In the production process of thin film transistors (TFTs), many researchers have discussed about crystallization of an amorphous silicon (a-Si) film by rapid thermal annealing for providing the polycrystalline silicon (poly-Si) film at a low temperature. The a-Si film was deposited on the glass substrate at a low temperature and annealed by rapid thermal annealing. It was important to reveal the mechanism of a silicon melt-solidification and grain growth for providing a poly-Si film having high crystallinity. So, they have proposed various measurement methods for finding the important information which is a lattice temperature, time-resolved electrical-conductance and optical-reflectance, and so on [1,2].

We have proposed our own analysis which is a high-speed camera (HSC) observation in real time in microsecond scale and a contactless temperature measurement of a substrate surface during an atmospheric pressure thermal plasma jet (TPJ) annealing [3,4]. We first disclosed leading wave crystallization (LWC) on the a-Si film during TPJ irradiation because of this HSC observation and revealed the two-dimensional temperature distribution for the surface of a glass or quartz substrate. However, we can’t obtain their information at the same time.

Here, we demonstrate a temperature distribution of the molten region on a-Si film when we simultaneously use the contactless temperature measurement and the HSC observation in a microsecond scale.

Amorphous silicon film with the thickness of 100 nm was formed on a 100-µm-thick flexible glass substrates by plasma-enhanced chemical vapor deposition at 250 °C and was dehydrated in N₂ ambient at 450 °C. We carried out the temperature measurement and real-time observation of the molten silicon region by the contactless temperature measurement and the HSC observation when the glass substrate and the prepared film on the substrate was rapidly annealed by TPJ irradiation.

In the result of the real-time reflectance by the contactless temperature measurement, we obtained a time resolved temperature on the surface of a glass substrate and a TPJ thermal profile by our own simulation analysis. Also, we got the three-dimensional temperature distribution from the obtained thermal information. We could get a cross-sectional image from the three-dimensional temperature distribution by cutting this distribution across the plane of the 1700 K parallel to a horizontal direction. We obtained isothermal lines from the cross-sectional image. We presumed that these isothermal lines correspond to the molten region on the a-Si film in the HSC image. So, we tried to superimpose the isothermal lines on the HSC image.

The black area in the HSC image is the molten silicon region and this area is in the width of ~0.43 mm and length of ~1.13 mm. TPJ irradiation at 1 m/s was scanned from right to left of this image. The crystallization area is located at the rear of TPJ irradiation. The high speed lateral crystallization (HSLC) which is dendrite crystal growth appears in this area. We carried out the overlap of the HSC image and the isothermal lines with the edge of the molten region and the isothermal line of 1700 K and recognized that these images most closely matched as shown in the figure. We found that there is a liquid area even though the temperature is lower than the melting point. Also, it was revealed that this area is supercooling area and supercooling degree is ~100 K. Also, it was obvious that the crystal growth direction didn’t exactly align parallel to the direction of a temperature gradient.

In this paper, we clarified that the temperature distribution in the molten region of the a-Si film can be visualized by the proposed method. This is a quite powerful tool to understand the crystalline growth in microsecond time regime.

Reference:

[1] G.J. Galvin, et al., Phys. Rev. B 27 (1983) 1079.

[2] B.C. Larson, et al., Appl. Phys. Lett. 42 (1983) 282.

[3] S. Hayashi, et al., Appl. Phys. Lett. 101 (2012) 172111.

[4] T. Okada, et al., Jpn. J. Appl. Phys., 45 (2006) 4355.