a) Field of the Invention
The invention is directed to an arrangement for the generation of EUV radiation based on a gas discharge plasma with a high radiation emission in the range between 12 nm and 14 nm. It is applied in industrial semiconductor fabrication and is conceived in particular for the process of EUV lithography under production conditions.
b) Description of the Related Art and Problems Addressed by the Invention
The generation of radiation from a gas discharge plasma has established itself in the field of plasma-based EUV radiation sources as a promising technology for excitation. Essentially, the following gas discharge concepts are known: Z-pinch arrangements with pre-ionization (e.g., U.S. Pat. No. 6,414,438 81), plasma focus arrangements (e.g., WO 03/087867 A2), hollow-cathode discharge arrangements e.g., U.S. Pat. No. 6,389,106 B1), star pinch discharge arrangements (e.g., U.S. Pat. No. 6,728,337 B1), and capillary discharge arrangements (e.g., U.S. Pat. No. 6,232,613 B1).
Further, there are variations of the above-named discharge types (e.g., hypocycloidal pinch discharge) and arrangements that combine elements of these different discharge types.
In all of these arrangements, a pulsed high-power discharge of >10 kA is ignited in a work gas of determined density, and a very hot (kT >30 eV), dense plasma is generated locally as a result of the magnetic forces and dissipated power in the ionized work gas.
Radiation sources must currently also satisfy the following specific requirements for use in semiconductor lithography under production conditions:
1. wavelength13.5 nm ± 1%2. radiation output in the intermediate focus115 W3. repetition frequency7-10 kHz4. Dose stability (averaged over 50 pulses)0.3%5. life of the collector optics6 months6. life of the electrode system6 months.
For sometimes different reasons, only certain aspects of these requirements are satisfied by the arrangements mentioned above. Above all, the radiation output, its stability, and the lifetime of the electrode system are generally insufficient.
It has been shown especially that the required radiation outputs can only be achieved through an efficient emitter substance. Such substances which emit radiation in the desired spectral range between 13 nm and 14 nm in a particularly intensive manner are xenon, lithium, and tin.
However, as described e.g. in WO 03/087867 A2, the latter two materials are difficult to manage in plasma generation because they are solid under normal conditions and, in addition, exhibit substantial debris emission. Further, the disadvantages of a successful handling of lithium and tin consist in the following difficulties:                in solid targets, discharge instabilities due to the formation of craters at the cathode;        formation of deposits at the electrodes (leads to a short-circuiting of the electrode system after prolonged operation);        with laser evaporation, poor dose distribution of the (preferably liquefied) target;        with gaseous targets, requirement for a high-power furnace for generating the necessary vapor pressure (with pure tin: temperatures T>1000° C.).        