Patent Description:
As is known, numerous pressure transducers are currently available, such as, for example, the so-called piezoelectric micromachined ultrasonic transducers (PMUT), which are devices of the category of micro-electromechanical systems (MEMS) which allow a pressure signal, such as, for example, an acoustic signal, to be transduced into an electrical signal, and vice versa. Furthermore, it is known that PMUT transducers typically include actuation piezoelectric structures, which in turn include regions of piezoelectric material; typically, this piezoelectric material is the so-called PZT.

As regards PZT, it is known that, assuming a PZT region having the shape of a parallelepiped and an orthogonal reference system XYZ, the value of the so-called parameter d<NUM> correlates the extent of the shortening along X of the PZT region with the extent of the electric field along Z. The value of the parameter d<NUM> of PZT is about ten times higher than the values of other possible piezoelectric materials, such as, for example, aluminum nitride AlN. For this reason, PZT is particularly suitable for the case in which the PMUT transducer mainly functions as a transducer of an electrical signal into an acoustic signal, that is, it functions as an acoustic source. However, PZT is also characterized by a particularly high value of the element ε<NUM> of the electrical permittivity tensor; this means that each actuation structure formed by PZT has a high electrical capacitance. Consequently, when the PMUT transducer is used in reception, that is to transduce an acoustic signal into an electrical signal, it occurs that, with the same mechanical stress induced by the acoustic signal, the actuation structure generates a lower voltage than what could occur in case of use of a piezoelectric material having a lower value of the element ε<NUM>.

In practice, PMUT transducers including actuation structures formed by PZT are not very sensitive when used, for example, as pressure sensors. More generally, regardless of the type of piezoelectric material, the need is felt to have pressure transducers that have good sensitivity in reception, without compromising the effectiveness during the transmission step.

The aim of the present invention is therefore to provide a solution which allows this need to be satisfied at least in part. Prior art documents relevant to the invention are patent documents <CIT>, <CIT>, and <CIT>.

According to the present invention, a pressure transducer and a manufacturing process are provided, as defined in the attached claims.

For a better understanding of the present invention, embodiments thereof are now described, purely by way of nonlimiting example, with reference to the attached drawings, wherein:.

<FIG> shows an array <NUM> of transducer devices <NUM>, which are equal to each other, are formed in an integrated manner in a same semiconductor body <NUM> (formed for example by silicon) of a die <NUM> and are arranged according to one matrix scheme. <FIG> also shows an orthogonal reference system XYZ.

In particular, the semiconductor body <NUM> forms a fixed body <NUM>, which is delimited at top and at bottom by, respectively, a top surface St and a bottom surface Sb (visible in <FIG>), which are parallel to the XY plane, and is shared between the transducer devices <NUM>.

In addition, for each transducer device <NUM>, the fixed body <NUM> laterally delimits a corresponding cavity <NUM>, which is referred to as the main cavity <NUM> hereinafter. The main cavity <NUM>, visible in <FIG>, is open downwardly and faces the bottom surface Sb; furthermore, without any loss of generality, the main cavity <NUM> is delimited by a side wall <NUM>, formed by the fixed body <NUM> and having a (for example) cylindrical shape. In addition, each transducer device <NUM> comprises a corresponding transduction structure <NUM>, which is suspended above the corresponding main cavity <NUM>, as described in greater detail below. In this regard, since the transducer devices <NUM> are equal to each other, only one of them is described hereinafter, shown for example in <FIG> and <FIG>.

In detail, the transduction structure <NUM> comprises a movable region <NUM>, formed by semiconductor material (for example, silicon), and three deformable structures, indicated with <NUM>.

The deformable structures <NUM> are equal to each other and are arranged symmetrically with respect to an axis of symmetry H parallel to the Z axis. In addition, the deformable structures <NUM> have elongated shapes along respective elongation directions, which are arranged in such a way that pairs of adjacent elongation directions are angularly spaced by <NUM>°. In other words, and without any loss of generality, the elongation directions are radial directions.

Each deformable structure <NUM> comprises a respective outer piezoelectric structure <NUM> and a respective inner piezoelectric structure <NUM>, which are piezoelectric transduction structures, and a respective support structure <NUM>. In rest conditions, the support structures <NUM> lie in a same plane parallel to the XY plane. Hereinafter, for the sake of brevity, only one deformable structure <NUM> is described.

In detail, the support structure <NUM> comprises a top beam <NUM> and a bottom beam <NUM>, which, without any loss of generality, in rest conditions are equal to each other and have the shape of parallelepipeds with longitudinal axes parallel to the elongation direction of the deformable structure <NUM>.

