Three-axis MEMS acceleration sensor

Abstract

The utility model discloses a three-axis MEMS acceleration sensor which is composed of three independent sensing units X, Y and Z. The X-axis sensing unit and the Y-axis sensing unit are made of epitaxial polycrystalline silicon independently. The Z-axis sensing unit is made of epitaxial polycrystalline silicon and lower-layer polycrystalline silicon, the lower-layer polycrystalline silicon is fixed on the substrate through the Z-axis electrode anchor point and serves as a Z-axis fixed electrode, the epitaxial polycrystalline silicon serves as a Z-axis movable electrode, and due to the fact that the MEMS structure is connected with the substrate only through the supporting anchor point, the influence of substrate stress on a device is small.

Description

Technical Field

[0001] The utility model relates to a structure of a three-axis MEMS acceleration sensor, belonging to the technical field of chip manufacturing. Background Art

[0002] MEMS (Micro-Electro-Mechanical System) chips are micro-mechanical chips manufactured through micro-machining technology, and MEMS sensors are the most important products. MEMS accelerometers are micro sensors that measure the linear acceleration of objects. Due to their small size, intelligence, and low cost, they have been widely used in automotive safety, industrial automation, consumer electronics, and other fields. A three-axis MEMS accelerometer chip can simultaneously measure the acceleration of X / Y / Z three-dimensional analysis. Its working principles include piezoelectric, piezoresistive, photoelectric, and resonant. However, the most widely used is the capacitive sensor structure designed based on the principle of Hooke's law. Silicon is usually used as the structural material, and a three-dimensional MEMS structure is processed through chip processing technology similar to integrated circuits. Among them, the X and Y axis sensor structures are relatively easy to process, and the movement direction of their movable structures is parallel to the chip surface. The Z axis sensor structure is relatively difficult to process, and the movement direction of its movable structure is perpendicular to the chip surface, requiring vertical electrodes. If the electrodes are made on the substrate or cover plate, the external stress during packaging or use will be transmitted to the vertical electrodes, and the electrodes will deform, thereby deteriorating the performance of the Z axis sensor. Patent US9134337B2 discloses a typical Z-axis MEMS acceleration sensor structure with upper and lower blocking blocks to protect the movable structure, but its entire sensing electrode is fixed on the substrate and cannot isolate the influence of substrate stress. Patent US9476905B2 also discloses a similar Z-axis MEMS acceleration sensor structure, with an insulating layer on the lower blocking block to protect the movable structure, but its entire sensing electrode is also fixed on the substrate and cannot isolate the influence of substrate stress. Patent US20210214213A1 is a Z-axis acceleration sensor structure with epitaxial polysilicon as the structural material, in which the lower thin polysilicon is combined with a part of the upper epitaxial polysilicon as a movable structure, and another part of the upper epitaxial polysilicon is used as a fixed sensing electrode, that is, the fixed electrode is above the movable electrode. The fixed sensing electrode of this structure is fixed on the substrate only by an anchor point, which can well isolate the substrate stress, but the disadvantage is that the upper and lower layers of polysilicon are used as movable structures at the same time. In some application scenarios, the movable structure is always in motion, and there are problems with the reliability of the joint of the two layers of polysilicon. Utility Model Content

[0003] The technical problem to be solved by the utility model is to overcome the deficiencies in the prior art and provide a three-axis MEMS acceleration sensor. Double-layer polysilicon is used as the structural material of the three-axis MEMS acceleration sensor. The lower layer of polysilicon is fixed on the substrate through the Z-axis electrode anchor point and serves as the fixed lower electrode of the Z-axis sensing unit. The upper layer of epitaxial polysilicon serves as a movable structure and serves as the movable upper electrode of the Z-axis sensing unit. Since the MEMS structure is connected to the substrate only through the supporting anchor point, the substrate stress has little effect on the device and the processing cost is low.

