Μm Wide Lines Research Articles

Uniform Metal-assisted Chemical Etching and the Stability of Catalysts

ABSTRACTRecently, metal-assisted chemical etching (MaCE) has been demonstrated as a promising technology in fabrication of uniform high-aspect-ratio (HAR) micro- and nanostructures on silicon substrates. In this work, MaCE experiments on 2 μm-wide line patterns were conducted using Au or Ag as catalysts. The performance of the two catalysts show sharp contrast. In MaCE with Au, a HAR trench was formed with uniform geometry and vertical sidewall. In MaCE with Ag, shallow and tapered etching profiles were observed, which resembled the results from isotropic etching. The sidewall tapering phenomena can be explained by the dissolution and re-deposition of the Ag catalyst in the etchant solution. The existence of Ag that was redeposited on the sidewall was further confirmed by energy dispersive spectrum. Also, etchant composition is found to play a profound role in influencing the etching profile by the Ag catalysts.

High frequency characterization and analysis of through silicon vias and coplanar waveguides for silicon interposer

A silicon interposer test vehicle with through silicon vias (TSVs) is evaluated under radio frequency (RF) application from DC to 10 GHz. TSVs with 30 μm diameter and 150 μm height were fabricated with one layer RDL of 20 μm line width. A group of coplanar waveguides (CPW) are designed and tested to analyze the electrical performance of the interposer under high frequency in both the time and frequency domains. Results show that the interposer cannot be used above 1 GHz because of excessive losses and high reflections. Based on the analysis of the test results and simulation results for different CPW (coplanar waveguides) structures, it is concluded that the poor performance was mainly caused by accumulated space-charge at the SiO2---Si interface and impedance mismatch of transmission lines. Solutions that were implemented included enhancing the thickness of the SiO2 insulation layer between the metal and silicon substrate and improving the design of the transmission line. Implementation of these changes led acceptable interposer performance up to 10 GHz.

Morphology and intrinsic excitability of regenerating sensory and motor neurons grown on a line micropattern.

Axonal regeneration is one of the greatest challenges in severe injuries of peripheral nerve. To provide the bridge needed for regeneration, biological or synthetic tubular nerve constructs with aligned architecture have been developed. A key point for improving axonal regeneration is assessing the effects of substrate geometry on neuronal behavior. In the present study, we used an extracellular matrix-micropatterned substrate comprising 3 µm wide lines aimed to physically mimic the in vivo longitudinal axonal growth of mice peripheral sensory and motor neurons. Adult sensory neurons or embryonic motoneurons were seeded and processed for morphological and electrical activity analyses after two days in vitro. We show that micropattern-guided sensory neurons grow one or two axons without secondary branching. Motoneurons polarity was kept on micropattern with a long axon and small dendrites. The micro-patterned substrate maintains the growth promoting effects of conditioning injury and demonstrates, for the first time, that neurite initiation and extension could be differentially regulated by conditioning injury among DRG sensory neuron subpopulations. The micro-patterned substrate impacts the excitability of sensory neurons and promotes the apparition of firing action potentials characteristic for a subclass of mechanosensitive neurons. The line pattern is quite relevant for assessing the regenerative and developmental growth of sensory and motoneurons and offers a unique model for the analysis of the impact of geometry on the expression and the activity of mechanosensitive channels in DRG sensory neurons.

High-density soft-matter electronics with micron-scale line width.

A method to produce soft and stretchable microelectronics composed of a liquid-phase Gallium-Indium alloy with micron-scale circuit features is introduced. Microchannels are molded onto the surface of a poly(dimethylsiloxane) (PDMS) elastomer and filled with EGaIn using a micro-transfer deposition step that exploits the unique wetting properties of EGaIn in air. The liquid-filled channels function as stretchable circuit wires or capacitor electrodes with a 2 μm linewidth and 1 μm spacing.

