Method of making an electret acoustic transducer

In the method of construction disclosed herein, posts for supporting the condenser transducer diaphragm in relation to a backplate are formed on the diaphragm film itself by selectively etching away a photoresist material laminated to the film. The electret material is then supported on the backplate rather than being carried by the diaphragm.

BACKGROUND OF THE INVENTION 
This invention relates to electret condenser acoustic transducers and more 
particularly to a method of providing supporting posts for spacing the 
diaphragm in a transducer with respect to a backplate which constitutes a 
fixed electrode. 
In the manufacture of electret condenser transducers, e.g. microphones, 
various methods have been proposed for precisely maintaining the desired 
small separation between the diaphragm which constitutes a moving 
electrode and a backplate which constitutes a fixed electrode. In addition 
to various schemes for clamping the diaphragm at its edges, methods have 
been proposed for forming regular or irregular patterns of ribs, bumps or 
posts which separate the two elements and effectively divide the diaphragm 
area into a plurality of regions. Several such methods are disclosed, for 
example, in the P. V. Murphy reissue Pat. Re. 28,420. As the usual 
diaphragm material comprises a thin film which is under tension, the 
straightforward and therefore apparently universally employed method of 
generating the post pattern is to form it on the backplate while the 
diaphragm is made comprising the material which can be polarized as an 
electret. 
In accordance with the present invention, however, it has been found that 
substantial unexpected advantages can be obtained if the post pattern is 
formed on the diaphragm, despite its thin film nature. A principal 
advantage is that the backplate, i.e. fixed electrode, may then be formed 
with a smooth, flat surface and the electret material can then be laid 
down on that surface. While most of the usual electret materials are 
plastics or resins, they do not have mechanical properties which are 
themselves ideal for use as a diaphragm. For example, Teflon, 
(polytetrafluoroethylene) one of the most stable and commonly used 
electrets, is not particularly tough or elastic so as to render it ideal 
for a highly compliant acoustically responsive diaphragm such as is 
desired in constructing a measurement grade condenser microphone. 
In accordance with another aspect of the invention, it has been found that 
supporting posts appropriate for defining the spacing between the 
diaphragm and backplate in an electret transducer may be expeditiously 
formed on a plastic film suitable for use as a diaphragm by conventional 
photo-etching techniques. 
SUMMARY OF THE INVENTION 
Briefly, the making of an electret acoustic transducer in accordance with 
the present invention involves laminating a plastic film with a 
photoresist material, exposing the photoresist material in a pattern 
corresponding to the desired post pattern, and then removing, e.g. by 
etching, the undesired portion of the material leaving the desired post 
pattern on the film. After metalizing the post side of the film, it is 
mounted on a supporting ring and the ring and a conductive backplate 
carrying a layer of polarized electret material are mounted in 
juxtaposition in a transducer assembly with the post pattern contacting 
the electret layer. Accordingly, the diaphragmatic film is spaced from the 
backplate by the post pattern. The metalization on the film provides a 
moving electrode while the backplate constitutes the fixed electrode. The 
electret material, being itself insulating, prevents shorting between the 
electrodes.

