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Due to the deformability of a microcantilever-based probe, there is an interesting and subtle interplay between the probe overtravel, the tip skate on the surface, and the ultimate tangency of the tip of the probe with the wafer surface. The relationship between these parameters is described here. The scalable model is tested using a macroscopic cantilever and found to be accurate in its predictions. In addition, to avoid potential skate-induced damage to metallisation, the idea of zero-skate using a cantilever-based probe has been introduced; minimal skate is demonstrated using a macroscopic cantileverâ€"the skate can be reduced by a factor of 0.176. As the model is scalable, this information could be of use to the designer of emerging miniature microelectromechanical systems (MEMS) microcantilever-based surface contact probes destined for on-wafer electrical measurements or the test engineer concerned with on-wafer probe contactingâ€"where skate and overtravel are important practical concerns having repercussions in electrical contact quality. Some predictions of the modelling for microcantilever-based probes are provided. [ABSTRACT FROM AUTHOR]

We demonstrate a straightforward surface micromachining process for the rapid prototyping of thin 'chip-edge' silicon microcantilevers protruding from the edge of a silicon-on-insulator (SOI) chip. The process uses a single photolithographic mask—with xenon difluoride used to both pattern the silicon microcantilevers and release them by etching part of the underlying silicon wafer. During the release step, the silicon microcantilevers are protected from the xenon difluoride by a combination of photoresist and buried silicon dioxide. The use of common microfabrication materials (SOI and positive photoresist) and chemicals (buffered hydrofluoric acid and xenon difluoride), along with a maximum process temperature of 100 °C, makes for a generic, soft micromachining process which is—in principle—compatible with preserving the integrity of any pre-patterned circuitry present on the silicon microcantilever top surface. Doppler vibrometry measurements of the silicon microcantilevers reveal a well-defined resonant frequency and a quality factor comparable with that of similar silicon microcantilevers fabricated using other means. Our enabling technological process allows the rapid prototyping of chip-edge silicon microcantilevers—potentially integrating sensitive circuitry for novel probe technologies—by avoiding the relatively cumbersome, expensive, and potentially circuit-damaging front-to-back processing/deep etching combination. [ABSTRACT FROM AUTHOR]

Attenuation has been studied in different-sized coplanar waveguides (CPW) patterned onto silicon-on-insulator (SOI) for three different surfaces between the CPW tracks: a silicon dioxide/silicon interface, a hydrogen-terminated silicon surface, and a native oxide. For large-gap CPW (signal track width = 100 µm, gap width = 63.5 µm), selective removal of the silicon dioxide from between the metal tracks reduces losses from 0.79 dB mm−1 to 0.69 dB mm−1 at 50 GHz. The subsequent growth of a native oxide on the silicon surface between the tracks results in losses equal to 0.67 dB mm−1. For small-gap CPW (= 2 µm, = 2.5 µm), losses are reduced from 5.6 dB mm−1 to 3.4 dB mm−1 at 50 GHz by selectively removing the silicon oxide between the metal tracks. However, when a native oxide is allowed to grow on the silicon surface between the tracks, the losses increase significantly to 5.8 dB mm−1. When the native oxide is removed, the losses decrease to those observed following removal of the silicon dioxide. The measurements suggest that the contribution of the intertrack surface losses is the result of free-carrier losses due to a surface inversion layer. The surface-associated losses are proportionally larger as the CPW dimensions shrink—as predicted by modelling. We suggest that technologies employing miniature CPW should take this into account. The native oxide should be routinely removed if possible—implying that appropriate chemically-resistant metallisation be chosen, or fabrication processes should incorporate stable passivation of silicon surfaces between CPW tracks to avoid native oxide growth. Conversely, the increased sensitivity of attenuation to surface effects by shrinking CPW dimensions suggests that CPW-based sensors could benefit from miniaturization. [ABSTRACT FROM AUTHOR]

A mathematical model is developed to calculate the topography of a mesa obtained by evaporation of matter via inclined rotating lift-off shadow masking. Two types of masking are considered: a circular mask and a cylindrical mask—the latter involves sidewall deposition. The model is able to predict various topographic profiles obtained via the evaporative deposition of matter, e.g., metals, when using a physical mask, e.g., a photoresist or an electron beam-sensitive resist patterned onto a flat wafer. The model predicts a range of profiles, e.g., sharp cones, round-tipped cones, spikes, irregular bumps, flat-topped features, "bagel-shaped" features, flat rings, and cylinders (fixed to the wafer and releasable)—depending on the aspect ratio of the circular opening, the deposition thickness, and the evaporation tilt angle. The ideas are extended to model an idealized resist-based lift-off mask involving overhang and undercut features. The model is simple to implement and should be of use for predicting the shape of deposited matter when using lift-off and stencil procedures—even at sub-micrometer dimensions. Despite its simplicity, the model goes some way in helping to understand the sensitivity of the various parameters on the final topography of the deposited matter. For example, the tilt angle—even when small—has an influence on the curvature radius of cone tips. In this way, the prediction—and even optimization—of the shape of the deposited material is possible prior to embarking on time-consuming, and perhaps costly, experimentation. [ABSTRACT FROM AUTHOR]

