Wavelength calibration of narrowband ArF laser with
iron hollow cathode lamp
Zhijun Yuan (袁志军)
1,2
, Haibo Zhang (张海波)
1,2
, Ren Ye (叶 韧)
1,2
,
Jun Zhou (周 军)
1,2,
*, Yunrong Wei (魏运荣)
1,2
, and Qihong Lou (楼祺洪)
1,2
1
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
2
Shanghai Key Laboratory of All Solid-state Laser and Applied Techniques, Shanghai 201800, China
*Corresponding author: junzhousd@siom.ac.cn
Received December 5, 2016; accepted March 30, 2017; posted online April 20, 2017
Accurate and precise wavelength controlling of narrowband excimer lasers is essential for the lithography of an
integrated circuit. High-precision wavelength tuning and calibration of a line-narrowed ArF laser are presented
in this work. The laser spectrum is narrowed to a sub-picometer with a line narrowing system. Absolute
wavelength calibration of the line-narrowed laser is performed based on the optogalvanic (OG) effect using iron
hollow cathode discharge (HCD). An sccuracy of better than 0.1 pm for wavelength tuning and calibration is
achieved with our homemade wavemeter.
OCIS codes: 140.2180, 150.1488, 300.6440, 300.3700, 110.5220.
doi: 10.3788/COL201715.071402.
ArF lasers emitting around 193 nm are widely used for
photolithographic processing of integrated circuit devi-
ces
[1]
. This kind of laser should provide a narrow spectral
band around a precisely determined and finely adjustable
absolute wavele ngth. The deviation of the wavelength
may result in the linewidth broadening and the defocus
on the exposure plane
[1]
. Therefore, it is desirable to cali-
brate the wavelength of the ArF lasers within a range of
0.05 pm
[2]
. Wavelength calibrations of the laser source
for lithography also require a compact wavemeter with
sub-picometer precision. Researches on high-precision
spectrum detection
[2–6]
in the ultraviolet (UV) region have
been extensively studied in recent years.
The optogalvanic (OG) effect described by Babin et al.
[7,8]
permits a very precise and reliable determination of an
absolute emission wavelength for an excimer laser system.
When the line-narrowed laser beam is incident into a see-
through hollow cathode discharge (HCD) lamp, the OG
effect originates from a change in the impedance in a
steady-state glow discharge when the discharge medium
resonantly absorbs laser photons
[9]
. Consequently, the
photodetector behind the HCD lamp can observe an inten-
sity drop of laser energy around the wavelength of absorp-
tion. This effect has been shown to be a powerful and
inexpensive technique for laser stabilization or wavelength
calibration
[10,11]
, especially in a lithography application
[2,7,12]
.
For example, emission lines of a platinum HCD lamp at
193.22433 and 193.43690 nm (vacuum wavelength) have
been used for the calibration of an ArF laser
[12]
.Never-
theless, very few researches on ArF laser wavelength cali-
bration with an iron lamp have been reported so far.
Moreover, the laser power incident into the HCD has to
be very carefully controlled for the ArF laser source because
the OG signal amplitude tends to saturation at relatively
high laser energy
[13]
. On the other hand, the ArF laser en-
ergy around 193.22433 nm is relatively low due to the
oxygen absorption; accordingly, a high signal-to-noise ratio
(SNR) OG signal around the absorption wavelength is
hardly accessible.
Because the OG signal is vulnerable to the interference
from the surrounding electromagnetic field, many re-
searchers since the 19th century have used the boxcar
averager
[8]
or lock-in amplifier
[14]
to get a high SNR signal.
However, the bulky and complicated structure of the box-
car averager or lock-in amplifier prevents its integration in
an optical lithography machine.
In our experiment, the wavelength of the ArF laser was
calibrated by measuring the OG voltage directly. Rela-
tively high SNR absorption for absolute wavelength cali-
bration was obtained. The ArF laser was line-narrowed
below 0.4 pm by a line narrowing system. Furthermore,
high-precision laser wavelength tuning and real-time mea-
surements with a homemade wavemeter wer e also demon-
strated in this work.
Before the wavelength calibration process, the broad-
band emission spectrum of nearly 500 pm of the ArF laser
spectrum has to be narrowed to the sub-picometer scale.
The narrow spectrum is obtained by an EX100 excimer
laser (provided by GAM LASER-INC, maxi mum pulse
energy of 20 mJ and repetition of 500 Hz) coupled with
a line narrowing system, which consists of three CaF
2
prisms and a grating. In order to reduce the energ y loss
in the beam path, the prisms and grating are equipped
in a sealed cavity purged with nitrogen. The design and
setup of the line narrowing system can be found in our
early works
[15–17]
. The bandwidth (full width at half-
maximum, FWHM) of the narrowband laser is about
0.38 pm, and the maximum pulse energy is 0.5 mJ. Figure
1
shows the typical laser spectrum recorded by a spectrom-
eter (provided by LTB Lasertechnik Berlin, model
ELIAS III, resolution of 0.022 pm for the linewidth, abso-
lute accuracy of 5 pm for wavelength calibration).
COL 15(7), 071402(2017) CHINESE OPTICS LETTERS July 10, 2017
1671-7694/2017/071402(5) 071402-1 © 2017 Chinese Optics Letters