Silicon Assembly Research Articles

Precise Capillary‐Assisted Nanoparticle Assembly in Reusable Templates

AbstractCapillary‐assisted particle assembly (CAPA) in predefined topographical templates is a scalable method for the precise positioning of nanoscale objects on various surfaces. High‐resolution CAPA templates are typically fabricated by expensive electron‐beam lithography and are used for a single assembly process. To increase the scalability and reduce the costs of the CAPA technique, the fabrication and characterization of reusable templates with nanoscale funnel‐shaped traps for repetitive precise nanoparticle placement are demonstrated. The yield of the first assembly of 100 nm gold nanoparticles (AuNPs) is as high as 94% with a median position offset of about 10 nm. The subsequent transfer process of the AuNPs from the silicon assembly template onto polymer surfaces, such as the elastic polydimethylsiloxane or the inelastic OrmoComp, shows a transfer yield larger than 99%. After the first transfer process, the assembly template is reused, resulting in a position offset and an assembly and transfer yield of this second assembly/transfer step that are comparable to the first ones. The obtained results demonstrate that the nanotemplates made by electron‐beam lithography can be reused for repeatable CAPA processes and thereby eliminate the need for recurring lithography steps for each assembly and thus make the CAPA technique more cost‐efficient.

Integrated microfluidic helium discharge photoionization detectors

Helium discharge photoionization detectors (HDPIDs) have been of increasing importance for detection of highly volatile compounds in gas chromatography (GC) and portable gas monitoring systems. The high ionization energy of these detectors (13.5–17.5 eV) allows for detection of virtually all compounds of interest, offering a distinct advantage compared to conventional PIDs, which can only detect compounds with ionization potentials below 10.6 (or 11.7, if using argon) eV. However, many current designs are bulky, power intensive, or helium intensive, restricting their usage to benchtop laboratory use. We recently developed a miniaturized HDPID that has low power and helium consumption, and small footprint. While offering suitable performance for portable GC applications, this design relies on hand assembly of silicon and Pyrex glass pieces, reducing fabrication yield, robustness, and repeatability. The current work improves on this prior device using an integrated, microfabricated μHDPID chip along with in-house designed plasma excitation and readout circuits. The μHDPID is characterized using permanent gases, light hydrocarbons, and formaldehyde, achieving low detection limits better than 10 pg, high linearity, rapid response time, short warm-up time, and high repeatability among devices. This integrated on-chip gas sensor offers advantages in ease of fabrication, yield and robustness, and repeatability. Therefore, it can be broadly used in portable GC for various applications.

Infinitesimal sulfur fusion yields quasi-metallic bulk silicon for stable and fast energy storage

A fast-charging battery that supplies maximum energy is a key element for vehicle electrification. High-capacity silicon anodes offer a viable alternative to carbonaceous materials, but they are vulnerable to fracture due to large volumetric changes during charge–discharge cycles. The low ionic and electronic transport across the silicon particles limits the charging rate of batteries. Here, as a three-in-one solution for the above issues, we show that small amounts of sulfur doping (<1 at%) render quasi-metallic silicon microparticles by substitutional doping and increase lithium ion conductivity through the flexible and robust self-supporting channels as demonstrated by microscopy observation and theoretical calculations. Such unusual doping characters are enabled by the simultaneous bottom-up assembly of dopants and silicon at the seed level in molten salts medium. This sulfur-doped silicon anode shows highly stable battery cycling at a fast-charging rate with a high energy density beyond those of a commercial standard anode.

Supersonic jet interactions with a micro-engineered skimmer

A micro-engineered, skimmer-based vacuum interface has been demonstrated and used to investigate gas dynamics on a sub-millimeter length scale. The interface is fabricated as a stacked assembly of silicon dies, based on an anisotropically etched inlet orifice and a pyramidal skimmer cone formed in electroplated nickel. Expansion of gas into vacuum, interaction of a supersonic jet with the skimmer and transmission of a collimated beam into a second vacuum stage have all been imaged with a schlieren microscope. Using a glass-walled vacuum chamber, flow patterns upstream and fully downstream of the skimmer have been imaged together for the first time. At low first-stage pressures, the 150–200 µm tall skimmers cannot fully penetrate the shock arising from interaction of the jet with the back wall. However, as the pressure is increased, a multiple shock cell structure evolves, the jet narrows and transmission rises sharply. Eventually, a collimated beam is transmitted to the second stage. When the skimmer aperture is smaller than the source aperture, a series of distinct peaks is evident in a plot of transmission against first-stage pressure. Imaging shows that at each successive peak, the number of shock cells increases by one and the skimmer inlet is coincident with a node.

