In general, the laser spectrum varies due to a fluctuation of the temperature in the resonator or deterioration of a module caused by running the laser unit. Introduction of the spectrum variable mechanism allows control of the spectrum at a certain value and elimination of the difference in the spectrum within a laser unit or between different laser units. Thus, it enables the impact of variation in the laser spectrum to be minimized.
Figure 7 shows the measurement results of E95 when the operating conditions of laser model GT61A were changed. In any of the cases that the uptime and heat load of the laser are changed, the output energy is changed and the repetition rate is changed. The spectrum is controlled at the target value of 0.3 pm, and its deviation ranges within a very small value of 6 fm (= 1/1000 pm). Even if the CD sensitivity of 3.5 nm/0.1 pm, measured when a contact hole of 40 nm in diameter is exposed by the laser beam of E95=0.35 pm, is applied, the change in the CD is only 0.2 nm.
Figure 8 shows the spectrum performance within a gas life of 100 M pulses/3 days*. The laser operating conditions vary depending on the exposure conditions. However, E95 is well controlled at the target value of 0.3 pm with a deviation of 23 fm, while the impact on the CD is approx. 0.8 nm.
*This is the case that the TGM option is not used.
Figure 9 shows E95 in four different units. In the same manner as in figure 7, E95 is well controlled at the target value of 0.3 pm with a deviation of 19 fm, while the impact on the CD is only approx. 0.7 nm.
With introduction of the BCM, laser spectrum performance is precisely controlled on the target surface. In the case of running a single laser unit, variation of the spectrum caused by variation in the laser life or laser operating conditions is suppressed, allowing its impact upon CD to be minimized. Even in the case of running multiple laser units, the spectrum performance does not differ between the laser units, thus causing no difference in the spectrum between lithography tools. Therefore, the BCM can be expected to become an indispensable major function for lithography processes as the device pattern becomes finer and finer.