WO2012007146A1

WO2012007146A1 - Method of improving the operation efficiency of a euv plasma discharge lamp

Info

Publication number: WO2012007146A1
Application number: PCT/EP2011/003467
Authority: WO
Inventors: Willi Neff; Dominik Vaudrevange; Christof Metzmacher
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2010-07-15
Filing date: 2011-07-11
Publication date: 2012-01-19
Also published as: TW201212726A

Abstract

The present invention relates to a method of improving the operation efficiency of a plasma discharge lamp generating EUV radiation and/or soft X- rays, said lamp comprising at least two electrodes (1, 2) being formed of a refractory metal and means for applying a liquid metal to a surface portion of at least one of said electrodes (1, 2) during operation of the lamp. At least one of said electrodes (1, 2) is pre-treated in a pre-treatment step, in which at least said surface portion of said electrode (1, 2) is brought into contact with the liquid metal and thermally annealed at a temperature of ≥ 800° C to cause a reaction between the refractory metal and the liquid metal in a reaction zone (9) of a controlled depth on said electrode (1, 2). Alternatively, a layer of a further material may be deposited on said surface portion, said further material being selected to improve the wetting behaviour of said surface portion for the liquid metal. With the proposed method the time required for in- situ wetting procedures of such a lamp is reduced resulting in an improvement of the overall efficiency of the lamp. Furthermore, the lifetime of the lamp is enhanced due to reduced electrode erosion.

Description

Method of improving the operation efficiency of a EUV plasma discharge lamp

Technical field and background

The present invention relates to a method of improving the operation efficiency of a plasma

discharge lamp generating EUV radiation and/or soft X- rays, said lamp comprising at least two electrodes being formed of a refractory metal and means for applying a liquid metal to a surface portion of at least one of said electrodes during operation of the lamp. The invention also relates to a plasma discharge lamp with improved operation efficiency.

Light sources emitting extreme ultraviolet (EUV) radiation and/or soft X-rays, i.e. in the wavelength region of between 1 and 20 nm, are required for example in the field of EUV lithography or metrology. These devices are by far the most promising candidates to be the high power light sources for the upcoming litho- graphy tools of semiconductor industry. It is known in the art how to create EUV light efficiently at a wavelength of 13.5 nm using highly ionized plasma. The excitation of such a plasma from an EUV emitting target material may be generated by means of a high power laser beam (laser produced plasma, LPP) or by an electrical gas discharge between electrodes (discharge produced plasma, DPP) .

A gas discharge light source comprises at least two electrodes (anode, cathode) separated from one another to form a gap in between. The required

electrical energy for a pulsed operation of such a discharge lamp can be supplied by means of a capacitor arrangement in which the energy is first stored and then discharged via the electrodes. The system is filled with a discharge material at pressures in the range of typically 1 to 100 Pa. By virtue of magnetic compression due to the application of a pulsed current of typically a few tens of kA to at most 100 kA and pulse durations of typically a few tens of ns to a few hundred ns, the pinch plasma is heated up to temperatures of few tens of eV.

In current state-of-the-art DPP based systems a change from gaseous supply to liquid metal based discharge material is realized. WO 2005/025280 A2 discloses such a system in which the liquid metal is applied to two rotating, circular electrodes. A pulsed laser triggers the discharge by evaporating at least partially some of the liquid metal, in particular tin, from the liquid metal layer formed on the electrodes. The capacitor bank is discharged through the metal vapour, creating a high current pinch discharge. With such a system higher conversion efficiencies and thus higher amounts of EUV photons are achieved than with gas based systems .

