MEMS transducer

Abstract

The present invention reduces noise that is superimposed on an output signal of a MEMS transducer without causing a problem that pertains to mechanical strength of the MEMS transducer.　A back plate of a MEMS microphone is configured to comprise: a plurality of beams that are arranged concentrically; an outer edge part that surrounds all of the plurality of beams; and a plurality of bridges that connect the outer edge part and the outermost beam among the plurality of beams and that also connect between the respective adjacent beams. The back plate is configured such that acoustic holes that are partitioned off by the outer edge part, the plurality of beams, and the plurality of bridges are each formed in a substantially quadrangular shape, and all of the acoustic holes have substantially the same opening area.

Description

MEMS transducer

[0001] The present disclosure relates to MEMS transducers, and more particularly to MEMS transducers with reduced noise levels.

[0002] A capacitive transducer has a back plate and a diaphragm arranged opposite each other. This diaphragm comes into contact with the fluid flowing through an acoustic space (back chamber, front chamber, or air gap) and vibrates in response to the fluid's vibrations. The diaphragm is made of a conductive film and is biased by a power source such as a charge pump. The back plate, on the other hand, has a front surface and a back surface, the back surface of which faces the diaphragm and has a conductive layer formed on it. Therefore, for example, if the transducer is placed in the open air, when air vibrations occur in the air due to audible sound, the diaphragm vibrates in response, changing the distance between the diaphragm and the conductive layer of the back plate. The change in capacitance due to this change in distance creates an electrical output signal. In other words, a capacitive transducer has the function of converting mechanical vibrations into an electrical signal. The volume of the space between the diaphragm and the back plate (the air gap) changes as the diaphragm vibrates. To prevent fluid from being trapped in the air gap and restricting the movement of the backplate, the backplate is provided with numerous acoustic holes that penetrate the front and back surfaces.

[0003] Among capacitive transducers, transducers formed using MEMS (Micro Electro Mechanical Systems) technology are called MEMS transducers, and are widely used as microphones and acoustic sensors in wearable devices, tablets, and other devices. Note that the MEMS transducers described in this disclosure are capacitive types and do not include piezoelectric types that do not have a backplate.

[0004] MEMS transducers are formed from semiconductor wafers and have, for example, a roughly cubic shape with sides less than a few millimeters long. Because the backplate and diaphragm are housed within this tiny semiconductor shell, both must be very thin and have delicate profiles. This means that their performance is easily affected by their mechanical strength. The mechanical strength of the backplate, in particular, varies depending on the shape and arrangement of the acoustic holes, and various attempts have been made to specify these. For example, in Patent Document 1, as shown in Figure 1, the acoustic holes 6 provided in the backplate 4 are circular and spaced apart at a predetermined interval. By making the acoustic holes circular in this way, corners are not formed at the edges, thereby avoiding stress concentrations.

[0005] Furthermore, Patent Document 2 discloses arranging adjacent circular acoustic holes at an angle of 60 degrees, as shown in Figure 2. This allows the acoustic holes to be arranged more densely.

[0006] JP 2016-002625 A JP 2011-250169 A

[0007] The back plate of a MEMS transducer is constantly exposed to external pressure. Particularly when a MEMS transducer is used as a microphone, external forces act on the back plate when a large volume of sound is input, causing the diaphragm to displace significantly, or when excessive wind pressure is applied. In this case, the viscous resistance of the air can cause the air passing through the acoustic hole to transition to a contracted or turbulent flow. In such cases, the back plate can be displaced or vibrate, and friction between the air and the back plate can also generate some heat.

[0008] As described above, the behavior of fluids such as air passing through the acoustic holes in the back plate can cause noise to be superimposed on the output of the MEMS microphone. Therefore, measures to prevent noise generation are necessary. However, the above-mentioned Patent Documents 1 and 2 do not take into consideration measures to prevent such noise.

[0009] The disclosed technology has been made in consideration of the above-mentioned problems, and aims to reduce noise superimposed on the output signal of a MEMS transducer while avoiding problems with the mechanical strength of the transducer.

[0010] According to one aspect of the present disclosure, a backplate includes a plurality of beams arranged concentrically, an outer edge portion surrounding all of the beams, and a plurality of bridges connecting the outer edge portion to the outer edge portion and connecting adjacent beams. All of the acoustic holes defined by the outer edge portion, the beams, and the bridges are substantially rectangular with substantially the same opening area. This allows for narrower spacing between the acoustic holes, improving the aperture ratio. The increased aperture ratio reduces external forces the backplate receives from the fluid, thereby reducing deformation and vibration of the backplate. Furthermore, by arranging the substantially rectangular acoustic holes at a substantially equal pitch in the circumferential direction of the concentric circles, stress concentration is reduced, thereby avoiding the impact of increased aperture ratio on the reduced mechanical strength.

