KOH Anisotropic Etching Research Articles

Surface termination and hydrogen bubble adhesion on Si(100) surfaces during anisotropic dissolution in aqueous KOH

The formation and growth of hydrogen bubbles on a Si(1 0 0) surface during its anisotropic etching in aqueous KOH has been investigated. Quantitative data on bubble size, lifetime and density on the etching surface was obtained and their dependence on KOH concentration, applied potential and temperature were measured. In situ FTIR measurements demonstrated a strong dependence of bubble attachment on surface termination and hence on the hydrophilicity of the Si(1 0 0) surface during etching. The formation of surface defects and the geometry of bubble imprints have been directly characterised with scanning probe microscopy. The analysis of hillock formation and statistical considerations show that the adhesion of hydrogen bubbles during anisotropic etching of silicon is a source of surface roughness and pyramid formation.

Design and Fabrication of a Micro Electro Mechanical Systems-Based Electrolytic Tilt Sensor

An electrolytic tilt sensor has been designed and fabricated using micro electro mechanical systems (MEMS) technique. The anisotropic KOH etching is used to form a deep Si cavity where conductive electrolyte solution is filled in. Au/NiCr electrodes are simultaneously deposited and patterned using e-beam evaporator with an aligned shadow mask, which is fabricated by through-wafer via etching on a 400-µm-thick Si wafer. A composite electrolyte of 4.8 µL in the volume is filled in the anisotropically-etched cavity that has a volume of 7.3 µL. The electrolyte solution is comprised of variable amount of KCl and a mixture of deionized (DI) water, ethanol and methanol (50, 25, and 25 vol %, respectively). When the electrolyte is injected into the cavity, a cover glass is bonded to seal the electrolyte solution. The fabricated electrolytic tilt sensor is excited by an alternating current to prevent electrolysis in the electrolyte-filled cavity and measured by Wheatstone bridge setup that reads the variation of resistance with respect to an incoming inclination. The measured output characteristic of the MEMS-based electrolytic tilt sensor is as good as that of a conventional electrolytic tilt sensor. Moreover, MEMS-based sensor prevails in its small size, low cost and possible mass production.

Characterisation of anisotropic etching in KOH using network etch rate function model: influence of an applied potential in terms of microscopic properties

Using the network etch rate function model, the anisotropic etch rate of p-type single crystal silicon was characterised in terms of microscopic properties including step velocity, step and terrace roughening. The anisotropic etch rate data needed have been obtained using a combination of 2 wagon wheel patterns on different substrate and 1 offset trench pattern. Using this procedure the influence of an applied potential has been investigated in terms of microscopic properties. Model parameter trends show a good correlation with chemical/electrochemical reaction mechanism and mono- and dihydride terminated steps reactivity difference. Results also indicate a minimum in (111) terrace roughening which results in a peak in anisotropic ratio at the non-OCP applied potential of −1250 mV vs OCP.

A fabrication process for a silicon tunnel barrier with self-aligned gate

A process for fabricating a device based on tunneling through a very thin vertical silicon membrane is presented. The process has been developed on a 〈1 1 0〉 oriented silicon wafer using high resolution e-beam lithography and KOH anisotropic etching to define the structure. A single evaporation step allows the fabrication of both the source–drain contacts and a control gate self aligned to the top of the silicon membrane. A vertical silicon membrane with a thickness of 15 nm has been obtained.

Silicon single-electron transistor fabricated by anisotropic etch and oxidation

In this work, we propose a fabrication process for a single-electron transistor on silicon. The process is developed on silicon on insulator wafer and it is based on electron beam lithography and KOH anisotropic etching. A structure composed by a small silicon isle connected to the leads by channels with triangular cross-section is obtained. Channel dimensions have been reduced by oxidation and the substrate has been used as backgate. Preliminary I– V characteristics show phenomena of charge/discharge at room temperature.

Fabrication of silicon vertical taper structures using KOH anisotropic etching

In order to exploit the full potential of silicon photonic devices we require an efficient way of coupling light from fibres to integrated photonic circuits. One of the most common solutions to this problem is the use of tapered silicon waveguides. We present a method of vertical silicon taper fabrication using KOH anisotropic etching of a silicon wafer with tilted 〈1 1 1〉 crystallographic orientation. The main advantage of this method is the low roughness of the tilted surface, which should lead to a good optical transmission of the tapered waveguide.