The top beam <NUM> is vertically (i.e., parallel to the Z axis) superimposed, at a distance, on the bottom beam <NUM>. Furthermore, both the top beam <NUM> and the bottom beam <NUM> have a respective first end, which is integral with the fixed body <NUM>, and in particular with the side wall <NUM>, and a respective second end, which is integral with the movable region <NUM>. In rest conditions, the top beams <NUM> lie in a respective plane parallel to the XY plane; the bottom beams <NUM> lie in a respective plane parallel to the XY plane.

Furthermore, the top beam <NUM> and the bottom beam <NUM> delimit, respectively at top and at bottom, a cavity <NUM>, which is referred to as the secondary cavity <NUM> hereinafter. In rest conditions, the secondary cavity <NUM> has approximately the shape of a parallelepiped and, as explained below, is laterally open.

With regard to the movable region <NUM>, it has a planar shape and is delimited at top by a first surface S<NUM>, which in rest conditions is coplanar with the top surface St of the fixed body <NUM>. Furthermore, the movable region <NUM> is delimited at bottom by a second surface S<NUM>, which is parallel to the XY plane and faces the underlying main cavity <NUM>.

In rest conditions the movable region <NUM> and the fixed body <NUM> laterally delimit three trench cavities <NUM>, which, without any loss of generality, are equal to each other and extend vertically through the entire thickness of the movable region <NUM>, so that they face the first and the second surfaces S<NUM>, S<NUM>. The trench cavities <NUM> are therefore open downwardly and upwardly; in particular, in rest conditions, the trench cavities <NUM> face downwardly corresponding portions of the main cavity <NUM>.

In greater detail, considering any trench cavity <NUM>, it comprises a respective peripheral portion 30A and a first and a second linear portion 30B, 30C, communicating with each other. The peripheral portion 30A has approximately the shape of a circumference portion, while the first and the second linear portions 30B, 30C have the shape of segments, are parallel to respective radial directions and extend from corresponding ends of the peripheral portion 30A; purely by way of example, the first linear portion 30B is arranged counterclockwise with respect to the second linear portion 30C. In practice, the peripheral portion 30A and the first and the second linear portions 30B, 30C laterally delimit a part of the movable region <NUM> which has approximately the shape of a corresponding circular sector, in top view. Furthermore, considering any pair formed by a first and a second trench cavity <NUM>, the second trench cavity <NUM> being arranged, for example, counterclockwise with respect to the first trench cavity <NUM>, it occurs that the first linear portion 30B of the first trench cavity <NUM> extends between the movable region <NUM> and a first side of a corresponding support structure <NUM>, while the second linear portion 30C of the second trench cavity <NUM> extends between the movable region <NUM> and a second side of this corresponding support structure <NUM>; furthermore, the aforementioned first and second linear portions 30B, 30C are parallel to the elongation direction of the deformable structure <NUM> whereto the aforementioned support structure <NUM> belongs. In addition, the top beam <NUM> and the bottom beam <NUM> of the support structure <NUM> laterally delimit the aforementioned first and second linear portions 30B, 30C, which extend on opposite sides of the support structure <NUM> and communicate laterally with the corresponding structure cavity <NUM>, which, as previously mentioned, is laterally open on both sides.

Again with reference to any deformable structure <NUM>, the respective outer piezoelectric structure <NUM> extends in part above the first end of the top beam <NUM> of the underlying support structure <NUM> and in part above the portion of fixed body <NUM> integral with the first end of this top beam <NUM>. The respective inner piezoelectric structure <NUM> is laterally offset, along the elongation direction of the deformable structure <NUM>, with respect to the outer piezoelectric structure <NUM>; furthermore, the inner piezoelectric structure <NUM> extends in part above the second end of the top beam <NUM> of the underlying support structure <NUM> and in part above the portion of the movable region <NUM> integral with the second end of this top beam <NUM>.

Without any loss of generality, the inner piezoelectric structure <NUM> and the outer piezoelectric structure <NUM> may be equal to each other. Furthermore, as shown in <FIG> with reference to the inner piezoelectric structure <NUM> (but the same considerations also apply to the outer piezoelectric structure <NUM>), the inner piezoelectric structure <NUM> may comprise a stack of regions, which includes: a dielectric region <NUM> of planar shape, formed for example by silicon oxide and arranged on the first surface S<NUM>; a bottom electrode region <NUM> of planar shape, formed for example by platinum and arranged on the dielectric region <NUM>, in direct contact; a piezoelectric region <NUM> of planar shape, formed for example by PZT and arranged on the bottom electrode region <NUM>, wherewith it is in direct contact; a top electrode region <NUM> of planar shape, formed for example by platinum (or for example by TiW or IrO<NUM>) and arranged on the piezoelectric region <NUM>, wherewith it is in direct contact; and a passivation region <NUM> of planar shape, formed for example by silicon nitride (SiN) and arranged on the top electrode region <NUM>, wherewith it is in direct contact.

