Yasuichi Mitsushima

Control of shape of silicon needles fabricated by highly selective anisotropic dry etching

The process to fabricate a needle-shaped silicon crystal (silicon needle) has been developed. These silicon needles are fabricated by highly selective anisotropic dry etching, where the etching mask is oxygen precipitation in the silicon substrate. In order to apply these silicon needles to field emitters, the shape of the silicon needle should be controlled. This is because a high aspect ratio can lead to electric field enhancement around tips and a flared base of the silicon needle can lead to mechanical and thermal stability. In this article we studied how to control the shape of the silicon needles. We found that the silicon needles with a high aspect ratio could be obtained by either lowering the deposition rate of sidewall passivation film or increasing etching depth, unlike conventional silicon cones fabricated by isotropic dry etching. Furthermore, a high aspect ratio could induce a flared base of the silicon needle, since incident ions were reduced in the vicinity of the base due to a shadowing effect by the silicon needle with a high aspect ratio.The process to fabricate a needle-shaped silicon crystal (silicon needle) has been developed. These silicon needles are fabricated by highly selective anisotropic dry etching, where the etching mask is oxygen precipitation in the silicon substrate. In order to apply these silicon needles to field emitters, the shape of the silicon needle should be controlled. This is because a high aspect ratio can lead to electric field enhancement around tips and a flared base of the silicon needle can lead to mechanical and thermal stability. In this article we studied how to control the shape of the silicon needles. We found that the silicon needles with a high aspect ratio could be obtained by either lowering the deposition rate of sidewall passivation film or increasing etching depth, unlike conventional silicon cones fabricated by isotropic dry etching. Furthermore, a high aspect ratio could induce a flared base of the silicon needle, since incident ions were reduced in the vicinity of the base due to a shadowing e...

Silicon Needles Fabricated by Highly Selective Anisotropic Dry Etching and Their Field Emission Current Characteristics

A new process to fabricate a silicon needle, whose tip radius is about 5 nm and aspect ratio is about 7, was developed. The silicon needles were fabricated by highly selective anisotropic dry etching. The etching mask was oxygen precipitation, which was formed by nitrogen ion implantation and the subsequent oxidation. The process is simple enough to be integrated with complementary metal-oxide-semiconductor (CMOS) circuits. The density of the silicon needle can be controlled by adjusting the dose for nitrogen ion implantation. The position of the silicon needle can be controlled by adjusting the position for nitrogen ion implantation, because silicon needles are formed only in the nitrogen ion implantation area. Furthermore, using these silicon needles as micro emitters, a field emission diode was fabricated. The Fowler-Nordheim plot shows that the field around the tip of the silicon needles was highly enhanced.

New 3-D lateral power MOSFETs with ultra low on-resistance

This paper presents a new 3D lateral power MOSFET which has a double gate and a trench gate/drain structure. The double gate structure decreased its channel resistance, and the trench gate/drain structure decreased its n drift resistance. We realized this structure using solid phase epitaxy and a conventional trench technology. From experimental results, a breakdown voltage of 49.5 V and a specific on-resistance of 42 m/spl Omega//spl middot/mm/sup 2/ were obtained.

Mechanism of Residue Formation in Silicon Trench Etching Using a Bromine-Based Plasma

The formation mechanism of residues observed in HBr-based Si trench etching has been investigated. The residues were characterized using an optical microscope and a scanning electron microscope (SEM), and the residue density dependence on the interstitial oxygen concentration in the Si substrate, the formation process of etching masks and the etching condition was examined. It was confirmed that needle-shaped silicon residues of various heights had a disorderly distribution at the trench bottom. The density of the silicon needles strongly depended on both the interstitial oxygen concentration in the Si substrate and the mask formation process: numerous needles were observed when the oxygen concentration was sufficiently high and the mask formation process included oxidation. It was also found that the residue density increased with etching depth and selectivity to silicon oxide controlled by the etching condition. All the experimental results were successfully explained by a model in which oxygen precipitates nucleate in the Si substrate during the mask formation process, acting as micromasks in trench etching.

Study on a condition for forming the high density of silicon needles with high aspect ratio

We have developed a process to fabricate needle-shaped silicon crystals (silicon needles) by highly selective anisotropic dry etching. The etching masks are fine SiOx clusters in the silicon substrate. Under high density of these clusters, side etching easily occurs at the silicon needles in the process of dry etching. This mechanism is as follows. As the dry etching proceeds, the etching area becomes smaller and smaller due to existing high density of silicon needles. This leads to reduction in the deposition rate of sidewall passivation film, because gas for this film comes from the etched silicon substrate. Accordingly, for forming high density of the silicon needles, the deposition rate must be kept higher than the etching rate of the sidewall passivation film during the dry etching. By adjusting the deposition rate, we confirmed this mechanism and obtained high density 3×108 cm−2 of the silicon needles with an aspect ratio of 5 without side etching. Such silicon needles can be expected to lead to hig...

