EP0331992A2 - Capacitive sound transducer - Google Patents

Capacitive sound transducer Download PDF

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Publication number
EP0331992A2
EP0331992A2 EP89103276A EP89103276A EP0331992A2 EP 0331992 A2 EP0331992 A2 EP 0331992A2 EP 89103276 A EP89103276 A EP 89103276A EP 89103276 A EP89103276 A EP 89103276A EP 0331992 A2 EP0331992 A2 EP 0331992A2
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EP
European Patent Office
Prior art keywords
membrane
electrode structure
capacitive
membrane unit
counter electrode
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Granted
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EP89103276A
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German (de)
French (fr)
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EP0331992B1 (en
EP0331992A3 (en
Inventor
Wolfgang Dipl.-Ing. Kühnel
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Sennheiser Electronic GmbH and Co KG
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Sennheiser Electronic GmbH and Co KG
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R19/00—Electrostatic transducers
    • H04R19/005—Electrostatic transducers using semiconductor materials
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T29/00—Metal working
    • Y10T29/43—Electric condenser making
    • Y10T29/435—Solid dielectric type

Definitions

  • the invention relates to a capacitive sound transducer, which consists of a membrane unit and at least one fixed counter-electrode structure made of semiconducting material.
  • the converter serves as a microphone for converting sound pressure changes into electrical signals.
  • Capacitive microphones based on the previous electrostatic principle consist of a membrane and at least one fixed counter electrode.
  • the membrane has a certain tensile stress with which the acoustic properties of the microphone capsule can be influenced.
  • the counter electrode is provided with channels and holes, on the one hand so that the air can flow out of the air gap delimited by the membrane and counter electrode into a back volume of the transducer and on the other hand to reduce the attenuation losses in the air gap, which reduce the sensitivity of the microphone and the frequency response influence.
  • the signal conversion is done by evaluating the relative change in capacitance of the converter.
  • the newer methods of semiconductor technology allow the production of miniature transducers in a micromechanical way, for example based on silicon.
  • the structure of a silicon microphone is described in the literature reference KAPAZITITVE SILICON SENSORS FOR HEARING SOUND APPLICATIONS, published in 1986 by VDI-Verlag, ISBN 3-18-14161o-9.
  • This transducer which is manufactured in a micromechanical way, has the dimensions of approx. 1.6 x 2 xo, 6 mm3.
  • the active membrane area exists from a silicon nitride layer coated with a metal layer, which, separated by an air gap, is opposed by a counterelectrode also made of silicon.
  • Miniature microphones manufactured using semiconductor technology have particular disadvantages which are caused by attenuation losses in the very narrow air gap. If the membrane is excited to oscillate by a periodic alternating pressure, a flow forms in the air gap. However, the narrower the air gap, the higher the flow resistance, since the losses are primarily caused by friction on the walls. The flow resistance is also frequency dependent; it increases with increasing frequencies, so that the sensitivity to higher frequencies drops sharply. Since the attenuation losses do not increase linearly with a gap narrowing but progressively, the negative influence on microphones of the type described is particularly high. The possibility of perforating the counter electrode is currently not available due to its small size and lack of technology. With the microphone specified in the literature reference, the sensitivity therefore drops to values below -6o dB due to air gap losses, based on 1 V / Pa, and the frequency response is limited to a few kilohertz.
  • Air gap damping that occurs between the membrane and counter electrode could be reduced by reducing the lateral dimensions of the counter electrode. Lateral dimensions are the dimensions perpendicular to the direction of air flow. Such reductions also reduce the converter's resting capacity. The lower limit thereof is approximately 1 pF with respect to the level of the signal obtained in a low-frequency circuit. A reduction in the counter-electrode dimensions, which could lead to a reduction in the flow resistance, is therefore no longer an option with this low resting capacity.
  • the invention has set itself the task of creating a miniature microphone produced using the means of semiconductor technology, in which the active surface of the membrane is good Efficiency as in previously known microphones is retained, but the attenuation losses occurring in the air gap are reduced by a suitable design of the counterelectrode in such a way that the disadvantages of previously known microphones are avoided.
  • This object is achieved with the features specified in the characterizing part of patent claim 1.
  • a counterelectrode which is significantly smaller in its lateral dimensions and inevitably also leads to lower attenuation losses, can be used if it is assumed that the output signal of the converter is obtained by the relative change in its quiescent capacitance. According to the invention, therefore, smaller resting capacities can be used if the input capacitance of an active element is controlled by the movements of the membrane.
  • Field effect transistors have gate-channel capacitances in the range of 1o ⁇ 15F, that is 1 / 1ooo of the above-mentioned membrane counterelectrode capacitance of 1 pF. If the drain-channel-source structure of a field effect transistor is arranged opposite a membrane, the flow losses are largely eliminated due to the very small dimensions of the counterelectrode structure required. This effect already occurs when the width of the counter electrode structure is approximately one tenth of the dimensions of the active membrane area.
  • FIG. 1 The basic structure of a capacitive sound transducer according to the invention, hereinafter called the FET microphone, is shown in FIG. 1.
  • a membrane metallized with aluminum, for example, is located, separated by an air gap d L, above a drain-channel-source structure, which is called the counter-electrode structure in the following.
  • the channel zone of this structure is covered with an oxide protective layer.
  • a weakly p-doped silicon substrate forms the channel zone L, the heavily n-doped electrodes form the drain and source of the FET. For example, this is an N-channel enhancement type.
  • the voltage U GS applied between the membrane and the source connection determines the operating point of the field effect transistor.
  • the FET microphone is advantageously operated in a source circuit. This is shown in FIG. 3, as is the associated small-signal equivalent circuit.
  • the operating voltage U B is supplied to the microphone via the drain resistor R d , which can be integrated directly on the chip forming the counter electrode.
  • the microphone output voltage U a is tapped at the drain connection; the membrane is biased against the source with the voltage U GS .
  • the current source with the mechanical-electrical slope S me is controlled by the membrane deflection X.
  • the impressed current produces a voltage drop in the drain resistor R d which corresponds to the output voltage U a .
  • the mechanical equivalent circuit shown in Fig. 2 is used to calculate the frequency response and sensitivity of the FET microphone.
  • R S (w) and M S (w) represent the radiation impedance Z mS of the membrane, M M the mass and C M the compliance of the membrane, which vibrates with the rapid v m .
  • the rear air volume is represented by the compliance C V.
  • the volume results from the wafer thickness, which represents the back volume height. It is 28o um.
  • C V 2.866 x 1o ⁇ 3 sec2 / kg.
  • the mass, compliance and frictional losses of the air in the air gap can be neglected, since the width of the air gap and the width of the drain-channel-source structure are considerably smaller than the lateral dimensions of the membrane and the openings in the back volume.
  • the microphone sensitivity increases proportionally with the mechanical-electrical slope S me and the drain resistance R D.
  • these cannot be increased arbitrarily, since the available level of the operating voltage U B and the maximum adjustable electrical membrane bias U GS (breakdown field strength in the channel) represent upper limits.
  • a large total resilience C ges requires a "soft" membrane (high resilience C M) and a large back volume (C V).
  • C M high resilience
  • C V back volume
  • the small membrane area A of subminiature transducers is an inherent problem.
  • Fig. 4 shows a graphical representation of the dependence of the sensitivity M e on the frequency for different mechanical membrane tensions and back volumes.
  • the FET microphone consists of two chips, the upper one carrying the membrane 2 as the membrane unit 1 and the lower one carrying the drain-channel-source structure 8 of the FET as the counter electrode structure 3.
  • the membrane 2 consists of a 150 nm thick layer 4 made of silicon nitride, the mechanical stress properties of which can be influenced by ion implantations during the manufacturing process.
  • the membrane 2 is held by a support frame 2.1, which surrounds the membrane in the form of a wall and consists of the semiconducting base material, preferably silicon. It is vapor-coated on its underside with a 100 nm aluminum layer 5. This vaporization represents the gate of the FET.
  • two trough-shaped pits 6 and 7 are introduced by plasma etching, which form the back volume of the microphone. Between the pits there is an 8 ⁇ m-wide web 8 which carries the drain-channel-source structure 9, 10 and 11 of the FET. The distance between the channel 10 and the aluminum layer of the membrane 5 is 2 ⁇ m.
  • a compensation hole for the static air pressure is located in the silicon oxide edge 12 of the counterelectrode chip, provided that the microphone capsule is to work as a pressure transducer with an acoustically closed volume.
  • the converter described in FIG. 5 can also be expanded to a push-pull converter by using a second counter-electrode structure with a suitably shaped web similar to the web 8 in the depression of the membrane unit 1 predetermined by the wall. In this case, the membrane 2 must then be metallized on both sides. If the transducer is to function as a push-pull transducer in the manner described or if a pressure gradient characteristic is to be obtained in accordance with another expedient embodiment, the volumes in front of or behind the diaphragm are to be connected to the external sound field via openings. 5, such openings are shown with the reference numerals 14 and 15, for example.
  • the N or P-channel enrichment principle was first used in the counter-electrode structure for the channel zone.
  • the depletion principle can also advantageously be used for the channel zone. Since an operating point is already specified here in the FET circuit, the separate bias for the gate can be omitted here, since it can itself be generated in a known manner via a resistor used in the source circuit.
  • a great advantage of a capacitive transducer according to the invention is that a relatively large active membrane area, which is required for good acoustic efficiency of the transducer, is only a small part of the membrane area opposite a counter-electrode structure, and thus the flow losses are negligibly small. This results in a large linear transmission range with very good sensitivity, as can be seen from FIG. 4. Furthermore, the noise behavior of the converter is extremely favorable, since the noise component caused by damping in the air gap is very low due to the principle. Capacitive converters are mostly operated in the so-called low-frequency circuit and therefore require a series resistor, the thermal noise of which also increases with increasing resistance. Decreasing converter quiescent capacitances in miniature microphones require increasing series resistances at the same lower cut-off frequency, which was an unsolvable problem in the previous versions. Since the FET microphone does not require a series resistor, the noise component has also been significantly reduced.
  • the noise behavior can also be improved by operating a plurality of FET microphones which have arisen jointly on the wafer in parallel as a microphone unit.

