Conventional approaches to chemical sensors have
traditionally made use of a “lock-and-key” design,
wherein a specific receptor is synthesized in order to
strongly and highly selectively bind the analyte of
interest.1-6 A related approach involves exploiting a
general physicochemical effect selectively toward a
single analyte, such as the use of the ionic effect in
the construction of a pH electrode. In the first
approach, selectivity is achieved through recognition
of the analyte at the receptor site, and in the second,
selectivity is achieved through the transduction
process in which the method of detection dictates
which species are sensed. Such approaches are appropriate
when a specific target compound is to be
identified in the presence of controlled backgrounds
and interferences. However, this type of approach
requires the synthesis of a separate, highly selective
sensor for each analyte to be detected. In addition,
this type of approach is not particularly useful for
analyzing, classifying, or assigning human value
judgments to the composition of complex vapor
mixtures such as perfumes, beers, foods, mixtures of
solvents, etc.

/pdf/cross-reactive-chemical-sensor-arrays-38dz1buu0b.pdf

Cross-reactive chemical sensor arrays.

Bulk silicon etching techniques, used to selectively remove silicon from substrates, have been broadly applied in the fabrication of micromachined sensors, actuators, and structures. Despite the more recent emergence of higher resolution, surface-micromachining approaches, the majority of currently shipping silicon sensors are made using bulk etching. Particularly in light of newly introduced dry etching methods compatible with complementary metal-oxide-semiconductors, it is unlikely that bulk micromachining will decrease in popularity in the near future. The available etching methods fall into three categories in terms of the state of the etchant: wet, vapor, and plasma. For each category, the available processes are reviewed and compared in terms of etch results, cost, complexity, process compatibility, and a number of other factors. In addition, several example micromachined structures are presented.

/pdf/bulk-micromachining-of-silicon-57ln84jsy8.pdf

Bulk micromachining of silicon

If you've not been involved in MEMS (MicroElectroMechanical Systems) technology or had the cause to use MEMS devices, then you may wonder what all the fuss is about. What are MEMS anyway? What's the difference between MEMS and MST (MicroSystems Technology)? What are the advantages over existing technologies? If you have ever found yourself pondering over such questions, then this book may be for you. As the title suggests, the main aim is to provide an introduction to MEMS by describing the processes and materials available and by using examples of commercially available devices. The intended readership are those technical managers, engineers, scientists and graduate students who are keen to learn about MEMS but have little or no experience of the technology. I was particularly pleased to note that Maluf has dedicated a whole chapter to the important (and often difficult) area of packaging. The first three chapters provide a general overview of the technology. Within the first three pages we are introduced to the MEMS versus MST question, only to discover that the difference depends on where you live! The United States prefer MEMS, while the Europeans use the handle MST. (Note to self: tell colleagues in MEMS group at Southampton). A good account is given of the basic materials used in the technology, including silicon, silicon oxide/nitride/carbide, metals, polymers, quartz and gallium arsenide. The various processes involved in the creation of MEMS devices are also described. A good treatment is given to etching and bonding in addition to the various deposition techniques. It was interesting to note that the author doesn't make a big issue of the differences between bulk and surface micromachined devices; the approach seems to be `here's your toolbag - get on with it'. One of the great strengths of this book is the coverage of commercial MEMS structures. Arising as they have, from essentially a planar technology, MEMS devices are often elaborate three-dimensional creations, and 2D drawings don't do them much justice. I have to say that I was extremely impressed with the many aesthetic isometric views of some of these wonderful structures. Pressure sensors, inkjet print nozzles, mass flow sensors, accelerometers, valves and micromirrors are all given sufficient treatment to describe the fundamental behaviour and design philosophy, but without the mathematical rigour expected for a traditional journal paper. Chapter 5 addresses the promise of the technology as a means of enabling a new range of applications. The concept of using MEMS devices as key elements within complex systems (or even microsystems!) is explored. The so-called `lab-on-a-chip' approach is described, whereby complex analytical systems are integrated onto a single chip together with the associated micropumps and microvalves. The design and fabrication of MEMS devices are important issues by themselves. A key area, often overlooked, is that of packaging. Painstaking modelling and intricate fabrication methodologies can produce resonator structures oscillating at precisely, say, 125 kHz. The device is then mounted in a dual-in-line carrier and the frequency shifts by 10 kHz because of the additional internal stresses produced. Packaging issues can't be decoupled from those of the micromachined components. Many of these issues, such as protective coatings, thermal management, calibration etc, are covered briefly in the final chapter. Overall, I found this book informative and interesting. It has a broad appeal and gives a good insight into this fascinating and exciting subject area. Neil White

