EP2214421B1 - Component with a micromechanical microphone structure and method for operating such a component - Google Patents

Description

State of the art

The invention relates to a component with a micromechanical microphone structure. This comprises at least one sound pressure deflectable membrane acting as a deflectable electrode, a fixed acoustically permeable counter element comprising a counter electrode, and means for applying a charging voltage between the membrane and the counter electrode.

The invention further relates to methods for operating such a device.

In the known from practice MEMS (micro-electro-mechanical system) microphones, the sound pressure is usually detected in the form of a capacitance change between an acoustically active membrane and a substantially rigid counter electrode.
If a relatively high charging voltage, for example of 10 V, is applied between the diaphragm and the counterelectrode and the measuring signal is read out via a high-impedance preamplifier, for example in the region of 10GOhm, the charge ratios between the diaphragm and counterelectrode change significantly more slowly than the frequency of the signal to be detected sound. Therefore, in this case, it can be assumed in a first approximation that the charge Q remains constant and there is a linear relationship between the capacitance C or capacitance change and the voltage V which can be tapped, namely Q = C * V.
When applying this measurement principle, therefore, a relatively high charging voltage between the membrane and the counter electrode must be applied in order to obtain a high measurement signal. However, a high charging voltage also leads to Strong attractive forces between the movable membrane and the rigid counter electrode. In order to avoid a short circuit and adhesion of the membrane to the counter electrode, in practice either the membrane is suspended relatively stiff or the distance between the membrane and the counter electrode is increased. Both measures have an adverse effect on the sensitivity of the microphone. Furthermore, such MEMS microphones are usually equipped with means by which the membrane can be returned to its rest position after such a collapse.

In the DE 10 2005 056 759 A1 a micromechanical structure for receiving and / or generating acoustic signals is described. In this case, a membrane is placed between two counter-elements such that it has a high mechanical stability. By using electrodes on or in the two counter-elements and the membrane, the two-sided arrangement of the counter-elements can be used to allow a differential evaluation of the capacitance change due to the membrane movement. The first counter-element can be used in addition to its function as an electrode for other mechanical or electrical functions, such as the electrical adjustment of sensitivity.

From the DE 10 2005 008 512 A1 is a micromechanical microphone module with a compensation circuit for preventing non-linear effects in the detection of large vibration amplitudes known. The deflection of the diaphragm caused by the compensation circuit counteracts the deflection caused by the acoustic pressure, so that the diaphragm oscillates at a reduced amplitude or not at all.

Disclosure of the invention

The present invention proposes a very space-saving and robust micromechanical microphone structure with high measuring sensitivity, in which a relatively high charging voltage can be applied to the measuring capacitance with a comparatively small electrode spacing.

For this purpose, the microphone structure according to the invention comprises a second fixed and acoustically permeable counter element comprising a compensation electrode. The membrane is disposed between the counter electrode and the compensation electrode. In addition, means for applying a compensation voltage between the counter electrode and the compensation electrode are provided.

With the aid of the compensation electrode, the microphone function of the component according to the invention can be realized in two metrologically different ways, both of which ensure a high measuring sensitivity and a low susceptibility to interference.

In a first operating variant, the compensation voltage between the counterelectrode and the compensating electrode is selected as a function of the charging voltage of the measuring capacitance in such a way that the electrical attraction between the diaphragm and the counterelectrode produced by the charging voltage is compensated by the compensation voltage. As a result, the movable membrane is in a virtually potential-free space where no electrostatic forces act on the membrane and membrane deflections caused solely by the sound pressure. Therefore, the charging voltage for the measuring capacitance can be set here relatively high, even with a small electrode gap, in order to obtain a high measuring signal in the form of the voltage change between the membrane and the counterelectrode. An electrostatic collapse of the microphone structure is not to be feared.