A chemically driven dewetting effect is demonstrated using sessile droplets of dilute hydrofluoric acid on chemically oxidized silicon wafers. The dewetting occurs as the thin oxide is slowly etched by the droplet and replaced by a hydrogen-terminated surface; the result of this is a gradual increase in the contact angle of the droplet with time. The time-varying work of adhesion is calculated from the time-varying contact angle; this corresponds to the changing chemical nature of the surface during dewetting and can be modeled by the well-known logistic (sigmoid) function often used for the modeling of restricted growth, in this case, the transition from an oxidized surface to a hydrogen-terminated silicon surface. The observation of the time-varying contact angle allows one to both measure the etch rate of the silicon oxide and estimate the hydrogenation rate as a function of HF concentration and wafer type. In addition to this, at a certain HF concentration, a self-running droplet effect is observed. In contrast, on hydrogen-terminated silicon wafers, a chemically induced spreading effect is observed using sessile droplets of nitric acid. The droplet spreading can also be modeled using a logistical function, where the restricted growth is the transition from hydrogen-terminated to a chemically induced oxidized silicon surface. The chemically driven dewetting and spreading observed here add to the methods available to study dynamic wetting (e.g., the moving three-phase contact line) of sessile droplets on surfaces. By slowing down chemical kinetics of the wetting, one is able to record the changing profile of the sessile droplet with time and gather information concerning the time-varying surface chemistry. The data also indicates a chemical interface hysteresis (CIH) that is compared to contact angle hysteresis (CAH). The approach can also be used to study the chemical etching and deposition behavior of thin films using liquids by monitoring the macroscopic droplet profile and relating this to the time-varying physical and chemical interface phenomena. [ABSTRACT FROM AUTHOR]

Exploiting pattern formation - such as that observed in nature - in the context of micro/nanotechnology could have great benefits if coupled with the traditional top-down lithographic approach. Here, we demonstrate an original and simple method to produce unique, localized and controllable self-organised patterns on elastomeric films. A thin, brittle silica-like crust is formed on the surface of polydimethylsiloxane (PDMS) using oxygen plasma. This crust is subsequently cracked via the deposition of a thin metal film - having residual tensile stress. The density of the mud-crack patterns depends on the plasma dose and on the metal thickness. The mud-crack patterning can be controlled depending on the thickness and shape of the metallization - ultimately leading to regularly spaced cracks and/or metal mesa structures. Such patterning of the cracks indicates a level of self-organization in the structuring and layout of the features - arrived at simply by imposing metallization boundaries in proximity to each other, separated by a distance of the order of the critical dimension of the pattern size apparent in the large surface mud-crack patterns. [ABSTRACT FROM AUTHOR]

ABSTRACT The fabrication of flexible electronics and systems, using rigid and brittle materials directly produced on stretchable substrate, leads to some issues and incompatibilities. These include rigidity for processing and modular flexibility for applications, macroscopic flexibility, and local rigidity to shield components from strain, compatibility with technological steps, and at the same time allowing patterning and machining. The development of smart substrate materials which meet such needs is therefore a promising route for flexible systems. Here, we demonstrate that by mixing polydimethylsiloxane (PDMS) and SU-8 photoresist, we obtain both a photo-hardenable and patternable stretchable hybrid material. A set of PDMS/SU-8 and baking process combinations have been tested to determine an effective photo-sensitive mixture. A standard photolithographic approach has been used on tensile test samples demonstrating a local hardening of millimeter-sized ultraviolet exposed features and a local strain reduction reaching 35%. In addition, surface topography analysis and wet-etching techniques have been used to demonstrate a light-induced molding process and a selective etching of micrometer-sized ultraviolet exposed patterns. The combined functional properties of the following material, its simplicity of implementation, and the well-known assets of PDMS and SU-8 make the PDMS/SU-8 material very interesting and promising for various applications, especially stretchable systems. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1281-1291 [ABSTRACT FROM AUTHOR]

