French company Saint-Gobain has developed a new patented technology to manufacture arrays of gradient index planar lenses in glass at the wafer scale. Due to its unique properties, the resulting product is very well appropriate for WLP and 3D packaging applications, in particular for LEDs, imaging or displays.

The optical function of a gradient index lens is not provided by the surface profile like standard spherical or aspherical lenses but by the gradual distribution of the refractive index inside the glass. As a consequence, the resulting optical element is perfectly flat. The bulk modification of the refractive index is achieved by ion exchange process, namely by the substitution of ions initially present in glass by other ions which present a different polarisability. If a low polarisability ion (e.g. Li+, Na+, K+) is replaced by an ion with a higher polarisability (e.g. Ag+, Tl+), the refraction index is increased. Reversely, the refraction index is decreased. For the first time, Saint-Gobain has developed a special technology which allows the fabrication of arrays of such gradient index lenses at the glass wafer-scale. The patented process, based on silver diffusion assisted by an external electric field (also called electromigration process), is reliable, repeatable, efficient and allows the creation of optical elements with a broad range of 3D geometries (spheres, hemispheres, cylinders, ellipsoids…).

Lateral sizes of the optical elements range from 200 μm to several cm whereas the vertical sizes (i.e. depth inside the glass) can reach up to few mm. In particular, the optical elements can go through the entire glass thickness. An example, namely a 8’’ glass wafer integrating 300 gradient index lenses, is presented in figure 1.

The glass thickness is 500 μm and the gradient index lenses have a pseudo hemisphere shape as presented in figure 2. At the current development stage, 1000 optical elements can be fabricated at 8’’ scale. In a near future, the goal is to reach 5000 gradient index lenses per 8’’ glass wafer. To succeed in controlling the process parameters as well as the final mechanical and optical quality (in particular transparency) of the wafers, special glasses have been formulated and synthesized. Among other properties, these patented special glasses present a large range of coefficients of thermal expansion from 40 to 100 10-7/°K.