Multilayer mirror for reflecting EUV radiation and method for producing the same

Abstract

A multilayer mirror for reflecting Extreme Ultraviolet (EUV) radiation and a method for producing the same are disclosed. In an embodiment a multilayer mirror includes a layer sequence having a plurality of alternating first layers and second layers, the first layers including lanthanum or a lanthanum compound and the second layers including boron, wherein the second layers are doped with carbon, and wherein a molar fraction of carbon in the second layers is 10% or less.

Claims

1. A multilayer mirror for Extreme Ultraviolet (EUV) radiation comprising: a layer sequence having a plurality of alternating first layers and second layers, the first layers comprising lanthanum or a lanthanum compound and the second layers comprising boron, wherein the second layers are doped with carbon, and wherein a molar fraction of carbon in the second layers is 10% or less.

2. The multilayer mirror according to claim 1, wherein the molar fraction of carbon in the second layers is 5% or less.

3. The multilayer mirror according to claim 1, wherein the molar fraction of carbon in the second layers is 3% or less.

4. The multilayer mirror according to claim 1, wherein the lanthanum compound is a lanthanum nitride, a lanthanum oxide or a lanthanum carbide.

5. The multilayer mirror according to claim 1, wherein the first layers and the second layers each have a thickness between 1 nm and 3 nm.

6. The multilayer mirror according to claim 1, further comprising thin barrier layers arranged at interfaces between the first layers and the second layers, wherein the thin barrier layers comprise B.sub.4C or C and have a thickness of not more than 1.0 nm.

7. The multilayer mirror according to claim 1, wherein the layer sequence is a periodic layer sequence, wherein a period comprises a layer pair of one of the plurality of first layers and one of the plurality of second layers, and wherein the period has a thickness in a range from 3 nm to 4 nm.

8. The multilayer mirror according to claim 1, wherein the layer sequence comprises between 100 and 400 layer pairs, each layer pairs having one of the first layers and one of the second layers.

9. A method for producing a multilayer mirror for an Extreme Ultraviolet (EUV) spectral range, the method comprising: alternately depositing first layers comprising lanthanum or a lanthanum compound and second layers comprising boron, wherein the second layers are doped with carbon, and wherein a molar fraction of carbon in the second layers is 10% or less.

10. The method according to claim 9, wherein depositing the first and second layers comprises DC magnetron sputtering the first layers and the second layers.

11. The method according to claim 10, wherein the DC magnetron sputtering uses a sputtering target comprising carbon-doped boron for sputtering the second layers.

12. The method according to claim 11, wherein the carbon in the sputtering target has a molar fraction of 10% or less.

13. The method according to claim 10, wherein the DC magnetron sputtering is performed at room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail in the following using examples in connection with FIGS. 1 to 4.

(2) In the figures:

(3) FIG. 1 shows a schematic representation of a multilayer mirror for EUV radiation;

(4) FIG. 2 shows a schematic representation of a process for producing the multilayer mirror for EUV radiation;

(5) FIG. 3 shows a schematic representation of a multilayer mirror for EUV radiation; and

(6) FIG. 4 shows a graphical representation of the reflection of multilayer mirrors for EUV radiation according to three different examples and a comparison example.

(7) Same or similarly acting components are provided in the figures with the same reference signs in each case. The represented components as well as the proportions of the components among each other are not to be regarded as true to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(8) The multilayer mirror 10 for EUV radiation shown schematically in cross-section in FIG. 1 has a layer sequence 5 with a plurality of alternating first layers 1 and second layers 2. The layer sequence 5 is applied to a substrate 6 which, depending on the application, can be a flat substrate or a curved substrate. A substrate 6 with a low surface roughness is preferably used to achieve the highest possible reflectivity. The substrate 6 can, for example, contain silicon or a glass ceramic.

(9) A first layer 1 and a second layer 2 each form a layer pair 4. To simplify the representation, only five layer pairs 4 are shown in FIG. 1, whereby the layer sequence 5 preferably has between about 100 and about 400 layer pairs 4 in order to achieve the highest possible reflection. For example, the multilayer mirror 10 can have about 250 layer pairs 4.

(10) The layer sequence 5 can in particular be a periodic layer sequence in which the layer pairs 4 have the same structure and the same layer thicknesses. In this case, the sum of the thickness of a first layer 1 and a second layer 2 is the period thickness, which is preferably between about 3 nm and about 4 nm. With a multilayer mirror 10 optimized for vertical incident radiation, the period thickness is approximately half the wavelength of the reflection maximum. For a mirror with a reflection maximum at the wavelength λ=6.7 nm, for example, the period thickness can be about 3.4 nm.

(11) The first layers 1 of the layer pairs 4 comprise or consist of lanthanum (La) or a lanthanum compound. In particular, the first layers may comprise or consist of a lanthanum nitride, a lanthanum oxide or a lanthanum carbide. The second layers 2 of the layer pairs 4 comprise boron (B) and additional carbon (C) in low concentrations. The molar fraction of carbon in the second layer 2 is not more than 10%, preferably not more than 5% and particularly preferably not more than 3%. The addition of carbon to the base material boron increases the electrical conductivity of the second layers 2 and makes it possible to manufacture them advantageously using DC magnetron sputtering. DC magnetron sputtering can be advantageously carried out at room temperature, especially without heating the sputtering target. An undesired heating of the substrate 6 during the coating process can thus be avoided. This results in the advantages that a high accuracy of the layer thicknesses and a good reproducibility are achieved.

(12) Furthermore, the reflectivity of the layer sequence 4 is greater than if boron carbide (B.sub.4C) were used for the second layers 2. In order to achieve good electrical conductivity of the second layers 2, it may be advantageous if the carbon molar fraction is at least 0.1%, preferably at least 1% or particularly preferably at least 2%.

