EP3552403B1 - Top-port mems microphone with reduced mechanical loads as well as a manufacturing process - Google Patents

Description

The invention relates to MEMS microphones in which electrical connections are subjected to less severe mechanical stress.

MEMS microphones (MEMS=Micro-Electro-Mechanical System) have microstructured functional elements that can be formed in or on the surface of a chip. The functional elements can, for. B. comprise a flexible membrane and a rigid backplate. One or more flexible membranes form an electrode of a capacitor. One or more stiffer backplates form the counter electrode. When sound waves hit such a chip, the membrane vibrates and the capacitance of the capacitor changes continuously due to the different distance between the capacitor electrodes. An evaluation circuit uses the variation in capacitance over time to create an electrical signal that corresponds to the acoustic signal received.

In order for a MEMS microphone to have good acoustic properties, the acoustic signals to be received should only hit the functional elements from one side. Viewed in the direction of the acoustic signal propagation, there should therefore be a rear volume behind the functional elements that is acoustically isolated from the area around the microphone.

In addition to the MEMS chip, MEMS microphones generally include other elements, e.g. B. a carrier and a cover. Carrier, cover and MEMS chip must be mechanically connected be. The MEMS chip must be connected—directly or indirectly—to external contacts of the microphone so that the microphone can be connected to an external circuit environment. In particular, the electrical connection points that connect the MEMS chip to its environment are sensitive to mechanical stress.

MEMS microphones can be embodied as so-called bottom-port microphones, in which a sound entry opening is arranged on the side that faces the object to which the microphone is attached. Such bottom-port microphones can have a poorer signal quality than so-called top-port microphones, whose sound entry opening is not covered by the object, since the sound entry opening of top-port microphones is located on the side facing away from the object. The type of microphone (top port or bottom port) is often determined by the installation-related requirements. However, top-port microphones usually require a higher design effort, especially with the acoustic seal between the front volume and the rear volume.

A MEMS microphone with an acoustic seal of a sound channel is used in DE 10 2011 087963 A1 shown. In particular, this document shows the features of the preamble of claims 1 and 7 respectively. Similar microphones are from DE 10 2010 026519 A1 and DE 10 2004 011148 B3 known. Multi-component materials that are used as seals, or are at least suitable as such, are made of U.S. 3,615,972 A and DE 10 2011 080142 A1 known.

There is therefore a desire for MEMS microphones that provide good signal quality, can be produced with as little construction effort as possible, and whose reliability is increased by reduced mechanical stress on electrical connections.

Such a MEMS microphone is specified in independent claim 1. A corresponding method for manufacturing a MEMS microphone is provided in claim 7. Dependent claims specify advantageous configurations of the microphone.

The MEMS microphone has a carrier, a cap on the carrier and a MEMS chip. The cap on the carrier encloses a cavity. The MEMS chip is placed in the cavity. The MEMS microphone also has a sound opening in the carrier or in the cap, with the arrangement in the cap being preferred. Furthermore, the microphone has a back volume and a channel that connects the sound port to the MEMS chip. The duct acoustically isolates the back volume from the sound port. The canal - more precisely: its wall - comprises a heterogeneous material. The heterogeneous material consists of at least two different components. The two different components have different thermomechanical properties.

It is possible that the cap completely encloses the cavity. It is also possible that the cap and the carrier together enclose the cavity. In this respect, the cap at least partially encloses the cavity.

The fact that the channel acoustically isolates the sound port from the back volume means that acoustic signals reaching the sound port are prevented from reaching the back volume directly, i.e. without passing through the MEMS chip. The channel - more precisely: its side walls - represent a barrier for acoustic signals.

The different thermomechanical properties of the two components of the material of the channel can include different thermally induced changes in length, different thermally induced changes in viscosity, different thermally induced changes in the acoustic Impedance, different temperature-dependent moduli of elasticity and different temperature-dependent densities and similar parameters include.

