Patent ID: 12215022

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

The same references are used to describe elements having substantially the same structure or substantially the same function.

InFIG.1an exemplary embodiment of a microphone MC1according to the invention can be seen.

The microphone includes a microphone unit2and a cover3.

Microphone unit2contains the elements for sensing and measuring the pressure variation. This microphone unit is made from a first substrate, the cover is made from a second substrate and the microphone is obtained directly after assembling the first and second substrates without requiring any additional step.

The microphone unit comprises a piston4or element sensitive to pressure variations, means10for measuring the displacement of the piston, means8for mechanically transmitting the displacement of the piston to the measurement means and an sealed insulation element between the part for sensing the pressure variation and the part for measuring this pressure variation.

In the example represented, the cover includes a first recess which forms the back volume and a second recess which, together with the microphone unit, delimits a controlled atmosphere cavity. In this example the microphone includes, by assembling the microphone unit and the cover, directly two cavities which are insulated in a sealed manner from each other.

Piston4is suspended between a chamber6, called the back volume formed by a first recess6.1of the cover, and a zone A, the pressure variations of which caused by sound waves are desired to be measured. The piston has a face4.1directed to the back volume and a face4.2directed to the zone A. Piston4displaces substantially in an out-of-plane Z direction. The plane of the microphone is a plane parallel to the directions in which the largest dimensions of the support and the cover extend.

Measurement means (sensor)10are disposed substantially in the plane of the piston and are located in a chamber12insulated from zone A and from the back volume, chamber12will be called the measurement chamber.

The mechanical transmission means8ensure transmission of the displacement of the piston to the measurement chamber12while at the same time ensuring insulation of measurement chamber12from zone A and from back volume6.

In the example represented, the measurement means (sensor)10are of the capacitive type and detect an air gap variation between a movable electrode EL1movably integral with the transmission means and a fixed electrode EL2attached to the support. For example, the movable electrode EL1is connected to ground and the fixed electrode EL2is connected to a voltage source, imposing a potential difference between both electrodes.

Measurement chamber12, formed in part by a second recess12.1of the cover, includes a controlled atmosphere, advantageously under vacuum. In the present application, by “controlled atmosphere chamber” it is meant a chamber in which the gas composition and pressure do not vary or vary only slightly over time, several months or even several years, and by “vacuum chamber”, it is meant a chamber under a rarefied atmosphere, i.e. at a pressure lower than the pressure of the external environment, preferably much lower than the pressure of the external environment, preferably in the order of 0.1 mbar and 10 mbar. For example, the pressure in measurement chamber12is at least lower than 0.1 bar or even lower than 0.01 bar. Reducing the pressure in measurement chamber12enables reduction of thermal noise and viscous damping intrinsic to air displacements between the electrodes.

As a variant, the measurement means are made by one or more resonator(s), for example resonating beams, of the guitar string type, advantageously arranged in a low-pressure atmosphere preferably between 0.1 mbar to 10 mbar. Operation in a low-pressure atmosphere enables reduction of friction, which improves the quality factor and transduction gain.

The transmission means include one or more parallel transmission elements extending at rest along an X axis contained in the plane and rotatably hinged by a pivot hinge22on support2. In this example, the transmission element(s) is (are) rigid or slightly deformable. As a variant, when several transmission elements are implemented, they can extend along the X axis, the Y axis or any intermediate angle.

Each transmission element has a first transmission arm20.1disposed in the external environment A and a second transmission arm20.2disposed in the measurement chamber12. In this example the two transmission arms20.1,20.2are aligned.

In the example represented, the first transmission arm20.1and the second transmission arm20.2are rigidly connected by a central portion20.3, called a transmission shaft.

In the example represented, the first transmission arm20.1and the second transmission arm20.2are arranged in two distinct planes, the first transmission arm20.1being located above a plane P containing the piston and the movable electrode, and the second transmission arm20.2being located below the plane P.

