Planar Silicon Substrates Research Articles

Nanotribological Investigation of Polymer Brushes with Lithographically Defined and Systematically Varying Grafting Densities.

Following controlled photodeprotection of a 2-nitrophenylpropyloxycarbonyl-protected (aminopropyl)triethoxysilane (NPPOC-APTES) film and subsequent derivatization with a bromoester-based initiator, poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes with various grafting densities were grown from planar silicon substrates using atom transfer radical polymerization (ATRP). The grafting density correlated closely with the extent of deprotection of the NPPOC-APTES. The coefficient of friction for such PMPC brushes was measured by friction force microscopy in water and found to be inversely proportional to the grafting density due to the osmotic pressure that resists deformation. Deprotection of NPPOC-APTES via near-field photolithography using a range of writing rates enabled the fabrication of neighboring nanoscopic polymeric structures with dimensions ranging from 100 to 1000 nm. Slow writing rates enable complete deprotection to occur; hence, polymer brushes are formed with comparable thicknesses to macroscopic brushes grown under the same conditions. However, the extent of deprotection is reduced at higher writing rates, resulting in the concomitant reduction of the brush thickness. The coefficient of friction for such polymer brushes varied smoothly with brush height, with lower coefficients being obtained at slower writing rate (increasing initiator density) because the solvated brush layer confers greater lubricity. However, when ultrasharp probes were used for nanotribological measurements, the coefficient of friction increased with brush thickness. Under such conditions, the radius of curvature of the tip is comparable to the mean spacing between brush chains, allowing the probe to penetrate the brush layer leading to a relatively large contact area.

Properties of Zirconium Oxide and Cobalt Ferrite Layered Nanocomposite

CoFe2O4 nanoparticles with 3–30 nm in diameter were synthesized by sol-gel method. The particles were spread as a solid discontinuous layer over planar silicon and TiN substrates by spin coating and covered by 15 nm thick ZrO2 films by atomic layer deposition. Crystal structures distinctively characteristic of CoFe2O4 and ZrO2 constituents were preserved. The nanocomposite CoFe2O4-ZrO2 layers demonstrated dielectric polarization, saturative magnetization, and implications of resistive switching behavior. Behavior most clearly attributed to memory materials was observed in the field admittance characteristic with two distinct states in susceptibility of the nanocomposite.

Degrafting of Poly(poly(ethylene glycol) methacrylate) Brushes from Planar and Spherical Silicon Substrates

Densely grafted, hydrophilic polymer brushes produced via surface-initiated controlled radical polymerization have been shown to undergo degrafting upon exposure to aqueous media. This degrafting process has been proposed to involve swelling-induced, mechanochemically facilitated hydrolysis of bonds located at the brush–substrate interface. While a number of reports have described degrafting of hydrophilic polymer brushes, only little is known about the key structural parameters of these thin films that dictate this process. Using a series of PPEGMA and PPEGMEMA brushes produced by surface-initiated atom transfer radical polymerization (SI-ATRP), this report investigates the influence of three parameters: (i) the chemical structure of the ATRP initiator, (ii) the molecular weight of the surface-grafted polymer chains, and (iii) surface curvature. Studies performed with PPEGMA and PPEGMEMA brushes grown from substrates modified with different ATRP initiators indicated that hydrolysis of both siloxane as we...

Porous, High Capacity Coatings for Solid Phase Microextraction by Sputtering.

