G. Jordan Maclay

THE ROLE OF THE CASIMIR EFFECT IN THE STATIC DEFLECTION AND STICTION OF MEMBRANE STRIPS IN MICROELECTROMECHANICAL SYSTEMS (MEMS)

We present an analysis describing how the Casimir effect can deflect a thin microfabricated rectangular membrane strip and possibly collapse it into a flat, parallel, fixed surface nearby. In the presence of the attractive parallel-plate Casimir force between the fixed surface and the membrane strip, the otherwise flat strip deflects into a curved shape, for which the derivation of an exact expression of the Casimir force is nontrivial and has not been carried out to date. We propose and adopt a local value approach for ascertaining the strength of the Casimir force between a flat surface and a slightly curved rectangular surface, such as the strip considered here. Justifications for this approach are discussed with reference to publications by other authors. The strength of the Casimir force, strongly dependent on the separation between the surfaces, increases with the deflection of the membrane, and can bring about the collapse of the strip into the fixed surface (stiction). Widely used in microelectromechanical systems both for its relative ease of fabrication and usefulness, the strip is a structure often plagued by stiction during or after the microfabrication process—especially surfacemicromachining. Our analysis makes no assumptions about the final or the intermediate shapes of the deflecting strip. Thus, in contrast to the usual methods of treating this type of problem, it disposes of the need for an ansatz or a series expansion of the solution to the differential equations. All but the very last step in the derivation of the main result are analytical, revealing some of the underlying physics. A dimensionless constant, K c , is extracted which relates the deflection at the center of the strip to physical and geometrical parameters of the system. These parameters can be controlled in microfabrication. They are the separation w 0 between the fixed surface and the strip in the absence of deflection, the thickness h, length L, and Young’s modulus of elasticity (of the strip), and a measure of the dielectricpermittivities of the strip, the fixed surface, and the filler fluid between them. It is shown that for some systems (K c >0.245), with the Casimir force being the only operative external force on the strip, a collapsed strip is inevitable. Numerical estimates can be made to determine if a given strip will collapse into a nearby surface due to the Casimir force alone, thus revealing the absolute minimum requirements on the geometrical dimensions for a stable (stiction-free) system. For those systems which do exhibit a stiction-free stable equilibrium state, the deflection at the middle of the strip is always found to be smaller than 0.48w 0 . This analysis is expected to be most accurately descriptive for strips with large aspect ratio (L/h) and small modulus of elasticity which also happen to be those most susceptible to stiction. Guidelines and examples are given to help estimate which structures meet these criteria for some technologically important materials, including metal and polymer thin films.

Impedance based sensing of the specific binding reaction between Staphylococcus enterotoxin B and its antibody on an ultra-thin platinum film

Immunobiosensing techniques to measure specific antigen-antibody binding reactions are important in the development of biosensor applications in biotechnology, in vitro diagnosis, medicine and food technology. An immunobiosensor was constructed to measure the specific binding reaction between Staphylococcus enterotoxin B (SEB) and anti-SEB antibodies. The biosensor comprised an anti-SEB bioactive layer covalently immobilized on an ultra-thin platinum (Pt) film sputtered onto a 100 nm thick silicon dioxide layer on a silicon chip. The Pt film was discontinuous with a normal thickness of 25 A. The impedance of the Pt film decreased during the binding of the anti-SEB to SEB in phosphate buffered saline (PBS) at room temperature. The impedance decreases were irreversible in PBS before saturation of the specific binding sites. When saturated, the impedance at 100 Hz was 14% of the value obtained for the fresh anti-SEB layer in PBS. The magnitude of the impedance (Z) decrease followed a simple relationship with SEB concentration in the range between 0.389 and 10.70 ng/ml SEB. The specificity of the biosensor was demonstrated by showing that no irreversible impedance decreases occurred when the sensor was exposed to 100 ng/ml kappa-casein, or alpha-lactalbumin, in PBS.

An integrated amperometric microsensor

Abstract An amperometric microsensor has been fabricated using techniques adapted from conventional microelectronic technology. The device was tested with 100 ppm of NO in humidified air, and gave a response an order of magnitude greater than a commercially available sensor and two orders of magnitude greater than a previously fabricated microsensor with special electrodes which were defined with metal masks (G. J. Maclay, W. J. Buttner and J. R. Stetter, IEEE Trans., 35 (1988) 793). This previous work indicated that improved signal/noise would be expected for an electrode with a larger ratio of the square of the perimeter to the area ( P2/ A). The sensor was fabricated on a thermally oxidized layer of silicon dioxide on a silicon wafer. A gold layer of 3000Athick was deposited by thermal evaporation and then patterned by photolithography and etching. In order to obtain good adhesion of the gold to the silicon dioxide it was necessary to first deposit a thin layer of chromium. The working electrode was etched to contain many windows about 50 micrometers square in order to obtain an improved sensor geometry with large P2/ A. A thin film of Nafion (a Du Pont trade mark) was spin-coated over the electrodes and served as the electrolyte (G. J. Maclay and J. R. Stetter, U.S. Patent applied for (1988)). A fixture was built to expose the Nafion surface of the sensor to gases. The response of the integrated microsensor to NO per gram of gold catalyst was about 5000 times greater than in the commercial sensor. Unfortunately the presence of the chromium adhesion layer led to corrosion in the electrodes which eventually destroyed the sensor operation.