Electrowetting (EW) has applications including displays, microactuation, miniaturized chemistry, adaptive optics, and energy harvesting--understanding the physics of EW junctions is of key importance. Here, the roles of semiconductor space-charge and electric double layer in continuous EW at an electrolyte-insulator-semiconductor junction are considered. A model is formulated in terms of experimental parameters--applied voltage, zero-bias wetting contact angle, semiconductor type and doping, insulator thickness and dielectric constant, and electrolyte concentration and dielectric constant. The model predicts, and experiments indicate, that the EW behavior is diminished for low concentration solutions (~1-10nM) and lowly doped silicon (1014-1015cm-3). [ABSTRACT FROM AUTHOR]

This short review focuses on the application of SU-8 for the microchip-based approach to the miniaturization of mass spectrometry. Chip-based mass spectrometry will make the technology commonplace and bring benefits such as lower costs and autonomy. The chip-based miniaturization of mass spectrometry necessitates the use of new materials which are compatible with top-down fabrication involving both planar and non-planar processes. In this context, SU-8 is a very versatile epoxy-based, negative tone resist which is sensitive to ultraviolet radiation, X-rays and electron beam exposure. It has a very wide thickness range, from nanometres to millimetres, enabling the formation of mechanically rigid, very high aspect ratio, vertical, narrow width structures required to form microfluidic slots and channels for laboratory-on-a-chip design. It is also relatively chemically resistant and biologically compatible in terms of the liquid solutions used for mass spectrometry. This review looks at the impact and potential of SU-8 on the different parts of chip-based mass spectrometry – pre-treatment, ionization processes, and ion sorting and detection. [ABSTRACT FROM AUTHOR]

A voltage polarity dependent, voltage-induced wetting transition is demonstrated using a liquid metal-oxide-semiconductor (MOS) junction. A droplet of mercury can be made to spread out on an oxidized silicon wafer upon the application of a voltage. The wetting is seen to depend on the doping type and doping density of the silicon-a voltage polarity related asymmetrical electrowetting is observed. Impedance measurements on Al-SiO2-Si MOS capacitors link the space-charge effects with the electrowetting on the Hg-SiO2-Si MOS junction. A modified Young-Lippmann equation for electrowetting at a MOS junction is derived-the model agrees with the experiments. [ABSTRACT FROM AUTHOR]

We demonstrate a proof-of-concept that microelectromechanical systems (MEMS) can be optically actuated using photoelectrowetting. A 30 µm thick aluminium cantilever is actuated using an ordinary white light source via the modulation of capillary forces in a liquid bridge on a Teflon® coated commercial silicon wafer. A deflection of 58 µm is observed using a light power of 100 mW at a bias of 7 V. The deflection of the cantilever relies on the photoelectrowetting effect [S. Arscott, Sci. Rep. 1, 184 (2011)]. Such wireless actuation could be useful for optical addressing and control of autonomous wireless sensors, MEMS, and microsystems. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate that GaAs spin injector microcantilever probes can be produced using a novel releasable “epitaxial patch technology” assembly approach as apposed to a monolithic fabrication route i.e. surface/bulk micromachining. The GaAs microcantilevers are attached to fused silica support wafers; thus taking advantage of the optical and spin properties of GaAs and the mechanical properties of the silica. The microcantilever probes have been characterized for their spin injection properties and also used as atomic force microscopy (AFM) probes to evaluate their imagining capabilities. The assembly involved the production of released epitaxial semiconductor “patches” having a micrometer thickness. These patches are then assembled onto a pre-fabricated fused silica support wafer. AFM characterization revealed a resonant frequency and quality factor equal to commercial silicon-based AFM probes together with high quality images; the spin injection characterization demonstrated injected photocurrents of tens of nA and a spin polarization of . [ABSTRACT FROM AUTHOR]

Abstract: A novel micromachining process for the fabrication of miniaturised polysilicon-based nanoelectrospray ionisation–mass spectrometry (nanoESI–MS) sources is developed in this paper. The nanoESI source topology is composed of two highly planar triangular free-standing cantilevers which form a microfluidic capillary slot having a width () and a height (h). A combination of low pressure chemical vapour deposition (LPCVD), pattern-transfer, reactive ion etching (RIE) and sacrificial layer etching is used to fabricate the nanoESI sources which project horizontally beyond the edge of a silicon substrate by a length of 800μm. NanoESI sources having two different capillary slot dimensions are tested: =1.8μm×2μm and 2.5μm×5μm. The sources have been tested on an ion trap mass spectrometer (MS) using a standard peptide sample (Glu-Fibrinopeptide B) at a concentration of 1μM. The resultant mass spectra show that the microfabricated capillary slot-based nanoESI sources presented here demonstrate state-of-the-art performances in terms of electrospray ionisation voltage (0.7kV) and test solution aqueous concentration (90% H2O). [Copyright &y& Elsevier]

