R.L. Smith

A physical model for drift in pH ISFETs

A physical model is presented which quantitatively describes the threshold voltage instability, commonly known as drift, in n-channel Si3N4-gate and as well as Al2O3-gate pH ISFETs. The origin of the so-called drift is postulated to be associated with the relatively slow chemical modification of the gate insulator surface as a result of exposure to the electrolyte. The chemical modification of the surface is assumed to result from a transport-limited reaction whose rate is modeled by a hopping and/or trap-limited transport mechanism known as dispersive transport. The change in the chemical composition of the insulator surface leads to a decrease in the overall insulator capacitance with time, which gives rise to a monotonic temporal increase in the threshold voltage.

A dry electrode for EEG recording

This paper describes the design, fabrication and testing of a prototype dry surface electrode for EEG signal recording. The new dry electrode has the advantages of no need for skin preparation or conductive paste, potential for reduced sensitivity to motion artifacts and an enhanced signal-to-noise ratio. The electrodes sensing element is a 3 mm stainless steel disk which has a 2000 A (200 nm) thick nitride coating deposited onto one side. The back side of the disk is attached to an impedance converting amplifier. The prototype electrode was mounted on a copper plate attached to the scalp by a Velcro strap. The performance of this prototype dry electrode was compared to commercially available wet electrodes in 3 areas of electroencephalogram (EEG) recording: (1) spontaneous EEG, (2) sensory evoked potentials, and (3) cognitive evoked potentials. In addition to the raw EEG, the power spectra of the signals from both types of electrodes were also recorded. The results suggest that the dry electrode performs comparably to conventional electrodes for all types of EEG signal analysis. This new electrode may be useful for the production of high resolution surface maps of brain activity where a large number of electrodes or prolonged recording times are required.

Preferential propagation of pores during the formation of porous silicon: a transmission electron microscopy study

A transmission electron microscopy study of porous silicon reveals that pores selectively propagate in the 〈100〉 crystallographic directions on both n‐ and p‐type silicon, independent of dopant concentration or anodization conditions.

Removable tubing interconnects for glass-based micro-fluidic systems made using ECDM

Reversible tubing connections for glass micro-fluidic systems are realized using electro-chemical discharge machining of three-dimensional glass vias. The connections reversibly connect standard sized plastic tubing to holes in borosilicate microscope slides. Tubing connections are demonstrated on a sealed, micro-fluidic channel which is fabricated between two glass slides using SU-8. The connections are experimentally tested to withstand up to 30 psi (~206 kPa) of air pressure without leaking.

Vaporizing liquid microthruster

MEMS technology is expanding into increasingly diverse applications. As part of a micropropulsion system, microthruster attitude controls have been micromachined in silicon. This paper presents the microthruster design, fabrication, and test results. Fluid injected into a microchamber is vaporized by resistive silicon heaters. The exiting vapor generates the thruster force as it exits a silicon micro-nozzle. The vaporization chamber, inlet and exit nozzles were fabricated using anisotropic wet etching of silicon. With a 5 W heater input, injected water could be vaporized for input flow rates up to a maximum of 0.09 cc/s. Experimental testing produced thruster force magnitudes ranging from 0.15 mN to a maximum force output of 0.46 mN depending on fabrication parameters: chamber length, nozzle geometries, heater power, and liquid flow rates.

A micromachined pressure sensor with fiber-optic interferometric readout

Abstract A high sensitivity, batch fabricated, fiber-optic pressure sensor has been fabricated using silicon micromachining technology. The transducer consists of a fiber positioning v-groove, a 45° stationary mirror and a silicon membrane, micromachined in silicon by anisotropic etching in KOH solution, and a single mode optical fiber. The membrane and optical fiber end form a Fabry-Perot cavity whose length varies with pressure. The generated optical interference fringes are used to detect and measure the change in membrane deflection. Pressure range of operation is dictated by the thickness, size and material of the membrane. The sensor described here was designed for low pressure range (0–25 mm Hg) applications. Temperature sensitivity and stability problems which are commonly encountered with currently available piezoresistive and capacitive pressure sensors are significantly reduced by the inherent differential nature of interferometric measurement and the use of all silicon construction. The fabrication, packaging and testing of the sensor are described in this paper. The performance of the sensor was evaluated and found to compare favorably with theoretical predictions.

Porous silicon microstructure as studied by transmission electron microscopy

Cross‐sectional and plan‐view transmission electron micrograph analysis of the structure of porous silicon reveals that the pore walls are also porous, yielding a large distribution of pore sizes for a given porous silicon sample. This infrastructure appears to be a universal morphological feature of porous silicon, independent of formation conditions and doping. It is proposed that the observed microstructure explains the recently reported results of adsorption isotherm experiments.

A microfabricated, electrochemiluminescence cell for the detection of amplified DNA

Abstract An electrochemical cell for the generation and detection of electrochemiluminescence of tris (2,2′-bipyridyl) ruthenium(II) has been microfabricated in silicon. The cell has been designed for use in the detection and quantification of DNA which has been amplified by the polymerase chain reaction. The cell is a vertical assembly of micromachined silicon and glass substrates, containing a gold, thin film cathode and an indium tin oxide, thin film anode. The anode is transparent, enabling the detection of the luminescence by an external photodetector. Preliminary tests have yielded the detection of free TBR concentrations from 10−9 M to 10−3 M with a cell volume of 85 μl.

DNA quantification with an electrochemiluminescence microcell

Abstract The detection and quantification of DNA strands using an electrochemiluminescence (ECL) instrument with a microfabricated excitation/detection cell is presented. The microcell is a vertical assembly of silicon and glass substrates, containing thin film Pt electrodes and micromachined fluid channels. A silicon PIN diode photodetector completes the cell assembly. The DNA strands are labeled with an electrochemically exited, luminescing molecule: tris (2,2′ bipyridyl) ruthenium (TBR). The labeled DNA strands are bound to paramagnetic beads with a biotin–streptavidin linkage. The magnetic beads are attracted to the Pt electrode surface with an external magnet, where ECL is excited. With a 1 cm 2 , Pt anode, a detection limit of 40 fmol of DNA is obtained with a silicon PIN diode photodetector operating at room temperature. The overall size of the microcell is only 1.5 cm×2 cm×0.2 cm and supports a sample volume of 150 μl. This is the first demonstration of DNA quantification using an ECL microinstrument.

MicroJoinery: concept, definition, and application to microsystem development

Abstract A new hybrid technology for fabricating, interconnecting, and assembling microdevices is introduced. This new technology is predicated on the creation of dimensionally scaled manufacturing and assembly techniques and has been coined ‘MicroJoinery’ as a descriptive designation. MicroJoinery, along with its collected ‘toolbox’ of associated microfabrication techniques, allows for the facile realization of a widely divergent collection of microsystem components including: fluidic microvalves, microfluidic interconnects, microreaction chambers, xyz positioning microstages, fiber-optic switches, etc. Additionally, MicroJoinery provides a new paradigm for design and practical assembly of microsystems. This paper explores the applications of MicroJoinery by presenting several examples of microdevices and microsystems tractable through MicroJoinery.