Patent Application
20210292158
The present invention relates to a microelectromechanical microphone and a method for manufacturing a microelectromechanical microphone.
Microelectromechanical or MEMS (Microelectromechanical system) microphones equip many objects, for example mobile phones, personal digital assistants. Their demand is increasing. Furthermore, it is attempted to produce microphones that are both more efficient and of less overall size. But the reduction in the size of the microphones affects their performance.
A MEMS microphone comprises an element sensitive to a pressure difference, one face of which is in contact with the zone in which the sound wave to be picked up is emitted, and one face of which is in contact with a cavity, called the back volume. The back volume is generally connected to the external environment to allow pressure equalisation to atmospheric pressure at low frequency. This cavity generally has a volume of a few mm3 to a few tens of mm3. The larger this volume, the more the acoustic noise associated with this volume decreases. Means measure the displacement of the sensitive element. These measurement means are generally of the capacitive type, the sensitive element forms an electrode and a counter-electrode which is disposed opposite to the sensitive element. The counter-electrode is perforated to allow the sound wave to reach the sensitive element. However, the counter-electrode forms an acoustic resistance, which degrades the detection threshold and also yields damping, which reduces the microphone bandwidth.
Generally, manufacturing a MEMS microphone comprises the step of manufacturing an assembly comprising a sensitive element suspended from a substrate, and means for measuring its displacement relative to the substrate, assembling on one face of the assembly with a support comprising at least one passage in line with the sensitive element to connect the sensitive element to the external environment, and transferring a cover on the support delimiting the back volume with the sensitive element. Such a microphone is of great overall size. It further has the problem of acoustic resistance due to the counter-electrode.
One possibility to solve the problem of resistance of the counter-electrode is to place the capacitive measurement means in a controlled atmosphere. However, it becomes relatively complex to provide a controlled atmosphere space and a back volume while offering a microphone having a reasonable overall size, and the packaging step of which would be relatively simple.
A purpose of the present invention is therefore to offer a high-performance microelectromechanical microphone, transduction means of which are placed in a controlled atmosphere, and the back volume of which is integrated into the MEMS, simplifying its integration and minimising its final size and a method for manufacturing such a microphone.
The purpose set out above is achieved with a microelectromechanical microphone made from two assembled substrates, a first one in which the microphone unit is made and a second substrate forming the cover, also referred to as the packaging. The second substrate includes at least one recess forming with the first substrate a first cavity receiving the sensitive element of the microphone and delimiting the back volume. The first and second substrates also delimit a space, in which there is a controlled atmosphere, advantageously at low pressure, receiving the means for measuring the displacement of the sensitive element, this space being insulated in a seal manner from the first cavity. A transmission means ensures transmission of the displacement of the piston to the controlled atmosphere space, while ensuring insulation of this space and the first cavity.
Thus the back volume and the controlled atmosphere space are directly formed upon assembling both substrates, so it is not required to manufacture it after assembling. The microphone is then directly functional after assembling the microphone unit and the cover.
By virtue of the invention, the microphone has a high level of integration and a reduced overall size.
The above purpose is also achieved with a method for manufacturing microelectromechanical microphones comprising manufacturing a first subassembly comprising at least one sensitive element and measurement means arranged substantially in the same plane and connected by an arm rotatably hinged about an axis contained in the plane, said arm transmitting the displacement of the sensitive element to the measurement means, and manufacturing a second subassembly including at least one first cavity for the sensitive element, and assembling both subassemblies so that the first cavity forms a back volume for the sensitive element and so that they delimit, for the measurement means, a space insulated from the external environment.
The method according to the invention only implements two subassemblies, i.e. it uses two substrates which are structured and then assembled. The manufacturing method is simplified. Further, the measurement means of the microphone are not facing the sensitive element and therefore do not reduce the microphone performance.
In other words, the microphone and the method implement a substrate including the microphone elements and a cover or packaging substrate.
Advantageously, the first subassembly comprises several pairs of sensitive elements/measurement means and the second subassembly at least several first cavities, enabling several microphones to be simultaneously manufactured. The microphones are then singularised, for example by cutting.
