WO2002090242A2 - Electroacoustic transducer for generating or detecting ultrasound, transducer array and method for the production of the transducer or the transducer array - Google Patents

Abstract

The invention relates to an electroacoustic transducer and to a transducer array for generating or detecting ultrasound, comprising a solid, deflection resistant substrate, a flexible membrane made of a semiconductor material, whose edge is affixed on the substrate, a cavity between the membrane and the substrate and electrodes for applying or tapping electric current between the membrane and the substrate. The invention is characterized in that at least two different local zones (Z1, Z2) of the membrane (8) have a different foreign matter concentration (N1, N2) and/or mass filling factor (M1,M2).

Description

Electroacoustic transducer for generating or detecting ultrasound, transducer array and method for producing the transducer or transducer array

The invention relates to an electroacoustic transducer for generating or detecting ultrasound, with a solid, rigid substrate, a flexible membrane made of a semiconductor material, the edge of which is attached to the substrate, a cavity between the membrane and the substrate, and electrodes for applying or tapping a voltage between the membrane and the substrate, the membrane being doped with solids. The invention further relates to an array constructed from such transducers and a method for producing such transducers or transducer arrays.

Such electroacoustic transducers are known and come in a number of commercial applications, for example in the non-destructive material testing or medical diagnostics. With the aid of such transducers, systems for distance measurement or for flow measurement can be implemented, or shape features can be recorded in acoustically transparent bodies, for example the human body, by means of imaging methods. In the case of such transducers, the flexible membrane is deflected by means of an electrical alternating voltage to generate ultrasound, at a frequency which corresponds to the mechanical resonance frequency of the system. The usable ultrasound is generated by the mechanical periodic deflection of the membrane and the alternating compression and dilution of the gas or liquid environment in the form of pressure gradients on the front of the membrane. The resulting ultrasound spreads from this membrane into the surrounding gas or liquid. In addition to the exciting AC voltage, a larger rest voltage is also applied between the two electrodes so that the membrane oscillates in the frequency of the exciting AC voltage. Conversely, if ultrasound occurs on the membrane from the outside and excites it in its mechanical resonance, this leads to the deflection of the membrane, and the distance between the two electrodes, ie the membrane and the substrate, is thereby changed. Using a suitable evaluation circuit, these changes in distance can be measured as changes in capacitance that are proportional to the incident ultrasound.

The mechanical resonance of the membrane is determined in particular by the mechanical rigidity of the membrane and its clamping and the nature of the cavity located below the membrane. The mechanical rigidity is in turn determined on the one hand by the geometric dimensions of the flexible membrane and on the other hand by its mechanical residual stress and other material parameters. It is disadvantageous that the known electroacoustic transducers, which work as membrane vibrators, can only be operated in a relatively low frequency range due to their design, which is determined by the width of the resonance increase of the transducer, that is to say by its usable bandwidth. However, converters with a low active bandwidth are disadvantageous for use as an image converter in imaging ultrasound diagnostic methods, since pulse-shaped signals and their different transit times have to be detected by different substances, for example body tissue, during image processing. The processing of pulse-shaped signals, however, relies on converters that have a large usable bandwidth. From US 5,870,351 an electroacoustic transducer of the type mentioned at the outset and a transducer array are known which comprise a large number of transducers of this type with membranes of different sizes. The size of the membrane area determines the mechanical resonance frequency of the transducer in question, and the sizes in the known transducer array are matched to one another in such a way that the resonance frequencies of the various transducers are at a predetermined distance from one another, that is to say transducers with very different resonance frequencies in the transducer. Arrays are present, so that the entire bandwidth of the transducer array results from a superposition of the individual - spaced - resonance curves of the individual transducers. The production of such known arrays with membranes of different sizes is not optimal in terms of area. In order to increase the usable bandwidth, only a few transducers with the same size membrane are arranged in the array - per unit area - that is, the local density of the respective membrane sizes is comparatively low, the local resolution is therefore unsatisfactory if converter arrays of this type are used, for example, for image processing. In addition, in the converter structure according to US Pat. No. 5,870,351, the flexible membrane is made of insulator material, and the electrode connected to the membrane is applied to the membrane as a metal layer, so that in addition to the cavity, the membrane thickness is also present between the first substrate-fixed electrode and the second electrode is, whereby the resting capacity around which the change in capacity due to a membrane deflection takes place is correspondingly smaller, the detection of the change in capacity and thus of the ultrasound is less sensitive.

