World Journal Of Engineering
Micro-Raman Characterization of MTMO
grayscale photomask
Hao Hu1, Jiwei Qi1, Liping Sun1, Penghong Liu1, Qi Wang2, Qian Liu2*,
Qian Sun1, Jingjun Xu1, and Xinzheng Zhang1†
1
The MOE Key Laboratory of Weak-Light Nonlinear Photonics, TEDA Applied Physics School and School of Physic,
Nankai University, Tianjin 300457, China
2
National Center for Nanoscience and Technology, No. 11, Beiyitiao, Beijing 100190, China
Corresponding author: *liuq@nanoctr.cn †zxz@nankai.edu.cn
Abstract: Laser direct writing based on metal-transparent-metallic-oxides (MTMO) in metal films provides an
effective way to fabricate grayscale photomask. Here we utilize micro-Raman spectroscopy to evaluate the MTMO
grayscale photomask of indium films. Raman spectra exhibit one characteristic band of In2O3 centered at 490 cm-1, whose
integrated intensity increases with the decreasing of optical density of the In-In2O3 grayscale mask. The concentration of
In2O3 was found to be linearly proportional to the Raman integrated intensity with the assistant of atomic force
microscopic analysis.
The boom in the field of information technology demands
the miniaturization and integration of electronic and
photonic devices, and therefore researches on micro/nano
fabrication technologies are essential and significant. Laser
direct writing (LDW) is one of the precise and convenient
methods to make grayscale photomasks for the fabrication
of three-dimensional microstructures [1-4]. Recently,
grayscale photomasks based on metal-transparentmetallic-oxide(s) (MTMO) systems by LDW in metal
films had been demonstrated [5,6]. The mechanism of the
gray levels is due to the coexistence of the metal and the
transparent metallic oxides formed in a laser-induced
thermal process.
There are many conventional methods to analyze the
fabricated photomasks, such as transmission electron
microscopy (TEM), selected area electron diffraction
(SAED), transmission spectroscopy, scanning electronic
microscopy, atomic force microscopy (AFM), and so on
[5-9]. SEM and AFM can show the surface topography of
film samples, but they cannot present the content of
samples directly. TEM and SAED can obtain the structure
and content of film samples, and their resolutions are
better than 100 nm. But both of them are not suitable to
scan a relatively large sample due to time-consuming
acquisition. It is convenient to obtain the optical density of
grayscale masks by transmission spectroscopy, but its
resolution cannot reach sub-micrometer scale. Because the
content of oxides fabricated by LDW (semi-)continuously
distributes among different gray scales, it is necessary to
find a precise and efficient method to explore the
composition and structure of the sample fabricated by
LDW at large scale. Though these conventional methods
are able to illustrate the properties of grayscale masks, they
also have different instinct drawbacks in characterizing
grayscale photomasks. In comparison, micro-Raman
spectroscopy is a non-destructive analytical technique
widely applied in the study of materials science, chemistry,
geology, environmental sciences. In general, micro-Raman
spectroscopy can provide the information about the
transformation of crystalline, the change of the crystal
grain size and the inner structure of samples. It is
unnecessary to pretreat or destruct the samples, or put
them in vacuum. Moreover, the resolution of this method
is relatively high (about 500 nm) and the scan scale is
relatively large.
In-In2O3 grayscale masks, for their excellent
photoelectrical properties, have significant applications in
the field of transparent conductive films such as gas
sensors, flat panel displays, transparent electrode materials,
solar cells, and etc. [10]. Indium films with a thickness of
25 nm and a roughness of 2 nm were sputtered on glass
substrates by radio-frequency magnetron sputtering
(ULVAC ACS400-C4). A Nd:YAG 532 nm pulsed laser
was used as the beam source of a laser direct writer and the
sample was placed on the focal plane of one objective lens
(NA 0.95, Nikon). The laser power ranged from 2.4 to 8.0
mW and the pulse width was from tens to hundreds of
nanoseconds. Laser irradiation will heat the local domain
of indium film and turn indium into transparent indium
oxide, a higher power yielded a higher transmittance. A
bitmap file, which was transformed automatically from an
original picture, defined laser power of each pixel, writing
path, and pixel stepping (50-200 nm, typically 150 nm).
Gray levels were realized by adjusting the writing power
with an acoustic-optic modulator. One grayscale mask with
five discrete gray levels was fabricated in indium films by
using LDW technique, as shown in Fig. 1: (a) 8.0 mW, (b)
6.6 mW, (c) 5.2 mW, (d) 3.8 mW, and (e) 2.4mW.
Fig. 1. Optical microscopy image of In-In2O3 mask with five
grayscales.
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World Journal Of Engineering
Micro-Raman scattering spectra were acquired by a
micro-Raman spectrometer (Renishaw inVia plus),
equipped with an argon ion laser (514.5 nm) and a triple
monochromator (1800 l/mm). A two-dimensional
charge-coupled device (CCD) detector, at the exit port of
the spectrometer, collected both spatial and spectral
information originating from the illuminated sample.
As shown in Fig. 2, there exists only one distinct
Raman band centered at 490cm-1 and the band width (Full
width at half maximum) is expanded to about 200cm-1. Fig.
3 is the line-scanning Raman signals of the mask, which
show that the signal intensities of the mask are almost
uniform in the same grayscale area.
linear relationship between the concentration of In2O3 and
the Raman signal intensity is shown in Fig. 3.
In conclusion, the grayscale photomask is fabricated
using direct laser writing technique. The relative thickness
of the film will be higher where the content of In2O3 is
more. The micro-Raman spectra showed that under the
same fabrication condition, the Raman scattering peaked at
490 cm-1 has the same intensity, which is linearly
proportional to the concentration of In2O3. We propose a
novel, convenient and precise method to analyze the
structure and concentration of In2O3 grayscale mask by
using micro-Raman scattering, which overcomes the
disadvantages of conventional methods.
Supported by National Science Foundation of China
(10874093, 10974037), National Basic Research Program
of China (2010CB934101, 2010CB934102).
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Fig. 2. Micro-Raman spectra of the In-In2O3 mask with five
different grayscales.
Bulk In2O3 has a body-centered cubic (BCC)
structure, whose Raman lines are at about 130 cm-1, 300
cm-1, 360 cm-1,490 cm-1 and 625 cm-1. In our experiment,
there exist only one distinct Raman band centered at about
490 cm-1 and the band width is expanded to about 200cm-1.
The polycrystalline effect and amorphous indium oxide
causes the Raman linewidths increased and become a
broad band.
Fig. 3. The concentration of In2O3 vs. Raman scattering intensity.
As has been clarified, the indium film is layered
oxidized as indicated from the Moiré fringes. Therefore,
the relative thickness of the film will be higher where the
content of In2O3 is more. As expected, AFM topographic
images agree well with optical images, which means the
laser exposed area not only has exact optical image
replication in the film plane but also controllable height in
the z direction. From the AFM images, we get the relative
thicknesses of mask with different grayscales. Moreover,
we know that the densities and molecular weights of In
and In2O3. Then we can obtain the dependence between the
concentration of In2O3 and the thickness of the mask. The
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