Microbridge Structures Research Articles

Design Rules for Capillary Force-Induced Clustering and Recovery of Monolithic Microbridges on Microapertured Polymer Membrane.

Microbridge structures have been widely used in microelectromechanical systems. When the devices with microbridges operate in diverse environments, including wet conditions, structural failures such as crumples, clustering, and collapse of micro/nanostructures occur due to the capillary force of liquid in this environment. It is necessary to establish comprehensive design criteria to address this. Herein, we investigate the structural stability of microbridges on microsized apertures in wet conditions. The multiscale structure is fabricated with microbridge width, spacing of 10 μm, and height of 6 μm while varying the supporting micro aperture size. The behavior of the microbridges is observed through an optical microscope during water dispensing on the bridges and evaporation. It is found that the microbridges remain stable on apertures of 100 μm in diameter, while clustering occurs on larger-sized apertures (300, 500 μm). Interestingly, in contrast to the 500 μm-sized aperture, the clustered microbridges on the 300 μm-sized aperture gradually recover to their original configuration after completely evaporating water. A simple theoretical model for capillary force-induced clustering and recovery is proposed to elucidate this phenomenon, which agrees with the experimental results. The microbridges constructed following the design rule can ensure robust and stable operation even in wet conditions. These findings contribute to advancing micro/nanoscale engineering and offer insights for developing innovative microdevices.

High-response humidity sensor based on silicon microbridge with edge-constrained sandwich sensing structure

Silicon microbridge structures have attracted the attention of researchers in humidity sensing area owing to their small size and ease of integration with the integrated circuit (IC). However, the preparation of silicon microbridge humidity sensors with high response are still a challenge. This paper proposed a silicon microbridge humidity sensor based on an edge-constrained sandwich sensing structure, in which a graphene oxide core layer with small area was covered on the surface of the silicon bridge by a larger area of Nafion film. Due to the constraint of the peripheral edge, the swelling deformations of this sandwich structure in the vertical direction were effectively magnified during humidity detection. Hence, the proposed humidity sensor exhibited a higher humidity sensitivity (109.97 µV/%RH) compared to the silicon microbridge sensors with general sandwich sensing structure or monolayer sensing film. Meanwhile, the novel sensor also demonstrated a small humidity hysteresis and good linearity in humidity range of 11.3–97.3%.

High-uniformity atomic layer deposition of superconducting niobium nitride thin films for quantum photonic integration

Atomic layer deposition (ALD) has been identified as a promising growth method for high-uniformity superconducting thin films for superconducting quantum photonic applications, offering superior uniformity, thickness control and conformality to techniques such as reactive sputtering. The potential scalability of ALD makes this method especially appealing for fabrication of superconducting nanowires and resonators across large areas. We report on the growth of highly uniform superconducting NbN thin films via plasma-enhanced atomic layer deposition (PEALD) with radio frequency substrate biasing, on a 200 mm (8 inch) Si wafer, specifically for superconducting nanowire single-photon detector applications. Niobium nitride films were grown using (tert-butylimido)-tris(diethylamido)-niobium(V) precursor and an H2/Ar plasma. The superconducting properties of a variable thickness series of films (5.9–29.8 nm) show critical temperature (T c) of 13.5 K approaching bulk thickness (28.8 nm) with low suppression down to the ultrathin regime (5.9 nm), with T c = 10.2 K. T c across the 200 mm wafer with 8 nm thick NbN, measured in 15 mm intervals, exhibits minimal variation (<7%). Microbridge structures fabricated on 8 nm thick NbN films also exhibit high critical current densities (J c), > 10 MA cm−2 at 2.6 K. PEALD could therefore be a pivotal technique in enabling large-scale fabrication of integrated quantum photonic devices across a variety of applications.

