Micropore x-ray optics using anisotropic wet etching of (110) silicon wafers

To develop x-ray mirrors for micropore optics, smooth silicon (111) sidewalls obtained after anisotropic wet etching of a silicon (110) wafer were studied. A sample device with 19 microm wide (111) sidewalls was fabricated using a 220 microm thick silicon (110) wafer and potassium hydroxide solution. For what we believe to be the first time, x-ray reflection on the (111) sidewalls was detected in the angular response measurement. Compared to ray-tracing simulations, the surface roughness of the sidewalls was estimated to be 3-5 nm, which is consistent with the atomic force microscope and the surface profiler measurements.     

Development of low-noise high value chromium silicide resistors for cryogenic detector applications

Extremely high sensitivity detectors, such as silicon bolometers are required in many NASA missions for detection of photons from the X-ray to the far infrared regions. Typically, these detectors are cooled to well below the liquid helium (LHe) temperature of 4.2 K to achieve the maximum detection performance. As photodetectors, they are generally operated with a load resistor and a pre-set bias voltage, which is coupled to the input gate of a source-follower field effect transistor (FET) circuit. It is imperative that the detector system signal-to-noise performance be limited by the noise of the detector and not by the noise of the external components. The load resistor value is selected to optimize the detector performance. These two criteria tend to be contradictory in that these detectors require load resistors in the hundreds of megaohms, which leads to a higher Johnson noise. Additionally, the physical size of the resistor must be small for device integration as required by such missions as the NASA High Resolution Airborne Wide-Band Camera instrument and the Submillimeter High-Angular Resolution Camera for the Caltech Submillimeter Observatory, both of which employ 384 detectors and resistors. We have designed, fabricated and characterized thin film resistors using a CrSi/TiW/Al metal system on optical quality quartz substrates. The resistor values range from 75 MΩ to over 650 MΩ and are Johnson noise limited to below LHe temperatures. The resistor film is sputtered with a sheet resistance ranging from 300 to 1600 Ω/□ and the processing sequence developed for these devices allows for chemically fine-tuning the sheet resistance in situ. The wafer fabrication process was of sufficiently high yield (>80%) providing clusters of good resistors for integrated multiple detector channels, a very important feature in the assembly of these two instruments.     

Fabrication of humidity sensors by multi-walled carbon nanotubes

Humidity sensors have multi-walled carbon nanotubes (MWNTs) as the sensing material is demonstrated. The sensor was fabricated on a silicon dioxide coated silicon wafer with metal electrodes. MWNTs were deposited and interlinked with the electrodes by means of the dielectrophoresis technique. The sensing device has the function of a hygrometer when measuring resistance variations to the local relative humidity percentage (RH%) through MWNTs. By measuring the MWNT resistances, we find that higher RH% results in a decrease of conductivity. The results indicate that electron transports in MWNTs are affected by water molecules adsorption on the outermost nanotube surface. A miniature thermocouple sensor was also fabricated and integrated with the humidity sensor. This allowed us to simultaneously sense environmental humidity and temperature. Hence, accurate humidity measurements were achieved with this prototype by calibrating the electrical resistance and temperature levels to carry out the tests with the humidity percentages.     

Characterization of the Temperature Dependence of the Pressure Coefficients of n- and p-Type Silicon Using Hydrostatic Testing

Piezoresistive stress sensors on the (111) surface of silicon offer the unique ability to measure the complete stress state at a point in the (111) material. However, four-point bending or wafer-level calibration methods can measure only four of the six piezoresistive coefficients for p- and n-type resistors required for application of these sensors. In this work, a hydrostatic test method has been developed in which a high-capacity pressure vessel is used to apply a triaxial load to a single die over the -25degC to+100 degC temperature range. The slopes of the adjusted resistance change versus pressure plots yield pressure coefficients for p- and n-type silicon that provide the additional information necessary to fully determine the complete set of piezoresistive coefficients.     

