Crystallization of Silicon Thin Films by Infrared Semiconductor Laser
Irradiation
T. Haba1, T. Sameshima1 and N. Sano2
1

Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo,
184-8588, Japan
2
Hightec Systems Corporation, 3-9-15, Shin Yokohama, Kohoku-ku, Yokohama, Kanagawa,
222-0033, Japan
E-mail : tsamesim@cc.tuat.ac.jp

Keywords: Infrared semiconductor laser, Carbon particles, Polycrystalline silicon, Laser
crystallization, Raman scattering spectrum

Abstract.
We report the crystallization of 40-nm-thick hydrogenated amorphous (a-Si:H) silicon films
using 976-nm continuous wave (CW) infrared semiconductor laser with photo absorption of
200-nm-diamater carbon particles. Laser beam was focused to the surface of carbon particles
with area of 500 by and 15μm. Laser irradiation for 50 μs at a power of 14 W heated thesurface region to about 3000 K, which was estimated by analysis of spectra of light emission
caused by laser irradiation when quartz substrates were placed on carbon particles. The siliconthin films coated with SiOx films were contacted to carbon particles. Laser irradiation for 50 μs
at laser power 12 W initiated crystallization. The crystallization volume ratio (CVR) increased
to 1 as the laser power increased to 14 W. The repetition of laser irradiation was also effective
to increase CVR and make it uniform.

Introduction
Polycrystalline silicon films have been applied to various kinds of devices such as thin filmtransistors (TFTs) and solar cells. Laser crystallization processes using pulsed excimer lasers
have been widely adopted for the mass production of low-temperature polycrystalline silicon(LTPS) TFTs used for switching transistors of pixels and the peripheral driver circuits of liquid
crystal displays (LCDs) [1,2]. However, thin silicon film has a low optical absorbance in thenear-infrared region. Therefore, infrared lasers have not been used for the crystallization ofsilicon films. We have proposed silicon crystallization using diamond-like carbon (DLC) films
as photo absorption layers by irradiation using a infrared laser [3,4]. DLC has outstanding
properties of high hardness, thermal conductivity, wear-resistance, thermal durability andchemical inertness. Therefore, it is widely utilized for technological and industrial applications
such as wear resistant and protective hard coats. DLC has also interesting optical properties,
which are low refractive indices from 1.3 to 1.9 and high extinction coefficients from 0.8 to 0.9
for wavelengths from 250 to 1100 nm. Therefore, a high power laser diodes with wavelengthsof 800 –1000 nm can be used for the crystallization of silicon films with the assistance of DLC.
The power of continues wave (CW) semiconductor laser is stable and easily modulated by theelectrical current. In particular, they have a markedly high durability with a lifetime longer than

10,000h. A laser annealing system with a laser power much higher than that of conventional
excimer lasers used for mass production of LCD panels will be established using commerciallyavailable cheap laser diodes with an energy conversion efficiency above 40%.
In order to look for a further possibility of crystallization process using a combination of
infrared laser with carbon, we report in this paper, crystallization of silicon films using carbon
particles mechanical contacting silicon samples. We demonstrate crystallization conditions of
the laser power and the laser dwell time. We also report crystalline properties using Raman
scattering and temperature increase in carbon particles by laser heating.