From the lecture publication "A new class of capacitive micromachined ultrasonic transducers" by Ahrens et al "held at" Ultrasonic Symposium 2000 ", Puerto Rico, Oct. 2000, transducers of the type mentioned are known, in which the flexible membrane consists of a uniformly doped polycrystalline silicon material. Due to a relatively high doping with dopants, the membrane has good conductivity and can therefore be used as a counter electrode to the base electrode fixed to the substrate. As a result, the ratio of change in capacity and resting capacity when the membrane is deflected during ultrasound generation / exposure is greater, and the sensitivity with which ultrasound can be detected is correspondingly greater. The disadvantage here is that due to those edge areas of the membrane that because of their proximity to the the clamped edge experiences no or almost no deflection, but non-usable capacity shares contribute to the resting capacity. In order to increase the active bandwidth, these known transducers with different diaphragm sizes must again be distributed next to one another on the surface, in accordance with the teaching of US Pat. No. 5,870,351.

The object of the invention is to develop a converter or a converter array of the type mentioned at the outset in such a way that the usable bandwidth of the converter or of the array is increased. It is also an object to specify a method for producing the converter or the converter array.

This object is achieved according to the invention in the converter of the type mentioned in the introduction in that at least two different local zones Z-ι, Z ₂ , the membrane have a different concentration of foreign substances Nj, N ₂ , and / or masses M _u M ₂ .

The advantages of the invention are, in particular, that the different doping of different local zones or areas of the membrane with foreign substances and / or the different masses of these zones, the material-specific or manufacturing-specific properties, such as the modulus of elasticity, the membrane density or the Poisson number Semiconductor material or the membrane thickness and the membrane internal stress changed so that zones of different mechanical material properties arise with the result that the different zones of the membrane have different discrete resonance frequencies, whereby the usable bandwidth of the transducer - with superposition of the individual resonances - is broadened. According to the invention, it is therefore possible, by means of doping processes or by chemical or physical etching processes or by chemical or physical material deposition (chemical vapor deposition CVD or physical vapor deposition PVD), which are mastered extremely precisely in semiconductor technology, the individual transducer and the latter Converters built transducers to give an increased usable bandwidth and thus improved properties.

According to a particularly preferred embodiment of the invention, a first electrode, also referred to as a base electrode, is arranged on the surface of the substrate which is adjacent to the membrane, that is to say adjoins the cavity. The substrate preferably consists of semiconductor material. Alternatively, the substrate can also be implemented as a base body made of insulator material with an upper semiconductor layer. On the side facing the cavity, the first electrode or base electrode is preferably realized by a highly doped conductive zone in the substrate.

Particular preference is given to using dopants, that is to say donors or acceptors, as foreign substances for doping the various membrane zones which, in addition to changing the mechanical properties of the semiconductor material, also change the electrical properties.

A central zone of the membrane is preferably particularly heavily doped with donors or acceptors and thus has a high conductivity, on the basis of which the membrane itself can be switched as a second electrode and forms the counter electrode to the base electrode. In this embodiment, the two electrodes, which are used to apply the generator voltage or to tap the voltage changes / capacitance changes generated, are located directly at the boundary layer with the cavity, and are therefore at a particularly small distance from one another, as a result of which the corresponding resting capacity or the capacitance changes are particularly large are. The transducer therefore has a greater sensitivity than in embodiments in which the electrodes are at a greater distance from one another. Due to the high dopant concentration N of individual local zones, i.e. Due to the defined, localized introduction of dopants, donors or acceptors, the corresponding zones become electrical conductors or electrodes. As a result, the capacitance present between the base electrode and the counter electrode is influenced, undesired capacitance components are reduced, which otherwise form in particular at the edge region of the membrane and reduce the signal-to-noise ratio.

The membrane is preferably attached or clamped to the substrate with its periphery all around an insulating intermediate layer.

