DE102010029570A1 - Substrate for optical elements - Google Patents

Abstract

In order to be able to measure their surface more precisely by interferometry, substrates (1) for optical elements with at least one base layer (3) and at least one doped cover layer (2), which serves as a polishing layer, are proposed, in which the cover layer (2) in relation to their doping has a concentration gradient across the thickness of the cover layer (2).

Description

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

In order to be able to produce ever finer structures in the production of semiconductor devices, for example, by means of lithographic methods, work is always carried out with shorter-wavelength light. If one works in the extreme ultraviolet (EUV) wavelength range, for example at wavelengths between approximately 5 nm and 20 nm, it is no longer possible to work with lenticular elements in transmission, but illumination and projection objectives are constructed from mirror elements with reflection coatings adapted to the respective working wavelength.

In both the EUV and ultraviolet (UV) wavelengths, the amount of scattered light in the optical systems, such as the illumination system and, in particular, the projection system, of lithographic projection exposure equipment has a significant impact on the performance of the respective projection exposure equipment. The proportion of scattered light is largely determined by the roughness of the optical elements. If the optical elements are reflective optical elements with a reflective coating, the roughness additionally has an influence on the actually achievable reflectivity. The lower the roughness, the higher the reflectivity.

As substrate material, in particular for reflective optical elements for EUV lithography, so-called zero-expansion materials are used whose coefficient of thermal expansion approaches zero during the lithography operation and the prevailing temperatures and room temperature. The focus is on glass ceramics and quartz glass doped with titanium dioxide, wherein glass ceramics are more economical, but have a higher microroughness. To compensate for the disadvantage of the glass-ceramics, glass-ceramic substrates are provided with a layer which can be polished to a lower micro-roughness than the glass-ceramic. Glass ceramics are preferably provided with a silicon dioxide layer. In order to match the thermal expansion coefficient of the silicon dioxide layer to the glass-ceramic, the silicon dioxide layer may be doped with titanium oxide.

In order to be able to use substrates with a polishing layer for high-precision optical elements, as used, for example, in EUV lithography, their pitch and roughness must be able to be surveyed interferometrically accurately. However, when using the transparent material for the light in which the interferometric measurements are made, there is a problem that radiation is reflected not only at the polishing layer-air interface but also at the substrate-polishing layer interface. Since often neither the thickness nor the homogeneity of the polishing layer are known with sufficient accuracy, this effect is also difficult to calculate.

An object of the present invention is to further develop the already known substrates in such a way that they can be measured interferometrically.

This object is achieved by a substrate for optical elements having at least one base layer and at least one doped cover layer, wherein the cover layer has a concentration gradient with respect to its doping over the thickness of the cover layer.

It has been found that by doping the cover layer with a concentration gradient over its thickness by changing the refractive index and / or changing the absorption within the cover layer, the interface between cover layer and base layer is optically no longer perceived as a sharp boundary surface and therefore the measurement of the interface Cover layer air is no longer impaired. By reducing the refractive index difference between the cover layer and the base layer, the reflection at its interface can be suppressed. By increasing the absorption in the cover layer, less light reaches the boundary layer base layer which could be reflected there or less possibly reflected light out of the cover layer.

In preferred embodiments, the doping concentration is higher on the side facing the base layer than on the opposite side. As a result, both an effective "smearing" or "invisibilization" of the interface base layer covering layer is achieved and ensures that the polishing properties of the cover layer at the interface surface layer air is as little as possible or not affected by the doping.

Preferably, the base layer comprises glass ceramic, in particular with the lowest possible coefficient of thermal expansion.

Advantageously, the cover layer comprises silicon dioxide. Silica can have comparable coefficients of thermal expansion as conventional substrate materials and has good polishability.

Preferably, the doping comprises one or more elements of the group carbon, hydrocarbon, silicon, oxide, boride, nitride and carbide. These materials allow a change in the refractive index of the cover layer in order to adapt it to the refractive index of the substrate material, or a change in the absorption coefficient in order to suppress light reflected at the interface base layer cover layer during an interferometric measurement.

In preferred embodiments, the doping concentration continuously decreases from the side facing the base layer to the opposite side. A steadily and monotonically decreasing concentration of the doping from the substrate-covering layer interface to the surface-air interface allows not only a particularly efficient suppression of the influence of the base-layer covering layer on interferometric measurements of the surface of the covering layer, but can also be achieved with conventional coating methods produce too much effort.

Advantageously, the maximum doping is up to 60 vol .-% in order to compensate for very large differences in refractive index between the substrate material and the outer layer material at the wavelength at which the interferometric measurement is performed on the graded doping as possible, while still a sufficiently low roughness reach the top surface.

In further embodiments, even below 10% by volume, a sufficient adjustment of the refractive index between customary substrates and customary readily polishable cover layers can be achieved so that the interfacial base-layer cover layer no longer has a negative effect on interferometric measurements on the surface of the cover layer. Up to 10% by volume, the material properties of the cover layer which are important for use as a polishing layer are particularly well preserved.