The generation of high radiation power requires very high average electrical powers which have to be fed into the source. This may lead to high electrode erosion and thus short lifetimes of the electrode system. Furthermore, the plasma will become spatially larger because of the change in electrode geometry, leading to the effect that only a small fraction of the produced light can be exploited and thus, optical performance is diminished. In a plasma discharge lamp being operated with liquid metal, the use of electrodes of a refractory metal, for example tungsten (W) or molybdenum (Mo) , in connection with the liquid metal film reduces the degree of electrode erosion. However, the liquid metal must show a reasonable wetting

behavior and sufficiently strong adhesion to the electrode material. If the liquid metal film is locally not permanently well adhered to the electrode surface, this may result in unstable thickness values of the liquid metal film, cathode spots, electrode erosion and many more unwanted effects, resulting finally in a decrease of electrode lifetime.

Before operating a plasma discharge lamp of the kind in which liquid metal is applied to at least one of the electrodes, preparation steps for ensuring a reliable wetting of the liquid metal on the electrode surfaces have to be carried out. These standard in-situ wetting procedures are complex, time consuming and block the EUV plasma discharge lamp a considerable amount of time, because several steps of some hours are required and have to take place always and repeatedly prior to common daily operation of the EUV lamp, thus reducing the overall efficiency of the lamp.

Description, of the invention

It is an object of the present invention to provide a method of improving the operation efficiency and lifetime of a liquid metal based plasma discharge lamp generating EUV radiation and/or soft X-rays and to provide such a plasma discharge lamp having increased efficiency.

The object is achieved with the method and plasma discharge lamp according to claims 1, 2, 11 and 12. Advantageous embodiments of the method and lamp are subject matter of the dependent claims or are described in the subsequent portions of the description and embodiments .

The proposed method improves the operation

efficiency and lifetime of a plasma discharge lamp generating EUV radiation and/or soft X-rays, which lamp comprises at least two electrodes formed of a refrac- tory metal and means for applying a liquid metal to a surface portion of at least one of said electrodes during operation of the lamp. In the proposed method according to a first alternative said at least one of said electrodes is pre-treated in a pre-treatment step in which at least said surface portion of said

electrode is brought into contact with the liquid metal and thermally annealed at a temperature of ≥ 800° C while in contact with the liquid metal, to cause a reaction between the refractory metal and the liquid metal in a reaction zone of a controlled depth on said electrode. In case of applying the liquid metal during operation of the lamp to surface portions of both electrodes, both electrodes are pre-treated in such a pre-treatment step. According to a second alternative, a layer of a further material is deposited on said surface portion of said electrode in a pre-treatment step, said further material being selected to improve the wetting behaviour of said surface portion for the liquid metal. In both alternatives, said surface portion of said electrode is pre-treated to generate a layer (reaction zone or layer of further material) which has a better wetting behaviour for the liquid metal than the electrode surface without such a layer.

In a preferred embodiment, the method is applied to a plasma discharge lamp having two electrode wheels which rotate during operation of the lamp. The

electrode wheels are mounted above two containers with liquid metal such that they partly dip into the liquid metal. As a result, the liquid metal is applied to the circumferential surfaces of the electrode wheels during rotation. In order to improve the efficiency of such a lamp according to the proposed method, the electrode wheels are dismounted in order to perform the pre- treatment step, in particular in an appropriate furnace for thermal annealing. During this annealing step, the electrode wheels may be completely submersed in a crucible containing the liquid metal. In order to avoid a reaction at the central portions of the electrode wheels, at which these wheels are mounted in the discharge lamp, these central portions may be masked with appropriate plates. After the pre-treatment step, the electrode wheels are mounted in the discharge lamp which can then be operated with these pre-treated electrode wheels.

Even if the above embodiment shows a preferred discharge lamp to which the method is applied, the proposed method may also be applied to other plasma discharge lamps in which at least one of the electrodes is wetted with a liquid metal during operation. The electrodes of such a lamp may have other geometric forms than electrode wheels and the liquid metal may also be applied in another way to the surface of the at least one electrode.