A High-Yield Fabrication Process for Silicon Neural Probes

There is a great need for silicon microelectrodes that can simultaneously monitor the activity of many neurons in the brain. However, one of the existing processes for fabricating silicon microelectrodes-reactive-ion etching in combination with anisotropic KOH etching-breaks down at the wet-etching step for device release. Here we describe a modified wet-etching sidewall-protection technique for the high-yield fabrication of well-defined silicon probe structures, using a Teflon shield and low-pressure chemical vapor deposition (LPCVD) silicon nitride. In the proposed method, a micro-tab holds each individual probe to the central scaffold, allowing uniform anisotropic KOH etching. Using this approach, we obtained a well-defined probe structure without device loss during the wet-etching process. This simple method yielded more accurate fabrication and an improved mechanical profile.

Superhydrophobic surfaces fabricated by nanoimprint lithography

Superhydrophobic surfaces were fabricated on silicon by nanoimprint lithography and wet chemical etching. Glass molds were used to imprint a positive photoresist layer. The residual layer in imprinted regions was removed by UV-ozone exposure and subsequent developing of the photoresist. The pattern is transferred to a thin SiO 2 layer by buffered hydrofluoric acid etching and then to Si by isotropic hydrofluoric-nitric-acetic acid (HNA) or by anisotropic KOH etching. After coating the samples with a hydrophobic self-assembled monolayer of octadecyltriclorosilane contact angles of water drops were measured and compared to model predictions.

Fabrication of poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) (PHBHHx) microstructures using soft lithography for scaffold applications

This paper reports two soft lithographic methods, micromolding and hot embossing, to produce biodegradable poly (3-hydroxybutyrate- co-3-ftydroxyhexanoate) (PHBHHx) arrays of microstructures for hosting and culturing cells in a local microenvironment by controlled shape. Silicon masters with high-aspect-ratio microfeatures were fabricated using KOH and DRIE anisotropic etching. These silicon masters were used as molds to construct PHBHHx microstructures using micromolding and hot embossing. Using silicon rather than conventional PDMS as molds allowed microstructures with feature size of 20 μm and height of 100 μm to be realized. PHBHHx microstructures with different configurations including circles, rectangles, and octagons were fabricated to investigate the effects of topography on cell culture. Mouse fibroblast cell lines L929 were cultured on PHBHHx microstructures in vitro to investigate the biocompatibility. This study demonstrates the feasibility of microfabrication of PHBHHx structures with micro-scale feature size using soft lithography, and the results show that PHBHHx microstructures can be created to mimic cellular microenvironment for cell culture, providing a convenient means to investigate relationships of microstructures and cell functions.

MICROMACHINED PIEZOELECTRIC POLYMER MEMBRANE ACOUSTIC SENSOR

ABSTRACT This paper reports on a 2.3 mm × 2.3 mm polymer micro-electromechanical systems (MEMS) for air transducer applications. The MEMS, used as a receiver of ultrasonic signal, has a suspended membrane structure. A 2.7 μ m thick poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] 70/30 mol% copolymer was spun on a silicon dioxide/silicon substrate and then poled in-situ. The Si substrate was then etched from the backside by anisotropic chemical etching in KOH to produce a P(VDF-TrFE)/SiO2/Si suspended membrane structure. The displacement of the membrane was measured by means of a laser vibrometer. The maximum deflection at the center of the membrane was 0.9 μ m at 40.8 kHz under an input drive of 7 V. A commercial air transducer was used as the transmitter to emit ultrasound at 40.8 kHz. The MEMS acoustic transducer, which has similar resonance frequency, was used as the receiver. The temporal response of the MEMS and its sensitivity were measured. The result suggested that the MEMS has potential for monitoring ultrasound emission.

Development of a low cost hybrid Si/PDMS multi-layered pneumatic microvalve

We report on the design and fabrication of a low cost active microvalve with a soft elastomer membrane driven by pneumatic actuation. The valve was made in two separate parts, a fluidic part in biocompatible and optically transparent material (PDMS) and a robust pneumatic interface in silicon, which were assembled together. The main issue of alignment and localized selective bonding of the PDMS parts to preserve the membrane mobility, hence the valving function, is described. In this work we also investigated two types of silicon moulds for PDMS casting, made by KOH anisotropic wet etching or DRIE.

