CN118243962A - Three-axis MEMS acceleration sensor and manufacturing method of Z-axis sensing unit thereof - Google Patents
Three-axis MEMS acceleration sensor and manufacturing method of Z-axis sensing unit thereof Download PDFInfo
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- 230000001133 acceleration Effects 0.000 title claims abstract description 51
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 15
- 229910021420 polycrystalline silicon Inorganic materials 0.000 claims abstract description 111
- 229920005591 polysilicon Polymers 0.000 claims abstract description 92
- 239000000758 substrate Substances 0.000 claims abstract description 65
- 238000000034 method Methods 0.000 claims abstract description 23
- 238000005530 etching Methods 0.000 claims abstract description 20
- 230000008569 process Effects 0.000 claims abstract description 11
- 230000007797 corrosion Effects 0.000 claims abstract description 7
- 238000005260 corrosion Methods 0.000 claims abstract description 7
- 238000000151 deposition Methods 0.000 claims abstract description 7
- 238000005259 measurement Methods 0.000 claims description 18
- 239000003990 capacitor Substances 0.000 claims description 16
- 230000003014 reinforcing effect Effects 0.000 claims description 10
- 238000002955 isolation Methods 0.000 claims description 6
- 239000000463 material Substances 0.000 claims description 6
- 238000001259 photo etching Methods 0.000 claims description 5
- 239000004020 conductor Substances 0.000 claims description 4
- 238000011065 in-situ storage Methods 0.000 claims description 3
- 238000012545 processing Methods 0.000 description 17
- 230000035882 stress Effects 0.000 description 11
- 238000013461 design Methods 0.000 description 8
- 238000005516 engineering process Methods 0.000 description 6
- 230000009471 action Effects 0.000 description 3
- 230000000903 blocking effect Effects 0.000 description 3
- 230000008859 change Effects 0.000 description 3
- 238000005229 chemical vapour deposition Methods 0.000 description 3
- 230000002787 reinforcement Effects 0.000 description 3
- 239000003351 stiffener Substances 0.000 description 3
- 230000004888 barrier function Effects 0.000 description 2
- 238000010276 construction Methods 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 230000007613 environmental effect Effects 0.000 description 2
- 230000005484 gravity Effects 0.000 description 2
- 238000009413 insulation Methods 0.000 description 2
- SLXKOJJOQWFEFD-UHFFFAOYSA-N 6-aminohexanoic acid Chemical compound NCCCCCC(O)=O SLXKOJJOQWFEFD-UHFFFAOYSA-N 0.000 description 1
- 230000005483 Hooke's law Effects 0.000 description 1
- 206010063385 Intellectualisation Diseases 0.000 description 1
- 229910004298 SiO 2 Inorganic materials 0.000 description 1
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000006355 external stress Effects 0.000 description 1
- 238000004518 low pressure chemical vapour deposition Methods 0.000 description 1
- 238000005459 micromachining Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000004806 packaging method and process Methods 0.000 description 1
- 238000000206 photolithography Methods 0.000 description 1
- 229920002120 photoresistant polymer Polymers 0.000 description 1
- 229910052710 silicon Inorganic materials 0.000 description 1
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B7/00—Microstructural systems; Auxiliary parts of microstructural devices or systems
- B81B7/02—Microstructural systems; Auxiliary parts of microstructural devices or systems containing distinct electrical or optical devices of particular relevance for their function, e.g. microelectro-mechanical systems [MEMS]
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
- B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
- B81C1/00015—Manufacture or treatment of devices or systems in or on a substrate for manufacturing microsystems
- B81C1/00023—Manufacture or treatment of devices or systems in or on a substrate for manufacturing microsystems without movable or flexible elements
- B81C1/00039—Anchors
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
- B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
- B81C1/00349—Creating layers of material on a substrate
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
- B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
- B81C1/00436—Shaping materials, i.e. techniques for structuring the substrate or the layers on the substrate
- B81C1/00523—Etching material
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
- G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
- G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
- G01P15/125—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values by capacitive pick-up
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
- G01P15/18—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration in two or more dimensions
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B2201/00—Specific applications of microelectromechanical systems
- B81B2201/02—Sensors
- B81B2201/0228—Inertial sensors
- B81B2201/0235—Accelerometers
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Abstract
The invention discloses a triaxial MEMS acceleration sensor, which consists of X, Y, Z independent sensing units, wherein X, Y axis sensing units are independently made of epitaxial polysilicon; the Z-axis sensing unit is manufactured by epitaxial polycrystalline silicon and lower polycrystalline silicon, the lower polycrystalline silicon is fixed on the substrate through a 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 the MEMS structure is connected with the substrate only through a supporting anchor point, so that the influence of substrate stress on a device is small. The manufacturing method of the Z-axis sensing unit comprises the steps of firstly forming a lower layer polycrystalline silicon pattern on a first oxide layer, then depositing a second oxide layer and forming a through hole pattern, then forming an epitaxial polycrystalline silicon layer through an epitaxial process, then etching the epitaxial polycrystalline silicon layer and etching the through hole polycrystalline silicon and the lower layer polycrystalline silicon for a longer time, finally removing part of the second oxide layer and the first oxide layer through HF corrosion, and releasing the MEMS structure.