With the first alternative of the proposed method, a thin layer, the so called reaction zone, forms on the dedicated surface portions of the electrode. This thin layer of a stable compound of the refractory metal and the applied metal shows an improved wetting behavior and adhesion to the liquid metal as well as a higher erosion resistance. The same applies to the deposited layer of the second alternative. Therefore, the in- situ wetting procedures, i.e. the initial and daily wetting as described in the introductory portion above, can be drastically reduced in time or even be skipped. The blocking times of such a plasma discharge lamp are thus reduced, which means a higher overall efficiency of the lamp in the sense of shorter downtimes. Due to the higher resistance the lifetime of the electrodes of such a lamp and thus the lifetime of the lamp is enlarged. The pre-treatment step has to be performed only once for the lifetime of the lamp or electrodes, but may also be repeated occasionally.

Typically, the liquid metal applied to the electrodes is tin (Sn) while the material of the electrodes is a refractory metal such as W or Mo. Liquid Sn, however, shows a generally poor adhesion on these refractory metals which is mainly due to the low mutual solubility between Sn and W or Mo, respectively. With the proposed method, a thin, uncritical reaction zone or layer is realized on the surface of the electrodes which shows good wetting and adhesion of the liquid metal on the surface of the electrodes. The reaction zone is formed with a controlled depth according to the present method in order to avoid progressing corrosion of the electrode through the reaction. Preferably, the pre-treatment step is performed such that the thickness (depth) of the reaction zone is between 100 nra and 5 μπι. To this end, the reaction parameters like gas atmosphere, annealing temperature and annealing time have to be controlled appropriately. Preferably, the annealing is performed at a temperature between 800 and 1600° C for a time period between 1 and 24 h.

The annealing may be performed in vacuum or in a dedicated gas atmosphere, for example in a gas atmosphere containing 95% N₂ / 5% H₂ or H₂ or Ar. The gas flow of the above mentioned gases is preferably set to between 100 and 1500 seem. In a further embodiment, the pre-treatment step may comprise two sub-steps. In the first sub-step the surface of the electrode is reduced in a reducing gas atmosphere, e.g. in a N₂/H₂ mixture, at elevated

temperatures in order to remove any oxide or oxide layer from the surface of the electrode. In the

following second sub-step the thermal annealing is performed in contact with the liquid metal as already described above . In a further embodiment, the pre-treatment is achieved by a dedicated ex- situ deposition process, and - optionally - subsequent delivery of Sn to the pre- treated region. The idea behind this is to apply a "wetting promoter" as a first layer on the refractory electrodes to allow for a more pronounced, subsequent wetting by Sn. The deposition process may be performed by a vacuum based technology, e.g. physical vapour deposition (PVD) such as arc evaporation, or a non- vacuum deposition such as galvanic processing, or, appropriately feasible in both configurations, by a brazing process. In any cases (alien) materials with good adhesion to refractory metals are preferred as materials ("wetting promoter") for the first layer on the electrode base. These materials may be Ni, Cr or Cu, e.g., and have standard thin film thickness values of a few μτα. This layer may work as a sacrificial layer, i.e., it may be consumed in the application due to further reaction and interdiffusion. Apart from that the formation of a stable interfacial phase between refractory base and Sn is aimed at and this stabilizes the structure as described in previous sections. With the proposed method, by pre-treatment of the electrodes, the wetting behavior and thus the control of the thickness of the liquid metal is improved, resulting in a minimization of electrode degradation, the preservation of general performance within life- time, a higher uptime of the EUV lamp, an enlargement of lifetime in general and a reduction of time for assembly or refurbishment of the source head. The method results in a reliably wetted electrode surface which reduces time for in-situ wetting procedures and thus leads to an improved overall efficiency of the usage of the lamp. Brief description of the drawings

The proposed method and discharge lamp are

described in the following by way of examples without limiting the scope of protection as defined by the claims. The figures show:

Fig. 1 a schematic view of an example of a

plasma discharge lamp according to the present invention; and

Fig. 2 a schematic illustration of a electrode wheel pre-treated according to the proposed method.

