Kenji Makihira

In situ observation of nickel metal-induced lateral crystallization of amorphous silicon thin films

The lateral crystallization of amorphous silicon thin films induced by nickel was studied in detail, performing in situ annealing experiments with a transmission electron microscope. The nickel-induced crystallization starts with the fast growth of thin needle-like crystallites of [110] orientation, which advance along the 〈111〉 directions within the film plane. The fast growth rate and the small probability of the crystallite exhibiting the [110] orientation result in large crystalline grains. These grains are, however, composed of many small misorientated subgrains. It is thought that this is because the needle-like crystallite does not grow continuously but grows by successive jumps. Our model is that after the nickel disilicide precipitate grows a thin crystalline slice epitaxially at the leading edge of the needle-like crystallite, the nickel moves to the new leading edge and forms the new nickel disilicide precipitates to maintain the needle-like crystalline growth.

Enhanced nucleation in solid-phase crystallization of amorphous Si by imprint technology

A method to enhance crystal nucleation at controlled sites in solid-phase crystallization of amorphous Si is demonstrated. The method uses imprint with Ni-coated Si tips prior to conventional furnace annealing of amorphous Si films deposited on SiO2 substrates. The incubation time for crystallization is found to be greatly reduced at sites imprinted with the tips. This enhanced nucleation can be used to form large crystal grains up to about 7 μm in diameter at controlled sites. Results obtained from imprint with SiO2-covered Si tips suggest that the enhanced nucleation results not from physical effects of indentation but from a chemical effect of metal transfered from the tip to the film surface.

Structural Properties of Nickel Metal-Induced Laterally Crystallized Silicon Films and Their Improvement Using Excimer Laser Annealing

Structural properties of nickel metal-induced laterally crystallized (Ni-MILC) silicon films are studied in detail mainly using transmission electron microscopy (TEM). Laterally grown crystalline grains can be as large as 17 µm, though the grains consist of small misorientated subgrains and, in addition, some subgrains are divided further into overlapping upper and lower subgrains. The excimer laser annealing (ELA) method definitely improves the Ni-MILC silicon film quality, enlarges the subgrains and removes the overlapping structure. As a result, fairly good polycrystalline silicon (polysilicon) thin film transistors (TFTs) are easily fabricated through a low-temperature process. It is difficult, however, to completely eliminate the subgrains by simply applying the ELA method to Ni-MILC silicon films.

CMOS Application of Single-Grain Thin Film Transistor Produced Using Metal Imprint Technology

A novel technology which is termed metal imprint has been developed for positioning and growing large thin-film Si grains on amorphous substrates. The metal imprint is carried out using Ni coated Si tip arrays and crystallization is carried out in solid phase. This technology enables us to fabricate a grain-boundary-free thin-film transistor (TFT) in a single-grain by aligning the position of the channel and that of the imprinted site. Single-grain TFTs having the channel in the single-grains which were fabricated using a high temperature process showed the field effect mobility up to 450 cm2/Vs for n-channel and up to 260 cm2/Vs for p-channel. Single-grain TFTs showed better threshold controllability than TFTs fabricated on Si films prepared by conventional solid phase crystallization (SPC). Complementary-metal-oxide-semiconductor (CMOS) circuits composed of single-grain TFTs have been fabricated. CMOS inverters composed of single-grain TFTs showed superior transfer characteristics to that composed of SPC-polycrystalline Si TFTs. The 3-µm CMOS ring oscillator which operated with the supply voltage down to 2 V showed the propagation delay time of less than 6.7 ns/stage at 7 V.

Field emission from an ion irradiated photoresist

The field emission from Ar ion irradiated photoresist material is demonstrated. A photoresist of novolac-type positive-tone is used as the test material. The electrical resistivity of the photoresist film is found to decrease after Ar ion implantation at doses on the order of 1016 cm-2. Raman spectroscopy shows that carbon-carbon bonds such as the graphite bond are produced due to ion bombardment. For the field emission, a pyramid-like structure is prepared using oxygen-plasma etching and Ar ions are implanted to the pyramid-like structured photoresist. Electron emission of the order of 10-6 A is observed for implanted samples, while no emission is detected from unimplanted samples.

