Micromechanical detection structure for a MEMS acoustic transducer and corresponding manufacturing process

A micromechanical structure for a MEMS capacitive acoustic transducer, has: a substrate made of semiconductor material, having a front surface lying in a horizontal plane; a membrane, coupled to the substrate and designed to undergo deformation in the presence of incident acoustic-pressure waves; a fixed electrode, which is rigid with respect to the acoustic-pressure waves and is coupled to the substrate by means of an anchorage structure, in a suspended position facing the membrane to form a detection capacitor. The anchorage structure has at least one pillar element, which is at least in part distinct from the fixed electrode and supports the fixed electrode in a position parallel to the horizontal plane.

BACKGROUND

1. Technical Field

The present disclosure relates to an improved micromechanical detection structure for a MEMS (Micro-Electro-Mechanical Systems) acoustic transducer, in particular a microphone of a capacitive type, and to a corresponding manufacturing process.

2. Description of the Related Art

As is known, a MEMS acoustic transducer of a capacitive type generally comprises a mobile electrode, provided as a diaphragm or membrane, set facing a substantially fixed electrode so as to form the plates of a detection capacitor.

The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a substrate, whereas a central portion is free to move or bend in response to acoustic-pressure waves impinging upon one of its surfaces. The mobile electrode and the fixed electrode provide a detection capacitor, and bending of the membrane that constitutes the mobile electrode causes a variation of capacitance of this detection capacitor. During operation, the variation of capacitance is converted, by suitable processing electronics, into an electrical signal, which is supplied as an output signal of the MEMS acoustic transducer.

A MEMS acoustic transducer of a known type is, for example, described in detail in US patent application No. 2010/0158279 A1 (which is incorporated by reference herein), filed in the name of the present Applicant.

FIG. 1shows by way of example, and in a simplified manner, a portion of the micromechanical detection structure of the acoustic transducer, designated as a whole by 1.

The micromechanical structure1comprises a substrate2made of semiconductor material, and a mobile membrane (or diaphragm)3. The membrane3is made of conductive material and faces a fixed electrode or rigid plate4, generally known as “back plate”, which is rigid, at least when compared to the membrane2, which is instead flexible and undergoes deformation as a function of incident acoustic-pressure waves.

Membrane3is anchored to substrate2by means of membrane anchorages5, formed by protuberances of the same membrane3, which extend from peripheral regions thereof towards the substrate2.

For instance, membrane3has in a top plan view, i.e., in a horizontal plane of main extension, a substantially square shape, and the membrane anchorages5, which are four in number, are set at the vertices of the square.

The membrane anchorages5perform the function of suspending the membrane3above the substrate2, at a certain distance therefrom. The value of this distance is a function of a compromise between the linearity of response at low frequencies and the noise of the acoustic transducer.

In order to enable relief of the residual stresses (tensile and/or compressive stresses) on the membrane3, for example deriving from the manufacturing process, through openings (not illustrated herein) may be formed through the membrane3, in particular in the proximity of each membrane anchorage5, in order to “equalize” the static pressure present on the surfaces of the same membrane3.

The rigid plate4is formed by a first plate layer4a, made of conductive material and facing the membrane3, and by a second plate layer4b, made of insulating material.

The first plate layer4aforms, together with the membrane3, the detection capacitor of the micromechanical structure1.

In particular, the second plate layer4bis set on the first plate layer4a, except for portions in which it extends through the first plate layer4aso as to form bumps6of the rigid plate4, which extend as far as the underlying membrane3and have the function of preventing adhesion of the membrane3to the rigid plate4, as well as of limiting the oscillations of the same membrane3.

For instance, the thickness of the membrane3is in the range 0.3-1.5 μm, for example equal to 0.7 μm; the thickness of the first plate layer4ais in the range 0.5-2 μm, for example equal to 0.9 μm; and the thickness of the second plate layer4bis in the range 0.7-2 μm, for example equal to 1.2 μm.

The rigid plate4moreover has a plurality of holes7, which extend through the first and second plate layers4a,4b, have, for example, a circular cross section, and perform the function of allowing, during the manufacturing steps, removal of underlying sacrificial layers. Holes7are, for example, set so as to form a lattice, in a horizontal plane, parallel to the substrate. Furthermore, in use, the holes7enable free circulation of air between the rigid plate4and the membrane3, making the rigid plate4acoustically transparent. Holes7hence act as acoustic access ports in order to enable the acoustic-pressure waves to reach and deform the membrane3.

