Gou Nakagawa

Inkjet printing of nickel nanosized particles for metal-induced crystallization of amorphous silicon

We demonstrated for the first time single-crystal grain growth in the solid-phase crystallization of amorphous silicon (a-Si) film induced by inkjet-printed colloidal solution containing Ni nanosized particles. The electrostatic inkjet nozzle with a needle can print dots of Ni colloidal solution, whose size ranges from submicron order to a few ten micrometers, on the a-Si film surface by controlling the needle apex radius and the number of applied voltage pulses. The dependence of the crystallization behavior of a-Si on dot size is investigated by electron backscattering pattern (EBSP) analysis. Crystallization behavior is categorized into three modes: single-crystal growth, polycrystal growth, and lateral-crystal growth. Statistical analysis suggests that, when the dot size is 0.86 µm in diameter, a single-crystal grain is grown at the inkjet-printed sites with a probability of 0.62.

Location and Orientation Control of Si Grain by Combining Metal-Induced Lateral Crystallization and Excimer Laser Annealing

A new technique of controlling the location and orientation of Si grain by combining metal-induced lateral crystallization (MILC) and excimer laser annealing (ELA) is proposed. A starting amorphous Si (a-Si) film is deposited on a SiO2 substrate having shallow pits. MILC is used to crystallize the a-Si film in a highly oriented polycrystal. ELA is used to recrystallize the highly oriented polycrystalline Si film. ELA produces large Si grains at the shallow pit sites because temperature gradient is generated by slanting Si surface on the slope of the pit. Si grains whose size is approximately 1.6 µm were formed at the pit sites. Electron backscatter diffraction pattern (EBSP) analysis showed that the crystal orientation aligned over the grain boundary.

Location Control of Si Thin-Film Grain Using Ni Imprint and Excimer Laser Annealing

A novel method of locating Si crystal grains by combining Ni imprint and excimer laser annealing (ELA) is demonstrated. Ni imprint is used for the purpose of creating Si crystal nuclei that act as the seed for the subsequent crystallization by ELA. Two Ni-imprint processes were investigated; (1) Ni imprint at the SiO2 substrate surface followed by the deposition of amorphous Si (a-Si), and (2) a-Si deposition on the SiO2 substrate followed by Ni imprint at the a-Si film surface. The annealing used to form nuclei at the imprinted sites was carried out at temperatures below 450 °C. XeCl-laser-based ELA of the sample thus prepared resulted in the formation of about 2 µm sized Si grains at the imprinted sites, whereas the grain consists of several sub-grains radially grown from the center of the grain. The application of Seccos etching to the location-controlled grains formed by Ni imprint at the SiO2 substrate delineated a cavity at the middle of the grain, whereas no cavity was observed in the grains formed by Ni imprint at the a-Si surface. Thus, Ni imprint at the a-Si surface produces higher-quality Si grains than Ni imprint at the SiO2 substrate surface. Details of the experimental conditions used to create the location-controlled grains are described.

Inkjet-Printed Metal-Colloid-Induced Crystallization of Amorphous Silicon

We demonstrate for the first time the crystallization of amorphous Si induced by Ni colloid printing using inkjet printing technology. An electrostatic inkjet nozzle having a needle inside a capillary was built in house, which was able to eject very fine droplets according to the applied electric voltage. Dots of Ni colloidal solution were formed in the prescribed position. Enhanced crystallization was observed at the Ni-colloid-printed sites. The probability of crystallization decreased with the Ni concentration in the solution. At and above 4 mass % Ni concentration, fully enhanced crystallization occurred. Furthermore, the orientation of crystallized Si grains was determined by electron-backscattering pattern (EBSP) analysis. The dependence of crystallization behavior on the Ni concentration was also investigated.

Orientation Control of Location-Controlled Si Crystal Grain by Combining Ni Nano-Imprint and Excimer Laser Annealing with Si Double-Layer Process

A new method of controlling the location and orientation of Si crystal grains by combining metal (Ni) nano-imprint and excimer laser annealing (ELA) using a double-layered Si thin-film structure was successfully demonstrated. Ni nano-imprint at the surface of the first amorphous silicon (a-Si) film (25 nm thick) was used for the purpose of creating {111}-oriented Si-crystal nuclei which act as the seed for the subsequent crystallization using ELA. The annealing to form nuclei at the imprinted sites was carried out at temperatures below 450 °C. After removal of Ni, the second a-Si film layer (75 nm thick) was deposited. XeCl-laser-based ELA of the sample resulted in the formation of approximately 2 µm sized Si crystal grains at controlled positions. Electron back-scattering pattern (EBSP) analysis showed that the surface-normal orientation of all the location-controlled grains was {111}, and that 87% of the boundaries in the grain interiors were the coincidence site lattice (CSL) boundaries.

