A fully integrated gas sensor microsystem is presented, which comprises for the first time a micro hot plate as well as advanced analog and digital circuitry on a single chip. The micro hot plate is coated with a nanocrystalline SnO2 thick film. The sensor chip is produced in an industrial 0.8-microm CMOS process with subsequent micromachining steps. A novel circular micro hot plate, which is 500 x 500 microm(2) in size, features an excellent temperature homogeneity of +/-2% over the heated area (300-microm diameter) and a high thermal efficiency of 6.0 degrees C/mW. A robust prototype package was developed, which relies on standard microelectronic packaging methods. Apart from a microcontroller board for managing chip communication and providing power supply and reference signals, no additional measurement equipment is needed. The on-chip digital temperature controller can accurately adjust the membrane temperature between 170 and 300 degrees C with an error of +/-2 degrees C. The on-chip logarithmic converter covers a wide measurement range between 1 kOmega and 10 MOmega. CO concentrations in the sub-parts-per-million range are detectable, and a resolution of +/-0.1 ppm CO was achieved, which renders the sensor capable of measuring CO concentrations at threshold levels.

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This paper presents two mixed-signal monolithic gas sensor microsystems fabricated in standard 0.8-m CMOS technology combined with post-CMOS micromachining to form the microhotplates. The on-chip microhotplates provide very high temperatures (between 200 C and 400 C), which are necessary for the normal operation of metal-oxide sensing layers. The first microsystem has a single-ended architecture comprising a microhotplate (diameter of 300 m) and a digital proportional-integral-derivative (PID) microhotplate temperature controller. The second microsystem has a fully-differential architecture comprising an array of three microhotplates (diameter of 100 m) and three digital PID microhotplate temperature controllers (one controller per microhotplate). The on-chip digital PID temperature controllers can accurately adjust the microhotplate temperatures up to 400 C with a resolution of 2 C. Further, both microsystems feature on-chip logarithmic converters for the readout of the metal-oxide resistors (which cover a measurement range between 1 k and 10 M ), 10-bit A/D converters, anti-aliasing filters, 10-bit D/A converters, 2 serial interfaces, and bulk-chip temperature sensors. Carbon monoxide (CO) concentrations in the sub-parts-per-million (ppm) range are detectable, and a resolution of 0.2 ppm CO has been achieved.Index Terms-CMOS-based microsystem, MEMS, metal-oxide gas sensors, PID control.

This paper presents a new system to measure the Intraocular Pressure (IOP) with very high accuracy (0.036 mbar) used for monitoring glaucoma. The system not only monitors the daily variation of the IOP (circadian IOP), but also allows to perform an spectral analysis of the pressure signal generated by the heartbeat (cardiac IOP). The system comprises a piezoresistive pressure sensor, an application-specific integrated circuit (ASIC) to read out the sensor data and an external reader installed on customized glasses. The ASIC readout electronics combines chopping modulation with correlated double sampling (CDS) in order to eliminate both the amplifier offset and the chopper ripple at the sampling frequency. In addition, programmable current sources are used to compensate for the atmospheric pressure ( 800-1200 mbar ) and the circadian component (± 7 mbar) thus allowing to read out the very weak cardiac signals (± 1.6 mbar) with a maximum accuracy of 0.036 mbar.

A modeling approach has been developed to support CMOS microhotplate optimization and to allow for sensor system simulations. All steps are detailed that are necessary to arrive at a geometric hotplate representation for nonlinear 3D-FEM simulations starting from a physical microhotplate layout. A lumped-model description of the microhotplate is discussed, which forms the basis for combined simulations of sensor and circuitry. FEM simulations were performed for two different microhotplate designs. Both types of microhotplates were fabricated in a 0.8 µm industrial CMOS process by post-CMOS micromachining. The first design includes a circular microhotplate with a Si-island underneath the heated area of the microhotplate. The characteristic figures such as thermal resistance and thermal time constant were extracted from the model, which showed an agreement within 5–10% with experimental values for temperatures up to 325 °C. The second design without Si-island featured an array of temperature sensors for assessment of the temperature distribution. Experimental data from the different sensor locations were compared to simulation results, and, again, showed excellent agreement with a maximum deviation of 5%. The influence of the nanocrystalline tin oxide thick-film layer on the temperature distribution was also experimentally investigated: better temperature homogeneity in the heated area and somewhat slower temperature dynamics.

This paper presents a monolithic chemical gas sensor system fabricated in industrial CMOS-technology combined with post-CMOS micromachining. The system comprises metal-oxide-covered (SnO 2) micro-hotplates and the necessary driving and signal-conditioning circuitry. The SnO 2 sensitive layer is operated at temperatures between 200 and 350 • C. The on-chip temperature controller regulates the temperature of the membrane up to 350 • C with a resolution of 0.5 • C. A special heater-design was developed in order to achieve membrane temperatures up to 350 • C with 5 V supply voltage. The heater design also ensures a homogeneous temperature distribution over the heated area of the hotplate (1-2% maximum temperature fluctuation). Temperature sensors, on-and off-membrane (near the circuitry), show an excellent thermal isolation between the heated membrane area and the circuitry-area on the bulk chip (chip temperature rises by max 6 • C at 350 • C membrane temperature). A logarithmic converter was included to measuring the SnO 2 resistance variation upon gas exposure over a range of four orders of magnitude. An Analog Hardware Description Language (AHDL) model of the membrane was developed to enable the simulations of the complete microsystem. Gas tests evidenced a detection limit below 1 ppm for carbon monoxide and below 100 ppm for methane.