EP2348503B1 - Ultrasound sensor for recording and/or scanning objects and corresponding manufacturing method - Google Patents

Ultrasound sensor for recording and/or scanning objects and corresponding manufacturing method Download PDF

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Publication number
EP2348503B1
EP2348503B1 EP10000489.4A EP10000489A EP2348503B1 EP 2348503 B1 EP2348503 B1 EP 2348503B1 EP 10000489 A EP10000489 A EP 10000489A EP 2348503 B1 EP2348503 B1 EP 2348503B1
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Prior art keywords
substrate
ultrasonic
elevations
depressions
sensor unit
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German (de)
French (fr)
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EP2348503A1 (en
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Thomas Dr. Herzog
Henning Dr. Heuer
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Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
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Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
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Priority to EP10000489.4A priority Critical patent/EP2348503B1/en
Priority to US12/987,514 priority patent/US8468892B2/en
Priority to JP2011004790A priority patent/JP5734673B2/en
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    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/002—Devices for damping, suppressing, obstructing or conducting sound in acoustic devices

Definitions

  • the present invention relates to an ultrasonic sensor for detecting and / or scanning objects according to the preamble of claim 1 and to a manufacturing method for such an ultrasonic sensor.
  • active piezoelectric thin films such as thin films of AlN or ZnO for ultrasonic sensors
  • these are usually deposited directly on suitable substrates or substrates such as silicon, sapphire, gallium nitride, etc. If these thin films with their carrier materials are to be used as ultrasonic sensors, then the propagation of the ultrasonic waves is in the coupled (to be detected and / or measured) medium (this will be referred to alternatively as an object hereinafter) and to evaluate the resulting echo reflected from a boundary layer in the medium or object.
  • an ultrasonic sensor (and a corresponding manufacturing method) for Provide available with the above disturbing echoes by the boundary layer between the back of the carrier and the adjacent medium as far as possible can be completely suppressed and yet the simplest possible, compact, in particular suitable for the use of active piezoelectric thin-film design and flexible use possible ,
  • the basic idea of the present invention is based on configuring the above-described rear surface of the carrier substrate (hereinafter also referred to simply as substrate) of the ultrasonic sensor such that no disturbing echoes of the boundary layer between the carrier substrate rear side and the adjacent medium lead to the piezoelectric sensor layer or to the sensor unit of the ultrasonic sensor Returned ultrasonic sensor.
  • This is done by forming the rear side of the substrate in such a way that a large number of needle-shaped elevations and depressions are introduced into this rear side, that is to say that a corresponding surface structuring of the substrate rear side takes place.
  • the substrate may also include multiple layers, so in this case a Surface structuring of the back of the sensor unit furthest away from the substrate layer takes place. (However, if required by the choice of material, even several surfaces or boundary layers of a multi-layered substrate can be surface or deep structured.)
  • the surface or back side of the substrate to be structured in this way can be formed in particular in the form of black silicon. Likewise, however, it is also possible, when using sapphire or gallium nitride as the substrate material, to deep-restructure their back sides accordingly.
  • a diffuse scattering is understood to mean a scattering of ultrasonic waves which is designed such that, after scattering, there is no more directed propagation of the ultrasonic waves in a preferred direction, but a further propagation of the ultrasonic energy in most different directions such that the scattered ultrasound waves can not be detected by the sensor unit (or only a slight echo).
  • a lateral direction is understood to mean a direction within the layer plane of the ultrasound sensor and / or its sensor unit.
  • the direction vertical For this purpose, that is to say the direction perpendicular to the sensor plane and / or to the plane of the substrate (eg wafer) is alternatively referred to below as the depth direction or as the height direction.
  • an average extent eg a mean lateral extent, ie an extension in the direction of the layer plane of the sensor or an average height extent of the elevations in the direction perpendicular to the layer plane
  • the arithmetic mean of the plurality of individual values is below the corresponding mean (eg of lateral extensions of individual needle-shaped elevations).
  • An inventive ultrasonic sensor comprises a substrate and a piezoelectric sensor unit arranged on or on this substrate and / or connected to this substrate.
  • the rear side of the substrate facing away from the piezoelectric sensor unit has a multiplicity of needle-shaped elevations and depressions, thus a surface structure is introduced in this rear side.
  • This surface structuring or surface structure is designed in such a way that it causes a diffuse scattering of the ultrasonic waves incident from the direction of the sensor unit (that is to say from the front side of the sensor) onto the structured rear side.
  • the elevations and / or depressions may have an average lateral extent in the range from 0.05 ⁇ m to 1 mm, preferably in the range from 0.1 ⁇ m to 200 ⁇ m and particularly preferably in the range from 0.2 ⁇ m to 20 ⁇ m. This average lateral extent may thus be less than or equal to the wavelength of an ultrasonic wave that can be generated by the piezoelectric sensor unit (on the front side of the substrate).
  • the piezoelectric sensor unit applied to the front surface of the substrate may be configured to emit and / or receive ultrasonic waves corresponding to a frequency in the range of 20 kHz to 1 GHz.
  • the piezoelectric sensor unit can also be composed of a plurality of subunits designed to receive or transmit ultrasound.
  • Corresponding embodiments as well as evaluation algorithms for evaluating the transmitted and / or received ultrasound signals are known to the person skilled in the art (for example, corresponding embodiments of US Pat DE 10 2006 005 048 A1 remove).
  • the surface structure patterned on the rear side of the substrate can be designed for the diffuse scattering of ultrasonic waves corresponding to the aforementioned frequency range.
  • the substrate is preferably silicon, in particular crystalline silicon.
  • the substrate may be a silicon wafer. However, it is also conceivable to use sapphire or gallium nitride as the substrate.
  • the back side and / or its surface structure is preferably formed in the form of black silicon.
  • black silicon is understood to mean a surface modification of the crystalline silicon.
  • the crystalline silicon is structured, for example by ultrashort laser pulses or by bombarding the silicon surface with high energy ions of the substrate back, so that optically optically active, preferably acicular structures (FIG. Surveys and recesses) be generated on the surface.
  • the needle-shaped depressions and elevations in the silicon can be produced using the reactive ion etching known to the person skilled in the art.
  • the ion etching process is a two-stage, alternating dry etching process in which an etching step and a passivation step alternate. The aim is to etch as anisotropically as possible, ie direction-dependent, perpendicular to the wafer surface.
  • sulfur hexafluoride SF6
  • carrier gas usually argon
  • an energy-rich high-frequency plasma is formed, with the SF6 producing a reactive gas (in plasma, SF6 + ions, activated SF6 molecules and fluorine-containing and oxygen radicals are formed).
  • a reactive gas in plasma, SF6 + ions, activated SF6 molecules and fluorine-containing and oxygen radicals are formed.
  • a chemical etching reaction isotropic on the substrate and a physical (anisotropic) material removal by means of argon ions are superimposed.
  • the process takes place at pressures of 50 Pa to 1 Pa, preferably in an RF plasma at 13.65 MHz, pressure range 10-50 Pa instead.
  • the etching process is stopped after a short time and a gas mixture of octaflourocyclobutane (C4F8) and argon is introduced.
  • the octafluorocyclobutane is activated as a plasma gas and the resulting fluorine-containing radicals and molecules form on the entire substrate a polymer-like passivation layer, ie both on the mask, as well as on the silicon and the vertical silicon sidewalls.
  • the passivation layer of the horizontal surfaces (trench bottom) is removed much faster by the directed physical component (ions) of the etching reaction than the layer on the sidewalls.
  • An essential feature of such a layer of black silicon on the back of the substrate is an increased absorption of incident visible light, by the formation of the aforementioned deep structure or Surface structure (the deep structure causes a gradual transition of the refractive index of the effective medium so that there is no sharp optical interface at which the light can be reflected) instead the light is "smoothly" guided into the material and hardly reflected, rendering the silicon black to appear).
  • the elevations and depressions of the surface structure can thus be produced (also with other substrates) by laser bombardment, by ion bombardment, in particular by reactive ion etching or reactive ion etching, and / or also by micromechanical chip removal machining of the back side of the substrate.
  • the elevations are needle-shaped.
  • the mean height of the elevations, the mean depth of the depressions and / or the mean extent of the elevations and / or the depressions perpendicular to the sensor plane is preferably in the range between 0.05 ⁇ m to 1 mm, preferably in the range from 0.1 pm to 200 pm, and more preferably in the range of 0.1 pm to 20 pm (ie, ultimately of the same order of magnitude as the lateral extent of the elevations and / or depressions in the sensor plane).
  • the aspect ratio a A / L from the aforementioned height, depth and / or extent and the average lateral extent of the elevations and / or depressions (which is also referred to below with the variable L) is preferably between 0.2 and 50, particularly preferably between 0.5 and 10.
  • the piezoelectric element of the piezoelectric sensor unit is preferably in the form of a piezoelectric thin layer formed.
  • This layer may consist of AlN or ZnO or contain this material.
  • the sensor unit preferably has a layer thickness in the range between 1 ⁇ m and 100 ⁇ m, preferably between 10 ⁇ m and 25 ⁇ m. As described above, the sensor unit can also consist of several subunits distributed over the layer plane, each of which has corresponding thin-film elements.
  • the piezoelectric sensor unit (or, in the case of several subunits each of these subunits) has two with the piezoelectric element for detecting and / or applying the electrical voltage connected electrical contacts.
  • the piezoelectric thin film is preferably sandwiched between these two electrical contacts and directly adjoins these electrical contacts.
  • the electrical contacts may be formed, for example, of copper.
  • the piezoelectric sensor unit or the corresponding sub-sensor units can be designed for emitting ultrasonic waves, for receiving ultrasonic waves or else combined for emitting and for receiving ultrasonic waves (transmitting and receiving unit).
  • the substrate with the / the sensor unit (s) formed thereon may be formed in sections as a thin membrane.
  • the ultrasonic sensor may be in the form of an ultrasonic probe be formed or integrated into such a probe.
  • trenches, depressions, holes, ... can be structured as elevations and depressions in the back side of the substrate, for example by reactive ion etching.
  • the depressions can for example have several 100 ⁇ m depth and be produced with a high aspect ratio (eg in the range from 2 to 50). This can be done by repeatedly alternating etching and passivation of the backside Substrate surface can be achieved. During etching, however, small deposits of the, passivation can remain on the ground and mask it. When the process is shifted to passivation, structures to be formed are formed which are not removed during the subsequent etching steps.
  • perpendicular (relative to the substrate plane) surfaces can arise there against which a polymer layer can deposit.
  • surveys for example, in the form of elongated silicon columns remain.
  • the reactive ion etching can be adjusted so that can form on 1 mm 2 million small needle-shaped columns.
  • the spatial structure of the backside of the substrate can also be changed by bombardment with extremely high-energy pulsed femtosecond lasers, resulting in a needle-shaped, deep-structured surface (eg needles with a mean length of 300 nm).
  • the processes are comparatively good and evenly reproducible.
  • Fig. 1 shows a section through an ultrasonic sensor according to the invention.
  • the back side 3 of a monocrystalline silicon wafer 1 is formed by reactive ion milling with one of a plurality of needle-shaped protrusions and depressions (see FIG. Fig. 2 ) comprehensive surface structure 4 provided.
  • the thickness of the wafer 1 here amounts to 500 ⁇ m, the depth of the depressions or the extent of the individual needle-shaped elevations A of the surface structure 4 on the back 3 of the substrate 1, that is to say the depth of the structures in the black silicon on the back 3 of the wafer 1 here 2 to 5 microns, and the lateral extent of these surveys (see. Fig. 2 ) is here 200 to 800 nm.
  • the individual elements of the electrical sensor unit 2 are subsequently applied to the front side 7 of the wafer opposite the rear side 3.
  • a here 1 to 2 microns thick insulating layer 8 of silicon oxide on the front side 7 of the wafer 1 is deposited.
  • a first electrode metallization or electrode layer 6 (here a 150 ⁇ m thick aluminum layer) is applied.
  • a piezoelectrically active thin film (piezoelectric layer 5) of A1N is coated.
  • ZnO can be used as a layer material.
  • the piezoelectric thin film here has a layer thickness of 5 to 25 microns.
  • the sensor unit 2 here comprises the elements 5, 6 and 9 (and depending on the view, the layer 8).
  • the following layer structure results from the rear side 3 with the surface structure 4 of the ultrasonic sensor towards the front side (electrical contact 9): back side 3 with surface structure 4, silicon wafer 1, insulation layer 8, first metal contact 6, piezoelectrically active thin layer 5 and second metal contact 9 ,
  • the sensor 1 to 9 shown can thus be placed on an external object 0 which is to be scanned or measured: in the piezoelectric sensor unit 2 of the ultrasonic sensor shown as a combined transmitting and receiving unit (the details of which are given to the person skilled in the art, for example, in accordance with FIG DE 10 2006 005 048 A1 is known) ultrasonic waves can be generated and coupled into the object O. The ultrasonic waves are reflected at interfaces in the object and the corresponding echo signals are detected and evaluated by the sensor unit 2.
  • Fig. 2 shows an example of a back 3 and a surface structure 4 of this page for a in Fig. 1 sketched ultrasonic sensor in an electron micrograph:
  • Fig. 2 left shows an electron micrograph at a magnification of 10,000, while
  • Fig. 2 right shows a higher magnification (magnification factor 50 000).
  • the individual needle-shaped elevations or the individual silicon needles of the black silicon formed on the rear side 3 of the silicon wafer 1 can easily be seen.
  • the average lateral distance L of two silicon needles here is about 2 to 5 microns, the average height A is here 10 to 20 microns, this corresponds to about 2 million needles per square millimeter.
  • a silicon absorber layer 1, 3, 4 for a piezoelectric sensor unit 2 or 2, 8 (the insulating layer 8 can be considered as part of the sensor unit 2) is thus on the silicon substrate 1 by means of the above-described processes on the bottom or the Back 3 applied a layer of black silicon.
  • the manufacturing process for the piezoelectric thin-film sensor unit 2, 8 After the deposition of the insulating layer 8 of silicon oxide, the first thin-film electrode metallization 6 is applied, followed by the active piezoelectric material 5. Finally This is followed by the application of the second thin-film electrode metallization 9.
  • the present invention it is thus possible to scatter the interfering ultrasonic echoes from the carrier substrate 1 for the layer sensor elements 2 in such a way that they have no great influence on the echo, which is from the medium or object coupled to the active surface (front side of the ultrasonic sensor) O returns.
  • the active surface front side of the ultrasonic sensor
  • much broader fields of application for piezoelectric ultrasonic thin-film sensors are possible. Since one no longer has to apply the sensor directly to the measurement object, does not necessarily have to realize air as a backside boundary layer and no longer has to use very thick carrier substrates, high-frequency ultrasound probes can also be produced without further ado with the present invention.
  • An essential core of the invention is thus the production of the electroacoustic absorber layer on the back side of a carrier substrate by a strongly fissured surface with feature widths of, for example, less than 1 ⁇ m and with feature depths of, for example, several 100 nm, in which case a piezoelectric sensor unit is located on the opposite front side or surface lies in thin-film technology.
  • Ultrasonic sensors or thin-layer ultrasonic sensors according to the invention can be implemented in the non-destructive testing of thin layers, in quality assurance, in process monitoring or, in general, for any ultrasonic, sensor tasks.
  • high-frequency ultrasonic probes can also be realized according to the invention.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Multimedia (AREA)
  • Transducers For Ultrasonic Waves (AREA)
  • Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)
  • Ultra Sonic Daignosis Equipment (AREA)

