Expanding Limit of Detection and Increasing Operating Resonant Frequency via Larger Anchor Widths for Capacitive Micromachined Resonator-Based Mass Sensors

An innovative technique is explored, aiming to expand the limit of detection (LOD) and increase the operating resonant frequency of the capacitive micromachined ultrasonic transducer (CMUT) working as a mass sensor through modifying anchor widths. The LOD is associated with the concentration range of target compounds and, therefore, their correspondent mass that can be measured using a CMUT mass sensor. The poly multiuser MEMS process (PolyMUMPs) sacrificial technique is used to fabricate three CMUT sensors with a range of anchor widths while keeping all other sensor design parameters and material properties constant. Experimental analysis is conducted to evaluate the influence of anchor width to resonant frequency and frequency shift at different biasing conditions. The experimental results demonstrate that a larger anchor width can increase CMUT operating resonant frequency and pull-in voltage, allowing for the expanded LOD. These results also agree with analytical expressions presented. In addition, it is shown that through employing a designed anchor width, resonant frequencies can be varied to fulfill specific sensor requirements with minimal fabrication adjustments. This study contributes to the evolving knowledge of the CMUT as a mass sensor, enabling more methods for variability of these devices.