A MEMS wafer integrated with stress gradient test units and a test method
By constructing two detection capacitors on a MEMS wafer and using the difference in capacitance values to determine the bending direction and degree, the problem of high cost in optical testing of stress gradients on MEMS wafers is solved, realizing low-cost and high-efficiency stress gradient testing.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- MT MICROSYST
- Filing Date
- 2026-01-16
- Publication Date
- 2026-06-02
AI Technical Summary
Existing MEMS wafer stress gradient optical testing equipment is costly and difficult to adapt to mass production.
The design integrates a stress gradient testing unit into a MEMS wafer. By constructing two detection capacitors on the wafer, the difference in capacitance values is used to determine the direction and degree of bending, thus replacing expensive optical equipment for testing.
It reduces manufacturing costs, improves testing efficiency, adapts to mass production needs, and is not limited by the field of view of optical equipment.
Smart Images
Figure CN121540314B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of MEMS testing technology, and in particular to a MEMS wafer with an integrated stress gradient testing unit and a testing method thereof. Background Technology
[0002] Microelectromechanical systems (MEMS) is a new, multidisciplinary, high-tech field that has emerged with the development of semiconductor integrated circuit microfabrication technology and ultra-precision machining technology. It refers to miniature devices or systems that can be mass-produced and integrate micro-mechanisms, micro-sensors, micro-actuators, signal processing and control circuits, and even interfaces, communication and power supplies, with feature sizes ranging from millimeters and micrometers to nanometers.
[0003] Thin film deposition and structure release are crucial steps in the fabrication of silicon-based MEMS devices to form movable structures. Due to lattice mismatch and thermal expansion coefficient mismatch between the structural layer thin film material and the substrate material, residual stress and stress gradients along the thickness direction may occur within the thin film. When a stress gradient exists along the thickness direction, it can cause structural deformation, directly affecting device performance. Excessively high stress gradients can even cause thin film rupture, leading to MEMS device fabrication failure. Therefore, accurately measuring the stress gradient of the structural layer thin film is essential for assessing process stability and improving the performance of MEMS devices.
[0004] The most common method for testing the stress gradient of thin films in MEMS wafer structures is to use sophisticated optical equipment and optical interferometry to obtain the bending deformation of the film caused by the stress gradient. However, the expensive optical testing equipment increases the process cost and difficulty, resulting in high manufacturing costs and hindering the mass production of MEMS wafers. Summary of the Invention
[0005] This invention provides a MEMS wafer with an integrated stress gradient testing unit and a testing method to solve the problem of high equipment cost in existing MEMS wafer stress gradient optical testing methods.
[0006] In a first aspect, embodiments of the present invention provide a MEMS wafer with an integrated stress gradient testing unit, wherein the testing unit comprises, from bottom to top: a substrate and an insulating layer; a first lower electrode plate and a second lower electrode plate disposed on the insulating layer, which are isolated from each other; a suspended upper electrode plate disposed above the first lower electrode plate and the second lower electrode plate; the vertical projection area of the upper electrode plate on the two lower electrode plates is the same; the upper electrode plate is grown and fabricated synchronously with the structure of the stress gradient to be tested; a fixing part of the upper electrode plate is fixedly connected to the insulating layer; the distance between the fixing part and the first lower electrode plate is less than the distance between the fixing part and the second lower electrode plate; the upper electrode plate and the first lower electrode plate form a first capacitor, and the upper electrode plate and the second lower electrode plate form a second capacitor.
[0007] In one possible implementation, the first lower electrode and the second lower electrode have the same area.
[0008] In one possible implementation, the fixing part is an anchor point; the anchor point is located directly below the upper electrode plate; the upper end of the anchor point is connected to the upper electrode plate, and the lower end is connected to the insulating layer.
[0009] In one possible implementation, the fixing part includes two support beams symmetrically arranged on both sides of the upper electrode plate, and anchor points corresponding to each support beam; the support beams extend in a horizontal direction away from the upper electrode plate, with their proximal ends integrally formed with the upper electrode plate and the anchor points located directly below their distal ends; the upper ends of the anchor points are fixedly connected to the support beams, and the lower ends are fixedly connected to the insulating layer.