Simple micro-patterning of high conductive polymer with UV-nano-imprinted patterned substrate and ethylene glycol-based second doping

We have developed a simple micro-patterning process for high conductive polymer (i.e., poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)) with a patterned substrate by using an ultraviolet (UV) nano-imprint and an ethylene glycol-based second doping technique. In the patterning process, the PEDOT:PSS water dispersion is first coated only on the hydrophilic area, which is fabricated by UV nano-imprinting, forming patterned PEDOT:PSS on the substrate. The patterned PEDOT:PSS film is then immersed in the ethylene glycol as a second doping technique for increasing its conductivity. The proposed process provides simplicity in terms of shorter process steps of the UV nano-imprinting and PEDOT:PSS coating and higher conductivity of patterned PEDOT:PSS film than existing complicated micro-fabrication processes for organic materials. The 200 nm wide nano-imprinted pillar structures change the wettability of the substrate where the contact angle of the substrate is decreased from 66.8° to 33.3°. The patterning resolution with the nano-imprinted pattern substrate is down to 100 µm, which is useful for sensor applications. The conductivity increase delivers a low sheet resistance (120 Ω sq−1) of patterned PEDOT:PSS film. Then, the patterning of PEDOT:PSS sensor shapes with its 300 µm wide feature line and high conductivity are demonstrated. Therefore, our process leads to applications to a variety of PEDOT:PSS-based sensors.

High aspect ratio electrodeposited Ni/Au contacts for GaAs‐based III–V concentrator solar cells

AbstractWe report on a photolithographic and electro‐deposition process that results in an optimized front grid structure for high efficiency multi‐junction III–V concentrator solar cells operating under flux concentrations up to 1000 suns. Two different thick photoresists were investigated to achieve a 6 µm wide grid line with an aspect ratio of 1:1. A positive photoresist, SPR220 manufactured by Rohm and Haas was compared with a negative photoresist, nXT15 manufactured by AZ. A gold sulfite electrolyte was employed to prevent underplating as well as for environmental and safety considerations. An initial layer of nickel was discovered to be necessary to prevent delamination of the fingers during the removal of the contact layer. When deposited on a purpose grown, heavily doped GaAs contact layer, this Ni/Au contact exhibits an acceptable specific contact resistance in the low 10−4 to mid 10−5 Ohm cm2 range along with excellent adhesion without sintering. Copyright © 2014 John Wiley & Sons, Ltd.

Cell Behavior on Surface Modified Polydimethylsiloxane (PDMS)

Designing complex tissue culture systems requires cell alignment and directed extracellular matrix (ECM) and gene expression. Here, a micro-rough, polydimethylsiloxane (PDMS) surface, that also integrates a micro-pattern of 50 µm wide lines of fibronectin (FN) separated by 60 µm wide lines of bovine serum albumin (BSA), is developed. Human fibroblasts cultured on the rough, patterned substrate have aligned growth and a significant change in morphology when compared to cells on a flat, patterned surface. The rough PDMS topography significantly decreases cell area and induces the upregulation of several ECM related genes by two-fold when compared to cells cultured on flat PDMS. This study describes a simple surface engineering procedure for creating surface architecture for scaffolds to design and control the cell-surface interface.

Design of a fine alignment system featuring active orientation adjustment for nano imprint lithography

The fine alignment between a template with nano patterns and a wafer substrate, especially the parallelism between the two surfaces, is critical to Nano Imprint Lithography. A fine alignment system featuring active orientation adjustment which is composed of an imprint unit and a 6-DOF micromanipulator is proposed in this work. Deformations of a compact flexure layer caused by imprint loads are measured by four identical force sensors embedded in the imprint unit. The tilt of the flexure layer can thus be eliminated by adjusting the orientation of the 6-DOF micromanipulator. Kinematics and stiffness analysis are then developed, followed by dynamic performance evaluations. Based on the proposed system, an imprint tool is further developed and corresponding experiments are conducted. A saw shape grating pattern with 1.6 μm linewidth and a lattice pattern with 0.9 μm period are both imprinted with a minimum feature of 30 nm well reserved on the substrate. A maximum parallelism error of 14 nm across the template surface is also demonstrated by further section analyses on the imprinted patterns, hence the feasibility and superiority of the proposed method is verified.

