Coherent light generators – Particular active media – Gas
Reexamination Certificate
2000-12-21
2003-11-04
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S038020, C372S060000, C372S086000
Reexamination Certificate
active
06643312
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a gas laser device that emits ultraviolet rays, especially a gas laser device that effects laser operation at long laser pulse width in an ArF excimer laser device and a fluoride laser device.
2. Description of Related Art
Higher resolution is demanded for projection exposure devices for the production of semiconductor integrated circuits as they become smaller and more integrated. Thus, the wavelength of exposure light emitted from an exposure light source becomes shorter, and gas laser devices that emit ultraviolet rays such as ArF excimer laser devices and fluoride laser devices would be effective as the next generation of light sources for semiconductor exposure.
A laser gas comprising a gas mixture of fluorine (F
2
) gas, argon (Ar) gas and a rare gas, such as neon (Ne), as buffer gas in an ArF excimer laser device or a gas mixture of fluorine (F
2
) and a rare gas, such as helium (He), as buffer gas in a fluorine laser device serving as the laser medium is excited by discharge within a laser chamber in which the laser gas is enclosed at several hundred kPa.
Since the spectral width of laser light that is emitted in an ArF excimer laser device is broad, at 400 pm, the spectral width must be set at a narrow band range below 1 pm to avoid the problem of color aberration in projection optical systems of exposure devices. The spectral width can be narrowed by disposing a band-restricting optical system comprising an expansion prism and a diffraction grating in a laser resonator.
Incidentally, the core oscillation wavelength in an ArF excimer laser device is 193.3 nm, which is shorter than the 248 nm core oscillation wavelength of a KrF excimer laser device currently used as an exposure light source. Consequently, the damage inflicted on quartz, the glass used in projection lens systems of exposure devices, such as steppers, is greater than that when using a KrF excimer laser device, which shortens the life of the lens system.
Quartz damage may be the formation of color centers, due to two photon absorption, and compaction (elevation of refractive index). The former appears as a decrease of the transmittance while the latter appears as a decrease of the resolution of the lens system. This effect is inversely proportional to the laser pulse width (Ti) defined by the following expression when the laser pulse energy is constant
T
is
=(∫
T
(
t
)
dt
)
2
/∫(
T
(
t
))
2
dt
(1)
where T(t) represents the periodic laser shape.
This definition of the laser pulse width T
is
is explained here. Assuming that damage to optical devices arises due to the absorption of two photons, the damage D that accumulates per pulse would be given by the following expression since damage is proportional to the square of the laser photointensity:
D=k·∫(
P
(
t
))
2
dt
(2)
where k represents a constant determined by the substance while P(t) is the periodic laser strength (MW).
Laser strength P(t) can be separated into time and energy via the following expression:
P
(
t
)=
I·T
(
t
)/∫(
T
(
t
′))
dt′
(3)
where, I represents energy (mJ) and T(t) represents the periodic laser shape.
I develops when P(t) is periodically integrated, and I would be 5 mJ in the case of an ArF excimer laser device.
Here, damage D would be represented by the following expression when expression (3) is substituted for expression (2):
D=k·I
2
·∫(
T
(
t
)/∫
T
(
t
′)
dt
′)
2
dt=k·I
2
·∫(
T
(
t
)
2
dt
/(∫
T
(
t
)
dt
)
2
When expression (1) is substituted, the result would be as follows:.
D=k·I
2
/T
is
(5)
Pulse width T
is
, which is inversely proportional to damage D, would be defined by expression (1) since k·I
2
(I is held constant) is a constant according to expression (5).
The laser pulse width has been defined in the past by the full-width half maximum (FWHM) of the periodic laser shape. When the laser pulse width is defined by the full-width half maximum, the value would remain the same even if the periodic laser shapes were to mutually differ, as shown by the model in FIG.
8
. However, the continuous duration of the actual laser pulse is longer if it is triangular than if it is rectangular in the example shown in FIG.
8
. In addition, the triangular shape is longer than the rectangular shape shown in
FIG. 8
of the laser pulse width T
is
defined in expression (1). For example, in the case shown in
FIG. 8
, the triangular laser pulse width T
is
is double the rectangular laser pulse width T
is
.
Extension of the laser pulse width T
is
(pulse stretch) is desirable since the decrease in the transmittance due to the absorption of two photons and the decrease in the resolution due to compaction are inversely proportional to the laser pulse width T
is
given by expression (1) when the laser pulse energy is constant.
The repetition rate of oscillation operation (hereinafter termed repetition rate) in a commercial, narrowed band range ArF excimer laser device for exposure is 1 kHz and the laser light output is commonly 5 W. Laser pulse width T
is
should exceed 30 ns to avoid damage to the optical system mounted on a semiconductor exposure device.
As indicated above, pulse stretch that extends laser pulse width T
is
is sought to reduce damage to the optical system mounted on an exposure device, but this pulse stretch is requested in light of the following points as well.
The resolution R of an image of a mask having a circuit pattern that is projected via a projection lens on a workpiece, such as a wafer having a photoresist applied to it, and the depth of focus DOF in a projection exposure device are represented by the following expressions:
R=k
1
·&lgr;/NA
(6)
DOF=k
2
·&lgr;/(
NA
)
2
(7)
Here, k
1
and k
2
represent the coefficients that reflect the resist characteristics, &lgr; represents the wavelength of exposure light emitted from the exposure light source and NA represents the numerical apertures.
To enhance resolution R, the wavelength of exposure light is shortened and the number of apertures is raised, as is clear from expression (6); but, the depth of focus DOF is diminished to the extent that these are implemented, as indicated in expression (7). The spectral line width of exposure light must be made narrower since the effects of color aberration are increased as a result. Specifically, still narrower spectral line width of laser light emitted from an ArF excimer laser device is sought.
The fact that the spectral line width of laser light becomes narrower as the laser pulse width is stretched was stated in Proc. SPIE Vol. 3679 (1999) 1030-1037, and experiments by the inventors have corroborated this point. Specifically, further narrowing of the spectral line width of laser light is sought to enhance resolution R, and pulse stretch of the laser pulse width is essential for that.
Laser pulse width T
is
must be elongated to enhance the resolution and that avoids damage to the optical system of an exposure device, as indicated above. Laser pulse width T
is
is known to be dependent on the concentration of fluorine gas in the laser gas enclosed in the laser chamber (source: Proc. SPIE Vol. 3679 (1999), 1030-1037), and laser pulse width T
is
can be stretched so that T
is
≧30 ns by adjusting the concentration of fluorine gas.
A method of forming a laser pulse such that T
is
≧30 ns was proposed by the inventors in Patent Application No. Hei-11-261628 by carrying out a laser oscillation operation by the initial half-cycle of the discharge oscillation current waveform of a pulse that reverses the polarity and by at least one subsequent half cycle.
Higher resolution, higher throughput, lower damage to quartz are required of ArF excimer laser devices and of fluoride laser devices that are viable candidates for the next generation of semiconductor exposure light sources. However, pulse stretch that has the effects of raising the resolution and lowering t
Kakizaki Koji
Sasaki Yoichi
Ip Paul
Nguyen Tuan
Ushiodenki Kabushiki Kaisha
LandOfFree
ArF excimer laser device and a fluoride laser device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with ArF excimer laser device and a fluoride laser device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and ArF excimer laser device and a fluoride laser device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3135622