Xerogel Films Research Articles

The effects of pre-aging and concentration of surface modifying agent on the microstructure and dielectric properties of SiO 2 xerogel film

Surface modification is a key process for the preparation of porous SiO 2 xerogel film by ambient pressure drying. The microstructure and surface terminal bonds which determine the properties of the film are closely related with the surface modification. Two kinds of pre-aged film were prepared to investigate the effect of wet-gel microstructure on the surface modification of film. The concentration of surface modifying agent (tetramethylchlorosilane; TMCS) in solvent ( n-hexane) was also varied. As concentration of TMCS increased, surface morphology was obtained from a two-dimensional structure to a three-dimensional network structure with pre-aged film for 1 h at 70°C. This is due to the increase in methyl groups as surface terminal bonds and the suppression of condensation reaction. Variation in film thickness, porosity, dielectric constants, and I– E characteristic according to the TMCS concentration in n-hexane modifying solution was explained in the view-point of the nature of wet-gel structure. However, because of the additional increase in the strength of network and the effective exchange of surface terminal –OH groups with –OSi(CH 3) 3 groups even with low concentration of TMCS, there were no serious changes in the properties of SiO 2 xerogel film pre-aged for 12 h at 70°C.

Etching of xerogel in high-density fluorocarbon plasmas

The etching of various xerogel films has been studied in high-density fluorocarbon plasmas. The xerogel etch rate is in part enhanced by the porosity. In discharges resulting in low surface polymerization, such as CF4 or oxygen-rich fluorocarbon plasmas, an additional enhancement up to 60% is observed. When the polymerization of the discharge is increased, this additional enhancement disappears and the xerogel etch rate becomes more suppressed. The suppression is more pronounced for xerogel films with a higher porosity and a larger pore size. X-ray photoelectron spectroscopy analysis on partially etched samples shows that the suppression in etch rate is accompanied by an increasing amount of fluorocarbon material at the xerogel surface, especially in the pores of the xerogel structure. Finally, a 30% porous xerogel film was patterned using CHF3 as an etching gas. Slight bowing of the sidewalls was observed.

Thermal conductivity study of porous low-k dielectric materials

An experimental method based on the 3ω technique has been developed to measure thermal conductivity of porous Xerogel films as a function of porosity. The results show that the thermal conductivity of these porous dielectric films can be an order of magnitude smaller than that of SiO2. To account for the porosity dependence of thermal conductivity, two porosity weighted semiempirical models are introduced. These models suggest the scaling rule expressing the thermal conductivity as a function of porosity. The decrease observed in thermal conductivity of porous films suggests that the tradeoff between thermal and electrical performance is an important consideration when implementing porous dielectric materials as interlevel dielectrics for on-chip interconnects.

EQCM measurements of solvent transport during Li + intercalation in V 2O 5 xerogel films

The electrochemical quartz crystal microbalance technique has been used to monitor the mass changes that occur in acetonitrile electrolytes during electrochemically driven Li + intercalation into V 2O 5 xerogel films prepared by aqueous sol–gel methods. The mass measurements made with the EQCM reveal that during reduction of the vanadium sites in the sol–gel oxide at low voltammetric scan rates (≤5 mV/s) the net compositional change corresponds to insertion of one Li + per electron injected, with no accompanying solvent (i.e. no net mass change that can be attributed to solvent transfer into or out of the film). However, at higher scan rates (>50 mV/s), increased mass changes during reduction are attributed to simultaneous acetonitrile transport into the V 2O 5 film. Based on previous descriptions of solvent swelling of V 2O 5 thin films in acetonitrile solutions, these results suggest the following: (1) during reduction at low scan rates the intercalation of solvated Li + takes place simultaneously with some solvent expulsion, such that the net mass change corresponds essentially exclusively to intercalation of one Li + per electron, (2) at higher scan rates, the rapid electromigration of Li + into the film drives simultaneous solvent insertion, such that the net compositional change corresponds to a net gain of considerable amounts of solvent during the Li + intercalation process. This solvent swelling leads to structural changes that allow larger amounts of solvent to transfer into the films during Li + intercalation for some time after the high scan rate perturbation. The implications of these findings for Li + secondary battery cathode materials are discussed.