[0004] In order to solve the above technical problems, the technical solution adopted by the utility model is: to provide a three-axis MEMS acceleration sensor, including an X-axis sensing unit, a Y-axis sensing unit and a Z-axis sensing unit, which are composed of two layers of polysilicon material and measure the acceleration of the three axes of X, Y and Z respectively;

[0005] The X-axis sensing unit is composed of an X-axis mass block, an X-axis fixed electrode, an X-axis movable electrode, an X-axis spring, an X-axis mass block anchor point, and an X-axis electrode anchor point. The X-axis mass block is fixed on the substrate through the X-axis spring and the X-axis mass block anchor point. The X-axis fixed electrode is fixed on the substrate through the X-axis electrode anchor point. The X-axis movable electrode is connected to the X-axis mass block and moves with the X-axis mass block. The X-axis fixed electrode and the X-axis movable electrode constitute an X-axis measuring capacitor. The size of the X-axis electrode spacing of the X-axis measuring capacitor changes with the movement of the X-axis mass block.

[0006] The Y-axis sensing unit and the X-axis sensing unit are arranged at 90 degrees to each other. The Y-axis sensing unit is composed of a Y-axis mass block, a Y-axis fixed electrode, a Y-axis movable electrode, a Y-axis spring, a Y-axis mass block anchor point, and a Y-axis electrode anchor point. The Y-axis mass block is fixed on the substrate through the Y-axis spring and the Y-axis mass block anchor point. The Y-axis fixed electrode is fixed on the substrate through the Y-axis electrode anchor point. The Y-axis movable electrode is connected to the Y-axis mass block and moves with the Y-axis mass block. The Y-axis fixed electrode and the Y-axis movable electrode constitute a Y-axis measurement capacitor. The size of the Y-axis electrode spacing of the Y-axis measurement capacitor changes with the movement of the Y-axis mass block.

[0007] The Z-axis sensing unit adopts a seesaw structure, including epitaxial polysilicon and lower polysilicon, wherein the epitaxial polysilicon includes a Z-axis frame, a Z-axis mass block, a Z-axis movable electrode, a Z-axis torsion spring and a Z-axis mass block anchor point, the Z-axis mass block is connected to the positive electrode of the Z-axis movable electrode, and a stress isolation groove is provided between the two, the Z-axis movable electrode is connected to the Z-axis torsion spring through the Z-axis frame, the Z-axis torsion spring is fixed on the substrate through the Z-axis mass block anchor point, and the Z-axis mass block, the Z-axis movable electrode and the frame constitute a Z-axis movable structure; the lower polysilicon includes a Z-axis fixed electrode and a Z-axis electrode anchor point, the Z-axis fixed electrode is fixed on the substrate through the Z-axis electrode anchor point, the Z-axis fixed electrode is suspended on the substrate, the Z-axis mass block performs a seesaw motion in the Z direction around the Z-axis torsion spring, the Z-axis movable electrode and the Z-axis fixed electrode constitute a Z-axis measurement capacitor, and the vertical spacing between the Z-axis fixed electrode and the Z-axis movable electrode constitutes a Z-axis electrode spacing of the Z-axis measurement capacitor.

[0008] When subjected to the acceleration in the +X-axis direction, the X-axis mass block moves in the -X direction relative to the X-axis mass block anchor point, driving the X-axis movable electrode to move in the -X direction, resulting in a smaller distance between the positive electrode of the X-axis fixed electrode and the X-axis movable electrode, and an increased capacitance of the positive electrode, and an increased distance between the negative electrode of the X-axis fixed electrode and the X-axis movable electrode, and a decreased capacitance of the negative electrode. The capacitance change value is calculated by the difference between the positive and negative capacitances of multiple groups of electrodes, which corresponds to the acceleration signal of the X-axis.

[0009] When subjected to the acceleration in the +Y direction, the Y-axis mass block moves in the -Y direction relative to the Y-axis mass block anchor point, driving the Y-axis movable electrode to move in the -Y direction, causing the distance between the positive electrode of the Y-axis fixed electrode and the Y-axis movable electrode to decrease, the capacitance of the positive electrode to increase, the distance between the negative electrode of the Y-axis fixed electrode and the Y-axis movable electrode to increase, the capacitance of the negative electrode to decrease, and the capacitance change value calculated by the difference of the positive and negative capacitances of multiple groups of electrodes is the acceleration signal of the Y-axis;

