Slot microphone

Details Slot microphone Slot microphone

1998-01-21

2001-06-19

Le, Huyen (Department: 2743)

Electrical audio signal processing systems and devices

Electro-acoustic audio transducer

Microphone capsule only

C381S191000

Reexamination Certificate

active

06249586

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention concerns a miniature microphone assembly advantageously based on silicon technology which can preferentially be manufactured using micromechanical methods.
Microphones transform sound into electrical signals. Certain applications require a reduction in the size of a microphone, for example for use in hearing aids. However, a reduction in the size of the microphone often leads to a reduction in the size of the microphone signal due to an associated reduction in the size of the sound-detecting diaphragm. The miniaturization of microphones based on silicon technology has been effected with the assistance of micromechanical techniques. Towards this end it has been possible to reduce the size of microphone membranes to less than 1 mm
2
. The sensitivity of such microphones is typically less than 1 mV/Pa. Current techniques allow for simultaneous manufacture of the microphones and integrated circuits on the same chip in order to be able to amplify the small signal directly at the location of signal production to thereby improve the signal-to-noise ratio. All microphones using this technology of prior art utilize a microphone membrane aligned perpendicular to the incident direction of the sound for maximizing the microphone sensitivity.
This technology has the disadvantage that, despite utilization of silicon chip technology, the chip area cannot be made as small as possible since signals of insufficient strength would thereby result for membrane sizes under 1 mm
2
. In addition, the membrane is surrounded by a frame needed for membrane support and the electronic amplification circuit requires additional surface area to be accommodated within this frame so that the microphone surface which is actually available for sound purposes cannot be less than several mm
2
in size.
Departing from this prior art, it is a primary object of the present invention to improve this arrangement for miniaturized microphone assemblies based, in particular, on silicon technology to overcome the problems associated with large external frontal areas while nevertheless maintaining a good signal-to-noise ratio for the microphone and a compact overall size.
SUMMARY OF THE INVENTION
The purpose of the invention is primarily achieved in a microphone system comprising a first plate having a first width, a second plate positioned across and at a first separation from the first plate, an acoustical membrane integrated in the first plate, and a first spacer means disposed between the first and the second plates to hold the first plate at the first separation from the second plate and to define an air gap between the first and the second plates, wherein the first separation is substantially less than the first width.
In accordance with the invention, an acoustical wave guide is formed between the first and second plates for passing an incident acoustical signal wave into and along the air gap to be incident on and detected by the acoustical membrane. The geometry of the inventive microphone system allows for use of small rectangular or circular moving diaphragm or plate electrodes having areas typically between 0.2×0.2 mm
2
to 0.5×0.5 mm
2
. This diaphragm is the acoustically active part of the transducer and is placed together with other parts of the system on a substrate plate for direct excitation by the incident acoustic pressure-wave. The diaphragm is situated at the front of the transducer and the size of the substrate plate determines the entire size of the assembly. The pressure sensitivity of the transducer thereby depends on the effective area of the diaphragm. The parallel geometry of the diaphragm along the air gap allows for the use of a plurality of diaphragm sensors to thereby increase the signal-to-noise ratio while maintaining the small overall size of the frontal portion of the microphone assembly.
In a particularly preferred embodiment of the microphone the first spacer means closes a first end of the air gap. This embodiment has the advantage of maintaining a stable separation between the first and second plates while providing a location for electronic circuit signal processing and for input and output leads for the electrical signals.
In an improvement of this embodiment, the first spacer means closes a first side of the air gap adjacent to the first end and a second side of the air gap adjacent to the first end and opposite the first side. This embodiment has the advantage that acoustical access to the air gap is from a front end of the device so that the air gap defines a closed wave guide system for the directed passage of the acoustical pressure wave into and through the air gap.
A preferred embodiment of the invention comprises a plurality of membranes integrated into the first plate. This embodiment has the advantage that an increased surface area for acoustical detection of the signal is facilitated without expanding the frontal dimensions of the device.
An improvement of this embodiment provides for a second plurality of membranes integrated into the second plate. This embodiment has the advantage that both the first and the second plates are acoustically active so that a larger fraction of the entire inner surface of the air gap defined by the first and second plates is utilized to produce acoustical signals for increasing the sensitivity of the device while largely maintaining its overall size.
A particularly preferred variation of this embodiment provides for a stacked system of microphone structures having a plurality of plates each pair of which having an air gap defined and separated by a spacer means. All of the plates adjacent to the respective gaps can thereby be configured with acoustically active membranes so that maximum usage of the inside area of the gaps for detection of the incident acoustical pressure wave is facilitated. This embodiment can be particularly envisioned as comprising a third plate having a third width, stacked above the first and the second plates, and having a third plurality of acoustical membranes as well as a fourth plate positioned above and at a second separation from the third plate, the fourth plate also having a fourth plurality of acoustical membranes. A second spacer means is then disposed between the third and the fourth plates to hold the fourth plate at the second separation above the third plate. As in the primary embodiment of the invention, the gap height is substantially smaller than the width of a typical dimension of the plates.
A preferred embodiment of the invention envisions silicon technology for processing the first and second plates. This embodiment allows for miniature silicon technology developments to be utilized, preferentially in a micromechanical fashion, to integrate mechanical and electronic requirements.
Another additional preferred embodiment envisions varying the separation between the first and second plates along a length of the plates. This embodiment allows for tailoring of the acoustical properties of the wave guide to, for example, compensate for attenuation effects along the length of the wave guide.
A preferred embodiment envisions an acoustical circular membrane having a diameter between 0.2 and 0.5 mm
2
or an acoustical rectangular membrane of dimensions between 0.2×0.2 and 0.5×0.5 mm
2
. This embodiment has the advantage of comprising a membrane which can be advantageously manufactured using current micromechanical technology.
In an additional embodiment an acoustically transparent material is positioned between the first and the second plates to protect, isolate and define the air gap. This embodiment has the advantage of preventing dust and other foreign objects from gaining entrance to the sensitive internal components, in particular, to the membrane located within the air gap.
In an improvement of the above mentioned embodiment the acoustically transparent material has a desired acoustical impedance. This embodiment has the advantage of double use of the acoustically transparent material both as a protective material as well as a materi

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