In a per se known manner, the bottom electrode region <NUM> and the top electrode region <NUM> may be put into electrical contact (for example, through corresponding pads) with an outer circuitry (not shown and formed, for example, by a semiconductive die other than the semiconductive die <NUM>) for applying a voltage between the bottom electrode region <NUM> and the top electrode region <NUM>, so as to control the transducer device <NUM> in transmission, and/or to receive and process the voltage that is established between the bottom electrode region <NUM> and the top electrode region <NUM> in the presence of deformations of the deformable structure <NUM> caused by an acoustic signal, in case the transducer device <NUM> is controlled in reception, as explained below.

In greater detail, the piezoelectric region <NUM> has a thickness for example lower than <NUM>, that is it forms a so-called PZT thin film. Furthermore, in a per se known manner, when subject to voltage (more precisely, to an electric field), the piezoelectric region <NUM> shortens along the elongation direction of the corresponding deformable structure <NUM>, with respect to the situation wherein it is not subject to voltage; this shortening induces a mechanical tension and a corresponding deformation of the inner piezoelectric structure <NUM>, and therefore also of the corresponding deformable structure <NUM>, as described hereinafter; the same considerations apply to the case in which the piezoelectric region belongs to an outer piezoelectric structure <NUM>.

In detail, when a voltage is applied to the inner piezoelectric structures <NUM>, that is when a voltage is applied between the bottom electrode regions <NUM> and the top electrode regions <NUM> of the inner piezoelectric structures <NUM>, and assuming that no voltage is applied to the outer piezoelectric structures <NUM>, the deformation of each inner piezoelectric structure <NUM> causes the top beam <NUM> and the bottom beam <NUM> of the underlying support structure <NUM> to bend downwards, in such a way that the second ends of the top beam <NUM> and of the bottom beam <NUM> lower, parallel to the Z axis, with respect to what occurs in rest conditions.

The lowering of the second ends of the top beams <NUM> and of the bottom beams <NUM> of the support structures <NUM> of the deformable structures <NUM> causes a downward translation, along the axis of symmetry H, of the movable region <NUM>, as shown in <FIG>, wherein for the sake of simplicity the secondary cavities <NUM> are not shown.

Similarly, when a voltage is applied to the outer piezoelectric structures <NUM>, and assuming that no voltage is applied to the inner piezoelectric structures <NUM>, the deformation of each outer piezoelectric structure <NUM> causes the top beam <NUM> and the bottom beam <NUM> of the underlying support structure <NUM> to bend upwards, in such a way that the second ends of the top beam <NUM> and of the bottom beam <NUM> rise, parallel to the Z axis, with respect to what occurs in rest conditions.

The raising of the second ends of the top beams <NUM> and of the bottom beams <NUM> of the support structures <NUM> of the deformable structures <NUM> causes a corresponding upward translation, along the axis of symmetry H, of the movable region <NUM>, as shown in <FIG>.

Both in case of upward translation and in case of downward translation, as a first approximation the movable region <NUM> is not subject to any rotation.

In greater detail, both in the case shown in <FIG> and in the case shown in <FIG>, the movable region <NUM> is subject to a translation parallel to the Z axis with respect to the rest conditions. In other words, the movable region <NUM> moves like a piston, as schematized in <FIG>, wherein the top part refers to the rest conditions, while the bottom part refers to the case of upward translation of the movable region <NUM> (for the sake of simplicity, the diagram of <FIG> shows only two support structures <NUM>). This is due to the fact that each support structure <NUM> comprises a pair of beams; in this manner, the movable region <NUM> is constrained to translate parallel to the Z axis, without being subject to rotations or deformations.

Operationally, it is therefore possible to control the outer piezoelectric structures <NUM> and the inner piezoelectric structures <NUM> so as to oscillate the movable region <NUM> around the rest position, so that this oscillation generates an acoustic signal. For example, it is possible to apply, to the outer piezoelectric structures <NUM> and to the inner piezoelectric structures <NUM>, a first and, respectively, a second series of unipolar voltage pulses, each pulse having, for example, a sinusoidal shape, the first and the second series of pulses having a period T and being temporally shifted by T/<NUM>, as shown in <FIG>.

