Patent Description:
Specifically, the MEMS device considered comprises two structural layers stacked on a substrate and forming at least one structure that is mobile out of the plane of one of the structural layers (the so-called out-of-plane mobile structure) and an out-of-plane stopper structure in the other structural layer. The mobile structure may, for example, form part of a motion sensor along Z and be equipped with a mobile mass formed in one of the structural layers, which oscillates in a sensing direction perpendicular to the plane of the structural layers and is capacitively coupled to fixed electrodes formed on the substrate.

In particular, in the ensuing description, reference will be made to a MEMS movement sensor and to the problems regarding its manufacture; however, the present invention may be applied in general to other types of MEMS devices.

For instance, the MEMS device may comprise one or more of the following structures, either single or coupled together (combo): accelerometer, gyroscope, geophone, inclinometer, and resonator. Moreover, the MEMS device may constitute a MEMS actuator.

Micromechanical devices of this type find wide use in consumer, automotive, and industrial applications.

As is known, in order to increase the mechanical strength of microelectromechanical devices, in particular of inertial devices, integrated stopper structures are frequently used, which limit the oscillations of the mobile structures. Stopper structures limit the free path of the mobile structures and prevent damage that might derive in the event of impact, for example due to high-speed impact or to overextension of the elastic connections. Of course, stopper structures must be designed in an appropriate way to prevent concentrations of forces and be able to absorb the impact, without undergoing or causing damage. Production of out-of-plane stopper structures may, however, prove not altogether effective. For instance, <FIG> illustrates a known microelectromechanical device <NUM>, in particular an accelerometer Z. The microelectromechanical device <NUM> comprises a substrate <NUM>, an epitaxial layer <NUM>, obtained from which are an external supporting structure <NUM>, anchorages <NUM>, a mobile mass <NUM>, and elastic connections <NUM> configured to enable oscillations of the mobile mass <NUM> in a sensing direction Z perpendicular to the substrate <NUM> and to a plane of the epitaxial layer <NUM>.

A cap <NUM> is joined to the supporting structure <NUM> by an adhesion layer <NUM>, normally a glass-frit layer. The cap <NUM> is shaped so as to form out-of-plane stopper structures 10a, which limit the oscillations of the mobile mass <NUM> in the sensing direction.

The solution described is not, however, without limitations, which depend mainly upon the thickness of the adhesion layer <NUM>. Given that the thickness of the adhesion layer <NUM> cannot be reduced beyond a certain limit, in fact, also the width of the gap between the mobile mass and the stopper structures 10a may not always have the desired value. The effectiveness of the out-of-plane stopper structures 10a may therefore not be optimal. On the other hand, the useful oscillation of the mobile mass <NUM> is much smaller than the width W, and consequently there are no advantages in terms of signal if this width is oversized. Furthermore, the cap <NUM> that incorporates the out-of-plane stopper structures 10a presents a considerable stiffness, and the capacity for dissipating energy may not be sufficient to prevent damage to the mobile mass <NUM>.

<CIT> discloses a microelectromechanical device comprising a substrate of semiconductor material, and at least one structural layer of semiconductor material on the substrate. A sensing mass extends in the at least one structural layer and is coupled to the substrate by first elastic connections, which are configured to enable the sensing mass to oscillate in a sensing direction, with a maximum elongation with respect to a resting position. An in-plane stopper structure comprises a stopper anchorage fixed to the substrate and a mechanical end-of-travel structure. The mechanical end-of-travel structure extends in the at least one structural layer, faces the sensing mass, and is separated from the sensing mass by a gap having a width smaller than the maximum elongation.

Other examples of known microelectromechanical devices are disclosed in <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

It is an aim of the present invention to provide a microelectromechanical device and a process for manufacturing a microelectromechanical device that enable the limitations described to be overcome or at least reduced.

According to the present invention, a microelectromechanical device and a process for manufacturing a microelectromechanical device are provided, as defined in claims <NUM> and <NUM>, respectively.

For a better understanding of the invention, some embodiments thereof will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein:.

