DE102009032751A1 - Reflective optical element for use as e.g. reflection mirror in projection system of extreme UV lithography device, has intermediate layer arranged at boundary surface between absorber and spacer layers of high and low refractive indexes - Google Patents

Abstract

The element has a multi-layer system made of an absorber layer (21) and a spacer layer (22). The absorber and spacer layers are made of materials e.g. molybdenum and silicon, with low and high refractive indexes. An intermediate layer (23) is arranged at a boundary surface between the absorber and spacer layers. The intermediate layer is made of molybdenum boride or molybdenum nitride. The multi-layer system is produced by successive coatings on a substrate. A protective layer is applied on the multi-layer system.

Description

The The present invention relates to a reflective optical Element for a working wavelength in the soft x-ray to extremely ultraviolet wavelength range, which is a multi-layer system from at least two alternating materials with different Have a real part of the refractive index at the working wavelength, and its use. Furthermore, the invention relates to a Projection system and a lighting system and on an EUV lithography device with at least one such reflective optical element.

In EUV lithography devices become the lithography of semiconductor devices Reflective optical elements for the soft X-ray to extreme ultraviolet (EUV) wavelength range (eg wavelengths between about 5 nm and 20 nm), such as photomasks or multilayer mirrors. Since EUV lithography devices usually more reflective must have optical elements this one as possible high reflectivity have to ensure a sufficiently high overall reflectivity. As usual in an EUV lithography device several reflective optical Elements are arranged one behind the other, already have an effect less deterioration in reflectivity for each single reflective optical element to a greater extent on the total reflectivity within the EUV lithography device.

reflective optical elements for the EUV and soft wavelengths generally have multi-level systems. It is about around alternately applied layers of a material with a higher real part the refractive index at the working wavelength (also called spacers) and a material with a lower real part of the refractive index the working wavelength (also called absorber), wherein an absorber-spacer pair a stack forms. This somehow simulates a crystal whose Lattice planes correspond to the absorber layers at which Bragg reflection takes place. The thicknesses of the individual layers as well as the repetitive ones Piles can over that entire multi-layer system to be constant or even vary, depending on which reflection profile is to be achieved.

in the Contrary to ideal multi-layer systems with clearly defined Layer boundaries occur in real multi-layer systems in most cases make to a mixing of the respective layer materials at the ply boundaries, so that the layer boundaries blur, so to speak. Depending on the technology, which is used to produce the multi-layer systems, can be a mixing already in the application of the individual layers not prevent. But even after longer periods or due to high heat load irradiation with EUV or soft X-rays often occurs to interdiffusion processes. They can lead to mixed layers form mixtures of absorber and spacer materials. These Lead mixed layers not only to reduce the maximum reflectivity, but also to a shift in the reflected wavelength, which the actual reflectivity further reduced.

It It is an object of the present invention to provide reflective optical Elements for Working wavelengths in soft x-ray and extreme ultraviolet wavelength range too put in the actual Use over a longer one Ensure sufficiently high maximum reflectivities.

In In a first aspect of the invention, this object is achieved by a reflective optical element for a Working wavelength in the soft x-ray solved to extremely ultraviolet wavelength range, the a multi-layer system of at least two alternating materials having different real part of the refractive index at the operating wavelength and at at least one interface between a layer Material with a larger real part the refractive index and a layer of material with a smaller real part of the computational index is arranged another layer, the molybdenum boride or molybdenum having.

It It has been proven that the provision of liners with molybdenum or molybdenum boride, especially in multi-layer systems with molybdenum as absorber, too thermal and thermodynamically more stable multilayer systems which also over longer Times or at elevated Temperatures no significant mixing of individual layers takes place. This leaves the optical properties such as maximum reflectivity and reflected Bandwidth of reflective optical elements, such multi-layer systems have, over the entire life in a reliable framework. Amazingly, has been found to be the insertion of liners of molybdenum boride or molybdenum for only small losses at the maximum reflectivity or bandwidth compared to multilayer systems without these liners leads. In particular, these losses less than the losses, the one due to the mixing of the absorber and Spacerlagen in Must take purchase.

In In a second aspect of the invention, this object is achieved by a Use of one of said reflective optical elements for Reflection of radiation in the soft X-ray to extreme ultraviolet wavelength range dissolved at an operating temperature of 20 ° C to 600 ° C.