Preferred Orientation in Ti Films Sputter-Deposited on SiO2 Glass: The Role of Water Chemisorption on the Substrate

Ti thin films have been grown on SiO2 glass layers at 350°C by ultrahigh vacuum magnetron sputtering with a small amount of H2O, H2 or O2 gas introduction to investigate the influence of adsorption on the crystallographic orientation of the Ti films. In situ reflection high-energy electron diffraction (RHEED) and ex situ X-ray diffraction (XRD) studies showed that a water vapor introduction at the beginning of sputter deposition promotes a highly preferred (002) orientation, whereas H2 gas or O2 gas introduction does not affect the orientation. These results indicate that the increase of water chemisorption on the substrate by H2O gas introduction and reduction of the surface free energy enhance the self-assembly of the Ti atoms toward the most stable C-axis orientation.

Effect of H2O partial pressure and temperature during Ti sputtering on texture and electromigration in AlSiCu/Ti/TiN/Ti metallization

The effect of residual gas constituents and substrate temperature during Ti sputtering on the texture of TiN/Ti films deposited on SiO/sub 2//Si substrates has been investigated. The Ti(002) and TiN(111) preferred texture of the films deposited at 350°C was improved drastically by increasing the H/sub 2/O partial pressure from lxl0-9 to 3xi0-8 Torr. Both the Ti(002) and TiN(lll) texture showed a similar H/sub 2/O partial pressure and substrate temperature dependence because of the epitaxial transfer between these planes. The improved Ti(002) texture was attributed to the self-assembly of Ti atoms on the SiO/sub 2/ surface, which had a low surface free energy due to the formation of surface OH groups. The AlSiCu/Ti/TiN/Ti film fabricated with the highly-textured TiN/Ti film showed a strong Al(111) texture and had a smooth surface. Moreover, interconnects from this improved film had longer EM lifetimes.

Field-emission characteristics of a silicon tip defined by oxygen precipitate

We have developed a silicon tip, fabricated by highly selective anisotropic dry etching using oxygen precipitates (SiOx,x∼2) in a silicon substrate as the etching mask. The gated field-emission device with the single silicon tip was characterized, under a pressure of 10−8-Torr range after an aging process. Although the turn-on voltage of as low as 20 V was obtained, it fluctuated in a range of 20–30 V. In order to reveal the origin of this fluctuation, we investigated the fluctuations of the emitting area (α) and the tip radius (r), using the Fowler-Nordheim theory. As a result, these two were found to fluctuate with a good relation of α∝r2e1.1r, assuming that the work function is constant. This demands that the fluctuation of the turn-on voltage was caused by the variations in the emitting area and the tip radius, not in the work function.

A Method to Detect Oxygen Precipitates in Silicon Wafers by Highly Selective Reactive Ion Etching

This work presents a new method for detecting oxygen precipitates in Si wafers. The method utilizes a highly selective reactive ion etching process. When a Si wafer containing oxygen precipitates is etched under the condition of high selectivity of silicon over silicon oxide, needle-shaped silicon cones can be formed below the oxygen precipitates, because they are hardly etched. Using this method, the depth distribution of the oxygen precipitates having diameters of at least 15 nm has been successfully evaluated by evaluating the number and height of the cones. It was also demonstrated that this method is effective in detecting oxygen precipitates remaining in the denuded zone.

Effect of water absorption of dielectric underlayers on crystal orientation in Al–Si–Cu/Ti/TiN/Ti metallization

The influence of the exposure of underlying dielectric (phophosilicate glass and borophosphosilicate glass) films to a humid air ambient on crystallographic orientations in Al–Si–Cu/Ti/TiN/Ti layered structures has been investigated as a function of the boron content and exposure time of the dielectric films. The Al(111) orientation in the layered structures was found to improve drastically with increasing boron content and exposure time of the dielectric films. The full width at half maximum value of an Al(111) x-ray rocking curve reached less than 1°. It was also found that the Al–Si–Cu surface becomes smoother and the average grain size increases as the Al(111) orientation improves. The improved Al(111) orientation was attributed to the improved Ti(002) orientation of the bottom Ti films. The mechanism of the improved Ti(002) orientation was investigated. It was confirmed that the improved orientation is closely related with the surface concentration of the absorbed water in the dielectric films. Further, it was demonstrated that interconnects fabricated from the improved layered structure have excellent electromigration performance.The influence of the exposure of underlying dielectric (phophosilicate glass and borophosphosilicate glass) films to a humid air ambient on crystallographic orientations in Al–Si–Cu/Ti/TiN/Ti layered structures has been investigated as a function of the boron content and exposure time of the dielectric films. The Al(111) orientation in the layered structures was found to improve drastically with increasing boron content and exposure time of the dielectric films. The full width at half maximum value of an Al(111) x-ray rocking curve reached less than 1°. It was also found that the Al–Si–Cu surface becomes smoother and the average grain size increases as the Al(111) orientation improves. The improved Al(111) orientation was attributed to the improved Ti(002) orientation of the bottom Ti films. The mechanism of the improved Ti(002) orientation was investigated. It was confirmed that the improved orientation is closely related with the surface concentration of the absorbed water in the dielectric films. Furth...