Opaque Thin Films Research Articles

Surface structure of the TiO2 thin film photocatalyst

Control of the surface structure of titanium dioxide thin film photocatalysts was successfully carried out by a polymer-doped dip-coating process. The thin films prepared were either transparent or opaque, depending on the molecular weight of the polymer doped. All the thin film photocatalysts had anatase form with similar crystallinity. The surface of the transparent thin films looked plain consisting of uniformly aggregated nanometer-size TiO2 particles, while the opaque thin films were formed of cubic crystalline TiO2 at the micrometer level. Both the transparent and opaque films showed catalytic activity for the elimination of NOx in air. The specific surface area and photocatalytic activity of the transparent thin film was almost the same as that of the opaque one. The activity of the thin films was almost equal to the commercial photocatalyst P25. Decrease in the film thickness led to a decrease of the elimination of NO in air by the thin films. The thin films were porous and the surface area was dependent on the film thickness. Adsorbed NO was photooxidized to NO2 by the thin films, while the NO2 formed was re-photooxidized to HNO3 before the desorption of NO2 from the film surface.

PREPARATION OF THE TIO2 THIN FILM PHOTOCATALYST BY THE DIP-COATING PROCESS

Titanium dioxide (TiO2) coated glass-plate thin film photocatalysts for elimination of air pollutants, were prepared by the dip-coating process with titanium alkoxide including polyethylene glycol (PEG). The surface structure of these thin films changed drastically with the size of the PEG. They were either transparent or opaque. Nitrogen oxides (NOx), one of the most hazardous of air pollutants, were found to be efficiently eliminated by the thin film photocatalyst. The photocatalytic activities of the transparent and opaque thin films were found to be almost equal. This may be due to the two films having the same surface area. The highest activity was obtained for thin films around 1 μm.

Theory of Transient Reflecting Grating in Fluid/Metallic Thin Film/Substrate Systems for Thin Film Characterization and Electrochemical Investigation

A general theory was developed to calculate the diffraction signals from a transient reflecting grating (TRG) at metallic thin film/substrate interfaces and liquid/solid interfaces. The TRG signals for an Au film/soda lime glass system were experimentally measured and theoretically calculated to examine their agreement. It was found theoretically and experimentally that longitudinal acoustic pulses and echoes with ultrahigh-frequency components (>10 GHz) propagating normal to the sample surfaces are generated and can be detected at the surfaces of thin films, using the TRG technique, along with surface acoustic waves. By using the time intervals of two neighboring longitudinal acoustic echoes, thicknesses or longitudinal acoustic wave velocities for opaque thin films were determinable. Furthermore, it was shown that this method is applicable to the analysis of electrochemical interfaces in situ, through changes in the thermal and longitudinal acoustic properties of electric double layers with electrochemical potentials.

Noncontacting measurement of opaque thin films using a dual beam thermal wave probe

A dual beam thermal wave probe has been used to measure the thickness of opaque thin films in a noncontact, nondestructive manner. The method relies on the measurement of the differential phase of two interferometrically determined photodisplacement signals. The technique does not require calibration against standard samples and can be used to determine film thicknesses from a few tens of nanometers up to several micrometers. Alternatively, if the film thickness is known, thermal material properties like diffusivity or conductivity can be determined. The conditions under which the system is expected to give the most accurate results are analyzed.

CHARACTERIZATION OF TRANSPARENT AND OPAQUE THIN FILMS USING LASER PICOSECOND ULTRASONICS

Abstract This paper describes new applications of laser picosecond ultrasonics to the nondestructive testing of transparent and opaque thin films of thickness in the range 20 nm to 10 μm. The optical pump-probe technique is used to excite ultrasonic stress pulses with acoustic wavelength of order 20 nm and detect time-dependent reflectance variations. Samples of practical importance such as silicon oxide anticorrosion coatings or thin film chromium electrodes are investigated. Transparent silicon oxide films on stainless steel substrates give reflectance variations characterized by beating oscillations superimposed on step-like changes. These effects are modelled as a sum of an interference contribution from light reflected by the moving stress pulse in the transparent film and a contribution from the ultrafast stressinduced film surface displacement of order 10-3 nm, detected for the first time. We show how both the longitudinal sound velocity and the film thickness can be derived, provided tha its refractive index is known. Results for echo detection in opaque films are also presented. The resolution for relative reflectance variations of order 10-7 allowed five echoes from an aluminium-sapphire interface to be resolved. An echo from the Cr-Cr interface of a double layer electrode of chromium from an amorphous silicon image sensor was also detected. This indicates the presence of a carbon polymer layer with thickness of atomic order left over after reactive ion etching.

Laser Picosecond Acoustics in Various Types of Thin Film

We describe the excitation and detection of picosecond stress pulses in a variety of thin films using an ultra fast laser pump-probe technique to measure the film reflectance variation. An opaque thin film of amorphous diamond-like carbon of known thickness is first investigated and the longitudinal sound velocity derived. Two methods of testing transparent films on opaque substrates are then considered, one a conventional technique in which an opaque transducer layer is deposited on the transparent film, and one a new technique in which the transparent film is uncoated. For the latter technique it is shown how both the sound velocity and the film thickness can be determined.

