Shingo Ichimura

Application of STM Nanometer-Size Oxidation Process to Planar-Type MIM Diode.

Ultra fine oxidized titanium (Ti) lines 18 nm wide and 3 nm high have been formed on the surface of a 4 nm Ti layer on a SiO 2 /Si substrate using the scanning tunneling microscope [STM] tip as a selective anodization electrode. The dependence of the size of the oxidized titanium line on the various parameters is investigated. The formed oxidized titanium line has resistivity of 2 × 10 4 ohm cm, which is a value seven orders of magnitude higher than that of the deposited Ti layer. The oxidized Ti line is used in the planar type metal-insulator-metal [MIM] diode, and works as an energy barrier for the electron. The energy barrier height of the oxidized Ti line is found to be δE g = 0.25 eV

Estimation of surface excitation correction factor for 200–5000 eV in Ni from absolute elastic scattering electron spectroscopy

We have determined the surface plasmon excitation correction (SEC) factor for nickel in the 200-5000 eV range from the ratios of the absolute elastic scattering electron intensities measured by a novel cylindrical mirror analyser and those by the Monte Carlo method. The inelastic mean free paths (IMFPs) of nickel used for the Monte Carlo method in the energy range specified were calculated by the Penn algorithm. The resulting SECs were smaller than the values calculated from Chen and Oswald general equations of surface excitation parameters (SEPs), which describe the influence of surface plasmon excitations by electrons crossing a solid surface. We also found that SEPs (obtained from SECs) could be fitted to the equation P s (α, E) = C/[E n cos(α) + C] or P s (α, E) = aE -b / cos(α) (<7% root-mean-square error) in the 200-5000 eV energy range, where P s is the SEP, a is the surface crossing angle of the electron to the surface normal, n(= 0.41), C(= 5.39), a(= 1.7) and b(= 0.29) are parameters and E is the electron energy.

Ozone jet generator as an oxidizing reagent source for preparation of superconducting oxide thin film

An ozone jet generator to supply precisely controlled ozone flux to a specimen was constructed with the particular aim of its application for the preparation of superconducting oxide thin films by an MBE (molecular beam epitaxy) method. The ozone jet is supplied to the thin film growth chamber by evaporating the liquid (or solid) ozone accumulated in the ozone vessel of the generator. The necessary condensed ozone is produced from the ozone‐oxygen mixture gas generated by a commercial ozonizer. The ozone flux can be changed by adjusting the temperature of the ozone vessel (i.e., the temperature of liquid ozone). Precise pressure and temperature control of the ozone vessel in liquefying ozone makes it possible to minimize dissolution of diatomic oxygen in the liquid ozone. As a result, the ozone jet generated from the liquid ozone possesses high purity. The precise temperature control also enables a very stable supply of the ozone jet, with a stability of less than 2.5% over 1.5 h. For the experimental con...

Tip characterizer for atomic force microscopy

A tip characterizer for atomic force microscopy (AFM) was developed based on the fabrication of multilayer thin films. Comb-shaped line and space (LS) and wedge-shaped knife-edge structures were fabricated on a GaAs substrate. GaAs∕InGaP superlattices were used to control the width of the structures precisely, and selective chemical etching was used to form sharp edges on the nanostructures. The minimum size of the LS structure was designed to be 10nm, and the radius of the knife edge was less than 5nm. These nanostructures were used as a well-defined tip characterizer to measure the shape of a tip on a cantilever from line profiles of AFM images.

Ultraviolet-ozone jet cleaning process of organic surface contamination layers

To understand the ultraviolet (UV)-ozone jet cleaning process of organic surface contamination layers, adventitious hydrocarbon layers on Si, self-assembled octadecyltrichlorosilane monolayers on Si, and self-assembled C60H–(CH2)12–SH monolayers on Au were cleaned with pure ozone jet and UV irradiation. Cleaned surfaces were analyzed with in situ x-ray photoelectron spectroscopy measurements. Ozone molecules could react with the unsaturated C–C bonds in self-assembled C60H–(CH2)12–SH monolayers on Au surfaces at room temperature. However, the saturated C–C bonds in OTS hydrocarbon molecules adsorbed on Au surfaces reacted not with ozone molecules but with oxygen radicals generated by the dissociation of ozone molecules under UV irradiation. For adventitious carbon contamination on Si surfaces, only a fraction could be cleaned by ozone at room temperature but it could be almost cleaned with UV-ozone jet.