Low Temperature Nafion Bonding of Silicon Wafers

The practical use of silicon wafer bonding in fabricating sensors, actuators, and microelectromechanical structures has grown rapidly and is presently receiving increasing attention. Currently, direct silicon bonding is achieved at temperatures which generally are not feasible after metallization in microelectromechanical system (MEMS) processing. 1-4 High temperature anneals (usually in excess of 700°C) may cause doping profile broadening, defect formation, induction of thermal stress between dissimilar materials, and dissociation between materials. Therefore, the high annealing temperature of silicon fusion bonding, typically >1000°C, restricts its application in the field of MEMS. In contrast to the high temperature fusion bonding, low temperature fusion bonding is not strong enough to guarantee high yielding devices during subsequent pro

The effect of hydrogen and carbon monoxide on the interface state density in MOS gas sensors with ultra-thin palladium gates

Abstract The admittance of 25 A palladium-gate MOS capacitors with 125 A of silicon dioxide has been studied as a function of hydrogen gas concentration and carbon monoxide gas concentration. The ultra-thin gate is a porous film, consisting of an array of partially connected islands. This MOS capacitor is sensitive to hydrogen (3 ppm to 1%) at room and elevated temperatures and to carbon monoxide (100 ppm to 10 000 ppm) at 150 °C. Using a bias scan conductance method at a fixed frequency, the interface state density (Nit) is determined at the surface potential corresponding to the peak in the conductance curve. It is shown that at the corresponding location in the bandgap, Nit increases with increasing concentrations of hydrogen, while Nit decreases with increasing concentrations of carbon monoxide.

Complex impedance measurements of capacitor structures on silicon with copper phthalocyanine dielectric

Measurements were made of the capacitance and the conductance of several capacitor structures with a copper phthalocyanine (CuPc) dielectric at different voltages and in the frequency range of 102–106 Hz. A thermally evaporated CuPc film about 1300 A thick was fabricated in a parallel‐plate capacitor between gold and aluminum electrodes (Al‖CuPc‖Au). Two distributions of relaxation times are observed in this frequency range. At low frequencies voltage‐dependent polarization (possibly interfacial polarization) appears to be the dominant mechanism of current conduction. At high frequencies a relatively voltage‐independent hopping within and/or between molecules appears to be dominant. The CuPc remains polarized for a long time. Measurements were also made of the capacitance‐voltage (C‐V) characteristic of a Al‖CuPc‖SiO2‖Si capacitor at 104, 105, and 106 Hz for different scanning rates. The basic features of the device’s C‐V characteristics are discussed.

Effects of substrate temperature, deposition pressure, and thickness on the morphology of ultrathin platinum film on SiO2/Si substrate

The dependence on thickness of the resistance of platinum film deposited by electron-beam evaporation at room temperature onto a thermally grown SiO2 substrate, shows that the platinum film is in a discontinuous island form when the thickness is less than 1.3 nm. At about 2 nm the film is very continuous electrically but porous. For platinum film deposited at 250 °C, it appears that the morphology is an island form when the thickness is less than 0.5 nm, and in a continuous porous form at about 1.8 nm. As the platinum deposition temperature increases, the film morphology, including the island dimensions or grain size and the gaps between islands or pores, becomes more uniform but has larger grains. Also, it appears that when both the background pressure and deposition temperature are varied, the increase in deposition temperature has a dominant effect in increasing the grains base area, but that the grains average height Zrms increases at lower pressure (10−8 Torr) and tends to decrease at higher pressure (10−6 Torr). n nThe morphology of an ultrathin metal film on an insulator substrate can also be determined by measuring the electrical properties of the film. For example, measuring the films impedance at different temperatures, frequencies, or applied bias can reveal that the film is either in discontinuous island form or in continuous form.

Electrical properties, stability, and applications of ultrathin porous Pt films on SiO2

The morphology of 25 A porous Pt films deposited on thermally grown SiO2 at room temperature and 250u2009°C is presented. The impedance of the films was measured over the frequency range 100 Hz to 1 MHz. At 1 kHz, the impedance was measured in electric fields up to 250 V/cm, over the temperature range −192–140u2009°C. The ac impedance of the 25 A Pt film deposited at room temperature decreased by 25% in 21 days at 150u2009°C in room air but the ac impedance of the Pt film deposited at 250u2009°C did not show any change. The stable 25 A film was used as a gate in a metal–oxide‐semiconductor (MOS) sensor, which was exposed to 800 ppm H2/air at 100u2009°C and 1% CO/air at 150u2009°C. Measurements were made of the change in the impedance of the gate film and the change in the MOS capacitance.

Measurement and wireless transmission of embedded capacitive microsensor's output using ΣΔ conversion and radio frequency identification (RFID) technology

This article concerns the design and post-fabrication testing of a CMOS integrated circuit (IC) for the Remote- Queried Embedded Microsensor (RQEM) system. The IC may be coupled to capacitive microsensors to measure the output of the sensors, to digitize this measured output, and to condition and encode the digital data. Wireless transmission of the code to a commercial Radio Frequency Identification (RFID) system reader is implemented using Differential Phase Shift Keying of a low-frequency signal, which inductively couples the RQEM antenna coil to the receiving antenna of the RFID reader. The IC extracts its own operating power and digital clock signal from the interrogating signal, which is transmitted by the RFID reader. The IC uses switched- capacitor techniques for acquisition and for A/D conversion of data. A first-order Sigma-Delta ((Sigma) (Delta) ) A/D converter is used with an output transconductance amplifier (OTA) in the balancing integrator and the comparator. The same OTA is also used in the acquisition circuit, which is a sample-and-hold offset-free circuit. Several fabricated chips were tested with on-chip test capacitors, used to calibrate the ICs output.