Description of preferred embodiments

The proposed method may be applied to a plasma discharge lamp generating EUV radiation and/or soft X- rays as schematically illustrated in figure 1. Such a gas discharge lamp comprises two electrode wheels 1, 2 (cathode, anode) which are separated from one another forming a gap in between. The two electrode wheels 1, 2 rotate during operation of the lamp while partly dipping into containers 3 containing a liquid metal like Sn. Due to the rotation in the liquid metal a tin film 4 forms on the outer circumferential surface of the electrode wheels. The electrode wheels are electrically connected through the tin bath to a capacitor bank 5, which supplies a pulsed current to the

electrode wheels 1, 2. The plasma discharge 8 is initiated by evaporating part of the liquid tin with a pulsed laser beam 6 of a laser source 7 as schematically indicated in the figure. The plasma 8 emits the desired EUV radiation and/or soft X-rays. The elec^¬ trodes are arranged in a vacuum chamber (not shown in the figure) . Furthermore, additional elements like wipers for ensuring a definite thickness of the tin film on the electrodes or shield elements are part of such a plasma discharge lamp. Examples for such

elements are shown for example in WO 2005/025280 A2.

In order to improve the overall efficiency of such a plasma discharge lamp, the electrode wheels 1, 2 are pre-treated, preferably prior to mounting these electrode wheels in the lamp, according to the present method. In this pre-treatment step in the present example, a dedicated thermal annealing in the tempera- ture range of 800 to 1000° C is performed with the electrodes of Mo in a gas atmosphere of 95% N₂/5% H₂. To this end the electrode wheels are submersed in a crucible containing the liquid Sn and thermally

annealed in contact with the liquid Sn in an appro- priate furnace. This allows for a controlled reaction between the liquid and solid materials leading to the formation of a Mo-Sn phase at the interfaces. For example, a reaction zone (Mo-Sn phase) with a depth of < 1 μτα forms after an annealing time of 3h at an annealing temperature of 850°C and a gas flow of 250 seem. The reaction time must be controlled to avoid any potentially detrimental effects upon pronounced recrys - tallization of the base material, i.e. the material of the electrodes.

Instead of dipping the electrode wheels in a crucible with the liquid metal, the reaction may also take place in any other way of delivering a liquid to a solid. For example, it is also possible to deliver the Sn component as an oxide powder material. This requires a preliminary reduction, e.g. at temperatures of about 700° C, to the metal state in the furnace under appro- priate atmospheric conditions, e.g. a N₂/H₂ atmosphere.

Figure 2 schematically illustrates the reaction zone 9 in which the Mo-Sn phase forms during the annealing step. In order to avoid the formation of a reaction zone on the central portions of the sides of the electrode wheel , prior to dipping the electrode wheel 1 into the crucible with the liquid Sn, plates of for example Mo, W, corundum or graphite are fixed to the sides of the electrode wheel 1. These plates are removed after the annealing step. The electrode wheels are subsequently mounted in the discharge lamp which can then be operated in the known manner .

The reaction zone is a thin layer which forms on the surface portions of the electrode wheels. The Mo-Sn phase forming this thin layer is more easily and permanently wetted by Sn. The material system is thus forced to overcome the general inability of the

refractory metal to form a strong surface complex or a metallic bond with Sn which improves the mutual

diffusion and dissolution in the interfacial region, i.e. improves wetting and adhesion of the liquid Sn on the electrode surface. Another advantage is that final wetting in systems with intense reactions at the interface - as in the present case - is usually less sensitive to environmental factors than in insoluble systems. Thus the electrode erosion is reduced also. In another embodiment a first part of the pre- treatment step comprises the removing of potential oxides on the electrode base material at elevated temperatures in a reducing gas atmosphere, in

particular N₂/H₂ mixtures. In the case of Mo as the base electrode material, usually temperatures above 1000° C under H₂ containing atmosphere are required to eliminate a layer of adsorbed impurities (O, C) . After that, the electrode base material is dipped into the liquid metal to allow for the interaction between species of the bare electrode material and the liquid metal. The annealing step is then performed as already described above . While the invention has been illustrated and described in detail in the drawings in forgoing

description, such illustration and description are to be considered illustrative or exemplary and not

restrictive, the invention is not limited to the disclosed embodiments. The reaction zone of Fig. 2 may also be substituted by the layer of a further material, like Ni, Cr or Cu, which is deposited on the electrode surface. The different embodiments described above and in the claims can also be combined. Other variations to the disclosed embodiments can be understood and

effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage . The reference signs in the claims should not be construed as limiting the scope of these claims.

