Gas sensors are widely used for detecting toxic gases for environmental protection, industrial monitoring, household safety, breath analysis and food deterioration. Apart from the electrochemical gas sensors, which have a short lifetime, and optical gas sensors with large volume size with high cost, semiconductor metal oxide (SMO) gas sensors as one of the chemiresistive type gas sensors are now developing fast owing to its low production cost, stable physical properties and chemical versatility. However, regarding the high operational temperature of SMO gas sensors, reduction of power consumption is extremely important for its application in smartphones and other portable devices. For this purpose, miniaturization of SMO gas sensor devices, primarily for the hotplate part acting as mechanical support of the sensing material and heater/electrode part, is an effective way to improve the power efficiency. Microelectromechanical systems (MEMS) offer an opportunity to achieve such goal. This dissertation addressed to miniaturization of the hotplate, was focused on hotplate fabrication by using Electron Beam Lithography (EBL) and Focused Ion Beam (FIB). Then two different approaches were studied and used at Bruno Kessler Foundation facilities to microfabricate the hotplates. First method combined EBL and FIB techniques to define the layout. EBL was used to exposure the micro-level size electrode part (or pad part), and FIB was used to mill the heater circuit part with fine and dense structure. The patterned hotplate structure was characterized by Scanning Electron Microscope (SEM), and the milling result was analyzed by Secondary-ion Mass Spectrometry (SIMS). By studying these results, the optimized parameters for EBL and FIB were selected. The second method used two-step EBL exposure. Low energy of electron beam with low dose and large writing field for the electrode part exposure and high energy of electron beam with high dose and small writing field for the dense heater circuit patterning. After these hotplates were fabricated, their electrical and thermal properties were experimentally evaluated. Subsequently, chemiresistive sensors based on the developed hotplates were developed. In particular, n-type sensing material ZnO nano film was deposited on MHP2 and NHP1 by magnetron sputtering technique. SEM revealed the nano size of ZnO particle, and the calcination condition effect on the size of ZnO. ZnO crystal structure was characterized by X-ray Powder Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS) proved the atom ratio of Zn and O. ZnO nanofilm did not show strong response to humidity, but humidity could decrease the response toward NO2, and increase the response toward ethanol. Thick films of SnO2 highly doped by antimony with concentration of 10 wt% (ATO1) and 15wt% (ATO2) were drop coated on MHP1. These materials were characterized by SEM, XRD and XPS. It suggested that antimony doping modified the morphology of SnO2 powder by preventing the growth of powder particles. The results of the XPS experiment demonstrated that the concentration of antimony was higher on the surface of SnO2 than its inside. It was found that ATO sensors led to a particularly high selectivity and sensitivity to NO2 when compared to the other gases at 400 °C in dry air. Additionally, the sensing response of ATO1 and ATO2 was only moderately affected by humidity, which made them ideal candidates to detect NO2 in the actual atmosphere.
I sensori di gas sono ampiamente utilizzati per rilevare gas tossici per la protezione ambientale, il monitoraggio industriale, la sicurezza domestica, l'analisi del respiro e il deterioramento degli alimenti. A parte i sensori di gas elettrochimici, che hanno una breve durata, e i sensori di gas ottici di grandi dimensioni con un costo elevato, i sensori di gas chemiresistivi basati su ossidi metallici semiconduttori (OMS) risultano essere una soluzione tecnologica estremamente interessante, grazie alla sua bassa produzione costo, proprietà fisiche stabili e versatilità chimica. Tuttavia, per via dell'elevata temperatura operativa dei sensori di gas OMS, la riduzione del consumo energetico è di fondamentale importanza per una loro futura integrazione su dispositivi portatili, quali gli smartphones. A tale scopo, la miniaturizzazione dei sensori di gas OMS, principalmente per quanto riguarda il microriscaldatore, che funge da supporto meccanico del materiale di rilevamento e della parte riscaldatore/elettrodo, è un modo efficace per migliorare l'efficienza energetica. I sistemi microelettromeccanici (MEMS) offrono l'opportunità di raggiungere tale obiettivo. Questa dissertazione, focalizzata principalmente alla miniaturizzazione del microriscaldatore, si è concentrata sulla simulazione della dissipazione del calore del microriscaldatore mediante analisi agli elementi finiti, e sulla fabbricazione degli stessi utilizzando la litografia a fascio di elettroni (EBL) e il fascio di ioni focalizzati (FIB) per lo sviluppo di sensori di gas a bassissimo consumo energetico. Quindi sono stati studiati e utilizzati due diversi approcci presso le strutture della Fondazione Bruno Kessler per fabbricare i microriscaldatori. Il primo metodo ha combinato le tecniche EBL e FIB per definire il layout del riscaldatore stesso. EBL è stato utilizzato per esporre la parte dell'elettrodo di dimensioni micrometriche, mentre il FIB è stato utilizzato per fresare la parte del circuito del riscaldatore con caratteristiche nanometriche. Nel secondo metodo, è stata utilizzata un'esposizione EBL in due fasi, senza utilizzo del FIB: i) bassa energia del fascio di elettroni con bassa dose e ampia area di scrittura per la definizione della struttura degli