The rigid plate4is anchored to the substrate2by means of plate anchorages8, which are connected to peripheral regions of the same rigid plate4.

In particular, plate anchorages8are formed by vertical pillars (i.e., extending in a direction orthogonal to the horizontal plane and to the substrate), which are made of the same conductive material as the first plate layer4aand hence form a single piece with the rigid plate4. In other words, the first plate layer4ahas prolongations that extend as far as the substrate2to define the anchorages of the rigid plate4.

The membrane3is moreover suspended and directly faces a first cavity9a, formed through the substrate2, by means of a through trench etched starting from a back surface2bof the substrate2, which is opposite to a front surface2athereof, on which the membrane anchorages5rest (the first cavity9ahence defines a through hole that extends between the front surface2aand the back surface2bof the substrate2). In particular, the front surface2alies in the horizontal plane.

The first cavity9ais also known as “back chamber”, in the case where the acoustic-pressure waves impinge first upon the rigid plate4and then upon the membrane3. In this case, the front chamber is formed by a second cavity9b, which is delimited at the top and at the bottom, respectively, by the first plate layer4aand by the membrane3.

Alternatively, it is in any case possible for the pressure waves to reach the membrane3through the first cavity9a, which in this case performs the function of acoustic access port, and, hence, of front chamber.

In greater detail, the membrane3has a first surface3aand a second surface3b, which are opposite to one another and face, respectively, the first cavity9aand the second cavity9b, to be in fluid communication, respectively, with the back chamber and with the front chamber of the acoustic transducer.

Furthermore, the first cavity9ais formed by two cavity portions9a′,9a′: a first cavity portion9a′ is set at the front surface2aof the substrate2and has a first extension in the horizontal plane; the second cavity portion9a′ is set at the back surface2bof the substrate2and has a second extension in the horizontal plane, greater than the first extension.

In a known way, the sensitivity of the acoustic transducer depends on the mechanical characteristics of the membrane3, as well as on the assembly of the membrane3and of the rigid plate4.

Furthermore, the performance of the acoustic transducer depends on the volume of the back chamber and the volume of the front chamber. In particular, the volume of the front chamber determines the upper resonance frequency of the acoustic transducer, and hence its performance at high frequencies. In general, indeed, the smaller the volume of the front chamber, the higher the upper cut-off frequency of the acoustic transducer. Furthermore, a large volume of the back chamber enables improvement of the frequency response and the sensitivity of the same acoustic transducer.

The present Applicant has found that the micromechanical structure1of a known type described previously is subject to some drawbacks, related in particular to manufacturing of the rigid plate4and to its anchorage to the substrate2.

In particular, it is known that in a capacitive microphone the rigid plate (or reference plate) should be as planar as possible (given that it forms the reference plate of the detection capacitor), and moreover as rigid as possible (in order to prevent movements correlated to the acoustic-pressure waves or other undesired movements).

However, in the micromechanical structure1described previously, the rigid plate4may undergo a bending stress, as shown by the curved arrow represented with a solid line inFIG. 1, on account of the conformation of the plate anchorages8, in particular of the aspect ratio, and of the suspended arrangement above the membrane3, as well as on account of the residual stresses in the constituent materials, which determine a force acting in the direction indicated by the dashed arrows.

The factors affecting the aforesaid residual stresses are multiple and are due, for example, to the properties of the materials used, to the techniques of deposition of the same materials, to the conditions (temperature, pressure, etc.) at which deposition is carried out, and to possible subsequent thermal treatments.

In other words, a sort of spring, or elastic element, is formed at the plate anchorages8, i.e., at the vertical pillar portions of the rigid plate4towards the substrate2.

On account of its mechanical deformation, the rigid plate4may have a lower stiffness and moreover may not be perfectly planar or horizontal, thus affecting, even significantly, the performance of the acoustic transducer, for example reducing its sensitivity.

Furthermore, from the standpoint of the manufacturing process of the micromechanical structure1, it is evident that formation of the plate anchorages8, given their conformation, requires a large thickness of a resist layer, during the step of lithographic definition (the so-called “patterning”) of the last layers, or levels, of material.

