MEMS device

Abstract

To provide a MEMS device whose characteristics do not change depending on temperature without using addition of dopant.SOLUTION: The MEMS device includes a substrate having a front surface and a back surface, a recessed part formed on the front surface of the substrate, and a movable electrode and a fixed electrode connected to the substrate and arranged opposite to each other in midair above the recessed part, wherein the movable electrode includes a buried oxide layer buried in a trench provided in the movable electrode. A method of manufacturing a MEMS device includes the steps of forming a trench by etching a substrate from a surface thereof, and forming a buried oxide layer in the trench by oxidizing a side surface and a bottom surface of the trench.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to MEMS devices, and more particularly to MEMS resonators. [Background technology]

[0002] In MEMS devices using single-crystal silicon, such as resonators and acceleration sensors, the characteristic parameters of the silicon material (e.g., frequency temperature coefficient and Young's modulus) change with temperature, which poses a problem in that the characteristics of the MEMS device also change depending on temperature.

[0003] In response to this, it has been proposed to suppress the temperature dependence of characteristic parameters by changing the electronic band structure of silicon by adjusting the type and concentration of dopants added to single crystal silicon. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2024-39416

[0005] [overview] When doping single-crystal silicon with dopants, the upper limit of the dopant that can be added is determined by the solubility limit of the dopant in silicon, so there is a limit to the doping concentration that can be achieved. Also, dopants in single-crystal silicon can diffuse within the single-crystal silicon due to thermal processes, etc., causing cross-contamination and adversely affecting the electrical characteristics of MEMS devices.

[0006] Therefore, an object of the present invention is to provide a MEMS device whose characteristics do not change depending on temperature without the addition of a dopant.

[0007] One aspect of the present disclosure is a substrate having a front surface and a back surface; a recess formed on a surface of a substrate; a movable electrode and a fixed electrode connected to the substrate and arranged opposite each other in the air above the recess; Including, The movable electrode is a MEMS device that includes a buried oxide layer buried in a trench provided in the movable electrode.

[0008] Another aspect of the present disclosure is 1. A method for manufacturing a MEMS device having a buried oxide layer buried in a trench provided in a movable electrode, comprising: providing a substrate having a front surface and a back surface; Etching the substrate from the surface to form a trench; oxidizing the side and bottom surfaces of the trench to form a buried oxide layer in the trench; and a step of etching the substrate from the surface to form a recess, and forming a movable electrode connected to the substrate and held in midair above the recess; A method for manufacturing a MEMS device comprising: [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a plan view of a MEMS device according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of the MEMS device of FIG. 1 as viewed in the II-II direction. [Figure 3A] FIG. 3A is a cross-sectional view of a manufacturing process of the MEMS device according to the first embodiment of the present invention. [Figure 3B] FIG. 3B is a cross-sectional view of a manufacturing process of the MEMS device according to the first embodiment of the present invention. [Figure 3C] FIG. 3C is a cross-sectional view of a manufacturing process of the MEMS device according to the first embodiment of the present invention. [Figure 3D] FIG. 3D is a cross-sectional view of a manufacturing process of the MEMS device according to the first embodiment of the present invention. [Figure 3E] FIG. 3E is a cross-sectional view of a manufacturing process of the MEMS device according to the first embodiment of the present invention. [Figure 3F]FIG. 3F is a cross-sectional view of a manufacturing process of the MEMS device according to the first embodiment of the present invention. [Figure 4] FIG. 4 is a plan view of a MEMS device according to a second embodiment of the present invention. [Figure 5] FIG. 5 is a cross-sectional view of the MEMS device of FIG. 4 when viewed in the VV direction. [Figure 6] FIG. 6 is a schematic diagram of a manufacturing process of a MEMS device according to the second embodiment of the present invention. [Figure 7] FIG. 7 is a plan view of a MEMS device according to a third embodiment of the present invention. [Figure 8] FIG. 8 is a cross-sectional view of the MEMS device of FIG. 7 as viewed in the direction VIII-VIII. [Figure 9] FIG. 9 is a cross-sectional view of another MEMS device according to the third embodiment of the present invention.

[0010] [Detailed explanation] <First Embodiment> Fig. 1 is a plan view of a MEMS device according to a first embodiment of the present invention, generally designated 100, and Fig. 2 is a cross-sectional view of the MEMS device of Fig. 1 as viewed in the II-II direction. Here, a resonator will be described as an example of the MEMS device 100.

