Chemical mechanical polishing of CW laser recrystallized Si thin films
with ethyl alcohol additive
Shin-Ichiro Kuroki, Masayuki Numata, Shuntaro Fujii,
Koji Kotani and Takashi Ito
Graduate School of Engineering, Tohoku University,
6-6-05 Aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8579.
E-mail : kuroki@ecei.tohoku.ac.jp

Keywords: CMP, polycrystalline silicon, laser crystallization
Abstract. The chemical mechanical polishing (CMP) on laser-crystallized poly-Si thin films
was carried out. After CMP with conventional colloidal silica slurry, the surface oflaser-crystallized poly-Si thin films became rougher than that of before CMP. Carbon
segregation in the grain boundary was occurred by the laser crystallization. It was consideredthat carbon segregation hardened the grain boundary, and lowered the wettability to the slurry.
For the improvement of this problem, a hydrophilic organic solution was added to conventionalslurry. The surface of the poly-Si thin film was coated with the organic additive, and thiscoating suppressed a polishing of concave portion, and increased the wettability of convex
portion in the grain boundary, and the pressure from the polishing pad was concentrated to theconvex portion. In this work, ethyl alcohol was used as a hydrophilic organic solution, ethylalcohol was soluble in the slurry and was stable in the room temperature, and planarized surface
with the average roughness of 1.13 nm was obtained.
Introduction
For the enhancement of the TFT performance, laser crystallization on silicon thin films has
been carried out. The laser-crystallized poly-Si thin films have a large surface roughness. Thelarge surface roughness restricts the thickness of the gate oxide of the TFTs, and lowers thecarrier mobility by the surface roughness scattering [1]. The CMP of LPCVD poly-Si thin films
was reported, and the dishing was inhibited by polymer additive [2]. In this work, continuouswave (CW) laser was applied to the laser crystallization [3-4], and the CMP of thelaser-crystallized poly-Si thin film was performed.
Experiments
A buffer SiO2 film was formed on a quartz substrate by PECVD. The thickness of the buffer
SiO2 was 1 µm. After the deposition of buffer SiO2, a non-doped amorphous silicon (a-Si) thinfilm was deposited by PECVD at the thickness of 150 nm. Thermal treatment on a-Si thin film
was performed in N2 ambient at 490°C for 20 minutes. After the thermal treatment, CW green
laser crystallization of a-Si thin film was carried out. The wavelength of the laser was 532 nm,and laser power was 10 W. Laser spot was elliptic form and had Gaussian profile withdimensions of 20 ×90 µm2 (FWHM). The sample wafer was placed on an X-Y linear stage, anda laser beam scanned over sample wafer. The scan speed was 30 cm/s. After the laser
crystallization, the CMP on the laser-crystallized Poly-Si thin films was carried out. In the CMP
process, NALCO2372 silicon slurry (Nitta-Haas Inc., average diameter of colloidal silica: 70nm, pH: 12) and a poly-urethane polishing pad (IC1000) were used. The silicon slurry wasRa= 3.25 nm
P-V=40.0 nm(a) Before CMPABA

0Ra= 5.83 nm
P-V=55.5 nm

50
D（µm）

0(b) CMP without EtOHCB（nm）

50C

20EtOH concentration: 0 %Polishing Time: 20 secResidueD（nm）

diluted by deionized water and theratio of the silicon slurry was 5.0%.
Ethyl alcohol (EtOH) was added to thediluted slurry. The ratio of ethyl
alcohol to the diluted slurry was varied
from 0 to 50%. The polishing pressure
was 6.7 kPa, and the rotation of thepressurized wafer stage and platen
were 15 and 10 rpm, respectively.
After the CMP process, the sample
wafer was cleaned by SPM(H2SO4:H2O2=4:1) for 5 min, andBuffered HF (Stella-Chemifa LAL800,
NH4HF2: 11.4%) for 2 min.

00（µm）

20

Fig.1. AFM images of laser crystallized Si thinfilms (a) before CMP, and (b) after CMP.

Results and Discussions
Figure 1 shows the surface morphologies of laser-crystallized poly-Si thin films before andafter the CMP without ethyl alcohol additive. These surface morphologies were measured byatomic force microscopy (AFM). Before the CMP process, the laser-crystallized poly-Si thinfilm had a surface morphology with the average roughness (Ra) and the peak-valley (P-V)
roughness of 5.83 nm and 40.0 nm, respectively. After the CMP process, the average roughnessand the P-V roughness increased to 5.83 nm and 55.5 nm, respectively. Especially, anomalous
residues at grain boundaries were observed as shown in Fig 1. (b). For the purpose ofexamination of the CMP residues, the transmission electron microscope (TEM) measurements
and the energy dispersive X-ray spectrometry (EDX) were carried out. Figure 2 shows the(a)EDX measuring point
Laser crystallized
Si thin film
100 nmGrain Boundary(b)GrainBoundary

Horizontal Direction

0.15
0.15
C/Si

0.1
0.1
0.05
0.0500-400-400

O/Si-200-20000200200Distance（nm）

400400600600

0.250.25

EDX Intensity (a. u.)

EDX Intensity (a. u.)