List of reference numerals

1 electrode wheel

2 electrode wheel

3 container with liquid Sn

4 Sn film

5 capacitor bank

6 pulsed laser beam

7 laser light source

8 plasma

9 reaction zone

Claims

Patent Claims

A method of improving the operation efficiency of a plasma discharge lamp generating EUV radiation and/or soft X-rays,

said lamp comprising at least two electrodes (1, 2) being formed of a refractory metal and means for applying a liquid metal to a surface portion of at least one of said electrodes (1, 2) during operation of the lamp,

wherein said at least one of said electrodes (1, 2) is pre-treated in a pre-treatment step in which at least said surface portion of said electrode (1, 2) is brought in contact with the liquid metal and thermally annealed at a temperature of ≥ 800°C to cause a reaction between the refractory metal and the liquid metal in a reaction zone (9) of a controlled depth on said electrode (1, 2) .

wherein said at least one of said electrodes (1, 2) is pre-treated in a pre-treatment step in which a layer of a further material is deposited on said surface portion of said electrode (1, 2) , said further material being selected to improve the wetting behaviour of said surface portion for the liquid metal .

The method according to claim 1 ,

wherein the annealing is performed at a

temperature between 800 and 1600°C for a time period between 1 and 24 h.

The method according to claim 1 or 3 ,

wherein the annealing is performed in a gas atmosphere at atmospheric pressure, said gas atmosphere being composed of Ar or of H₂ or of a mixture of N₂ and H₂.

The method according to claim 4 ,

wherein the annealing is performed at a gas flow between 100 and 1500 seem.

The method according to one of claims 1, 3 to 5, wherein the annealing is performed such that the depth of the reaction zone (9) is between 100 nm and 5 μπι.

The method according to one of claims 1, 3 to 6, wherein in a first part of the pre-treatment step, prior to bringing the surface portion of said electrode (1, 2) in contact with the liquid metal, oxides are removed from the surface portion at elevated temperatures in a gas atmosphere of a reducing gas or gas mixture . The method according to one of claims 1, 3 to 7, wherein said electrode (1, 2) is dipped into a crucible with said liquid metal in the pre- treatment step.

The method according to claim 8 ,

wherein said electrode (1, 2) is an electrode wheel which is rotatable in a rotational direction around a rotational axis and has an outer circumferential surface between two side surfaces, and wherein central portions of said side surfaces are masked in the pre-treatment step to prevent a contact of the liquid metal with the central portions .

The method according to claim 2 ,

wherein the further material is one of Ni, Cr or Cu.

A plasma discharge lamp generating EUV radiation and/or soft X-rays, comprising at least two electrodes (1, 2) being formed of a refractory metal and means for applying a liquid metal to a surface portion of at least one of said electrodes (1, 2) during operation of the lamp,

wherein said at least one of said electrodes (1, 2) comprises a reaction layer at least at said surface portion, said reaction layer being formed by a process in which said surface portion of said electrode (1, 2) is brought in contact with the liquid metal and thermally annealed at a

temperature of ≥ 800°C to cause a reaction between the refractory metal and the liquid metal in a reaction zone (9) of a controlled depth forming said reaction layer.

wherein said at least one of said electrodes (1, 2) comprises a layer of a further material at least at said surface portion, said further material being selected to improve the wetting behaviour of said surface portion for the liquid metal .

The lamp according to claim 11 or claim 12, wherein said electrode (1, 2) is an electrode wheel which is rotatable in a rotational direction around a rotational axis and has an outer

circumferential surface between two side surfaces, said surface portion being formed by the outer circumferential surface.