JP2004245844A - Semiconductor dynamic-quantity sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a beam excitation type yaw rate sensor of a capacitance detection system, to easily manufacture it, and to detect a movable state in two directions and further in three directions (by using two). <P>SOLUTION: This sensor has two sensor units arranged along directions orthogonal each other. In the each sensor unit, a recess and a trench are formed on a main surface of a single crystal silicon substrate 101, n+ diffusion layers 117 as a pair of counter electrodes are formed to sandwich the trench therebetween along a substrate face direction, and the n+ diffusion layers 117 are formed along a direction perpendicular to the substrate face direction. The recess and the trench are filled by polysilicon films, and n+ polysilicon films are formed to sandwich the polysilicon films. The main surface of the silicon substrate 101 is smoothed, and the main surface of the silicon substrate 101 is joined to the silicon substrate 101. A reverse face side of the silicon substrate 101 is polished by a prescribed amount to form the silicon substrate 101 into a thin film, and the polysilicon films are etchedly removed from the reverse face side of the silicon substrate 101 to form an overhung beam having a weight 139. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a semiconductor dynamic sensor and a method of manufacturing the same, and particularly suitable for a yaw rate sensor and its manufacture.

  The present applicant has already proposed a semiconductor yaw rate sensor in Japanese Patent Application No. 223072 in 1992. As shown in FIG. 17, a beam structure is formed on a part of the semiconductor substrate so as to be separated from the substrate, and the AC power is applied to one surface of the weight formed at the tip of the beam and the wall surface of the substrate facing the same weight surface. To excite the weight by static electricity, and dispose electrodes on the substrate wall surface facing the one surface of the weight and the surface of the weight in an axial direction orthogonal to the excitation direction of the weight to change the capacitance between the opposed electrodes. The yaw rate acting in the same direction by electrical detection is detected.

  However, a semiconductor dynamic sensor that can move in two directions, such as a semiconductor yaw rate sensor, is insufficient from the viewpoint of its specific structure, and the manufacturing method of the sensor is not mentioned at all. I have.

  Therefore, an object of the present invention is to detect a movable state in two directions or even three directions (by using two) as well as a yaw rate sensor using a capacitance detection method of a beam excitation type. It is to provide a semiconductor dynamic sensor capable of performing the above.

In order to solve the above problems, in the invention according to claim 1,
A semiconductor physical quantity sensor having a plurality of sensor units on a substrate, wherein the sensor unit includes:
A silicon layer formed on a substrate via an insulating film, and insulated from the substrate;
A movable portion formed on the silicon layer and having a first surface and a second surface which are perpendicular to each other;
A first counter electrode formed insulated and separated from the substrate and facing the first surface of the movable portion;
A second counter electrode that is insulated and separated from the substrate and faces the second surface of the movable portion, and at least one of the first and second counter electrodes is the silicon layer that is insulated and separated from the substrate. Formed using
An electric path in the thickness direction of the silicon layer from the first opposing electrode to an electrode extraction portion electrically connected to the first opposing electrode, wherein the silicon is insulated and separated from its periphery by an insulator. The electrode is formed using a layer, and all of the electrode extraction portions are formed on the surface of the silicon layer and substantially on the same plane.

  According to a second aspect of the present invention, in the first aspect, the insulator isolation is an insulator isolation including a trench.

  According to a third aspect of the present invention, in the first or second aspect, the electric path is insulated and separated from the movable portion.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, any one of the first and second counter electrodes is formed parallel to a main surface of the silicon layer. It is characterized by the following.

  Also, in the invention according to claim 5, in any one of claims 1 to 4, one of the first and second counter electrodes is an excitation electrode for exciting the movable portion, and the other is the other. Wherein the opposite electrode is a detection electrode for detecting the displacement of the movable portion.

  Further, in the invention according to claim 6, according to claim 5, further comprising a sensing electrode for detecting excitation of the movable portion, and a sensing electrode for detecting an excitation of the movable portion with respect to an opposing surface of the movable portion corresponding to the excitation electrode. The sensing electrode and the excitation electrode are both parallel to each other.

  Further, in the invention according to claim 7, in claim 1 or 2, any one of the first and second counter electrodes is used as the substrate itself, and a desired potential is applied to the substrate. It is characterized.