The minimum doping is preferably up to 0% by volume. In particular, it is preferred that the doping is as low as possible for the boundary layer covering layer air, so that the polishing ability of the covering layer remains as unchanged as possible.

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

Finally, this object is achieved by a mask having a substrate as described, a reflective layer and an absorbent layer. The smaller the wavelengths used, the more important it is with masks whose structures are imaged by lithographic methods on an object to be exposed, to keep the roughness as low as possible in order to avoid scattered light. This can be achieved particularly well by using the substrates described here. Depending on whether the mask is formed as negative or positive, either the absorbing layer between the substrate and a structured reflective layer or the reflective layer between the substrate and a structured absorbent layer is arranged.

The above and other features are apparent from the claims and from the description and drawings, wherein the individual features each alone or more in the form of sub-combinations in an embodiment of the invention and in other fields be realized and advantageous as well protectable versions.

The present invention will be explained in more detail with reference to a preferred embodiment. Show this

1 schematically an embodiment of the substrate;

2 schematically a possible concentration over the cover layer of the substrate;

3 schematically an embodiment of the mirror; and

4 schematically an embodiment of the mask.

In 1 is schematically a substrate 1 represented for an optical element. It includes a base layer 3 from in this example a zero expansion material based on glass ceramic. Commercially available Zerodur ^®, for example, the company Schott AG. Glass-ceramics have crystals of an extent of generally 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 coefficient of thermal expansion of the glass-ceramic in this temperature range is essentially zero. However, in polishing to form a passport for use as a component of a reflective optical element, there is a problem that the abrasion on the crystals and the glass matrix is different. If you describe the roughness as Power density curve (PSD power spectral density) can degrade roughness on a length scale of 1 mm to 10 nm during polishing.

To reduce the roughness is on the base layer 3 a cover layer 2 provided as a polishing layer. In the example shown here, the cover layer is based 2 On silica in the form of quartz glass, that as a very homogeneous, highly viscous liquid can be polished well to very low roughness even on a length scale of 1 mm to 10 nm. In addition to silicon dioxide, it is also possible to use other readily polishable materials.

Before applying the topcoat 2 on the base layer 3 becomes the base layer 3 initially brought as close as possible to the desired final shape. The cover layer 2 can be applied by conventional coating methods, such as CVD (chemical vapor deposition), in particular plasma-enhanced CVD method, or PVD (physical vapor deposition), in particular ion-assisted PVD method. In particular, in a cover layer based on quartz glass and sol-gel method are possible. If necessary, the topcoat becomes 2 polished to the desired roughness. Possible shape corrections in order to obtain the desired pass can be carried out, for example, using IBF (ion beam figuring) methods.

Before the substrate 1 can be further processed by applying optical layers to form an optical element, especially in the context of the production of high-precision reflective optical elements for EUV lithography, the surface of the substrate 1 on the side with the topcoat 2 interferometrically to determine if the specifications are met with respect to the desired pass and roughness values. In order to avoid that the measurement is falsified thereby, not only at the interface 21 Cover layer air, but also at the interface 20 Base layer cover layer is reflected, is the cover layer 2 doped, with a concentration gradient across the thickness D of the cover layer 2 , For a cover layer 2 On the basis of quartz glass, the doping is preferred already during application of the cover layer 2 carried out. When using other materials as a cover layer, the doping can also be carried out subsequently.

In 2 schematically a possible concentration gradient is shown. For this purpose, the concentration C of the doping material over the thickness D in the x direction (see also 1 ), wherein the coordinate origin of the x-axis at the interface 20 lies. The doping concentration is at the interface 20 higher than at the interface 21 , It assumes a maximum concentration C _max at the interface 20 to a minimum concentration C _min at the interface 21 from. In the example of a concentration gradient shown here, the concentration C decreases steadily and strictly monotonically. In other concentration gradients, the concentration may also decrease in steps or monotonically. In further variants, the maximum concentration C _{max may} also be from the interface 20 be removed, especially if the material of the cover layer better adheres to the material of the base layer when it is undoped. Im in 2 As illustrated, the concentration of doping already decreases before the interface 21 to a minimum concentration C _min of zero. In further variants, the minimum concentration C _min at the interface 21 be achieved. Likewise, the minimum concentration C _{min may} also be greater than zero.

The material for the doping may, for example, be carbon, hydrocarbon, silicon, an oxide, in particular a metal oxide, a boride, a nitride or a combination thereof. Hydrocarbons can be used as doping material, in particular in the case of cover layers based on quartz glass, which, starting from monomers comprising silicon and hydrocarbons, for example silanes or siloxanes, are used. The influence on the refractive index of the covering layer is based primarily on the carbon content and the hydrogen content can be neglected.

