Experimental investigations of liquid flow in rib-patterned microchannels with different surface wettability

The effects of rib-patterned surfaces and surface wettability on liquid flow in microchannels were experimentally investigated in this study. Microchannels were fabricated on single-crystal silicon wafers by photolithographic and wet-etching techniques. Rib structures were patterned in the silicon microchannel, and the surface was chemically treated by trichlorosilane to create hydrophobic condition. Experiments with water as the working fluid were performed with these microchannels over a wide range of Reynolds numbers between 110 and 1914. The results for the rib-patterned microchannels showed that the friction factor with the hydraulic diameter based on the rib-to-upper-wall height was lower than that predicted from incompressible theory with the same height. The friction factor-Reynolds number products for the hydrophobic condition increased as Reynolds number increased in the laminar flow regime. The experimental results were also compared with the predictive expressions from the literature, and it was found that the experimental data for the small rib/cavity geometry was in good agreement with those in the literature.     

Design and fabrication of a silicon probe for surface profilers

A micromachining probe for surface profilers is reported in this paper. This probe employs single crystal silicon double-ended tuning forks (DETFs) as the sensitive unit to achieve high resolution and frequency read-out. The frequency shift of the DETFs caused by the induced axial stress is directly proportional to the undulation of the measured surface. Moreover, one-stage and two-stage microleverage amplification mechanisms are respectively implemented to improve the probe performance. The silicon on glass (SOG) process which can provide high aspect ratio single crystal silicon structures has been adopted to enhance the performance of the probes. The testing results indicate that the probe with two-stage mechanical amplifiers has a frequency-displacement sensitivity of 257.1 Hz/μm with a nominal frequency 27.3 kHz at room temperature in atmosphere, which is in close agreement with the simulation results.     

Fabrication of high frequency two-dimensional nanoactuators forscanned probe devices

The authors have developed an integrated silicon process that uses suspended single crystal silicon (SCS) structures to fabricate x-y capacitive translators and high aspect ratio conical tips for scanned probe devices. The integrated nanomechanical device design and the process sequence include methods to form integrated tunneling tip pairs and to produce electrical isolation, contacts, and conductors. Each device occupies a nominal area of 40 μm×40 μm. These devices include a novel self-aligned tip-above-a-tip tunneling structure and capacitive x-y translators defined by electron beam lithography and the thermal oxidation of silicon. The x-y translators produce a maximum x-y displacement of ±200 nm for an applied voltage of 55 V. The low mass (2×10^-13 kg), rigid structure has a measured fundamental mechanical resonant frequency of 5 MHz     

Direct Fabrication of Nanoscale Light Emitting Diode on Silicon Probe Tip for Scanning Microscopy

We have fabricated a silicon microprobe integrated with a nanometer-sized light emitting diode (Nano-LED) on the tip. This paper describes the fabrication procedure and preliminary topographic testing results. The silicon probe with electrode pattern was made by wet-etching a silicon-on-insulator wafer using oxide as the mask. Subsequently, the probe tip was cut using a focused ion beam (FIB) to form a 150 nm-wide gap. Semiconductor nanoparticles (CdSe/ZnS core-shell nanoparticles) were electrostatically trapped and excited within the electrode gap made on the probe tip. The LED-tip is approximately 150 nm 150 nm. The nano-LED light intensity and current were measured as a function of the driving voltage up to 25 V. In addition to the electroluminescence peaks from the CdSe particles, possible emission from silicon dioxide doped in the FIB milling process was also observed in the measured spectra. Basic mechanical characteristics of the silicon probe were measured by mounting the probe on a tuning fork in a standard near-field scanning optical microscopy (NSOM) set up. It was observed that the drag force reduces the probe oscillation as the vibrating tip approached the near-field of the sample surface. The topographic images of a chromium test pattern on a glass substrate were successfully acquired by keeping the probe tip within roughly 5 nm from the sample surface. Although the probe tip shape and the location of the Nano-LED are yet to be further optimized before realizing near-field optical scanning experiment, the result showed its great promise as a new type of NSOM tip with the ldquoon-proberdquo light-source.     