[0010] In one possible implementation, the first lower electrode plate and the second lower electrode plate are distributed on the same side of the fixing part.
[0011] In one possible implementation, the first lower electrode plate and the second lower electrode plate are distributed on different sides of the fixing part.
[0012] In one possible implementation, the test unit further includes a cover layer; the cover layer and the substrate form a sealed cavity; the cover layer is disposed above the upper electrode plate and has a gap between them.
[0013] In one possible implementation, the upper electrode, the first lower electrode, and the second lower electrode are all made of polycrystalline silicon.
[0014] In a second aspect, embodiments of the present invention provide a stress gradient testing method, applied to testing a MEMS wafer with an integrated stress gradient testing unit as described in any of the first aspects; the method includes: obtaining the capacitance values of a first capacitor and a second capacitor; if the capacitance value of the first capacitor is greater than the capacitance value of the second capacitor, then the upper electrode plate bends upward; if the capacitance value of the first capacitor is less than the capacitance value of the second capacitor, then the upper electrode plate bends downward; based on the ratio of the capacitance values of the first capacitor and the second capacitor, obtaining the degree of bending of the upper electrode plate; and obtaining the stress gradient of the MEMS wafer according to the bending direction and the degree of bending.
[0015] In one possible implementation, after obtaining the capacitance values of the first capacitor and the second capacitor, the method further includes: if the capacitance value of the first capacitor is equal to the capacitance value of the second capacitor, then the upper plate is not bent.
[0016] This invention provides a MEMS wafer with an integrated stress gradient testing unit and a testing method. By setting a first and second lower electrode plate fixed integrally to the upper surface of an insulating layer, and a suspended upper electrode plate, a first capacitor and a second capacitor with the same area are formed with the electrode plates facing each other. The suspended design of the upper electrode plate allows it to bend freely under the action of a stress gradient. Bending changes the electrode spacing between the upper and lower electrode plates, thereby changing the capacitance value. Since the distances between the two lower electrode plates and the fixing part of the upper electrode plate are different, the degree of bending of the upper electrode plate above the two lower electrode plates is also different; the farther away from the fixing part, the greater the degree of bending. Therefore, the difference in the two capacitance values is only related to the difference in bending of the upper electrode plate. Based on the relationship between the capacitance values of the two capacitors, the bending direction and degree of bending of the upper electrode plate can be determined, thereby obtaining the stress gradient of the structural layer film in the same layer as the upper electrode plate in the MEMS wafer. This invention adopts a capacitance testing method compatible with existing MEMS wafer manufacturing processes. By simultaneously testing the two capacitance values to determine the bending direction and degree of bending, and thus obtaining the stress gradient, the use of optical equipment for testing can be avoided, reducing manufacturing costs. Attached Figure Description
[0017] Figure 1 This is a schematic diagram of the planar structure of a MEMS wafer with an integrated stress gradient testing unit provided in an embodiment of the present invention;
[0018] Figure 2 This is a schematic diagram of the cross-sectional structure of the test unit provided in an embodiment of the present invention;
[0019] Figure 3 This is a schematic diagram of the cross-sectional structure of the upper electrode plate bending upwards, provided in an embodiment of the present invention;
[0020] Figure 4 This is a schematic diagram of the cross-sectional structure of the upper electrode plate bending downwards, provided in an embodiment of the present invention;
[0021] Figure 5 This is a schematic diagram of the cross-sectional structure of another test unit provided in an embodiment of the present invention;
[0022] Figure 6 This is a schematic diagram of another cross-sectional structure of the upper electrode plate bending upwards, provided in an embodiment of the present invention;
[0023] Figure 7 This is a schematic diagram of another cross-sectional structure of the upper electrode plate bending downwards, provided in an embodiment of the present invention;
[0024] Figure 8 This is a flowchart illustrating the implementation of a stress gradient testing method provided in an embodiment of the present invention. Detailed Implementation
[0025] To enable those skilled in the art to better understand this solution, the technical solutions in the embodiments of this solution will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this solution, not all of them. Based on the embodiments of this solution, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this solution.