The local doping of a defined, locally restricted zone of the membrane is carried out using the known methods for structured material introduction. By doping individual zones with impurity atoms in different concentrations, N becomes influences the mechanical properties of these zones differently. In particular, the modulus of elasticity and / or the Poisson number and / or the membrane density and the mechanical residual stress of the membrane clamped circumferentially on the edge of the substrate are changed locally accordingly, and in this particular embodiment is reduced with increasing foreign substance concentration. The residual stress is a measure of the rigidity of the membrane zone in question. Due to the changed stiffness, the highly doped zone in question has a different mechanical resonance than the entire membrane, including the highly doped zone. Overall, a resulting residual stress occurs in the membrane, which differs from the residual stress of the membrane, which is homogeneous over the entire surface.

According to a further particularly preferred embodiment of the invention, either the membrane on its underside or the substrate on its top carries an insulating layer. This prevents the membrane and substrate from touching each other and forming electrical contact that could damage the transducer.

According to the invention, a circumferential third zone can be provided between the heavily doped central zone and the weakly doped or undoped edge zone, which circulates concentrically around the central zone and whose foreign substance concentration N is different from that of the other two zones. An intermediate zone can also be arranged between the concentrically rotating third zone and the central first zone, which has its own foreign substance concentration N which differs from the other foreign substance concentrations, but which, like the peripheral zone, is preferably undoped or weakly doped. Instead of concentrically arranged zones, the differently doped zones can also be arranged arbitrarily, for example next to one another or as islands within other zones. The shape of the membrane is preferably round or square or hexagonal or polygonal.

In a preferred embodiment, monocrystalline silicon is used as the material for the substrate, the surface of which adjoins the cavity has a heavily doped and thus highly conductive zone, which represents the base electrode. In this embodiment of the invention, the base electrode can itself be electrically contacted via a highly doped semiconductor structure introduced into the substrate. In this embodiment, the membrane consists of polycrystalline silicon, which is weakly or not doped at the edge and also has a high doping concentration N in the central zone.

In the present patent application, N means the impurity concentration when the membrane is doped with any suitable impurity atoms, but in particular also when doping with dopants, ie donors or acceptors. In the latter case, N is also referred to as the doping concentration or dopant concentration. Nj, with i = 2, 3 ... means the foreign substance concentration in the local

M |, with i = 2, 3 ... means the mass assignment of the membrane, which is defined as mass / area unit, in each case in the local zone Zj.

The invention also relates to an array of a plurality of capacitive membrane oscillators, all of which are arranged on a common, rigid, rigid substrate and have a structure according to one of Claims 1 to 16.

According to one embodiment of the invention, the membranes in such an array of ultrasonic transducers are all of the same size. Alternatively, the membranes have different sizes and / or different thicknesses in order to design the resonance frequencies of the individual transducers differently. In addition - or alternatively in the case of membranes of the same size - the membranes of the individual transducers can also be doped to different extents with foreign substances and / or be provided with different mass assignments, in order in this way to differentiate the mechanical residual stress ranges of the individual transducers even more strongly.

In a particularly preferred embodiment of the transducer array according to the invention, the membranes and the transducers all have the same structure and the same size. In neighboring membranes, however, local zones of comparatively different sizes are provided with a high concentration of foreign matter and / or different masses, which can also be of different heights in neighboring membranes, as a result of which the mechanical residual stresses of the adjacent transducers differ from one another. As a result, the entire active bandwidth of the converter array is spread out and enlarged. The invention also relates to a method for producing a converter or a converter array, the construction of which is designed according to one of Claims 1 to 24. In the process, a flexible membrane made of semiconductor material is attached all around at its edge to a rigid and rigid substrate. The method according to the invention is characterized in that at least two different local zones Zj, i = 2, 3 ... of the membrane are doped to different extents with foreign substances. The dopants known in semiconductor technology are preferably used as foreign substances, which change their electrical properties in addition to the mechanical properties of the membrane when they are introduced.

