Novel Radiation Source Developed for EUV

-- Semiconductor International, 10/1/1999

Collaborative work between the Lehrstuhl für Lasertechnik (Aachen, Germany) and the Fraunhofer Institut für Lasertechnik (Aachen, Germany) has developed a compact and economical gas discharge source emitting in the extreme ultraviolet (EUV) range. It is being studied as an alternative to laser-produced plasma EUV sources for lithography. In most of the initial work oxygen has been used to produce the EUV-emitting plasma. The 2p-4d transitions in oxygen produce EUV wavelengths in the 13 nm region, which are suitable for use with molybdenum-silicon multi-layer mirror optics.

A pinch plasma is formed in the discharge tube by rapidly discharging a capacitor through the gas using a low-inductance circuit. The current pulses rise to several kA in about 100 nsec. This causes the gas to be compressed to a density in the 10¹⁸ cm^-3 range while its temperature rises to that corresponding to a particle energy of some tens of eV. The discharge chamber, including the electrode system and storage capacitor, measures about 600 x 600 x 200 mm. Pressure in the chamber is less than 100 Pa, so the EUV radiation can be extracted into a vacuum through thin windows, such as silicon nitride or beryllium. A beryllium filter can be used to restrict the radiation's spectral spread to the 11 nm - 17 nm region. The lifetime of the plasma, and thus the duration of the emission, is about 30 nsec. The pulse repetition rate has been limited only by the power available from the supply.

An electrical to EUV power conversion efficiency of some 0.1% was achieved using about 1 J of electrically stored energy. The formation of debris and electrode erosion, which are major problems with laser plasma sources, have not been observed after more than 10⁷ pulses were passed via the specially designed electrodes. An electrical input power of 40 W results in an electrode temperature of less than 100°C with uncooled electrodes.

The workers see a potential increase in the conversion efficiency to EUV through the use of other gases with more intense transitions than oxygen. Fluorine, neon and xenon are good candidates, xenon having a broadband emission characteristic. The group plans further studies with other promising gases to try to maximize power conversion efficiency. A xenon plasma resulted in a broadband emission over the 10 nm - 16 nm wavelength range. The electrical input power of 400 W produced an emission of some 0.24 W over 2f sr. The wavelength was centered at 13.4 nm with a 2% bandwidth and a pulse repetition rate of 100 Hz.

Recently, operation at a pulse repetition rate of more than 2 kHz has been demonstrated. The lack of electrode cooling and the limited power available from the supply restricted the operation to only short bursts. The plasma technology group of the Aachen Institute plans further basic studies of the discharge itself and also of the handling of kHz repetition rates. An electrical input of several kW will be tried with continuous operation of the source to achieve the EUV output power required for EUV lithography.