(Digital Presentation) Fabrication of Strained Ge Microbridges with Mesh-Patterned Pads and Their Optical Properties

IntroductionRecently, Ge has attracted attention as a luminescent material on Si. As fabrication of Ge-on-Si structures can induce tensile strain into Ge owing to thermal mismatch between Si and Ge and resultantly modify its band structure, the direct transition probability can be improved, offering strong luminescence. However, amounts of the strain induced in the Ge-on-Si are quite small (~0.2 %) and the larger strain is highly desired. And then, microbridge structures have been studied to increase the tensile strain and luminescence intensity [1,2]. Usually, electron beam lithography, which is not suitable for mass production, is used for making sub mm scale microbridges. We attempt to fabricate relatively large, several mm scale, microbridges by simple photolithography process. To provide large tensile strain in the Ge microbridge, it is necessary to enlarge areas of the pad region (both side areas of the bridge) due to the relatively large bridge size. However, the lower etching rate of the (111) plane makes it difficult to completely remove the Si underneath the Ge in the pad area. Therefore, we attempt to fabricate microbridge structures with mesh-type holes on the pads to enhance the etching of the Si in the pad areas and obtain a significant increase in room temperature PL luminescence intensity.ExperimentalA low-temperature Ge layer (Tg=350℃,40 nm) and high-temperature Ge layer (Tg=600℃, 500nm) were grown on Si(100) substrates using solid source MBE, and annealed to improve crystallinity. Subsequently, microbridge structures with mesh holes on the pad areas were patterned with the bridge along <110> direction by standard photolithography. The Ge and Si were etched down by dry etching. Then the Si underneath the Ge was removed by selective etching with KOH to fabricate freestanding microbridges. In addition, since amounts of strain are expected to vary depending on the area of the holes in the pad area, samples with different areas of holes were fabricated and compared. PL measurements at room temperature (excitation wavelength: 980nm) were performed to evaluate luminescent characteristics. Moreover, Raman measurements were performed to evaluate the amounts of strain (Laser wavelength: 514.5nm).Results and discussionsLaser microscope measurements confirmed that microbridges with mesh-holes on the pads were fabricated and that the Si underneath the Ge in the pad areas was fully etched and the pad areas are freestanding. From the PL spectrum obtained from the central position of the microbridge, a strong PL peak is observed, and a PL intensity of the microbridge with the mesh structure is significantly higher than that without the mesh structure. The peak is seen to shift to longer wavelength around 1800 nm, indicating that the tensile strain in the Ge microbridge with the mesh pattern is larger than that without the mesh structure, which was confirmed by Raman measurements. It is notable that the Ge-on-Si without patterning hardly yield light emission, indicating that the fabrication of microbridges with mesh-pattern by the simple photolithography process is expected to realize Si-based high-efficiency light sources.This work was supported in part by Grant-in-Aid for Scientific Research (19H02175, 19H05616, 20K21009).Ref1.M.J.Süess et al. Nature Photonics7 46-472(2013) 2. F.T. Armand Pilon et al. Nature Communications10 2742 1-8(2019) Figure 1

(Digital Presentation) Fabrication of Microbridges Based on Ge-on-SOI and Observation of Strong Resonant Light Emission