0.20.2SiO2Depth Direction(c)

0.2
0.2

0.15
0.15

C/Si

0.1
0.1
O/Si

0.05
0.050000

5050

100100150150200200

Distance from substrate（nm）

Fig.2. EDX measurement of laser crystallized Si thin films: (a) cross-sectional TEM of laser
crystallized Si thin films, (b) EDX distribution in the horizontal direction and (c) in the depth
direction.

（nm）

Peak-Valley Roughness （nm）

Average Roughness Ra （nm）

Average Roughness Ra （nm）

5.0results of the TEM-EDX measurements.
5.0
Platen rotation ： 10 rpm
80 secHead rotation ： 15 rpm
4.5
Figure 2 (a) shows the TEM micrographs andPressure： 6.7 kPa
4.0
4.0
120 sec
the measuring points of the EDX. Figure 2 (b)3.5 Beforeand (c) show carbon and oxygen distributions
3.0 CMP
3.0
in the horizontal and depth direction in the2.5
laser-crystallized poly-Si film, respectively. In
2.0
2.0
Fig. 2 (b) and (c), the background noise level
1.5
was 0.02. In this measurement, carbon was1.01.0102030405060
observed near the grain boundary. This result
00102030405060
EtOH concentration （%）
shows the carbon segregation in the grainboundary was occurred. It was considered thatFig.3. Average Roughness as a function of ethylcarbon segregation hardened the grain
alcohol concentration.
boundary, and lowered the wettability to the4.0
4.0
Platen rotation： 10 rpm
slurry.
Head rotation： 15 rpm
3.5
Pressure： 6.7 kPa
Ethyl alcohol was added to the slurry. It
3.0
3.0
EtOH concentration ： 35%was considered that the surface of the poly-Si
2.5
2.02.0thin film was coated with ethyl alcohol, and1.5
this coating suppressed a polishing of concave
1.01.0portion, and increased the wettability of0.5
convex portion in the grain boundary, and the000.0
pressure from the polishing pad was
50100150200
250
050100150
250
Polishing Time（sec）
concentrated to the convex portion. The ratioof ethyl alcohol to the diluted slurry wasFig.4. Average Roughness (Ra) as a function
varied from 0 to 50%. Figure 3 shows theof polishing time.
average roughness of the laser-crystallized
poly-Si thin films after the CMP with ethyl
45
Platen rotation： 10 rpm
alcohol. As increasing ethyl alcohol additive,
Head rotation： 15 rpm
40Pressure： 6.7 kPa
35EtOH concentration ： 35%
the average roughness became smaller, and at30
the polishing time of 80 and 120 s average
25roughnesses had minimal values at ethyl
2020
alcohol concentration of 25 and 35%,1515
respectively.
105Figure 4 shows the average roughness of00
laser-crystallized poly-Si thin films as a050
100
15020050100200250
Polishing Time （sec）
function of polishing time at the ethyl alcohol
concentration of 35%. As increasing polishingFig.5. Peak-valley roughness as a function oftime, an average roughness decreased, andpolishing time.
took a minimal value of 1.13 nm at
EtOH concentration: 35%
Ra= 1.13 nm
CMP with EtOH
160 s. Figure 5 shows the P-V
P-V=19.9 nm
Polishing Time: 160 sec
roughness as a function of polishing
EFE50time. As increasing polishing time, aFP-V roughness also decreased, and0
took values of 19.9 nm and 12.4 nm at0（µm）
20160 and 200 s, respectively. Figure 6Fig.6. AFM image of laser crystallized Si thin film aftershows the surface morphology of thethe CMP with ethyl alcohol additive.
laser-crystallized poly-Si thin film
after CMP with ethyl alcohol additive. Figure 7 shows the TEM micrograph of thelaser-crystallized poly-Si thin film after CMP with ethyl alcohol additive. The TEM micrographof the poly-Si thin film shows the surface was planarized by the CMP with ethyl alcoholadditive, and the thickness of the poly-Si thin film varied only slightly. It was considered thatG.B
Si
SiO2

G.B

G.B

G.B
200 nm

Fig.7. Cross-sectional TEM image of laser crystallized Si thin film after the CMP with ethyl alcohol
additive.

ethyl alcohol was adsorbed at the surface of the poly-Si thin films, and the adsorbed layer
inhibited alkali etching of the poly-Si thin films. In the convex portion at the grain boundaries,
the ethyl alcohol adsorbed layer was removed by the colloidal silica, and the CMP wasperformed.
Summary
The CMP of the CW laser-crystallized poly-Si thin films was carried out. Before the CMP,
the laser-crystallized poly-Si thin film had the average and P-V roughness of 3.25 nm and 40.0nm, respectively. After the CMP with ethyl alcohol additive, the laser-crystallized poly-Si thinfilms were planarized, and the average and P-V roughness were 1.13 nm and 19.9 nm,
respectively. It was considered that the ethyl alcohol was adsorbed at the surface of the poly-Si
thin films, and in the convex portion at the grain boundaries the CMP was selectivelyperformed.
Acknowledgments
A part of this work was supported by Special Coordination Fund for Promoting Science andTechnology of Ministry of Education, Culture, Sports, Science and Technology (MEXT). Theauthors would like to thank Dr. A. Ishikawa of Nikon Corp. and T. Miyazaki of Tohoku Univ.
for their useful discussions and helpful supports.
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