The ultimate goal of silicon photonics development is the monolithical integration of a photonic layer with optical functions onto silicon IC chips. This 3-D stacking can be obtained by wafer-to-wafer bonding leading to the photonic layers to be embedded into the last levels of metalization above the IC layer. We present a method that takes advantage of this integration process in order to realize enhanced fiber grating couplers. Indeed, grating structures are very attractive to couple the light between tiny silicon wires to the external world, which is dominated by optical fibers. Moreover, the insertion of a mirror below the grating is a well-known solution to increase significantly the coupling efficiency. We propose a method to obtain this bottom mirror by anticipating the grating coupler flip due to the wafer-to-wafer bonding. To this end, the mirror is formed above the encapsulation layer of the fiber grating coupler in order to appear below it after the optical layers integration onto an IC wafer. In this way, the thickness between the grating and the bottom mirror, which is a very sensitive parameter, remains under control whatever wafer-to-wafer bonding process tolerances. Experimental results with one single silicon layer as a bottom mirror exhibit coupling efficiency up to 69%.
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