Description
Technical Field
The invention relates to a structure of a triaxial MEMS acceleration sensor and a manufacturing method of a Z-axis sensing unit thereof, belonging to the technical field of chip manufacturing.
Background
MEMS (Micro-Electro-MECHANICAL SYSTEM) chips are micromechanical chips manufactured by micromachining techniques, and MEMS sensors are among the most important products. The MEMS acceleration sensor is a miniature sensor for measuring linear acceleration of an object, and has been widely applied to the fields of automobile safety, industrial automation, consumer electronics and the like due to small volume, intellectualization and low cost. A chip of the triaxial MEMS acceleration sensor can simultaneously measure acceleration analyzed by X/Y/Z three-dimensional degree, the working principle of the triaxial MEMS acceleration sensor comprises piezoelectric type, piezoresistive type, photoelectric type, resonant type and the like, but the capacitive sensor structure designed based on Hooke's law principle is most widely adopted, silicon is generally used as a structural material, a stereoscopic MEMS structure is processed by a chip processing technology similar to an integrated circuit, wherein X, Y-axis sensor structures are easier to process, the moving direction of movable structures of the sensor structures is parallel to the surface of the chip, the Z-axis sensor structure is harder to process, the moving direction of movable structures of the sensor structures is perpendicular to the surface of the chip, electrodes in the perpendicular direction are required, if the electrodes are manufactured on a substrate or a cover plate, external stress can be conducted to the perpendicular electrodes in the packaging or using process, and the electrodes can deform, so that the performance of the Z-axis sensor is deteriorated. Patent US9134337B2 discloses a typical Z-axis MEMS acceleration sensor structure with upper and lower barrier blocks protecting the movable structure, but its entire sensing electrode is fixed on the substrate, unable to isolate the effect of substrate stress. Patent US9476905B2 discloses a similar Z-axis MEMS acceleration sensor structure, with an insulating layer on the lower barrier to protect the movable structure, but its entire sensing electrode is also fixed on the substrate, which cannot isolate the effect of the substrate stress. Patent US20210214213A1 is a Z-axis acceleration sensor structure using epitaxial polysilicon as a structural material, in which a lower thin polysilicon layer and a part of an upper epitaxial polysilicon layer are combined together to form a movable structure, and the other part of the upper epitaxial polysilicon layer is used as a fixed sensing electrode, that is, the fixed sensing electrode is above the movable electrode, and the fixed sensing electrode of the structure is fixed on a substrate only through one anchor point, so that the substrate stress can be well isolated, but the defect is that the upper polysilicon layer and the lower polysilicon layer are simultaneously used as the movable structure, the movable structure is in a motion state in some application scenarios, and the reliability of the combination part of the two polysilicon layers is problematic.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provide the triaxial MEMS acceleration sensor, wherein double-layer polysilicon is used as a structural material of the triaxial MEMS acceleration sensor, lower-layer polysilicon is fixed on a substrate through a Z-axis electrode anchor point and used as a fixed lower electrode of a Z-axis sensing unit, upper-layer epitaxial polysilicon is used as a movable structure and used as a movable upper electrode of the Z-axis sensing unit, and the MEMS structure is connected with the substrate only through a support anchor point, so that the substrate stress has little influence on devices and the processing cost is low.
In order to solve the technical problems, the invention adopts the following technical scheme: the triaxial MEMS acceleration sensor comprises an X-axis sensing unit, a Y-axis sensing unit and a Z-axis sensing unit, wherein the X-axis sensing unit, the Y-axis sensing unit and the Z-axis sensing unit are made of two layers of polysilicon materials, and acceleration in three axial directions of X, Y, Z is measured respectively;
The X-axis sensing unit consists 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, wherein the X-axis mass block is fixed on a 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 along with the X-axis mass block, the X-axis fixed electrode and the X-axis movable electrode form an X-axis measurement capacitor, and the size of the X-axis electrode distance of the X-axis measurement capacitor changes along with the movement of the X-axis mass block;
The Y-axis sensing unit and the X-axis sensing unit are mutually distributed at 90 degrees, 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 a 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 on the Y-axis mass block and moves along with the Y-axis mass block, the Y-axis fixed electrode and the Y-axis movable electrode form a Y-axis measurement capacitor, and the size of the Y-axis electrode spacing of the Y-axis measurement capacitor changes along with the movement of the Y-axis mass block;
The Z-axis sensing unit adopts a teeterboard structure and comprises epitaxial polycrystalline silicon and lower polycrystalline silicon, wherein the epitaxial polycrystalline silicon comprises 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 with a positive electrode of the Z-axis movable electrode, a stress isolation groove is arranged between the Z-axis movable electrode and the positive electrode, the Z-axis movable electrode is connected with the Z-axis torsion spring through the Z-axis frame, the Z-axis torsion spring is fixed on a substrate through the Z-axis mass block anchor point, and the Z-axis mass block, the Z-axis movable electrode and the frame form a Z-axis movable structure; the lower polysilicon layer comprises 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 moves in a seesaw mode around the Z-axis torsion spring in the Z direction, the Z-axis movable electrode and the Z-axis fixed electrode form a Z-axis measurement capacitor, and the vertical distance between the Z-axis fixed electrode and the Z-axis movable electrode forms the Z-axis electrode distance of the Z-axis measurement capacitor.