With reference to <FIG>, a microelectromechanical device according to an embodiment of the present invention is illustrated schematically and designated by the number <NUM>. In the example illustrated, in particular, the microelectromechanical device <NUM> is a Z-axis accelerometer. It is understood, however, that the invention is not limited to this type of devices and applies to any microelectromechanical device with out-of-the-plane movements, including, in particular, sensors of a differential type for out-of-plane movement, gyroscopes with movements of pitch and roll, and multiaxial gyroscopes and accelerometers.

The microelectromechanical device <NUM> comprises a substrate <NUM>, coated with an insulating layer <NUM>, a first structural layer <NUM>, a second structural layer <NUM>, and a cap <NUM>, joined to the second structural layer <NUM> through an adhesion layer <NUM>, for example a glass-frit layer.

The first structural layer <NUM> and the second structural layer <NUM> are made of semiconductor material, for example, respectively, a first epitaxial layer and a second epitaxial layer grown in succession on the substrate <NUM> and on the insulating layer <NUM>, as explained in greater detail hereinafter. Consequently, the first structural layer <NUM> is interposed between the substrate <NUM> and the second structural layer <NUM>.

Components of the microelectromechanical device <NUM> are obtained from the first structural layer <NUM> and/or from the second structural layer <NUM>.

In particular, an external supporting frame <NUM> extends along the perimeter of the microelectromechanical device <NUM> and comprises respective portions <NUM>', <NUM>" of the first structural layer <NUM> and of the second structural layer <NUM>. The cap <NUM> is joined to the supporting frame <NUM> by the adhesion layer <NUM>.

A sensing mass <NUM> extends in the first structural layer <NUM> and is supported and connected to the substrate <NUM> by anchorages <NUM> and first elastic connections <NUM>. Also the anchorages <NUM> and the first elastic connections <NUM> are formed by the first structural layer <NUM>. The first elastic connections <NUM> are configured to enable the sensing mass <NUM> to oscillate in a sensing direction Z perpendicular to the substrate <NUM> and to a plane of the first structural layer <NUM> and of the second structural layer <NUM>, with a maximum elongation with respect to a resting position (in the absence of rotations and/or external forces applied). The elastic connections <NUM> are for simplicity represented in <FIG> as linear springs in the direction Z, but may be formed as flexible beam elements from portions of the first structural layer <NUM>, as shown purely by way of example in <FIG>.

A sensing electrode <NUM> arranged on the insulating layer <NUM> faces, and is capacitively coupled to, the sensing mass <NUM>.

An out-of-plane stopper structure <NUM> extends both in the first structural layer <NUM> and in the second structural layer <NUM> and comprises an anchorage <NUM> fixed to the substrate <NUM> and a mechanical end-of-travel structure <NUM>. More precisely, the anchorage <NUM> is adjacent to the anchorage <NUM> of the sensing mass <NUM> and in turn comprises portions <NUM>', <NUM>" of the first structural layer <NUM> and of the second structural layer <NUM>, respectively, which are set on top of one another.

The mechanical end-of-travel structure <NUM> is defined by a plate that extends in the second structural layer <NUM>, at least in part faces the sensing mass <NUM>, and is separated from the sensing mass <NUM> by a gap <NUM> having a width W smaller than the maximum elongation of the sensing mass <NUM> from the resting position. The mechanical end-of-travel structure <NUM> is coupled to the anchorage <NUM> by second elastic connections <NUM> configured to enable displacements of the mechanical end-of-travel structure <NUM> in the sensing direction Z in response to an impact of the sensing mass <NUM>. As shown in greater detail in <FIG>, in one embodiment, the second elastic connections <NUM> comprise an elastic twisting element <NUM>, which extends along an axis A parallel to the substrate <NUM> and perpendicular to the sensing direction Z. The elastic twisting element <NUM> has ends connected to the anchorage <NUM>, in particular to the portion <NUM>" that extends in the second structural layer <NUM>, and a central portion connected to the mechanical end-of-travel structure <NUM>. In this way, in the event of impact of the sensing mass <NUM> against the mechanical end-of-travel structure <NUM>, the mechanical end-of-travel structure <NUM> rotates about the axis A. Rotation about the axis A, which is an off-centre axis and not centroidal in one embodiment, results in a displacement with respect to the sensing axis Z (variation of position in height) of the portion of the mechanical end-of-travel structure <NUM> that faces the sensing mass <NUM>. The energy of the impact is consequently in part converted into energy of strain of the second elastic connections <NUM> and/or kinetic energy of the mechanical end-of-travel structure <NUM>, thus reducing the risk of microcracks in the sensing mass <NUM> and in the mechanical end-of-travel structure <NUM> itself.

