WO2011151381A1

WO2011151381A1 - Substrate for optical elements

Info

Publication number: WO2011151381A1
Application number: PCT/EP2011/059052
Authority: WO
Inventors: Günther SEITZ; Gennady Fedosenko; Frank Eisert
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2010-06-01
Filing date: 2011-06-01
Publication date: 2011-12-08
Also published as: DE102010029570A1

Abstract

To enable the more precise interferometric measurement of their surfaces, substrates (1) for optical elements comprising at least one base layer (3) and at least one doped cover layer (2), serving as a polishing layer, are suggested, wherein the dopant concentration is such that the refractive index of the doped cover layer (2) at the interface (20) cover layer - base layer differs not more than 5% from the refractive index of the base layer (2) or wherein the dopant concentration is such that the absorption coefficient of the doped cover layer (2) at the interface (20) cover layer - substrate is more than 15% for a given wavelength in the visible wavelength.

Description

Substrate for optical elements

The present invention relates to a substrate for optical elements, comprising at least one base layer and at least one doped cover layer. Furthermore, the invention relates to a mirror, and to a mask comprising such a substrate.

To be able to produce ever finer structures in the production of semiconductor components, for example, with lithographic methods, light of an increasingly shorter wavelength is used. When working in the extreme ultraviolet (EUV) wavelength range, at a wavelength, for example, between about 5 nm and 20 nm, it is no longer possible to work with lens-like elements in the transmission mode, rather exposure and projection objectives of mirror elements are constructed, having reflective coatings adapted to the respective working wavelength. Both in the EUV and ultraviolet (UV) wavelength ranges, the proportion of scattered light in the optical systems, such as illumination systems and, in particular, projection systems, of projection exposure apparatus for lithographic methods, has a substantial influence on the performance of each projection exposure apparatus. The proportion of scattered light is substantially determined by the roughness of the optical elements. If the optical elements are reflective optical elements with a reflective coating, the roughness has an additional influence on the actually achievable reflectivity. The smaller the roughness the higher is the reflectivity.

As a substrate material, in particular for reflective optical elements for EUV lithography, so- called zero-expansion materials are used whose thermal expansion coefficient approaches zero at the temperatures prevailing during the lithography operation and at room

temperature. Primarily, glass-ceramic materials and titanium dioxide-doped quartz glass is used, the glass-ceramic materials being more economical but having higher micro roughness. To compensate the drawback of the glass-ceramic materials, substrates of glass-ceramic are provided with a layer which can be polished to smaller micro roughness than the glass-ceramic material. Preferably, glass-ceramic materials are provided with a silicon dioxide layer. To adapt the thermal expansion coefficient of the silicon dioxide layer to the glass-ceramic, the silicon dioxide layer can be doped with titanium dioxide. To be able to use substrates comprising a polishing layer for high-precision optical elements, as they are used, for example, in EUV lithography, their dimensional fit and roughness must be measurable with interferometric precision. When using material transparent to the light used for the interferometric measurement, there is a problem, however, that radiation is not only reflected at the interface between polishing layer and air, but also on the interface between the substrate and the polishing layer. Since often neither the thickness nor the homogeneity of the polishing layer is sufficiently well known, it is very hard to compensate for this effect computationally. It is an object of the present invention to further develop the already known substrates in such a way that they can be interferometrically measured.

In a first aspect, this object is achieved by a substrate for optical elements, comprising at least one base layer and at least one doped cover layer, wherein the dopant concentration is such that the refractive index of the doped cover layer at the interface cover layer - base layer differs not more than 5% from the refractive index of the base layer for a given wavelength in the visible wavelength range.

It has been found, that this approach can lead to a lessened contrast at the interface cover layer - base layer, such that the reflection at this interface can be sufficiently suppressed compared with the reflection at the interface between cover layer and air to not adversely affect the interferometric measurement of the surface of the substrate, i.e. the interface between cover layer and air. Preferably, the dopant concentration is such that the refractive index of the doped cover layer at the interface cover layer - base layer differs not more than 3% from the refractive index of the base layer for a given wavelength in the visible wavelength range.

In a second aspect, this object is achieved by a substrate for optical elements, comprising at least one base layer and at least one doped cover layer, wherein the dopant concentration is such that the absorption coefficient of the doped cover layer is more than 15% for a given wavelength in the visible wavelength range.