In In a third aspect, this object is achieved by using a said reflective optical elements as a collector of radiation in the soft x-ray to extreme ultraviolet wavelength range in an EUV lithography device.

In In further aspects of the invention, this object is achieved by a projection system, especially for one EUV lithography apparatus, with at least one of the aforementioned reflective optical elements, by a lighting system, in particular for one EUV lithography apparatus, with at least one of the aforementioned reflective optical elements, by a beam-shaping system, in particular for an EUV lithography device, with at least one of the aforementioned reflective optical elements, as well by an EUV lithography apparatus having at least one of solved before mentioned reflective optical elements.

advantageous Embodiments can be found in the dependent claims.

The The present invention is intended to be better understood with reference to a preferred embodiment be explained in more detail. Show this

1 schematically an embodiment of an EUV lithography device;

2a D schematically shows the construction of a reflective optical element and various multi-layer systems; and

3 the reflectivity of different reflective optical elements as a function of the wavelength.

In 1 schematically is an EUV lithography device 100 shown. Essential components are the beam-forming system 110 , the lighting system 120 , the photomask 130 and the projection system 140 ,

As a radiation source 111 For example, a plasma source or a synchrotron can serve for the wavelength range from 5 nm to 20 nm. The emerging radiation is first in a collector mirror 112 bundled. Also, with the help of a monochromator 113 filtered out by varying the angle of incidence, the desired operating wavelength. In the aforementioned wavelength range are the collector mirror 112 and the monochromator 113 Usually formed as reflective optical elements, which, in order to achieve a reflection of the radiation of the operating wavelength, a multilayer system of at least two alternating materials with different real part of the refractive index at the operating wavelength. Collector mirrors are often cup-shaped reflective optical elements to achieve a focusing or collimating effect. Both the collector mirror 112 as well as the monochromator 113 can be designed as reflective optical elements with intermediate layers of molybdenum nitride or molybdenum boride or with spacer layers of silicon carbide, as will be explained in detail later. Since collector mirrors are arranged in the beam path very close to the radiation source, they are exposed to a high heat load. Therefore, collector mirrors having a multilayer system comprising molybdenum nitride or molybdenum boride spacers or silicon carbide spacer layers are particularly suitable.

The in the beam-forming system 110 In terms of wavelength and spatial distribution processed operating beam is then in the lighting system 120 introduced. Im in 1 illustrated example, the lighting system 120 two mirrors 121 . 122 on, which are configured in the present example as a multi-level mirror. The mirror 121 . 122 direct the beam onto the photomask 130 that has the structure on the wafer 150 should be displayed. At the photomask 130 It is also a reflective optical element for the EUV and soft wavelengths, which is changed according to the manufacturing process. With the help of the projection system 140 becomes that of the photomask 130 reflected beam on the wafer 150 projected and thereby imaged the structure of the photomask on him. The projection system 140 In the example shown, there are two mirrors 141 . 142 on, which are also configured in the present example as a multi-level mirror. It should be noted that both the projection system 140 as well as the lighting system 120 also each may have only one or even three, four, five or more mirrors.

Im in 1 example shown are all mirrors 121 . 122 . 141 . 142 designed as reflective optical elements with intermediate layers of molybdenum nitride or molybdenum boride or with spacer layers of silicon carbide, as will be explained in detail later. Optionally, it can also be the photomask 130 to act such a reflective optical element.

2a -D show by way of example a reflective optical element 1 for the extreme ultraviolet and soft x-ray wavelength range, especially for use in EUV lithography devices, e.g. B. as a mirror of the projection or illumination system or as a photomask, collector mirror or monochromator. 2a schematically shows the parent structure of the multi-layer system 2 , The multi-day system 2 is in the present example by successively coating a substrate 3 made with different materials with different complex refractive indices. In addition, on the multi-day system 2 in addition a protective layer 4 applied for protection against external influences such as contamination, which may be composed of several different layers of material.