Experimental
quartz substrate
Figure 1 shows experimental steps for
a-Si 40nm
the present method. 40-nm-thick undopedhydrogenated amorphous silicon (a-Si:H)
quartz substratefilms with a hydrogen concentration of
a-Si 40nm
16% were formed on quartz glass
SiOｘ 130nm
substrates by plasma enhanced chemical
vapor deposition. 200-nm-thick SiOx films
were subsequently formed on the a-Si:H
Infrared semiconductor laser
films using Polysilazane precursor heatedat 260℃ with 1.3x106 Pa H2O vapor for
a-Si 40nm
quartz substrate
3h. A stainless-steel container with a
SiOｘ 130nm
diameter of 6 cm and a depth of 1cm was
prepared. It was filled with carbon
particles with a mean diameter of 200 nm.
The top surface of samples was carefully
Carbonand mechanically contacted with noStainless-steel
particles
substantial air gaps. The samples were
container
placed a moving stage for laser irradiation.
During samples moving, the samples wereExperimental steps
forinfrared
irradiated with 976 nm infrared Fig.1.
semiconductor line beam laser from the semiconductor laser annealing using carbon
rear side of the transparent substrates. Line particles
beam laser was focused at the top surfaceof the samples with a 500-μm-width and 15-μm-length. The laser dwell time was given by alaser beam length divided by the sample velocity. It ranged from 3 ms to 15 μs. Carbon
particles absorbed a laser light and were heated themselves. The heating energy propagated tothe samples and silicon films were heated. There must be small air gaps among carbon particles.
If thermal diffusion length in air during laser heat duration for example 100 μs is much larger
than the gaps, silicon films would be heated uniformly. The thermal diffusion length wasestimated 47μm in the case of laser irradiation for 100 μs and thermal diffusion coefficient of
air of 0.217 cm2/s [5]. We believe that there is a possibility of uniform heating andcrystallization of silicon films using carbon particles when they are closed packed. Carbon
particles can effectively absorb the infrared light because the optical reflectivity at the surfaceof carbon surface was low about 10% due to a low refractive index [6]. Moreover, there is lightscattering in the case of carbon particles. However, there was a reflection loss at the quartz
glass/Si layer interface when the laser light was irradiated from the rear side. Because thereflectivity was 35% at 976 nm for quartz glass/40-nm-a-Si layer, we interpret the opticalabsorption efficiency was about 60% for the present experimental system.

In order to estimate temperature increase of carbon particles by laser heating, black bodyradiation was observed using multichannel spectrometer from 250 to 800 nm, as shown byinsert in Fig. 2. Quartz substrates were placed on carbon particles. Laser beam was irradiated at
a dwell time of 50 μs by sample moving and at different laser powers ranging from 8 to 14 W.
light emission from quartz/carbon interfaces were measured using spectrometer during laser
irradiation.

Results and discussions
18

Temperature [K]Emission intensity [a.u.]

Figure 2 shows spectra of light emission at
spectrometer
16
14W 3000K
dwell time of 50 μs and at laser power of 8
14
laser
radiationand 14 W. The sensitivity of the detector
light12
was calibrated. The emission showed aquartz10
broad spectra and it intensity increased as
8
the wavelength increased. It was also
8W 2865K
increased as the laser power increased. The
6emission spectra was analyzed by black
4
experimental
body radiation model using Planck’s
2
calculated-5
radiation equation, I=8πhcλ {exp(hc/λ0kT)-1}-1, where h is Planck’s constant, c is
200 300 400 500 600 700 800
the light velocity, λ is the wavelength, k is
Wavelength [nm]the Boltzmann’s constant and T istemperature. The emissivity of carbon Fig.2. Spectra of light emission at dwell time
particles was supposed to be 1. Calculated of 50 μs and at laser power of 8 and 14 W
spectra reasonably explained characteristics
of experimental spectra. The fitting
3100
calculated spectra to experimental spectra
3050
gave temperature each laser irradiation
condition, as shown in Fig. 2. Figure 3
3000
summarizes temperature of carbon particles
as a function of laser power at dwell time of2950
50 μs. The temperature increased from 2865
to 3000 K as laser power increased from 8 to2900
14 W. It is important that laser heating of
2850
carbon particles at temperature higher thanthe melting point of silicon was achieved.
2800
Quartz substrates were not broken by laser
8
9
10
11
12
13
14
irradiation. They kept the smooth surface
Laser power [W]after laser heating.
Figure 4 shows Raman scattering spectra of Fig.3. Temperature of carbon particles as a
40-nm-thick silicon films irradiated for function of laser power at dwell time of 50 μs
dwell time 50 μs and at different laser
powers ranging from 11 to 14 W. The
crystalline silicon phonon peaks were observed around wavenumber of 515 cm-1 in the most ofregion irradiated with laser beam above 12 W. Crystalline silicon peak was observed in only
partial regions at 11 W. We interpret a threshold laser power of crystallization of 12 W in thiscase. The silicon films were crystallized by heat diffusion from carbon particles heated by laser
irradiation. The peak of crystalline silicon increased and the peak of amorphous silicon aroundwavenumber of 480cm-1 decreased as laser power increased from 12 to 14 W. The line shape ofIntensity [a.u.]

