Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-11-18
2001-06-19
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S328000, C029S025350
Reexamination Certificate
active
06249075
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to acoustic transducers and more specifically to, micro-machined acoustic transducers.
DESCRIPTION OF THE RELATED ART
Transducers are devices which convert input energy in one form into output energy in another form. For example, microphones are acoustic transducers that convert input acoustic energy into output electrical energy. Micro-machined acoustic transducers are miniature transducers (sub-centimeter in size) fabricated with techniques commonly used for making integrated circuits (e. g., material deposition/growth, lithography, and etching). Potential uses for micro-machined acoustic transducers include microphones for hearing aids and pressure sensors for automobiles. Scheeper, P. R., et al. “A Review of Silicon Microphones”,
Sensors and Actuators
, Vol. A 44 (1994) pp. 1-11, describes several micro-machined transducers suitable for converting acoustic signals into electrical signals. A cross-sectional view of one such micro-machined acoustic transducer
1
is shown in FIG.
1
. Micro-machined acoustic transducer
1
includes a membrane
4
and substrate
3
. The substrate
3
has an acoustic cavity
7
formed in a surface thereof. The membrane
4
is attached to the substrate
3
and covers the acoustic cavity
7
.
Micro-machined acoustic transducer
1
converts input acoustic signals into output electrical signals. In particular, when acoustic signals
5
(e. g., sound waves) impinge on membrane
4
, the impinging acoustic signals apply a force thereto. The force applied by the acoustic signals impinging on the membrane
4
causes the portion of the membrane
4
covering the acoustic cavity
7
to vibrate. The membrane
4
vibrates relative to the surface of the substrate
3
. Thus, the impinging acoustic signals are converted into mechanical energy (i. e. membrane vibration).
The amount of acoustic energy that is transformed into mechanical energy depends on the amount of force applied to the membrane
4
by the impinging acoustic signals
5
as well as the physical properties associated with the membrane material (e. g., thickness, elastic properties, and tensile strength).
The mechanical displacement imparted to the membrane
4
by the impinging acoustic signals
5
is converted to an electrical signal using for example strain gauges (not shown) or by detecting changes in the capacitance between the membrane
4
and an electrode (not shown). Thereafter, the electrical signal is amplified and filtered.
One problem with some micro-machined transducers is related to the depth of the acoustic cavity
7
. Since the acoustic cavity
7
is formed in the surface of the substrate
3
, the thickness of the substrate
3
limits the depth of such cavity
7
. The depth of the acoustic cavity
7
is related to the compressibility of a gas (e.g., air) confined therein. The compressibility of a confined gas refers to the ability of such gas to be displaced in response to the application of a force. For example, when an impinging acoustic signal applies a force to a membrane confining a gas in a cavity formed in a substrate, the membrane is displaced x(t) as
x
(
t
)
=
V
(
p
(
t
)
-
p
0
)
p
0
(
1
)
where V is the volume of the cavity, p
0
is the initial pressure both in and out of the cavity, and p(t) is the pressure of the sound wave at time t. Based on equation 1, as the depth of the cavity becomes smaller, the membrane vibrates less. When the membrane vibrates less in response to acoustic signals impinging thereon, the amount of input acoustic energy that gets converted to mechanical energy and output as electrical energy is reduced.
Thus, micro-machined transducers continue to be sought.
SUMMARY OF THE INVENTION
The present invention is directed to an acoustic micro-machined transducer having a structure wherein an acoustic enclosure is formed on a substrate above the plane of the substrate surface. Forming the acoustic enclosure on the substrate above the plane of the substrate surface, rather than an acoustic cavity in a surface of the substrate, provides an acoustic cavity with a size that is not limited by the thickness of the substrate.
The acoustic enclosure formed on the substrate surface is defined by a plurality of enclosure sides. Since the acoustic enclosure is on the surface of the substrate, the bottom enclosure side is a portion of the substrate surface. Examples of suitable geometrical shapes for the acoustic enclosure include polyhedrons such as tetrahedrons and cubes.
At least two enclosure sides are in hinged attachment with each other to form the enclosure. At least one enclosure side is in hinged attachment with the substrate.
At least one enclosure side is adapted to receive acoustic energy by having an acoustic membrane formed therein. The acoustic membrane is attached with beams to the enclosure side. Attaching the acoustic membrane to the enclosure side with beams, means that the physical properties associated with the membrane (e. g., tensile strength, elastic properties, dependence, thereby increasing the conversion of acoustic signals into electrical signals.
The acoustic membrane moves rigidly in response to acoustic signals impinging thereon. The acoustic membrane is made of one or more layers of material. Examples of suitable materials for the acoustic membrane include polysilicon, silicon nitride, and silicon dioxide.
When the acoustic membrane moves in response to acoustic signals impinging thereon, the beams attaching such membrane to the enclosure side are displaced. transforming the acoustic energy of the acoustic signals into mechanical energy. The amount of acoustic energy transformed into mechanical energy depends on the physical properties (e.g., tensile strength, elastic properties, layer thickness) associated with the beams.
A detector is coupled to the acoustic membrane. The detector measures the mechanical energy imparted to the beams from the impinging acoustic signals. Piezoelectric devices, piezoresistive devices, and capacitive devices are examples of detectors suitable for measuring the mechanical energy imparted to the acoustic membrane.
The detector is also coupled with electronics which convert the mechanical energy measured thereby into electrical energy. Examples of electronics suitable for converting mechanical energy into electrical energy include amplifiers, modulators/demodulators, and filters.
In one embodiment of the present invention, the enclosure sides are formed on a surface of a substrate. The enclosure sides are formed on the surface of the substrate by depositing one or more material layers on the substrate followed by defining in them a desired geometrical shape which can be assembled to form the enclosure. The one or more material layers are deposited on the substrate and defined in the desired geometrical shape with techniques (e. g., lithography, evaporation, etching) typically used for making integrated circuits.
The one or more material layers used to form the enclosure sides have a tensile strength which prevents the enclosure sides from significantly bowing. The tensile strength of the one or more material layers depends on the composition as well as the thickness thereof. Examples of suitable materials for the one or more material layers include polysilicon, silicon nitride, silicon dioxide, and metals.
The substrate is made of a material typically used for integrated circuit fabrication. Examples of such substrate materials include silicon and quartz.
The geometrical shape of each enclosure side defined in the one or more material layers depends on the shape of the acoustic enclosure. Examples of suitable shapes for the enclosure sides are square, rectangular, and triangular.
The enclosure sides are in hinged attachment with each other and with the substrate. The hinges permit the enclosure to be formed by pivoting the enclosure sides with respect to each other and with respect to the substrate surface. The hinges are formed with fabrication techniques (e. g., evaporation, lithography and etching) typically used for making integrate
Bishop David John
Boie Robert Albert
Pardo Flavio
Botos Richard J.
Dougherty Thomas M.
Lucent Technologies - Inc.
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