the independent patent claim 1 and the method according to the features of the independent claim 14.

An inventive arrangement for measuring the intensity of electromagnetic radiation in an optical system, wherein the optical system is designed for a working wavelength less than 100nm, comprises a detector material, wherein the measurement of the intensity is based on an interaction of the electromagnetic radiation with this detector material, and wherein the detector material comprises a group III nitride.

The invention is based in particular on the concept, in an arrangement or a method for measuring the intensity of electromagnetic radiation in a designed for operation in EUV microlithographic projection exposure apparatus or a mask inspection system, the "detector material", which depending on the measurement principle for generating a photocurrent or for generating of secondary radiation is used, from a group III nitride (ie, a nitride having one or more elements of the third main group in the periodic table), such as Gallium nitride (GaN), aluminum nitride (AlN) or boron nitride (BN).

The invention is u.a. from the consideration that a material such as e.g. Gallium nitride (GaN) due to the relatively high binding energy or the (for example compared to amorphous silicon) high energy barrier to the displacement of an atom from its grid position ("Atomic Displacement Energy") is relatively insensitive to electromagnetic radiation and thus a correspondingly high radiation resistance which, in turn, can advantageously be utilized in the inventive use as a detector material in conjunction with electromagnetic EUV radiation.

Specifically, the bonding energy for silicon is about 3.6eV, for amorphous silicon (a-Si) about 4eV to 4.8eV, for gallium nitride (GaN) about 8.9eV and for aluminum nitride (AlN) about 11.5eV. Irradiation experiments with ions, electrons, etc., carried out with such group III nitrides have already resulted in higher stability compared to conventional detector materials.

The group III nitride may in particular be a binary or a ternary compound. In one embodiment, the group III nitride is selected from the group consisting of gallium nitride (GaN), aluminum nitride (AlN), boron nitride (BN), and aluminum gallium nitride (AlGaN).

According to one embodiment, the interaction of the electromagnetic radiation with the detector material comprises the generation of a photocurrent. This photocurrent can be generated in particular in a space charge zone which has a thickness in the range of 20 nm to 40 nm.

According to one embodiment, the arrangement further comprises a cover layer of electrically conductive material on this space charge zone. This cover layer may in particular have a thickness in the range from 2 nm to 10 nm, so that an absorption-related loss of EUV radiation prior to its entry into the respective "detection zone" formed by the space charge zone is kept as low as possible. Furthermore, the cover layer may be made porous to minimize absorption.

According to a further embodiment, the interaction of the electromagnetic radiation with the detector material comprises the generation of electromagnetic secondary radiation.

According to one embodiment, the projection exposure apparatus is designed for a working wavelength smaller than 30 nm, in particular smaller than 15 nm.

• – wobei die Messung der Intensität auf Basis einer Wechselwirkung der elektromagnetischen Strahlung mit einem Detektormaterial erfolgt; und
• – wobei das Detektormaterial ein Gruppe-III-Nitrid aufweist.

The invention further relates to a method for measuring the intensity of electromagnetic radiation in a microlithographic projection exposure apparatus, wherein the projection exposure apparatus is designed for a working wavelength of less than 100 nm,

• - Wherein the measurement of the intensity is based on an interaction of the electromagnetic radiation with a detector material; and
• - wherein the detector material comprises a group III nitride.

According to an embodiment of the method, the interaction of the electromagnetic radiation with the detector material comprises the generation of a photocurrent.

According to a further embodiment of the method, the interaction of the electromagnetic radiation with the detector material comprises the generation of electromagnetic secondary radiation.

• – Ermitteln der Intensitätswerte dieser Sekundärstrahlung für wenigstens zwei voneinander verschiedene Wellenlängen; und
• – Abschätzen einer strahlungsbedingten Alterung bzw. Degradation des Detektormaterials auf Basis eines Vergleichs dieser Intensitätswerte.

According to one embodiment, the method further comprises the steps:

• Determining the intensity values of this secondary radiation for at least two mutually different wavelengths; and
• Estimation of a radiation-induced aging or degradation of the detector material on the basis of a comparison of these intensity values.

Thus, according to this aspect, the intensities of the secondary radiation are compared at two mutually different wavelengths, preferably one of these wavelengths being the "main emission line" (eg, band edge luminescence) and the other of these wavelengths chosen to be defective at that wavelength contribute to the luminescence (eg at 0.1 eV below the band edge luminescence).

The group III nitride for the detector material of the invention may be e.g. by crystal growth methods (e.g., MBE, MOCVD, HVPE).

The invention further relates to an optical system of a microlithographic projection exposure apparatus, in particular an illumination device or projection objective, wherein the optical system has at least one arrangement with the features described above.

