Transducer element and MEMS microphone

Abstract

The present application relates to a transducer element (1) which comprises: a movable diaphragm (2, 2a, 2b) which has a border (4), a frame (5) to which the border (4) of the diaphragm (2, 2a, 2b) is attached, and a reinforcement element (10) which connects to one another a first sub-section of the frame (5) and a second sub-section of the frame (5) which lies opposite the first sub-section.

Claims

1. A transducer element comprising a movable diaphragm which has a border, a frame to which the border of the diaphragm is attached, and a reinforcement element which connects to one another a first sub-section of the frame and a second sub-section of the frame which lies opposite the first sub-section, wherein the reinforcement element has a height corresponding to the height of the frame minus a minimum distance between the reinforcement element and the movable diaphragm, wherein the first sub-section of the frame and the second sub-section of the frame extend from a lower edge of the frame, which is arranged on a side pointing away from the diaphragm, as far as a height corresponding to the height of the frame minus a minimum distance between the reinforcement element and the diaphragm, such that the reinforcement element ensures that the first sub-section and the second sub-section of the frame are at the same distance from one another along the entire height of the reinforcement element.

2. The transducer element according to claim 1, wherein the reinforcement element has a height which is less than a height of the frame.

3. The transducer element according to claim 1, wherein the reinforcement element and the frame are composed of the same material.

4. The transducer element according to claim 1, wherein the reinforcement element connects to one another a third sub-section of the frame and a fourth sub-section of the frame which lies opposite the third sub-section.

5. The transducer element according to claim 1, wherein the reinforcement element is in the shape of a strip.

6. The transducer element according to claim 1, wherein the reinforcement element is in the shape of a cross.

7. The transducer element according to claim 1, wherein the reinforcement element is in the shape of a star.

8. The transducer element according to claim 1, wherein the diaphragm is elliptical or rectangular.

9. The transducer element according to claim 1, wherein the reinforcement element has a height between 150 m and 700 m.

10. An MEMS microphone having a transducer element according to claim 1.

11. The transducer element according to claim 1, wherein the transducer element comprises a fixed back plate and wherein the reinforcement element is arranged on a side of the diaphragm opposite of the fixed back plate.

12. The transducer element according to claim 1, wherein the frame is composed of silicon.

13. The transducer element according to claim 2, wherein the reinforcement element connects to one another a third sub-section of the frame and a fourth sub-section of the frame which lies opposite the third sub-section.

14. The transducer element according to claim 3, wherein the reinforcement element connects to one another a third sub-section of the frame and a fourth sub-section of the frame which lies opposite the third sub-section.

15. The transducer element according to claim 14, wherein the reinforcement element and the frame are composed of the same material.

16. The transducer element according to claim 2, wherein the reinforcement element is in the shape of a strip or of a cross or of a star.

17. The transducer element according to claim 2, wherein the diaphragm is elliptical or rectangular.

18. The transducer element according to claim 2, wherein the reinforcement element has a height between 150 m and 700 m.

19. The transducer element according to claim 13, wherein the reinforcement element has a height between 150 m and 700 m.

Description

(1) In the drawings:

(2) FIG. 1 shows a cross section through a transducer element having a reinforcement element according to a first exemplary embodiment,

(3) FIG. 2 shows a cross section through the transducer element shown in FIG. 1,

(4) FIG. 3 shows a simulation of the mechanical stress which occurs in an oval diaphragm in a transducer element which does not have a reinforcement element,

(5) FIG. 4 shows a cross section through a transducer element having a reinforcement element according to a second exemplary embodiment,

(6) FIG. 5 shows a simulation of the mechanical stress which occurs in an oval diaphragm in a transducer element which has a reinforcement element according to the first exemplary embodiment,

(7) FIG. 6 shows a simulation of the mechanical stress which occurs in an oval diaphragm in a transducer element which has a reinforcement element according to the second exemplary embodiment,

(8) FIG. 7 shows a further exemplary embodiment of the transducer element having a reinforcement element according to the first exemplary embodiment,

(9) FIG. 8 shows a further exemplary embodiment of the transducer element having a reinforcement element according to the second exemplary embodiment, and

(10) FIG. 9 shows a detail of a transducer element.