Introduction Ge is one of the most attractive material for silicon-based optoelectronics because it can be epitaxially grown on a Si substrate, allowing us to integrate Ge based light emitters and photodetectors monolithically on the Si platform. Although Ge is an indirect band gap semiconductor like Si, by epitaxially growing Ge on the Si substrate, tensile strain is introduced into Ge, resultantly leading to the Γ-valley shifts and increase in the light emission efficiency via the direct transition. The amount of the strain induced in Ge-on-Si, however, is not sufficiently large, demanding other methods of applying large tensile strain to Ge. Fabrication of Ge microbridge structure is expected to induce the larger strain and further improve light emission efficiency [1]. In general, microbridges reported so far have been fabricated by the electron beam lithography process, which is not appropriate for mass production. In this study, we fabricate Ge microbridge by simple photolithography process based on Ge-on-Si and Ge-on-SOI structures and very strong room temperature photoluminescence is obtained and strong resonant peaks can be observed. Experimental Method Ge layers were grown on a Si (110) wafer (Ge-on-Si) or a SOI (100) wafer (Ge-on-SOI) using a two-step growth method by solid source MBE. Firstly, a 40nm thick buffer layer at a low temperature of 350 ~ 400℃. Secondly, a 500 nm thick Ge layer was grown at a high temperature of 550 ~ 650℃. Post-growth-annealing was carried out at 800℃ for 10 minutes. The microbridge structures were defined by the standard photolithography process and mesa etching was performed by the reactive ion etching. Subsequently, the Si under the bridge and pads was selectively removed by KOH etchant, leading to free-standing Ge microbridge. For the Ge-on-SOI, the underlying SiO2 layer (buried oxide) was additionally removed by HF. Results and Discussion It was observed by SEM measurements that the Ge microbridges fabricated from Ge-on-Si were slightly warped from both ends to the center of the bridges. Hydrogen gases generated due to the reaction of Si and KOH during the selective etching are considered to be responsible for the warping of the microbridges, which makes it difficult to control the strain of the Ge. The selective etching time, therefore, has to be minimized to reduce the effects of the gas generation. For this purpose, the microbridges were fabricated from Ge-on-SOI, where the floating structure can be easily formed by the etching of the buried SiO2 layer with HF and the etching time of Si with KOH was reduced by a factor of four. As a result, the warping was completely suppressed and very flat microbridges were fabricated without any warping.Photoluminescence measurements at the center of the microbridge on Ge-on-SOI were performed at room temperature. Strong PL emissions were obtained in the wavelength range of 1630-2000 nm, where several strong resonant peaks appear. This resonance is considered to occur due to reflection at the side wall of the bridge and the peaks can be well fitted with the cavity length of the bridge width. Also, we carried out PL measurements from one end to another end of the bridge, and investigated the bridge position dependence of the PL emission. It was found that the light emission intensity increased toward the center of the bridge, and the highest intensity was obtained at the center. Moreover, the strain amounts were measured by micro Raman line scan along the bridge and strain dependence of PL emission was investigated. It was also shown that the strain amount increased toward the center of the bridge and became the highest at it. It was demonstrated that the higher PL intensity can be obtained due to the higher tensile strain at the center of the microbridge. In summary, we fabricated Ge microbridge structures by photolithography process and obtained strong room temperature resonant light emission from the Ge microbridges formed from Ge-on-SOI where bridge-warping was completely suppressed by shortening the etching time. From this result, we can say that the Ge microbridge light emitting devices are very promising for applications to the light sources on the Si platform.This work was partially supported by JSPS KAKENHI (Nos. 19H02175, 19H05616 and 20K21009).[1] M. J. Suess et al, Nat. Photonics 7: 466, 2013 Figure 1

(Digital Presentation) Strain Engineering of Heteroepitaxial SiGe/Ge on Si with Various Crystal Orientations

In recent years, Ge has emerged as one of the most promising materials toward next-generation high-performance and low-power-consumption optoelectronic devices. Moreover, SiGe/Ge heterostructures, consisting of strained SiGe with high Ge contents and Ge, have high potential for various applications. In terms of crystal orientations, there are various attractive choices for Ge and Ge-rich SiGe differently from Si. Particularly, the (111) orientation is very attractive for spintronics applications owing to capability of lattice matched epitaxial growth of high-quality ferromagnetic materials on the Ge(111) and SiGe(111). To create the SiGe/Ge heterostructures on the Si substrate, high quality Ge virtual substrates firstly have to be formed on the Si. The so-called Ge-on-Si is simple but very important templates for various applications, such as Ge-MOSFETs, LEDs, LDs, photo detectors, micro structures and Ge-on-Insulator (GOI) wafers etc. Especially the tensile strain induced in the Ge due to the thermal mismatch between the Ge and Si plays very important roles. For example, very efficient light emission can be obtained through strain-induced Γ-valley shift and resultant increase in the direct transition probabilities. We fabricated both Ge-on-Si(100) and Ge-on-Si(111) p-i-n LEDs and strong room-temperature EL emissions were obtained [1]. Epitaxial growth of the high quality strained SiGe on the Ge-on-Si is the most important for strain engineering of SiGe/Ge heterostrucutures. As the growth of the strained layer is limited by the critical thickness, we systematically investigated the critical thickness of the strained SiGe and clarified that it depends on the crystal orientations [2]. It was also shown that the critical thickness is markedly reduced when grown on the Ge-on-Si compared to the Ge substrate due to dislocations in the Ge-on-Si. We proposed to grow the SiGe on the Ge-on-Si on which the mesa patterning is performed. As a result, the critical thickness is drastically increased owing to the suppression of defect generation and propagation [3]. This patterning method enables us to increase the strained layer thickness and highly widen potential applicabilities of the strained SiGe/Ge heterostructures. One of the attractive applications is strained SiGe/Ge multi quantum wells (MQWs). We obtained strong roome temperature light emissions from the relatively thick MQWs formed on the Ge-on-Si (111) and Si(100) with patterning. Based on the Ge-on-Si, the larger tensile strain can be induced by the fabrication of microbridge structures, where the freestanding Ge is realized and elastic tensile stress is applied to the narrow microbridge area via the local strain relaxation of large pad areas sandwiching the bridge. In many reports, standard Si(100) substrates have been used with the bridge direction set to be <001> direction, whereas the Γ-valley shift via the tensile strain is expected to be the largest with the <111> directional uniaxial strain [4]. We attempt to fabricate the Ge microbridge along the <111> direction on the Ge-on-Si(110) substrate and very strong room-temperature PL was obtained. Moreover, when microbridge side walls are formed vertical, the generated light is well reflected by the side walls and very strong resonant light emissions are observed. Technically, the selective wet-etching of the underlying Si of the Ge-on-Si is a key process of importance to fabricate completely freestanding structures. As the crystal orientation highly affects the etching rate, a choice of the orientation is very important. Additionally, we are attempting other fabrication methods. One is making many holes in the pad areas, which enables to enhance the under-etching. Use of Ge-on-SOI or Ge-on-Insulator (GOI) substrates is another way to facilitate the under-etching as the buried oxide can be easily removed by the fast etching. Moreover, we show recent results on SiGe/Ge heterostructure imbedded microbridge, where the light emission wavelength becomes controllable and emission efficiency can be enhanced owing to the strain and band engineering.[1] K. Yamada et al. Appl. Phys. Express 14, 045504 (2021). [2] M. M. Alam, Appl. Phys. Express 12, 081005 (2019). [3] Y. Wagatsuma et al. Appl. Phys. Express 14, 025502 (2021). [4] H. Tahini et al. J. Phys. Condens. Matter 24, 195802 (2012).