When the acceleration in the +X axis direction acts, the X axis mass block moves to the-X direction relative to an X axis mass block anchor point to drive the X axis movable electrode to move to the-X direction, so that the distance between the positive electrode of the X axis fixed electrode and the X axis movable electrode is reduced, the positive electrode capacitance is increased, the distance between the negative electrode of the X axis fixed electrode and the X axis movable electrode is increased, the negative electrode capacitance is reduced, the capacitance change value is calculated by the positive and negative capacitance difference of a plurality of groups of electrodes, and the corresponding acceleration signal of the X axis is obtained;
When the acceleration in the +Y axis direction acts, the Y-axis mass block moves to the-Y direction relative to the Y-axis mass block anchor point to drive the Y-axis movable electrode to move to the-Y direction, so that the distance between the positive electrode of the Y-axis fixed electrode and the Y-axis movable electrode is reduced, the positive electrode capacitance is increased, the distance between the negative electrode of the Y-axis fixed electrode and the Y-axis movable electrode is increased, the negative electrode capacitance is reduced, and the capacitance change value is calculated by the positive and negative capacitance difference of a plurality of groups of electrodes, namely the acceleration signal of the Y axis;
When the acceleration in the Z-axis direction is zero, the epitaxial polysilicon and the lower polysilicon are parallel, and the vertical distance between the epitaxial polysilicon and the lower polysilicon is a fixed value; when acceleration in the Z axis direction is applied to the MEMS acceleration sensor, for example, a force which is equal to the gravity of the earth and opposite to the direction is applied to generate an acceleration with the unit of the gravity of the earth, the Z axis movable structure is called +1G, the Z axis movable structure rotates along the Z axis torsion spring relative to the substrate, the Z axis mass block moves towards the-Z direction with the positive electrode of the Z axis movable electrode, and the negative electrode of the Z axis movable electrode moves towards the +Z direction; the Z-axis mass block anchor point and the Z-axis electrode anchor point and the Z-axis fixed electrode fixed on the Z-axis mass block anchor point do not move relative to the substrate, so that the distance between the sensing capacitor C+ formed by the positive electrode of the Z-axis movable electrode and the positive electrode of the Z-axis fixed electrode becomes smaller, the capacitance value C=epsilon S/d, epsilon is the dielectric constant, S is the area, both of which are unchanged, d is the electrode distance, when the Z-axis mass block anchor point and the Z-axis electrode anchor point are smaller, the capacitance value C+ is increased, and similarly, the electrode distance between the sensing capacitor C-formed by the negative electrode of the Z-axis movable electrode and the negative electrode of the Z-axis fixed electrode is increased, the capacitance value C-is reduced, and delta C= (C+) - (C-) is calculated, so that the acceleration value of the Z-axis can be output.
According to the invention, double-layer polysilicon of lower polysilicon and upper epitaxial polysilicon is used as structural materials of the triaxial MEMS acceleration sensor, the lower polysilicon is fixed on a substrate through a Z-axis electrode anchor point and used as a fixed lower electrode of a Z-axis sensing unit, the upper epitaxial polysilicon is used as a movable structure and used as a movable upper electrode of the Z-axis sensing unit, and as the MEMS structure is only connected with the substrate through an anchor point, the rest part of the MEMS structure is suspended in the substrate, the influence of substrate stress on a device is small, and the measurement accuracy is high; and the expensive SOI wafer is not needed, and the processing cost is low.
Because of the limitation of the MEMS wafer processing technology, the thickness of the lower layer polysilicon is generally between 0.5 and 3 mu m, the lower layer polysilicon is relatively thin, the mechanical rigidity is insufficient, a certain amount of movement can be generated under the action of acceleration when the lower layer polysilicon is independently used as a suspended fixed lower electrode, that is, the Z-axis measurement capacitance C can become unstable, so that a cantilever is manufactured by using a part of epitaxial polysilicon as a reinforcing rib for fixing the lower layer polysilicon, and the cantilever is connected with a reinforcing area of a Z-axis fixed electrode through-hole polysilicon and used as the reinforcing rib to increase the rigidity of the lower electrode area.
