Method of making a variable capacitor microphone

DETAILED DESCRIPTION One of the objects of our invention is to provide a method of making a variable capacitor microphone comprising that maintains its polarization after a long period of time, even in moist environments.
Another object of out invention is to provide a variable capacitor microphone comprising an electret that satisfactorily adheres to a substrate to which it is applied.
Other and further objects will be apparent from the following description.
Our invention contemplates a method of making a variable capacitor microphone in which the fixed capacitor element is formed by first coating the metallized surface of a relatively rigid insulating substrate with a polarizable dielectric material which preferably is a fluorocarbon and more particularly a fluorinated ethylene-prophylene copolymer such as the one sold under the trademark TEFLON.
The coating is preferably applied to a plurality of conductive supports carried by a single substrate, preferably by spraying through a mask.
Next, the dielectric carried by the substrate is permanently polarized.
More particularly, a pair of electrodes, one of which preferably supports the substrate carrying the dielectric and is heated, define a region therebetween capable of containing an ionizable gas such as air.
The region is exposed to alpha radiation to generate charged particles of a given polarity from the gas, and a potential is applied to the electrodes to effect electrostatic deposition of the particles on a dielectric disposed in the region.
Preferably, the alpha radiation source, which may comprise polonium 210 as noted above, is disposed relatively adjacent to the charging electrode and is shielded from the heating source.
We have found that by using an alpha source such as polonium 210 as an ion generator, we can effectively polarize dielectrics to form electrets.
At the same time, by avoiding the use of a corona generator with its attendant high-intensity field, we avoid the undesirable result of generating ozone.
We have found that by forming the dielectric layer in situ in the manner described above on the surface of the conductive support, we ensure satisfactory adhesion between the dielectric and the support