Enabling a novel approach to a controlled fabrication of 1D crystalline nanowires on suspended microstructures of arbitrary geometries using two direct-writing technologies

Recent progresses in additive manufacturing have inspired new technologies, such as direct laser writing technique, based on two-photon polymerization (2 PP), which complements and further enriches the nanofabrication tools portfolio. In this work, we combine 2 PP and our mask-free scanning probe assisted ‘direct-write patterning’ (DWP) method to allow for: a) the fabrication of micro-bridge structures with sub-micrometer resolution, b) selective synthesis of crystalline ZnO nanowires at predefined locations, respectively. This synergistic approach enables cantilever probe patterning of catalysts directly on suspended micro-bridges for in-situ CVD growth of nanoscale material, in a templated manner. The study reported here represents the first proof-of-concept experiments demonstrating versatile and scalable methodology, which can be applied and straightforwardly extended to grow a variety of other nanomaterials, in a controlled and selective fashion, on freestanding micro/nanoscale structures, whose size and geometry can be conveniently varied via templating of sacrificial 2 PP polymeric scaffolds. Finally, the demonstration of the possibility to integrate this new approach with the conventional lithography techniques provides a step forward to the development of the novel class of hybrid polymer-silicon-1D or -2D materials, and systems. The quality of the produced ZnO nanowire assemblies was assessed using several physical characterization methods.

Tough Mechanically Interlocked Transparent Interface of Hydrogels and Elastomers for Biomedical Applications

AbstractHydrogel‐based soft, wet devices receive a great deal of attention in advanced medical engineering, which will require integration of hydrogels and elastomers through a transparent and tightly bound interface. Here, the strength of the mechanical interlock of the hydrogel–elastomer interface is studied using microstructured poly(dimethylsiloxane) (PDMS) sheets embedded in a polyvinyl alcohol (PVA) hydrogel. The PDMS sheet, which exhibits micro‐protrusions (cuboid and frustum‐shape), has a tensile strength of 10 kPa, which can be sufficient for gentle handling of the PVA/PDMS assembly. The PDMS protrusions (even the frustum‐shape) are found to drag out from the deformed PVA hydrogel without any destruction. The tensile strength of the embedded PDMS is further increased by attaching a thin roof film to bridge the protrusions. These microbridge structures are more than ten times stronger than those with only protrusions, and extraction of the PDMS microbridges caused the destruction of PVA hydrogel. It is also confirmed by a 90°‐peeling test that the microbridge structure is effective for tough lamination of the PVA on PDMS. The PVA/PDMS assembly with the microbridged interface exhibits transparency and deformability that will be sufficient for biomedical applications.

