16. Microhotplates and Integration with Metal-Oxide Nanomaterials

The current scenario of metal-oxide gas sensing shows, on one side, highly innovative silicon-based platforms, as outcomes of microelectronic and micromachining manufacturing processes, while on the other side, several techniques and methods for the synthesis of the metal-oxide active layers in the form of nanoporous-nanostructured coatings. The high specific surface area of nanoporous coatings improves the interaction with the atmosphere, while the nanostructure offers characteristic surface-dependent electrical properties. Changes in these electrical properties upon gas exposure and interfacial chemical reactions allow for the development of novel, nano-enhanced gas sensors. The base element of innovative micromachined platforms for gas sensing is the microhotplate. Although microhotplates have the same functional parts of traditional devices (integrated heater, electrodes for resistance readout), micromachining provides considerable improvements. These include, for example, the 2–3 orders of magnitude reduction in power consumption for heating: a feature that may disclose the possibility for remote powering through batteries or photovoltaic cells. Moreover, microhotplates originate from the manufacturing track of microelectronics, hence the concept of “system integration” turns out straightforwardly. Within this perspective, the microhotplate may be considered as just an individual component of a many-element sensing platform, including for example, other transducers, or even on-board front-end electronics. Integration concepts are also needed for optimizing the functionalization of the microhotplate with the metal-oxide nanostructured sensing layer, whose batch deposition should become one step of a device production pipeline. As two beautiful countries separated by the sea, with just few bridges in between, difficulties still exist from the point of view of the integration of metal-oxide nanomaterials on microhotplates and micromachined platforms in general. In fact, although many different techniques for the production of metal-oxide nanomaterials have been developed so far, each one of them suffers difficulties, at various degrees, with respect to the fundamental step of microhotplate functionalization. For example, the high temperature step required by certain techniques for stoichiometric oxide synthesis, may be incompatible with microhotplate safety, while the mechanical stress during deposition may result in microhotplate destruction and a subsequent low production yield. The chapter will describe the concepts and the technologies behind microhotplates manufacturing with respect to drawings adopted, chosen materials, and system integration approaches. Techniques and methods for metal-oxide nanomaterial production will be reviewed, highlighting weaknesses and strength points, once they would be employed for microhotplate functionalization. Recent developments on the use of nanoparticle beams to directly deposit nanoporous coatings on microhotplate batches will be included: besides providing thermal and mechanical compatibility with microhotplates, these methods also offer the possibility to synthesize a wide group of different metal-oxides, which is beneficial for an array approach to gas sensing. Relevant examples of sensing performances of microhotplates-based devices will be reported as well.