Derek L. Ho

Chain conformation in ultrathin polymer films

Polymer thin films are used in a variety of technological applications—for example, as paints, lubricants and adhesives. Theories that predict the properties of molten polymers in confined geometries (as in a thin film) generally start from the premise that the chains maintain their unperturbed gaussian conformation in the direction parallel to the surface. This assumption has been questioned, however, by recent experiments. Here we use small-angle neutron scattering to characterize the chain structure and conformation in ultrathin (less than 100 nm) polymer films. The conformation can be deduced directly from the scattering from mixtures of protonated and perdeuterated polystyrenes. We find that the gaussian conformation is retained parallel to the surfaces in all cases. Chain sizes equal the bulk value, within experimental uncertainty, although there is a systematic trend towards chain swelling in the thinnest films.

Effect of Pigment Dispersion on Durability of a TiO 2 Pigmented Epoxy Coating During Outdoor Exposure

The effect of pigment dispersion on durability of a TiO 2 pigmented epoxy coating during outdoor exposure has been investigated. Well-dispersed and poorly dispersed coating samples were prepared through the addition or absence of a dispersant in the coating formulation. Ultra small angle neutron scattering (USANS) and scanning electron microscopy (SEM) showed that pigment aggregation occurs in the absence of dispersant. A thin, clear layer of epoxy was observed at the air/exposed surface interface in both the dispersed and non-dispersed samples. Chemical degradation and physical changes during UV exposure were measured by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), atomic force microscopy (AFM), and laser scanning confocal microscopy (LSCM). Results showed that the degree of pigment dispersion and the thickness of the clear layer contributed to weathering. Changes in surface topography and gloss loss during UV degradation were correlated with degree of pigment dispersion. Ripples and bumps on the top surface of the poorly dispersed coating greatly affected gloss. Bulk and surface mechanical properties were investigated using dynamic mechanical thermal analysis (DMTA) and instrumented indentation, respectively. Relative to the neat epoxy coatings, the addition of TiO 2 particles into the epoxy coatings increased elastic modulus but decreased the glass transition temperatures (T g ) of both of the pigmented coatings. Relationships between surface and bulk mechanical property changes and chemical degradation are discussed.

EFFECT OF COMPOSITION AND PROCESSING CONDITION ON MICROSTRUCTURAL PROPERTIES AND DURABILITY OF FLUOROPOLYMER/ACRYLIC BLENDS

Fluoropolymer blends have been widely used as binders for exterior coatings because of their excellent resistance to ultra-violet (UV) radiation as well as to many corrosive chemical agents. It is known that the fluorinated component usually has a lower glass transition temperature and easily crystallizes in the final structure depending upon the blend composition and sample annealing condition. We investigated the effect of blend composition and annealing process (slow and fast cooling) on the surface morphology and microstructure a poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blend before and after UV exposure. Surface and subsurface microstructures were studied by atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM). Bulk microstructure of PVDF-coatings before and after UV exposure were characterized using small angle neutron and light scattering. Higher PVDF content and a slow cooling process result in larger spherulite crystallite structure and rougher surface morphology. Significant ordering in the spherulite crystallite structure has been observed on the surface and the bulk films after UV exposure.

Characterization of Metal-Oxide Nanoparticles: Synthesis and Dispersion in Polymeric Coatings

Abstract : Metal-oxide nanoparticles can be used to optimize UV absorption and to enhance the stiffness, toughness, and probably the service life of polymeric materials, Characterization of the nano- and microstructure dispersion of particles is necessary to optimize the structure-property relationships. Characterizations of both TiO2 particles dispersed in an acrylic-urethane matrix and TiO2 nanostructured films obtained through sol-gel synthesis are discussed. Experimental methods include microscopy (confocal, AFM) and small angle neutron scattering (SANS). Results from SANS experiments, which yield information about the cluster size of the nano-TiO2 particles and the spatial dispersion in various nanoparticle/polymer samples are presented and compared to the results of microscopy studies.

