The present invention relates to an acoustic sensor element having at least one diaphragm and at least one fixed counter element. The diaphragm of the sensor element is situated in a cavity between a substrate and the counter element and acts as movable electrode of a capacitor system, while the counter element functions as fixed counter electrode of this capacitor system. At least one through hole is formed in the substrate, via which sound pressure is able to act upon the diaphragm.
Micromechanical microphones are available which convert the sound waves into an electrical signal with the aid of such a sensor element. The conventional sensor elements include a capacitor system having at least two electrodes, between which an air gap of 0.5 μm to 10 μm is situated. Ideally, one electrode is rigid while the other electrode is movable, so that it is induced to oscillate when sound waves arrive. This causes a change in the capacitance between the two electrodes in accordance with the varying sound pressure.
The quality of such a micromechanical transducer element generally depends on the immovability of the counter electrode. In practice, the counter electrode is therefore frequently provided with relatively great thickness in that it is either structured out of the carrier substrate of the transducer element, or in that it is retroactively provided with a thick layer made of epi polysilicon, for example. However, high rigidity of the counter electrode may also be achieved if the counter electrode is produced under high tensile stress. Both the structuring of the carrier substrate and the producing of high layer thicknesses or the producing of highly stretched layers is labor-intensive and correspondingly costly.
An acoustic sensor or transducer element is described in U.S. Pat. No. 6,535,460 B2. The design of this sensor element includes a substrate having a through hole, which is spanned by a diaphragm. A perforated counter element is situated above the diaphragm, at a distance therefrom, and is connected to the substrate in the edge region of the through hole. Diaphragm and counter element jointly form a capacitor, the diaphragm acting as movable electrode while the counter element constitutes the rigid electrode. Via the through hole in the substrate, sound waves are acting upon the diaphragm, which causes the diaphragm to oscillate. The movement of the diaphragm is then detected with the aid of the counter element as capacity fluctuations of the capacitor. Special measures for affixing and/or strengthening the perforated counter element are not described in U.S. Pat. No. 6,535,460 B2.
The present invention provides simple constructive measures for improving the transducer characteristics of a micromechanical acoustic sensor element of the type mentioned in the introduction. These measures concern the fixation and strengthening of the counter element or the counter electrode of the capacitor system, in particular.
According to example embodiments of the present invention, the counter element is connected to the substrate by at least one support element for this purpose, the support element being situated in the region of the cavity. Moreover, an opening is formed in the diaphragm for the support element, so that the diaphragm is able to swing freely within the cavity.
According to the present invention it was realized that the rigidity of the counter element is able to be increased simply in that the counter element is supported at one or a plurality of locations on an existing firm structure of the substrate, and the wing span of the counter element is therefore reduced. This measure provides the opportunity to realize the counter element also in the form of a thin layer that need not necessarily be under tensile stress. The wing span of the diaphragm, and thus also the sensitivity of the sensor element, are not affected by the support element to any important extent since the diaphragm according to the present invention is provided with openings through which the support elements extend from the counter element to the substrate structure, so that the diaphragm is able to move freely between the counter element and the substrate structure.
Because the counter element of the example sensor element according to the present invention is able to be realized in a thin layer, which need not be designed for high tensile stressing, overall the example sensor element according to the present invention may be produced with the aid of standard semiconductor processes, which are cost-effective and allow high volume production.
There are basically different possibilities for designing a sensor element according to the present invention and, in particular, for the placement of the support elements in the region of the cavity between the counter element and the substrate.
In one preferred variant of the present invention, a substrate structure having a substrate base for the support element is formed in the area below the cavity. Therefore, the substrate base is situated underneath the cavity and thus connected to the “substrate mainland”, so that the substrate base is fixed in place and forms an excellent support point for the support element and the counter element.
In an advantageous manner, the substrate structure underneath the cavity, or the through hole in the substrate delimited by the substrate structure, is designed in such a way that the diaphragm is able to be acted upon by sound pressure on the largest surface possible. It is advantageous in this context if the substrate base is connected to the substrate in the edge region of the cavity via relatively narrow webs. The stability of the substrate structure required for the fixation of the counter element is able to be achieved in an uncomplicated manner in that the substrate base and the webs essentially have the thickness of the unstructured substrate.
In an advantageous development of the example sensor element according to the present invention, the counter element is provided with perforation holes, which reduce damping of the diaphragm oscillation. In addition, a pressure compensation between the cavity above the diaphragm and the environment is able to take place via these perforation holes.