[0011] 1 and 2, the MEMS device 100 includes a substrate 10 made of, for example, single crystal silicon. A recess 20 is formed in the substrate 10, and a vibrator 30 and electrodes 40 are provided on top of the recess 20 so as to sandwich the vibrator 30. The vibrator 30 and the electrodes 40 are arranged in parallel and supported in midair above the recess 20.

[0012] The vibrator 30 is formed by etching a substrate 10 made of single crystal silicon, and is insulated from the substrate 10 by an isolation joint (IJ) 32 made of silicon oxide. Furthermore, a passivation film 34 made of, for example, silicon oxide is provided across the IJ 32, and a wiring layer 36 made of, for example, AlCu is provided thereon. The wiring layer 36 electrically connects the vibrator 30 to pad electrodes (not shown) and the like.

[0013] Like the vibrator 30, the electrode 40 is also formed by etching the substrate 10 made of single crystal silicon, and is insulated from the substrate 10 by an isolation joint (IJ) 42. Furthermore, a passivation film 44 is provided across the IJ 42, and a wiring layer 46 is provided thereon. The wiring layer 46 electrically connects the electrode 40 to a pad electrode (not shown) or the like.

[0014] A plurality of silicon oxide layers 38 are embedded in the vertical direction (Z-axis direction) in the vibrator 30. Here, the silicon oxide layers 38 have a structure in which the lower portions thereof protrude from the vibrator 30, but they may have an embedded structure in which the lower portions are not exposed from the vibrator 30.

[0015] In a MEMS device 100 such as a resonator, applying an AC voltage to the vibrator 30 causes the vibrator 30 to resonate, and applying a DC voltage to the electrode 40 changes the capacitance between the vibrator 30 and the electrode 40, thereby converting the mechanical vibration of the vibrator 30 into an electrical signal.

[0016] In the MEMS device 100 according to the first embodiment of the present invention, the temperature-dependent deformation of the resonator 30, which has a significant impact on the device characteristics, is offset and prevented or reduced by forming a silicon oxide layer 38 having a temperature dependence (e.g., Young's modulus) opposite to that of silicon, rather than by adding a dopant.

[0017] In particular, MEMS resonator 30 made of single crystal silicon has a relatively large frequency fluctuation due to temperature of 20 to 30 ppm / ° C., and therefore, by forming silicon oxide layer 38, frequency fluctuation can be effectively reduced or prevented.

[0018] Here, the linear temperature coefficient of frequency (TCf) will be used as an example of a device characteristic that changes with temperature.

[0019] For example, the frequency temperature coefficient of the vibrator 30 made of a composite material such as silicon and silicon dioxide is a combination of the frequency temperature coefficients of the silicon portion (main body) and the silicon dioxide portion (silicon oxide layer 38) of the vibrator 30. That is, the frequency temperature coefficient (TCf1) of the vibrator 30 is expressed by the following formula (1): TIFF2026017742000002.tif1870...Formula (1) (TCf1)SiO2: temperature coefficient of frequency of amorphous silicon oxide constituting the silicon oxide layer 38 embedded in the resonator 30, (TCf2)Si: frequency temperature coefficient of single crystal silicon constituting the resonator 30, It is expressed as:

[0020] r in formula (1) is expressed by the following formula (2): TIFF2026017742000003.tif2144...Formula (2) mSi: mass of the single crystal silicon constituting the oscillator 30, mSiO2: mass of silicon dioxide constituting the silicon oxide layer 38 embedded in the vibrator 30, fSi: Resonance frequency of the single crystal silicon constituting the oscillator 30, fSiO2: the resonant frequency of the silicon dioxide constituting the silicon oxide layer 38 embedded in the oscillator 30, It is expressed as:

[0021] When the temperature dependency of the vibrator 30 disappears, that is, when the frequency temperature coefficient (TCf1) becomes 0, r is obtained from the formulas (1) and (2) as follows: TIFF2026017742000004.tif2081...Formula (3) It can be calculated as follows.

[0022] Here, the temperature coefficient of frequency (TCf1) corresponds directly to the temperature coefficient of expansion (TCE), so (TCf1) TCE of SiO2 as SiO2: +179 (ppm / ℃), (TCf1) TCE of Si as Si: -63.82 (ppm / ℃), Substituting these into equation (3), we get r=-(+179) / (-63.82)=2.8 This becomes:

[0023] That is, by selecting the mass (mSi) of the single crystal silicon that constitutes the vibrator 30 and the mass (mSiO2) of the silicon dioxide that constitutes the silicon oxide layer 38 embedded in the vibrator 30 so that r = 2.8 from equation (2), the frequency temperature coefficient (TCf1) of the vibrator 30 can be set to 0, that is, temperature dependency can be eliminated.