  In the invention described in claim 8, in any one of claims 1 to 4, the first and second counter electrodes each detect a capacitance formed between the movable portion and the first and second counter electrodes. The displacement of the movable electrode in two directions can be detected based on the capacitance values detected by the first and second counter electrodes.

  In the invention according to claim 9, the plurality of sensor units are two sensor units arranged in directions orthogonal to each other.

(First embodiment)
An embodiment of the present invention will be described below with reference to the drawings.

  In this embodiment, two sets of sensor units each including a semiconductor yaw rate sensor described below are arranged in a direction orthogonal to each other to detect an angular velocity (yaw rate) in two axial directions.

  FIG. 2 is a schematic plan view of the semiconductor yaw rate sensor according to the present embodiment. That is, in this sensor, the cantilever 102 is formed on the single crystal silicon substrate 101, and the weight 139 is formed at the tip. In addition, three projections 103, 104, and 105 are extended at the tip of the weight 139 in the direction in which the beam extends. On the side of the single crystal silicon substrate 1 facing the tip surface of the cantilever 102 (weight 139), two projections 106 and 107 are separated from each other between the projections 103 and 104, and the projections 103 and 104 are extended. It extends in a state parallel to the direction. Similarly, on the silicon substrate 101 side facing the tip end surface of the cantilever 102 (weight 139), two projections 108 and 109 are separated between the projections 104 and 105, and the extending direction of the projections 104 and 105. Are extended in parallel with the.

  FIG. 3 is a plan view of a semiconductor yaw rate sensor including electrodes. FIG. 1 is a sectional view taken along the line AA of FIG. Note that IC circuits, wirings, and the like formed in the SOI circuit are omitted, and only aluminum electrodes for external extraction are shown with respect to only electrodes for extracting capacitance and vibration electrodes in the present sensor. That is, all the electrode extraction portions are formed on the main surface of single crystal silicon substrate 101.

As shown in FIG. 1, a single-crystal silicon substrate 101 is bonded on a single-crystal silicon substrate 110 via an SiO ₂ film 111, and the beam structure described above is formed on the single-crystal silicon substrate 101.

  1 and 3, a movable electrode 112 is formed on the surface of a weight 139 of the cantilever 102. The movable electrode 112 includes three projections 103, 104, and 105 of the weight 139. Below the weight 139, two electrodes 113 and 114 are arranged in parallel. The excitation electrode 114 is for applying AC power to excite the weight 139 by static electricity. In other words, the movable electrode 112 and the excitation electrode 114 form an excitation counter electrode.

  On the other hand, the sensing electrode 113 is for detecting the excitation of the weight 139, and the predetermined weight 139 is excited by feedback control based on an output signal accompanying the excitation of the weight 139. That is, the movable electrode 112 and the sensing electrode 113 form a feedback feedback counter electrode.

  As shown in FIG. 3, fixed electrodes 133 and 134 (projections 106) are formed with the protrusion 103 of the cantilever 102 interposed therebetween, and fixed electrodes 135 (projections 107) and 136 (projection) with the protrusion 104 interposed therebetween. 108) is formed. Further, fixed electrodes 137 (projections 109) and 138 are formed with the projection 105 interposed therebetween. That is, an opposing electrode is formed by the projection 103 (movable electrode 112) and the fixed electrodes 133 and 134, and an opposing electrode is formed by the projection 104 (movable electrode 112) and the fixed electrodes 135 and 136. Further, the projection 105 (movable electrode 112) and the fixed electrodes 137 and 138 form an opposing electrode.

4 to 8 show the manufacturing process. Hereinafter, the manufacturing process will be described. As shown in FIG. 4, an n-type (100) single-crystal silicon substrate 101 of 1 to 20 Ω · cm is prepared, and a recess 115 is formed on the main surface of the single-crystal silicon substrate 101 by dry etching or wet etching to a predetermined depth. For example, it is formed at a depth of 0.1 to 5 μm. Then, an SiO ₂ film is formed on the main surface of the single crystal silicon substrate 101, and a pattern is formed by a photolithography technique. Subsequently, a trench 116 of about 0.1 to 30 μm is formed on the main surface of the single crystal silicon substrate 101 including the bottom of the recess 115 by dry etching or the like.

