Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-12-10
2001-10-30
Young, Lee (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S025410, C029S412000, C029S025350, C029S886000, C310S322000, C310S324000
Reexamination Certificate
active
06308398
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to electroacoustic transducers, such as microphones, and particularly to capacitive electroacoustic transducers fabricated in batches by means of a wafer manufacturing process.
2. Background Art
Capacitive electroacoustic transducers are widely used for the measurement of static and dynamic pressures. Traditionally, these capacitive transducers, such as employed in a microphone, have been made in such a manner that one electrode of a capacitor structure is formed by an electrically conductive diaphragm. This diaphragm is disposed adjacent to, but insulated from, a stationary electrode forming the other electrode of the capacitor structure. The two electrodes are spaced apart with an air gap in-between. A relatively high DC bias voltage is then applied between the electrodes. Variations in the electrode spacing caused by deflections of the diaphragm in response to the force of acoustic wave energy incident on the diaphragm, produce a change in capacitance. A detection network is connected to the capacitive transducer such that the change in capacitance is detected and transformed into an electrical signal proportional to the force of the acoustic wave energy applied to the diaphragm.
The sensitivity and performance of a capacitive electroacoustic transducer is closely tied to the at-rest spacing between the diaphragm and the stationary electrode. Thus, this spacing must be accurately controlled. To achieve accurate spacing, close machining tolerances are required for the parts making up the transducer. The required tolerances can be extremely difficult to hold in production. As a result, these devices are often hand crafted from machined parts in an attempt to meet the response and sensitivity characteristics imposed by the particular application in which the transducer is to be employed. This hand crafting tends to increase the cost of the transducers. Additionally, each transducer so produced exhibits a slightly different response in phase and magnitude.
The sensitivity and response of a capacitive electroacoustic transducer is also closely tied to its thermal stability. This thermal stability is partially dependent upon the change in the separation between the diaphragm and the stationary electrode caused by expansion or contraction of the transducer components when subjected to changing temperatures. The critical electrode spacing in existing capacitive transducers has been difficult to maintain over a widely varying temperature environment. This is especially true where the differential axial expansion length of the components is large in the first place. For instance, many existing transducers have expansion lengths on the order of 0.25 inch. Large expansion lengths mean that expansion and contraction of the transducer elements produce significant changes in the electrode separation distance. A significant change in this separation distance alters the response of the transducer. Additionally, changes in the tension on the diaphragm resulting from differing rates of expansion for the case than for the diaphragm, also affect the thermal stability of the transducer. When the tension of the diaphragm is allowed to change with temperature, the sensitivity of the transducer is altered.
Therefore, what is needed is a capacitive electroacoustic transducer which can be batch produced with consistent and reproducible response and sensitivity performance characteristics, and which maintains these characteristics even over a widely varying temperature environment.
SUMMARY
Wherefore, it is an object of the present invention to provide a capacitive electroacoustic transducer made by a repeatable process that produces a desired at-rest spacing between the diaphragm and planar electrodes of the transducer without the necessity of hand crafting.
Wherefore, it is another object of the present invention to provide a capacitive electroacoustic transducer which can be batch produced with repeatable and consistent response and sensitivity performance characteristics between the individual transducers so produced.
Wherefore, it is still another object of the present invention to provide a capacitive electroacoustic transducer which maintains consistent response and sensitivity performance characteristics over a widely varying temperature environment.
The foregoing objects have been attained by a capacitive electroacoustic transducer which includes an electrically insulative substrate, a layer of conductive material disposed on a portion of a top surface of the substrate forming a first electrode of the transducer, a conductive diaphragm forming a second electrode of the transducer which is deflectable in relation to the first electrode, and a structure for electrically and physically separating the first and second electrodes in a spaced relationship so as to constitute a capacitor. This electrical and physical separation allows an electric field formed between the first and second electrodes to vary in relationship with deflections of the second electrode to permit conversion between electrical and acoustic signals. In addition, the substrate and first electrode can include at least one through-hole for allowing air trapped in the space formed between the diaphragm and the top surfaces of the substrate and first electrode to escape to a region adjacent a back surface of the substrate. The number and diameter of these holes determines the resistance to the aforementioned air flow, and thus partially determines the response characteristics of the transducer. Also, the diaphragm includes a vent hole for equalizing relative pressure between ambient air exterior of the diaphragm and air interior of the diaphragm. This equalization is required to provide stable transducer performance characteristics in the face of variations in the external air pressure. In addition, the vent hole size can be varied to tune the response characteristics of the transducer.
Preferably, the separating structure is a diaphragm mounting ring disposed about the periphery of the top surface of the substrate and separated from the first electrode. The ring is thicker than the first electrode by an amount corresponding to a desired separation between the diaphragm and the first electrode. The diaphragm is also peripherally bonded to this diaphragm mounting ring. In addition, a compensation ring can be disposed on an opposite side of the substrate in an area corresponding to the diaphragm mounting ring on the top surface of the substrate. This compensation ring has the same physical size as the diaphragm mounting ring and is made of the same material. The purpose of the compensation ring is to balance out any stress caused in the substrate by the thermal expansion and contraction of the diaphragm mounting ring. Further, the diaphragm mounting ring and compensation ring can be electrically conductive and electrically connected, thereby allowing connection of the mounting ring to ground or to electronic components from the backside of the substrate.
A layer of conductive material is disposed on the sides of the through-holes and on a bottom surface of the substrate to provide an electrical pathway between the first electrode and the layer of conductive material on the bottom surface of the substrate. This pathway facilitates the connection of the first electrode to the electronics of the transducer.
The above-described transducer exhibits a high degree of thermal stability. The stability is partly due to the substrate and diaphragm being made of materials having closely matched thermal expansion coefficients. This feature ensures that the tension in the diaphragm stays constant even with varying temperatures, thereby maintaining a constant transducer sensitivity. Preferably, the substrate is made of FORSTERITE ceramic material and the diaphragm is made of titanium foil, which have closely matched thermal expansion coefficients. In addition, the distance separating the first and second electrodes is minimized so as to create a short thermal expansion path. Thi
Anderson Terry J.
Hoch, Jr. Karl J.
Northrop Grumman Corporation
Tugbang A. Dexter
Young Lee
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