Corresponding reference characters indicate corresponding parts throughout 
the several views of the drawings. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, the completed microphone illustrated there by way 
of an exemplary transducer employs a thin, low mass diaphragm 11 which is 
supported in close proximity to a backplate 13. As is explained in greater 
detail hereinafter, the film of diaphragm 11 is metalized to render it 
conductive and this diaphragm acts as a moving element varying the 
capacitance of the transducer system. The backplate 13 is likewise 
conductive and functions as a fixed electrode in the system. 
Spacing between the diaphragm 11 and the backplate 13 is controlled to 
great extent by a series of separators 15 of predetermined thickness. The 
separators 15 are conventionally designated as posts in spite of their 
rather shallow height. 
Diaphragm 11 is held at its periphery, together with a thin insulating 
washer 17, between inner and outer supporting rings 18 and 19, 
respectively. The rings 18 and 19 together with the diaphragm 11 and 
washer 17 are preferably bonded together, e.g. by an epoxy adhesive. The 
rings 18 and 19 are clamped against the upper rim 21 of an otherwise 
generally cylindrical housing 23 by an insulating sleeve 25 which is in 
turn urged upwardly by a lock nut 27 which engages internal threads 29 on 
the lower part of the housing 23. A fiber washer 31 may be interposed 
between the lock nut 27 and the sleeve 25. 
The sleeve 25 is internally threaded and carries, in these threads, a 
backplate terminal assembly 33 which carries the backplate 13. 
Accordingly, the height of the backplate with respect to the mounting 
rings 19 and 21 and the diaphragm 11 carried thereby may be adjusted by 
appropriately rotating the assembly 33 with respect to the sleeve 25. The 
assembly 33 may be maintained at the desired level by tightening an inner 
locknut 35 against the assembly. 
The backplate carrier 33 is vented as indicated at 37 to allow pressures on 
the back side of the diaphragm 11 to equalize. The microphone capsule is 
completed by a perforated cover 39, the shape and the orientation of the 
perforations in the cover being selected in relation to the desired 
acoustic characteristics desired of the microphone. 
As indicated previously, the formation of the separating posts 15 on the 
diaphragm allows the electret material to be laid down on the essentially 
flat top surface of the backing plate 13. This layer is indicated at 36 in 
FIG. 5 which diagrammatically illustrates, to enlarged scale, the 
diaphragm 11 and backplate 13 in juxtaposition. The presently preferred 
material for the electret layer 36 is polytetrafluoroethylene (Teflon). 
This material may be polarized with a high degree of stability as taught 
in U.S. Pat. No. 3,644,605, the Sessler et al patent. 
In order to ultimately obtain the desired acoustic properties, the 
diaphragm 11 preferably comprises a relatively tough and elastic plastic 
film such as that sold under the tradename Mylar. A thickness of about one 
quarter mil is appropriate for the film itself. In constructing the 
diaphragm with its spacing posts in accordance with the present invention, 
the film is laminated with a conventional photoresist material. A 
presently preferred photoresist material of this type is that available 
commercially under the designation Dynachem type AX resist film. Adhesion 
of this material with the Mylar film is improved if the Mylar is initially 
metalized. Such metalized film is available commercially from a variety of 
sources. The metalized film is illustrated diagrammatically in FIG. 2, the 
Mylar itself being designated by reference character 41, its metalization 
by reference character 43. It should be understood that the thicknesses of 
the various layers are not shown to scale but have been exaggerated for 
the purpose of illustration. In FIG. 3, the laminated structure is 
illustrated, the photoresist material being indicated at 45 while the 
adhesive layer is indicated at 47. The photoresist material is supplied 
with a mylar protective overlay which is indicated at 47 in FIG. 3. This 
overlay is, however, removed before the resist material is developed. The 
composite material is exposed and etched using conventional printed 
circuit techniques to remove all of the photoresist material from the film 
except those portions constituting the desired post pattern. In the case 
of a one inch diameter standard microphone, a circular array of posts 15 
may be appropriate, as illustrated in FIG. 6, whereas in the case of a 
half inch diameter standard microphone, a single post 52 in the center of 
the diaphragm may be appropriate, as illustrated in FIG. 7. 
After etching, the underside of the diaphragm is metalized, e.g. by 
evaporating gold under vacuum onto the surface of the plastic diaphragm to 
provide a coating which is essentially conducting within the context of 
the charge movement experienced within a condenser microphone. While 
metalization in this fashion will typically cover the underside of the 
posts 15 as well as the intervening surfaces of the Mylar film, e.g. as 
indicated at 55 in FIG. 5, it should be understood that this conductive 
coating will not short the transducer capacitance owing to the 
non-conducting Teflon layer 36 on the top surface of the backing plate 
against which posts 15 ultimately rest. 
In assembling the microphone, the height of the backing plate is adjusted 
with respect to the diaphragm mounting rings so that the posts lift the 
diaphragm slightly, i.e. distorting it slightly, and thereby themselves 
establish the nominal spacing between the two plates of the capacitor. 
With the layer 36 polarized, a charge distribution within the transducer 
exists such that movement of the diaphragm 11 will cause a voltage to be 
generated between the metalization on the diaphragm and the metal backing 
plate 13. The diaphragm is connected through the rings 18 and 19 to the 
housing 23 which constitutes one output terminal while the backing plate 
13 is connected directly to the carrier 33 which comprises the other, i.e. 
center, output terminal, a conventional arrangement with regard to 
standard microphones. Thus, a signal generated by vibration of the 
diaphragm can be conducted out of the transducer to a suitable high input 
impedance pre-amplifier. In the same fashion, if an alternating electrical 
voltage is applied between the center terminal 33 and the housing 23, an 
electrostatic force will be produced between the metalization on the 
diaphragm and the backplate 13. This force will cause the diaphragm to 
move thus producing an acoustic output. 
In view of the foregoing, it may be seen that several objects of the 
present invention are achieved and other advantageous results have been 
attained. 
As various changes could be made in the above constructions without 
departing from the scope of the invention, it should be understood that 
all matter contained in the above description or shown in the accompanying 
drawings shall be interpreted as illustrative and not in a limiting sense.