MEMS sensor

Abstract

To provide a highly sensitive MEMS sensor in which an interval between a fixed electrode and a movable electrode is narrow and which is easily manufactured.SOLUTION: An MEMS sensor includes: a pair of movable electrodes 20 arranged in parallel with each other across a space part 40 over a cavity part provided in a substrate; and fixed electrodes 10 arranged, on opposite sides to the space part of the movable electrode, in parallel with each other across groove parts 30, respectively. The space part includes: a central part having a first space width Z1; and a terminal part having a second space width Z2. The first space width Z1 is shorter than the second space width Z2. The space width Z1 of the first space part and a groove width X of the groove part satisfy the following formula: 3.4≤(Z1 / X)≤4.9. The movable electrodes in one pair have finger parts on the space sides, respectively, for example, and the first space width Z1 is an interval between the finger parts.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a MEMS sensor, and more particularly to a capacitive acceleration sensor using a MEMS structure.

Background Art

[0002] A capacitive acceleration sensor that detects acceleration by arranging a fixed electrode and a movable electrode opposite to each other and detecting a change in the capacitance between the two electrodes is known. As such a capacitive acceleration sensor, a sensor using a MEMS (Micro Electro Mechanical System) structure in which a silicon substrate is processed using semiconductor microfabrication technology to fabricate a fixed electrode and a movable electrode has been proposed (for example, see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a capacitive acceleration sensor, acceleration is detected by detecting a change in the capacitance between two electrodes as the position of the movable electrode changes with respect to the fixed electrode. Therefore, by reducing the distance between the fixed electrode and the movable electrode to increase the capacitance, the sensitivity of the acceleration sensor can be improved.

[0005] However, on the other hand, in a MEMS structure, it is necessary to form a cavity by etching below the fixed electrode and the movable electrode to make the electrodes float from the semiconductor substrate. Therefore, when the distance between the fixed electrode and the movable electrode is reduced, it becomes difficult for the etchant to flow into the semiconductor substrate from between the two electrodes, resulting in a problem that electrode formation becomes difficult.

[0006] Therefore, the present invention aims to provide a highly sensitive MEMS sensor that has a narrow gap between the fixed electrode and the movable electrode and is easy to manufacture. [Means for solving the problem]

[0007] That is, one aspect of this disclosure is, A MEMS sensor comprising a pair of movable electrodes arranged parallel to each other on a cavity provided in a substrate, with a space between them, and a fixed electrode arranged parallel to the movable electrodes on the opposite side of the space, with a groove between them, The space portion includes a central portion having a first space width Z1 and an end portion having a second space width Z2. The first space width Z1 is shorter than the second space width Z2 for the MEMS sensor.

[0008] Other aspects of this disclosure are: A MEMS sensor comprising a pair of movable electrodes arranged parallel to each other on a cavity provided in a substrate, with a space between them, and a fixed electrode arranged parallel to the movable electrodes on the opposite side of the space, with a groove between them, wherein each pair of movable electrodes has a finger portion on the space side, When the groove width X is 2.0 μm or more and 2.8 μm or less, the following formula (2): 0≦b / ((Z1 / 2)+(Y+b)+X)<0.125......(2) However, Z1 is the distance between the finger sections, Y is the width of the movable electrode, and b is the width of the finger section. This is a MEMS sensor that satisfies the following conditions.

[0009] Other aspects of this disclosure are: A MEMS sensor comprising a pair of movable electrodes arranged parallel to each other on a cavity provided in a substrate, with a space between them, and a fixed electrode arranged parallel to the movable electrodes on the opposite side of the space, with a groove between them, wherein each pair of movable electrodes has a finger portion on the space side, When the groove width X is 1.5 μm or more and less than 2.0 μm, the following formula (3): 0.027≦b / ((Z1 / 2)+(Y+b)+X)≦0.054....(3) However, Z1 is the distance between the finger sections, Y is the width of the movable electrode, and b is the width of the finger section. This is a MEMS sensor that satisfies the following conditions. [Effects of the Invention]

[0010] As described above, the MEMS sensor according to the present invention makes it possible to provide a highly sensitive and easy-to-manufacture MEMS sensor by adjusting the first space width Z1 of the space portion and the balance between the first space width Z1 and the groove width X of the groove portion. [Brief explanation of the drawing]

