Microdevices For Applications Research Articles

PECVD SiO2/Si3N4 Double-layer Electrets for Application in Micro-devices

In this paper, the influence of different environmental conditions, substrates, micromachining processes, areas and patternings on the performances of PECVD SiO2/Si3N4 double-layer electrets were studied for its application in micro-devices. Various samples were prepared and then charged by the negatively corona charging method. The charge decays at either 250°C or 95%RH were observed to reveal the chargeability and charge stability. Finally, a micro power generator with patterned PECVD SiO2/Si3N4 double-layer electret was presented briefly as demonstration. The results show that all these conditions that conventionally or even unavoidably happen in the fabrication process of electret micro-devices had more or less impact on the electret performance. It is crucial that the electret should be as large area as possible and kept from the micromachining processes and humid condition to the best. Our micro power generator with 2mm rectangular array of PECVD SiO2/Si3N4 double-layer electret had the 5μW output power at 20Hz and 0.7g.

Stepwise Optimized 3D Printing of Arbitrary 3D Structures at Millimeter Scale with High Precision Surface

Abstract3D printing based on additive manufacturing has attracted widespread attention in the fields of microbiology and microelectronics due to its advantages of waste reduction, arbitrary manufacturing, and rapid prototyping in potential applications. These techniques can create structures at the centimeter scale, however, there are some limitations in terms of resolution and geometric constraints. Here, a micro–nano 3D printing protocol based on additive manufacturing to achieve the 3D structure (3DS) not only possessing millimeter scale structural dimensions but also nanometer features are proposed. A theory is verified to assist the design and fabrication of the 3DS with millimeter scale and nanometer precision. The structures are predesigned and the scanning strategy is optimized before 3D printing to improve the manufacturing efficiency and precision. A customized 3DS with a height of 2.2 mm is obtained, which is a challenge for the conventional two‐photon polymerization fabrication. Furthermore, a 1.2 mm 3DS with inside scaffold and smooth surface is efficiently achieved within 2.7 h with a nanometer surface roughness by using the proposed stepwise optimized 3D printing process. This study offers a flexible and low‐cost technology to generate highly customizable, precisely controllable 3DS for potential applications in microelectronics and microdevices.

Smoothed particle hydrodynamics simulation: a tool for accurate characterization of microfluidic devices

This work presents a two-dimensional (2D) model employing the mesh-free smoothed particle hydrodynamics (SPH) method for accurate characterization of microdevices. The simulator was validated by comparing analytical and numerical results from literature with the cases of Poiseuille, Couette, and biphase flow in microfluidic devices. Finally, a test case using two immiscible fluids in a cross-like device with three inputs and one output was computed. This device produces droplets in a flow-focusing configuration. The simulations produced similar flows when compared with theoretical, numerical, and experimental data reported in literature. When handling two liquid phases, such as water and oil, properties such as surface tension must be taken into account. These properties can be well modeled using the continuum surface force method, commonly applied for modeling capillarity in microdevice and microliquid applications. Thus, the implementation of the SPH method demonstrated that it represents a novel and promising alternative for simulation of emulsion formation in microfluidic devices.

A Facile One-Step Approach for Constructing Multidimensional Ordered Nanowire Micropatterns via Fibrous Elastocapillary Coalescence.

Nanowire (NW) based micropatterns have attracted research interests for their applications in electric microdevices. Particularly, aligning NWs represents an important process due to the as-generated integrated physicochemical advantages. Here, a facile and general strategy is developed to align NWs using fibrous elastocapillary coalescence of carbon nanotube arrays (ACNTs), which enables constructing multidimensional ordered NW micropatterns in one step without any external energy input. It is proposed that the liquid film of NW solution is capable of shrinking unidirectionally on the top of ACNTs, driven by the dewetting-induced elastocapillary coalescence of the ACNTs. Consequently, the randomly distributed NWs individually rotate and move into dense alignment. Meanwhile, the aggregating and bundling of ACNTs is helpful to produce carbon nanotube (CNT) yarns connecting neighboring bundles. Thus, a micropatterned NW network composed of a top-layer of horizontally aligned NWs and an under-layer of vertical ACNT bundles connected by CNT yarns is prepared, showing excellent performance in sensing external pressure with a sensitivity of 0.32 kPa-1 . Moreover, the aligned NWs can be transferred onto various substrates for constructing electronic circuits. The strategy is applicable for aligning various NWs of Ag, ZnO, Al2 O3 , and even living microbes. The result may offer new inspiration for fabricating NW-based functional micropatterns.