Photodisplacement Measurement by Interferometric Laser Probe

A highly sensitive optical heterodyne interferometer was developed for the purpose of measuring the photodisplacement of a sample induced by the absorption of modulated light. The sensitivity is 1 pm in the 1 Hz bandwidth, sufficient to measure the photodisplacement. A good correlation was obtained between the amplitude of the photodisplacement and the subsurface structures of aluminum thin-film samples. The experimental results revealed that this technique is applicable to thickness measurement and to imaging of subsurface structures for opaque thin films.

Quartz crystal microbalance thin-film dissolution rate monitor

We describe the details of construction and operation of an instrument useful for the characterization of dissolution kinetics of thin films. This device, based on a quartz crystal microbalance operating in contact with a liquid, avoids the limitations associated with the use of optical, electrical, and mechanical dissolution rate measurement techniques. The QCM rate monitor has general application to the measurement of the kinetics of dissolution of transparent and opaque thin films such as dielectrics, metals, and polymeric resists.

Picosecond acoustics as a non-destructive tool for the characterization of very thin films

A new technique for the non-destructive evaluation of film thickness, adhesion and homogeneity is presented. We describe how a subpicosecond laser pulse generates a high frequency acoustic pulse in opaque thin films of a material. The pulse traverse the film, reflects at the substrate interface and returns to the front surface, where it is optically detected by a second laser pulse. From the propagation time we can determine the thickness; the amplitude gives us information about the quality of the bond between the film and the substrate. We present examples of measurements on amorphous hydrogenated silicon ( d = 722 nm) and aluminum ( d = 5 nm) to demonstrate the capability and accuracy of the technique.

Average Rayleigh-wave velocity of a computer-simulated crystallographic plane

A model is proposed for calculating the Rayleigh-wave velocity as obtained by an acoustic lens, taking into account the anisotropy of the material under study and the profile of the miniature acoustic probe. The results obtained by this model on different `cuts' of single-crystal silicon agree with both those published in the literature and our own experimental results. In addition, the model has been applied to the non-destructive thickness measurement of opaque thin films deposited on different anisotropic substrates. More significantly, using the model and experimental acoustic material signature (AMS) curves obtained with this acoustic measurement system, the thickness of tungsten thin films deposited on silicon (100) have been measured. The results are in good agreement with measurements made using a Rutherford back-scattering technique.

Rapid nondestructive thickness measurement of opaque thin films on anisotropic substrates

A rapid, nondestructive opaque thin-film thickness measurement system is presented which takes into account the anisotropy of the sample under study and the geometry of the minature acoustic probe. Experimental results obtained using this system on W/Si (100) samples are in good agreement with those made using the Rutherford backscattering technique.

Thermal-wave detection and thin-film thickness measurements with laser beam deflection

A new technique has been developed that employs highly focused laser beams for both generating and detecting thermal waves in the megahertz frequency regime. This technique includes a comprehensive 3-D depth-profiling theoretical model; it has been used to measure the thickness of both transparent and opaque thin films with high spatial resolution. Thickness sensitivities of ±2% over the 500–25,000-A range have been obtained for Al and SiO2 films on Si substrates.

Prototype sodium and potassium sensitive micro ISFETS

Design and fabrication methods, along with the results of a preliminary evaluation, are described for microprobe ISFET sensors intended for extracellular ion-activity measurements. The devices are 5 mm long with a gradual taper to the tip where the cross-sectional area is 30 × 30 μm 2. Since the ion-sensing area is located at the end of the tip, a very small sensing region is achieved. Photo-induced noise generated in the depletion zone of the long drain diffusion layer was found to be particularly troublesome. This effect was greatly reduced by the use of an opaque tantalum thin film deposited on the probe surface. Preliminary tests of micro pH, pNa and pK ISFETs have shown that it is possible to measure extracellular ion activities using ISFETs.

Internal-Reflection-Spectroscopy Studies Of Thin Films And Surfaces

Internal-reflection spectroscopy has found considerable use in the infrared for studying vibration-band properties, optical constants, surface electromagnetic waves, and guided modes in thin-film channels. This paper discusses the principal ideas, including the forms of the evanescent waves and the effects of refractive indices and absorption coefficients of various magnitudes, for various prism-sample combinations--for example, prism pressed against bulk sample, prism separated slightly from sample and prism with thin film. Also discussed are examples of applications of the techniques to the studies of (a) vibration bands in opaque materials and thin films, (b) molecules adsorbed to surfaces, (c) Raman scattering in thin films, (d) fluorescence in thin films and (e) surface plasmons on bulk metal surfaces and on thin films.

Optical Properties of Thin Films of Zirconium Diboride

The optical constants of the intermetallic compound ZrB2 were obtained at different angle of incidences over the wavelength range 100–600 nm. These were calculated using a computer code based on the Fresnel equations which solves for n and k from reflectance at two angles of incidence and by the well-known Kramers-Kronig relation. Opaque thin films of about 3000 Å were deposited on quartz substrates employing electron-beam melting technique under a vacuum of 10−7 Torr. The material used was very high-purity ZrB2 powder. Although the samples were exposed to air briefly before measurement, precautions were taken to observe the growth, if any, of an oxide-type layer on the ZrB2 films. No such layer growth was observed. The real and imaginary parts of the dielectric constant, and the energy loss function were calculated from n and k. The significant features of the dispersion curves and the energy loss function are discussed.