Control of Si Solid Phase Nucleation by Surface Steps for High-Performance Thin-Film Transistors

The solid phase nucleation process of amorphous Si (a-Si) deposited by vacuum evaporation on thermally grown SiO2 layers on Si substrates having steps has been investigated. Steps were formed by either isotropic wet chemical etching of the SiO2 layer or anisotropic wet chemical etching of Si(100) followed by thermal oxidation. It has been found that solid phase nucleation is enhanced at the steps and that nucleation sites can be controlled by changing the step shape and a-Si thickness. Grain growth up to about 3 µm from the step edge has been observed. n-channel MOSFETs (metal-oxide-semiconductor field-effect-transistors) which had steps at the source/drain edge were fabricated. They showed channel electron mobility of about 200 cm2/Vs, which is approximately one order higher than that obtained from MOSFETs fabricated in Si films formed by solid phase crystallization on flat SiO2/Si substrates.

Characteristics of thin-film transistors fabricated on nucleation-controlled poly-Si films by surface steps

Polycrystalline Si films were prepared on SiO2/Si substrates by solid-phase crystallization of vacuum-evaporated amorphous Si with a nucleation-controlled process by steps formed at the substrate surface. Crystallinity of the Si films and characteristics of n- and p-channel thin-film transistors fabricated on the Si films have been investigated. The Si films were found to be composed of stripe-shaped grains elongated in the direction perpendicular to the step. The vast majority of the grains had the axis in the direction perpendicular to the substrate surface. Concerning the in-plane crystal orientation, a tendency for the axis to align in the direction perpendicular to the step was found. Both n- and p-channel transistors fabricated on the Si films showed improved channel carrier mobility compared to those fabricated on Si films prepared by solid-phase crystallization without nucleation control. The improvement was more pronounced in n-channel transistors, which suggested that p-channel transistor performances were less sensitive to grain boundaries.

Structural Properties of Nickel-Metal-Induced Laterally Crystallized Silicon Films

The nickel metal-induced-lateral-crystallized silicon films are studied in detail. Laterally grown crystalline grains can be as large as 17 μm, though the grains consist of small misorientated sub-grains and some sub-grains are divided further into overlapping upper and lower sub-grains. The nickel-induced crystallization starts with thin needle-like cryst allites, which advance along the <111> directions within the film plane. These thin needle-like crystalli tes grow by successive jumps with a very fast growth rate. The fast growth rate and the small probability that the <111> growth directions are within the film plane result in the laterally grown large grains.

High-performance thin-film transistor fabricated on poly-Si films prepared by metal imprint technology

One promising approach for fabricating high-performance poly-Si TFTs is to place entire channel within a single grain of poly-Si films. To realize these devices, large grains must be formed at controlled positions. In solid phase crystallization (SPC) of amorphous Si, there have been proposed methods such as ion implantation and metal imprint technology to induce nucleation at desired sites. In this work, we demonstrate TFTs fabricated in single-grains of poly-Si films prepared by the metal- imprint technology. It is shown that the single-grain TFTs have superior performance in terms of switching behavior and uniformity in characteristics to those fabricated on conventional SPC poly-Si films.

Vaporization and nucleation on microheater in microchannel with nozzle

A vaporization microchamber is fabricated by the MEMS technology in order to improve and check the concept of vaporizing liquid microthruster for nanosatellite. This chamber is a part of microchannel with 2-10 micrometers height made by silicon and glass substrates. Nozzle is fabricated in silicon just above a thin film ITO heater deposited on glass. Liquid propellant is repetitively pulse-heated by the heater and the flow pattern is recorded thorough glass and ITO by high-speed video camera. Four types of flow patterns are found depending on the employed voltage, pulse width and frequency. For example, large heat flux per pulse of high frequency does not allow the liquid to fill the chamber. Even with lower heat flux, no droplet emission from the nozzle is observed because the nozzle itself is heated enough in this geometry. The nanoscale cavity or roughness on heating surface is found to play an important role for the nucleation in microchannel. The preferable thermal design for microthruster is also discussed.