A New Method for Microlens Fabrication by a Heating Encapsulated Air Process

This letter reports a new method to fabricate thin microlens array on a silicon substrate by a heating encapsulated air process. We use silicon bulk micromachining, wafer-to-wafer bonding, and photoresist (PR) spin coating to achieve the air sealing process. Under a heating process, the PR filled in the micro-through-hole of cap wafer is compressed by the thermal expansion of the sealed air to form a thin microlens with out-of-plane sphere shape. By adjusting the heating temperature and the sealed air volume, the curvature and size of the lens are controllable. A typical microlens with a diameter of 1475 $\mu$m and sag height of 486 $\mu$ m was fabricated. The calculated radius of curvature and focal length are about 800 and 1200 $\mu$m, respectively. The fabrication provides an alternative way to manufacture thin microlens or microlens mold serve as master elements for replication.