Patent GB-1414977-A (GB)
Title: Electret transducer
IPC: H01G7/02
Filed: 1972-12-01
Abstract: 1414977 Electret transducers SONY CORP 1 Dec 1972 [2 Dec 1971] 55634/72 Heading H4J [Also in Division G1] A microphone or other transducer comprises an electret member comprising two layers of different inorganic insulating materials and an electrode spaced from and parallel to the electret member. Either the electret member or electrode is movable relative to the other. A microphone (Fig. 4) comprises a gold layer 26 and a duel layer electret member 27 which together form a diaphragm separated from a fixed electrode 23 on a cyclindrical synthetic resin support 12. A slotted insulating plate 13 and a slotted metallic shield plate 16 are pressed against the support 12 by an inwardly turned rim at the lower end of the cylindrical skirt of a cylindrical metal cap 8. A terminal 21 extends from metal plate 16 for connection to earth. A terminal 22 makes electrical contact with electrode 23. The gate of a FET (not shown) is connected to terminal 22 while the source and drain electrodes are connected to the terminals 18 and 19 respectively. An alternative microphone (Fig. 5) has a fixed electrode 33 provided with apertures 34 that communicate with a chamber 36 and, via channels 37, with the rear opening. In another embodiment (Fig. 7) a FET (of either an MOS type or junction type) incorporates the fixed electrode 41 of the transducer as part of the FET structure. The FET comprises a substrate of semiconductor material attached to a header 39 by solder terminal posts 46, 47, 48 are held by the header 39, posts 46 and 48 being connected to the source and drain respectively white post 47 is connected to the gate. A duel layer electret member 27 and metallic film 26 form the diaphragm. In other arrangements the dual layer electret member is formed on the fixed electrode, such as electrode 41 (Fig. 7). In that case the metallic film 26 may be used above as the diaphragm. Preferably the electret structure is made using aluminium sheet 24 (Fig. 6c), thickness 2 mm. on which is vapour deposited a gold layer 26 of thickness 0À1 Á. On top of this layer 26 the electret member is formed. This comprises two separate laminee of different materials, preferably silicon dioxide, aluminium oxide, titanium dioxide or an adduct consisting of phosphorus pentoxide and silicon dioxide. Thus member 27 may be formed by vapour depositing a layer of aluminium oxide of thickness 1500Š to 2000Š and a layer of silicon dioxide of thickness 100Š to 300Š. The member 27 may be permanently polarized by connecting a 70 volt source between the sheet 24 and the exposed surface of the member 27 for about 5 seconds. Alternatively the member 27 is exposed to an electron beam having a voltage of about 5 kV emanating from an electron gun located about 8 cm. from the surface of the member 27. In a further method the member 27 is charged positively by diffusion of sodium or potassium into it. After the member 27 has been polarized a layer 28 of aluminium of thickness about 10 Á is vapour deposited. Subsequently holes 29, 31 (Fig. 6D) are chemically etched through the metal layers 24, 28 to leave a deformable central area consisting of electret member 27 and gold film 26. Thereafter, individual structures are cut out of a large sheet by cutting along lines 32 to form circular structures of diameter to fit cup 8. Remainder portions 24, 28 support the diaphragm layers 26, 27. Specification 1,414,978 claims the electrets and methods of making them.