We develop a 2D model which predicts the contact angle hysteresis (CAH) introduced by smooth micropatterned defects. The defects are modeled by a smooth function, and the CAH is explained using a tangent line solution. When the liquid micro-meniscus touches both sides of the defect simultaneously, depinning of the contact line occurs and the droplet 'pops-up.' The defects are fabricated using the photoresist SU-8. The experimental results, using common liquids (water, isopropyl alcohol, and ethylene glycol), agree well with the predictions of the model. The profile of the defect has a large influence on the CAH. [ABSTRACT FROM AUTHOR]

Lithography is used to form circular micropatterns which govern the evaporation of a water droplet. The surfaces are composed of concentric circular defects having a smooth indentation profile. When a droplet encounters a micropattern, evaporation occurs with distinct discontinuities in the droplet wetting contact angle and base radius. The addition of gaps into the patterns enables modification of the contact angle hysteresis; the receding angle of fluorocarbon coated SU-8 can be tuned between 34.6° and 89.1° and that of SU-8 from 5.6° to 43.3° depending on the gap length. A model is developed which accurately predicts the observed behavior. [ABSTRACT FROM AUTHOR]

The absorption coefficient of thick-films of the negative photoresist SU-8 is observed to be time dependent during photolithographic exposure by I-line ultraviolet light (λ=365 nm); varying linearly from 38±1 cm-1 to 49±1 cm-1 for a surface exposure dose of 415 mJ/cm2. We develop a general model which enables the exposure dose to be calculated at a given photoresist depth for a given exposure time. We determine the critical exposure dose for the subsequent polymerization of SU-8 having an arbitrary thickness to be 49.4±3.9 mJ cm-2. [ABSTRACT FROM AUTHOR]

We present here an original electrospray emitter tip based on a nanofluidic capillary slot. The nanofabrication involves focused ion beam etching to form the slot which has a cross-section of 50×300 nm and a length of 4 μm. The liquid is deformed into the nanofluidic capillary slot by capillary action; the electrospray is produced by the application of a low voltage. Using a water–methanol–formic acid solution, we have observed an electrospray onset voltage of 125 V. At a measured electrospray current of 1 nA, the unforced flow-rate is estimated to be 110 pL min-1. [ABSTRACT FROM AUTHOR]

We report electrowetting at a liquid metal-semiconductor (Schottky) junction using of a mercury droplet resting on silicon. This is demonstrated using n-type and p-type single-crystal silicon wafers of different doping levels. The voltage-dependent wetting contact angle variation of the mercury droplet is observed to depend on both the underlying semiconductor doping density and type. The electrowetting behavior can be explained by the voltage-dependent modulation of the capacitance of a Schottky junction; current-voltage and capacitance-voltage measurements indicate this to be the case. A modified Young-Lippmann electrowetting equation-formulated using a well-established metal-semiconductor junction model-agrees well with the observations. [ABSTRACT FROM AUTHOR]

A proof-of-concept demonstration of the electrowetting-on-dielectric of a sessile soap bubble is reported here. The bubbles are generated using a commercial soap bubble mixture-the surfaces are composed of highly doped, commercial silicon wafers covered with nanometer thick films of Teflon®. Voltages less than 40 V are sufficient to observe the modification of the bubble shape and the apparent bubble contact angle. Such observations open the way to inter alia the possibility of bubble-transport, as opposed to droplet-transport, in fluidic microsystems (e.g., laboratory-on-a-chip)-the potential gains in terms of volume, speed, and surface/volume ratio are non-negligible. [ABSTRACT FROM AUTHOR]

Wetting of sessile bubbles on various wetting surfaces (solid and liquid) has been studied. A model is presented for the apparent contact angle of a sessile bubble based on a modified Young's equation--the experimental results agree with the model. Wetting a hydrophilic surface results in a bubble contact angle of 90° whereas using a superhydrophobic surface one observes 134°. For hydrophilic surfaces, the bubble angle diminishes with bubble radius whereas on a superhydrophobic surface, the bubble angle increases. The size of the plateau borders governs the bubble contact angle, depending on the wetting of the surface. [ABSTRACT FROM AUTHOR]