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Publication Abstracts

Here are the abstracts of our recent publications introducing ellipsometry, reflectometry and their applications. All these articles are available upon request.

  1. Characterization of Thin Films and Stack in MOEMS Structure with Ellipsometry and Reflectometry Techniques. Proceedings of SPIE Volume: 5715, Micromachining and Microfabrication Process Technology X , Editor(s): Mary-Ann Maher, Harold D. Stewart, 2005, pp. 176-183

    Abstract: Nondestructive characterization on thin films and their stack in MOEMS device is highly desirable. But, it is often a challenging task because the area is usually small. During processing of thin films, the deposition rates, optical properties, and mechanical properties must be fully understood to fabricate a device with desired performance. With the patterned surface, deposition rate of a typical physical vapor deposition (PVD) technique, such as electron-beam evaporation and sputtering, varies at different location due to shadow effect. In this study, spectroscopic ellipsometry and reflectometry were used to characterize the optical properties of electron-evaporation thin films on a flat substrate. On the other hand, microreflectometer was used to monitor the spectrum of deposited multi-stack of optical thin films inside via-holes. Combination of these two techniques provides a practical way to qualify the processing and ensure the device performance.

  2. Spectroellipsometric characterization of Au-Y2O3-stabilized ZrO2 nanocomposite films, Journal of Materials Research (JMR), December 2005 (with George Sirinakis, Rezina Siddique, Kathleen A. Dunn, Harry Efstathiadis, Michael A. Carpenter, Alain E. Kaloyeros)

Abstract: Nanocomposite thin films consisting of Au nanoparticles embedded in yttria-stabilized zirconia (YSZ) were synthesized at room temperature by radio frequency magnetron co-sputtering from YSZ and Au targets and subsequently annealed in an argon atmosphere. Au microstructure and particle size were characterized as a function of annealing temperature from 600 to 1000 C by x-ray diffraction, transmission electron microscopy, scanning electron microscopy, and Rutherford backscattering spectroscopy. Spectroscopic ellipsometry was also used to determine the optical constants of the resulting films. In particular, the refractive index of the nanocomposites was found to undergo an anomalous dispersion in the spectral region where the extinction coefficient achieves its maximum. Additionally, the incorporation of Au in the YSZ matrix was found to increase the refractive index in comparison to that of YSZ. At annealing temperatures higher than 800 C, a good agreement was found between experimental findings and theoretical models using bulk dielectric functions for Au, as modified to account for a reduced mean free path for scattering than that for free electrons. However, for annealing temperatures below 800 C, an additional offset was required for the optical constants of Au to obtain good agreement between theory and experiment. This behavior was attributed to a relatively high atomic Au concentration in the YSZ matrix.

  1. Synthesis and Spectroellipsometric Characterization of Y2O3-stabilized ZrO2-Au Nanocomposite Films for Smart Sensor Applications. MRS Fall Meeting Proceedings, Volume 846, 2004

    Abstract: Noble metal nanoparticles exhibit significant potential in all-optical, smart-sensing applications due to their unique optical properties. In particular, gold (Au) nanoparticles exhibit a strong surface plasmon resonance (SPR) band, the spectral position and shape of which depends on the size, shape, and density of the nanoparticles and the physical and chemical properties of surrounding environment. Embedding the nanoparticles in an yttria-stabilized zirconia (YSZ) matrix is believed to expand their range of operation to temperatures above 500°C. YSZ is a material that has been proven suitable for optical applications due to its high refractive index, low absorption coefficient and high transparency in the visible and infrared regions. Thus, its use as a base platform for nanocomposite thin films is expected to provide significant benefits in the development of harsh environment multifunctional sensors. In this work YSZ-Au nanocomposite films were synthesized from a YSZ and a Au target by the radio frequency magnetron co-sputtering technique in combination with a post-deposition annealing treatment in an argon atmosphere, with the annealing temperature being varied from 500-1000°C in steps of 100°C. The microstructure and the optical properties of the resulting films were characterized by x-ray diffraction spectroscopy, scanning electron microscopy and spectroscopic ellipsometry. Results on the effect of the Au particle size on the real and the imaginary part of the refractive index of the nanostructured composites are presented. Future smart sensor systems utilizing these multifunctional material sets for harsh environment sensing applications will likewise be outlined.