We describe a new process for preparing porous solid phase microextraction (SPME) coatings by the sputtering of silicon onto silica fibers. The microstructure of these coatings is a function of the substrate geometry and mean free path of the silicon atoms, and the coating thickness is controlled by the sputtering time. Sputtered silicon structures on silica fibers were treated with piranha solution (a mixture of concd H2SO4 and 30% H2O2) to increase the concentration of silanol groups on their surfaces, and the nanostructures were silanized with octadecyldimethylmethoxysilane in the gas phase. The attachment of this hydrophobic ligand was confirmed by X-ray photoelectron spectroscopy and contact angle goniometry on model, planar silicon substrates. Sputtered silicon coatings adhered strongly to their surfaces, as they were able to pass the Scotch tape adhesion test. The extraction time and temperature for headspace extraction of mixtures of alkanes and alcohols on the sputtered fibers were optimized (5 min and 40 °C), and the extraction performances of SPME fibers with 1.0 or 2.0 μm of sputtered silicon were compared to those from a commercial 7 μm poly(dimethylsiloxane) (PDMS) fiber. For mixtures of alcohols, aldehydes, amines, and esters, the 2.0 μm sputtered silicon fiber yielded signals that were 3-9, 3-5, 2.5-4.5, and 1.5-2 times higher, respectively, than those of the commercial fiber. For the heavier alkanes (undecane-hexadecane), the 2.0 μm sputtered fiber yielded signals that were approximately 1.0-1.5 times higher than the commercial fiber. The sputtered fibers extracted low molecular weight analytes that were not detectable with the commercial fiber. The selectivity of the sputtered fibers appears to favor analytes that have both a hydrophobic component and hydrogen-bonding capabilities. No detectable carryover between runs was noted for the sputtered fibers. The repeatability (RSD%) for a fiber (n = 3) was less than 10% for all analytes tested, and the between-fiber reproducibility (n = 3) was 0-15%, generally 5-10%, for all analytes tested. The repeatabilities of our sputtered fibers and the commercial 7 μm PDMS fiber are essentially the same. Fibers could be used for at least 300 extractions without loss of performance. More than 50 compounds were identified in a gas chromatography-mass spectrometry headspace analysis of a real world botanical sample with the 2.0 μm fiber.

Periodic nanostructured Au arrays on an Si electrode for high-performance electrochemical detection of hydrogen peroxide without an enzyme

Periodic gold nanosphere arrays were prepared on a planar silicon substrate, which could be directly developed as an electrode to sensitively and selectively detect H2O2 without an enzyme via an electrochemical method.

Structure and field emission of graphene layers on top of silicon nanowire arrays

Monolayer graphene was grown on copper foils and then transferred on planar silicon substrates and on top of silicon nanowire (SiNW) arrays to form single- to quadruple-layer graphene films. The morphology, structure, and electron field emission (FE) of these graphene films were investigated. The graphene films on the planar silicon substrates were continuous. The single- to triple-layer graphene films on the SiNW arrays were discontinuous and while the quadruple-layer graphene film featured a mostly continuous area. The Raman spectra of the graphene films on the SiNW arrays showed G and Gʹ bands with a singular-Lorentzian shape together with a weak D band. The D band intensity decreased as the number of graphene layers increased. The FE efficiency of the graphene films on the planar silicon substrates and the SiNW arrays varied with the number of graphene layers. The turn-on field for the single- to quadruple-layer graphene films on planar silicon substrates were 4.3, 3.7, 3.5 and 3.4V/μm, respectively. The turn-on field for the single- to quadruple-layer graphene films on SiNW arrays decreased to 3.9, 3.3, 3.0 and 3.3V/μm, respectively. Correlation of the FE with structure and morphology of the graphene films is discussed.

Reversible structural transformation and enhanced performance of PEDOT:PSS-based hybrid solar cells driven by light intensity.

PSS) and appropriate amounts of a cosolvent and a fluorosurfactant on planar n-type silicon substrates showed a photoconversion efficiency (PCE) of above 13%. These cells also exhibited stable, reproducible, and high external quantum efficiency (EQE) that was not sensitive to light-bias intensity (LBI). In contrast, solar cells made of pristine PSS showed low PCE and high EQE only under certain measurement conditions. The EQE was found to degrade with increasing LBI. Here we report that the LBI-sensitive variation of EQE of the low-PCE cells is related to a reversible structural transformation from a quinoid to a benzoid structure of PEDOT.

Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin films grown by atomic layer deposition

Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin films were grown by atomic layer deposition on silicon substrates. For depositing dysprosium and titanium oxides Dy(thd)3-O3 and TiCl4-O3 were used as precursors combinations. Appropriate parameters for Dy(thd)3-O3 growth process were obtained by using a quartz crystal microbalance system. The Dy2O3 films were deposited on planar substrates and on three-dimensional substrates with aspect ratio 1:20. The Dy/Ti ratio of Dy2O3-doped TiO2 films deposited on a planar silicon substrate ranged from 0.04 to 0.06. Magnetometry studies revealed that saturation of magnetization could not be observed in planar Dy2O3 films, but it was observable in Dy2O3 films on 3D substrates and in doped TiO2 films with a Dy/Ti atomic ratio of 0.06. The latter films exhibited saturation magnetization 10−6 A cm2 and coercivity 11 kA/m at room temperature.