Development and investigation of sol–gel solutions for the formation of TiO 2 coatings

We show a technological approach for the sol–gel processing of stabilized xerogel colloidal titanium oxide films. Glycerol was used as a drying control additive agent. Glycerol helped in stabilizing the solution. The thermal transformation of a xerogel film was studied by differential thermal analysis (DTA) and powder X-ray diffraction. The optical index of the annealed coatings was evaluated using UV–VIS–NIR spectrophotometry. The results showed that a nanocrystalline titania anatase film of high optical quality ( n=2.34 at 600 nm) can be obtained by the sol–gel process.

Mechanical properties of xerogel silica films derived from stress versus temperature and cracking experiments

The biaxial modulus, coefficient of thermal expansion, and crack energy release rate of sol–gel derived silica, xerogel, films are presented. Using stress versus temperature measurements for xerogel on Si and quartz substrates, the biaxial modulus and linear thermal expansion coefficient of the film were determined. The film thickness at the onset of cracking and the corresponding stress were also found. For the critical thickness sample, the cracks initiated in the xerogel film and propagated into the Si substrate. The depth of the cracks in the substrate was roughly equal to the thickness of the film. From these data, the crack energy release rate for steady state channeling in the film was estimated; it was found to be roughly five times 2γs, where γs is the reported surface energy of the film. This is an indication of a plastic zone at the crack tip.

Scandia optical coatings for application at 351 nm

The recent interest in high-power UV lasers, as energy source to study nuclear fusion in confinement, has motivated the design of new high-index materials highly resistant to UV radiation. Such materials were to be deposited on optical surfaces and used as components for lenses, mirrors, protective layers, polarizers, etc., devices. Quarter-wavelength thickness (i.e. 351/4 nm) optical thin films of scandia, hafnia and zirconia xerogels, produced from sol–gel chemistry, have been prepared on fused silica substrates by dip coating. The initial scandia-based particles were lozenge-shaped platelets (i.e. 70×40×5 nm). These were highly stable in suspension in alcohols, and did not change their morphology after calcination at 773 K. Hafnia- and zirconia-based particles were spherical and less than 5 nm in dimension. Structures and surface topographies of coatings were characterized by atomic force microscopy and photothermal deflection. X-ray photoemission spectroscopy investigation allowed for the determination of the thin film chemical composition. Optical properties of coatings were deduced from UV-transmittance and ellipsometry measurements. Resistance to laser radiation at 351 nm in N-on-1 and R-on-1 modes are reported in detail. Each xerogel thin film was of high optical quality, exhibited a low surface roughness, and had excellent transparency at 351 nm. Scandia thin films were successfully purified by solvent extraction and thermal treatment. Annealing the scandia-based coatings at 773 K induced a densification of the xerogel network to cubic Sc2O3, exhibiting a refractive index of 1.85. Except for the dense scandia layer, other coatings exhibited laser-induced damage threshold (LIDT) above the resistance of the bare fused silica substrate (LIDT>11 J/cm2), suggesting that they could all be used as component for UV optical devices.

Determination of pore size distribution in thin films by ellipsometric porosimetry

We show that ellipsometric porosimetry can be used for the measurement of the pore size distribution in thin porous films deposited on top of any smooth solid substrate. In this method, in situ ellipsometry is used to determine the amount of adsorptive, which is adsorbed/condensed in the film. Changes in refractive index and film thickness are used to calculate the quantity of adsorptive present in the film. Room temperature porosimetry based on adsorption of vapor of organic solvents has been developed. In this article, a method of calculation of pore size distribution and results of measurements on mesoporous and microporous xerogel films is discussed. Examination of the validity of the Gurvitsch rule for various organic adsorptives (toluene, heptane, and carbon tetrachloride) is carried out to assess the reliability of measurements of pore size distributions by ellipsometric porosimetry.

Metal diffusion barriers for porous SiO2

The diffusion characteristics of metal barrier materials in porous films having low dielectric constant are studied using the Rutherford backscattering spectrometry technique. It is shown that barrier materials such as TaN and TiN do not diffuse into the xerogel film with porosity of 70%–75% after 450 °C annealing in N2 ambient for 2 h. This result points to an interesting possibility that one may not require a dielectric liner for a porous film (to prevent the diffusion of metal barrier materials into the porous material) in the Cu porous film interconnect technology.

