Patent Grant
7897948
The present invention relates to a plasma discharge lamp for generating EUV radiation and/or soft X-rays by means of an electrically operated discharge, comprising at least two electrodes arranged in a discharge space at a distance from one another to form a gap which allows the ignition of a plasma in a gaseous medium between said electrodes, a device for applying a metal to a surface of said electrodes in said discharge space and an energy beam device adapted to direct an energy beam onto at least one of said surfaces in the gap evaporating said applied metal at least partially thereby producing said gaseous medium.
Plasma discharge lamps for generating EUV radiation (EUV: extreme ultraviolet) or soft X-rays, i.e. radiation in the wavelength region of around 1 nm to 20 nm, are required in the field of EUV lithography, microscopy or metrology. For most applications high conversion efficiency together with a long life time of the lamps is desirable.
The above requirements are fulfilled by the EUV plasma discharge lamp disclosed in WO 2005/025280 A2. The EUV lamp of this document comprises two electrode wheels arranged in a discharge space at a distance from one another to form a gap which allows the ignition of a plasma in a gaseous medium between the electrodes, as can be seen in
Such a EUV plasma discharge lamp has the following advantages. Since tin can be used as plasma fuel, a high conversion efficiency of the energy stored on the capacitor bank to EUV is obtained. Since the electrodes rotate, the heat generated by the plasma is spreading over a large surface, which allows high average input powers. The tin layer on the wheels is continuously regenerated, so that electrode erosion does not change the shape of the electrodes. Hence, a very long life time of the lamp is obtained. The liquid tin used for the electrical contact between the capacitor bank and the rotating electrode wheels avoids the requirement of sliding contacts or of a rotating capacitor bank.
The critical region around the plasma is cooled by rotating the electrodes, which means that the input power scales proportionally with the rotation frequency. However, the rotation frequency is limited for the following reason. The centrifugal forces accelerate the tin outwards and at high rotation frequencies droplets are created, i.e. the tin layer tears off. This process can be shifted toward higher rotation frequencies by reducing the thickness of tin film, for instance by appropriate wipers 9. Another possibility is to increase the diameter of the electrodes, which reduces the centrifugal forces (ω2R) at the same velocity (ωR). The drawback of this solution is that extremely large wheels are necessary which improves neither the mechanical stability nor the compactness of the lamp.
It is an object of the present invention to provide a plasma discharge lamp for generating EUV radiation and/or soft X-rays by means of an electrically operated discharge, which can use metal vapor for plasma generation and allows a more compact design for high input powers.
The object is achieved with the plasma discharge lamp according to claim 1. Advantageous embodiments of the lamp are subject of the sub claims and are furthermore disclosed in the following description and examples for carrying out the invention.
The proposed plasma discharge lamp for generating EUV radiation and/or soft X-rays by means of an electrically operated discharge comprises at least two electrodes arranged in a discharge space at a distance from one another to form a gap which allows the ignition of a plasma in a gaseous medium between said electrodes. A metal applying device is arranged to apply a metal to a surface of said electrodes. Preferably, said metal applying device comprises two containers with a metal melt and each of said electrodes dips into one of said containers to apply the metal melt to the surface of said electrodes. The metal applying device can also be formed, for example, of one or several evaporating or sputtering devices or of one or several rollers for applying the metal or metal melt. In the present plasma discharge lamp the electrodes are formed of conveyer belts driven to transport the metal to said gap. For each of the electrodes a shaper element is provided at the gap to ensure a proper form and distance of the electrodes at the gap. The conveyer belt is moved over the shaper element to transport the metal on its surface to the gap. An energy beam device, in particular a laser, is adapted to direct an energy beam onto at least one of said surfaces in the gap evaporating the applied metal at least partially thereby producing said gaseous medium.
With such conveyer belts as electrodes, i.e. as cathode and anode, of the plasma discharge lamp it is possible to achieve a compact lamp design and at the same time a sufficient cooling of the electrodes for a higher input power.
The proposed plasma discharge lamp is preferably designed like the plasma discharge lamp of WO 2005/025280 A2, which is incorporated herein by reference, except of the design and movement of the two electrodes. The use of conveyer belts instead of electrode wheels has the further advantage that it allows more flexibility in the mechanical design. For instance, the contact surface between the belt and the melted metal can be easily increased by making several passes or turns under the surface of the melted metal in the corresponding metal bath. This improves the cooling of the belt significantly.