The microelectromechanical device <NUM> further comprises a voltage-balancing structure <NUM>, configured to prevent voltage differences between the sensing mass <NUM> and the mechanical end-of-travel structure <NUM>. In particular, the voltage-balancing structure <NUM> comprises a conductive line that contacts both the sensing mass <NUM> and the mechanical end-of-travel structure <NUM>. More precisely, the voltage-balancing structure <NUM> extends on the insulating layer <NUM>. The anchorages <NUM> of the sensing mass <NUM> and the anchorage <NUM> of the stopper structure <NUM> are formed directly on the voltage-balancing structure <NUM>. Equalization of the voltage between the sensing mass <NUM> and the mechanical end-of-travel structure <NUM> is obtained by the fact that the anchorages <NUM> and the first elastic connections <NUM> on the one hand and the anchorage <NUM> and the second elastic connections <NUM> on the other are of doped semiconductor material and form continuous conductive structures. <FIG> shows schematically the voltage-balancing structure <NUM> and the connections to the sensing mass <NUM> and to the mechanical end-of-travel structure <NUM>. Moreover, illustrated in <FIG> are a biasing circuit <NUM>, which applies a voltage operative desired to the sensing mass <NUM>, and a sensing circuit <NUM>, which reads the capacity present between the sensing mass <NUM> and the sensing electrode <NUM> according to of the position of the sensing mass <NUM>. Thanks to the voltage-balancing structure <NUM>, the contact between the sensing mass <NUM> and the mechanical end-of-travel structure <NUM> does not cause any potentially harmful transfer of charge.

<FIG>, where parts that are the same as parts already illustrated are designated by the same reference numbers, show a microelectromechanical device <NUM> according to a different embodiment of the invention. The microelectromechanical device <NUM> comprises the substrate <NUM>, the insulating layer <NUM>, a first structural layer, and a second structural layer, here designated by <NUM> and <NUM> and defined, respectively, by a first epitaxial layer and a second epitaxial layer grown in succession on the substrate <NUM>.

Extending in the first structural layer <NUM> is the sensing mass <NUM> with the anchorages <NUM>. As already described, the sensing mass <NUM> and the anchorages <NUM> are obtained from the first structural layer <NUM>.

An out-of-plane stopper structure <NUM> extends in the second structural layer <NUM> and comprises an anchorage <NUM> and a mechanical end-of-travel structure <NUM>. More precisely, the anchorage <NUM> extends in the second structural layer <NUM> and is fixed to one of the anchorages <NUM> of the sensing mass <NUM>. A same anchorage is thus obtained in part from the first structural layer <NUM> and in part from the second structural layer <NUM> and has the function of supporting both the sensing mass <NUM> and the mechanical end-of-travel structure <NUM>.

The mechanical end-of-travel structure <NUM> extends in the second structural layer <NUM>, at least in part faces the sensing mass <NUM>, and is separated from the sensing mass <NUM> by a gap <NUM> having a width W' smaller than the maximum elongation of the sensing mass <NUM> from the resting position. The mechanical end-of-travel structure <NUM> is coupled to the anchorage <NUM> by second elastic connections <NUM> configured to enable the mechanical end-of-travel structure <NUM> to translate in the sensing direction Z in response to an impact of the sensing mass <NUM>. As shown in greater detail in <FIG>, in a non-limiting embodiment, the second elastic connections <NUM> comprise flexible beam elements obtained from portions of the second structural layer <NUM>, as shown purely by way of example in <FIG>. In this way, in the event of impact of the sensing mass <NUM> against the mechanical end-of-travel structure <NUM>, the mechanical end-of-travel structure <NUM> translates in the sensing direction Z, absorbing part of the energy of the impact and reducing the risk of microcracks in the sensing mass <NUM> and in the mechanical end-of-travel structure <NUM> itself.

