EP2145505A1

EP2145505A1 - Acoustic transducer

Info

Publication number: EP2145505A1
Application number: EP08748345A
Authority: EP
Inventors: Christoph Nölle
Original assignee: Baumer Electric AG
Current assignee: Baumer Electric AG
Priority date: 2007-05-07
Filing date: 2008-05-02
Publication date: 2010-01-20
Also published as: WO2008134909A1; US20110026367A1

Abstract

Disclosed is an acoustic transducer comprising one or more membranes which cover hollow spaces (9) on a substrate (3) and are provided with an evenly thick polymer layer (7) that is produced using a vapor deposition process. In the area of the hollow spaces (9), the vapor is deposited on the surface of a liquid that can then be evacuated from the hollow spaces (9) through ducts (10).

Description

Acoustic transducer

The invention relates to an acoustic transducer and a method for producing such a transducer according to the features of claims 1 and 3.

In ultrasonic sensors, which operate on the pulse-echo principle, are widely used ultrasonic transducers with a piezoceramic disk and a matching layer. The matching layer has an acoustic characteristic impedance which is between that of the piezoceramic disk and that of the surrounding medium (usually air or water). Such ultrasonic transducers are relatively narrow band. They are excited by electrical impulses or transmit bursts to emit wave packets. These sound waves are reflected on objects. Upon impact of such echo signals on the ultrasonic transducer they are evaluated by a detection electronics. The transit time between the emission of the ultrasonic bursts and the

Receiving the echo signals is a measure of the respective object distance.

For many applications, miniaturization of ultrasonic sensors is desired. In sensors with ultrasonic transducers, which include a piezoceramic disk and a matching layer connected to it, miniaturization is limited. In medical imaging applications, where ultrasonic waves are transmitted between a sensor head and the human body by means of a gel, it is known to form the sensor head with one or two-dimensional array-like arranged transducer elements. By controlling the transducer elements with defined variable relative phase positions propagation properties such as the propagation direction or the focal range of the ultrasonic waves can be influenced.

It is also known to produce the transducer elements of such ultrasonic sensors as capacitive transducers with micromechanical methods. DE-Al-10 2005 051604 discloses a method for producing such capacitive ultrasonic transducer arrays, which are also referred to as CMUT (Capacitive Micromechanical Ultrasonic Transducer). Due to the lower acoustic impedance of the thin transducer membranes, such transducers can also be operated in gaseous environments (air). When using a semiconductor material such as silicon as a substrate, it is possible to arrange electronic components in the immediate vicinity of the transducer element on this substrate. Conventional production methods of such micromechanical capacitive ultrasonic transducers require a large number of process steps. In particular, the formation of a solid sacrificial layer is provided in each case, which in one of the following Process steps must be etched away again. While in other known methods, the membranes are prepared by depositing a hard nitride layer and / or by PECVD or plasmachemic DampfabScheidung (in these methods disturbing mechanical stresses must be removed by thermal aftertreatment), DE-Al-10 2005 051604 proposes a method for Producing a polymer-based capacitive ultrasonic transducer before. It essentially comprises the following steps:

(a) providing a substrate; (b) forming a first conductor on the substrate; (c) coating the substrate with a sacrificial layer to cover the first conductor through the layer; (d) etching the sacrificial layer to form an island which makes it possible to contact the island with the first conductor; (e) coating the substrate with a first polymer-based material to cover the island therethrough; (f) forming a second conductor on the first polymer-based material; (g) forming a passage opening on the first polymer-based material to allow the passage opening to be led to the island; and (h) using the passageway to etch away and remove the island, thereby forming a cavity.

The substrate made of silicon, the conductors of sputtered copper or platinum, the sacrificial layers of Metal and the polymer-based material to be made of a photoresist.

The openings introduced from the front side into the polymer-based material are closed in a further method step by spin-coating with a further layer of the polymer-based material.