Pressure-Compensated Chip Attach: A Low-Stress Silicon Bump Reflow Process

Assembly of silicon onto FR4-based organic substrates using flip-chip process continues to pose substantial thermomechanical challenges largely due to the disparate coefficients of thermal expansion (CTEs) of silicon and substrate. In flip-chip packaging, this CTE mismatch causes mechanical stresses in the chip-to-substrate interconnection bumps and the mechanically fragile interlayer dielectric (ILD) within the chip, leading to fracture and delamination. A novel process called pressure-compensated chip attach (PCCA) is presented, where the silicon chip is attached to the substrate by reflowing the solder bumps while maintaining the chip and substrate in a controlled hydrostatic pressure environment. Experiments show that the new PCCA process reduced the silicon test-chip warpage by 40% and eliminated ILD cracking.

Facile, Water-Based, Direct–Deposit Fabrication of Hybrid Silicon Assemblies for Scalable and High–Performance Li–ion Battery Anodes

This study employs water-based electrospinning to directly deposit silicon nanocomposite anode on the copper current collector. This process creates layers of polyvinyl alcohol (PVA) nanofibers filled with silicon nanoparticles (Si NPs) and carbon nanotubes (CNTs), while eliminating conventional slurry steps such as sonication, grinding, ball milling and doctor blading or slot and reverse roll coating. This facile direct deposition approach utilizes water and thus is free of toxic-solvent. Half cells fabricated with these directly deposited nanofibers exhibited an outstanding initial discharge capacity of 4,200mAhg−1 at 0.18Ag−1, which is attributed to the synergic effects of well dispersed Si NPs and CNTs. Composite silicon nanofibers also showed an excellent capacity retention of 93% over 300 cycles at 2.5Ag−1. Furthermore, these nanocomposites have a great performance at fast charge/discharge rates (1193, 945 and 650mAhg−1 at 5, 7.5 and 12.5Ag−1, respectively), due to incorporation of highly conductive CNTs in the nanocomposite matrix. Full cells with LiCoO2 cathode and directly deposited silicon anodes showed no capacity fading when subjected to a cutoff value of 1200mAhg−1anode .These results demonstrate that direct deposition via water-based electrospinning can be an attractive route to the production of high-performance silicon anodes.

Synthesis of Silicon and Germanium Nanowire Assemblies by Template-Assisted Electrodeposition from an Ionic Liquid

We report on the template-assisted synthesis of silicon and germanium nanowires from the air- and water-stable ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4] TFSA). The synthesis was done by electrochemical deposition in the pores of a commercial track-etched polycarbonate membrane. After chemical dissolution of the polycarbonate membrane in dichloromethane, nanowire assemblies with a regular arrangement were obtained. Different lengths of nanowires can be obtained by varying the applied potential and the time of deposition. The nanowire assemblies were characterised by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDX). Our results show that the template-assisted electrochemical deposition approach in ionic liquids has the potential to easily synthesise germanium and silicon nanowire assemblies.

Development of LAMBDA: Large Area Medipix-Based Detector Array

The Medipix3 photon counting readout chip has a range of features — small pixel size, high readout rate and inter-pixel communication — which make it attractive for X-ray scattering and imaging at synchrotrons. DESY have produced a prototype large-area detector module that can carry a 6 by 2 array of Medipix3 chips (1536 by 512 pixels), which can be used with a single large silicon sensor (85mm by 28mm) or two ``hexa'' high-Z sensors. The detector head is designed to be tilable and compatible with low temperatures, and will allow high speed parallel readout of the Medipix3 chips. It consists of a ceramic board, on which the sensor assembly is mounted, and a secondary board for signal routing and voltage regulators. A prototype DAQ board using USB2 readout has also been produced. A ``quad'' Medipix3 sensor assembly has been mounted on the detector head, and successfully configured and read out by the DAQ board. Development has begun on a high-speed readout board, and large-area silicon assemblies are in production.