Abstract

Capacitive sound transducer of a very small construction, in particular a microphone has at least two joint semiconductor chips, which embody a membrane unit and a fixed counter-electrode structure. The acoustic active portion of the membrane unit 1 with at least one counter-electrode structure 3, which is separated from the membrane unit by means of an air gap, forms a system which is comparable to a field effect transistor. The membrane unit which is formed of a semiconducting ground material encompasses an acoustically active membrane surface (2), one side 5 of which confronts the counter-electrode structure is electrically conductive. The counter-electrode structure 3 has a semiconductive base material out of which there is machined a channel length which has been limited by a source-drain arrangement, the geometric width measurrement of which is on the order of magnitude of a tenth of the lateral measurement of the active membrane surface.

Description

Die Erfindung bezieht sich auf einen kapazitiven Schallwandler, welcher aus einer Membraneinheit und mindestens einer feststehenden Gegen­elektrodenstruktur aus halbleitenden Material besteht. Der Wandler dient als Mikrofon der Umsetzung von Schalldruckänderungen in elektri­sche Signale. Kapazitive Mikrofone nach dem bisherigen elektrostati­schen Prinzip bestehen aus einer Membran und zumindest einer festste­henden Gegenelektrode. Die Membran besitzt eine bestimmte Zugspannung, mit der die akustischen Eigenschaften der Mikrofonkapsel beeinflußt werden können. Die Gegenelektrode ist mit Kanälen und Bohrungen verse­hen, einerseits, damit die Luft aus dem vom Membran und Gegenelektrode begrenzten Luftspalt in ein Rückvolumen des Wandlers abströmen kann und andererseits, um die Dämpfungsverluste im Luftspalt zu reduzieren, die die Empfindlichkeit des Mikrofons herabsetzen und den Frequenzgang un­günstig beeinflussen. Die Signalwandlung geschieht durch Auswertung der relativen Kapazitätsänderung des Wandlers.The invention relates to a capacitive sound transducer, which consists of a membrane unit and at least one fixed counter-electrode structure made of semiconducting material. The converter serves as a microphone for converting sound pressure changes into electrical signals. Capacitive microphones based on the previous electrostatic principle consist of a membrane and at least one fixed counter electrode. The membrane has a certain tensile stress with which the acoustic properties of the microphone capsule can be influenced. The counter electrode is provided with channels and holes, on the one hand so that the air can flow out of the air gap delimited by the membrane and counter electrode into a back volume of the transducer and on the other hand to reduce the attenuation losses in the air gap, which reduce the sensitivity of the microphone and the frequency response influence. The signal conversion is done by evaluating the relative change in capacitance of the converter.