The following problem of common MEMS microphones was identified: The MEMS chip is connected to its environment via electrical connections and wired up. In addition, the MEMS chip is connected to a sound opening in such a way that the acoustic signals to be received should hit the functional elements, but not the rear volume directly. Temperature changes can occur during operation, but especially during the manufacture of such a MEMS microphone. In general, MEMS microphone materials have different coefficients of thermal expansion. When the temperature changes, therefore, distances change, e.g. B. between MEMS chip and cap and / or between MEMS chip and carrier. Electrical interconnections between the MEMS chip and its circuit environment, e.g. B. contact pads on the top of the carrier can be made via bump connections. The solder material used and the corresponding connection pads on the top of the carrier or on the surface of the MEMS chip can tear off if the tensile stress is too high, rendering the microphone inoperable. Even at a low level, these forces affect the component characteristics.

The channel is shaped and the heterogeneous material with its two different components with different thermal properties is chosen in such a way that mechanical stresses induced by temperature changes on the electrical circuitry of the chip are reduced.

At the same time, the specified material of the duct enables a good acoustic seal between the sound opening and the rear volume.

The acoustic sealing between the sound opening and the rear volume should not be achieved by arranging an elastic or compressible element between the cap and the MEMS chip under pretension. This would exert a temperature dependent force on the MEMS chip. In addition, solder bumps, with which the MEMS chip can be mounted, could be deformed by this force when they are melted in a soldering process, with the prestress being reduced at the same time and the sealing being eliminated.

Rather, the seal should be made by gluing the cap and MEMS chip. Nevertheless, unavoidable forces due to temperature changes are kept low by a very low modulus of elasticity of the heterogeneous sealing material. The modulus of elasticity can be less than 100 MPa, preferably less than 10 MPa. Suitable sealants are also gel-like sealants such as viscoelastic fluids.

Accordingly, it is possible for the MEMS chip to be connected and mechanically connected to the carrier or the cap via an electrically conductive connection. Furthermore, the MEMS chip is arranged between the channel and the carrier.

In both cases, thermally induced changes in length of the channel, the electrically conductive connection and the thickness of the chip as well as the distance between the carrier and the top of the cap mean that the mechanical stress on the electrical connection would increase if the channel were to comprise conventional materials.

According to the invention, the heterogeneous material of the channel comprises a first component as a matrix and a second component with elements embedded in the matrix.

Through a heterogeneous composition of the material of the duct, ducts with new thermomechanical properties can be obtained. The matrix material of the first component can have a first temperature-dependent viscosity and a first temperature-dependent density. The elements of the second material can correspondingly have different second temperature-dependent viscosities or stiffnesses and a second temperature-dependent density. The ratio of these two components in the heterogeneous material then determines the resulting thermomechanical properties, e.g. B. the temperature-dependent viscosity or the temperature-dependent density of the wall material of the acoustic channel.

In particular, it is possible that the different thermomechanical properties of the first and the second component include a different thermal expansion behavior. The first component and the second component can differ accordingly in their thermal expansion behavior. Temperature-induced changes can be changes along the vertical direction perpendicular to the surface of the carrier or horizontal changes in length parallel to the top of the carrier. It is also possible that the temperature-dependent change in volume of the various components is different.

The first component comprises a thermoplastic material, an elastomer and/or a silicone gel

The second component includes polymer beads. Spheres may be filled with a hydrocarbon prior to heat treatment. In an expanded state, the spheres may have a polymer shell and be hollow on the inside.

The hydrocarbon in the polymer beads can undergo a phase transition, e.g. B. a boiling point, which is in a preferred temperature range. The spheres have a shell made of polymer, the rigidity of which is so low that changes in the volume of the hydrocarbon in the environment of the polymer spheres, i. H. to the matrix material of the first component.

It is also possible that the heterogeneous material has a non-linear thermal expansion behavior.

In particular, the second component can have a non-linear thermal expansion behavior.