Description

Die vorliegende Erfindung bezieht sich auf einen Ultraschallsensor zum Erfassen und/oder Abtasten von Objekten gemäß dem Oberbegriff des Anspruchs 1 sowie auf ein Herstellungsverfahren für einen solchen Ultraschallsensor.The present invention relates to an ultrasonic sensor for detecting and / or scanning objects according to the preamble of claim 1 and to a manufacturing method for such an ultrasonic sensor.

Bei der Verwendung von aktiven piezoelektrischen Dünnschichten wie beispielsweise Dünnschichten aus AlN oder ZnO für Ultraschallsensoren werden diese meist direkt auf geeignete Trägermaterialien bzw. -substrate wie beispielsweise Silizium, Saphir, Galliumnitrid usw. abgeschieden. Sollen diese Dünnschichten mit ihren Trägermaterialen als Ultraschallsensoren eingesetzt werden, so ist die Ausbreitung der Ultraschallwellen in das angekoppelte (zu erfassende und/oder zu vermessende) Medium (dieses wird nachfolgend alternativ auch als Objekt bezeichnet) und das daraus resultierende, von einer Grenzschicht im Medium bzw. Objekt reflektierte Echo auszuwerten.When using active piezoelectric thin films such as thin films of AlN or ZnO for ultrasonic sensors, these are usually deposited directly on suitable substrates or substrates such as silicon, sapphire, gallium nitride, etc. If these thin films with their carrier materials are to be used as ultrasonic sensors, then the propagation of the ultrasonic waves is in the coupled (to be detected and / or measured) medium (this will be referred to alternatively as an object hereinafter) and to evaluate the resulting echo reflected from a boundary layer in the medium or object.

Da sich bei der Messung jedoch gleichzeitig auch Ultraschallwellen im Trägersubstrat ausbreiten, können durch Reflektionen an der Grenzschicht zwischen der Trägersubstratrückseite und dem angrenzenden Medium (z.B. Luft) störende Echos erzeugt werden, die dann (ebenso wie die im eigentlichen Messobjekt erzeugten Echos) von der Sensoreinheit nachgewiesen werden. Im Sinne einer möglichst hohen Messegenauigkeit sind solche Echos somit zu vermeiden.However, since ultrasound waves also propagate in the carrier substrate during the measurement, disturbing echoes can be generated by reflections at the boundary layer between the carrier substrate rear side and the adjacent medium (eg air), which then (as well as the echoes generated in the actual measurement object) from the sensor unit be detected. In the sense of the highest possible accuracy of measurement such echoes should be avoided.

Dazu ist es aus dem Stand der Technik bekannt, die piezoelektrischen Dünnschichten direkt auf das Messobjekt mittels eines Halbleiterprozesses abzuscheiden, wobei dann die umgebende Luft als rückseitige Schicht die Echos unterdrückt. Alternativ dazu ist es bekannt, die Trägersubstrate der Sensoren in ihrer Dicke so zu gestalten, dass die Echos von deren Rückseite (also von der Grenzschicht zwischen der Trägerrückseite und dem angrenzenden Medium) erst an der piezoelektrischen Schicht des Sensors wieder eintreffen, wenn das Nutzecho aus dem vorderseitig angekoppelten Medium bzw. Objekt bereits registriert worden ist. Schließlich ist es auch aus dem Stand der Technik bekannt, die Piezoschwinger von Ultraschallköpfen durch impedanzangepasste, z.B. blockförmige Dämpfungskörper so abzudecken, dass die Ultraschallwellen in diesen separaten Dämpfungskörpern absorbiert werden, um die störenden Echos mittels des Dämpfungskörpers zu verhindern.For this purpose, it is known from the prior art to deposit the piezoelectric thin films directly on the measurement object by means of a semiconductor process, in which case the surrounding air as the back layer suppresses the echoes. Alternatively, it is known to make the support substrates of the sensors in their thickness so that the echoes from the back (ie from the boundary layer between the back of the carrier and the adjacent medium) only arrive again at the piezoelectric layer of the sensor when the useful echo from the front-coupled medium or object has already been registered. Finally, it is also known in the art to control the piezoelectric transducers of ultrasonic transducers by impedance-matched, e.g. To cover block-shaped damping body so that the ultrasonic waves are absorbed in these separate damping bodies to prevent the disturbing echoes by means of the damping body.