Cell Adhesion Response on Femtosecond Laser Initiated Liquid Assisted Silicon Surface

Silicon substrates were irradiated at normal incidence with a femtosecond Ti:sapphire laser (Quatronix, 90 fs pulse duration, 1 kHz repetition rate, M(2) ~ 1.2, maximum energy peak 350 mJ ) operating at a wavelength of 400 nm and focused via a microscope objective (Newport; UV Objective Model, 37x 0.11 N.A.). The laser scanning was assisted by liquids precursors media such as methanol and 1,1,2-trichlorotrifluoroethane. By altering the processing parameters, such as incident laser energy, scanning speed, and different irradiation media, various surface structures were produced on areas with 1 mm(2) dimensions. We analyzed the dependence of the surface morphology on laser pulse energy, scanning speed and irradiation media. Well ordered areas are developed without imposing any boundary conditions for the capillary waves that coarsens the ripple pattern. To assess biomaterial-driven cell adhesion response we investigated actin filaments organization and cell morphological changes following growth onto processed silicon substrates. Our study of bone cell progenitor interaction with laser nanoprocessed silicon lines has shown that cells anchor mainly to contact points along the nanostructured surface. Consequently, actin filaments are stretched towards the 15 µm wide parallel lines increasing lateral cell spreading and changing the bipolar shape of mesenchymal stem cells.

Evaluation of X-Ray Microanalysis for Characterization of Dental Enamel

Elemental analysis of dental hard tissues is of importance. The aim of this study is to evaluate X-ray microanalysis (XRMA) of bovine enamel in a scanning electron microscope (SEM) with different coatings. The buccal surface of bovine incisors was polished flat, one-third was coated with carbon, one-third with gold, leaving one-third uncoated for XRMA in an SEM equipped with an energy-dispersive microanalysis system. The elements oxygen, sodium, magnesium, phosphorous, chlorine, potassium, and calcium were analyzed using their respective characteristic K X-ray series. Comparisons were made with analyses of glass produced by fusion of the bovine enamel, showing that oxygen analyses using the K X-ray series are reliable and preferable to calculating oxygen by stoichiometry for natural enamel. For the gold-coated and uncoated analyses, carbon was also measured using the K X-ray series. Small area Analyses in small areas (80 × 80 μm) in variable pressure-SEM mode with low vacuum (20 Pa), without any coating, midway between 40 μm wide gold lines 140 μm apart to avoid build-up of electrostatic charge is the preferred method, especially if carbon is included in the analysis. The analyses of bovine enamel are sufficiently reproducible to be regarded as quantitative for all elements except carbon.

Selective surface modification of PET substrate for inkjet printing

We present the results of inkjet printing of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulphonate) (PEDOT/PSS) conductive ink on selective surface-modified polyethylene terephathalate substrate via vapor coating of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FOTS) and atmospheric oxygen plasma. Selective surface modification was carried out using a nickel stencil with ~50 μm line width and spacing. PEDOT/PSS-conductive ink was found to wet the hydrophilic surface-treated line widths spontaneously while avoiding the hydrophobic-treated regions.

Migration speed and directionality switch of normal epithelial cells after TGF-β1-induced EMT (tEMT) on micro-structured polydimethylsiloxane (PDMS) substrates with variations in stiffness and topographic patterning

The epithelial to mesenchymal transition (EMT) involves several physiological and pathological phenomena and endows cells with invasive and migratory properties. However, the effects of substrate stiffness and topography on the migration of cells before or after transforming growth factor-β1 (TGF-β1)-induced EMT (tEMT) are unknown. Herein, we seed control or tEMT NMuMG cells on the 2D patterns consisted of 1 μm or 5 μm line-widths and groove or cone patterns on either 2 MPa (1.96 ± 0.48 MPa) or 4 MPa (3.70 ± 0.74 MPa) polydimethylsiloxane (PDMS) substrates. After tEMT, the increased expression of α-SMA with vinculin in focal adhesion (FA) sites led to an acceleration of tEMT cell motility. On the 2 MPa substrate, the most influenced substrate was the 1 μm, cone-patterned substrate, where the tEMT cells' motility decelerated by 0.13 μm/min (36% slower than the cells on groove pattern). However, on the 5 μm, groove-patterned substrate, where the tEMT cells demonstrated the most rapid motility relative to the control cells, with an increment of 0.18 μm/min (100%). Among the different physical cues from substrate, the cone pattern could impede the migration speed of tEMT cells. Furthermore, we recommend the groove-patterned with a 5 μm line-width substrate as a useful tool to differentiate control and tEMT cells by migration speed.

