Modeling and characterization of capacitive micromachined ultrasonic transducers

Abstract

The Capacitive Micromachined Ultrasonic Transducer (cMUT) is a device used for the generation and detection of ultrasonic sound waves. The device is constructed on a silicon substrate using a microfabrication process. Individual cells constituting the device are membranes which have dimensions in the order of tens of microns, and are made up of a mechanicalh^ strong compound of silicon. The transducer itself has dimensions measured in centimeters, thus the total number of cells that make up a transducer is in the order of thousands. The excitation/detection of acoustic waves relies on the capacitance between the substrate and membrane: The presence of acoustic waves induces a small -AlC variation on the DC bias on the device, which can be used for detection, while a small -A.C component added to the DC bias by the drive circuit changes the electro-static attraction force on the membrane causing it to vibrate, producing acoustic waves. Basic advantages of cMUT devices include easy patterning of array structures, integration of drive/detection electronics with mechanical structures, and low cost. In this study, basic theory describing the characteristics of cMUT devices were developed. The analytic formulation was used to test the validity of a Finite Element Method (FEM) model. The FEM model, then, was emplo3'ed in the analysis of structures for which no analytical models are present. Specific problems solved using the FEM model included the characterization of cMUT devices with judiciously patterned electrodes. A more specific study showed that the bandwidth of an immersion device with an active area of radius 25 /¿m can be increased by 100% by simply setting the electrode radius to 10 /rm. The FEM analysis was, then, extended to handle the effects of substrate loss, which required the incorporation of an Absorbing Boundary Condition (ABC) into the model. A Normal Mode Theory analysis was conducted to give better insight to the physical nature of the effect of substrate loss to device characteristics. The dominant wavemode for a transducer of central frequency 2.5 MHz was found to be the lowest order anti-symmetric lamb wave mode (AO), for a silicon substrate of thickness 500 //m. A microfabrication process was developed for the production of cMUT devices. Hexagonally shaped transducers of radius 40 p.m were fabrictated on a conducting silicon substrated with silicon nitride as the sacrificial la.j'er and amorphous silicon as the membrane material. Both the gap and membrane thicknesses are set to 0.5 //m. 8, 16, and 24 /im gold plates were deposited as top eletrodes. The total number of active cells were 24 thousand for a substrate size of 0.7x0.7 cm^. Some experimental results were obtained from the fabricated transducers to support the analytical cMUT model. The device is found to have a central frequency of 2 MHz.