Transparent SiOxCyHz Barrier Film Prepared by Roll to Roll PECVD

This paper uses plasma-enhanced chemical vapor deposition (PECVD) to successfully fabricate silicon oxide films on silicon dioxide substrates for supporting layers of uncooled infrared focal plane array microbridge structures. In this paper, the thickness, refractive index, growth rate, and other parameters of the films were measured using the XP-2 step meter and ellipsometer. At the same time, atomic force microscopy (AFM) was used to study the surface morphology of the films. The experimental results show that deposition temperature and ICP power are the main factors affecting the growth rate of thin films. Deposition temperature is a crucial factor affecting the surface morphology of thin films.

Frequency Modulation and Absorption Improvement of THz Micro-bolometer with Micro-bridge Structure by Spiral-Type Antennas

Antenna-coupled micro-bridge structure is proven to be a good solution to extend infrared micro-bolometer technology for THz application. Spiral-type antennas are proposed in 25 μm × 25 μm micro-bridge structure with a single separate linear antenna, two separate linear antennas, or two connected linear antennas on the bridge legs, in addition to traditional spiral-type antenna on the support layer. The effects of structural parameters of each antenna on THz absorption of micro-bridge structure are discussed for optimized absorption of 2.52 THz wave radiated by far infrared CO2 lasers. The design of spiral-type antenna with two separate linear antennas for wide absorption peak and spiral-type antenna with two connected linear antennas for relatively stable absorption are good candidates for high absorption at low absorption frequency with a rotation angle of 360*n (n = 1.6). Spiral-type antenna with extended legs also provides a highly integrated micro-bridge structure with fast response and a highly compatible, process-simplified way to realize the structure. This research demonstrates the design of several spiral-type antenna-coupled micro-bridge structures and provides preferred schemes for potential device applications in room temperature sensing and real-time imaging.

Passive Regulation of Thermally Induced Axial Force and Displacement in Microbridge Structures

The analytical model of a mechanism for regulating the thermally induced axial force and displacement in a fixed–fixed microbeam is presented in this article. The mechanism which consists of a set of parallel chevron beams replaces one of the fixed ends of the microbeam. The thermomechanical behavior of the system is modeled using Castigliano’s theorem. The effective coefficient of thermal expansion is used in the analytical model. The analytical model takes into account both the axial and bending deformations of the chevron beams. The model provides a closed-form equation to determine the thermally induced axial force and displacement in the microbeam. In addition, the model is used to derive the equations for the sensitivities of the microbeam’s axial force and displacement to the variations of the design parameters involved. Moreover, the model produces the stiffness of the chevron beams. The effect of the stiffness of the chevron beams on the dynamic behavior of the microbeam is discussed. The analytical model is verified by finite element modeling using a commercially available software package. Using the analytical model, two special cases are highlighted: a system with thermally insensitive axial force and a system with thermally insensitive axial displacement. The main application of the model presented in this article is in the design of sensors and resonators that require robustness against changes of temperature in the environment. The analytical model and the sensitivity equations can be easily integrated into optimization algorithms.

Direct writing of three-dimensional Cu-based thermal flow sensors using femtosecond laser-induced reduction of CuO nanoparticles

We have demonstrated the fabrication of two types of thermal flow sensors with Cu-rich and Cu2O-rich microheaters using femtosecond laser-induced reduction of CuO nanoparticles. The microheaters in the shape of microbridge structures were formed to thermally isolate from the substrates by four layer-by-layer laminations of two-dimensional micropatterns. First, we evaluated the patterning properties such as dispensing coating conditions and degree of reduction for the selective fabrication of three-dimensional Cu-rich and Cu2O-rich microstructures. Then, a hot-film flow sensor with a Cu-rich microheater and a calorimetric flow sensor with a Cu2O-rich microheater were fabricated using their respective appropriate laser irradiation conditions. The hot-film sensor with the Cu-rich microbridge single heater enabled us to measure the flow rate in a wide range of 0–450 cc min−1. Although a large temperature dependence of the Cu2O-rich microbridge heaters caused a large error for the hot-film flow sensors with single heaters, they showed higher heat-resistance and generated heat with a lower drive power. The temperature coefficient of resistance of the Cu2O-rich microstructures had a semiconductor-like large absolute value and was less than −4.6 × 10−8 °C−1. The higher temperature sensitivity of the Cu2O-rich microstructures was useful for thermal detection. Based on these advantages, a calorimetric flow sensor composed of the Cu2O-rich microbridge single heater and two Cu2O-rich thermal detectors was proposed and fabricated. The calorimetric flow sensor was driven by a circuit for measuring the temperature difference. The Cu2O-rich flow sensor could detect bi-directional flow with a small output error.

