Alan Mathewson

Application of magnetohydrodynamic actuation to continuous flow chemistryElectronic supplementary information (ESI) available: figures depicting a silicon MHD microreactor, finite element solution for velocity profile in the silicon microreactor annulus, and the effect of MHD actuation conditions on the PCR product previously generated by conventional amplification methods and on the PCR reagents prior to thermocycling by conventional methods. See http://www.rsc.org/suppdata/lc/b2/b206756k/

Continuous flow microreactors with an annular microchannel for cyclical chemical reactions were fabricated by either bulk micromachining in silicon or by rapid prototyping using EPON SU-8. Fluid propulsion in these unusual microchannels was achieved using AC magnetohydrodynamic (MHD) actuation. This integrated micropumping mechanism obviates the use of moving parts by acting locally on the electrolyte, exploiting its inherent conductive nature. Both silicon and SU-8 microreactors were capable of MHD actuation, attaining fluid velocities of the order of 300 microm s(-1) when using a 500 mM KCl electrolyte. The polymerase chain reaction (PCR), a thermocycling process, was chosen as an illustrative example of a cyclical chemistry. Accordingly, temperature zones were provided to enable a thermal cycle during each revolution. With this approach, fluid velocity determines cycle duration. Here, we report device fabrication and performance, a model to accurately describe fluid circulation by MHD actuation, and compatibility issues relating to this approach to chemistry.

Optimizing MOS transistor mismatch

An investigation of MOS transistor mismatch is undertaken and a methodology is developed for optimizing mismatch without increasing layout area. Dramatic improvements of up to 300% in matching can be realized by selecting the optimum W/L ratio without changing the overall WL area product. The theoretical basis for the obtainable improvements is fully described and an expression is derived and verified by experiment to predict the W/L ratio which gives optimum matching.

Dielectric Reliability Measurement Methods: A Review

Reliability of thin dielectric films such as silicon dioxide grown on single crystalline silicon is of great importance for integrated circuits of present and future technologies. For the characterization of the quality of dielectric films, it is essential to have measurement methods available which can give a measure of dielectric reliability in a relatively short time. Stress biases are usually highly accelerated and cause destructive dielectric breakdown. Testing for dielectric reliability has been performed for more than 30 years, and in that time many different stress methods have been established. This article reviews that most common dielectric reliability measurement methods and gives practical guidelines to the reliability engineer in the field of dielectric characterization. The examples and data shown here are mainly from MOS gate oxides. The aim of this review paper is to emphasize advantages and disadvantages of the various stress methods. Appropriate dielectric stress methods are pointed out for applications such as process development, process characterization, pocess control and screening (burn-in). A broad number of different measurement techniques are described in detail for which the set up of the measurement and its stress parameters are clarified. Suitable dielectric test structures and the determination of the correct voltage and thickness of the dielectric are discussed; they are essential to determine the electric field across the thin film. The identification of dielectric breakdown and the interpretation and significance of the measurement results are reviewed. A good understanding of the stress method and the various measured parameters is essential to draw correct conclusions for the lifetime of the dielectric at operating conditions. The commonly used, basic analysis techniques for the measurement results are illustrated. Finally, the influence of stress-induced leakage currents on the dielectric reliability characterization is discussed and other aspects relating to very thin oxides of future technologies are briefly described. The paper also includes a large bibliography of more than 250 references.

Characterization of micromechanical structures using white-light interferometry

As microelectromechanical systems (MEMS) move rapidly towards commercialization, the issue of mechanical characterization has emerged as a major consideration in device design and fabrication. It is now common to include a set of test structures on a MEMS wafer for extraction of thin film material properties (in particular, residual stress, stress gradient and Youngs modulus), and for process and device monitoring. These structures usually consist of micromachined beams and strain gauges. Measurement techniques include tensile testing, scanning electron microscopy (SEM) imaging, atomic force microscopy (AFM) analysis, surface profiling and Raman spectroscopy. However, these tests are often destructive and may be difficult to carry out at the wafer scale. Instead of these methods, this paper uses white-light interferometry surface profiling for material characterization and device inspection. Interferometry is quick, non-destructive, non-contact, and can offer a high density lateral resolution with extremely high sensitivities to the surface in the z-direction—all essential requirements for high volume manufacturing. A range of devices is employed to illustrate the capabilities of white-light interferometry as a measurement and process characterization tool.It is shown that residual stress may be determined by using electrostatic actuation to pull fixed–fixed beams towards the substrate, and interferometry to record the beam deflection profile. Finite-element simulation software is employed to model this deflection, and to estimate the material properties which minimize the difference between the measured and simulated profiles. The results agree well with blanket film measurements.

