Akiyoshi Baba

DEEP LEVEL OF IRON-HYDROGEN COMPLEX IN SILICON

Deep levels related to iron in n-type silicon have been investigated using thermally stimulated capacitance (TSCAP) combined with minority carrier injection. The TSCAP measurement reveals two traps of EV+0.31 and EV+0.41 eV. The trap of EV+0.41 eV is a donor due to interstitial iron. The trap of EV+0.31 eV, due to a complex of interstitial iron and hydrogen, is observed in the sample etched chemically with an acid mixture containing HF and HNO3 and annihilates after annealing at 175u2009°C for 30 min. It is demonstrated that interstitial 3d transition metals such as vanadium, chromium, and iron tend to form complexes with hydrogen in n-type silicon, and the complexes induce donor levels below the donor levels of the isolated interstitial species. This trend is related to the interaction between the metals and hydrogen in the complexes.

Field Emission from an Ion-Beam-Modified Polyimide Film

We demonstrate the field emission of electrons from an ion-beam-modified polyimide material. A 25-µm-thick polyimide sheet is used as the starting material. The electrical resistivity of this polyimide film is found to abruptly decrease after Ar ion irradiation at doses higher than 5×1015 cm-2. To prepare a field emitter array, a pyramid-like structure is fabricated directly on the polyimide sheet using oxygen-plasma etching, and Ar ions are subsequently irradiated. An emission current of the order of µA is observed at relatively low electric fields for the irradiated samples, while no emission is detected from unirradiated samples. An emission current stability of 1.9±0.3 µA is observed. The current-voltage characteristics of the polyimide field emitters are compared with those of a field emitter made from a photoresist.

Growth kinetics of CoSi formed by ion beam irradiation at room temperature

Growth kinetics of cobalt silicide layers formed by ion beam irradiation was investigated at a temperature between room temperature and 100u2009°C. The CoSi phase was identified by x-ray diffraction of Co/Si samples irradiated with 25 keV argon ions to a dose of 2.0×1015u2009cm−2. The number of intermixed silicon atoms in the CoSi layers was evaluated as a function of dose, dose rate, and nuclear energy deposition rate at the Co/Si interface for samples irradiated with 40 keV focused silicon ion beams. The growth is shown to be diffusion-limited and attributed to radiation-enhanced diffusion with an activation energy of 0.16 eV. The number of intermixed silicon atoms is approximately proportional to the nuclear energy deposition rate at the initial Co/Si interface, while it is independent of dose rate, which shows that the CoSi phase is formed without contribution of the sample heating caused by irradiation.

Behavior of radiation-induced defects and amorphization in silicon crystal

Abstract We have investigated the dose rate dependence of the lateral amorphization of silicon crystals irradiated with 40 keV Si2+ focused ion beams (FIB) as a function of sample temperature. The recovery time of point defects, τ, and the extent of their distribution, d, around the collision cascades produced by impinging ions were evaluated. The amorphous line-width was measured with scanning electron microscopy (SEM) after a selective etching. We have obtained a critical dose rate 1 (τd 2 ) of 1.0 × 1015 cm−2s−1 for radiation at 100°C. The temperature dependence of the critical dose rate suggests that the lateral amorphization is controlled by a simple kinetics of the defects with an activation energy of 0.85 eV. From the value of the activation energy, we speculate that the recovery process of radiation-induced defects is controlled by the migration of interstitial Si atoms.

Fabrication of micro field emitter tip using ion-bearn irradiation-induced self-standing of thin film

We propose a novel standing technique of fabricating a high-aspect-ratio tip structure. The technique utilizes the bending of films induced by ion irradiation. Using this technique, the position and aspect ratio can be easily controlled by the conventional thin-film deposition and photolithography processes. We found that the standing is realized when ion energy is chosen at which the film internal stress at the medium depth of the film relaxes, while low- and high-energy irradiations simply bend up and bend down the film, respectively. We demonstrate the application of the thin-film standing technique to field emitter fabrication.

