Ralph W. Bernstein

Capacitive differential pressure sensor for harsh environments

Abstract A capacitive differential pressure sensor for the pressure range of 0–1 bar has been developed. The primary field of application is hydrodynamic flow measurements in hot petroleum wells. In these harsh environments the sensor has to survive high common mode pressure in the range of 1000 bar and temperatures up to 180°C. The pressure element is formed in a triple-stack of fusion-bonded silicon wafers. A bossed diaphragm etched in the upper wafer bends due to the differential pressure across it. The capacitance to the middle wafer is measured. A reference capacitor insensitive to the differential pressure enables compensation for capacitance shifts caused by ambient pressure and temperature changes. The lower silicon wafer is included to minimise the diaphragm stress from the package. An ASIC, certified for 230°C, which is developed at SINTEF, is used for signal read-out. The sensor is measured to have a low zero pressure signal drift, smaller than 2.5% of full-scale output, when the temperature is varied in the range 0–200°C and the ambient pressure in the range 0–1000 bar. The sensitivity of the sensor is sufficient for the application.

Boron etch-stop in TMAH solutions

Abstract Etch rates of 〈100〉 single-crystal silicon in tetramethyl ammonium hydroxide (TMAH) solutions have been measured as a function of boron doping concentration with the purpose of studying the feasibility of an etch-stop. The boron concentration has been varied up to 2.5 × 1020 cm−3. An etch ratio of 1:40 between the heavily and lightly boron-doped silicon has been obtained. This ratio may depend slightly on the temperature of the etch, but no significant variation with etchant concentration has been observed for TMAH concentrations in the range 23–32 wt.%. Preliminary experiments on the effect of adding pyrazine to the etch solution indicate that pyrazine increases the etch rate slightly and seems to have the effect of reducing surface roughness.

Microoptical characterization and modeling of positioning forces on drosophila embryos self-assembled in two-dimensional arrays

In this paper, we describe high-precision experimental and numerical characterization of the positioning forces acting on Drosophila embryos that have self-assembled onto 2-D arrays of hydrophobic sites on a silicon substrate in water. The forces measured using a surface micromachined optical-encoder force sensor operating in reflection, are in good agreement with numerical simulations based on an extended surface energy model for the oil-based fluidic system. The positioning forces of ellipsoidal embryos on flat sites show a linear-spring-like relationship between the force and displacement on rectangular as well as cross-shaped sites. An average detachment force of 8.9 /spl mu/N/spl plusmn/1.3 /spl mu/N was found for the immobilized embryos on 250 /spl mu/m/spl times/100 /spl mu/m sites. The cross-shaped site has only 19.85% of the area of the rectangular site, but provides a comparable positioning force with a significant reduction in embryo clustering. In contrast, the positioning forces of flat silicon chips, similar in size to the embryos, are linear in the displacement only over a limited range (0/spl sim/40 /spl mu/m), and are then constant up to the detachment force (25.0 /spl mu/N/spl plusmn/3.5 /spl mu/N). Our measurements also show significant hysteresis in the force vs. displacement, indicating that variations in the surface properties play an important role in the self-assembly process.

Capping and decapping of MBE grown GaAs(001), Al0.5Ga0.5As(001), and AlAs(001) investigated with ASP, PES, LEED, and RHEED

Abstract Arsenic capping and regeneration of MBE-grown GaAs(001), Al 0.5 Ga 0.5 As(001), and AlAs(001) epilayer surfaces were examined with Auger sputter profiling (ASP), synchrotron radiation and X-ray photoelectron spectroscopy (PES), LEED, and RHEED. It is found that clean, ordered surfaces of different As/Ga(Al) compositions and different surface reconstructions can be prepared in a controlled manner after long-term storage in air, by thermal desorption of the As cap at appropriate annealing temperatures. A protective film of amorphous arsenic was deposited in situ with both As 2 and As 4 molecular beams onto cold substrates. The recorded Auger depth profiles unveil capping layer thicknesses from 0.3 to 3 μm, the thicker for depositions using the As 2 dimer source. The As 3+ surface oxide, formed immediately upon exposure of the passivated wafers to air, remains on the order of 10Athick, even after storage in atmosphere for several months. Core level photoemission shows selective desorption of this oxide upon annealing in UHV at 250°C. Further heating at 350°C evaporates the protective arsenic cap, and clean, As-terminated Al x Ga 1− x As(001) surfaces with a regular arrayof chemisorbed excess As As dimers prevail. The recorded LEED and RHEED patterns show a c(4 × 4) surface reconstruction for GaAs(001) and Al 0.5 Ga 0.5 As(001), whereas this structural phase was observed with RHEED only for the highly reactive AlAs(001) surface. Subsequently annealing in UHV at 450°C causes desorption of the chemisorbed surface arsenic and a concurrent transition from c(4 × 4) to the (2 × 4)/c(2 × 8) surface of As stabilized MBE-grown Al x Ga 1− x As(001). With AlAs(001), surface Al oxidation was observed immediately after annealing at 450°C, in spite of carefully controlled UHV environments

