Coherent light generators – Particular active media – Gas
Patent
1998-09-22
2000-11-21
Font, Frank G.
Coherent light generators
Particular active media
Gas
372 57, H01S 322
Patent
active
061513504
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a gas laser which is used as a light source for material machining, material modification, projection exposure, and the like, and in particular, to an excimer laser.
BACKGROUND ART
Gas lasers are used as light sources for material machining, such as marking on products, and boring, cutting, and modification of material. In particular, rare-gas halogen excimer lasers are used as light sources for marking on organic material, ablation processing, microfabrication of general material, surface modification, photochemical reaction, and the like. Such rare-gas halogen excimer lasers are also used in the processor manufacturing semiconductor products. For example, they can be used as light sources of projection aligners used in optical lithography for forming circuit patterns on semiconductors. Further, fluorine excimer lasers, which have mechanisms comparable with those of the rare-gas halogen excimer lasers and are provided for generating ultraviolet rays shorter in wavelength than those from the rare-gas halogen excimer lasers, are expected to be used in the same fields as the rare-gas halogen excimer lasers are used. Hereinbelow, the rare-gas halogen excimer lasers and the fluorine excimer lasers are generically called the excimer lasers.
FIG. 15 is a perspective view illustrating the structure of a typical conventional discharge-pumped gas laser. The following description is made based on the drawing. A laser chamber 1 is a container filled with laser medium gas (hereinafter, referred to as laser gas) for oscillating laser light. Provided inside the laser chamber 1 are a main discharge electrode 2, for performing glow discharge to excite or pump the laser gas, and a preionization electrode 3, for performing preionized discharge to generate initial electrons in a main discharge space of the main discharge electrode 2. Also provided inside the laser chamber 1 are a fan 7, for circulating the laser gas to supply laser gas into the main discharge space, and a heat exchanger 8, for cooling the laser gas whose temperature has risen due to discharge energy. Provided outside the laser chamber 1 is a high-voltage pulsed power source 4 for supplying the main discharge electrode 2 and the preionization electrode 3 with discharge energy. A capacitor is provided inside the high-voltage pulsed power source 4 for storing the discharge energy. The discharge energy is controlled by controlling a charging voltage across the capacitor. The laser gas used can be a mixture of carbon dioxide gas, helium gas and nitrogen gas in case of carbon dioxide gas lasers; a mixture gas of fluorine, krypton, and a buffer gas (helium or neon) in case of Kr F excimer lasers; and a mixture of fluorine, argon, and a buffer gas (helium or neon) in case of Ar F excimer lasers.
In typical gas lasers, a component gas showing relatively high reactiveness, e.g., a fluorine gas, reacts while adhering to the inside of the laser chamber 1 or to the surfaces of the fan 7 and the heat exchanger 8. The component gas also reacts while being adsorbed onto the surfaces of metal particles caused by sputtering of electrode material at the time of discharge-pumping. Thus, the concentration of the component gas showing relatively high reactiveness decreases with time. On the other hand, the component gas reacts with moisture in the laser chamber 1, a hydrogen atom occluded into the metal or a hydrogen atom in a high polymer such as a seal material or lubricant used in the laser chamber 1, to generate an impurity gas, resulting in an increase in the concentration of the impurity gas.
When the concentration of the component gas in the laser gas decreases to a prescribed value or less, or the concentration of the impurity gas increases to a prescribed value or more, the laser output power is reduced. In many cases, applications for the gas laser require that the laser output power be kept constant. In general, in order to keep the laser output power constant, the gas concentration is controlled by injecting a prope
REFERENCES:
patent: 5090020 (1992-02-01), Bedwell
patent: 5440578 (1995-08-01), Sandstrom
patent: 5450436 (1995-09-01), Mitzoguchi et al.
patent: 5642374 (1997-06-01), Wakabayashi et al.
Amada Yoshiho
Komori Hiroshi
Wakabayashi Osamu
Font Frank G.
Komatsu Ltd.
Rodriguez Armando
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