Pattern recognition methods for thermal drift correction in Atomic Force Microscopy imaging

Atomic Force Microscopy (AFM) is a fundamental tool for the investigation of a wide range of mechanical properties on nanoscale due to the contact interaction between the AFM tip and the sample surface. The information recorded with AFM is stored and synthesized by imaging the sample properties to be studied. Distortions and unwanted effects in AFM images can be produced both due to instrumental sources or sample unknown bad responses. The focus of this paper is on an algorithm for distortion corrections for AFM recorded images due to the convolution of thermal drift and unknown polymer compliance. When a sequence of AFM images correspondent to the same polymeric area is acquired, it is common to observe the convolution of thermal drift with surface modifications due to the AFM tip stresses. The surface modifications are material properties and add knowledge to the response of the materials on nanoscale. As a consequence, a suitable de-convolution of the thermal drifts on the recorded images needs to be developed. Because soft polymeric samples can present unwanted height alteration due to the stressing AFM tip contact, we present a method that combines a thermal drifts correcting tool (where the original images are modified using a suitable mapping function) with a height rescaling method. In turn, an image matching method based on a Tikhonov functional is developed between topography data and the surface elastic maps, respectively. The precision achieved and the fast computation time required make our methods particularly useful for image analysis on soft polymeric samples as well as in a wide range of other scanning probe microscopy applications.     

Dimensional measurement of high aspect ratio micro structures with a resonating micro cantilever probe

 In non-destructive dimensional measurement of high aspect ratio micro structures (HARMS), optical methods cannot offer full three dimensional information due to the lack of observation light. Again, conventional mechanical measurement, such as a surface profiler or a coordinate measurement machine, cannot be applied because their stylus is too large. Furthermore, the AFM, though popular among the semiconductor industry, is also limited in terms of dimensional measurement, because its system is usually designed for planar samples. Thus, we have developed a new sensor-integrated micro resonating cantilever probe and a new dimensional measurement machine, which allows the probe's vertical access to microstructures in a sample. The new probe is made of tungsten carbide super hard alloy and possesses design flexibility according to its intended application. Validity of the system is confirmed through the measurement experiment of EDM drilled and chemically etched micro holes.     

Numerical simulation of flow through microchannels with designed roughness

A three-dimensional numerical simulation of flow through serpentine microchannels with designed roughness in form of obstructions placed along the channels walls is conducted here. CFD-ACE+ is used for the numerical simulations. The effect of the roughness height (surface roughness), geometry, Reynolds number on the friction factor is investigated. It is found that the friction factor increases in a nonlinear fashion with the increase in obstruction height. The friction factor is more for rectangular and triangular obstructions and it decreases as the obstruction geometry is changed to trapezoidal. It is observed that the obstruction geometry, i.e., aspect ratio plays an important role in prediction of friction factor in rough channels. It is also found that the pressure drop decreases with the increase in the roughness pitch. Hence, the roughness pitch is an important design parameter for microchannels.     

A single crystal silicon 3 dimensional processing technique with applications in large displacement electrostatic actuation

 A new process has been developed to produce mechanical structures of multiple levels on a silicon substrate. The process lends itself to high aspect ratio bulk micromachining of single crystal silicon. Structures can be fabricated at multiple levels with a single lithography step and without alignment requirements. Any required difference in height between levels is defined precisely at intermediate etch steps. One application of this process utilizes the out of plane asymmetries produced by the multiple levels to create a large displacement, large force and highly linear out of plane actuator stage. This technique is easily combined with single level high aspect ratio MEMS processing methods. Received: 10 August 2001/Accepted: 24 September 2001     

