Abstract Due to their special shapes, compositions, chemical and physical properties, one dimensional (1D) or quasi 1D metal oxide nanostructures are widely employed in gas sensor devices and many others. Tetrapods (TPs) nanostructures are one of the many different ZnO shapes, that could be exploited for high-performance chemical sensors. TPs structures can be obtained by vapor phase growth, which is a simple and cost-effective approach for growing micro and nanoparticles on a relatively large scale. Microsystems technology intrinsically offers a powerful tool to obtain low cost, high efficiency and long-term devices. In Microsystems technology, micromachined hotplates are mainly used for sensor applications where the active sensing material is deposited onto the membrane integrated with electrical stimuli and readout. The microhotplate presents a series of advantages such as miniaturized size, fast response, high sensitivity, low power consumption and implementable selectivity. In this view, a nanostructured layer on a thin membrane allows for increasing the sensing performance and obtaining a highly sensitive sensor, while decreasing power consumption. The current efforts to integrate nanostructures on a microdevices, lack to find out a cost-efficient method. In this thesis the process for the fabrication of a Micro Electro-Mechanical System (MEMS) based gas sensor that integrates a ZnO TPs layer is successfully developed. In particular, the integration of the nanostructures is achieved using a cost-effective patterning method. TPs are dispersed in a 2-propanol (IPA) suspension and precipitated onto the membrane. The nanostructures form a sensitive layer patterned through a polydimethylsiloxane (PDMS) physical mask with a central window. The deposition of TPs is performed by centrifugation in order to obtain a homogeneous dispersion and a as thin as possible packed layer on the membrane. The final device is a square shaped die, with a suspended Si3N4 membrane in the centre. The interdigitated Au/Ti electrodes and a Pt/Ta resistor are fabricated on the suspended Si3N4 membrane. In particular, the Pt/Ta resistor is embedded in a dielectric layer in order to optimize the temperature distribution and is used both for heating the microhotplate and getting temperature readout. A highly porous film of entangled ZnO TPs is deposited between the interdigitated electrodes ensuring the electrical contact. Such a device is able to respond as a variable resistance as a function of gas target concentration at determinate temperatures. The device is tested by varying the concentration of target gases in the atmosphere of a test chamber. The sensor response proves the device functionality and, with the top-down integration approach, enables Very Large Scale Integration (VLSI) production.

Integration of ZnO nanostructures onto a microhotplate for gas sensing / Tommasi, Alessio. - (2016).

Integration of ZnO nanostructures onto a microhotplate for gas sensing

TOMMASI, ALESSIO
2016

Abstract

Abstract Due to their special shapes, compositions, chemical and physical properties, one dimensional (1D) or quasi 1D metal oxide nanostructures are widely employed in gas sensor devices and many others. Tetrapods (TPs) nanostructures are one of the many different ZnO shapes, that could be exploited for high-performance chemical sensors. TPs structures can be obtained by vapor phase growth, which is a simple and cost-effective approach for growing micro and nanoparticles on a relatively large scale. Microsystems technology intrinsically offers a powerful tool to obtain low cost, high efficiency and long-term devices. In Microsystems technology, micromachined hotplates are mainly used for sensor applications where the active sensing material is deposited onto the membrane integrated with electrical stimuli and readout. The microhotplate presents a series of advantages such as miniaturized size, fast response, high sensitivity, low power consumption and implementable selectivity. In this view, a nanostructured layer on a thin membrane allows for increasing the sensing performance and obtaining a highly sensitive sensor, while decreasing power consumption. The current efforts to integrate nanostructures on a microdevices, lack to find out a cost-efficient method. In this thesis the process for the fabrication of a Micro Electro-Mechanical System (MEMS) based gas sensor that integrates a ZnO TPs layer is successfully developed. In particular, the integration of the nanostructures is achieved using a cost-effective patterning method. TPs are dispersed in a 2-propanol (IPA) suspension and precipitated onto the membrane. The nanostructures form a sensitive layer patterned through a polydimethylsiloxane (PDMS) physical mask with a central window. The deposition of TPs is performed by centrifugation in order to obtain a homogeneous dispersion and a as thin as possible packed layer on the membrane. The final device is a square shaped die, with a suspended Si3N4 membrane in the centre. The interdigitated Au/Ti electrodes and a Pt/Ta resistor are fabricated on the suspended Si3N4 membrane. In particular, the Pt/Ta resistor is embedded in a dielectric layer in order to optimize the temperature distribution and is used both for heating the microhotplate and getting temperature readout. A highly porous film of entangled ZnO TPs is deposited between the interdigitated electrodes ensuring the electrical contact. Such a device is able to respond as a variable resistance as a function of gas target concentration at determinate temperatures. The device is tested by varying the concentration of target gases in the atmosphere of a test chamber. The sensor response proves the device functionality and, with the top-down integration approach, enables Very Large Scale Integration (VLSI) production.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11583/2641068
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