Genshiro Kawachi

A 10-s doping technology for the application of low-temperature polysilicon TFTs to giant microelectronics

A bucket-type high-density (0.25-1.2-mA/cm/sup 2/) low-energy (500-2000 V) ion source was utilized for high-speed phosphorus doping directly into a thin polysilicon layer without cap SiO/sub 2/. Doping gas with He dilution was selected to reduce etching of polysilicon film. Excimer laser (XeCl, 8 mm*8 mm) pulse annealing was introduced to activate effectively the doped impurity. The combination of these techniques provided a practically low sheet resistance for the TFT source, drain, and gate with a short time doping. The low-temperature polysilicon TFT fabricated with a doping time of 10 s had characteristics comparable to those of that fabricated by a longer time doping or conventional ion implantation, showing the practicality of this technology and its promise for giant microelectronics. >

A novel technology for a-Si TFT-LCD's with buried ITO electrode structure

A novel process technology for a-Si TFT-LCDs with the buried ITO electrode (BI) structure was developed and applied to 10-in-diagonal LCD panels. By employing the BI structure, an aperture ratio of 29% was achieved in high resolution panels with a pixel size of 192 /spl mu/ and the pixel defect density was reduced to about one third of the conventional structure. The defect reduction effect of the BI structure was also confirmed theoretically. The BI structure provides significant advantages for high-performance TFT-LCDs. >

Characterization of high-performance polycrystalline silicon complementary metal-oxide-semiconductor circuits

Polycrystalline silicon (poly-Si) complementary metal–oxide–semiconductor (CMOS) circuits have been fabricated by using an advanced excimer-laser annealing method and a plasma-oxidation method. The 1-µm-long thin-film transistors (TFTs) were fabricated on arrays of laterally grown long and narrow grains, so that the majority of carriers were free from scattering at grain boundaries during propagation through the channel. The propagation delay time measured by a 21-stage ring oscillator was 175 ps and a power-delay product of 9×10-13 J/gate was obtained at a supply voltage of 3.3 V. The obtained propagation delay time was almost the same as those of bulk Si devices having the same gate length. Furthermore, we expect that 1-µm-long CMOS TFT circuits on glass will have a performance superior to that of 1-µm-long bulk Si devices when the short channel effect and threshold voltage fluctuation are controlled well.

Large-Area Doping Process for Fabrication of poly-Si Thin Film Transistors Using Bucket Ion Source and XeCl Excimer Laser Annealing

A large-area doping process for polycrystalline Si (poly-Si) thin-film transistor addressed liquid crystal displays (TFT/LCDs) has been developed. A large ion beam that was extracted from the bucket ion source and a XeCl excimer laser were utilized for impurity doping and impurity activation. A sufficiently low value of sheet resistance (500±25 Ω/\Box) was obtained for an implantation time of 10 s. The poly-Si TFTs fabricated by using this technique have good characteristics and uniformity. This technique seems suitable for the fabrication of large area poly-Si TFT/LCDs.

Saturation measurements of electrically detected magnetic resonance in hydrogenated amorphous silicon based thin-film transistors

The saturational broadening of an electrically detected magnetic resonance signal in hydrogenated amorphous silicon has been observed in thin-film transistor structures. It was found that broadening of the resonance spectrum with increasing microwave power is caused by an enhanced local microwave field in the transistor due to strong coupling of the microwave field with the microstrip-like structure of the transistor. The field enhancement factor, which was estimated from saturation measurements, can reach 33 in a transistor with a channel width-to-length ratio of 500/10, demonstrating that the thin-film resonator is an effective tool for improving the detection sensitivity.

Analysis of threshold voltage of short channel polycrystalline silicon thin-film transistors fabricated on large grains

Drain bias dependence of threshold voltages of short channel thin-film transistors (TFTs) fabricated on polycrystalline silicon (poly-Si) films with large grains has been investigated. The drain coefficient of the threshold voltage was found to be substantially larger in poly-Si TFTs than that in metal-oxide-semiconductor field-effect transistors on a silicon-on-insulator wafer. In addition, the drain coefficient of poly-Si TFTs shows asymmetric behavior with respect to source-drain swapping. The observed results can be explained in terms of body potential modulation caused by accumulation of excess holes generated by impact ionization and drain junction leakage. The asymmetry in drain coefficient with source-drain swapping can be attributed to the difference in the bipolar gain between the source-body and drain-body junctions.

