Mitsuharu Tai

Performance of poly-Si TFTs fabricated by SELAX

Selectively enlarging laser crystallization (SELAX) has been proposed as a new crystallization process for use in the fabrication of thin-film transistors (TFTs). This method is capable of producing a large-grained and flat film of poly-Si. The average grain size is 0.3/spl times/5 /spl mu/m, and the surface roughness of the poly-Si layer is less than 5 nm. The TFTs fabricated with this method have better performance and are more uniform than those produced with the conventional excimer laser crystallization (ELC) method. The average values of field-effect mobility are 440 cm/sup 2//Vs (n-type), and 130 cm/sup 2//Vs (p-type). The subthreshold slope for both types is 0.20 V/dec. Values for standard deviation of threshold voltage are 0.03 V (n-type) and 0.20 V (p-type). The delay time of the CMOS-inverter of SELAX TFTs is less than half that of ELC TFTs.

Scalable 3-D vertical chain-cell-type phase-change memory with 4F2 poly-Si diodes

A three-dimensional (3-D) vertical chain-cell-type phase-change memory (VCCPCM) for next-generation large-capacity storage was developed. The VCCPCM features formation of memory holes in multi-layered stacked gates by using a single mask and a memory array without a selection transistor. As a result of this configuration, the number of process steps for fabricating the VCCPCM is reduced. The excellent scalability of the VCCPCMs new phase-change material makes it possible to reduce the cell size beyond the scaling limit of flash memory. In addition, a poly-silicon selection diode makes it possible to reduce the cell factor to 4F2. Consequently, relative cost of the VCCPCM compared to 3-D flash memory is reduced to 0.2.

1T-1R pillar-type topological-switching random access memory (TRAM) and data retention of GeTe/Sb 2 Te 3 super-lattice films

A 1T-1R pillar-type “topological-switching RAM” (TRAM) and the data retention of GeTe/Sb₂Te₃ super-lattice were investigated. Reset voltage of TRAM, 2 V, was 40 % of that of the conventional PCM with Ge₂Sb₂Te₅. From data retention evaluation, the TRAM was found to endure the retention at 260 °C for 18 hours.

A 50-nm 1.2-V Ge x Te 1−x /Sb 2 Te 3 superlattice topological-switching random-access memory (TRAM)

A 50nm topological-switching random-access memory (TRAM) was fabricated for the first time. A high-quality Ge_xTe_1-x/Sb₂Te₃ superlattice film enabled set and reset voltages of TRAM to be less than 40% of those of PRAM. Statistical analysis of 16kb data showed the reset voltage to be less than 1.2 V, the lowest as a TRAM test chip.

Novel high-performance TFTs fabricated by selectively enlarging laser x'tallization (SELAX) technology

We have developed a new crystallization technique using the EO-modulated CW-laser (LD-pumped Nd:YVO4 SHG λ=532nm). Enlarging of lateral crystallization is attained by rapid laser-scanning on Si surface where the large (4μm × 0.5μm in average) columnar grains are uniformly obtained. Sequential and step-by-step scanning makes large crystallization areas. During the each scanning, the irradiation is frequently suspended by pulse-like modulation. By using this technique, in-plane tensile strain in the irradiated areas is relieved. The size and the crystal orientation ({110} normal to the substrate and {100} normal to the growth direction) of obtained grains are, therefore, rather homogeneous. We have developed the crystallization technique, which can obtain high crystal quality as well as large grains in the selected areas. We have fabricated TFTs (typically W/L=4μm/2-4μm, tox=100nm, tSi=50nm) in the irradiation areas on the glass-substrate. The field-effect mobility is 480 cm2/Vs for n-channel devices and 130 cm2/Vs for p-channel devices, respectively. The sub-threshold swing (S-value) is less than 0.2 V/dec for both types. This technology gives possibility to integrate electronic systems on the glass.

Vision-based autonomous micro-air-vehicle control for odor source localization

This paper presents a novel control method for autonomous-odor-source localization using vision and odor sensing by micro air vehicles (MAVs). Our method is based on biomimetics, which enable highly autonomous localization. Our method does not need any instruction signals, including even global positioning system (GPS) signals. An experimenter just blows a whistle, and the MAV starts to hover, to seek an odor source, and to keep hovering near the source. The GPS-signal-free control based on vision enables indoor/underground use. Moreover, the MAV is light-weight (85 grams) and does not cause harm to others even if it accidentally falls. Experiments conducted in the real world were successful in enabling odor source localization using the MAV with a bio-inspired searching method. The distance error of the localization was 63 cm, more accurate than the target distance of 120 cm for individual identification. Our odor source localization is the first step to a proof of concept for a danger warning system. These results will be applied to the system to enable a safer and more secure society.

Post-Annealing Effects on a Laterally Grown Polycrystalline Silicon Layer

A polysrystalline silicon (poly-Si) layer prepared by selectively enlarging laser crystallization (SELAX) was post-annealed by excimer laser irradiation. The average grain width increased 26% because of partly merged grains, while the morphological trace of the laterally grown poly-Si layer was maintained. The difference in field-effect mobility resulting from thin-film-transistor (TFT) configuration was reduced by post-annealing. The field-effect mobility of TFTs parallel to the lateral growth direction decreased slightly (within 4.5% for n-type and 11.5% for p-type). On the other hand, TFTs configured perpendicular to the lateral growth direction increased as the energy density during post-annealing increased (up to 200% for both types). For n-type TFTs these results were attributed to the increase of the effective width of the grain and the reduction of the density of trapping states at grain boundaries.