[0010] When the acceleration in the Z-axis direction is zero, the epitaxial polysilicon and the lower polysilicon are parallel, and the vertical spacing between them is a fixed value; when there is an acceleration in the Z-axis direction applied to the MEMS acceleration sensor, for example, a force equal to the earth's gravity and opposite in direction is applied, generating an acceleration of one unit of earth's gravity, which we call +1G, the Z-axis movable structure rotates relative to the substrate along the Z-axis torsion spring, the Z-axis mass block and the positive electrode of the Z-axis movable electrode move in the -Z direction, and the negative electrode of the Z-axis movable electrode moves in the +Z direction; the Z-axis mass block anchor point and Z The Z-axis electrode anchor point and the Z-axis fixed electrode fixed on it do not move relative to the substrate, so that the spacing between the inductive capacitor C+ composed of the positive electrode of the Z-axis movable electrode and the positive electrode of the Z-axis fixed electrode becomes smaller, and the capacitance value C=εS / d, ε is the dielectric constant, S is the area, both of which remain unchanged, d is the electrode spacing, when it becomes smaller, the capacitance value C+ increases, similarly, the electrode spacing between the inductive capacitor C- composed of the negative electrode of the Z-axis movable electrode and the negative electrode of the Z-axis fixed electrode becomes larger, and the capacitance value C- decreases. By calculating ΔC=(C+)-(C-), the acceleration value of the Z-axis can be output.

[0011] The utility model adopts double-layer polysilicon of a lower polysilicon layer and an upper epitaxial polysilicon layer as the structural material of the three-axis MEMS acceleration sensor. The lower polysilicon layer is fixed on the substrate through the Z-axis electrode anchor point and serves as the fixed lower electrode of the Z-axis sensing unit. The upper epitaxial polysilicon layer serves as a movable structure and serves as the movable upper electrode of the Z-axis sensing unit. Since the MEMS structure is only connected to the substrate through the anchor point and the rest of the structure is suspended on the substrate, the influence of the substrate stress on the device is small, and the measurement accuracy is high. In addition, no expensive SOI wafer is required, and the processing cost is low.

[0012] Due to the limitations of MEMS wafer processing technology, the thickness of the lower layer of polysilicon is generally between 0.5 and 3μm, which is relatively thin and has insufficient mechanical rigidity. When used independently as a suspended fixed lower electrode, it will produce a certain amount of movement under the action of acceleration, that is, the Z-axis measurement capacitor C will become unstable. Therefore, a part of the epitaxial polysilicon is used to make a cantilever as a reinforcement rib to fix the lower layer of polysilicon. The cantilever is connected to the reinforcement area of ​​the Z-axis fixed electrode through the through-hole polysilicon, which serves as a reinforcement rib to increase the rigidity of the lower electrode area.

[0013] Preferably, the X-axis mass block is provided with an X-axis release hole; the Y-axis mass block is provided with a Y-axis release hole; the Z-axis frame, the Z-axis mass block, and the Z-axis movable electrode are provided with an epitaxial release hole; the Z-axis fixed electrode is provided with a lower layer release hole; and the cantilever is provided with a cantilever release hole, and the release hole is used to release the MEMS structure by removing the oxide layer by HF solution or gaseous HF corrosion in the wafer processing step. Due to the design layout, some gaps are generated inside the structures of the X-axis sensing unit, the Y-axis sensing unit, and the Z-axis sensing unit, which are used for insulation between electrodes or to provide a movable space for the movable structure, such as the X-axis electrode spacing between the X-axis fixed electrode and the X-axis movable electrode, the Y-axis electrode spacing between the Y-axis fixed electrode and the Y-axis movable electrode, the vertical spacing between the Z-axis fixed electrode and the Z-axis movable electrode, and the Z-axis gap between the cantilever and the Z-axis movable structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a top view of the three-axis MEMS acceleration sensor.

[0015] Figure 2 It is a top view of the negative electrode structure of the Z-axis sensing unit.

[0016] Figure 3 is along Figure 1 A cross-sectional view along the dashed line.

[0017] Figure 4 Schematic diagram of the structural movement of the Z-axis sensing unit when sensing acceleration in the first embodiment.

[0018] Figure 5 In the first embodiment, Figure 2 A cross-sectional view along the dashed line B.

