Analysis of μ-Czochralski Technique Using Two-Dimensional Crystallization Simulator

doi:10.4236/jcpt.2012.21002

Paper Menu >>

Journal Menu >>

Journal of Crystallization Process and Technology, 2012, 2, 12-15

http://dx.doi.org/10.4236/jcpt.2012.21002 Published Online January 2012 (http://www.SciRP.org/journal/jcpt)

Analysis of μ-Czochralski Technique

Using Two-Dimensional Crystallization Simulator

Kuniaki Matsuki1, Ryusuke Saito1, Shuji Tsukamoto1, Mutsumi Kimura1,2, Ryoichi Ishihara3

1Department of Electronics and Informatics, Ryukoku University, Otsu, Japan; 2Joint Research Center for Science and Technology,

Ryukoku University, Otsu, Japan; 3Delft Institute of Microelectronics and Submicrontechnology, Delft University of Technology,

Delft, Netherlands.

Email: [email protected]

Received December 4th, 2011; revised January 5th, 2012; accepted January 16th, 2012

ABSTRACT

-Czochralski technique has been analyzed using two-dimensional crystallization simulator. It is observed that the

temperature is relatively uniform in the entire Si region after the laser irradiation because the heat conductivity of the Si

region is much higher than that of the underneath SiO2. Grain growth advances from the grain filter to the channel re-

gion and continues until it collides with what advances from random nucleation in the channel region. When the initial

temperature is high, the random nucleation rarely occurs even under the supercooling condition, and the grain size be-

comes large. Moreover, it is qualitatively reproduced that the grain size increases as the irradiated energy of the laser

irradiation increases.

Keywords: -Czochralski Technique; Two Dimensional Crystallization Simulator; Grain Growth; Grain Filter;

Grain Size

1. Introduction

-Czochralski technique is a crystallization technique to

enlarge poly-Si grains in thin-film transistors (TFTs) not

only for flat panel displays and but also for general elec-

tronics [1,2]. In the -Czochralski technique, grain filters

are bored in underneath SiO2 films, amorphous-Si films

are deposited and filled into the grain filters, excimer laser

are irradiated to the amorphous-Si films, and grain growth

advances from the grain filter and to the channel regions.

Although the -Czochralski technique has been experi-

mentally analyzed in detail [3-6], the crystallization pro-

cess should be theoretically clarified.

Recently, a two-dimensional (2-D) crystallization simu-

lator has been developed and proposed as a practical eva-

luation tool for poly-Si TFTs [7]. In the 2-D crystallization

simulator, random nucleation, crystal growth velocity, la-

tent heat emission, and partial crystallization are modeled.

In this paper, we analyze the -Czochralski technique

using the 2-D crystallization simulator. We evaluate the

temperature, grain growth, nucleation, etc. We try to repro-

duce the dependence of the grain size on the irradiated

energy of the laser irradiation.

2. 2-D Crystallization Simulator

The 2-D crystallization simulator is minutely explained

in a previous paper [7]. Roughly speaking, the nucleation

rate is defined as a function of temperature following a

classical nucleation theory, and the crystal growth veloc-

ity is also defined as a function of temperature following

a classical crystal growth theory. The partial crystalliza-

tion model is used allowing the co-existence of the liquid

and crystal phases even in a finite element. The simulation

algorithm is composed of the phase transition and heat

transfer algorithms. The thermal properties of Si and SiO2

are listed in the previous paper.

The grain filters are located in underneath SiO2 films,

and grain seeds are put at the bottoms of the grain filters.

The depth and diameter of the grain filters are 250 nm and

50 nm. The grain seeds consist of fine grains formed dur-

ing explosive crystallization at the beginning of the laser

irradiation [3]. The channel regions are melt to the liquid-

Si, and initial temperature of the liquid-Si is varied. The

thickness of the channel regions is 250 nm. The nuclea-

tion, grain growth, and temperature are successively cal-

culated with the time after the laser irradiation. The cal-

culation area is more than 2.5 m. It should be noted that

the initial temperature is set instead of giving the irradi-

ated energy of the laser irradiation. The initial tempera-

ture is not the temperature at the laser irradiation, but the

temperature at the start of the crystallization simulation.