  2. Atmospheric Stability of E-Beam Deposited Optical Thin Film Stacks. MRS Proceedings, Fall Meeting, Volume 854, 2004

    Abstract: Optical thin films with SiO2-TiO2 stack were prepared by the technology of electron beam (e-beam) evaporation with ion beam assistant deposition (IBAD). The mechanical (stress) and optical properties of as-deposited thin films were studied as a function of exposure time in the atmospheric environment. Exposing to the air at the ambient temperature causes incremental compressive stress and spectrum profile changes, which is related to the absorption of water moisture into the films. Making a dense film is, therefore, a practical approach to improve structural stability of thin films and then the performance of optical devices.

  3. Spectroscopic Ellipsometry Study on E-Beam Deposited Titanium Dioxide (TiO2) Films. Thin Solid Films, Vol. 455-456, pp. 525-529

    Abstract: Spectroscopic ellipsometry (SE) was applied to study the optical inhomogeneity of TiO2 films deposited by the technique of ion-beam assisted e-beam evaporation. A three-sub-layer model of ellipsometry analysis can successfully simulate the structure variation of TiO2 films, which consists of a dense amorphous layer near the substrate, a crystal columnar layer, and a very thin roughness layer on top. Such a theoretical model is further confirmed by microscopic observations with SEM, TEM, and AFM. The structure variation of TiO2 films is related to the increase of chamber temperature during the e-beam evaporation process. Annealing evolution of as-deposited TiO2 films in atmospheric environment at elevated temperatures of 150°C and 250°C was also investigated. The ellipsometry analysis shows that the total film thickness of TiO2 film increases with the annealing time at both temperatures. A more detailed analysis further reveals that thickness of the top sub-layer increased; whereas the region of the bottom amorphous sub-layer shrunk when the film was annealed at 250°C.

  4. Evaluation on Stress and Optical Property of Thin Films Used in Optical MEMS Device. Thin Films - Stresses and Mechanical Properties X MRS Proceedings Vol. 795, pp. 479-484, 2003

    Abstract: Control of the film stress and optical property has long been considered as an issue in the tunable optical MEMS devices. In this paper, the atmospheric evolution of TiO2 and SiO2 thin films for the optical MEMS devices were studied. These films were prepared by the way of ion-assisted e-beam evaporation. It is found that as-deposited SiO2 films exhibit compressive stress; whereas, it is tensile in the TiO2 films under present processing conditions. When annealed at 150°C, both SiO2 and TiO2 films show slight changes in stress with annealing time. However, increasing the anneal temperature to 250°C caused an apparent change of film stresses with time, in which SiO2 film turns into less compressive and TiO2 film appears to be more tensile. The optical properties after annealing were also investigated by measuring the thickness and the refractive index changes using the technique of spectroscopic ellipsometry analysis. At both experimental temperatures, the film thickness increases slightly and the refractive index at 1550 nm decreases a little at the initial annealing stage for SiO2 films. For TiO2 films, it is found that the refractive index increases after annealed at 250°C. This might be caused by the TiO2 film densification process of amorphous-to-crystalline phase transformation. Because most of the significant film evolutions occur during the initial 12 hours of annealing, a practical way of stabilizing the film properties in a MEMS device is to pre-anneal the as-deposited thin films.

  5. Properties of Titanium Oxide Thin Film Prepared With E-Beam Evaporation. MRS Fall Meeting Proceedings: Morphological and Compositional Evolution of Thin Films, Vol. 749, pp. 101-106

    Abstract: The optical and mechanical properties of TiO2 film prepared by ion-assisted e-beam evaporation have been examined in this research. Spectroscopic ellipsometry analysis revealed an inhomogeneous behavior in both optical property and growth structure, vertically from substrate to the top surface of the film. This phenomenon was further confirmed with the electron microscopic analyses. The effects of deposition rate, chamber pressure, anode voltage and current on the stress of TiO2 films were also investigated and reported. Further study showed that a structural homogeneous film could be obtained through TiO2-SiO2 co-evaporation.

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