High-quality patterning technique for polarization-enhanced (Ba0.67Sr0.33TiO3/LaNiO3)n multi-interface multilayer hetero-structure

In this paper, a fabrication method for a patterned (Ba0.67Sr0.33TiO3, BST/LaNiO3, LNO)n thin-film multilayer is reported. The multilayer films were deposited by radio-frequency (RF) magnetron sputtering. A patterning technique of the as-prepared multilayer thin films was developed using ultrasonic-assisted wet etching. The scanning electron microscopy (SEM) surface images show that the developed technique can effectively etch the multilayer thin films deposited on SiO2 and Pt substrates without any residues. The etched multilayer thin film units of the size of 50 μm × 50 μm display clear and smooth etching edges, low distortions (<1 μm) and high uniformity. The dielectric properties of these units for the (BST/LNO)2 multilayer thin films include a high dielectric constant (>900) and a relatively low loss tangent (<0.035) at frequencies below 10 kHz. The capacitance density ranges from 9 nF/mm2 for (BST/LNO)1–24.5 nF/mm2 for (BST/LNO)5 multilayer thin films at 1 kHz, and all these multilayer thin film units withstand a DC voltage of 24 V. This study can facilitate the development of the application of multi-interface multilayer ferroelectric micro-devices.

Controllable synthesis of Y doped AlN microtubes, prism-arrayed microstructures and microflowers via an improved DC arc discharge plasma method

The yttrium doped AlN (AlN:Y) microtubes, prism-arrayed microstructures and microflowers were fabricated through the direct reaction of Al:Y alloy and N2 without extra catalyst. AlN microstructures can be controlled by varying deposition temperature or changing deposition area. The structure and morphology of AlN:Y microstructures were characterized by X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy. The results indicate that Y3+ ions incorporate inside the AlN nanostructures. The photoluminescence spectra of the AlN:Y microstructures show strong defect-induced emissions in the visible range of 400–750 nm. Room-temperature ferromagnetism is observed in AlN:Y microstructures which is caused by high concentration of Al vacancies. The AlN:Y microstructures with interesting geometries possess excellent optical and magnetic properties, suggesting potential applications in optoelectronic and spintronic microdevices.

Surface compressive and softening effect on deformation mode transition in Ni-Nb metallic glassy thin films: A molecular dynamics study

Size-dependent deformation mode transition in metallic glasses (MGs) attracts a lot of interest due to potential application in micro-devices, but the underlying mechanisms are still unclear from the perspective of structure, e.g., how the chemical composition affects the deformation mode transition in a particular system is mysterious as well and needs to be addressed. Here, a series of NixNb100−x (x = 30, 50, 62, and 70 at. %) MG thin films has been studied by molecular dynamics simulations for better understanding the thickness-dependent tensile behaviors. The results show that the deformation mode transition from highly-localized to non-localized occurs as the film thickness (t) approaches the critical size, tc, which strongly correlates with the chemical composition, i.e., a Ni-rich specimen with higher modulus has smaller tc. It is revealed that the softening and compressive effect of surface layers with about 0.4 nm thickness in terms of Voronoi volume is the key factor for this transition regardless of composition. We illustrate the surface softening effect in various MG thin films by introducing a softening coefficient (Ψ) parameter reflecting the structural difference between the surface layer and the internal part. It is found that the higher the Ψ, the severer the surface softening effect, and the larger the tc in the Ni-depleted specimen, indicating the importance of Ψ as an indicator for the deformation mode transition.

Potassium Ion Selective Electrode Using Polyaniline and Matrix-Supported Ion-Selective PVC Membrane

Solid-state potassium ion selective electrode (K+ ISE) has been the most studied chemical sensors due to its practical importance in biomedical applications. One of the major obstacles that prevented widespread use of solid-state K+ ISE has been output potential drift problem. In this paper, we developed an electrochemical sensing unit in which working, counter, and reference electrodes are integrated in a single plane as all-solid-state form. In order to mitigate the output potential drift, a polyaniline intermediate layer and salt-saturated polyvinylebutyral top coating are introduced in the working and reference electrodes, respectively. Using cyclic voltammetry (CV), uniform layers of polyaniline are deposited on carbon electrode, as confirmed by scanning electron microscope observation. Potentiometry and electrochemical impedance spectroscopy measurement on our K+ ISE show high sensitivity (60.5 mV/decade), low concentration for the limit of detection ( $10^{-5.8}$ M), and large range of linear detection ( $10^{-5}-1$ M), and superior selectivity of K+ ISE against NH4+, Na+, Mg2+, Ca2+, and Fe3+. With its high potential to be miniaturized, we foresee that our solid-state K+ ISE will motivate the future applications in microdevices for clinical analysis, agricultural, and environmental applications.