The microelectromechanical device <NUM> further comprises the external supporting frame <NUM> and the cap <NUM>, joined to the supporting frame <NUM> through the adhesion layer <NUM>.

The microelectromechanical device <NUM> of <FIG> may be obtained using the process described in what follows with reference to <FIG>. As already mentioned, in practice, the sensing mass <NUM>, the anchorages <NUM> of the sensing mass, and the out-of-plane stopper structure <NUM> are obtained from two structural layers grown epitaxially on one another, as described in detail hereinafter.

With reference to <FIG>, a wafer <NUM> of semiconductor material, for example monocrystalline silicon, initially comprises the substrate <NUM>, grown on which is the insulating layer <NUM>, for example of silicon oxide. A conductive layer, not shown entirely, for example of polycrystalline silicon, is deposited on the insulating layer <NUM> and patterned to provide the sensing electrode <NUM> and the voltage-balancing structure <NUM>. A first sacrificial layer <NUM>, for example, of silicon oxide grown thermally or deposited, is formed on the first dielectric layer <NUM> and incorporates the sensing electrode <NUM> and the voltage-balancing structure <NUM>. The first sacrificial layer <NUM> is selectively etched in positions corresponding to the anchorages <NUM> of the sensing mass and to the anchorage <NUM> of the mechanical end-of-travel structure <NUM>, which will then be formed. The sensing electrode <NUM> and the voltage-balancing structure <NUM> remain exposed where the first sacrificial layer <NUM> has been removed.

Then (<FIG>), the first structural layer <NUM> is grown epitaxially on the first sacrificial layer <NUM> from a deposited seed layer <NUM>' and contacts the sensing electrode <NUM> and the voltage-balancing structure <NUM> through the openings in the first sacrificial layer <NUM>. The first structural layer <NUM> has a thickness that is determined on the basis of the characteristics of the desired microelectromechanical structures and may be comprised, for example, between <NUM> and <NUM>. After structural growth, the first structural layer <NUM> is planarized and brought to the desired final thickness, for example by CMP (Chemical Mechanical Polishing).

The first structural layer <NUM> (<FIG>) is etched to define bottom portions of the desired structures and other regions envisaged. In particular, formed in this step from the first structural layer <NUM> are the anchorages <NUM>, the sensing mass <NUM>, for first elastic connections <NUM>, bottom portions <NUM>' of the anchorages <NUM>, and bottom portions <NUM>' of the supporting frame <NUM>. For this purpose, the wafer <NUM> is coated with a resist mask not shown (first trench mask) and subject to a dry etch, to form trenches <NUM>, which traverse the first structural layer <NUM> completely. The etch stops automatically on the first sacrificial layer <NUM>, which is then removed to release the sensing mass <NUM> and the first elastic connections <NUM>.

Then (<FIG>), a second sacrificial layer <NUM> is deposited, for example of TEOS (TetraEthyl OrthoSilicate) for a thickness equal to the desired width W of the gap <NUM> between the sensing mass <NUM> and the mechanical end-of-travel structure <NUM>. The second sacrificial layer <NUM> partially fills the trenches <NUM>, for example for a third of their depth even though this filling, as well as the degree and depth of filling, are not important. The second sacrificial layer <NUM> is then planarized.

The second sacrificial layer <NUM> is etched and removed selectively, using a masking layer not shown (second anchorage mask) to form anchorage openings <NUM>, as illustrated in <FIG>. Etching of the second sacrificial layer <NUM> terminates automatically on the first structural layer <NUM>. In general, the anchorage openings <NUM> are provided in the areas where it is desired to form connection regions between the first structural layer <NUM> and the second structural layer <NUM> that will be subsequently formed. In particular, here, the anchorage openings <NUM> are formed in positions corresponding to the supporting frame <NUM> and to the anchorage <NUM> of the mechanical end-of-travel structure <NUM>.

