Integrated-Circuit-Compatible Electret and Condenser Ultrasonic Transducers

This dissertation presents research on the design, analysis and construction of thin film electret and condenser ultrasonic transducers which are compatible with integrated circuit fabrication. In addition, materials research is presented concerning the electromc properties of anodically formed aluminum oxide (both barrier and porous) which results in new uses for these materids in ultrasonic detection.
A review of the fundamental theory of electrets is given, including derivations of surface effective charge and surface potential difference. An improved non-contacting apparatus for measuring the surface potential difference (SPD) across electrets is described. A review is given of the analysis of muitilayer acoustic transducers, based on the use of voltage current transmission matrices. This analysis allows calculation of the voltage output per unit acoustic stress (V/Pa) as a function of frequency. Several transducer configurations are then analyzed, including some that are physically realizable (stress-free top surface, air-gap and thick top-contact transducers) and some which are idealizations (infinitely thick top-contact and fixed top surface transducers). Except for the air-gap transducer, these all use a soft polymer gap layer.
Comparisons are made among these transducers, and it is concluded that the air-gap transducer has the highest output, followed by the thick top-contact transducer. The use of a polymer foam gap is predicted to greatly improve transducer performance.
We have built working I.C.-compatible and non-I.C.-compatible transducers, and complete fabrication process specifications are included. For thin film condenser transducers, a graph of output voltage vs. bias voltage is given. Experimental results for transducer sensitivity at 10 MHz are (in V/Pa): 1.26*10^-7 for an air-gap transducer using an electret, 1.82*10^-8 for an air-gap transducer using a Teflon electret, and 1.5*10-8 for a thin-film condenser transducer with a compliant polymer gap and a thick top electrode
Existing thin-film piezoeIectric transducers are analyzed for comparison with the electret and condenser transducers. Performance of existing sputtered ZnO piezoelectric film transducers is shown to be inferior to that of the air-gap transducers (at low frequencies), and comparable to that of the thick top-contact condenser transducers. The design and analysis of an improved thin-film ZnO transducer with a thick top-contact is given.
A potential thin-film electret material, plasma-deposited fluorocarbon, is evaluated. Films deposited from CF4 had pinholes and did not retain charge when irrabated by an electron beam. Films deposited from other gases such as C2F6 might make better electrets.  
A theory for charge decay in a nonlinearly-conducting dielectric is derived, and applied to barrier type anodic Al2O3 electret films. Measurements of the decay of SPD are given for anodic films produced in a variety of electrolytes. -5. new rinsing procedure for avoiding charge compensation on these electrets is given. Charge decay is experimentally shown to be dominated by surface conduction, and measurements of charge decay in Al2O3 electrets in vacuum yield extrapolated decay times of 8 years.
A new phenomenon, the piezo-electro-capillary (PEC) effect, is observed in wet, porous, anodic Al2O3 films. An explanation for ths effect is given in terms of the electro-osmotic effect and the propagation of acoustic waves in a fluid-tilled capillary. In same porous anodic oxides, the pore diameter is comparable to the Debye length in the liquid in the pore, therefore the liquid in the pore carries an electric charge which is equal and opposite to the charge in the porous oxide. The motion relative to the pore wall of this charged liquid results in an electrical current and an externally measurable signal. The observed output voltage correlates with derived functions of the compressibility, viscosity and resistivity of the fluid.
Publication date: 
November 30, 1983
Publication type: 
Ph.D. Dissertation
Bernstein, J. J. (1983). Integrated-circuit-compatible Electric and Condenser Ultrasonic Transducers. United States: University of California, Berkeley.

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