The second component can have a non-reversible thermal expansion behavior. When hollow spheres inflate, the wall thickness drops so much that there is hardly any restoring force left. The expansion can therefore conclude with a stable new, permanent state.

The heterogeneous material as a whole can have non-reversible thermal expansion behavior.

The heterogeneous material can behave as follows when the temperature changes, in particular when the temperature increases: The matrix material of the first component has a certain viscosity and can be deformed relatively easily.

When the temperature increases, the filling of the elements of the second component expands. In the case of polymer balls as the second component, their volume increases relatively sharply. The heterogeneous material, in which the elements are preferably distributed as evenly as possible in the matrix, is inflated.

It is possible that in the range of a characteristic temperature, a phase transition of the filling of the elements takes place. A large increase in volume takes place within a relatively narrow temperature range.

At temperatures above this phase transition, a transformation of the material of the first component begins. The viscosity of the matrix material decreases. The heterogeneous material solidifies as the volume increases. Should the temperature drop again, the heterogeneous material essentially retains its volume and shape.

$${\mathrm{T}}_1<{\mathrm{T}}_3;$$

$$\left|{\mathrm{T}}_3-{\mathrm{T}}_1\right|\ge 50\mathrm{K;}$$

$$100°\mathrm{C}\le {\mathrm{T}}_1,{\mathrm{T}}_2,{\mathrm{T}}_3,\le 220°\mathrm{C}$$

The use of such a material solves a variety of intrinsic stress problems in manufacturing. A temperature treatment may be necessary to bond the cap to the carrier, e.g. B. to glue. A reflow process establishes the electrical and mechanical connection between the chip and its circuitry. The critical temperatures of the heterogeneous material, e.g. B. the phase transition temperature and the solidification temperature can be selected so that the mechanical stress on the electrical interconnections is minimized despite significantly different temperatures. So it is possible that a silver comprehensive connecting material that connects the cap to the carrier, or an adhesive that connects the cap to the carrier, a sufficiently strong connection between the cap and Carrier from a temperature T ₁ allows. Correspondingly, a material can be selected for the material of the first component that begins to stiffen above a further temperature T ₂ , while the elements of the second component expand at a temperature T ₃ . The following applies: $${\mathrm{T}}_1<{\mathrm{T}}_2;$$

$${\mathrm{T}}_1<{\mathrm{T}}_3;$$

$$\left|{\mathrm{T}}_3-{\mathrm{T}}_1\right|\ge 50\mathrm{K;}$$

$$100°\mathrm{C}\le {\mathrm{T}}_1,{\mathrm{T}}_2,{\mathrm{T}}_3,\le 220°\mathrm{C}$$

In this way, the cap can first be firmly connected to the carrier. The channel is then formed by foaming the heterogeneous material between chip and cap or between chip and carrier. For this purpose, the heterogeneous material can have been applied in a raw state before the volume expansion in a closed curve around the functional elements on the chip or around the sound opening on the carrier substrate or cap before the cap is put on.

The matrix material preferably still has reactivity or stickiness when the swelling front reaches the opposite surface. This will ensure a good seal.

Due to the foaming, the density and the rigidity of the material of the acoustic channel are so low that the mechanical tensile or compressive stress passed on to the electrical wiring does not exceed critical values.

If the first component comprises an elastomer or a silicone gel, its viscosity is initially preferably very low in order to ensure easy application, e.g. B. by applying using dispensing needles with an inner diameter between 0.09 mm and 0.11 mm. From a transition temperature Components of the first component can crosslink so that the viscosity then assumes a sufficiently high value when the heterogeneous material has assumed the desired shape, in particular the desired height, and the acoustic seal is achieved over a wide temperature range without critical stresses on the electrical circuitry .