Characterization of Novel Polycrystalline Silicon Thin-Film Transistors with Long and Narrow Grains

Excellent characteristics of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with long and narrow grains aligned one-dimension have been experimentally clarified for the first time. The field effect mobility and on-off transition slope of n-channel and p-channel devices were as high as 685 cm2 V-1 s-1 and 190 mV/decade and 145 cm2 V-1 s-1 and 104 mV/decade, respectively. Fluctuations of characteristics were considerably reduced by widening the channel, and uniform characteristics were observed when there were approximately twenty long grains within the channel. These results were obtained when the TFT channel was formed within a region free from grain boundaries formed by head-on collision of laterally growing grains and seeds used to initiate lateral grain growth. Material properties are discussed from the viewpoint of device characteristics.

Application of Ion Doping and Excimer Laser Annealing to Fabrication of Low-Temperature Polycrystalline Si Thin-Film Transistors

Feasibility of the non-mass-separated ion implantation (ion doping) technique followed by excimer laser annealing for fabrication of low-temperature polycrystalline Si thin-film transistors (TFTs) is studied. High-speed doping, less than 10 s for formation of the source and drain of TFT, can be achieved by using a bucket-type ion source. Hydrogen ions incorporated in a non-mass-separated ion beam induce undesirable etching of Si films during implantation. To avoid this, He-diluted gas is used. The fabricated TFTs exhibit excellent characteristics comparable to those of TFTs fabricated conventionally. There is no instability due to contamination which may be introduced from a non-mass-separated ion beam. OFF-state characteristics of TFTs can be improved by increasing laser energy for impurity activation. However, also the avalanche-induced short channel effect is enhanced by increasing laser energy. Adjustment of laser energy is required to optimize the device characteristics. It is concluded that ion doping and excimer laser annealing techniques are promising solutions to problems plaguing poly-Si TFTs.

Ultrahigh-Performance Polycrystalline Silicon Thin-Film Transistors on Excimer-Laser-Processed Pseudo-Single-Crystal Films

Thin-film transistors (TFTs) were fabricated on polycrystalline silicon (poly-Si) films formed by position-controlled large-grain growth technology using an excimer laser. The field-effect mobility, on-off transition slope, and threshold voltage were 914 cm2 V-1 s-1, 93 mV/decade, and 0.58 V for the n-channel device, and 254 cm2 V-1 s-1, 122 mV/decade, and -0.43 V for the p-channel device, respectively. These values indicate that TFTs had an ultrahigh performance comparable to that of {100}-oriented crystal-silicon metal–oxide–semiconductor (MOS) transistors. Furthermore, their effective mobilities had the same effective field and temperature dependences as those of MOS transistors, indicating that electrons and holes were predominantly scattered not by random grain boundaries or defects in the Si film, but by phonons at the SiO2–Si interface, similarly to those of crystal-silicon MOS transistors. These attractive results were obtained as a result of the fact that the TFT channel region was made up of nearly {100}-oriented single grains.

Interaction of Hydrogenated Silicon Nitride Films with Indium Tin Oxide

The whitening mechanism of hydrogenated silicon nitride (SiNx:H) film deposited on indium tin oxide (ITO) film was investigated by means of optical emission spectroscopy (OES), scanning electron microscopy (SEM) and X-ray photoelectron spectrosscopy (XPS) analysis. The degree of whitening of the SiNx:H film on ITO depends on the deposition conditions of SiNx:H, i.e., the SiH4 flow rate and the substrate temperature. Reactive species such as SiHn, decomposed from SiH4 gas, preferentially caused the reduction of ITO. This was followed by formation of In metal and a Si-rich porous layer containing SiO2. An abnormal growth of SiNx:H films caused by these successive reactions led to the whitening.