Shuntaro Fujii

Analysis of Continuous-Wave Laser Lateral Crystallized Polycrystalline Silicon Thin Films with Large Tensile Strain

Randomly oriented polycrystalline silicon (poly-Si) thin films with a typical grain size of 20×2 µm2 grown by continuous-wave laser lateral crystallization (CLC) were obtained. It was found that CLC poly-Si thin films have a large tensile strain corresponding to 0.6% of single-crystalline Si lattice in the in-plane direction. These results suggest that there is a possibility that not only grain size but also the large tensile strain in in-plane direction can affect thin film transistor (TFT) performance.

Enlargement of Crystal Grains in Thin Silicon Films by Continuous-Wave Laser Irradiation

The cw laser recrystallization of amorphous silicon (a-Si) thin films deposited on SiO2 films with Si(100) substrates was investigated by controlling irradiation conditions. The effects of the laser spot shape and laser-irradiated region overlap were found important resulting in two dimensional crystal growth different from conventional lateral crystallization. A laser beam spot was designed to realize a gradual temperature slope in the laser-irradiated region. The crystal grain growth of the Si thin films was effectively enhanced. Raster scanning was performed to make the crystal growth direction regular and effectively enhanced the crystallization of the recrystallized Si thin films. The laser-irradiated region overlap also effectively enhanced the crystallization of the recrystallized Si thin films. Consequently, (100) well-oriented polycrystalline Si thin films with an average grain size of 1.60 µm, which is about 5 times larger than that obtained by conventional excimer laser recrystallization, were obtained.

Strain-Induced Back Channel Electron Mobility Enhancement in Polycrystalline Silicon Thin-Film Transistors Fabricated by Continuous-Wave Laser Lateral Crystallization

Four-terminal (4T) polycrystalline silicon (poly-Si) thin-film transistors (TFTs) having both front and back gates were fabricated to investigate the effect of the internal tensile strain induced by continuous-wave (CW) laser lateral crystallization (CLC) on the carrier mobility. The tensile strain values at the surfaces and back interfaces were estimated to be approximately 0.3% and over 0.4%, respectively. In both front and back channel operations, the successful operation of a variable threshold voltage (Vth) scheme was confirmed. Front and back channel effective electron mobilities of 4T CLC poly-Si TFTs were evaluated under bias conditions so as not to form an inversion layer on the Vth-control gate side. Because of the larger tensile strain at the back interface, the back channel effective electron mobility was 1.2 times larger than the front channel effective mobility.

Roughness Reduction in Polycrystalline Silicon Thin Films Formed by Continuous-Wave Laser Lateral Crystallization with Cap SiO2 Thin Films

In continuous-wave (cw) laser lateral crystallization of amorphous silicon (a-Si) thin films, the effects of cap SiO2 thin films were investigated. The thickness of the cap SiO2 film was 10 nm, which is sufficiently thin to exclude anti reflection effect. The cap SiO2 thin films suppressed the generation of voids during the cw laser crystallization, and the available crystallization condition to form lateral-crystallized polycrystalline silicon (poly-Si) thin films was expanded. By using the cap SiO2 thin films, the surface of the lateral-crystallized poly-Si thin films became smooth, and an average roughness of 1.3 nm was achieved.

Tri-Gate Polycrystalline Silicon Thin-Film Transistors Fabricated by Continuous-Wave Laser Lateral Crystallization with Improved Electron Transport Properties

Tri-gate channel structures were applied to polycrystalline silicon (poly-Si) thin-film transistors (TFTs) fabricated by continuous-wave (CW) laser lateral crystallization (CLC). We had two objectives in using tri-gate structures in CLC poly-Si TFTs. One was the enhancement of effective electron mobility (µeff) by using the tensile strain induced by the CLC process and the lateral-strain-relaxation effect in tri-gate structures. The other was the reduction of µeff variation caused by increasing the number of surfaces with different crystal orientations by up to a factor of three. By applying tri-gate structures to CLC poly-Si TFTs, both 8% µeff enhancement and 41% reduction of µeff variation were achieved at the surface carrier density of 5×1012 cm-2. These results are expected to be useful for the device size shrinkage of high-performance poly-Si TFT circuits.