In particular, this makes control of the manufacturing process problematical and moreover generates markedly vertical geometries, which may be particularly critical for chemical etchings (in particular, dry etches), considerably increasing the time required for execution of the same etchings.

BRIEF SUMMARY

According to one or more embodiments of the present disclosure, a micromechanical detection structure for a MEMS acoustic transducer, and a corresponding manufacturing process are provided.

One embodiment includes a micromechanical structure comprising a semiconductor substrate having a first surface and an anchorage structure including a plurality of pillar elements. The micromechanical structure further comprises a detection capacitor that includes a membrane coupled to said substrate. The membrane is configured to deform in response to acoustic-pressure waves. The detection capacitor further includes a fixed electrode that is rigid with respect to said acoustic-pressure waves and is coupled to said substrate by the anchorage structure. The anchorage structure is configured to support the fixed electrode in a suspended position facing said membrane.

DETAILED DESCRIPTION

As will be described in detail hereinafter, the general idea underlying the present disclosure envisages providing, during the manufacturing process of the micromechanical detection structure of the acoustic transducer, a supporting and anchoring structure for the rigid plate, distinct from the same rigid plate, such as to support the rigid plate and create an equivalent force that will reduce the bending stresses to which it is subjected on account of the residual stresses in the materials, and moreover such as to enable a reduction in the thickness of the resist layers required in the last process steps for definition of the same rigid plate.

The manufacturing process of a micromechanical detection structure for a MEMS acoustic transducer according to one aspect of the present solution is now described, with reference first toFIG. 2a(for reasons of illustration,FIG. 2a, as likewise the subsequent figures, are not drawn to scale; moreover, in what follows, same reference numbers will generally be used to designate elements that are similar to ones previously described with reference toFIG. 1).

In an initial step (FIG. 2a) a substrate12is provided in a wafer13of semiconductor material, in particular silicon, having a thickness between 400 μm and 800 μm, for example 725 μm; substrate12is subjected to a polishing step at the front and at the back, hence at a front surface12aand a back surface12bthereof.

As illustrated inFIG. 2b, a first sacrificial layer14, which is made, for example, of silicon oxide and has a thickness for example of 2.6 μm, is thermally deposited on the wafer13and subjected to a first chemical etching, for example a timed etching, for opening first anti-adhesion recesses15. As will be described hereinafter, the first anti-adhesion recesses15have the function of molds, for formation of anti-adhesion elements (also referred to as “bumps”) on the membrane of the micromechanical structure of the acoustic transducer. The first anti-adhesion recesses15may, for example, have a height in a vertical direction (i.e., orthogonal to a horizontal plane of main extension of the substrate12) of 0.5 μm.

The first sacrificial layer14is subjected to a chemical etching with etch-stop on the silicon, such as to open membrane-anchorage openings17throughout the thickness of the first sacrificial layer14, in desired positions (for example, at the vertices of what will be the square occupied in plan view by the membrane).

According to one aspect of the present solution, the mask used to execute the chemical etching for definition of the first sacrificial layer14is moreover configured for enabling formation, substantially at a same time, of first plate-anchorage openings18, laterally shifted with respect to the membrane-anchorage openings17, on the opposite side with respect to the first anti-adhesion recesses15.

As shown also in the schematic plan view ofFIG. 3, the first plate-anchorage openings18are more than, or equal to, two in number (in the example, five) and are set laterally side-by-side in the horizontal plane, henceforth designated by xy. Furthermore, the first plate-anchorage openings18define in plan view a closed perimeter having a generically polygonal shape, designed to surround entirely the membrane of the micromechanical detection structure of the acoustic transducer.

The position of the membrane anchorages5is moreover shown inFIG. 3, which are set at the corners of the aforesaid closed perimeter, and the area occupied at the center by the membrane3is also represented with a dashed line.

As shown inFIG. 2c, a first conductive layer19is deposited on the wafer13, for example made of optimized polysilicon, having a thickness of between 0.3 μm and 1.5 μm, for example 0.75 μm, which coats the wafer13and fills in particular the anti-adhesion recesses15, the membrane-anchorage openings17, and the first plate-anchorage openings18.

Accordingly, the membrane anchorages5are formed, as well as membrane anti-adhesion elements20(also referred to as “bumps”), designed to define protuberances of the membrane, once again designated by3, in order to prevent adhesion thereof to the underlying substrate12. In particular, membrane anchorages5comprise vertical portions of the first conductive layer19formed within the membrane-anchorage openings17, in direct contact with the front surface12aof the substrate12, and a portion of the first sacrificial layer14between the same vertical portions.

