Development of resonant mass sensors for MEMS based real time cell detection applications

Kangül, Mustafa
This thesis represents design and implementation of MEMS based resonant mass sensors for cell detection applications. The main objective of the thesis is real-time detection inside liquid medium and obtaining the results by electronic means, without the assistance of bulky optical instruments. Novel resonant based mass sensor architectures that have various improvements over selected benchmark design are presented. Purpose of the new structures is to establish real-time mass detection by improving the quality factor, increasing the sensor gain and eliminating the feedthrough current effect. Proposed sensors oscillate in the lateral direction and are coated with a thin parylene layer to prevent liquids flow through the narrow gaps of the device, further improving the quality factor. The resonator is located on top of a microchannel. A thin gold film on the oscillating proof mass is employed as an antibody based cell capture surface. Theoretical background of the physical resonators is investigated to diagnose the design issues of the sensors in the literature. Capacitive actuation, sensing optimization, and feedthrough current elimination methods are presented. In the light of these theoretical studies and finite element modeling simulations, four new resonating structures are proposed for the purpose of real-time mass sensing by enabling self-oscillation. A new process flow consisting SOI, glass, and polymer micromachining methods has been utilized for resonator fabrication. Each device has a foot print area of 18 x 6 mm2, a significant percent of which is used for the inlet and outlet connections of the microchannels. Resonance characterization results in air are presented to see the improvements over the benchmark design, which is previously developed in METU BioMEMS research group. Total sensor mass has been decreased without any loss in the sensor gain. Even two orders of magnitude increase in the gain is achieved with the same mass. There is also quality factor enhancement up to ten times. These improvements are established without changing the anchor structure or cell capturing surface area of the benchmark design. Feedthrough current is also eliminated to make self-oscillation possible. According to the conducted mass measurement results, the mass of a single Sigma-Aldrich® polystyrene microbead is 0.53 ng, whereas the mass is given as 0.55 ng ±30% in its datasheet. In-liquid functionality was also tested. The quality factor of the sensor is reduced from 402, in air, to only to 183 in wet environment. Reported improvements enable real time cell detection in liquid environment.