The multi-day system 2 consists essentially of repetitive stacking 20 , whose structure for various preferred embodiments schematically in the 2 B -D is shown. The essential layers of a pile 20 , in particular through the multiple repetition of the pile 20 lead to sufficiently high reflection at a working wavelength, are the so-called Spacerlagen 22 . 22 ' made of material with a higher real part of the refractive index and the so-called absorber layers 21 made of a material with a lower real part of the refractive index. As a result, a crystal is simulated, the absorber layers 21 correspond to the lattice planes within the crystal, the one through the respective Spacerlagen 22 . 22 ' have defined distance to each other and where reflection of incident EUV or soft X-rays takes place. The thicknesses of the layers are chosen such that at a certain operating wavelength at each absorber layer 21 reflected radiation superimposed constructively so as to achieve a high reflectivity of the reflective optical element. It should be noted that the thicknesses of the individual layers 21 . 22 . 22 ' . 23 . 24 as well as the repeating stack 20 be constant over the entire multi-layer system or even vary, depending on which reflection profile is to be achieved. In particular, multi-layer systems for certain wavelengths can be optimized in which the maximum reflectivity and / or the reflected bandwidth is greater than at other wavelengths. For radiation of this wavelength, the corresponding reflective optical element 1 z. As used in EUV lithography, which is why this wavelength, for the reflective optical element 1 was optimized, also called working wavelength.

For example In EUV lithography one likes to work at wavelengths 12 nm and 15 nm. In this wavelength range can be particularly high reflectivities with multilayer systems based on molybdenum as absorber material and Silicon obtained as a spacer material. For example, at a wavelength of 13.5 nm theoretically reflectivities in the range of over 75% possible. In this case, one often uses 50 to 60 stacks of a thickness of 7 nm and a relationship from absorber layer thickness to stack thickness of 0.4.

In real molybdenum-silicon multilayer systems, however, a mixed layer of molybdenum silicide forms at the layer boundaries, which leads to a significant reduction in the maximum achievable reflectivity. Table 1 gives the reflectivity values which result when assuming a mixed layer thickness of 1 nm. Since layers of such a small thickness can not give completely unambiguous stoichiometric data, the reflectivities at 13.5 nm were determined for various stable molybdenum-silicon compounds, namely MoSi ₂ , Mo ₅ Si ₃ and Mo ₃ Si (see also Table 1) ). However, different methods of investigation and reflectivity measurements indicate that the stoichiometric ratio in the mixed layer is closest to MoSi ₂ , resulting in a significantly lower maximum reflectivity of 70.9%.

In order to counteract the deterioration of the optical properties, according to the in 2 B example proposed, at the transition from absorber 21 to Spacer 22 an additional intermediate layer 23 to provide molybdenum boride or molybdenum nitride, preferably consists of molybdenum boride or molybdenum nitride. Molybdenum boride and molybdenum nitride have the advantage of being thermodynamically more stable than the mixed layers formed of absorber and spacer material. In particular, during production and immediately thereafter, there is less mixing with the adjacent layers. Their thickness therefore remains constant, rather than in the case of mixed layers forming without intermediate layers, so that the contrast between the spacer and absorber layers remains substantially constant. Also their thermal stability is higher. With the same thickness as the mixed layer of absorber and spacer material, the contrast can be compared to systems without these intermediate layers 23 even slightly increase.

In preferred embodiments, the reflective optical element 1 absorber layers 21 made of molybdenum. In this case, the influence of the intermediate layers on the reflectivity and the thermodynamic and thermal stability is particularly good, which may possibly be due to the fact that the intermediate layer has or even consists of a boride or nitride of the absorber material. In particularly preferred embodiments, the reflective optical element 1 absorber layers 21 made of molybdenum and Spacerlagen 22 made of silicon. It has been found that molybdenum boride or nitride liners 23 have a particularly contrast and thus reflectivity-enhancing effect compared with mixed layers of molybdenum silicide.

The intermediate layer with molybdenum boride or molybdenum nitride has in particular at the interfaces of spacer 22 on absorber 21 a particularly clear contrast-increasing effect. In particular, liners are sufficient 23 at the interfaces of Spacer 22 on absorber 21 already in place to achieve a significant improvement in thermodynamic and thermal stability. Likewise, alternatively or additionally, intermediate layers at the interfaces of absorber 21 on spacer 22 be provided.

In the present, in 2c Example shown are both at interfaces of spacer 22 on absorber 21 liners 23 with molybdenum boride or molybdenum nitride as well as at interfaces of absorber 21 on spacer 22 an intermediate layer 24 with molybdenum boride or molybdenum nitride to increase the thermodynamic and thermal stability of the multilayer system.