[0011] FIG. 1 is a diagram showing a backplate of a conventional MEMS transducer; FIG. 2 is a diagram showing a backplate of a conventional MEMS transducer; FIG. 3 is a diagram showing a cross section of a MEMS transducer according to a first embodiment; FIG. 4 is a diagram showing a backplate of a MEMS transducer according to a first embodiment; FIG. 5 is a diagram showing a backplate of a MEMS transducer according to a second embodiment; FIG. 6 is a diagram showing a backplate of a MEMS transducer according to a third embodiment; FIG. 7 is a diagram showing a backplate of a MEMS transducer according to a fourth embodiment; FIG. 8 is a diagram showing a backplate of a MEMS transducer according to other embodiments.

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In the following embodiments, identical or equivalent parts will be denoted by the same reference numerals.

[0013] First Embodiment The first embodiment will be described with reference to FIGS. 3 and 4. FIG. 3 shows a cross section of a MEMS transducer, and FIG. 4 shows a plan view of the MEMS transducer, i.e., a view from the backplate side. First, the structure of the MEMS transducer of this embodiment will be described using the cross section of FIG. 3. The cross section of FIG. 3 is shown as seen from the direction of arrows A-A in FIG. 4. The MEMS transducer of this embodiment includes a substrate 1, a diaphragm 2 whose periphery is supported by the substrate 1 via an insulating film 3 (spacer layer), and a backplate 4 arranged parallel to the diaphragm 2. The back surface of the backplate 4 faces the diaphragm 2 and has a fixed electrode 5 made of a conductive film formed thereon. This conductive film is generally made of the same material as the diaphragm 2, for example, a doped polysilicon layer, and its conductivity is adjusted by the amount of impurities contained therein.

[0014] The diaphragm 2 is biased to a predetermined potential via an electrode (not shown), and the fixed electrode 5 is connected to an electrode 7. The fixed electrode 5 is provided in an area corresponding to at least a part of the area in which the diaphragm 2 can vibrate, and therefore forms a parallel plate capacitor together with the diaphragm 2. The change in capacitance due to the displacement of the diaphragm 2 is output via the electrode 7.

[0015] On the other hand, acoustic holes 6 are formed in the back plate 4, penetrating from the front surface to the back surface, and a fluid can move through these acoustic holes 6. This fluid is air when the MEMS transducer is placed in the atmosphere, and it becomes the specified gas when placed in a specified gas atmosphere. The most common application of MEMS transducers is a microphone that picks up sounds in the audible sound range.

[0016] Next, the acoustic holes of the backplate according to this embodiment will be described with reference to FIG. 4 . As shown in FIG. 4 , when the backplate 4 is viewed from its surface, a plurality of substantially rectangular acoustic holes 6 are formed. The term "substantially rectangular" is used here because a pair of opposite sides is not a straight line but an arc, and thus the backplate 4 is not geometrically defined as a rectangle. The backplate 4 includes an outer edge 10, beams 8 arranged concentrically around the outer edge 10, and bridges 9 connecting these. The bridges 9 connect the outer edge 10 to the outermost beam 8 (the outermost beam from the center of the concentric circle) and further connect the beams 8 to each other. Therefore, the acoustic holes 6 are defined within an area defined by the outer edge 10, the beams 8, and the bridges 9. The widths and numbers of the beams 8 and bridges 9 are adjusted so that the acoustic holes 6 have approximately the same opening area. The outer edge 10 is the part of the back plate 4 excluding the beams 8 and bridges 9. The fixed electrode 5 is disposed on the back surface of the back plate 4, which is not visible in a plan view, but its area is shown by a dotted line to facilitate understanding of the drawing.

[0017] In this embodiment, the widths of the beams 8, 8... and the bridges 9, 9... are approximately constant. The spacing between the outer edge 10 and the outermost beam 8, the spacing between adjacent beams 8, and the spacing between adjacent bridges 9 are also approximately constant. As a result, the acoustic holes are approximately trapezoidal near the center and gradually change to a square shape as they move away from the center. By appropriately selecting the widths and spacing between the beams and bridges, and the spacing between the outer edge and the beams, the acoustic holes can be positioned without creating unnecessary space. Furthermore, thinning the beams and bridges reduces the external force caused by the viscous resistance of the fluid passing through the acoustic holes and thus reduces noise. However, if the beams and bridges are too thin, the area of ​​the fixed electrodes required for a parallel plate capacitor will be insufficient, making it difficult to obtain a sufficient output level. In other words, it is important to set the ratio of the total area of ​​the acoustic holes 6, 6... to the area of ​​the backplate 4 facing the diaphragm 2 (i.e., the aperture ratio) to a predetermined value. According to the inventors' experiments and other experiences, the aperture ratio is preferably between 65% and 75%, and 70% is most suitable.

