Fabrication technologies for chirped refractive microlens arrays

Abstract

Conventional microlens arrays consist of a repetitive arrangement of a unit cell on a fixed pitch. In a chirped array, the inflexibility of a regular structure has been overcome. Here, the array consists of individually shaped lenses which are defined by a parametric description of the cells optical function. We propose different fabrication methods for chirped microlens arrays and present experimentally obtained data. Reflow of photoresist is an established technology for the fabrication of microlenses with superior optical performance. For the generation of a chirped microlens array the photolithographic mask for patterning the resist to be melted has to be chirped. We present an algorithm for mask generation with an example of an ultra-thin camera objective. Inherent to the reflow process stringent limitations to viable surfaces apply. For achieving more arbitrary surfaces, laser lithography and also 2-photon polymerization are employed. In both methods the structures are decomposed into pixels. In laser lithography the local height is converted into an intensity value for the exposure. This variable dose writing locally changes the solubility of the resist in the development process leading to the required surface profile. We propose a writing scheme enabling structure heights of several ten microns with sufficient height discretization. 2-photon polymerization is a rapid prototyping method. Here, a small volume of a UV-curing organic-inorganic co-polymer is hardened in the tight focus of the writing beam. The volume pixel to be exposed is addressed by piezoelectric translation stages. Experimentally obtained structures and performed tests of the optical quality are presented

Recent advancements in optical microstructure fabrication through glass molding process

“…1) Freeform microstructure fabrication Optical systems using freeform lenses are becoming a viable solution to both imaging and nonimaging optics because of their capability to reduce the number of elements in an optical system and accurately control light irradiation. A freeform lens is loosely defined as an optical element that is not symmetric around its optical axis [76][77][78][79][80].…”

Fabrication of microoptical freeform arrays on wafer level for imaging applications

“…al. [11]. Therefore, he applied two photon photopolymerization and laser lithography to fabricate the intended structures but could not achieve sufficient shape and surface quality for imaging applications.…”

Wafer-Level Hybrid Integration of Complex Micro-Optical Modules

“…We used this enhanced design freedom in the randomization of arrays for homogenization [17], the channel-wise aberration correction of multichannel imaging optics [18] or simply the lateral arrangement of elements with different parameters on the same wafer [19]. A means to simulate -generalized‖ chirped or stochastic lens arrays in the ray tracing design and routines to calculate the corresponding mask data for each lenslet were implemented in our fabrication process [20]. RIE assisted lens mastering: Reactive ion etching (RIE) of polymer structures like reflow lenses is a well established technology for proportional transfer of lenses to fused silica or other inorganic substrates [21].…”