[0026] The term "comprising" and any other variations thereof in the specification, claims, and accompanying drawings of this invention mean "including but not limited to," and are intended to cover a non-exclusive inclusion, not limited to the examples listed herein. Furthermore, the terms "first" and "second," etc., are used to distinguish different objects, not to describe a specific order.
[0027] The implementation of the present invention will be described in detail below with reference to the accompanying drawings:
[0028] In materials science, stress gradient refers to the rate of change of stress with spatial location within a material or structure, used to describe the degree of difference in stress distribution at different locations. For MEMS wafers, stress gradients are most commonly found along the thickness direction. For example, the stress gradient of a thin film along its thickness direction is a key parameter affecting the performance and reliability of MEMS devices.
[0029] For example, in the field of MEMS, surface silicon technology achieves microstructure fabrication by depositing, patterning, and etching functional and sacrificial layers layer by layer on the silicon wafer surface. Suspended or movable microstructures are constructed using surface-grown thin film layers. Stress gradients are typically generated during the thin film growth process. Thin film growth is essentially a layer-by-layer stacking process. Due to process fluctuations, the growth state of each layer in the thickness direction varies, easily causing uneven density distribution in the thickness direction, resulting in stress variations, i.e., stress gradients. The presence of stress gradients causes the thin film to spontaneously bend under internal stress. Especially in MEMS wafers, after the removal of the sacrificial layer, the movable structure is released, and without material constraints in the vertical direction, it bends under the influence of stress gradients, affecting the performance and reliability of MEMS devices. Therefore, stress gradient testing of MEMS wafers mainly focuses on the thin film stress gradient of the movable structure layer. Stress gradient testing can be performed after the sacrificial layer is removed and the movable structure is released.
[0030] Existing optical methods for testing stress gradients require expensive optical interferometry equipment, resulting in high manufacturing costs. Furthermore, optical testing has a limited field of view, restricting the range of materials that can be measured in a single test, leading to low efficiency and making it unsuitable for mass production testing.
[0031] In this embodiment of the invention, two detection capacitors are constructed through a test unit. The two capacitance values can be used to test both the bending direction and the degree of bending, which avoids the use of optical equipment for testing and reduces manufacturing costs.
[0032] Figure 1 This is a schematic diagram of the planar structure of a MEMS wafer with an integrated stress gradient testing unit provided in an embodiment of the present invention. (Refer to...) Figure 1 The test unit, from bottom to top, includes: a substrate and an insulating layer; a first lower electrode plate and a second lower electrode plate, isolated from each other, are disposed on the insulating layer; a suspended upper electrode plate is disposed above the first lower electrode plate and the second lower electrode plate; the vertical projection area of the upper electrode plate on the two lower electrode plates is the same; the upper electrode plate is grown and fabricated synchronously with the structure of the stress gradient to be measured; the fixing part of the upper electrode plate is fixedly connected to the insulating layer; the distance between the fixing part and the first lower electrode plate is less than the distance between the fixing part and the second lower electrode plate; the upper electrode plate and the first lower electrode plate form a first capacitor, and the upper electrode plate and the second lower electrode plate form a second capacitor; the capacitance values of the first capacitor and the second capacitor are used to detect the bending direction and degree of bending of the upper electrode plate, thereby obtaining the stress gradient of the MEMS wafer.
[0033] Figure 1 This is a schematic diagram of the planar structure of a MEMS wafer. (Refer to...) Figure 1 For example, MEMS chip 2 and stress gradient testing units are distributed in different regions of MEMS wafer 1 in a plane direction parallel to the substrate. MEMS wafer 1 is a MEMS wafer that integrates stress gradient testing units.