The sealed insulation element16is located in plane P and ensures sealed insulation between measurement chamber12and zone A. The sealed insulation element16is adapted to withstand the pressure difference between the external environment and the pressure in measurement chamber12, especially when the pressure in measurement chamber12is reduced as compared to the external environment.

The drive shaft20.3passes through the sealed insulation element16.

In the example represented, the first transmission arm20.1connects to a first zone of the side face of the transmission shaft20.3and the second transmission arm20.2connects to the transmission shaft in a second zone of its side face, opposite to the first zone.

The free end of the first transmission arm20.1can be connected to the piston4by a hinge transmitting the displacement along the Z direction, while allowing rotation about the Y axis and translation along the X axis. This connection allows rotation of the arm and translation along Z axis of the piston to co-exist. The piston can also be directly connected to the end20.1. It is then held in rotation by the arm, as represented inFIG.1.

Likewise, the free end of the transmission arm20.2can be connected to the movable electrode by a hinge transmitting the displacement along the Z direction while allowing rotation about the Y axis and translation along the X axis, thus allowing implementing an electrode which is translationally mobile along Z. The movable electrode can also be directly connected to the end of the arm20.2and be rotationally displaced, as represented inFIG.1.

The drive shaft is rotatably hinged about the Y axis in connection with support2, in the zone that passes through the sealed insulation element. The hinge is made, for example, by means of blades (not visible) which are aligned with the Y axis and able to be torsionally deformed about the Y axis.

Preferably, the blades ensuring rotatable hinge of the transmission arms in connection with the support have a large dimension in the Z direction, thus providing a high degree of rigidity in the out-of-plane Z direction, which advantageously limits the out-of-plane displacement of the transmission element. Thus the pressure difference between the external environment and the measurement chamber does not tend to displace the transmission element and does not influence the measurement.

Preferably, piston4is suspended from the support by suspension elements including blades which are deformable in the out-of-plane direction and allowing the piston to displace in the Z direction.

By implementing several transmission arms, forces are recovered at several places on the piston, making it easier to make the piston. Conversely, the device could have several diaphragms connected to a single transmission arm.

Transmission means using a single transmission arm do not depart from the scope of this application.

In the example represented, the piston is trimmed and rigidified by one or more reinforcements. The piston has a rigidifying structure26with a thin layer28collecting the pressure difference over the entire surface, the thin layer28coming from the layer which also forms the sealed insulation element. The layer28has a thickness of, for example, a few hundred nanometres to a few micrometres.

The rigidifying structure advantageously includes a rim30extending in the Z direction on its external contour so as to lengthen air path between zone A and chamber6all around the piston and thus reduce leaks between the outside and the back volume used as a reference.

By virtue of the structure implemented, the pressure is collected over the entire surface and the energy lost in deforming the piston is negligible.

Implementing a thin layer28for sealing a rigidifying structure26enables a piston with high rigidity to be made, which limits energy losses during deformation, while at the same time advantageously limiting the mass. Indeed, an increase in mass causes a loss of bandwidth.

Further, the piston can take any shape to optimise the size of the microelectromechanical and/or nanoelectromechanical system.

In addition, the sealed insulation element16limits displacement in the plane XY.

The sealed insulation element16is such that it deforms under the effect of the rotational displacement of the transmission arms, the stiffness of the insulation element is low enough not to provide additional stress, and especially it does not require an increase in the piston surface area.

The sealed insulation element16and torsion blades easily deform to allow the transmission arms to rotate about Y and oppose the movements in X, Y and Z directions at this axis of rotation. The amount of energy lost during transmission of the useful movement is limited, so the hinge has a very good mechanical efficiency.

As described for piston4, the movable electrode can be connected to several transmission elements with different axes of rotation by means of a mechanical connection which allows out-of-plane rotation between the arm and the movable electrode. The movable electrode thus has a translational displacement along the Z axis, provided that the transmission elements all transmit the same displacement.