elettrodi; ii) alta energia del fascio di elettroni con dose elevata e piccolo campo di scrittura per la definizione del circuito del riscaldatore. Dopo che questi microriscaldatori sono stati fabbricati, le loro proprietà elettriche e termiche sono state valutate sperimentalmente. Successivamente sono stati sviluppati sensori chemiresistivi sfruttando i microriscaldatori sviluppati. In particolare, il nanofilm ZnO di materiale sensibile di tipo n è stato depositato su MHP2 e NHP1 mediante magnetron sputtering. Il SEM ha rivelato le dimensioni nanometriche delle particelle di ZnO. La struttura cristallina di ZnO è stata caratterizzata dalla diffrazione della polvere di raggi X (XRD) e la spettroscopia fotoelettronica a raggi X (XPS) ha dimostrato il rapporto atomico di Zn e O. Il nanofilm di ZnO non ha mostrato una forte risposta all'umidità, mentre ha mostrato una buona sensibilità nei confronti del NO2. Successivamente, i microriscaldatori MHP1 sono stati testati anche come substrati per sensori chemiresistivi a film spesso, utilizzando come materiale sensibile SnO2 altamente drogate con antimonio (ATO), concentrazione atomica del 10% e 15% in peso. Questi materiali sono stati caratterizzati da SEM, XRD e XPS, il che ha suggerito che il drogaggio di antimonio ha modificato la morfologia rispetto alla polvere di SnO2 non drogata, prevenendo la crescita delle particelle di polvere e diminuendo quindi la dimensione media delle nanoparticelle. La caratterizzazione XPS ha dimostrato che la concentrazione di antimonio era maggiore sulla superficie delle nanoparticelle di SnO2 rispetto al bulk. È stato riscontrato che i sensori ATO hanno portato a un’alta selettività e sensibilità all'NO2.
Electron Beam Lithography and Focused Ion Beam Techniques for the Development of Low Power Consumption Microelectromechanical Systems-based Chemiresistive Gas Sensors
FENG, Zhifu
2023
Abstract
Gas sensors are widely used for detecting toxic gases for environmental protection, industrial monitoring, household safety, breath analysis and food deterioration. Apart from the electrochemical gas sensors, which have a short lifetime, and optical gas sensors with large volume size with high cost, semiconductor metal oxide (SMO) gas sensors as one of the chemiresistive type gas sensors are now developing fast owing to its low production cost, stable physical properties and chemical versatility. However, regarding the high operational temperature of SMO gas sensors, reduction of power consumption is extremely important for its application in smartphones and other portable devices. For this purpose, miniaturization of SMO gas sensor devices, primarily for the hotplate part acting as mechanical support of the sensing material and heater/electrode part, is an effective way to improve the power efficiency. Microelectromechanical systems (MEMS) offer an opportunity to achieve such goal. This dissertation addressed to miniaturization of the hotplate, was focused on hotplate fabrication by using Electron Beam Lithography (EBL) and Focused Ion Beam (FIB). Then two different approaches were studied and used at Bruno Kessler Foundation facilities to microfabricate the hotplates. First method combined EBL and FIB techniques to define the layout. EBL was used to exposure the micro-level size electrode part (or pad part), and FIB was used to mill the heater circuit part with fine and dense structure. The patterned hotplate structure was characterized by Scanning Electron Microscope (SEM), and the milling result was analyzed by Secondary-ion Mass Spectrometry (SIMS). By studying these results, the optimized parameters for EBL and FIB were selected. The second method used two-step EBL exposure. Low energy of electron beam with low dose and large writing field for the electrode part exposure and high energy of electron beam with high dose and small writing field for the dense heater circuit patterning. After these hotplates were fabricated, their electrical and thermal properties were experimentally evaluated. Subsequently, chemiresistive sensors based on the developed hotplates were developed. In particular, n-type sensing material ZnO nano film was deposited on MHP2 and NHP1 by magnetron sputtering technique. SEM revealed the nano size of ZnO particle, and the calcination condition effect on the size of ZnO. ZnO crystal structure was characterized by X-ray Powder Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS) proved the atom ratio of Zn and O. ZnO nanofilm did not show strong response to humidity, but humidity could decrease the response toward NO2, and increase the response toward ethanol. Thick films of SnO2 highly doped by antimony with concentration of 10 wt% (ATO1) and 15wt% (ATO2) were drop coated on MHP1. These materials were characterized by SEM, XRD and XPS. It suggested that antimony doping modified the morphology of SnO2 powder by preventing the growth of powder particles. The results of the XPS experiment demonstrated that the concentration of antimony was higher on the surface of SnO2 than its inside. It was found that ATO sensors led to a particularly high selectivity and sensitivity to NO2 when compared to the other gases at 400 °C in dry air. Additionally, the sensing response of ATO1 and ATO2 was only moderately affected by humidity, which made them ideal candidates to detect NO2 in the actual atmosphere.File | Dimensione | Formato | |
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Electron Beam Lithography and Focused Ion Beam Techniques for the Development of Low Power Consumption Microelectromechanical Systems-based Chemiresistive Gas Sensors.pdf
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https://hdl.handle.net/20.500.14242/73689
URN:NBN:IT:UNIFE-73689