The carrier can be a printed circuit board material, e.g. B. PCB, or a ceramic material. The carrier can consist of one or more layers. The carrier can comprise dielectric layers and metallization layers arranged in between. Signal conductors and/or circuit elements, e.g. B. inductive elements or capacitive elements. Contact areas on the upper side or on the underside of the carrier and metallization structured inside the carrier can be interconnected via vias.

The cap can consist of a metal or at least include a metallic layer for shielding.

In particular, if the sound opening in the cap is structured above the carrier and the channel is arranged between the upper segment of the cap and the MEMS chip, a top-port microphone with good acoustic properties is obtained, in which the mechanical stress on the electrical Interconnection is minimized, thereby reducing the probability of a defect during manufacture and increasing the service life during operation.

The materials specified for forming the channel essentially do without solvents, so that contamination is avoided.

The two components of the heterogeneous material can be matched to one another in such a way that a slight solidification of the first component begins even as the volume increases, but this does not appreciably impair the expansion of the second component.

The acoustic sealing compound cured in this way retains a certain elasticity (E≧100 MPa, preferably E≦10 MPa) and can absorb temperature fluctuations. Due to the low density, acoustic energy is absorbed and not transmitted.

The mechanical force that the bloated heterogeneous material exerts on the cap and MEMS chip is extremely low and e.g. controllable via the degree of expansion. The shear modulus of the heterogeneous material can be selected to be less than 1.5 MPa.

The rate of expansion of the heterogeneous material can be three or more, where the rate of expansion is the ratio of the volumes in the inflated state and in the applied state.

Heat-expanding elements for the second component are e.g. B. from the U.S. Patent 3,615,972 known. Suitable balls are z. B. the microspheres sold under the brand name Expancel ^® .

According to the invention, the matrix material has a thermal setting mechanism which is activated above the temperature at which the expansion of the second component begins.

A method for producing a corresponding MEMS microphone therefore includes applying the heterogeneous material in a ring shape to the MEMS chip, the carrier and/or to the underside of the cap. Afterwards, e.g. B. after assembling the carrier, MEMS chip and cap, the material is inflated by heating to form the final acoustic seal. Ideally, such channeling occurs after the cap is secured to the carrier.

It is possible that the connecting material between cap and carrier is solidified at a temperature that is below the temperature required for the expansion of the heterogeneous material.

It is possible for a conductive adhesive to be used as the connecting material between the cap and the carrier.

Central aspects of the MEMS microphone and details of exemplary embodiments are explained below with reference to the schematic figures.

Fig. 1:

eine mögliche relative Anordnung von Kappe, MEMS-Chip, Träger und Kanal,

Fig. 2:

das Material des Kanals vor der thermischen Aktivierung,

Fig. 3:

die Wirkung des Aufblähens der Elemente der zweiten Komponente,

Fig. 4:

die Ausrichtung der Funktionselemente zur Schallöffnung,

Fig. 5:

eine mögliche Anordnung einer Auswertschaltung,

Fig. 6:

Wärmestromkurven für verschiedene zweite Komponenten mit unterschiedlicher Übergangstemperatur,

Fig. 7:

eine Wärmestromkurve eines Silberleitklebers.

Show it:

Figure 1:

a possible relative arrangement of cap, MEMS chip, carrier and channel,

Figure 2:

the material of the channel before thermal activation,

Figure 3:

the effect of inflating the elements of the second component,

Figure 4:

the alignment of the functional elements to the sound opening,

Figure 5:

a possible arrangement of an evaluation circuit,

Figure 6:

Heat flow curves for different second components with different transition temperatures,

Figure 7:

a heat flow curve of a silver conductive adhesive.

figure 1 shows a possible arrangement of the elements of a MEMS microphone MM, in which a cap KP is arranged on a carrier TR and encloses a cavity together with the carrier TR. A preferably large area of the cavity forms the rear volume RV, which is arranged behind the functional elements, here MEMS structures MS on the MEMS chip MC, as viewed in the direction of sound. Acoustic signals can reach the microphone MM via a sound opening SO. The MEMS structures MS include a rigid backplate and a flexible membrane. These two elements form the electrodes of a capacitor whose capacitance varies with the frequency of the acoustic signals received. The sound entry opening SO and the rear volume RV are formed by an acoustic seal AI in the form of the channel K. The channel K includes the expanded heterogeneous material HM. figure 1 shows the representation of a section through a microphone MM.

