Anisotropic Wet Etching Technique Research Articles

Selective area epitaxy of gallium phosphide-based nanostructures on microsphere lithography-patterned Si wafers for visible light optoelectronics

In this study, we present the selective area plasma-assisted molecular beam epitaxial growth of GaP-based nanoheterostructures (nanostubs), incorporating direct bandgap GaAsP or GaPN segments, on patterned SiO2/Si(001) wafers. A microsphere optical lithography and anisotropic Si wet-etching techniques were employed for wafer-scale surface patterning through SiO2 growth mask, allowing to obtain either planar or pyramidal pit nucleation site morphologies. X-ray diffraction reciprocal space mapping and Raman microspectroscopy studies confirm compositional homogeneity of the nanostub arrays. The dilute nitride nanostubs display the narrowest and most intense visible red photoluminescence response at room temperature, which is an order of magnitude higher compared to the GaAsP ones. The formation of the nitrogen sub-band in GaPN alloy was confirmed in the framework of density functional theory, providing insights for interpreting the experimental results. These findings demonstrate the feasibility of the proposed approach for fabricating the ordered arrays of nanoscale visible light emitters on silicon.

Producing Microscale Ge Textures via Titanium Nitride‐ and Nickel‐Assisted Chemical Etching with CMOS‐Compatibility

AbstractAs an emerging anisotropic wet etching technique, metal‐assisted chemical etching (MacEtch) has been widely employed for fabricating nano‐ and micro‐structures of germanium (Ge) for potential infrared (IR) photonics and optoelectronics. However, traditional noble metal catalysts such as Au and Ag limit its application in complementary metal oxide semiconductor (CMOS) processes, as Au is considered as a detrimental deep‐level impurity in Ge. In this work, the feasibility of exploring TiN and Ni as CMOS‐compatible catalysts for Ge MacEtch is investigated. Both TiN and Ni catalysts exhibit inverse MacEtch behavior, resulting in formation of inverted pyramid and v‐groove Ge microscale textures which exhibit outstanding IR antireflection performance. No catalyst delamination of TiN is observed during etching, while it can be avoided by inserting a Ti adhesive layer beneath Ni catalyst. Schottky contact barrier of metal catalyst with Ge is also investigated indicating similar hole injection efficiency among TiN, Ni, and Ti/Ni with Ge junction due to strong Fermi level pinning effect. The TiN‐ and Ni‐assisted chemical etching of Ge shed light on CMOS‐compatible Ge‐based photonic and optoelectronic applications.

Bactericidal Characteristics of Bioinspired Nontoxic and Chemically Stable Disordered Silicon Nanopyramids.

Controlling bacterial growth using artificial nanostructures inspired from natural species is of immense importance in biomedical applications. In the present work, a low cost, fast processing, and scalable anisotropic wet etching technique is developed to fabricate the densely packed disordered silicon nanopyramids (SiNPs) with nanosized sharp tips. The bactericidal characteristics of SiNPs are assessed against strains implicated in nosocomial and biomaterial-related infections. Compared to the bare silicon with no antibacterial activities, SiNPs of 1.85 ± 0.28 μm height show 55 and 75% inhibition of Escherichia coli (Gram-negative) and Bacillus subtilis (Gram-positive) bacteria, whereas the silicon nanowires (SiNWs) fabricated using a metal-assisted chemical etching method show 50 and 58% inhibition of E. coli and B. subtilis. The mechanistic studies using a scanning electron microscope and live/dead bacterial cell assay reveal cell rupture and predominance of dead cells on contact with SiNPs and SiNWs, which confirms their bactericidal effects. Chemical stability and cell viability studies demonstrate the biocompatible nature of SiNP and SiNW surfaces. Owing to their capability to kill both Gram-negative and positive bacteria and minimal toxicity to murine fibroblast cells, SiNPs can be used as an antibacterial coating on medical devices to prevent nosocomial and biomaterial-related infections.

Silicon slot fin waveguide on bonded double-SOI for a low-power accumulation modulator fabricated by an anisotropic wet etching technique.

We propose a new low VπL, fully-crystalline, accumulation modulator design based on a thin horizontal gate oxide slot fin waveguide, on bonded double Silicon-on-Insulator (SOI). A combination of anisotropic wet etching and the mirrored crystal alignment of the top and bottom SOI layers allows us for the first time to selectively pattern the bottom layer from above. Simulations presented herein show a VπL = 0.17Vcm. Fin-waveguides and passive Mach-Zehnder Interferometer (MZI) devices with fin-waveguide phase shifters have been fabricated, with the fin-waveguides having a transmission loss of 5.8dB/mm and a 13.5nm thick internal gate oxide slot.