One subject-matter of the present application is then a microelectromechanical microphone including:
the microphone unit and the cover being assembled so that they delimit between them a controlled atmosphere space, advantageously a vacuum space, housing the measurement means, and a first cavity from the first recess, the movable element partly closing said first cavity, the controlled atmosphere space and the first cavity being insulated in a sealed manner from each other, said microphone also including means for mechanically transmitting the displacement of the movable element to the measurement means and an sealed insulation element through which the transmission means pass and ensuring sealed passage of the transmission means between the external environment and the measurement chamber while maintaining the sealed insulation.
In one advantageous example, the cover has a second recess which, together with the microphone unit, delimits the vacuum space.
Preferably, the sealed space and the first recess are on a same side of the plane of the sealed insulation element.
In one exemplary embodiment, the connection means are in the cover and are formed by at least one via and/or connection tracks extending in the plane of the cover.
For example, the connection means are in the microphone unit and are formed by at least one via.
The control unit can be an ASIC integrated into the first substrate or into the second substrate.
For example, the measurement means are capacitive and comprise a fixed electrode on the first substrate and an electrode integral with the transmission means.
In another example, the measurement means are measurement means implementing resonating beams.
Another subject-matter of the present application is a method for manufacturing at least one microphone including a movable element configured to displace under the effect of a pressure difference between the external environment and a first cavity, the movable element comprising a face directed to the external environment and a face directed to the first cavity, said first cavity being fluidly connected to the external environment, means for measuring the displacement of the movable element, said measurement means being housed in a measurement chamber insulated inn a sealed manner from the external environment, means for mechanically transmitting the displacement of the movable element to the measurement means connecting the movable element and at least part of the measurement means, an sealed insulation element through which the transmission means pass and ensuring sealed passage of the transmission means between the external environment and the measurement chamber while maintaining the sealed insulation, said method including:
a) manufacturing a first subassembly including the movable part, part of the mechanical transmission means, the sealed insulation element and at least part of the measurement means,
(b) manufacturing a second subassembly comprising at least the first cavity,
(c) manufacturing first electrical connection means for connecting the measurement means to a control unit,
(d) assembling the first subassembly and the second subassembly so that they delimit a controlled atmosphere space and so that the first cavity faces the second face of the movable element and the first electrical connection means are in electrical contact with the measurement means, and so as to form the measurement chamber insulated in a sealed manner from the external environment,
(e) structuring the first subassembly to form the other part of the transmission means and releasing the movable element, the mechanical transmission means and the measurement means,
f) connecting the measurement means to a control unit.
Upon assembling the first subassembly and second subassembly, preferably a controlled atmosphere is created in the measurement chamber.
A getter material can be formed on the second subassembly so that the getter material is in the measurement chamber.
Advantageously, during step b), a second cavity adjacent to the first cavity is formed and delimits, together with the second subassembly, the measurement chamber.
In one exemplary embodiment, manufacturing the first connection means includes making a via through the first subassembly opening into the front face of the first subassembly and connecting directly to the measurement means.
The first connection means can be manufactured on the second subassembly and step f) can include cutting part of the first subassembly to gain access to the first electrical connection means of the second subassembly, and connecting the first electrical connection means to the control unit.
Step f) advantageously includes a step of encapsulating the control unit, the first electrical connection means and part of the first subassembly and the second subassembly.
The control unit may be an ASIC (AS) and the method may include a step of integrating the ASIC into the first or second subassembly. The method may then include manufacturing a housing in the front face of the first or second subassembly and placing the ASIC into said housing.
The method may also include making second means for connecting to the ASIC and making a via in the first or second subassembly so that it connects to the second connection means.
In another exemplary embodiment, the second subassembly is formed from a substrate, said substrate being an ASIC.
Preferably, several microphones are simultaneously manufactured, said first subassemblies being made from the same substrate and said second subassemblies being made from the same second substrate, said method comprising a step of separating the microphones after step e).
The present invention will be better understood based on the following description and on the appended drawings in which:
The same references are used to describe elements having substantially the same structure or substantially the same function.
