Po-Kai Chiu

Design and fabrication of optical thin films for remote sensing instruments

Optical thin films designed for multispectral band-pass filters and a primary Ag mirror were deposited on radiation-resistant glass by ion-beam-assisted deposition. The filters and mirror are for use in the optical payload of a remote sensing instrument. The optical parameters of the films and mirror were optimized by using admittance loci analysis. The simulation shows that the band-pass filters can achieve average transmittances of 95% in the blue (B1), green (B2), red (B3), near-infrared (B4), and panchromatic (400–900 nm) spectral ranges and an average reflectance of 99% for the primary Ag mirror in the visible spectrum. The corresponding properties of the films were investigated by in situ optical monitoring, spectrometry, and high-resolution transmission electron microscopy. It was found that the average transmittances are above 85% for all five band-pass filters, with a rejection transmittance below 1% in the spectral range of 350–1100 nm. The average reflectance of the primary Ag mirror (with a pr...

Effect of substrate temperature on the properties of ZrB2 film on Si(111) deposited by pulsed DC magnetron sputtering

The wurtzite GaN and related nitride films are typically grown on sapphire or SiC substrates. However, large lattice-mismatches between GaN and sapphire or SiC can lead to degradation of quality of grown GaN by 16 and 4%, respectively. ZrB2 is a semi-metal compound and has a hexagonal crystal structure. The a-axis lattice constant of ZrB2 is 3.168 A, which is almost lattice-matched to GaN. The thickness of ZrB2 film is 228 nm which was prepared on a Si(111) substrate by pulsed DC magnetron sputtering deposition. The effects of substrate temperatures ranging from 400 to 550 °C on the microstructure, resistivity and surface roughness of the deposited ZrB2 films were investigated by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), the four-point probe technique and atomic force microscopy (AFM). XRD analysis revealed that the ZrB2 film had a polycrystalline structure and the preferred orientation was along (001) when the substrate temperature was above 450 °C. The relationship between the average grain size of the ZrB2 film and substrate temperatures ranging from 450 to 550 °C forms a downward parabolic function. The minimum average grain size of the ZrB2 film was 10.6 nm when the substrate temperature was 500 °C. The minimum resistivity of the ZrB2 film was 229 µΩ cm when the substrate temperature was 550 °C. The AFM measurements showed the ZrB2 films to have a uniform morphology with a very low surface roughness value of 0.2 nm. In this work, it was found that a uniform surface morphology and preferred orientation along (001) of the ZrB2 film on Si(111) could be obtained through pulsed DC magnetron sputtering when the substrate temperature was above 450 °C. This mirror-polished ZrB2 buffer layer is a good candidate for GaN film growth on Si(111) wafers.

WO 3 Electrochromic Thin Films Doped With Carbon

(WO₃)_100-XC_X thin films (x=0-9.3 at%) were cosputtered on indium-tin-oxide glass substrate with W and C targets by reactive dc magnetron sputtering at room temperature. The effects of doped C on the microstructure and optical properties of the (WO₃)_100-XC_X thin film were examined by field emission scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffraction, and a spectrophotometer. It was found that all of the as-deposited films were amorphous. The average transmittances of bleaching and colored WO₃ film in the 400-700 nm range were 75% and 65.9%, respectively. The variation in average transmittance of the bleaching and colored states of (WO₃)_90.7C_9.3 film was 68.3%. The average transmittance of bleaching (WO₃)_100-XC_X thin films did not vary with C doping. However, the average transmittance of colored (WO₃)_100-XC_X film decreased to <;48% after C doping. In addition, the optical transmittance contrast of (WO₃)_100-XC_X thin films was found to be suitable for blue laser wavelength recording.

Investigation on optical and electrical properties of ZnO sandwich structure with metal interlayer

The dielectric–metal–dielectric (DMD) sandwich structure is fabricated by the RF magnetron sputtering. The effects of sputtering voltage, thickness of the metal layer, and process temperatures on the optical, crystalline structure, and electrical properties of the zinc oxide (ZnO) films were investigated by UV–vis spectrometry, X-ray diffraction and electron energy loss spectroscopy (EELS). Silver, copper and aluminum are used for the DMD sandwich metal layer in this research. The thickness of the dielectric films is decided by admittance loci analysis base on the selected metal characteristic. The optical average transmittance in the visible spectrum and bulk resistivity of the DMD with intermediate Ag layer can reach 85% and 3 × 10−5 Ω cm, respectively.

Relative reflectivity uncertainty evaluation for a broadband spectrophotometer system

This study aims to analyze the reflectivity measurement uncertainty of a spectrophotometer, with considerations of the perpendicular incidence/reflection, scattering, and artificial manipulation. The operation procedure is designed to utilize a double beam spectrophotometric system, taking advantage of the direct comparison of the luminous intensity to evaluate the uncertainty. The environmental range of the temperature and the relative humidity for the measuring process are controlled to be 23±1.5oC and 50%±10%, respectively. The range for correction reflectivity is 10%-100%. High-reflectivity standard parts are used with the integrating sphere for the calibration. Low-reflectivity standard parts are used along with an integrating sphere for the evaluation when the measurement reflectivity is 0%-9%. The optimal expanded composition uncertainty of the system is found to be 1.2% in the wavelength range of 400 nm-700 nm.

