Periodic Porous Silica Research Articles

Exchange Bias in Iron-Oxide Particles Nanocasted in Periodic Porous Silica

Iron-oxide nanoparticles were nanocasted in the periodic mesoporous silica matrix, consisting of two-dimensional hexagonally ordered channel system with the mean diameter of the channels about 7 nm. The magnetic measurements of dc magnetization confirm behavior typical of a superparamagnetic system, such as the irreversibility of the zero-field-cooled and field-cooled curves, presence of a maximum in zero-field-cooled curve related with blocking temperature TB and revealing of coercivity HC below TB. The existence of negative exchange bias effect below TB was confirmed in our system represented by value of exchange bias field HEB = −970 Oe measured at the temperature 2 K.

Development of Synthesis Methods for Periodic Nano-structured Porous Silica Films

Development of Synthesis Methods for Periodic Nano-structured Porous Silica Films

Theoretical Investigation of Dielectric Constant and Elastic Modulus of Three-Dimensional Isotropic Porous Silica Films with Cubic and Disordered Pore Arrangements

The dielectric constant (k) and elastic modulus (E) of self-assembled three-dimensional porous silica films were investigated by analytical and numerical calculations to reveal the relationship between k and E. It was found that cubic pore arrangements have E values higher than those of random pore arrangements and two-dimensional periodic hexagonal pore arrangements for the same k. It was also found that disordered isotropic porous silica films having cylindrical pores with well-controlled pore size distributions exhibit an E vs k relationship similar to that of two-dimensional hexagonal periodic porous silica films. The elastic modulus of the skeletal silica was determined to be 40 GPa from the combination of the calculated results and experimental data on ultralow-k disordered porous silica film with a k value of 2.0 and a modulus of 8 GPa.

Influence of Humidity on Electrical Characteristics of Self-Assembled Porous Silica Low-k Films

The influence of humidity on the dielectric constant and leakage current of self-assembled porous silica films which have two-dimensional hexagonal periodic porous structures was investigated quantitatively by proposing a new water adsorption model. The amount of adsorption was calculated by the modified Rayleigh model, where molecules are assumed to be adsorbed on the inner surface of cylindrical porous silica structures and form a dispersal concentric double-layer dielectric cylinder system. The amount of calculated by the proposed model was consistent with the measured dielectric constant and thermogravimetry data. It suggests that inner-surface coverage of the cylindrical porous silica wall with a hydrophobic group is the most effective way to suppress water adsorption. Hexamethyldisilazane as a surface coverage molecule was introduced to the periodic cylindrical porous silica film and the leakage current was suppressed by a factor of even below 0.5% relative humidity, resulting in the improvement of time-dependent dielectric breakdown lifetime by a factor of 30.

Theoretical Investigation into Effects of Pore Size and Pore Position Distributions on Dielectric Constant and Elastic Modulus of Two-Dimensional Periodic Porous Silica Films

The effects of the pore size distribution (psd) and pore position distribution (ppd) of two-dimensional (2D) periodic porous silica films on dielectric constant (k) and elastic modulus (E) were investigated by theoretical calculations. It was found that such porous silica films with a wider psd or a wider ppd show lower E value than 2D hexagonal periodic porous silica films with an identical pore size. Careful control of pore structure is necessary for obtaining ultralow-k porous silica films with a high E.

Control of Pore Structures in Periodic Porous Silica Low-k Films

The structure of nanometer-size pores in periodic porous silica low-dielectric-constant (low-k) films was investigated by in-plane and out-of-plane X-ray scattering and diffraction. The periodic porous silica films were prepared by spin coating of a tetraethoxysilane (TEOS)-based precursor solution mixed with self-organizing surfactant sacrificial template molecules. TEOS vapor treatment before the removal of the sacrificial templates was shown to help maintain the periodically ordered structure during the template removal process at 673 K. It was demonstrated that a successful control of pore structure is feasible by the selection of pore-generating surfactant template molecules. Namely, the pore diameter, porosity and dielectric constant were controlled successfully by varying the alkyl chain length in the surfactant template molecules.

