Micromachined Silicon Wafers Research Articles

Fabrication and embedded sensors characterization of a micromachined water-propellant vaporizing liquid microthruster

Rapid advances in micro/nanotechnology have enabled to achieve high levels of miniaturization, promoting the development of low cost and highly efficient microsystems for specific applications. In the space sector, the miniaturization of satellites has led to a renewed interest in the research and development of advanced micro-propulsion technologies able to generate small and accurate thrust forces and high specific impulse.This work presents the design and fabrication of a silicon-based water-propellant Vaporizing Liquid Microthurster (VLM) equipped with embedded microsensors for real-time monitoring of in-channel vapor/liquid fraction and fluid temperature during its operation. Anisotropic dry etching of silicon wafer and thermo-compressive bonding were chosen as key fabrication steps: the former process was used to better control the surface roughness on microchannels inner walls, the latter was used to guarantee the fluidic tightness and complete the fabrication process of the device. Borofloat 33 glass, used to seal the micromachined silicon wafer, allows the optical inspection of the fluid flow and vaporization within the different chambers during the device operation. A platinum resistive heater placed on the bottom of the chip was exploited for the heating of the propellant. A set of Resistive Temperature Detectors (RTDs) and, for the first time, capacitive sensors were designed and integrated inside the microthruster chip to add distributed sensing capabilities for flow instability control. Further, a secondary low-power platinum thin film resistive heater was placed inside each of the eight channels, in order to allow for localized precision fluid heating and flow control. The operational feasibility of the fabricated microthruster was assessed by means of a preliminary characterization of the embedded sensors, based on experimental tests supported by numerical investigations; the results demonstrated that the designed microsensor devices are able to maximize the microthruster efficiency, in terms of microtexture-enhanced in-channel water vaporization and reduced power consumption.

Microelectromechanical system-based, high-finesse, optical fiber Fabry–Perot interferometric pressure sensors

A high-finesse, optical fiber, extrinsic Fabry–Perot interferometric (EFPI) pressure sensor based on a microelectromechanical system (MEMS) technique is proposed and experimentally demonstrated. The essential element in the pressure sensor is the high-finesse EFPI cavity that consists of a Pyrex glass wafer, a micromachined silicon wafer, and highly reflective dielectric films. Another Pyrex glass is used for fixing an optical fiber collimator, which allows the realization of the alignment of the incident light. Experimental results show that the proposed sensor exhibits a pressure sensitivity of 1.598 μm/MPa and a high-pressure sensing resolution of 0.002% of the full scale. This sensor is expected to benefit many applications that require high-accuracy pressure measurements, and especially atmospheric pressure applications.

Roles of hydroxyapatite allocation and microgroove dimension in promoting preosteoblastic cell functions on photocured polymer nanocomposites through nuclear distribution and alignment.

This study clarifies how hydroxyapatite (HA) allocation and microgroove dimension affect mouse preosteoblastic MC3T3-E1 cell functions on microgrooved substrates of polymer nanocomposites. Using replica molding from micromachined silicon wafer templates, we fabricated photocured poly(ε-caprolactone) triacrylate (PCLTA)/HA nanocomposite substrates with parallel microgrooves (two groove widths of 5 and 15 μm and one groove depth of 5 μm). Four types of microgrooved substrates were made: "homogeneous" ones of PCLTA and PCLTA/HA with uniform distribution and two "heterogeneous" laminated microgrooved substrates with HA only in the PCLTA matrix in the ridges or bottom. These substrates were used to regulate MC3T3-E1 cell attachment, proliferation, alignment, nuclear circularity and distribution, and mineralization. MC3T3-E1 cell attachment and proliferation were much higher on the microgrooved substrates of PCLTA/HA than on those of PCLTA, in particular, on the 5 μm wide microgrooved substrate with PCLTA/HA ridges and PCLTA bottom. The shape and distribution of MC3T3-E1 cytoskeleton and nuclei were altered by the substrate topography and HA allocation. For 5 μm wide heterogeneous microgrooved substrates with HA only in the ridges, MC3T3-E1 cells exhibited better spreading perpendicular to the microgrooves but tended to extend along the microgrooves containing HA in the bottom. The widest cells and the roundest/largest cell nuclei were observed on the heterogeneous substrate with PCLTA/HA ridges, while the narrowest cells with the best elongation were found on the homogeneous PCLTA/HA substrate. The trend in MC3T3-E1 cell mineralization on the substrates was consistent with that in cell/nuclear elongation. Osteocalcin mRNA expression was significantly higher on the PCLTA/HA substrates than on the PCLTA ones and also on the microgrooved substrates of PCLTA/HA than on the flat ones, regardless of the groove width of 5 or 15 μm.

