Publicação
Microlenses for optical microsystems
| Resumo: | Lenses have been used by mankind for thousands of years for innumerous different reasons and applications. More recently, lenses in the micro scale dimension, so called microlenses (MLs), have been designed and fabricated using semiconductor technology. These new lenses are used for collimation, focusing or imaging and are an appealing alternative for applications where miniaturization and alignment simplicity are important requirements. Moreover, they also opened a large number of new applications for optical structures and, at the same time, reducing the mechanical and electrical complexity of the existing systems. In this context, the presented thesis has as main purposes, the design and development of a process that allows the fabrication of different sized plano-convex MLs with minor intervention on the process parameters. The MLs were fabricated using a photoresist, the AZ4562, through classical photolithography and the thermal reflow process. Another achievement was the fabrication of MLs directly on the surface of a silicon die containing complementary metal–oxide–semiconductor (CMOS) photodiodes (PDs) for quantifying the differences in their photocurrents generation capacity. The MLs’ optimum fabrication process was achieved when a 128k dots per inch (dpi) super high-resolution chrome on soda lime glass 3×3-0.060” photomask was employed. This photomask allows the design pattern to be transferred into the photoresist with very high precision. Nevertheless, for actually obtaining the desired lens profile, it is necessary to apply a thermal treatment to the fabricated microstructures. When the photoresist is submitted to a temperature higher than its glass transition temperature, it softens allowing the shape change to occur. For MLs, the major external force acting during this process is the surface tension. The fabricated MLs were structurally characterized using a profilometer and scanning electron microscope (SEM) images. For measuring the focal length, an optomechanical alignment system was assembled and a difference of just 4% was found between the measured and the theoretical values. An additional improvement was achieved by introducing a rehydration step in the fabrication process. The prebake stage used during the fabrication serves for evaporating the solvent off the photoresist but also, all of its water content. As a result, it was demonstrated that the AZ4562 needs rehydration in order to obtain excellent results by preventing structural damages in the MLs which are crucial for achieving efficient optical properties. The main advantage of this new optimized process is the further improvement of well-established standard microfabrication processes, i.e., photolithography combined with photoresist thermal reflow. Then, three approaches for integrating the MLs with the photodetecting substrate were tested. The first was using a polydimethylsiloxane (PDMS) intermediate layer for controlling the thickness between the MLs and the photodetecting substrate for allowing different focal lengths to be used depending on the application. The second one is setting the MLs’ focal length within the photodetectors’ depletion region using a 150 μm thin glass substrate for demonstrating that the current generation is enhanced for the same active area. Finally, the third approach consists on a setup composed by a MLs array fabricated directly on top of the PDs and in this approach, two solutions are presented. One is the fabrication of a ML on a square PD with the side measuring 24 μm. This setup enables the capture of light that would otherwise fall outside the photodiodes’ active area resulting in an overall photocurrent generation gain. The other is the fabrication of a MLs array using the same photomask but on a square PD with the side measuring 240 μm for determining the level of photocurrent generation. Moreover, two light sources (red and white lights) were used for evaluating the light acquisition enhancement capacity. From the results that were obtained under different integration solutions, the direct fabrication of MLs on PDs was the one with the better results concerning photocurrent generation by improving it by more than 14% and 2% for red and white lights, respectively. The red light has the ideal penetration depth in silicon for achieving the most prominent enhancement in photocurrent generation presented in this thesis. The MLs that were designed and fabricated, as well as their integration solutions with a photosensitive substrate, show interesting potential in applying them on industry standard fabrication processes for optical microsystems, from light-acquisition enhancement applications to image sensors. |
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| Autores principais: | Rocha, R. P. |
| Assunto: | Microlenses Thermal reflow Rehydration Photodiodes Current generation Microlentes Refluxo térmico Reidratação Fotodíodos Geração de corrente |
| Ano: | 2013 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade do Minho |
| Idioma: | inglês |
| Origem: | RepositóriUM - Universidade do Minho |
| Resumo: | Lenses have been used by mankind for thousands of years for innumerous different reasons and applications. More recently, lenses in the micro scale dimension, so called microlenses (MLs), have been designed and fabricated using semiconductor technology. These new lenses are used for collimation, focusing or imaging and are an appealing alternative for applications where miniaturization and alignment simplicity are important requirements. Moreover, they also opened a large number of new applications for optical structures and, at the same time, reducing the mechanical and electrical complexity of the existing systems. In this context, the presented thesis has as main purposes, the design and development of a process that allows the fabrication of different sized plano-convex MLs with minor intervention on the process parameters. The MLs were fabricated using a photoresist, the AZ4562, through classical photolithography and the thermal reflow process. Another achievement was the fabrication of MLs directly on the surface of a silicon die containing complementary metal–oxide–semiconductor (CMOS) photodiodes (PDs) for quantifying the differences in their photocurrents generation capacity. The MLs’ optimum fabrication process was achieved when a 128k dots per inch (dpi) super high-resolution chrome on soda lime glass 3×3-0.060” photomask was employed. This photomask allows the design pattern to be transferred into the photoresist with very high precision. Nevertheless, for actually obtaining the desired lens profile, it is necessary to apply a thermal treatment to the fabricated microstructures. When the photoresist is submitted to a temperature higher than its glass transition temperature, it softens allowing the shape change to occur. For MLs, the major external force acting during this process is the surface tension. The fabricated MLs were structurally characterized using a profilometer and scanning electron microscope (SEM) images. For measuring the focal length, an optomechanical alignment system was assembled and a difference of just 4% was found between the measured and the theoretical values. An additional improvement was achieved by introducing a rehydration step in the fabrication process. The prebake stage used during the fabrication serves for evaporating the solvent off the photoresist but also, all of its water content. As a result, it was demonstrated that the AZ4562 needs rehydration in order to obtain excellent results by preventing structural damages in the MLs which are crucial for achieving efficient optical properties. The main advantage of this new optimized process is the further improvement of well-established standard microfabrication processes, i.e., photolithography combined with photoresist thermal reflow. Then, three approaches for integrating the MLs with the photodetecting substrate were tested. The first was using a polydimethylsiloxane (PDMS) intermediate layer for controlling the thickness between the MLs and the photodetecting substrate for allowing different focal lengths to be used depending on the application. The second one is setting the MLs’ focal length within the photodetectors’ depletion region using a 150 μm thin glass substrate for demonstrating that the current generation is enhanced for the same active area. Finally, the third approach consists on a setup composed by a MLs array fabricated directly on top of the PDs and in this approach, two solutions are presented. One is the fabrication of a ML on a square PD with the side measuring 24 μm. This setup enables the capture of light that would otherwise fall outside the photodiodes’ active area resulting in an overall photocurrent generation gain. The other is the fabrication of a MLs array using the same photomask but on a square PD with the side measuring 240 μm for determining the level of photocurrent generation. Moreover, two light sources (red and white lights) were used for evaluating the light acquisition enhancement capacity. From the results that were obtained under different integration solutions, the direct fabrication of MLs on PDs was the one with the better results concerning photocurrent generation by improving it by more than 14% and 2% for red and white lights, respectively. The red light has the ideal penetration depth in silicon for achieving the most prominent enhancement in photocurrent generation presented in this thesis. The MLs that were designed and fabricated, as well as their integration solutions with a photosensitive substrate, show interesting potential in applying them on industry standard fabrication processes for optical microsystems, from light-acquisition enhancement applications to image sensors. |
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