[0024] In this way, the silicon dioxide that makes up the silicon oxide layer 38 embedded in the resonator 30 is amorphous, and its temperature expansion coefficient, for example, does not depend on the crystal orientation as in single-crystal silicon, but has a positive temperature dependence, in contrast to single-crystal silicon. Therefore, by adjusting the amount of silicon oxide layer 38 embedded in the resonator 30, the temperature characteristics of both materials can be offset, thereby providing a MEMS device 100 in which the temperature dependence of the frequency temperature coefficient and the like is reduced or eliminated.

[0025] Next, a method for fabricating the vibrator 30 of the MEMS device 100 will be described with reference to Figures 3A to 3F. Figures 3A to 3F are cross-sectional views taken along the line III-III in Figure 1, and in Figures 3A to 3F, the same reference numerals as in Figures 1 and 2 indicate the same or corresponding parts. The method for fabricating the vibrator 30 includes the following steps 1 to 6.

[0026] Step 1: As shown in FIG. 3A, a substrate 10 made of single crystal silicon having a front surface and a back surface is prepared.

[0027] 3B, the surface of the substrate 10 is thermally oxidized to form a silicon oxide film 12. Subsequently, the silicon oxide film 12 is patterned / etched to form openings, and then the substrate 10 is etched using the silicon oxide film 12 as a mask to form trenches 14 and 24.

[0028] 3C, after removing the silicon oxide film 12, the surface of the substrate 10 and the side and bottom surfaces of the trenches 14, 24 are oxidized by, for example, thermal oxidation. As a result, a silicon oxide layer is formed on the surface of the substrate 10, and an IJ 32 and a silicon oxide layer 38 are formed in the trenches 14, 24.

[0029] Step 4: As shown in FIG. 3D, the silicon oxide layer is partially etched by patterning / etching using a photoresist (not shown) to form a passivation film 34 and an opening 16 made of, for example, silicon oxide.

[0030] Step 5: As shown in FIG. 3E, an AlCu film is formed on the surface side of the substrate 10 by, for example, a sputtering method, and then an AlCu wiring layer 36 is formed on the passivation film 34 and in the opening 16 by patterning / etching using a photoresist (not shown).

[0031] Step 6: As shown in Fig. 3F, the substrate 10 is etched from the surface to form a recess 20. This completes the vibrator 30, which includes an IJ 32 and a buried silicon oxide layer 38 and is held in the air above the recess 20, as shown in Fig. 2. The substrate 10 around the IJ 32 is removed by etching (see Figs. 1 and 2), so the substrate 10 and the vibrator 30 are insulated by the silicon oxide IJ 32.

[0032] In the MEMS device 100 according to the first embodiment, a structure has been described in which the lower end of the silicon oxide layer 38 protrudes from the bottom of the vibrator 30, but the silicon oxide layer 38 may also be embedded in the vibrator 30 without protruding from it.

[0033] <Embodiment 2> Fig. 4 is a plan view of a MEMS device according to a second embodiment of the present invention, generally designated 200, and Fig. 5 is a cross-sectional view of the MEMS device of Fig. 4 as viewed in the VV direction. In Figs. 4 and 5, the same reference numerals as in Figs. 1 and 2 indicate the same or corresponding parts.

[0034] As shown in FIGS. 4 and 5, in the MEMS device 200, the silicon oxide layer 38 is formed from two types of silicon oxide layers 38a, 38b having different cross-sectional shapes in a plane (see FIG. 4). The two types of silicon oxide layers 38a, 38b are alternately arranged in the longitudinal direction (X-axis direction) of the vibrator 30. The two types of silicon oxide layers 38a, 38b have the same depth. The other configurations are the same as those of the MEMS device 100 according to the first embodiment.

[0035] When a silicon oxide layer 38 is formed in a resonator 30 made of single-crystal silicon by internal oxidation of a trench, stress is applied from the silicon dioxide to the silicon due to the difference in the coefficient of thermal expansion (CTE) between silicon and silicon dioxide. The silicon oxide layers 38a and 38b, which have different sizes, generate different amounts of stress around them.

