Arrays of millimeter-sized glass lenses for miniature inspection systems
Résumé
In this paper, we adapt a technique employed for glass microlenses fabrication in order to obtain matrices of millimeter size lenses having high quality. The use of microfabrication processes and Micro ElectroMechanical Systems (MEMS) compatible materials allow the integration of lenses larger than usual in microsystems. Since the presented lenses can have 2 mm in diameter or more, some aspects apparently irrelevant when diameters are lower than 500 µm must be reviewed and taken into account. Indeed, when the lenses are in the millimeter range, problems such as size non-uniformities within a matrix and asymmetric shapes of each lens are dependent on parameters such as mask design, depth of the silicon cavities, enclosed vacuum control after anodic bonding, reflow temperature and even position of the lenses on the substrate. In the fabrication process, a mask formed of circular shapes is patterned with positive photoresist on a silicon wafer by standard UV photolithography. Then, cylindrical cavities about 300 µm deep are obtained in silicon by deep reactive ion etching (DRIE). After removing the photoresist mask, the silicon cavities are sealed under vacuum with a borosilicate glass wafer by anodic bonding. When placing the silicon-glass stack inside a furnace at high temperature, the glass melts with spherical shape by getting aspired into the cavity thanks to the pressure differences between the sealed cavity and the outer environment. After melting, the glass backside is polished in order to obtain a flat surface with optical quality. Silicon can be afterwards selectively removed by dry or wet etching and the plano-convex lenses are then released. Issues related to this fabrication flow-chart are addressed in this paper and few solutions are proposed. Results consecutive to optimization are shown to prove the pertinence of this technique to fabricate MEMS-compatible millimeter-sized lenses to be integrated in miniature microscopes.