Fabrication and optical characterization of hybrid antireflective structures with zinc oxide nanorods/micro pyramidal silicon for photovoltaic applications

We investigated the hybrid antireflective (AR) structures with zinc oxide nanorods (ZnO NRs) grown on micro pyramidal silicon (MP-Si) structures. The MP-Si structures were fabricated by a simple and cost-effective anisotropic wet chemical etching technique under different ratios between potassium hydroxide (KOH) and isopropyl alcohol (IPA). For the MP-Si structures etched at 10:9 vol% of KOH and IPA, the relatively low average reflectance (Ravg) value of 14.5% was obtained in the wavelength range of 300-1100 nm. Its solar weighted reflectance (SWR) value was also estimated to be 12.6% in the wavelength range of 400-1100 nm. The ZnO NRs were hydrothermally grown on the MP-Si structures at various solution concentrations. For the ZnO NRs (25 mM)/MP-Si, the Ravg and SWR values were further decreased to 3.6% and 3.8%, respectively. The omnidirectional AR behavior was also observed in the wide incident angle range of 20-70°. The hybrid ZnO NRs/MP-Si AR structures revealed a superhydrophilic surface with water contact angles of < 5 o.

Low-Loss Slot Waveguides with Silicon (111) Surfaces Realized Using Anisotropic Wet Etching

We demonstrate low-loss slot waveguides on silicon-on-insulator (SOI) platform. Waveguides oriented along the (11-2) direction on the Si (110) plane were first fabricated by a standard e-beam lithography and dry etching process. A TMAH based anisotropic wet etching technique was then used to remove any residual side wall roughness. Using this fabrication technique propagation loss as low as 3.7dB/cm was realized in silicon slot waveguide for wavelengths near 1550nm. We also realized low propagation loss of 1dB/cm for silicon strip waveguides.

Low-Loss Silicon Waveguides and Grating Couplers Fabricated Using Anisotropic Wet Etching Technique

We report low-loss silicon waveguides and efficient grating coupler to couple light into them. By using anisotropic wet etching technique, we reduced the side wall roughness down to 1.2nm. The waveguides were patterned along the [112] direction on a [110] SOI substrate. The waveguide boundaries are decided by the [111] planes which are normal to the [110] surface. Fabricated waveguides show minimum propagation loss of 0.85 dB/cm for TE polarization and 1.08dB/cm for TM polarization. The fabricated grating couplers show coupling efficiency of -4.16dB at 1570nm with 3dB bandwidth of 46nm.

Towards nanometer-spaced silicon contacts to proteins

A vertical nanogap device (VND) structure comprising all-silicon contacts as electrodes for the investigation of electronic transport processes in bioelectronic systems is reported. Devices were fabricated from silicon-on-insulator substrates whose buried oxide (SiO2) layer of a few nanometers in thickness is embedded within two highly doped single crystalline silicon layers. Individual VNDs were fabricated by standard photolithography and a combination of anisotropic and selective wet etching techniques, resulting in p+ silicon contacts, vertically separated by 4 or 8 nm, depending on the chosen buried oxide thickness. The buried oxide was selectively recess-etched with buffered hydrofluoric acid, exposing a nanogap. For verification of the devices’ electrical functionality, gold nanoparticles were successfully trapped onto the nanogap electrodes’ edges using AC dielectrophoresis. Subsequently, the suitability of the VND structures for transport measurements on proteins was investigated by functionalizing the devices with cytochrome c protein from solution, thereby providing non-destructive, permanent semiconducting contacts to the proteins. Current–voltage measurements performed after protein deposition exhibited an increase in the junctions’ conductance of up to several orders of magnitude relative to that measured prior to cytochrome c immobilization. This increase in conductance was lost upon heating the functionalized device to above the protein’s denaturation temperature (80 °C). Thus, the VND junctions allow conductance measurements which reflect the averaged electronic transport through a large number of protein molecules, contacted in parallel with permanent contacts and, for the first time, in a symmetrical Si–protein–Si configuration.

Mechanical losses of oscillators fabricated in silicon wafers

Investigation of mechanical dissipation in silicon ribbons is important for the development of low noise silicon test mass suspension of the future interferometric gravitational wave detectors. Ribbon-like oscillators were fabricated in commercial silicon wafers using the anisotropic chemical wet etching technique. Results of measurement of the mechanical loss of such oscillators in the temperature range from 90 K to 300 K are presented. Suppression of thermoelastic loss down to at the temperature of about 124 K is observed.