Die neueren Verfahren der Halbleitertechnologie erlauben die Herstel­lung von Miniaturwandlern auf mikromechanischem Wege, beispielsweise auf der Basis von Silizium. In der Literaturstelle KAPAZITITVE SILI­ZIUMSENSOREN FÜR HÖRSCHALLANWENDUNGEN, erschienen 1986 im VDI-Verlag, ISBN 3-18-14161o-9, wird der Aufbau eines Silizium-Mikrofones beschrie­ben. Dieser auf mikromechanischem Wege hergestellte Wandler besitzt die Abmessungen von ca. 1,6 x 2 x o,6 mm³. Die aktive Membranfläche besteht aus einer mit einer Metallschicht überzogenen Siliziumnitrid-Schicht, der, durch einen Luftspalt getrennt, eine ebenfalls aus Silizium herge­stellte Gegenelektrode gegenübersteht.The newer methods of semiconductor technology allow the production of miniature transducers in a micromechanical way, for example based on silicon. The structure of a silicon microphone is described in the literature reference KAPAZITITVE SILICON SENSORS FOR HEARING SOUND APPLICATIONS, published in 1986 by VDI-Verlag, ISBN 3-18-14161o-9. This transducer, which is manufactured in a micromechanical way, has the dimensions of approx. 1.6 x 2 xo, 6 mm³. The active membrane area exists from a silicon nitride layer coated with a metal layer, which, separated by an air gap, is opposed by a counterelectrode also made of silicon.

Bei halbleitertechnologisch hergestellten Miniaturmikrofonen ergeben sich besondere Nachteile, die durch Dämpfungsverluste im sehr engen Luftspalt bedingt sind. Wird die Membran von einem periodischen Wech­seldruck zu Schwingungen angeregt, so bildet sich im Luftspalt eine Strömung. Der Strömungswiderstand ist jedoch umso höher, je schmaler der Luftspalt ist, da die Verluste in erster Linie durch Reibung an den Wänden zustande kommen. Der Strömungswiderstand ist außerdem frequenz­abhängig; er nimmt mit steigenden Frequenzen zu, so daß die Empfind­lichkeit zu höheren Frequenzen hin stark absinkt. Da die Dämpfungsver­luste nicht linear mit einer Spaltverengung zunehmen sondern progres­siv, so ist der negative Einfluß bei Mikrofonen der beschriebenen Art besonders hoch. Die Möglichkeit, die Gegenelektrode zu durchlöchern ist wegen ihrer geringen Größe und wegen fehlender Technologie zur Zeit nicht gegeben. Bei dem in der Literaturstelle angegebenen Mikrofon sinkt daher die Empfindlichkeit aufgrund von Luftspaltverlusten auf Werte unter -6o dB, bezogen auf 1V/Pa und der Frequenzgang ist auf ei­nige Kilohertz begrenzt.Miniature microphones manufactured using semiconductor technology have particular disadvantages which are caused by attenuation losses in the very narrow air gap. If the membrane is excited to oscillate by a periodic alternating pressure, a flow forms in the air gap. However, the narrower the air gap, the higher the flow resistance, since the losses are primarily caused by friction on the walls. The flow resistance is also frequency dependent; it increases with increasing frequencies, so that the sensitivity to higher frequencies drops sharply. Since the attenuation losses do not increase linearly with a gap narrowing but progressively, the negative influence on microphones of the type described is particularly high. The possibility of perforating the counter electrode is currently not available due to its small size and lack of technology. With the microphone specified in the literature reference, the sensitivity therefore drops to values below -6o dB due to air gap losses, based on 1 V / Pa, and the frequency response is limited to a few kilohertz.

Luftspaltdämpfungen, die zwischen Membran und Gegenelektrode auftreten, ließen sich durch Verringerung der lateralen Abmessungen der Gegenelek­trode verringern. Laterale Abmessungen sind hier die Abmessungen senk­recht zur Strömungsrichtung der Luft. Durch solche Verkleinerungen sinkt jedoch auch die Ruhekapazität des Wandlers. Die untere Grenze derselben liegt im Hinblick auf die Höhe des in einer Niederfrequenz-­Schaltung gewonnenen Signals bei etwa 1 pF. Eine Verkleinerung der Ge­genelektrodenmaße, die zu einer Verringerung des Strömungswiderstandes führen könnte, kommt daher bei dieser geringen Ruhekapazität nicht mehr in Betracht.Air gap damping that occurs between the membrane and counter electrode could be reduced by reducing the lateral dimensions of the counter electrode. Lateral dimensions are the dimensions perpendicular to the direction of air flow. Such reductions also reduce the converter's resting capacity. The lower limit thereof is approximately 1 pF with respect to the level of the signal obtained in a low-frequency circuit. A reduction in the counter-electrode dimensions, which could lead to a reduction in the flow resistance, is therefore no longer an option with this low resting capacity.

Die Erfindung hat sich die Aufgabe gestellt, ein mit den Mitteln der Halbleitertechnologie hergestelltes Miniaturmikrofon zu schaffen, bei welchem die aktive Fläche der Membran hinsichtlich eines guten Wirkungsgrades wie bei bisher bekannten Mikrofonen erhalten bleibt, die im Luftspalt auftretenden Dämpfungsverluste jedoch durch eine geeignete Gestaltung der Gegenelektrode so verringert werden, daß die Nachteile bisher bekannter Mikrofone vermieden werden. Diese Aufgabe wird mit den im kennzeichnenden Teil des Patentanspruchs 1 angegebenen Merkmalen ge­löst.The invention has set itself the task of creating a miniature microphone produced using the means of semiconductor technology, in which the active surface of the membrane is good Efficiency as in previously known microphones is retained, but the attenuation losses occurring in the air gap are reduced by a suitable design of the counterelectrode in such a way that the disadvantages of previously known microphones are avoided. This object is achieved with the features specified in the characterizing part of patent claim 1.

