ACS preview: Research reveals work on nanoimprint, porous low-k, PV

Among the literally hundreds of presentations lined up for this week’s American Chemical Society fall national meeting (Boston, MA, Aug. 19-23) are talks about chemical advancements in nearly every field imaginable, from medicine to petroleum and agriculture, and materials science. Here’s a quick rundown of several papers of interest to the semiconductor industry, including research into nanoimprint lithography, porous low-k dielectrics, and photovoltaic cells.

Nanoimprint lithography

A new strategy to achieve an anti-adhesion surface for the simple fabrication of nanostructures with high fidelity, applicable to all types of molds, will be discussed by Mihee Kim et al., Division of Nano Sciences and Department of Chemistry, Ewha Womans U., Seoul, South Korea. Although perfluoro-groups have the lowest surface energy, the adhesion energy, represented by peel fracture energy, of an intrinsic PDMS surface is lower than that of a surface treated with perfluoro-groups. This presentation will introduce a simple fabrication route to a highly transparent super-hydrophobic flexible mold coated with PDMS.

Burcin Erenturk and Kenneth R. Carter, Department of Polymer Science and Engineering, U. of Massachusetts/Amherst, MA, will discuss their successful imprint into porogen-containing low molecular weight, chain extendible polymethylsilsesquioxanes (PMSSQ) SOG. The thermally decomposable porogens are aliphatic polyester-based.

Many monomers that have shown promise for low wavelength lithography have proven difficult to incorporate into polymers of the structure typically used in the lithography industry. Jacob R. Adams et al. from the Willson Lab in the Department of Chemistry and Biochemistry, U. of Texas/Austin, will show polymers synthesized by the formation of a norbornene based acetal backbone, which offers an alternate route to incorporation of these materials. The synthesis and evaluation of a series of new, imageable acetal polymers for general lithography will be presented.

Porous low-k dielectrics

CheongYang Koh et al. from the Institute for Soldier Nanotechnologies and Department of Materials Science and Engineering, Massachusetts Institute of Technology, will review periodic structures in 1,2 and 3 dimensions at the sub-micron and nanoscale, which exhibit interesting behavior with photons and phonons at these length scales. Presenters will discuss interference lithography to form such periodic polymer/air structures for photonics and phononics.

Methods for porosity characterization of porous SiLK dielectric films will be examined by Brian G. Landes et al. from the Dow Chemical Co., Veeco, and the Synchrotron Research Center at Northwestern U. Due to the complex nature of the porous structure, multiple on-wafer methods are being investigated to quantify the porosity: ellipsometry, small angle X-ray scattering (SAXS), X-ray reflectivity (XRR), and atomic force microscopy. Void fraction, pore size and size distribution, pore morphology and uniformity across a porous SiLK film can be measured.

Mikhail R. Baklanov et al., IMEC, Leuven, Belgium, will look at plasma damage to low dielectric constant (low-k) films formed in different plasma reactors, with observed phenomena “well described” by a diffusion-recombination model. The depth of damage, they will report, can be significantly reduced by generation of surface active centers — by VUV photons emitted in a He plasma — that increase the probability of recombination of active radicals.

Barry J. Bauer et al., National Institute of Standards and Technology (NIST), Gaithersburg, MD; Penn State U.; and Technion-Israel Institute of Technology, Haifa, Israel will discuss their work examining pore size distribution (PSD) using small angle neutron scattering (SANS) and x-ray reflectivity porosimetry (XRP). SANS measures the angular dependence of scattered neutrons, and models are applied to the data to extract PSD information. XRP measures the adsorption of a probe molecule as a function of pressure and PSD can be extracted by application of thermodynamic models.

Photovoltaic solar cells

Prashant V. Kamat et al. from the Department of Chemistry and Biochemistry, U. of Notre Dame, and IN Argonne National Laboratory, Center for Nanoscale Materials, Argonne, IL, will explore the photoresponse of TiO2 nanoarray in the visible spectrum by attaching CdSe quantum dots.

Lead sulfide (PbS) quantum dots (QDs) offer potential as an alternative sensitizer in a Graetzel solar cell, because of the many advantages they have over organic dyes, including multiple electron generation (MEG), photo stability, and a controllable spectral absorption range which is tunable through particle size. Chunrong Xiong et al., NanoTech Institute, U. of Texas/Dallas, Richardson, TX, will discuss the preparation of TiO2 nanotubes and nanofibers, which were then doped with PbS QDs of controlled size to control the spectral absorption range.

Work with SnS nanocrystals synthesized from a single source precursor in oleylamine at elevated temperature will be discussed by Dmitry S. Koktysh et al., Vanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt U., Nashville, TN. The SnS nanocrystals’ shape and size can be tuned by controlling the reaction temperature and time, and the nature of the stabilizing ligands. Comparison between experimental optical band gap values shows evidence of quantum confinement of SnS nanocrystals, and prepared low toxicity SnS nanocrystals display strong absorption in the visible and near-infrared spectral regions — making them promising candidates for solar cell energy conversion devices with tunable optical properties. — E.K. and J.M.

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