Integration of capacitive micromachined ultrasound transducers to microfluidic devices

The design and manufacturing flexibility of capacitive micromachined ultrasound transducers (CMUT) makes them attractive option for integration with microfluidic devices both for sensing and fluid manipulation. CMUT concept is introduced here by presenting simplified equivalent circuit model; while finite element analysis (FEA) and near-field CMUT operation experiments were used as the main methods of research. The transient time FEA was used to simulate the transmission and reception of the short pulse and then analysis of the CMUT membrane motion in the time and frequency domains was made. 200 nm finite element spacing and 0.2 ns sampling time was used during the simulations for increased precision and reliability of the results. Surface micromachined CMUT devices with two-phase interdigital arrangement of the transducer elements were fabricated for experiments. The microchannels of different heights were modeled by attaching the adhesive tape of various thicknesses at the sides of the CMUT devices to form the bead for the liquid. The drop of the transformer oil was put in the microchannel before attaching the upper microchannel wall. Afterwards the electromechanical impedance of CMUT was measured with the network analyzer to evaluate the CMUT operating regime. The finite element analysis showed that when the microchannel cross-section dimensions are comparable with the CMUT cell dimensions, the transducer is subjected to the pressure waves bouncing between the microchannel walls and comparatively compliant CMUT membranes. This situation causes the distortion of the CMUT operating regime and has to be accounted during the design and use of CMUT-coupled microfluidic devices. Corrections for the amount of the near-field distortions are to be made already during the design phase. We found significant relationship between the CMUT membrane motion characteristics and the microchannel height. For example: CMUT having 5 MHz central frequency in immersion exhibits the broad band frequency spectra when in normal operating conditions, but the central frequency decreases by 60-70 % with decreasing the microchannel height from 400 microns to 2 microns, and the frequency band becomes narrower. It is also shown here how the simulated results can be experimentally verified.