An Introduction to Microelectromechanical Systems Engineering

This paper describes a prototype of an integrated fluorescence detection system that uses a microavalanche photodiode (μAPD) as the photodetector for microfluidic devices fabricated in poly(dimethylsiloxane) (PDMS). The prototype device consisted of a reusable detection system and a disposable microfluidic system that was fabricated using rapid prototyping. The first step of the procedure was the fabrication of microfluidic channels in PDMS and the encapsulation of a multimode optical fiber (100-μm core diameter) in the PDMS; the tip of the fiber was placed next to the side wall of one of the channels. The optical fiber was used to couple light into the microchannel for the excitation of fluorescent analytes. The photodetector, a prototype solid-state μAPD array, was embedded in a thick slab (1 cm) of PDMS. A thin (80 μm) colored polycarbonate filter was placed on the top of the embedded μAPD to absorb scattered excitation light before it reached the detector. The μAPD was placed below the microchannel an...

An Integrated Fluorescence Detection System in Poly(dimethylsiloxane) for Microfluidic Applications

This paper presents a low-dropout regulator (LDO) for portable applications with an impedance-attenuated buffer for driving the pass device. Dynamically-biased shunt feedback is proposed in the buffer to lower its output resistance such that the pole at the gate of the pass device is pushed to high frequencies without dissipating large quiescent current. By employing the current-buffer compensation, only a single pole is realized within the regulation loop unity-gain bandwidth and over 65deg phase margin is achieved under the full range of the load current in the LDO. The LDO thus achieves stability without using any low-frequency zero. The maximum output-voltage variation can be minimized during load transients even if a small output capacitor is used. The LDO with the proposed impedance-attenuated buffer has been implemented in a 0.35-mum twin-well CMOS process. The proposed LDO dissipates 20-muA quiescent current at no-load condition and is able to deliver up to 200-mA load current. With a 1-muF output capacitor, the maximum transient output-voltage variation is within 3% of the output voltage with load step changes of 200 mA/100 ns.

/pdf/a-transient-enhanced-low-quiescent-current-low-dropout-119enznkif.pdf

A Transient-Enhanced Low-Quiescent Current Low-Dropout Regulator With Buffer Impedance Attenuation

A low power, hand-held system has been developed for the measurement of heavy metal ions in aqueous solutions. The system consists of an electrode array sensor, a high performance single chip potentiostat and a microcontroller circuit. The sensor is a microfabricated array of iridium electrodes, onto which a thin film of mercury is electroplated. Quantitative heavy metal analysis is performed using square-wave anodic stripping voltammetry. Measured results show a one part-per-billion sensitivity and multiple use capability.

Microfabricated electrochemical analysis system for heavy metal detection

This paper describes a two-stage CMOS amplifier that is stable for any capacitive load. This is achieved through the use of an optimized cascoded compensation topology. A new level shifting technique allows independent optimization of drive capability, noise and systematic offset voltage. The circuit is 0.1 mm/sup 2/ in a 2 /spl mu/m technology and has a quiescent current consumption of 110 /spl mu/A. >

An unconditionally stable two-stage CMOS amplifier

This paper details a post-processing technique by which circuitry in an unmodified IC technology is thermally and electrically isolated from the silicon substrate. This method enables new applications for micromachining, including temperature regulation of analog ICs, to be carried out. The process is discussed in detail, along with improved tetramethyl ammonium hydroxide (TMAH) etching chemistries that use strong oxidizers to eliminate hillock formation. Also presented is an electrochemical biasing method that uses circuitry on the silicon being etched during the micromachining step. Work is currently underway to evaluate the use of these micromachining techniques for commercial analog circuit applications, several of which are discussed.

Micromachined thermally isolated circuits

Temperature-sensitive transducers and other circuitry are manufactured by an electrochemical post-processing etch on an integrated circuit fabricated using a conventional CMOS process. Tetramethyl ammonium hydroxide or another anisotropic etchant having similar characterisics is used to selectively etch exposed front-side regions of a p-type silicon substrate, leaving n-type wells suspended from oxide beams. Circuits in the n-wells are thermally and electrically insulated from the substrate.

Suspended single crystal silicon structures and method of making same

Two major limiting factors in mass production and field use of electroanalytical instruments have been the size and cost of potentiostats. This monolithic CMOS potentiostat has performance comparable to bench-top instruments at a fraction of the size, power consumption and cost. A block diagram of the potentiostat chip is shown. An integrating DAC sets the voltage between the working electrode, where the electrochemical reactions of interest take place, and a chemically stable reference electrode. The control amplifier regulates this voltage using feedback to drive a third (counter) electrode. The working electrode current is measured with a current-input dual-slope ADC. >

R.J. Reay

Papers

Microfabricated electrochemical analysis system for heavy metal detection

An unconditionally stable two-stage CMOS amplifier

Micromachined thermally isolated circuits

Suspended single crystal silicon structures and method of making same

An integrated CMOS potentiostat for miniaturized electroanalytical instrumentation