Preferably, an X-axis release hole is formed in the X-axis mass block; y-axis release holes are formed in the Y-axis mass block; the Z-axis frame, the Z-axis mass block and the Z-axis movable electrode are provided with epitaxial release holes, the Z-axis fixed electrode is provided with a lower layer release hole, the cantilever is provided with a cantilever release hole, and the release holes are used for removing an oxide layer through HF solution or gaseous HF corrosion in the wafer processing process step to release the MEMS structure. Some gaps may be generated inside the structures of the X-axis sensing unit, the Y-axis sensing unit, and the Z-axis sensing unit for insulation between electrodes or to provide a movable structure with a movable space, such as an X-axis electrode gap between the X-axis fixed electrode and the X-axis movable electrode, a Y-axis electrode gap between the Y-axis fixed electrode and the Y-axis movable electrode, a vertical gap between the Z-axis fixed electrode and the Z-axis movable electrode, and a Z-axis gap between the cantilever and the Z-axis movable structure, due to design layout.
In order to solve the technical problems, the invention also provides a manufacturing method of the Z-axis sensing unit of the triaxial MEMS acceleration sensor, which comprises the following steps:
(1) Manufacturing a first oxide layer on a substrate wafer, depositing lower polysilicon on the first oxide layer, and forming a Z-axis fixed electrode and a lower release hole by photoetching and etching methods;
(2) Continuing to deposit a second oxide layer, and forming a through hole on the second oxide layer by a photoetching and etching method;
(3) Depositing a polysilicon seed layer, epitaxial thick polysilicon and CMP flattening on the second oxide layer to form epitaxial polysilicon, and filling the epitaxial polysilicon into the through hole to form through hole polysilicon;
(4) Etching the epitaxial polysilicon by using a mask to form an epitaxial release hole and a Z-axis gap, exposing the second oxide layer, lengthening etching time until part of the first oxide layer is exposed, and dividing the epitaxial polysilicon into a Z-axis mass block, a Z-axis movable electrode and a frame to form a Z-axis movable structure;
(5) Removing the second oxide layer and the first oxide layer by using HF solution or gaseous HF corrosion, forming a vertical interval between the epitaxial polysilicon and the lower polysilicon, wherein the vertical interval is used as an electrode interval of a Z-axis measurement capacitor, and simultaneously forming a vertical gap between the lower polysilicon and the substrate, so that a Z-axis fixed electrode is suspended on the substrate; and controlling HF corrosion time, and forming a support anchor point for supporting the MEMS structure on the substrate.
The method is simple to operate, does not need to use an expensive SOI wafer, and has low processing cost.
Preferably, step (4) further forms a cantilever on the epitaxial polysilicon, a Z-axis gap is formed between the Z-axis movable structure and the cantilever, and the cantilever is connected to the reinforced region of the Z-axis fixed electrode through the through-hole polysilicon.
Preferably, the reinforcing region of the Z-axis fixed electrode is fixed on the substrate by a part of the first oxide layer, and the area of the reinforcing region of the Z-axis fixed electrode is 10% of the area of the Z-axis fixed electrode.
Specifically, in the step (2), the thickness of the second oxide layer is 0.5-5 μm, and the thickness of the second oxide layer determines the vertical distance between the Z-axis movable electrode and the Z-axis fixed electrode, that is, the size of the Z-axis electrode distance.
Specifically, in the step (3), the epitaxial polysilicon becomes a conductor through an in-situ doping process, and the resistivity is less than 0.02 Ω & cm.
Drawings
Fig. 1 is a top view of a triaxial MEMS acceleration sensor.
Fig. 2 is a top view of the negative electrode structure of the Z-axis sensing unit.
Fig. 3 is a sectional view along a broken line a in fig. 1.
Fig. 4 is a schematic diagram of the structural motion of the Z-axis sensing unit in sensing acceleration according to the first embodiment.
Fig. 5 is a sectional view taken along a broken line B of fig. 2 in the first embodiment.
Fig. 6 is a sectional view taken along a broken line B in fig. 2 in the second embodiment.
Fig. 7-11 are flowcharts illustrating the manufacturing process of the Z-axis sensing unit according to the first embodiment.
Detailed Description
The invention is further described below with reference to the drawings and examples.
Example 1
The triaxial MEMS acceleration sensor, as shown in fig. 1-5, is composed of X, Y, Z independent sensing units, and their movable structural parts can be connected by signal lines or independent from each other.