Functional PDMS Composite Microbridges for Temperature Sensing Applications

Microstructures provide excellent sensing and actuating capabilities due to their high surface to volume ratio and have brought about important advancements in life sciences research [1-3]. The development of electrically-conductive polymer composites for such sensors and actuators as well as rapid fabrication techniques will result in microstructures with better transduction, low cost of production and flexibility [4, 5]. Conventional microfabrication techniques for such transducers are photolithography, reactive ion etching (RIE), laser ablation, and focussed ion beam etching. However these techniques are limited by the requirement of multiple processing steps (photolithography and RIE) or serial processing with specialized equipment (laser ablation and focussed ion-beam etching). In this work we report a low-cost, rapid and convenient technique to microfabricate electrically-conductive Iron-Polydimethylsiloxane (Fe-PDMS) microbridges using agar as the sacrificial material and further demonstrate the temperature sensing property of this polymer composite. Fabrication of the Fe-PDMS microbridges by the sacrificial agar technique is illustrated schematically in Fig. 1. A rectangular mold containing a 20mm×0.7mm×0.2mm channel and three pairs of sidewall through-holes (800µm in diameter) was manufactured via 3D printing (Fig.1a-i). Glass capillary guide rods with diameters of 65µm, 240µm and 350µm were inserted into the sidewall though-holes and passed through the channel (Fig.1a-ii). The guide rods functioned as master molds for the sacrificial agar, creating cavities for the Fe-PDMS composite to flow through. A 4% agar solution was prepared and poured into the rectangular mold (Fig.1a-iii). Following the room-temperature curing of the agar, the guide rods were removed horizontally and the agar replica was carefully de-molded, exposing the cylindrical cavities to be filled with Fe-PDMS composite (Fig.1a-iv). The agar replica was transferred into a petri dish (Fig.1a-v). The Fe-PDMS composite was prepared by mixing 80wt% iron particles (200 mesh size) with Sylgard 184 pre-polymer (10:1 elastomer-curing agent ratio). This composite was carefully casted into the aforementioned cylindrical cavities using assistive capillary flow. Undoped Sylgard 184 pre-polymer (10:1 elastomer-curing ratio) was then casted on the entire structure and cured at 37oC for 24 hours (Fig. 1a-vi). The cured structure was then immersed in a 100˚C water bath (Fig. 1a-vii) to dissolve the sacrificial agar and dried to form the suspended microbridge structures (Fig. 1a-viii) before plasma bonding to another flat PDMS layer (Fig. 1a-ix). The scanning electron microscope images of the fabricated microbridges are shown in Fig. 2. The thickness of the fabricated suspended bridges were measured and compared against their respective guide rod thicknesses (Fig. 3). The average thicknesses of the 65µm and 240µm microbridges showed a high precision in fabrication with a standard deviation of ~12µm from the guide rods. The larger deviation of the 350µm microbridge may be attributed to the size range (1-75 µm) of the iron particles in the Fe-PDMS composite which is currently under investigation. Uniform particle size would ensure better consistency in microbridge thicknesses. The temperature sensing ability of the Fe-PDMS composite was experimentally verified as well. A 24 AWG copper wire was used to provide electrical interconnection with the composite. The current-voltage (IV) characteristics of the Fe-PDMS composite in the input voltage range of 1-20V was measured by a Keithley 2410 source meter at four equilibrium temperatures of 45, 50, 60 and 70˚C applied externally via a hot plate. Each equilibrium temperature level produced a distinctive near-ohmic IV curve (Fig. 4a) with a positive correlation between the temperature and electrical conductivity. The obtained mean resistivities (Fig. 4b) imply a positive coefficient of resistance that is analogous to metals. The sacrificial agar fabrication technique in conjunction with the properties of the developed Fe-PDMS polymer composite will be suitable for development of low-cost and flexible electrodes and microstructures in microfluidic devices for thermo-electric temperature sensing and actuating applications. References F. Mei, S. P. J. Fancy, Y.-A. a Shen, J. Niu, C. Zhao, B. Presley, E. Miao, S. Lee, S. R. Mayoral, S. a Redmond, A. Etxeberria, L. Xiao, R. J. M. Franklin, A. Green, S. L. Hauser and J. R. Chan, Nat. Med., 2014, 20, 954–960.F. Liu, Y. Piao, J. S. Choi and T. S. Seo, Biosens. Bioelectron., 2013, 50, 387–392.S. Ito, T. Yasui, Y. Okamoto, N. Kaji and M. Tokeshi, Proc. 16th Int. Conf. Miniaturized Syst. Chem. Life Sci., 2012, 1234–1236.Gong X, Wen W, Polydimethylsiloxane-based conducting composites and their applications in microfluidic chip fabrication, Biomicrofluidics 2009, 3, 012007, pp1-14Calvert P, Deepak D, Patra P, Agrawal A, Sawhney A, Conducting Polymer and Conducting Composite Strain Sensors on Textiles. Molecular Crystals and Liquid Crystals, 2008; 484 (1), pp291-302 Figure 1

