Method for Fabricating Ferroelectric Ultrasonic Transducers

1. Field of the Invention

The present invention relates to a method of fabricating acoustic transducers, and more particularly to a method of fabricating ferroelectric ultrasonic transducers by using an electton beam.

2. Description of the Prior Art

Ferroelectric acoustic transducers are currently fabricated by placing a thin disk of the ferroelectric material in an oven at the Curie temperature of the ferroelectric. A high DC voltage is imposed on the disk to polarize it and make it acoustically active in a manner similar to piezoelectric crystals. The thickness of the disk determines the resonant frequency of the transducer and damping is determined by a backing member.

The backing member must be impedance matched as closely as possible to the ferroelectric disk to minimize the amplitude of back reflected waves. These back reflected waves result in decreased sensitivity.

For high frequencies (greater than 10 megahertz) a very thin disk is required. However, the ferroelectric materials are very brittle and the thicknesses required to reach these frequencies are very difficult to achieve in useful diameters.

Accordingly, the present invention provides a method for fabricating ferroelectric ultrasonic transducers which uses the bulk ferroelectric material as the backing member while a plate area within the material serves as the transducer. A ferroelectric disk is placed in a vacuum and a beam of electrons is directed at the disk. An aperture confines the beam to a desired diameter, and the depth of heating is controlled by the energy of the beam and the time of exposure to the beam to produce a plate in which the Curie temperature is everywhere exceeded.

A high DC voltage is applied across the faces of the disk while the Curie temperature of the plate is exceeded to polarize the plate, which will then act as an acoustic generator and receiver.

Therefore, it is an object of the present invention to provide a method for fabricating ferroelectric ultrasonic transducers which polarizes ferroelectric solids to a specified depth without polarizing the bulk material.

Another object of the present invention is to provide a method for fabricating ferroelectric ultrasonic transducers which do not create back reflected waves.

Yet another object of the present invention is to provide a method for fabricating a ferroelectric ultrasonic transducer which is not brittle.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing wherein:

FIG. 1 is a diagrammatic view of the method of the present invention.

FIG. 2 is a sectional view of the transducer fabricated by the method of the present invention.

Referring now to FIG. 1 a ferroelectric disk 10 or other flat shape is fabricated with flat parallel faces. These faces are provided with conductive surfaces 12, 14 which are electrically insulated from each other. The conductive surfaces 12, 14 are a thin conductive layer from 0.02 to 0.1 microns thick of an electrical conducting material such as aluminum, gold, platinum, silver, etc. The disk 10 is placed in a vacuum and a source of electrons 16 provides a beam of electrons 18 which is directed at the disk. An aperture 20 confines the electron beam 18 to a desired diameter.

As the electrons from the electron beam 18 are absorbed into the surface of the ferroelectric disk 10, the disk surface heats. The depth to which the ferroelectric disk 10 is heated is controlled by the energy of the electron beam 18 and the time of exposure to the ferroelectric material. The plate 22 is defined by the region within the disk in which the Curie temperature of the ferroelectric material is everywhere exceeded. The conductive surface 12 facing the source of electrons 16 is maintained at a high positive potential relative to the source of electrons to control the depth of the plate 22, and thereby determine the operating frequency of the finished transducer which is inversely proportional to the thickness, or depth, of the plate. A high DC voltage source 24 imposes a potential difference between the conductive surfaces 12, 14 such that an electric field of at least 10 million volts per meter (10.sup.7 vdc/m) exists in the ferroelectric disk 10. The high potential difference between the conductive surfaces 12, 14 is maintained while the plate 22 is maintained above the Curie temperature, and results in polarizing the plate, i.e., enabling the plate to act as an acoustic generator and receiver.

To complete the transducer no further preparation is required except for enclosure in a case 26 and appropriate wiring 28 attached to the conductive surface 14 opposite the plate 22, as shown in FIG. 2.

The surface of the ferroelectric material exposed to the electron beam 18 may be curved rather than flat to achieve focusing of the sound, the curvature being determined by the conventional criteria used for shaped transducer elements.

Thus, the transducer fabricated according to the present invention eliminates the conventional backing member since the buld ferroelectric material provides an exact imedance match which eliminates back reflected waves. The transducer also eliminates the requirement of a very thin, brittle ferroelectric disk for high frequencies since the disk may have an arbitrary thickness, the frequency being determined by the thickness, or depth, of the plate.