Photolithographic molecular fluorine laser system

Details Photolithographic molecular fluorine laser system Photolithographic molecular fluorine laser system

2002-12-27

2004-05-25

Scott, Jr., Leon (Department: 2828)

Coherent light generators

Particular active media

Gas

C375S213000, C375S213000

Reexamination Certificate

active

06741627

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to a photolithographic molecular fluorine laser system, and more particularly to a two-stage laser mode of photolithographic molecular fluorine laser system in which the center wavelength of an oscillation-stage laser is matched to the center wavelength of an amplification-stage laser.
For photolithographic technologies to achieve on semiconductors a semiconductor integrated circuit having a marking width of 70 nm or less, there are demanded exposure light sources of wavelengths of 160 nm or less. F
2
(molecular fluorine) laser systems that give out ultraviolet rays of wavelengths of around 157 nm are now thought of as a promising candidate for those light sources.
As shown in
FIG. 16
, a typical F
2
laser system has two primary oscillation wavelengths (&lgr;
1
=157.6299 nm and &lgr;
2
=157.5233 nm: Sov. J. Quantum Eelectron. 16(5), May 1986), with its spectral linewidth (FWHM: full width at half maximum) being of the order of about 1 pm. The intensity ratio of two such oscillation lines is I(&lgr;
1
)/I(&lgr;
2
)≈78. For exposure purposes, usually, the oscillation line of wavelength &lgr;
1
(=157.6299 nm) having stronger intensity is used.
By the way, the semiconductor photolithography technology is roughly broken down into the following two types that use:
1) a dioptric system, and
2) a catadioptric system.
The use of the catadioptric system for photolithography ensures that chromatic aberrations are reduced. For this reason, an aligner using such a catadioptric system holds great promising in a wavelength range of the order of current 157 nm. However, the catadioptric systems are more troublesome than conventional dioptric systems in terms of the optical axis alignment of an aligner.
On the other hand, the dioptric system is a projection optical system generally used on heretofore known semiconductor aligners. One grave problem with the semiconductor photolithographiy technology is how to correct an optical system for chromatic aberrations. In the dioptric system, correction of chromatic aberrations has been achieved by some combinations of optical elements such as lenses having varying refractive indices Because of considerable restrictions on the types of possible optical materials that are transparent to a wavelength range in the vicinity of 157 nm, however, there is still no option but to use CaF
2
.
Thus, some line-narrowing means is needed for F
2
laser systems used as a light source for the dioptric type aligners. For instance, the spectral full width at half maximum of a laser beam should be narrowed to 0.3 pm or less.
On the other hand, the average output necessary for an F
2
laser used as a photolithographic light source, for instance, is 20 W. To put it another way, when the repetitive frequency of the F
2
laser is 2 kHz, a pulse energy per pulse is 10 mJ, and at the repetitive frequency of 4 kHz, a pulse energy per pulse is 5 mJ.
However, when one wishes to obtain laser outputs of 5 to 10 mJ while, for instance, a coated etalon is located as line-narrowing means in a laser resonator, the coating of the etalon is damaged; there is no option but to use an etalon without any coating thereon. Accordingly, it is still impossible to make spectral linewidths narrow. In addition, only linewidths of 0.4 to 0.6 pm are merely obtained at best because of the presence of much ASE (amplified spontaneous emission) component. Thus, it is still difficult to achieve any narrow band at pulse energies of 5 to 10 mJ.
Situations being like this, for instance, it is reasonalbe to rely on a two-stage laser system comprising an oscillation-stage laser and an amplification-stage laser for the purpose of obtaining laser beams having spectral linewidths of 0.3 pm or less and pulse energies of 5 mJ or greater. To be specific, the oscillation-stage laser gives out a laser beam that has low outputs yet spectral linewidths of 0.3 pm or less. Then, this laser beam is amplified at the amplification-stage laser to obtain a laser beam that has spectral linewidths of 0.3 pm or less and pulse energies of 5 mJ or greater.
A typical two-stage laser system operates in two modes, the injection locking mode and the master oscillator power amplifier (MOPA) mode. In the injection locking mode, the amplification-stage laser is provided with a laser resonator for which an unstable resonator is used. In the MOPA mode, no laser resonator is used. In the MOPA mode where no laser resonator is provided to the amplification-stage laser, the amplification-stage laser functions as a one-pass amplifier for a laser beam coming from the oscillation-stage laser.
For a line-narrowing element for the oscillation-stage laser, some combinations of one or more combined beam expansion and dispersion prism groups with gratings, a combination of an etalon with a total reflecting mirror, or the like are used. In what follows,-exemplary constructions of two-stage laser systems operating in the injection lock and the MOPA modes are explained.
FIG.
17
(
a
) is illustrative of the injection locking mode where a prism and a diffraction grating are used as line-narrowing element and FIG.
17
(
b
) again of the injection locking mode where an etalon is used as a line-narrowing element. FIG.
18
(
a
) is illustrative of the MOPA mode where a prism and a diffraction grating are used as line-narrowing element and FIG.
18
(
b
) again of the MOPA mode where an etalon is used as a line-narrowing element. Throughout FIGS.
17
(
a
) to
18
(
b
), reference numeral
10
represents an oscillation-stage laser,
20
an amplification-stage laser,
1
a laser chamber in the oscillation-stage laser
10
,
1
′ a laser chamber in the amplification-stage laser
20
,
2
a line-narrowing module (line-narrowing element),
3
an output mirror in the oscillation-stage laser
10
,
4
an aperture for limiting a laser beam in the oscillation-stage laser
10
,
5
a diffraction grating that forms a part of the line-narrowing module
2
,
6
a combined beam expansion and dispersion prism that forms a part of the line-narrowing module
2
,
7
a concave mirror that forms a part of an unstable resonator in the amplification-stage laser
20
,
8
a convex mirror that forms a part of the unstable resonator in the amplification-stage laser
20
,
9
a reflecting mirror interposed between the oscillation-stage laser
10
and an amplification-stage laser
20
,
11
an etalon that forms a part of the line-narrowing module
2
, and
12
a total reflecting mirror that forms a part of the line-narrowing module
2
.
Specific reference is here made to the injection locking mode of FIG.
17
(
a
), wherein the prism
6
and diffraction grating
5
are used as the line-narrowing element
2
. The oscillation-stage laser
10
has a function of giving out a seed laser (seed laser light) for the laser system, and the amplification-stage laser
20
has a function of amplifying that seed laser. Namely, the overall spectral characteristics of the laser system are determined by the spectral characteristics of the oscillation-stage laser
10
. Then, laser outputs (energy or power) from the laser system are determined by the amplification-stage laser
20
. The oscillation-stage laser
10
comprises the line-narrowing module
2
including the expanding prism
6
and diffraction grating
5
, so that laser beams having narrowed spectra linewidths are produced from the oscillation-stage laser
10
.
It is here noted that the line-narrowing module
2
may be made up of the etalon
11
and total reflecting mirror
12
, as shown in FIGS.
17
(
b
) and
18
(
b
).
The laser beam (seed laser beam) from the oscillation-stage laser
10
is guided to and poured into the amplification-stage laser
20
via a laser propagating system including the reflecting mirror
9
or the like. In the injection locking mode (FIG.
17
), the amplification-stage laser is built up of the concave mirror
7
and convex mirror
8
so that the laser can be amplified even at a small input. For instance, an unstable res

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