Howard D. Goldberg

Screen printing: a technology for the batch fabrication of integrated chemical-sensor arrays

Abstract The commercialization of integrated chemical sensor has been slowed by the difficulty of device encapsulation and membrane application. For reasons of both cost and reproducibility, the sensor-specific structures should be mass fabricated, as are the microelectronics. The challenge lies in merging the standard semiconductor process sequence with the non-standard steps used to form the transducers. We demonstrate that screen printing can be used to partition the fabrication into two distinct sequences, semiconductor processing and sensor-specific steps. This simplifies process development and evolution, and makes semiconductor foundry services available for manufacturing sensors. After conventional semiconductor processing, our wafers have silver epoxy contacts screen printed on the aluminum sensor pads; the silver forms a stable chemical interface to the membranes, and the epoxy makes a strong physical bond to them. Next, the polymeric membranes are applied and patterned with screen printing. Membranes of different compositions can be deposited on the various sites of a multisensor chip by simply repeating the screen print/cure cycle. We show that the electrochemical performance of mass-fabricated passive and active sensor arrays is comparable to that of conventional liquid-junction ion-selective electrodes.

A wafer-bonded floating-element shear stress microsensor with optical position sensing by photodiodes

This paper discusses a noninvasive sensing technique for the direct measurement of low-magnitude shear stresses in laminar and turbulent air flows. The sensing scheme detects the flow-induced in-plane displacement of a microfabricated floating-element structure (500 /spl mu/m/spl times/500 /spl mu/m/spl times/7 /spl mu/m), using integrated photodiodes. The wall-mounted floating-element sensors were fabricated using a wafer-bonding technology. The sensors were calibrated in a custom-designed laminar flow cell and subsequently shown to be able to transduce shear stresses of 0.01 Pa during tests in a low-speed wind tunnel.

Potentiometric ion- and bioselective electrodes based on asymmetric polyurethane membranes

The potentlometnc eon responses of ammomum- and proton-selective electrodes prepared by incorporating appropnate neutral tamers wlthm novel asymmetrlc polyurethane membranes are reported The membranes are formed by first castmg a plastlclzed polyurethane(PU)/ terpoly(vmy1 chlonde/vmyl acetate/vmyl alcohol) (PVA)based Ion-selectwe membrane and then applymg a thm second layer of a more hydrophlhc polyurethane (HPU) contammg polylysme The resulting asymmetnc membranes function equwalently to normal PU/PVA membranes and conventlonal poly(vmy1 chloride) type membranes m terms of potentlometnc ion selectlvlty and dynamic response propertles The large amount of amme functlonal groups from the polylysme wthm the outer hydrophlhc layer can be further activated for direct enzyme unmobdlzatlon As examples, adenosme deammase and urease are munobrllzed on ammomum- and proton-selectlve membranes, respectwely, to yield adenosme and urea electrodes with good dynanuc responses and sensltMtles Advantageous use of this new membrane system for preparation of sohd-state mlcrofabrlcated enzyme-based sensors IS also described kizywords Blosensors, Ion-selectwe electrodes, Potentlometry, Asymmetnc membranes, Polyurethane membranes, Sohd-state electrodes

Unique MEMS characterization solutions enabled by laser Doppler vibrometer measurements

Micro-Electro-Mechanical Systems (MEMS) devices present many difficult characterization challenges. In an environment where dimensions are measured in microns and mechanical resonant frequencies are measured in kilohertz, conventional measurement and characterization techniques cannot be used. Laser Doppler Vibrometer (LDV) technology offers many unique advantages for MEMS characterization and troubleshooting. One of the key problems in characterizing and troubleshooting MEMS devices is the separation of electrical and mechanical effects. By definition, MEMS devices have integrated electrical and mechanical components to form electro-mechanical systems. When characterizing and troubleshooting these devices it is often difficult to determine whether an observed behavior is purely mechanical, purely electrical, or inherently electro-mechanical. Because LDV measurements are electrically inert and do not introduce mechanical artifacts, they are ideally suited for this application. Applied MEMS and Polytec PT have successfully developed LDV based measurement techniques that allow detailed characterization and rapid troubleshooting of MEMS devices. Three real-world examples of MEMS characterization using a LDV are presented including, an optical micro-mirror, a robust low-noise accelerometer and a hermetic ceramic sensor package.

Multiionophore-based solid-state potentiometric ion sensor as a cation detector for ion chromatography

A new solvent/polymeric multiion-selective membrane electrode is described for use as a flow-through detector in ion chromatography. Unlike conventional ion-selective membranes that utilize only one type of ionophore, the multiion-selective membranes incorporate several different ionophores. In this study, a K+iNH,+/Na+/Ca*+ selective membrane has been prepared by incorporating four different neutral-carrier ionophores in a strongly adhesive polyurethane matrix. This membrane is cast on solid electrode surfaces, with no internal electrolyte solution, to form solid-state multiion sensors. The detection characteristics of the muhiionophore membranes, which are controlled by the amounts and ratios of the ionophores they contain, are examined in a flow-injection arrangement. The sensors have been demonstrated in the target application, a flow-through ion chromatography system.

Screen printing: a technology for partitioning integrated microsensor processing

Potentiometric chemical sensors are presented as a case study in the use of screen printing for process partitioning. After CMOS processing, the authors print silver epoxy over the openings in the overglass layer to the aluminum input pads; the silver forms a stable chemical interface to the membranes, and the epoxy forms a strong physical bond to them. The screen-printed silver epoxy allows the use of conventional aluminum metallization in the microelectronic circuits. The polymeric membranes are applied and patterned with screen printing. Membranes of different compositions can be deposited on the various sites of a multisensor chip by simply repositioning the mask and repeating the screen print/cure cycle. There is no cross-contamination of membranes.<<ETX>>

Ion-selective sensors incorporating strongly adhesive polymeric membranes

Chemical-selective membranes having matrices of polyvinyl chloride (PVC)/(hydroxylated PVC), polyurethane/(hydroxylated PVC), and moisture-curable silicone rubber are compared to conventional PVC membranes in terms of adhesion to silicon nitride. A well-controlled peel test, developed for this evaluation, yields repeatable, quantitative results for both wet and dry membranes. Polyurethane and silicon membranes have much better adhesion to the sensor surface than do PVC or hydroxylated PVC. The hydroxylated PVC- and polyurethane-based membranes have electrochemical performance equivalent to that of PVC membranes in terms of slope, detection limit, and selectivity. Though the electrochemical properties of the silicone-matrix membrane are degraded somewhat by its high resistance, it has extremely good adhesion.<<ETX>>

MEMS for geophysicists

Input/Output, Inc. has developed a MEMS accelerometer to use as a seismic sensor for oil and gas exploration. Currently, moving coil inductive geophones are used as seismic sensors. Geophone design and performance have evolved for more than 50 years to the point that modern geophones are small, rugged, highly sensitive to motion, and produce minimal background noise. Achieving performance superior to a modern geophone with a MEMS accelerometer has been a significant technical challenge, but other benefits enabled by MEMS and accelerometer technology, such as direct digital output at the sensor, inherent high vector fidelity, and superior low-frequency response, justify the effort.