[0011] [Figure 1] This is a plan view of an acceleration sensor according to an embodiment of the present invention. [Figure 2] This is an enlarged view of region A of the accelerometer in Figure 1. [Figure 3A] This is a cross-sectional view of the electrode fabrication process for an acceleration sensor according to an embodiment of the present invention. [Figure 3B] This is a cross-sectional view of the electrode fabrication process for an acceleration sensor according to an embodiment of the present invention. [Figure 3C] This is a cross-sectional view of the electrode fabrication process for an acceleration sensor according to an embodiment of the present invention. [Figure 3D] This is a cross-sectional view of the electrode fabrication process for an acceleration sensor according to an embodiment of the present invention. [Figure 3E] This is a cross-sectional view of the electrode fabrication process for an acceleration sensor according to an embodiment of the present invention. [Figure 3F] This is a cross-sectional view of the electrode fabrication process for an acceleration sensor according to an embodiment of the present invention. [Figure 3G] This is a cross-sectional view of the electrode fabrication process for an acceleration sensor according to an embodiment of the present invention. [Figure 4A] This is a plan view of the electrode structure of an acceleration sensor according to an embodiment of the present invention. [Figure 4B] This is a schematic diagram of the electrode structure of an acceleration sensor according to an embodiment of the present invention. [Figure 5]It is a plan view of the electrode structure of the acceleration sensor according to an embodiment of the present invention. [Figure 6] It is a schematic diagram of the acceleration sensor when the size of the finger portion is changed.

Embodiments for Carrying Out the Invention

[0012] FIG. 1 is a plan view of a capacitive acceleration sensor of a MEMS structure according to an embodiment of the present invention, which is represented by 100 as a whole, and FIG. 2 is an enlarged view of region A in FIG. 1. In the acceleration sensor 100, movable electrodes 20 are provided in parallel on both sides of a fixed electrode 10 extending linearly in the Y-axis direction. The fixed electrode 10 and the movable electrode 20 extend in a stripe shape with a certain width, and a groove portion 30 is formed therebetween.

[0013] Two adjacent movable electrodes 20 are connected by a connection portion 23, and the inside thereof is a space portion 40. The two movable electrodes 20 are insulated from each other by an isolation joint (IJ) 25 provided at the center of the connection portion 23.

[0014] The connection portion 23 including the fixed electrode 10, the movable electrode 20, and the isolation joint 25 is held in a floating state with respect to the silicon substrate on top of a cavity provided in the silicon substrate. Therefore, when the acceleration sensor 100 receives a certain acceleration, the distance between the fixed electrode 10 and the movable electrode 20 changes, and accordingly, the capacitance between the two electrodes 10 and 20 also changes. By detecting this change in capacitance, acceleration can be detected.

[0015] Since the fixed electrode 10 and the movable electrode 20 form a parallel plate capacitor, the smaller the distance (distance in the X-axis direction) between the fixed electrode 10 and the movable electrode 20, the larger the capacitance, and the detection accuracy of acceleration also improves. Therefore, in the acceleration sensor 100 of the MEMS structure, the detection accuracy is improved by narrowing the distance between the fixed electrode 10 and the movable electrode 20 using semiconductor microfabrication technology.

[0016] Figures 3A to 3G are schematic diagrams of the electrode fabrication process for the acceleration sensor 100 using semiconductor microfabrication technology. Electrode fabrication is carried out in the following steps 1 to 6.

[0017] Step 1: As shown in Figure 3A, a silicon oxide film 2 is formed over the entire surface of the silicon substrate 1 by thermal oxidation, and then patterned using lithography technology. In Figure 3A, the silicon oxide film 2 extends in a straight line with a constant width perpendicular to the plane of the paper.

[0018] Step 2: As shown in Figure 3B, the silicon substrate 1 is etched using reactive ion etching (RIE) with a mixed gas of SF6 and C4F8, for example, using the silicon oxide film 2 as a mask, to form grooves. The depth of the grooves is, for example, 30 μm.

[0019] Step 3: As shown in Figure 3C, a TEOS (tetraethoxysilane) film 3 is formed over the entire surface using the CVD method.

[0020] Step 4: As shown in Figure 3D, the entire surface is etched using sputtering with, for example, a CF-based gas to remove the TEOS film 3. As a result, the TEOS film 3 remains only on the side walls of the grooves formed on the silicon substrate 1.

[0021] Step 5: As shown in Figure 3E, the silicon substrate 1 is further etched to deepen the grooves. The further etching is performed using RIE with a mixed gas of SF6 and C4F8, similar to Step 2, with the silicon oxide film 2 used as a mask. As a result, the groove depth becomes approximately 5 μm. The TEOS film 3 remains on the sidewall from the top to partway up the groove.