[0019] Figure 6 In the second embodiment, Figure 2 A cross-sectional view along the dashed line B.

[0020] Figure 7 — Fig.11 4 is a manufacturing flow chart of the Z-axis sensing unit in the first embodiment. DETAILED DESCRIPTION

[0021] The utility model is further described below in conjunction with the accompanying drawings and embodiments.

[0022] Embodiment 1

[0023] Three-axis MEMS accelerometer, such as Figure 1 — Figure 5 As shown, it is composed of three independent sensing units, X, Y, and Z. Their movable structural parts can be connected by signal lines or be independent of each other.

[0024] The X-axis sensing unit is composed of an X-axis mass block 10, an X-axis fixed electrode 12, an X-axis movable electrode 14, an X-axis spring 16, and an X-axis electrode anchor point 24 and an X-axis mass block anchor point 26; the X-axis mass block 10 is in the shape of a square, and the X-axis movable electrode 14 is connected to the X-axis mass block 10 and moves with the X-axis mass block 10; the X-axis mass block 10 is fixed on a substrate 70 through the X-axis spring 16 and the X-axis mass block anchor point 26, and when there is acceleration in the X-axis direction, the X-axis mass block 10 moves along the X-axis direction relative to the X-axis mass block anchor point 26; the X-axis fixed electrode 12 is fixed on the substrate 70 through the X-axis electrode anchor point 24, and the X-axis fixed electrode 12 and the X-axis movable electrode 14 form an X-axis measurement capacitor, and the size of the X-axis electrode spacing 18 changes with the movement of the X-axis mass block 10. When the X-axis sensing unit is subjected to the acceleration in the +X-axis direction, the X-axis mass block 10 moves in the -X direction relative to the X-axis mass block anchor point 26, driving the X-axis movable electrode 14 to move in the -X direction, resulting in a smaller spacing between the X-axis fixed positive electrode 12a and the X-axis movable electrode 14, a larger capacitance of the positive electrode, a larger spacing between the X-axis fixed negative electrode 12b and the X-axis movable electrode 14, a smaller capacitance of the negative electrode, and a capacitance change value calculated by differentially calculating the positive and negative capacitances of the multiple groups of electrodes, which corresponds to the acceleration signal; an X-axis release hole 20a is made on the X-axis mass block 10, which is used to remove the sacrificial oxide layer by HF solution or gaseous HF in the wafer processing step to release the MEMS structure; due to the design layout, some X-axis gaps 22a are generated inside the structure, which are used for insulation between electrodes or to provide activity space for the X-axis movable structure. In actual product design, a blocking structure can also be set between the X-axis movable electrode 14 and the X-axis fixed electrode 12 to prevent the X-axis movable electrode 14 from moving too far and causing the parallel plate electrodes to be attracted.

[0025] The Y-axis sensing unit has the same structure as the X-axis sensing unit and is arranged at 90 degrees to each other. The structure of the Y-axis sensing unit will not be described in detail here.

[0026] The Z-axis sensing unit adopts a seesaw structure and is composed of the following parts: a cantilever 32, a Z-axis movable electrode, a Z-axis mass block anchor point 36, a Z-axis electrode anchor point 38, a Z-axis torsion spring 40, a frame 42, a Z-axis mass block 48 and a Z-axis fixed electrode 60, wherein the Z-axis movable electrode is divided into a Z-axis movable positive electrode 44 and a Z-axis movable negative electrode 34, and the Z-axis fixed electrode is Figure 1The Z-axis movable electrode is hidden in the middle; the Z-axis mass block 48 is connected to the Z-axis movable positive electrode 44, and there is a stress isolation groove 46 between the two, which is used to reduce the influence of the deformation of the Z-axis mass block 48 caused by the stress and stress gradient of the epitaxial polysilicon 28 itself on the Z-axis movable positive electrode 44; the outer sides of the Z-axis movable positive electrode 44 and the Z-axis movable negative electrode 34 are connected to the frame 42, and the middle of the frame 42 is fixed to the substrate 70 through the Z-axis torsion spring 40 and the Z-axis mass block anchor point 36; in fact, the Z-axis movable structure including the Z-axis movable positive electrode 44, the Z-axis movable negative electrode 34, the frame 42 and the Z-axis mass block 48 can be regarded as a movable mass block as a whole. For the sake of simplicity, only the asymmetric part of the Z-axis movable positive electrode 44 and the Z-axis movable negative electrode 34 is named as the Z-axis mass block 48. The Z-axis fixed electrode 60 is at Figure 1 Only the cantilever 32 can be seen in the figure. The Z-axis fixed electrode is connected to the bottom of the cantilever 32 and fixed to the substrate 70 through the Z-axis electrode anchor 38. The Z-axis fixed electrode is suspended on the substrate 70, and the cantilever 32 acts as a mechanical reinforcement rib of the Z-axis fixed electrode 60. Similar to the structure of the X-axis and Y-axis sensing units, a Z-axis release hole 20c is made on the Z-axis sensing unit, which is used to remove the sacrificial oxide layer through HF solution or gaseous HF in the wafer processing step to release the MEMS structure. In the actual product design, a blocking structure is also provided to prevent the movable parts from moving too much in the six directions of +X, -X, +Y, -Y, +Z, and -Z, causing the parallel plate electrodes to be attracted or the structure to be damaged.