A New Contactless Assembly Method for Paper Substrate Antennas and UHF RFID Chips

This paper deals with a low-cost method for the assembly of flexible substrate antennas and UHF RF identification silicon (Si) chips. Such a method exploits a magnetic coupling mechanism, thus not requiring for galvanic contacts between the Si chip and antenna itself. The magnetic coupling is established by a planar transformer, the primary and secondary windings of which are implemented on flexible substrate and Si chip, respectively. As a result, the Si chip can be assembled on the antenna with a mere placing and gluing process. First, the idea has been validated by theory. Electromagnetic simulations of a square heterogeneous transformer (1.0-mm side) show a maximum available power gain (MAG) of -0.4 dB at 868 MHz. In addition, the heterogeneous transformer is also quite tolerant with respect to misalignment between primary and secondary. An offset error of 150 μm reduces the MAG to - 0.5 dB. A sub-optimal matching strategy, exploiting a simple on-chip capacitor, is then developed for antennas with 50- Ω input impedances. Finally, the idea has been experimentally validated exploiting printed circuit board (PCB) prototypes. A PCB transformer (1.5-mm side) and a transformer rectifier (two-diode Dickson multiplier) have been fabricated and tested. Measurements indicates a MAG of -0.3 dB at 868 MHz for the transformer and the capability of the developed rectifier to supply a 220-kΩ load at 1.5 V with a - 2-dBm input power.

Development of silicon and carbon foam low mass interposers

We propose fabrication methods and present prototyping results for a new wafer integration component called a low mass interposer (LMI). The LMI prototype is an assembly of silicon and carbon foam resulting in a composite 4 inch wafer of 4 mm thickness and average density 10% that of silicon. Rows of vertical copper contacts traverse the bulk on 4mm pitch. Each row can have a dense contact pitch, such that average contact densities can be or order order 25 mm−2. Actual pitch parameters can be varied by factors of 2 or more within the construction process proposed. The LMI could be used for copper-copper bonding to IC wafers in a 3D integration process. Present prototyping does not match any specific wafer design and was produced to develop a fabrication procedure and quantify the results. No 3D integration tests have been performed at this stage.

Electrically tunable Yb-doped fiber laser based on a liquid crystal photonic bandgap fiber device

We demonstrate electrical tunability of a fiber laser using a liquid crystal photonic bandgap fiber. Tuning of the laser is achieved by combining the wavelength filtering effect of a tunable liquid crystal photonic bandgap fiber device with an ytterbium-doped photonic crystal fiber. We fabricate an all-spliced laser cavity based on the liquid crystal photonic bandgap fiber mounted on a silicon assembly, a pump/signal combiner with single-mode signal feed-through and an ytterbium-doped photonic crystal fiber. The laser cavity produces a single-mode output and is tuned in the range 1040-1065 nm by applying an electric field to the silicon assembly.

Carbon Nanotubes Anchored to Silicon for Device Fabrication

This report highlights recent progress in the fabrication of vertically aligned carbon nanotubes (VA-CNTs) on silicon-based materials. Research into these nanostructured composite materials is spurred by the importance of silicon as a basis for most current devices and the disruptive properties of CNTs. Various CNT attachments methods of covalent and adsorptive nature are critically compared. Selected examples of device applications where the VA-CNT on silicon assemblies are showing particular promise are discussed. These applications include field emitters, filtration membranes, dry adhesives, sensors and scaffolds for biointerfaces.