In contrast, the compensation voltage is regulated in a second advantageous operating variant, so that the movable membrane is kept as possible in its rest position even with sound effects. In this case, the voltage between the counter electrode and the diaphragm, which changes due to the sound pressure with the electrode spacing, is used as a control variable for the compensation voltage control. As a microphone signal here serves the compensation voltage. In this variant too, it is possible to work with relatively high charging voltages with comparatively small electrode spacing. In addition, it proves to be particularly insensitive to electromagnetic noise.

In addition to the electrical metrological advantages of the device according to the invention discussed above, it should also be mentioned at this point that the two counter elements additionally form a mechanical protection for the movable and thus also sensitive diaphragm of the microphone structure arranged therebetween. The microphone structure proposed here is therefore also advantageous in mechanical terms.

In principle, there are various possibilities for the realization of the microphone structure according to the invention.

Thus, the microphone structure may be constructed, for example, mirror-symmetrical to the membrane, in the sense that the membrane is arranged centrally between the two counter-elements and the two counter-elements have a substantially identical structure.

In certain applications, it may also be beneficial to provide different distances between the counter electrode and the membrane and between compensation electrode and membrane in order to achieve a higher microphone sensitivity.

It may also be advantageous for the application to realize the counter electrode and the compensation electrode with different hole or grid geometries.

As already explained, the component structure according to the invention enables different operating variants for the realization of the microphone function. Both variants described above are based on a suitable choice or regulation of the compensation voltage. Advantageously, in addition to electrical parameters, such as charging voltage or tapped voltage between counter electrode and membrane, and structural parameters, such as distance to the rest position of the membrane and hole or grid structure, taken into account. In particular, in an asymmetric structure of the microphone structure, it is often useful to put the counter electrode and the compensation electrode to different electrical potentials.

In an advantageous embodiment of the component according to the invention, the membrane surfaces and / or the membrane facing surface of the first and / or second counter element are provided with a dielectric coating to avoid a short circuit within the microphone structure even in overload situations.

Furthermore, the microphone structure can also comprise an overload protection in the form of stops, which are formed in the surfaces of the membrane and / or in the membrane-facing surface of the first and / or second counter-element. It is particularly advantageous if these stops are arranged in a region, such as the edge region of the microphone structure, where the stops and the surface opposite them can be set to a defined potential. In this case, there is no short-circuit between the membrane and the counter electrode or compensation electrode when placing the stops.

Brief description of the drawings

Fig. 1

zeigt eine schematische Schnittdarstellung eines Bauelements 10 mit einer erfindungsgemäßen Mikrofonstruktur,

Fig. 2

veranschaulicht die Potentialverhältnisse innerhalb der Mikrofon- struktur des Bauelements 10, und

Fig. 3a bis 3h

veranschaulichen die einzelnen Verfahrensschritte zur Herstel- lung des Bauelements 10 anhand von schematischen Schnitt- darstellungen.

As already discussed above, there are various possibilities for embodying and developing the teaching of the present invention in an advantageous manner. For this purpose, reference is made, on the one hand, to the claims subordinate to independent claim 1 and, on the other hand, to the following description of an embodiment of the invention. The claimed production method will also be explained in more detail with reference to the figures.

Fig. 1

shows a schematic sectional view of a component 10 with a microphone structure according to the invention,

Fig. 2

illustrates the potential relationships within the microphone structure of the device 10, and

Fig. 3a to 3h

illustrate the individual process steps for the production of the device 10 by means of schematic sectional representations.

Embodiments of the invention

This in Fig. 1 illustrated component 10 includes a micromechanical microphone structure, which is formed in a layer structure. This microphone structure consists essentially of a deflectable by the sound pressure membrane 11, which is arranged between two fixed and acoustically permeable counter-elements 12 and 13. The membrane 11 is electrically insulated by insulation layers against both counter elements 12 and 13. Both the membranes 11 and the two counter-elements 12 and 13 are at least partially made of an electrically conductive material, such as a correspondingly doped polysilicon or silicon substrate. The counter element 12, which is arranged in the layer structure over the membrane 11, here comprises a counter electrode 22 for the membrane 11, which acts as a deflectable electrode. Together they form a measuring capacity which is charged by means not shown here for applying a charging voltage. So can deflections the membrane 11 are detected as capacitance changes or fluctuations of a voltage tapped at the measuring capacitance.