Design and Fabrication of a Tunable Two-fluid Micro-lens Device with a Large Deflection Polymer Actuator

A large deflection polymer bending actuator was employed in the design of an integrated micro-lens with electrically adjustable focal length. This micro-lens consisted of micro-fluidic chambers bonded together and sealed by an elastomeric lens membrane to which the polymer actuator has been attached. For focal length optimization, the chambers could be filled with fluids of different refractive indices. The multilayered actuator of radial shape and the variable micro-lens chip design were optimized using 2D FEM simulations. Fabrication steps included bonding of polymer actuator stack with the elastomeric lens membrane on a micro-machined silicon wafer. For a tunable micro-lens with 3mm aperture, a tuning range in optical power between 0 and more than 15 dpt was achieved.

Carbon nanotube based miniaturized electron gun and column assembly

We report on the application of silicon micromachining for the fabrication of miniaturized electron gun (MEG) assembly using vertically aligned carbon nanotubes. The proposed MEG consists of two main parts of electron gun and the accelerating column. While the electron gun consists of carbon nanotubes grown on a silicon substrate acting as an electron emission source, the accelerating column is made of micromachined silicon wafers with 5 μm thick membranes operating as objective lenses. These two wafers are placed together and sealed using a three-dimensional packaging technique. The simulation and experimental results show the evolution of a narrow electron beam by applying a proper voltage to the anodes and objective lens. The diverging and focusing of the beam can be controlled by applying the proper voltage on electrostatic lenses. This structure could be suitable for low energy SEM devices and surface physics applications.

A MEMS-based pneumatic micro-conveyor for planar micromanipulation

This paper present a two dimensional pneumatic actuator based on silicon MEMS technology for objects micro-manipulation using tilted air jets. The device is composed of three layers stacked together, two micro-machined silicon wafers and a Pyrex glass wafer. The system is composed of a set of micro-conveyors in about 9mm×9mm area. Each micro-conveyor has four nozzles and can generate tilted air-jets which allow four conveyance directions. An experiment of the conveyance of a silicon chip of 3mm diameter and weighing approximately 2mg was performed with pulsed air flow.

Fabrication of WO 3 nanodot-based microsensors highly sensitive to hydrogen

A novel approach that involves electrochemical anodising for fabricating WO 3 nanodot-based microsensors is presented. The formation of gas sensing film is assisted by a nanoporous anodic alumina layer grown from a thin layer of aluminium sputtering-deposited over a thin layer of tungsten (Al/W bilayer). The anodic film annealed at 500 °C consists of a self-ordered array of WO 3 nanodots (∼50 nm), each composed of few nanocrystallites (∼9 nm) having thermostable tetragonal structure. The processes for forming the WO 3 nanodot active films are fully compatible with standard technology for fabrication of sensor chips on a micromachined silicon wafer. Characterization of the WO 3 nanodot film was performed by field-emission scanning electron microscopy, atomic force microscopy and X-ray diffraction analysis. A study of the ability of the WO 3 nanodot-based microsensors to detect H 2, CO, ethanol and relative humidity was presented for the first time. High sensitivity to hydrogen, with linear response characteristic was obtained in the range of low H 2 concentrations (5–20 ppm). The gas sensing mechanism of the WO 3 nanodot-based sensors was discussed.

