Cheon-Hong Kim

A poly-Si TFT fabricated by excimer laser recrystallization on floating active structure

This letter reports a. new excimer laser annealing (ELA) method to produce large polycrystalline silicon (poly-Si) lateral grains exceeding 4 /spl mu/m. A selectively floating amorphous silicon (a-Si) flint with a 50 nm-thick air-gap was irradiated by a single-pulse XeCl excimer laser and uniform lateral grains were grown due to the lateral thermal gradient caused by the low thermal conductivity of the air. A poly-Si thin-film transistor (TFT) with two high-quality 4.6 /spl mu/m-long lateral grains was fabricated by employing the proposed ELA and high field-effect mobility of 331 cm/sup 2//Vsec was obtained due to. the high-quality grain structure.

Poly-Si TFT fabricated by laser-induced in-situ fluorine passivation and laser doping

A new poly-Si TFT has been fabricated by employing laser-induced in-situ fluorine passivation and a laser-doping method. With only one excimer laser annealing, we have successfully fabricated the device using one step to crystallize, passivate and dope simultaneously. Although no additional plasma post-passivation was performed, the on-state and the off-state leakage properties of TFTs with fluorine passivation were improved compared with those without fluorine passivation. The device with in-situ fluorine passivation has the maximum transconductance of 13.3 /spl mu/A/V for a C/sub 2/F/sub 6/ flow rate of 100 sccm, whilst that for a device without fluorine passivation is 8.4 /spl mu/A/V. The device reliability under electrical stress was remarkably improved in the in-situ fluorine-passivated devices due to the fluorine passivation of trap states in the poly-Si channel and at the SiO/sub 2//poly-Si interface.

Excimer-Laser-Induced In-Situ Fluorine Passivation Effects on Polycrystalline Silicon Thin Film Transistors

We report a new fluorine passivation method without ion implantation and additional annealing step by low-temperature process and its effects on p-channel polycrystalline silicon (poly-Si) thin film transistors (TFTs). The proposed method is not post-passivation, but in-situ passivation because fluorine passivation is simultaneous with excimer-laser-induced crystallization by employing excimer laser annealing of fluorine-doped silicon oxide (SiOxFy) on amorphous silicon (a-Si) structure. From experimental results, it has been shown that in-situ fluorine passivation is effective to improve the electrical characteristics, specially field-effect mobility, and the stability of p-channel poly-Si TFTs. The improvement is due to fluorine passivation, which reduces the trap state density and forms the strong Si-F bonds in place of the weak Si-H bonds in poly-Si channel and SiO2/poly-Si interface.

A simple low-temperature laser-doping employing phosphosilicate glass and borosilicate glass films for the source and drain formation in poly-Si thin film transistors

Abstract A simple low-temperature laser-doping process employing phosphosilicate glass (PSG) and borosilicate glass (BSG) films as dopant sources is proposed in order to fabricate the source and drain in polycrystalline silicon thin film transistors (poly-Si TFTs). The sheet resistance and the dopant depth profile have been successfully controlled by the PH 3 /SiH 4 flow ratio, the laser energy density and the number of laser pulses. The sheet resistances of 450 Ω/□ for phosphorus doping and 1.1 kΩ/□ for boron doping were obtained. The devices fabricated by the proposed laser doping method showed a good ohmic property without any current crowding effect. Our experimental result shows that the proposed laser-doping process which does not require any post-annealing is suitable for the source and drain formation of poly-Si TFTs.

Excimer laser recrystallization of selectively floating a-Si thin film

Abstract We report a new excimer laser annealing (ELA) method which increases the grain size of poly-Si film considerably by employing a selectively floating a-Si active layer structure. An air-gap, with low thermal conductivity, blocks the heat conduction to the underlying substrate in the floating a-Si region so that the lateral temperature gradient, which is responsible for a lateral grain growth, is successfully induced by the proposed simple air-gap structure. Our experimental result shows that large grains were grown in the lateral direction from the edge to the center of the floating active region. The large lateral grains exceeding 4 μm were successfully obtained with only one laser irradiation and uniform lateral grains have been grown with a wide laser annealing process window.

Low Temperature Laser-Doping Process Using PSG and BSG Films for Poly-Si TFTs

A simple low-temperature excimer-laser doping process employing phosphosilicate glass (PSG) and borosilicate glass (BSG) films as dopant sources is proposed in order to form source and drain regions for polycrystalline silicon thin film transistors (poly-Si TFTs). We have successfully controlled sheet resistance and dopant depth profile of doped poly-Si films by varying PH 3 /SiH 4 flow ratio, laser energy density and the number of laser pulses. The penetration depth and the surface concentration of dopants were increased with increasing laser energy density and the number of laser pulses. The minimum sheet resistance of 450ω/ for phosphorus (P) doping and 1100ω/ for boron (B) doping were successfully obtained. Our experimental results show that the proposed laser-doping process is suitable for source/drain formation of poly-Si TFTs.

Enlargement of on/off current ratio in poly-silicon TFT by selective crystallization method

Abstract We have proposed a new polycrystalline silicon thin film transistor, in which the leakage current is reduced without the reduction of the on current, by employing resistive amorphous silicon region in the channel. The proposed device has a crystallized active layer, where both sides of the channel near the source/drain are not recrystallized and remained as highly resistive amorphous silicon. In the proposed device, the amorphous silicon region behaves like an offset to reduce the leakage current and acts as the conduction channel of carriers under the on state, so that the on current can be maintained. The crystallized active layer was successfully fabricated by irradiating with an excimer laser over the patterned laser beam-blocking material. The experimental results show that the on/off current ratio of the proposed device is 5×10 6 while that of conventional one is 1×10 5 .

28.1: High-Mobility Poly-Si TFTs Employing XeCl Excimer Laser Annealing on Selectively Floating a-Si Thin Film

High-mobility poly-Si TFTs have been fabricated by a simple ELA method which employs a selectively floating a Si active layer. Large lateral grains exceeding 4μm have been grown. The proposed poly-Si TFT exhibits a high field—effect mobility of 459cm2/Vsec due to the reduction of grain boundaries perpendicular to current path.

Poly-Si TFT Fabricated by Ion Beam Sputtering and Excimer Laser Annealing at 100?C

Polycrystalline silicon (poly-Si) thin film transistors (TFTs) have been fabricated by ion beam sputtering (IBS) and XeCl excimer laser annealing (ELA) at 100?C. The amorphous silicon (a-Si) films to be crystallized into poly-Si were deposited by IBS and they had very low argon content (less than 1 at. %). No explosive gas evolution was observed during the instantaneous crystallization of the a-Si film by ELA, while the a-Si film deposited by conventional RF/DC magnetron sputtering or PECVD underwent severe silicon film ablation due to the explosive argon or hydrogen evolution during ELA. The poly-Si TFT fabricated by IBS and ELA exhibited an ON/OFF current ratio of 4 ? 104. The measured sheet resistance of the n+-doped source/drain resistance was less than 700??/sq. and the output ID-VDS curves showed that good ohmic contacts were formed

Excimer Laser Recrystallization of Selectively Floating a-Si Active Layer for Large-Grained Poly-Si Film

We report a new excimer laser annealing method by employing selectively floating a-Si active layer structure in order to increase the grain size of poly-Si film. The floating a-Si region blocks the heat conduction into the underlying substrate due to high thermal-insulating property of an air so that the lateral temperature gradient is successfully induced by the proposed simple air-gap structure. Our experimental results show that large grains were grown in the lateral direction from the edge to the center of the floating active region. The large grains exceeding 4 m were successfully obtained with only one laser irradiation.