[0022] Step 6: As shown in Figure 3F, the silicon substrate 1 is etched using plasma isotropic etching with, for example, SF6 gas, with the silicon oxide film 2 and TEOS film 3 used as etching masks. As a result, the silicon substrate 1 is etched below the areas covered by the silicon oxide film 2 and TEOS film 3, creating cavities and forming an electrode structure that floats above the silicon substrate 1. Here, the left side becomes the movable electrode 20 and the right side becomes the fixed electrode 10, with a groove 30 formed between them.

[0023] However, if the gap between the movable electrode 20 and the fixed electrode 10 (the width of the groove 30) becomes narrow, it becomes difficult for SF6 gas to enter the groove 30 in step 6. As a result, as shown in Figure 3G, etching below the electrode becomes insufficient, leaving a fragile protrusion, such as the one shown at B (fragile structure). Such protrusions can break off when the movable electrode 20 moves, causing malfunctions in the acceleration sensor. Furthermore, if the width of the groove 30 becomes even narrower and isotropic etching becomes insufficient, the space between the movable electrode 20 and the silicon substrate 1 may not be completely etched, and the movable electrode 20 may not be able to float away from the silicon substrate 1 (cannot be released).

[0024] Although not shown in Figure 3G, similar etching defects can occur when etching the silicon substrate 1 below the fixed electrode 10, resulting in protrusions remaining near the fixed electrode 10 or the fixed electrode 10 not being able to float away from the silicon substrate 1.

[0025] In an embodiment of the present invention, as shown in Figure 2, even when the distance between the fixed electrode 10 and the movable electrode 20 (the width of the groove 30) is narrowed, good etching as shown in Figure 3F is made possible by adjusting the amount of etchant supplied from the space 40 between the two movable electrodes 20.

[0026] Figure 4A is a plan view of the electrode structure of the acceleration sensor 100, and Figure 4B is a schematic diagram of the electrode structure of the acceleration sensor 100 corresponding to Figure 4A. In Figure 4A, 10 is a fixed electrode, 20 is a movable electrode, 30 is the groove between the fixed electrode 10 and the movable electrode 20, and 40 is the space between the two movable electrodes 20. Also, 27 indicates a rectangular finger portion provided on the movable electrode 20.

[0027] As described above, reducing the distance between the fixed electrode 10 and the movable electrode 20, i.e., the groove width X of the groove 30, in order to increase the sensitivity of the acceleration sensor 100, reduces the amount of etchant (e.g., SF6) that enters the groove 30 and etches the silicon substrate 1. On the other hand, the distance between the two movable electrodes 20 (the width of the space) is sufficiently large compared to the groove width X of the groove 30, so the amount of etchant that enters the space 40 and etches the silicon substrate 1 also increases. Therefore, reducing the groove width X of the groove 30 disrupts the balance between the amount of etchant supplied from the space 40 and the amount of etchant supplied from the groove 30. As a result, for example, etching of the region C surrounded by the two grooves 30 becomes insufficient, and fragile protrusions remain unetched.

[0028] Therefore, in this embodiment of the present invention, the movable electrode 20 is provided with a finger portion 27 that protrudes toward the space portion 40, thereby narrowing the first space width Z1 in the center of the space portion 40 and limiting the amount of etchant supplied from the space portion 40. This balances the amount of etchant supplied from the groove portion 30, thereby enabling good etching. As shown in Figure 4A, the space portion 40 includes a central portion having a first space width Z1 and terminal portions on both sides having second space widths Z2, with the first space width Z1 being shorter than the second space width Z2.

[0029] Figure 5 is a plan view showing the dimensions of the electrode structure of the acceleration sensor 100 of the present invention, and an etching diagram (lower right) for half of a unit cell. In the acceleration sensor 100, the fixed electrode 10 and the movable electrode 20 extend in a stripe shape in the Y-axis direction with a constant width, and are arranged parallel to each other with a groove portion 30 of groove width X in between. Two adjacent movable electrodes 20 are connected by a connecting portion 23 and electrically insulated from each other by an isolation joint 25. The area surrounded by the two movable electrodes 20 and the connecting portion 23 is open and forms a space portion 40. The connecting portion 23, which includes the fixed electrode 10, the movable electrode 20, and the isolation joint 25, is held above a cavity provided in the silicon substrate, floating above the silicon substrate.

[0030] Furthermore, in the acceleration sensor 100, rectangular finger sections 27 are provided extending from the movable electrode 20 toward the space section 40. The length (Y-axis direction) of the finger section 27 is a, the width (X-axis direction) is b, and the thickness (Z-axis direction) is the same as that of the movable electrode 20. The distance (Y-axis direction) between the two connection sections 23 is c. The width (X-axis direction) of the space section 40 is a first space width Z1 in the central part where the finger section 27 is provided, and a second space width Z2 at the ends on both sides. W represents the width of a unit cell.