As a variant, the capacitive measurement means could be surface area varying means using interdigital combs.

InFIG.10, another example of a microphone MC2can be seen. The microphone inFIG.10differs from that inFIG.1in that the hinge20.3′ of the transmission arms20.2′ is located in the centre of the movable electrode EL2′ of the measurement means, as a result the movable electrode EL2′ pivots about a Y axis located in the middle thereof. During the displacement of4′ piston, a differential measurement is carried out because there is both an increasing capacitance and a decreasing capacitance. This exemplary embodiment has the advantage of a balanced movable electrode.

An example of a manufacturing method according to the invention of a microphone with improved performance will now be described.

The manufacturing method consists of the following steps of:manufacturing a first subassembly, forming the microphone unit, including the piston, the measurement means and at least part of the transmission means, also referred to as the microphone subassembly E1,manufacturing a second subassembly E2, forming the cover, for delimiting, together with the microphone subassembly, the back volume of the microphone and the measurement chamber,making first electrical connection means,assembling the first E1and second E2subassemblies,structuring the first subassembly through the back face to complete the transmission means,connecting the measurement means to a control unit UC.

An exemplary embodiment of the first subassembly E1will now be described in connection withFIGS.2A to2D. This example is not limiting.

For example, a silicon-on-insulator (SOI) substrate100is used, comprising a thick silicon layer102, a SiO2 layer104and a single crystal silicon layer106.

The substrate is represented inFIG.2A.

The layer106is structured, for example, by photolithography and etching. Then a SiO2 layer108is formed on the structured layer106, for example by deposition, for example by chemical vapour deposition or any other suitable type of deposition. The layer106forms the thin part of the piston and the sealed insulation element.

Layer108is also structured for example by photolithography and etching. The etching of layer108can also result in the etching of layer104where layer106has been previously etched, as is the case inFIG.2B.

During a next step, a thick silicon layer110is formed, for example by epitaxial growth. Then layer110is etched, for example by deep reactive ion etching (DRIE).

The element obtained is represented inFIG.2C.

During a next step, the piston, the movable electrode and the second transmission arm20.2are released by etching SiO2from layers104and108, for example by etching with hydrofluoric acid in the vapour phase. This is a time-controlled etching method.

The element thus obtained is represented inFIG.2D, it is the first subassembly E1.

An example of manufacturing the second subassembly E2will now be described in connection withFIGS.2E to2I. This example is not limiting.

A silicon substrate112represented inFIG.2Eis for example used.

A full-plate SiO2layer114is formed successively by thermal deposition or oxidation, a metal layer116, by chemical vapour deposition or any other type of deposition, which is structured, and then a full-plate SiO2layer118is formed.

The metal layer is for forming the first electrical connection means.

The element obtained is represented inFIG.2F.

During a next step, a layer120is formed on layer118and then structured, for example by etching, to form two trenches to gain access to the metal layer116. It can also be structured in such a way as to create extra thicknesses at some places. For this, a partial time controlled etching or the addition of a stop layer, for example of SiN within layer120can be contemplated, to stop etching during etching thereof.

The element obtained is represented inFIG.2G.

During a next step, a metal layer122is formed and structured so as to form contacts in it on the first electrical connection means formed in the metal layer116at the trenches. Advantageously, metal layer122, in addition to making contacts, also ensures mechanical assembling of both subassemblies and ensures sealed insulation of the measurement chamber. For example, layer122is structured in order to form sealing beads for providing eutectic sealing.

The element obtained is represented inFIG.2H.

During a next step, the element inFIG.2His structured to form cavities117,119to form the back volume and the measurement chamber respectively. For example, the SiO2layers and substrate112are etched by deep reactive ion etching or DRIE.

In the example represented, a getter material121is deposited in the bottom of cavity119to confirm low pressure in the measurement chamber.

The element obtained is represented inFIG.2I, this is the second subassembly E2.