In the present exemplary embodiment, the second counter element 13 is formed in the component substrate 1 below the diaphragm 11 and comprises a compensation electrode 33. For this purpose, the component 10 comprises means for applying and regulating a compensation voltage between the counter electrode 22 and the compensation electrode 33. These means are likewise not shown here ,
In a first operating variant, the compensation voltage is selected such that the electrostatic attraction between the membrane 11 and the counterelectrode 22 caused by the state of charge of the measuring capacitance is canceled by a corresponding potential difference to the compensation electrode 33 and the diaphragm 11 is deflected exclusively due to the sound pressure. In an alternative operating variant, the compensation voltage is regulated as a function of the voltage tapped off at the measuring capacitance, ie as a function of the diaphragm deflection, in such a way that the diaphragm 11 is held in its rest position as far as possible. In this case, the sound pressure acting on the membrane 11 is also compensated with the aid of the compensation voltage. As a microphone signal here the compensation voltage is used.

The counter-elements 12 and 13 are in the illustrated embodiment significantly thicker than the membrane 11 and thus substantially rigid. Both counter-elements 12 and 13 have the same lattice structure with passage openings 121 and 131, so that the counter-elements 12 and 13 are equally acoustically permeable and only the centrally disposed between the counter-elements 12 and 13 membrane 11 is acoustically active.
In order to achieve that the counter elements react much less on sound waves than the membrane, however, a counter element can also be realized in a thinner polysilicon layer, which is more rigidly suspended than the membrane in the layer structure. Another possibility is to realize a counter element in the form of a layer stack of polysilicon and oxide or nitride, which is under tensile stress.

The microphone structure of the component 10 includes a mechanical overload protection in the form of stops 122 and 112. These are to prevent sticking and burning of the membrane 11 on the counter electrode 22 on the one hand or on the compensation electrode 33 on the other. Thus, the stops 122 are formed in the edge region 124 of the counter element 12 on its underside, while the stops 112 are formed in the edge region of the membrane 11 on its underside. In the case of overload situations, the stops 122 and 112 form the contact surfaces between the diaphragm 11 and the counter-elements 12 and 13 of the microphone structure. The smaller these contact surfaces, the lower the adhesive force between these components and, accordingly, the force required to bring the membrane 11 back to its original position. The stops 122 and 112 may be made of, or at least coated with, a dielectric material such as SiN or silicon-rich nitride so as to prevent a short circuit between the diaphragm 11 and one of the mating elements 12 or 13 in overload situations. As a result, not only the microphone structure but also the electronics of the component 10 are protected.

Another way to prevent a short circuit between the individual components of the microphone structure is through Fig. 2 illustrating the potential conditions within the microphone structure. In both operating variants described above, the measuring capacitance is charged with a relatively high charging voltage. Accordingly, the counter electrode 12 and the diaphragm 11 with the stops 112 are at different electrical potentials. The size of the potential difference is in Fig. 2 represented by the different hatch width. Fig. 2 also clarifies that the edge regions 124 and 134 of the counter-elements 12 and 13 by isolation trenches 125 and 135 of the electrode portions of the counter-elements 12 and 13 are electrically decoupled but mechanically connected to each other. These edge regions 124 and 134 can now either be placed on the membrane potential, or be placed from the outside to a potential which is very close to the membrane potential. Since the stops 122 of the counter element 12 are formed in the edge region 124 and the stops 112 of the membrane 11 in overload situations only with the edge region 134 of the counter element 13 into contact, at most small currents between the membrane 11 and the counter-elements 12 and 13 can flow that neither cause a short circuit still sufficient for welding the membrane 11.

