Coherent light generators – Particular active media – Gas
Reexamination Certificate
2002-05-23
2004-04-13
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S032000
Reexamination Certificate
active
06721344
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an injection locking type or MOPA (Main Oscillator Power Amplifier) type of laser device.
BACKGROUND ART
An injection locking type of laser device in which seed laser light oscillated from an oscillator is amplified in an amplifier is conventionally known and disclosed in, for example, Japanese Patent Laid-open No. 2000-357838.
FIG. 8
shows a block diagram of an injection locking type of laser device according to a prior art, and the prior art will be explained based on
FIG. 8
hereinafter. In
FIG. 8
, an injection locking type of laser device
11
includes an oscillator
11
A for oscillating seed laser light
21
A with narrow-banded wavelength, and an amplifier
11
B for amplifying the seed laser light
21
A and emitting amplified laser light
21
B.
The oscillator
11
A causes discharge between oscillator electrodes
14
A and
15
A inside an oscillator chamber
12
A in which a laser gas including fluorine and neon or helium as a buffer gas, and excites the laser gas to generate the seed laser light
21
A. The seed laser light
21
A thus occurring is incident on a band narrowing unit
30
placed at a rear of the oscillator chamber
12
A (the left side of the paper surface in FIG.
8
), and is expanded by prisms
32
and
32
, and only predetermined wavelength is reflected at a grating
33
. This is called band-narrowing.
The seed laser light
21
A emitted from the oscillator
11
A passes through a rear window
19
B from an injection hole
45
of a concave mirror
36
with a hole of the amplifier
11
B, and is incident on the amplifier chamber
12
B containing a laser gas. Amplification discharge is caused between amplifier electrodes
14
B and
15
B synchronously with the seed laser light
21
A in the amplifier chamber
12
B. Thus, pulse output of the seed laser light
21
A is amplified with center wavelength &lgr;c and spectral line width &Dgr;&lgr; that is the same meaning as a spectral band width &Dgr;&lgr; (they are called wavelength characteristics) being kept, and the seed laser light
21
A passes through a window
17
B and is emitted from around a convex mirror
37
as the amplified laser light
21
B. The amplified laser light
21
B is incident on an aligner
25
to be a light source for exposure. In the above Laid-open Patent, the explanation is made with an ArF excimer laser device, but the same explanation is also applied to KrF excimer laser devices and fluorine laser devices.
However, the following disadvantages exist in using the injection locking type of laser device
11
according to the aforementioned prior art as the light source for exposure of the aligner
25
such as a stepper. Specifically, during exposure, it is sometimes necessary to suspend discharge for a specified period of time to stop the laser oscillation in order to replace wafers or reticles. When the laser oscillation is stopped, optical components such as a prism
32
and the like are not irradiated with laser light, so that temperature of the optical components and gas around the optical component decreases.
As a result, at the time of resuming oscillation after the suspension, there arises the disadvantage that the amplified laser light
21
B having the center wavelength &lgr;c, which is deviated from a target value, is emitted and resolution of exposure is deteriorated. Further, there arises the disadvantage that it takes time to carry out a control to improve a change in the wavelength characteristics, and during that period of time, exposure cannot be carried out, which reduces availability of the aligner
25
.
Especially when the injection locking type of fluorine molecular laser device is used as a source for exposure, the wavelength of the amplified laser light
21
B is short, and thus the optical material that can be used in the aligner
25
is limited, which makes it difficult to remove chromatic aberration according to the optical component. Therefore, it is necessary to conform the center wavelength &lgr;c of the amplified laser light
21
B to the target value more strictly as compared with the case of the excimer laser device. Consequently, in the case of a fluorine molecular laser device, it is difficult to conform the wavelength characteristic to a target value after oscillation is resumed, thus causing the disadvantage that more time is required as compared with the excimer laser device.
SUMMARY OF THE INVENTION
The present invention is made in view of the above-described disadvantages, and its object is to provide an injection locking type or MOPA type of laser device capable of promptly obtaining stable output energy and a wavelength characteristic.
In order to attain the above-described object, the injection locking type or MOPA type of laser device according to the present invention has a structure including
an oscillator for exciting a laser gas by oscillator discharge and oscillating seed laser light with wavelength being band-narrowed by a band-narrowing unit,
an amplifier for amplifying the seed laser light by amplification discharge and emitting the amplified laser light, a first wavelength monitor for detecting at least a wavelength characteristic of the seed laser light, and
a laser controller for performing adjustment oscillation to contain the wavelength characteristic of the seed laser light within a predetermined allowable range, at a time of startup or when laser oscillation is suspended for a predetermined period of time or more.
According to the above structure, at the time of starting laser oscillation or when resuming the laser oscillation after suspending it, the wavelength characteristic of the seed laser light is within the predetermined allowable range, and therefore the wavelength characteristic of the amplified laser light is near a target value just after starting or resuming the oscillation, thus reducing time required for the wavelength control.
Further, in the laser device,
the laser controller may drive the band-narrowing unit to contain the wavelength characteristic of the seed laser light within the predetermined allowable range on an occasion of the adjustment oscillation.
According to the above structure, the band-narrowing unit is actually driven, and therefore the wavelength characteristic of the seed laser light to be oscillated can be surely and promptly contain within the allowable range.
In the laser device,
a second wavelength monitor for detecting at least a wavelength characteristic of the amplified laser light may be further included, and
the laser controller may carry out a control so that the amplified laser light oscillates in synchronization with the seed laser light on an occasion of the adjustment oscillation.
According to the above structure, since the amplified laser light is synchronized with the seed laser light just before startup or during the suspension, the amplified laser light oscillates in synchronization with the seed laser light from a time just after startup or resuming oscillation, and thus the amplified laser light with an improper wavelength characteristic is not emitted.
Further, in the laser device,
a delay circuit for setting a delay time from light emission of the seed laser light to a start of the amplification discharge may be further included, and
the laser controller may output a command to the delay circuit to change the delay time and oscillate the amplified laser light in synchronization with the seed laser light.
According to the above structure, the delay time is adjusted, and therefore the amplified laser light can be surely synchronized with the seed laser light.
REFERENCES:
patent: 5142543 (1992-08-01), Wakabayashi et al.
patent: 5237331 (1993-08-01), Henderson et al.
patent: 5715269 (1998-02-01), Shinji
patent: 5856991 (1999-01-01), Ershov
patent: 6005880 (1999-12-01), Basting et al.
patent: 6327286 (2001-12-01), Ness et al.
patent: 6370174 (2002-04-01), Onkels et al.
patent: 6381257 (2002-04-01), Ershov et al.
patent: 2002/0154668 (2002-10-01), Knowles et al.
patent: 2002/0154671 (2002-10-01), Knowles et al.
Nakao Kiyoharu
Wakabayashi Osamu
Ip Paul
Komatsu Ltd.
Nguyen Tuan
Rader & Fishman & Grauer, PLLC
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