Patent ID: 12234144

DETAILED DISCLOSURE OF SPECIFIC EMBODIMENTS

The present invention proposes a method for sealing cavities, for example arranged in a matrix, using membranes. The method according to the present invention particularly comprises the transfer of a sealing film, intended to form membranes.

Moreover, in order to ensure a limited and more uniform deformation of the membranes sealing the cavities, the method according to the present invention comprises a step of forming a first contour, which comprises an advantageously continuous first trench surrounding the arrangement of cavities.

FIGS.7a-7cá8a-8cillustrate the different steps of the method for sealing cavities according to a first embodiment of the present invention.

The method particularly comprises a step a) of forming a plurality of cavities11opening at a front face10aof a support substrate10(FIG.7a).

The remainder of the disclosure also relates to the use of a sealing film intended to be transferred using one of the faces thereof, referred to as sealing face, on the front face10a. According to another aspect of this first embodiment, it may be considered to form the plurality of cavities11on the sealing film, and particularly cavities opening at the sealing face. A person skilled in the art, merely by reading the following description, will be capable of implementing this other aspect.

The cavities11can, for example, be arranged in a matrix (FIG.9a).

“Matrix arrangement” denotes a regular arrangement along two different directions, for example orthogonal, of identical cavities11. The matrix arrangement can particularly comprise n rows and m columns of the cavities11.

A matrix arrangement according to the present invention is thus not limited to a periodic arrangement along two orthogonal directions of rows and columns. The matrix arrangement can comprise other matrix shapes, such as for example a hexagonal arrangement, or a circular symmetrical arrangement organised in concentric circles.

The cavities11have a depth p, a characteristic dimension a, and are spaced apart by a spacing b. For example, once an arrangement according to a rectangular matrix is considered, the spacing b is defined along one and the other of the directions defined by the columns and rows of the cavity matrix (FIG.9b).

The spacing b, according to the present invention, corresponds to a mean spacing of the cavities of a disorganised arrangement for example.

“Characteristic dimension” denotes the greatest dimension of the cavity opening. The opening may be of circular, square, rectangular shape, or more generally with n sides, and for example polygonal, particularly triangular or hexagonal.

The shape of a cavity according to the present invention is defined by the shape of the opening thereof.

For example, the cavities11can have a square shape of side a between 0.5 μm and 500 μm, and a depth p between 10 nm and 10 μm.

The spacing b is advantageously between 1 μm and 500 μm.

The matrix arrangement can comprise several hundred, or several thousand, cavities.

It is understood that an arrangement is delimited by a contour defined by the peripheral cavities of said arrangement (FIG.9a).

The support substrate can comprise a semiconductor material, advantageously the semiconductor material comprises at least elements chosen from: silicon, germanium, silicon and germanium alloy, indium phosphide, a III-V semiconductor arsenide, a III-V semiconductor phosphide, or a III-V semiconductor nitride, silicon carbide.

The support substrate can comprise a metallic material, advantageously the metallic material comprises at least elements chosen from: aluminium, copper, titanium, tungsten, tungsten silicide, gold.

The support substrate can comprise an insulating material, advantageously the insulating material comprises at least elements chosen from: silicon dioxide, silicon nitride.

The sealing film can comprise a piezoelectric material, advantageously the piezoelectric material comprises at least elements chosen from: lithium tantalate, lithium niobate, aluminium nitride, zinc oxide, zinc and lead tantalate.

Step a) can particularly comprise a masking step for delimiting the contour of the cavities11, and an etching step.

The masking step can involve the formation of a hard mask, and particularly a silicon dioxide mask.

The etching step can comprise dry etching (for example etching involving a plasma) or wet etching.

The method also comprises a step a1) of forming a first contour.

“First contour” denotes a path, advantageously a closed path, formed by one or more structures recessed in relation to the face of the support whereon it/they is/are formed. The first contour particularly has a width L defined by the structure(s).