CSi2Ga2: a neutral planar tetracoordinate carbon (ptC) building block

Ever being a large curiosity, the "anti-van't Hoff/Le Bel" realm that is associated with tetracoordinate or hypercoordinate planar centers has made rapid progress. In particular, it has been disclosed that silicon and gallium can be embedded in various planar species. However, to our best knowledge, assembly of silicon- and gallium-embedded planar units has never been reported, though such assembled species might be used as potential nanoscale devices. Here we report the first attempt on how to design assembled molecular compounds featuring silicon- and gallium embedded planar tetracoordinate carbon (ptC) units. Taking the special silicon- and gallium-embedded ptC unit CSi2Ga2as an example, we performed density functional calculations on a series of model compounds [DM(CSi2Ga2)](q+) as well as the saturated compounds (Cl-)q[CpM(CSi2Ga2)](q+) (D = CSi2Ga2, Cp-(C5H5-); M = Li, Na, K, Be, Mg, Ca)and the more extended sandwich-like species. For the six metals, CSi2Ga2 can only be assembled in the "heterodecked sandwich" scheme (e.g., [CpM(CSi2Ga2)](q+)) so as to avoid cluster fusion. Interestingly, among all the designed sandwich species, CSi2Ga2 generally prefers to interact with the partner deck at the corner (Ga atoms) or face (CSi2Ga2 planes) sites. Such interaction types serve as an interesting growth pattern that might be applicable to the assembly of Si- and Ga-embedded ptC unit CSi2Ga2 into highly extended sandwich-like complexes. Our results for the first time showed that the Si- and Ga-embedded ptC unit CSi2Ga can act as a new type of building block. The present results are expected to enrich planar tetracoordinate carbon chemistry and metallocenes.

Micromechanical behaviour of Si-based particulate assemblies: A comparative study using DEM and atomistic simulations

In this paper, we investigate the micromechanical behaviour of Si-based particulate systems subjected to tri-axial compression loading. The investigations are based on three-dimensional discrete element modelling (DEM) and simulations. At first, we compare the variation of mean compressive stress for a silicon assembly subjected to tri-axial compression, predicted at two different scales: at the particulate scale, using the DEM simulation (mono-dispersed particulates) and at the atomistic scale using the molecular dynamics (MD) simulation results for silicon mono-crystal reported by Mylvaganam and Zhang (2003) [K. Mylvaganam, L. Zhang, Key Eng. Mater. 233–236 (2003) 615–620]. Both the simulation methods considered the silicon assembly subjected to an identical (tri-axial) loading condition. We observed a good qualitative agreement between the DEM and MD simulation results for the mean compressive stress when the assembly was subjected to small volumetric strain. However, at large volumetric strain, the mean stress of the silicon assembly predicted from MD simulation did not scale-up with the DEM results. This discrepancy could be due to that MD simulation is only valid for particle contacts, which are independent of one another and does not consider the inherent ‘discrete’ nature of particulates and the induced anisotropy prevailing at particulate scale. The micromechanical behaviour of particulate assemblies strongly depends on the inherent discrete nature of the particles, their single-particle properties and the induced anisotropy during mechanical loading. At the second stage, using DEM, we present the evolution of macroscopic compressive stress and several micromechanical features for four cases of the commonly used Si based poly-dispersed particulate assemblies (Si, SiC,Si 3N 4 and SiO 2) under tri-axial compression loading. We also present the evolution of several other phenomena occurring at particulate scale, such as the energy dissipation characteristics due to sliding contacts and the features of fabric structures developed during mechanical loading. The study shows that the single-particle properties of the Si based assemblies considered here significantly affect the micromechanical behaviour of the assemblies and DEM is a powerful tool to get insights on the internal behaviour of discrete particulates under mechanical loading.

Electrostatic Printing of Functional Liquid Toners for Electronic Manufacturing Applications

Strategies for the making functional toners are examined as well as means to convert particulate images into solid structures. Limitations on the imaging and fusing process imposed by the substrates are reviewed. Examples of electronic components printed on glass, paper, and PET film are shown.Finally the converting of amorphous silicon layers to the polycrystalline state for high performance transistors using a catalytic toner is presented. Also the Electrophoretic Self Assembly of silicon die using toner-imaging techniques is discussed.

ELECTROPHORETIC ASSEMBLY OF POROUS SILICON

Electrophoretic assembly of porous silicon (PS) has been studied in the present paper. A toluene/porous silicon suspension was prepared by the sonication of anodically etched porous silicon wafers and showed a PL property similar to the as-prepared PS, giving 620nm orange-red photoluminescence emissions. A broad optical absorption band about 286nm arising from scattering and absorption of the light by the PS particles has been observed. The PS particles suspended in toluene drifted towards the positive electrode and have been assembled on the surface of P+Si or indium-tin oxide (ITO) electrodes under a electric field of 100~600V/cm. The assembled PS films showed a quite stable PL property after ageing for 280 days even though there were some oxides revealed by XPS analysis.