Silver nanowire array-polymer composite as thermal interface material

Silver nanowire arrays embedded inside polycarbonate templates are investigated as a viable thermal interface material for electronic cooling applications. The composite shows an average thermal diffusivity value of 1.89×10−5 m2 s−1, which resulted in an intrinsic thermal conductivity of 30.3 W m−1 K−1. The nanowires’ protrusion from the film surface enables it to conform to the surface roughness to make a better thermal contact. This resulted in a 61% reduction in thermal impedance when compared with blank polymer. An ∼30 nm Au film on the top of the composite was found to act as a heat spreader, reducing the thermal impedance further by 35%. A contact impedance model was employed to compare the contact impedance of aligned silver nanowire-polymer composites with that of aligned carbon nanotubes, which showed that the Young’s modulus of the composite is the defining factor in the overall thermal impedance of these composites.

Analysis of electromechanical boundary effects on the pull-in of micromachined fixed-fixed beams

Using a commercial finite-element simulation tool, this work considers some of the electromechanical effects commonly neglected during the analysis of electrostatically actuated fixed–fixed beams. These structures are used in many applications of micromechanical systems, from relay switches and RF resonators to thin film characterization tests, but much of the analytical modelling of the device behaviour disregards the effects of electrostatic field fringing, plane-strain conditions and anchor compliance. It is shown that the cumulative total of these errors can be substantial, and may lead to large discrepancies in the expected operational characteristics of the device. We quantify the influence of these effects on the electrostatic pull-in of fixed–fixed beams, and illustrate some of the limitations of ideal pull-in theory. In order to more accurately predict the pull-in voltage for a real structure, a model is developed that combines ideal case theory with anchor compliance correction factors extracted using finite-element analysis. Three common anchor types (ideal, step-up and cup-style) are characterized. The final model takes account of the compliance of the beam anchors, electrostatic field fringing and plane-strain effects, and agrees well with simulated results.

Toward integrated single-photon-counting microarrays

Silicon, shallow junction, Geiger-mode avalanche photo- diodes (APDs) can be manufactured with complementary metal-oxide semiconductor (CMOS) compatible processing steps and provide single- photon-counting sensitivity. As we move toward providing integrated de- tection of increasingly nanoscopic-sized emissions, small-area detectors and arrays that can be easily integrated into marketable systems will be required. Geiger-mode diodes with diameters of 10, 15, and 20 mm are manufactured and the dark counts measured at 10 V above breakdown are 9, 95, and 990, respectively, at room temperature. The simulated and measured optical crosstalk is found to be significantly reduced for detec- tor pixel pitches beyond 300 mm. The activation energy of the dark count with temperature is found to be 0.58 eV, representing an order of mag- nitude drop in dark count for every 27°C decrease in temperature. The responsivity of the detectors, without antireflection coatings, is found to peak between 550 and 650 nm with a photon detection probability of 43% at 10 V above the breakdown voltage. The low dark counts of the detectors and high photon detection probability highlight the potential these detectors have for fluorescence decay experiments and also in future integrated photonic detection systems.

Modelling of lateral bipolar devices in a CMOS process

In spite of the emergence of CMOS technology, the well-controlled characteristics of bipolar transistors retain many advantages over those of CMOS transistors for some critical analog applications. This is the reason why special technologies have been proposed to combine both types of transistors on the same chip. An inexpensive and widely applicable approach lies in using bipolars that are realisable with existing CMOS technologies. Bipolar transistors occur as parasitic devices in CMOS and it is not necessary to use additional processing steps in their manufacture. These bipolar transistors, therefore, provide cost effective devices which are relatively simple to fabricate. The extraction of a DC parameter set for the lateral device is more complicated than for a vertical device because of the presence of two parasitic vertical bipolar transistors which are formed by the emitter/collector, the base and the substrate regions. This paper proposes a method which involves the use of subcircuits incorporating three SPICE Gummel-Poon models. The development of this model, its implementation and the results obtained are outlined and discussed.

Influence of aluminum nitride crystal orientation on MEMS energy harvesting device performance

Aluminum nitride (AlN) is a widely researched piezoelectric material due to its CMOS compatibility. One of the most common applications for AlN is in the area of vibrational energy harvesting. The piezoelectric quality of AlN is related to the crystal orientation of the film and optimal conditions are obtained when AlN is c-axis aligned with a (0 0 2) orientation. AlN can be a challenging material to integrate into a fabrication process due to orientation dependency of the fabrication process. This paper reports on the effects of non-(0 0 2) oriented AlN peaks on an energy harvesting MEMS cantilever structure. Results show that FWHM values of the AlN films from different wafers were approximately the same 8.5°, 8.7°, and 9°, however wafer 1 had additional peaks at (1 0 2) and (1 0 3), which significantly affected the piezoelectric constants and the amount of power generated. The measured d31 value for the wafers were 2.04, 1.97, and 0.84 pm V−1, and the power generated was 0.67, 0.64, and 0.24 µW respectively. These values show that non-peaks of AlN can cause a significant decrease in the piezoelectric constant, which causes significant decrease in the ability to generate power from an AlN film.

A novel silicon Geiger-mode avalanche photodiode

Dark count nonlinearity in CMOS compatible, single photon counting, Geiger-mode avalanche photodiodes (GM-APD) has been investigated. A novel structure was designed, fabricated, and characterized to allow dark count optimization. Dark count levels for the proposed structure are shown to scale linearly with area.