The width L of the contour throughout the description corresponds to a mean length. More specifically, the lateral walls of the trench forming the contour can have varying spacing. For example, one and/or the other of these walls can outline a saw-toothed profile, or have irregularities causing width variations.

“Recessed” denotes that the structure(s) has/have a first depth p1.

The first contour can comprise one or more advantageously continuous first trenches.

FIGS.21aand21brepresent a first contour formed by a first discontinuous trench, and which can particularly comprise four sections, each in the vicinity of one side of the arrangement of cavities.

The remainder of the disclosure of this first embodiment will be confined to a first contour formed by a first continuous trench.

According to this first embodiment, the first trench21is formed on the support substrate10, and opens at the front face10a.

In particular, the first trench surrounds the arrangement of cavities.

The first trench21has a width L, and is at an essentially constant distance G from the arrangement of cavities11.

In other words, the first trench has the same shape as the contour of said arrangement.

The distance G, throughout the description, represents a mean distance. More specifically, distance variations between the first contour and the arrangement of cavities can be observed.

The first trench21has a first depth p1 advantageously equal to the depth p.

Advantageously, step a1) and step a) are performed simultaneously.

According to a preferred embodiment of the present invention, the distance G is between one-fifth of b (b/5) and five b (5×b), advantageously between one-half of b (0.5×b) and two b (2×b), more advantageously between 0.9×b and 1.1×b.

The method according to the present invention also comprises a step b) of forming membranes19sealing each of the cavities11.

Step b) of forming membranes19particularly comprises transferring the sealing film16on the front face10aof the support substrate10, and the first trench21(FIGS.7b,7c,8a,8bet8c).

More specifically, the transfer comprises assembling the sealing face16aof the sealing film16with the front face10a.

In other words, the sealing film16hermetically seals the first trench21and the cavities11.

The membranes19are thus suspended above the cavities11.

The sealing film16can comprise a semiconductor material, advantageously the semiconductor material comprises at least elements chosen from: silicon, germanium, silicon and germanium alloy, indium phosphide, a III-V semiconductor arsenide, a III-V semiconductor phosphide, or a III-V semiconductor nitride, silicon carbide.

The sealing film16can comprise a metallic material, advantageously the metallic material comprises at least elements chosen from: aluminium, copper, titanium, tungsten, tungsten silicide, gold.

The sealing film16can comprise an insulating material, advantageously the insulating material comprises at least elements chosen from: silicon dioxide, silicon nitride.

The sealing film16comprises a piezoelectric material, advantageously the piezoelectric material comprises at least elements chosen from: lithium tantalate, lithium niobate, aluminium nitride, zinc oxide, zinc and lead tantalate.

The sealing film16can also comprise a stack of layers, and particularly a stack of LTO and Si.

The transfer of the sealing film can particularly involve a step b1) of bonding a substrate, referred to as donor substrate17, on the front face10a, and a step b2) of removing a first part, referred to as handle substrate17a, of the donor substrate17so as to only retain a second part of said substrate forming the sealing film16(FIGS.7band8a).

It is understood that bonding step b1) can comprise molecular (or direct) bonding or thermocompression bonding or eutectic bonding.

Bonding step b1) can also be performed in a vacuum or in a controlled atmosphere particularly to apply a predetermined gas and pressure in the cavities11sealed using the sealing film16.

More generally, any bonding method known to a person skilled in the art can be used within the scope of the present invention.

According to a first alternative embodiment of this first embodiment, step b2) can comprise a thinning step (FIGS.7ato7c).

The thinning step can particularly comprise wet etching, dry etching, or mechanical abrasion (grinding).

For example, a handle substrate17acomprising silicon can be etched using a wet process with a KOH or TMAH solution.

Particularly advantageously and within the scope of this first embodiment, the donor substrate17comprises an intermediate layer17cinserted between the handle substrate17aand the sealing film16.

The intermediate layer17cparticularly has a selectivity facing the etching in relation to the sealing film16. It is thus possible to consider a sealing film16thickness less than 20 μm, or 10 μm, for example equal to 5 μm.