Silicon tip sharpening based on thermal oxidation technology

Thermal oxidation at low temperatures (below 1050 °C) is widely used in the microfabrication of sharp silicon tips. However, the influences of the oxidation temperature on morphology of the tips have not been investigated in detail. This work systematically studied the dependence of tip profile on the oxidation temperature. Thermal oxidation were performed in four groups at 900, 950, 1000 and 1050 °C. The results show that a trade-off between the tip aspect ratio and diameter should be taken into account when choosing the oxidation temperature. The optimized oxidation temperature to make tips with small apex, high aspect ratio, and smooth surface is around 1000 °C. The tip with a diameter of 6.3 nm was realized through oxidation at 1000 °C.

     

Design and fabrication of a novel microfluidic nanoprobe

The design and fabrication of a novel microfluidic nanoprobe system are presented. The nanoprobe consists of cantilevered ultrasharp volcano-like tips, with microfluidic capabilities consisting of microchannels connected to an on-chip reservoir. The chip possesses additional connection capabilities to a remote reservoir. The fabrication uses standard surface micromachining techniques and materials. Bulk micromachining is employed for chip release. The microchannels are fabricated in silicon nitride by a new methodology, based on edge underetching of a sacrificial layer, bird's beak oxidation for mechanically closing the edges, and deposition of a sealing layer. The design and integration of various elements of the system and their fabrication are discussed. The system is conceived mainly to work as a "nanofountain pen", i.e., a continuously writing upgrade of the dip-pen nanolithography approach. Moreover, the new chip shows a much larger applicability area in fields such as electrochemical nanoprobes, nanoprobe-based etching, build-up tools for nanofabrication, or a probe for materials interactive analysis. Preliminary tests for writing and imaging with the new device were performed. These tests illustrate the capabilities of the new device and demonstrate possible directions for improvement.     

Design and batch fabrication of probes for sub-100 nm scanningthermal microscopy

A batch fabrication process has been developed for making cantilever probes for scanning thermal microscopy (SThM) with spatial resolution in the sub-100 nm range. A heat transfer model was developed to optimize the thermal design of the probes. Low thermal conductivity silicon dioxide and silicon nitride were chosen for fabricating the probe tips and cantilevers, respectively, in order to minimize heat loss from the sample to the probe and to improve temperature measurement accuracy and spatial resolution. An etch process was developed for making silicon dioxide tips with tip radius as small as 20 nm. A thin film thermocouple junction was fabricated at the tip end with a junction height that could be controlled in the range of 100-600 nm. These thermal probes have been used extensively for thermal imaging of micro- and nano-electronic devices with a spatial resolution of 50 nm. This paper presents measurement results of the steady state and dynamic temperature responses of the thermal probes and examines the wear characteristics of the probes     

Fabrication of AFM probe with CuO nanowire formed by stress-induced method

A novel method has been proposed to fabricate an atomic force microscope (AFM) probe using CuO nanowire and a stress-induced method that can form the nanowire easily. By heating a commercial AFM probe with a film coating of Ta and Cu, a Cu hillock with CuO nanowires on its surface could be formed at the end of the probe. The thickness of the coating films, the heating temperature, and the heating time were investigated to obtain CuO nanowires with a high aspect ratio for use as an AFM probe tip. It was found that a suitable probe tip can be fabricated using the a Cu film thickness of 700 nm, a heating temperature of 380 °C and a heating time of 6 h. Probe tips (~5 μm high) and nanowires of ~25 nm diameter were obtained successfully. In the range evaluated, the measurement resolution of the CuO nanowire probe was slightly worse than that of a commercial AFM probe. However, both probes had almost the same dimensional measurement precision.     