In Tables 2 and 3, the maximum reflectivities at a wavelength of 13.5 nm are given for molybdenum-silicon multi-layer systems which have at all layer boundaries between absorber and Spacerlagen intermediate layers of molybdenum nitride (Table 3) or molybdenum boride (Table 2). The reflectivities are compared with the reflectivity of a molybdenum-silicon multilayer system without an intermediate layer, in which a mixed layer of MoSi _{2 has} formed. For better comparison, both the mixed layer and the intermediate layers are 1 nm thick. As with the liners 23 . 24 due to their small thickness of 1 nm, no fully definite stoichiometric data can be given, the values in Tables 2 and 3 for MoB ₂ , MoB and Mo ₂ B on the one hand and Mo ₂ N and MoN on the other hand were given. The stoichiometric composition can be influenced by the particular coating process by adding more or less nitrogen or boron during molybdenum coating. The reflectivity as a function of the wavelength in angstroms for a multilayer system with mixed layers of MoSi ₂ , a multilayer system with interlayers of Mo ₂ B and a multilayer system with interlayers of Mo ₂ N is also in 3 shown.

Molybdenum boride interlayers not only increase the thermodynamic and thermal stability of reflective optical elements, but also result in higher reflectivity. The molybdenum borides are predominantly MoB ₂ and partly MoB, which is also supported by the comparable enthalpies of these two borides, which are more negative than the enthalpy for Mo ₂ B, but also influence the actual stoichiometry via the respective coating process can be taken.

Especially with multi-layer systems based on molybdenum as absorber and silicon as spacer, maximum reflectivities are achieved with molybdenum boride interlayers, which are 1.9 to 3.5% above the reference value of 70.9% for a corresponding molybdenum-silicon multilayer system MoSi ₂ mixed layers lie. As well as out 3 As can be seen, the bandwidth reflected from multilayer systems with molybdenum boride interlayers is larger than in multilayer systems with molybdenum silicide mixed layers. This leads to a significantly higher overall reflectivity, which is maintained over long periods of time and when used at some 100 ° C above room temperature and up to 600 ° C thanks to the good thermodynamic and thermal stability.

A statement for the good stability in both thermodynamic and thermal terms, in the different enthalpies of molybdenum silicides and molybdenum borides lie. The lower enthalpies compared to the molybdenum silicides the molybdenum boride prove (see Tables 1 and 2) that the molybdenum borides form more preferred than the molybdenum silicides. That's why they work probably like a diffusion barrier between molybdenum and silicon layers and thus prevent over longer Time as well as at elevated Temperatures diffuse the silicon into the molybdenum layers.

Reflective optical elements with a multilayer system comprising molybdenum nitride interlayers exhibit higher thermal stability than multilayer systems without a molybdenum nitride interlayer. In particular, in multi-layer systems based on molybdenum as absorber and silicon as spacer, molybdenum nitride intermediate layers allow use at temperatures which are significantly above room temperature. in the range of 200 ° C to 300 ° C and up to 600 ° C. Comparing the maximum reflectivities given in Tables 1 and 3, it can be seen that the reflectivities for multilayer systems with molybdenum silicide mixed layers and for multilayer systems with molybdenum nitride interlayers are in similar ranges depending on the stoichiometry. However, the reflected bandwidth in multilayer systems with molybdenum nitride liners, such as 3 clearly larger than in multilayer systems with molybdenum silicide mixed layers. This leads to a higher overall reflectivity in reflective optical elements with multilayer systems with molybdenum nitride spacers. Because of the higher thermal stability, this higher overall reflectivity is substantially maintained even when used at higher temperatures.

Im in 2d Example shown instead of an additional intermediate silicon carbide as spacer layer 22 ' used. It has also been found that the use of silicon carbide as a spacer material reduces the mixing with the layers of absorber material effectively and in particular at higher temperatures. Reflective optical elements with silicon carbide as a spacer material are thermodynamically and thermally stable, without sacrificing the maximum reflectivity compared to real multi-layer systems, in which the individual layers of absorber and spacer material mix at their interfaces with each other would have to be accepted. In preferred embodiments, the in 2d illustrated reflective optical element 1 as material of the absorber layers 21 Molybdenum on. It has been found that intermixing by diffusion between silicon carbide on the one hand and molybdenum on the other hand hardly takes place at the ply boundaries as compared with silicon and molybdenum. This effect can also be observed over longer periods in the range of days and weeks, as well as when heated to temperatures of 200 ° C to 300 ° C and even to 600 ° C.

