Preparation of packaging cap with binary glass microlenses for wafer level packaging of Focal Plane Arrays (FPAs)

Abstract

To improve the image resolution of thermal imagers, which image objects by uncooled Focal Plane Arrays (FPAs) made up of two-dimension Infra-red (IR) detector arrays, detector size is decreased, and this results in the reduction of detector IR absorber area . However, reduction of IR absorber area may affect IR detector's sensitivity accordingly. In order to reduce the detector size without reducing the detector's sensitivity, the microlenses which can focus the infrared ray on the IR absorber area need to be prepared. A hot forming process of fabricating binary-optics microlenses for wafer-level-packaging of IR FPAs using Pyrex7740 glass is introduced in the paper. Pyrex7740 glass is a widely-used material in the field of packaging of micro system due to its coefficient of thermal expansion is similar to that of silicon and good optical performance for optical devices. The process is based on etching stepwise structures in silicon, and binary-optics silicon-mold wafer is fabricated by a multilevel reactive-ion etching (RIE), followed by anodic bonding of a thin glass wafer to the etched silicon wafer under vacuum. The bonded wafers are then heated inside a furnace at a temperature above the softening point of the glass, and the glass is blown into the stepwise structure due to the atmospheric pressure. Besides, the filling rate of glass into the silicon cavity mold is also discussed. After the silicon mold is removed, glass packaging caps with wafer-level binary-optics microlenses for vacuum packaging are fabricated. Results show that packaging caps, which are of 620um×620um with focal lengths ranging from 1mm to 100 mm at 2.2 um wavelength, are prepared. While the FPAs are encapsulated by the packaging glass caps, the microlenses combined with the detector concentrate light sufficiently to increase the effective collection area, and IR detector's sensitivity is improved. Due to the excellent transmission efficiency in visible and near IR light of the Pyrex 7740 glass, the binary-optics microlenses may have potential application in the other optic devices including Light Emitting Diode (LED).