In the present embodiment, a groove is formed by the concave portion 115 and the trench 116. Then, n ⁺ diffusion layer 117 is formed on the main surface of single crystal silicon substrate 101 including the inner wall of trench 116, and SiO ₂ film 118 is formed on the surface by thermal oxidation.

Thereafter, as shown in FIG. 5, a polysilicon film 119 is buried in the recess 115 and the trench 116 by the LPCVD method. Subsequently, the surface of the polysilicon film 119 is polished using the SiO ₂ film 118 as a stopper to smooth the surface. At this time, it is desirable that the surfaces of the polysilicon film 119 and the SiO ₂ film 118 be smooth.

Subsequently, a SiO ₂ film 120 having a thickness of about 0.3 to ₂ μm is formed on the surface by, for example, a CVD method or the like, and a lower contact 121 for electrical connection with the n ⁺ diffusion layer 117 is formed at a predetermined position. .

Further, n ⁺ polysilicon 122 containing As and P (phosphorus) as impurities is formed in a thickness of 0.2 to 1 μm, and this is used as a predetermined electrode pattern and a shield layer. Next, on the surface, for example, a BGSP film 123 as an insulating film is formed with a thickness of 0.2 to 1 μm. Then, the surface of the BGSP film 123 is flattened and polished.

On the other hand, as shown in FIG. 6, a silicon substrate 110 is prepared, and a 0.2-1 μm SiO ₂ film 111 is formed on the surface of the silicon substrate 110 by thermal oxidation. Subsequently, as shown in FIG. 7, the silicon substrates 101 and 110 are joined via the SiO ₂ film 111 in, for example, 1000 ° C. in N ₂ . Then, the back surface of the single crystal silicon substrate 101 is selectively polished using the SiO ₂ film 118 as a stopper. By this polishing, the polysilicon 119 and the region of the silicon substrate 101 separated thereby are exposed on the surface.

  Subsequently, an IC substrate and other devices (not shown) are manufactured in a region of the single crystal silicon substrate 101 by a known method, and an aluminum wiring, a passivation film, and a pad window (all are not shown) are formed.

Subsequently, as shown in FIG. 8, the SiO ₂ film 118 in the predetermined region is removed, and the polysilicon film 119 in the predetermined region is removed using the etching holes 124 shown in FIG. As an example, a TMAH (tetramethylammonium hydroxide) etching solution is used. By this etching, a movable electrode (beam portion) is formed.

In the semiconductor yaw rate sensor manufactured in this manner, a single crystal silicon substrate 101 thinned via a SiO ₂ film 111 is bonded on a silicon substrate 110, and a weight 139 is provided at the tip of the single crystal silicon substrate 101. A cantilever beam 102 is formed. An n ⁺ diffusion layer 117 is formed on one surface of the weight 139 (the lower surface in FIG. 1), and an n ⁺ polysilicon 122 (excitation electrode 114) is formed on the lower surface of the single crystal silicon substrate 101 facing the same weight surface. Then, an excitation counter electrode is formed by n ⁺ diffusion layer 117 and n ⁺ polysilicon 122. Then, AC power is applied to the excitation counter electrode, and the weight 139 is excited by static electricity. Further, in the axial direction perpendicular to the driving direction of the weight 139, ^{n +} diffusion layer 117 on one surface of the weight 139, also has ^{n +} diffusion layer 117 on the wall surface of the single crystal silicon substrate 101 facing the same weight surface is formed, the yaw rate detection electrode is formed by the ^{n +} diffusion layer 117 of the weight 139 side and the ^{n +} diffusion layer 117 of the wall surface side of the single crystal silicon substrate 101. The yaw rate detecting electrode detects a change in electric capacity and detects a yaw rate acting in the same direction.

That is, AC power is applied to the excitation opposite electrode (the n ⁺ diffusion layer 117 and the n ⁺ polysilicon 122) to excite the weight 139 by static electricity. In this state, the capacitance (the ^{n +} diffusion layer 117 of the weight 139 side, a single wall side of the ^{n +} diffusion layer 117 of crystalline silicon substrate 101) yaw rate detecting electrodes in the axial direction perpendicular to the driving direction of the weight 139 by Is detected, and the yaw rate acting in the same direction is detected.