The X-axis sensing unit consists of an X-axis mass block 10, an X-axis fixed electrode 12, an X-axis movable electrode 14, an X-axis spring 16, an X-axis electrode anchor point 24 and an X-axis mass block anchor point 26; the X-axis mass block 10 is shaped as a square frame, and the X-axis movable electrode 14 is connected to the X-axis mass block 10 and moves together with the X-axis mass block 10; the X-axis mass block 10 is fixed on the substrate 70 through the X-axis spring 16 and the X-axis mass block anchor point 26, and when acceleration exists 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 to the substrate 70 by an X-axis electrode anchor 24, and the X-axis fixed electrode 12 and the X-axis movable electrode 14 form an X-axis measurement capacitance whose size of the X-axis electrode spacing 18 changes with the movement of the X-axis mass 10. When the X-axis sensing unit receives acceleration action 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 to drive the X-axis movable electrode 14 to move in the-X direction, so that the distance between the X-axis fixed positive electrode 12a and the X-axis movable electrode 14 is reduced, the positive electrode capacitance is increased, the distance between the X-axis fixed negative electrode 12b and the X-axis movable electrode 14 is increased, the negative electrode capacitance is reduced, the capacitance change value is calculated by the positive and negative capacitance difference of a plurality of groups of electrodes, and the corresponding acceleration signal is obtained; an X-axis release hole 20a is formed in the X-axis mass block 10 and is used for removing a sacrificial oxide layer through HF solution or gaseous HF in the wafer processing process step to release the MEMS structure; because of the design layout, some X-axis gaps 22a are generated inside the structure for insulation between electrodes or for providing active space for the X-axis movable structure, and a blocking structure can be arranged between the X-axis movable electrode 14 and the X-axis fixed electrode 12 in the practical product design to prevent the parallel plate electrodes from being attracted due to too large moving distance of the X-axis movable electrode 14.
The Y-axis sensing units and the X-axis sensing units have the same structure and are mutually distributed at 90 degrees, and the structure of the Y-axis sensing units is not repeated here.
The Z-axis sensing unit adopts a wane structure and is composed of the following parts: cantilever 32, Z-axis movable electrode, Z-axis mass anchor 36, Z-axis electrode anchor 38, Z-axis torsion spring 40, frame 42, Z-axis mass 48, and Z-axis fixed electrode 60, wherein the Z-axis movable electrode is divided into Z-axis movable positive electrode 44 and Z-axis movable negative electrode 34, which are obscured in FIG. 1 by the Z-axis movable electrode; the Z-axis mass block 48 is connected with the Z-axis movable positive electrode 44, and a stress isolation groove 46 is arranged between the Z-axis mass block 48 and the Z-axis movable positive electrode 44, so that the influence of the deformation of the Z-axis mass block 48 on the Z-axis movable positive electrode 44 caused by the stress and stress gradient of the epitaxial polysilicon 28 per se is reduced; the outer sides of the Z-axis movable positive electrode 44 and the Z-axis movable negative electrode 34 are connected with a frame 42, and the middle of the frame 42 is fixed on a substrate 70 through a Z-axis torsion spring 40 and a Z-axis mass anchor point 36; the Z-axis movable structure, which actually includes several parts of the Z-axis movable positive electrode 44, the Z-axis movable negative electrode 34, the frame 42, and the Z-axis mass 48, can be regarded as one movable mass as a whole, and for simplicity of description, only the part of the Z-axis movable positive electrode 44 that is asymmetric to the Z-axis movable negative electrode 34 will be referred to herein as the Z-axis mass 48. The Z-axis fixed electrode 60 is only visible under the cantilever 32 in fig. 1, the Z-axis fixed electrode is connected under the cantilever 32 and is fixed on the substrate 70 by the Z-axis electrode anchor point 38, the Z-axis fixed electrode is suspended on the substrate 70, and the cantilever 32 acts as a mechanical stiffener for the Z-axis fixed electrode 60. Similar to the structure of X, Y-axis sensing unit, the Z-axis sensing unit is fabricated with a Z-axis release hole 20c for removing the sacrificial oxide layer by HF solution or gaseous HF during the wafer processing process step, releasing the MEMS structure. The practical product design is also provided with a blocking structure, and the movable part is prevented from being moved too much in six directions of +X, -X, +Y, -Y, +Z and Z to cause attraction or collision between parallel plate electrodes.
The X-axis sensing unit and the Y-axis sensing unit are in the same plane, the relative technology is mature, and the Z-axis sensing unit needs to use electrodes in the vertical direction, so that the difficulty is high. To further illustrate the structure of the Z-axis sensing unit, taking the Z-axis negative electrode region 50 within the dashed-line box in fig. 1 as an example, as shown in fig. 2, 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 frame first side 42a and the frame second side 42b, are located at the outer side of the cantilever 32 and connected to both ends of the Z-axis torsion spring 40, and the Z-axis torsion spring 40 is fixed to the substrate 70 through the Z-axis mass anchor point 38; z-axis release hole 20c is formed in both Z-axis movable negative electrode 34 and frame 42; the Z-axis movable negative electrode 34 and the frame 32 constitute the Z-axis movable structure 30. The dashed line in FIG. 2 is the occluded Z-axis fixed electrode 60, the Z-axis fixed electrode 60 being fixed to the cantilever 32, with the +Y direction being connected to the cantilever first end 32a, -the Y direction being connected to the cantilever second end 32b, and being connected to the cantilever third end 32c in the-X direction, and being suspended in the +X direction, the cantilever 32 being fixed to the substrate 70 by the Z-axis mass anchor 36; cantilever 32 has cantilever release holes 20d formed therein for releasing the MEMS structure during the wafer processing steps; the cantilever 32 and the Z-axis movable structure 30 have a Z-axis gap 22b therebetween for electrical isolation therebetween and the Z-axis movable structure provides a movable space.
