A. Kulkarni, V. Stenchly, J. Quenzer, R. Dudde
Fraunhofer Institute Silicon Technology,
Keywords: IR optics, silicon Lens, wafer level packaging
Summary:A new process technology has been developed for a cost efficient production of refractive convex lenses made of silicon. These lenses are produced as lens arrays on standard 8 inch silicon wafers to allow the capping of IR sensors in a wafer level packaging (WLP) process. WLP is a cost efficient procedure for a hermetically tight package of uncooled IR-sensors. Silicon is an interesting material for IR-optical elements due to its large refractive index of approx. 3.4. Processes have therefore been developed to manufacture silicon lens arrays by dry etching of Si-wafers. Due to the necessary optical quality of the lens surface the dry etching is a time-consuming and expensive process. Here we describe an alternative manufacturing process using a glass forming process with thin (50µm) AF32 glass wafers , . Silicon spheres of well-defined diameter are attached to a thin glass carrier wafer and transferred on top of a silicon wafer with prefabricated cylindrical openings. After this the silicon-spheres are looped back by a grinding process until the desired lens shape is generated. Glass residues are removed by wet etching. In this manner an array of silicon lenses on a silicon wafer is realized to be used as cover wafer for wafer level packaging of IR-sensor devices . First designs of Si-lens arrays have been fabricated. Starting with Si-spheres of radius 0,9mm convex lenses of 0,824mm diameter (aperture) and a sagittal height of 0,1mm were produced. A focal length of 0,375mm has been calculated for this lens dimensions resulting in a F-number of 0,45.