Ausgehend vom Stand der Technik ist es die Aufgabe der vorliegenden Erfindung, einen Ultraschallsensor (sowie ein entsprechendes Herstellungsverfahren) zur Verfügung zu stellen, mit dem die vorbeschriebenen störenden Echos durch die Grenzschicht zwischen der Trägerrückseite und dem angrenzenden Medium weitestmöglich bis vollständig unterdrückt werden können und der dennoch eine möglichst einfache, kompakte, insbesondere auch zum Einsatz von aktiven piezoelektrischen Dünnschichten geeignete Bauform und möglichst flexible Einsatzmöglichkeiten erlaubt.Starting from the prior art, it is the object of the present invention, an ultrasonic sensor (and a corresponding manufacturing method) for Provide available with the above disturbing echoes by the boundary layer between the back of the carrier and the adjacent medium as far as possible can be completely suppressed and yet the simplest possible, compact, in particular suitable for the use of active piezoelectric thin-film design and flexible use possible ,

Die vorliegende Erfindung wird nachfolgend zunächst allgemein, dann in Form eines Ausführungsbeispiels im Speziellen beschrieben. Die im Ausführungsbeispiel in Kombination miteinander verwirklichten einzelnen, ggf. auch vorteilhaften Merkmale der vorliegenden Erfindung müssen dabei nicht genau in der gezeigten Kombination verwirklicht werden, sondern können im Rahmen der Erfindung bzw. der Patentansprüche auch in anderen Kombinationen verwirklicht sein. Insbesondere können einzelne Merkmale des Ausführungsbeispiels auch weggelassen werden.The present invention will be described below first in general, then in the form of an embodiment in particular. The realized in the embodiment in combination with each other, possibly also advantageous features of the present invention need not be realized exactly in the combination shown, but may be realized in the context of the invention or the claims in other combinations. In particular, individual features of the embodiment can also be omitted.

Die Grundidee der vorliegenden Erfindung basiert darauf, die vorbeschriebene, rückseitige Oberfläche des Trägersubstrats (nachfolgend auch vereinfacht als Substrat bezeichnet) des Ultraschallsensors so auszugestalten, dass keine störenden Echos der Grenzschicht zwischen der Trägersubstratrückseite und dem angrenzenden Medium hin zur piezoelektrischen Sensorschicht bzw. zur Sensoreinheit des Ultraschallsensors zurückgelangen. Dies geschieht durch Ausbilden der Rückseite des Substrats derart, dass in diese Rückseite eine Vielzahl von nadelförmigen Erhebungen und Vertiefungen eingebracht werden, dass also eine entsprechende Oberflächenstrukturierung der Substratrückseite erfolgt. Selbstverständlich kann das Substrat auch mehrere Schichten umfassen, sodass in diesem Falle eine Oberflächenstrukturierung der Rückseite der der Sensoreinheit am weitesten abgewandten Substratschicht erfolgt. (Es können jedoch, sofern durch die Materialwahl notwendig, auch mehrere Oberflächen bzw. Grenzschichten eines mehrschichtigen Substrats oberflächen- bzw. tiefenstrukturiert werden.)The basic idea of the present invention is based on configuring the above-described rear surface of the carrier substrate (hereinafter also referred to simply as substrate) of the ultrasonic sensor such that no disturbing echoes of the boundary layer between the carrier substrate rear side and the adjacent medium lead to the piezoelectric sensor layer or to the sensor unit of the ultrasonic sensor Returned ultrasonic sensor. This is done by forming the rear side of the substrate in such a way that a large number of needle-shaped elevations and depressions are introduced into this rear side, that is to say that a corresponding surface structuring of the substrate rear side takes place. Of course, the substrate may also include multiple layers, so in this case a Surface structuring of the back of the sensor unit furthest away from the substrate layer takes place. (However, if required by the choice of material, even several surfaces or boundary layers of a multi-layered substrate can be surface or deep structured.)

Die so zu strukturierende Oberfläche bzw. Rückseite des Substrats kann insbesondere in Form von schwarzem Silizium ausgebildet werden. Ebenso ist es jedoch auch möglich, bei Einsatz von Saphir oder Galliumnitrid als Substratmaterial, deren Rückseiten entsprechend tiefenzustrukturieren.The surface or back side of the substrate to be structured in this way can be formed in particular in the form of black silicon. Likewise, however, it is also possible, when using sapphire or gallium nitride as the substrate material, to deep-restructure their back sides accordingly.

Wird im Rahmen der vorliegenden Erfindung und der nachfolgenden Beschreibung davon gesprochen, dass ein Element (z.B. die piezoelektrische Sensoreinheit) auf oder an einem anderen Element (z.B. Substrat) angeordnet ist und/oder mit diesem anderen Element verbunden ist, so schließt dies nicht aus, dass sich zwischen den beiden Elementen ein oder mehrere weitere Elemente (z.B. Passivierungsschichten, Schutzschichten o.ä.) befinden. Unter einer diffusen Streuung wird im Rahmen der Erfindung eine Streuung von Ultraschallwellen verstanden, die so ausgebildet ist, dass, nach erfolgter Streuung, keine gerichtete Ausbreitung der Ultraschallwellen in eine Vorzugsrichtung mehr erfolgt, sondern eine weitere Ausbreitung der Ultraschallenergie in unterschiedlichste Richtungen so, dass durch die gestreuten Ultraschallwellen kein Echo (oder nur noch ein geringfügiges Echo) durch die Sensoreinheit nachgewiesen werden kann.If in the context of the present invention and the following description it is said that one element (eg the piezoelectric sensor unit) is arranged on or on another element (eg substrate) and / or is connected to this other element, this does not exclude that there are one or more further elements between the two elements (eg passivation layers, protective layers or the like). In the context of the invention, a diffuse scattering is understood to mean a scattering of ultrasonic waves which is designed such that, after scattering, there is no more directed propagation of the ultrasonic waves in a preferred direction, but a further propagation of the ultrasonic energy in most different directions such that the scattered ultrasound waves can not be detected by the sensor unit (or only a slight echo).

Unter einer Lateralrichtung wird eine Richtung innerhalb der Schichtebene des Ultraschallsensors und/oder dessen Sensoreinheit verstanden. Die Richtung senkrecht hierzu, also die Richtung senkrecht zur Sensorebene und/oder zur Ebene des Substrates (z.B. Wafer) wird nachfolgend alternativ auch als Tiefenrichtung oder als Höhenrichtung bezeichnet. Wird nachfolgend von einer mittleren Ausdehnung (z.B. einer mittleren lateralen Ausdehnung, also einer Ausdehnung in Richtung der Schichtebene des Sensors oder einer mittleren Höhenausdehnung der Erhebungen in Richtung senkrecht zur Schichtebene) gesprochen, so ist unter dem entsprechenden Mittel das arithmetische Mittel aus einer Vielzahl von Einzelwerten (z.B. von lateralen Ausdehnungen einzelner nadelförmiger Erhebungen) zu verstehen.A lateral direction is understood to mean a direction within the layer plane of the ultrasound sensor and / or its sensor unit. The direction vertical For this purpose, that is to say the direction perpendicular to the sensor plane and / or to the plane of the substrate (eg wafer) is alternatively referred to below as the depth direction or as the height direction. If an average extent (eg a mean lateral extent, ie an extension in the direction of the layer plane of the sensor or an average height extent of the elevations in the direction perpendicular to the layer plane) is subsequently referred to, the arithmetic mean of the plurality of individual values is below the corresponding mean (eg of lateral extensions of individual needle-shaped elevations).

Ein erfindungsgemäßer Ultraschallsensor umfasst ein Substrat und eine auf oder an diesem Substrat angeordnete und/oder mit diesem Substrat verbundene, piezoelektrische Sensoreinheit. Die der piezoelektrischen Sensoreinheit abgewandte Rückseite des Substrats weist eine Vielzahl von nadelförmigen Erhebungen und Vertiefungen auf, in dieser Rückseite ist somit eine Oberflächenstruktur eingebracht. Diese Oberflächenstrukturierung bzw. Oberflächenstruktur ist so ausgebildet, dass durch sie eine diffuse Streuung der aus Richtung der Sensoreinheit (also von der Vorderseite des Sensors) auf die strukturierte Rückseite einfallenden Ultraschallwellen erfolgt. Die Erhebungen und/oder Vertiefungen können eine mittlere laterale Ausdehnung im Bereich von 0.05 pm bis 1 mm, bevorzugt im Bereich vom 0.1 pm bis 200 pm und besonders bevorzugt im Bereich von 0.2 pm bis 20 pm aufweisen. Diese mittlere laterale Ausdehnung kann somit kleiner oder gleich der Wellenlänge einer durch die piezoelektrische Sensoreinheit (auf der Vorderseite des Substrats) erzeugbaren Ultraschallwelle sein.An inventive ultrasonic sensor comprises a substrate and a piezoelectric sensor unit arranged on or on this substrate and / or connected to this substrate. The rear side of the substrate facing away from the piezoelectric sensor unit has a multiplicity of needle-shaped elevations and depressions, thus a surface structure is introduced in this rear side. This surface structuring or surface structure is designed in such a way that it causes a diffuse scattering of the ultrasonic waves incident from the direction of the sensor unit (that is to say from the front side of the sensor) onto the structured rear side. The elevations and / or depressions may have an average lateral extent in the range from 0.05 μm to 1 mm, preferably in the range from 0.1 μm to 200 μm and particularly preferably in the range from 0.2 μm to 20 μm. This average lateral extent may thus be less than or equal to the wavelength of an ultrasonic wave that can be generated by the piezoelectric sensor unit (on the front side of the substrate).

