Output controller for laser device

Details Output controller for laser device Output controller for laser device

1995-06-05

1998-01-20

Scott, Jr., Leon

Coherent light generators

Particular beam control device

Control of pulse characteristics

372 57, 372 31, 372 29, 372 38, H01S 310

Patent

active

057107874

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to an output control system for a laser device which is used mainly as a light source of a stepping reduction projection aligner (which will be referred to as the stepper, hereinafter) for causing discharging excitation of the laser device and oscillation thereof.


BACKGROUND ART

In a stepper, for the purpose of maintaining the resolution of a circuit pattern at a constant or higher level, strict control of the quantity of exposure light is necessary. Meanwhile, an excimer laser used as a light source of the stepper is of a so-called pulse discharging excitation laser gas type, for which reason the pulse energy per pulse varies from one another. Thus, in order to improve an accuracy in the control of the quantity of exposure light, it is necessary to make the above variations small. In addition, since the exposure light is of an intermittent type, the control of the exposure light quantity must be carried out in a way different from the prior art shutter control when a mercury lamp emitting continuous light is used as a light source.
For the purpose of realizing an improvement in the exposure light quantity control, there has been proposed a so-called plural pulse exposure method in which a plurality of pulses are continuously oscillated for exposure, as disclosed in a literature entitled "Excimer Laser Lithography", written by Miyaji, et al., International Laser/Application '91, Seminar L-5/pp. 36-51.
This method utilizes the fact that, since variations in the oscillation pulse energy of an excimer laser can approximate nearly a normal distribution, a dispersion in the energy accumulated after n times of pulse oscillation for exposure corresponds to 1/(n) of the dispersion in the energy per pulse. That is, assuming that a dispersion in the energy per pulse is denoted by .DELTA.P/P and a necessary exposure light quantity control accuracy is denoted by A, then the number N of exposure pulses necessary for it is expressed by the following relationship.
For example, when the dispersion .DELTA.P/P in the energy per pulse is 15%(3.sigma.) and the exposure quantity control accuracy A is 1.5%(3.sigma.), a relationship of N.gtoreq.100 is satisfied so that a desired accuracy can be attained with continuous oscillation of 100 or more pulses.
Meanwhile, a stepper repeats its exposure and stage shift operations alternately. Thus, the running state of the excimer laser as a light source becomes inevitably a so-called burst mode. In the burst mode, such a cycle is repeated that a laser beam is pulse-oscillated a predetermined number of times and thereafter the pulse oscillation is paused for a predetermined time. In other words, a continual pulse oscillation period for a short time and an oscillation pause period for a short time are alternately repeated.
In this specification, the terms "continual pulse" and "continual pulse oscillation" are used with such a meaning that pulse discharging is repetitively carried out to obtain repetitive intermittent pulse laser beams. Accordingly, the above terms are used in the meaning different from that usually used in the general words "continual oscillation laser" and "CW oscillation".
As mentioned above, since the excimer laser is of the pulse discharging excitation laser gas type, it is difficult to continue to oscillate always with a predetermined constant level of pulse energy. This is because discharging causes a disturbance of concentration in laser gas within a discharging space, which results in that the next discharging becomes heterogeneous or unstable, causing a temperature rise to take place locally on discharging electrodes with the result of the next discharging further degraded, heterogeneous and unstable. This tendency is remarkable, in particular, at the beginning of the above continual pulse oscillation period, so that, as shown in FIGS. 5 and 6, stable discharging and a relatively high level of pulse energy are obtained with the first pulse PL1 after passage of the oscillation pause period but thereafter, the disc

REFERENCES:
patent: 5463650 (1995-10-01), Ito et al.

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