Design and fabrication of a micro fuel cell array with “flip-flop” interconnection

A design configuration is presented for integrated series connection of polymer electrolyte fuel cells in a planar array. The design is particularly favorable for miniature fuel cells and has been prototyped using a variety of etch and deposition techniques adopted from microfabrication. The series path is oriented in a “flip-flop” configuration, presenting the unique advantage of a fully continuous electrolyte requiring absolutely no interconnecting bridges across or around the membrane. Electrical interconnections are made by thin-film metal layers that coat etched flow channels patterned on an insulating substrate. Both two-cell and four-cell prototypes have successfully demonstrated the expected additive performance of the integrated series, and peak power in a four-cell silicon assembly with hydrogen and oxygen has exceeded 40 mW/cm 2. Factorial experimentation has been applied to investigate the adequacy of metal film conduction over etched topology, and results conclude that film thickness dominates over other design parameters. The design effort and subsequent testing has uncovered new topics for extended study, including the possibility of lateral ionic conduction within the membrane as well as the effects of non-uniform reactant distribution.

TEM and HREM study of 3D silicon and platinum nanoscale assembliesin dielectric opal matrix

In this paper, regular systems of silicon and platinumassemblies have been fabricated in athree-dimensional (3D) void sublattice of synthetic opal.Silicon was introduced into opal pores bythermal CVD.The filling of the opal pores was varied by changing theparameters of the CVDprocess.The samples were also filled with platinum from a solution of platinumtetrachloride inethanol in order to fabricate metal contact to silicon.A detailed TEM and HREM structural study ofopal-Si and opal-Pt-Si composites was carried out using JEM4000EXIIand JEM2010 electronmicroscopes.It was found that silica spheres in regular opal-Si compositesare uniformly coveredwith a nanocrystalline silicon layer up to 25-30 nm thick.To form the Pt-Si contact,the silica sphereswere coated with platinum before being embedded in silicon.The presence of a substructurein the silica spheres ensures a uniform distribution of platinumover the sphere surface.

Basic: Bio-Inspired Assembly of Semiconductor Integrated Circuits

AbstractIn the recent years, biologically-inspired self-assembly of artificial structures, some with useful optical properties, has been demonstrated. However, to date there has been no demonstration of self-assembly of useful electronic devices for the construction of complex systems. In this paper, a new process called BASIC (Bio-Inspired Assembly of Semiconductor Integrated Circuits) is proposed. The main theme is to use the mutual binding (hybridization) and specificity of DNA strands (oligonucleotides) for the assembly of useful silicon devices on silicon or other substrate. These devices need to be ‘released’ from their host substrate into a liquid medium where they can be functionalized with single stranded DNA. Silicon-on-insulator (SOI) substrates, which naturally lend themselves for such application, due to the presence of an oxide layer underlying the silicon layer, are used. These devices can vary in size and have a thin gold layer on one surface. This approach can be used to assemble micro and nano-scale devices and circuits and can also be a powerful technique for heterogeneous integration of materials (e.g. Si on Glass or polymer). The general idea of the BASIC process can also be extended to be used with any antibody/antigen complex. Preliminary results regarding the fabrication and release of the device islands will be presented. In addition, surface AFM characterization of the gold surfaces, prior to attachment of bio-molecules, is also presented.

TEM and HREM study of silicon and platinum nanoscale ensembles in 3D dielectric opal matrix

ABSTRACTIn the present paper regular systems of silicon and platinum assemblies have been fabricated in a three-dimensional (3D) void sublattice of synthetic opal. The detailed TEM and HREM structure study of ‘opal-Si’ and ‘opal-Pt-Si’ composites was carried out. It was found that in regular composites ‘opal-Si’ the silica spheres were covered uniformly with a nanocrystalline silicon layer of up to 25-30 nm in thickness. To form the Pt-Si contact the silica spheres were coated with platinum layer before embedding silicon. The results obtained demonstrate a possibility of creating 3D multilayer semiconductor structure (p-n junctions, Schottky barriers etc.) on the inner surface of opal voids.