Spectroscopic and electrochromical properties of metallophthalocyanines in silicate bulks and thin films prepared by the sol–gel method

Magnesium phthalocyanine (MgPc) and praseodymium bisphthalocyanine (Pr(Pc) 2) have been entrapped in bulks and thin films of porous silicate xerogels prepared via the sol–gel method. The electronic absorption data obtained for the complexes immobilized in bulk sol–gel matrices indicate the formation of complex equilibria between their monomeric forms and respective agglomerates. Thin xerogel films doped with metallophthalocyanines and connecting two slides of transparent ITO electrodes display changes in the absorption spectra caused by varying of the applied voltage. These changes appear to be reversible in certain ranges of the applied voltages.

Grazing Incidence Small Angle X-Ray Scattering Study on Low Dielectric Thin Films

AbstractHighly porous silica films with pore size in the nanometer scale are being extensively studied as potential candidates for interlevel dielectrics. Because these dielectric materials appear in the form of thin films with a thickness of only several thousand Angstroms, conventional techniques are difficult to be readily applied to study their structure and porosity. We employed small angle scattering in the grazing incidence geometry in this study. Using high resolution xray beamline with synchrotron radiation source, we demonstrate that the small angle x-ray scatteirng (SAXS) data of the porous films can be obtained. The structure of sol-gel derived silica - xerogel films on silicon substrate studied by specular reflectivity and grazing incidence small angle x-ray scattering (GISAXS) will be presented.

Processing, Characterization and Reliability of Silica Xerogel Films for Interlayer Dielectric Applications

AbstractSurface modified silica xerogel films of high porosity (25-90 %) and uniform thickness (0.4-2 µm) were fabricated at ambient pressure on silicon and other substrates. Mechanical reliability of the films was determined by measuring fracture toughness (adhesive) as a function of aging time and temperature using the modified edge-lift-off technique. There is an optimum aging time at 60 °C aging to obtain maximum fracture toughness for the procedure used here.Cu/xerogel/Si and Ta/xerogel/Si structures were annealed at different temperatures and in different ambient environments were analyzed using RBS and optical microscopy to assess the extent of interaction with the xerogel film. When annealed in N2 with trace amounts of O2 (equivalent to 10-7-10-6 Torr vacuum), RBS analysis does not show diffusion of Cu or Ta through the xerogel up to 450 °C. At higher temperatures, or in the presence of larger concentrations of O2, Cu and Ta oxidize. Cu oxidation leads to significant diffusion through the xerogel. Ta oxidation also results in diffusion-like RBS spectra. Using the micron-size ion beam to probe the Ta surface, this was found to be solely due to buckling of Ta films on xerogel. A thin SiNx layer on top of Cu and Ta prevents metal oxidation up to 640 °C, Cu diffusion, and Ta buckling.

Effects of Processing Temperature on the Oxygen Quenching Behavior of Tris(4,7′-diphenyl-1,10′-phenanthroline) Ruthenium (II) Sequestered Within Sol-Gel-Derived Xerogel Films

Sol-gel processing methods offer novel pathways for tailoring glasses. Amongst the issues that have received the least attention are the effects of the curing temperature on the behavior and photophysics of a dopant molecule sequestered within a sol-gel-derived xerogel. Of particular interest to our group are the effects of processing variables on the ability of a dopant molecule, that is sequestered within a xerogel glass, to be accessed by an analyte and the distribution of the dopant sites within the xerogel. The thermal stability of the luminophore tris(4,7′-diphenyl-1,10′-phenanthroline) ruthenium (II) ([Ru(dpp)3]2+) provides a convenient way to address these issues and develop an understanding of how one might best exploit curing temperature to construct improved chemical sensors. This paper focuses on quantifying how the film curing temperature affects the spectroscopy and O2 quenching of ([Ru(dpp)3]2+) sequestered within sol-gel-derived xerogel thin films. Our quenching data on films once they have been cured demonstrate that there is a dramatic increase in the sensitivity of the ([Ru(dpp)3]2+) molecules to O2 quenching when the films have been cured at elevated temperatures. This arises primarily because there are two main types of ([Ru(dpp)3]2+) microenvironments within the glass and higher temperature curing leads to an increase in the bimolecular quenching rate between O2 and ([Ru(dpp)3]2+). This is accomplished as follows. Below a curing temperature of 100–150°C, ∼15% of the xerogel-doped ([Ru(dpp)3]2+) molecules are not accessed to any detectable degree by the O2 molecules during the ([Ru(dpp)3]2+) excited-state luminescence lifetime. However, as the xerogel is cured at or above 150°C, residual silanol-bound waters (or other impurities) dissociate from the xerogel and those ([Ru(dpp)3]2+) molecules that were initially inaccessible become accessible to O2. The dissociation of these water molecules, plus other events, also causes the originally inaccessible ([Ru(dpp)3]2+) population to ultimately exhibit a quenching rate that is greater than the fraction of initially accessible ([Ru(dpp)3]2+) molecules that were formed under ambient curing conditions.