A 60-GHz CPW-Fed High-Gain and Broadband Integrated Horn Antenna

An integrated horn antenna is presented for 60-GHz WPAN applications. Compared with other types of antenna for 60-GHz WPAN applications, an integrated horn antenna features wide bandwidth and high gain. This integrated H-plane horn is elevated on the top of the substrate using CMOS-compatible microfabrication steps. Antenna efficiency is greatly improved after eliminating dielectric loss. This antenna is excited using an integrated vertical current probe connected with a coplanar-waveguide (CPW) by surface micromachining technologies. The lower part of the horn is constructed by rows of metallized pillars. The upper part and the top wall are built by stacking two layers of micromachined silicon wafers. The horn bottom is formed by metalizing the substrate's top surface. A prototype antenna is designed, fabricated, and characterized. Simulation and measurement results have shown wide input matching bandwidth and radiation bandwidth. The measured radiation pattern agrees well with the simulated one, demonstrating a gain as high as 14.6 dBi.

Nanoscale Pattern Definition on Nonplanar Surfaces Using Ion Beam Proximity Lithography and Conformal Plasma-Deposited Resist

Ion beam proximity lithography (IBL) is a technique where a broad beam of energetic light ions floods a stencil mask and transmitted beamlets transfer the mask pattern to resist on a substrate. With a depth-of-field up to 20000 times larger than the minimum feature size and the high-throughput potential of a parallel exposure tool, IBL is very attractive for prototyping and manufacturing nanoelectromechanical systems over the steep topography of micromachined silicon wafers. This paper reports a conformal resist coating process that unlocks this potential. This negative-tone resist, plasma-polymerized methyl methacry- late, has a sensitivity of 27 muC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a contrast of 1.3 for 30-keV He <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion exposures and amyl acetate developer. Sub-100-nm features have been printed down the sidewall and across a membrane at the bottom of a 500-mum-deep anisotropically etched pit in a silicon wafer. Pattern fidelity is near 2 nm for 10-nm features. Lines have also been formed on unpolished substrates, including rolled titanium foils and coarse-ground silicon wafers. Patterns on ground silicon have been etched into the surface using a nickel hard mask and SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> /O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> reactive ion etching.

A Mold and Transfer Technique for Lead-Free Fluxless Soldering and Application to MEMS Packaging

This paper demonstrates a technique to premold and transfer lead-free solder balls for microelectrocmechanical systems (MEMS)/electronics packaging applications. A reusable bulk micromachined silicon wafer is used to mold a solder paste and remove excess flux prior to transfer to a host wafer that may contain released MEMS. This technique has been used to fabricate low temperature thin film MEMS vacuum packages. Long term (>5 months) reliability of these packages at room temperature and pressure is demonstrated through integrated Pirani gauges. These packages have survived over 600 hours in an autoclave (130degC, 85% RH, 2 atm) and more than 1300 temperature cycles (55degC to 125degC)

Coupling bulge testing and nanoindention to characterize materials properties of bulk micromachined structures

Nanoindentation and the continuous stiffness method was used to obtain the elastic modulus of Pb(ZrxTi1−x)O3 (X:1−XPZT) thin films that can be utilized in high aspect ratio structures. PZT films were deposited onto bulk micromachined silicon wafers by sol–gel deposition and two annealing treatments. Conventional annealing produced films that had an elastic modulus of 80 GPa for both the 52:48 and 40:60 PZT, while a 52:48 PZT deposited using rapid thermal annealing demonstrated a modulus of 70 GPa. These moduli allowed accurate stress measurements of both the composite membranes by bulge testing and also the individual PZT layers by X-ray diffraction. The residual tensile stress in the PZT film ranged from 190 to 400 MPa. Residual stresses were shown to affect both the strain to failure and the resonance frequency of the membrane generators. The applied strain to failure decreased with increasing residual stress. A tungsten underlayer was added to reduce the composite residual stress and increase the compliance, lowering the resonant frequency from 23 to 18 kHz.