The piston movement of the movable region <NUM> also occurs when the transducer device <NUM> is used in reception, that is when the transducer device <NUM> is used to transduce a pressure signal (for example, an acoustic signal) which impinges on the transducer device <NUM> into corresponding electrical signals, which are generated by the outer piezoelectric structures <NUM> and by the inner piezoelectric structures <NUM> following the deformations of the respective support structures <NUM> (in particular, of the respective top beams <NUM>) induced by the translation of the movable region <NUM>, this translation being precisely caused by the acoustic signal.

More precisely, owing to the constraint mechanism described, the acoustic signal causes, regardless of the direction of origin, an oscillation of the movable region <NUM> along the axis of symmetry H, around the position assumed in rest conditions. The piston translation mechanism of the movable region <NUM> allows the dimensions of the piezoelectric regions <NUM>, and therefore the capacitance value of the corresponding piezoelectric structures to be contained, since it optimizes the energy exchange between the acoustic signal and the movable region <NUM>, and consequently maximizes the deformation of the piezoelectric regions, increasing the sensitivity. In fact, unlike what occurs for example in the (known) case wherein the piezoelectric structures are coupled to a suspended membrane, no portions of the movable region <NUM> exist which maintain, in the presence of the acoustic signal, the position assumed in rest conditions. Conversely, if the piezoelectric structures are coupled to a suspended membrane, and with the same area of the piezoelectric regions, the sensitivity is limited by the fact that, in the presence of the acoustic signal, only the central portion of the membrane is subject to a deformation, while the peripheral portion is not subject to any deformation, as it is fixed to the fixed body; in other words, the piezoelectric regions are not mechanically excited efficiently.

For example, it is possible to size the outer piezoelectric structures <NUM> and the inner piezoelectric structures <NUM> so that each has a capacitance equal to about 2pF. Furthermore, the transducer device <NUM> may reach sensitivity values of the order of mV per Pascal, much higher than what is currently obtainable in case of transducers wherein the piezoelectric structures are applied to membranes.

Different embodiments are also possible, wherein the number of deformable structures <NUM> is different from three and/or the deformable structures have a different arrangement; for example, <FIG> shows an embodiment still including three deformable structures <NUM>, which extend along corresponding elongation directions which are parallel to each other. The shapes of the trench cavities (here indicated with <NUM>) modify accordingly.

Again with reference to the number of deformable structures <NUM>, in general embodiments (not shown) are possible including only two deformable structures <NUM>. Furthermore, embodiments (not shown) are possible wherein each support structure <NUM> comprises more than two beams.

The array <NUM> of transducer devices <NUM> may be manufactured through the manufacturing process described hereinbelow, which for the sake of simplicity refers to the operations relating to the manufacturing of a single transducer device <NUM>, of the type shown in <FIG>. Furthermore, the following <FIG> and <FIG> refer to section line W-W shown by way of example in <FIG>.

Initially, as shown in <FIG>, the semiconductor body <NUM> is formed, so that it is delimited at top by a temporary surface Stemp and includes a main buried cavity <NUM>, intended to form the main cavity <NUM>. The semiconductor body <NUM> is delimited at bottom by the bottom surface Sb.

The buried cavity <NUM> extends at a distance below the temporary surface Stemp, that is, it is overlaid by a corresponding portion of the semiconductor body <NUM>. For example, the formation of the main buried cavity <NUM> may occur in a manner known per se, and therefore not illustrated in detail, as explained in <CIT>, that is by initially forming a plurality of laterally offset trenches in a semiconductive substrate and subsequently by performing an epitaxial growth, to close the trenches at top, and finally by performing a thermal treatment that causes the migration of semiconductor material and the formation of the main buried cavity <NUM>.

Subsequently, as shown in <FIG>, secondary buried cavities <NUM> are formed, which are intended to form corresponding secondary cavities <NUM> and overlay the main buried cavity <NUM>. To this end, the same method used to form the main buried cavity <NUM> may be performed, from the aforementioned portion of semiconductor body <NUM> which overlays the main buried cavity <NUM>. This step of the manufacturing process may entail an increase in the thickness of the semiconductor body <NUM>, which, once the formation of the secondary buried cavities <NUM> is completed, is delimited at top by the top surface St.

In practice, the secondary buried cavities <NUM> are substantially equal to each other, extend at a same height (measured along the Z axis) with respect to the underlying main buried cavity <NUM>, that is, they are coplanar, and are laterally offset. Each secondary buried cavity <NUM> is therefore overlaid by a corresponding top portion <NUM> of semiconductor body <NUM>, which extends between the top surface St and the underlying secondary buried cavity <NUM>; furthermore, a corresponding portion of the semiconductor body <NUM>, which is referred to as the bottom portion <NUM> of the semiconductor body <NUM>, extends between each secondary buried cavity <NUM> and the underlying main buried cavity <NUM>.