The production of the in the FIGS. 1 and 2 illustrated component 10 will be described below with reference to the FIGS. 3a to 3h explained.

The manufacturing method is based on a substrate 1, such as a silicon wafer, on which first a first sacrificial layer 2 is deposited and patterned. In this case, a negative impression of the stops 112 of the membrane 11 is generated. Typically, the sacrificial layer material is a thermal oxide or a TEOS oxide, which also forms an electrical insulation for individual layer regions within the scope of the component 10. Fig. 3a shows the layer structure after the application of a further sacrificial layer 3, in which the structuring of the sacrificial layer 2 has transferred.

Now, a polysilicon layer is deposited, doped and patterned as the membrane layer 4, as in FIG Fig. 3b shown. In this case, a spring suspension for the membrane 11 can be realized in order to favor membrane deflections and thus to increase the microphone sensitivity.

On the structured membrane layer 4, a further sacrificial layer 5 is deposited and patterned, wherein a negative impression of the stops 122 of the counter-element 12 is generated. Fig. 3c shows the layer structure after the application of a further sacrificial layer 6, in which the structuring of the sacrificial layer 5 has been transferred.

In the exemplary embodiment described here, a polysilicon starter layer 7 is now deposited and patterned, which results in Fig. 3d is shown. This starting layer 7 is subsequently used for the generation of the counter element 12.

The definition of the edge regions 124 and 134 of the counter-elements 12 and 13 and the definition of the contacts for the substrate 1, for the edge regions 124 and 134, for the compensation electrode 33 and the membrane 11 takes place here in an etching step following the structuring of the starting layer 7 Alternatively, you can but this also be done in advance by an appropriate structuring of the sacrificial layers 2, 3 and 5, 6.

In the exemplary embodiment described here, a thick epi-polysilicon layer 8 is deposited on the layer structure after this etching step, which results in Fig. 3e is shown. From this epi-polysilicon layer 8, the fixed counter-element 12 is formed. For this purpose, the epi-polysilicon layer 8 is doped and can also be planarized for better further processing in a CMP step.
Alternatively, the mating element can also be realized in a thinner polysilicon layer that is stiffer than the membrane, or in a layer stack of polysilicon and oxide or nitride, which is under tensile stress. In this way it can also be achieved that the counter element reacts less to sound waves than the membrane.

For subsequent electrical contacting, a metal layer or a metal layer stack can now be deposited and patterned. This can be done before the structuring of the counter-element 12 or the epi-polysilicon layer 8 or at a later time.

The epi-polysilicon layer 8 is patterned in a trench step starting from the front or upper side of the layer structure. In this case, the passage openings 121 and the isolation trenches 125 are generated, which is in Fig. 3f is shown. The sacrificial layer 6 acts as a stop layer for this trench process.

The following describes the processing of the substrate rear side, which can also be brought forward within the process sequence.
As a rule, the substrate 1 is first thinned. Then, the size and position of the second mating member 13 are defined by means of back masking. Finally, in a backside trenching process, the thickness of the second counter-element 13 or of the compensation electrode 33 is adjusted via the trenching depth. The counter-element 13 is then patterned in a further trenching step, wherein on the one hand the through-openings 131 are produced and on the other hand the electrical decoupling of the edge region 134 via the isolation trenches 135. In this trenching step, the sacrificial layer serves 2 as an etch stop layer. The resulting layer structure is in Fig. 3g shown.

Finally, the membrane 11 is exposed by the sacrificial layer material, for example by means of HF or in a Gasphasenätzverfahren around the membrane 11 is removed. In this case, the passage openings 121, 131 and the isolation trenches 125 and 135 are used as Ätzzugänge. Fig. 3h shows the resulting component structure and corresponds essentially to the Fig. 1 ,

It should be noted at this point that deviations from the process sequence described above are also possible. Thus, the backside trench process can also be run before the front side trench process. In addition, it may prove advantageous to remove part of the sacrificial layer material after the first trenching step. The membrane is then completely released after the second trench step in a second etching step. A dielectric coating of the membrane and / or the surfaces of the counter-elements facing the membrane may be realized by depositing a suitable material before or after the respective sacrificial layers are formed. Suitable coating material is, for example, SiN or silicon-rich nitride, since its etching rate is significantly lower than that of the sacrificial layer material and the adhesive force between SiN or silicon-rich nitride and silicon is very low.