[0034] For example, the stress gradient testing unit is used to test the stress gradient in the thickness direction of the movable structural layer thin film on MEMS wafer 1. The stress gradient testing unit is hereinafter referred to as testing unit 3.
[0035] In chip manufacturing, PCM stands for Process Control Monitor. PCMs are typically process testing units located at the edge of the wafer or on the chip dicing track. They do not occupy the core functional area of the chip, but can directly reflect the execution effect of key processes such as photolithography, etching, and deposition.
[0036] Thus, in the planar direction, the MEMS chip 2 and the test unit 3 are distributed in different regions of the MEMS wafer 1. For example, in the vertical direction, the materials of each layer of the MEMS chip 2 and the test unit 3 are identical, so that the material stress gradient of the test unit 3 can characterize the material stress gradient of the MEMS chip 2. Furthermore, the test unit 3 is compatible with the manufacturing process of the MEMS chip 2 and is manufactured synchronously with the MEMS chip 2. For example, the material of each layer is the same, but the pattern layout is different. It should be noted that the test unit 3 does not necessarily have to be manufactured synchronously with the chip. If it is only for verifying the thin film growth process, the test unit 3 can also be fabricated only on the MEMS wafer 1. Of course, to avoid manufacturing a separate photomask for the test unit and to reduce costs, the test unit is usually manufactured synchronously with the MEMS chip 2.
[0037] The above explains the relationship between the test unit and MEMS wafer 1 and MEMS chip 2. Figure 2 This is a schematic diagram of the cross-sectional structure of the test unit provided in an embodiment of the present invention; refer to Figure 2 The following describes the structure of each layer in the vertical direction of test unit 3.
[0038] In some embodiments, the test unit 3 includes, from bottom to top, a substrate 31 and an insulating layer 32.
[0039] For example, substrate 31 may be a silicon substrate. For instance, substrate 31 may be a single-crystal silicon wafer. Substrate 31 provides structural support for the entire test unit.
[0040] For example, the insulating layer 32 can be silicon oxide or silicon nitride. The main function of the insulating layer 32 is to achieve insulation between the lower electrodes. The actual function of the insulating layer 32 will be explained below with reference to a specific structure.
[0041] In some embodiments, the insulating layer 32 is provided with a first lower electrode plate 33 and a second lower electrode plate 34 that are isolated from each other.
[0042] For example, the first lower electrode 33 and the second lower electrode 34 are made of the same material. Further, both the first lower electrode 33 and the second lower electrode 34 are made of polysilicon. For instance, a layer of polysilicon can be grown on an insulating layer, and then the first lower electrode 33 and the second lower electrode 34 can be obtained through patterned etching. In the MEMS field, mutually isolated conductive structures are typically achieved by depositing conductive materials, patterning by photolithography, and etching to remove excess material. For example, after depositing a polysilicon layer on an insulating layer, the shapes of the two lower electrodes are defined by photolithography, and then the material between the lower electrodes is removed by dry etching to form a mutually isolated structure. This is consistent with the layer-by-layer growth and patterning method of surface silicon processes.
[0043] It should be noted that the supporting substrate for the lower electrode is an insulating layer, not a substrate. The lower surface of the lower electrode is fixed to the upper surface of the insulating layer. The insulating layer blocks the electrical conduction between the lower electrode and the substrate, ensuring that the electrical signal of the lower electrode is independent. The first lower electrode 33 and the second lower electrode 34 are physically isolated from each other, also preventing short circuits through the substrate.
[0044] In some embodiments, a suspended upper electrode plate 35 is disposed above the first lower electrode plate 33 and the second lower electrode plate 34; the vertical projection area of the upper electrode plate 35 on the two lower electrode plates is the same.
[0045] For example, the upper electrode 35, the first lower electrode 33, and the second lower electrode 34 are all made of polycrystalline silicon.