A particular advantage of the reflective optical elements described here is that they can be operated not only at room temperature with a sufficiently long life, but also at operating temperatures of 200 ° C to 300 ° C and even up to 600 ° C. Even with high incident on the reflective optical radiation intensity therefore no cooling of the reflective optical elements is necessary. Of particular advantage is the use of the reflective optical elements described here as the collector mirror of an EUV lithography device. Such a collector mirror, which is usually exposed to the highest radiation intensity within an EUV lithography device, can still be operated without expensive cooling. Intermediate layer, 1 nm Enthalpy [kJ / cm ³ ] Reflectivity R [%] MoSi2 (Ref.) -5.4 70.9 Mo ₅ Si ₃ -4.5 72.2 Mo ₃ Si -3.1 73.2 Table 1: Molybdenum-silicon multilayer system, optimized for a working wavelength of 13.5 nm with mixed layers of molybdenum silicide Intermediate layer, 1 nm Enthalpy [kJ / cm ³ ] Reflectivity R [%] Reflektivitätsgewinn MoB ₂ -10.4 72.8 1.9 Mob -10.1 73.8 2.9 Mo ₂ B -6.0 74.4 3.5 Table 2: Molybdenum-silicon multilayer system optimized for a working wavelength of 13.5 nm with molybdenum boride spacers Intermediate layer, 1 nm Reflectivity R [%] Reflektivitätsgewinn Mo ₂ N 71.7 0.8 MoN 69.6 -1.3 Table 3: Molybdenum-silicon multilayer system, optimized for a working wavelength of 13.5 nm with molybdenum nitride interlayer

1

reflective optical element

2

Multilayer system

3

substratum

4

protective layer

20

periodically recurring stack of layers

21

absorber

22 22 '

spacer

23

liner

24

liner

100

EUV lithography device

110

Beam shaping system

111

radiation source

112

collector mirror

113

monochromator

120

lighting system

121 122

mirror

130

photomask

140

projection system

141 142

mirror

150

wafer

Claims (12)

Reflective optical element for a working wavelength in the soft X-ray to extreme ultraviolet wavelength range, comprising a multilayer system of at least two alternating materials with different real part of the refractive index at the operating wavelength, characterized in that at least one interface between a layer of material ( 22 ) with a larger real part of the refractive index and a layer of material ( 21 ) with a smaller real part of the calculation index another position ( 23 ) having molybdenum boride or molybdenum nitride. Reflective optical element according to claim 1, characterized in that the further layer ( 23 ) at at least one interface of a layer of material ( 22 ) with a greater real part of the refractive index on a layer of material ( 21 ) is arranged with a smaller real part of the refractive index. Reflective optical element according to claim 1 or 2, characterized in that the further layer ( 23 ) at all interfaces of a layer of material ( 22 ) with a greater real part of the refractive index on a layer of material ( 21 ) is arranged with a smaller real part of the refractive index. Reflective optical element according to one of claims 1 to 3, characterized in that the further layer ( 23 ) at at least one interface of a layer of material ( 21 ) with a smaller real part of the refractive index on a layer of material ( 22 ) is arranged with a larger real part of the refractive index. Reflective optical element according to one of claims 1 to 4, characterized in that the material ( 21 ) with smaller real part of the refractive index is molybdenum. Reflective optical element according to one of claims 1 to 5, characterized in that the material ( 22 ) with a larger real part of the refractive index is silicon. Use of a reflective optical element according to any one of claims 1 to 6 for the reflection of radiation in the soft X-ray to extreme ultraviolet wavelength range at an operating temperature from 20 ° C to 600 ° C. Use of a reflective optical element after one of the claims 1 to 6 as a collector of radiation in the soft X-ray to extreme ultraviolet Wavelength range in an EUV lithography apparatus. Projection system ( 120 ), in particular for an EUV lithography apparatus, having at least one reflective optical element ( 121 . 122 ) according to claim 1 to 6. Lighting system ( 140 ), in particular for an EUV lithography apparatus, having at least one reflective optical element ( 141 . 142 ) according to claim 1 to 6. Beam shaping system ( 110 ), in particular for an EUV lithography apparatus, having at least one reflective optical element ( 112 . 113 ) according to claim 1 to 6. EUV lithography apparatus ( 100 ) with at least one reflective optical element ( 112 . 113 . 121 . 122 . 141 . 142 ) according to claim 1 to 6.