Laser-nanostructured metal oxide semiconductors for conductometric gas sensors.

Los materiales nanoestructurados presentan diferentes propiedades físicas si se comparan con su material “bulk” equivalente. La integración de este tipo de materiales en dispositivos convencionales, tales como sensores de gas, puede mejorar algunas de sus características como la sensibilidad, la selectividad y la respuesta, lo que es importante para el desarrollo de sensores de gas fiables. En particular, la nanoestructuración de óxidos metálicos semiconductores se ha investigado ampliamente para su aplicación en sensores de gas conductométricos. Los dos principales inconvenientes de la mayoría de las técnicas de nanoestructuración son la baja velocidad de los procesos, no escalable a la producción en la industria, y la necesidad de transferir las nanoestructuras al dispositivo (procesos ex-situ). Por este motivo, el presente trabajo estudia la detección de gases de semiconductores nanoestructurados mediante dos técnicas top-down que son rápidas, de bajo coste, de proceso in-situ y automatizables: patrón directo por interferencia láser (direct laser interference patterning (DLIP)) y nanoestructuración mediante láser de femtosegundos (femtosecond laser subwavelength patterning). DLIP es una técnica sin contacto, que utiliza los patrones de interferencia generados por dos o más haces láser coherentes para estructurar directamente los materiales. Por otro lado, la nanoestructuración mediante láser de femtosegundos genera estructuras periódicas inducidas por láser (LIPSS) cuando la radiación polarizada linealmente interactúa con un sólido. Este trabajo se centra en la detección de dióxido de nitrógeno (NO2), ya que es uno de los contaminantes más comunes, del que es necesario medir concentraciones muy bajas. De hecho, la recomendación del Comité Científico de Límites de Exposición Ocupacional para el Dióxido de Nitrógeno de La Comisión Europea establece 0.5 ppm como el TWA de 8 horas. En particular, esta tesis recoge el estudio de tres tipos diferentes sensores de gas nanoestructurados por láser para la detección de bajas concentraciones de NO2: sensores de ZnO procesados por DLIP, sensores de ZnO nanostructurado con LIPSS y sensores de WO3 procesados por DLIP. En todos los casos estudiados, se ha obtenido una mejora de la respuesta en los sensores nanoestructurados en comparación con dispositivos a los que se ha realizado un recocido, lo cual indica el potencial de las tecnologías láser. Además, se ha estudiado el efecto de las condiciones de detección (flujo y posición del sensor dentro de la cámara) en el rendimiento de los sensores comparando resultados experimentales con simulaciones de flujo de gas. Por último, se ha incluido la integración de los sensores fabricados en una plataforma inalámbrica.Nanostructured materials present different physical properties in comparison to their bulk counterparts and the integration of this type of materials in conventional devices, such as gas sensors, can improve some of their characteristics such as sensitivity, selectivity and response. These enhanced features are important for the development of reliable gas sensors. In particular, nanostructuration of semiconductor metal oxides has been widely researched to be applied in conductometric gas sensors. The two major drawbacks of most nanostructuring techniques are the low velocity of the process, not scalable for mass production and the need to transfer the nanostructures to the sensing device (ex-situ approaches). Hence, the present work studies the gas sensing performance of semiconductors nanostructured by two top-down techniques that are fast, inexpensive, in-situ process and automatable: direct laser interference patterning (DLIP) and femtosecond laser subwavelength patterning. The DLIP is a non-contact technique that uses the interference patterns generated by two or more coherent laser beams to directly structure materials. On the other hand, femtosecond laser subwavelength patterning generates laser-induced periodic structures (LIPSS) when linearly polarized radiation interacts with a solid. This work focuses on the detection of NO2, since it is one of the most common pollutants, and needs to be detected in very low concentrations. In fact, the recommendation from the Scientific Committee on Occupational Exposure Limits for Nitrogen Dioxide of the European Commission establishes 0.5 ppm as the 8-hour TWA . In particular, this thesis gathers the study of three different type of laser nanostru ctured semiconductor gas sensors for the detection of low concentration of NO2: ZnO based sensors processed by DLIP, ZnO based sensors nanostructured with LIPSS and WO3 based sensors processed by DLIP. In all the approaches, a response improvement has been obtained by the nanostructured sensors compared with classically annealed devices, pointing out the laser technologies potential. Furthermore, the study of the operating conditions influence (flow and position of the sensor inside the chamber) on the sensors performance is investigated comparing experimental results with gas flow simulations. Finally, the integration of the fabricated sensors into a wireless platform is included in this dissertation

Influence of the test-chamber shape on the performance of conductometric gas sensors

In this article, CFD simulations results are presented as a key tool to the comprehension of the target gas concentration evolution in a test chamber, at different working conditions. The simulation results are compared with the experimental data, which shows a qualitative good correlation with the evolution of the concentration gradient detected. The experiments were carried out using an aluminum gas test chamber, where a WO3 based conductometric sensor is introduced. The results demonstrate how the response time is dependent on the sensor working conditions. Analyzing the CFD and experimental results, some assumptions for this behavior are proposed. The WO3 sensor needs a Pt heating element, which is heated up to 300°C. As the response is highly temperature-dependent, the temperature distribution on the sensor surface was measured by an IR thermographic camera. The simulation results show that the temperature distribution matches with those obtained experimentally. To validate the model, a mesh and time step convergence study was also implemented

