Coherent light generators – Particular active media – Gas
Reexamination Certificate
2001-10-03
2003-10-21
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular active media
Gas
Reexamination Certificate
active
06636546
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to ArF and KrF excimer laser apparatus and fluorine laser apparatus for lithography. More particularly, the present invention relates to gas laser apparatus for lithography, e.g. ArF excimer laser apparatus, KrF excimer laser apparatus, and fluorine laser apparatus, which perform a lasing operation with a long laser oscillation pulse width.
With the achievement of small, fine and high-integration semiconductor integrated circuits, it has been demanded that projection exposure systems for the manufacture of such highly integrated circuits be improved in resolution. Under these circumstances, the wavelength of exposure light emitted from light sources for lithography is becoming shorter. At present, KrF excimer laser apparatus are used as light sources for lithography. ArF excimer laser apparatus and fluorine laser apparatus are promising as next-generation light sources for semiconductor lithography.
In these excimer laser apparatus, a laser gas is sealed in a laser chamber under several hundred kPa. That is, in the KrF excimer laser, a mixed gas of fluorine (F
2
) gas, krypton (Kr) gas and a rare gas, e.g. neon (Ne), as a buffer gas is sealed in the laser chamber as a laser gas. In the ArF excimer laser, a mixed gas of fluorine (F
2
) gas, argon (Ar) gas and a rare gas, e.g. neon (Ne), as a buffer gas is similarly sealed in the laser chamber as a laser gas. In the fluorine laser, a mixed gas of fluorine (F
2
) gas and a rare gas, e.g. neon (Ne), as a buffer gas is similarly sealed in the laser chamber as a laser gas. In these apparatus, the laser gas as a laser medium is excited by generating an electric discharge in the laser chamber.
These laser apparatus emit laser beams having a wide spectral linewidth. Therefore, in order to avoid the problem of chromatic aberration in the projection optical system mounted in the exposure system, it is necessary that the spectral linewidth be narrowed down to 1 pm or less. Narrowing of the spectral linewidth is realized by placing a line-narrowing optical system comprising, for example, a magnifying prism and a diffraction grating, in the laser resonator.
Incidentally, the ArF excimer laser apparatus have an oscillation center wavelength of 193.3 nm, which is shorter than the oscillation center wavelength of the KrF excimer laser apparatus presently used as light sources for lithography, i.e. 248 nm. Accordingly, quartz used as a vitreous material in the projection lens system of a stepper or other exposure system is damaged to a larger extent than in the case of using KrF excimer laser apparatus, resulting in a reduction in lifetime of the lens system.
Damage to quarts includes color-center formation due to two-photon absorption and a compaction (an increase in refractive index). The former appears as a reduction in transmittance, and the latter as a reduction in resolution of the lens system. The influence of the damage is in inverse proportion to the laser pulse width (T
is
), which is defined by the following equation, when the laser pulse energy is assumed to be constant:
T
is
=(∫
T
(
t
)
dt
)
2
/∫(
T
(
t
))
2
dt
(1)
where T(t) is a temporal laser pulse shape.
Let us describe the definition of the laser pulse width T
is
. Assuming that an optical element is damaged by two-photon absorption, because the damage is proportional to the square of the laser light intensity, the damage D accumulated per pulse is given by
D=k
·∫(
P
(
t
))
2
dt
(2)
where k is a constant determined by a substance, and P(t) is a temporal laser intensity (MW).
The laser intensity P(t) may be separated into time and energy by the following equation:
P
(
t
)=
I·T
(
t
)/∫
T
(
t′
)
dt′
(3)
where I is energy (mJ), and T(t) is a temporal laser pulse shape.
Temporally integrating P(t) gives I. In the case of ArF excimer laser, I is 5 mJ, for example.
If Eq. (3) is substituted into Eq. (2), the damage D is expressed by
D=k·I
2
·∫(
T
(
t
)/∫
T
(
t
′)
dt
′)
2
dt =k·I
2
·∫(
T
(
t
))
2
dt
/(∫
T
(
t
)
dt
)
2
(4)
Substituting Eq. (1) into Eq. (4), we obtain
D=k·I
2
/T
is
(5)
From Eq. (5), the pulse width T
is
, which is in inverse proportion to the damage D, is defined by Eq. (1) because k·I
2
is constant (I is maintained at a constant value).
There have heretofore been cases where the laser pulse width is defined by the full width at half maximum (FWHM) of the temporal laser pulse shape. When the laser pulse width is defined by the full width at half maximum, different temporal laser pulse shapes may become equal to each other in laser pulse width as shown in the model diagram of FIG.
8
. In the example shown in
FIG. 8
, however, the actual laser pulse durations of the two temporal laser pulse shapes are different from each other. That is, the pulse duration of the triangular laser pulse shape is longer than that of the rectangular laser pulse shape. Meanwhile, in the case of the laser pulse width T
is
defined by Eq. (1), the laser pulse width T
is
of the triangular laser pulse shape shown in
FIG. 8
is longer than that of the rectangular laser pulse shape. In the example shown in
FIG. 8
, for instance, the laser pulse width T
is
of the triangular laser pulse shape is twice as long as the laser pulse width Tof the rectangular laser pulse shape.
As has been stated above, the reduction in transmittance due to two-photon absorption and the reduction in resolution due to a compaction are in inverse proportion to the laser pulse width T
is
, which is given by Eq. (1), when the laser pulse energy is assumed to be constant. Therefore, it is demanded that the laser pulse width T
is
be stretched (i.e. a longer pulse width should be achieved).
Narrow-linewidth ArF excimer laser apparatus for lithography commercially available at present in general perform an oscillating operation at a repetition frequency (hereinafter referred to as “repetition rate”) of 1 kHz and provide a laser output of 5 W. In order to avoid damage to the optical system mounted in the semiconductor exposure system, it is necessary that the laser pulse width T
is
be 30 ns or longer.
As has been stated above, it is demanded in ArF excimer laser apparatus that the laser pulse width T
is
be stretched to achieve a longer pulse width in order to reduce the damage to the optical system mounted in the exposure system. The achievement of a longer pulse width is also demanded for KrF excimer laser apparatus and fluorine laser apparatus from the following points of view.
In a projection exposure system, an image of a mask provided with a circuit pattern or the like is projected through a projection lens onto a work, e.g. a wafer, coated with a photoresist. The resolution R of the projected image and the depth of focus DOF are expressed by
R=k
1
·&lgr;/NA
(6)
DOF=k
2
·&lgr;/(
NA
)
2
(7)
where k
1
and k
2
are coefficients reflecting the characteristics of the resist and so forth; &lgr; is the wavelength of exposure light emitted from a light source for lithography; and NA is a numerical aperture.
To improve the resolution R, the wavelength of exposure light is reduced, and the NA is increased, as will be clear from Eq. (6). However, the depth of focus DOF decreases correspondingly, as shown by Eq. (7). Consequently, the influence of chromatic aberration increases. Therefore, it is necessary to further narrow the spectral linewidth of exposure light. In other words, it is demanded that the spectral linewidth of the laser beam emitted from the gas laser apparatus for lithography be further narrowed.
It is stated in Proc. SPIE Vol. 3679. (1999) 1030-1037 that according as the laser pulse width increases, the spectral linewidth of the laser beam narrows. This was actually proved by an experiment conducted by the present inventors. In other words, it is demanded in order to improve the resolution R that the spectral linewidth of the laser beam be further narrowe
Kakizaki Koji
Sasaki Yoichi
Dellett and Walters
Jr. Leon Scott
Ushio Denki Kabushiki Kaisya
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