[0046] The upper electrode 35 is located directly above the two lower electrodes, forming a parallel plate capacitor structure consisting of the upper electrode, an air gap, and the lower electrodes. The gap between the upper electrode 35 and the two lower electrodes is the dielectric layer of the capacitor. The upper electrode 35 is suspended, meaning that it does not directly contact the lower electrodes, nor does it directly contact the structure above it. The upper electrode can be fixed to the insulating layer below via a fixing part. There is a preset gap between the surfaces of the upper electrode 35 and the lower electrodes, the size of which can be controlled by the thickness of the sacrificial layer. For example, the upper electrode can be suspended by etching away the sacrificial layer below the upper electrode 35. The suspended state means that the upper electrode 35 is only mechanically constrained by the fixing part. When there is a stress gradient in the thickness direction of the MEMS wafer, the upper and lower surfaces of the upper electrode 35 will bend due to the stress difference.
[0047] For example, the fabrication steps of the suspended upper electrode 35 are as follows: After the lower electrode is fabricated, a sacrificial layer is deposited and patterned, retaining the sacrificial layer in the area below the upper electrode 35; upper electrode material is deposited on top of the sacrificial layer, and the planar shape of the upper electrode is defined by photolithography; the sacrificial layer is removed with a selective etchant, at which point the upper electrode is only connected to the insulating layer through a fixing part, ultimately realizing the upper electrode structure suspended above the lower electrode. Furthermore, in subsequent processes, a cover plate layer may be provided above the upper electrode, with a gap between the cover plate layer and the upper electrode to ensure that the upper electrode can be bent upwards or downwards.
[0048] The vertically projected area refers to the overlapping area projected vertically downwards from the upper surface of the upper electrode 35 onto the surfaces of the first lower electrode 33 and the second lower electrode 34, i.e., the effective facing area of the capacitor. For example, the vertically projected area of the upper electrode 35 on the first lower electrode 33 is the same as the vertically projected area of the upper electrode 35 on the second lower electrode 34. It should be noted that this refers to the area projected onto each lower electrode being the same, not that the areas of the two lower electrodes are the same. The areas of the two lower electrodes can be different, as long as they are equal to the overlapping area with the upper electrode 35. Ensuring that the vertically projected areas are the same is to ensure that the difference in capacitance value is caused by stress gradient.
[0049] For example, the first lower electrode plate 33 and the second lower electrode plate 34 have the same area. Furthermore, the vertical projection of the upper electrode plate 35 completely covers the first lower electrode plate 33 and the second lower electrode plate 34, so that the vertical projection area of the upper electrode plate 35 on the two lower electrode plates is the same.
[0050] In some embodiments, the fixing portion of the upper electrode plate 35 is fixedly connected to the insulating layer; the distance between the fixing portion and the first lower electrode plate 33 is less than the distance between the fixing portion and the second lower electrode plate 34.
[0051] The fixing part is connected to the insulating layer, rather than to the lower electrode plate or the substrate. The insulating layer provides reliable support for the fixing part and prevents the fixing part from forming an electrical short circuit with the substrate or the lower electrode plate. For example, the fixing part and the upper electrode plate 35 are integrally formed, that is, they are prepared by photolithography and etching using the same material.
[0052] For example, the distance between the fixing part and the lower electrode plate refers to the distance between the geometric center of the fixing part and the geometric center of the lower electrode plate.
[0053] As a further example, the distance between the fixing part and the lower electrode plate represents the distance between the geometric center of the horizontal cross-section of the fixing part and the geometric center of the horizontal cross-section of the effective capacitance region of the lower electrode plate. That is, through layout design, the two lower electrodes plate are asymmetrically distributed relative to the fixing part, forming a closer first lower electrode plate 33 and a farther second lower electrode plate 34.
[0054] In some embodiments, the upper electrode 35 and the first lower electrode 33 form a first capacitor C1, and the upper electrode 35 and the second lower electrode 34 form a second capacitor C2.
[0055] The aforementioned structural constraints regarding the upper and lower plates are ultimately intended to form two capacitors. Furthermore, the aforementioned constraints regarding the vertically projected area and the asymmetrically distributed lower plate are designed to ensure that the capacitance values of the two capacitors can characterize the bending direction and degree of the upper plate caused by the stress gradient.