An embodiment of the method according to the invention is further characterized in that in different zones Zj, i = 2, 3... Of the membrane, the thickness of the membrane is set differently by material application or material removal, and thus the mass assignment of the membrane. For this purpose, the thickness of the membrane is either removed locally by chemical or physical etching, or material is applied locally by chemical or physical deposition from the vapor phase.

Advantageous developments of the invention are characterized by the features of the subclaims.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawing. Show it:

1 shows a diagram which shows the mechanical residual stress of a membrane and the specific surface resistance as a function of time and type of treatment, which are present in a membrane during a recrystallization phase and a subsequent doping phase;

2a to 2d show a cross section of an electroacoustic transducer in different phases of its manufacture;

3 shows a perspective view of a transducer array comprising a plurality of transducers with rectangular membranes arranged in rows and columns;

4 the membrane of an electroacoustic transducer, in each case in supervision and with differently doped zones; 5 shows the membrane of an electroacoustic transducer in supervision, different transducer shapes and different zones with different doping are shown; and

Fig. 6 different transducer arrays, each with associated schematically shown resulting bandwidth.

1 shows the mechanical residual stress, measured in MPa, and the specific surface resistance, measured in ohm / sq, as is typically present in the case of crystallization or recrystallization of a membrane and in a subsequent doping step. In the measured exemplary embodiment, the membrane consists of a thin semiconductor layer of approximately 0.5 to 2 μm. It is remarkable that during a recrystallization phase the mechanical internal stress of the membrane, i.e. the voltage in the membrane plane can be negative at first and increases sharply with increasing crystallization time, so that the membrane becomes more and more tense with increasing crystallization and therefore always stiffer. If the membrane is subsequently doped, the mechanical residual stress as a function of the doping time and concentration decreases again sharply. In addition, the specific resistance decreases. The physical effect shown in Fig. 1 can be used according to the invention to determine the mechanical properties of the membrane of a capacitive electroacoustic transducer manufactured on a semiconductor basis, i.e. to be set differently in different subareas or zones. With a low doping concentration N, there is a comparatively high mechanical residual stress; with increasing doping concentration, this residual stress is reduced.

2a to 2c show a cross section through a capacitive electroacoustic transducer at different times during its manufacture. 2d shows an alternative embodiment to the representation according to FIG. 2c. A semiconductor substrate 2 is provided on its surface with a highly doped zone 4, which serves as the first electrode or base electrode of the converter and is led to the outside with a connecting line. One or more sacrificial layers are arranged above the first electrode 4, an insulating layer 12 is arranged outside the base electrode 4 and outside the sacrificial layer. A thin semiconductor layer 7 is applied over the sacrificial layer 5, which can consist of several material layers, which in the example shown is undoped or has only a weak doping concentration N. The semiconductor layer 7 is then in the outer, the sacrificial layers 5 partially overlapping zones, for example by means of a Photoresists 30 covered. The photoresist-free central zone of the semiconductor layer 7, which is located above the base electrode 4 and the sacrificial layers 5 above, is then subjected to a doping step and thereby receives a high doping concentration of dopants, for example donors or acceptors. The photoresist 30 is then removed. The membrane layer is then structured and thereby receives its final lateral dimensions, for example a round or square, hexagonal or polygonal shape. Then the sacrificial layers 5 are removed, in this way the cavity 6 is created. Finally, the membrane 8 is connected circumferentially to the substrate at its edge by means of already existing or still to be applied insulating layers 12 and thus completely clamped in at the edge. Finally, a metallic connecting line 20 is applied, which is connected either via a highly conductive web at the edge of the membrane to the central, highly doped and thus highly electrically conductive zone Z ^ cf. 2c and 4b or directly to the central zone Zj and contacted with this, cf. Fig. 2d.

2c shows the typical structure of a capacitive electroacoustic ultrasound transducer based on semiconductor material in cross section. The substrate 2 contains a first electrode 4, the base electrode, which is implemented as a highly doped zone in the substrate 2. A cavity 6 is located above it, and above the cavity is the thin, flexible membrane 8, which likewise consists of semiconductor material and is clamped against the substrate 2 at its edge by means of suitable insulating layers 12.

