Suspended Microstructures Research Articles

Pre-buried mask wet etching for suspended silicon microstructures applied in rocking mass micro-gyroscope

A pre-buried mask method to fabricate suspended silicon microstructures by using pure wet etching in TMAH solution is presented in this paper. Pre-buried mask method adopts a common mask and a pre-buried mask before etching, greatly simplifies etching process, and also enables suspended silicon microstructures fabrication with finite thickness to be achieved by using pure wet etching. Pre-buried mask etching method can fabricate suspended silicon microstructures with arbitrary thickness in theory. The 40 μm thick suspended silicon beam-mass microstructures have been conducted on 500 μm thick silicon wafer by using pre-buried mask method, and a 60 μm thick suspended silicon beam-mass microstructures also have been fabricated on 500 μm thick silicon wafer for rocking mass micro-gyroscope to effectively improve Q-factors and sensitivity. Pre-buried mask etching method can also be widely used to fabricate various thin microstructures on thick silicon wafers and bulk silicon devices by using pure wet etching, especially for low-cost industrial applications.

Suspended 3D pyrolytic carbon microelectrodes for electrochemistry

Carbon microelectrodes have a wide range of applications because of their unique material properties and biocompatibility. This work presents the fabrication and characterization of suspended pyrolytic carbon microstructures serving as three-dimensional (3D) carbon microelectrodes for electrochemical applications. A 3D polymer template in epoxy based photoresist (SU-8) was fabricated with multiple steps of UV photolithography and pyrolysed at 900 °C to obtain 3D carbon microelectrodes. The pyrolytic carbon microstructures were characterized by SEM, Raman spectroscopy and XPS to determine the mechanical stability, shrinkage and material properties. The smallest feature size fabricated in the suspended carbon layer was 2 μm. A three electrode microelectrode chip with 3D pyrolytic carbon microstructures as the working electrode was designed and fabricated. The electrodes were characterized with cyclic voltammetry (CV) and impedance spectroscopy (EIS) using potassium ferri-ferrocyanide redox probe in a custom made batch system with magnetic clamping. Different 3D pyrolytic carbon microelectrodes were compared and the optimal design displayed twice the peak current and half the charge transfer resistance as compared to 2D carbon electrodes. The higher sensitivity of 3D carbon microelectrodes for electrochemical sensing was illustrated by dopamine detection.

Lagrangian Mesh Model with Regridding for Planar Poiseuille Flow

AbstractMany biological settings involve complex fluids that have non-Newtonian mechanical responses that arise from suspended microstructures. In contrast, Newtonian fluids are liquids or mixtures of a simple molecular structure that exhibit a linear relationship between the shear stress and the rate of deformation. In modeling complex fluids, the extra stress from the non-Newtonian contribution must be included in the governing equations.In this study we compare Lagrangian mesh and Oldroyd-B formulations of fluid-structure interaction in an immersed boundary framework. The start-up phase of planar Poiseuille flow between two parallel plates is used as a test case for the fluid models. For Newtonian and Oldroyd-B fluids there exist analytical solutions which are used in the comparison of simulation and theoretical results. The Lagrangian mesh results are compared with Oldroyd-B using comparable parameters. A regridding algorithm is introduced for the Lagrangian mesh model. We show that the Lagrangian mesh model simulations with regridding produce results in close agreement with the Oldfoyd-B model.

Temperature dependence of the biaxial tensile strain in suspended Ge cross-shaped microstructures

We fabricate suspended Ge cross-shaped microstructures to biaxially enhance the residual tensile strain induced by a difference of thermal expansion coefficients. The strain and optical properties are characterized by means of micro-Raman spectroscopy and micro-photoluminescence. A maximum biaxial strain of 1.05% is obtained by cooling the sample to a temperature of 100 K.

Suspended microstructures of epoxy based photoresists fabricated with UV photolithography

In this work we present an easy, fast, reliable and low cost microfabrication technique for fabricating suspended microstructures of epoxy based photoresists with UV photolithography. Two different fabrication processes with epoxy based resins (SU-8 and mr-DWL) using UV exposures at wavelengths of 313nm and 405nm were optimized and compared in terms of structural stability, control of suspended layer thickness and resolution limits. A novel fabrication process combining the two photoresists SU-8 and mr-DWL with two UV exposures at 365nm and 405nm respectively provided a wider processing window for definition of well-defined suspended microstructures with lateral dimensions down to 5μm when compared to 313nm or 365nm UV photolithography processes.