As shown in FIG. 3, when the Z-axis acceleration is zero, the epitaxial polysilicon 28 and the underlying polysilicon 58 are parallel, and the Z-direction spacing therebetween, i.e., the vertical spacing 68, is a fixed value determined by the design and wafer processing accuracy, typically 0.5-5 μm; the lower polysilicon 58 is fixed on the substrate 70 through the first oxide layer 62, and is patterned to manufacture a lower electrode region, wherein the-X direction is a positive lower electrode 60a, the +X direction is a negative lower electrode 60b, and the middle three blocks are anchor point regions 60c, 60d and 60e; other portions of the underlying polysilicon 58, except the anchor region, are not in contact with the substrate 70, and have a vertical gap 64 therebetween, typically 0.5-5 μm in length; anchor point gaps 52 are arranged among the anchor point areas 60c, 60d and 60e and are used for electric isolation; the anchor point 60c is connected with the positive lower electrode 60a, and the anchor point 60e is connected with 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 7 by the first oxide layer 62, so as to form a Z-axis fixed electrode 60; the lower polysilicon layer 58 is formed with a plurality of Z-axis relief holes 20c for releasing the positive lower electrode 60a and the negative lower electrode 60b during the wafer processing steps. Epitaxial polysilicon 28 is connected to underlying polysilicon 58 by via polysilicon 72a, and patterned to form Z-axis movable structure 30 and three anchor regions 38a, 38b, 36; 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 48; there is a Z-axis gap 22b between the three anchor regions 38a, 38b, 36 and with the Z-axis movable structure 30 for electrical isolation and to provide room for movement of the Z-axis movable structure 30. Anchor 60d of lower polysilicon 58 connects Z-axis torsion spring 40 and Z-axis movable structure 30 through via polysilicon 72a and Z-axis mass anchor 36.
When there is acceleration in the Z-axis direction, for example, the triaxial MEMS acceleration sensor is placed on a horizontal table surface in the direction shown in fig. 4, and a force equal to the gravitational force is applied in the opposite direction to generate an acceleration in the gravitational force unit, which is called +1g, the Z-axis movable structure 30 rotates along the Z-axis torsion spring 40 relative to the substrate 70, the Z-axis mass 48 moves along the Z-axis direction with the Z-axis movable positive electrode 44, and the Z-axis movable negative electrode 34 moves along the +z direction; the anchor structure 76 and the positive and negative lower electrodes 60a, 60b fixed thereto do not move relative to the substrate 70, so that the distance 68a of the induced capacitance c+ composed of the Z-axis movable positive electrode 44 and the positive lower electrode 60a becomes small, the capacitance value c=εs/d, ε is the dielectric constant, S is the area, both of which are unchanged, d corresponds to the electrode distance 68a, and when it becomes small, the capacitance value c+ increases, and similarly, the induced capacitance C-electrode distance 68b composed of the Z-axis movable negative electrode 34 and the negative lower electrode 60b becomes large, the capacitance value C-decreases, and Δc= (c+) - (C-) is calculated, so that the Z-axis acceleration value can be output.
Because of the limitation of the 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, and when the lower polysilicon 58 is independently used as a suspended lower electrode structure, a certain amount of movement is generated under the action of acceleration, that is, the measurement capacitance C becomes unstable, so that a cantilever 32 needs to be made from a part of the epitaxial polysilicon 28 as a reinforcing rib to fix the lower polysilicon 58, and as shown in fig. 5, the cantilever 32 is a part of the epitaxial polysilicon 28, and the thickness is generally between 10 and 60 μm, which is more than one order of magnitude larger than the thickness of the lower polysilicon 58; cantilever 32 is connected to negative bottom electrode reinforcement region 60f through via polysilicon 72a as a stiffener to increase the rigidity of the bottom electrode region, resembling a "counter-suspended beam" structure in construction. A cantilever release hole 20d is formed in a negative bottom electrode structure 78 (dashed line portion in fig. 5) composed of the cantilever 32, the via polysilicon 72a and the negative bottom electrode 60b 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 by a vertical spacing 68 in the Z-direction; the negative bottom electrode structure 78 is isolated from the substrate 70 by a vertical gap 64, suspended above the substrate 70.