crystalline peak was sharp with a band width2000
of 7 cm-1 at 14 W and no amorphous peak
was observed. Surface roughness was
14W
observed in the case of laser irradiation at
1500
14W. It probably resulted from movementof silicon atoms during melting of silicon
13W
films caused by high power laser irradiation.
1000
On the other hand, no surface roughness was
11W
12W
observed below observed in the case of laser
500
irradiation below 13W. We interpret thatsilicon films were probably crystallized in
the solid state.
0
Figure.5 shows CVR distribution in the
400
450
500
550
600
normal direction of laser scanning of silicon-1
Wavenumber [cm ]films at dwell time of 47 μs, at 13 W andrepetition of laser irradiation from 1 and 3
Fig.4. Raman scattering spectra oftimes. The average CVR increased as
40-nm-thick silicon films irradiated for dwell
number of laser scans increased. Moreover
time 50 μs and at different laser powers
fluctuation of CVR became low in the caseranging from 11 to 14 W
of 3 times laser irradiation. The result showscarbon particles played a role of heating
source at every laser irradiation occasion. Total heating duration increased and crystallization
proceeded. We believe that there was a possibility of moving of carbon particles during laser
irradiation because high thermal energy. The results of Fig.5 indicates that the average moving
distance of carbon particles was much smaller than heat diffusion length and that carbon
particle contributed multiple heating of silicon films. There is therefore a possibility that silicon
film with high uniformity of CVR can be achieved by overlapping irradiation.
1
1 scan
CVR

0.8
0.6
.

0.4
0.2
0

3 scans
CVR

0.8
0.6
.

0.4
0.2
0
0

50

100

150

200

250 300 350
Position [μm]

400

450

500

550

Fig.5. CVR distribution in the normal direction of laser scanning of silicon films at dwell
time of 47 μs, at laser power of 13 W and repetition of laser irradiation from 1 and 3 times

Summary
Rapid thermal crystallization of silicon films by CW infrared semiconductor laser irradiation
using carbon particles as photoabsorption layer was investigated. The photoabsorption layers
were fabricated by stainless-steel container filled with carbon particles with a mean diameter of
200 nm. Quartz substrates were placed on carbon particles and irradiated with 976-nm infrared
semiconductor line beam laser at dwell time of 50 μs and at different laser powers ranging from
8 to 14 W. During laser irradiation, spectra of light emission were observed using multichannel
spectrometer from 250 to 800 nm. Temperature of carbon particles were estimated by fitting
spectra. The temperature increased from 2865 to 3000 K as laser power increased from 8 to 14
W. 40-nm-thick undoped hydrogenated amorphous silicon (a-Si:H) films were formed onquartz glass substrates by plasma enhanced chemical vapor deposition. 200-nm-thick SiOx films
were subsequently formed on the a-Si:H films using Polysilazane precursor heated at 260℃
with 1.3x106 Pa H2O vapor for 3h. The top surface of samples was contacted to carbon particles.
The samples were irradiated by laser from the rear side of the transparent substrates. Raman
scattering spectra were measured in order to investigate crystallization of silicon films.
Crystallization of silicon films were confirmed at dwell time of 50 μs and at laser power of 12
W. Crystalline volume ratio (CVR) increased as laser power increased. CVR of 1 was achievedat dwell time of 50 μs and laser power of 14 W. The average CVR increased and fluctuation of
CVR became low as number of laser scans increased. This result shows carbon particles played
a role of heating source at every laser irradiation occasion.

Acknowledgement
This work was partially supported by CASIO Science Promotion Foundation.
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panel Displays and Devices (2006) p329.
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[5]The Physical society of Japan, Physics data dictionary, Asakura Publishing Co.Ltd.
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