In
The microphone includes a microphone unit 2 and a cover 3.
Microphone unit 2 contains 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 piston 4 or element sensitive to pressure variations, means 10 for measuring the displacement of the piston, means 8 for 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.
Piston 4 is suspended between a chamber 6, called the back volume, and a zone A, the pressure variations of which caused by sound waves are desired to be measured. The piston has a face 4.1 directed to the back volume and a face 4.2 directed to the zone A. Piston 4 displaces 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 10 are disposed substantially in the plane of the piston and are located in a chamber 12 insulated from zone A and from the back volume, chamber 12 will be called the measurement chamber.
The mechanical transmission means 8 ensure transmission of the displacement of the piston to the measurement chamber 12 while at the same time ensuring insulation of measurement chamber 12 from zone A and from back volume 6.
In the example represented, the measurement means 10 are of the capacitive type and detect an air gap variation between a movable electrode EL1 movably integral with the transmission means and a fixed electrode EL2 attached to the support. For example, the movable electrode EL1 is connected to ground and the fixed electrode EL2 is connected to a voltage source, imposing a potential difference between both electrodes.
Measurement chamber 12 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 chamber 12 is at least lower than 0.1 bar or even lower than 0.01 bar. Reducing the pressure in measurement chamber 12 enables 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 hinge 22 on support 2. 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 arm 20.1 disposed in the external environment A and a second transmission arm 20.2 disposed in the measurement chamber 12. In this example the two transmission arms 20.1, 20.2 are aligned.
In the example represented, the first transmission arm 20.1 and the second transmission arm 20.2 are rigidly connected by a central portion 20.3, called a transmission shaft.
In the example represented, the first transmission arm 20.1 and the second transmission arm 20.2 are arranged in two distinct planes, the first transmission arm 20.1 being located above a plane P containing the piston and the movable electrode, and the second transmission arm 20.2 being located below the plane P.
The sealed insulation element 16 is located in plane P and ensures sealed insulation between measurement chamber 12 and zone A. The sealed insulation element 16 is adapted to withstand the pressure difference between the external environment and the pressure in measurement chamber 12, especially when the pressure in measurement chamber 12 is reduced as compared to the external environment.
The drive shaft 20.3 passes through the sealed insulation element 16.
In the example represented, the first transmission arm 20.1 connects to a first zone of the side face of the transmission shaft 20.3 and the second transmission arm 20.2 connects to the transmission shaft in a second zone of its side face, opposite to the first zone.
The free end of the first transmission arm 20.1 can be connected to the piston 4 by 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 end 20.1. It is then held in rotation by the arm, as represented in
Likewise, the free end of the transmission arm 20.2 can 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 arm 20.2 and be rotationally displaced, as represented in
The drive shaft is rotatably hinged about the Y axis in connection with support 2, 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, piston 4 is 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 structure 26 with a thin layer 28 collecting the pressure difference over the entire surface, the thin layer 28 coming from the layer which also forms the sealed insulation element. The layer 28 has a thickness of, for example, a few hundred nanometres to a few micrometres.
The rigidifying structure advantageously includes a rim 30 extending in the Z direction on its external contour so as to lengthen air path between zone A and chamber 6 all 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 layer 28 for sealing a rigidifying structure 26 enables 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 element 16 limits displacement in the plane XY.
The sealed insulation element 16 is 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 element 16 and 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 piston 4, 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.
In
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:
An exemplary embodiment of the first subassembly E1 will now be described in connection with
For example, a silicon-on-insulator (SOI) substrate 100 is used, comprising a thick silicon layer 102, a SiO2 layer 104 and a single crystal silicon layer 106.
The substrate is represented in
The layer 106 is structured, for example, by photolithography and etching. Then a SiO2 layer 108 is formed on the structured layer 106, for example by deposition, for example by chemical vapour deposition or any other suitable type of deposition. The layer 106 forms the thin part of the piston and the sealed insulation element.