In one embodiment of the proposed plasma discharge lamp the shaper elements are formed to provide a curved surface at the gap, wherein the curved surface has a sufficiently large smallest radius of curvature to allow a high driving speed of the belt without the risk that liquid metal tears off at the shaper elements due to centrifugal forces. Since the remaining portion of the shaper elements is not in contact with the conveyer belts, this portion can be formed to occupy a minimum space. Such shaper elements may have the profile of a segment of a circle, for example made of a cut off portion a circular disk with a thickness of the width of the belt or smaller. Also other curved forms are possible. Compared with rotating wheels, therefore, such shaper elements can provide a large radius of curvature of the electrodes at the gap without occupying the same space. Further required deflection elements for the guidance of the belt with a far smaller curvature are then positioned in the baths of the liquid metal or are combined with an appropriate cover which prevents the tearing off of the liquid metal film due to centrifugal forces.
The electrical connection between the capacitor bank of the power supply and the electrodes can be achieved through the baths of the liquid metal in the same manner as already disclosed in WO 2005/025280 A2. Nevertheless, since the shaper elements in the above embodiment are fixed and do not rotate, the electrical connection to the electrodes can also be achieved through the shaper elements. This has the advantage that the containers with the metal melt can be separated by a sufficient distance from one another to allow the emission of plasma generated EUV radiation and/or soft X-rays in this direction, i.e. the radiation can pass between the two containers. Electrical connection of the electrodes through the containers in such a case may result in an undesirably high inductance of the system.
In another arrangement the moving plane of the conveyor belts is inclined with respect to a vertical plane, i.e. inclined with respect to the paper plane of
In a further embodiment of the present plasma discharge lamp, the shaper elements are formed of rotating rollers with a smaller radius of curvature as that of the above embodiment. In order to avoid the tearing off of liquid metal from the surface at these shaper elements, additional pairs of cooled rollers are provided upwards the shaper elements with respect to the driving direction of the belts. These additional rollers are cooled such that the liquid metal film on the surface of the belt, which passes between the pair of cooled rollers is cooled down below the melting temperature, thereby forming a solid layer on the conveyer belt. With this solid layer, the metal melt does not move and the problem of the centrifugal forces at higher driving speeds does not occur so that the curvature of the shaper elements and other deflection elements downwards of the cooled rollers can be held small even at higher driving speeds.
The conveyer belts of the present plasma discharge lamp are made of a material which can be wetted by liquid metal, in particular tin, and has a sufficiently high heat resistance to withstand the temperatures during operation of the lamp. Preferably this material has also a high heat conductance. The belts may be made for example of Mo, W or Nb. The belts can be closed or open belts. In case of open belts, for each belt two carrier rollers are provided between which the belt is winded forward and backward.
These and other aspects of the invention will be apparent from an elucidated with reference to the embodiments described herein after.
The proposed plasma discharge lamp is described in the following by way of examples in connection with the accompanying figures without limiting the scope of protection as defined by the claims. The figures show:
The EUV plasma discharge lamp of
In order to achieve a high rotational speed of the electrode wheels of
In the present example, the conveyer belt 15 is driven by the deflection rollers 18 which are connected to an appropriate driving motor. The conveyer belts are guided through the container 19 containing a tin bath 2, thereby wetting the surface of the conveyer belt 15 with a thin tin film. The thickness of this film is controlled by wipers 9 which are appropriately arranged upwards of the shaper elements 13. In
Some EUV illumination systems require that the EUV radiation produced by the plasma is directed downwards, i.e. in the direction of the tin baths 2. This is rather difficult to realize with the wheel based system of
In the examples of
The exemplary embodiments of
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. For example, although the figures suggest that the driving planes of the two electrodes are identical, it is also possible that these driving planes are not parallel to each other. Furthermore, the number of deflection wheels is not limited to the number shown in the figures and can be set appropriately. Different embodiments described above can also be combined, for example the embodiments of
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 undefined article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of these claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06120170 | Sep 2006 | EP | regional |
| 06120419 | Sep 2006 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2007/053480 | 8/29/2007 | WO | 00 | 3/3/2009 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2008/029327 | 3/13/2008 | WO | A |
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| 102006027856 | Nov 2007 | DE |
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| Number | Date | Country | |
|---|---|---|---|
| 20090250638 A1 | Oct 2009 | US |