The channel K encloses the sound opening SO along a closed curve. If the rear volume RV were acoustically coupled to the sound opening SO, the microphone MM would be acoustically short-circuited and practically no signal would be present. Due to a deflection of the membrane of the MEMS structures MS directed towards the rear volume RV, the rear volume RV is compressed and a restoring force on the membrane is increased. In order to obtain the best possible signal quality, a rear volume RV that is as large as possible is therefore advantageous.

The MEMS chip MC is connected and interconnected to the carrier TR via bump connections BU. Thermal expansion jeopardizes the integrity of the electrical circuitry. With conventional top-port microphones, there is always a risk that an acoustic seal will damage or completely destroy the electrical circuitry if the temperature changes.

figure 2 shows an intermediate step during the manufacture of a corresponding microphone. The heterogeneous material HM is still in its original state, ie before thermal activation. In this state, the cap KP can be firmly connected to the carrier TR without exerting thermally induced stresses on the electrical circuitry, since the cap KP is not yet connected to the MEMS chip.

figure 3 shows on the left a lot of the heterogeneous material HM before activation. On the right is the state after thermal activation has taken place. The increase in volume is based essentially on the thermally induced increase in volume of the second component in the form of the expandable elements E, represented here by balls KG. The matrix material M essentially retains its volume. After the increase in volume of the balls K, the material of the matrix can be stiffened in order to keep the final state, ie the final shape, essentially independent of temperature.

figure 4 shows a possible embodiment in which the MEMS structures MS are not as in figure 1 shown facing the carrier TR but facing the cap KP. As a result, the volume within the MEMS chip also contributes to the back volume RV, as a result of which the back volume RV is enlarged. A disadvantage of this construction is that additional electrical lines are required from the top of the chip to the carrier. For this purpose, vias DK can be structured in the chip.

figure 4 also shows the possibility of making the microphone connectable to an external circuit environment via external connections EA. The external connections EA can include connection pads on the underside of the carrier and additional bump connections.

figure 5 also shows the possibility of providing an evaluation circuit AS in the form of an additional chip. The evaluation circuit can be arranged on the carrier TR. Chip and evaluation circuit AS can be interconnected via vias DK and structured signal conductors in a metallization layer ML. The chip and/or evaluation circuit can also be connected to external contacts on the underside of the carrier via analog vias.

figure 6 shows heat flow curves of different second components. Heat flow curves provide information about exothermic or endothermic processes within a material and show the corresponding temperature dependency. In a possible version for the material of the second component, an endothermic process takes place at about 125 °C (lower curve). The middle curve shows a process that takes place between 130 °C and 150 °C. The upper curve shows a transition at around 175 °C.

Depending on which materials are chosen for the second component, z. B. different hydrocarbons with different phase transition temperatures, different temperatures at which the inflation process begins can be selected.

figure 7 shows the WSK heat flow curve for a silver-containing conductive adhesive with which the cap can be attached to the carrier. An exothermic reaction takes place in the temperature range around 150 °C, during which the adhesive hardens.

The transition temperatures of the swelling component (cf. figure 6 ) and the connecting means between cap and carrier (cf. figure 7 ) can be chosen in such a way that the acoustic isolation essentially takes place after the cap has been attached to the carrier and the attachment of the cap to the carrier does not thereby impair the electrical connections.

The MEMS microphone and the method for producing a MEMS microphone are not limited by the technical features and embodiments shown. Microphones that include additional circuit elements and/or attachment elements and methods that include additional manufacturing steps also fall within the scope of protection defined by the following claims.