Effect of wall roughness on performance of microchannel applied in microfluidic device

The influence of wall roughness on flow and heat transfer performance in microchannels at low Reynolds number is investigated in this paper. Two sizes of PMMA microchannels are fabricated by microinjection molding and the width is 20 μm and 800 μm respectively. The surface profile of bottom wall is described by the two-dimensional fractal geometry method and it is found there is error within 5% between surface roughness obtained by the fractal geometry method and actual roughness. Then, the effects of dimensionless relative roughness (5–7.5%), fractal dimension (1.5–1.8), aspect ratio (0.025–4) on the flow resistance and heat transfer performance are analyzed by numerical and experimental method respectively. Reynolds number considered here are 10–60. The results show that the better flow performance and heat transfer performance can be obtained with high aspect ratio of rectangular microchannel. However, increasing surface roughness not only increases the heat transfer performance, but also introduces a large flow resistance, which makes the friction coefficient rise sharply. As a result, surface roughness has great influence on the flow and heat transfer performance, and the most suitable surface morphology should be obtained according to the specific application.

     

Polysilicon sacrificial layer etching using ClF3 for thin film encapsulation of silicon acceleration sensors with high aspect ratio

We present a new thin film encapsulation technique for surface micromachined sensors using a polysilicon multilayer process. The main feature of the encapsulation process is that both the sacrificial layer above the silicon sensor structure and the cap layer consist of epitaxial polysilicon. The sacrificial layer is removed by chlorine trifluoride (ClF₃) plasmaless gas-phase etching through vents within the cap layer. The perforated cap membrane is sealed by a nonconformal oxide deposition. The method has been applied to a silicon surface micromachined acceleration sensor with high aspect ratio structures, but is broadly applicable. Capacitance–voltage measurements have been performed to show the electrical functionality of the accelerometer.     

A methodology for quantitative evaluation of local electrical conductivity: from micron to submicron

The local electrical conductivity of aluminum thin film with dimensions from micron to submicron was quantitatively measured by a four-point atomic force microscope (AFM) technique. The technique is a combination of the principles of four-point probe method and standard AFM. A silicon nitride based AFM probe with a V-shaped two-dimensional sliced structure tip was patterned by using conventional photolithography method. The probe was then etched to four parallel electrodes isolated from each other, for the purpose of performing current input and electrical potential drop measurement. The spacing between electrodes is smaller than 1.0 μm, which facilitates the quantitative electrical conductivity measurement of ultrathin film. The four-point AFM probe technique is capable of measuring surface topography together with local conductivity simultaneously. The technique was applied to a series of 99.999% aluminum thin films with thicknesses from micron to submicron. The repeatable measurements demonstrate the capability of this technique and its possible extension to be used for fast in situ electrical properties characterization of submicron interconnects that widely applied in nanosensors and nanodevices.     

Design, fabrication, and characterization of thermally actuated probe arrays for dip pen nanolithography

In Dip Pen Nanolithography (DPN), arbitrary nanoscale chemical patterns can be created by the diffusion of chemicals from the tip of an atomic force microscope (AFM) probe to a surface. This paper describes the design, optimization, fabrication, and testing of an actuated multi-probe DPN array. The probe array consists of 10 thermal bimorph active probes made of silicon nitride and gold. The probes are 300 /spl mu/m long and the tips are spaced 100 /spl mu/m apart. An actuation current of 10 mA produces a tip deflection of 8 /spl mu/m, which is enough to remove individual tips from the surface independent of the adjacent probes. An analytical probe model is presented and used to optimize the design against several possible failure modes. The array is demonstrated by using it to simultaneously write 10 unique octadecanethiol patterns on a gold surface. Pattern linewidth as small as 80 nm has been created at a maximum write speed of 20 /spl mu/m/sec. By writing multiple, distinctly different patterns in parallel, this device provides a significant improvement in throughput and flexibility over conventional AFM probes in the DPN process.     