Fabrication and testing of bulk micromachined silicon carbide piezoresistive pressure sensors for high temperature applications

This paper explores the development of high-temperature pressure sensors based on polycrystalline and single-crystalline 3C-SiC piezoresistors and fabricated by bulk micromachining the underlying 100-mm diameter (100) silicon substrate. In one embodiment, phosphorus-doped APCVD polycrystalline 3C-SiC (poly-SiC) was used for the piezoresistors and sensor diaphragm, with LPCVD silicon nitride employed to electrically isolate the piezoresistor from the diaphragm. These piezoresistors fabricated from poly-SiC films deposited at different temperatures and doping levels were characterized, showing -2.1 as the best gauge factor and exhibited a sensitivities up to 20.9-mV/V*psi at room temperature. In a second embodiment, epitaxially-grown unintentionally nitrogen-doped single-crystalline 3C-SiC piezoresistors were fabricated on silicon diaphragms, with thermally grown silicon dioxide employed for the piezoresistor electrical isolation from the diaphragm. The associated 3C-SiC/SiO/sub 2//Si substrate was fabricated by bonding a (100) silicon wafer carrying the 3C-SiC onto a silicon wafer with thermal oxide covering its surface. The 3C-SiC handle wafer was then etched away in KOH. The diaphragm was fabricated by time etching the silicon substrate. The sensors were tested at temperatures up to 400/spl deg/C and exhibited a sensitivity of 177.6-mV/V*psi at room temperature and 63.1-mV/V*psi at 400/spl deg/C. The estimated longitudinal gauge factor of 3C-SiC piezoresistors along the [100] direction was estimated at about -18 at room temperature and -7 at 400/spl deg/C.     

All-(111) surface silicon nanowires: selective functionalization for biosensing applications

We demonstrate the utilization of selective functionalization of carbon-silicon (C-Si) alkyl and alkenyl monolayers covalently linked to all-(111) surface silicon nanowire (Si-NW) biosensors. Terminal amine groups on the functional monolayer surfaces were used for conjugation of biotin n-hydroxysuccinimide ester. The selective functionalization is demonstrated by contact angle, X-ray photoelectron spectroscopy (XPS), and high-resolution scanning electron microscopy (HRSEM) of 5 nm diameter thiolated Au nanoparticles linked with streptavidin and conjugated to the biotinylated all-(111) surface Si-NWs. Electrical measurements of monolayer passivated Si-NWs show improved device behavior and performance. Furthermore, an analytical model is presented to demonstrate the improvement in detection sensitivity of the alkyl and alkenyl passivated all-(111) Si-NW biosensors compared to conventional nanowire biosensor geometries and silicon dioxide passivation layers as well as interface design and electrical biasing guidelines for depletion-mode sensors.     

Fabrication of silicon nanowire for detecting p-amyloid (1-42) by nanoimprint lithography

Ultraviolet nanoimprint lithography (UV-NIL) is a high volume and cost-effective patterning technique with sub-10 nm resolution. It has great potential as a candidate for next generation lithography. Using UV-NIL, nanowire patterns were successfully fabricated on a four-inch silicon-on-insulator (SOI) wafer under moderate conditions. The fabricated nanowire patterns were characterized by FE-SEM. Its electrical properties were confirmed by semiconductor parameter analysis. Monoclonal antibodies against beta-amyloid (1-42) were immobilized on the silicon nanowire using a chemical linker. Using this fabricated silicon nanowire device, beta-amyloid (1-42) levels of 1 pM to 100 nM were successfully determined from conductance versus time characteristics. Consequently, the nanopatterned SOI nanowire device can be applied to bioplatforms for the detection of proteins.     

Application of nanoporous silicon for a metal-semiconductor-metal visible light photodetector

In this paper we present a study on the application of nanoporous silicon to an optoelectronic device called a nanoporous silicon metal-semiconductor-metal (MSM) visible light photodetector. This device was fabricated on a nanoporous silicon layer which was formed by electrochemical etching of a silicon wafer in a hydrofluoric acid solution under various anodization conditions such as the resistivity of the silicon wafer, current density, concentration of the hydrofluoric acid solution and anodization time. The structure of this device has two square Al/nanoporous silicon Schottky-barrier junctions on the silicon substrate and the electrode spacing is 500 microm. The experiment will study photoresponse and the response time of a nanoporous silicon MSM photodetector which was fabricated on the various porosity of a nanoporous silicon layer. It is found that when devices are fabricated on a higher porosity nanoporous silicon layer, the photoresponse of the device will expand toward the short-wavelength and the bandwidth of the spectrum response will cover visible light. In addition, it is found that the response time of the device decreases.     