The production of capacitive ultrasonic transducers according to the method described in DE-Al-10 2005 051604 comprises a plurality of process steps and is correspondingly expensive. The removal of openings in the membrane for wet-chemical etching away of a sacrificial layer arranged behind it and the application of a second polymer layer which closes these openings again can impair the mechanical properties of the membrane and its reproducibility. From EP-Al-1672394 a method for producing filters, lenses and waveguides is known wherein polymer membranes are produced by deposition of plastic on a substrate and on the surface of a liquid located on the substrate. For example, the substrate may comprise a plate covered by a patternable layer, such as a photoresist or a protective layer, such as the blue protective film used with silicon wafers. In this layer, structures such as channels or round holes can be formed by known methods which are then filled with a liquid such as an optical oil. The used materials of the structured layer and the liquid lead due to the surface tension to a characteristic curvature of the liquid surface. The liquid is preferably selected so that it is repelled or does not adhere to the structured layer. In a further step, substrate and liquid are coated with parylene in a low-pressure gas deposition process at about 7 Pa chamber pressure, wherein the substrate and the liquid can be kept at ambient or room temperature. Here, di-para-xylenes is pyrolyzed, then polymerized at 600 ° and at room temperature on the substrate or the

Liquid surface deposited. The liquid is not reactive and has a significantly lower saturation vapor pressure than the pressure in the reaction chamber. Subsequently, the liquid is discharged through openings. Depending on the design of the structured layer, these openings may, for example, be formed at the end of a channel embedded in the substrate or be released by removing a pin from a hole in the plate. Furthermore, EP-Al-1672394 discloses the possibility of producing a channel with piezoelectric or capacitive actuators produced according to the described method equip. These actuators comprise rectangular electrodes or piezoelectric regions arranged along the channel on the membrane-like sheath and / or on the substrate plate. A staggered periodic activation of these actuators can trigger peristaltic contractions of the envelope for transporting liquids in the channel.

In a further application of the proposed method cylindrical recesses are formed in a layer, which are then filled with liquid and covered with a membrane. The liquid is left in the cavities between the membrane and the structured layer, resulting in microlenses. By means of transparent heating resistors, the temperature of the liquid can be changed. Due to the associated relatively slow volume change, the focal lengths of the microlenses can be changed.

It is an object of the present invention to provide a method for producing an acoustic transducer having at least one membrane and such an acoustic transducer.

This object is achieved by a method for producing an acoustic transducer and by an acoustic transducer according to the features of patent claims 1 and 3. With the method according to the invention, acoustic transducers with one or more membranes can be produced simply and inexpensively. The membranes are made by depositing and / or depositing a plastic and / or other materials on a surface of a high viscosity liquid or gel and the adjacent surface of a substrate. Preferably, parylene is used to make the membranes. Depending on the composition, this may be inert and / or mechanically stable and / or optically transparent and / or biocompatible. For example, by vapor deposition or sputtering, means for exciting and / or detecting vibrations or deformations or mechanical stresses of the membrane or parts of such agents can be applied to the membrane. Such means are, for example, planar electrodes of capacitors, metallizations acting as optical mirrors, gratings or the like, multilayer systems such as DFB structures, piezoelectric or piezoresistive structures. If required, these detection and / or stimulation means can be isolated and protected from the environment by a further plastic or parylene layer. The substrate is preferably plate-like and may comprise one or more layers. Preferably, the uppermost layer of the substrate is prepared by known methods such as anisotropic etching, laser processing, mechanical processing or embossing process structured. In this case, cavities or depressions with different dimensions, shapes and surface textures can be formed. These depressions can then be filled with the liquid, on the surface of which the plastic deposit should take place. If necessary, means for exciting and / or detecting, for example, by means of coating and / or structuring techniques

Membrane vibrations and / or deformations or parts of such agents are applied to the substrate or formed on this. Such means are, for example, planar electrodes, which together with such electrodes on the associated membranes form capacitors whose capacitances are variable depending on the membrane deflections. In particular, when the substrate comprises a semiconductor plate or layer, for example, in the region below the diaphragm any sensor elements and / or actuator elements for detecting vibrations or deflections of the membrane can be arranged, so for example, the aforementioned electrodes or light emitting or laser diodes and Photodiodes which detect light emitted by the light-emitting diodes and reflected at the mirror-finished membrane. Such sensor and / or actuator elements can eg for control and / or control tasks (closed loop Feedback). The use of a substrate with a semiconductor layer (eg a wafer) also has the advantage that even very low signal levels can be detected and amplified largely without interference immediately after the respective sensor element. The electronics for driving and / or evaluating the actuator elements and / or sensor elements can thus be arranged to save space on the substrate. In particular, in more complex arrangements with multiple arranged to a one- or two-dimensional array