Further embodiments of the invention are described in the description and the dependent claims.

The invention will be explained in more detail with reference to embodiments shown in the accompanying drawings.

Show it:

1a -B are schematic illustrations for explaining possible embodiments of the invention;

2 a diagram in which the thickness-dependent transmission is shown for different applicable within the scope of the invention group-III nitrides;

3a -B are schematic illustrations for explaining further embodiments of the invention;

4 a schematic representation for explaining a further embodiment of the invention; and

5 a schematic representation of an exemplary construction of a microlithographic projection exposure apparatus.

1a shows a schematic representation for explaining a first embodiment of a measuring arrangement according to the invention. In this embodiment, the radiation sensor used in the measuring arrangement is designed as a Schottky diode, according to 1a a metallic contact or a metal layer 12 this Schottky diode made of platinum (Pt) with a thickness in the range of 2 to 10 nm and the semiconductor 11 this Schottky diode is made of gallium nitride (GaN). In one embodiment, this gallium nitride (GaN) may have an electron concentration of n = 10 ¹⁸ cm ^-3 . In further embodiments, the gallium nitride (GaN) may also be n-doped to achieve a higher electrical conductivity and, for example, have an electron concentration of n = 10 ¹⁹ cm ^-3 . Which is in the semiconductor 11 forming space charge zone (= "depletion zone") may have a width of about 30 nm to 40 nm in the embodiment.

In further embodiments, suitable doping profiles can also be selected in order to achieve the largest possible space charge zone (30-40 nm). In this case, the space charge zone is preferably almost undoped, wherein immediately afterwards a high doping concentration can be set for a charge carrier density of, for example, n = 10 ¹⁹ cm ^-3 . In this way, a wide space charge zone can be obtained, from the limits of which the charge carriers generated can be dissipated well.

1b shows a schematic representation for explaining a further embodiment of a radiation sensor used in the inventive arrangement in the form of a pn diode of an n-doped semiconductor 22 and a p-type semiconductor 21 ,

In the embodiment of 1b For example, the n-doped semiconductor is formed of n-doped gallium nitride having an electron concentration of n = 10 ¹⁹ cm ^-3 and a thickness in the range of 2 nm to 10 nm. The p-doped semiconductor is formed of p-doped gallium nitride with a hole concentration of p = 10 ¹⁸ cm ^-3 . The between the p-doped semiconductor 21 and the n-type semiconductor 22 formed space charge zone may in the embodiment have a width in the range of 20 nm to 40 nm, in particular in the range of 30 nm to 40 nm.

Basically, in the embodiments described above, the respective thicknesses are chosen so that an undesirable "loss" of EUV radiation due to absorption is minimized. In particular, care must be taken to ensure that the least possible loss of EUV radiation occurs before the EUV radiation reaches the respective "detection zone", the latter being formed in each case by the space charge zone. On the one hand, the respective "inactive" cover layer (eg the metal layer 12 in 1a or the n-doped semiconductor 22 in 1b ) as thin as possible (eg with a thickness in the range of 2 nm to 10 nm), whereby the said loss of EUV radiation to less than 25% (corresponding to a transmission of more than 75%) can be limited.

In other embodiments, the said "inactive" capping layer may also be porous with still good electrical conductivity (e.g., as a porous metal layer) to minimize absorption.

At the same time, the respective embodiments are designed such that the largest possible space charge zone (eg with a width in the range of 20 to 40 nm) is generated. In particular, an undoped region for enlarging the space charge zone can also be added for this purpose. As a result, absorption in the region of the space charge region of more than 50% can be achieved. 2 shows a diagram in which the thickness-dependent transmission for different usable in the invention Group III nitrides and for platinum (Pt) is shown as an exemplary metallic material.

3a shows a schematic representation for explaining a structure of a measuring arrangement according to the invention in a further embodiment. This embodiment is based on the functional principle of a "quantum converter", wherein the effect is exploited that the electromagnetic EUV radiation impinging on a semiconductor material "strikes" electrons from the valence band of the semiconductor material and generates holes thereon, whereupon electrons from the conduction band into the valence band accordingly energetically "fall down" and recombine with the holes, with the result that the energy released in the form of secondary radiation (with a band gap of the semiconductor corresponding photon energy) is emitted, so that in particular the corresponding band edge luminescence can be used to measure the EUV intensity ,

The wavelength of the secondary radiation or of the luminescent light depends on the choice of the group III nitride, the band gap for example for indium nitride (InN) at 0.7 eV, for gallium nitride (GaN) at 3.4 eV and for aluminum nitride (AlN) at 6.1 eV , Furthermore, by alloying the respective material, e.g. to aluminum gallium nitride (AlGaN), the band gap and thus the wavelength of the luminescent light are continuously tuned. The measurement of band edge luminescence can be carried out in the case of gallium nitride (GaN) e.g. be performed with a photodiode, which is "blind" for energies below 3.35 eV.