As described above, in the present embodiment, on the main surface of the single-crystal silicon substrate 101, the concave portion 115 and the trench 116 are formed as grooves of a predetermined depth for forming the cantilever 102 having the weight 139 (first step). ), An n ⁺ diffusion layer 117 as a pair of opposing electrodes across the trench 116 in the substrate surface direction (left-right direction in FIG. 4) in the substrate surface region serving as the weight 139 and the inner wall of the recess 115 and the trench 116 surrounding the weight 139. And an n ⁺ diffusion layer 117 (first electrode) is formed in a direction perpendicular to the substrate surface direction (the vertical direction in FIG. 5; the thickness direction of the silicon substrate 101) in the substrate surface region serving as the weight 139 ( 2nd process). The recesses 115, ^{n +} polysilicon layer 122 (electrode opposed to each other across the polysilicon film 119 ^{n +} diffusion layer 117 (the first electrode) to fill polysilicon film 119 of the trench 116 as a filler Is formed, and the main surface of the single crystal silicon substrate 101 is smoothed (third step), and the main surface of the single crystal silicon substrate 101 and the silicon substrate 110 are joined (fourth step). Further, the rear surface of the single-crystal silicon substrate 101 is polished by a predetermined amount to make the single-crystal silicon substrate 101 thinner (fifth step), and the polysilicon film 119 is etched away from the rear surface of the single-crystal silicon substrate 101 to remove the weight 139. Is formed (sixth step).

As a result, the single-crystal silicon substrate 101 bonded to the silicon substrate 110 via the SiO ₂ film 111 (insulating film) and thinned, and the beam 102 formed on the single-crystal silicon substrate 101 and having the weight 139, The movable electrode 112 and the excitation electrode 114 (first opposing electrode) formed on one surface of the weight 139 and the wall surface corresponding to the same weight surface are orthogonal to the movable electrode 112 and the excitation electrode 114 of the weight 139. The projections 103 to 105 and fixed electrodes 133 to 138 (second counter electrodes) formed on one surface of the weight 139 and the wall surface facing the same weight surface in the axial direction are provided.

One of the opposing electrodes, that is, the movable electrode 112 and the excitation electrode 114 are formed parallel to the main surface of the single-crystal silicon substrate 101. Further, all the electrode take-out portions were formed on the same surface of the thinned single-crystal silicon substrate 101.
As described above, the single crystal silicon substrate 101 bonded to the silicon substrate 110 via the SiO ₂ film 111 and thinned, and the cantilever beam formed on the single crystal silicon substrate 101 and having the weight 139 at the tip end 102, one surface of the weight 139 and a counter electrode for excitation formed on the wall surface of the single-crystal silicon substrate 101 facing the same weight surface and applying an AC power to excite the weight 139 by static electricity; An electrode for detecting a yaw rate formed on one surface of the weight 139 and a wall surface of the single-crystal silicon substrate 101 facing the surface of the weight 139 in a direction perpendicular to the axis, and detecting a change in capacitance and detecting a yaw rate acting in the same direction. And a semiconductor yaw rate sensor having the following.

  In this way, by using the surface micromachining technology, heat treatment, photolithography, and the like are not performed during wafer processing, particularly during IC circuit fabrication, in the presence of wafer recesses, through holes, etc. Therefore, it is possible to stabilize the device and improve the accuracy.

  As an application of this embodiment, in the above-described embodiment, the structure in which the excitation electrode and the sense electrode are embedded in the substrate has been described. However, the sense electrode may be omitted for cost reduction. In this case, in addition to the above structure, a silicon substrate can be used as it is as the excitation electrode.

  In this embodiment, the electrodes formed in parallel with the wafer surface are used as sense electrodes and excitation electrodes, and the electrodes in the vertical direction are used as fixed electrodes for detecting Coriolis force. it can. That is, one of the fixed electrodes formed in the vertical direction on the silicon substrate 101 is used as an excitation electrode, the other vertical electrode is used as a sensing electrode for applying feedback, and a horizontal electrode on the wafer surface is used as a Coriolis force. May be used as an electrode for detecting

  Further, as the polysilicon film 119 (that is, a polycrystalline silicon film) for filling the recess 115 and the trench 116, an amorphous silicon film or a mixed silicon film of polycrystalline and amorphous may be used.