Effect of ultraviolet illumination and ambient gases on the photoluminescence and electrical properties of nanoporous silicon layer for organic vapor sensor

The purpose of this research, the nanoporous silicon layer were fabricated and investigated the physical properties such as photoluminescence and the electrical properties in order to develop organic vapor sensor by using nanoporous silicon. The Changes in the photoluminescence intensity of nanoporous silicon samples are studied during ultraviolet illumination in various ambient gases such as nitrogen, oxigen and vacuum. In this paper, the nanoporous silicon layer was used as organic vapor adsorption and sensing element. The advantage of this device are simple process compatible in silicon technology and usable in room temperature. The structure of this device consists of nanoporous silicon layer which is formed by anodization of silicon wafer in hydrofluoric acid solution and aluminum electrode which deposited on the top of nanoporous silicon layer by evaporator. The nanoporous silicon sensors were placed in a gas chamber with various organic vapor such as ethanol, methanol and isopropyl alcohol. From studying on electrical characteristics of this device, it is found that the nanoporous silicon layer can detect the different organic vapor. Therefore, the nanoporous silicon is important material for organic vapor sensor and it can develop to other applications about gas sensors in the future.     

Replication of surfaces of natural leaves for enhanced micro-scale tribological property

In this paper, we report on the replication of surfaces of Lotus and Colocasia leaves onto thin polymeric films using a capillarity-directed soft lithographic technique. The replication was carried out on poly(methyl methacrylate) (PMMA) film spin coated on silicon wafer using poly(dimethyl siloxane) (PDMS) molds. The friction properties of the replicated surfaces were investigated at micro-scale in comparison with those of PMMA thin film and uncoated silicon wafer. The coefficients of friction of the replicated surfaces were almost five times lower than those of the PMMA thin film and four times lower than those of the uncoated silicon wafer. The superior micro-tribological properties of the replicated surfaces could be attributed to the reduced real area of contact projected by the surfaces.     

Microfiber-directed boundary flow in press-fit microdevices fabricated from self-adhesive hydrophobic surfaces

We report a rapid microfluidic device construction technique which does not employ lithography or stamping methods. Device assembly physically combines a silicon wafer, an elastomer (poly(dimethylsiloxane) (PDMS)), and microfibers to form patterns of hydrophobic channels, wells, elbows, or orifices that direct fluid flow into controlled boundary layers. Tweezers are used to place glass microfibers in a defined pattern onto an elastomeric (PDMS) hydrophobic film. The film is then manually pressed onto a hydrophobic silicon wafer, causing it to adhere to the silicon wafer and form a liquid-tight seal around the fibers. Completed in 15 min, the technique results in an operable microdevice with micrometer-scale features of nanoliter volume. Microfiber-directed boundary flow is achieved by use of the surface wetting properties of the hydrophilic glass fiber and the hydrophobicity of surrounding surfaces. The simplicity of this technique allows quick prototyping of microfluidic components, as well as complete biosensor systems, such as we describe for the detection of pathogenic bacteria.     

A High-Resolution MEMS Piezoelectric Strain Sensor for Structural Vibration Detection

This paper presents the modeling, fabrication, and testing of a high-performance dynamic strain sensor. Using microelectromechanical systems (MEMS) technology, ZnO piezoelectric microsensors are directly fabricated on silicon and steel substrates. The sensors are intended to be used as point sensors for vibration sensing without putting an extra burden on the host structures. A model that incorporates piezoelectric effects into an RC circuit, representing the sensor architecture, is developed to describe the voltage output characteristics of the piezoelectric microsensors. It is shown that the sensitivity of microplanar piezoelectric sensors that utilize the $e_{31}$ effect is linearly proportional to sensor thickness but unrelated to sensor area. Sensor characterization was performed on a cantilever beam cut from a fabricated silicon wafer. The experimental data indicate that the overall sensor and circuit system is capable of resolving better than 40.3 nanostrain time domain signal at frequencies above 2 kHz. The corresponding noise floor is lower than 200 femto-strain per root hertz and the sensitivity, defined as the sensor voltage output over strain input, is calculated to be 340 V/$varepsilon$ . Micro ZnO piezoelectric sensors fabricated on steel hard disk drive suspensions also show excellent results. The sensor not only has a better signal-to-noise ratio but also detects more vibration information than the combination of two laser-doppler-vibrometer measurements in different directions.      