Stress and thermal characterization of 4H-SiC microelectromechanical structures

In this letter, the potential of single crystalline 4H-polytype silicon carbide (4H-SiC) based microelectromechanical structures as resistance thermometer for high temperature sensing were explored. A dopant-selective photoelectrochemical etching process was applied to release the sensing element – suspended microstructures on 4H-SiC substrate. Residual stress and stress gradient in the microstructure before and after release was examined by micro-Raman spectroscopy. Electrical resistance of the suspended microstructures at different temperatures were characterized and analyzed by a temperature-dependent electron mobility model.

Fabrication and characterisation of suspended microstructures of tantalum

An investigation of the influence of deposition and post-fabrication processes on the final mechanical structure of tantalum beams is reported in the present study. The final deflection profiles of doubly supported beams made from compressive and tensile-stressed films have been studied experimentally. An optimum finite element model has been developed to predict the buckling behaviour of the doubly supported beams by considering the boundary conditions in the form of a compressive stress and an applied load. No matter which etch release method has been used, the initial stress state of the as-deposited films has been found to have a significant impact on the final deflection profile of the fabricated device. The compressive-stressed films have presented larger deflection in the final released beams than the tensile-stressed films. Taking into account the type of etch release methods, the beams that have been released in the dry etch release processes have been found to deform more vertically than those released in the wet-etch release method. Moreover, it has been found that the amplitude of vertical deflection increases with the increase of the beam length and thickness. The results indicate that optimum profiles of tantalum suspended structures can be obtained from the tensile-stressed films that have been released by the wet etching method with an aspect ratio of 1:48.

Superior thermal conductivity in suspended bilayer hexagonal boron nitride.

We reported the basal-plane thermal conductivity in exfoliated bilayer hexagonal boron nitride h-BN that was measured using suspended prepatterned microstructures. The h-BN sample suitable for thermal measurements was fabricated by dry-transfer method, whose sample quality, due to less polymer residues on surfaces, is believed to be superior to that of PMMA-mediated samples. The measured room temperature thermal conductivity is around 484 Wm−1K−1(+141 Wm−1K−1/ −24 Wm−1K−1) which exceeds that in bulk h-BN, providing experimental observation of the thickness-dependent thermal conductivity in suspended few-layer h-BN.

Suspended germanium cross-shaped microstructures for enhancing biaxial tensile strain

We fabricate a suspended germanium cross-shaped microstructure to biaxially enhance residual tensile strain using a germanium epilayer directly grown on a silicon-on-insulator substrate. Such a suspended germanium system with enhanced biaxial tensile strain will be a promising platform for incorporating optical cavities toward the realization of germanium lasers. We demonstrate systematic control over biaxial tensile strain and photoluminescence peaks by changing structural geometry. The photoluminescence peaks corresponding to the direct recombination between the conduction Γ valley and two strain-induced separated valence bands have been clearly assigned. A maximum biaxial strain of 0.8% has been achieved, which is almost half of that required to transform germanium into a direct band-gap semiconductor.

The Fringe-Capacitance of Etching Holes for CMOS-MEMS

Movable suspended microstructures are the common feature of sensors or devices in the fields of Complementary-Metal-Oxide-Semiconductors and Micro-Electro-Mechanical Systems which are usually abbreviated as CMOS-MEMS. To suspend the microstructures, it is commonly to etch the sacrificial layer under the microstructure layer. For large-area microstructures, it is necessary to design a large number of etching holes on the microstructure to enhance the etchant uniformly and rapidly permeate into the sacrificial layer. This paper aims at evaluating the fringe capacitance caused by etching holes on microstructures and developing empirical formulas. The formula of capacitance compensation term is derived by curve-fitting on the simulation results by the commercial software ANSYS. Compared with the ANSYS simulation, the deviation of the present formula is within ±5%. The application to determine the capacitance of an electrostatic micro-beam with etching holes is demonstrated in a microstructure experiment, which agrees very well with the experimental data, and the maximum deviation is within ±8%. The present formula is with simple form, wide application range, high accuracy, and easy to use. It is expected to provide the micro-device designers to estimate the capacitance of microstructures with etching holes and predominate in the device characteristics.

Realizing Area efficient Silicon Micro Structures Using Only Front End Bulk Micromachining

Front-end bulk micro machining is one of the proven techniques of making suspended microstructures and is highly adapted due to its simple and cost effective way of fabricating the devices. In this paper we propose novel geometric mask designs for achieving area efficient microstructures by front-end Silicon (Si) bulk micromachining. In this work we adapt the geometric mask design having a microstructure between two rectangular openings. These openings are aligned at 45° to wafer prime flat of (100) silicon wafer and act as etch openings for front-end bulk micromachining. However rectangular openings lead to high silicon area consumption which makes the process unworthy. Therefore we proposed different geometries to minimize area consumption for achieving the same dimension of suspended structure. All these different geometries are simulated using Intellisuite FABSIM based physical simulator. We have observed more than 28 % reduction in foot print over recent literature and 80 % over the basic design used in this paper.

