Takanobu Ishihara

Laser-produced plasma source development for EUV lithography

We are developing a laser produced plasma light source for high volume manufacturing (HVM) EUV lithography. The light source is based on a high power, high repetition rate CO2 laser system, a tin droplet target and a magnetic plasma guiding for collector mirror protection. This approach enables cost-effective high-conversion efficiency and EUV power scaling. The laser system is a master oscillator power amplifier (MOPA) configuration. We have achieved a maximum average laser output power of more than 10 kW at 100 kHz and 20 ns pulse by a single laser beam with good beam quality. EUV in-band power and out-of-band characteristics are measuring with high power CO2 laser and Sn droplet target configuration. This light source is scalable to more than 200 W EUV in-band power based on a 20-kW CO2 laser. Collector mirror life can be extended by using droplet target and magnetic plasma guiding. Effectiveness of the magnetic plasma guiding is examined by monitoring the motion of fast Sn ion in a large vacuum chamber. The ion flux from a Sn plasma was confined along the magnetic axis with a maximum magnetic flux density of 2 T.

Ultranarrow-bandwidth 4-kHz ArF excimer laser for 193-nm lithography

We have developed a 4-kHz ArF excimer laser with ultra-narrow bandwidth, which is applicable to high-NA scanners for sub-0.13-micrometers microlithography. In this paper, we describe a 4-kHz ArF excimer laser for mass production: the model G40A, which has an output power of 20 W and energy dose stability of less than +/- 0.3% (20-ms window) at 4 kHz. This dose stability is comparable to the performance of an existing 2-kHz ArF excimer laser, the model G20A. The new laser also has the following specifications: a long pulse duration of over 40 ns, spectral bandwidth of less than 0.35 pm (FWHM), and spectral purity of less than 1.0 pm (95%). These characteristics are better than those of the G20A. A lifetime test of over 7 billion pulses has been conducted at 4-kHz operation. The new laser has maintained an energy dose stability of less than +/- 0.3% (20-ms windows) and demonstrated performance suitable for mass production even after over 7 billion pulses.

Stability of krypton fluoride laser in real stepper mode operation

The stable performance of the line-narrowed Krypton Fluoride Excimer Laser for production steppers, the KLES-G6, in real stepper mode operation is presented. Wavelength stability of < +/- 0.1 pm and pulse-to-pulse energy stability of < 2% ((sigma) ) were achieved in the real stepper mode operation. Further the durability test was made at 6 W(10 mJ, 600 Hz) in 1 sec. ON - 1 sec. OFF (50%) burst mode and in cw mode. The central wavelength stability < +/- 0.2 pm including the drift at the head of burst, spectral bandwidth < 1.5 pm, and pulse-to-pulse energy stability < 2.5% ((sigma) ) over 1.6 X 109 shots were achieved. The KLES-G6 will assist us to utilize the excimer stepper in real commercial production successfully.

Feasibility study of 6 kHz ArF excimer laser for 193 nm immersion lithography

A feasibility study of next generation 6 kHz ArF laser for lithography is presented. High repetition rate operation of excimer lasers faces two technical challenges: 1) the occurrence of acoustic waves caused by the discharge in the laser chamber and 2) the huge energy consumption of the large gas flow fans. This paper describes our approach to dampen the acoustic waves. A computer simulation of acoustic wave generation inside the discharge chamber was done. The simulation correlates well with Schlieren photography measurements that visualized the acoustic waves. Based on these results, a chamber for 6 kHz repetition rate was newly designed. Measured spectral data (FHWM and E95) proved that the acoustic wave perturbation was remarkably reduced. A very efficient design method for high repetition rate laser chamber has therefore been established.

Narrow band KrF excimer laser for mass production of ULSI ICs

A narrow band KrF excimer laser system for lithography has been developed. Its durability has been proved up to 2 billion shots that correspond to one year operation in mass production line. The system has kept the average power of 6W with the power stability within +- 3% and the spectral line-width 1.2 pm (FWHM) with the wavelength stability within +- 0.2 pm. The integration of the spectrum has indicated that 95% of the spectral energy lay inside 4.6 pm band during 2 billion shots

Investigation of a step-like output energy decrease observed in an ArF excimer laser for microlithography

A step-like decrease of the output energy of an ArF excimer laser for microlithography has been observed during high-repetition rate and high-power operation. The reason for the step-like energy decrease was traced to oxygen by adding 50 ppm of various gas impurities to the laser gas. It is supposed that the energy decrease is caused by laser light absorption due to the oxygen or to some kinds of oxygen compounds because no change was observed in ArF and XeF emission intensities which were monitored as measures of excimer formation, Xe content, and discharge states.

Recent advances of a KrF excimer laser on a plant's practical requirements

In this paper we describe the performance of the newest model of line-narrowed KrF excimer laser KLES-G6 (1995 model) developed in factory on the practical requirements. The KLES- G6 exhibits: (1) spectral bandwidth < 0.8 pm; (2) wavelength stability < +/- 0.1 pm; (3) pulse-to-pulse energy stability < 1.8% ((sigma) ); (4) output power equals 6 W at 600 Hz; (5) gas life > 100 million pulses or 7 days; (6) window cleaning or exchange > 1 billion pulses; (7) laser chamber exchange > 2 billion pulses; (8) mean time between failures > 1500 hours; (9) running cost per a billion pulses is about 29 thousand dollars. These advanced performances will save the running cost and guarantee the high uptime ratio needed to satisfy the plants practical requirements.

Elucidation of steplike output energy decrease observed in arf excimer laser for microlithography

A steplike decrease in the output energy of an ArF excimer laser for microlithography is caused by the laser light absorption in the laser gas. To identify the absorbing gas component, the light absorption characteristics of the laser gas inside the discharge chamber were investigated under various gas conditions using a continuous xenon light source in the ultraviolet region and an ArF excimer laser at 193 nm. As a result, the oxygen–fluorine compound FxOy generated in the laser discharge was identified as the absorbing molecule, because a strong steplike absorption of the 193 nm laser light was only observed when oxygen and fluorine were added together to the discharge gas. The lifetime of this absorbing molecule was examined against gas temperature during the operation of the laser discharge. The data obtained were useful in setting the best operational conditions for the ArF excimer laser for microlithography.