Membranes, which are to be excited and / or coordinated synchronously or coordinated, a driving and / or evaluation electronics integrated in the substrate is very advantageous. The deposition technique used for membrane fabrication does not require high temperatures and is compatible with the semiconductor structures used. In alternative embodiments, the substrate may comprise one or more layers of any other materials and with the same or different layer thicknesses, such as glass, ceramic, metal, semiconductors or plastics. Such layers may be polycrystalline, amorphous, organic or inorganic, for example. The means on the membrane and / or the substrate for detecting and / or generating deflections or vibrations of the membrane are each connected via insulated tracks to a controller and / or connected via a suitable interface with such a controller. Depending on the intended application, the liquid used for producing the membrane on the substrate can be left in the cavity or can be removed from the cavity via one or more openings in the substrate from the rear side or laterally. Preferably, a plurality of openings are formed in each cavity, which are sealed during the manufacturing process by a film or pin on the back of the substrate. When emptying the cavities, gas can thus flow into the cavity through at least one of these openings. , Alternatively or additionally, the liquid can also be drained from the cavity through openings, such as pores in the deposited or applied membrane, sucked off or removed in any other way. In particular, a porous membrane can subsequently be further coated or post-treated, so that the pores close.

Based on some figures, the invention will be described in more detail below. Show

Figure 1 shows an embodiment of a capacitive

Ultrasonic transducer according to the prior art,

FIG. 2 shows a cross section of a first embodiment of an ultrasound transducer with a two-layered substrate, FIG. 3 shows a semiconductor substrate layer for a capacitive ultrasonic transducer with integrated electronics and with a plurality of electrodes for transducer elements arranged along a line,

4 shows a cross section of an ultrasonic transducer in a further embodiment, FIG. 5 shows a cross section of a further transducer with a single-layered substrate,

FIG. 6 shows a cross-section of a transducer of an annular rib structure for supporting the membrane and / or for increasing the sensitivity or the efficiency.

FIG. 1 shows a cross section through a capacitive micromechanically produced ultrasonic transducer 1, as is known from DE-Al-10 2005 051604. On a substrate 3 made of silicon, a first planar conductor region 5a is applied. From a first polymer layer 7 covering the substrate 3 and the conductor region 5a, a cavity 9 is excluded which has been formed by etching away a sacrificial layer (not shown) previously applied to the conductor region 5a and laterally overlapping it. For etching away the sacrificial layer, passage openings through the first polymer layer 7 have to be exposed. After applying a second Conduction region 5b on the first polymer layer 7 above the cavity 9 is applied by spin coating a second polymer layer 11, which closes the through holes again, but leaves the cavity 9 as such. In this case, the penetration of the second polymer layer 11 through the passage openings into the cavity 9 must be prevented. FIG. 2 shows a cross section of a first embodiment of a capacitive ultrasonic transducer 1 that can be produced according to the invention. A substrate 3 comprises a composite of a planar first substrate layer 3a or plate, which can be made, for example, from an electrically insulating plastic or an electrically conductive metal or a semiconductor and a thickness sl of, for example, about 1 mm, and a planar second substrate layer 3b or plate, which may be made, for example, from an oleophobic plastic such as polyethylene, PVC or Teflon and has a thickness s2 of, for example, about 0.1 mm. In the second substrate layer 3b, one or more depressions or recesses 4 produced by known structuring methods, such as anisotropic etching, are formed. These may, for example, have a circular cross section with a diameter d 1 of, for example, 2 mm. Alternatively, the cross section of the recesses 4 could also have a different shape, for example elliptical or polygonal, in particular square, rectangular or hexagonal. Depending on the design and Scope of the ultrasonic transducer 1 or in general of the acoustic transducer, the layer thicknesses sl 'and s2 of the substrate layers 3a, 3b and the dimensions of the recesses 4 can be set within a wide range. Thus, the first substrate layer 3a may be formed, for example, as a thin, flexible plastic film or as a solid metal, glass or ceramic body. Accordingly, layer thicknesses sl in the range of preferably about 0.1mm to about 10mm or more may be provided. Capacitive ultrasonic transducer 1 with one or more too