Example two
In some special application environments, even the negative bottom electrode structure 78 of the first embodiment cannot meet the product requirements, and in the structural design of the tri-axis MEMS acceleration sensor, the negative bottom electrode structure needs to be considered in terms of performance and environmental adaptability. As shown in fig. 6, cantilever 32 is connected to negative bottom electrode reinforcement region 60f by through-hole polysilicon 72a, acting as a stiffener to increase the rigidity of the bottom electrode region, resembling a "counter-suspended beam" structure in construction. The negative bottom electrode structure 80 consisting of the cantilever 32, the through-hole polysilicon 72a and the negative bottom electrode 60b 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 by a vertical spacing 68 in the Z-direction; the negative bottom electrode 60b is isolated from the substrate 70 by a vertical gap 64, but the negative bottom electrode reinforcing region 60f is fixed on the substrate 70 by the first oxide layer 62, the area of the negative bottom electrode reinforcing region 60f is about 10% of the area of the negative bottom electrode 60b, and compared with the design that the whole bottom electrode region is fixed on the substrate, the structure of the second embodiment is less affected by the substrate stress; the substrate stress is more greatly affected than in the first embodiment, but the environmental adaptability is stronger than in the first embodiment.
Example III
The design of the MEMS structure requires a wafer processing technology to be matched with the wafer processing technology, and the steps for forming the Z-axis sensing unit in the wafer processing flow of the triaxial MEMS acceleration sensor in the first embodiment are as follows:
(1) Manufacturing a first oxide layer 62 on a substrate wafer 70 by using a CVD (chemical vapor deposition) process, wherein the thickness of the first oxide layer 62 is 0.5-5 mu m, depositing lower polysilicon 58 on the first oxide layer 62 by using an LPCVD process, and forming a Z-axis fixed electrode 60 pattern and a Z-axis release hole 22c by using a photoetching and etching method, wherein the thickness of the lower polysilicon 58 is 0.5-3 mu m, as shown in FIG. 7;
(2) A second oxide layer 66 is deposited on the wafer shown in fig. 7 by a CVD process to a thickness that determines the vertical spacing 68 between the Z-axis movable electrode and the Z-axis fixed electrode, typically to a thickness of 0.5-5 μm, and a portion of the second oxide layer 66 above the anchor region 60c is removed by photolithography and etching to form a via 72, as shown in fig. 8. In the actual wafer processing process, there is usually a step of forming a blocking block in the Z direction;
(3) Depositing a polysilicon seed layer on the wafer shown in FIG. 8, epitaxially growing thick polysilicon, flattening by CMP to form epitaxial polysilicon 28 with the thickness of 10-60 μm, and filling the epitaxial polysilicon 28 in the through holes 72 to form through hole polysilicon 72a as shown in FIG. 9; the epitaxial polysilicon 28 is made into a conductor by an in-situ doping process, and the resistivity of the conductor is less than 0.02 Ω & cm;
(4) Forming photoresist or hard mask patterns on the wafer shown in fig. 9 through corresponding process steps, etching epitaxial polysilicon 28 to form a Z-axis release hole 20c and a Z-axis gap 22b, exposing a second oxide layer 66 in the Z-axis release hole 20c and the Z-axis gap 22b, and lengthening etching time until a cantilever release hole 20d is formed, exposing a first oxide layer 62, wherein the Z-axis fixed electrode 60 is protected by the second oxide layer 66 and cannot be etched during lengthening etching time because the etching rate of polysilicon is far greater than that of the oxide layer; at this time, the epitaxial polysilicon 28 is divided into a cantilever 32 and a Z-axis movable structure 30, which are separated by a Z-axis gap 22 b; cantilever 32 is connected to negative bottom electrode reinforcement region 60f through via polysilicon 72a, and cantilever release hole 20d is located in cantilever 32, extending through these three layers of polysilicon;
(5) Etching the wafer shown in fig. 10 with HF solution or gaseous HF, which reacts with the oxide layer (SiO 2) and does not react with the polysilicon, the HF etching being a isotropic etching, i.e., the etching rates in all directions are substantially equal, the HF removing 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 in the vertical direction and a vertical gap 64 between the underlying polysilicon 58 and the substrate 70, as shown in fig. 11, at which time the Z-axis movable structure 30 is truly movable; and controlling HF corrosion time, and when the movable structure and the lower electrode region are not in the movable structure, only the oxide layers around the movable structure and the lower electrode region are partially corroded due to the fact that the anchor points are not provided with release holes, and the anchor points are still fixed on the substrate 70 and support the MEMS structure.
The foregoing is only the best mode of carrying out the invention. It should be noted that it is also possible for those skilled in the art to make several modifications or equivalent substitutions to the technical solution of the present invention without departing from the principle of the present invention, and shall be considered as falling within the protection scope of the present invention.