Eine in ihren lateralen Abmessungen wesentlich verkleinerte Gegenelek­trode, die zwangsläufig auch zu geringeren Dämpfungsverlusten führt, kann verwendet werden, wenn man davon abgeht, das Ausgangssignal des Wandlers durch die relative Änderung seiner Ruhekapazität zu gewinnen. Erfindungsgemäß lassen sich daher kleinere Ruhekapazitäten verwenden, wenn man durch die Bewegungen der Membran die Eingangskapazität eines aktiven Elementes steuert.A counterelectrode, which is significantly smaller in its lateral dimensions and inevitably also leads to lower attenuation losses, can be used if it is assumed that the output signal of the converter is obtained by the relative change in its quiescent capacitance. According to the invention, therefore, smaller resting capacities can be used if the input capacitance of an active element is controlled by the movements of the membrane.

Feldeffekttransistoren besitzen Gate-Kanal-Kapazitäten im Bereich von 1o⁻¹⁵F, also von 1/1ooo der oben beispielsweise genannten Membran-­Gegenelektrodenkapazität von 1 pF. Wird also die Drain-Kanal-Source-­Struktur eines Feldeffekttransistors einer Membran gegenüber angeord­net, so werden die Strömungsverluste aufgrund der benötigten sehr ge­ringen Abmessungen der Gegenelektrodenstruktur weitgehend eleminiert. Dieser Effekt tritt bereits auf, wenn die Breite der Gegenelektroden­struktur ungefähr ein Zehntel der Abmessungen der aktiven Membranfläche beträgt.Field effect transistors have gate-channel capacitances in the range of 1o⁻¹⁵F, that is 1 / 1ooo of the above-mentioned membrane counterelectrode capacitance of 1 pF. If the drain-channel-source structure of a field effect transistor is arranged opposite a membrane, the flow losses are largely eliminated due to the very small dimensions of the counterelectrode structure required. This effect already occurs when the width of the counter electrode structure is approximately one tenth of the dimensions of the active membrane area.

Ein kapazitiver Schallwandler nach der Erfindung wird anhand einer Zeichnung nachfolgend und beispielsweise beschrieben. Es zeigen

  • die Fig. 1 den prinzipiellen Aufbau eines nach der Erfindung arbeitenden Schallwandlers,
  • die Fig. 2 ein Kleinsignal-Ersatzschaltbild
  • die Fig. 3 ein mechanisches Ersatzschaltbild
  • die Fig. 4 eine Frequenzgangdarstellung
  • die Fig. 5 eine schematische Darstellung eines Schallwandlers nach der Erfindung
  • die Fig. 6 eine beispielweise Anordnung mehrerer Schallwandler auf einem Wafer.
A capacitive sound transducer according to the invention is described below and, for example, with reference to a drawing. Show it
  • 1 shows the basic structure of a sound transducer according to the invention,
  • Fig. 2 is a small signal equivalent circuit
  • 3 shows a mechanical equivalent circuit diagram
  • 4 shows a frequency response
  • 5 is a schematic representation of a sound transducer according to the invention
  • 6 shows an example of an arrangement of a plurality of sound transducers on a wafer.

Der prinzipielle Aufbau eines kapazitiven Schallwandlers nach der Er­findung, im folgenden FET-Mikrofon genannt, ist in der Fig. 1 darge­stellt. Eine beispielsweise mit Aluminium metallisierte Membran befin­det sich, getrennt durch einen Luftspalt dL über einer Drain-Kanal-­Source-Struktur, die im folgenden Gegenelektrodenstruktur genannt wird. Die Kanalzone dieser Struktur ist mit einer Oxid-Schutzschicht überzo­gen. Ein schwach p-dotiertes Siliziumsubstrat bildet die Kanalzone L, die stark n-dotierten Elektroden bilden Drain und Source des FETs. Es handelt sich hier beispielsweise um einen N-Kanal-Anreicherungstyp. Die Spannung UGS, angelegt zwischen der Membran und dem Source-Anschluß be­stimmt den Arbeitspunkt des Feldeffekttransistors.The basic structure of a capacitive sound transducer according to the invention, hereinafter called the FET microphone, is shown in FIG. 1. A membrane metallized with aluminum, for example, is located, separated by an air gap d L, above a drain-channel-source structure, which is called the counter-electrode structure in the following. The channel zone of this structure is covered with an oxide protective layer. A weakly p-doped silicon substrate forms the channel zone L, the heavily n-doped electrodes form the drain and source of the FET. For example, this is an N-channel enhancement type. The voltage U GS , applied between the membrane and the source connection determines the operating point of the field effect transistor.

Das FET-Mikrofon wird zweckmäßigerweise in einer Source-Schaltung be­trieben. Diese ist in der Figur 3 ebenso dargestellt, wie das dazugehö­rigende Kleinsignal-Ersatzschaltbild. Die Betriebsspannung UB wird dem Mikrofon über den Drain-Widerstand Rd zugeführt, der auf dem die Gegen­elektrode bildenden Chip gleich integriert werden kann. Am Drain-­Anschluß wird die Mikrofonausgangsspannung Ua abgegriffen; die Membran ist gegenüber Source mit der Spannung UGS vorgespannt. In der darge­stellten Kleinsignalersatzschaltung der Fig. 3 wird die Stromquelle mit der mechanisch-elektrischen Steilheit Sme durch die Membranauslenkung X gesteuert. Der eingeprägte Strom erzeugt im Drain-Widerstand Rd einen Spannungsabfall, der der Ausgangsspannung Ua entspricht.The FET microphone is advantageously operated in a source circuit. This is shown in FIG. 3, as is the associated small-signal equivalent circuit. The operating voltage U B is supplied to the microphone via the drain resistor R d , which can be integrated directly on the chip forming the counter electrode. The microphone output voltage U a is tapped at the drain connection; the membrane is biased against the source with the voltage U GS . In the small signal equivalent circuit shown in FIG. 3, the current source with the mechanical-electrical slope S me is controlled by the membrane deflection X. The impressed current produces a voltage drop in the drain resistor R d which corresponds to the output voltage U a .

Zur Berechnung von Frequenzgang und Empfindlichkeit des FET-Mikrofons wird das in Abb. 2 gezeigte mechanische Ersatzschaltbild zugrunde ge­legt. RS(w) und MS(w) stellen die Strahlungsimpedanz ZmS der Membran dar, MM die Masse und CM die Nachgiebigkeit der Membran, die mit der Schnelle vm schwingt. Das rückwärtige Luftvolumen wird durch die Nach­giebigkeit CV repräsentiert. Die Eingangskraft K = p x A setzt sich aus der Membranfläche A und dem vor der Membran herrschenden Wechseldruck p zusammen.The mechanical equivalent circuit shown in Fig. 2 is used to calculate the frequency response and sensitivity of the FET microphone. R S (w) and M S (w) represent the radiation impedance Z mS of the membrane, M M the mass and C M the compliance of the membrane, which vibrates with the rapid v m . The rear air volume is represented by the compliance C V. The input force K = px A is composed of the membrane area A and the alternating pressure p in front of the membrane.