In this process step, a bottom portion is moreover formed of what will become, at the end of the manufacturing process, the anchorage structure of the rigid plate of the acoustic transducer. In particular, this bottom portion comprises vertical portions19′ of the first conductive layer19formed within the plate-anchorage openings18, in direct contact with the front surface12aof the substrate12, and portions14′ of the first sacrificial layer14between vertical portions19′ adjacent to, and overlaid by, horizontal portions19″ of the same first conductive layer19(which are joined to the aforesaid vertical portions19′).

As shown in the sameFIG. 2c, the first conductive layer19is defined (i.e., selectively etched and removed) by means of lithography and chemical etching step, with etch-stop on the first sacrificial layer14in order to define the conformation of the membrane3(which may, for example, be square or generally polygonal in the horizontal plane xy). In particular, the membrane3is contained, in the horizontal plane xy, within the closed perimeter defined by the plate-anchorage structure.

After definition of the first conductive layer19, also the horizontal portions19″ remain, arranged in the area of the plate-anchorage structure, whereas the remaining part of the first conductive layer19is removed.

In this etching step, one or more through openings21are moreover defined throughout the thickness of the membrane3, which are aimed at equalizing the front and back surfaces of the membrane3.

As shown inFIG. 2d, a second sacrificial layer22, made, for example, of silicon oxide with a thickness of approximately 1.1 μm, is deposited on the wafer13.

The second sacrificial layer22is then defined, for example etching with etch-stop on the first conductive layer19, so as to form a plurality of anti-adhesion openings23, which have the function of enabling, during subsequent steps of the manufacturing process, formation of anti-adhesion elements for the rigid plate of the micromechanical structure of the acoustic transducer.

A third sacrificial layer24is then deposited on the wafer13(FIG. 2e), made for example of USG (undoped silicon glass), and having a thickness, for example, of 2 μm; the third sacrificial layer24in particular fills the second anti-adhesion openings23, assuming a surface shape that follows, at least partially, the shape of the underlying second sacrificial layer22, hence having depressions25at the second anti-adhesion openings23.

As shown inFIG. 2fenvisages, according to one aspect of the present solution, definition of the third sacrificial layer24and of the underlying second sacrificial layer22in an area corresponding to what will become the anchorage structure of the rigid plate.

In particular, by means of a chemical etching with etch-stop on the first conductive layer19, second plate-anchorage openings26are formed, which are arranged vertically in a position corresponding to the position previously assumed by the first plate-anchorage openings18, and have a width, in the horizontal plane xy, greater than, or equal to, the width previously assumed by the same first plate-anchorage openings18.

As shown inFIG. 2ga second conductive layer28, for example made of polysilicon with a thickness of 0.9 μm, is deposited on the wafer13.

The second conductive layer28is then selectively removed by chemical etching with etch-stop on the underlying second sacrificial layer24in order to expose the depressions25in the same second sacrificial layer24. The remaining portions of the second conductive layer28, arranged above the membrane3, are designed to form the first plate layer4aof what will be the rigid plate4of the micromechanical detection structure.

Furthermore, the second conductive layer28, in an area corresponding to the anchorage structure of the rigid plate4, fills the second plate-anchorage openings26, coming into contact with the underlying portions of the first conductive layer19.

The top portion of the anchorage structure of the rigid plate4of the acoustic transducer is thus formed, here designated as a whole by30.

This top portion comprises: vertical portions28′ of the second conductive layer28, formed within the second plate-anchorage openings26; the residual portions22′,24′ set on top of one another of the second and third sacrificial layers22,24between adjacent vertical portions28′; and the overlying horizontal portions28″ of the second conductive layer28(which are joined to the aforesaid vertical portions28′). In particular, the vertical portions28′ hence constitute prolongations of the first plate layer4aof the rigid plate4, being integral with the rigid plate4itself at an edge portion thereof. These vertical portions28′ terminate at a distance from the front surface12aof the substrate12, in particular at a distance comparable (i.e., substantially equivalent) to the height in the vertical direction of the same vertical portions28′.

As shown inFIG. 2h, a passivation layer32is deposited on the wafer13, for example by LPCVD (low-pressure chemical vapor deposition), made of insulating material, for example silicon nitride, with a thickness, for example, of 2 μm.