Miniaturized acceleration sensors with in-plane polarized piezoelectric thin films produced by micromachining

Miniaturized acceleration sensors employing piezoelectric thin films were fabricated through batch micromachining with silicon and silicon-on-insulator (SOI) wafers. The acceleration sensors comprised multiple suspension beams supporting a central seismic mass. Ferroelectric (Pb,La)(Zr,Ti) O(3) (PLZT) thin films were coated and in-plane polarized on the surfaces of the suspension beams for realizing electromechanical conversion through the piezoelectric effect. Interdigital electrodes were formed on the PLZT films and connected in parallel. Finite element analyses were conducted for the stress and strain distributions, providing guidance to the structural design, including optimizing electrode positioning for collecting the electrical output constructively. Uniformity of the beam thickness and sample consistency were significantly improved by using SOI wafers instead of silicon wafers. The measurement results showed that all the sensor samples had fundamental resonances of symmetric out-of-plane vibration mode at frequencies in the range of 8 to 35 kHz, depending on the sample dimensions. These sensors exhibited stable electrical outputs in response to acceleration input, achieving a high signal-to-noise ratio without any external amplifier or signal conditioning.     

Fracture toughness measurement of thin-film silicon

The fracture toughness of single‐crystal silicon thin films oriented to (100) and (110) was investigated by tensile testing under both 〈100〉 and 〈110〉 loading conditions. The specimen was fabricated from a p‐type Czochralski (CZ)‐grown wafer and passed through a thermal process during the fabrication of the test device. The measured fracture toughness is dependent on the loading direction in the tensile test and independent of the specimen surface orientation. The test results were 1.94 MPa√m in the 〈100〉 direction and 1.17 MPa√m in the 〈110〉. In these tests, no longitudinal size effect on the fracture stress or fracture toughness was observed. The SEM photographs obtained from the fracture specimens after the tensile test show that the crack initiated from the notch tip and propagated straight in the across‐the‐width direction on the (110) or (111) cleavage plane.     

Optical nonreciprocal devices with a silicon guiding layer fabricated by wafer bonding

Optical nonreciprocal devices with a silicon guiding layer fabricated by wafer bonding are proposed. The optical nonreciprocal devices are composed of a magneto-optic waveguide with a magnetic garnet/Si/SiO2 structure. Nonreciprocal characteristics are obtained by an evanescent field penetrating into the upper magnetic garnet cladding layer. Several kinds of the optical nonreciprocal device are investigated with the magneto-optic waveguide and designed at a wavelength of 1.55 microm. As a preliminary experiment, wafer bonding between Gd3Ga5O12 and Si was studied. Wafer bonding was successfully achieved with heat treatment at 220 degrees C in H2 ambient.     

An X band RF MEMS switch based on silicon-on-glass architecture

Communication systems such as those used on satellite platforms demand high performance from individual components that make up the various systems and sub-systems. Switching and routing of RF signals between various modules is a routine and critical operation that determines the overall efficiency of the entire system. In this paper, we present the design and fabrication aspects of a direct contact RF MEMS switch designed to operate in the X band (8–12 GHz) with a target insertion of about 0·5 dB and isolation better than 30 dB. The actuation voltage is expected to be around 50 V. The die size is designed to be 3 mm (H) × 3 mm(W) × 2 mm(H). The switch is built from a low residual stress device layer of a highly conducting (0·005 Ohms-cm) silicon on insulator (SOI) wafer. After subsequent lithographic steps, the wafer is bonded to a Pyrex glass wafer which has been previously patterned with gold transmission lines and pull in electrodes. Being built from a single crystal silicon structure, the mechanical robustness of the actuator is much greater than the those in similar membrane-based devices. A 6 mask fabrication process utilizing Deep Reactive Ion Etching to achieve high aspect ratio stiction free structures was developed and implemented. Devices from the first fabrication run are being analysed in our laboratory.     