With the aid of the example sensor system according to the present invention, it is possible to detect sound waves in differential manner as well. For this purpose, the example sensor element according to the present invention is simply provided with an additional fixed counter electrode, which is realized in the substrate or in the substrate structure underneath the diaphragm.
As previously discussed already, there are different possibilities for realizing and further developing the teaching of the present invention in an advantageous manner. In this context, reference is made to the following description of a plurality of exemplary embodiments of the present invention with reference to the figures.
a shows a plan view of the substrate of an example sensor element according to the present invention.
b shows a plan view of the diaphragm of this sensor element.
c shows a plan view of the counter element of this sensor element.
The layer structure of acoustic sensor element 10 shown in
Micromechanical components like the afore-described sensor element 10 are produced on the basis of a semiconductor substrate, e.g., a silicon wafer. Counter element 3, functioning as fixed electrode, of sensor element 10 is developed in a polysilicon layer, for example, with a thickness of 0.5 μm-4 μm. This layer is able to be produced in a simple standard LPCVD process and doped. The layer tension that comes about in such a process typically lies between 10-100 mPa of pressure. Because counter element 3 of sensor element 10 according to the present invention is stabilized and fixed in place with the aid of support element 7, no special measures have to be taken to increase or influence the layer tension. Support element 7 advantageously is made from an electrically insulating material in order to decouple substrate 1 and counter element 3 electrically. For instance, support element 7 may be made of oxide, which is selectively left to remain as residual oxide during the sacrificial layer etching for the purpose of exposing diaphragm 2 and for producing cavity 4. However, other electrically insulated variants are also possible, such as a polysilicon support element having nitride insulation, for instance.
Sensor element 20 shown in
The layer structure of a sensor element according to the present invention is explained once more in the following text with the aid of
a shows the plan view of substrate 1 in the region of the capacitor system. In this region substrate 1 is provided with through holes 6 for the application of pressure to a diaphragm, which acts as movable electrode and is disposed above substrate 1. Through holes 6 are in the shape of annular segments in this case and separated from each other by eight webs 9 of a corresponding substrate structure. In the center of the substrate structure, at the intersection of the eight webs 9 and in the center of each web 9 between the intersection and the outer circular edge of through holes 6, substrate bases 8 have been formed in the substrate structure. It should be pointed out here that the shape of the through holes is advantageously adapted to the diaphragm shape in order to obtain the best possible sound application of the diaphragm. To achieve excellent fixation of the fixed electrode, substrate bases 8 are distributed as evenly as possible across the wing span of the counter element to be supported.
b shows substrate 1 after a circular diaphragm 2 has been placed above through holes 6 and the substrate structure delimiting them. As mentioned earlier already, diaphragm 2 functions as movable electrode of the capacitor system of the sensor element. For this purpose, diaphragm 2 is electrically contacted via circuit track 14, which is developed in the same layer as diaphragm 2. Furthermore,
c finally shows a plan view of the layer structure of the sensor element after counter element 3 has been produced above diaphragm 2. Counter element 3 has been provided with perforation holes 12 in substrate 1 in the region above diaphragm 2 and through holes 6. The only region where the structure of counter element 3 is without perforations is in the region above substrate bases 8. In this location there are support elements 7, via which counter element 3 is connected to substrate bases 8. Because of this support construction, the free wing span of counter element 3 is reduced and therefore also the deflection of counter element 3 in response to occurring sound waves.
Sensor elements 10 and 40 generally differ in the realization of support elements 7 or 47 for counter element 3 or 43, respectively. Three infoldings 47 are developed in counter element 43, whose bottom regions are connected to substrate 41 or substrate structure 45 underneath cavity 44 by an insulation layer 48. These infoldings 47 form support elements for counter element 43, which are disposed in the region of cavity 44. Diaphragm 42 has been provided with openings 49 for infoldings 47, so that diaphragm 42 is able to swing freely inside cavity 44 when corresponding sound pressure is acting upon diaphragm 2. Perforation holes 50 are developed in counter element 43 in the region above cavity 44.
Number | Date | Country | Kind |
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10 2007 029 911 | Jun 2007 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2008/057211 | 6/10/2008 | WO | 00 | 11/5/2009 |
Publishing Document | Publishing Date | Country | Kind |
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WO2009/000641 | 12/31/2008 | WO | A |
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20100135123 A1 | Jun 2010 | US |