The passivation layer32is hence deposited on the second conductive layer28to form the second plate layer4bof the rigid plate4and fills, in particular, the depressions25, thus forming the bumps6of the rigid plate4, which extend towards the underlying membrane3with anti-adhesion function.

The passivation layer32moreover extends above the anchorage structure30, by which it is supported at the same level (with respect to the horizontal plane xy) as the rigid plate4.

A subsequent etching step of the passivation layer32and of the underlying second conductive layer28, with etch-stop on the third sacrificial layer24, provides formation of the holes7, which traverse the entire thickness of the rigid plate4. As previously mentioned, holes7may be arranged to form a lattice in the horizontal plane xy.

Furthermore, a further etching step (by means of a further lithographic step) for etching the passivation layer32, with etch-stop on the underlying second conductive layer28, enables formation of at least one contact opening36through the passivation layer32, laterally with respect to the underlying anchorage structure30.

As shown in the sameFIG. 2h, at least one contact pad37is formed within the contact opening36, in electrical contact with the second conductive layer28and hence with the rigid plate4of the micromechanical detection structure of the acoustic transducer.

The contact pad37is, for example, made of gold, by means of the cathode-sputtering technique.

As shown inFIG. 2i, a step of machining of the back of the wafer13is carried out.

In particular, by means of successive steps of etching and mechanical polishing (the so-called “grinding” operation), the back of the wafer13(at the back surface12bof the substrate12) is polished and thinned until a thickness of, for example, 400 μm is reached. To protect the front of the wafer13during these steps of polishing and thinning, it may be advantageous to deposit a protective layer (not shown), which is then removed.

By means of successive lithography and etching steps, the first cavity9aof the micromechanical structure10of the acoustic transducer is then formed, in particular, using a double dry etching.

First, a layer of TEOS oxide is grown on the back of the wafer13and is defined to form first mask regions40, by etching with etch-stop on the underlying substrate12.

This is followed by a first deep dry etch of the substrate12, starting from the back surface12b.

The area of the substrate12subjected to etching is defined by the first mask regions40, whereas the depth of the portion of substrate12etched is equal to the depth that it is desired to obtain for the first cavity portion9a′.

As shown inFIG. 2j, the first mask regions40are partially removed to form second mask regions41that define the area of the second cavity portion9a′, with an amplitude greater than the area of the first cavity portion9a′. In particular, a second deep dry etching on the back of the wafer13with etch-stop on the first sacrificial layer14allows removal of the substrate12in an area corresponding to the first cavity9a′, partially exposing the first sacrificial layer14; at the same time, the second cavity portion9a′ is formed. The first cavity portion9a′ may, for example, have in plan view a circular or square shape, and the second cavity portion9a′ a generically square shape.

The second mask portions41are then removed.

As shown inFIG. 2k, the wafer13is subjected to a wet etching, for example with hydrofluoric acid (HF), to remove the first, second, and third sacrificial layers14,22and24, where they are not protected, thus defining the resulting micromechanical structure, designated as a whole by10, of the acoustic transducer, and in particular releasing the membrane3, which is suspended over the first cavity9a, and the rigid plate4, which is separated from the same membrane3by a gap defined by the second cavity9b.

It should be noted that, during the aforesaid etching step, the anchorage structure30operates like a sort of a dyke, which is not involved by the wet etching. In particular, due to the presence of the vertical portions19′,28′ of the first and second conductive layers19,28, the portions of the sacrificial layers inside the anchorage structure30itself are not removed.

In the resulting micromechanical structure10, the rigid plate4is coupled to the substrate12by means of the anchorage structure30, which hence comprises a plurality of pillar elements45, each formed by stacked portions of the first, second, and third sacrificial layers14,22and24and by the horizontal portions19″,28″ of the first and second conductive layers19,28(which together constitute a body portion45aof each pillar element45). Adjacent pillar elements45are separated laterally by the vertical portions19′,28′ of the first and second conductive layers19,28(which, being stacked in a vertical direction, together constitute wall portions45bof the pillar elements45).

In particular, as previously mentioned, the wall portions45bof the pillar elements45have a protection and confinement function with respect to the body portions45aof the same pillar elements45, during the wet etching step leading to removal of the sacrificial regions, where they are not protected.