A novel MOSFET with vertical signal-transfer capability for 3D-structured CMOS image sensors

We have developed a novel MOSFET that can transfer signals vertically without through-silicon vias but by using a fully depleted silicon-on-insulator (FDSOI) structure with its source region connected to the back electrodes as well as the front ones. A prototype MOSFET fabricated using the backside anisotropic wet etching technique has confirmed that the electrical characteristics measured from the front and the back electrodes are identical. The subthreshold factor S of the prototype was found to be 64.5 mV/decade, suggesting a good switching performance. Since the double-sided MOSFET has vertical signal-transfer capability and excellent operating characteristics, it is expected to contribute to developing a More-than-Moore type device of three-dimensional integration such as pixel-parallel image sensors. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Simultaneous Formation of Non-coplanar Resonant Beams and Supporting Beams of Bulk Micromachining Resonant Accelerometers by Maskless Anisotropic Wet Etching

The paper presents the design principles and fabrication sequence of a newly developed mechanical structure of bulk micromachining resonant Z axis accelerometers on the same silicon substrate by maskless anisotropic wet etching technique. Four resonant beams are located at the top surface of silicon substrates in order to facilitate the patterning of excited resistors, piezoresistive resistors or electrode plates, whereas the gravity centre of the proof mass lies within the neutral plane of eight supporting beams to minimize the rotating of proof mass under in-plane acceleration. Compared with early reported mechanical structures, the simple structures can eliminate the bending moments caused by in-plane acceleration, as a result, avoid the rotating of the proof mass.

Fabrication and application of ordered Si nanopore array induced by Au

Size-controlled Si nanopore array with a pore size less than 100 nm is fabricated on Si (100) substrates by using monolayer self-assembled and KOH anisotropic wet etching technique. Morphology and structure of the pores are characterized by SEM and AFM. Results show that a large area of defect-free polystyrene (PS) monolayer film can be obtained when the volume ratio of PS solution to methanol solution is 9:11. A larger volume ratio or a smaller volume ratio will induce similar bilayer structure and defects (point and line) in the PS film, respectively. The lateral size and depth of the nanopore will increase with the etching time, and its morphology will change from circular to inverted pyramid type gradually. But the orderly arranged structure will be destroyed as the etching time is over 10 min. On the other hand, ordered Ge/Si nanoislands and nanorings will be grown on nanopore-patterned Si (100) substrates (inverted pyramid and circular nanopores, respectively) by ion beam sputtering. In addition, reasonable interpretations have been proposed for the formation mechanism of the ordered Ge/Si nanostructure.

(Invited) Extremely Short Channel Si-MOSFETs Prepared on SOI Substrates Using Anisotropic Wet Etching

Electrical properties of extremely short channel Si-MOSFETs are examined. Nanometer-scale channels are fabricated by the V-shape groove formation on SOI substrate using anisotropic wet etching technique. The channel length, defined as the width of the Vgroove bottom, is as short as 3 nm. Source, channel, and drain regions are uniformly highly doped by the same dopant species. Excellent transistor characteristics with threshold voltages that are optimal for low-power operation are obtained for both n-FETs and p-FETs when the thicknesses of both the channel and gate dielectric film thickness are reduced to 1 nm. Contributions of quantum effects in the nanometer-scale channel are discussed.

Development of Novel MOSFET with Front and Back Side Electrodes for 3D-Structured Image Sensors

We have studied a three-dimensional (3D)-structured image sensor, which is capable of pixel-parallel signal processing, thereby meeting the demand for high resolution and high frame rate. Conventional 3D-stacked devices have been developed by using through silicon via (TSV), which is not applicable to image sensor pixels of a few micrometers or less, because its hole diameter usually is more than the pixel size. We have developed novel MOSFET that can transfer signals vertically without TSV using a fully depleted silicon-on-wafer (FDSOI) structure with its source region connected to the back electrodes as well as the front ones. A prototype MOSFET fabricated using anisotropic wet etching technique has confirmed that the characteristics from the front and back electrodes are identical. Since the double-sided MOSFET has vertical signal-transfer capability and excellent operating characteristics, it is expected to contribute to develop a More-than-Moore type device of 3D integration such as pixel-parallel image sensors.

Fabrication of size-tunable, periodic Si nanohole arrays by plasma modified nanosphere lithography and anisotropic wet etching

In this study, we propose a new, facile and efficient method to define the desired patterns on Si surfaces and to fabricate periodic arrays of size- and position-controllable Si nanoholes on blank- and pre-patterned (001)Si substrates, which is based on the O2 plasma modified nanosphere lithography with an anisotropic wet etching technique. In contrast to earlier approaches, no additional thin-film deposition equipments and metal film hard masks are needed for this method. The Si nanohole arrays exhibit the same hexagonal arrangement as the initial colloidal nanospheres template. The sizes of the Si nanoholes can be tuned from 150nm to 480nm by varying the etching time. The Si substrates with nanohole-textured surfaces exhibit strong antireflection properties. UV–Vis spectroscopic measurements revealed that by increasing the nanohole size, the optical reflectance of the nanohole-structured (001)Si surfaces was gradually decreased down to less than 8%. Since the nanohole sizes, shapes and spacings can be readily controlled by adjusting the diameters of the colloidal nanospheres and anisotropic wet etching conditions, the combined approach presented here provides the capability to fabricate a variety of nanohole array structures on various Si-based substrates without complex lithography.