Design, Fabrication, and Characterization of a Polymer-Based MEMS Uncooled Infrared Focal Plane Array

This paper presents the design, fabrication, and characterization of a polymer-based uncooled infrared (IR) focal-plane-array (FPA). In order to achieve high-temperature resolution, a thin-film IR absorber made of multiwalled carbon nanotubes (MWNTs) was adopted to absorb more than 90% of incident IR radiation. Furthermore, novel bimaterial microbeams composed of polyvinyl chloride and gold (Au), whose thermal expansion coefficient mismatch is up to 140 ppm/K, were utilized to convert the IR absorption-induced temperature rise to a thermal deflection with an ultrahigh sensitivity. The FPA was successfully fabricated with a low residual stress based on polymer surface micromachining techniques, and the MWNTs were integrated onto suspended microstructures in the FPA using a method combining screen printing with liftoff. To obtain a thermal image, an optical readout system utilizing an interferometric displacement detection based on integrated gratings was designed and implemented, which simultaneously measured the displacements of all pixels. Thermomechanical sensitivity of the fabricated FPA was measured to be 230 nm/K. Experimental results reveal that the current FPA is capable of detecting IR radiation power and temperature change in a target object with 27-nW and 14-K resolutions, respectively. [2014-0348]

Microfabrication of conical microfunnels for drug delivery applications

A cost-effective fabrication methodology for the mass production of conical microfunnels for drug delivery applications is presented. The fabrication methodology originates from a technique previously used for fabricating suspended or hollow microstructures. The process comprises six stages: in stage 1, a UV-opaque resin substrate is formed by mixing photocurable SU-8 with a UV-opaque impurity; in stage 2, an embossing stamp with an array of microrods is fabricated; in stage 3, the upper surface of the UV-opaque resin substrate (from the first stage) is mechanically deformed by stamping with the embossing stamp from stage 2; in stage 4, the regions in which microfunnels are photolithographically defined are selectively exposed to UV light; in stage 5, the wafer from stage 4 is subjected to heat and then developed in 1-methoxy-2-propanol acetate; finally, a hole on the tip of each microfunnel is formed by dry etching with oxygen plasma. The sharpness of the funnel tips can be flexibly adjusted by changing the stroke speed of the stamping in stage 3. The overall process can be carried out photolithographically with conventional photolithographic equipment. Using the proposed fabrication methodology, a matrix of polymeric microfunnels tapering from the base measuring 300 μm to the tip measuring 50 μm over a funnel height of 100 μm has been fabricated. Each fabricated microfunnel features a wide base and a sharp tip so that it has sufficient mechanical strength and sharpness to pierce the human skin. The conical geometry used in this study had a volume in excess of 24.4 nl.

Phononic crystal strip based anchors for reducing anchor loss of micromechanical resonators

Micromechanical resonators must be clamped to the substrate via anchors to support the suspended microstructure. However, these anchors will introduce anchor loss, and decrease quality factors (Qs) of the micromechanical resonators. To reduce the anchor loss, one dimensional phononic crystal based strips are employed as anchors of the microresonators in this paper. The dispersion relations and eigenmodes of the phononic crystal strips are presented. Flexural mode ring resonator and Lamé mode square plate resonator are designed to verify the effect of phononic crystal strips. The calculated results and finite-element simulations indicate that the leaky energy could be effectively reduced by the phononic crystal strip anchor design. Resonators with different anchor designs are also fabricated and characterized. The measured Qs of the microresonators show that the phononic crystal strips could reduce the energy dissipated through anchors, and with increasing the number of phononic strip periods, Qs of the resonators could be further enhanced. Furthermore, the experimental result also shows that compared with the Lamé mode square plate resonator, the flexural mode ring resonator is more susceptible to the anchor loss.

Localized Synthesis of Carbon Nanotube Films on Suspended Microstructures by Laser-Assisted Chemical Vapor Deposition

A laser-assisted chemical vapor deposition (LCVD) method has been developed for in situ synthesis of carbon nanotube (CNT) films on suspended microstructures. Focused laser beams are used to heat locally the suspended microstructures with low thermal mass and low thermal dissipation to high temperatures for localized CNT growth. Other substrate areas than the microstructures remain at low temperatures, preventing the devices on the substrate from being destroyed by high temperatures. The synthesizing parameters and the influences on CNT morphology and structures are systematically investigated and optimized, and solutions for uniform temperature distribution are proposed. Upon optimization, uniform, localized, and rapid growth of CNT synthesis has been achieved on suspended microstructures, and aligned CNTs with length and uniformity comparable to conventional hot-wall CVD have been successfully obtained. The experimental results show LCVD is a promising technology for in situ and localized synthesis of CNT films on suspended microstructures for CNT-CMOS (complementary metal oxide semiconductor) integration.