3. Simulation Results

The distribution of the temperature in the grain filter,

Analysis of μ-Czochralski Technique Using Two-Dimensional Crystallization Simulator 13

channel region, and underneath SiO2 is shown in Figure

1. Here, the initial temperature is 1800 K, and the time after

the laser irradiation is 80 ns. It is observed that the tem-

perature is relatively uniform in the entire Si region. This

is because the heat conductivity of the Si region, 0.25

WK–1·cm–1, is much higher than that of the underneath

SiO2, 0.014 WK–1·cm–1. As seen in Figure 1, the tempera-

ture at the top of the grain filter is slightly high owing to

the latent heat released during the grain growth from the

grain filter. Moreover, the temperature at the right side in

the calculation area is also slightly high owing to the latent

heat released during the grain growth from the random

nucleation, as seen in Figure 2(b).

The advance of the grain growth in the -Czochralski

process is shown in Figure 2. Here, the initial temperature

is 1800 K, and the time after the laser irradiation is 40 -

200 ns. It is observed that the grain growth advances from

the grain filter to the channel region. The random nuclea-

tion occurs, as seen in Figures 2(b) and (c), and the grain

growth continues until it collides with what advances from

random nucleation in the channel region, as seen in Fig-

ures 2(c) and (d). As a result, the center grain from the

grain filter is surrounded by the many grains from random

nucleation, as seen in Figure 2(d).

Figure 1. Distribution of the temperature in the grain filter, channel region, and underneath SiO2.

Figure 2. Advance of the grain growth in the -Czochralski process.

Analysis of μ-Czochralski Technique Using Two-Dimensional Crystallization Simulator

The dependence of the grain growth on the initial tem-

perature is shown in Figure 3. Here, the initial tempera-

ture is varied from 1700 K to 2200 K. The grain size is

small for the initial temperatures of 1700 K and 1800 K,

as seen in Figures 3(a) and (b). This is because the ran-

dom nucleation occurs in the channel region during the

grain growth from the grain filter. On the other hand, the

grain size increases for the initial temperature more than

1900 K as the initial temperature increases, as seen in Fi-

gures 3(c)-(e). This is because the grain growth advances

from the grain filter while the temperature in the channel

region is high and the Si stays liquid-Si. Since the nuclea-

tion rate is small considering that the time scales of the

laser irradiation and following cooling process are short,

the random nucleation rarely occurs even under the su-

percooling condition, and the grain size becomes large.

The dependence of the grain size on the initial tempe-

rature is shown in Figure 4. Here, the initial temperature

is varied from 1700 K to 2400 K. It is qualitatively repro-

duced that the grain size increases as the initial tempera-

ture increases, which corresponds to the irradiated energy

of the laser irradiation. This result is consistent with ex-

perimental results [8].

4. Conclusions

-Czochralski technique has been analyzed using 2-D cry-

stallization simulator. It was observed that the tempera-

ture is relatively uniform in the entire liquid-Si after the

laser irradiation because the heat conductivity of the Si

region is much higher than that of the underneath SiO2.

Grain growth advances from the grain filter to the channel

region and continues until it collides with what advances

from random nucleation in the channel region. When the

initial temperature is high, the random nucleation rarely

occurs even under the supercooling condition, and the grain

size becomes large. Moreover, it was qualitatively repro-

duced that the grain size increases as the irradiated energy

of the laser irradiation increases.

It is obvious that the lateral grain growth such as -Czo-

chralski technique cannot be reproduced using one-dimen-

sional crystallization simulator, whereas it is expected that

Figure 3. Dependence of the grain growth on the initial temperature.

