Coherent light generators – Particular active media – Gas
Patent
1996-12-13
1998-05-19
Scott, Jr., Leon
Coherent light generators
Particular active media
Gas
372 38, 372 86, H01S 322
Patent
active
057545790
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a discharge-excitation type laser device that is employed as a light source for the processing of materials or for reduction-projection, etc., and more particularly to a device that controls the supply of the laser gas, and to a charging/discharging device therefor.
BACKGROUND ART
Discharge-excitation type laser devices are employed not only for material processing such as marking, hole-forming or annealing, but also as light sources of photolithography for producing the circuit patterns of large-scale integrated circuits (LSIs).
Among gas lasers, excimer lasers in particular provide powerful ultraviolet light sources; this property is exploited in their use for material processing, chiefly in marking and hole-forming of organic materials such as resins. Also, in photolithography, the reduction-projection method is chiefly employed, in which a circuit pattern is formed by projecting on to a photosensitive substance on a semiconductor substrate by a reduction-projection optical system the light that passes through an original image (reticle) pattern illuminated by an illuminating light source.
The resolution of this projected image is limited by the wavelength of the light source that is used, so there has been a gradual trend to decrease the wavelength of the light source that is used from the visible region to the ultraviolet region. Conventionally, as the light source for the ultraviolet region, use is made of the g ray (436 nm) and i ray (365 nm) generated by a high-voltage mercury lamp. However, with minimum pattern line widths of less than 0.35 .mu.m being demanded at 64 MB, the limits in terms of wavelength are already being approached even for the i ray.
Deep ultraviolet laser light sources are viewed as promising for solving this technical limitation. In particular, with excimer lasers, strong oscillations can be obtained at the wavelengths of KrF (248 nm) or ArF (193 nm) etc., depending on the composition of the gaseous medium, with high output and high efficiency. On the other hand, in the deep-UV region, the glass or crystalline materials that may be used to constitute the reduction-projection lens system are extremely restricted, so the chromatic aberration correction that was employed in a reduction-projection lens system using a mercury lamp is difficult to implement.
Rather than by correcting the chromatic aberration of the lens system, this difficulty is therefore eliminated by reducing the spectral bandwidth of the output light to such an extent that chromatic aberration of the lens material can be neglected, by arranging a wavelength selection element within the laser resonator. By this method, the spectral bandwidth, which is a few nm in the case of natural oscillation, can be reduced to a few pm.
The laser output beam of an excimer laser device employed in such bandwidth reduction is obtained by throwing the electrical energy stored on a capacitor for laser excitation into the discharge space to create electrical discharge in the gaseous laser medium.
FIG. 24 shows the layout of a conventional excimer laser device, and FIG. 25 shows in particular the charging/discharging electrical circuit therefor. Usually an automatic pre-ionisation capacitative transfer circuit is employed for the charging/discharging circuit, on account of its straightforward construction.
In more detail, primary capacitor C1 for charge storage is charged up to the prescribed voltage by charger 26, and charging (charge transfer) on to secondary capacitors (peaking capacitors) C2 is commenced when switch element Q conducts, being made conductive by discharge of a thyratron etc. in response to a trigger pulse that is subsequently output from a trigger pulse generator 25. On this charge transfer, the charging current il is conducted to a few tens of pre-ionisation electrodes 6, which are arranged on both side faces of main electrodes (discharging electrodes within the laser chamber) 5. Pre-ionising discharge is thereby generated, so that automatic pre-ionisation is a
REFERENCES:
patent: 5450436 (1995-09-01), Mizoguchi et al.
Komori Hiroshi
Mizoguchi Hakaru
Nishisaka Toshihiro
Jr. Leon Scott
Komatsu Ltd.
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