Nanocharacterization of Surface and Interface of Different Epoxy Networks

The effect of network changes on the surface and interface properties of amine-cured epoxy has been investigated. Samples of different crosslinked epoxies are prepared by mixing stoichiometrically pure diglycidyl ether of bisphenol A (n=0.03) with different ratios of 1,3-bis(aminomethyl)cyclohexane (terafunctional amine) and cyclohexylmethylamine (difunctional amine). All samples are cured in CO 2 -free air. Both the film surface in contact with air and that in contact with the silicon substrate (the interface) are analyzed using atomic force microscopy (AFM) and nanoindentation. Small angle neutron scattering (SANS), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and contact angle measurements, are used to assist in the interpretation of AFM results. Substantial morphological and mechanical differences are observed between the surface and the interface for different crosslinked epoxies. The findings have strong implications on the wettablity, adhesion, and durability of amine-cured epoxies.

Structure development in aerogel-processed nanocrystalline alkaline earth oxides as revealed by SANS

Nanocrystalline MgO, CaO and SrO were prepared according to a modified aerogel process (AP). Small-angle neutron scattering (SANS) was used to probe the nanoscale structural features of these materials after each stage of the synthetic process, including hydrolysis, supercritical drying and calcining. SANS data were interpreted using a classical analysis involving power-law and Guinier regimes, and by application of the maximum entropy method. Results are compared with previously published structural data based on X-ray diffraction, electron microscopy and gas adsorption. It is found that the gel hydrolysis product suspended in methanol and toluene exhibits rod-like scattering at small length scales. This is consistent with a filiform morphology previously reported for air-dried Mg(OH) 2 alcogel, yet SANS data for air-dried alcogels tested in this study indicate no evidence for low-dimensional structure on any length scale. A previous assertion of mass fractal structure in the AP aerogels and oxides was not confirmed by the present data. Instead, surface fractal scattering was found to be the most dominant characteristic feature associated with the SANS data for all AP powders examined. Additionally, MgO and CaO exhibited a correlation peak that corresponds to liquid-like ordering at Bragg length scales of 5.9 nm and 20.3 nm, respectively. These values are roughly consistent with previous independent estimates of primary particle size, suggesting that local packing of primary crystallites is facilitated by the calcination/dehydration process. An alternative interpretation treats these features as Guinier scattering regions. Fitting of results using the unified Guinier/power-law equation yields sphere-equivalent radii for the primary particles that are nearly identical to the Bragg lengths calculated from the positions of the maxima. Air-dried alcogels produced very weak maxima that could be interpreted either as correlation peaks or as Guinier regions. No maxima were observed for aerogel samples. Maximum entropy analysis using a spherical shape factor produced interesting but complex results for the calculated volume size distributions of these materials. Overall, the observed trend shows an increase in structural feature size with increasing metal cation size.

CD-SAXS measurements using laboratory-based and synchrotron-based instruments

Critical dimension small angle X-ray scattering (CD-SAXS) is a metrology platform capable of measuring the average cross section and line width roughness (LWR) with a sub-nm precision in test patterns with line widths ranging from 10 to 500 nm. The X-ray diffraction intensities from a collimated X-ray beam of sub-Angstrom wavelength were collected and analyzed to determine line width, pitch, sidewall angle, LWR, and others structural parameters. The capabilities of lab-scale and synchrotron-based CD-SAXS tools for LWR characterization were tested by measuring a set of identical patterns with designed roughness amplitude and frequency. These test patterns were fabricated using EUV lithography with sub-50 nm linewidths. To compensate for the limited photon flux from the lab-based X-ray source, the incident beam of the lab system was collimated to a less extent than the synchrotron beam-based tool. Consequently, additional desmearing is needed to extract information from data obtained from lab-based equipment. We report the weighted nonlinear least-squares algorithm developed for this purpose, in addiiton to a comparison between the results obtained from our lab system and the synchrotron beam-based tool.