New method for testing hermeticity of silicon sensor structures

Abstract A method for testing the hermeticity of different wafer-bonding processes used in silicon sensor devices is proposed. The method is based on measuring the gas concentration in a sealed silicon cavity by Fourier-transform infrared spectroscopy (FTIR). The gas concentration and thereby the leakage into the sealed silicon cavity after external pressure exposure is measured by FTIR absorbance. In this work the method has been evaluated by measuring a test silicon cavity filled with a controlled amount of test gas. N 2 O is evaluated as the test gas in this experiment.

Integrated optical diffractive micrograting-based injection force sensor

We present a micrograting-based injection force sensor integrated with a surface micromachined silicon-nitride injector. The injector is supported by springs of known spring constant, and the injection force is determined from displacement measurements using a high-resolution, miniaturized optical encoder. The sensor exhibits configurable sensitivity and dynamic range, allowing monitoring over a wide range of forces. The periodicity of the encoder response can be used for calibration and to obtain information about the deformation of the target. We used an injector with a measured spring constant of 1.85 N/m for penetration experiments on Drosophila embryos, and found a penetration force of 52.5/spl plusmn/13.2% /spl mu/N and a membrane displacement of 58/spl plusmn/5.2% /spl mu/m.

GaAs(100) substrate cleaning by thermal annealing in hydrogen

The removal of residual surface contaminants from chemically polished GaAs wafers has been examined with x‐ray photoelectron spectroscopy. Al Kα‐excited As 2p3/2 and Ga 2p3/2 core level spectra with a shallow probing depth show complete desorption of As3+ surface oxide after annealing in a stream of pure hydrogen at atmospheric pressure and 400 °C, whereas a temperature of 600 °C is required for entire removal of the Ga3+ surface oxide. Carbonaceous surface impurities are also shown to be efficiently reduced by this hydrogen processing. The recorded core level peak intensities indicate formation of a fully stoichiometric substrate surface, with no depletion of arsenic. The chemical structure of the hydrogen processed surface compares favorably with that found for substrates annealed in an ultrahigh vacuum environment. In contrast, annealing at 600 °C in 1 atm of high‐purity inert gas (argon) is shown to promote formation of a thick Ga–oxide on the substrate surface, thus emphasizing the reactive nature of...

BioMEMS for high-throughput handling and microinjection of embryos

Technologies for handling, sorting, and positioning of embryos are increasingly important in biomedicine. In this paper the status for ongoing projects aimed at developing instrumentation for high-throughput treatment of embryos is reviewed. Techniques for positioning of Drosophila (fruit-fly) embryos in 2-D arrays for use in microinjection experiments are especially focused. A method based on fluidic micro assembly is discussed, and important parameters such as immobilization yield, the number of misplaced embryos, and adhesion force of the embryos are reported. A model for the assembly process is described, and simulation results are in good agreement with adhesion force measurements. A fully automated MEMS based system for fruit-fly embryo injection has recently been demonstrated at Stanford University. The first experiments with double-stranded RNA injection proved successful, and the expected genetic modification of the embryos was observed.

New Method For Testing Hermeticity Of Silicon Sensor Structures

A method for testing hermeticity of different wafer bonding processes used in silicon sensor devices is proposed. The method is based on measuring the gas concentration in a sealed silicon cavity by Fourier Transform Infrared spectroscopy (FTIR). The gas concentration and thereby the leakage into the sealed silicon cavity after external pressure exposure is measured by FTIR-absorbance. In this work the method was evaluated by measuring a test silicon cavity filled with a controlled amount of test gas. N/sub 2/O as testing gas was evaluated in this experiment.

Sensitive and Selective Photo Acoustic Gas Sensor Suitable for High Volume Manufacturing

Sensitive and selective gas measurements are crucial for a large variety of applications. This paper describes the manufacturing and characterisation of a photo acoustic gas sensor system. The system is based on a pressure sensor element with a sensitivity of 10 muV/V/Pa. 12 prototypes for measuring CO2 have been characterised. Detection limits ranging from 92 ppm to below 6 ppm CO2 were obtained, depending on the measurement time and photo acoustic cell design. No cross-sensitivity towards CO, CH4, or humidity could be observed in any of the sensors. The temperature drift of the uncompensated raw signal of two sensor designs was below 117 ppm CO2 in the range from 25degC to 50degC.