Force sensing integrated tip and active readout structures with improved dynamics and detection range

Abstract

We introduce a sensor which uses phase shifted dual diffraction gratings in order to increase the detectable range of motion when using phase sensitive diffraction for optical detection of displacement in probe microscopy. The modified FIRAT structure is comprised of a micro-machined bridge structure with integrated diffraction gratings for use in optical detection. With this new design and modified geometry/fabrication process, we will demonstrate force sensing structures with improved displacement detection range and dynamic response. The structure is based upon a previous implementation of force sensor modified for use in AFM imaging. It is built upon a transparent substrate and the bridge / grating serve as an integrated electrostatic actuator. The previously introduced an AFM sensor structure, with integrated actuator, can feasibly obtain Pico Newton force resolution along with increased bandwidth. The advantages of the integrated phase-sensitive diffraction grating and electrostatic actuator over other cantilever based implementations are well developed, yet the FIRAT structure suffered from limited displacement detection range of about ë/4 (167.5 nm for ë = 670 nm), its dynamics were dominated by squeeze film damping and the stiffness was not suitable for many imaging applications. This limitation in range was not due to the device physical structure or actuator, but was inherent to the optical detection scheme used. Modifications to the previous structure design, and sensor detection scheme, are implemented in order to increase the detectable range of the sensor implementation, reduce damping and stiffness, and custom tailor our devices to imaging applications