RF sputtered polytetrafluoroethylene—a potential masking material for MEMS fabrication process

Microsystems technology has received great attention lately, creating new fields of application. Many new smart materials have been proposed and investigated, and many processing methods for ultrafine fabrication have been developed. Polymers have attracted much attention as promising masking materials due to their inherent properties. Polytetrafluoroethylene (PTFE) is a very unique polymeric material in this category. The present study deals with the deposition of PTFE by RF (13.56 MHz) sputtering on silicon substrate. The deposited polymer film is characterized for stoichiometry and adhesion after deposition by Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and contact angle measurements and more importantly test of masking (masking properties). The masking properties include the measurement of etch rate of PTFE film (at various deposition conditions) in KOH and TMAH anisotropic etchants at various temperatures and concentrations of the etchant solution. The masking properties are then compared with the conventional and widely used masking materials such as SiO2 and Si3N4. For confirmation of the presence of PTFE film on the silicon substrate after etching in KOH and TMAH, FTIR and XPS characterizations were again done. Also, a diaphragm structure is realized in the silicon by anisotropic etching in KOH (40 wt% at 90 °C) using sputtered PTFE film as the masking material.

Templated cluster assembly for production of metallic nanowires in passivated silicon V-grooves

Wires with meso-scale and nano-scale widths have been fabricated using a novel templated cluster assembly technique. Soft-landed Sb clusters are assembled into wires at the apexes of V-grooves (widths 2–7 μm) formed in Si substrates using anisotropic KOH etching. Two methods for forming nano-wires using templated cluster assembly have been demonstrated. Depending on the chosen method of wire formation, the Si V-grooved substrates are either thermally passivated with SiO 2 , or passivated with SiO 2 and then coated with evaporated Ti/Au. On passivated substrates, the metallic wires are formed directly by aggregation of the Sb clusters. On metallised substrates, Ar-plasma etching was used to remove the Ti/Au film around the Sb cluster wires to produce nano-scale width (∼100 nm) Ti/Au wires. Following a selective wet chemical etch, Energy Dispersive X-ray analysis confirmed the complete removal of the Sb cluster assembled mask from the substrate, and the presence of Au wires within the passivated V-grooves.

Mold-type SiC field emitters with heavily boron-doped gates

A new fabrication process for mold-type SiC sharp tips for field-emission arrays has been presented. Gated emitters have been fabricated in a multi-step process using the transfer-mold technique. Gates were made of heavily boron-doped silicon, and a boron etch-stop technique for the KOH anisotropic etching was applied. The critical points of the fabrication process: anodic bonding of the silicon covered with a thin-film multi-layer of silicon oxide and boron-doped silicon carbide have been discussed in detail. The new method can be applied in the fabrication of double-gate mold-type emitters able to emit convergent electron beams.

Templated cluster assembly for production of metallic nanowires in passivated silicon V-grooves

Wires with meso-scale and nano-scale widths have been fabricated using a novel templated cluster assembly technique. Soft-landed Sb clusters are assembled into wires at the apexes of V-grooves (widths 2–7 μm) formed in Si substrates using anisotropic KOH etching. Two methods for forming nano-wires using templated cluster assembly have been demonstrated. Depending on the chosen method of wire formation, the Si V-grooved substrates are either thermally passivated with SiO 2, or passivated with SiO 2 and then coated with evaporated Ti/Au. On passivated substrates, the metallic wires are formed directly by aggregation of the Sb clusters. On metallised substrates, Ar-plasma etching was used to remove the Ti/Au film around the Sb cluster wires to produce nano-scale width (∼100 nm) Ti/Au wires. Following a selective wet chemical etch, Energy Dispersive X-ray analysis confirmed the complete removal of the Sb cluster assembled mask from the substrate, and the presence of Au wires within the passivated V-grooves.