A temperature dependent measurement of the carrier velocity vs. electric field characteristic for as-grown and H-intercalated epitaxial graphene on SiC

A technique for the measurement of the electron velocity versus electric field is demonstrated on as-grown and H-intercalated graphene. Van der Pauw, coplanar microbridge, and coplanar TLM structures are fabricated in order to assess the carrier mobility, carrier concentration, sheet resistance, and contact resistance of both epi-materials. These measurements are then combined with dynamic IV measurements to extract a velocity-field characteristic. The saturated electron velocity measurements indicate a value of 2.33 × 107cm/s for the as-grown material and 1.36 × 107cm/s for the H-intercalated material at 300 K. Measurements are taken as a function of temperature from 100 K to 325 K in order to estimate the optical phonon energy Eso of 4H-SiC by assuming an impurity scattering model. The extracted values of Eso are 97 meV for the as-grown sample and 115 meV for the H-intercalated sample. The H-intercalated result correlates to the anticipated value of 116 meV for 4H-SiC, while the as-grown value is significantly below the expected value. Therefore, we hypothesize that the transport properties of epitaxial graphene on SiC are influenced both by intercalation and by remote phonon scattering with the SiC substrate.

A passive mechanism for thermal stress regulation in micro-machined beam-type structures

In this article, a passive mechanism for thermal stress regulation in micro-bridge structures is proposed. The mechanism is essentially a set of precisely designed parallel chevron beams that replace one of the fixed ends of the micro-bridge. The axial stress/force of the beam is determined by utilizing a sensing mechanism that makes use of two side-electrodes and the electromechanical buckling of the micro-bridge toward one of them. A combination of analytical and finite element methods has been employed for the modeling of the proposed mechanism. It has been demonstrated that the regulator mechanism has negligible dynamic effect on the sensing micro-beam. The model takes into account the effect of electric field fringes between the electrically charged bodies. It has been shown that there is a time delay between the moment that the pull-in occurs and the instant that the sensing beam rests on a side-electrode. The results of the modeling are verified by experiment on two devices fabricated using MetalMUMPs® fabrication technology. The thermal stress sensitivities of the devices are positive and their residual stresses are in excellent agreement.

Stress Monitoring of Post-processed MEMS Silicon Microbridge Structures Using Raman Spectroscopy

Inherent residual stresses during material deposition can have profound effects on the functionality and reliability of fabricated Micro-Electro-Mechanical Systems (MEMS) devices. Residual stress often causes device failure due to curling, buckling, or fracture. Typically, the material properties of thin films used in surface micromachining are not well controlled during deposition. The residual stress; for example, tends to vary significantly for different deposition methods. Currently, few nondestructive techniques are available to measure residual stress in MEMS devices prior to the final release etch. In this research, micro-Raman spectroscopy is used to measure the residual stresses in polysilicon MEMS microbridge devices. This measurement technique was selected since it is nondestructive, fast, and provides the potential for in-situ stress monitoring. Raman spectroscopy residual stress profiles on unreleased and released MEMS microbridge beams are compared to analytical and FEM models to assess the viability of micro-Raman spectroscopy as an in-situ stress measurement technique. Raman spectroscopy was used during post-processing phosphorus ion implants on unreleased MEMS devices to investigate and monitor residual stress levels at key points during the post-processing sequences. As observed through Raman stress profiles and verified using on-chip test structures, the post-processing implants and accompanying anneals resulted in residual stress relaxation of over 90%.