Aufgrund der Frequenzabhängigkeit der Strahlungsimpedanz müssen für das Ersatzschaltbild zwei Gültigkeitsbereiche unterschieden werden. Unter­halb von etwa 155 kHz gilt für die Strahlungsimpedanz ZmS:
ZmS= RS+jwMS,
mit RS = 2,245 x 1o⁻¹⁶kg sec x w² und MS = 3,163 x 1o⁻¹⁰kg.Due to the frequency dependence of the radiation impedance, two areas of validity must be distinguished for the equivalent circuit diagram. Below approximately 155 kHz, the following applies to the radiation impedance Z mS :
Z mS = R S + jwM S ,
with R S = 2.245 x 1o⁻¹⁶kg sec x w² and M S = 3.163 x 1o⁻¹⁰kg.

Oberhalb 155 kHz ergibt sich für die Strahlungsimpedanz:
ZmS = RS+jwMS,
mit RS = 2,84o x 1o⁻⁴kg/sec und MS = (24o,5 kg/sec²) / w².Above 155 kHz, the radiation impedance is:
Z mS = R S + jwM S ,
with R S = 2.84o x 1o⁻⁴kg / sec and M S = (24o, 5 kg / sec²) / w².

Die Membranelemente dynamische Masse MM und Nachgiebigkeit CM haben die Werte:
MM = 7,384 x 1o⁻¹⁰ kg und
CM = 1/3oT (Zugspannung T in N/m im Bereich 2o...2oo N/m).The membrane elements dynamic mass M M and compliance C M have the values:
M M = 7.384 x 1o⁻¹⁰ kg and
C M = 1 / 3oT (tension T in N / m in the range 2o ... 2oo N / m).

Für die Nachgiebigkeit des rückwärtigen Luftvolumens V gilt:
CV = V/ρOC²Aeff²·The following applies to the flexibility of the rear air volume V:
C V = V / ρ O C²A eff ² ·

Als effektive Querschnittsfläche Aeff wird die Membranfläche angesetzt, Aeff = a². Das Volumen ergibt sich durch die Waferdicke, die die Rück­volumenhöhe darstellt. Sie beträgt 28o um. Somit folgt für CV:
CV = 2,866 x 1o⁻³ sec²/kg.
The effective cross-sectional area A eff is the membrane area, A eff = a². The volume results from the wafer thickness, which represents the back volume height. It is 28o um. Thus for C V follows:
C V = 2.866 x 1o⁻³ sec² / kg.

Masse, Nachgiebigkeit und Reibungsverluste der Luft im Luftspalt können vernachlässigt werden, da die Breite des Luftspaltes, der Breite der Drain-Kanal-Source-Struktur entsprechend wesentlich kleiner ist als die lateralen Abmessungen der Membran und der Öffnungen des Rückvolumens.The mass, compliance and frictional losses of the air in the air gap can be neglected, since the width of the air gap and the width of the drain-channel-source structure are considerably smaller than the lateral dimensions of the membrane and the openings in the back volume.

Die Rückwirkung des elektrischen Teils des FET-Mikrofons auf seine me­chanischen Eigenschaften entfällt, da die Membran das elektrische Feld im Luftspalt durch die Vorspannung UGS niederohmig treibt. Bei herkömm­lichen Kondensatormikrofonen in Niederfrequenzschaltung kann jedoch die Wirkung der angeschlossenen Schaltung auf das mechanische Verhalten des Wandlers nicht vernachlässigt werden. Eingangswiderstand und -kapazität des Vorverstärkers erzeugen eine Dämpfung und eine transformierte "elektrische" Nachgiebigkeit, die in das Schwingungsverhalten der Mem­bran und damit in das Verhalten des gesamten Wandlers eingehen.The reaction of the electrical part of the FET microphone to its mechanical properties is eliminated since the membrane drives the electrical field in the air gap with low resistance due to the bias voltage U GS . With conventional condenser microphones in a low-frequency circuit, however, the effect of the connected circuit on the mechanical behavior of the converter cannot be neglected. The input resistance and capacitance of the preamplifier generate damping and a transformed "electrical" compliance, which are reflected in the vibration behavior of the membrane and thus in the behavior of the entire transducer.

Für die mechanische Impedanz Zm folgt:
Zm= K/vm= ZmS+jwMM+1/jwCges,
wobei Cges= (1/CM+1/CV)⁻¹.For the mechanical impedance Z m it follows:
Z m = K / v m = Z + mS JWM M + 1 / JWC ges,
where C tot = (1 / C M + 1 / C V ) ⁻¹.

Mit vm= jwx und Membranfläche A folgt:
Ua=-SmexRD=-SmeRDVm/jw = -SmeRDpA/jwZm.With v m = jwx and membrane area A follows:
U a = -S me xR D = -S me R D V m / jw = -S me R D pA / jwZ m .

Für die Mikrofonemepfindlichkeit Me und ihren Freuquenzgang folgt daraus:
Me = Ua/p = -SmeRDA/jwZm
= -SmeRDACgesx 1/(1-w²MMCges+jwZmSCges)For the microphone sensitivity M e and its frequency response follows from this:
M e = U a / p = -S me R D A / jwZ m
-S = me R D AC ges x 1 / (1-C w²M M ges + jwz mS C ges)

Man erkennt, daß die Mikrofonempfindlichkeit proportional mit der me­chanisch-elektrischen Steilheit Sme und dem Drainwiderstand RD an­steigt. Diese lassen sich jedoch nicht beliebig vergrößern, da die ver­fügbare Höhe der Betriebsspannung UB und die maximal einstellbare elektrische Membranvorspannung UGS (Durchschlagfeldstärke im Kanal) Obergrenzen darstellen. Eine große Gesamtnachgiebigkeit Cges bedingt eine "weiche" Membran (hohe Nachgiebigkeit CM) und ein großes Rückvolu­men (CV). Auch hier sind gewisse Grenzen gesetzt. Die kleine Membran­fläche A von Subminiaturwandlern stellt ein inhärentes Problem dar.It can be seen that the microphone sensitivity increases proportionally with the mechanical-electrical slope S me and the drain resistance R D. However, these cannot be increased arbitrarily, since the available level of the operating voltage U B and the maximum adjustable electrical membrane bias U GS (breakdown field strength in the channel) represent upper limits. A large total resilience C ges requires a "soft" membrane (high resilience C M) and a large back volume (C V). Here, too, there are certain limits. The small membrane area A of subminiature transducers is an inherent problem.