According to the invention, the membrane 8 according to FIG. 2 c has differently doped zones Z which have a different doping concentration N either from acceptors A or donors D. In the exemplary embodiment shown in FIG. 2c, the central zone Z _{1 is provided} with a high doping concentration Ni. This zone Zi therefore has - according to the diagram in FIG. 1 - a low mechanical residual stress. The edge zone Z ₂ , on the other hand, is not or only weakly doped, it has the doping concentration N ₂ . This edge zone Z ₂ therefore has a comparatively high mechanical internal stress and therefore rigidity, cf. Fig. 1.

In the embodiment shown in FIG. 2c, the highly conductive central zone Zi is used as the second electrode - also called the membrane electrode - because of the highly conductive properties of this central area can be dispensed with the otherwise usual additional application of a metal electrode to the membrane 8. The two electrodes 4, Zi are aligned one above the other, they end laterally at a predetermined distance in front of the clamped and thus immovable edge of the membrane 8. This ensures that the capacitance between the electrodes 4 and only in that area of the membrane 8 that is defined takes part in the movement of the membrane 8, caused by an applied voltage or by incident ultrasound. Unused portions of the quiescent capacitance and also parasitic capacitances of supply lines are not detected via the two electrodes 4, Z _λ , and therefore do not reduce the sensitivity.

2c shows the embodiment of the converter in which the highly doped central zone Z contains a narrow, web-shaped section which extends to the edge of the membrane 8 and is contacted there directly by an electrical connecting line 20. Such a configuration of the membrane is, for example, also shown in a top view in FIG. 4b.

vollständig von der schwach bzw. nicht dotierten Randzone Z₂ umgeben ist. Bei dieser Ausführungsform ist die Randzone Z₂ an einer Stelle - in dem dargestellten Querschnitt links - von einer Isolierschicht 12 abgedeckt, und über diese Isolierschicht 12 ist die elektrische Anschlussleitung 20 bis in die zentrale Zone Zi geführt, um die zentrale Zone Z-i dort zu kontaktieren.In contrast, FIG. 2d shows an embodiment of the converter in which the highly doped central zone

is completely surrounded by the weak or undoped edge zone Z ₂ . In this embodiment, the edge zone Z _{2 is covered} at one point - on the left in the cross section shown - by an insulating layer 12, and the electrical connection line 20 is guided via this insulating layer 12 into the central zone Zi in order to contact the central zone Zi there ,

3 shows an array of several transducers, which is arranged on a semiconductor substrate 2 and each has a first electrode 4 and a rectangular membrane 8 above it. Electrical connection lines 22 run between the membranes in order to contact the electrodes 4, Zi.

Z₂ ... mit unterschiedlichen Fremdstoffkonzentrationen N_1τ N₂ ... versehen sind. Während die Membranen gemäß Fig. 4 eine quadratische Gesamtfläche besitzen, sind die Membranen 8 gemäß Fig. 5 entweder rund, hexagonal oder quadratisch ausgebildet.FIG. 4 shows plan views of various membranes 8, all of which have a quadratic basic shape and various, differently doped zones Zi, Z _2, Z ₃ and _Z. ... each with the associated different foreign substance concentrations Ni, N ₂ , N ₃ and N ₄ ..., whereby in principle all chemical elements can be used as foreign substances. Fig. 5 also shows top views of different membranes in different zones

Z ₂ ... are provided with different _{concentrations of} foreign substances N _1τ N ₂ ... While the membranes according to FIG. 4 have a square total area, the membranes 8 according to FIG. 5 are either round, hexagonal or square.

4 and 5, highly doped zones Z are shown with different hatching, weak or undoped zones are not hatched. It is common to all membranes according to FIGS. 4 and 5 that the edge zone Z _{2 is} undoped or has a weak doping N ₂ . In certain cases, the lowering of the mechanical residual stress in the peripheral zone can be aimed at by doping with foreign substances. In contrast, in almost all of the illustrated embodiments, a central zone Z _{π is} provided, which has a high concentration of foreign substances N _T and which, for example in FIG. 4b, is even led out with a web to the edge of the membrane, on which a metallic connecting line the central zone Z ^ contacted.