(Second embodiment)
Next, the second embodiment will be described focusing on the differences from the first embodiment.

  This embodiment is intended to further increase the output as compared with the first embodiment and to prevent the beam from being broken by excessive impact or the like. 9 to 15 show a manufacturing process of the sensor. Hereinafter, the manufacturing process will be described.

In FIG. 4 of the first embodiment, as shown in FIG. 9, after the formation of the SiO ₂ film 118, a 200-2000 ° Si ₃ N ₄ film 125 is formed by LPCVD. In this embodiment, the thickness of the Si ₃ N ₄ film 125 is set to 500 °.

  The surface flattening polishing as shown in FIG. 7 of the first embodiment is performed by a process similar to that of the first embodiment. Subsequently, a predetermined pattern is formed with the resist 126 of FIG. 9 by photolithography. Then, as shown in FIG. 10, a region to be a sensor portion of the single crystal silicon substrate 101 is partially removed by dry etching or the like.

Next, using the resist 126 as a mask, the SiO ₂ film 118 is removed by wet etching mainly using hydrofluoric acid, for example. Subsequently, the resist 126 is removed.

  Hereinafter, the description will be made using an enlarged view of the sensor unit B in FIG. 10 for easy understanding. FIG. 11 is an enlarged portion thereof.

As shown in FIG. 12, an SiO ₂ film 127 is formed at 500 to 10000 ° using the Si ₃ N ₄ film 125 as a mask for thermal oxidation. In this embodiment, the thickness of the SiO ₂ film 127 is set to 1000 °.

Subsequently, as shown in FIG. 13, the Si ₃ N ₄ film 125 used as a mask during thermal oxidation is removed by plasma etching or hot phosphoric acid etching. Subsequently, a polysilicon 128 is formed on the surface by LPCVD or the like, and the surface of the polysilicon 128 is removed by selective polishing using the SiO ₂ film 127 as a stopper.

  Further, the surface is finished with a TMAH (tetramethylammonium hydroxide) solution. Here, a process of forming an IC circuit or the like is performed on the peripheral portion (not shown).

Then, as shown in FIG. 14, a Si ₃ N ₄ film 129 is formed on the surface to a thickness of 500 to 2000 °, and an n ⁺ polysilicon layer 130 is formed as a stopper against an excessive amplitude of the electrode layer and the sensor. Subsequently, a BPSG film 131 is formed as a surface protection film. Note that this film can also be formed of a Si ₃ N ₄ film or the like. Subsequently, the window 132 is opened.

  Subsequently, as shown in FIG. 15, the polysilicon 119 and the polysilicon 128 are etched and removed from the window 132 using a TMAH solution. In this way, a sensor having a movable portion (cantilever) surrounded all around by the electrodes and the stopper is formed. Further, in this structure, when the weight portion is excited in the direction perpendicular to the substrate, as shown in FIG. 15, since a> b and b are within the range of a, the capacity of detecting the yaw rate by the excitation is small. Little change. Further, the relationship between a and b can be made in the first embodiment.

  FIG. 16 is a diagram in which the overall appearance can be understood in more detail. As described above, in this embodiment, since the stopper member is disposed above the cantilever 102, the output is further increased as compared with the first embodiment, and the destruction of the cantilever 102 due to excessive impact or the like is prevented. it can.

  Note that the present invention is not limited to a cantilever. Further, the present invention is not limited to the yaw rate detection. For example, the electrode used as the excitation electrode in the above-described embodiment may be used as a capacitance sensor for detecting displacement in the vertical direction and used as a dynamic sensor capable of detecting displacement in two directions. It is.

  As described above in detail, according to the present invention, it is possible to easily manufacture the yaw rate sensor based on the beam excitation type capacitance detection method and the yaw rate sensor thereof, and it is also possible to detect the movable state in two directions or even three directions. A semiconductor dynamic sensor and an excellent effect capable of manufacturing the same are exhibited.

