Amorphous WO3 Films Research Articles

Lithographic properties of amorphous WO3 films exposed to photons, electrons, and hydrogen plasma

Lithographic properties of amorphous WO3 films exposed to photons, electrons, and glow-discharge hydrogen plasma have been investigated. It has been observed that the etching rates of the films in an alkaline solution are lower compared to the unexposed ones, giving rise to a negative tone behavior of the material. The etching characteristics as a function of exposure time and their correlation with the optical transmission have been studied. The contrast (γ) values obtained are 2.3 for photons, 4.0 for electrons, and 2.7 for hydrogen plasma. The sensitivity (S) values obtained are∼1021 photons /cm2 for photons and 10−3 C/cm2 for electrons.

Mechanism of a High-Contrast Inorganic Ion Resist Using Amorphous WO3

Optical absorption spectra of thin amorphous WO3 films irradiated by a beam of sodium ions at ∼ 10 kV have been measured in order to determine the properties of these films as a negative-type ion resist. A characteristic color band of amorphous tungsten bronze was observed in a relatively low-dose region. The resist behavior at the threshold dose Dth was linked with the appearance of a metallic luster at the film surface and with a corresponding change in the spectral shape. The surface sodium content at Dth was nearly equal to the value at the insulator-metal transition of sodium tungsten bronze. The mechanism of this inorganic resist is based on a change in the chemical potential; this is mainly due to doping effects by ion implantation and partly due to some structural change by energy deposition.

ChemInform Abstract: Colouring of Amorphous WO3 with Hydrogen Penetrated Through an LaNi5 Thin Film.

AbstractBy applying a hydrogen pressure of 2.0‐ 10°C Pa to a transparent amorphous WO3 film, which is coated with a LaNi5 thin film, in a stainless steel vessel at 350 K (l0min), the colour of the amorphous WO3 becomes blue.

Electron Microscopy and Diffraction Studies of the Structure of Amorphous WO3 Film

High-resolution electron-microscopic observation and analysis of the amorphous halos in electron diffraction have been carried out on WO3 film prepared by vacuum deposition. Crossed fringes forming a region about 1 nm in size are frequently seen in the micrographs, and each of their spacings corresponds to that of the lattice spacing of the WO3 crystal. On tilting the film, the lattice fringes in some regions disappeared and new fringes appeared at different positions. This shows that the micro-crystallites are distributed at random in the film. The experimental electron diffraction intensity is explained by a micro-crystal model in which microcrystallites are distributed at random with sizes of 0.8, 1.0 and 1.2 nm at ratios of 23, 54 and 23%, respectively.

Ion-Beam Modification of Amorphous WO3 Film and Its Properties as a High-Contrast Inorganic Ion Resist

The effect of ion-beam irradiation on amorphous WO3 films was studied experimentally and in detail using thermionic Na+ ions with an accelerating voltage of 4∼12 kV. The films exhibit a resistive property of extremely high contrast (γ≃8) at ion doses beyond a threshold value Dth. The sensitivity (Dth ≃5 ×10-4 C/cm2 at 5 kV), which is almost independent of the film thickness, decreases gradually with increasing accelerating voltage. The behavior of the optical absorption spectra and the ion-induced secondary electron emission yield of the irradiated films suggests the formation of sodium tungsten bronze, NaxWO3, by ion implantation. The resistive property can thus be explained from the metal transition of NaxWO3 at the threshold dose.

アモルファスＷＯ３薄膜のイオンビームレジスト効果

Ion irradiation effect on amorphous WO3 (a-WO3) films was studied using thermionic Na+ ions with accelerating voltages V=412 kV. These films become insoluble in alkaline solution at values of ion dose beyond a threshold Dth. Exposure characteristics show that the contrast is considerably higher than that of conventional organic polymer electron-beam resists. The threshold dose, which was almost independent of the film thickness, increased gradually with increasing accelerating voltage of ions (Dth∝V2/3). These results were compared with exposure characteristics for electron irradiation reported in the literature. A-WO3 films are potentially useful as inorganic ion-beam resists.