Electrochemical Formation of Porous Silicon Carbide for Micro-Device Applications

Anodization of silicon carbide (SiC) in hydrofluoric acid (HF) solutions is a promising way to etch this material which is very resistant against traditional chemical etching methods. Moreover, it has been shown that several reproducible porous SiC morphologies can be performed varying anodization conditions (current density, electrolyte composition, UV lighting) and/or substrate properties (doping type and level). This paper proposes a state of the art of porous SiC etching in GREMAN and a presentation of the morphologies achievable using anodization in HF based electrolytes.

Electrohydrodynamic Printing of Microscale PEDOT:PSS-PEO Features with Tunable Conductive/Thermal Properties.

Electrohydrodynamic (EHD) printing has been recently investigated as an effective technique to produce high-resolution conductive features. Most of the existing EHD printing studies for conductive features were based on metallic nanoparticle inks in a microdripping mode, which exhibited relatively low efficiency and commonly required high-temperature annealing process to achieve high conductivity. The EHD printing of high-resolution conductive features at a relatively low temperature and in a continuous cone-jetting mode is still challenging because the conductive inks might connect the charged nozzle, and the grounded conductive or semiconductive substrates to cause discharge and terminate the printing process. In this study, the EHD printing process of conductive polymers in a low-temperature cone-jetting mode was explored to fabricate conductive microstructures. The smallest width of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) lines was 27.25 ± 3.76 μm with a nozzle diameter of 100 μm. It was interesting to find that the electrohydrodynamically printed PEDOT:PSS-PEO features exhibited unique thermal properties when a dc voltage was applied. The conductive and thermal properties of the resultant features were highly dependent on the printing layer number. Microscale PEDOT:PSS features were further encapsulated into electrospun nanofibrous mesh to form a flexible sandwich structure. The EHD printing of PEDOT:PSS features with tunable conductive and thermal properties might be useful for the applications of flexible and wearable microdevices.

All-Solid-State Potentiometric Sensors for Potassium Ion Detection with Enhanced Stability By Interlayer Incorporation

Solid state potassium ion selective electrode (K+ ISE) has been the epitome of the most studied chemical sensors in recent years due to its practical importance in biomedical applications. In fact, potassium monitoring in food and serum, urine, and potentially in brain has been carried out in the clinical and medical fields to reveal physical conditions of the patients such as renal diseases, hypopotassemia, alkalosis, cirrhosis of liver, etc. In this report, we developed an electrochemical sensing unit which working, counter and reference electrodes are integrated in a single plane as all-solid-state form. Here, a polyaniline intermediate layer and salt-saturated polyvinylebutyral top coating are introduced in the working and reference electrodes, respectively, in order to mitigate the output potential drift problem that prevented widespread use of solid-state K+ ISE. Morphological and electrochemical properties of K+ ISE are studied by using a scanning electron microscope (SEM) and a potentiostat, respectively. Various measurement modes of electrochemical measurements, including cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS), are implemented. The K+ ISE show desirable properties including high sensitivity (60.5 mV/decade), low concentration for the limit of detection (10-5.8 M), and large range of linear detection (10-5 – 1 M). Selectivity of our of K+ ISE against NH4 + , Na+ , Mg2+ , Ca2+ , and Fe3+ was studied. With its high potential to be miniaturized, we foresee that our solid-state K+ ISE will find future applications in microdevices for clinical analysis, agricultural and, environmental applications.

Simulation of a falling droplet in a vertical channel with rectangular obstacles

Droplet microfluidic systems have attracted a large amount of research due to their numerous applications in biomedical micro-devices and drug discovery/delivery platforms. One of the most important problems in such systems is to investigate deformation, coalescence, and breakup of droplets within the channel. The present study demonstrates numerical simulation of a falling droplet subject to gravitational force in a channel with embedded rectangular obstacles. The lattice Boltzmann method incorporated using He–Chen–Zhang method for two phase flow is employed. Two rectangular obstacles with inverse aspect ratios are introduced to investigate the mechanism of breakup and deformation of the droplet. The influence of gravity magnitude, viscosity and surface tension on the deformation rate of droplet for two different aspect ratios of the obstacle is studied. It is observed that increasing the gravity force, decreasing the viscosity or surface tension increase droplet deformation rate resulting in more stretched filaments and so breakup occurs in a shorter time.