Computational study of the effect of finger width and aspect ratios for the electrostatic levitating force of MEMS combdrive

Since the width ratio between movable and fixed fingers, and the aspect ratio between the height and width of fingers, can play very important roles for combdrive levitation control, computational study of variations in those parameters for electrostatic levitating force acting on the movable finger is indispensable for MEMS performance. For diverse finger width and aspect ratios of MEMS combdrive design, the BEM has become a better method than the domain-type FEM because BEM can provide a complete solution in terms of boundary values only, with substantial saving in modeling effort. DBEM still has some advantages over conventional BEM for singularity, so the DBEM was used to simulate the fringing of field around the edges of the fixed finger and movable finger of MEMS combdrive for diverse finger width and aspect ratios. Results show that the less the finger width ratio is, the larger the levitating force acting is. Furthermore, the levitating force becomes more dominant as the aspect ratio increases, but it will be kept constant while the aspect ratio becomes larger.     

Analysis and measurements of mixing in pressure-driven microchannel flow

The mixing phenomena for two fluid streams in pressure-driven rectangular microchannels are analyzed and directly compared with the measurements of mixing intensity for a wide range of aspect ratio (width/depth = 1–20). In the analysis, the three-dimensional transport equation for species mixing was solved using the spectral method in a dimensionless fashion covering a large regime of the normalized downstream distance. The analysis reveals the details of non-uniform mixing process, which originates from the top and bottom walls of the channel and stretches out toward the center of the channel, and its transition to uniformity. Employing different length scales for the non-uniform and uniform mixing regimes, the growth of mixing intensity can be expressed in a simple relationship for various aspect ratios in the large range. The mixing experiments were carried out on silicon- and poly(methyl methacrylate) (PMMA)-based T-type micromixers utilizing fluids of pH indicator (in silicon channel) and fluorescent dye (in PMMA channel) to evaluate the mixing intensity based on flow visualization images. Using conventional microscopes, the experiments demonstrate the mixing intensity as a power law of the stream velocity for all the microfluidic channels tested. The variations of measured mixing intensity with the normalized downstream distance are found in favorable agreement with the numerical simulations. The comparison between the experiments and simulations tells the capabilities and limitations on the use of conventional microscopes to measure the mixing performance.     

Design and characterization of MEMS-based flow-rate and flow-direction microsensor

This study designs and characterizes a novel MEMS-based flow-rate micro-sensor consisting of a platinum resistor deposited on a silicon nitride-coated silicon cantilever beam. Due to the difference between the thermal conductivities of the silicon nitride film and the silicon beam, the tip of the cantilever structure bends slightly in the upward direction. As air travels across the upper surface of the sensor, it interferes with the curved tip and displaces the beam in either the upward or the downward direction. The resulting change in the resistor signal is then used to calculate the velocity of the air. A flow-direction micro-sensor is constructed by arranging eight cantilever structures on an octagonal platform. Each cantilever is separated from its neighbors by a tapered baffle plate connected to a central octagonal pillar designed to attenuate the aerodynamic force acting on the cantilever beams. By measuring the resistor signals of each of the cantilever beams, the micro-sensor is capable of measuring both the flow rate and the flow direction of the air passing over the sensor. A numerical investigation is performed to examine the effects of the pillar height and pillar-to-tip gap on the airflow distribution, the pressure distribution, the bending moment acting on each beam, and the sensor sensitivity. The results show that the optimum sensor performance is obtained using a pillar height of 0.75 mm and a pillar-to-tip gap of 5 mm. Moreover, the sensitivity of the octagonal sensing platform is found to be approximately 90% that of a single cantilever beam.     

A method for precision patterning of silicone elastomer and its applications

This paper presents a general microfabrication method for precision patterning of thin-film poly(dimethylsiloxane) (PDMS). The method enables PDMS microstructures with controlled lateral dimensions and thickness on a silicon or glass substrate. Two applications based on this new method are discussed. First, a scanning probe microscopy (SPM) probe with PDMS tip is developed and used for scanning probe contact printing (SPCP). Second, this paper demonstrates surface micromachined membranes with integrated silicone gaskets.