Then the subassemblies E1and E2are assembled, for example by eutectic sealing, for example aluminium-germanium through their front faces. Sealing is selected, for example, from metal-to-metal sealing, metal eutectic sealing, welding and conductive adhesive sealing.

The element obtained is represented inFIG.2J.

A step of thinning the layer102is then performed by grinding to obtain a reduced thickness, typically a thickness of about 100 microns. During one step, the first transmission arm20.1is formed by structuring the substrate102, for example by etching into the back face of the first subassembly.

The element obtained is represented inFIG.2K.

During a next step, the first transmission arm and the face4.2of the piston are released, for example by etching with hydrofluoric acid in the vapour phase. This is a time-controlled etching.

The element obtained is represented inFIG.2L.

During a next step, the substrate is structured to ensure connection of the measurement means to a control unit UC, for example carried by an ASIC (Application Specific Integrated Circuit).

For example, the first subassembly E1is cut opposite to the piston so as to disengage a contact carried by the second subassembly and a wire connection is made between the ASIC and the contact.

The element obtained is represented inFIG.2M.

Next, a plastic material is overmoulded onto the ASIC, the wire and the exposed portion of the second subassembly. This overmoulding provides the packaging function and protects the ASIC and the connection.

InFIG.3A, a plurality of microphones MC1.1, MC1.2, MC1.3simultaneously made can be seen.

InFIG.3B, the microphones MC1.1, MC1.2, MC1.3are separated from each other, for example by cutting symbolized by dashed lines.

In the example represented, the cavities etched in substrate112have the same depths, but it can be contemplated that they have different depths. Indeed, it is preferable to have a large back volume, the cavity for partly delimiting this volume is preferably large. As regards the measurement chamber, it is preferable to have a low pressure, which is made easier when the volume is large. As a variant, it can be contemplated not to make a cavity119if the front face of the second subassembly is structured in such a way as to allow the displacement of the transmission means.

InFIG.4, another exemplary embodiment can be seen, of a microphone MC3formed directly by assembling a microphone unit (subassembly E1) and a cover (E2subassembly), in which the control unit, for example the ASIC, is integrated into the substrate112before the SiO2 layer114is formed. For example, the front face of substrate112is structured to form a housing124to house the ASIC. Then, after the layer114is formed on substrate112, it is structured to provide access to the ASIC and also to a zone of substrate112. Then, when forming metal layer116, vias117.1,117.2connected to the ASIC are formed simultaneously and one via117.3opening into substrate112. The ASIC is connected to the connectors formed in the metal layer116. When forming the contacts from layer122, the ASIC is connected to the front face of the second subassembly and will be connected to the fixed electrode EL2upon assembling with the first subassembly.

A TSV (Through Silicon Via) type via126is formed through substrate112in line with via117.3and allows a connector from metal layer116to be connected to the back face of the substrate for connection to the outside to recover the signal and supply electric power.

As the ASIC is integrated into the second subassembly, it is no longer necessary to cut the first assembly to gain access to the electrical contact, nor is it necessary to encapsulate the ASIC to protect it. The microphone is self-contained.

InFIG.12, a variant of a microphone MC9can be seen, in which the control unit UC is external to the assemblies E1and E2, and a via126passing through substrate112directly above the vacuum cavity is formed. The via126connects to the metal layer122which is in electrical contact with the fixed electrode EL2. The control unit UC is connected to via126. The metal layer122forms the contact, routing and eutectic sealing at the same time.

InFIG.5, a variant of the microphone MC4inFIG.4can be seen, in which the TSV is replaced by a doped silicon via. For this, the substrate is made of doped silicon and a trench is formed so as to delimit a silicon column128in line with via117.3, which will allow the ASIC to be connected to the outside.