In-Plane Grain Orientation Alignment of Polycrystalline Silicon Films by Normal and Oblique-Angle Ion Implantations

Random crystallographic orientations of polycrystalline silicon (poly-Si) grains in the films grown on a SiO2 substrate by chemical vapor deposition were laterally aligned by maintaining the 110 restricted pillar texture through double Si+ self-ion implantations. The in-plane X-ray diffraction pattern and rocking curve clearly indicate the lateral alignment. The oblique-angle Si+ self-ion implantation was also found to be useful for increasing the amount of the 110 pillar texture. The electron backscatter diffraction (EBSD) pattern supports the increase in the amount of the 110 pillar texture and the lateral crystal orientation alignment. The transmission electron micrography and EBSD results also suggest that grain size is increased by double Si+ self-ion implantations. Although further systematic optimization may be required, the technique will be useful for improving the electrical characteristics of poly-Si devices for future electronic systems on insulators.

Self-seeding Crystallization of Silicon Thin Films Using Continuous-Wave Laser

Low temperature crystallization of amorphous silicon thin films using continuous-wave laser was investigated. An overlap region of laser irradiation was designed for a Si crystal nucleation. From the silicon nucleation region, lateral-crystallization along laser spot grew. Consequently, (111) well-oriented silicon thin films were obtained. Low-temperature polycrystalline-silicon (LTPS) thin film transistors (TFTs) are used in active-matrix liquid crystal display technologies, and now are developed into key technology for new applications, such as paper-like displays. Diode-pumped solid state (DPSS) continuous wave (CW) laser lateral crystallization is one of LTPS technologies, and achieved one dimensional long and narrow grain [1]. The Purpose of this work is to form two dimensional large grain of Si thin films. Overlap effect of laser irradiation on Si crystal orientation was investigated. Figure 1 shows a schematic of the sample used for the laser crystallization experiments. An amorphous silicon (a-Si) thin film was deposited by LPCVD method using SiH4 on a plasma-CVD SiO2 (800 nm) / thermal SiO2 (100 nm) / Si(100) substrate. The thickness of the aSi film was 150 nm. After the a-Si film deposition, a cap SiO2 film was deposited by plasma-CVD. The purpose of the cap SiO2 deposition is anti-reflection for laser irradiation. The DPSS-CW laser with λ= 532 nm was used for silicon melting. Laser power was varied from 5.0 to 10.0 W. Scanning speed was also varied from 1 to 100 cm/s. Figure 2 shows the schematic diagram of laser scanning. The width of Si melted region at 7.0 W laser power and 10 cm/s speed was 48 μm. The overlap region of laser irradiation was varied from overlap ratio 0 to 90 %, and orientation of Si films was investigated by XRay Diffraction (XRD). The optical constants for Si thin films were measured by spectroscopic ellipsometry with TaucLorentz calculation. A refractive index of the a-Si thin film was 4.70. After the laser annealing, the refractive index decreased to 4.139. A conventional poly-Si film deposited by LPCVD and a conventional Si single crystal had refractive index 4.189 and 4.134, respectively and then the refractive index after laser annealing was smaller than that of a conventional poly-Si film. Optical adsorption coefficient at λ= 532 nm was also measured. Adsorption coefficient (imaginary part of complex refractive index) of the a-Si film was 0.574, and the a-Si film has high optical adsorption for 532 nm, which means the a-Si film melts easily by the 532 nm CW laser. On the other hand, after the laser annealing, adsorption coefficient of a re-crystallized Si film was zero, therefore a re-crystallized Si film was transparent to 532 nm laser light. The orientation of Si crystal was measured by XRD. X-ray source was 0.154 nm Cu Kα. Figure 3 shows the result of XRD measurement. Before laser annealing, the a-Si film had no orientation of Si crystal. After the laser annealing, orientations of Si crystal were observed. The diffraction peaks of Si Miller indices (111), (220) and (311) appeared in 2theta angle 28.48 °, 47.40 ° and 56.28 °, respectively. The peak intensity depended on the overlap ratio. From the overlap ratio 0 to 40 %, the (220) orientation increased. This means that crystallization of the Si films grew. Above 40 % overlap ratio, the (220) orientation decreased, and (111) orientation increased. Consequently, the (111) well –oriented Si thin film was developed, and the (111)/(220) diffraction intensity ratio reached 17 as shown in Fig. 4. A part of this work was supported by Special Coordination Fund for Promoting Science and Technology of Ministry of Education, Culture, Sports, Science and Technology (MEXT). [1] A. Hara, M. Takei, F. Takeuchi, K. Suga, K. Yoshino, M. Chiba, T. Kakehi, Y. Sano, and N. Sasaki, Jpn. J. Appl. Phys.,43(4A) ,1820-1824(2004).