Laser Crystallization Produces High-Quality TFTs
Brian Dance, Contributing Editor -- Semiconductor International, 2/1/2001
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The excimer laser crystallization of amorphous silicon on glass has been tested by irradiating each part of the silicon film with many excimer laser pulses. TFTs of the highest electron mobility reported so far (640 cm2/Vsec) have been obtained by the deposition and crystallization of the material in an ultra-high vacuum. However, the grain size in the polycrystalline silicon layer is dependent on the excimer laser's local energy density, and therefore the electrical properties of the deposited material vary widely.
Sequential lateral solidification (SLS) may be used to decrease grain-size dependence on local energy density, since it can be employed to reduce the number of grain boundaries in the material. This method can significantly improve the electrical properties of the deposited film. In the past, only excimer lasers had been used to perform the SLS technique, but the crystallization process is slow because the maximum repetition rate of excimer lasers is ~300 Hz.
A French-German collaboration created TFTs using polycrystalline silicon crystallized by a non-vacuum SLS process based on an all-solid-state laser system. Ralf Dassow and associates from the Institut für Physikalische Elektronik at the University of Stuttgart (Germany) have developed an SLS process that can yield polycrystalline silicon with grains longer than the channel length of the TFTs.Y. Helen and researchers of the Groupe de Microélectronique et Visualisation at the University of Rennes (France) processed TFTs using these films.
The crystallization system employed by the German group uses a frequency-doubled, diode-pumped Nd:YVO4 laser. The repetition frequency can be varied over a range of 1-100 kHz. The 150 nm thick undoped amorphous silicon film was deposited by low-pressure chemical vapor deposition on glass substrates. The sample was scanned by a translation stage moving in the x direction at a maximum velocity of 10 mm/sec. After each scan, the sample was moved 130 µm in the y direction.
Each 18.5 µJlaser pulse completely melted an elliptical area of the silicon film some 5 µm in the x direction and 200 µm in the y direction. After the 10 nsec laser pulse was applied, heat diffusion into the substrate resulted in the cooling and recrystallization of the melt to form polycrystalline silicon with a grain size dependent on the position within the molten area. Fine-grained material is produced at the center of the melt. As it cools, crystallites grow laterally from each edge of the completely molten area into the center of this area. Crystallites obtained in one step act as seeds in the next crystallization step. Thus, they grow in the x direction. Repeated steps of laser-induced melting and translation produce grains with a length far exceeding their lateral growth length. The lateral growth is stopped at a length of ~1 µm by the homogeneous nucleation-induced growth of crystallites in the heavily under-cooled liquid silicon.
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The French group produced n-channel TFTs using these polycrystalline silicon films. The characteristic (Figure) was obtained using a TFT in which the channel was formed in the same direction as that used for the laser scanning during crystallization. The minimum leakage current per unit channel width is about 5 × 10-13 A/µm — more than an order of magnitude greater than the leakage current of TFT devices produced by the excimer laser crystallization of 40 nm thick silicon films. Hydrogen passivation could be employed to reduce the leakage current. The use of the SLS process increases the grain length from about 1 µm to more than the 25 µm channel length. The long grains increase the mobility from about 150 to 470 cm2/Vsec, and the homogeneity of the films decreases the standard deviation of the mobility to less than ±5%.
"We calculate that our process could attain a maximum crystallization rate of 35 cm2 per second and with a pulse repetition frequency of 100 kHz," Dassow said. "Commercially available high-power, solid-state laser systems should make it possible to outperform traditional crystallization using excimer laser systems."