用于气体检测的热激励MEMS悬臂梁谐振器

笔者设计了一种电热激励压阻检测的微悬臂梁谐振器．悬臂梁上的敏感层吸附特定的气体后,可以通过测量质量变化所导致的悬臂梁谐振频率的变化而得出待测气体的浓度．该谐振器的加工基于SOI硅片和ICP刻蚀技术．通过有限元仿真得出了悬臂梁的应力温度分布曲线,从而证实了该方案可以提供比普通结构更适宜于气体敏感材料工作的温度分布．从理论和实验上分析了谐振器在不同激励电压下的频率响应．实验结果表明,该谐振器具有较高的谐振频率和品质因数,其激励电压与振动幅值成线性关系．     

A fully integrated monolithic microchip electrospray device for mass spectrometry

A novel microfabricated nozzle has been developed for the electrospray of liquids from microfluidic devices for analysis by mass spectrometry. The electrospray device was fabricated from a monolithic silicon substrate using deep reactive ion etching and other standard semiconductor techniques to etch nozzles from the planar surface of a silicon wafer. A channel extends through the wafer from the tip of the nozzle to a reservoir etched into the opposite planar surface of the wafer. Nozzle diameters as small as 15 microm have been fabricated using this method. The microfabricated electrospray device provides a reproducible, controllable, and robust means of producing nano-electrospray of a liquid sample. The electrospray device was interfaced to an atmospheric pressure ionization time-of-flight mass spectrometer using continuous infusion of test compounds at low nanoliter-per-minute flow rates. Nozzle-to-nozzle signal intensity reproducibility using 10 nozzles was demonstrated to be 12% with single-nozzle signal stability routinely less than 4% relative standard deviation (RSD). Solvent compositions have been electrosprayed ranging from 100% organic to 100% aqueous. The signal-to-noise ratio from the infusion of a 10 nM cytochrome c solution in 100% water at 100 nL/min was 450:1. Microchip electrospray nozzles were compared with pulled capillaries for overall sensitivity and signal stability for small and large molecules. The microchip electrospray nozzles showed a 1.5-3-times increase in sensitivity compared with that from a pulled capillary, and signal stability with the microchip was 2-4% RSD compared with 4-10% with a pulled capillary. Electrospray device lifetimes achieved thus far have exceeded 8 h of continuous operation and should be sufficient for typical microfluidic applications. The total volume of the electrospray device is less than 25 pL, making it suitable for combination with microfluidic separation devices.     

Residual stress in piezoelectric poly(vinylidene-fluoride-co-trifluoroethylene) thin films deposited on silicon substrates

The residual stress and its evolution with time in poly(vinylidene-fluoride-co-trifluoroethylene) (P(VDF-TrFE) (72/28)) piezoelectric polymer thin films deposited on silicon wafers were investigated using the wafer curvature method. Double-side polished silicon wafers with minimized initial wafer warpage were used to replace single-side polished silicon wafers to obtain significantly improved reliability for the measurement of the low residual stress in the P(VDF-TrFE) polymer thin films. Our measurement results showed that all the P(VDF-TrFE) films possessed a tensile residual stress, and the residual stress slowly decreased with time. Our analysis further indicates that the tensile stress could arise from the thermal mismatch between the P(VDF-TrFE) film and the silicon substrate. Besides possible viscoelastic creep mechanism in thermoplastic P(VDF-TrFE) films, microcracks with widths in the range of tens of nanometers appeared to release the tensile residual stress.     

STW gas sensors using plasma-polymerized allylamine

Gas sensors generally consist of two major components: a gas recognition element which provides the specificity and selectivity of the measurement and a physical transducer which translates the gas absorption or desorption event into electronic signal. In this paper, plasma polymerized allylamine (PPAa) film is used as a gas recognition element and a surface transverse wave (STW) device is used as a physical transducer. It is confirmed that STW sensor devices coated with PPAa films provide high sensitivity for moisture. The STW sensor device with a 63 nm PPAa film provided twenty four times higher sensitivity than that of a non-coated STW device. In addition, the chemical structure of PPAa films is characterized by the FT-IR and the contact angle measurement.