Tomoyuki Nohda

Enlargement of Poly-Si Film Grain Size by Excimer Laser Annealing and Its Application to High-Performance Poly-Si Thin Film Transistor

By both numerical simulation and experimental investigation, we found it possible to enlarge the grain size (?3000 ?) of polycrystalline silicon (poly-Si) films by excimer laser annealing, using a new method to control the solidification process of molten Si - low-temperature (?400?C) substrate heating during laser annealing. Poly-Si thin-film transistors (TFTs) fabricated by this new excimer laser annealing method showed a high field-effect mobility of 230 cm2/V?s, and good uniformity of field-effect mobility (?10%) within the effective laser irradiation area.

Lateral Grain Growth of Poly-Si Films with a Specific Orientation by an Excimer Laser Annealing Method

Dramatic lateral grain growth of nondoped poly-Si films (maximum grain size: ~4.5 µm, film thickness: 500 A) with strong crystallographic (111) orientation on glass substrates has been achieved using an excimer laser annealing method, namely at low temperature below 400°C and in a processing time shorter than a second, for the first time. The surface morphology of these poly-Si films was very smooth and the crystallinity was excellent with minimal internal defects. These poly-Si films have monomodally distributed grain sizes, with an average grain size of around 1.5 µm. As a result of experimental study, we speculate that the basic driving force behind this lateral grain growth was surface free energy anisotropy, as the same mechanism was observed in high-temperature furnace annealing of doped poly-Si thin films.

Comprehensive study of lateral grain growth in poly-Si films by excimer laser annealing and its application to thin film transistors

Lateral grain growth in nondoped poly-Si films was studied by using Si thin films (500 A) with different structures as a starting material for excimer laser crystallization. It was clarified that the lateral grain growth phenomenon (micron-size grains with strong (111) orientation) upon excimer laser annealing was strongly affected by both the microstructure and the orientation of the initial Si thin films. This result supports our previous speculation that the principal driving force of the lateral grain growth phenomenon is surface energy anisotropy. Poly-Si thin-film transistors using these films show a high field effect mobility of 440 cm2/Vs, achieved through a low-temperature process below 600° C. This excellent electrical characteristic is thought to be due to the large grain size of poly-Si thin film with controlled orientation, good crystallinity, and a smooth surface.

High mobility poly-Si TFT by a new excimer laser annealing method for large area electronics

A high-mobility (280 cm/sup 2//V-s), high-throughput poly-Si TFT (thin-film transistor) formed using a low-temperature process (<or= 600 degrees C) has been achieved through a novel excimer laser annealing method. The method uses thin, active-layer poly-Si film (500 AA) and involves controlling the solidification process of molten Si by low-temperature (<or= 400 degrees C) substrate heating during laser annealing. Poly-Si film grain formed by this process is radically larger in size, and has minimal internal defects. The maximum grain size is over 5000 AA, and uniformity in field effect mobility was found to be +or-10% within the effective laser irradiation area.<<ETX>>

Improving the uniformity of poly-Si films using a new excimer laser annealing method for giant-microelectronics

Film uniformity is the main problem when applying laser-recrystallised poly-Si films to thin film transistors (TFTs) in giant micro electronics. However, this has been dramatically improved by a new excimer laser annealing method in which the solidification process of molten Si is controlled by low-temperature (400°C) substrate heating during excimer laser annealing. Poly-Si TFT fabricated around the laser irradiation overlap region exhibited a high field effect mobility uniformity of within ±8%.

Lateral Grain Growth in the Excimer Laser Crystallization of Poly-Si

We have succeeded in obtaining nondoped, thin poly-Si film (thickness ∼500A) with excellent crystallinity and large grain size (Maximum grain size ∼4.5 μ m) by an excimer laser annealing Method, which offers the features of low-temperature processing and a short processing time. The grain size distribution shrinks in the region around 1.5 μ m and this poly-Si film exhibits a strong (111) crystallographic orientation. Poly-Si thin film transistors using these films show quite a high field effect mobility of 440cm 2 /V · s below 600°C process.

Directional Crystallizing Si Films with a Fundamental Continuous Wave Yttrium Aluminum Garnet Laser (Zone Melting for Film)

To form large-grained polycrystalline-silicon (poly-Si) films on glass substrates, we propose a new lateral crystallization method, zone melting for film, which uses a high-power fundamental continuous wave yttrium aluminum garnet (CW-YAG) laser and an absorption layer. The lasers infrared light is changed into thermal energy at the absorption layer and heats the amorphous Si (a-Si) film. Next, the Si film is zone-melted and solidified in one direction with a scanning laser beam. By this method, columnar structured Si films were successfully formed with scanning velocities from 400 to 1000 mm/s. Very large typical grains of 1 or 2 µm by few hundreds of µm were obtained on both glass and quartz substrates, and the long axes of the grains were almost parallel to the laser scanning direction. Textures were also observed in these columnar structured Si films. Recombination carrier lifetimes of these films were several times longer than those of conventional low-temperature-processed poly-Si films.

Low-temperature activation method of poly-Si films using rapid thermal annealing

Doped polysilicon (poly-Si) films with a low resistivity have been successfully obtained at a low temperature using novel processing technology, which was combined with the rapid thermal annealing (RTA) and ion-doping methods. P- doped poly-Si films with a sheet resistance of 3k(Omega) /

Analysis of Weighting Function in Transient Spectroscopy for Precise Measurement of Deep States

DAL were achieved with a process temperature 220 degrees C lower than that of the conventional process which combined the RTA and ion implantation methods. The uniformity of sheet resistance for P-doped poly-Si films prepared by the novel process was better that for the excimer laser annealing. The threshold voltage, subthreshold swing and field effect mobility for n-channel thin film transistors with a lightly doped drain structure using the novel process were 2.0V, 0.3V/dec., and 70cm2/V s, respectively.

Lateral Solid-Phase Recrystallization from the Crystal Seed Selectively Formed by Excimer Laser Annealing in Ge-Ion-Implanted Amorphous Silicon Films

The weighting function in transient deep-level spectroscopy has been analyzed for precise measurement of deep states. The relation among weighting functions of different measurement modes is discussed. It is shown that unless the deep energy level is a well-defined discrete one, reconstruction of the density-of-states from the observed quantity is not straightforward and the weighting function greatly depends upon the energy dependence of attempt-to-escape frequency or capture cross section of the states. Approximate and exact methods to deduce the density-of-states are presented emphasizing that the neglect of energy dependence of the attempt-to-escape frequency brings a large error in the density-of-states estimation.