[0027] The X-axis sensing unit and the Y-axis sensing unit are in the same plane, and the technology is relatively mature. The Z-axis sensing unit requires electrodes in the vertical direction, which is more difficult. Figure 1 The Z-axis negative electrode region 50 in the dashed line frame is taken as an example to explain in detail. Figure 2 As shown, the outer side (+X direction side) of the Z-axis movable negative electrode 34 is connected to the frame 42, and two parts of the frame 42, namely the first side 42a of the frame and the second side 42b of the frame are located on the outer side of the cantilever 32, and are connected to the two ends of the Z-axis torsion spring 40. The Z-axis torsion spring 40 is fixed on the substrate 70 through the Z-axis mass block anchor point 38; Z-axis release holes 20c are made on the Z-axis movable negative electrode 34 and the frame 42; the Z-axis movable negative electrode 34 and the frame 32 constitute the Z-axis movable structure 30. Figure 2The dotted line portion in the figure is the blocked Z-axis fixed electrode 60, which is fixed to the cantilever 32, wherein the cantilever is connected to the first end 32a of the cantilever in the +Y direction, to the second end 32b of the cantilever in the -Y direction, and to the third end 32c of the cantilever in the -X direction, and is suspended in the +X direction. The cantilever 32 is fixed to the substrate 70 through the Z-axis mass block anchor point 36; a cantilever release hole 20d is made on the cantilever 32, which is used to release the MEMS structure in the wafer processing step; there is a Z-axis gap 22b between the cantilever 32 and the Z-axis movable structure 30, which is used for electrical isolation between the two and to provide activity space for the Z-axis movable structure.

[0028] like Figure 3 As shown, when the acceleration in the Z-axis direction is zero, the epitaxial polysilicon 28 and the lower polysilicon 58 are parallel, and the spacing between them along the Z direction in the figure, that is, the vertical spacing 68 is a fixed value determined by the design and the wafer processing accuracy, usually 0.5 to 5 μm; the lower polysilicon 58 is fixed on the substrate 70 through the first oxide layer 62, and is patterned to form a lower electrode area, wherein the -X direction is the positive lower electrode 60a, the +X direction is the negative lower electrode 60b, and the three middle blocks are anchor point areas 60c, 60d, and 60e; except for the anchor point area, the other parts of the lower polysilicon 58 do not contact the substrate 70, There is a vertical gap 64 between them, and its length is generally 0.5 to 5 μm; there is an anchor gap 52 between each anchor area 60c, 60d, and 60e for electrical isolation; the anchor 60c is connected to the positive lower electrode 60a, and the anchor 60e is connected to the negative lower electrode 60b, so that the positive lower electrode 60a and the negative lower electrode 60b are suspended and fixed on the substrate 70, but are electrically isolated from the substrate 70 by the first oxide layer 62, forming a Z-axis fixed electrode 60; a plurality of Z-axis release holes 20c are made on the lower layer of polysilicon 58, which are used to release the positive lower electrode 60a and the negative lower electrode 60b in the wafer processing process. The epitaxial polysilicon 28 is connected to the lower polysilicon 58 through the through-hole polysilicon 72a, and after patterning, a Z-axis movable structure 30 and three anchoring areas 38a, 38b, and 36 are formed; wherein the Z-axis movable structure 30 includes a Z-axis movable positive electrode 44, a Z-axis movable negative electrode 34, and a Z-axis mass block 48; there is a Z-axis gap 22b between the three anchoring areas 38a, 38b, and 36 and between the Z-axis movable structure 30, which is used for electrical isolation and providing a movement space for the Z-axis movable structure 30. The anchor point 60d of the lower polysilicon 58 is connected to the Z-axis torsion spring 40 and the Z-axis movable structure 30 through the through-hole polysilicon 72a and the Z-axis mass block anchor point 36.

