Akito Hara

High-performance polycrystalline silicon thin film transistors on non-alkali glass produced using continuous wave laser lateral crystallization

We have developed high-performance polycrystalline silicon (poly-Si) thin film transistors (TFTs) with a field-effect mobility of 566 cm2/Vs for n-channel TFT and 200 cm2/Vs for p-channel TFT on 300 mm×300 mm non-alkali glass substrate. The TFTs were developed using a stable diode pumped solid state (DPSS) continuous-wave laser lateral crystallization (CLC) method at a temperature below 450°C. The high performance of the TFTs was attributed to the very large predominantly (100)-oriented grain. This crystallization method will enable high-performance Si-LSI circuits to be fabricated on large non-alkali glass substrates.

High Performance Low Temperature Polycrystalline Silicon Thin Film Transistors on Non-alkaline Glass Produced Using Diode Pumped Solid State Continuous Wave Laser Lateral Crystallization

High performance low temperature polycrystalline silicon (poly-Si) thin film transistors (TFTs) with large grains were created using diode pumped solid state (DPSS) continuous wave (CW) laser lateral crystallization (CLC), employing fabrication processes at 450°C. Field-effect mobilities of 566 cm2/Vs for the n-channel and 200 cm2/Vs for the p-channel were obtained for a thick Si film (100–150 nm) on a 300×300 mm non-alkaline glass substrate. The high performance of the TFTs is attributed to the predominantly (100)-oriented very large grains. With a decreasing Si-film thickness, the grain size decreases, and the surface orientation of the grain changes from (100) to other orientations. These effects lead to reduced field-effect mobility with decreasing Si-film thickness, but it is easy to obtain a high field-effect mobility of over 300 cm2/Vs, even with a 50 nm thick Si film, without special processing techniques. A complementary metal oxide semiconductor (CMOS) ring oscillator was fabricated using a thin Si film 65 nm thick to demonstrate the high circuit performance of CLC poly-Si TFTs by applying the simplest CMOS process technology. A delay of 400 ps/stage at a gate length of 1.5 µm and a supply voltage of Vdd=5.0 (V) was produced on a large non-alkaline glass substrate utilizing a fabrication temperature of 450°C. This crystallization method will lead to the fabrication of high-performance and cheap Si-LSI circuits on large non-alkaline glass substrates.

Electron Spin Resonance of Oxygen-Nitrogen Complex in Silicon

We observed the electron spin resonance of oxygen-nitrogen complexes (ONCs) and found that they have C2V symmetry. Although they contain nitrogen, hyperfine interaction (hf) with nitrogen cannot be clearly observed. These characters of ONCs resemble thermal donors (TDs) very closely.

Selective Single-Crystalline-Silicon Growth at the Pre-defined Active Region of a Thin Film Transistor on Glass by Using Continuous Wave Laser Irradiation

We have developed a new silicon (Si) crystallization method that makes it possible to form single-crystalline Si in the channel regions of thin-film transistors (TFTs) on non-alkali glass without introducing thermal damage. The method includes using a frequency-doubled (2ω) diode-pumped solid-state (DPSS) Nd:YVO4 continuous-wave (CW) laser (λ=532 nm). The unique characteristics of this crystallization method are the introduction of a pre-defined thick capping-Si layer on a pre-patterned channel region and laser irradiation from the back surface through the glass substrate. We succeeded in forming 2-µm-wide and 20-µm-long single-crystalline Si in the channel region of a TFT. A high-performance n-channel TFT on a glass substrate was obtained using a 450°C fabrication process. The TFT had a field-effect mobility of 400 cm2/Vs, a subthreshold swing of 0.16 V/dec, and a threshold voltage of 0.24 V.

Influence of Grown-in Hydrogen on Thermal Donor Formation and Oxygen Precipitation in Czochralski Silicon Crystals

Using hydrogen-enhanced thermal donor formation and hydrogen-enhanced oxygen precipitate nuclei formation, we confirmed the presence of hydrogen in as-grown Czochralski (CZ) silicon (Si) ingots. Hydrogen concentrations in as-grown ingots were very low at 2.5×1011 cm-3. We found that even such a small amount of hydrogen influences the quality of as-grown CZ Si crystals due to hydrogen-enhanced oxygen precipitate nuclei formation caused by in situ annealing during crystal growth. Reducing hydrogen contamination during crystal growth is important in obtaining high-quality CZ Si crystals.