(11) FIG. 1 shows a cross section through a transducer element 1. The transducer element 1 has a movable diaphragm 2 and a fixed back plate 3. A voltage can be applied between the diaphragm 2 and the back plate 3, with the result that the diaphragm 2 and the back plate 3 form a capacitor. If the diaphragm 2 moves relative to the back plate 3 owing to a pressure fluctuation, the capacitance of this capacitor changes. In particular, sound waves can give rise to pressure fluctuations which change the capacitance of the capacitor. The transducer element 1 is configured to convert pressure fluctuations into an electrical signal. In particular, the transducer element 1 can convert an acoustic signal into an electrical signal.

(12) The transducer element 1 forms a front volume and a rear volume. The front volume is suitable for communicating in terms of pressure with the surroundings of the transducer element 1. The transducer element 1 correspondingly has a sound inlet opening (not shown) via which the front volume can communicate in terms of pressure with the surroundings and via which sound waves or other pressure waves can travel to the diaphragm 2. The rear volume of the transducer element 1 is a reference volume which is acoustically isolated from the front volume. The transducer element 1 is suitable for measuring a time-variant difference between the sound pressure in the front volume and the pressure in the rear volume.

(13) In addition, the transducer element 1 has a ventilation opening for static pressure equalization between the front volume and the rear volume. There is therefore no constant invariable pressure in the rear volume. Instead, the pressure in the rear volume is adapted slowly to an ambient pressure via the ventilation opening.

(14) The ventilation opening has high acoustic impedance. Correspondingly, sound waves cannot penetrate the rear volume through the ventilation opening.

(15) In addition, the movable diaphragm 2 has a border 4 which is attached to a frame 5 of the transducer element 1. The border 4 of the diaphragm 2 is attached in such a way that it cannot move in a direction toward the back plate 3 or away from the back plate 3. Just one internal region 6 of the diaphragm 2, which internal region 6 is not directly attached to the frame 5, is movable in the direction toward the back plate 3 and away from the back plate 3. The frame 5 of the transducer element 1 is composed of silicon.

(16) FIG. 2 shows a cross section through the transducer element along the line AA shown in FIG. 1.

(17) The shape of the frame 5 is adapted to the shape of the diaphragm 2. The frame 5 can be divided into numerous sub-sections. In particular, the frame 5 has a first sub-section 7 and a second sub-section 8, wherein the first and the second sub-sections 7, 8 of the frame 5 lie opposite one another.

(18) In addition, the transducer element 1 has a reinforcement element 10. The reinforcement element 10 connects the first sub-section 7 of the frame 5 to the second sub-section 8 of the frame 5. The reinforcement element 10 has a height which is somewhat less than the height of the frame 5. For example, the height of the reinforcement element 10 can be 5 to 25 m less than the height of the frame 5. Correspondingly, the minimum distance 16 remains between the diaphragm 2 and the reinforcement element 10, with the result that the diaphragm 2 does not rest directly on the reinforcement element 10. The reinforcement element 10 extends from a lower edge 9 of the frame 5, which lower edge 9 is located on the side of the frame 5 lying opposite the diaphragm 2, as far as an upper limit 15 which is spaced apart from the diaphragm 2 by a minimum distance 16.

(19) According to a first exemplary embodiment, the reinforcement element 10 is in the shape of a strip. The method of functioning of the reinforcement element becomes clearer from the cross section shown in FIG. 2.

(20) The reinforcement element 10 connects the first sub-section 7 of the frame 5 and the second sub-section 8 of the frame 5. The reinforcement element 10 has the effect that smaller forces are applied to the diaphragm 2, and that, in particular, no asymmetrical forces act on the diaphragm 2, or at least the portion of the forces acting asymmetrically on the diaphragm 2 is reduced considerably.

(21) Asymmetrically acting forces can arise, in particular, in the way described below: the fixed back plate 3 has a high stress. Correspondingly, the fixed back plate 3 applies to the frame 5 a force which contracts the frame 5 at its upper edge 17 at which the fixed back plate is arranged. At the same time, this force causes the frame 5 to be forced apart at its lower edge 9. As a result of the contraction of the frame 5 at the upper edge 17, the diaphragm 2, whose border 4 is attached to the upper edge 17 of the frame 5, also becomes warped. In the first exemplary embodiment shown in FIG. 2, the diaphragm 2 is in the shape of an ellipse. The ellipse shape defines a main axis 11 and a secondary axis 12 which is at a right angle to the main axis 11 and is shorter than the main axis 11.