Eine grafische Darstellung der Abhängigkeit der Empfindlichkeit Me von der Frequenz zeigt die Abb. 4 für verschiedene mechanische Membranspan­nungen und Rückvolumina.Fig. 4 shows a graphical representation of the dependence of the sensitivity M e on the frequency for different mechanical membrane tensions and back volumes.

Eine zweckmäßige Ausführungsform eines kapazitiven Schallwandlers nach der Erfindung wird anhand der Fig. 5 beschrieben. Das FET-Mikrofon be­steht aus zwei Chips, von denen der obere als Membraneinheit 1 die Mem­bran 2 trägt und der untere als Gegenelektrodenstruktur 3 die Drain-Ka­nal-Source-Struktur 8 des FETs trägt. Die Membran 2 besteht aus einer 15o nm starken Schicht 4 aus Siliziumnitrid, deren mechanische Span­nungseigenschaften durch Ionenimplantationen während des Herstellungs­prozesses beeinflußt werden können. Die Membran 2 wird von einem Stütz­rahmen 2.1 gehalten, welcher die Membran wallförmig umgibt und aus dem halbleitenden Grundmaterial, vorzugsweise Silizium besteht. Sie ist auf ihrer Unterseite mit einer 1oo nm-starken Aluminiumschicht 5 bedampft. Diese Bedampfung stellt das Gate des FETs dar. In dem unteren Chip wer­den durch Plasmaätzen zwei wannenförmige Gruben 6 und 7 eingebracht, die das Rückvolumen des Mikrofons bilden. Zwischen den Gruben befindet sich ein 8oµm-breiter Steg 8, der die Drain-Kanal-Source-Struktur 9, 1o und 11 des FETs trägt. Der Abstand des Kanals 1o zur Aluminium­schicht der Membran 5 beträgt 2 µm. Auf der Gegenelektrodenstruktur 3 sind ferner drei nicht weiter im einzelnen dargestellte Anschlußpads 11 für Drainkontakt, Sourcekontakt und die Aluminiumschicht der Membran, welche den Gate-Kontakt darstellt, angebracht. Eine Ausgleichsbohrung für den statischen Luftdruck befindet sich im Siliziumoxid-Rand 12 des Gegenelektrodenchips, sofern die Mikrofonkapsel als Druckwandler mit akustisch abgeschlossenen Volumen arbeiten soll.An expedient embodiment of a capacitive sound transducer according to the invention is described with reference to FIG. 5. The FET microphone consists of two chips, the upper one carrying the membrane 2 as the membrane unit 1 and the lower one carrying the drain-channel-source structure 8 of the FET as the counter electrode structure 3. The membrane 2 consists of a 150 nm thick layer 4 made of silicon nitride, the mechanical stress properties of which can be influenced by ion implantations during the manufacturing process. The membrane 2 is held by a support frame 2.1, which surrounds the membrane in the form of a wall and consists of the semiconducting base material, preferably silicon. It is vapor-coated on its underside with a 100 nm aluminum layer 5. This vaporization represents the gate of the FET. In the lower chip, two trough-shaped pits 6 and 7 are introduced by plasma etching, which form the back volume of the microphone. Between the pits there is an 8 µm-wide web 8 which carries the drain-channel-source structure 9, 10 and 11 of the FET. The distance between the channel 10 and the aluminum layer of the membrane 5 is 2 μm. On the counterelectrode structure 3, three connection pads 11 for drain contact, source contact and the aluminum layer of the membrane, which represents the gate contact, are also not shown in detail. A compensation hole for the static air pressure is located in the silicon oxide edge 12 of the counterelectrode chip, provided that the microphone capsule is to work as a pressure transducer with an acoustically closed volume.

Die Prozeßschritte zur Herstellung sowohl des Chips für die Membranein­heit als auch des Chips für die Gegenelektrodenstruktur sind dem in der Halbleitertechnologie bewanderten Fachmann bekannt und brauchen hier somit nicht weiter beschrieben zu werden. Um das Zusammenfügen der bei­den Halbleiterchips zu ermöglichen, wird noch auf die Siliziumoxid­schicht 12 eine Aluminiumschicht 13 aufgebracht. Die beiden Chips wer­den nun durch Erwärmung miteinander verbunden, wobei sich die gegen­überliegenden Aluminiumflächen der Membraneinheit 5 und der Gegenelek­trodenstruktur 13 miteinander verschmelzen.The process steps for producing both the chip for the membrane unit and the chip for the counterelectrode structure are known to the person skilled in semiconductor technology and therefore do not need to be described further here. In order to enable the joining of the two semiconductor chips, an aluminum layer 13 is also applied to the silicon oxide layer 12. The two chips are now connected to one another by heating, the opposite aluminum surfaces of the membrane unit 5 and the counterelectrode structure 13 fusing together.

Der in Fig. 5 beschriebene Wandler kann auch zu einem Gegentaktwandler erweitert werden, indem eine zweite Gegenelektrodenstruktur mit einem geeignet geformten Steg ähnlich dem Steg 8 in der durch den Wall vorge­gebenen Vertiefung der Membraneinheit 1 eingesetzt wird. In diesem Fall muß dann die Membran 2 auf beiden Seiten eine Metallisierung erhalten. Soll der Wandler in der beschriebenen Weise als Gegentaktwandler arbei­ten oder gemäß einer anderen zweckmäßigen Ausbildungsform eine Druck­gradientencharakteristik erhalten, so sind die vor beziehungsweise hin­ter der Membran liegenden Volumina über Öffnungen mit dem äußeren Schallfeld zu verbinden. In der Fig. 5 sind solche Öffnungen mit den Bezugsziffern 14 und 15 beispielsweise eingezeichnet.The converter described in FIG. 5 can also be expanded to a push-pull converter by using a second counter-electrode structure with a suitably shaped web similar to the web 8 in the depression of the membrane unit 1 predetermined by the wall. In this case, the membrane 2 must then be metallized on both sides. If the transducer is to function as a push-pull transducer in the manner described or if a pressure gradient characteristic is to be obtained in accordance with another expedient embodiment, the volumes in front of or behind the diaphragm are to be connected to the external sound field via openings. 5, such openings are shown with the reference numerals 14 and 15, for example.