Nickel Metal Induced Lateral Crystallization of Patterned Amorphous Silicon Thin Film

Metal-induced lateral crystallization (MILC) of patterned amorphous silicon(a-Si) thin film using Ni as a catalyst has been investigated. Ni-MILC grains are based on the growth of needle-like crystals due to the migration of NiSi2 precipitates, which located at the crystalline front, along the <111> directions. In the case where the needle-like crystallites collided at the a-Si pattern edge, not only “turn” or “branch” of the needle-like crystallites toward one of the possible <111> directions but also the growth along the pattern edge were observed. By limiting the growth area, the competitive growth of dendrite crystals that originated in needle-like crystallites was found to appear. This phenomenon resulted in the orientation alignment of MILC crystals in a wide area. Besides, the grain-filtering of MILC crystals was found to be possible by narrowing the pattern width.

Oriented Growth of Location-Controlled Si Crystal Grains by Ni Nano-Imprint and Excimer Laser Annealing

A new method of controlling the location and orientation of Si crystal grains by combining metal (Ni) nano-imprint and excimer laser annealing (ELA) using a double-layered Si thin-film structure was successfully demonstrated. Ni nano-imprint at the surface of the first amorphous Si (a-Si) film (25 nm thick) was performed to create {111}-oriented Si-crystal nuclei that act as the seed for the subsequent crystallization using ELA. The annealing that induces the formation of nuclei at the imprinted sites was carried out at temperatures below 450 °C to meet the requirement of low-temperature process. After the removal of Ni or Ni–silicide, the second a-Si film (75 nm thick) was deposited. XeCl-laser-based ELA of the sample resulted in the formation of approximately 2 µm sized Si crystal grains at controlled positions. Electron backscattering pattern (EBSP) analysis showed that the surface-normal orientation of all the location-controlled grains distributed less than 10° from crystal axis.

Single-grain TFTs on location-controlled crystal grains formed by excimer laser crystallization of Si thin films

Single-grain Si thin-film transistors with no grain boundary in the channel are fabricated on location-controlled crystal grains formed by excimer-laser crystallization of Si thin films. Both of the n-channel and p-channel single-grain TFTs exhibit superior performance and single-crystal-like characteristics, compared to those over conventional random poly-Si TFTs fabricated by solid-phase crystallization or melting-crystallization on the same substrate, and by the same device process and configuration.

Impact of Rapid Crystallization of Si Using Nickel-Metal-Induced Lateral Crystallization on Thin-Film Transistor Characteristics

Metal-induced lateral crystallization (MILC) of amorphous Si using a nickel disilicide catalyst at temperatures up to 770 °C is investigated to produce high-quality polycrystalline Si films in a short period, while 670 °C is the maximum temperature allowed for processing using non alkaline glass substrates. Investigation of crystallization kinetics by isothermal annealing experiments provides activation energy values of 2.3 and 3.6 eV for MILC growth and spontaneous nucleation of amorphous Si, respectively. These values indicate that a MILC region of about 30 µm, which is large enough to place transistor circuits, can be grown by annealing for 15 min at 670 °C, which almost agrees with experimental results. N-channel thin-film transistors are fabricated on MILC films. An average carrier mobility of about 200 cm2V-1s-1 is obtained from the MILC film crystallized at 670 °C.

Grain filtering in MILC and its impact on performance of n- and p-channel TFTs

Metal induced lateral crystallization (MILC) using nickel di-silicide catalyst is able to grow poly-Si films having a preferential crystal orientation along surface normal direction. The poly-Si film prepared by MILC, however, contains randomly distributed sub-grain boundaries which may degrade the performance of poly-Si TFT fabricated using the MILC film. We have investigated effects of patterning of the a-Si film prior to MILC on the growth characteristics and TFT performance. When the width of a-Si film pattern was narrowed, grain filtering effect occurred and, as a result, poly-Si islands whose active region for TFT is mostly composed of single oriented crystal were successfully grown. We characterized the film thus prepared by fabricating n and p-channel TFTs. TFTs were fabricated using the standard high temperature process. The results indicated that TFT performance is very much improved in terms of carrier mobility, on-current, and sub-threshold swing. We conclude that the pre-growth pattering of a-Si in MILC is useful technique to improve the performance of MILC TFTs.