Analysis of μ-Czochralski Technique Using Two-Dimensional Crystallization Simulator 15

Figure 4. Dependence of the grain size on the initial tempe-

rature.

it can be reproduced using three-dimensional simulator,

which consumes terribly long computation time. It is mean-

ingful that the lateral grain growth can be at least qualita-

tively reproduced using two-dimensional crystallization,

which consumes acceptable computation time, although a

certain cross section is only considered.

REFERENCES

[1] P. C. van der Wilt, B. D. van Dijk, G. J. Bertens, R. Ishi-

hara and C. I. M. Beenakker, “Formation of Location-

Controlled Crystalline Islands Using Substrate-Embedded

Seeds in Excimer-Laser Crystallization of Silicon Films,”

Applied Physics Letters, Vol. 79, No. 12, 2001, pp. 1819-

1821. doi:10.1063/1.1402641

[2] N. Saputra, M. Danesh, A. Baiano, R. Ishihara, J. R. Long,

N. Karaki and S Inoue, “An Assessment of µ-Czo-chral-

ski, Single-Grain Silicon Thin-Film Transistor Techno-

logy for Large-Area, Sensor and 3-D Electronic Integra-

tion,” IEEE Journal of Solid-State Circuits, Vol. 43, No.

7, 2008, pp. 1563-1576. doi:10.1109/JSSC.2008.922404

[3] R. Ishihara, , P. C. van der Wilt, B. D. van Dijk, A. Burt-

sev, J. W. Metselaar and C. I. M. Beenakker, “Advanced

Excimer-Laser Crystallization Process for Single-Crys-

talline Thin Film Transistors,” Thin Solid Films, Vol. 427,

No. 1-2, 2003, pp. 77-85.

doi:10.1016/S0040-6090(02)01250-6

[4] R. Ishihara, Y. Hiroshima, D. Abe, B. D. van Dijk, P. C.

van der Wilt, S. Higashi, S. Inoue, T. Shimoda, J. W.

Metselaar and C. I. M. Beenakker, “Single-Grain Si TFTs

with ECR-PECVD Gate SiO2,” IEEE Transactions on

Electron Devi ces, Vol. 51, No. 3, 2004, pp. 500-502.

doi:10.1109/TED.2004.823326

[5] V. Rana, R. Ishihara, Y. Hiroshima, D. Abe, S. Inoue, T.

Shimoda, W. Metselaar and K. Beenakker, “Dependence

of Single-Crystalline Si TFT Characteristics on the Chan-

nel Position inside a Location-Controlled Grain,” IEEE

Transactions on Electron Devices, Vol. 52, No. 12, 2005,

pp. 2622-2628. doi:10.1109/TED.2005.859689

[6] R. Ishihara, D. Danciu, F. Tichelaar, M. He, Y. Hiroshima,

S. Inoue, T. Shimoda, J.W. Metselaar and C. I. M. Be-

enakker, “Microstructure Characterization of Location-

Controlled Si-Islands Crystallized by Excimer Laser in

the μ-Czochralski (Grain Filter) Process,” Journal of

Crystal Growth, Vol. 299, No. 2, 2007, pp. 316-321.

doi:10.1016/j.jcrysgro.2006.12.010

[7] K. Matsuki, R. Saito, S. Tsukamoto and M. Kimura,

“Two-Dimensional Simulator of Laser Crystallization for

Polycrystalline-Silicon Thin-Film Transistors,” IEE E Tran-

sactions on Semiconductor Manufacturing, Vol. 24, No. 3,

2011, pp. 472-476. doi:10.1109/TSM.2011.2154368

[8] R. Vikas, R. Ishihara, Y. Hiroshima, D. Abe, S. Inoue, T.

Shimoda, W. Metselaar and K. Beenakker, “Capping

Layer on Thin Si Film for µ-Czochralski Process with

Excimer Laser Crystallization,” Japanese Journal of Ap-

plied Physics, Vol. 45, No. 5B, 2006, pp. 4340-4343.

doi:10.1143/JJAP.45.4340