It has been found, that this approach can suppress the light reflected at the interface between base layer and cover layer compared with the light reflected at the interface between cover layer and air by absorbing it in the cover layer, such that the interferometric measurement of the substrates surface is less impaired than with conventional substrates.

Preferably, the given wavelength is in the violet wavelength range to provide a high resolution of the interferometric measurement.

In a third aspect, this object is achieved by a substrate for optical elements, comprising at least one base layer and at least one doped cover layer, wherein the dopant concentration of the cover layer is below 5 vol.-% or more than 20 vol.-%.

It has been found that these concentrations can lead for most common combinations of base layer materials and cover layer materials to concentrations that cause the refractive index of the cover layer to be nearly the same as the refractive index of the base layer for a given wavelength in the visible wavelength range or that increase the absorption of light of the given wavelength in the cover layer. Due to these effects, which can be concurrent, the light reflected at the interface between the base layer and the cover layer during an interferometric measurement of the substrate's surface can be sufficiently suppressed compared with the light being reflected at the interface between the cover layer and air to not to much or not at all impede this interferometric measurement.

In a fourth aspect, this object is achieved by a substrate for optical elements, comprising at least one base layer and at least one doped cover layer, wherein the dopant of the cover layer consists of one or more materials of the group carbon, hydrocarbon, silicon, boride, nitride, metal oxide - except titanium oxide.

It has been found that doping the cover layer of substrate for optical elements can, for most common combinations of base layer materials and cover layer materials, influence the resulting refractive index and/or absorption coefficient of the cover layer to be nearly the same as the refractive index of the base layer for a given wavelength in the visible wavelength range or to increase the absorption of light of the given wavelength in the cover layer. As explained before, due to these effects, the light reflected at the interface between the base layer and the cover layer during an interferometric measurement of the substrate's surface can be sufficiently suppressed to not to much or not at all impair the interferometric measurement of the surface of the substrate at the interface between cover layer and air. In preferred embodiments, the cover layer, as regards its doping, has a concentration gradient across the thickness of the cover layer. It has been found that by doping the cover layer with a concentration gradient across its thickness by changing the refractive index and/or changing the absorption within the cover layer, the interface between the cover layer and the base layer is no longer optically discernible as a sharply defined interface, and thus the measurement of the interface between the cover layer and air is no longer adversely affected. By reducing the refractive index difference between the cover layer and the base layer, the reflection can be suppressed at its interface. By increasing the absorption in the cover layer, less light passes to the interface between the base layer and the cover layer, which could be reflected there, or less reflected light, if any, passes back out of the cover layer.

Advantageously, the doping concentration on the side facing the base layer is higher than on the opposite side. By these means, effective "smearing", or making "invisible" of the interface between the base layer and the cover layer is achieved, and it is ensured that the polishing properties of the cover layer at the interface between the cover layer and the air is influenced as little as possible or not at all by the doping.

Preferably, the base layer comprises glass-ceramics, in particular with as small a thermal expansion coefficient as possible.

Advantageously the cover layer comprises silicon dioxide. Silicon dioxide can have thermal expansion coefficients that are comparable to the ones of the usual substrate materials, and has excellent polishability.

Preferably, the doping comprises one or more elements of the group comprising carbon, hydrocarbon, silicon, oxide, boride, nitride and carbide. These materials allow the refractive index of the cover layer to be changed to adapt them to the refractive index of the substrate material, or the absorption coefficients to be changed in order to suppress light, if any, reflected at the interface between the base layer and the cover layer during interferometric measurement.

In preferred embodiments, the doping concentration continuously decreases from the side facing the base layer to the opposite side. A continuously and monotonically decreasing concentration of the doping from the interface between the substrate and the cover layer to the interface between the cover layer and the air not only allows the influence of the interface between the base layer and the cover layer on interferometric measurements of the surface of the cover layer to be particularly efficiently suppressed, but can also be created without excessive effort with the aid of conventional coating methods.

Advantageously, the dopant concentration is up to 60 vol.-%, in particular the maximum concentration in case there is a concentration gradient over the thickness of the cover layer, so that even huge differences in the refractive index between the substrate material and the cover layer material at the wavelength at which the interferometric measurement is carried out, can be largely compensated by means of graduated doping, while still achieving sufficiently low roughness of the cover layer surface. A dopant concentration between 20 vol.-% and 60 vol.-% is particularly preferred for cover layers showing a high absorption in the wavelength range of interferometric measurements of the substrate's surface. In further embodiments, even less than 5 vol.-% can achieve sufficient adaption of the refractive index between common substrates and common cover layers having good polishability, so that the interface between the base layer and the cover layer does no longer have a negative effect on interferometric measurements at the surface of the cover layer. At values up to 5 vol.-%, the material properties of the cover layer essential for its use as a polishing layer remain excellently intact.