Self-termination in the gas-phase layer-by-layer growth of an aza silane and water on planar silicon and nylon substrates

The authors report the gas phase, layer-by-layer deposition of an organosilane (N-n-butyl-aza-2,2-dimethoxysilacyclopentane, 1) and either water or aqueous ammonium hydroxide onto two substrates: Si/SiO2 and nylon. This process results in smooth, water resistant, inorganic-organic barrier layers. The layer-by-layer deposition of 1 appears to be self-limiting to a few nanometers, which may make it useful where ultrathin films of controllable dimensions and uniformity are desired. The authors are unaware of another thin film system that has these properties. Films were characterized by spectroscopic ellipsometry, water contact angle goniometry, x-ray photoelectron spectroscopy, and atomic force microscopy. Interestingly, film thicknesses on nylon were much higher than on silicon, and films prepared in the presence of the ammonia “catalyst” were thinner than those prepared with water. Test circuits coated only with a fluorosilane showed higher penetration of water compared to those coated with a barrier layer of 1/H2O and the fluorosilane.

Potentiometric Response Characteristics of Oxide-Coated Gallium Electrodes in Aqueous Solutions

This paper reports on the potentiometric response characteristics of gallium electrodes coated with native or anodic oxide layers in aqueous solutions. Solid metallic gallium electrodes were prepared by cooling liquid gallium monoliths supported either on planar silicon substrates or incorporated within glass capillaries. These metallic gallium electrodes showed potentiometric pH responses with a slope of ca. -38mV/pH in the pH range between 3 and 9. The potentials measured could originate from redox processes involving surface oxide that was formed as a result of reactions between the metallic gallium and water (and/or OH−), as observed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Similar potentiometric measurements were examined for planar and capillary-incorporated gallium electrodes that were anodized in ice-cooled aqueous H2SO4 at 10V. Although the resulting nanoporous anodic oxide-coated electrodes did not initially show potentiometric pH responses, they exhibited pH responses with a smaller response slope (ca. -30mV/pH; pH 3∼9) after potentiometric measurements in a solution of pH ≤ 3. This observation could be explained by the acid-induced dissolution of an oxide-based barrier layer that passivated the underlying gallium electrode. This explanation was compatible with the observation of an additional gallium oxide layer over the nanoporous anodic oxide after the potentiometric pH measurements. Interestingly, in contrast to the metallic gallium electrodes, the anodic oxide-coated electrodes exhibited potentiometric Nernstian responses to SO42− at ≥ 10−4M, reflecting the presence of sulfate doped into the oxide during the anodization. These results provide fundamental knowledge required to design miniaturized electroanalytical probes based on gallium electrodes.

Formation Mechanism of High-Density, Flattened Polymer Nanolayers Adsorbed on Planar Solids

Thermal annealing is one of the most indispensable polymer fabrication processes and plays essential roles in controlling morphologies and properties of polymeric materials. We here report that thermal annealing also facilitates polymer adsorption from the melt on planar silicon (Si) substrates, resulting in the formation of a high-density polymer nanolayer with flattened chain confirmations. Three different homopolymers (polystyrene, poly(2-vinylpyridine), and poly(methyl methacrylate)), which have similar inherent stiffness and bulk glass transition temperature (Tg), but have different affinities with Si substrates, were chosen as models. Spin-cast films (∼50 nm in thickness) with the three polymers were prepared on cleaned Si substrates and then placed in a vacuum oven set at a temperature far above the bulk Tg. In order to monitor the polymer adsorption process at the solid-polymer melt interface during thermal annealing, we used the protocol that combines vitrification of the annealed films (via rapi...

Fabrication of hierarchical nanostructures using free-standing trilayer membrane

Hierarchical nanostructures are typically fabricated with multiple lithography steps on prepatterned micro- or nanostructures. However, such conventional multiple lithography steps result in poor coverage and uniformity, especially when patterning on high-aspect-ratio micro- or nanostructures. In this work, the authors present a new fabrication method which can make hierarchical nanostructures even on high-aspect-ratio prepatterns, using a free-standing trilayer membrane. The nanoporous trilayer composite films, consisting of metal/photoresist/antireflective coating, is created on a planar silicon substrate by using laser interference lithography, reactive ion etching, and e-beam deposition. Then, a customized solution of ammonia (NH3), hydrogen peroxide (H2O2), and water (H2O) is used to release the composite film from the supporting substrate. The free-standing composite membrane is then transferred on a prepatterned silicon substrate. Showing no major defects on the membrane surface, the flexible and soft trilayer membrane allows a good, uniform contact with the top surface of the prepatterns and is utilized as a robust etch mask in the deep reactive ion etching (DRIE) process of silicon to fabricate hierarchical nanopore structures. Since the membrane covers the top surfaces of the prepatterns seamlessly, the nanopore patterns of the membrane are transferred onto the top surfaces with great fidelity during the etching process. The membrane can also be used as a stencil for deposition processes. For example, metal is deposited through the nanoporous membrane by a lift-off process. Then, the transferred metal nanodot patterns are used as an etch mask in the DRIE process to achieve high-aspect-ratio hierarchical nanopillar structures. Compared to conventional methods to fabricate hierarchical nanostructure, the introduced new approach is more reliable and versatile to realize large-area hierarchical nanostructures with a good uniformity, in both nanopore and nanopillar array by using the same single membrane.