Claims (10)

1. Component (10) with a micromechanical microphone structure, at least comprising

   - a membrane (11), which can be deflected by the acoustic pressure and acts as a deflectable electrode,

   - a fixed acoustically permeable counter element (12), which comprises at least one counter electrode (22), and

   - means for applying a charging voltage between the membrane (11) and the counter electrode (22); and

   - a second fixed and acoustically permeable counter element (13), which comprises at least one compensation electrode (33), the membrane (11) being arranged between the counter electrode (22) and the compensation electrode (33); and

   - means for applying a compensation voltage between the counter electrode (22) and the compensation electrode (33);

   characterized in that
   the compensation voltage (33) has a dependence

   - on the charging voltage applied between the membrane and the counter electrode or

   - on the voltage present between the membrane and the counter electrode, changed by the deflection.
2. Component according to Claim 1, characterized in that the microphone structure is substantially mirror-symmetrical in relation to the membrane.
3. Component according to Claim 1, characterized in that the distance between the counter electrode and the membrane and the distance between the compensation electrode and the membrane are different.
4. Component according to Claim 3, characterized in that the counter electrode and the compensation electrode are realized with different hole or grid geometries.
5. Component according to one of Claims 1 to 4, characterized in that the membrane surfaces and/or the surface of the first and/or second counter element that is facing the membrane is/are provided with a dielectric coating.
6. Component (10) according to one of Claims 1 to 5, characterized in that the microphone structure comprises overload protection in the form of stops (112, 122), which are formed in the surfaces of the membrane (11) and/or in the surface of the first and/or second counter element (12) that is facing the membrane (11).
7. Component according to Claim 6, characterized in that the stops (122) are arranged in an electrically insulated region (124), so that they can be set to a defined potential, in particular to the potential of the membrane (11) or a potential that differs only a little from the potential of the membrane (11).
8. Method for operating a component with a micromechanical microphone structure

   - with an acoustically active membrane (11), which is arranged between two fixed and acoustically permeable counter elements (12, 13), the one counter element (12) comprising at least one counter electrode (22) and the other counter element (13) comprising at least one compensation electrode (33),

   - with means for applying a charging voltage between the membrane (11) and the counter electrode (22) and

   - with means for applying a compensation voltage between the counter electrode (22) and the compensation electrode (33);
   characterized in that the compensation voltage is chosen such that the electrostatic forces of attraction caused by the charging voltage between the counter electrode (22) and the membrane (11) are nullified, and in that the voltage between the counter electrode (22) and the membrane (11), changing on account of the deflection of the membrane (11), is picked off as a microphone signal.
9. Method for operating a component with a micromechanical microphone structure

   - with an acoustically active membrane (11), which is arranged between two fixed and acoustically permeable counter elements (12, 13), the one counter element (12) comprising at least one counter electrode (22) and the other counter element (13) comprising at least one compensation electrode (33),

   - with means for applying a charging voltage between the membrane (11) and the counter electrode (22) and

   - with means for applying and controlling a compensation voltage between the counter electrode (22) and the compensation electrode (33);
   characterized in that the voltage between the counter electrode (22) and the membrane (11), changing on account of the deflection of the membrane (11), is picked off as a manipulated variable for the control of the compensation voltage and in that the compensation voltage is controlled such that the membrane (11) is kept as far as possible in its position of rest.
10. Method according to Claim 8 or 9, characterized in that the distances of the counter electrode (22) and the compensation electrode (33) from the position of rest of the membrane (11) and/or their hole or grid geometries are taken into consideration in the choice or control of the compensation voltage.

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