[0036] In this way, by forming the silicon oxide layers 38a, 38b, it is possible to reduce or eliminate the temperature dependence of the frequency temperature coefficient, etc., and by forming two types of silicon oxide layers 38a, 38b with different sizes, it is possible to locally control the stress inside the vibrator 30, and it is possible to locally correct or prevent undesired deformation of the vibrator 30.

[0037] FIG. 6 is a schematic diagram showing the steps of forming two types of silicon oxide layers 38a and 38b according to the second embodiment of the present invention, and shows a plan view similar to FIG.

[0038] In the process of fabricating the silicon oxide layers 38a and 38b, in fabrication process 2 of the MEMS device 100 of the first embodiment described above, the substrate 10 is etched using the silicon oxide film 12 formed by thermally oxidizing the surface of the substrate 10 as a mask to form trenches 24a and 24b with different cross-sectional shapes, as shown in Fig. 6(a). Although not shown in Fig. 6, the trench 14 for forming the IJ 32 is formed in the same manner as in the first embodiment.

[0039] Subsequently, as shown in FIG. 6(b), after removing the silicon oxide film 12, the side and bottom surfaces of the trenches 24a and 24b are oxidized by, for example, thermal oxidation.

[0040] 6(c), silicon oxide layers 38a and 38b are formed. In this process, a greater stress is applied to the periphery of silicon oxide layer 38a with a smaller cross-sectional area than to the periphery of silicon oxide layer 38a with a larger cross-sectional area.

[0041] Therefore, by forming two types of silicon oxide layers 38a and 38b with different cross-sectional areas, the stress applied to the vibrator 30 can be locally changed, and thereby deformation of the vibrator 30 can be locally corrected or prevented.

[0042] Although the two types of silicon oxide layers 38a and 38b having different cross-sectional areas are alternately arranged in the longitudinal direction in the above description, three or more types of silicon oxide layers may be used, and the order of arrangement is not limited to this.

[0043] <Third Embodiment> FIG. 7 is a plan view of a MEMS device according to a third embodiment of the present invention, generally designated 300, and in FIG. 7, the same reference numerals as those in FIGS. 1 and 2 indicate the same or corresponding parts.

[0044] 7, in the MEMS device 300, electrodes 50 are provided in parallel on only one side of the vibrator 30. The electrodes 50 are connected to the substrate 10 at two points and are held in the air above the recess 20. The substrate 10 and the electrodes 50 are insulated by an IJ 52, and the electrodes 50 are connected to electrode pads (not shown) by a wiring layer 56 provided on a passivation film 54. The rest of the structure is the same as that of the MEMS device 100 according to the first embodiment.

[0045] In the MEMS device 300, by providing the resonator 30 with the silicon oxide layer 38, it is possible to provide a MEMS device 300 in which temperature dependence such as the frequency temperature coefficient is reduced or eliminated.

[0046] In the MEMS device 300 as well, the silicon oxide layer 38 may be a plurality of silicon oxide layers with different cross-sectional shapes.

[0047] Figure 8 is a cross-sectional view seen in the VIII-VIII direction of Figure 7, where Figure 8(a) is a cross-section of the MEMS device 300 of Figure 7, Figure 8(b) shows a case where an insulating layer 72 of silicon oxide or the like is provided on the vibrator 30, and Figure 8(c) shows a case where a conductive layer 74 of metal or polycrystalline silicon is provided on the vibrator 30. The arrows indicate the direction of the electric field.

[0048] The electric field between the vibrator 30 and the electrode 50 is preferably formed in a direction perpendicular to the side surfaces of the vibrator 30 and the electrode 50 (Y-axis direction), and since the vibrator 30 and the electrode 50, both made of single-crystal silicon, are conductive, it is formed parallel to the Y-axis direction.

[0049] As described above, in the MEMS device 300 according to the third embodiment (as well as the MEMS devices 100 and 200), the silicon oxide layer 38 is embedded inside the vibrator 30 made of conductive single crystal silicon, and the surface is covered with conductive single crystal silicon, so that no charge accumulation occurs on the side surface of the vibrator 30 and the electric field is not affected.

[0050] On the other hand, since the silicon oxide layer 38 embedded in the vibrator 30 is an insulator, electric charges accumulate on its surface. As a result, curved electric fields are generated from the silicon oxide layer 38 toward the electrodes 50 at the upper and lower ends of the vibrator 30, as indicated by the curved arrows in FIG. 8(a). Such electric fields may adversely affect the characteristics of the vibrator 30.