Micro-fabrication method of graphite mesa microdevices based on optical lithography technology

Graphite mesa microdevices have incommensurate contact nanometer interfaces, superlubricity, high-speed self-retraction, and other characteristics, which have potential applications in high-performance oscillators and micro-scale switches, memory devices, and gyroscopes. However, the current method of fabricating graphite mesa microdevices is mainly based on high-cost, low efficiency electron beam lithography technology. In this paper, the processing technologies of graphite mesa microdevices with various shapes and sizes were investigated by a low-cost micro-fabrication method, which was mainly based on optical lithography technology. The characterization results showed that the optical lithography technology could realize a large-area of patterning on the graphite surface, and the graphite mesa microdevices, which have a regular shape, neat arrangement, and high verticality could be fabricated in large batches through optical lithography technology. The experiments and analyses showed that the graphite mesa microdevices fabricated through optical lithography technology basically have the same self-retracting characteristics as those fabricated through electron beam lithography technology, and the maximum size of the graphite mesa microdevices with self-retracting phenomenon can reach 10 µm × 10 µm. Therefore, the proposed method of this paper can realize the high-efficiency and low-cost processing of graphite mesa microdevices, which is significant for batch fabrication and application of graphite mesa microdevices.

Understanding the electro-stimulated deformation of PVC gel by in situ Raman spectroscopy

Electro-responsive materials are of promising applications in microdevices. A dioctyl terephthalate (DOTP) plasticized polyvinyl chloride (PVC) gel actuator driven by unilateral electrodes was proposed, with which the dynamical creeping deformation of the gel around the anode by electro-stimulation was investigated by in situ Raman spectroscopy. The obtained results reveal that the intensity of the band at 1612 cm−1 was linearly correlated to the ratio of DOTP in PVC gel. The plasticizer concentration evolved after an external electro-stimulation was applied, which indicates the migration of plasticizer around the anode. The migration of the plasticizers to the two sides of the anode consequently drives the deformation of the PVC gel. The memory effect of deformation is due to the residuals of the plasticizers out of their original positions after the removal of electro-stimulation. The understanding of the dynamical deformation of PVC gel in response to electro-stimulations may be helpful for the designing of novel electric actuators.

Creation of antifouling microarrays by photopolymerization of zwitterionic compounds for protein assay and cell patterning

Nonspecific binding or adsorption of biomolecules presents as a major obstacle to higher sensitivity, specificity and reproducibility in microarray technology. We report herein a method to fabricate antifouling microarray via photopolymerization of biomimetic betaine compounds. In brief, carboxybetaine methacrylate was polymerized as arrays for protein sensing, while sulfobetaine methacrylate was polymerized as background. With the abundant carboxyl groups on array surfaces and zwitterionic polymers on the entire surfaces, this microarray allows biomolecular immobilization and recognition with low nonspecific interactions due to its antifouling property. Therefore, low concentration of target molecules can be captured and detected by this microarray. It was proved that a concentration of 10ngmL−1 bovine serum albumin in the sample matrix of bovine serum can be detected by the microarray derivatized with anti-bovine serum albumin. Moreover, with proper hydrophilic-hydrophobic designs, this approach can be applied to fabricate surface-tension droplet arrays, which allows surface-directed cell adhesion and growth. These light controllable approaches constitute a clear improvement in the design of antifouling interfaces, which may lead to greater flexibility in the development of interfacial architectures and wider application in blood contact microdevices.