Structural and electrochromic properties of nanosized Fe/V-oxide films with FeVO 4 and Fe 2V 4O 13 grains: comparative studies with crystalline V 2O 5

Various mixed Fe/V-oxides can be used as anodes in Li + rocking chair batteries, however, their small optical modulation during the insertion/extraction of Li + ions makes them candidates for the counter electrodes in electrochromic (EC) devices. The sol–gel route in combination with dip-coating deposition was used for the preparation of Fe/V-oxide films with molar ratios Fe:V=0.1:1, 1:2, 1:1 and 2:1. X-ray diffraction combined with Fourier transform infrared (FT-IR) spectroscopy studies of films and powders reveal that heating of xerogel films at 400°C produces films with nanosized FeVO 4 (Fe:V=1:1) and Fe 2V 4O 13 (Fe:V=1:2) grains, while the corresponding crystalline powders were obtained at 500°C (8 h). Charge capacities ( Q) of Fe/V-oxide films (300 and 400°C) were determined using cyclic voltammetry (CV) from 1.5 to −1.5 V vs. Ag/AgCl (4.8 to 1.8 V vs. Li) in 1 M LiClO 4/propylene carbonate (PC) electrolyte. Our results revealed that Q values of Fe/V-oxide films are up to 20 mC cm −2 depending on the thickness (40–100 nm), temperature of heating and the Fe:V molar ratio (1:2, 1:1). During the first 300 cycles the cycling stability of the Fe-containing films is better than that of V 2O 5 crystalline films. UV-visible spectra of charged/discharged films revealed that these films, similar to V 2O 5 films, exhibit a mixed anodic/cathodic electrochromism. It was established that with regard to the colouring/bleaching changes of V 2O 5 crystalline films, the Fe/V-oxide films exhibit smaller cathodic colouring at wavelengths λ>600 nm and higher visible transmittance. IR spectroscopy of charged/discharged Fe/V-oxide films confirmed that the reduction of Fe 3+ prevents the overreduction of V 5+ to V 3+, which takes place in V 2O 5 films cycled in the same potential range.

Effect of sol rheology on the uniformity of spin-on silica xerogel films

Spin-on xerogels, which are promising candidates for use as interlayer dielectric materials in future microelectronic devices, change from a Newtonian liquid to a solid gel during processing. Since the rheology of the sol may affect the uniformity of the xerogel films produced, here we relate the rheology of a two-step, acid-base catalyzed, sol-gel system to the thickness and porosity profiles across xerogel films of importance to the microelectronics industry. We also analyze the effect of spin speed on the thickness and porosity of the films. Our rheological studies of the xerogel sol demonstrated that the sol changes from Newtonian far from the gel point, to shear thinning close to the gel point. On films spin coated with shear-thinning sols there is a region of uniformity extending for a distance of about 5 mm from the center. The film thickness and porosity are highest in this region and both quantities decrease towards the edge. If the sol is spun in its Newtonian regime, the resulting films are uniform (<3% thickness and porosity). The predictions of film thickness and uniformity based on simple models for spin coating a Newtonian and truncated power law fluid were found to be in good agreement with the experimental observations. The film thickness was varied from 0.485 to 1.9 μm by adjusting the spin coating speed from 5900 to 1000 rpm. Over this sixfold range of speed, the porosity was almost constant and varied by less than 10% while the thickness varied by about a factor of 4.

