High-Power EUV Source Breaks Barriers

Extreme ultraviolet (EUV) lithography faces several challenges, not least of which is the current level of source power. To meet requirements for high-volume production (M 80 wph), in-band source power needs to reach 50-150 W, a range that has proved difficult.

There are several techniques in development — primarily plasma-based techniques — but they are mostly in the same boat in terms of power capabilities. To achieve the needed in-band source power (which refers to the power captured by the condenser optics, at 2% bandwidth), total source power must be in the tens of kilowatts range. Researchers at JILA at the University of Colorado (Boulder, Colo.) has combined some relatively simple principles to develop a tightly focused EUV source with peak powers approaching 1 MW. Although the researchers hope to develop the source to reach wavelengths below 4 nm, a region suited to biological imaging, the team also points to the achievement as a potential 13 nm coherent source for EUV lithography.

Led by JILA fellows Margaret Murnane and Henry Kapteyn, the team developed the source by applying quasi-phase matching (QPM) techniques to high-harmonic generation (HHG). HHG — which focuses a femtosecond laser into a gas to produce high harmonics of the fundamental laser frequency — is a well-known method of producing coherent EUV light, but conversion efficiencies are severely limited at high energies because of ionization. QPM uses a modulated hollow-core waveguide to overcome efficiency problems, periodically varying the laser intensity to phase match the laser and EUV light waves.

The experiment, described in the Jan. 2 issue of Nature, uses a 25 fsec Ti:sapphire laser, focused into 150-µm-diameter hollow fibers filled with various gases. The intense laser light rips the gas atoms apart, resulting in charged ions and electrons. The electrons then accelerate to high energies, releasing EUV radiation when they slam back into the ions. Without the phase-matching technique, the strength and coherence of the beam would be weakened because of some of the EUV waves being out of phase with the laser. So in the JILA technique, some of the fibers (waveguides) are periodically modulated — the diameter changed every ~1 mm by a depth of ~10 µm. The waveguide produces a highly efficient, tightly focused EUV beam.

The entire system for creating EUV beams in the JILA lab fits within a space of less than 2 m2. In this image, the setup is configured for creating holograms. (Source: JILA, University of Colorado)

For additional information on lithography, go to www.semiconductor.net/lithography.