Die auf der Vorderseite des Substrats aufgebrachte piezoelektrische Sensoreinheit kann zum Aussenden und/oder zum Empfangen von Ultraschallwellen entsprechend einer Frequenz des Bereichs von 20 kHz bis 1 GHz ausgebildet sein. Die piezoelektrische Sensoreinheit kann dabei auch aus mehreren, zum Empfangen oder zum Senden von Ultraschall ausgebildeten Untereinheiten aufgebaut sein. Entsprechende Ausgestaltungen sowie Auswertealgorithmen zum Auswerten der gesendeten und/oder empfangenen Ultraschallsignale sind dem Fachmann dabei bekannt (beispielsweise lassen sich entsprechende Ausgestaltungen der DE 10 2006 005 048 A1 entnehmen).The piezoelectric sensor unit applied to the front surface of the substrate may be configured to emit and / or receive ultrasonic waves corresponding to a frequency in the range of 20 kHz to 1 GHz. The piezoelectric sensor unit can also be composed of a plurality of subunits designed to receive or transmit ultrasound. Corresponding embodiments as well as evaluation algorithms for evaluating the transmitted and / or received ultrasound signals are known to the person skilled in the art (for example, corresponding embodiments of US Pat DE 10 2006 005 048 A1 remove).

Ebenso kann die der Rückseite des Substrats einstrukturierte Oberflächenstruktur zur diffusen Streuung von Ultraschallwellen entsprechend dem vorgenannten Frequenzbereich ausgebildet sein.Likewise, the surface structure patterned on the rear side of the substrate can be designed for the diffuse scattering of ultrasonic waves corresponding to the aforementioned frequency range.

Bei dem Substrat handelt es sich bevorzugt um Silizium, insbesondere um kristallines Silizium. Das Substrat kann ein Silizium-Wafer sein. Ebenso ist es jedoch auch denkbar als Substrat Saphir oder Galliumnitrid zu verwenden.The substrate is preferably silicon, in particular crystalline silicon. The substrate may be a silicon wafer. However, it is also conceivable to use sapphire or gallium nitride as the substrate.

Im Falle von Silizium als Substrat ist die Rückseite und/oder deren Oberflächenstruktur bevorzugt in Form von schwarzem Silizium ausgebildet. Unter schwarzem Silizium wird im Rahmen der vorliegenden Erfindung eine Oberflächenmodifikation des kristallinen Siliziums wie folgt verstanden: Das kristalline Silizium wird, beispielsweise durch ultrakurze Laserpulse oder durch den Beschuss der Siliziumoberfläche mit hochenergetischen Ionen der Substratrückseite so strukturiert, dass bevorzugt lichtoptisch wirksame, bevorzugt nadelförmige Strukturen (Erhebungen und Vertiefungen) auf der Oberfläche erzeugt werden.In the case of silicon as a substrate, the back side and / or its surface structure is preferably formed in the form of black silicon. For the purposes of the present invention, black silicon is understood to mean a surface modification of the crystalline silicon. The crystalline silicon is structured, for example by ultrashort laser pulses or by bombarding the silicon surface with high energy ions of the substrate back, so that optically optically active, preferably acicular structures (FIG. Surveys and recesses) be generated on the surface.

Die nadelförmigen Vertiefungen und Erhöhungen im Silizium können mit dem dem Fachmann bekannten reaktiven Ionentiefätzen hergestellt werden: Der Ionentiefätz-Prozess ist ein zweistufiger, alternierender Trockenätzprozess bei dem sich ein Ätz- und ein Passivierungsschritt abwechseln. Ziel ist es, möglichst anisotrop zu ätzen, d.h. richtungsabhängig, senkrecht zur Wafer-Oberfläche. Nach einer Maskierung zu schützender Bereiche auf dem Silizium, z.B. mittels Aluminium, wird Schwefelhexafluorid (SF6) und als Trägergas (meist Argon) in den Reaktor mit dem darin befindlichen Substrat eingeleitet. Nach elektrischer Energiezufuhr (z.B. induktiv ICP oder mittels Mikrowellen Elektronenzyklotronresonanz) bildet sich ein energiereiches Hochfrequenzplasma, wobei aus dem SF6 ein reaktives Gas entsteht (im Plasma entstehen SF6+-Ionen, aktivierte SF6-Moleküle sowie fluorhaltige und Sauerstoff-Radikale). Zusammen mit der Beschleunigung der Argon-Ionen in einem elektrischen Feld wird eine chemische Ätzreaktion (isotrop) auf dem Substrat und ein physikalischer (anisotroper) Materialabtrag mittels Argon-Ionen überlagert. Je nach Anlagentyp findet der Prozess bei Niederdrücken von 50 Pa bis 1 Pa, bevorzugt in einem HF-Plasma mit 13,65 MHz, Druckbereich 10-50 Pa statt.The needle-shaped depressions and elevations in the silicon can be produced using the reactive ion etching known to the person skilled in the art. The ion etching process is a two-stage, alternating dry etching process in which an etching step and a passivation step alternate. The aim is to etch as anisotropically as possible, ie direction-dependent, perpendicular to the wafer surface. After masking areas to be protected on the silicon, for example by means of aluminum, sulfur hexafluoride (SF6) and as a carrier gas (usually argon) is introduced into the reactor with the substrate therein. After electrical energy supply (eg, inductively ICP or by means of microwave electron cyclotron resonance), an energy-rich high-frequency plasma is formed, with the SF6 producing a reactive gas (in plasma, SF6 + ions, activated SF6 molecules and fluorine-containing and oxygen radicals are formed). Together with the acceleration of the argon ions in an electric field, a chemical etching reaction (isotropic) on the substrate and a physical (anisotropic) material removal by means of argon ions are superimposed. Depending on the type of plant, the process takes place at pressures of 50 Pa to 1 Pa, preferably in an RF plasma at 13.65 MHz, pressure range 10-50 Pa instead.

Der Ätzprozess wird nach kurzer Zeit gestoppt und ein Gasgemisch aus Octaflourocyclobutan (C4F8) und Argon eingeleitet. Im Reaktor wird das Octaflourocyclobutan als Plasmagas aktiviert und die entstehenden fluorhaltigen Radikale und Moleküle bilden auf dem gesamten Substrat eine polymerartige Passivierungsschicht, d.h. sowohl auf der Maske, als auch auf dem Silizium und den vertikalen Silizium-Seitenwänden. Durch den anschließend wiederholten Ätzschritt mit SF6 wird die Passivierungsschicht der horizontalen Flächen (Grabenboden) durch die gerichtete physikalische Komponente (Ionen) der Ätzreaktion deutlich schneller entfernt als die Schicht an den Seitenwänden.The etching process is stopped after a short time and a gas mixture of octaflourocyclobutane (C4F8) and argon is introduced. In the reactor, the octafluorocyclobutane is activated as a plasma gas and the resulting fluorine-containing radicals and molecules form on the entire substrate a polymer-like passivation layer, ie both on the mask, as well as on the silicon and the vertical silicon sidewalls. By the subsequently repeated etching with SF6, the passivation layer of the horizontal surfaces (trench bottom) is removed much faster by the directed physical component (ions) of the etching reaction than the layer on the sidewalls.

Mit diesem Verfahren können erfindungsgemäß, abgedeckt durch die Ablagerung von oben und dem Polymer von den Seiten, lange Siliziumsäulen stehen bleiben. Der Prozess kann dabei so eingestellt werden, dass sich auf einem Quadratmillimeter Millionen Nadeln bilden können.With this method, covered by the deposition of the top and the polymer from the sides, long silicon columns remain according to the invention. The process can be adjusted so that millions of needles can form on a square millimeter.

Dem Fachmann ist auch bekannt, dass Silizium im halogengasgefüllten Vakuumrezipient durch Beschuss der Siliziumoberfläche mit extrem energiereichen gepulsten Femtosekunden-Laser (Laser, die Lichtpulse aussenden, deren Dauer im Femtosekunden-Bereich 1 fs = 10-15 s liegt mit Spitzenenergien im Giga- bzw. Terrawatt-Bereich) die räumliche Struktur durch höchstenergetische Energieeinträge derart verändert, dass das schwarze Silizium entsteht. Auch durch den Laserbeschuss (mehrere Hundert Pulse) kann erfindungsgemäß eine nadelförmige Oberfläche hergestellt werden.The person skilled in the art is also aware that silicon in the halogen gas-filled vacuum recipient by bombarding the silicon surface with extremely high-energy pulsed femtosecond laser (laser emitting light pulses whose duration in the femtosecond range 1 fs = 10 -15 s with peak energies in Giga or Terrawatt range), the spatial structure is changed by high-energy energy inputs in such a way that the black silicon is formed. Also, by the laser bombardment (several hundred pulses) can be prepared according to the invention, a needle-shaped surface.