Current-Biased Transition Edge Detector of $ \hbox{MgB}_{2}$ Nanowires for Neutrons: Imaging by Scanning Laser

The superconducting MgB2 nanowire detector is biased by a current by a battery-class low voltage, and uses the nuclear reactions with a single neutron and the 10B contained in MgB2. The MgB2 detector consists of a 200-nm-thick MgB2 thin film meander structure of 3 μm line width protected by 300-nm-thick SiO layer. We use enriched boron 10B to enhance the sensitivity. The thermal energy of 2.3 MeV released by the reaction 10 B(n, α)7Li gives 1.83 MeV to 4He and 0.47 MeV to 7Li. The energy dissipation in a restricted space causes a local resistive state in the MgB2 wire, which yields a subtle change in bias current. Through a mutual inductance connected in series with current-biased detector, we are able to measure the event as a voltage pulse. The position-dependent response of the MgB2 detector is investigated by scanning a focused laser spot with the aid of an XYZ piezo-driven stage and an optical fiber with a focusing lens. A clear two-dimensional imaging pattern of the sensor signal well corresponds to the detector geometry. According to our results, the inhomogeneity and position-sensitive nature of the detector are supposed to be critical in governing the detection efficiency of the current-biased transition edge detector, which works only in the narrow temperature regime near Tc.

Fine-Pitch Copper Wiring Formed with Super-Inkjet and Oxygen Pump

We have successfully printed wires of 5 µm line widths and spaces in copper nanoparticle ink with a super-inkjet printer. The wires show resistivity as low as 8.1 µΩ·cm after sintering at 250 °C in an extremely low oxygen atmosphere generated by an oxygen pump. To our knowledge, this is the first report of micron-scale copper wiring formed by a direct printing method without any masks or templates.

Adaptive Fabrication of a Flexible Electrode by Optically Self‐Selected Interfacial Adhesion and Its Application to Highly Transparent and Conductive Film

A novel adaptive electrode fabrication method using optically self-selected interfacial adhesion between a laser-processed metal layer and polymer film is introduced to fabricate cost-effectively a high-resolution arbitrary electrode with high conductivity. The quality is close to that from vacuum deposition on a highly heat sensitive polymer film, with active response to various design requirements. A highly conductive metal film (resistivity: 3.6 μΩ cm) below a 5 μm line width with a uniform stepwise profile and mirror surface quality (R(rms) : 5-6 nm) is fabricated on a cheap polymer film with a heat resistance limit of below 100 °C. Severe durability tests are successfully completed without using any adhesion promoters. Finally, a highly transparent and conductive electrode with a transparency above 95% and sheet resistance of less than 10 Ω sq⁻¹ is fabricated on a polymer film and on glass by using this method. These results can help realize a potential high-throughput, low-cost, solution-processable replacement for transparent conductive oxides.

Air-stable, surface-oxide free Cu nanoparticles for highly conductive Cu ink and their application to printed graphene transistors

Air-stable, surface-oxide free Cu nanoparticles are, for the first time, synthesized by surrounding completely the Cu surface with oleic acid incorporated as a capping molecule. XPS analysis, in conjunction with TEM analysis, revealed that the oleic acid is chemisorbed to the Cu surface via a chemical interaction wherein a monodentate bond is included, without leaving behind free (non-interacting) oleic acid, thereby providing complete surface protection against oxidation. By eliminating the surface oxide layer that critically degrades the electrical properties, the surface-oxide free Cu nanoparticle ink facilitates the realization of a solution-processed Cu electrode layer with resistivity as low as 4 μΩ cm, comparable to the resistivity of noble metal-based, solution-processed counterparts. In addition, high resolution Cu electrode patterns with 5 μm line-width are directly printed using an electrohydrodynamic inkjet technique, and graphene transistors with the printed Cu electrodes demonstrate potential applications in printed electronics.

