Technology / Sensors / MOS

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1 Technology / Sensors / MOS The metal oxide semiconductor (MOS) sensors developed by AppliedSensor use tin oxide based sensing thick films deposited onto two different types of substrates: alumina substrates (thick-film sensors) and Si-micromachined substrates (micro sensors). Both substrates are provided with electrodes that enable measurement of the resistance of the sensing layer, and heaters that allow for the heating of the sensing layer at a temperature in the range 200 to 400 C. Photographs of MOS sensors: left a micro sensor, right a thick-film sensor The sensor response to changes in the composition of the ambient atmosphere is a change in the resistance of the sensing layer. A large number of e.g. toxic and explosive gases can be detected, even at very low concentrations. CHEMICAL PRINCIPLE The sensing layer is a porous thick film of polycrystalline SnO 2. In normal ambient air, oxygen and water vapor related species are adsorbed at the surface of the SnO 2 grains. The sensing of target gases takes place as follows. For reducing gases such as CO or H 2, a reaction takes place with the preadsorbed oxygen and water vapor related species, which decreases the resistance of the sensor. For oxidizing gases such as NO 2 and O 3, the resistance increases. The magnitude of the changes depends on the microstructure and composition/doping of the base material, on the morphology and geometrical characteristics of the sensing layer and of the substrate, as well as on the temperature at which the sensing takes place. Alterations in any of these parameters allow for the tuning of the sensitivity towards different gases or classes of gases.

2 chemisorption, e.g. NO 2 O 2 NO 2 CO 2 CO heterogeneous catalysis: (e.g. CO oxidation) electrodes e - e - grain boundaries substrate U I Sketch of a MOS sensor illustrating the detection principle. The resistance of the sensing layer changes when molecules react on the surface. SnO 2 Si 3 N 4 membrane 0.5 m sensitive layer electrodes Al SnO 2 Al 50 m 5 m 0.7 mm 5 m 3.6 mm silicon wafer heater Sketches of a micro sensor (left) and a thick-film sensor (right) TRANSDUCER PRINCIPLE The changes in composition of the ambient atmosphere will determine changes in resistance of the sensing layers. In practice, the relationship between sensor resistance and concentration of the target gas usually follows a power law. Over a large range of concentrations, it can be described by: R K*c ±n where c is the concentration of the target gas, K is a measurement constant, and n has values between 0.3 and 0.8 and the positive sign is to be used for oxidizing gases, the negative sign for reducing gases. The following figure shows a simple electrical circuit, which can be used for measurement of the sensor resistance R S. The heating voltage V H is applied between and 3; typical values for both types of sensor are between 2 and 5 V.

3 The measuring voltage V S is applied between 2 and 4; it is recommended that the value should not exceed 5 V. For determination of the R S, V out is measured and R L is known. The relationship between R S and V out is: VS R = S RL Vout R S Vs V H R L V out Electronic circuitry for operation of a MOS sensor with heater TYPICAL RESPONSE CURVES The figure below shows a typical behavior for a thick-film MOS sensor when exposed to a series of CO pulses. The sensor resistance drops very quickly immediately after CO exposure, and after removal of CO from the ambient atmosphere, the sensor resistance will recover to its original value after a short time. The speed of response and recovery will vary according to the operation temperature, the type of sensing layer, and the gases involved.

4 resistance (Ω) CO concentration (ppm) CO (ppm) ppm 50ppb AppliedSensor TFS sensor 50ppm time (hours) Response of a thick-film MOS sensor to CO Sensor signal CO concentration (ppm) CO concentration versus sensor signal for a thick-film MOS sensor.

5 IMPORTANT FEATURES Gases detectable by using AppliedSensor MOS sensors include alcohols, ammonia, butane, carbon monoxide, chlorine, ethylene, heptane, hexane, hydrogen, hydrogen sulfide, LPGs, methane, nitrogen dioxide, ozone, propane, sulfur dioxide, toluene, etc. The operation temperature of the MOS sensors is in the range 200 to 400 C. The MOS sensors show high sensitivity, good stability, long lifetime, and short response/recovery times. The sensors can be operated in a wide ambient temperature range (-40 C to +70 C) and humidity range (0 to 00%RH without condensation), and do not require maintenance. The micro sensors have a small size and low power consumption, making them suitable for applications in hand-held devices. MINIATURIZATION POSSIBILITIES The micro sensors are fabricated in a CMOS and micro-machining compatible process. A gas sensor and a heating element have been integrated on a thermally isolated membrane, resulting in low power consumption and a low thermal time constant. APPLICATION IDEAS The application range for MOS sensors covers various fields such as: automotive packaging and food control medical security process control LINKS TO RESEARCH ACTIVITIES