Vibrations of excitable membranes 2 preferably have a low thickness s 2 of the second substrate layer 3 b or the depth of the recesses 4 or of the structures in the second substrate layer 3 b. For converters with optical or piezoresistive deflection or

Vibration detection, however, the thickness s2 of the second substrate layer s2 be significantly greater. Accordingly, layer thicknesses s2 can be provided in the range from about 0.05 mm to about 5 mm or more. In high-frequency ultrasonic transducers 1 with a plurality of (eg three) or a plurality (eg 16 or more) of coordinately controllable and / or evaluable transducer elements, the membrane areas of the individual transducer elements can be very small; By contrast, acoustic transducers which are intended to generate sound signals with relatively high sound levels in the audible range preferably comprise a single transducer element with one relatively large membrane area. Accordingly, the membrane surfaces covering the recesses 4 may be of the order of about 0.001 mm ² to about 1000 mm ² or more. The material of the second substrate layer 3b is preferably completely removed in the region of the recesses 4, so that the upper side of the first substrate layer 3b or a metallization applied to the first substrate layer 3a at least in the region of the recesses 4 (henceforth called the first conductor region 5a) is exposed there , Alternatively, the first conductor region 5a could also be formed on the surface of the second substrate layer 3b facing the first substrate layer 3a or within the second substrate layer 3b. The first conductor region 5 a can be produced, for example, in the case of insulators or semiconductors by vapor deposition of the first substrate layer 3 a with a thin metal layer of, for example, 0.05 mm, wherein the regions which are not to be metallized are masked in a conventional manner by means of a photoresist layer. In the case of electrically conductive first substrate layers 3a, these can be used directly as first conductor regions 5a. Alternatively, electrically conductive first substrate layers 3a can be covered with a thin insulator layer on which the first conductor region 5a is then applied. The first conductor region 5a, in addition to the planar electrodes in the region of the recesses 4, also comprises electrical connection lines 6a to a connection interface (eg connection plug or cable) and / or to an electronic control 8 (FIG. 3) for exciting and / or evaluating membrane oscillations or deformations. The controller 8 or parts thereof can be arranged directly on the substrate 3 or alternatively outside the converter. FIG. 3 schematically shows a first substrate layer 3 a made of silicon for an ultrasonic transducer 1 comprising five transducer elements, the electrodes or first conductor regions 5 a being connected via connecting lines 6 a to the controller 8 integrated in the substrate layer 3 a.

As shown in FIG. 2, the second substrate layer 3b and the recess 4 are covered by a homogeneous polymer layer 11, preferably a parylene layer, such that each of the recesses 4 is covered or covered by a membrane 2 delimiting a cavity 9. Analogous to the first substrate layer 3a, a second conductor region 5b with planar electrodes in the region of the membranes 2 and with connecting lines 6b is formed on the second substrate layer 3b. If required, these can be connected, for example via plated-through holes 6c, with parts of the first conductor region 5a and / or with a possible controller 8 or a connection interface. In an alternative embodiment of the converter, the second substrate layer 3b and the second conductor region 5b may be covered by a further polymer layer 11-preferably a further parylene layer-which acts as an electrical insulator and protect against mechanical and / or chemical environmental influences. Such an arrangement is shown in FIG.

On or in at least one of the substrate layers 3a, 3b, one or more channels 10 are formed, which open into the cavity 9 and allow a connection of the cavity 9 with the environment. The Channels In the embodiment of the transducer according to FIG. 2, the channels 10 penetrate the first substrate layer 3a and the electrodes on this first substrate layer 3a. The channels 10 may be formed in the first substrate layer 3a before or after the application of the first conductor region 5a, for example by mechanical, micromechanical or chemical processing. The production of the channels 10 can take place before or after the connection of the two substrate layers 3a, 3b. The channels 10 can be sealed, for example by applying a self-adhesive plastic film (not shown) on the underside of the first substrate layer 3a sealing. The channels 10 are preferably arranged in the peripheral region of the cavities 9 or of the electrodes located there. There are the vibration amplitudes of the respective recess 4 covering membrane 2 and thus interference in capacitive excitation / evaluation of membrane vibrations minimal.