  Further, according to the present invention, since all of the electrode lead-out portions are formed on the surface of the silicon layer and substantially on the same plane, signal lines input to and output from the sensor can be accurately and easily formed.

  Further, a silicon layer formed through the insulating film on the substrate to form a movable portion in the thickness direction of the silicon layer from the first counter electrode to a specific electrode extraction portion of the electrode extraction portion. And the side of the electric path is insulated and separated from its surroundings by insulator separation. Therefore, the electric path in the thickness direction of the silicon layer from the first counter electrode to the specific electrode take-out part is separately formed by aluminum. It is possible to efficiently form an electrical path in the thickness direction of the silicon layer without using a wiring such as the above.

It is sectional drawing of the semiconductor yaw rate sensor in 1st Example. FIG. 2 is a schematic plan view of a semiconductor yaw rate sensor according to the first embodiment. FIG. 3 is a plan view of a semiconductor yaw rate sensor including electrodes. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows the manufacturing process of the semiconductor yaw rate sensor of 2nd Example. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. It is sectional drawing which shows a manufacturing process. FIG. 3 is an explanatory diagram for explaining the principle of a sensor.

Explanation of reference numerals

Reference Signs List 101 single-crystal silicon substrate 102 cantilever 103 to 105 protrusion forming second counter electrode 110 silicon substrate 111 SiO ₂ film as insulating film 112 movable electrode forming first counter electrode 114 first counter electrode Exciting electrode to be configured 115 Depressed portion to configure a groove 116 Trench to configure a groove 117 n ⁺ Diffusion layer 119 Polysilicon film 122 n ⁺ Polysilicon film as a filler 133 to 138 Fixed electrode to configure a second counter electrode 139 Weight

Claims (9)

A semiconductor physical quantity sensor having a plurality of sensor units on a substrate, wherein the sensor unit includes:
A silicon layer formed on a substrate via an insulating film, and insulated from the substrate;
A movable portion formed on the silicon layer and having a first surface and a second surface which are perpendicular to each other;
A first counter electrode formed insulated and separated from the substrate and facing the first surface of the movable portion;
A second counter electrode that is insulated and separated from the substrate and faces the second surface of the movable portion, and at least one of the first and second counter electrodes is the silicon layer that is insulated and separated from the substrate. Formed using
An electric path in the thickness direction of the silicon layer from the first opposing electrode to an electrode extraction portion electrically connected to the first opposing electrode, wherein the silicon is insulated and separated from its periphery by an insulator. A semiconductor dynamic quantity sensor, wherein the semiconductor physical quantity sensor is formed using a layer, and furthermore, all of the electrode extraction portions are formed on the surface of the silicon layer and substantially on the same plane. The semiconductor physical quantity sensor according to claim 1, wherein the insulator isolation is an insulator isolation including a trench. The semiconductor physical quantity sensor according to claim 1, wherein the electric path is insulated and separated from the movable part. 4. The semiconductor dynamic sensor according to claim 1, wherein one of the first and second counter electrodes is formed parallel to a main surface of the silicon layer. Either one of the first and second counter electrodes is used as an excitation electrode for exciting the movable part, and the other counter electrode is used as a detection electrode for detecting displacement of the movable part. The semiconductor dynamic sensor according to any one of claims 1 to 4, wherein Furthermore, it has a sensing electrode for detecting the excitation of the movable portion, and the sensing electrode and the excitation electrode are both parallel to a facing surface of the movable portion corresponding to the excitation electrode. The semiconductor dynamic sensor according to claim 5, wherein: 3. The semiconductor dynamic sensor according to claim 1, wherein one of the first and second counter electrodes is the substrate itself, and a desired potential is applied to the substrate. 4. The first and second counter electrodes each detect a capacitance formed between the movable portion and the first and second counter electrodes. Based on the capacitance values detected by the first and second counter electrodes, the first and second counter electrodes detect the capacitance of the movable electrode. The semiconductor dynamic sensor according to any one of claims 1 to 4, wherein displacement detection in two directions is enabled. 9. The semiconductor dynamic sensor according to claim 1, wherein the plurality of sensor units are two sensor units arranged in directions orthogonal to each other.