Toshiyuki Sameshima

XeCl Excimer Laser Annealing Used to Fabricate Poly-Si TFT's

Polycrystalline silicon thin-film transistors (poly-Si TFTs) with a high carrier mobility were fabricated at low processing temperatures of 150 and 250°C. A hydrogenated amorphous silicon (a-Si:H) film was successfully crystallized at room temperature by multistep irradiation of XeCl-308 nm excimer laser pulses without explosive evaporation of hydrogen. The poly-Si TFTs fabricated by the 250°C process had a carrier mobility of 54 cm2/Vs and a low potential barrier height at a grain boundary of 0.01 eV.

High performance poly-Si TFTs fabricated using pulsed laser annealing and remote plasma CVD with low temperature processing

Key technologies for fabricating polycrystalline silicon thin film transistors (poly-Si TFTs) at a low temperature are discussed. Hydrogenated amorphous silicon films were crystallized by irradiation of a 30 ns-pulsed XeCl excimer laser. Crystalline grains were smaller than 100 nm. The density of localized trap states in poly-Si films was reduced to 4/spl times/10/sup 16/ cm/sup -3/ by plasma hydrogenation only for 30 seconds. Remote plasma chemical vapor deposition (CVD) using mesh electrodes realized a good interface of SiO/sub 2//Si with the interface trap density of 2.0/spl times/10/sup 10/ cm/sup -2/ eV/sup -1/ at 270/spl deg/C. Poly-Si TFTs were fabricated at 270/spl deg/C using laser crystallization, plasma hydrogenation and remote plasma CVD. The carrier mobility was 640 cm/sup 2//Vs for n-channel TFTs and 400 cm/sup 2//Vs for p-channel TFTs. The threshold voltage was 0.8 V for n-channel TFTs and -1.5 V for p-channel TFTs. The leakage current of n-channel poly-Si TFTs was reduced from 2/spl times/10/sup -10/ A//spl mu/m to 3/spl times/10/sup -13/ A//spl mu/m at the gate voltage of -5 V using an offset gate electrode with an offset length of 1 /spl mu/m. >

Pulsed laser-induced amorphization of silicon films

The reversible transition from crystalline to amorphous silicon was observed on films deposited on quartz substrates through laser‐induced melting using a 30‐ns pulsed XeCl excimer laser. A 20‐nm‐thick silicon film was completely melted and then amorphized. The melt duration exceeded 70 ns when the film was amorphized. The transition from liquid to the amorphous state would occur homogeneously throughout the film because the temperature gradient in molten silicon could be reduced to 1.0×105 K/cm (0.2 K/20 nm) at 70 ns after initiation of melt. The laser‐amorphized film had a large mid‐gap density of states of 5.3×1019 cm−3 eV−1. The density of states was remarkably reduced using a hydrogen plasma treatment at 250 °C for only 1 min. Thin‐film transistors fabricated in a laser‐amorphized film showed good characteristics with a carrier mobility of 0.6 cm2/V s after hydrogenation.

Laser‐induced melting of predeposited impurity doping technique used to fabricate shallow junctions

Shallow p+ junctions have been fabricated using a new technique in which doping is accomplished by depositing boron film on a silicon surface by radio‐frequency glow discharge (rf‐GD) and melting locally with a pulsed XeCl excimer laser. Sheet resistivity as low as 15 Ω/⧠ was obtained by irradiation of a single laser pulse. The junction depth was 0.07–0.3 μm, depending on the laser energy density. Electrical characteristics of p+‐n junctions which were formed by the laser doping technique subsequently subjected to a 400 °C, 1‐h furnace anneal show essentially ideal diode behavior with an ideality factor of 1.03.

Pulsed laser‐induced melting followed by quenching of silicon films

Amorphization and crystallization were studied through laser‐induced melting of silicon films formed on quartz substrates induced by irradiation with a 30 ns XeCl excimer laser. Homogeneous and rapid solidification occurs and amorphous solid state can be formed when the melt duration is long enough to make the temperature gradient in liquid silicon lower than 1×105 K/cm at the Si/quartz interface. The solid state after homogeneous solidification is governed by recalescence caused by latent heat released at solidification. A completely amorphous state is formed when film thickness is thinner than 24 nm because latent heat reduces as film thickness decreases. Both crystalline and amorphous states were observed for film thickness above 24 nm because recalescence can cause crystalline grain growth. Complete crystallization occurs through interface controlled growth when the temperature gradient is higher than 1×105 K/cm. The velocity of liquid/solid interface is 0.6 m/s, which is too low to cause amorphization.