[0029] When there is acceleration in the Z-axis direction, for example, a three-axis MEMS acceleration sensor Figure 4The Z-axis movable structure 30 is placed on a horizontal table in the direction shown, and a force equal to and opposite to the earth's gravity is applied, generating an acceleration of one unit of earth's gravity, which we call +1G. The Z-axis movable structure 30 rotates along the Z-axis torsion spring 40 relative to the substrate 70, the Z-axis mass block 48 and the Z-axis movable positive electrode 44 move in the -Z direction, and the Z-axis movable negative electrode 34 moves in the +Z direction; the anchor point structure 76 and the positive lower electrode 60a and the negative lower electrode 60b fixed thereon do not emit an acceleration relative to the substrate 70. Movement occurs, so that the spacing 68a of the induction capacitor C+ composed of the Z-axis movable positive electrode 44 and the positive lower electrode 60a becomes smaller, and the capacitance value c=εs / d, ε is the dielectric constant, and S is the area, both of which remain unchanged. d corresponds to the electrode spacing 68a, and when it becomes smaller, the capacitance value C+ increases. Similarly, the electrode spacing 68b of the induction capacitor C- composed of the Z-axis movable negative electrode 34 and the negative lower electrode 60b becomes larger, and the capacitance value C- decreases. Calculate ΔC=(C+)-(C-), and the Z-axis acceleration value can be output.

[0030] Due to the limitation of MEMS wafer processing technology, the thickness of the lower polysilicon 58 is generally between 0.5 and 3 μm, which is relatively thin and has insufficient mechanical rigidity. When used as a suspended lower electrode structure, it will move to a certain extent under the action of acceleration, that is, the measurement capacitor C will become unstable. Therefore, it is necessary to use a part of the epitaxial polysilicon 28 to make a cantilever 32 as a reinforcing rib to fix the lower polysilicon 58, such as Figure 5 As shown, the cantilever 32 is a part of the epitaxial polysilicon 28, and its thickness is generally 10 to 60 μm, which is more than one order of magnitude thicker than the lower polysilicon 58; the cantilever 32 is connected to the negative lower electrode reinforcement area 60f through the through-hole polysilicon 72a, and serves as a reinforcing rib to increase the rigidity of the lower electrode area, similar to the "inverted hanging beam" structure on the building. The negative lower electrode structure 78 ( Figure 5 A cantilever release hole 20d is made in the middle dotted line part) for releasing the MEMS structure in the wafer processing step. The negative lower electrode structure 78 is isolated from the Z-axis movable negative electrode 34 and the frame 42 by a Z-axis gap 22b in the Y direction and a vertical spacing 68 in the Z direction; the negative lower electrode structure 78 is isolated from the substrate 70 by a vertical gap 64 and is suspended above the substrate 70.

