The Research Of Controllable Long Focal Length Microlens Based On Thermal Expansion

Microlens and microlens array are important optical component,and have been widely used in many application fields,such as optical sensors and imaging.For microlens with long focal length,it has played an indispensable role in many applications,such as Shack-Hartmann wavefront sensors.During past years,various fabrication methods were proposed to obtain long focal length microlens.However,it is challenging for these methods to fabricate microlens with long focal length in millimeter order.Moreover,these methods lead to poor reproducibility and uniformity of the microlens,which results in an uncontrollable long focal length.Thus,it is difficult to fabricate a microlens with a certain long focal length under a determinate linewidth,which is required in practical applications.In order to achieve a microlens with a controllable long focal length,a novel method based on the reflow and thermal expansion of negative photoresist has been proposed in this thesis.The main contents of this thesis are summarized as follows:1.The domestic and foreign progress of microlens fabrication methods has been investigated and analyzed.In order to overcome drawbacks in conventional fabrication methods,a novel method to fabricate microlens with a controllable long focal length is presented here.It is based on the fabricating and heating of a mold microlens consisting of two materials with a great difference in coefficient of thermal expansion.The principle of the formation mechanism has been analyzed,and the controllability and consistency of the focal length in this method has been conducted.Based on the formation mechanism of this method,the fabrication process has been designed,and the corresponding fabrication materials have been selected.2.In order to verify the formation mechanism of this method,the temperature dependence of microlens geometry and focal length were obtained by three methods,which are theoretical calculation,simulation by the finite element method and direct measurement of fabricated microlens.To direct measure the fabricated microlens,the geometry of the microlens was obtained by a surface profiler and optical microscope,and the focal length was obtained by an optical setup.According to the analysis of a large amount of data,the formation mechanism has been demonstrated.Based on the focal lengths of microlens with different linewidths and types,the applicability of this method was analyzed.3.To investigate the uniformity of the microlens,the relative standard deviation of focal length has been calculated.The results show that this method provides good uniformity.The important performance parameters of microlenses,such as fill factor,numerical aperture,are explored.The results indicate that this method could be used to fabricate microlen with high fill factor and high numerical aperture.It is concluded that this fabrication method has the following advantages:1.Microlens array with long focal length in millimeter order could be fabricated in this method,which has the potential to be applied in many other fields such as wavefront sensors,laser systems and functional biomimetics.2.The focal length in this method is controllable with the processing temperature.This method has shown excellent performance in fabricating microlens with a certain long focal length under a determinate linewidth,which is urgently needed in the practical application.3.This method has strong applicability and could be applied to fabricate microlens in different linewidths and types.It could also be used for microlens with high fill factor and numerical aperture.