Claims (8)
1. A triaxial MEMS acceleration sensor comprises an X-axis sensing unit, a Y-axis sensing unit and a Z-axis sensing unit, wherein the X-axis sensing unit, the Y-axis sensing unit and the Z-axis sensing unit are made of two layers of polysilicon materials, and acceleration in three axial directions of X, Y, Z is measured respectively;
The X-axis sensing unit consists 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, wherein the X-axis mass block is fixed on a 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 along with the X-axis mass block, the X-axis fixed electrode and the X-axis movable electrode form an X-axis measurement capacitor, and the size of the X-axis electrode distance of the X-axis measurement capacitor changes along with the movement of the X-axis mass block;
The Y-axis sensing unit and the X-axis sensing unit are mutually distributed at 90 degrees, 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 a 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 on the Y-axis mass block and moves along with the Y-axis mass block, the Y-axis fixed electrode and the Y-axis movable electrode form a Y-axis measurement capacitor, and the size of the Y-axis electrode spacing of the Y-axis measurement capacitor changes along with the movement of the Y-axis mass block;
The method is characterized in that:
The Z-axis sensing unit adopts a teeterboard structure and comprises epitaxial polycrystalline silicon and lower polycrystalline silicon, wherein the epitaxial polycrystalline silicon comprises 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 with a positive electrode of the Z-axis movable electrode, a stress isolation groove is arranged between the Z-axis movable electrode and the positive electrode, the Z-axis movable electrode is connected with the Z-axis torsion spring through the Z-axis frame, the Z-axis torsion spring is fixed on a substrate through the Z-axis mass block anchor point, and the Z-axis mass block, the Z-axis movable electrode and the frame form a Z-axis movable structure; the lower polysilicon layer comprises 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 moves in a seesaw mode around the Z-axis torsion spring in the Z direction, the Z-axis movable electrode and the Z-axis fixed electrode form a Z-axis measurement capacitor, and the vertical distance between the Z-axis fixed electrode and the Z-axis movable electrode forms the Z-axis electrode distance of the Z-axis measurement capacitor.
2. The three-axis MEMS acceleration sensor of claim 1, wherein: and a cantilever is also formed on the epitaxial polycrystalline silicon, a Z-axis gap is formed between the cantilever and the Z-axis movable structure, and the cantilever is connected with a reinforcing area of the Z-axis fixed electrode through the through hole polycrystalline silicon.
3. The triaxial MEMS acceleration sensor according to claim 1 or 2, characterized in, that: an X-axis release hole is also formed in the X-axis mass block; y-axis release holes are formed in the Y-axis mass block; an epitaxial release hole is formed in the Z-axis frame, the Z-axis mass block and the Z-axis movable electrode, and a lower layer release hole is formed in the Z-axis fixed electrode.
4. The triaxial MEMS acceleration sensor according to claim 2, characterized in that: the cantilever is provided with a cantilever release hole.
5. A method for manufacturing a Z-axis sensing unit of a triaxial MEMS acceleration sensor, comprising the steps of:
(1) Manufacturing a first oxide layer on a substrate wafer, depositing a layer of lower polysilicon on the first oxide layer, and forming a Z-axis fixed electrode and a lower release hole by photoetching and etching;
(2) Continuing to deposit a second oxide layer, and forming a through hole on the second oxide layer by a photoetching and etching method;
(3) Depositing a polysilicon seed layer, epitaxial thick polysilicon and CMP flattening on the second oxide layer to form epitaxial polysilicon, and filling the epitaxial polysilicon into the through hole to form through hole polysilicon;
(4) Etching the epitaxial polysilicon by using a mask to form an epitaxial release hole and a Z-axis gap, exposing the second oxide layer, lengthening etching time until part of the first oxide layer is exposed, and dividing the epitaxial polysilicon into a Z-axis mass block, a Z-axis movable electrode and a frame to form a Z-axis movable structure;
(5) Removing the second oxide layer and the first oxide layer by using HF solution or gaseous HF corrosion, forming a vertical interval between the epitaxial polysilicon and the lower polysilicon, wherein the vertical interval is used as an electrode interval of a Z-axis measurement capacitor, and simultaneously forming a vertical gap between the lower polysilicon and the substrate, so that a Z-axis fixed electrode is suspended on the substrate; and controlling HF corrosion time, and forming a support anchor point for supporting the MEMS structure on the substrate.
6. The method of manufacturing a Z-axis sensing unit of a three-axis MEMS acceleration sensor of claim 5, wherein: and (4) forming a cantilever on the epitaxial polycrystalline silicon, wherein a Z-axis gap is formed between the Z-axis movable structure and the cantilever, and the cantilever is connected with a reinforcing area of the Z-axis fixed electrode through the through hole polycrystalline silicon.
7. The method of manufacturing a Z-axis sensing unit of a three-axis MEMS acceleration sensor of claim 5, wherein: the thickness of the second oxide layer in the step (2) is 0.5-5 μm.
8. The method of manufacturing a Z-axis sensing unit of a three-axis MEMS acceleration sensor of claim 5, wherein: in the step (3), the epitaxial polysilicon becomes a conductor through an in-situ doping process, and the resistivity is less than 0.02 ohm cm.
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