[0031] Embodiment 2

[0032] In some special application environments, even the negative bottom electrode structure 78 of the first embodiment still cannot meet the product requirements. In the structural design of the three-axis MEMS acceleration sensor, the negative bottom electrode structure in particular needs to be compromised in terms of performance and environmental adaptability. Figure 6As shown, the cantilever 32 is connected to the negative lower electrode reinforcement area 60f through the through-hole polysilicon 72a, which serves as a reinforcement rib to increase the rigidity of the lower electrode area, similar to the "inverted hanging beam" structure on the building. The negative lower electrode structure 80 composed of the cantilever 32, the through-hole polysilicon 72a and the negative lower electrode 60b is isolated from the Z-axis movable negative electrode 34 and the frame 42 by the Z-axis gap 22b in the Y direction and by the vertical spacing 68 in the Z direction; the negative lower electrode 60b is isolated from the substrate 70 by the vertical gap 64, but the negative lower electrode reinforcement area 60f is fixed on the substrate 70 through the first oxide layer 62, and the area of ​​the negative lower electrode reinforcement area 60f is about 10% of the area of ​​the negative lower electrode 60b. Compared with the design in which the entire lower electrode area is fixed on the substrate, the structure of the second embodiment is less affected by the substrate stress; although it is more affected by the substrate stress than the first embodiment, it has stronger environmental adaptability than the first embodiment.

[0033] Embodiment 3

[0034] The design of a MEMS structure requires a wafer processing technology to match it. The steps of forming the Z-axis sensing unit in the wafer processing flow of the three-axis MEMS acceleration sensor in the first embodiment are as follows:

[0035] (1) A first oxide layer 62 with a thickness of 0.5 to 5 μm is formed on a substrate wafer 70 by a CVD process, a lower layer of polysilicon 58 with a thickness of 0.5 to 3 μm is deposited on the first oxide layer 62 by an LPCVD process, and a Z-axis fixed electrode 60 pattern and a Z-axis release hole 22c are formed by photolithography and etching. Figure 7 As shown;

[0036] (2) Using CVD process Figure 7 A second oxide layer 66 is deposited on the wafer, and its thickness determines the size of the vertical spacing 68 between the Z-axis movable electrode and the Z-axis fixed electrode. The thickness is generally 0.5 to 5 μm. Then, a portion of the second oxide layer 66 above the anchor point region 60 c is removed by photolithography and etching to form a through hole 72, as shown in FIG. Figure 8 In the actual wafer processing process, there is usually a step of forming a blocking block in the Z direction;

[0037] (3) Figure 8 A polysilicon seed layer is deposited on the wafer, and a thick polysilicon is grown. The polysilicon is planarized by CMP to form an epitaxial polysilicon 28 with a thickness of 10 to 60 μm. Fig. 9 As shown, the through hole 72 is filled with epitaxial polysilicon 28 to form a through-hole polysilicon 72a; the epitaxial polysilicon 28 is made into a conductor through an in-situ doping process, and its resistivity is less than 0.02Ω·cm;

[0038] (4) Fig. 9The wafer shown in the figure is subjected to corresponding process steps to form a photoresist or hard mask pattern, and the epitaxial polysilicon 28 is etched to form a Z-axis release hole 20c and a Z-axis gap 22b, exposing the second oxide layer 66 in the Z-axis release hole 20c and the Z-axis gap 22b, and then the etching time is prolonged until the cantilever release hole 20d is formed to expose the first oxide layer 62. Since the etching rate of polysilicon is much greater than that of the oxide layer, the Z-axis fixed electrode 60 is protected by the second oxide layer 66 and will not be etched when the etching time is prolonged; at this time, the epitaxial polysilicon 28 is divided into a cantilever 32 and a Z-axis movable structure 30, and the two are separated by the Z-axis gap 22b; the cantilever 32 is connected to the negative lower electrode reinforcement area 60f through the through-hole polysilicon 72a, and the cantilever release hole 20d is located in the cantilever 32 and penetrates the three layers of polysilicon;

[0039] (5) Etching with HF solution or gaseous HF Fig.10 As shown in the wafer, HF reacts with the oxide layer (SiO2) but not with the polysilicon. The HF etching is isotropic, that is, the etching rate in each direction is substantially equal. HF removes the second oxide layer 66 and the first oxide layer 62 through the Z-axis gap 22b, the Z-axis release hole 20c and the cantilever release hole 20d, forming a Z-axis electrode spacing 68 and a vertical gap 64 between the lower polysilicon 58 and the substrate 70 in the vertical direction, as shown in FIG. Fig.11 As shown, the Z-axis movable structure 30 can actually move at this time; when controlling the HF corrosion time, when determining whether the movable structure and the lower electrode area, since there are no release holes on the anchor points, only the oxide layer around them is partially corroded, and the anchor points are still fixed on the substrate 70 to support the MEMS structure.