Bei der beschriebenen Wandlerausführung ist zunächst in der Gegenelek­trodenstruktur für die Kanalzone das N- oder P-Kanal-Anreicherungsprin­zip verwendet worden. In vorteilhafter Weise kann jedoch auch für die Kanalzone das Verarmungsprinzip eingesetzt werden. Da hier bereits ein Arbeitspunkt in der FET-Schaltung vorgeben ist, kann hier die gesonder­te Vorspannung für das Gate entfallen, da sie in bekannterweise über einen im Source-Stromkreis eingesetzten Widerstand selbst erzeugt wer­den kann.In the converter design described, the N or P-channel enrichment principle was first used in the counter-electrode structure for the channel zone. However, the depletion principle can also advantageously be used for the channel zone. Since an operating point is already specified here in the FET circuit, the separate bias for the gate can be omitted here, since it can itself be generated in a known manner via a resistor used in the source circuit.

Wie aus den Herstellungsverfahren für integrierte Schaltungen bekannt geworden ist, werden sehr viele einander gleiche Baueinheiten auf ei­nem sogenannten Wafer gleichzeitig hergestellt und nach abgeschlossenem Herstellungsverfahren auseinandergetrennt. Bei der Herstellung von ka­pazitiven Schallwandlern nach der Erfindung ist es nun ebenfalls mög­lich, sehr viele Kleinstmikrofone auf einem Wafer herzustellen, sie aber nicht zu vereinzeln, sondern in besonders geformten Gruppen her­auszutrennen. Durch die Reihenanordnung mehrerer nebeneinanderliegender Mikrofonsysteme und deren elektrische Zusammenschaltung ist es möglich, beispielsweise ein Interferenz-Richtmikrofon zu erhalten.As has become known from the production processes for integrated circuits, a large number of identical structural units are produced simultaneously on a so-called wafer and separated after the production process has been completed. In the production of capacitive sound transducers according to the invention, it is now also possible to manufacture a large number of microphones on a wafer, they but not to separate them, but to separate them in specially shaped groups. The series arrangement of several microphone systems lying next to one another and their electrical interconnection makes it possible to obtain, for example, an interference directional microphone.

Ein großer Vorteil bei einem kapazitiven Wandler nach der Erfindung liegt darin, daß einer relativ großen aktiven Membranfläche, die für einen guten akustischen Wirkungsgrad des Wandlers gefordert wird, nur ein kleiner Teil der Membranfläche einer Gegenelektrodenstruktur gegen­über liegt und somit die Strömungsverluste vernachlässigbar klein wer­den. Daraus ergibt sich ein großer linearer Übertragungsbereich bei sehr guter Empfindlichkeit, wie aus der Fig. 4 zu erkennen ist. Weiter­hin ist auch das Rauschverhalten des Wandlers außerordentlich günstig, da der durch Dämpfungen im Luftspalt hervorgerufene Rauschanteil prin­zipbedingt sehr niedrig ausfällt. Kapazitive Wandler werden zumeist in der sogenannten Niederfrequenzschaltung betrieben und benötigen daher einen Vorwiderstand, dessen thermisches Rauschen ebenfalls mit wachsen­dem Widerstandswert zunimmt. Sinkende Wandlerruhekapazitäten bei Mi­niaturmikrofonen bedingen bei gleicher unterere Grenzfrequenz jedoch größer werdende Vorwiderstände, worin bei den bisherigen Ausführungen ein unlösbares Problem bestand. Da das FET-Mikrofon keinen Vorwider­stand benötigt, ist damit ebenfalls der Rauschanteil wesentlich ver­ringert worden.A great advantage of a capacitive transducer according to the invention is that a relatively large active membrane area, which is required for good acoustic efficiency of the transducer, is only a small part of the membrane area opposite a counter-electrode structure, and thus the flow losses are negligibly small. This results in a large linear transmission range with very good sensitivity, as can be seen from FIG. 4. Furthermore, the noise behavior of the converter is extremely favorable, since the noise component caused by damping in the air gap is very low due to the principle. Capacitive converters are mostly operated in the so-called low-frequency circuit and therefore require a series resistor, the thermal noise of which also increases with increasing resistance. Decreasing converter quiescent capacitances in miniature microphones require increasing series resistances at the same lower cut-off frequency, which was an unsolvable problem in the previous versions. Since the FET microphone does not require a series resistor, the noise component has also been significantly reduced.

Das Rauschverhalten kann auch dadurch verbessert werden, daß mehrere auf dem Wafer gemeinsam entstandene FET-Mikrofone parallel geschaltet als eine Mikrofoneinheit betrieben werden.The noise behavior can also be improved by operating a plurality of FET microphones which have arisen jointly on the wafer in parallel as a microphone unit.

Claims (10)

