Patent Application
20140374625
The present invention relates to an apparatus for generating EUV radiation and/or soft X-rays by means of an electrically operated discharge, said apparatus comprising a protective component arranged and shaped to prevent at least a direct passage of metal vapor or metal droplets from the discharge gap into slits between parts of the apparatus having different electrical potential. The invention also relates to a method of reducing a local heating of the protective component.
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.
Such a discharge lamp is disclosed for example 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 plasma in a gaseous medium between the electrodes, as can be seen in
In order to avoid transport of evaporated tin or tin droplets to other parts of the lamp, a protective metal shield 10 is arranged inside the lamp below the discharge gap. This so called wedge is crucial for a high conversion efficiency as well as a long life time of the lamp. A high conversion efficiency requires the inductivity of the electrode system to be low, i.e. <10 nH, which means that the gaps or slits between the parts at different electrical potential should be small, in particular of the order of several millimeters. Those small gaps can be easily bridged by some tin that accumulates over the life time of the lamp head. If this happens, effectively a short circuit is made so that the capacitor bank can not be charged again and no EUV can be generated. For the repair action, the lamp head needs to be replaced and thus at least the vacuum needs to be broken, which results in significant down time of the discharge lamp. To avoid that tin splashes into the crucial slit the metal shield 10, a special roof above the slit, is provided in the lamp of
WO 2005/025280 A2 also discloses an embodiment of a plasma discharge lamp in which the metal shield is formed as a rotating disk used for transporting the liquid metal close to the discharge gap. This rotating disk dips into a reservoir with the liquid tin during rotation. This restricts the shape of this shield so that it can not cover the slits to be effectively protected from liquid metal vapor or drops.
The metal shield, in the present patent application also called protective component, must be located close to the plasma discharge, the distance varying from a few millimeters to a few centimeters, so that it is exposed to high average light intensity emitted by the plasma. When the average input power of the lamp or apparatus is scaled up the protective component could become too hot resulting in the following problems. The tin that is deposited on this protective component then evaporates to a high degree and can have a negative impact on the plasma generating the EUV. Also at such higher temperatures, the material of the protective component will react much faster with the tin so that the protective component will be corroded away.
It is thus an object of the present invention to provide an apparatus for generating EUV radiation and/or soft X-rays by means of an electrically operated discharge in which the heating effect on the protective component is reduced. It is also an object of the present invention to provide a method for reducing the temperature of the protective component in such an apparatus.
The object is achieved with the apparatus and method according to claims 1 and 14. Advantageous embodiments of the apparatus and method are subject matter of the dependent claims or are disclosed in the subsequent portions of the description.
The proposed apparatus for generating EUV radiation and/or soft X-rays by means of an electrically operated discharge at least comprises
two electrodes arranged at a distance from one another to form a discharge gap which allows ignition of a plasma in a gaseous medium between said electrodes,
a device for providing a metal or metal melt on a surface at the discharge gap,
an energy beam device adapted to direct an energy beam onto said surface evaporating said metal or metal melt at least partially thereby generating at least part of said gaseous medium, and
a protective component arranged and shaped to prevent at least a direct passage of metal vapor or metal droplets generated by evaporating said metal or metal melt into at least one slit between parts of the apparatus having different electrical potential during operation, said parts being covered at least partly by the protective component. In the proposed apparatus the protective component is mounted allowing to be placed in movement, in particular in rotation, during operation of the apparatus.
With such a movement of the protective component, i.e. the protective shield or wedge, the heat on the protective component is distributed over a larger surface area during operation. Therefore, the local heating of the protective component is lowered compared to a static component. On the other hand, the protective component may be appropriately shaped to sufficiently cover the one or several slits between components of different electrical potential during operation and thus provides an effective protection against the deposition of liquid metal in such slits. Also additional static shields or wedges can be used to shield the slits for droplets reflected via other surfaces, i.e. there will be no direct line-of-sight from the discharge, so the heating of these additional parts will be much smaller.
The proposed method of reducing the heating effects on the protective component in such an apparatus accordingly comprises a movement, preferably a rotation, of this protective component during operation of the apparatus. The movement or rotational speed may be selected dependent on the heat load on the protective component.
The proposed apparatus is preferably designed like the discharge lamp known from WO 2005/025280 A2. Therefore, the electrodes are formed by electrode wheels placed in rotation during operation of the apparatus and dipping into containers with the metal melt while rotating. The electrodes are electrically connected to a capacitor bank via the metal melt. The metal melt is thus applied to the outer surfaces of the rotating electrodes and conveyed with the rotation to the discharge gap. The proposed device for providing a metal melt on a surface at the discharge gap is thus formed by the two containers with the liquid metal melt and the corresponding driving arrangement of the electrodes. Instead of the containers also other types of devices may be provided, which apply the liquid metal to the outer surface of the rotating electrodes. An energy beam, preferably a laser beam, is focused onto the surface of at least one of the electrodes at the discharge gap to evaporate the liquid metal, preferably liquid tin, for generating at least part of the gaseous medium. The operation of such an apparatus is explained in detail in the above mentioned WO 2005/025280 A2. In order to avoid the deposition of the liquid metal in the gap or slit between the two containers having a different electrical potential, the protective component is arranged between the discharge gap and the to be protected gap or slit and shaped to cover the complete gap or slit. The term “cover” in this context means that the gap or slit is masked by the protective component in a region of a parallel projection of the protective component in the direction from the discharge to the gap or slit.