Photopatterning Based on a Chemically Amplified Mechanism Nano-Sheet Films

In the paper, an approach for introducing a photoacid generator (PAG) into Langmuir- Blodgett (LB) films to draw photopatterns is described. The chemically amplified positive-tone resist system used here consisted of two components: a copolymer, poly(iso-pentylmethacrylamide- co-4-t-butyloxylvinylphenylcarbonate) [poly(iPMA-t-BVPC39)] and a photoacid generator (PAG), tri(2,3-dibrompropyl)iso-cyanvrate (TDBPIC). In the two-component system, the acid generated by the photoacid generator (PAG) catalyzes the deprotection reaction of poly(iPMA-t-BVPC39) to re- move the tert-butoxycarbonyl group (t-BOC) in the exposed region during the post exposure baking process, thus rendering the exposure region soluble to alkali aqueous to form a positive tone. Phhot- olithographic properties of the LB films have been evaluated. The patterns can be resolved with a resolution of 1 μm line width by UV irradiation,followed by development with an alkali solution. Quantum yield is determined to be 0.065 and catalytic chain length is also calculated to be 2460. The LB films used above can be used to generate etched gold patterns on a glass substrate using an aqueous iodide, ammonium iodide in alcohol /water，as the etchant. The etch resistance of such LB films is sufficiently good, allowing patterning of a gold film suitable for photomask fabrication.

Directed cell growth and alignment on protein-patterned 3D hydrogels with stereolithography

The stereolithography apparatus (SLA) is a computer-assisted, three-dimensional (3D) printing system that is gaining attention in the medical field for the fabrication of patient-specific prosthetics and implants. An attractive class of implantable biomaterials for the SLA is photopolymerisable hydrogels because of their resemblance to soft tissues and intrinsic support of living cells. However, most laser-based SLA machines lack the minimum feature size required to imitate cell growth and alignment patterns in complex tissue architecture. In this study, we demonstrate a simple method for aligning cells on 3D hydrogels by combining the micro-contact printing (µCP) technique with the stereolithographic process. Fibronectin modified with acrylate groups was printed on glass coverslips with unpatterned, 10, 50, and 100 µm wide line patterns, which were then transferred to hydrogels through chemical linkages during photopolymerisation. Fibroblasts cultured on protein-printed 3D hydrogels aligned in the direction of the patterns, as confirmed by fast Fourier transform and cell morphometrics.

Improvement of ampacity of LTCC conductors

Improvement of the current load-carrying capacity can be achieved by combining different technologies in Low Temperature Cofired Ceramics (LTCC). Screen and stencil printing, full tape thickness features (FTTF) and embossing in LTCC were investigated. Silver paste conductors with 80 – 200 μm line width are included in the study. The structures on the test substrates are fed with an increasing current until the surface reaches an equilibrium temperature of 100°C measured by an infrared camera. 200 μm lines created by screen printing and stencil printing are able to bear 4.3 A and 14 A, respectively. For the FTTF conductor with a thickness of 190 μm the estimated maximum current is about 30 A and for the embossed conductor (thickness approx. 125μm) about 20 A. FTTF allowed the highest current without strong LTCC heating. However, such conductors are relatively difficult to process by laser and to fill with paste. Therefore the line resolution cannot be much smaller than 200 μm. Furthermore, spiral structures are not compatible to this technology. Hot embossing supports both structural resolution and a high current load-carrying capacity but requires additional tooling. Embossing can be used for the production of curved conductors and coils. With the laser cut stencil, structures with small line width and considerable line thickness can be achieved. Screen printing gives a good possibility to produce small conductors, complex structures and coils.

Formulation and screen printing of water based conductive flake silver pastes onto green ceramic tapes for electronic applications

Abstract Environmentally friendly, water-based silver pastes, adapted for screen printing, were formulated with different silver contents (67–75%). These pastes allowed screen printing onto low temperature co-fired ceramic (LTCC) of narrow conductive tracks with a 60 μm line width and a 3 × 10−8 Ω m electrical resistivity. Inks were formulated with a mixture of spherical and flake shape silver particles with 2–4 μm mean diameter. Rheological behaviour of pastes was studied in order to determine its effect on printed lines properties. Prepared inks were then screen printed and sintered under normal atmosphere at 875 °C. As expected, electrical properties depended on silver content. Resistivity values varying from 1.6 × 10−8 to 3.3 × 10−8 Ω m were calculated over 36.3 cm line length. These values are very close to bulk silver resistivity (1.6 × 10−8 Ω m). Compared to previous research and commercial pastes, the newly formulated pastes reached equivalent or even better conductivities with lower silver content (70% by weight).