To produce the membranes 2, the recesses 4 are filled with a liquid. In general, the outwardly limited channels 10 are filled with the liquid. The volume of liquid that can be received by the channels 10 is generally small compared to the volume of liquid that can be received by the recesses 4. At least the channel widths are small in comparison to the corresponding dimensions of the recesses 4. Subsequently, the polymer deposition is carried out analogously to the process described in EP-Al-1672394. In this case, the membranes 2 are formed, which cover the recesses 4 or cavities 9. In a further step, the pins or the film which seals off the channels 10 are removed and the liquid is discharged from the cavity 9. This process can e.g. by movements of the substrate 3 (in particular by spinning), by suction,

Evaporation, adsorption or chemical reactions and by the repulsive effect of one or both substrate layers 3a, 3b are supported on the liquid. In alternative embodiments of the transducer, channels 10 may also be provided, for example, as grooves or trenches in the surface of the first surface facing cavity 9 Substrate layer 3a and / or be formed in one of the surfaces of the second substrate layer 3b, as shown in Figure 4. Such channels 10 formed on the surface of one of the substrate layers 3a, 3b protrude laterally beyond the cavity 9 or the one for the

Cavity 9 provided by a small length bl or b2, but without reaching to the edge of the respective substrate layer 3a, 3b. After applying the polymer layer 11 and optionally further process steps, the channels 10 may be e.g. by means of

Separations, as required for separating a plurality of arranged on a common substrate 3 ultrasonic transducer 1, or by local mechanical, thermal or chemical removal of the polymer layers 7 and 11 and optionally further layers in the end portions of the channels 10 openings (not shown) for removing the Liquid can be created from the cavities 9. When installing such transducers in a housing, these openings can optionally be kept sealed or protected from the environment. It is also possible to connect the channels 10 to pressure chambers or other means for controlling or regulating the pressure in the cavity 9. The type of connections of the cavities 9 to the outside (closed or connected to a pressure chamber or open), for example, properties such as attenuation, radiation angle or bandwidth or affect usable frequency spectrum of an acoustic transducer.

The possibility of simultaneously producing a large number of transducers on a substrate 3 (this naturally also applies to transducers having a plurality of transducers)

Transducer elements or membranes 2), and then to separate these transducers by separation processes, allows a cost-effective production of such transducers.

Instead of a two-layer substrate 3, acoustic transducers can also be produced with a plurality of substrate layers 3a, 3b or with only one substrate layer 3a. A possible embodiment is shown in FIG. The surface of the substrate layer 3a is first structured with recesses 4 or depressions. The substrate layer 3a is metallized with a first conductor region 5a, wherein a planar electrode is formed at the bottom of the recess 4 and connected by means of over the edges of the recess 4 protruding connecting lines 6a with an interface and / or optionally with an electronics 8. The side surfaces of the recess 4 may be angled in a cone or pyramid (not shown), so that a proper electrical connection between the electrode in the recess and the connecting lines 6a is ensured. The wells are analogous to the method described for two-layer substrates 3 with filled with a liquid, coated with a polymer layer 7 and provided with a second conductor portion 5b. The materials for the substrate 3 and the liquid are preferably chosen so that the adjacent to the side edges of the substrate

Liquid surface and thus the membrane formed thereon 2 has no or only a slight curvature. In analogy to transducers with two-layer substrates 3, the liquid can be removed from the cavity 9 via channels 10 or left in the cavity 9, for example in ultrasonic transducer arrays for medical diagnostics or applications in liquids. Of course, the application of a second polymer layer 11 is also possible here. In further alternative embodiments, the recesses 4 or depressions can comprise pillars, webs or other structures for supporting the membrane 2 and / or for locally reducing the distance between the membrane 2 and the substrate 3, ie island-like or contiguous regions which are from below are in contact with the membrane 2 or have only a small distance from the membrane 2 and are not firmly connected to the membrane 2. Such structures may include metallizations which are connected to or belong to the first conductor region 5a.