Nondestructive Characterization of a Series of Periodic Porous Silica Films by in situ Spectroscopic Ellipsometry in a Vapor Cell

Pore size distributions in a series of periodic porous silica low-dielectric-constant (low-k) films were determined by the analysis of adsorption/desorption isotherms of heptane. The isotherms were obtained from in situ spectroscopic ellipsometric measurements at room temperature in a vapor cell. The porous silica low-k films were prepared by the spin coating of a mixture of silica precursor and alkyltrimethylammoniumchloride surfactant template solutions. A systematic increase in pore diameter with an increase in the alkyl chain length of the template molecules was observed. It is concluded that the in situ spectroscopic ellipsometry in a vapor cell as a nondestructive technique of characterizing pore structures in porous low-k films has the potential to clearly distinguish such a slight difference in pore diameter as is caused by only –(CH2)2– in a template molecular structure.

Theoretical Analysis of Elastic Modulus and Dielectric Constant for Low-k Two-Dimensional Periodic Porous Silica Films

To lower the dielectric constant k of interlayer-dielectric films with two-dimensional pore structures while maintaining their mechanical strength, the influences of pore arrangement on the elastic modulus E and k of the films were investigated. It was found that periodicity in pore structure enhances E with constant k. Periodic porous silica films having a hexagonal arrangement of circular cylindrical pores with k<2.0 and E>3 GPa were demonstrated to be feasible at a porosity of 0.614 using a bulk material with a k of 4.0 and E>21 GPa.

Mechanical properties of periodic porous silica low-k films determined by the twin-transducer surface acoustic wave technique

Low dielectric constant (k) materials qualified for interconnect integration of high speed devices are required in the microelectronic industry. The introduction of nanopores into the dielectrics reduces their k but also deteriorates their mechanical properties. An accurate measurement technique for determining the mechanical properties of such thin fragile low-k films is needed. We report on a designed piezoelectric twin-transducer detector for accurate and nondestructive characterization of mechanical properties for porous low-k films by the laser-generated surface acoustic wave. Experimental results of several low-k films demonstrate that influences from the fabricating processes and material treatments on Young’s elastic modulus can be sensitively detected by this technique.

Effects of Surfactants on the Properties of Ordered Periodic Porous Silica Films

We report the effects of surfactants on the properties of ordered periodic porous silica films. Silica films with a periodically ordered structure were synthesized on crystalline silicon substrates by spin-coating an alkoxysilane solution mixed with a self-assembling template and a cationic alkyltrimethylammonium chloride (ATMACl, CH3(CH2)n-1N(CH3)3 Cl) surfactant, where n was 12, 14, or 16. The effects of surfactants on the properties of porous silica films with a periodic pore structure were investigated. Both dielectric constant and refractive index decreased with the length of alkylchain in the template molecule ATMACl. The pore structure was successfully controlled by the alkylchain length of the template molecule when the tetraethyl-orthosilicate (TEOS) vapor treatment was applied.

Advanced characterization of ultra-low-k periodic porous silica films – pore size distribution, pore-diameter anisotropy, and size and macroscopic isotropy of domain structure

AbstractWe have shown previously the results from out-of-plane and in-plane X-ray scattering /diffraction measurements together with transmission electron microscope and X-ray reflectance measurements and shown that they are effective in characterization of a periodic porous silica low-k film [1]. In the present work, we report the results on pore-size distribution, pore-diameter anisotropy, and size and macroscopic isotropy of domain structure.

