SUMMARY
It was previously shown that a Force Sensing Integrated Tip and Active Readout Structure (FIRAT), with its integrated actuator and phase-sensitive diffraction grating, can feasibly obtain piconewton force resolution along with increased bandwidth. The advantages over cantilever based implementations have been developed, yet the FIRAT structure suffered from a limited interferometric displacement detection range of about quarter of the laser wavelength, its dynamics were dominated by squeeze film damping, and the stiffness was not suitable for some imaging applications. Modifications to the previous structure design and sensor detection scheme are implemented in order to increase the detectable displacement range, improve dynamic response and stiffness, and custom tailor these devices for particular imaging applications. A sensor structure is introduced, which uses phase shifted dual diffraction gratings in order to increase the detectable range of motion when using phase sensitive diffraction for optical interferometric detection of displacement in probe microscopy. The structure is based on a previous implementation of FIRAT sensor used in AFM imaging. With a new design and modified geometry/fabrication process, FIRAT structures with improved displacement detection range and dynamic response are demonstrated. Increased detection range is obtained by imparting a 90 degree phase shift, between gratings. The phase shift is introduced via a micro-machined step in the quartz substrate below one of the gratings. An increase in detectable motion of 4 times that of previous FIRAT structures is demonstrated via the modified fabrication process. Bridge shape devices were characterized, that possessed natural frequency ranging from 30 kHz to 1.2 MHz, with Q from 1 to 15 and stiffness ranging from 1 to 100 N/m. Actuation displacement range using the electrostatic actuator was increased 3 times, via the increased gap thickness.