Preferably, the dopant concentration is down to 0 vol.-% , in particular the minimum concentration in case there is a concentration gradient over the thickness of the cover layer. In particular, it is preferred for the doping to be as weak as possible toward the interface between the cover layer and air, so that the polishability of the cover layer remains intact as far as possible.

Furthermore, this object is achieved by a mirror comprising a substrate as described above, and a reflective layer. Mirrors with such a substrate can have both a particularly precise dimensional accuracy and very low roughness, so that they are suitable not only for use in UV lithography, but also, in particular, for use in EUV lithography.

Finally, this object is achieved by a mask with a substrate as described above, a reflective layer and an absorbing layer. The smaller the wavelengths used are, the more important it is for masks, whose structures are imaged onto an object to be exposed by means of lithographic methods, to keep the roughness as small as possible to avoid scattered light. The substrates described here are particularly well suited to achieve this. Depending on whether the mask is configured as a negative or a positive, either the absorbing layer is arranged between the substrate and a structured reflective layer, or the reflective layer is arranged between the substrate and a structured absorbing layer.

The above and further features can also be derived from the description and the drawings, as well as from the claims, wherein the individual features either alone or in combination in the form of subcombinations can be realized in an embodiment of the invention, and also in other fields, and can also represent embodiments advantageous and capable of protection as such.

The present invention will be explained in more detail with reference to a preferred exemplary embodiment, wherein:

Fig 1 schematically shows an embodiment of the substrate;

Fig. 2 schematically shows a possible concentration profile across the cover layer of the substrate;

Fig. 3 schematically shows an embodiment of the mirror; and

Fig. 4 schematically shows an embodiment of the mask.

Fig. 1 schematically shows a substrate 1 for an optical element. It comprises a base layer 3 of, in the present example, a zero-expansion material on the basis of glass-ceramic.

Zerodur^® is commercially available, for example, from Schott AG. Glass-ceramic materials have crystals of an extension of usually less than 100 nm in a glass matrix. In certain temperature ranges, these crystals contract while, at the same time, the glass matrix expands. These two effects cancel each other out, so that the thermal expansion coefficient of the glass-ceramic in this temperature range is essentially zero. During polishing to achieve a dimensionally accurate fit for the use as a component of a reflective optical element, there is the problem, however, that different amounts of material are removed from the crystals and from the glass matrix. If roughness is defined as a power spectral density (PSD) curve, the roughness can deteriorate on a longitudinal scale from 1 mm to 10 nm during polishing.

In order to reduce roughness, a cover layer 2 is provided as a polishing layer on base layer 3. In the example shown here, cover layer 2 is based on silicon dioxide in the form of quartz glass which, as a homogeneous, highly-viscous liquid, is well suited for polishing onto very small roughnesses also in a longitudinal scale from 1 mm to 10 nm. Apart from silicon dioxide, other materials having good polishability can also be used. Prior to the application of cover layer 2 on base layer 3, base layer 3 is first brought to the desired final shape as closely as possible. Cover layer 2 can be applied by the usual coating methods, such as chemical vapor deposition (CVD) methods, in particular plasma-based CVD methods, or physical vapor deposition (PVD) methods, in particular ion-based PVD methods. With a cover layer on the basis of quartz glass, in particular, sol-gel methods are also possible. If necessary, cover layer 2 is polished to the desired roughness. Any shape corrections to achieve the desired dimensionally accurate fit can be carried out e.g. by means of ion-beam figuring (IBF) methods.

Before substrate 1 can be further processed to an optical element by applying optical layers, the surface of substrate 1 must be interferometrically measured on the side comprising cover layer 2, in particular in the context of the production of high-precision reflective optical elements for EUV lithography, to determine whether the specifications with respect to the desired dimensional accuracy and the desired roughness values are fulfilled. To prevent the measurement being falsified by reflections not only at the interface 21 between the cover layer and air but also at the interface 20 between the base layer and the cover layer, cover layer 2 is doped, in some embodiments with a concentration gradient across thickness D of cover layer 2. With a cover layer 2 on the basis of quartz glass, it is preferably already doped during the application of cover layer 2. If other materials are used as the cover layer, doping can also be carried out later.