Imprint Lithography Using Triple-Layer-Resist and Its Application to Metal-Oxide-Silicon Field-Effect-Transisor Fabrication

Imprint lithography using a triple-layer resist technique has been investigated. The triple-layer resist consisted of either novolac resin or polymethylmethacrylate (PMMA) for the top layer, Al for the intermediate layer, and novolac resin for the bottom layer. Molds made of crystal Si or porous-Si covered Si were prepared. The pattern transfer characteristic depending on pressure, substrate temperature during imprint, and mold material was investigated. It is found that imprinting at a substrate temperature close to the pre-bake temperature for the top resist is preferred for pattern transfer with high accuracy. The use of porous Si covered molds is shown to be effective for improving the uniformity. PMMA which has a relatively low glass-transition temperature is found to decrease the pressure required for the pattern transfer. It is also shown that mechanical stress applied to metal-oxide-silicon (MOS) capacitors does not affect the gate oxide integrity, as it is evaluated by time-dependent dielectric-breakdown. MOS field-effect transistors (MOSFETs) on SOI silicon-on-insulator (SOI) having the gate length of about 0.1 µm are fabricated using the imprint lithography. It is shown that imprint lithography does not degrade the performance of MOSFETs.

Field Electron Emission from Inkjet-Printed Carbon Black

We demonstrate the field electron emission from a carbon black dot array prepared by inkjet printing. A commercially available inkjet printer and drawing software were used to pattern the carbon-black-containing ink on the tungsten silicide surface which was deposited on a Si wafer. Field electron emission characteristics were measured using a diode configuration. It was found that oxygen reactive ion etching (RIE) of the printed carbon black is effective for obtaining the field emission. The turn-on voltage was 500 V and an emission current over 100 µA was obtained at a voltage of 1400 V with a 25 µm-wide gap between anode and cathode. It was also found that a rapid increase of the voltage is effective in increasing the emission current and the number of emission sites.

Stamp technology for fabrication of field emitter from organic material

We propose and demonstrate a novel processing technique for fabrication of field electron emitter arrays. A stamp technology was used, in which the emitter tips are prepared by transferring the shape of a mold to an organic material through pressing the mold against the organic material. The transformed organic material is then modified by ion irradiation to produce a carbon based emitter material. The starting material tested was a polyimide film spin coated and cured on a Si substrate. The mold was prepared using anisotropic etching of Si(100). It has been found that the mold shape can be completely transferred to the polyimide by applying a pressure of 1 GPa at 250u200a°C. A field emission current up to the order of μA was obtained from a wedge-shaped emitter array made using this new technology with modification using Ar ion irradiation at an energy of 100 keV, a dose of 3×1016u200acm−2.

Field emission from metal particles bound with a photoresist

A film composed of metal particles was evaluated as a material for the fabrication of field electron emitters on large substrates. Two test films were prepared on silicon substrates from either a mixture of fine palladium particles or spin-on glass and a mixture of fine palladium particles and a positive tone photoresist. Field electron emission characteristics were evaluated with a diode. It was found that the insulating film on the surface of the electron emitting material acted as a barrier to electron emission. It was also found that, with the application of a constant voltage, the field emission current abruptly increased with time in the initial stage of field electron emission. Following this phenomenon the threshold voltage for electron emission was drastically reduced and the emission current fluctuation was less than 10%.

Deep states in silicon on sapphire by transient-current spectroscopy

It is demonstrated that deep states in silicon on sapphire (SOS) films can be evaluated by transient-current spectroscopy (TCS). In the TCS spectra, a broad peak extending over 100–200 K was observed for the 6000-A-thick n-type SOS film. Assuming the value of capture cross section to be 10−15 cm2 and independent of temperature, the density distribution of deep states was estimated. The density distribution shows a peak of 1.2×1012 cm−2u2009eV−1 at EC−0.25 eV. Raman backscattering spectroscopy was also performed to evaluate the stress in the silicon film. It was concluded that the defects detected by TCS should be caused by the compressive stress of 6.2×108 Pa in the silicon film.