Sub‐Micron‐Wide Surficial Trench Frames to Define the Coating Areas of Sensitive Layers on Silicon MEMS Resonant Chemical Sensors

SUMMARYWe propose a new processing method to form thin films on the predefined micro‐areas of movable parts of microdevices by using trench frames on the device surfaces in the coating of solutions containing the membrane materials. We fabricated 500‐nm‐wide surficial trench frames additively on silicon microcantilevers to define the areas coated with the thin films by increasing the tolerance to protrusion of a liquid droplet from the edge in the coating process. The trench frames were fabricated on the silicon surfaces by deep‐reactive ion etching with the resist mask patterned by electron beam lithography. We also fabricated the same trench frames on planar silicon substrates, and we examined the proper conditions for the formation of polybutadiene (PBD) thin films on the framed surfaces by coating PBD solution using a microdispenser system. When the n‐dodecane solution of PBD was applied to the micro‐area framed by the trenches, PBD film was successfully defined within the framed area. The trench frames fabricated on the cantilevers indicated the practical effectiveness to define the PBD thin films within the framed micro‐areas. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(10): 60–66, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11379

Toward Three-Dimensional Microelectronic Systems: Directed Self-Assembly of Silicon Microcubes via DNA Surface Functionalization

The huge and intelligent processing power of three-dimensional (3D) biological "processors" like the human brain with clock speeds of only 0.1 kHz is an extremely fascinating property, which is based on a massively parallel interconnect strategy. Artificial silicon microprocessors are 7 orders of magnitude faster. Nevertheless, they do not show any indication of intelligent processing power, mostly due to their very limited interconnectivity. Massively parallel interconnectivity can only be realized in three dimensions. Three-dimensional artificial processors would therefore be at the root of fabricating artificially intelligent systems. A first step in this direction would be the self-assembly of silicon based building blocks into 3D structures. We report on the self-assembly of such building blocks by molecular recognition, and on the electrical characterization of the formed assemblies. First, planar silicon substrates were functionalized with self-assembling monolayers of 3-aminopropyltrimethoxysilane for coupling of oligonucleotides (single stranded DNA) with glutaric aldehyde. The oligonucleotide immobilization was confirmed and quantified by hybridization with fluorescence-labeled complementary oligonucleotides. After the individual processing steps, the samples were analyzed by contact angle measurements, ellipsometry, atomic force microscopy, and fluorescence microscopy. Patterned DNA-functionalized layers were fabricated by microcontact printing (μCP) and photolithography. Silicon microcubes of 3 μm edge length as model objects for first 3D self-assembly experiments were fabricated out of silicon-on-insulator (SOI) wafers by a combination of reactive ion etching (RIE) and selective wet etching. The microcubes were then surface-functionalized using the same protocol as on planar substrates, and their self-assembly was demonstrated both on patterned silicon surfaces (88% correctly placed cubes), and to cube aggregates by complementary DNA functionalization and hybridization. The yield of formed aggregates was found to be about 44%, with a relative fraction of dimers of some 30%. Finally, the electrical properties of the formed dimers were characterized using probe tips inside a scanning electron microscope.