1. Kapazitiver Schallwandler, bestehend aus mindestens zwei zusammen­gefügten Halbleiterchips, welche eine Membraneinheit und eine feststehende Gegenelektrodenstruktur verkörpern und mittels be­kannter Methoden der Halbleitertechnologie hergestellt werden,
dadurch gekennzeichnet,
daß der akustisch aktive Teil der Membraneinheit mit mindestens einer Gegenelektrodenstruktur, welche von der Membraneinheit durch einen Luftspalt getrennt ist, ein einem Feldeffekttransistor ver­gleichbares System bildet derart, daß einerseits die aus halbleitendem Grundmaterial gebildete Mem­braneinheit eine akustisch aktive Membranfläche umfaßt, deren der Gegenelektrodenstruktur zugewandte Seite elektrisch leitend ist, und andererseits die Gegenelektrodenstruktur aus einer aus halb­leitenden Grundmaterial herausgearbeiteten, durch eine Source-­Drain-Anordnung begrenzten Kanalstrecke besteht, deren geometri­sche Breitenabmessung in der Größenordnung von einem Zehntel der lateralen Abmessung der aktiven Membranfläche liegt.1. capacitive sound transducer, consisting of at least two assembled semiconductor chips, which embody a membrane unit and a fixed counter-electrode structure and are produced by means of known methods of semiconductor technology,
characterized,
that the acoustically active part of the membrane unit with at least one counterelectrode structure, which is separated from the membrane unit by an air gap, forms a system comparable to a field effect transistor such that, on the one hand, the membrane unit formed from semiconducting base material comprises an acoustically active membrane surface, the side of which facing the counterelectrode structure is electrical is conductive, and on the other hand the counterelectrode structure consists of a channel section worked out from semiconducting base material and delimited by a source-drain arrangement, the geometrical width dimension of which is in the order of magnitude of one tenth of the lateral dimension of the active membrane surface. 2. Kapazitiver Schallwandler nach Anspruch 1,
dadurch gekennzeichnet,
daß als Grundmaterial für die Membraneinheit und die Gegenelektro­denstruktur Silizium eingesetzt wird, und die aktive Fläche der Membraneinheit aus einer Siliziumnitrid-Schicht besteht, welche mit Aluminium bedampft und deren mechanische Spannung durch Ionen­implantation bestimmt ist.2. Capacitive sound transducer according to claim 1,
characterized,
that silicon is used as the base material for the membrane unit and the counterelectrode structure, and the active surface of the membrane unit consists of a silicon nitride layer which is vapor-deposited with aluminum and whose mechanical stress is determined by ion implantation. 3. Kapazitiver Schallwandler nach einem der vorhergehenden Ansprüche,
dadurch gekennzeichnet,
daß nach Art eines Gegentaktwandlers beide Seiten der aktiven Fläche der Membran metallisiert sind und jeder Seite eine Gegen­elektrodenstruktur zugeordnet ist.3. Capacitive sound transducer according to one of the preceding claims,
characterized,
that in the manner of a push-pull converter both sides of the active surface of the membrane are metallized and each side is assigned a counter electrode structure. 4. Kapazitiver Wandler nach einem der vorhergehenden Ansprüche,
dadurch gekennzeichnet,
daß in der Gegenelektrodenstruktur für die Kanalzone das N- oder P-Kanal-Anreicherungsprinzip verwendet wird.4. Capacitive converter according to one of the preceding claims,
characterized,
that the N or P channel enrichment principle is used in the counter electrode structure for the channel zone. 5. Kapazitiver Wandler nach einem der vorhergehenden Ansprüche,
dadurch gekennzeichnet,
daß in der Gegenelektrodenstruktur für die Kanalzone das N- oder P-Kanal-Verarmungsprinzip verwendet wird.5. Capacitive converter according to one of the preceding claims,
characterized,
that the N or P channel depletion principle is used in the counter electrode structure for the channel zone. 6. Kapazitiver Wandler nach einem der vorhergehenden Ansprüche,
gekennzeichnet
durch eine durch ein abgeschlossenes Volumen der Gegenelektroden­struktur bedingte Druckwandlercharakteristik.6. Capacitive converter according to one of the preceding claims,
featured
by a pressure transducer characteristic caused by a closed volume of the counter electrode structure. 7. Kapazitiver Wandler nach einem der vorhergehenden Ansprüche,
gekennzeichnet
durch eine durch in der Gegenelektrodenstruktur außerhalb des Kanalbereichs angeordnete Öffnungen bedingte Druckgradienten­charakteristik.7. Capacitive converter according to one of the preceding claims,
featured
by a pressure gradient characteristic caused by openings arranged in the counter electrode structure outside the channel region. 8. Mehrfachwandler unter Verwendung von kapazitiven Schallwandlern nach einem der vorhergehenden Ansprüche,
gekennzeichnet
durch die elektrische Zusammenschaltung mehrerer auf einem Wafer in Reihe angeordneter und gleichzeitig hergestellter Wandler zu einem Interferenz-Richtmikrofon.8. multiple transducers using capacitive sound transducers according to one of the preceding claims,
featured
by the electrical interconnection of several transducers arranged in series on a wafer and simultaneously manufactured to form an interference directional microphone. 9. Mehrfachwandler unter Verwendung von kapazitiven Wandlern nach einem der vorhergehenden Ansprüche,
gekennzeichnet
durch die elektrische Parallelschaltung mehrerer auf einem Wafer gemeinsam herausgetrennter Wandlersysteme.9. Multiple converter using capacitive converters according to one of the preceding claims,
featured
through the electrical parallel connection of several transducer systems that are jointly separated on a wafer. 10. Kapazitiver Wandler nach einem der vorhergehenden Ansprüche,
dadurch gekennzeichnet,
daß auf der Gegenelektrodenstruktur weitere, Verstärkerschaltungen bildende Bauelemente integriert sind.10. Capacitive converter according to one of the preceding claims,
characterized,
that further components forming amplifier circuits are integrated on the counter electrode structure.
EP89103276A 1988-03-05 1989-02-24 Capacitive sound transducer Expired - Lifetime EP0331992B1 (en)

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DE3807251A DE3807251A1 (en) 1988-03-05 1988-03-05 CAPACITIVE SOUND CONVERTER
DE3807251 1988-03-05

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FR2697675A1 (en) * 1992-11-05 1994-05-06 Suisse Electronique Microtech Method of manufacturing integrated capacitive transducers
EP0596456A1 (en) * 1992-11-05 1994-05-11 CSEM, Centre Suisse d'Electronique et de Microtechnique S.A. Method of manufacturing an integrated capacitive transductor
US5408731A (en) * 1992-11-05 1995-04-25 Csem Centre Suisse D'electronique Et De Microtechnique S.A. - Rechere Et Developpement Process for the manufacture of integrated capacitive transducers
WO2007062975A1 (en) 2005-11-29 2007-06-07 Robert Bosch Gmbh Micromechanical structure for receiving and/or generating acoustic signals, method for producing a micromechanical structure, and use of a micromechanical structure
US7902615B2 (en) 2005-11-29 2011-03-08 Robert Bosch Gmbh Micromechanical structure for receiving and/or generating acoustic signals, method for producing a micromechanical structure, and use of a micromechanical structure

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CA1298396C (en) 1992-03-31
ATE110919T1 (en) 1994-09-15
EP0331992B1 (en) 1994-08-31
DE58908250D1 (en) 1994-10-06
DE3807251A1 (en) 1989-09-14
JPH01316099A (en) 1989-12-20
EP0331992A3 (en) 1991-07-03
US4922471A (en) 1990-05-01

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