Bevorzugt besitzen die im Silizium erzeugten, "schwarzen" Strukturen eine Länge (senkrecht zur Substratebene) von einigen µm bis > 10 µm bei einem Durchmesser von etwa 1 µm oder darunter auf einkristallinem Silizium, weshalb die Struktur auch als "silicon grass" oder "RIE grass" bezeichnet wird (DRIE = deep reactive ion etching). Wesentliches Merkmal einer solchen Schicht aus schwarzem Silizium auf der Rückseite des Substrats ist eine erhöhte Absorption von einfallendem sichtbaren Licht, die durch die Ausbildung der vorgenannten Tiefenstruktur bzw. Oberflächenstruktur bewirkt wird (die Tiefenstruktur bewirkt einen stetigen Übergang der Brechzahl des effektiven Mediums, sodass keine scharfe optische Grenzfläche existiert, an der das Licht reflektiert werden kann; stattdessen wird das Licht "sanft" in das Material geleitet und kaum reflektiert, was das Silizium schwarz erscheinen lässt).Preferably, the "black" structures produced in the silicon have a length (perpendicular to the substrate plane) of a few microns to> 10 microns with a diameter of about 1 micron or less on monocrystalline silicon, which is why the structure as "silicon grass" or "RIE grass "(DRIE = deep reactive ion etching). An essential feature of such a layer of black silicon on the back of the substrate is an increased absorption of incident visible light, by the formation of the aforementioned deep structure or Surface structure (the deep structure causes a gradual transition of the refractive index of the effective medium so that there is no sharp optical interface at which the light can be reflected) instead the light is "smoothly" guided into the material and hardly reflected, rendering the silicon black to appear).

Die Erhebungen und Vertiefungen der Oberflächenstruktur können (auch bei anderen Substraten) somit durch Laserbeschuss, durch Ionenbeschuss, insbesondere durch reaktives Ionenätzen oder reaktives Ionentiefätzen, und/oder auch durch mikromechanische, spanabtragende Bearbeitung der Rückseite des Substrats hergestellt sein. Wie vorbeschrieben, sind die Erhebungen nadelförmig ausgebildet.The elevations and depressions of the surface structure can thus be produced (also with other substrates) by laser bombardment, by ion bombardment, in particular by reactive ion etching or reactive ion etching, and / or also by micromechanical chip removal machining of the back side of the substrate. As described above, the elevations are needle-shaped.

Die mittlere Höhe der Erhebungen, die mittlere Tiefe der Vertiefungen und/oder die mittlere Ausdehnung der Erhebungen und/ oder der Vertiefungen senkrecht zur Sensorebene (nachfolgend auch mit der Variablen A bezeichnet) liegt bevorzugt im Bereich zwischen 0.05 µm bis 1 mm, bevorzugt im Bereich von 0.1 pm bis 200 pm und besonders bevorzugt im Bereich von 0.1 pm bis 20 pm (also letztendlich in derselben Größenordnung wie die laterale Ausdehnung der Erhebungen und/oder Vertiefungen in der Sensorebene). Somit beträgt das Aspektverhältnis a = A/L aus vorgenannter Höhe, Tiefe und/oder Ausdehnung und der mittleren lateralen Ausdehnung der Erhebungen und/oder Vertiefungen (die nachfolgend auch mit der Variablen L bezeichnet ist) bevorzugt zwischen 0.2 und 50, besonders bevorzugt zwischen 0.5 und 10.The mean height of the elevations, the mean depth of the depressions and / or the mean extent of the elevations and / or the depressions perpendicular to the sensor plane (also referred to below as variable A) is preferably in the range between 0.05 μm to 1 mm, preferably in the range from 0.1 pm to 200 pm, and more preferably in the range of 0.1 pm to 20 pm (ie, ultimately of the same order of magnitude as the lateral extent of the elevations and / or depressions in the sensor plane). Thus, the aspect ratio a = A / L from the aforementioned height, depth and / or extent and the average lateral extent of the elevations and / or depressions (which is also referred to below with the variable L) is preferably between 0.2 and 50, particularly preferably between 0.5 and 10.

Das Piezoelement der piezoelektrischen Sensoreinheit ist bevorzugt in Form einer piezoelektrischen Dünn schicht ausgebildet. Diese Schicht kann aus AlN oder ZnO bestehen oder dieses Material enthalten. Bevorzugt weist die Sensoreinheit eine Schichtdicke im Bereich zwischen 1 µm und 100 µm, bevorzugt zwischen 10 µm und 25 µm auf. Die Sensoreinheit kann wie vorbeschrieben auch aus mehreren über die Schichtebene verteilten Untereinheiten, die jeweils entsprechende Dünnschichtelemente aufweisen, bestehen.The piezoelectric element of the piezoelectric sensor unit is preferably in the form of a piezoelectric thin layer formed. This layer may consist of AlN or ZnO or contain this material. The sensor unit preferably has a layer thickness in the range between 1 μm and 100 μm, preferably between 10 μm and 25 μm. As described above, the sensor unit can also consist of several subunits distributed over the layer plane, each of which has corresponding thin-film elements.

Zum Anregen des Piezoelementes bzw. der piezoelektrischen Dünnschicht zu Schwingungen und/oder zum Messen der durch mechanischen Druck im Piezoelement bzw. der Dünnschicht erzeugten elektrischen Spannung weist die piezoelektrische Sensoreinheit (oder, bei mehreren Untereinheiten jede dieser Untereinheiten) zwei mit dem Piezoelement zum Erfassen und/oder Anlegen der elektrischen Spannung verbundene elektrische Kontakte auf. Bevorzugt ist die piezoelektrische Dünnschicht dabei sandwichartig zwischen diesen beiden elektrischen Kontakten angeordnet und grenzt unmittelbar an diese elektrischen Kontakte an. Die elektrischen Kontakte können beispielsweise aus Kupfer ausgebildet sein.To excite the piezoelectric element or the piezoelectric thin layer to oscillate and / or measure the electrical voltage generated by mechanical pressure in the piezoelectric element or the thin film, the piezoelectric sensor unit (or, in the case of several subunits each of these subunits) has two with the piezoelectric element for detecting and / or applying the electrical voltage connected electrical contacts. In this case, the piezoelectric thin film is preferably sandwiched between these two electrical contacts and directly adjoins these electrical contacts. The electrical contacts may be formed, for example, of copper.

Die piezoelektrische Sensoreinheit oder die entsprechenden Unter-Sensoreinheiten können zum Aussenden von Ultraschallwellen, zum Empfangen von Ultraschallwellen oder auch kombiniert zum Aussenden und zum Empfangen von Ultraschallwellen (Sende- und Empfangseinheit) ausgebildet sein. Um z.B. ein freies Schwingen der Sensoreinheit und/oder der Untereinheiten zu ermöglichen, kann das Substrat mit der/den darauf ausgebildeten Sensoreinheit(en) abschnittweise als dünne Membran ausgebildet sein.The piezoelectric sensor unit or the corresponding sub-sensor units can be designed for emitting ultrasonic waves, for receiving ultrasonic waves or else combined for emitting and for receiving ultrasonic waves (transmitting and receiving unit). For example, To allow free swinging of the sensor unit and / or the subunits, the substrate with the / the sensor unit (s) formed thereon may be formed in sections as a thin membrane.

Der Ultraschallsensor kann in Form eines Ultraschallprüfkopfs ausgebildet sein oder in einen solchen Prüfkopf integriert sein.The ultrasonic sensor may be in the form of an ultrasonic probe be formed or integrated into such a probe.

Erfindungsgemäß wird somit für aktive piezoelektrische Dünnschichten, die auf geeigneten Trägermaterialien abgeschieden sind (insbesondere: Silizium) eine akustisch stark streuende Rückseite des Substrats realisiert. Insbesondere lässt sich dies über die Schwarze-Silizium-Technologie mittels Ionenätzen, durch Strukturieren mittels Laserbearbeitung oder durch spanabtragende Verfahren wie beispielsweise Wafersägen realisieren. Die Vorgehensweise bei den einzelnen vorgenannten Bearbeitungsverfahren ist dem Fachmann grundsätzlich bekannt, beispielsweise wie folgt:

  • Silizium-Ätzen:
    1. 1. F. Lärmer, A. Schilp: Verfahren zum anisotropen Ätzen von Silicium, Patent DE 4241045 , Deutschland, angemeldet am 5. Dezember 1992, erteilt am 26. Mai 1994.
    2. 2. W. Menz, J. Mohr: Mikrosystemtechnik für Ingenieure, VCH-Verlag, Weinheim 1997, ISBN 352730536X .
    3. 3. Gary S. May, Simon M. Sze: Fundamentals of Semiconductor Fabrication, Wiley & Sons, 2003, ISBN 0-47145238-6 .
    4. 4. Kanechika M., Sugimoto N., Mitsushima Y., Control of shape of silicon needles fabricated by highly selective anisotropic dry etching, Jour of Vacuum Science & Technology B: Microelectronics and Nanometer Structures - July 2002 - Vol. 20, I. 4, pp. 1298-1302 .
    5. 5. H. V. Jansen et al, the black silicon method: a universal method for determining the parameter setting of a fluorine based reactive ion etcher in deep silicon trench etching with profile control, Journal of Micromechanical Microengineering 5 (1995), pp. 115-120 .
  • Laserbearbeitung:
    • 6. Fritz Kurt Kneubühl, Markus Werner Sigrist: Laser, 6. Auflage, Teubner, Wiesbaden 2005, ISBN 3-8351-0032-7 .
    • 7. J. Eichler, H. J. Eichler: Laser, Bauformen, Strahlführung, Anwendungen, 5. Auflage, Springer-Verlag, ISBN 3-540-00376-2 .
  • Silizium-Mikrotechnik-Mikromechanik:
    • 8. Ulrich Hilleringmann: Mikrosystemtechnik: Prozesschritte, Technologien, Anwendungen, 1. Auflage, Vieweg + Teubner, 2006, ISBN 3-835-10003-3 .
    • 9. Brück, Rainer [Hrsg.] Bauer, Hans-Dieter: Angewandte Mirkotechnik; LIGA, Laser, Feinwerktechnik/München; Wien: Hanser, 2001 - ISBN 3-446-21471-2 .
According to the invention, therefore, an acoustically strongly scattering rear side of the substrate is realized for active piezoelectric thin films deposited on suitable carrier materials (in particular: silicon). In particular, this can be realized via the black-silicon technology by means of ion etching, by structuring by means of laser processing or by chip-removing methods, such as, for example, wafer saws. The procedure in the individual aforementioned processing methods is known in principle to the person skilled in the art, for example as follows:
  • Silicon etching:
    1. 1. F. Lärmer, A. Schilp: Process for anisotropic etching of silicon, patent DE 4241045 , Germany, filed on 5 December 1992, granted on 26 May 1994.
    2. Second W. Menz, J. Mohr: Microsystem Technology for Engineers, VCH-Verlag, Weinheim 1997, ISBN 352730536X ,
    3. Third Gary S. May, Simon M. Sze: Fundamentals of Semiconductor Fabrication, Wiley & Sons, 2003, ISBN 0-47145238-6 ,
    4. 4th Kanechika M., Sugimoto N., Mitsushima Y., Control of shape of silicon needles fabricated by highly selective anisotropic dry etching, Jour of Vacuum Science & Technology B: Microelectronics and Nanometer Structures - July 2002 - Vol. 20, I. 4, pp. 1298-1302 ,
    5. 5th HV Jansen et al., The black silicon method: a universal method for determining the parameter setting of a fluorine-based reactive ion etcher in deep silicon trench etching with profile control, Journal of Micromechanical Microengineering 5 (1995), p. 115-120 ,
  • Laser processing:
    • 6th Fritz Kurt Kneubühl, Markus Werner Sigrist: Laser, 6th edition, Teubner, Wiesbaden 2005, ISBN 3-8351-0032-7 ,
    • 7th J. Eichler, HJ Eichler: Laser, types, beam guidance, applications, 5th edition, Springer-Verlag, ISBN 3-540-00376-2 ,
  • Silicon micromachining technology micromechanics:
    • 8th. Ulrich Hilleringmann: Microsystems Technology: Process Steps, Technologies, Applications, 1st Edition, Vieweg + Teubner, 2006, ISBN 3-835-10003-3 ,
    • 9th Brück, Rainer [ed.] Bauer, Hans-Dieter: Applied Micro Technology; LIGA, Laser, Precision Engineering / Munich; Vienna: Hanser, 2001 - ISBN 3-446-21471-2 ,

Die Herstellung eines Substrates mit einer Ultraschall streuenden Rückseite ("Substrat-Absorberschicht") kann folglich erfindungsgemäß für piezoelektrische Dünnschichtsensoreinheiten derart erfolgen, dass das Substrat (beispielsweise der Sililiziumwafer) vor den Beschichtungsprozessen (Beschichtung mit der dünnen, piezoelektrischen Schicht sowie mit entsprechenden elektrischen Kontakten) zunächst auf der Rückseite mit einer entsprechenden Oberflächenstruktur (z.B. einer Oberfläche aus schwarzem Silizium) versehen wird. Dies kann wie vorbeschrieben durch Laserpulsen oder reaktives Ionenätzen erfolgen. Die Beschichtung mit der piezoelektrischen Sensorschicht und den Elektroden ist dem Fachmann dabei grundsätzlich bekannt, als Beschichtungsverfahren können beispielsweise Kathodenzerstäubungsverfahren (Sputtern) eingesetzt werden. Theoretisch können alle PVD-Verfahren, wie HF-Sputtern eingesetzt werden, bevorzugt geeignet sind Puls-Magnetron-SputterVerfahren. Siehe hierzu beispielsweise:

  • 10. Leyens, Christoph: Wechselwirkung zwischen Herstellungsparametern und Schichteigenschaften ausgewählter metallischer und keramischer Systeme bei der Magnetron-Kathodenzerstäubung / Düsseldorf: VDI-Verl., 1998. ( Fortschrittberichte VDI: Reihe 5, Grund- und Werkstoffe, Kunststoffe; 534) ISBN 3-18-353405-3 .
  • 11. U. Krause: Das Verhalten der elektrischen Parameter beim bipolaren Puls-Magnetron-Sputtern am Beispiel von Zinn- und Zinkoxid Jahr 2002, Hochschulschrift Magdeburg, Univ., Diss., 2001 .
  • 12. D. Glöß, Einfluss von Beschichtungsparametern auf den Teilchen- und Energiestrom zum Substrat und Auswirkungen auf ausgewählte Eigenschaften von Titanoxidschichten beim reaktiven Puls-Magnetron-Sputtern, Diss. Fakultät für Naturwissenschaften der Technischen Universität Chemnitz, 2006 .
  • 13. D. Depla: Reactive sputter deposition; Berlin, Heidelberg [u.a.]: Springer, 2008 (Springer series a in materials science, 109) ISBN 978-3-540-76662-9 .
The production of a substrate with an ultrasound-scattering rear side ("substrate absorber layer") can thus take place according to the invention for piezoelectric thin-film sensor units such that the substrate (for example the silicon silicon wafer) before the coating processes (coating with the thin, piezoelectric layer and with corresponding electrical contacts) initially on the back with a corresponding surface structure (eg a surface of black silicon) is provided. This can be done as described above by laser pulses or reactive ion etching. The coating with the piezoelectric sensor layer and the electrodes is the expert basically known, as a coating method, for example, sputtering can be used. Theoretically, all PVD methods, such as RF sputtering may be used, preferably pulse magnetron sputtering methods are suitable. See for example:
  • 10. Leyens, Christoph: Interaction between production parameters and layer properties of selected metallic and ceramic systems in magnetron sputtering / Düsseldorf: VDI-Verl., 1998. ( Progress reports VDI: Series 5, basic materials and materials, plastics; 534) ISBN 3-18-353405-3 ,
  • 11th U. Krause: The behavior of the electrical parameters in bipolar pulse magnetron sputtering using the example of tin and zinc oxide Year 2002, University of Magdeburg, Univ., Diss., 2001 ,
  • 12th D. Glöß, Influence of coating parameters on the particle and energy flow to the substrate and effects on selected properties of titanium oxide layers in reactive pulse magnetron sputtering, Diss. Faculty of Natural Sciences of Chemnitz University of Technology, 2006 ,
  • 13th D. Depla: Reactive sputter deposition; Berlin, Heidelberg [ua]: Springer, 2008 (Springer series a in materials science, 109) ISBN 978-3-540-76662-9 ,

Erfindungsgemäß können beispielsweise durch reaktives Ionentiefätzen Gräben, Vertiefungen, Löcher, ... als Erhebungen und Vertiefungen in die Rückseite des Substrats strukturiert werden. Die Vertiefungen können beispielsweise mehrere 100 µm Tiefe aufweisen und mit einem hohen Aspektverhältnis (z.B. im Bereich von 2 bis 50) erzeugt werden. Dies kann durch wiederholtes Wechseln von Ätzen und Passivieren der rückseitigen Substratoberfläche erreicht werden. Beim Ätzen können allerdings kleine Ablagerungen der,Passivierung am Boden verbleiben und diesen maskieren. Bei einer Verlagerung des Prozesses hin zur Passivierung entstehen so auszuformende Strukturen, die auch bei den folgenden Ätzschritten nicht abgetragen werden.According to the invention, trenches, depressions, holes, ... can be structured as elevations and depressions in the back side of the substrate, for example by reactive ion etching. The depressions can for example have several 100 μm depth and be produced with a high aspect ratio (eg in the range from 2 to 50). This can be done by repeatedly alternating etching and passivation of the backside Substrate surface can be achieved. During etching, however, small deposits of the, passivation can remain on the ground and mask it. When the process is shifted to passivation, structures to be formed are formed which are not removed during the subsequent etching steps.

Hierdurch können dort senkrechte (relativ zur Substratebene) Flächen entstehen, an denen sich eine Polymerschicht ablagern kann. So können, abgedeckt durch die Ablagerung von oben und abgedeckt durch das Polymer an den Seiten, Erhebungen beispielsweise in Form von länglichen Siliziumsäulen stehen bleiben. Das reaktive Ionenätzen kann dabei so eingestellt werden, dass sich auf 1 mm2 Millionen kleiner nadelförmiger Säulen bilden können. Auch durch Beschuss mit extrem energiereichen gepulsten Femtosekundenlasern kann die räumliche Struktur der Rückseite des Substrats verändert werden, sodass eine nadelförmige, tiefenstrukturierte Oberfläche entsteht (z.B. Nadeln in einer mittleren Länge von 300 nm). Die Prozesse sind vergleichsweise gut und gleichmäßig reproduzierbar.As a result, perpendicular (relative to the substrate plane) surfaces can arise there against which a polymer layer can deposit. Thus, covered by the deposition from above and covered by the polymer on the sides, surveys, for example, in the form of elongated silicon columns remain. The reactive ion etching can be adjusted so that can form on 1 mm 2 million small needle-shaped columns. The spatial structure of the backside of the substrate can also be changed by bombardment with extremely high-energy pulsed femtosecond lasers, resulting in a needle-shaped, deep-structured surface (eg needles with a mean length of 300 nm). The processes are comparatively good and evenly reproducible.