Intrinsic Gettering of Iron Impurities in Silicon Wafers

We present a new experimental approach to using intrinsic gettering to remove Fe impurities. We annealed samples, followed by quenching, and measured the Fe concentration near the surface using deep-level transient spectroscopy. The supersaturation of Fe impurities is necessary for the intrinsic gettering of Fe. However, for a higher supersaturation, Fe impurities precipitate faster than gettering. The optimum degree of supersaturation is one order of magnitude. Gettering is limited by the reaction of Fe with the oxygen precipitates in the defect region, rather than by Fe diffusion to the defect region.

Control of Nucleation and Solidification Direction of Polycrystalline Silicon by Excimer Laser Irradiation

The nucleation site and solidification direction of polycrystalline silicon were controlled by excimer laser crystallization. The sidewall and the top of the amorphous silicon island, which includes a gradually narrowing region, were covered with a thick layer of polycrystalline silicon, and single-shot irradiation was performed from the back surface. The formation of only one nucleus was observed in a gradually narrowing region of width two times that of the lateral growth distance. Solidification from the nucleus toward a narrower width region was then effected in a region 2 µm in width and 3 µm in length. The growth mechanism is explained on the basis of the temperature gradient formed by the covering of the polycrystalline silicon and the gradually narrowing structure.

Phase Variation of Amorphous-Si and Poly-Si Thin Films with Excimer Laser Irradiation

Excimer laser induced crystallization of Si thin films was studied by Raman spectroscopy and transmission electron microscopy. Single pulses of laser with various energies were irradiated on two types of starting materials to analyze complicated effects of the laser crystallization. For energies higher than a threshold value, an obvious difference in crystallization was found between starting materials of amorphous Si (a-Si) and laser-crystallized Si (lc-Si). Microcrystals were generated in the former but not in the latter, which is attributed to the presence of nuclei which remained deep within the lc-Si film during the film melting. Phase variation of a-Si and degradation of lc-Si induced by small-energy irradiation were also demonstrated.

Impact of the Hydrogenation Process on the Performance of Self-Aligned Metal Double-Gate Low-Temperature Polycrystalline-Silicon Thin-Film Transistors

We investigated hydrogenation of low-temperature (LT) polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) from the point of view of the gettering phenomenon, specifically, using self-aligned metal double-gate p-channel LT poly-Si TFTs that had a small subthreshold swing value and a high field-effect mobility. Hydrogenation of TFTs was carried out by forming gas annealing. Our results indicate that the conventionally used hydrogenation temperature of 400 °C is considerably high because annealing at this temperature results in the re-emission of gettered hydrogen. Moreover, when annealing in forming gas, hydrogenation actually occurs during cooling from 400 °C, but not at 400 °C. The most important parameter for effective hydrogenation is the rate of cooling from 400 °C, but not the hydrogenation temperature of 400 °C.

Structural Elements of Shallow Thermal Donors Formed in Nitrogen-Gas-Doped Silicon Crystals

It has been reported that shallow thermal donors (STDs) are formed in oxygen-rich silicon (Si) crystals preannealed in nitrogen gas (N-gas-doped) and also in hydrogen-doped (H-doped) oxygen-rich Si crystals. The STDs formed in these crystals exhibit very similar electronic structures. Experiments using far-infrared optical absorption showed that several hydrogen-like STDs were formed at the same time and their energy levels in both the above-mentioned crystals were very similar. It has also been reported that the g-values of the STDs formed in both the crystals were identical. On the basis of electron–nucleus double resonance results, it has been strongly suggested that a hydrogen impurity is incorporated as a structural element of the STDs formed in the H-doped Si crystals. However, the origin of the STDs that are formed in N-gas-doped Si crystals is still unclear. To clarify this point, hydrogen detection in N-gas-doped Si was conducted and the annealing behaviors of STDs in N-gas-doped Si and H-doped Si were compared by electron spin resonance and far-infrared optical absorption measurement. It was concluded that the origin of the STDs formed in N-gas-doped Si crystals is not related to the hydrogen impurity.