The donor substrate17, according to this alternative, can particularly be a silicon-on-insulator (SOI) substrate, the insulating layer being the intermediate layer. In one embodiment, the silicon layer of the SOI substrate is strongly doped. The barrier layer can be etched using abrasion or using a selective chemical etching step.

According to a second alternative embodiment of the first embodiment, step b1) can be preceded by a step of forming an embrittlement zone17ddelimiting the sealing film in the donor substrate. The embrittlement zone is particularly a zone at which a fracture of the donor substrate17is liable to occur, during the execution of step b2), under the effect of a heat treatment and/or a mechanical action (FIGS.8ato8c).

The embrittlement zone17dcan be an amorphised zone or an implanted zone, particularly a zone implanted with hydrogen atoms.

Regardless of the alternative embodiment considered, step a1) can also comprise the formation of at least a second contour31. The second contour can particularly comprise, like the first contour21, an advantageously continuous second trench.

The second trench31forms a contour, for example closed, around the first trench21advantageously at all points at an essentially constant distance from the first trench21(FIGS.10aand10b).

The depth of the at least a second trench31, referred to as second depth p2, can be equal to the depth p.

The first trench21and the at least a second trench31can be linked by one or more interconnection channels40. In particular, one or more interconnection channels can link two adjacent corners, respectively, of the first trench and the second trench (FIG.10b).

Particularly advantageously, the sealing film transferred during step b) also seals the at least a second trench.

Considering the first trench21, and optionally the at least a second trench31, makes it possible to reduce the non-uniformity in terms of deformation of the membranes liable to be observed between the centre and the edge of the matrix arrangement.

In particular, the inventors demonstrated that the distance G has a direct impact on the uniformity of deflection of the membranes circumscribed by the first contour21. In particular, after sealing the cavities, and more specifically during heat treatments, species are liable to diffuse at the bonding interface. In this regard, the cavities can act as reservoirs. Additionally, the greater the bonded surface area surrounding the cavity, the greater the deformation of the transferred membrane will be. In other words, the distance G can be defined, according to the integration constraints (subsequent contacts, etc.), the distance b separating the cavities of the array and the sought uniformity. Thus, a distance G close to b will make it possible to obtain a better uniformity of deformation of the membranes. On the other hand, considering a distance G greater than b will have little or no beneficial effect.FIG.22illustrates this perfectly. The latter graphically represents a mean membrane deformation (in “nm” on the vertical axis) according to the spacing b. It is in this regard worth noting that the deformation increases with the spacing b.

Moreover,FIGS.11aand11bare images obtained using optical microscopy making it possible to observe the effect of the first trench21on the deformation of the membranes in relation to the deformation observed in the absence thereof.

The comparison of these figures clearly reveals the beneficial effect of the first trench21on non-uniformity of the deformation of the membranes. In particular, the first trench makes it possible to reduce non-uniformity of the deformation of the membranes.

FIGS.12aand12b, which lead to the same conclusions, represent measurements using optical interferometry of the zones respectively relative toFIG.11aand toFIG.11b. In particular, the amplitude of deformation of the membranes is much more homogeneous from one membrane to another inFIG.12b.

The method according to the present invention can also comprise a step c), performed after step b), of removing a section of the sealing film overlapping with the first trench and/or the at least a second trench.

In other words, after step c), the sealing film is retained on the cavities, and is removed at the first trench and/or the at least a second trench.

This removal step c) can comprise wet etching or dry etching.

FIGS.13a,13b,14a,14band15illustrate the different steps of the method for sealing cavities according to a first aspect of a second embodiment of the present invention.

The second production embodiment differs, in this regard, from the first embodiment in that the first contour21, and the second contour31if considered, are formed in the sealing film16. In particular, the first contour21is confined hereinafter in the disclosure to the first trench.

A first aspect of this second embodiment is illustrated inFIGS.13a,13b,14a,14band15.