Silicon Photomultipliers technologies for 3D integration

Progress in 3D interconnecting technologies paved the way to a new generation of Silicon Photomultipliers (SiPM) by combining the integrated functionalities of the digital SiPM with the high performance, in terms of noise and efficiency, of the analog SiPM. Recently, FBK has been developing new 3D integration technologies, specifically designed for SiPMs, to improve performances and functionalities by using backside-illuminated (BSI) devices and Through Silicon Vias (TSV) interconnections. Two different technology platforms have been identified: a BSI design for NIR and TSV interconnections for NUV/VUV SiPMs. Two R&D batches are under development to demonstrate the feasibility as well as robustness and reliability of both the technologies. For NIR applications, electrical characterization of ultra-thin SiPM wafers with a metal reflector on the front side has shown an improved photon detection efficiency when operated in BSI configuration compared with thinned front-side illuminated (FSI) devices, allowing at the same time high-segmentation access to the SiPM output from the front-side. Instead, for NUV/VUV applications, a FSI stacked approach is more suitable since the junction depth needs to be shallower to absorb short wavelengths. In this case, TSV interconnections have been implemented allowing to place the contacts on the backside of the wafer

3D integration technologies for custom SiPM: From BSI to TSV interconnections

Progress in 3D interconnecting technologies paved the way for a new generation of Silicon Photomultipliers (SiPM) and Single Photon Avalanche Diode (SPAD): hybrid devices which combine the integrated functionalities of the digital SiPM with the high performance of custom technologies, like low noise and high detection efficiency. Recently, Fondazione Bruno Kessler (FBK) has been working on the implementation of recently developed 3D integration technologies, on SiPMs devices, to improve both performances and functionalities by creating backside-illuminated (BSI) devices and Through Silicon Vias (TSV) interconnections. Two different technology platforms have been investigated: a BSI design for near-infrared (NIR) sensitive SiPMs and TSV interconnections for near- and vacuum-ultraviolet (NUV/VUV) sensitive detectors. For NIR applications, electrical characterization of ultra-thin (about ) SiPM wafers with a metal reflector on the frontside has shown an improved photon detection efficiency (PDE) when operated in BSI configuration compared with non-thinned front-side illuminated (FSI) devices, allowing at the same time full high-segmentation access to the SiPM output from the front-side. Instead, for NUV/VUV applications, a FSI stacked approach is considered more suitable since the junction depth needs to be shallower. In this case, TSV interconnections using two different approaches (named Via-Mid and Via-Last) have been implemented allowing the placement of the contacts on the backside of the wafer

Filling nanoporous polymer thin films: an easy route toward the full control of the 3D nanostructure

A novel approach enabling the full control of the 3D nanostructure of polymer thin films is presented. An ordered nanoporous polymer film is obtained using a nanoparticlemonolayer as template. The pores are then filled with a second organic component via spin coating which, by modulating the deposition parameters, enables the control of the filling degree with nanometric precision.JRC.I.4-Nanobioscience

Filling nanoporous polymer thin films: an easy route toward the full control of the 3D nanostructure

A novel approach enabling the full control of the 3D nanostructure of polymer thin films is presented. An ordered nanoporous polymer film is obtained using a nanoparticle monolayer as template. The pores are then filled with a second organic component via spin coating which, by modulating the deposition parameters, enables the control of the filling degree with nanometric precision

Study of sputtered ZnO modified by Direct Laser Interference Patterning: Structural characterization and temperature simulation

ZnO thin film sputtered on alumina substrate is processed by Direct Laser Interference Patterning (DLIP). The heat transfer equation has been simulated for interference patterns with a period of 730 nm and two different fluences (85 mJ/cm2 and 165 mJ/cm2). A thermal threshold of 900 K, where crystal modification occurs has been calculated, indicating a lateral and depth processing around 173 nm and 140 nm, respectively. The experimentally reproduced samples have been analyzed from the structural and composition point of view and compared to conventional thermal treatments at three different temperatures (600 °C, 700 °C and 800 °C). Promising properties have been observed for the laser treated samples, such as low influence on the thin film/substrate interface, an improvement of the crystallographic structure, as well as a decrease of the oxygen content from O/Zn = 2.10 to 1.38 for the highest fluence, getting closer to the stoichiometry. The DLIP characteristics could be suitable for the replacement of annealing process in the case of substrates that cannot achieve high temperatures as most of flexible substrates

Influence of the test-chamber shape on the performance of conductometric gas sensors

In this article, CFD simulations results are presented as a key tool to the comprehension of the target gas con- centration evolution in a test chamber, at different working conditions. The simulation results are compared with the experimental data, which shows a qualitative good correlation with the evolution of the concentration gradient detected. The experiments were carried out using an aluminum gas test chamber, where a WO3 based conductometric sensor is introduced. The results demonstrate how the response time is dependent on the sensor working conditions. Analyzing the CFD and experimental results, some assumptions for this behavior are proposed. The WO3 sensor needs a Pt heating element, which is heated up to 300 ◦C. As the response is highly temperature-dependent, the temperature distribution on the sensor surface was measured by an IR thermo- graphic camera. The simulation results show that the temperature distribution matches with those obtained experimentally. To validate the model, a mesh and time step convergence study was also implemente

Influence of the test-chamber shape on the performance of conductometric gas sensors

In this article, CFD simulations results are presented as a key tool to the comprehension of the target gas con- centration evolution in a test chamber, at different working conditions. The simulation results are compared with the experimental data, which shows a qualitative good correlation with the evolution of the concentration gradient detected. The experiments were carried out using an aluminum gas test chamber, where a WO3 based conductometric sensor is introduced. The results demonstrate how the response time is dependent on the sensor working conditions. Analyzing the CFD and experimental results, some assumptions for this behavior are proposed. The WO3 sensor needs a Pt heating element, which is heated up to 300 ◦C. As the response is highly temperature-dependent, the temperature distribution on the sensor surface was measured by an IR thermo- graphic camera. The simulation results show that the temperature distribution matches with those obtained experimentally. To validate the model, a mesh and time step convergence study was also implemente