Micromachined metal-oxide gas sensors

In the main, two types of metal-oxide gas sensitive films have been integrated into micromachined hotplate transducers thin and thick films. The different developments will be presented in this section. The integration of a third type of structure - nanowires, into which considerable efforts are 

In this section, for better readability and to allow comparison between results, all responses are given as if R or as R JR if R,  

Other techniques have been used for the fabrication of thin-film metal-oxide gas sensors. At NIST in the USA, Cavicchi et al. (1995) and Semancik et al. (2001) produced gas sensors by chemical vapor deposition (CVD). By applying a current and thus heating the hotplate, sensing films could be deposited locally (i.e. only on heated active areas) using an adequate organ-ometaUic precursor. SnOj and ZnO films were obtained with tetramethyltin and diethylzinc in an oxygen atmosphere. They were deposited onto different seed layers, which played a significant role in terms of gas selectivity. 

6 Gas measurement obtained from the sensor array. The sensing materials exhibited different behavior toward the analytes. From Wollenstein et al. (2003). 

7 (a) SEM image of a drop-coated metal-oxide gas sensor from AppliedSensor GmbH, (b) Three-dimensional schematic drawing of the sensor structure. From Blaschke et al (2006). 

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I. Simon, N. Barsan, M. Bauer, and U. Weimar. Micromachined metal oxide gas sensors opportunities to improve sensor performance . Sensors and Actuators B73 (2001), 1-26. 

Cross-sectional diagram of a micromachined metal-oxide gas sensor. 

Briand et al. (2007) reported on a higher level integration of wafer-level packaged micromachined metal-oxide gas sensors. The concept was based... 

Simon, I., Barsan,N., Bauer, M. and Weimar, U. (2001), Micromachined metal oxide gas sensors Opportunities to improve sensor performance . Sens. Actuators B, 73,1-26. 

The central topic of the book was the integration of microhotplate-based metal-oxide gas sensors with the associated circuitry to arrive at single-chip systems. Innovative microhotplate designs, dedicated post-CMOS micromachining steps, and novel system architectures have been developed to reach this goal. The book includes a multitude of building blocks for an application-specific sensor system design based on a modular approach. 

Lorenzelli, L., Benvenuto, A., Adami, A. et al. (2005) Development of a gas chromatography silicon-based microsystem in clinical diagnostics. Biosens Bioelectron, 20 (10), 1968-1976. Zampolli, S., Ehni, I., Stiirmann J. et al. (2005) Selectivity enhancement of metal oxide gas sensors using a micromachined gas chromatographic column. Sens Actual B, 105 (2), 400-406. Bessoth, F.G., Naji, O.P., Eijkel, J.C.T. and Manz, A. (2002) Towards an on-chip gas chromatograph the development of a gas injector and a dc plasma emission detector. J Anal Atom Spectrom, 17 (8), 794-799. 

Zampolli S., Elmi I., Sturmann J., Nicoletti S., Dori L., and Cardinal G. C., Selectivity enhancement of metal oxide gas sensors using a micromachined gas chromatographic column, Sens. Actuators B, 105(2), 400, 2005. 

A diagram of a typical cross-sectional view of a silicon micromachined metal-oxide (MOX) sensor is presented in Fig. 6.2. Their development has evolved towards silicon substrates to produce devices suitable for commercialization due to their low-cost, low-power consumption and high reliability. To lower the resistivity of the gas sensitive film, as well as to improve the kinetics of the chemical reactions, the metal-oxide layer is heated with a micro-heater. The heated area is usually embedded in a thin dielectric membrane to improve the thermal insulation and to reduce the power consumption of the device, which is typically in the order of a few tens of milliwatts at 300°C, and its thermal time constant (few to tens of milliseconds). Thermal programming allows kinetically controlled selectivity. 

Nanowires are seen as a solution with which to improve the sensitivity, selectivity, stability and response time of metal-oxide gas sensors. Meier et al (2007) grew Sn02 nanowires of 100 nm in diameter by the vapor-solid growth method. For testing, they were deposited onto micromachined hotplates with a focused ion beam scanning electron microscope (FIB-SEM), as shown in Fig. 6.19. Due to their diameter being similar to the Debye length, a completely depleted conduction channel can be obtained. Maximum response to CO and NH3 occurred at about 260°C. 

The third block in Fig. 2.1 shows the various possible sensing modes. The basic operation mode of a micromachined metal-oxide sensor is the measurement of the resistance or impedance [69] of the sensitive layer at constant temperature. A well-known problem of metal-oxide-based sensors is their lack of selectivity. Additional information on the interaction of analyte and sensitive layer may lead to better gas discrimination. Micromachined sensors exhibit a low thermal time constant, which can be used to advantage by applying temperature-modulation techniques. The gas/oxide interaction characteristics and dynamics are observable in the measured sensor resistance. Various temperature modulation methods have been explored. The first method relies on a train of rectangular temperature pulses at variable temperature step heights [70-72]. This method was further developed to find optimized modulation curves [73]. Sinusoidal temperature modulation also has been applied, and the data were evaluated by Fourier transformation [75]. Another idea included the simultaneous measurement of the resistive and calorimetric microhotplate response by additionally monitoring the change in the heater resistance upon gas exposure [74-76]. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

It then addresses the micro-hotplates concept that has led to the development of different types of micromachined gas sensor devices. The different reahzations of micromachined semiconductor gas sensors are presented thin- and thick-film metal-oxide, field effect, and those using complementary metal-oxide semiconductors (CMOSs) and silicon-on-insulator (SOI) technologies. Finally, recent developments based on gas sensitive nanostructures, polymers, printing and foil-based technologies are highlighted. 

Key words silicon micromachining, micro-hotplates, semiconductor, metal-oxide, field-effect, gas sensors, CMOS and SOI, nanowires, printing, polymeric, plastic. 

This chapter therefore focuses on silicon micromachined semiconductor gas sensors. After a brief history of silicon hotplates and metal-oxide... 

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