Ph.D. Dissertation Defense by Jeffrey J. McLean
Tuesday, August 17, 2004
(Dr. Levent Degertekin, Chair)
"Interdigital Capacitive Micromachined Ultrasonic Transducers for Microfluidic Applications"
The goal of this research was to develop acoustic sensors and actuators for microfluidic applications. To this end, capacitive micromachined ultrasonic transducers (cMUTs) were developed which generate guided acoustic waves in fluid half-spaces and microchannels. An interdigital transducer structure and a phased excitation scheme were used to selectively excite guided acoustic modes which propagate in a single lateral direction. Analytical models were developed to predict the geometric dispersion of the acoustic modes and to determine the sensitivity of the modes to changes in material and geometric parameters. Coupled field finite element models were also developed to predict the effect of membrane spacing and phasing on mode generation and directionality.
After designing the transducers, a surface micromachining process was developed
which has a low processing temperature of 250ºC and has the potential for
monolithically integrating cMUTs with CMOS electronics. The fabrication process
makes extensive use of PECVD silicon nitride depositions for membrane formation
and sealing. The fabricated interdigital cMUTs were placed in microfluidic channels
and demonstrated to sense changes in fluid sound speed and flow rate using Scholte
waves and other guided acoustic modes. The minimum detectable change in sound
speed was 0.25m/s, and the minimum detectable change in flow rate was 1mL/min.
The unique nature of the Scholte wave allowed for the measurement of fluid properties
of a semi-infinite fluid using two transducers on a single substrate. Changes
in water temperature, and thus sound speed, were measured and the minimum detectable
change in temperature was found to be 0.1ºC. For fluid pumping, interdigital
cMUTs were integrated into microchannels and excited with phase-shifted, continuous
wave signals. Highly directional guided waves were generated which in turn generated
acoustic streaming forces in the fluid. The acoustic streaming forces caused
the fluid to be pumped in a single, electronically-controlled direction. For
a power consumption of 43mW, a flow rate of 410nL/min was generated against
a pressure of 3.4Pa; the thermodynamic efficiency was approximately 5x10-8%.
Although the efficiency and pressure head are low, these transducers can be
useful for precisely manipulating small amounts of fluid around microfluidic