Nachfolgend wird die Erfindung anhand eines Ausführungsbeispiels beschrieben.The invention will be described with reference to an embodiment.

Es zeigen:

  • Fig. 1 einen Schnitt senkrecht zur Substratebene durch einen erfindungsgemäßen Ultraschallsensor (Schemazeichnung).
  • Fig. 2 eine elektronenmikroskopische Aufnahme einer Oberflächenstruktur der Rückseite eines in dem Sensor gemäß Fig. 1 eingesetzten Siliziumwafers (Schwarzes Silizium).
Show it:
  • Fig. 1 a section perpendicular to the substrate plane by an ultrasonic sensor according to the invention (schematic drawing).
  • Fig. 2 an electron micrograph of a surface structure of the back of one in the sensor according to Fig. 1 used silicon wafer (black Silicon).

Fig. 1 zeigt einen Schnitt durch einen erfindungsgemäßen Ultraschallsensor. Wie vorbeschrieben wird die Rückseite 3 eines einkristallinen Siliziumwafers 1 durch reaktives Ionentiefätzen mit einer eine Vielzahl von nadelförmigen Erhebungen und Vertiefungen (vgl. Fig. 2) umfassenden Oberflächenstruktur 4 versehen. Die Dicke des Wafers 1 beträgt hier 500 µm, die Tiefe der Vertiefungen bzw. die Ausdehnung der einzelnen nadelförmigen Erhebungen A der Oberflächenstruktur 4 auf der Rückseite 3 des Substrats 1, also die Tiefe der Strukturen im schwarzen Silizium auf der Rückseite 3 des Wafers 1 beträgt hier 2 bis 5 µm, und die laterale Ausdehnung dieser Erhebungen (vgl. Fig. 2) beträgt hier 200 bis 800 nm. Fig. 1 shows a section through an ultrasonic sensor according to the invention. As described above, the back side 3 of a monocrystalline silicon wafer 1 is formed by reactive ion milling with one of a plurality of needle-shaped protrusions and depressions (see FIG. Fig. 2 ) comprehensive surface structure 4 provided. The thickness of the wafer 1 here amounts to 500 μm, the depth of the depressions or the extent of the individual needle-shaped elevations A of the surface structure 4 on the back 3 of the substrate 1, that is to say the depth of the structures in the black silicon on the back 3 of the wafer 1 here 2 to 5 microns, and the lateral extent of these surveys (see. Fig. 2 ) is here 200 to 800 nm.

Mithilfe eines Magnetron-Sputter-Prozesses werden anschließend auf der der Rückseite 3 gegenüberliegenden Vorderseite 7 des Wafers die einzelnen Elemente der elektrischen Sensoreinheit 2 aufgebracht. Zunächst wird jedoch eine hier 1 bis 2 µm dicke Isolationslage 8 aus Siliziumoxid auf der Vorderseite 7 des Wafers 1 abgeschieden. Auf dieser elektrischen Isolationslage 8 wird zunächst eine erste Elektrodenmetallisierung bzw. Elektrodenschicht 6 (hier eine 150 µm dicke Aluminiumschicht) aufgebracht. Auf dieser ersten Elektrodenmetallisierung 6 wird eine piezoelektrisch aktive Dünnschicht (piezoelektrische Schicht 5) aus A1N aufbeschichtet. Alternativ dazu kann beispielsweise auch Zn0 als Schichtmaterial verwendet werden. Die piezoelektrische Dünnschicht hat hier eine Schichtdicke von 5 bis 25 µm. Auf der der ersten Metallisierung 6 gegenüberliegenden Seite der piezoelektrischen Schicht 5 wird schließlich der zweite elektrische Kontakt 9 der piezoelektrischen Dünnschicht 5 aufbeschichtet. Auch bei diesem Kontakt handelt es sich um einen Aluminiumschichtkontakt, dessen Dicke der Dicke des ersten Metallkontakts 6 entspricht. Die Sensoreinheit 2 umfasst hier die Elemente 5, 6 und 9 (und je nach Sichtweise auch die Schicht 8).By means of a magnetron sputtering process, the individual elements of the electrical sensor unit 2 are subsequently applied to the front side 7 of the wafer opposite the rear side 3. First, however, a here 1 to 2 microns thick insulating layer 8 of silicon oxide on the front side 7 of the wafer 1 is deposited. On this electrical insulation layer 8, first a first electrode metallization or electrode layer 6 (here a 150 μm thick aluminum layer) is applied. On this first electrode metallization 6, a piezoelectrically active thin film (piezoelectric layer 5) of A1N is coated. Alternatively, for example, ZnO can be used as a layer material. The piezoelectric thin film here has a layer thickness of 5 to 25 microns. On the opposite side of the first metallization 6 of the piezoelectric layer 5 is finally the second electrical contact 9 of the piezoelectric thin film 5 coated. Also in this contact is an aluminum layer contact whose thickness corresponds to the thickness of the first metal contact 6. The sensor unit 2 here comprises the elements 5, 6 and 9 (and depending on the view, the layer 8).

Somit ergibt sich von der Rückseite 3 mit der Oberflächenstruktur 4 des Ultraschallsensors hin zur Vorderseite (elektrischer Kontakt 9) gesehen der folgende Schichtaufbau: Rückseite 3 mit Oberflächenstruktur 4, Siliziumwafer 1, Isolationslage 8, erster Metallkontakt 6, piezoelektrisch aktive Dünnschicht 5 und zweiter Metallkontakt 9.Thus, the following layer structure results from the rear side 3 with the surface structure 4 of the ultrasonic sensor towards the front side (electrical contact 9): back side 3 with surface structure 4, silicon wafer 1, insulation layer 8, first metal contact 6, piezoelectrically active thin layer 5 and second metal contact 9 ,

Der gezeigte Sensor 1 bis 9 kann somit auf ein externes Objekt 0, das abgetastet bzw. vermessen werden soll, aufgesetzt werden: In der als kombinierte Sende- und Empfangseinheit ausgebildeten piezoelektrischen Sensoreinheit 2 des gezeigten Ultraschallsensors (deren Detailaufbau dem Fachmann z.B. gemäß der DE 10 2006 005 048 A1 bekannt ist) können Ultraschallwellen erzeugt und in das Objekt O eingekoppelt werden. An Grenzflächen im Objekt werden die Ultraschallwellen reflektiert und von der Sensoreinheit 2 werden die entsprechenden Echosignale erfasst und ausgewertet. Die gleichzeitig mit der Einkopplung von Ultraschallwellen bzw. von Ultraschallenergie ins Objekt 0 erfolgende Einkopplung von Ultraschallenergie bzw. von Ultraschallwellen in den Substratträger 1 führt aufgrund der diffusen Reflektion dieser Wellen an der tiefenstrukturierten 4 Rückseite 3 des Trägersubstrats 1 nicht zu messbaren Echos (ungerichtete Rückstreuung der an der Oberfläche 3 reflektierten Ultraschallwellen). Bei dem gezeigten Ultraschallsensor 1 bis 9 werden somit störende Echosignale vermieden und die Messgenauigkeit beim Abtasten des Objekts O wird erhöht.The sensor 1 to 9 shown can thus be placed on an external object 0 which is to be scanned or measured: in the piezoelectric sensor unit 2 of the ultrasonic sensor shown as a combined transmitting and receiving unit (the details of which are given to the person skilled in the art, for example, in accordance with FIG DE 10 2006 005 048 A1 is known) ultrasonic waves can be generated and coupled into the object O. The ultrasonic waves are reflected at interfaces in the object and the corresponding echo signals are detected and evaluated by the sensor unit 2. The simultaneous coupling of ultrasonic waves or ultrasonic energy into the object 0 coupling of ultrasonic energy or of ultrasonic waves in the substrate carrier 1 leads due to the diffuse reflection of these waves on the deep structured 4 back 3 of the support substrate 1 not measurable echoes (non-directional backscatter of on the surface 3 reflected ultrasonic waves). In the illustrated ultrasonic sensor 1 to 9 disturbing echo signals are thus avoided and the measurement accuracy when scanning the object O is increased.

Fig. 2 zeigt ein Beispiel für eine Rückseite 3 bzw. eine Oberflächenstruktur 4 dieser Seite für einen in Fig. 1 skizzierten Ultraschallsensor in einer elektronenmikroskopischen Aufnahme: Fig. 2 links zeigt eine elektronenmikroskopische Aufnahme mit einer Vergrößerung von 10 000, während Fig. 2 rechts eine stärkere Vergrößerung (Vergrößerungsfaktor 50 000) zeigt. Gut zu erkennen sind die einzelnen nadelförmigen Erhöhungen bzw. die einzelnen Siliziumnadeln des an der Rückseite 3 des Siliziumwafers 1 ausgebildeten schwarzen Siliziums. Der mittlere laterale Abstand L zweier Siliziumnadeln beträgt hier ca. 2 bis 5 µm, die mittlere Höhe A beträgt hier 10 bis 20 µm, dies entspricht ca. 2 Millionen Nadeln pro Quadratmillimeter. Fig. 2 shows an example of a back 3 and a surface structure 4 of this page for a in Fig. 1 sketched ultrasonic sensor in an electron micrograph: Fig. 2 left shows an electron micrograph at a magnification of 10,000, while Fig. 2 right shows a higher magnification (magnification factor 50 000). The individual needle-shaped elevations or the individual silicon needles of the black silicon formed on the rear side 3 of the silicon wafer 1 can easily be seen. The average lateral distance L of two silicon needles here is about 2 to 5 microns, the average height A is here 10 to 20 microns, this corresponds to about 2 million needles per square millimeter.