Endothelialized Networks with a Vascular Geometry in Microfabricated Poly(dimethyl siloxane)

One key challenge in regenerating vital organs is the survival of transplanted cells. To meet their metabolic requirements, transport by diffusion is insufficient, and a convective pathway, i.e., a vasculature, is required. Our laboratory pioneered the concept of engineering a vasculature using microfabrication in silicon and Pyrex. Here we report the extension of this concept and the development of a methodology to create an endothelialized network with a vascular geometry in a biocompatible polymer, poly(dimethyl siloxane) (PDMS). High-resolution PDMS templates were produced by replica-molding from micromachined silicon wafers. Closed channels were formed by bonding the patterned PDMS templates to flat PDMS sheets using an oxygen plasma. Human microvascular endothelial cells (HMEC-1) were cultured for 2 weeks in PDMS networks under dynamic flow. The HMEC-1 cells proliferated well in these confined geometries (channel widths ranging from 35 mum to 5 mm) and became confluent after four days. The HMEC-1 cells lined the channels as a monolayer and expressed markers for CD31 and von Willebrand factor (vWF). These results demonstrate that endothelial cells can be cultured in confined geometries, which is an important step towards developing an in vitro vasculature for tissue-engineered organs.

Leak-Tight Piezoelectric Microvalve for High-Pressure Gas Micropropulsion

This paper describes the results of our development of a leak-tight piezoelectric microvalve, operating at extremely high upstream pressures for microspacecraft applications. The device is a normally closed microvalve assembled and fabricated primarily from micromachined silicon wafers. The microvalve consists of a custom-designed piezoelectric stack actuator bonded onto silicon valve components (such as the seat, boss, and tether) with the entire assembly contained within a stainless steel housing. The valve seat configurations include narrow-edge seating rings and tensile-stressed silicon tethers that enable the desired, normally closed, leak-tight operation. Leak testing of the microvalve was conducted using a helium leak detector and showed leak rates of 5/spl times/10/sup -3/ sccm at 800 psi (5.516 MPa). Dynamic microvalve operation (switching rates of up to 1 kHz) was successfully demonstrated for inlet pressures in the range of 0/spl sim/1000 psi. The measured static flow rate for the microvalve under an applied potential of 10 V was 52 sccm at an inlet pressure of 300 psi. The measured power consumption, in the fully open state, was 3 mW at an applied potential of 30 V. The measured dynamic power consumption was 180 mW for 100 Hz continuous operation at 100psi.

Bidirectional Reflection Measurements of Periodically Microstructured Silicon Surfaces

Surface modifications have a great potential for selective emission and absorption for applications in photonics, energy conversion, and biosensing. Pattern-induced radiative property changes can be an important issue in the manufacturing and diagnostics of microelectronic devices. This work investigates the polarized diffraction of micromachined silicon wafers. Both one-dimensional (1-D) and two-dimensional (2-D) periodic microstructures are manufactured by plasma-assisted anisotropic etching. The rotating mask method is used to produce 2.25 × 106 2-D structures in a single sample (7.5 × 7.5 mm2). Surface topography is characterized by using a scanning electron microscope (SEM). A bidirectional scatterometer with high accuracy and angular resolution measures the diffraction patterns from the microstructured silicon surfaces at a wavelength of 635 nm. The diffraction patterns follow the grating equation, which are caused by microstructures and their orientations. Predicted diffraction angles are in excellent agreement with the experimental results.

MEMS-based design and fabrication of a new concept micro direct methanol fuel cell (μ-DMFC)

A design for a novel micro direct methanol fuel cell (μ-DMFC) of 0.018 cm 2 active area is described. The μ-DMFC was prepared using a series of fabrication steps from micro-machined silicon wafer including photolithography, deep reactive ion etching, and electron beam deposition. The novelty of this structure is that we have fabricated the anodic and cathodic micro-channels arranged in plane, dissimilar to the conventional bipolar structure. The first objective of the experimental trials was to verify the feasibility of this novel structure on basis of MEMS technology. Preliminary testing results show that this new concept μ-DMFC generates electricity.