The dopant concentration can be such that the refractive index of the doped cover layer at the interface cover layer - base layer differs not more than 5%, preferably not more than 3% from the refractive index of the base layer for a given wavelength in the visible wavelength range. Alternatively, the dopant concentration can be such that the absorption coefficient of the doped cover layer at the interface cover layer - substrate is more than 15% for a given wavelength in the visible wavelength range. Alternatively, the dopant concentration is below 5 vol.-% or more than 20 vol.-%. Alternatively, the dopant consists of one or more materials of the group carbon, hydrocarbon, silicon, boride, nitride, metal oxide - except titanium oxide. It should be noted that these measures can be cumulated in various combinations.

Fig. 2 schematically shows a possible concentration gradient. For this purpose,

concentration C of doping material applied across thickness D is plotted in the x direction (cf. Fig. 1 ), wherein the origin of the x axis lies in interface 20. The doping concentration is higher at interface 20 than at interface 21. It decreases from a maximum concentration C_max at interface 20 to a minimum concentration C_min in the area of interface 21 . In the example of a concentration gradient shown here, concentration C decreases continuously and strictly monotonically. In other concentration gradients, the concentration can also decrease in a stepwise or monotonic fashion. In further variants, the maximum concentration C_max can also be at a distance from interface 20, in particular if the material of the cover layer has better adhesion to the material of the base layer if it is undoped. In the example shown in Fig. 2, the concentration of doping already falls to a minimum concentration C_min of zero prior to reaching interface 21 . In further variants, the minimum concentration C_min can be reached at interface 21. The minimum concentration C_min can also be greater than zero. The material for doping can be, for example, carbon, hydrocarbon, silicon, an oxide, in particular a metal oxide, a boride, a nitride or a combination thereof. Hydrocarbons can be used as a doping material, in particular, with cover layers on the basis of quartz glass, applied on the basis of monomers, comprising silicon and hydrocarbons, for example silanes or siloxanes. Herein, the influence on the refractive index of the cover layer is primarily due to the carbon content while the hydrogen content is negligible. Titanium oxide is disregarded in particular for embodiments based only on dopant material and eventually on

concentration gradient of the cover layer for suppressing the light reflected at the interface between base layer and cover layer during interferometric measurements of the substrate#s surface.

When quartz glass is used as a material for cover layer 2, in particular, as in the example shown here, carbon, or hydrocarbons, silicon or a mixture of carbon, or hydrocarbon, and silicon are preferred as a doping material. Due to their size and coordination number, they are particularly well suited to be inserted in the silicon dioxide matrix. Depending on the concentration range used therein, the negative effect of reflections on the interface between the base layer and the cover layer during interferometric measurement at the interface between the cover layer and air can be suppressed, for example, by approximating the refractive index of the cover layer to the refractive index of the base layer, the refractive index being the real part of the complex refractive index in the present context. For example, in the visual wavelength range, in which interferometric measurements are usually carried out, such as at 400 nm, the refractive index of quartz glass is at about 1 .45 to 1 .5 and the refractive index of Zerodur^® is about 1.57 to 1 .55. By doping quartz glass with carbon, also on the basis of hydrocarbons, and/or silicon at about 1 vol.-%, the refractive index of quartz glass, for example, can be adapted to the refractive index of Zerodur^® so that, in the visible wavelength range, hardly any reflection occurs at the interface between the Zerodur^® base layer and the doped quartz glass cover layer. In particular, doping of quartz glass with carbon and/or hydrocarbons and/or silicon with a concentration of around 1 vol.-% can lead to a refractive index of quartz glass of about 1 ,53 at a wavelength of 400 nm. Depending on the substrate material used for the base layer, higher concentrations can also be used to adapt the refractive index in the cover layer to the base layer. Additionally, as the concentration of the doping increases, the reflection at the interface between the base layer and the cover layer can also be suppressed by increasing the absorption in the cover layer so that less radiation used for interferometric measurement reaches the interface between the base layer and the cover layer and radiation reflected by the interface between the base layer and the cover layer is largely absorbed within the cover layer. When a substrate material having a very high refractive index is used as a base layer, a maximum carbon concentration of 55 vol.-% in the area of the interface between the base layer and the cover layer could be used in an excellently polishable cover layer of quartz glass to achieve a refractive index of 2.14 at 400 nm which, at the same time, would lead to an absorption of about 20% at 400 nm.

