Atomic force microscopy with integrated on-chip interferometric readout

The most common readout technique used in atomic force microscopy (AFM) is based on optical beam deflection (OBD), which relies on monitoring deflection of the cantilever probe by measuring the position of the laser beam reflected from the free end of the AFM cantilever. Although systems using the OBD readout can achieve subnanometre displacement resolution and video rate imaging speeds, its main limitation is size, which is difficult to minimise, thus limiting multiprobe imaging capability. Currently, system miniaturisation has been accommodated by adopting on-chip electrical readout solutions, often at the expense of measurement sensitivity. To date, no cost-effective AFM readout solution exists without sacrificing either measurement sensitivity, system miniaturisation, or multiprobe array scalability. In this paper we present an AFM probe with integrated on-chip optical interferometric readout based on silicon photonics. Our AFM probe combines the advantages of subnanometre resolution of optical readouts with on-chip miniaturisation. The adopted approach determines deflection of the cantilever using an integrated on-chip photonics waveguide by monitoring the separation between the sensing cantilever and an interrogating grating. The implemented methodology provides ultimate interferometric resolution and sensitivity, on-chip miniaturisation, and array scalability, which makes possible ultrafast multiprobe-array AFM imaging. Using a Digital Instruments D3000 AFM retrofitted with our cantilever probe and integrated readout, we report sub-nanometre AFM topography images obtained on reference samples. We demonstrate RMS static AFM noise level of 19 pm, outperforming the operation of this system in its standard, optical beam deflection configuration (51 pm). The noise spectrum measurements of our probe indicate that the integrated readout is shot noise limited, achieving a deflection noise density (DND) of 36fm/Hz.