Akihito Ikedo

Vertically aligned silicon microwire arrays of various lengths by repeated selective vapor-liquid-solid growth of n-type silicon/n-type silicon

Repeated vapor-liquid-solid (VLS) growth with Au and PH3–Si2H6 mixture gas as the growth catalyst and silicon source, respectively, was used to construct n-type silicon/n-type silicon wire arrays of various lengths. Silicon wires of various lengths within an array could be grown by employing second growth over the first VLS grown wire. Additionally, the junction at the interface between the first and the second wires were examined. Current-voltage measurements of the wires exhibited linear behavior with a resistance of 850 Ω, confirming nonelectrical barriers at the junction, while bending tests indicated that the mechanical properties of the wire did not change.

Heterogeneously Integrated Vapor–Liquid–Solid Grown Silicon Probes/(111) and Silicon MOSFETs/(100)

In this letter, we report the heterogeneous integration of vertically aligned silicon (Si) microprobe arrays/(111) with MOSFET circuits/(100) by IC processes and subsequent selective vapor-liquid-solid (VLS) growth of Si. A hybrid Si-on-insulator (SOI) substrate with different species of Si layers, e.g., a (100)-top-Si/buried oxide/(111)-handle-Si system, was utilized for the heterogeneous integration technique. MOSFETs were fabricated on (100) top Si, and vertical VLS probes were synthesized at the selectively exposed (111) handle Si. The different Si layers of the MOSFETs and probes were electrically connected by a 3-D metallization technique. In addition, the electrical properties of 3-D metallization and the MOSFETs were investigated. The results indicate potential for heterogeneous integration of VLS probes/(111) and MOSFETs/(100), promising further integrations of numerous microdevices/different species of substrates and CMOS/(100), including fully depleted SOI-CMOS for high-performance electronics, on the same chip.

Nanoscale sharpening tips of vapor–liquid–solid grown silicon microwire arrays

We developed out-of-plane, high aspect ratio, nanoscale tip silicon microwire arrays for application to penetrating, multisite, nanoscale biological sensors. Silicon microwire arrays selectively grown by gold-catalyzed vapor-liquid-solid growth of silicon can be formed to create sharpened nanotips with a tip diameter of less than 100 nm by utilizing batch-processed silicon chemical etching for only 1-3 min. The tip angles achieved ranged from 11 degrees to 38 degrees. The nanotip silicon microwires can perform gelatin penetration without wire breakdown, indicating their potential penetrating capability for measurements inside biological tissues.

Fabrication of force sensitive penetrating electrical neuroprobe arrays

MEMS-based penetrating micro-scale probe electrodes have been used in numerous electrophysiological measurements. However, it is necessary to quantitatively study probe-induced stress on the tissue/neurons and the damage during the probe penetration. Here we propose a vertically-aligned electrical recording neuroprobe array each with the force detection capability during the probe penetration into biological samples (Fig. 1). We fabricated the force sensitive probe array based on piezoresistive p-type silicon probes (~0.9 Ω·cm) with the length of 60 μm and the diameter of 5 μm. The p-type silicon probe array was vertically assembled over an n-type silicon island by using in-situ doping vapor-liquid-solid (VLS) growth technology. Each probe has two terminal interconnections at the tip and the base formed by three-dimensional metallization process. The probe resistance for the piezoresistance effect in silicon was confirmed by measuring the current-voltage characteristics of vertically assembled individual probes. During the probe penetration, the probe array can also be used for multi-site electrical recordings of neural activities via Pt-black electrodes at probe-tips with a low impedance of 14 KΩ at 1 kHz.

Vertically Aligned Various Lengths Doped-Silicon Microwire Arrays by Repeated Selective Vapor-Liquid-Solid Growth

We have proposed a growth technique of various lengths, 2-4¿m diameter, conductive-silicon micowire arrays, by repeated vapor-liquid-solid (VLS) growth of n-type silicon, using Au as the growth catalyst and a mixture gas of 1% PH3 with 100% Si2H6 as the silicon gas source. We obtained a longer 100¿m-length silicon wire by both the first growth of 50¿m-length wire and an additional growth of 50¿m-length wire over the first wire, while a shorter 50¿m-length silicon wire had simultaneously been grown from the substrate by the additional growth. We investigated the junction existing at the interface between the first and the second n-type silicon wire bodies. Current I-voltage V measurements on a two-step grown n-type/n-type silicon wire exhibit linear behavior with the overall resistance of 850¿, confirming no electrical barrier at the interface junction. Several bending tests on the wires with the junction confirmed no significant change in the mechanical properties of the wire. We developed the microwire arrays for a potential application to investigations of multiple cell layers in brain cortex or retina (Fig. 1). We also believe that the proposed technique becomes new approach to construct three-dimensional devices in MEMS fields.