Durable Modification of Silica Aerogel Monoliths with Fluorescent 2,7-Diazapyrenium Moieties. Sensing Oxygen near the Speed of Open-Air Diffusion

Silica sol−gels were covalently modified with N-(3-trimethoxysilylpropyl)-2,7-diazapyrenium bromide (DAP). Luminescent aerogels are created in which none of the fluorophore leaches from the gels during either the washing or supercritical drying necessary to prepare DAP-modified aerogels. The bulk density (0.17 g/cm3), N2-adsorption surface area (870 m2/g), and thermogravimetric and scanning electron micrographic characteristics of the dye-modified aerogels remain identical to those of the unmodified silica aerogels. The bulk concentration of the dye in the aerogels was ≤6.6 mM; at these loadings the aerogels demonstrate bulk photoluminescence. As based on the mesoporous surface area, the surface coverage of the dye is 7−8% of a monolayer. The absorption, emission, and O2-quenching characteristics of the diazapyrenium dye in the aerogels parallel those obtained in alcoholic (rather than aqueous) solution, which further indicates that the dopant molecules are isolated from each other and that they see an environment with a ∼OH polarity. Time-resolved emission studies indicate that all DAP moieties reside in a single type of microenvironment. Emission quenching of ∼1-cm-diameter monoliths of DAP−silica aerogel is complete in <15 s, which compares very favorably with the best response times for pyrene guests in micrometer-thick xerogel films. The apparent diffusion coefficient of O2 or Ar in the DAP−aerogel monoliths was estimated at ≥0.01−0.02 cm2/s, which is only 10× less than the unimpeded diffusion coefficient of Ar in air.

Electrochemical conditioning of vanadium(V) pentoxide xerogel films

The electrochemical behavior of layered vanadium(V) pentoxide xerogel films has been studied, focusing on the lithium intercalation and reorganization effects accompanying the electrode conditioning process. The films at the initial condition consist of electrochemically heterogeneous regions, which collapse into a uniform band structure as the lithium ions are inserted into the interlamellar space. At the saturation point, a single broad intervalence-transfer band is observed in the visible–near infrared region, and the electrochemical behavior becomes independent of the diffusion process.

The sol-gel transformation of TIPT coatings: a FTIR study

The sol-gel transformation mechanisms of tetraisopropyl orthotitanate (TIPT) derived coatings were qualitatively studied by Fourier transform infrared (FTIR) spectroscopy. Kinetics studies were carried out in the liquid state for pure and diluted precursor. Xerogel film resulting from a stable isopropoxide-derived sol with low water content and acidic pH was also studied as a function of time. The absorption band evolution was followed, from the non-hydrolysed metal isopropoxide until the xerogel thin film formation. This evolution was interpreted and discussed in terms of TiO2 sol-gel reactions (hydrolysis, reesterification or transesterification, condensation).

Spectroscopic properties of pure and Coumarin 153-doped thin films of sol-gel silica xerogels

Thin-films doped with Coumarin 153, an important laser dye, have been prepared by the sol-gel method. Spectroscopic properties of the entrapped dye are studied by electronic absorption, diffuse reflectance, electronic excitation, emission and time-resolved emission techniques. The obtained results indicate that the entrapment of the dye results in formation of molecular aggregates having absorption and emission characteristics different from those of the monomeric dye. The blue matrix emission band (at approx. 450nm) is relatively short-lived while the red band (at approx. 700nm) displays a very long lifetime. Electronic excitation spectra of the sol-gel thin-films suggest that the two matrix-related luminescence bands possess the same electronic origin.

Surface modified spin-on xerogel films as interlayer dielectrics

SiO 2 -based xerogels are highly porous materials that may enhance the performance of microelectronic devices due to their extremely low dielectric constants (ε=1.36–2.2). Conventional xerogel and aerogel manufacturing techniques include an expensive and hazardous supercritical drying step to deposit crack free, high porosity films. Ambient drying techniques have recently been developed and in this article, we discuss how the process parameters in the ambient drying process affect the properties of a spin-coated film. Successful spin-on deposition of highly porous (&amp;gt;70%), thick (&amp;gt;1 μm), crack-free, xerogel films was accomplished using a solvent saturated atmosphere during spinning and aging. The saturated atmosphere allowed for the isolation of each processing step and a better understanding of the effects of process variable changes. The film porosity was controlled by varying the extent of silylation (surface modification), the aging time, or the initial water/silane ratio. Fourier transform infrared spectra demonstrated that silylation of xerogel films helps eliminate bound moisture in these films and renders them hydrophobic. Finally, the dielectric constants extrapolated from refractive index measurements were in good agreement with those obtained from our conventional electrical measurements.