4c, two separate zones Z ₃ , Z _{4 are provided} with a high concentration of dopants N ₃ , N ₄ , the remaining areas of the membranes, also called base zone Z _G , enclose the edge zone Z ₂ and have the weak doping concentration N. ₂ or are not endowed.

4 and 5, at least one highly doped zone, for example the central zone Z ^ with the doping concentration, is introduced at the base zone Z _G , which also contains the edge zone Z ₂ with the weak doping concentration N ₂ . In the examples according to FIGS. 4d, 4e and 4f, further circumferential zones Z ₃ with the high concentration N ₃ and zone Z ₄ with the high concentration N ₄ are at a short distance from the central zone Z ^ arranged from the adjacent zones, the doping of the highly doped zones having an identical concentration or different concentrations N of foreign substances.

6 shows the schematic top view of several transducers within different transducer arrays. The array A is designed in accordance with the prior art, in which all transducers have an identical membrane which is identical in terms of size, shape and doping. All individual converters therefore have a sharp resonance frequency at which the membrane can be set in mechanical vibrations, ie the bandwidth B of the transducer array is small, cf. Figure 6b.

6b shows a transducer array in which many transducers according to the invention are attached to a substrate, the individual membranes having highly doped central zones Zi with the foreign substance concentration Ni and lightly doped edge zones Z ₂ with the low foreign substance concentration N ₂ . According to the representation according to FIG. 6d, each transducer thus has two different resonance frequencies, first the entire membrane, ie the edge zone together with the central zone, resonates, with a different frequency the less rigid central zone Zi then resonates alone. The overlaying of two resonances results in a broadening of the active bandwidth of the array, cf. Fig. 6d.

6e shows a transducer array in which the central zones Zi, Z _{3 are} again heavily doped with the concentration N, while the edge zones Z _{2 have} a weak doping. In addition, the highly doped zones Z have different sizes in different converters. This constellation results in at least two and up to four discrete mechanical resonances, the total usable bandwidth resulting from a superimposition of these resonances is increased, cf. 6f.

FIG. 6g shows a transducer array in which the membranes all have the same area and in which the central, highly doped zones Zi also all have the same size. However, while some transducers have a high doping concentration Nu, other transducers have a different high doping concentration N ₂ . For this reason, the transducer field shown has at least two and up to four discrete mechanical resonance frequencies. The usable total bandwidth resulting from superimposition is correspondingly large, cf. Fig. 6h.