Reference List

AI:

acoustic isolation

AS:

evaluation circuit

BU:

bump connection

DK:

via

DM:

dielectric material of a dielectric layer

E:

elements of the second component embedded in the matrix

EA:

external connections

HM:

heterogeneous material

K:

channel

CP:

cap

KG:

Sphere, a possible shape of the embedded elements

M:

Matrix of the first component

MC:

MEMS chip

ML:

metallization layer

MM:

MEMS microphone

MS:

MEMS structures

RV:

back volume

SO:

sound port

TR:

carrier

VM:

Connection material between cap and strap

value chain:

heat flow curve

Claims (9)

1. MEMS microphone (MM), comprising

   - a carrier (TR),

   - a cap (KP) on the carrier (TR), which at least partially encloses a cavity,

   - a MEMS chip (MC) in the cavity,

   - a sound opening (SO) in the cap (KP),

   - a rear volume (RV),

   - a channel (K), which connects the sound opening (SO) to the MEMS chip (MC) and acoustically insulates it from the rear volume (RV),

   wherein

   - the channel (K) comprises a heterogeneous material (HM), which consists of at least two different components with different thermomechanical properties, characterized in that

   - the heterogeneous material (HM) comprises a first component as matrix (M) and a second component with elements (KG) embedded in the matrix,

   - the first component comprises a thermoplastic material, an elastomer and/or a silicone gel,

   - the second component comprises polymer beads (KG),

   - the matrix material (M) has a thermal hardening mechanism which is activated above that temperature at which the expansion of the second component is activated.
2. MEMS microphone according to the preceding claim, wherein

   - the MEMS chip (MC) is interconnected and mechanically connected to the carrier (TR) via an electrically conductive connection (BU), and

   - the MEMS chip (MC) is arranged between the channel (K) and the carrier (TR).
3. MEMS microphone according to either of the preceding claims, wherein the different thermomechanical properties of the first and the second component entail a different thermal expansion behaviour.
4. MEMS microphone according to one of the preceding claims, wherein the heterogeneous material has a nonlinear thermal expansion behaviour.
5. MEMS microphone according to the preceding claim, wherein the second component has a nonlinear thermal expansion behaviour.
6. MEMS microphone according to the preceding claim, wherein the heterogeneous material has a non-reversible thermal expansion behaviour.
7. Method for producing a MEMS microphone (MM), wherein the microphone comprises:

   - a carrier (TR),

   - a cap (KP) on the carrier (TR), which at least partially encloses a cavity,

   - a MEMS chip (MC) in the cavity,

   - a sound opening (SO) in the cap (KP),

   - a rear volume (RV),

   - a channel (K), which connects the sound opening (SO) to the MEMS chip (MC) and acoustically insulates it from the rear volume (RV),

   wherein

   - the channel (K) comprises a heterogeneous material (HM), which consists of at least two different components with different thermomechanical properties, characterized in that

   - the heterogeneous material (HM) comprises a first component as matrix (M) and a second component with elements (KG) embedded in the matrix,

   - the first component comprises a thermoplastic material, an elastomer and/or a silicone gel,

   - the second component comprises polymer beads (KG),

   - the matrix material (M) has a thermal hardening mechanism which is activated above that temperature at which the expansion of the second component is activated, and

   wherein

   - the heterogeneous material (HM) is applied in a ring around the MEMS chip (MC) and/or to the bottom side of the cap (KP), and

   - the material (HM) is expanded by heating to form an acoustic seal after the carrier (TR), MEMS chip (MC) and cap (KP) have been assembled.
8. Method according to the preceding claim, wherein a connecting material (VM) between the cap (KR) and carrier (TR) is solidified at a temperature which is below the temperature required for expansion of the heterogeneous material (HM).
9. Method according to the preceding claim, wherein a conductive adhesive is used as connecting material (VM) between the cap (KP) and carrier (TR).

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