A vertical micro-scale light guiding silicon dioxide tube array for optical neurostimulator

In this paper, we present out-of-plane micro-scale diameter light guiding silicon dioxide tube arrays for use in optical stimulation of neurons. An optical propagation through the dioxide tube was calculated by using finite-difference time-domain (FDTD) simulation. Based on the calculated result, we designed and fabricated 3-µm-inner diameter 24-µm-height silicon dioxide tube arrays with the wall thickness of 0.5 µm, using selective vapor-liquid-solid (VLS) growth of silicon microwire, followed by microfabricating processes. The optical transmission capability of the fabricated tube was experimentally confirmed by using light of 470 nm, 525 nm, and 595 nm in wavelength.

Layer-by-layer nanoassembly of iridium oxide/platinum-black for low impedance, high charge injecting microelectrode applications

We report an electrode device with a low impedance and high charge injecting characteristics for a powerful application to micro/nano-scale electrophysiological measurements of neuron/cells. Due to the small effective electrode area, conventional microelectrodes exhibit high interfacial electrode impedance (~10 MΩ at 1 kHz) and low charge injection characteristics, making the targeted cells impossible to record/stimulate. To overcome these limitations, we propose enhanced surface-area of an electrode with a low impedance material, based on layer-by-layer assembled iridium oxide (IrOx)/platinum-black (Pt-black) with nano-scale roughness. The assembled nanorough-IrOx/Pt-black electrode exhibits 2 times lower impedance and 2.4 times larger injection delivery capacity (QCDC) compared to a planer-IrOx electrode with the same size. Additionally, we fabricated nanorough-Ir/Pt-black tipped microprobes and demonstrated in saline, while improved stimulating currents were observed.

Integration and 3D fabrication techniques to nanoscale-tip silicon high-aspect-ratio microprobe arrays

We developed integration and three-dimensional (3D)-fabrication techniques to nanoscale-tip silicon-microprobe arrays for multiple electrical nano-measurement systems with a high aspect ratio. Vapor-liquid-solid (VLS) grown vertically-aligned 120µm-length silicon microprobe arrays (2µm-diameter), each with nanoscale-tip by controlling the silicon-etching (less than 100-nm-diameter, radius of curvature 50nm), have been integrated with IC-processed interconnections. Subsequently, the nanotip silicon probe is entirely covered with Pt/Ti and encapsulated with an insulator, SiO2. In addition, herein we proposed the use of a spray-coating of photoresist and cycled etchings of the photoresist/SiO2 at the probe-tips. Consequently, the nanotips can precisely be patterned and etched, resulting in the exposed Pt/Ti/silicon-nanotip with a controlled height of 2µm.

Batch Fabrication of Out-of-Plane, IC-Compatible, Nanoscale-Tip Silicon Neuroprobe Arrays

We developed a batch-fabrication of nanoscale-tip silicon microprobe arrays for use in multipoint nanoscale investigations of cell/neuron in-vivo/in-vitro. Sharpened tips, less than 100nm diameter, can be formed at the tips of out-of-plane three-dimensional silicon microprobe (length ≫10¿m) arrays, by silicon wet etching-based batch-process within only 1-3min, providing precisely controlled tip angles ranging from 15° to 50°. The penetration capability of the nanoscale-tip microprobes was demonstrated, using finite element modeling (FEM) simulations and penetration tests with a gelatin as tissue/cell.

Electrical catching and transfer of nanoparticles via nanotip silicon probe arrays

We demonstrated batch, pinpoint, and multisite-transfer of nanoparticles via vertically-aligned nanotip silicon probe arrays for powerful applications of nanoinjectors, including local drug delivery and DNA transfer into biological samples (e.g., cell, neuron and tissue). Although several nanoinjections have been demonstrated using “nanoprobe” (e.g., glass pipette, silicon nanowire and carbon nanotube [1–3]), the common issues of these devices are the limitation of the probe number, short length and robustness of the probe section. In order to solve these issues, we propose a nanoinjector, based on a vertical nanotip probe array with a high aspect ratio. Here, we demonstrated electrical batch trappings of nanoparticles at positive biased nanotips, and the trapped nanoparticles were released by ultrasonic vibration. These preliminary results indicate the nanoparticle transfer capability of the nanotip probes for use in multisite deep nanoinjections on numerous biological samples.