[0051] 8(b), when the top surface of the vibrator 30 is covered with an insulating layer 72, charges are accumulated on the insulating layer 72, and the electric field (curved arrow) caused by this increases. Such an electric field may adversely affect the characteristics of the vibrator 30.

[0052] On the other hand, as shown in FIG. 8(c), if the upper surface of the vibrator 30 is covered with a conductive layer 74 such as polycrystalline silicon or metal, charge is not accumulated on the silicon oxide layer 38, and the electric field caused by the accumulated charge can be reduced or eliminated (dashed curved arrow).

[0053] Unlike the structure of Figure 8, Figure 9 shows the electric field when the electrode 50 is located above (in the Z-axis direction) the vibrator 30. In Figure 9, the same reference numerals as in Figure 8 indicate the same or corresponding parts.

[0054] As shown in FIG. 9(a), when the surface of the vibrator 30 is exposed, the silicon oxide layer 38 may affect the electric field.

[0055] As shown in FIG. 9(b), when the top surface of the vibrator 30 is covered with an insulating layer 72, the electric field becomes stronger, but the accumulated charge on the insulating layer 72 affects the behavior of the electric field.

[0056] On the other hand, as shown in FIG. 9(c), when the upper surface of the vibrator 30 is covered with a conductive layer 74, charges are not accumulated on the silicon oxide layer 38, and the electric field caused by the accumulated charges can be reduced or eliminated.

[0057] 8 and 9, the influence of charge accumulation increases in the following order: (c) the configuration in which the silicon oxide layer 38 is covered with the conductive layer 74; (a) the configuration in which the silicon oxide layer 38 is exposed; and (b) the configuration in which the silicon oxide layer 38 is covered with the insulating layer 72. Therefore, in order to prevent the influence of the accumulated charge, the configuration (c) is most preferable, followed by the configuration (b).

[0058] Although the above first to third embodiments have been described using a resonator as an example, in other MEMS devices such as acceleration sensors, by providing a silicon oxide layer 38 in a single-crystal silicon movable electrode (for example, vibrator 30) and by forming silicon oxide layers 38a and 38b with different cross-sectional areas, it is possible to prevent or reduce temperature-dependent changes in the characteristics of the MEMS device. Furthermore, by providing a conductive layer on silicon oxide layer 38, it is possible to prevent or reduce the effect of stored charges on the electric field.

[0059] <Additional Notes> The present disclosure provides: a substrate having a front surface and a back surface; a recess formed on a surface of a substrate; a movable electrode and a fixed electrode connected to the substrate and arranged opposite each other in the air above the recess; Including, The movable electrode is a MEMS device that includes a buried oxide layer buried in a trench provided in the movable electrode. In the MEMS device disclosed herein, the temperature-dependent deformation of the resonator, which has a significant impact on device characteristics, is offset not by adding dopants but by forming a silicon oxide layer that has temperature dependence (e.g., Young's modulus) opposite to that of silicon, thereby preventing or reducing temperature-dependent changes in the characteristics of the MEMS device.

[0060] In this disclosure, The movable electrode is made of silicon and the buried oxide layer is made of silicon dioxide and has the following formula: TIFF2026017742000005.tif1870However, (TCf1)SiO2: Temperature coefficient of frequency of silicon dioxide, (TCf2)Si: Temperature coefficient of frequency of silicon, TIFF2026017742000006.tif2144mSi: mass of silicon, mSiO2: mass of silicon dioxide, fSi: resonant frequency of silicon, fSiO2: resonant frequency of silicon dioxide, The mass of silicon and the mass of silicon dioxide are adjusted so that TCf1, expressed as: In this way, by forming a buried oxide layer in the resonator and adjusting the mass ratio of silicon and silicon dioxide that make up the resonator, it is possible to provide a MEMS device whose characteristics do not change depending on temperature.

[0061] In the present disclosure, the buried oxide layer may be a plurality of buried oxide layers arranged in the longitudinal direction of the movable electrode, and by forming a plurality of buried oxide layers in the longitudinal direction, temperature-dependent deformation can be prevented across the entire movable electrode.

[0062] In the present disclosure, the multiple buried oxide layers may have different cross-sectional sizes parallel to the surface of the substrate, which allows the stress applied to the movable electrode to be locally changed, thereby locally correcting or preventing deformation of the movable electrode.

[0063] In the present disclosure, the buried oxide layer may have its side and bottom surfaces surrounded by the movable electrode, or its side surfaces surrounded by the movable electrode and its bottom surface protruding from the movable electrode. The buried oxide layer may have a trench shape with its bottom surface buried in the movable electrode, or an isolation joint shape with its bottom surface protruding from the movable electrode.