Fabrication of sharp knife-edged micro probe card combined with shadow mask deposition

We introduce a micro-machined cantilever type probe card with a special tip shape made of single crystal silicon. A novel process for the micro-machined probe card with a sharp tip formation was developed and micro probe arrays of 40 μm in pitch were also realized. Probe cantilevers including the sharp tip were formed with anisotropic KOH etching and LOCOS, and thermally deposited with chrome (Cr) and gold (Au). In the final post-process, the tip area of the realized probes was covered with hard material like tungsten (W) using a micro-machined shadow mask. This probe could endure considerable force because both the tip and cantilever were entirely fabricated from single crystal silicon, and the shape of the sharp probe tip was specifically fabricated to break the oxidized surface of the IC chip with ease. This probe is also expected to have a long life-time and sufficient tip hardness since the end of the tip is coated with hard metal.

Modelling and fabrication of step height control of a multilevel Si(100) structure in KOH solution

A new model of the step height control of a micromachined multilevel Si(100) terraced structure by KOH anisotropic etching with one photomask is proposed. The main factors affecting the step height of the terraces include the etching rate and etching time of the KOH solution, the angle of misalignment from the ⟨110⟩ direction of the silicon crystallography and the geometry of the photomask pattern, especially the relation between the width array [bn] and the interval array [an] of the masked areas. The different step heights of the multilevel structure can be obtained by the different dimensions of the mask pattern. If the multilevel structure has the same step height, the widths [bn] and intervals [an] of the masked areas must be an arithmetic progression. This will simplify the experimental design for application. The fabrication of three-level to eight-level Si(100) terraced structures has been demonstrated on this model and experiments.

Advanced sacrificial poly-Si technology for fluidic systems

Sacrificial poly-Si etching can be used to create thin cavities and channels. By combining it with anisotropic KOH etching of a mono-Si substrate, important components for fluidic systems, such as V-grooved channels, thin sandwiched channels, channel crossings and membrane filters and injectors, have been fabricated in a single etch step.

PZT thin films integration for the realisation of a high sensitivity pressure microsensor based on a vibrating membrane

Abstract Pb(Zr,Ti)O3 (PZT) thin films are widely used both to make actuator and sensor. In this paper, a vibrating pressure sensor based on a (PZT thin film)/Si membrane is presented. This sensor is based on a 3 mm ×3 mm, 15.5 μm thick silicon membrane actuated by a Pt/Ti/PZT/Pt/Ti/SiO2 multilayer. All depositions have been performed by magnetron RF-sputtering. After a RTA (700 °C, 30 s in air), a PZT perovskyte phase [1 1 1] oriented has been obtained as observed by XRD. In order to improve this preferential orientation for PZT, the bottom Pt electrode has been annealed by RTA (400 °C, 30 s in argon). The electrodes have been patterned by lift-off. The silicon membranes have been realised by anisotropic chemical etching in KOH. A finite element analysis has been developed to optimise the top electrode and to determine the fundamental resonant frequency of our device. The mechanical behaviour of the sensor under pressure has been analysed by means of a laser vibrometer. The fundamental resonant frequency without additional pressure was 18.6 kHz. A frequency shift has been observed for an applied pressure from 15 mbar up to 140 mbar with a good linearity and a high sensitivity of 115 Hz mbar−1. The normalised sensitivity observed is 6800 ppm mbar−1.

Mechanical characterization and reliability study of bistable SiO2/Si membranes for microfluidic applications

Abstract The thermoelastic stresses in the SiO2 films on silicon membranes are known to lead to buckling effects. For very thin membranes, the deflection is as important as a bistable state can be observed under pressure. We have studied the mechanical behaviour and the reliability of bistable SiO2/Si membranes with various thicknesses. The 3 mm ×3 mm silicon membranes have been micromachined from thermally oxidized (1 0 0) silicon substrates by anisotropic chemical etching in an aqueous KOH (40%, 60 °C) solution. Oxide thickness from 1 to 1.5 μm have been measured by profilometry. Silicon thicknesses from 5 to 25 μm have been measured with a 0.1 μm accuracy by FT-IR spectrometry after oxide removal. For a 1.5 μm thick oxide, a bistable behaviour has been obtained for silicon thickness lower than 15 μm. A reliability test system using a mechanized metallic bellows has been fabricated to perform positive/negative pressure cycles and low pressure measurements. The deflection and the volume change between the two equilibrium states of the bistable membranes have been measured by optical profilometry as a function of the Si thickness. A 3 mm side length, 5.9 μm thick, silicon membrane covered with a 1 μm oxide has been successfully tested for 2,000,000 cycles without deterioration.