Bei dem in den Fig. 1 und 2 beschriebenen Aufbau einer Silizium-Absorberschicht 1, 3, 4 für eine piezoelektrische Sensoreinheit 2 bzw. 2, 8 (die Isolationslage 8 kann als Teil der Sensoreinheit 2 angesehen werden) wird somit auf dem Siliziumsubstrat 1 mittels der vorbeschriebenen Prozesse auf der Unterseite bzw. der Rückseite 3 eine Schicht aus schwarzem Silizium aufgebracht. Danach erfolgt auf der Oberseite bzw. Vorderseite 7 des Siliziumwafers 1 der Herstellungsprozess für die piezoelektrische Dünnschicht-Sensoreinheit 2, 8: Nach dem Aufbringen der Isolationslage 8 aus Siliziumoxid wird die erste Dünnschicht-Elektrodenmetallisierung 6 aufgebracht, gefolgt von dem aktiven piezoelektrischen Material 5. Schließlich folgt das Aufbringen der zweiten Dünnschicht-Elektrodenmetallisierung 9.In the in the Fig. 1 and 2 described construction of a silicon absorber layer 1, 3, 4 for a piezoelectric sensor unit 2 or 2, 8 (the insulating layer 8 can be considered as part of the sensor unit 2) is thus on the silicon substrate 1 by means of the above-described processes on the bottom or the Back 3 applied a layer of black silicon. Thereafter, on the upper side or front side 7 of the silicon wafer 1, the manufacturing process for the piezoelectric thin-film sensor unit 2, 8: After the deposition of the insulating layer 8 of silicon oxide, the first thin-film electrode metallization 6 is applied, followed by the active piezoelectric material 5. Finally This is followed by the application of the second thin-film electrode metallization 9.

Mit der vorliegenden Erfindung ist es somit möglich, die störenden Ultraschallechos aus dem Trägersubstrat 1 für die Schichtsensorelemente 2 derart zu streuen, dass sie keinen großen Einfluss auf das Echo, welches aus dem an der aktiven Oberfläche (Vorderseite des Ultraschallsensors) angekoppelten Medium bzw. Objekt O zurückkehrt, haben. Somit sind wesentlich breitere Anwendungsfelder für piezoelektrische Ultraschall-Dünnschichtsensoren möglich. Da man den Sensor nicht mehr direkt auf das Messobjekt auftragen muss, nicht unbedingt Luft als rückseitige Grenzschicht realisieren muss und keine sehr dicken Trägersubstrate mehr verwenden muss, können ohne Weiteres auch Hochfrequenz-Ultraschallprüfköpfe mit der vorliegenden Erfindung hergestellt werden.With the present invention, it is thus possible to scatter the interfering ultrasonic echoes from the carrier substrate 1 for the layer sensor elements 2 in such a way that they have no great influence on the echo, which is from the medium or object coupled to the active surface (front side of the ultrasonic sensor) O returns. Thus, much broader fields of application for piezoelectric ultrasonic thin-film sensors are possible. Since one no longer has to apply the sensor directly to the measurement object, does not necessarily have to realize air as a backside boundary layer and no longer has to use very thick carrier substrates, high-frequency ultrasound probes can also be produced without further ado with the present invention.

Ein wesentlicher Kern der Erfindung ist somit das Herstellen der elektroakustischen Absorberschicht auf der Rückseite eines Trägersubstrats durch eine stark zerklüftete Oberfläche mit Strukturbreiten von beispielsweise unter 1 µm und mit Strukturtiefen von beispielsweise mehreren 100 nm, wobei dann auf der gegenüberliegenden Vorderseite bzw. Oberfläche eine piezoelektrische Sensoreinheit in Dünnschichttechnik liegt.An essential core of the invention is thus the production of the electroacoustic absorber layer on the back side of a carrier substrate by a strongly fissured surface with feature widths of, for example, less than 1 μm and with feature depths of, for example, several 100 nm, in which case a piezoelectric sensor unit is located on the opposite front side or surface lies in thin-film technology.

Erfindundungsgemäße Ultraschallsensoren bzw. Dünnschicht-Ultraschallsensoren können in der zerstörungsfreien Materialprüfung dünner Schichten, in der Qualitätssicherung, in der Prozessüberwachung oder auch ganz allgemein für beliebige Ultraschall-, Sensoraufgaben realisiert werden. Insbesondere können auch Hochfrequenz-Ultraschallprüfköpfe erfindungsgemäß realisiert werden.Ultrasonic sensors or thin-layer ultrasonic sensors according to the invention can be implemented in the non-destructive testing of thin layers, in quality assurance, in process monitoring or, in general, for any ultrasonic, sensor tasks. In particular, high-frequency ultrasonic probes can also be realized according to the invention.

Claims (8)

  1. An ultrasonic sensor for detecting and/or scanning an object (O) with a substrate (1) and
    a piezoelectric sensor unit (2) arranged on this substrate and/or connected to this substrate, wherein
    the rear side (3) of the substrate (1) which is remote from the piezoelectric sensor unit (2) has a surface structure (4) comprising a plurality of elevations and depressions,
    this surface structure (4) being formed such that it causes diffuse scattering of ultrasonic waves which are incident on the rear side (3) from the direction of the sensor unit (2) to occur; characterised in that
    their elevations and/or depressions are formed in a needle shape; and
    these elevations and/or depressions have an average lateral extent (L) in the range from 0.05 µm to 1 mm, preferably of 0.1 µm to 200 µm, preferably of 0.2 µm to 20 µm, and/or an average lateral extent (L) which is less than or equal to the wavelength of an ultrasonic wave which can be generated by the piezoelectric sensor unit (2);
    with
    the average height (A) of the elevations, the average depth of the depressions and/or the average extent of the elevations and/or depressions perpendicular to the sensor plane lying in the range between 0.05 µm to 1 mm, preferably from 0.1 µm to 200 µm, preferably from 0.2 µm to 20 µm,
    and/or
    the aspect ratio a=A/L of the aforementioned height (A), depth and/or extent and the average lateral extent (L) of the elevations and/or depressions being between 0.2 and 50, preferably between 0.5 and 10.
  2. An ultrasonic sensor according to the preceding claim,
    characterised in that
    the piezoelectric sensor unit (2) for transmission and/or for reception of and/or in that the surface structure (4) for diffuse scattering of ultrasonic waves is/are configured corresponding to a frequency in the range of 20 kHz to 1 GHz.
  3. An ultrasonic sensor according to one of the preceding claims.
    characterised in that
    the substrate (1) contains or consists of silicon, in particular crystalline silicon, and/or is a silicon wafer, or in that the substrate contains or consists of sapphire or gallium nitride,
    and/or
    in that the rear side (3) and/or the surface structure (4) thereof contains or consists of black silicon
    and/or
    in that the elevations and depressions of the surface structure (4) are produced by laser bombardment, by ion bombardment, in particular by reactive ion etching or deep reactive ion etching, and/or by mechanical, chip-removing machining of the rear side (3) of the substrate (1).
  4. An ultrasonic sensor according to one of the preceding claims,
    characterised in that
    the piezoelectric sensor unit (2) at least one piezo element (5), preferably in the form of a piezoelectric thin film with a layer thickness in the range between 1 µm and 100 µm, preferably between 10 µm and 25 µm, and/or preferably contains or consists of AlN or ZnO.
  5. An ultrasonic sensor according to one of the preceding claims,
    characterised in that
    the piezoelectric sensor unit (2) comprises at least one piezo element (5) and at least two electrical contacts (9, 6) connected to this piezo element (5) for detecting an electric voltage occurring in this piezo element (5) due to an external pressure and/or for applying an electric voltage to this piezo element (5), with preferably at least one of these piezo elements (5) being arranged between at least two electrical contacts (9, 6) which are connected thereto.
  6. An ultrasonic sensor according to one of the preceding claims,
    characterised by
    a piezoelectric sensor unit (2) formed as a transmitting unit for transmitting ultrasonic waves, as a receiving unit for receiving ultrasonic waves or as a transmitting and receiving unit for transmitting and receiving ultrasonic waves
    and/or
    by a piezoelectric sensor unit (2) which together with the substrate (1) at least in sections is formed as a membrane.
  7. An ultrasonic sensor according to one of the preceding claims,
    characterised by
    a plurality of piezoelectric sensor units (2), with preferably a plurality of or all these sensor units (2) being formed in accordance with one of the three preceding claims, and/or
    by formation of the ultrasonic sensor as an ultrasonic probe.
  8. A method for manufacturing an ultrasonic sensor according to one of the preceding claims,
    characterised in that
    the elevations and depressions of the surface structure (4) are produced in the rear side (3) of the substrate (1), in particular in the rear side of a crystalline silicon wafer, initially by laser bombardment, by ion bombardment, in particular by reactive ion etching or deep reactive ion etching, and/or by mechanical, chip-removing machining, before then the front side (7) of the substrate (1) which is located opposite the rear side is provided with at least one piezoelectric layer (5) and with at least two electrical contacts (9, 6) in layer form by means of a coating process, preferably by means of cathode sputtering, by means of a PVD method, and/or by means of pulsed magnetron sputtering.
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