[0064] In the present disclosure, the top surface of the buried oxide layer is preferably covered with a conductive layer, which prevents charge accumulation on the buried oxide layer and ensures a uniform electric field.

[0065] In the present disclosure, an insulating layer (IJ) may be sandwiched between the substrate and the movable electrode to provide insulation. This is also applicable to the SCREAM process using an insulating layer (IJ).

[0066] In the present disclosure, the movable electrode is, for example, a vibrator of a resonator, and it is possible to prevent characteristics such as the resonant frequency from changing depending on temperature, thereby obtaining an accurate resonator.

[0067] The present disclosure provides: 1. A method for manufacturing a MEMS device having a buried oxide layer buried in a trench provided in a movable electrode, comprising: providing a substrate having a front surface and a back surface; Etching the substrate from the surface to form a trench; oxidizing the side and bottom surfaces of the trench to form a buried oxide layer in the trench; a step of etching the substrate from the surface to form a recess, and forming a movable electrode connected to the substrate and held in midair above the recess; The manufacturing method includes: By using a general manufacturing process, it is possible to fill the trench provided in the movable electrode with a buried oxide layer.

[0068] In the present disclosure, the buried oxide layer may be surrounded by the movable electrode on its side and bottom, or may be surrounded by the movable electrode on its side and have its bottom protrude from the movable electrode. This method can be applied to fabricating a trench-shaped buried oxide layer and forming an isolation joint-shaped buried oxide layer. [Industrial Applicability]

[0069] The MEMS device structure of the present invention can be applied to MEMS devices such as resonators and acceleration sensors. [Explanation of symbols]

[0070] 10 Substrate 16 Opening 20 recess 30 oscillators 32 Isolation Joint (IJ) 34 Passivation film 36 wiring layer 38 silicon oxide layer 40 electrodes 42 Isolation Joint (IJ) 44 Passivation film 46 Wiring layer 50 electrodes 72 Insulating layer 74 Conductive Layer 100 MEMS devices

Claims

1. a substrate having a front surface and a back surface; a recess formed on a surface of the substrate; a movable electrode and a fixed electrode connected to the substrate and arranged opposite each other in the air above the recess; Including, The movable electrode comprises a buried oxide layer buried in a trench provided in the movable electrode.

2. The movable electrode is made of silicon, the buried oxide layer is made of silicon dioxide, and has the following formula: however, (TCf １ ) SiO ２ : temperature coefficient of frequency of silicon dioxide, (TCf ２ ) Si: temperature coefficient of frequency of silicon, mSi: mass of silicon, mSiO ２ : mass of silicon dioxide, fSi: resonant frequency of silicon, fSiO ２ : the resonant frequency of silicon dioxide, TCf expressed as １ 2. The MEMS device according to claim 1, wherein the mass of silicon and the mass of silicon dioxide are adjusted so that .lambda.

3. The MEMS device according to claim 1 , wherein the buried oxide layer comprises a plurality of buried oxide layers arranged in the longitudinal direction of the movable electrode.

4. The MEMS device according to claim 3 , wherein the plurality of buried oxide layers have different cross-sectional sizes parallel to the surface of the substrate.

5. The MEMS device according to claim 1 , wherein the buried oxide layer has a side surface and a bottom surface surrounded by the movable electrode, or a side surface surrounded by the movable electrode and a bottom surface protruding from the movable electrode.

6. The MEMS device of claim 1 , wherein a top surface of the buried oxide layer is covered with a conductive layer.

7. The MEMS device according to claim 1, wherein an insulating layer (IJ) is sandwiched between the substrate and the movable electrode for insulation.

8. The MEMS device according to claim 1 , wherein the movable electrode is an oscillator of a resonator.

9. 1. A method for manufacturing a MEMS device having a buried oxide layer buried in a trench provided in a movable electrode, the method comprising: providing a substrate having a front surface and a back surface; Etching the substrate from the surface to form a trench; oxidizing the side and bottom surfaces of the trench to form the buried oxide layer in the trench; a step of etching the substrate from the surface to form a recess, and forming a movable electrode connected to the substrate and held in midair above the recess; A manufacturing method comprising:

10. The manufacturing method according to claim 9 , wherein the buried oxide layer has a side surface and a bottom surface surrounded by the movable electrode, or a side surface surrounded by the movable electrode and a bottom surface protruding from the movable electrode.

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