Elasticity through Nanoscale Distortions in Periodic Surfactant-Templated Porous Silica under High Pressure

High-pressure infrared absorption spectroscopy is used to examine changes in local bonding upon hydrostatic compression in both ordered surfactant-templated mesoporous silica and sintered sol−gel silica with a goal of connecting atomic scale structural changes with variations in nanoscale periodicity. High-pressure IR absorption spectra are analyzed on the basis of a noncentral force model. It is found that the intertetrahedral bond angle and its distribution width in both the dense and the mesoporous silica decrease at elevated pressure up to 4 GPa. With increasing pressure above this value, decreases in the average bond angle and distribution width cease in the mesoporous silica, while they continue in the bulk material. The results suggest that in the mesoporous silica the nanometer length scale of the silica framework makes it energetically unfavorable to form high-density atomic scale structures at higher pressures (>4 GPa). Instead, further compression of the mesoporous silica takes place by distortion of the periodic pore structures on the nanometer scale (deformation of pores). Surprisingly, upon the release of pressure, structural changes on both the nanometer and atomic length scales are reversible. The results suggest that reversible nanometer scale distortions in periodic porous materials can replace the irreversible atomic scale distortions observed in bulk amorphous silica.

Macroporous silica and titania obtained using poly[styrene-co-(2-hydroxyethyl methacrylate)] as template

Ordered macroporous silica and titania have been prepared using poly[styrene-co-(2-hydroxyethyl methacrylate)] (PS-HEMA) latex arrays as templates. Polystyrene (PS) particles have also been used to template macroporous titania. Arrays of spherical latex particles, with average diameters of 260 nm (PS-HEMA) and 390 nm (PS), have been obtained by either evaporation of the solvent at room temperature or by deposition on filtration membranes. In both cases, 3D extended regions with cubic close packing of microspheres are observed, but in the samples obtained after evaporation of the solvent the self-assembled particles show a higher degree of order. The oxides were synthesized by hydrolysis of sodium silicate or titanium isopropoxide (diluted in n-propanol or hexane) which were infused into the cavities of the latex arrays. Two different methods have been used to fill the cavities with the precursor: filtration with suction and capillary forces. When solidification of the ceramic matrix took place, the template was eliminated by heating at 500 °C. As replica of the latex assemblies, periodic porous silica and titania were obtained. Sheets of amorphous silica, as thin as 4–7 nm, form the wall of the interconnected pores whose diameters range from 200 to 220 nm. In macroporous titania obtained from titanium isopropoxide diluted in n-propanol and imprinted on PS-HEMA particles, the pore diameters range from 160–180 nm. The porous network is shaped by walls that are 8–12 nm thick and are formed from anatase nanocrystals with a major dimension of 8–20 nm. Pore diameters, crystallite sizes and thicknesses are similar when hexane is used as the solvent of the inorganic precursor. However, in macroporous titania imprinted on PS particles, nanocrystals with a major dimension of 6–10 nm form the pore wall with a thickness of 6 to 8 nm. Interactions between the titania precursor and the latex particles seem to affect the microstructural parameters. The porous skeleton of titania is preserved after treatment at 600 °C for 16 hours but the pore wall thickness slightly increases with crystallite growth and in some particles disordered areas appear.

Novel Periodic Nanoporous Silicate Glass With High Structural Stability as Low-k Thin Film

AbstractWe have recently developed novel periodic nanoporous silicate glass with high structural stability as low-k thin film by spin-coating method. Periodic porous silicate glass films developed so far cause structural shrinkage (10&gt;∼20% or more) by annealing the spin-coated films. In this investigation we adopt vapor-phase TEOS (tetraethoxysilane)-treatment before anneal. Our novel nanoporous film shows little shift of XRD peak position after annealed at 673K, indicating both the ultimate mechanical strength and the minimization of stress in the interface between the prepared film and the underlying substrate. Such a shrinkage-free periodic nanoporous silica film can possess higher VBD (break down voltage) and lower ILeak (leakage current). In this article we estimate structural properties (including information on pores introduced intentionally) by XRD and TEM observation, and electrical properties (dielectric constant, VBD and ILeak) by IV and CV measurement of this special-treated periodic nanoporous silica film. The dielectric constant of the thus prepared periodic porous silica film with silylation after calcination was evaluated to be around 1.8 at 100kHz.