Research on Preparation Method and Etching Technique of Large Scale Free P+ Diaphragm

This paper introduces the preparation method of large scale free P+ diaphragm based on the boron doped and anisotropic wet etching techniques. It also explores the impact of the RF power configuration, pressure and SF6 flow rate on etching rate during ICP etching. It is indicated that silicon can be used as mask to solve the problems of large scale film break and the adhesion between mask and film which are caused by vacuumization during ICP etching. In order to avoid over etching, twice etching technique can be employed to control etching time and end point. Results of the experiment show that through etching, free P+ diaphragm can be fabricated into sensitive free beam which can be widely used in MEMS.

Generation of uniaxial tensile strain of over 1% on a Ge substrate for short-channel strained Ge n-type Metal–Insulator–Semiconductor Field-Effect Transistors with SiGe stressors

Tensile strain of over 1% in Ge stripes sandwiched between a pair of SiGe source-drain stressors was demonstrated. The Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET)-like structures were fabricated on a (001)-Ge substrate having SiO2 dummy-gate stripes with widths down to 26nm. Recess-regions adjacent to the dummy-gate stripes were formed by an anisotropic wet etching technique. A damage-free and well-controlled anisotropic wet etching process is developed in order to avoid plasma-induced damage during a conventional Reactive-ion Etching process. The SiGe stressors were epitaxially grown on the recesses to simulate strained Ge n-channel Metal–Insulator–Semiconductor Field-Effect Transistors (MISFETs) having high electron mobility. A micro-Raman spectroscopy measurement revealed tensile strain in the narrow Ge regions which became higher for narrower regions. Tensile strain of up to 1.2% was evaluated from the measurement under an assumption of uniaxial strain configuration. These results strongly suggest that higher electron mobility than the upper limit for a Si-MOSFET is obtainable in short-channel strained Ge-nMISFETs with the embedded SiGe stressors.

Study on atomistic model for simulation of anisotropic wet etching

The silicon anisotropic wet chemical etching technique is widely employed for fabricating various microelectromechanical system structures for advantages such as simple etching equipment and its capability to fabricate special microstructures. In this paper, we consider one of the most frequently used equations of atomistic model of simulation of anisotropic wet etching. Then using a trial and error method we solve the mentioned equations and derive a new equation for calculating the microscopic etch rates of some surface atoms for a new range of concentrations and temperatures. Furthermore, a new equation is presented for the calculation of the microscopic activation energies for some typical surface atoms and a new comparison of activation energies between some atoms is made dividing atoms into three categories. This classification makes it easier to study microscopic etch rates and activation energies of other atoms. The results demonstrate a good agreement with the experimental results.

Fluid Control MEMS constructed with Polymer Materials

A polymer material indicates higher durability and resistance properties against to reactive gases, acid and alkaline solutions. Fluid control MEMS (micro electro mechanical systems) with micro diaphragm pumping system used for reactive gas control is constructed. A resist film made of novolac resin as a diaphragm material is employed. An Au/Si/resist multilayer structure as a diaphragm of 1mm diameter is formed by optical lithography and anisotropic wet etching techniques. A micro channel structure of 50μm width is also fabricated by employing a polymer thick film. The mechanical strength of a diaphragm is tested by applying static load using a probe system. By applying 20V bias between diaphragm and base electrodes, Coulomb attractive force acts to operate the diaphragm motion. As the fluid flow control, a silane-coupling vapor gas of HMDS (hexamethyldisilazane) is employed. A contact angle of water indicates hydrophobic of a glass substrate by HMDS vapor control using the MEMS.

A novel quartz micro-structure based on shear stress detection and its gyroscopic effect

In order to simplify sidewall electrode pattern of quartz gyroscope, a novel micro-structure used to sense Coriolis’ force based on shear stress detection is demonstrated. The approach of shear stress detection is analyzed and the structure of sidewall electrode is a single polarity electrode. A tapered beam with 15° taper designed as the sensing beam can increase its sensitivity. A prototype structure is fabricated by processing z-cut quartz by anisotropic wet etching technique. The prototype has a drive mode frequency of about 5.6 kHz, and the quality factor is over 5000 in atmosphere. The spectrum analysis of the structure’s output signal reveals the Coriolis force, which validates our new design scheme.