A Laboratory Scale Critical-Dimension Small-Angle X-ray Scattering Instrument

New methods for critical dimension (CD) measurements may be needed to enable the detailed characterization of nanoscale structures produced in the semiconductor industry and for nanotechnology applications. In earlier work, small angle x‐ray scattering (SAXS) measurements with synchrotron sources have shown promise in meeting several grand challenges for CD metrology. However, it is not practical to depend upon x‐ray synchrotron sources, which are large national facilities with limitations in the number of available instruments. To address this problem, a laboratory scale SAXS instrument for critical dimension measurements on periodic nanoscale patterns has been designed, installed, and tested. The system possesses two configurations, SAXS and ultra‐small‐angle x‐ray scattering (USAXS), with a radiation target of either copper or molybdenum. With these configurations, the instrument is capable of accessing scattering angles that probe length scales ranging from ca. 0.5 nm to 2 μm. In this work, we compare CD‐SAXS measurements taken from a synchrotron‐based SAXS at the Advanced Photon Source of the Argonne National Laboratory with those from the National Institute of Standards and Technology laboratory‐scale SAXS instrument. The results from standard line/space gratings possessing periodic line‐space patterns with CDs of tens to hundreds of nanometers show that the laboratory‐scale system can quantitatively measure parameters, such as the pitch, line width, height, line‐width roughness and sidewall angle. These results show that laboratory‐scale measurements are feasible and can be used for research and development purposes or to assist calibration of optical scatterometry and CD‐scanning electron microscopy instruments. The primary limitation of the measurement is that the data collection rate is unacceptably slow for production metrology because of the significantly lower x‐ray beam fluxes currently available.New methods for critical dimension (CD) measurements may be needed to enable the detailed characterization of nanoscale structures produced in the semiconductor industry and for nanotechnology applications. In earlier work, small angle x‐ray scattering (SAXS) measurements with synchrotron sources have shown promise in meeting several grand challenges for CD metrology. However, it is not practical to depend upon x‐ray synchrotron sources, which are large national facilities with limitations in the number of available instruments. To address this problem, a laboratory scale SAXS instrument for critical dimension measurements on periodic nanoscale patterns has been designed, installed, and tested. The system possesses two configurations, SAXS and ultra‐small‐angle x‐ray scattering (USAXS), with a radiation target of either copper or molybdenum. With these configurations, the instrument is capable of accessing scattering angles that probe length scales ranging from ca. 0.5 nm to 2 μm. In this work, we compare...

Line Edge Roughness and Cross Sectional Characterization of Sub-50 nm Structures Using Critical Dimension Small Angle X-ray Scattering

The need to characterize line edge and line width roughness in patterns with sub‐50 nm critical dimensions challenges existing platforms based on electron microscopy and optical scatterometry. The development of x‐ray based metrology platforms provides a potential route to characterize a variety of parameters related to line edge roughness by analyzing the diffracted intensity from a periodic array of test patterns. In this study, data from a series of photoresist line/space patterns featuring programmed line width roughness are measured by critical dimension small angle x‐ray scattering (CD‐SAXS). For samples with designed periodic roughness, CD‐SAXS provides the wavelength and amplitude of the periodic roughness through satellite diffraction peaks. For real world applications, the rate of decay of intensity, termed an effective “Debye‐Waller” factor in CD‐SAXS, provides an overall measure of the defects of the patterns. CD‐SAXS data are compared to values obtained from critical dimension scanning electron microscopy (CD‐SEM). Correlations between the techniques exist, however significant differences are observed for the current samples. A tapered cross sectional profile provides a likely explanation for the observed differences between CD‐SEM and CD‐SAXS measurements.The need to characterize line edge and line width roughness in patterns with sub‐50 nm critical dimensions challenges existing platforms based on electron microscopy and optical scatterometry. The development of x‐ray based metrology platforms provides a potential route to characterize a variety of parameters related to line edge roughness by analyzing the diffracted intensity from a periodic array of test patterns. In this study, data from a series of photoresist line/space patterns featuring programmed line width roughness are measured by critical dimension small angle x‐ray scattering (CD‐SAXS). For samples with designed periodic roughness, CD‐SAXS provides the wavelength and amplitude of the periodic roughness through satellite diffraction peaks. For real world applications, the rate of decay of intensity, termed an effective “Debye‐Waller” factor in CD‐SAXS, provides an overall measure of the defects of the patterns. CD‐SAXS data are compared to values obtained from critical dimension scanning electr...