[0018] Next, advantages of the MEMS transducer of this embodiment compared to conventional examples will be described. First, the first advantage, i.e., the reduction of the impact on the diaphragm, will be described. In the case of conventional circular acoustic holes such as those shown in FIG. 1 , protrusions remain on the end surface of the spacer layer exposed to the air gap in a planar view, making the diaphragm susceptible to damage. This is because etching of the spacer layer (sacrificial layer etching) is performed by isotropic wet etching, in which an etchant is poured through the acoustic holes, and the shape of the end surface of the spacer layer is affected by the shape and arrangement of the outermost acoustic holes. The shape of the protrusions narrows toward the tip, and in some cases the tip may become pointed. However, in this embodiment, the outermost acoustic holes are approximately rectangular, and the spacing between adjacent acoustic holes is extremely narrow. This prevents the passage of the etchant during sacrificial layer etching, and prevents protrusions that could damage the diaphragm from being formed.

[0019] Second, we will discuss reducing the effects of stress concentration. In the conventional arrangement shown in Figure 2, in which adjacent circular acoustic holes are arranged at a 60-degree angle (a so-called regular hexagonal arrangement), to prevent the edges of the fixed electrodes from overlapping the acoustic holes, the fixed electrodes must either include all of the acoustic holes 6, 6... as indicated by the dotted line 5A in Figure 2, or have a geometric shape that weaves between the acoustic holes as indicated by the dotted line 5B, or a complex shape as indicated by the dashed line 5C. In the area indicated by 5A, the capacitance distribution becomes uneven. In the area indicated by 5B, stress concentration occurs at the corners of the geometric shape. In the complex shape indicated by 5C, the increased number of corners disperses stress, but stress concentration at the corners remains unavoidable. The stress concentration referred to here is due to the difference in material between the fixed electrode portion of the backplate and the adjacent beams, bridges, and other components, resulting in different mechanical properties for each material. As mentioned above, the typical material for the fixed electrode is doped polysilicon, and the typical material for the backplate, excluding the fixed electrode, is silicon nitride. These materials have different rigidities and thermal expansion coefficients. Therefore, in the case of fixed electrodes with corners on their outer surface, delamination at the interface can occur.

[0020] In this embodiment, the fixed electrode has a circular outer shape as shown by the dotted line 5 in Figure 4. This circle is arranged concentrically with the beams 8, 8... The acoustic holes surrounded by the dotted line 5 are distributed almost evenly. Therefore, there is no bias in the capacitance distribution, and the absence of corners prevents stress concentration. Note that the outer shape of the fixed electrode is not limited to this and may be a narrower area. The fixed electrode may be formed only in areas where the diaphragm displacement is particularly large. For example, a conductive layer may be provided on the back surface of all of the beams 8, 8 and all of the bridges 9, 9..., and these conductive layers may serve as the fixed electrodes.

[0021] Second Embodiment Next, a second embodiment will be described with reference to FIG. 5. FIG. 5 is a schematic diagram of the surface of the backplate, omitting the outer edge and the bridges connected thereto. As shown in this figure, the acoustic holes 6, 6... are all approximately trapezoidal in shape. The term "approximately trapezoidal" is used here because the upper and lower bases are not straight lines but arcs, and therefore are not geometrically defined as trapezoids. The upper base of the approximately trapezoidal acoustic hole 6 is defined by the edge of the beam 8 closest to the center, and the two sides connecting both ends of the upper base are defined by the edges of the adjacent bridges 9, 9. The lower base is defined by the edge of the beam 8 farthest from the center. The arrangement of the acoustic holes 6, 6... will now be described. First, the spacing between the beams 8, 8... is determined, and they are arranged concentrically. Next, the bridges 9, 9... are arranged so that their widths W are approximately the same. As shown in the figure, this width W is the circumferential distance between the longitudinal midpoints of the bridges 9, 9. Strictly speaking, the width W cannot be made exactly the same value, so the bridges 9, 9... are arranged at approximately the same pitch, taking into account slight increases and decreases. In this way, the acoustic holes 6, 6... are arranged at approximately the same pitch, and their opening areas are approximately the same. In this embodiment, because the acoustic holes 6, 6... are approximately trapezoidal, the spacing can be narrowed compared to shapes such as squares with right-angled corners, which has the effect of increasing the opening ratio.