(22) FIG. 3 shows a simulation of the mechanical stress which acts on an oval diaphragm 2a without reinforcement elements 10. The oval diaphragm 2a is very similar to the ellipse-shaped diaphragm 2 shown in FIG. 2. The left-hand illustration shows the mechanical stress acting in the x direction, and the right-hand illustration shows the mechanical stress acting in the y direction. In this context, the x direction is defined by the connecting line of the two points on the diaphragm 2a which are furthest away from one another, and the y direction is perpendicular to the x direction. In the case of the elliptical diaphragm 2, the main axis 11 extends in the x direction, and the secondary axis 12 extends in the y direction.

(23) In FIG. 3 it is clearly apparent that a significantly higher mechanical stress occurs along the x direction of the diaphragm 2a. The average mechanical stress is 49.6 MPa along the x direction. The average mechanical stress is 38.7 MPa along the y direction. Overall, the difference between the average mechanical stresses in the x and y directions in the oval diaphragm 2a without a reinforcement element 10 is 10.9 MPa.

(24) The reason for the non-uniform distribution of the mechanical stress in the x and y directions is that the frame 5 is weaker in the x direction, owing to the relatively large extent of the diaphragm 2, than in the y direction. Correspondingly, the diaphragm 2 becomes warped to a greater extent in the x direction than in the y direction under the force applied to the frame 5 by the fixed back plate 3.

(25) The reinforcement element 10 ensures that the first and second sub-sections 7, 8 of the frame 5 are held at a fixed distance from one another. The frame 5 is therefore held fixedly from its lower edge 9 up to the height which corresponds to the minimum distance between the diaphragm 2 and the reinforcement element 10, in such a way that the sub-sections 7, 8 are at a fixed distance from one another here. This fixed distance is predefined by the length of the reinforcement element 10. This prevents the frame 5 from being able to move to a great extent at its upper edge. Therefore, fewer forces are applied to the diaphragm 2, 2a. As a result of the arrangement of the reinforcement element 10 at a mechanical weak point of the frame 5 as described here, in particular the asymmetrical portions of the force acting on the diaphragm 2, 2a can be reduced.

(26) FIG. 4 shows a second exemplary embodiment of the reinforcement element 10a. Here, the reinforcement element 10a is configured in the shape of a cross. The reinforcement element 10a correspondingly connects not only the first and second sub-sections 7, 8 of the frame 5 but now also a third sub-section 13 of the frame 5 to a fourth sub-section 14 of the frame 5 which lies opposite the third sub-section 13. The third and fourth sub-sections 13, 14 are also held at a fixed distance from one another. In addition, the third and fourth sub-sections 13, 14 are now also held in a defined position relative to the first and second sub-sections 7, 8 by the reinforcement element 10a. The third and fourth sub-sections 13, 14 of the frame 5 also each extend from the lower edge 9 of the frame 5 up to the upper limit 15, with the result that the minimum distance 16 between the reinforcement element 10a and the diaphragm 2 remains free.

(27) FIGS. 5 and 6 each show simulations of the mechanical forces which occur and which act on the oval diaphragm 2a, wherein a reinforcement element 10 in the shape of a strip according to the first exemplary embodiment is provided in FIG. 5, and a reinforcement element 10a in the shape of a cross according to the second exemplary embodiment is provided in FIG. 6. In FIG. 5 and FIG. 6, the mechanical stresses acting in the x direction are respectively illustrated in a left-hand illustration, and the mechanical stresses acting in the y direction are respectively illustrated in a right-hand illustration.

(28) It is apparent that the mechanical stresses which occur and which act on the diaphragm 2a can be reduced significantly compared to an exemplary embodiment without a reinforcement element 10. In the embodiment shown in FIG. 5 with the reinforcement element 10 in the shape of a strip, the average mechanical stress along the x direction is 47.4 MPa. Along the y direction, the average mechanical stress is 42.4 MPa. Overall, the difference between the average mechanical stresses in the x and y directions in the oval diaphragm 2a with the reinforcement element 10 in the shape of a strip is 5.0 MPa.