Then, as shown in <FIG>, in a per se known manner, a corresponding outer piezoelectric structure <NUM> and a corresponding inner piezoelectric structure <NUM> (shown schematically) are formed above each top portion <NUM> of the semiconductor body <NUM>.

Subsequently, an etch (for example, of the dry type) is performed, from the top surface St, so that it selectively removes portions of semiconductor body <NUM> and forms the trench cavities <NUM>, and therefore so that it forms the support structures <NUM> and the movable region <NUM>.

In particular, the effect of this etch is visible for example in <FIG> and <FIG>, which refer to section line K-K indicated by way of example in <FIG> and show a portion of semiconductor body <NUM> respectively before and after the aforementioned etch.

In greater detail, referring to the intermediate surface S<NUM> to indicate the top surface of the main buried cavity <NUM>, portions of the semiconductor body <NUM> are removed which extend vertically between the top surface St of the semiconductor body <NUM> and the intermediate surface S<NUM>, to form the peripheral portions 30A of the trench cavities <NUM>; furthermore, for each secondary buried cavity <NUM>, parts of the corresponding top portion <NUM> of the semiconductor body <NUM> are removed which are laterally offset with respect to the corresponding outer piezoelectric structure <NUM> and the corresponding inner piezoelectric structure <NUM>, and underlying parts of the corresponding bottom portion <NUM> of the semiconductor body <NUM>; the remaining parts of the top portion <NUM> and of the bottom portion <NUM> of the semiconductor body <NUM> respectively form the corresponding top beam <NUM> and the corresponding bottom beam <NUM>. The first linear portions 30B and the second linear portions 30C of the trench cavities <NUM> are thus formed, and therefore the support structures <NUM> and the movable region <NUM> are formed. This entails that each portion of secondary buried cavity <NUM> interposed between a corresponding top beam <NUM> and a corresponding bottom beam <NUM> forms a corresponding secondary cavity <NUM>, as shown in <FIG>, which refers again, as does the following <FIG>, to section line W-W.

Then, as shown in <FIG>, an etch (for example, of dry type) is performed from the back, that is from the bottom surface Sb, so that it removes portions of the semiconductor body <NUM> arranged between the main buried cavity <NUM> and the bottom surface Sb. In this manner, the main buried cavity <NUM> is opened downwardly and forms the main cavity <NUM>. The transducer device <NUM> is thus formed.

The advantages that the present solution affords are clear from the preceding description.

In particular, the present solution allows piezoelectric regions having a reduced area, and therefore a reduced capacitance, to be adopted, with consequent optimization of the sensitivity. This solution, while being particularly useful in case the piezoelectric regions are formed by PZT, due to the high value of the element ε<NUM>, is also useful in case the piezoelectric material is different, even in case it is desired to optimize the sensitivity and contain the dimensions of the transducer.

Finally, it is clear that modifications and variations may be made to the transducer device and the manufacturing process described and illustrated herein without thereby departing from the scope of the present invention, as defined in the attached claims.

For example, each piezoelectric structure may comprise, instead of a single piezoelectric region, a stack formed by two or more piezoelectric regions, having intermediate conductive regions therebetween, in order to increase the force applied to the movable region during the transmission step.

Claim 1:
A micromachined pressure transducer comprising:
- a fixed body (<NUM>) of semiconductor material, which laterally delimits a main cavity (<NUM>);
- a transduction structure (<NUM>), which is suspended on the main cavity (<NUM>) and comprises at least a pair of deformable structures (<NUM>) and a movable region (<NUM>), which is formed by semiconductor material and is mechanically coupled to the fixed body (<NUM>) through the deformable structures (<NUM>);
characterized in that each deformable structure (<NUM>) comprises:
- a support structure (<NUM>) of semiconductor material, which includes a first and a second beam (<NUM>,<NUM>), each of which has ends respectively fixed to the fixed body (<NUM>) and to the movable region (<NUM>), the first beam (<NUM>) being superimposed, at a distance, on the second beam (<NUM>); and
- at least one piezoelectric transduction structure (<NUM>,<NUM>), mechanically coupled to the first beam (<NUM>);
and wherein the piezoelectric transduction structures (<NUM>,<NUM>) are electrically controllable so as to cause corresponding deformations of the respective support structures (<NUM>) and a consequent translation of the movable region (<NUM>) along a translation direction (H).