[0031] The finger portion 27 is preferably positioned on the inside of the two movable electrodes 20, in opposing positions. In Figure 5, the finger portion 27 is provided with its end portion separated from the connection portion 23 in the Y-axis direction, making it easier to etch the area below the connection portion 23. In the XY plane, the finger portion 27 is preferably rectangular in shape, but it may be other shapes such as a semicircle as long as the amount of etchant can be controlled.

[0032] The finger portion 27 is integrally formed with the movable electrode 20 and can be formed, for example, by patterning the silicon oxide film 2 into the shape shown in Figure 5 in the process shown in Figure 3A.

[0033] Table 1 below shows the etching results when the finger width b is varied for electrode structures with groove widths X of 1.5 μm and 2.0 μm. Samples No. 1 to 4 are for groove width X of 1.5 μm, and samples No. 5 to 9 are for groove width X of 2.0 μm. Since the unit cell width W is constant for all samples, the width Y of the movable electrode 20 is narrower in samples No. 5 to 9 (4.7 μm) compared to 5.2 μm in samples No. 1 to 4. Z1 is the distance between opposing finger portions 27 (first space width Z1), and S is the area of ​​the region sandwiched between opposing finger portions 27.

[0034] Table 1 TIFF0007846536000001.tif78158

[0035] Figure 6 is a schematic diagram showing the etching results of the electrodes in Table 1, where the numbers in the figure (e.g., No. 1) correspond to the numbers in Table 1. The etching conditions of electrode fabrication steps 1 to 6 described above were used for etching. When the groove width X was 1.5 μm, the width Y of the movable electrode 20 was 5.2 μm (constant), and the finger width b was varied to 0 μm, 0.3 μm, 0.6 μm, and 0.9 μm. The combined width Y+b of the movable electrode 20 and the finger portion 27 was 5.2 μm (No. 1), 5.5 μm (No. 2), 5.8 μm (No. 3), and 6.1 μm (No. 4).

[0036] On the other hand, when the groove width X is 2.0 μm, the width Y of the movable electrode 20 is 4.7 μm (constant), and the finger width b is varied to 0 μm, 0.5 μm, 0.8 μm, 1.1 μm, and 1.4 μm. The combined width Y+b of the movable electrode 20 and the finger portion 27 is 4.7 μm (No. 5), 5.2 μm (No. 6), 5.5 μm (No. 7), 5.8 μm (No. 8), and 6.1 μm (No. 9).

[0037] As can be seen from the schematic diagram in Figure 6, good etching is obtained when Y+b is 5.5 μm and 5.8 μm (No. 2, No. 3, No. 7, No. 8) for both groove widths X of 1.5 μm and 2.0 μm. When the width b of the finger portion 27 is increased to make Y+b 6.1 μm, etching becomes insufficient for both groove widths X of 1.5 μm (No. 4) and 2.0 μm (No. 9), and the movable electrode 20 cannot be released from the silicon substrate 1 (unreleaseable). On the other hand, when the width b of the finger portion 27 is decreased to make Y+b 5.2 μm, over-etching occurs in the structure with a groove width X of 1.5 μm (No. 1), and a fragile protrusion remains below the fixed electrode 10 (fragile).

[0038] Table 2 summarizes the results from Figure 1 and shows the dimensions when good etching results are obtained.

[0039] Table 2 TIFF0007846536000002.tif31139

[0040] Thus, in an electrode structure where the groove width X is narrowed to 2.0 μm or less, for example, 2.0 μm or 1.5 μm, by setting the first space width Z1 to a range of 6.8 μm to 7.4 μm, or in other words, by setting the ratio Z1 / X of the first space width Z1 to the groove width X to a range of 3.4 to 4.9, the balance between the amount of etchant entering from the groove portion 30 and the amount of etchant entering from the space portion 40 can be adjusted well, and good electrode etching results can be obtained.

[0041] Considering the etching mechanism, in the electrode structure shown in Figure 5, the etchant contributing to the etching of the pair of fixed electrodes 10 and movable electrodes 20 is supplied from half the area of ​​the space portion 40 (width: Z1 / 2) and from one groove portion 30 (width: X). In the etching diagram (bottom right) of Figure 5, the white area is the area to which the etchant is supplied, the hatched area is the mask area, and the total length is half the unit cell width W (hereinafter referred to as "half cell width"). The smaller the groove width X of the groove portion 30, the more difficult it becomes to balance the etchant supplied from the two white areas.