Figure 6 shows an example of such a transducer having structures in the form of concentric rings. These are during the deposition of the polymer layer 7 covered with liquid, so that no adhesive bond is formed between the polymer layer 7 and the protruding on the substrate 3 rings. The capacitance of the dielectric comprising the polymer layer 7 through the two conductor regions 5a and 5b and the interposed therebetween is relatively large due to the short distance between the membrane 2 and the structures. The conductor regions 5a, 5b can be charged by applying electrical voltages. Depending on the relative polarity of the charges on the two opposing electrodes, the membrane 2 bulges outwards or inwards and is thereby mechanically tensioned. As a result, parameters such as bandwidth, resonance frequency or directional characteristic of the acoustic transducer can be influenced. By driving with an AC signal, the acoustic transducer can be used as a sound generator for generating sound waves or ultrasonic waves. If the capacitance of the transducer is connected to a reinforcing evaluation electronics (this is usually part of the electronic control 8), it can be used as a microphone, wherein incident on the transducer sound waves lead to corresponding vibrations of the membrane 2, which then detects as a capacitance change can be. Alternatively, other physical principles may be used to excite and / or detect vibrations or static pressures. Thus, for example, a piezoelectric layer, for example PVDF, may be applied to the membrane for this purpose. In a further variant, piezoresistive structures are preferably formed in the transition region between the recess 4 and the substrate 2 carrying the membrane 2 on the membrane 2, with which membrane vibrations or deflections can be detected as resistance or change in resistance. In a further embodiment, a light-emitting diode or a laser diode and a photodiode or a CCD line or corresponding other optical elements are formed on the substrate 3 below the metallized and thus reflective membrane 2. The light emitted by the light source is reflected differently at the reflecting membrane 2 as a function of its deflection or oscillation behavior. This can be detected and evaluated with the optical detectors. In particular, it is possible to

Stimulating membrane vibrations and evaluating such vibrations to use different physical principles. This decoupling makes it possible, in particular in the case of ultrasonic sensors, where signals must be transmitted and echoes recorded in very short time intervals, that significant improvements are made. Further possible applications of the acoustic transducers according to the invention are, for example, microphone / loudspeaker combinations, mobile telephones, earphones with integrated microphone, hearing aids. Furthermore, not only acoustic transducers can be produced with the method according to the invention, but also a multiplicity of other sensors which operate on different physical principles and make use of the advantages of a mechanically stable, chemically resistant membrane 2.

The explained in different embodiments features of the invention can be combined with each other.

Claims

claims

1. A method for producing an acoustic transducer having at least one membrane (2), characterized in that the membrane (2) is formed by a portion of the surface of a substrate (3) covered with a liquid and a plastic or other material the surface of this liquid and the adjoining surface of the substrate (3) is deposited and / or applied and / or integrated, and in that on the membrane (2) means for exciting and / or sensory detection of vibrations or deformations of the membrane (2 ) be formed.

2. The method according to claim 1, characterized in that the liquid through one or more channels (10) or openings in the substrate (3) and / or the

Membrane (2) is removed from the converter.

3. Acoustic transducer having at least one membrane (2), characterized in that the membrane (2) a uniformly thick, on a substrate (3) adhering and a cavity (9) in the substrate (3) covering layer of a polymer or a includes other material.

4. Acoustic transducer according to claim 3, characterized in that the polymer layer (7) is formed by vapor deposition of a plastic on the substrate (3).

5. Acoustic transducer according to one of claims 3 or 4, characterized in that the polymer layer (7) is made of parylene.

6. Acoustic transducer according to one of claims 3 to 5, characterized in that the substrate (3) in the region of the cavity (9) uprising structures with a smaller distance to the membrane (2).

7. Acoustic transducer according to one of claims 3 to 6, characterized in that on the membrane (2), a device or parts of a device for exciting and / or detecting deformations and / or vibrations of the membrane (2) are formed.

8. Acoustic transducer according to claim 7, characterized in that the device or parts of the device for exciting and / or detecting deformations and / or vibrations of the membrane (2) a planar capacitor electrode and / or a piezoelectric or piezoresistive material and / or an optical element.

9. Acoustic transducer according to one of claims 3 to 8, characterized in that on the substrate (3) below the membrane (2), a device or parts of a device for exciting and / or detecting deformations and / or vibrations of the membrane (2 ) are formed.

10. Acoustic transducer according to claim 9, characterized in that the device or parts of the device for exciting and / or detecting deformations and / or vibrations of the membrane (2) a capacitor electrode and / or a piezoelectric or piezoresistive material and / or comprise an optical element.

11. Acoustic transducer according to one of claims 3 to 10, characterized in that the substrate (3) comprises a semiconductor substrate layer (3a), and that a controller (8) for driving and / or evaluating vibrations and / or deflections of Membrane (2) is at least partially disposed in or on the substrate layer (3a).