Coherent light generators – Particular active media – Gas
Patent
1996-06-20
2000-10-10
Healy, Brian
Coherent light generators
Particular active media
Gas
372 55, 372 57, H01S 322
Patent
active
061309044
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an excimer laser apparatus employed for example in light sources for reduced-size projection exposure, microprocessing of materials, or surface improvement of materials; in particular, it relates to improvements for enabling the laser to oscillate in stable fashion over a long period.
BACKGROUND ART
Conventionally, when an excimer laser apparatus employing halogen gas is operated, the halogen gas is consumed, during the process of operation, by evaporation of the electrode materials and chemical reaction with the constituent material of the laser chamber. Conventionally therefore, control was performed as follows in order to compensate for the lowering of laser output produced by consumption of halogen gas.
Specifically, the laser output is obtained by passing through a discharge space electrical energy for laser excitation that was accumulated on a capacitor, the discharge being effected in laser medium gas; if the charging voltage of this capacitor is raised, laser output is increased. Conventionally therefore laser output was stabilized by detecting the laser output and controlling the value of the charging voltage in accordance with the results of this detection. Such control is called "power lock control" and this charging voltage will hereinbelow be called the "power lock voltage".
However, even with such control, if operation is continued for a long time, the efficiency of oscillation is lowered by consumption of halogen gas, with the result that the prescribed output cannot be maintained unless the charging voltage (power lock voltage) is progressively raised.
Conventionally therefore arrangements were made to attempt to cope with this consumption of halogen gas by arranging for supplementation with a fixed quantity of halogen gas when the charging voltage increased above some prescribed voltage.
Such a method of halogen gas supplementation according to the prior art will be described with reference to FIG. 29 to FIG. 31.
In more detail, FIG. 29 shows structural parts pertaining to gas supplementation of a typical fluorine-based excimer laser apparatus; in this case, there are provided, as a gas feed cylinder, a cylinder 41 that is charged with a halogen gas (F2, HCl etc) diluted with a buffer gas (Ne or He etc), a cylinder 42 charged with a diluent gas such as Kr, and a cylinder 43 charged with a buffer gas such as Ne or He; when effecting gas feed on start-up, gas feed to laser chamber 47 is effected by open/shut control of on/off valves 44, 45, 46, and, when supplementing the halogen gas after operation, gas feed is effected through on/off valves 48, 49 and "subtank" 50.
In more detail, when introducing new gas into laser chamber 47 before start-up, first of all, the old gas in laser chamber 47 was discharged by means of on/off valve 51 and vacuum pump 52.
Next, Kr gas is introduced into laser chamber 47 to a pressure of 40 torr from cylinder 42 through on/off valve 45; next, F2 gas diluted by Ne gas is introduced to a pressure of 80 torr from cylinder 41 through on/off valve 44; finally Ne gas is introduced from cylinder 43 through on/off valve 46 to make the overall pressure in laser chamber 47 2500 torr.
By such gas feed control, the gas composition within laser chamber 47 in this apparatus becomes F2:Kr: Ne=4:40:2456 (torr), i.e. F2:Kr:Ne=0.16:1.60:98.24 (%) in terms of concentration ratios.
Thus, when laser chamber 47 is charged with new gas, gas supplementation control is performed by the procedure shown in the flow chart of FIG. 30 during subsequent laser operation.
First of all, before operating the excimer laser apparatus, the target laser output Ec, optimum control charging voltage range Vm (Vmin to Vmax), the increase/decrease charging voltage .DELTA.V when control is exercised once, and the one-time supplementation gas amount .DELTA.G are set beforehand (step 500).
When operation is then started, controller 55 gets the laser output E detected by laser output monitor 53 and the charging voltage V detected by charging voltage detecto
REFERENCES:
patent: 5396514 (1995-03-01), Voss
patent: 5440578 (1995-08-01), Sandstrom
patent: 5450436 (1995-09-01), Mizoguchi et al.
patent: 5463650 (1995-10-01), Ito et al.
Fujimoto Junichi
Ishihara Takanobu
Mizoguchi Hakaru
Healy Brian
Komatsu Ltd.
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