An experimental setup for photonic integrated circuit alignment sensors

Abstract

The ever-growing demand for smaller microchip feature sizes and thus more powerful chips, has fuelled innovation in photolithography machines for decades. Currently, within this context, one possibility that is explored at ASML involves an increase in the number of alignment markers on a wafer, for a more accurate mapping of local wafer deformations. The current alignment marker metrology system consists of a diffraction-based sensor that is space consuming and measures the markers one-by-one. To prevent a longer total measurement time due to a larger number of markers, parallel read-out is pursued by miniaturisation of the sensor. The technology that is explored for this miniaturisation is integrated optics, in which classical optical components are replaced by their waveguide-based equivalent structures on a chip-sized sensor. Contributing to the feasibility study of integrated optics for ASML’s wafer metrology, this work focuses on the design, construction and verification of an experimental setup for photonic integrated circuit (PIC) alignment sensors. The setup is capable of characterizing the output beams of grating couplers, important PIC components, by imaging their intensity profiles at different locations along their propagation direction. Spots of 40µm can be imaged with a spatial resolution of 0.3µm and an expected Z-position accuracy of 6µm. The setup is also able to scan alignment marks with a PIC alignment sensor, with an expected repeatability of 1.1nm. Anticipating completion of the first PIC alignment sensor designed by the TU Eindhoven, measurements on an alternative PIC containing elementary structures were conducted. Successful coupling between angle polished fibers and 15x15µm grating couplers was achieved, with a verified coupling efficiency of 21.4±3.6%, compared to a coupling efficiency of 28% with the conventional flat end-face fiber coupling method.