Electrical and structural properties of poly-SiGe film formed by pulsed-laser annealing

Electrical and structural properties of polycrystalline silicon germanium (poly-SiGe) films fabricated by pulsed-laser annealing were investigated. Observation of laser-induced melt-regrowth of SiGe films using transient conductance measurement revealed that the melt depth and the crystallization velocity increased as Ge concentration increased. The increase of the crystallization velocity resulted in increase of the average size of crystalline grains from 66 to 120 nm at the laser energy density of 360 mJ/cm2 with increasing Ge concentration from 0 to 60%. The crystalline volume ratio obtained by reflectivity spectra in the ultraviolet region also increased from 0.83 to 1.0. Numerical analysis revealed that the density of electrically active defects decreased from 3.5×1018 to 1.1×1018 cm−3 as Ge concentration increased from 0 to 80%. The density of defect states of Si0.8Ge0.2 films were reduced from 3.5×1018 to 1.9×1018 cm−3 by 13.56-MHz hydrogen plasma treatment at 250 °C, 30 W, and 130 Pa for 30 s. How...

Improvement of SiO 2 Properties by Heating Treatment in High Pressure H 2O Vapor

Properties of SiO2 and SiO2/Si interfaces formed by plasma chemical vapor deposition were improved by heating at 270°C in high pressure H2O vapor. The treatment reduced the fixed oxide charge density from 2.5×1012 cm-2 (initial) to 8×1010 cm-2 as the H2O vapor pressure increased to 54 bar. The peak frequency of the absorption band caused by the Si-O antisymmetric stretching vibration mode was increased to 1078 cm-1 for treatment with 54 bar H2O vapor, while it was 1062 cm-1 before the treatment. The full width at half-maximum of the absorption band was reduced to 65 cm-1.

Analysis of Microwave Absorption Caused by Free Carriers in Silicon

Microwave absorption caused by free carriers was investigated. A 9.35 GHz microwave interferometer was constructed. The transmissivity of 525-µm-thick silicon substrates decreased from 60.4 to 3.8% as the resistivity decreased from 1000 to 4 Ω cm. This characteristic was explained well by a numerical analysis using the free carriers absorption theory. Microwave free carrier photo absorption caused by light-induced carriers was also investigated for p-type silicon samples coated with 100 nm thermally grown SiO2 layers as well as SiOx layers deposited by the vacuum evaporation method. The effective minority carrier lifetime and recombination velocity were analyzed in the case of the photo induced carrier generation with 532 nm light illumination. The effective minority carrier lifetime was increased from 360 to 540 µs and the recombination velocity was decreased from 78 to 30 cm/s by 1.3 ×106 Pa H2O vapor heat treatment at 260 °C for 3 h for light illumination at 0.315 mW/cm2 in the case of the thermally grown SiO2/Si because of the passivation of SiO2/Si interfaces. They were markedly increased from 30 to 380 µs and from 1300 to 60 cm/s, respectively, by the H2O vapor heat treatment in the case of the vacuum-evaporated SiOx/Si.

High quality SiO/sub 2//Si interfaces of poly-crystalline silicon thin film transistors by annealing in wet atmosphere

A new post-metallization annealing technique was developed to improve the quality of metal-oxide-semiconductor (MOS) devices using SiO/sub 2/ films formed by a parallel-plate remote plasma chemical vapor deposition as gate insulators. The quality of the interface between SiO/sub 2/ and crystalline Si was investigated by capacitance-voltage (C-V) measurements. An H/sub 2/O vapor annealing at 270/spl deg/C for 30 min efficiently decreased the interface trap density to 2.0/spl times/10/sup 10/ cm/sup -2/ eV/sup -1/, and the effective oxide charge density from 1/spl times/10/sup 12/ to 5/spl times/10/sup 9/ cm/sup -2/. This annealing process was also applied to the fabrication of Al-gate polycrystalline silicon thin film transistors (poly-Si TFTs) at 270/spl deg/C. In p-channel poly-Si TFTs, the carrier mobility increased from 60-400 cm/sup 2/ V/sup -1/ s/sup -1/ and the threshold voltage decreased from -5.5 to -1.7 V.<<ETX>>

Mechanism of pulsed laser‐induced amorphization of silicon films

Amorphization and crystallization were studied through homogeneous solidification of molten silicon layers on quartz substrates induced by irradiation with a 30 ns XeCl excimer laser. The crystalline nucleation rate was obtained to be 8×1030 m−3 s−1. Silicon films were completely amorphized for films thinner than 18 nm due to the fact that grain growth is reduced as film thickness decreases. It was also experimentally determined that recalescence caused by latent heat released at solidification can cause grain growth.