Radiant energy – Irradiation of objects or material
Patent
1998-02-25
1999-12-28
Nguyen, Kiet T.
Radiant energy
Irradiation of objects or material
362259, B23K 2600
Patent
active
060084979
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a laser apparatus to output a laser beam for a machining apparatus to apply prescribed machining, using a laser beam, to semiconductors, polymer materials, or inorganic materials and particularly to an improvement to attain uniform pulse energy values continuously, when effecting burst mode operation, which is switching alternately between a continuous oscillation action for continuous pulse oscillation of a laser beam at a prescribed frequency and a stopping action for stopping the pulse oscillation for a prescribed time, and an improvement to make uniform the shifting cumulative exposure when effecting machining with a step and scan system.
BACKGROUND ART
In the field of microfinishing using ultraviolet light, such as with semiconductor exposure apparatuses, strict exposure control is necessary in order to maintain the resolution of the circuit pattern at a constant level or better. However, excimer lasers used as the light source in semiconductor exposure apparatuses have variations in the pulse energy of each pulse since these lasers are so-called pulse excitation gas discharge lasers. It is necessary to reduce these variations in order to improve the precision of exposure control.
Therefore, a method for decreasing the variation in the cumulative value of the irradiated energy, by decreasing the light energy output with one pulse oscillation and irradiating the same location to be machined with a plurality of successive pulses, is being considered.
In consideration of production, however, a large number of pulses is not preferable. Also, in the field of semiconductor exposure, the sensitivity of photosensitive materials applied to wafers has improved in recent years and exposure with a small number of pulses is becoming possible. For this reason, a method to increase the number of pulses and reduce variations in total energy of the irradiated light is unavoidable.
However, semiconductor exposure apparatuses repeatedly alternate between exposure and stepped movement. Therefore, the operating state of the excimer laser, the light source, as shown in FIG. 23, necessarily becomes the burst mode of switching between the action of continuous pulse oscillation to oscillate the laser beam continuously at a fixed frequency, and the action of stopping the pulse oscillation for a prescribed time. In effect, the burst mode switches alternately between the continuous pulse oscillation period and the oscillation stoppage period. In effect, in FIG. 23, one IC chip formed on a semiconductor wafer undergoes machining with a pulse group constituting a single continuous pulse oscillation period. Moreover, FIG. 23 shows the energy intensity of each pulse when the excitation intensity (charging voltage) is set at a constant value.
As discussed above, since the excimer laser is a pulse excitation gas discharge laser, it is difficult for it to continuously oscillate at a pulse energy of a constant size. The cause of this is as follows: the discharge disturbs the density of the laser gas within the discharge space, making the next discharge irregular and unstable; because of this irregular discharge, etc., a localized temperature increase occurs on the surface of the discharge electrode, deteriorating the next discharge; and discharge becomes irregular and unstable.
In particular, that trend is marked in the initial phase of the continuous pulse oscillation period; the so-called spiking phenomenon appears as shown in FIGS. 23 and 24. In the spiking phenomenon, a comparatively high pulse energy is attained initially in the spike zone, including the first few pulses after the oscillation stoppage period t, and afterwards pulse energy gradually decreases. When this spike zone is finished, the pulse energy passes through a plateau zone where a stable value at a comparatively high level is maintained, and then enters a stable zone (stationary zone).
This type of excimer laser apparatus with a burst mode operation has such problems as variations in the energy of each pulse, disc
REFERENCES:
patent: 4302782 (1981-11-01), Gunning et al.
"Advanced Krypton Fluoride Excimer Laser for Microlithography" by Ishihara et al., in SPIE vol. 1674.
Optical/Laser Microlithography V (1992), pp. 437-485 and including cover page, copyright page & Table of Contents iii-viii.
Amada Yoshiho
Kobayashi Yukio
Mimura Tatsuo
Mizoguchi Hakaru
Wakabayashi Osamu
Komatsu Ltd.
Nguyen Kiet T.
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