[0022] Third Embodiment Next, a third embodiment will be described with reference to FIG. 6 . FIG. 6 is a diagram schematically illustrating the surface of a back plate, omitting the illustration of the outer edge and the bridges connected thereto. As shown in this diagram, the acoustic holes 6 are all substantially rectangular. The term "substantially rectangular" is used here because the pair of long sides are arcs rather than straight lines, and the acoustic holes 6 are not geometrically defined as rectangles. In this embodiment, the acoustic holes 6 are arranged in the same manner as in the second embodiment. The two short sides of the acoustic hole 6 are defined by the edges of the bridges 9. The two long sides of the acoustic hole 6 are defined by the edges of the beams 8. The length of the beams 8 is longer than the length of either of the bridges 9. In this embodiment, the acoustic holes 6 are substantially rectangular, which provides the same effect as a substantially trapezoidal shape.

[0023] Fourth Embodiment Next, a third embodiment will be described with reference to FIG. 7 . FIG. 7 is a diagram schematically illustrating the surface of a back plate, omitting the illustration of the outer edge and the bridges connected thereto. As shown in this diagram, the acoustic holes 6, 6... are all substantially rectangular in shape, with pairs of diagonal angles forming an acute angle and an obtuse angle. In this embodiment, the acoustic holes 6, 6... are arranged in the same manner as in the second embodiment. The four corners of the acoustic hole 6 are formed by the intersections of sides defined by the edges of the beams 8 and the bridges 9, with one diagonal angle being an acute angle and the other diagonal angle being an obtuse angle. In this embodiment, the acoustic holes 6, 6... are substantially rectangular in shape, with pairs of diagonal angles forming an acute angle and an obtuse angle, so that the lines extending from the bridges 9 do not intersect at the center of the back plate, which has the effect of dispersing the direction of stress when an external force acts on the back plate.

[0024] Although the present disclosure has been described with reference to the embodiments, the present disclosure is not limited to the embodiments or structures. The present disclosure also encompasses various modifications and equivalent modifications. In addition, various combinations and forms, as well as other combinations and forms including only, more than, or less than the positional elements, are also within the scope and spirit of the present disclosure.

[0025] In the above-described embodiment, the structure of the diaphragm can be modified as appropriate. For example, the diaphragm may not be fixed at its periphery, but may be supported at several points or at a central point by a central pillar.

[0026] In the above embodiment, the MEMS transducer exemplified is of a back chamber type, but it may also be of a front chamber type.

[0027] In the above embodiment, the MEMS transducer is a single-membrane type, but it may be a double-membrane type, in which case a back plate is interposed between the two membranes. Alternatively, it may be a dual-membrane type, in which case multiple back plates are arranged opposite each membrane.

[0028] Furthermore, in the above embodiment, the exemplified MEMS transducer does not include a signal processing circuit, but it may also include an input capacitor, an amplifier, a charge pump, a quantization circuit, etc. mounted on the same substrate.

[0029] The above-mentioned embodiments may also be combined as appropriate. The above-mentioned second and fourth embodiments may be combined to form a structure in which a substantially trapezoidal acoustic hole 6a and an acoustic hole 6b in which a pair of diagonal corners are aligned at an acute angle and an obtuse angle are mixed, as shown in Fig. 8.

[0030] 1: Substrate, 2: Diaphragm, 3: Insulating film, 4: Back plate, 5: Fixed electrode, 6: Acoustic hole

Claims

1. A backplate for a MEMS microphone comprising: a plurality of beams arranged concentrically; an outer edge portion surrounding all of the plurality of beams; and a plurality of bridges connecting the outer edge portion to the outermost beam of the plurality of beams and connecting adjacent beams; wherein the acoustic holes defined by the outer edge portion, the plurality of beams, and the plurality of bridges are approximately rectangular in shape, and all of the acoustic holes have approximately the same opening area.

2. The backplate of a MEMS microphone according to claim 1, wherein the acoustic hole is approximately trapezoidal in shape, and the upper base of the approximately trapezoidal shape is defined by the edge of the beam closest to the center.

3. The back plate of the MEMS microphone according to claim 1, wherein the acoustic hole is substantially rectangular, and the length of the side defined by the edge of the beam is longer than the length of the side defined by the edge of the bridge.

4. The backplate of a MEMS microphone according to claim 1, wherein the acoustic hole has four corners defined by edges of the beam and edges of the bridge, and one diagonal angle of the four corners is an acute angle, and the other diagonal angle is an obtuse angle.

5. A backplate for a MEMS microphone as described in claim 1, characterized in that the acoustic holes are a mixture of those that are approximately trapezoidal in shape, with the upper base defined by the edge of the beam closest to the center, and those that have four corners defined by the edges of the beam and the edges of the bridge, with one diagonal angle of the four corners all being acute angles and the other diagonal angles all being obtuse angles.

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