Claims

Expectations 1. Electroacoustic transducer for generating or detecting ultrasound, with a rigid, rigid substrate, a flexible membrane with its edge attached to the substrate made of semiconductor material, a cavity between the membrane and the substrate, and with Electrodes for applying or tapping a voltage between the membrane and the substrate, the membrane being in particular doped with foreign substances, characterized in that at least two different local zones Zi, Z ₂ , the membrane (8) have a differently high foreign substance concentration N, N ₂ , and / or have a different mass allocation Mi, M ₂ . 2. Transducer according to claim 1, characterized in that a first electrode is arranged on the surface of the substrate (2) adjacent to the cavity (6). 3. Converter according to claim 1 or 2, characterized in that a central zone Zi of the membrane (8) has a high impurity concentration Ni of dopants, and serves as a second electrode. 4. Transducer according to one of the preceding claims, characterized in that the central zone Zi of the membrane (8) has a low mass occupancy Mi and the peripheral zone Z _{2 has} a larger mass movement M ₂ . 5. Converter according to one of the preceding claims, characterized in that the edge zone Z _{2 of} the membrane (8) has a low or no foreign substance concentration, and the central zone Zi has a high foreign substance concentration. 6. Converter according to one of the preceding claims, characterized in that dopants, ie donors and / or acceptors, are introduced into the different local zones Z i = 2, 3, ... in different concentrations Nj as foreign substances. 7. Converter according to claim 6, characterized in that the edge zone Z ₂ ) of the membrane (8) is undoped or weakly doped, and that the central zone Zi has a high doping concentration Ni of dopants. 8. Converter according to claim 6 or 7, characterized in that the highly doped central zone Zi by means of a highly doped narrow connecting web of the membrane (8) or by means of a metallic line to the edge of the membrane (8) with a metallic connecting line (20) electrically connected is. 9. Converter according to one of claims 6 to 8, characterized in that the central zone Z1 has a lower doping concentration N-ι than the doping concentration N _{2 of} the peripheral zone Z ₂ , and that a metal layer is arranged on the central zone Z1, which forms a second electrode, which is contacted by a metallic connecting line. 10. Converter according to one of the preceding claims, characterized in that the membrane (8) on its underside facing the cavity (6) carries an insulating layer. 11. Converter according to one of the preceding claims, characterized in that the substrate (2) consists of semiconductor material. 12. Converter according to one of claims 1 to 10, characterized in that the substrate (2) contains an insulating base body, on which a semiconductor layer is arranged adjacent to the cavity (6). 13. Converter according to one of the preceding claims, characterized in that the substrate (2) on its surface facing the cavity (6) carries an insulating layer. 14. Transducer according to one of the preceding claims, characterized in that the edge of the membrane (8) is circumferentially attached to the substrate (2) by means of an insulating layer (12) of different heights. 15. Converter according to one of the preceding claims, characterized in that the different zones Zi, Z ₂ , Z ₃ , ..., the membrane (8) are doped with the same or different foreign substances. 16. Converter according to one of the preceding claims, characterized by a peripheral intermediate zone Z ₃ between the central zone Zi and the peripheral zone Z ₂ , and a foreign substance concentration N ₃ , which of the foreign substance concentrations N-ι, N _{2 of} the adjacent zones Zi, Z _2nd is different. 17. Converter according to one of the preceding claims, characterized in that a plurality of spaced-apart zones heavily doped with foreign substances are provided, and that all intermediate zones between the highly doped zones have the same low foreign substance concentration. 18. Converter according to one of the preceding claims, characterized in that the first electrode (4) is designed as a highly doped zone in the substrate (2). 19. Converter according to one of the preceding claims, characterized in that the membrane has a round, square, hexagonal or polygonal shape. 20. Converter according to one of the preceding claims, characterized in that the substrate (2) consists of monocrystalline silicon, that the first electrode (4) in the substrate (2) has a high donor concentration N _D ⁺ , that the membrane (8) polycrystalline silicon, and that the different zones Zi, Z ₂ , Z _{3 of} the membrane (8) have different concentrations of donors N _D. 21. Array of several electroacoustic transducers on a common substrate, characterized in that the transducers are designed according to one or more of claims 1 to 16. 22. Array according to claim 21, characterized in that the membrane (8) of the transducers have different shapes and / or different sizes and / or different thicknesses. 23. Array according to claim 21 or 22, characterized in that zones of different sizes Z _1f Z ₂ , ... in membranes of adjacent transducers are provided with a high foreign substance concentration N. 24. Array according to one of claims 21 to 23, characterized in that the membranes of adjacent transducers with foreign substances have differently doped central zones which are doped much more strongly than the neighboring zone or the peripheral zone. 25. A method for producing a transducer or a transducer array according to one of claims 1 to 24, in which a flexible membrane made of semiconductor material is circumferentially attached to its edge on a rigid, rigid substrate, characterized in that at least two different local zones Zj , with i = 2, 3 ... the membrane (8) with different amounts of foreign substances. 26. The method according to claim 25, characterized in that dopants are used as foreign substances. 27. A method for producing a transducer or a transducer array according to one of claims 1 to 24, in which a flexible membrane made of semiconductor material is circumferentially attached to its edge on a rigid, rigid substrate, characterized in that in at least two different local zones Zj , with i = 2, 3 ... the membrane mass is applied to the membrane or removed from the membrane. 28. The method according to claim 27, characterized in that the central zone Z- has a different occupancy from the peripheral zone Z ₂ . 29. The method according to claim 27 or 28, characterized in that in the different zones Z | a different mass coverage M _{f is} applied by chemical or physical vacuum deposition (chemical vapor deposition CVD or physical vapor deposition PVD). 30. The method according to claim 27, 28 or 29, characterized in that the thickness of the membrane in the different zones Zi, i = 2, 3 ... is removed differently by chemical or physical etching processes.