Layer 108 is also structured for example by photolithography and etching. The etching of layer 108 can also result in the etching of layer 104 where layer 106 has been previously etched, as is the case in
During a next step, a thick silicon layer 110 is formed, for example by epitaxial growth. Then layer 110 is etched, for example by deep reactive ion etching (DRIE).
The element obtained is represented in
During a next step, the piston, the movable electrode and the second transmission arm 20.2 are released by etching SiO2 from layers 104 and 108, for example by etching with hydrofluoric acid in the vapour phase. This is a time-controlled etching method.
The element thus obtained is represented in
An example of manufacturing the second subassembly E2 will now be described in connection with
A silicon substrate 112 represented in
A full-plate SiO2 layer 114 is formed successively by thermal deposition or oxidation, a metal layer 116, by chemical vapour deposition or any other type of deposition, which is structured, and then a full-plate SiO2 layer 118 is formed.
The metal layer is for forming the first electrical connection means.
The element obtained is represented in
During a next step, a layer 120 is formed on layer 118 and then structured, for example by etching, to form two trenches to gain access to the metal layer 116. 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 layer 120 can be contemplated, to stop etching during etching thereof.
The element obtained is represented in
During a next step, a metal layer 122 is formed and structured so as to form contacts in it on the first electrical connection means formed in the metal layer 116 at the trenches. Advantageously, metal layer 122, in addition to making contacts, also ensures mechanical assembling of both subassemblies and ensures sealed insulation of the measurement chamber. For example, layer 122 is structured in order to form sealing beads for providing eutectic sealing.
The element obtained is represented in
During a next step, the element in
In the example represented, a getter material 121 is deposited in the bottom of cavity 119 to confirm low pressure in the measurement chamber.
The element obtained is represented in
Then the subassemblies E1 and E2 are 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 in
A step of thinning the layer 102 is then performed by grinding to obtain a reduced thickness, typically a thickness of about 100 microns. During one step, the first transmission arm 20.1 is formed by structuring the substrate 102, for example by etching into the back face of the first subassembly.
The element obtained is represented in
During a next step, the first transmission arm and the face 4.2 of 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 in
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 E1 is 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 in
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.
In
In
In the example represented, the cavities etched in substrate 112 have 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 cavity 119 if the front face of the second subassembly is structured in such a way as to allow the displacement of the transmission means.
In
A TSV (Through Silicon Via) type via 126 is formed through substrate 112 in line with via 117.3 and allows a connector from metal layer 116 to 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.
In
In
In
Then a metal layer 122′ is formed, which is also structured so as to have only portions on the structured layer 114′. Advantageously, the oxide layer 114′ and metal layer 122′ are formed and the layers 114′ and 122′ are simultaneously structured.
The element obtained is represented in
During a next step, the element in
In the example represented, a Getter material 121′is deposited in the bottom of the cavity 119′ to confirm the low pressure in the measurement chamber.
The obtained element is represented in
Then the subassemblies E1 and E2′ are assembled, for example by eutectic sealing through their front faces.
In
The fixed electrode EL2 is connected to the ASIC AS by a via 140 through the SiO2 layers 102. 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 E1 is made for example of doped silicon and the via 140 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 layers 104 and 106, it is provided to open layers 104 and 106 to gain access to substrate 102 in order to make a silicon via 142 through layer 104 upon forming the Si layer 108, and thus ensure Si continuity through the entire thickness of the first subassembly. As a variant a TSV is provided instead of via 140.
In
In
In this example, housing 135 for the ASIC is formed at the front face of the first subassembly by structuring layers 104, 106 and 108.
The fixed electrode is connected to the ASIC AS by means of a conductive track 137 formed on the front face of the second subassembly E2′ and the ASIC is connected to the outside by means of a via 136 formed through the entire thickness of the first subassembly. A conductive track 138 is formed on the front face of the second subassembly E2′ and connects the ASIC and the end of via 136 opening into the front face of the first subassembly.
During step 11B, the oxide layer 114′ is structured so as not to reveal the front face of the substrate 112′ and to allow creation of conductive tracks extending from the front face of second subassembly E2′ and which are insulated from substrate 112′.
In
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20 02826 | Mar 2020 | FR | national |