(13) It is possible that the multilayer mirror 10 has at least one cover layer (not shown) on the side facing away from the substrate 6, which may have a different material and/or layer thickness than the first layers 1 and second layers 2. Such a cover layer may, for example, be intended to improve the resistance of the multilayer mirror 10 to oxidation and/or incident ions, in particular when used in the vicinity of an EUV radiation source. Suitable cover layer materials are in particular oxides, nitrides or carbides, such as oxides, nitrides or carbides of La, Ce, U or Th.

(14) In the production of the multilayer mirror 10, the entire layer sequence 5 is preferably deposited by DC magnetron sputtering. FIG. 2 schematically shows the sputtering process in a DC magnetron sputtering system 20. During the deposition process, the substrate 6 is arranged on a substrate holder 23, which is preferably movable in a lateral direction and can thus be moved under different sputtering targets 21, 22 for the deposition of different layer materials. Between the substrate holder 23, which acts as the anode, and the sputtering targets 21, 22, which act as the cathode, a DC voltage 25 is applied during the sputtering process. The DC magnetron sputtering system 20 also has a gas inlet for the sputtering gas, preferably argon, and a high vacuum pump (not shown). A magnetron 24 for generating a magnetic field is arranged above the sputtering targets 21, 22. In addition to the two sputtering targets 21, 22 shown as examples, at least one further sputtering target can be provided, e.g., for the production of barrier layers from a third material.

(15) A sputtering target 22 containing carbon-doped boron is used to apply the second layers 2. The sputtering target 22 may in particular contain boron with a carbon molar fraction of not more than 10%, preferably not more than 5% and particularly preferably not more than 3%. For example, the carbon molar fraction may be 2%.

(16) The foreign atoms of carbon are already introduced into the second layers 2 during production by using the sputtering target 22, which in addition to boron contains the foreign atoms of carbon in the desired amount. In this way, the material carbon is distributed over the entire thickness of the second layer 2. In particular, apart from possible diffusion-related deviations at the interfaces, the molar fraction of carbon in the second layers 2 can be essentially constant.

(17) FIG. 3 shows another example of the multilayer mirror 10. This essentially corresponds to the first example, but a thin barrier layer 3 is additionally inserted at the interfaces between the first layers 1 and the second layers 2. In the example in FIG. 3, a barrier layer 3 is inserted at all interfaces, regardless of whether a first layer 1 follows a second layer 2 in the direction of growth or, conversely, a second layer 2 follows a first layer 1 in the direction of growth. In this case, a layer pair 4 in the layer sequence is a pair of a first layer 1 and a second layer 2 including the barrier layers 3 in between. In this example, a period of the multilayer mirror 10 has four sublayers each.

(18) Alternatively, however, it would also be possible for the barrier layers 3 to be arranged at only one of the interfaces where a first layer 1 follows a second layer 2 in the direction of growth, or vice versa. In this case, a period of the multilayer mirror 10 would have three sublayers.

(19) The barrier layers 3 are preferably only between 0.1 nm and 1.0 nm thick. Particularly suitable materials for the barrier layers are carbon (C) and boron carbide (B.sub.4C). It has been found that the reflectivity of the multilayer mirror 10 can be further increased by inserting the thin barrier layers 3 between the first layers 1 and the second layers 2. It can be assumed that the insertion of the thin barrier layers results in 3 smoother interfaces in layer sequence 5, which have a positive effect on the reflectivity of layer sequence 5.

(20) FIG. 4 shows the measured reflection R for s-polarized radiation at an angle of incidence of 10° for three examples of multilayer mirrors and a comparison example not in accordance with embodiments of the invention. In the inserted detail magnification, the area around the reflection maximum is enlarged to illustrate the differences in the maximum reflectivity. The measurements were carried out on the EUV reflectometer of PTB Berlin.

(21) The reflection curve 11 shows the measured reflectivity R of a comparative example which does not correspond to embodiments of the invention of a multilayer mirror with 250 layer pairs consisting of alternating first layers of LaN and second layers of B.sub.4C as a function of the wavelength λ. The maximum reflectivity measured is 57.8%. The reflection curve 12 shows the measured reflectivity of an example of a multilayer mirror with 250 layer pairs consisting of alternating first layers of LaN and second layers of boron doped with carbon, where the mass fraction of carbon is 2%. It can be seen that the measured reflectivity of 58.4% is higher than in the comparison example of the reflection curve 11.

(22) The reflection curve 13 shows the measured reflectivity of another example of a multilayer mirror with 250 layer pairs which have alternating first layers of LaN and second layers of boron doped with carbon, where the amount of carbon is 2%. In this example, barrier layers of carbon (C) are additionally inserted at all interfaces between the first layers and the second layers. With 59.1%, the measured reflectivity at the maximum is even greater than in the example of the reflection curve 12.

(23) The reflection curve 14 shows the measured reflectivity of another example of a multilayer mirror with 250 layer pairs, which have alternating first layers of LaN and second layers of boron doped with carbon, whereby the amount of carbon is 2%. In this example, barrier layers of boron carbide (B.sub.4C) are additionally inserted at all interfaces between the first layers and the second layers. With 60.6%, the measured reflectivity at the maximum is even greater than in the example of curve 13.

(24) The invention is not limited by the description of the examples. Rather, the invention includes each new feature as well as each combination of features, which in particular includes each combination of features in the patent claims, even if that feature or combination itself is not explicitly stated in the claims or examples.

(25) The research work that led to these results was funded within the framework of the internal programs of the Fraunhofer-Gesellschaft.