(29) In the embodiment shown in FIG. 6 with the reinforcement element 10a in the shape of a cross, the average mechanical stress along the x direction is 47.3 MPa. Along the y direction, the average mechanical stress is 42.2 MPa. Overall, the difference between the average mechanical stresses in the x and y directions in the oval diaphragm 2a with the reinforcement element 10a in the shape of a cross is 5.1 MPa.

(30) Both the reinforcement element 10 in the shape of a strip and the reinforcement element 10a in the shape of a cross therefore bring about a significant reduction in the difference between the average mechanical stresses in the x and y directions from 10.9 MPa to 5.0 MPa, and 5.1 MPa, respectively. Accordingly, the reinforcement element 10 in the shape of a strip and the reinforcement element 10a in the shape of a cross ensure that the mechanical stress is distributed more uniformly in the diaphragm 2a. There is no significant improvement to be seen here between the first and second exemplary embodiments of the reinforcement element 10, 10a. This is attributable to the particular shape of the frame 5, which is significantly less stable in one direction than in the other direction. Along the x axis, the frame 5 has a virtually straight part which deforms comparatively easily. Along the y axis, the frame 5 is in the shape of a semicircle and is therefore comparatively difficult to deform. In the case of frames 5 or diaphragms 2 which are shaped in some other way, a configuration of the reinforcement element 10a in the shape of a cross can, in contrast, significantly increase the mechanical stability compared to a configuration in the shape of a strip.

(31) FIG. 7 and FIG. 8 show further exemplary embodiments of the transducer element 1. In FIG. 7 and in FIG. 8, the diaphragm 2b is configured in each case as a rectangle. In FIG. 7, the reinforcement element 10 is in the shape of a strip, and in FIG. 8 the reinforcement element 10a is in the shape of a cross.

(32) In addition, other shapes of the reinforcement element are also possible, for example it can be in the shape of a star. The selected shape of the reinforcement element should always be adapted to the shape of the diaphragm.

(33) FIG. 9 shows a detail of the transducer element 1 on the basis of which the manufacturing method of the transducer element 1 is outlined.

(34) The frame 5 and the reinforcement element 10 are manufactured in a common etching step in which a mask is applied to a silicon wafer, and part of the silicon wafer is subsequently etched away, with the result that the front volume of the transducer element 1 is formed. The frame 5 and the reinforcement element 10 are therefore fabricated photolithographically from the silicon wafer.

(35) The reinforcement element 10 is therefore manufactured with the etching step which generates a cavity in a silicon block. This method is referred to as deep reactive ion etching (DRIE). Depending on the selection of the process parameters, it can give rise to a negative angle of inclination at the side walls of the cavity, which angle is also found on the sides of the reinforcement element 10. The height H of the reinforcement element 10 is set by means of the etching angle and the width W of the mask which is used.

(36) Various configurations of the reinforcement element 10 are shown in FIG. 9. In the case of a width W1, W2 and W3, the reinforcement element 10 has in each case the height H. In the case of a width W4 or W5 of the reinforcement element 10, a height of H4 or H5 is produced.

(37) Depending on the mask used and depending on the etching angle , the reinforcement element 10 can be in the shape of a wedge with a blunt tip or can taper in the direction towards the diaphragm 2. The method is set in such a way that the reinforcement element 10 is spaced apart from the diaphragm 2 by the minimum distance. The etching process can also be modified in such a way that the etching angle can be changed in order to fabricate reinforcement elements 10 of different widths with the same height.

LIST OF REFERENCE NUMBERS

(38) 1 Transducer element 2, 2a, 2b Diaphragm 3 Back plate 4 Border of the diaphragm 5 Frame 6 Internal region of the diaphragm 7 First sub-section 8 Second sub-section 9 Lower edge of the frame 10, 10a Reinforcement element 11 Main axis 12 Secondary axis 13 Third sub-section 14 Fourth sub-section 15 Upper limit 16 Minimum distance 17 Upper edge of the frame