[0042] First, let's consider the case where the groove width X of the groove portion 30 is relatively large, between 2.0 μm and 2.8 μm. In this case, etching is good even when b=0 μm (no finger portion), as in No. 5 of Table 1, while etching is not good when b=1.4 μm, as in No. 9. In other words, when the groove width X occupies a ratio of 0.18 (2.0 μm / 11.2 μm) to 0.25 (2.8 μm / 11.2 μm) relative to the half-cell width (W / 2) of 11.2 μm, good etching can be obtained when the ratio of the width b of the finger portion 27 is between 0 (0 μm / 11.2 μm) and less than 0.125 (1.4 μm / 11.2 μm).

[0043] In contrast, when the groove width X of the groove portion 30 became small, such as 1.5 μm or more and less than 2.0 μm, etching was good in the range of No. 2 to No. 3 in Table 1, i.e., when b was in the range of 0.3 μm to 0.6 μm, while etching was not good in the other ranges of No. 1 and No. 4. In other words, when the ratio of the groove width X to the half-cell width (W / 2) of 11.2 μm is 0.13 (1.5 μm / 11.2 μm) to 0.18 (2.0 μm / 11.2 μm), good etching can be obtained when the ratio of the finger portion width b is 0.027 (0.3 μm / 11.2 μm) or more and 0.054 (1.4 μm / 11.2 μm) or less.

[0044] Here, we have shown the range in which etching is good based on the ratio of the finger width b to the half-cell width (W / 2). However, etching is also good with a similar ratio for the finger area (a × b) relative to half the area of ​​the space (S / 2). Therefore, even if the shape of the finger portion 27 is not rectangular, good etching can be obtained as long as the area satisfies the predetermined ratio. For example, the finger portion 27 may be semicircular, wave-shaped, or the like.

[0045] As described above, in the MEMS sensor according to the embodiment of the present invention, by, for example, providing a finger portion on the movable electrode to control the supply of etchant, it is possible to provide a highly sensitive and easy-to-fabricate MEMS sensor, particularly a capacitive acceleration sensor. [Industrial applicability]

[0046] A MEMS sensor equipped with the electrode structure according to the present invention can be applied to small acceleration sensors and the like. [Explanation of Symbols]

[0047] 1. Silicon substrate 10 Fixed electrode 20 Movable electrodes 23 Connection part 25 Isolation Joints 27 Finger section 30 groove 40 Space section 100 Accelerometer

Claims

1. A MEMS sensor comprising a pair of movable electrodes arranged parallel to each other on a cavity provided in a substrate, with a space between them, and a fixed electrode arranged parallel to the movable electrodes on the opposite side of the space, with a groove between them, The space portion includes a central portion having a first space width Z1 and an end portion having a second space width Z2. The first space width Z1 is shorter than the second space width Z2, and furthermore, The first space width Z1 and the groove width X of the groove portion are given by the following equation (1): 3.4≦(Z1 / X)≦4.9...(1) A MEMS sensor that meets the following criteria.

2. The MEMS sensor according to claim 1, wherein the set of movable electrodes each have a finger portion on the space portion side, and the first space width Z1 is the distance between the finger portions.

3. A MEMS sensor comprising a pair of movable electrodes arranged parallel to each other on a cavity provided in a substrate, with a space between them, and a fixed electrode arranged parallel to the movable electrodes on the opposite side of the space with a groove between them, wherein each of the pair of movable electrodes has a finger portion on the side facing the space, The groove width X of the groove portion is 2.0 μm or more and 2.8 μm or less, and furthermore, the following formula (2): 0≦b / ((Z1 / 2)+(Y+b)+X)<0.125...(2) However, Z1 is the distance between the finger sections, Y is the width of the movable electrode, and b is the width of the finger section. A MEMS sensor that meets the following criteria.

4. A MEMS sensor comprising a pair of movable electrodes arranged parallel to each other on a cavity provided in a substrate, with a space between them, and a fixed electrode arranged parallel to the movable electrodes on the opposite side of the space with a groove between them, wherein each of the pair of movable electrodes has a finger portion on the side facing the space, The groove width X of the groove portion is 1.5 μm or more and less than 2.0 μm, and furthermore, the following formula (3): 0.027≦b / ((Z1 / 2)+(Y+b)+X)≦0.054...(3) However, Z1 is the distance between the finger sections, Y is the width of the movable electrode, and b is the width of the finger section. A MEMS sensor that meets the following criteria.

5. The MEMS sensor according to claim 3 or 4, wherein the finger portion is a rectangular-shaped protrusion.

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