More specifically, the method illustrated inFIGS.13aand13buses the terms of the first alternative embodiment of the first embodiment, whereas the method illustrated inFIGS.14aand14buses the terms of the second alternative embodiment of the first embodiment.

FIGS.13aand14arepresent a donor substrate17on a face whereof (the sealing face16a) the first contour21is formed.

According to this alternative embodiment, the first contour21has a first depth less than the thickness of the sealing film.

The support substrate10, at which the arrangement of cavities11is formed is then assembled with the donor substrate17(FIGS.13band14b).

After step b), represented inFIG.15, the arrangement of cavities11is circumscribed by the first trench21.

Regardless of the embodiment or alternative embodiment envisaged, “circumscribed” denotes that the projection of the first trench on the front face surrounds the arrangement of cavities.

It is possible to thin the sealing film16so as to uncover the first trench21.

A second alternative embodiment of this second embodiment is illustrated inFIGS.16a,16b,17a,17band18.

According to this second alternative embodiment, the first trench21has a depth greater than the thickness of the sealing film such that after the transfer step, said first trench21is exposed to the external environment (FIG.21).

The method illustrated inFIGS.16aand16buses the terms of the first alternative embodiment of the first embodiment, whereas the method illustrated inFIGS.17aand17buses the terms of the second alternative embodiment of the first embodiment.

FIGS.16aand17arepresent a donor substrate17on a face whereof (the sealing face16a) the first contour21is formed.

The support substrate10, at which the arrangement of cavities11is formed is then assembled with the donor substrate17(FIGS.16band17b).

FIGS.19a,19b,20illustrate the different steps of the method for sealing cavities according to a third aspect of the second embodiment of the present invention.

According to this third aspect, the depth of the first trench is greater than the thickness of the sealing film16.

In particular, the steps of the method illustrated inFIGS.19aand19buses the terms of the method illustrated inFIGS.16aand16b. Considering the first trench and/or the at least a second trench makes it possible to damp the deformation of the membranes liable to occur when transferring the sealing film, and hence also reduce the non-uniformity of deformation.

The method according to the present invention is advantageously used for manufacturing MEMS, and particularly MEMS involving resonators each formed by a membrane/cavity pair.

Reducing the deformation sustained by the membranes results in an increase in the combined quality factor of the resonator matrix arrangement, and thus makes it possible to manufacture a much more sensitive matrix arrangement of capacitive ultrasound transducers (cMUT or capacitive micromachined ultrasound transducers) than the arrangements known from the prior art.

The method according to the present invention can also be used for manufacturing removable substrates of which the sealing layer overlapping with the cavities can be detached. The energy involving the detachment between the receiving substrate and the sealing film can be adjusted according to the dimensional characteristics of the matrix arrangement and the cavities.

Regardless of the application envisaged, the method according to the present invention makes it possible to perform subsequent technological steps (such as photolithography, bonding, etching, film deposition steps).

It is finally understood that the first contour and/or the second contour can be formed both on the front face10aand the sealing face16a.

The present invention also relates to a device provided with a plurality of cavities11sealed using a plurality of membranes19, the device comprising:a) a plurality of cavities11opening at a front face10aof a support substrate10or at a sealing face of a sealing film, the cavities11, advantageously arranged in a matrix, have a depth p, a characteristic dimension a, and are spaced apart by a spacing b;b) a plurality of membranes19, sealing each of the cavities11, formed by the sealing film16assembled with the front face10ausing the sealing face thereof;the device further comprises a first contour of a width L on one and/or the other of the front face10aand the sealing face, the first contour comprising an advantageously continuous first trench21, and is arranged such that the plurality of cavities11is circumscribed by the first contour, said first contour being at an advantageously essentially constant distance G from the plurality of cavities and between one-fifth of b (b/5) and five b (5×b), advantageously between one-half of b (0.5×b) and two b (2×b), more advantageously between 0.9×b and 1.1×b.