An additional advantage of doping the cover layer, in particular of doping quartz glass with carbon and/or silicon, is that the cover layer can carry more mechanical load. In particular it becomes harder and more scratch resistant, so that the substrate becomes more easily manageable and transportable.

Fig. 3 schematically shows an example of a mirror 4 comprising a reflective layer 5 on a substrate 1 including a base layer 3 and a doped cover layer 2. Doped cover layer 2 was polished to the required roughness prior to applying the reflective layer 5. In mirrors for EUV lithography, in particular, reflective layer 5 is a multilayer system of alternating layers of material having different real parts of the complex refractive index, essentially simulating a crystal with lattice planes, on which Bragg refraction occurs. Fig. 4 schematically shows an example of a mask 6 comprising a reflective layer 5 and an absorbing layer 7 on a substrate 1 including a base layer 3 and doped cover layer 2 polished to the desired roughness. In masks for EUV lithography, in particular, reflective layer 5 also comprises multilayer systems of alternating layers of material having different real parts of the complex refractive index, essentially simulating a crystal with lattice planes, at which Bragg refraction occurs. Materials are chosen for the absorbing layer 7, which show particularly high absorption in the wavelength range in which lithography is carried out. In the example shown in Fig. 4, absorbing layer 7 is arranged above reflective layer 5, and is structured. In other embodiments, this can also be reversed. It should be noted that the substrates described here, having a doped cover layer as explained before are also suitable for further processing to optical elements for UV lithography, for example at wavelengths of 248 nm or 193 nm, or also other applications.

Claims

1. A substrate for optical elements, comprising at least one base layer and at least one doped cover layer, wherein the dopant concentration is such that the refractive index of the doped cover layer (2) at the interface (20) cover layer - base layer differs not more than 5% from the refractive index of the base layer (3) for a given wavelength in the visible wavelength range.

2. The substrate according to claim 1 , wherein the dopant concentration is such that the refractive index of the doped cover layer(2) at the interface (20) cover layer - base layer differs not more than 3% from the refractive of the base layer (3) for a given wavelength in the visible wavelength range.

3. A substrate for optical elements, comprising at least one base layer and at least one doped cover layer, wherein the dopant concentration is such that the absorption coefficient of the doped cover layer (2) at the interface (20) cover layer - substrate is more than 15% for a given wavelength in the visible wavelength range.

4. The substrate according to any of claims 1 to 3, wherein the given wavelength is in the violet wavelength range.

5. A substrate for optical elements, comprising at least one base layer and at least one doped cover layer, wherein the dopant concentration in the cover layer (2) is below 5 vol.-% or more than 20 vol.-%.

6. A substrate for optical elements, comprising at least one base layer and at least one doped cover layer, wherein the dopant of the cover layer (2) consists of one or more materials of the group carbon, hydrocarbon, silicon, boride, nitride, metal oxide - except titanium oxide.

7. The substrate according to any of claims 1 to 6, wherein the cover layer (2) has a concentration gradient across the thickness (D) of the cover layer with respect to its doping.

8. The substrate according to claim 7, wherein the doping concentration on the side (20) facing the base layer (3) is higher than on the opposite side (21 ).

9. The substrate according to any of claims 1 to 8, wherein the base layer (3) comprises glass-ceramic material.

10. The substrate according to any one of claims 1 to 9, wherein the cover layer (2) comprises silicon dioxide.

1 1 . The substrate according to any one of claims 1 to 6, 7 to 10, characterized in that the doping comprises one or more elements of the group comprising carbon, hydrocarbon, silicon, oxide, boride, nitride and carbide.

12. The substrate according to any one of claims 1 to 1 1 , characterized in that the doping concentration continuously decreases from the side (20) facing the base layer to the opposite side (21 ).

13. The substrate according to any one of claims 1 to 12, characterized in that the dopant concentration is up to 60 vol.-%.

14. The substrate according to any one of claims 1 to 13, characterized in that the dopant concentration is down to 0 vol.-%.

15. A mirror, comprising a substrate (1 ) according to any one of claims 1 to 14, and a reflective layer (5).

16. A mask, comprising a substrate (1 ) according to any one of claims 1 to 14, a reflective layer (5), and an absorbing layer (6).