Especially when using quartz glass as the material for the cover layer 2 As in the example shown here, preference is given to carbon or hydrocarbons, silicon or a mixture of carbon or hydrocarbons and silicon as doping material. Because of their size and coordination number, they can be inserted particularly well into the silicon dioxide matrix. Depending on the concentration range used, it is possible to suppress the negative effect of reflection at the base layer covering layer interface during an interferometric measurement at the cover layer air interface, for example, by approximating the refractive index of the cover layer to the refractive index of the base layer In this context, refractive index is understood to be the real part of the complex refractive index. Thus, the refractive index of fused silica at about 1.45 to 1.5, and the refractive index of Zerodur ^® is approximately in the visible wavelength range, are usually in the interferometric measurements carried out, for example at 400 nm, at about 1.57 to 1.55 , By doping quartz glass with carbon, also based on hydrocarbons, and / or silicon by about 1% by volume, it is possible to adapt the refractive index of the quartz glass to the refractive index of Zerodur®, so that practically no ^{effect is achieved} in the visible wavelength range reflection at the interface between Zerodur ^® -Basischicht and doped silica glass cladding layer takes place.

Depending on the substrate material used for the base layer, it is also possible to use higher concentrations in order to adapt the refractive index in the cover layer to the base layer. In addition, with increasing concentration of the doping, the reflection at the interface base layer covering layer is also suppressed by increasing the absorption in the covering layer so that even less penetrates from the radiation used for the interferometric measurement to the interface base layer covering layer and at the Boundary layer of base layer cover layer reflected radiation in the cover layer is largely absorbed. For example, when using a substrate material as a base layer with a very high refractive index could reach a refractive index of 2.14 at 400 nm, with a maximum carbon concentration of 55 vol .-% in the region of the interface layer base layer of a well-polishable top layer of quartz glass, which at the same time lead to an absorption of about 20% at 400 nm.

An additional advantage of the doping of the covering layer, in particular when doping quartz glass with carbon and / or silicon, is that the covering layer becomes more resilient. In particular, it is harder and more scratch-resistant, so that the substrate is easier to handle and transport.

In 3 is schematically an example of a mirror 4 shown on a substrate 1 with base layer 3 and doped cover layer 2 a reflective layer 5 having. The doped cover layer 2 was before applying the reflective layer 5 polished to the required roughness. In particular, mirrors for EUV lithography is the reflective layer 5 multilayer systems of alternating layers of material with different real part of the complex refractive index, over which a kind of crystal with lattice planes is simulated, where Bragg diffraction takes place.

In 4 is a schematic example of a mask 6 shown on a substrate 1 with base layer 3 and doped and polished to the desired roughness topcoat 2 a reflective layer 5 and an absorbing layer 7 having. Especially with masks for EUV lithography, it is in the reflective layer 5 also to multi-layer systems of alternating layers of material with different real part of the complex refractive index, over which a kind of crystal with lattice planes is simulated, where Bragg diffraction takes place. For the absorbing layer 7 For example, materials are selected which absorb particularly well in the wavelength range in which the lithography is carried out. Im in 4 The example shown is the absorbent layer 7 over the reflective layer 5 arranged and structured. In other embodiments, this may also be the other way round.

It should be noted that the graded-gradient doped cover substrates described herein are also suitable for further processing into optical elements for UV lithography, for example at wavelengths of 248 nm or 193 nm, or other applications.

LIST OF REFERENCE NUMBERS

1

substratum

2

topcoat

3

base layer

4

mirror

5

reflective layer

6

mask

7

absorbing layer

20

page

21

page

D

cover layer thickness

C

concentration

C _min

minimal concentration

C _max

maximum concentration

Claims (10)

Substrate for optical elements with at least one base layer and at least one doped cover layer, characterized in that the cover layer ( 2 ) has a concentration gradient across the thickness (D) of the cover layer with respect to its doping. Substrate according to Claim 1, characterized in that the doping concentration on the base layer ( 3 ) facing side ( 20 ) is higher than on the opposite side ( 21 ). Substrate according to claim 1 or 2, characterized in that the base layer ( 3 ) Glass ceramic. Substrate according to one of claims 1 to 3, characterized in that the cover layer ( 2 ) Comprises silica. Substrate according to one of claims 1 to 4, characterized in that the doping comprises one or more elements of the group carbon, hydrocarbon, silicon, oxide, boride, nitride and carbide. Substrate according to one of claims 1 to 5, characterized in that the doping concentration of the base layer facing side ( 20 ) to the opposite side ( 21 ) decreases continuously. Substrate according to one of claims 1 to 6, characterized in that the maximum doping is up to 60 vol .-%. Substrate according to one of claims 1 to 7, characterized in that the minimum doping is up to 0 vol .-%. Mirror comprising a substrate ( 1 ) according to one of claims 1 to 8 and a reflective layer ( 5 ). Mask comprising a substrate ( 1 ) according to one of claims 1 to 8, a reflective layer ( 5 ) and an absorbent layer ( 6 ).

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