[0040] The above is only the best implementation of the present invention. It should be pointed out that, for ordinary technicians in this field, without departing from the principle of the present invention, the technical solution of the present invention can be modified or replaced by equivalents, which should also be considered to belong to the protection scope of the present invention.

Claims

1. A three-axis MEMS acceleration sensor, comprising an X-axis sensing unit, a Y-axis sensing unit and a Z-axis sensing unit, composed of two layers of polysilicon material, and measuring acceleration in the three axes of X, Y and Z respectively; The X-axis sensing unit is composed of an X-axis mass block, an X-axis fixed electrode, an X-axis movable electrode, an X-axis spring, an X-axis mass block anchor point, and an X-axis electrode anchor point. The X-axis mass block is fixed on the substrate through the X-axis spring and the X-axis mass block anchor point. The X-axis fixed electrode is fixed on the substrate through the X-axis electrode anchor point. The X-axis movable electrode is connected to the X-axis mass block and moves with the X-axis mass block. The X-axis fixed electrode and the X-axis movable electrode constitute an X-axis measuring capacitor. The size of the X-axis electrode spacing of the X-axis measuring capacitor changes with the movement of the X-axis mass block. The Y-axis sensing unit and the X-axis sensing unit are arranged at 90 degrees to each other. The Y-axis sensing unit is composed of a Y-axis mass block, a Y-axis fixed electrode, a Y-axis movable electrode, a Y-axis spring, a Y-axis mass block anchor point, and a Y-axis electrode anchor point. The Y-axis mass block is fixed on the substrate through the Y-axis spring and the Y-axis mass block anchor point. The Y-axis fixed electrode is fixed on the substrate through the Y-axis electrode anchor point. The Y-axis movable electrode is connected to the Y-axis mass block and moves with the Y-axis mass block. The Y-axis fixed electrode and the Y-axis movable electrode constitute a Y-axis measurement capacitor. The size of the Y-axis electrode spacing of the Y-axis measurement capacitor changes with the movement of the Y-axis mass block. Features: The Z-axis sensing unit adopts a seesaw structure, including epitaxial polysilicon and lower polysilicon, wherein the epitaxial polysilicon includes a Z-axis frame, a Z-axis mass block, a Z-axis movable electrode, a Z-axis torsion spring and a Z-axis mass block anchor point, the Z-axis mass block is connected to the positive electrode of the Z-axis movable electrode, and a stress isolation groove is provided between the two, the Z-axis movable electrode is connected to the Z-axis torsion spring through the Z-axis frame, the Z-axis torsion spring is fixed on the substrate through the Z-axis mass block anchor point, and the Z-axis mass block, the Z-axis movable electrode and the frame constitute a Z-axis movable structure; the lower polysilicon includes a Z-axis fixed electrode and a Z-axis electrode anchor point, the Z-axis fixed electrode is fixed on the substrate through the Z-axis electrode anchor point, the Z-axis fixed electrode is suspended on the substrate, the Z-axis mass block performs a seesaw motion in the Z direction around the Z-axis torsion spring, the Z-axis movable electrode and the Z-axis fixed electrode constitute a Z-axis measurement capacitor, and the vertical spacing between the Z-axis fixed electrode and the Z-axis movable electrode constitutes a Z-axis electrode spacing of the Z-axis measurement capacitor.

2. The three-axis MEMS acceleration sensor according to claim 1, characterized in that: A cantilever is also formed on the epitaxial polysilicon, a Z-axis gap is provided between the cantilever and the Z-axis movable structure, and the cantilever is connected to the reinforcement area of ​​the Z-axis fixed electrode through the through-hole polysilicon.

3. The three-axis MEMS acceleration sensor according to claim 1 or 2, characterized in that: The X-axis mass block is also provided with an X-axis release hole; the Y-axis mass block is provided with a Y-axis release hole; the Z-axis frame, the Z-axis mass block, and the Z-axis movable electrode are provided with an epitaxial release hole, and the Z-axis fixed electrode is provided with a lower layer release hole.

4. The three-axis MEMS acceleration sensor according to claim 2, characterized in that: A cantilever release hole is made on the cantilever.

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