Atomic force microscopy using a novel on-chip interferometric readout

Currently, the most common readout approaches for atomic force microscopy (AFM) systems are based on the optical beam deflection (OBD) technique. OBD relies on monitoring the deflection of the sensing cantilever probe by measuring the position of the laser beam reflected from the free end of the cantilever using two or more detectors. Alternative AFM readouts consist of electrical readouts, including piezoelectric solutions and tuning fork probes. Although the OBD readout approach achieves subnanometer resolution and video rate imaging speeds, it is hindered in terms of size of the reflected laser beam and the noise originating from mechanical vibration of the free-space optics. A limitation of the OBD readout is its size, which is difficult to minimise as the extent of leveraging is directly associated with the distance travelled by the reflected light beam. System miniaturisation can be accommodated by adopting electrical readout solutions, often at the expense of measurement sensitivity. Electrical readouts can be integrated on-chip and provide scalability for ultrafast imaging using multiprobe arrays. However, imaging in liquids might be challenging, requiring additional electrical isolation. To date, no cost-effective AFM readout solution exists without sacrificing either the measurement sensitivity, system miniaturisation, or multiprobe array scalability. This gap is addressed in this thesis. We present the realisation of an AFM probe with integrated on-chip optical interferometric readout based on silicon photonics. Our AFM probe merges the advantages of subnanometer resolution of optical readouts with miniaturisation previously available only via electrical readout solutions. Our readout does not require any alignment and can operate in air and liquid. The adopted approach determines the deflection of the cantilever in contact with the imaged surface using an integrated on-chip photonics waveguide monitoring the distance to the sensing cantilever via an interrogating Bragg diffraction grating. The implemented methodology provides ultimate interferometric resolution, on-chip miniaturisation, and scalability, to ultrafast multiprobe-array imaging. The AFM probe response was characterised by electrostatic actuation, achieving the expected interferometric response and measurement resolution. We present AFM imaging of reference samples using the developed probe, demonstrating a significant reduction of the measured noise level compared to a modern commercial AFM tool (Bruker Dimension ICON). We achieve an AFM static image root-mean-square (RMS) noise floor of 19 pm surpassing the performance of OBD (25 pm) and piezoresistive readouts (32 pm). The noise spectrum measurements of our probe indicate that our readout is shot noise limited, achieving a deflection noise density (DND) of 36 fm/ √ Hz. In this thesis, we present the design, fabrication and characterisation of a micromachined integrated on-chip AFM probe operating in contact mode, with the prospect of tapping mode operation. We analyse light coupling to the silicon photonics chip viable for our application, and demonstrate device fabrication as well as probe assembly. In the mechanical design of the sensing cantilevers, we adopt a double anchor geometry to eliminate spring effects of the surface micromachined anchors. To achieve a useful mechanical excursion and to minimise the effects of squeezed film damping, thick sacrificial layers around 8 μm are required, which are not commonly used for microelectromechanical systems (MEMS) fabrication. Accordingly, effort is devoted to the development of a flexible device fabrication process utilising material of either inorganic (amorphous silicon) or organic (polyimide) alternatives for the 8 μm thick sacrificial layers. In addition, the attachment of a sharp standard AFM tip using a focused ion beam (FIB) is detailed.