Mutiscale substrates based on hydrogel-incorporated silicon nanowires for protein patterning and microarray-based immunoassays

Here, protein micropatterns were prepared on micropatterned nanostructures for potential applications in microarray-based multiplex bioassays with enhanced protein-loading capacity and detection sensitivity. Vertically-aligned silicon nanowires (SiNWs) that were about 8μm in height and 150nm in diameter were prepared using an etching process and were surface-modified with aminopropyltriethoxysilane (APTES) to allow them to covalently immobilize proteins. The SiNW substrate was then overlaid with a micropattern of poly(ethylene glycol) (PEG) hydrogel to create defined arrays of microwells consisting of APTES-modified SiNW on the bottom of the wells, with hydrogel on the walls of the wells. Due to the non-adhesiveness of PEG hydrogels toward proteins, proteins were selectively immobilized on the surface-modified SiNW regions to create protein micropatterns. The increase in surface area increased the protein loading capacity of the SiNWs by more than 10 times the capacity of a planar silicon substrate. Immunobinding assays between IgG and anti-IgG and between IgM and anti-IgM that were performed on micropatterned SiNWs emitted stronger fluorescent signals and showed higher sensitivity than assays performed on planar silicon substrates. Finally, microfluidic channels were successfully integrated into the micropatterned SiNWs to enable the simultaneous performance of multiple immunoassays on a single microarray platform.

First Demonstration of Quasi-Planar Segmented-Channel MOSFET Design for Improved Scalability

Quasi-planar segmented-channel MOSFETs (SegFETs) with gate lengths down to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $\sim$</tex></formula> 45 nm are fabricated using a conventional process flow by starting with a corrugated-silicon substrate. In comparison with control devices (fabricated using the same process flow, but with a planar-silicon substrate), the SegFETs show reduced short-channel effect due to enhanced electrostatic integrity. Despite having a smaller physical channel width, the SegFET can achieve comparable drive current per unit layout area as the conventional planar MOSFET design.

Silicon Platform With Vertically Aligned Carbon Nanotubes for Enhancing Thermal Conduction in Hybrid Optoelectronic Integration

Silicon platforms with high-number-density vertically aligned carbon nanotubes (VACNTs) on patterned substrates are designed and fabricated to provide efficient thermal conduction for hybrid optoelectronic integration and device packaging. The simulation shows that the platform can reduce the thermal resistance by >;40% , resulting from the help of the high-thermal-conductivity VACNTs and the increase in the heat spreading area of the patterned platform geometry. The platform can also provide the flexibility in device attachment and alignment. Applications of this platform to packaged light-emitting diodes (LEDs) are used to demonstrate the feasibility of this approach. The experimental results verify that an increase in the saturation power and optical output power of LEDs can be obtained by packaging with the VACNT-integrated platform. The thermal resistance measurement also indicates that the proposed platform has lower thermal resistance than the planar silicon substrate.

Growth of [010] oriented α-MoO3 nanorods by pulsed electron beam deposition

Arrays of vertically aligned, [010] oriented α-MoO3 nanorods were grown on planar silicon substrates by pulsed electron beam deposition. The morphology and structure of the nanorods as well as their growth mechanism were investigated. These [010] oriented nanorods exhibited a good field emission performance with a turn-on field of 3.6 V/μm. As the space between layers of the [010] oriented nanorods is large, their formation may be also meaningful for exploring the gas sensing and energy storage properties of layered oxides like α-MoO3.

Germanium photodetectors on Silicon-on-insulator grown with differential molecular beam epitaxy in silicon wells

In this paper we propose a method for growing fast Germanium pin photodetectors in pre-patterned areas on a Silicon-on-insulator substrate. The layers are deposited by means of molecular beam epitaxy and structured by chemical mechanical polishing. A comparison of the electrical and optical characteristics between a photodetector grown with the proposed method and a reference detector grown on a planar Silicon substrate is made indicating only minor differences.

共振型マイクロ化学センサへの感応膜塗布特性の制御のための表面微細パターンの利用

SUMMARY We propose a new processing method to form thin films on the predefined micro-areas of movable parts of microdevices by using trench frames on the device surfaces in the coating of solutions containing the membrane materials. We fabricated 500-nm-wide surficial trench frames additively on silicon microcantilevers to define the areas coated with the thin films by increasing the tolerance to protrusion of a liquid droplet from the edge in the coating process. The trench frames were fabricated on the silicon surfaces by deep-reactive ion etching with the resist mask patterned by electron beam lithography. We also fabricated the same trench frames on planar silicon substrates, and we examined the proper conditions for the formation of polybutadiene (PBD) thin films on the framed surfaces by coating PBD solution using a microdispenser system. When the n-dodecane solution of PBD was applied to the micro-area framed by the trenches, PBD film was successfully defined within the framed area. The trench frames fabricated on the cantilevers indicated the practical effectiveness to define the PBD thin films within the framed micro-areas. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(10): 60–66, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11379