In embodiments of the invention, the aging / degradation of the particular detector material used can also be monitored by e.g. In the case of gallium nitride (GaN), the secondary radiation or the luminescent light is measured both at an energy of 3.4 eV and at an energy of 3.3 eV, the ratio of the corresponding measured values being sensitive to crystal defects.

Here, the fact is exploited that the generation of crystal defects also leads to a change in the electronic density of states, in particular in the vicinity of the band edge, which in turn leads to "foothills" of the band edge. The reason for this is, for example, deviations of the atomic bond lengths or of bond angles. This effect leads to the absorption or emission showing an exponential dependence on the energy E below the bandgap E _gap (also called "Urbach tail"), the exponent ("Urbach energy E ₀ ") being sensitive to the defect density , The absorption coefficient or emission coefficient is in this case proportional to exp ((EE _gap ) / E ₀ ) (for this see the publication "Urbach Tails and Disorder", Comments Cond. Mat Phys. 1987, Vol. 13, no. 1, pp. 35-48 ).

In the embodiment of 3a For example, the semiconductor layer used is formed of gallium nitride (GaN) and, in the illustrated embodiment, is on a transparent substrate 31 , which may be formed by way of example only sapphire (Al ₂ O ₃ ). The semiconductor layer 32 Gallium nitride may, in embodiments of the invention, for example, have a thickness in the range of 50 nm to 100 nm, wherein the electron concentration may be, for example, in the range of n = 10 ¹⁷ cm ^-3 to n = 10 ¹⁹ cm ^-3 . In the case of quantum converter Embodiment, it is preferred if the bulk material and doped (and thus has fewer defects and thus less self-absorption), the surface (eg over a thickness of a few nm, eg 5nm) can be highly doped (eg n = 10 ¹⁹ cm ^-3 ) in order to minimize charge effects due to the fact that electrons can be knocked out by photons. This reduces the risk that electrons are attracted to the environment and then induce secondary radiation, which would reduce the accuracy of measurement.

In 3a is the on the semiconductor layer 32 incident EUV radiation with " 35 "And that of the semiconductor layer 32 through the transparent substrate 31 emerging secondary radiation (luminescent light) with " 36 " designated. This secondary radiation 36 meets according to 3a on a detector 33 , which can be configured for example as a CCD camera or as a photodiode (eg GaN / InGaN photodiode).

In another configuration, according to 3b the detection of the secondary radiation or the luminescent light also on the on the light entry surface of the semiconductor layer 32 located side of the semiconductor layer 37 respectively.

In another, in 4 schematically illustrated embodiment, a semiconductor layer 42 also directly on a (so to speak, an "integrated" detector forming) photodiode 41 (Eg GaN / InGaN) be formed. The semiconductor layer 42 may be undoped here and, for example, have a thickness of at least 50 nm. The photodiode 41 may be formed in the embodiment as a GaN / InGaN photodiode.

Quantitatively, the increased radiation resistance achieved according to the invention by the embodiments described above can be described such that the lifetime of the measuring arrangement is increased by at least a factor of 3 compared to an analogously constructed silicon-based sensor. In this case, to define the "lifetime", e.g. the criterion that at the end of the life of the quantum efficiency or the photocurrent is only 10% of the initial value.

5 shows a schematic representation of an exemplary designed for operation in the EUV projection exposure equipment in which the present invention is feasible.

According to 5 has a lighting device in a designed for EUV projection exposure system 500 a field facet mirror 503 and a pupil facet mirror 504 on. On the field facet mirror 503 becomes the light of a light source unit, which is a plasma light source 501 and a collector mirror 502 includes, steered. In the light path after the pupil facet mirror 504 are a first telescope mirror 505 and a second telescope mirror 506 arranged. In the light path below is a deflection mirror 507 arranged, which reflects the radiation impinging on an object field in the object plane of a six mirror 551 - 556 comprehensive projection lens steers. At the location of the object field is a reflective structure-bearing mask 521 on a mask table 520 arranged, which is imaged by means of the projection lens in an image plane in which a substrate coated with a photosensitive layer (photoresist) 561 on a wafer table 560 located.