Microbridge structures for uniform interval control of flowing droplets in microfluidic networks

Precise temporal control of microfluidic droplets such as synchronization and combinatorial pairing of droplets is required to achieve a variety range of chemical and biochemical reactions inside microfluidic networks. Here, we present a facile and robust microfluidic platform enabling uniform interval control of flowing droplets for the precise temporal synchronization and pairing of picoliter droplets with a reagent. By incorporating microbridge structures interconnecting the droplet-carrying channel and the flow control channel, a fluidic pressure drop was derived between the two fluidic channels via the microbridge structures, reordering flowing droplets with a defined uniform interval. Through the adjustment of the control oil flow rate, the droplet intervals were flexibly and precisely adjustable. With this mechanism of droplet spacing, the gelation of the alginate droplets as well as control of the droplet interval was simultaneously achieved by additional control oil flow including calcified oleic acid. In addition, by parallel linking identical microfluidic modules with distinct sample inlet, controlled synchronization and pairing of two distinct droplets were demonstrated. This method is applicable to facilitate and develop many droplet-based microfluidic applications, including biological assay, combinatorial synthesis, and high-throughput screening.

Experimental investigation of the optimal laser-induced microbridges

Abstract Three new microbridge structures, which are able to form lateral-and-vertical links simultaneously in the metal layers of ICs for low resistance interline connections, are presented in order to solve the issue of undesirable links occurred after laser processing. Comprehensive laser processing experiments were performed to verify these designs. The results show that there exits different performance (electrical resistances) and laser energy windows for these structures. There is no perfect design so that one structure (S2) is selected as an optimal structure by considering of the production reality with respect to the highest yield first and then wider energy window, though it only presents the second low resistance. In addition, the same experiments were performed with the scaled-down structure sizes and two additive factors, relative process window and minimum resistance and its variation, are applied as the criteria to evaluate the scalability of these structures. The structure (S2) is selected as an optimal structure since it is the best design for scalability and it has a low resistance in the small scale. Therefore, structure 2 is determined to be the optimal microbridge design for all structures and scales due to the wide laser energy window, low resistance, high yield, and the best scalability. Moreover, it is able to achieve the fundamental requirements of low power consumption and current leakage for most applications of ASICs.

A bistable magnetically enhanced shape memory microactuator with high blocking forces

A novel approach of combining nonmagnetic shape memory alloy (SMA) microactuators with electroplated magnetic layers (ML) is presented, which enables the use of two intrinsic actuation principles in a single SMA-ML component allowing considerable gain in functionality. In the present work, we demonstrate local electroplating of Ni-81.75at.%Fe discs onto the surface of Ti-49at.%Ni microbridge structures. Antagonistic SMA-ML microactuators are fabricated by mechanically coupling two SMA-ML microbridges in their center by a spacer and prestraining them with respect to each other. The SMA-ML microactuators show bistable performance in the presence of an inhomogenous magnetic field generated by permanent magnets near the magnetic discs. Compared to antagonistic SMA microactuators fabricated without electroplated discs, a considerable improvement of mechanical performance is achieved. Magnetically enhanced SMA-ML microactuators of 7×7×4mm3 overall size show a blocking force and bistable stroke of about 33 mN and 84μm, respectively, while corresponding conventional SMA microactuators show no blocking force and a considerably smaller bistable stroke of 15μm.

Surface micromachined freestanding ZnO microbridge and cantilever structures on Si(111) substrates

In this study, the authors report on the fabrication processes to realize the freestanding ZnO micromechanical structures on Si(111) substrates. Arrays of freestanding cantilevers and microbridges have been fabricated using a combination of dry etching techniques. The mechanical properties of the released ZnO structures are characterized by micro-Raman spectroscopy. The residual stress in these freestanding micromechanical structures is determined from the E2(2) phonon peak shift. Such a method to realize the freestanding structures on Si platform would be useful for the fabrication of ZnO-based microelectromechanical systems and sensors.