Mark Michaelian

Xenon Target and High-Power Laser Module Development for LPP Sources

This chapter gives an overview of LPP EUV source development work at Northrop Grumman Corporation (NGC). The chapter covers development of the laser module, xenon target, and overall system. The volume editor (V. Bakshi) prepared this chapter as a summary of information provided to him by NGC. Lasers for LPP EUV sources are expected to produce tens of kilowatts of high-pulse-rate, high-pulse-energy, short-pulse-width, near-diffraction-limited output. Such lasers will be focused onto a condensed jet of cryogenic xenon or tin targets to produce a plasma with sufficient temperature to generate EUV radiation. For the generation of the EUV-producing plasma, pulse widths of around 10 ns and pulse energies in the range of 0.5 to 1 J are required. High beam quality and low pointing error are required to maintain constant high intensity on the EUV source target so that the radiated EUV power and consequent exposure doses on the semiconductor wafer are uniform. Depending on the choice of target material, eventually pulse rates of at least 7500 Hz and laser powers of 10–30 kW will be required to ensure the required power collection at the intermediate focus (IF). In 1999, NGC constructed a 1700-W diode-pumped Nd:YAG phase-conjugated master oscillator-power amplifier (MOPA) laser, designated EUV-Alpha, which was used in a lithography testbed at Sandia Labs in Livermore (see Chapter 24 for further description). Later NGC built an EUV-Beta laser (Fig. 25.1) that produced 4500 W and was operated at NGCs EUV source development facility at Cutting Edge Optronics (CEO). The Beta laser, a modular design for better maintainability, was twice as efficient and had two-thirds the footprint of the Alpha laser. For this laser, NGC selected a MOPA architecture (Fig. 25.2) using stimulated Brillouin scattering (SBS) phase conjugation to compensate for aberrations, figure error, and thermal distortions in the Nd:YAG gain media. The output of a custom 12-W master oscillator (MO) was split in two with a polarizer and directed to two amplifier trains. Each amplifier train consisted of two diode-pumped zigzag slab amplifiers, image relay telescopes, shaping optics, and an SBS cell. After round trips through the two slab amplifiers, the two MO beams were brought to their full 750-W power in each train, and then polarization-combined for a total of 1500 W. In the Beta laser, there were three such 1500-W modules, which yielded a system total of 4500 W at 7500 Hz.

A mechanism for erosion of optics exposed to a laser-generated EUV plasma

This paper introduces a theory for material erosion in proximity to a laser driven EUV source, with a xenon target. The mechanism hypothesized is x-ray induced damage. A semi empirical photo ablation model is developed using the laser induced damage threshold at 1.06 microns to set the critical energy density for material removal. The model also includes absorption of the plasma generated xrays and is shown to agree well with experiment. With the theory validated, the paper concludes with a calculation of a safe operating distance and how this distance could be calculated for other optic materials and plasma targets.

Laser produced plasma EUV sources

Progress in laser-produced plasma EUV generation for advanced micro-lithography is reviewed and compared with industry requirements. Performance of a high power diode-pumped Nd:YAG laser and a xenon target is presented, demonstrating that intense emission and clean operation are feasible, and these sources can meet EUV power requirements for high-volume manufacturing.