The arrangement according to the invention can in principle be used at any suitable positions in the illumination device or the projection objective, wherein positions are selected which can indeed be illuminated during operation of the projection exposure device but in which there is no optical component of the projection exposure device. For example, the placement is suitable instead of a mirror element or micromirror of the field facet mirror 503 or the pupil facet mirror 504 or in the region between two adjacent mirror elements or micromirrors, wherein the "overshoot" of the mirror elements taking place during operation of the projection exposure apparatus and the small installation space (for example 5 mm × 5 mm) required for an arrangement according to the invention can be utilized for the intensity measurement according to the invention.

In embodiments, during operation of the projection exposure apparatus, it is also possible to decouple EUV radiation onto a separate system or module having an arrangement according to the invention. In other embodiments, an e.g. grid-like arrangement or an array of a plurality of arrangements according to the invention are used to measure the illuminance or intensity in a larger area. In addition, use of the arrangement according to the invention can in principle also take place in the projection objective, wherein only an example of an arrangement according to the invention can be pivoted in to measure the illuminance or intensity at the wafer level.

Although the invention has also been described with reference to specific embodiments, numerous variations and alternative embodiments will be apparent to those skilled in the art, eg, by combining and / or replacing features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments of the present invention are included, and the scope of the invention is limited only in terms of the appended claims and their equivalents.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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.

Cited patent literature

• DE 2010006326 A1 [0010]
• US 2013/0032724 A1 [0010]
• US 2013/0099249 A1 [0010]

Cited non-patent literature

• A. Ionascut-Nedelcescu et al .: "Radiation Hardness of Gallium Nitrides", IEEE Transactions on Nuclear Science Vol. 49 (2002), pp. 2733-2738 [0010]
• Xueping Xu et al .: "Fabrication of GaN wafers for electronic and optoelectronic devices", Optical Materials 23 (2003), p. 1-5 [0010]
• PJ Sellin et al .: "New materials for radiation hard semiconductor detectors", CERN-OPEN-2005-005, pp. 1-24 [0010]
• "Urbach Tails and Disorder", Comments Cond. Mat Phys. 1987, Vol. 13, no. 1, pp. 35-48 [0047]

Claims (17)

Arrangement for measuring the intensity of electromagnetic radiation in an optical system, wherein the optical system is designed for a working wavelength of less than 100 nm, with A detector material, wherein the measurement of the intensity is based on an interaction of the electromagnetic radiation with this detector material; • wherein the detector material comprises a group III nitride. An assembly according to claim 1, characterized in that the group III nitride is selected from the group consisting of gallium nitride (GaN), aluminum nitride (AlN), boron nitride (BN) and aluminum gallium nitride (AlGaN). Arrangement according to claim 1 or 2, characterized in that the interaction of the electromagnetic radiation with the detector material comprises the generation of a photocurrent. Arrangement according to claim 3, characterized in that this photocurrent is generated in a space charge zone having a thickness in the range of 20 nm to 40 nm. Arrangement according to claim 4, characterized in that it further comprises a cover layer of electrically conductive material in this space charge zone. Arrangement according to claim 5, characterized in that this cover layer has a thickness in the range of 2nm to 10nm. Arrangement according to claim 5 or 6, characterized in that this cover layer is porous. Arrangement according to claim 1 or 2, characterized in that the interaction of the electromagnetic radiation with the detector material comprises the generation of electromagnetic secondary radiation. Arrangement according to one of the preceding claims, characterized in that the projection exposure apparatus is designed for a working wavelength of less than 30 nm, in particular less than 15 nm. Arrangement according to one of claims 1 to 9, characterized in that the optical system is an optical system of a microlithographic projection exposure apparatus. Arrangement according to one of claims 1 to 9, characterized in that the optical system is an optical system of a mask inspection system. Optical system with at least one arrangement according to one of the preceding claims. Optical system according to claim 12, characterized in that it is an optical system for microlithography, in particular a lighting device or a projection objective of a microlithographic projection exposure apparatus ( 500 ) or a mask inspection system. Method for measuring the intensity of electromagnetic radiation in an optical system, wherein the optical system is designed for a working wavelength of less than 100 nm, Wherein the measurement of the intensity is based on an interaction of the electromagnetic radiation with a detector material; • wherein the detector material comprises a group III nitride. A method according to claim 14, characterized in that the interaction of the electromagnetic radiation with the detector material comprises the generation of a photocurrent. A method according to claim 14, characterized in that the interaction of the electromagnetic radiation with the detector material comprises the generation of electromagnetic secondary radiation. A method according to claim 16, characterized in that it further comprises the steps of: - determining the intensity values of this secondary radiation for at least two mutually different wavelengths; and - estimating a radiation-induced aging or degradation of the detector material on the basis of a comparison of these intensity values.

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