Ultrathin SiO2 film growth on Si by highly concentrated ozone

The growth mechanism of SiO 2 thin film on Si(100) and Si(111) by ozone was investigated using various surface/interface analytical techniques such as X-ray photoelectron spectroscopy (XPS), second harmonic generation (SHG) and medium energy ion scattering spectroscopy (MEIS). Two different ozone generators were fabricated and used for the investigation. The first ozone generator, which was used for the study of initial oxidation, supplies low pressure ( 80%) ozone gas by vaporization of pure liquid ozone at low temperature (< 100 K). The second ozone generator, used mainly for ultrathin SiO 2 film growth, supplies high pressure (1 atm) ozone gas with concentration < 30% by desorbing ozone adsorbed on silica-gel. Through the comparison of ozone oxidation to the oxidation with molecular oxygen, followings features of the ozone oxidation were made clear. (i) Atomic oxygen dissociated from ozone molecules at Si surface directly attacks the back bond of Si, hence it can oxidize hydrogen-terminated Si which oxygen molecules cannot. (ii) The oxide thin film growth proceeds in layer-by-layer manner, especially at the initial stage of oxidation. (iii) Formation of suboxids at and/or near the SiO 2 /Si interface was suppressed, leading to a stable Si-O-Si network formation even at low pressure and low temperature condition. In addition to these features, the existence of no (or very thin) structural transition layer was suggested for ozone oxide film from MEIS experiments and etching experiment with dilute HF solution, while those experiments for thermally grown oxide showed the existence of the transition layers with thickness of approximately 1 nm.

Silicon oxidation by ozone

Understanding the oxidation of silicon has been an ongoing challenge for many decades. Ozone has recently received considerable attention as an alternative oxidant in the low temperature, damage-free oxidation of silicon. The ozone-grown oxide was also found to exhibit improved interface and electrical characteristics over a conventionally dioxygen-grown oxide. In this review article, we summarize the key findings about this alternative oxidation process. We discuss the different methods of O(3) generation, and the advantages of the ozone-grown Si/SiO(2) interface. An understanding of the growth characteristics is of utmost importance for obtaining control over this alternative oxidation process.

High-quality SiO2 film formation by highly concentrated ozone gas at below 600°C

Highly concentrated (>93 vol %) ozone (O3) gas was used to oxidize silicon for obtaining high-quality SiO2 film at low temperature. Compared to O2 oxidation, more than 500 °C lower temperature oxidation (i.e., from 830 to 330 °C) has been enabled for achieving the same SiO2 growth rate. A 6 nm SiO2 film, for example, could be grown at 600 °C within 3 min at 900 Pa O3 atmosphere. The temperature dependence of the oxidation rate is relatively low, giving an activation energy for the parabolic rate constant of 0.32 eV. Furthermore, a 400 °C grown SiO2 film was found to have satisfactory electrical properties with a small interface trap density (5×1010 cm−2/eV) and large breakdown field (14 MV/cm).

Evaluation of new ozone generator designed for oxide film formation by molecular beam epitaxy method

The quality of ozone vapor produced with a new ozone generator was evaluated. The ozone vapor was produced by evaporation of liquid ozone after accumulating it in a ozone vessel at various temperatures between 75 and 95 K. Mass analysis of the ozone vapor revealed that the purity of ozone in the vapor depended on both the ozone vessel temperature during liquefaction and that during evaporation. The purity was ∼70% when liquid ozone accumulation temperatures higher than 90 K were used. The activity of the ozone vapor was investigated by oxidation of a Cu foil under low pressure condition. The formation of a CuO layer by exposure to the ozone vapor (pressure: ∼5×10−4 Pa) at 300 °C was confirmed through x‐ray photoelectron spectroscopy analysis.

Reduction of the interfacial Si displacement of ultrathin SiO2 on Si(100) formed by atmospheric-pressure ozone

We examined the structure around the interface of SiO2 and Si using medium-energy ion scattering spectroscopy (MEIS) to investigate the interfacial Si displacement of an ultrathin silicon dioxide formed by oxidation of a Si(100) substrate with atmospheric-pressure ozone at a substrate temperature of 375 °C. A thermally grown oxide with the same thickness as an ozone-formed oxide was also measured with MEIS for comparison. The ozone-formed oxide exhibited considerably less Si displacement in the oxide layers near the interface than a thermally grown oxide, which indicates that an ozone oxide is homogenous. These results explain well our previous findings that an ozone oxide exhibits a constant HF etching rate of silicon dioxide while a thermally grown oxide slows the etching rate near the interface.