A new approach to control a deflection of an electroplated microstructure: dual current electroplating methods

We propose and demonstrate a simple and novel method to control the deflection in a suspended microstructure by using a dual current electroplating (DuCE) method. The key concept of this method is to divide the structure into two layers—a bottom layer and a top layer—and then apply respective current densities in electroplating to those two layers while all other conditions are kept the same. In addition to a flat structure, the direction of structure bending is freely controlled by virtue of the DuCE method. Cantilever Ni beams with a length of 400 µm, which were electroplated by the conventional single current electroplating method, bent downward with a deflection of 3.4 µm. On the contrary, by the DuCE method, cantilever beams with a length of 400 µm showed an almost flat structure as desired. (The current densities of the bottom layer, the top layer, and the ratio of the two current densities, are 0.15, 1.24 A dm−2, and 8.3, respectively.) Consequently, a nickel electroplated spiral structure with a length of 8600 µm was suspended flat with an end deflection of less than 0.7 µm (the ratio between the deflection and length is 0.007%). This work therefore represents the unprecedented ultra-long suspended microstructure with submicrometer deflection.

A Slot Micro/Nano Fiber With Elliptical Low-Index Core

A slot micro/nano optical fiber with elliptical low-index core, referred to as the elliptical-slot micro/nano fiber (ESMNF), is studied systematically in great detail in this paper. It is demonstrated that the ESMNF can confine most of the light in the low-index region and hence lead to large field enhancement. Benefiting from the large low-index power confinement factor and the field enhancement, the ESMNF also yields significant nonlinearity enhancement by the proper selection of materials. In addition, large birefringence between the two fundamental polarized modes is observed, due to the breakage of the circular symmetry for the core. The nonlinearity and low-index confinement properties of a more practical elliptical-slot embedded suspended microstructure fiber are also studied.

Electron transport in suspended semiconductor structures with two-dimensional electron gas

We study electron transport in suspended semiconductor microstructures fabricated from AlAs/GaAs membranes containing a high mobility two-dimensional electron gas. In quantizing magnetic fields, a reflection of edge current channels from the border of suspended area is observed resulting in the absence of vanishing magnetoresistance in the quantum Hall effect (QHE) regime. Relocation of this border out of the Hall bar revives the QHE. We have also found that the critical current of the breakdown of QHE in suspended samples is three times lower than in non-suspended samples due to the peculiarity of heat transport in the membranes.

Polymer as the protecting passivaton layer in fabricating suspended SCS structures in both anisotropic and isotropic etching

This paper presents a rapid bulk micromachining process named polymer passivation layer for suspended structures etching by using a polymer as a protecting passivation layer at both anisotropic and isotropic etching steps. Without using silicon-dioxide (SiO2) deposition or boron doping as a protection layer at the releasing step, the proposed method can fabricate suspended single-crystal silicon structures in an inductively coupled plasma reactive ion etching chamber directly, which would simplify the fabrication process and save fabrication time. The current study systematically investigates critical fabrication parameters to verify the feasibility of the proposed method, and discusses the polymer passivation time and removal time of a polymer at the base of a substrate at four different opening gaps of 5, 10, 30 and 50 µm with the 30 µm deep trench to establish suitable recipes for fabricating suspended microstructures. It is also shown that the proposed method can fabricate not only the suspended microstructures with the same thickness, but also suspended microstructures with different thicknesses, as well as in sub-micro scale.

In-Situ Heat Capacity Measurement of Carbon Nanotubes Using Suspended Microstructure-Based Microcalorimetry

This paper reports a method for measuring the heat capacity of as-grown carbon nanotubes (CNTs) using a microcalorimeter. The microcalorimeter consists of a double-layer suspended silicon dioxide microstructure and two silicon resistors sandwiched in-between the silicon dioxide layers. CNTs for heat capacity measurement are locally synthesized on the surface of the microstructure using laser-assisted chemical vapor deposition. The CNTs and the microcalorimeter are heated to a high temperature with a silicon resistor, and the temperature of the microcalorimeter with CNTs is measured with using the other silicon resistor. The heat capacity of the CNTs is obtained by dividing the measured effective heating power by the rate of temperature increase. The suspended configuration and the low thermal conductivity of silicon dioxide achieve low thermal dissipation, which together with the minute thermal capacity enable significant temperature changes. Using this sensitive microcalorimeter, the heat capacity of as-grown CNTs, 6-14 nJ/K from 340 to 440 K, is measured in-situ without the need of CNT transfer and pretreatment, avoiding damage to the CNT samples. This microcalorimeter is also applicable to in-situ measurement of structure-related thermal properties of porous materials.