As is evident from what has been illustrated inFIG. 3, the anchorage structure30surrounds the membrane3entirely, and each pillar element45defines a closed perimeter, of a substantially polygonal shape in plan view, around the same membrane3. In particular, the pillar elements45are set side-by-side in the horizontal plane xy and each defines a perimeter that is, for example, substantially square (but for projections set at the vertices of the square and extending along the diagonals of the square itself).

As shown schematically in the aforesaidFIG. 2k, the presence of the anchorage structure30induces in the rigid plate4of the micromechanical structure10a compensation force, designated by F′ and acting in the horizontal plane xy, which counters the bending force F, which is generated in the rigid plate4itself on account of the residual stresses in the constituent materials. The anchorage structure30hence supports the rigid plate4in the horizontal plane xy, preventing, or in any case reducing, its deformation and keeping the rigid plate itself substantially parallel to the front surface12aof the substrate12and to the horizontal plane xy.

FIG. 2lshows termination of the anchorage structure30, at a scribe line SL of the wafer13, along which the wafer13is subjected to dicing during the dicing steps for formation of dice starting from the same wafer.

In a possible embodiment, a termination pillar element46at the scribe line SL has a lateral extension that is greater (i.e., a greater extension of the corresponding body portion, designated by46a) as compared to that of the pillar elements45of the anchorage structure30, and supports the contact pad37at the top.

In any case, it is clear that the manufacturing steps described may be used for manufacturing a plurality of micromechanical detection structures10for corresponding acoustic transducers on one and the same wafer13.

FIG. 4shows an electronic device100that uses one or more MEMS acoustic transducers101(just one MEMS acoustic transducer101is shown in the figure), each comprising a micromechanical detection structure10and a corresponding electronic circuit102for processing the transduced electrical signals.

The electronic device100comprises, in addition to the MEMS acoustic transducer101, a microprocessor104, a memory block105, connected to the microprocessor104, and an input/output interface106, for example including a keyboard and a display, which is also connected to the microprocessor104. The MEMS acoustic transducer101communicates with the microprocessor104via the electronic circuit102. Furthermore, a speaker108for generating sounds on an audio output (not shown) of the electronic device100may be present.

The electronic device100is preferably a mobile communication device, such as for example a mobile phone, a PDA, a notebook, but also a voice recorder, a reader of audio files with voice-recording capacity, etc. Alternatively, the electronic device100may be a hydrophone, capable of working under water. The electronic device may be a wearable device.

The advantages of the solution described emerge clearly from the foregoing discussion.

It is in any case once again emphasized that the particular anchorage structure30for the rigid plate4of the micromechanical detection structure10affords the dual advantage of: eliminating, or in any case markedly reducing, the bending stresses to which the rigid plate4is subjected, thus improving the electrical characteristics of the corresponding acoustic transducer101; and simplifying the manufacturing process, given the greater “horizontality” of the micromechanical detection structure10, and the absence of vertical elements that have to be coated with thick resist layers.

In particular, tests made by the present Applicant have shown that in the micromechanical structure10of the present solution the rigid plate4is subjected to deformations not greater than 0.2 μm, much less (in particular, by at least one order of magnitude) than the deformations present in traditional solutions, which have values in the region of a few microns.

Furthermore, the deformations in the micromechanical structure10of the present solution are altogether repeatable in different production lots, unlike traditional solutions, where the deformations vary even markedly in different production lots (being linked to the residual stresses in the materials, which are also markedly dependent upon the specific production lot).

Furthermore, the manufacturing process described does not involve additional process steps as compared to known solutions, only using different conformations of the lithography and chemical-etching masks that lead to definition of the various layers and levels of the micromechanical detection structure10.

Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present disclosure.

In particular, it is clear that the number of pillar elements45, of which the anchorage structure30is made, may vary with respect to what has been illustrated, according to the specific requirements of the micromechanical structure10. In this regard, by increasing the number of pillar elements45the resulting mechanical strength may be increased; moreover, the redundancy thus introduced would prove advantageous in the presence of defectiveness that would enable permeation of the wet etching in the internal area of the anchorage.

Furthermore, also the geometrical conformation of the anchorage structure30in the horizontal plane may possibly differ from what has been illustrated. In particular, in a position corresponding to the contact pads37, the anchorage structure30may have a smaller number of pillar elements45, for example just one pillar element45having a larger horizontal extension.