Harry Shields

Xenon target performance characteristics for laser-produced plasma EUV sources

Laser-produced plasmas (LPPs) are being developed as light sources for EUV lithography. To meet the requirements for high-volume manufacturing, LPP EUV sources must generate intense EUV output in the 13.5 nm band, and minimize source-induced degradation of EUV optics allowing hundreds of hours of clean operation. Xenon has been identified as a promising target material for LPP EUV light sources, with the potential for both high-efficiency EUV generation, and low optics contamination. Several dense xenon target configurations have been tested including aerosol sprays, continuous liquid streams, condensed xenon droplets, and frozen solid xenon. Important LPP performance characteristics, such as conversion efficiency, EUV radiation distribution, EUV optics degradation by material erosion and/or deposition, and the physical interface to the EUV optical system, are strongly influenced by the xenon target design. The performance of xenon targets with measured conversion efficiencies in the 0.4 percent to 1.4 percent range is reported. Prospects for xenon targets to reach the EUV power generation and contamination goals for production lithography tools are addressed.

Absorption of extreme ultraviolet light in a laser produced gas-jet plasma source

Laser produced plasmas (LPPs) provide a stable source of extreme ultraviolet (EUV) light making them well suited for use in next-generation lithography tools. The plasma is generated by directing a laser at a target composed of a partially condensed gas after it undergoes a supersonic expansion through a nozzle and enters a vacuum chamber. The expansion process results in very cold temperatures such that the gas partially condenses forming a mixture of gas and small clusters. The clusters absorb the laser energy leading to the formation of the plasma, but the excess gas absorbs some of the emitted EUV light reducing the net output of the LPP. Calculations were carried out to determine the amount of EUV light absorbed by the gaseous xenon that surrounds the plasma. The Navier–Stokes equations were solved to obtain the gas density field. Observations from experiments were used for the shape of the plasma, which showed it to be approximately that of a prolate spheroid. The relative EUV signal strength was ob...

Laser-produced plasma (LPP) scale-up and commercialization

An EUV light source, created when a high-average power (750 W) Nd:YAG laser forms a plasma in a xenon liquid-spray jet, has been characterized. This source has shown improved conversion from laser to EUV, and a more uniform angular distribution, as the laser pulse energy and average power are increased. System performance has been analyzed and compared with the requirements for future EUV microlithography tools for semiconductor manufacturing. EUV power scaling requirements and factors influencing Cost-of-Ownership are discussed.

High-power source and illumination system for extreme ultraviolet lithography

A clean, high-power Extreme Ultraviolet (EUV) light source is being developed for Extreme Ultraviolet Lithography (EUVL). The source is based on a continuous jet of condensable gas irradiated with a diode-pumped solid state laser producing a time-averaged output power of 1700 W at 5000 - 6000 Hz. An illumination system is being assembled to collect and deliver the EUV output from the source and deliver it to a reticle and projection optics box to achieve an EUV exposure rate equivalent to ten 300-mm wafers per hour.

System and process learning in a full-field, high-power EUVL alpha tool

Full-field imaging with a developmental projection optic box (POB 1) was successfully demonstrated in the alpha tool Engineering Test Stand (ETS) last year. Since then, numerous improvements, including laser power for the laser-produced plasma (LPP) source, stages, sensors, and control system have been made. The LPP has been upgraded from the 40 W LPP cluster jet source used for initial demonstration of full-field imaging to a high-power (1500 W) LPP source with a liquid Xe spray jet. Scanned lithography at various laser drive powers of >500 W has been demonstrated with virtually identical lithographic performance.

Performance upgrades in the EUV Engineering Test Stand

The EUV Engineering Test Stand (ETS) has demonstrated the printing of 100-nm-resolution scanned images. This milestone was first achieved while the ETS operated in an initial configuration using a low power laser and a developmental projection system, PO Box 1. The drive laser has ben upgraded to a single chain of the three-chain Nd:YAG laser developed by TRW. The result in exposure time is approximately 4 seconds for static exposures. One hundred nanometer dense features have been printed in step-and-scan operation with the same image quality obtained in static printing. These experiments are the first steps toward achieving operation using all three laser chains for a total drive laser power of 1500 watts. In a second major upgrade the developmental wafer stage platen, used to demonstrate initial full-field imaging, has been replaced with the final low-expansion platen made of Zerodur. Additional improvements in the hardware and control software have demonstrated combined x and jitter from 2 to 4 nm RMS Over most of the wafer stage travel range, while scanning at the design scan speed of 10 mm/s at the wafer. This value, less than half of the originally specified jitter, provides sufficient stability to support printing of 70 nm features as planned, when the upgraded projection system is installed. The third major upgrade will replace PO Box 1 with an improved projection system, PO Box 2, having lower figure error and lower flare. In addition to these upgrades, dose sensors at the reticle and wafer planes and an EUV- sensitive aerial image monitor have been integrated into the ETS. This paper reports on ETS system upgrades and the impact on system performance.

Laser-produced plasma light source for extreme ultraviolet lithography

Pulsed Nd:YAG lasers have been developed to achieve high peak power and high pulse repetition rate. These systems are being used as drivers for laser-produced plasmas which efficiently convert the 1064-nm laser output to extreme ultraviolet (EUV) light at 13.5 nm for future microlithography systems. The requirements for laser-produced plasma EUV light sources and their integration in lithography tools for high-volume manufacturing are reviewed to establish the key design issues for high-power lasers and plasma targets. Xenon has been identified as a leading target material to realize the goals of intense EUV emission and clean operation. Recent progress in high-power diode-pumped Nd:YAG lasers and xenon targets for EUV generation is reviewed, showing that laser-produced plasma sources meet the needs for current EUV lithography development tools. Future directions to meet EUV source requirements for high-volume manufacturing tools are discussed.

EUV absorption in a laser-produced plasma source

Laser produced plasmas (LPP) provide a stable source of EUV making them well suited for use in next-generation lithography tools. The plasma is generated by directing a laser at a target composed of a partially condensed gas after it undergoes a supersonic expansion through a nozzle and enters a vacuum chamber. The expansion process results in very cold temperatures such that the gas partially condenses forming a mixture of gas and small clusters. The clusters absorb the laser energy leading to the formation of the plasma, but the excess gas absorbs some of the emitted EUV reducing the net output of the LPP. Calculations were carried out to determine the amount of EUV absorbed by the gaseous xenon that surrounds the plasma. The Navier-Stokes equations were solved to obtain the gas density field. Observations from experiments were used for the shape of the plasma, which showed it to be approximately that of a prolate spheroid. The relative EUV signal strength was obtained as a function of the direction angle by calculating the absorption of EUV in the gas surrounding the plasma and integrating over the plasma surface. Calculated results for the normalized EUV energy distribution compare well with measurements of the EUV angular distribution obtained in experiments.

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.