5 - MEMS devices

Combustion type gas sensors detect the concentration and type of desired gases by the measurement of the heat produced by the oxidation reaction. The resistance of a heater-filament changes with temperature. The micro-heater resistor can reach surface temperature in excess of 800K. This offers the possibility of measuring flammable gases without the risk of explosion even above the low-explosion-limit (LEL).

SGFET (Suspended Gate Field Effect Transistor) uses the change of electron work function of a chemically sensitive polymer layer in response to interaction with a gas or vapour. The changes are detected by the threshold voltage shift or drain current change of the FET.

The cross section of a micromachined NO sensor structure is shown here. The gas reaches the working electrode from the back side through a porous substrate rather than diffusing through the polymer electrolyte. The pores were made by reactive ion etching. The electrodes were deposited by sputtering thin metallization films. The polymer electrolyte was applied in gel form by mechanical spreading.

The structure of a micromachined Clark-type oxygen sensor is shown below. A glass substrate with a silver working electrode, gold counter-electrode, and Ag/AgCl reference electrode is bonded to a silicon substrate. The Si substrate has anisotropically etched grooves to provide small cavities to accommodate an electrolyte solution. The electrolyte is incorporated by dipping the whole sensor in the electrolyte solution in a centrifuge tube, placing it in a chamber and evacuating.

In Lamb-wave (Flextural Plate Wave) device, waves propagate in the bulk of plates whose thickness is small compared with the ultrasonic wavelength: the thickness/wavelength ratio is less than one. Particle motions of Lamb waves are similar to those of Rayleigh waves; however, in a thin plate, the waves give rise to a series of symmetric and antisymmetric plate modes. The lowest order antisymmetric modes have a unique flexural character, hence, the name "flexural plate wave" or FPW. The core of the device is an ultrasonic delay line consisting of a composite plate of a low stress insulation, metallization, and piezo-electric film. IDTs on the piezoelectric layer launch and receive the waves and, together with the amplifier, form a feedback oscillator whose output frequency depends on the mass per unit area of the membrane, including the chemically sensitive film. The advantages are in the low-MHz operation frequency range, and the possibility of operation while immersed in a liquid.

The light-addressable potentiometric sensor (LAPS) is a semiconductor-based chemical sensor with an electrolyte-insulator-semiconductor (EIS) structure. By depositing ion-selective materials on the sensing surface, it can be used as an ion-selective sensor. A dc bias is applied across the EIS structure so that a depletion layer appears at the insulator-semiconductor interface. The width and the capacitance of the depletion layer change with the surface potential, which is a function of the ion concentration in the electrolytic solution. To read out the capacitance, the semiconductor substrate is illuminated with a modulated light and the generated ac photocurrent is measured in the external circuit. The "light-addressability" of the LAPS makes it possible to obtain a spatially resolved map of the distribution of the ion concentration in the specimen.