Alloying effects on the microstructure and mechanical properties of nanocrystalline Cu-based alloyed thin films: Miscible Cu-Ti vs immiscible Cu-Mo

Tuning the microstructure to optimize the mechanical performance of nanocrystalline Cu thin films via the alloying strategy is quite important for their application in microdevices. In this work, we prepared nanocrystalline miscible Cu-Ti and immiscible Cu-Mo alloyed thin films to investigate alloying effects on the microstructure and mechanical properties of Cu thin films in terms of mixing enthalpies. It is found that the dopants of both Ti and Mo can notably refine the grains, and in particular promote the formation of nanotwins below a critical content of solute, beyond which the formation of nanotwins is notably suppressed. The nonmonotonic solute concentration-dependent twinning behavior observed in Cu-Ti and Cu-Mo alloyed thin films is explained by the coupling effects between grain size and grain boundary segregated dopants that affects the stimulated slip process of partials. The increased hardness of both Cu-Ti and Cu-Mo systems with increasing the solute contents are quantitatively explained by combining several strengthening mechanisms, including solid solution strengthening, grain/twin boundary (GB/TB) strengthening, solute segregation-induced strengthening. It unexpectedly appears that with increasing the solute contents, the Cu-Ti system exhibits monotonically reduced positive strain rate sensitivity (SRS, m), whereas the Cu-Mo system manifests almost constant negative SRS. The fundamental difference in SRS m between Cu-Ti and Cu-Mo is rationalized in terms of the interactions between solute atomic clusters and dislocations based on the cross-core diffusion mechanism.

Designed Microdevices for In Vitro Diagnostics

AbstractIn vitro diagnostics (IVD) refers to in vitro tests of biological specimens and contributes to two‐thirds of clinical diagnosis. There are two major components to consider for the construction of devices for IVD, including: i) capture and recognition; and ii) detection methods. Herein, the principle, design, and application of microdevices regarding these two routes in recent progress for IVD are shown. Microarrays, microfluidics, and nanodevices are reviewed for capture and recognition. Mass spectrometry, fluorescence, electrochemistry, and so on are summarized as detection methods. The roles of materials in devices toward diverse applications are elucidated with the aims of designing materials for high‐performance device toward precision medicine and initiating the development of advanced diagnostic tools involving various biomarkers.

Mechanical behavior of PDMS at low pressure

Polydimethylsiloxanes (PDMS) are used in a wide range of soft devices including lab-on-chip, soft robots and flexible electronics. These technologies are currently considered for space exploration applications. We experimentally study the effects of vacuum pressure on the dynamics of PDMS resonator and demonstrate that, unlike hard materials, it exhibits an appreciable change in modulus manifested as a shift in resonance frequency with varying pressure. To reveal this dependence, we carefully probe the dissipation due to air pressure damping acting on the surface of the membrane. For a 1 µm thick membrane, the modulus of the PDMS decreases when the pressure is below Pt = 3.175 Torr, whereas at pressures above the dynamics are dominated by gas damping in the free molecular flow and viscous regimes. We conjecture that the observed effect is a consequence of a change in shear modulus resulting in a logarithmic linear relationship between pressure and stiffness for a circular PDMS membrane. These results are important for the application of PDMS microdevices at low pressure.

Resistive switching in TiO2 nanocolumn arrays electrochemically grown

Resistive switching in metal oxides, especially in TiO2, has been intensively investigated for potential application in non-volatile memory microdevices. As one of the working mechanisms, a conducting filament consisting of a substoichiometric oxide phase is created within the oxide layer. With the aim of investigating the filament formation in spatially confined elements, we fabricate arrays of self-ordered TiO2 nanocolumns by porous-anodic-alumina (PAA)-assisted anodizing, incorporate them into solid-state microdevices, study their electron transport properties, and reveal that this anodizing approach is suitable for growing TiO2 nanostructures exhibiting resistive switching. The electrical properties and resistive switching behavior are both dependent on the electrolytic formation conditions, influencing the concentration and distribution of oxygen vacancies in the nanocolumn material during the film growth. Therefore, the PAA-assisted TiO2 nanocolumn arrays can be considered as a platform for investigating various phenomena related to resistive switching in valve metal oxides at the nanoscale.

Residual stress modeling and analysis for micro electroforming layer

Large residual stress in the micro electroforming layer has been considered to be the main reason for causing warpage, delamination and micro cracking of the microdevice. These undesirable phenomena dramatically influence the productivity, dimensional accuracy and service life of the microdevice. Reasonable control of the residual stress is a direct and efficient way to improve the production rate of microdevice and expand the application of microdevice. In this paper, the residual stress finite element analysis (FEA) simulation model for Ni micro electroforming layer was established by the method of “Equivalent Reference Temperature (ERT)”. The simulation result shows that the ERT for Ni micro electroforming layer is 238.15 K. The error of residual stress value is 0.37% between the simulation result and the experimental result. This error value proves the accuracy of the FEA model.