COL 11(4), 041405(2013) CHINESE OPTICS LETTERS April 10, 2013
Effects of prism beam expander and slits on excimer
laser linewidth narrowing module
Haibo Zhang (
ÜÜÜ
°°°
ÅÅÅ
)
1,2
, Zhijun Yuan (
)
1,2∗
, Jun Zhou (
±±±
)
1,2∗∗
,
Yunrong Wei (
$$$
JJJ
)
1,2
, and Qihong Lou (
¢¢¢
ööö
)
1,2
1
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai 201800, China
2
Shanghai Key Laboratory of All Solid-state Laser and Applied Techniques, Shanghai 201800, China
∗
Corresponding author: zhijunyuan@gmail.com;
∗∗
corresponding author: junzhousd@siom.ac.cn
Received September 29, 2012; accepted December 7, 2012; posted online March 20, 2013
Theoretical simulation and experiments based on a prism beam expander and an echelle grating are con-
ducted to study the dependence of linewidth and pulse energy on incidence angle and slit width. With a
larger prism incident angle or narrower slit width, the linewidth becomes narrower while the laser pulse
energy becomes lower. However, the pulse energy can be improved by optimally designing the prism beam
expander. In addition, a subpicometer linewidth ArF laser is obtained with a double-prism beam expander
and an echelle grating.
OCIS codes: 140.2180, 220.2740, 230.5480, 300.3700.
doi: 10.3788/COL201311.041405.
Deep ultraviolet radiation based on ArF excimer lasers
is extensively used in ultralarge-scale integrated circuit
lithography
[1−3]
, stimulated Raman sc atte ring
[4−6]
, and
fiber grating writing
[7−9]
. ArF excimer laser s that are
characterized by short wavelengths and high photon en-
ergy can be precisely focus e d
[10]
. However, before the
advantages of the lasers can be realized, a free running
linewidth of approximately 500 pm should be substan-
tially narrowed. The critical dimension and re solution of
a projection system are determined by the linewidth o f
a lithography source. The chromatic aberration of ex-
posure tools can be effectively mitigated by linewidth
narrowing of laser sources. Thus, spectral linewidth nar-
rowing of ArF excimer lasers significantly reduces the
lithography node.
Effective optics used for linewidth narrowing include
etalons, prisms, and gratings
[11]
. An ultranarrow
linewidth can be obtained using narrow-band filtering
etalons in excimer lasers. However, the output pulse en-
ergy of the laser is limited by the low damage threshold
of etalons
[12−17]
. Obtaining an ultranarrow linewidth by
separately using prisms or grating is difficult
[18−20]
, and
prism grating configurations characterize high cavity dis-
persion and high o ptical da mage threshold
[21−23]
. Previ-
ous studies
[24−27]
reported that an ultranarrow linewidth
laser could be demonstrated through a pris m beam ex-
pander and a grating. Laser efficiency is low be c ause
of the limited diffraction efficiency of the g rating work-
ing in extremely high orders and the reflective losse s of
prisms
[28]
. However, whether optimizing the prism beam
expander could enhance the pulse energy of linewidth-
narrowed lasers has yet to be determined. In addition,
the re lationship between the slit width and linewidth of
excimer lasers remains unknown.
In this letter, the sp e c tral linewidth narrowing of an
ArF excimer laser based on a prism be am expander and
an echelle grating is reported. Linewidth and pulse en-
ergy are sensitive to beam magnification a nd divergence
angle. The magnification and divergence angle are deter-
mined primarily by the incident angle of the prism and
slit width in the cavity, respective ly. Thus, the depen-
dence of linewidth and output energy on incident angle
and slit width is investigated.
The cavity dispersion equation is extensively used to
estimate the dispersive linewidth in high-gain pulsed
lasers that incorporate multiple dispersive optical ele-
ments. The dispersion can be high in multiple-prism
grating arrangements because grating dispersion is mul-
tiplied by the large beam magnification provided by the
multiple-prism beam expander. For achromatic prism
beam expanders
[29]
, the total dispersion for line narrow-
ing results mainly from grating. Therefore, the linewidth
of a linewidth-nar rowed laser can be e xpressed as
[30,31]
∆λ
FWHM
=
θ
div
2M
√
N
R
tan α
B
λ, (1)
where θ
div
and λ are the horizontal divergence angle and
laser wavelength, respective ly, M is the magnification of
the prism beam expander, α
B
denotes the blazed angle
of the echelle grating, and N
R
is the number of r ound
trips in the laser cavity.
Based on Eq. (1), a narrower linewidth can be obtained
by increasing the beam magnification of the pris m beam
expander, selecting an echelle grating with a large blazed
angle, or increasing the number of round trips. However,
the practical approach to linewidth narrowing is to en-
hance the magnification of the prism beam expander.
The total magnification of the N -prism beam expander
can be expr essed as
M =
N
Y
k=1
cos φ
k
cos ν
k
cos θ
k
cos µ
k
, (2)
where θ
k
and φ
k
are the incidence and refra c tion angles
on the incidence plane of the kth prism, respectively, and
µ
k
and ν
k
are the incidence and refraction angles on the
exit plane of the prisms, respectively.
The parameters in the simulation a re the same as those
used in the following experimental c onfigurations. Con-
sidering fused silica r ight angle prisms with an apex angle
1671-7694/2013/041405(4) 041405-1
c
2013 Chinese Optics Letters