Miniature fuel cells fabricated on silicon substrates

AbstractNovel miniature fuel cells were fabricated from micromachined silicon wafers. The cells used methanol and air as reactants, and a thin polymer electrolyte as separator. The assembled cells had a working volume of 12 mm3 and could be scaled down in size by three orders of magnitude by simple adjustments of the masking and etching procedures. Electrodeposition of Pt‐Ru as the anode catalyst (oxidation of methanol) was successful in lowering the loading to 0.25 mg/cm2 without loss of performance. Cell performance approached that of the best, state‐of‐the‐art, large fuel cells, when scaled for size. In particular, single miniature cells yielded 822 Wh/kg and 924 Wh/L when operated at 70°C. The same chip design was also used for the hydrogen/air system, and the cell current, power, and specific energy density were higher than those of methanol/air, Further tailoring of the chips for spefici fuels could lead to further improvements.

Patterning of Polymeric Hydrogels for Biomedical Applications

Surface-patterned 2-hydroxyethyl methacrylate hydrogels were produced by using a method similar to the silicon-rubber stamp fabrication for microcontact printing. Polymerization and network formation were carried out in contact with a micromachined silicon wafer. The polymeric patterns retained their shape during isotropic swelling/deswelling cycles. The use of microstructured hydrogels in tissue engineering can be envisaged.

Infrared micro-spectrometer based on a diffraction grating

The optical properties of crystalline silicon in the IR spectral range have been used for the realisation of a grating type micro-spectrometer. An aluminium metallisation on a double side polished silicon wafer is used for fabrication of the multi-slit gratings. The dispersed spectrum propagates through this silicon wafer and is projected on an array of polysilicon thermopiles, integrated in a second bulk micromachined silicon wafer. The two wafers are bonded at low temperature. An IR spectrometer results, which is simple to fabricate and remains compatible with standard IC processing.

Characterization of Chemically and Topographically Modified Siloxane Elastomer for Controlled Cell Growth

ABSTRACTA main limitation of biomedical devices is the inability to start, stop, and control cell growth making it crucial to develop biomaterial surfaces that induce a desired cellular response. Micropatterns of ridges and pillars were created in a siloxane elastomer (Dow Corning) by casting against epoxy replicates of a micromachined silicon wafer. Silicone oils were incorporated to determine the change in modulus and surface energy caused by these additives. SEM and white light interference profilometry verified that the micropatterning process produced high fidelity, low defect micropatterns. Mechanical analysis indicated that varying the viscosity, weight percent and functionality of the added silicone oil could change the elastic modulus by over an order of magnitude (0.1-2.3 MPa). As a self-wetting resin, silicone oils migrate to the surface, hence changing the surface properties from the bulk. Both topographical and chemical features define the surface energy, which in combination with elastic modulus, dictate biological activity. The results imply that the morphology, mechanical properties and surface energy of the siloxane elastomer can be modified to elicit a specific cell response as a function of engineered topographical and chemical functionalization.

Anodic Bond Quality and impact on Pressure Sensor Long-Term Stability

ABSTRACTSilicon pressure sensors have historically been fabricating by bonding a glass wafer to a micro-machined silicon wafer. The sensor may be sealed as an absolute pressure sensor by using planar glass and can then be used for detection of barometric pressure changes.It has generally been assumed that as long as the glass and silicon are reasonable clean, then the silicon-glass seal is good and the part becomes a reliable, stable sensor. This paper addresses a low-level drift that was identified in such an absolute pressure sensor. A Zero drift in the range of 0.1% FS was detectable under humidity stresses. The stress always caused drift in the same direction, indicating an effective increased pressure in the sealed cavity.The impact of various cleaning processes in reducing drift are reported. The improved process assure reliable product for applications such as automotive and altimeter applications.