Required Laser Power

The required laser power is determined by the sensitivity of the photoresist to be used, the transmittance of the lithography tool optics, and the stage speed of the lithography tool. For KrF lithography, the maximum photoresist sensitivity, transmittance of the optics, and maximum stage speed are 50 mJ/cm2, approx. 15%, and 500 mm/s, respectively. Therefore, the laser power required for KrF lithography is 40 W. Laser power of this level can be attained by increasing the repetition rate of the laser.

For ArF lithography, on the other hand, the photoresist sensitivity is higher than that for KrF lithography, while the stage speed is almost the same. The biggest difference is the transmittance of the optics, which for ArF lithography is 1/2 to 1/3 of that for KrF lithography. This is one of the reasons that an ArF laser requires approx. twice the power of a KrF laser. Also, the ArF laser has a problem in that the emission efficiency is low. If the same electrical power is applied to both ArF and KrF lasers, the emission efficiency of the ArF laser is approx. 1/2 that of the KrF laser since the transmittance of the laser optics is also bad. As the result, the technological hurdle for an ArF laser is approx. 4 times that of a KrF laser. In order to meet such challenges, simply increasing the repetition rate is not sufficient; a breakthrough technology is required.

The GigaTwin Platform

In order to meet such a high power requirement, we at Gigaphoton have developed the GigaTwin platform that uses an injection locking method as a breakthrough technology. The laser unit using this method incorporates two laser chambers (twin-chamber configuration), each of which is mounted with an optical resonator. In this configuration, one chamber emits a low-output, narrow-bandwidth laser while the other chamber amplifies the laser output from the first chamber to the maximum level.

Development of the injection locking technology at Gigaphoton dates back to the 1990s. In 1993 through 1994, we joined a big project sponsored by the Ministry of International Trade and Industry, and developed an ArF laser with a repetition rate of 200 Hz and an output of 300 W. In 2000 through 2002, we further developed a F2 laser with a repetition rate of 5 kHz and an output of 30 W in the ASET F2 lithography project.

Figure 1. Injection-Locking System and MOPA System

Injection Locking and MOPA Methods

Figure 1 shows a typical twin-chamber laser system. Both of the injection locking and MOPA methods use two chambers. The big difference is that the injection locking method uses an amplifier chamber mounted with an optical resonator, thus enabling the amplifier chamber itself to also function as a resonator.

In the injection locking method, the line-narrowing chamber (oscillator) first starts to discharge to form a narrow-spectral-bandwidth laser by resonating the laser beam between the line-narrowing module (LNM) and the mirror. The laser beam is then introduced into the amplifier chamber via the laser transfer system, and the amplifier chamber starts discharge synchronizing with the output from the line-narrowing chamber. In the amplifier chamber, the laser beam is amplified multiple times by the optical resonator, for as long as discharge continues, and then it is output from the output mirror as the final beam. Mounting of the optical resonator on the amplifier chamber allows highly effective amplification to produce sufficiently large power even if the output from the line-narrowing chamber is small. In addition, the beam stay time in the line-narrowing chamber is short, allowing the start of discharge in the amplifier chamber to be timed easily.

The operation of the MOPA method is the same as the injection locking method for the line-narrowing chamber (Master Oscillator), while it limits the number of beam amplifications to twice in the amplifier chamber (Power Amplifier). Therefore, the MOPA method requires an output from the line-narrowing chamber that is several times higher than that of the injection locking method. Also, the beam stay time in the line-narrowing chamber is short, which is likely to cause the final output power to vary depending on the discharge start time.

In such a comparison as above, the injection locking method seems to be superior to the MOPA method. However, it has been indicated that there are two critical challenges in application of the injection locking method to lithography. They are related to ASE and coherence.
We’ll discuss these in the next issue…

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