Optical Characteristics of WO 3 Electrochromic Cells under Heavy Li Ion Injection

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光吸収スペクトルによる非晶質酸化タングステン膜の評価

Absorption spectra of evaporated amorphous WO3 films were measured under various conditions. Slope of the Urbach tail of fundamental absorption edge was found to decrease with increasing temperature of annealing. Decrease of the slope at the annealing temperature of 100°C is caused by the release of water from the films, while the decrease at 300 °C is originated from crystallization. Shift of optical gap was also observed with increase of annealing temperature. These effects are considered to be caused from the change in the disorder of atomic configurations.Electrolytic coloration procedure induces a near-infrared absorption band. The absorption band produced by H+ injection was narrower than that by Li+ injection. This kind of difference in absorption spectra is believed to relate with the difference of frequency of atomic vibrations in the color centers. Optical gap was found to shift to higher energies during electrolytic coloration. This shft is brought about by incorporation of H+ or Li+ ions into WO3 matrix.

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO3 films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be −2×10−4 eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×1021 cm−3 when H+ electrolyte is used. If Li+ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H+ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li+ during coloration.

Desities of Amorphous Thin Films

The densities of amorphous Ge, C, SiO, Se, and WO3 films prepared by vacuum-evaporation were measured using multiple beam interferometry in combination with an oscillating crystal microbalance. Optically polished quartz crystals were used as both the microbalance sensing elements and the substrates for interferometry. The film densities were found to be independent of thickness. The measurements indicated that the amorphous film densities were 23–31% less than the crystalline bulk densities and 13–14% less than the amorphous or vitreous bulk densities. This density decrease may be attributed to voids, the presence of which is deduced from a microcrystalline model of the amorphous structure confirmed by recent high-resolution electron microscopic observation.

Chemical and electrochemical stability of WO3 electrochromic films in liquid electrolytes

Chemical and electrochemical stability of thermally evaporated amorphous films of WO3 on conductive glass substrates have been studied in various electrolytes in order to determine the intrinsic lifetime or WO3-based dis-play devices. The thickness loss of the WO3 films measured under storage conditions in the glycerin-H2SO4 (10 : 1) mixture was a linear function of time for devices held at 50‡ C with a dissolution rate of 20 –25 a/day. Under cycling at room temperature an average dissolution rate of about 18 a/day was observed during the first 30 days at 0.5 Hz and 40 % con-trast [defined as 100 (Bcolored - Bbleached / Bbleached) where B = brightness]. After that time the dissolution rate increased and the WO3 film began to crumble. The dissolution rate of WO3 has also been meas-ured in various non-aqueous solvents. The electrochemical response of WO3 in the various electrolytes has been investigated.

Electronic structure of tetragonal tungsten bronzes and electrochromic oxides

X-ray photoemission spectra of the band structures of WO3, crystalline HxWO3 and the tetragonal and cubic bronzes MxWO3 (M=Li, Na) exhibit great similarity. In the bronzes tungsten 5d conduction band states are occupied. The tungsten 4f core level spectra of these materials have an unusual, but characteristic structure attributed to a combination of final state screening and hydrogen or alkali ion neighbor effects. The band structure of amorphous electrochromic WO3 films differs in characteristic ways from that of the crystalline bronzes.

Dynamics of coloration of amorphous electrochromic films of WO3 at low voltages

Measurements of the time dependence of the coloration (formation of HxWO3) of amorphous films of WO3 are described and analyzed in terms of a barrier for current flow at the WO3-electrolyte interface. A novel feature is that as coloration proceeds, the chemical potential of hydrogen in the HxWO3 increases, opposing further coloration.

Model for the bleaching of WO3 electrochromic films by an electric field

Measurements have been made of the current flow in amorphous WO3 films containing electrons and mobile cations. In a configuration in which electrons are extracted at one contact and cations at the other, the current decays as t−3/4 over many decades of time. By using space-charge current flow ideas, we develop a theory that gives the correct time dependence and magnitude of the current for this double-extraction phenomenon.