In a preferred embodiment, the rotational axis of the protective component is arranged which respect to the electrodes such that the protective component and/or its driving axis can extend without any restriction by the electrodes so that the required length of the protective component in this direction and the connection of the driving axis with a driving motor can be easily realized.
Nevertheless, the proposed apparatus may also be designed differently from the above preferred design. The metal or metal melt may also be provided on another surface close to the discharge gap, from which surface the metal or metal melt is then evaporated. The provision of the metal or metal melt may thus be realized not only by means of the electrodes itself but also by other transporting means, for example by a transporting belt or by an appropriately arranged nozzle. It is also possible to provide the metal in a solid form which is then melted by the energy beam and evaporated. The electrodes itself may also be formed in a different manner. It is obvious to the skilled person that with such other designs of the apparatus the proposed movement or rotation of the protective component fulfills the same task, i.e. protects any slit or gap between parts of different electrical potential against deposition of the metal while reducing the local heat load of this protective component.
The proposed protective component preferably has a rotation-symmetric elongated shape, i.e. the cross section perpendicular to the rotational axis is circular. The diameter of the protective component may however vary along the rotational axis which allows an effective protection of the gaps or slits arranged underneath the protective component.
The protective component may additionally be mounted on a rotational driving axis that allows an oscillation or movement of the component along this axis. Such an oscillation further increases the area of heat deposition and thus further reduces the local heat load of the protective component during operation of the apparatus.
In a preferred embodiment the protective component may additionally be cooled by integrating at least one cooling channel in the component. The cooling channel is connected to a cooling circuit which allows the flow of a cooling liquid through the cooling channel during operation of the apparatus. With this additional forced cooling the temperature of the protective component can be controlled. The protective component can for example be cooled with water as the cooling liquid. The cooling with water can be implemented very easy but requires special precautions. The water cooling can freeze the liquid metal at low power and on the other hand can easily boil if the power dissipation in the protective component becomes high. Therefore, instead of water cooling preferably a liquid metal, in particular the liquid metal used for generation of the gaseous medium, is used as the cooling liquid. Although a separate circuit for this liquid metal may be required for such a solution, it does not have the disadvantages of water cooling.
In order to prevent liquid metal deposited on the protective component to uncontrolled fly off during rotation of the component, a droplet catcher or a skimmer may be arranged on the side of the protective component to which the surface of this component after passing the pinch region of the discharge moves first. The droplet catcher is an element arranged and shaped such that droplets flying off the rotating protective component deposit on this element and are safely guided with this element to a corresponding reservoir, for example to the container containing the liquid metal for the gas discharge. This droplet catcher may for example have a concave surface on the side of the protective component, in particular a spherical or near spherical cross section. The droplet catcher avoids a deflection of impinging droplets towards positions where the liquid metal will cause problems. Additionally or instead of the droplet catcher at least one skimmer may be provided to remove an excess of the liquid metal on the rotating protective component and to guide this excess metal to the corresponding surface of the droplet catcher or to a corresponding reservoir.
With the proposed apparatus and corresponding method the local temperature or heat load of the protective component is reduced compared to other known arrangements and shapes of such a component. If the surface of the protective component is kept only slightly above the melting point temperature of the metal, the metal will evaporate only very slowly and will thus not have a negative impact on the EUV or soft X-ray generation. Also the reaction speed of the liquid metal with the material of the protective component will be much slower at such a temperature.
The proposed apparatus and method will be described in the following by way of examples in connection with the accompanying figures. The figures show:
The EUV plasma discharge lamp of
The driving axis of the protective component can be extended outside of the vacuum vessel 8 in the same manner as the driving axis 3 of the electrode wheels. The protective component can thus be driven in the same manner as the electrode wheels by an appropriate motor.
The diameter of the protective component can vary along the rotational axis as shown in the perspective view of
An extension of the idea to limit the damage due to erosion caused by the high temperature is to introduce an additional oscillation of the protective component 13 in the direction of the rotation axis.
The rotating protective component is placed in the region between the anode and cathode wheels in case of the embodiment of
In case of the rotating protective component,
A further measure is also indicated in the figure. This measure includes a skimmer 22 that is removing the excess of tin from the protective component, so that the rotating protective component is covered only by a thin layer of tin. The skimmer 22 is arranged such that the liquid tin gathered by this skimmer flows to the inner surface of the droplet catcher 21 and from there into the corresponding container. The protective component 13 could rotate at a high rotation speed, e.g. above 20 m/s, such that the droplets fly off close to the point where they hit a sensitive component, i.e. the gap or the electrode wheels. These wheels 1 are located on the top part as shown in
While the invention has been illustrated and described in detail in the drawings and 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. Other variations of the disclosed embodiments can be understood and affected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. For example, the protective component may also be a continuous belt which is rotated around several guide rolls. The movement of the protective component may also be a linear movement without rotation. 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 certain 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13003177.6 | Jun 2013 | EP | regional |