Next (<FIG>), the second structural layer <NUM> is grown, also in this case epitaxially, for a thickness comprised, for example, between <NUM> and <NUM>. The thickness of the second structural layer <NUM> is linked to the desired microelectromechanical structures, in particular the structure of the mechanical end-of-travel plate <NUM>. In general, the second structural layer <NUM> may be thinner than the first structural layer <NUM>, even though the opposite may be the case, and the invention is not limited to any particular ratio between the thicknesses of the epitaxial layers <NUM>, <NUM>. The thickness of the second structural layer <NUM> is greater where the second structural layer <NUM> itself is joined to the first structural layer <NUM>. Elsewhere, the difference is represented by the thickness of the second sacrificial layer <NUM> and corresponds to the width of the gap <NUM> to be created between the sensing mass <NUM> at rest and the mechanical end-of-travel structure <NUM>.

After epitaxial growth, the second structural layer <NUM> is planarized and brought to the desired final thickness, for example by CMP (Chemical Mechanical Polishing).

Next, the second structural layer <NUM> is etched, as shown in <FIG>. For this purpose, the wafer <NUM> is coated with a resist mask (not illustrated) and subjected to a dry etch. In this step, the portions of the second structural layer <NUM> not covered by the resist mask are removed for the entire thickness, and the etch stops on the second sacrificial layer <NUM>.

In particular, defined in this step are the sensing mass <NUM>, the top portion <NUM>" of the anchorage <NUM>, the second elastic connections <NUM>, and the top portion <NUM>" of the supporting frame <NUM>.

Then, the residual portions of the second sacrificial layer <NUM> are removed, thus releasing the mechanical end-of-travel structure <NUM>.

Finally, a cap wafer is bonded to the wafer <NUM> by the adhesion layer <NUM>, and the composite wafer thus obtained is diced to form the microelectromechanical device <NUM> of <FIG>.

Advantageously, the thickness of the second sacrificial layer <NUM> may be selected in a flexible way on the basis of the design preferences and may be controlled with high precision. Consequently, also the width W of the gap <NUM> can be selected and defined precisely according to the design preferences.

<FIG> illustrate a variant of the process described and a microelectromechanical device thus obtained.

In a semiconductor wafer <NUM>, initially the processing steps already described with reference to <FIG> are carried out up to formation of anchorage openings, here designated by <NUM>, in the second sacrificial layer <NUM>. Prior to the second epitaxial growth, a further mask (not shown) is used for providing recesses <NUM> on the sensing mass <NUM>, for example by a time-controlled etch.

When the second structural layer is grown, here designated by <NUM>, formed within the recesses <NUM> are protuberances <NUM> that project from the second structural layer <NUM> towards the sensing mass <NUM>, at a distance W" determined by the depth of the recesses <NUM>. The process proceeds as already described with etching of the second structural layer <NUM> to define the mechanical end-of-travel structure (connected to which are the teeth <NUM>) and the second elastic connections, removal of the second sacrificial layer <NUM>, bonding of the cap <NUM>, and cutting of the wafer <NUM> into dice, each of which contains a microelectromechanical device <NUM>. Each microelectromechanical device <NUM> comprises an out-of-plane stopper structure <NUM>, where the mechanical end-of-travel structure <NUM> is provided with teeth <NUM>.

According to another embodiment of the invention (illustrated in <FIG>), in a microelectromechanical device <NUM> the supporting frame <NUM> for the cap <NUM> defines anchorages <NUM> for the sensing mass <NUM> and anchorages <NUM> for the mechanical end-of-travel structure <NUM> of an out-of-plane stopper structure <NUM>. More precisely, the microelectromechanical device <NUM> comprises a first structural layer <NUM> and a second structural layer <NUM> defined, respectively, by a first epitaxial layer grown on a substrate <NUM> and a second epitaxial layer grown on the first epitaxial layer. Defined in the first structural layer <NUM> are the sensing mass <NUM>, first elastic connections <NUM>, and the anchorages <NUM>, which also form bottom portions of the supporting frame <NUM>. Defined in the second structural layer <NUM> are the mechanical end-of-travel structure <NUM>, second elastic connections <NUM>, and the anchorages <NUM>, which also form top portions of the supporting frame <NUM>. The anchorages <NUM> are joined without any discontinuity to the anchorages <NUM> and laid thereon.