InFIGS.6to9, further examples of microphones also formed directly by assembling a microphone unit (subassembly E1) and a cover (subassembly E2′) can be seen. The manufacturing steps for the second subassembly E2′ of these examples of microphone differ from those for the second subassembly E2′. InFIGS.11A to11D, an example of manufacturing the second subassembly E2′ can be seen. From the substrate112′, for example of Si, an oxide layer114′ which is structured is formed. The element obtained is represented inFIG.11B.

Then a metal layer122′ is formed, which is also structured so as to have only portions on the structured layer114′. Advantageously, the oxide layer114′ and metal layer122′ are formed and the layers114′ and122′ are simultaneously structured.

The element obtained is represented inFIG.11C.

During a next step, the element inFIG.11Cis structured to form cavities117′,119′ for forming the back volume and the measurement chamber respectively. For example, the substrate112′isetched by Deep Reactive Ion Etching (DRIE).

In the example represented, a Getter material121′isdeposited in the bottom of the cavity119′ to confirm the low pressure in the measurement chamber.

The obtained element is represented inFIG.11D, this is the second subassembly E2′.

Then the subassemblies E1and E2′ are assembled, for example by eutectic sealing through their front faces.

InFIG.6, an example of the microphone MC5made from the subassemblies E1and E2′ can be seen.

The fixed electrode EL2is connected to the ASIC AS by a via140through the SiO2layers102. In this example, the shape of the fixed electrode and the shape of the movable electrode are different from those of the electrodes in the previous examples. In this example, the movable electrode does not surround the fixed electrode.

In this example, the first subassembly E1is made for example of doped silicon and the via140is formed by digging a trench so as to delimit a doped silicon column. It is noted that upon manufacturing the first subassembly, especially upon structuring layers104and106, it is provided to open layers104and106to gain access to substrate102in order to make a silicon via142through layer104upon forming the Si layer108, and thus ensure Si continuity through the entire thickness of the first subassembly. As a variant a TSV is provided instead of via140.

InFIG.7, an example of the microphone MC6made from the subassemblies E1and E2′ can be seen. Between the steps inFIGS.11A and11B, before forming the oxide layer114′, a housing is formed in the front face of the subassembly E2′ to house an ASIC, and during step11C, the layers114′ and122′ are structured to provide a contact132on the ASIC. The connection to the outside is made through the first subassembly. A via130is made through the entire thickness of the first subassembly E1and opens in line with contact132connected to the ASIC. In this exemplary embodiment, the first subassembly E1is made of doped silicon and the via is formed by digging a trench so as to delimit a doped silicon column. It is noted that upon manufacturing the first subassembly, especially upon structuring layers104and106, it is provided to open layers104and106to gain access to the substrate102in order to make a silicon via134through layer104upon forming the Si layer108and thus ensure Si continuity through the entire thickness of the first subassembly. As a variant a TSV is provided instead of via130.

InFIG.8, another exemplary embodiment of a microphone MC7can be seen, into which the ASIC is integrated in the first subassembly.

In this example, housing135for the ASIC is formed at the front face of the first subassembly by structuring layers104,106and108.

The fixed electrode is connected to the ASIC AS by means of a conductive track137formed on the front face of the second subassembly E2′ and the ASIC is connected to the outside by means of a via136formed through the entire thickness of the first subassembly. A conductive track138is formed on the front face of the second subassembly E2′ and connects the ASIC and the end of via136opening into the front face of the first subassembly.

During step11B, the oxide layer114′ is structured so as not to reveal the front face of the substrate112′ and to allow creation of conductive tracks extending from the front face of second subassembly E2′ and which are insulated from substrate112′.

InFIG.9, another exemplary embodiment of a microphone MC8according to the invention can be seen, in which the substrate of the second subassembly is an ASIC AS. The fixed electrode EL2is connected to the ASIC AS by a contact144formed in step11C, and the ASIC is connected to the outside by a via146through the substrate of the first subassembly and a contact148formed in step11C.

It will be understood that any measurement means may be implemented in the sealed chamber such as piezoresistive or piezoelectric means including one or more strain gauges or detection means using resonating beams.