Investigation of the Microstructure, Porosity, Adhesion, and Optical Properties of a WO 3 Film Fabricated Using an E-Beam System With Ion Beam-Assisted Deposition

The relationship between the oxygen pressure, porosity, adhesion, and ion beam-assisted deposition (IAD) conditions for WO₃ electrochromic films prepared using an electron-beam (E-beam) system was investigated. The adhesion of thin films on the substrate by sole E-beam system is very weak. The best method is proposed to obtain good adhesion and better porosity by combining the E-beam evaporation, and the microstructure and coloration efficiency were determined using a field emission scanning electron microscope (SEM) and a spectrometer, respectively. Notably, the effect of the oxygen pressure on the coloration efficiency and porosity of Ta₂O₅/WO₃ films was found to be different. The optimal average transmittance of the electrochromic film in the bleached state was 11%, and in the colored state was 68%, when a Ta₂O₅ thin film was deposited in oxygen at a pressure at 9 × 10^-4 torr using IAD, and a WO₃ thin film was deposited at an oxygen pressure of 7.5 × 10^-5 torr. The studied results are useful for the design of more efficient photoelectrochemical devices employing evaporated WO₃ films.

Effect of electron-beam deposition process variables on the film characteristics of the CrOx films

The film characteristics and optical properties of the chromium oxide films on the glass substrates prepared by electron-beam deposition with different process variables were investigated. The process variables included are the various oxygen flow rates, the different applied substrate temperatures, and the preparation process in Ar or O2 surrounding environment with and without ion-assisted deposition. The optical constants of the deposited films are determined from the reflectance and transmittance measurements obtained using a spectrophotometer with wavelengths ranging from 350 nm to 2000 nm. The microstructures of the films were examined by the XRD, SEM, and XPS. The electrical conductivity was measured by a four-point probe instrument. The resulting microstructures of all the prepared films are amorphous and the features of the films are dense, uniform and no pillar structure is observed. The refractive index of deposited films decrease with oxygen flow rate increase within studied wavelengths and th...

Development of high-performance parallel exposure I-line UV light source

UV light source and high-pressure mercury lamps are usually used as a projecting light source of foreign exposure light source recently, including a spherical mirror ellipse, an optical integrator, and a parallel lens. The light source system is the most crucial technology to achieve better image quality and better uniformity of light field. High-stability, high-uniformity, and high-parallelism light source should be created to ensure the stability of all processes. In this research, two sets of Flys eye lens are used as an optical integrator for combining with the redesigned parallel lens, which are evaporated AR coating to increase the transmittance of overall optical system up to 80% and the uniformity of light field. Equivalent doublets are also proposed to improve the original design of single parallel lenses for reducing the thickness, curvature, and divergent angle of a single lens so as to enhance the efficiency of light irradiation.

Optical properties and crystallinity of silver mirrors under a 35 krad cobalt-60 radiation

This study addresses the effects of thin film optical design and environmental radiation on the optical properties of silver mirrors. Different experimental thin film optical designs are selected, and the film stack is built using Macleods approach. Mirror elements are exposed to the same dose of radiation and their properties are characterized using a spectrophotometer equipped with an integration sphere and an x-ray diffractometer. Spectrophotometric analyses of mirrors exposed to about 35 krad of 60Co radiations overall show that the B270 glass substrates coated with titanium oxide (TiO2), silicon dioxide (SiO2), pure chrome, and pure silver effectively reduces radiation damage. The absorption spectrum of the TiO2 film in the visible region decreases after radiation and displays drifting. As thin metal films comparison, the silver thin film exhibits higher radiation resistance than the chrome thin film. The x-ray diffraction analysis on metal film layers reveals that crystallinity slightly increases w...

Fabrication of highly ordered nanohoneycomb (ZnO/Pt) arrays

The double nanohoneycombs (ZnO/Pt) on glass and Si substrates were fabricated by the two-step process of nanosphere lithography (NSL). The working parameters of the fabrication processes on nanohoneycomb performances were analysed and the factors affecting the size of nanohoneycombs, such as controlling nanosphere size, lift-off nanosphere and the second NSL process, were elucidated. In the first-step NSL process, an ordered single layer of polystyrene (PS) nanospheres with an average diameter of 600 nm was spin-coated on the substrates. Then, the PS nanospheres were modified and reduced by oxygen plasma and performed as deposition masks. By ZnO film deposition and lift-off processes sequentially, we fabricated ordered periodic nanohoneycomb (ZnO) arrays. After repeating the second-step NSL process and Pt film deposition on the ZnO arrays, we fabricated an ordered structure with periodic double nanohoneycomb (ZnO/Pt) arrays. This technique for forming double nanohoneycombs, especially with desired period, is expected to be a candidate for manufacturing wide nanostructures used in different fields, such as photonic crystals, optoelectronic devices, sensors, data storage and fuel cells.