<FIG> shows an electronic system <NUM> that may be of any type, in particular, but not exclusively, a wearable device, such as a watch, a bracelet or a smartband; a computer, such as a mainframe, a personal computer, a laptop or a tablet; a smartphone; a digital music player, a digital camera, or any other device designed to process, store, transmit, or receive information. The electronic system <NUM> may be a general-purpose processing system or embedded in a device, an apparatus, or some other system.

The electronic system <NUM> comprises a processing unit <NUM>, memory devices <NUM>, a microelectromechanical gyroscope according to the invention, for example the microelectromechanical gyroscope <NUM> of <FIG>, and may moreover be provided with input/output (I/O) devices <NUM> (for example, a keyboard, a pointing device, or a touchscreen), a wireless interface <NUM>, peripherals <NUM>,. N, and possibly further auxiliary devices, here not shown. The components of the electronic system <NUM> may be coupled together in communication directly and/or indirectly through a bus <NUM>. The electronic system <NUM> may further comprise a battery <NUM>. It should be noted that the scope of the present invention is not limited to embodiments necessarily having one or all of the devices listed.

The processing unit <NUM> may comprise, for example, one or more microprocessors, microcontrollers, and the like, according to the design preferences.

The memory devices <NUM> may comprise volatile memory devices and non-volatile memory devices of various kinds, for example SRAMs, and/or DRAMs for volatile memories, and solid-state memories, magnetic disks and/or optical disks for non-volatile memories.

Finally, it is evident that modifications and variations may be made to the microelectromechanical device and to the process described, without thereby departing from the scope of the present invention, as defined in the annexed claims.

In the first place, the microelectromechanical device is not limited to a particular type of sensor, transducer, or actuator, but may be any microelectromechanical device that can be integrated in a semiconductor body.

Moreover, further microstructures may be provided in the first and second structural layers for further sensors, transducers, and actuators, in addition to what has been described.

Claim 1:
A microelectromechanical device comprising:
a substrate (<NUM>; <NUM>) of semiconductor material;
a first structural layer (<NUM>; <NUM>; <NUM>) of semiconductor material on the substrate (<NUM>; <NUM>);
a second structural layer (<NUM>; <NUM>; <NUM>; <NUM>) of semiconductor material on the first structural layer (<NUM>; <NUM>; <NUM>):
a sensing mass (<NUM>; <NUM>), coupled to the substrate (<NUM>; <NUM>) by first elastic connections (<NUM>; <NUM>), which are configured to enable the sensing mass (<NUM>; <NUM>) to oscillate in a sensing direction (Z) perpendicular to the substrate (<NUM>; <NUM>), with a maximum elongation with respect to a resting position;
an out-of-plane stopper structure (<NUM>; <NUM>; <NUM>; <NUM>) comprising a stopper anchorage (<NUM>; <NUM>; <NUM>) fixed to the substrate (<NUM>; <NUM>) and a mechanical end-of-travel structure (<NUM>; <NUM>; <NUM>; <NUM>); and
a cap (<NUM>), joined to the second structural layer (<NUM>) through an adhesion layer (<NUM>);
wherein the mechanical end-of-travel structure (<NUM>; <NUM>; <NUM>; <NUM>) faces the sensing mass (<NUM>; <NUM>), and is separated from the sensing mass (<NUM>; <NUM>) by a gap (<NUM>; <NUM>) having a width (W; W'; W") smaller than the maximum elongation;
characterized in that the sensing mass (<NUM>; <NUM>)extends in the first structural layer (<NUM>; <NUM>; <NUM>) and the mechanical end-of-travel structure (<NUM>; <NUM>; <NUM>; <NUM>) extends in the second structural layer (<NUM>; <NUM>; <NUM>; <NUM>) and is coupled to the stopper anchorage (<NUM>; <NUM>; <NUM>) by second elastic connections (<NUM>; <NUM>; <NUM>) configured to enable displacements of the mechanical end-of-travel structure (<NUM>; <NUM>; <NUM>; <NUM>) with respect to the sensing direction (Z) in response to an impact of the sensing mass (<NUM>; <NUM>).