[0056] In some embodiments, the capacitance values of the first capacitor C1 and the second capacitor C2 are used to detect the bending direction and degree of bending of the upper plate, thereby obtaining the stress gradient of the MEMS wafer.
[0057] When a MEMS wafer exhibits a stress gradient along its thickness, the upper electrode will bend in a certain direction due to the stress difference between the upper and lower surfaces. For example, if tensile stress is on the upper surface, the upper electrode will bend upwards; if compressive stress is on the upper surface, the upper electrode will bend downwards. Since the distance between the fixing part and the two lower electrodes is unequal, the distance between the upper electrode and the two lower electrodes will change differentially when the upper electrode bends. However, the area of the upper electrode facing the two lower electrodes remains unchanged when the upper electrode bends. Therefore, the difference in capacitance between the two capacitors is only related to the difference in distance between the upper and lower electrodes. Thus, the bending direction and degree of the upper electrode can be determined based on the capacitance values of the first capacitor C1 and the second capacitor C2, thereby obtaining the stress gradient of the MEMS wafer.
[0058] Figure 3 This is a schematic diagram of the cross-sectional structure of the upper electrode plate bending upwards, provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the cross-sectional structure of the upper electrode plate bending downwards, provided in an embodiment of the present invention. (Refer to...) Figure 3 , Figure 4 It should be noted that the distance between the upper electrode and the first lower electrode is the electrode spacing of the first capacitor; the distance between the upper electrode and the second lower electrode is the electrode spacing of the second capacitor. When the upper electrode bends upward, the electrode spacing of the first capacitor is smaller than that of the second capacitor. Because the electrode spacing and capacitance are inversely proportional, the capacitance of the first capacitor is greater than that of the second capacitor. When the upper electrode bends downward, the electrode spacing of the first capacitor is greater than that of the second capacitor, making the capacitance of the first capacitor smaller than that of the second capacitor.
[0059] Furthermore, in the same bending direction, the farther away from the fixed part, the greater the degree of bending of the upper electrode plate, and the smaller the distance between the upper electrode plate and the lower part. For the same change in bending of the upper electrode plate, the farther away from the fixed part, the greater the change in the distance between the upper electrode plate and the lower part. Therefore, based on the ratio of the first capacitor to the second capacitor, the degree of bending of the upper electrode plate can be determined.
[0060] Additionally, it should be noted that the test unit also includes leads and external pads. The leads connect the capacitor plates to the external pads. The external pads are used to connect test equipment, such as probe stations.
[0061] This invention, through the provision of a first and second lower electrode plate fixed integrally to the upper surface of an insulating layer, and a suspended upper electrode plate, forms a first capacitor and a second capacitor with the same area facing each other. The suspended design of the upper electrode plate allows it to bend freely under stress gradient, changing the electrode spacing between the upper and lower electrodes, and thus altering the capacitance value. Because the distances between the two lower electrodes and the fixing portion of the upper electrode plate differ, the degree of bending of the upper electrode plate above the two lower electrodes also differs; the farther from the fixing portion, the greater the degree of bending. Therefore, the difference in the two capacitance values is only related to the difference in bending of the upper electrode plate. Based on the relationship between the capacitance values of the two capacitors, the bending direction and degree of bending of the upper electrode plate can be determined, thereby obtaining the stress gradient of the structural layer film in the same layer as the upper electrode plate in the MEMS wafer. This invention employs a capacitance testing method compatible with existing MEMS wafer manufacturing processes. By simultaneously testing the two capacitance values to determine the bending direction and degree of bending, and thus obtaining the stress gradient, it avoids the use of optical equipment for testing, reducing manufacturing costs.
[0062] The stress gradient testing structure used in this embodiment of the invention can be completed simultaneously with the actual MEMS chip without the need for additional process steps.
[0063] The speed of optical methods is limited by the field of view. The field of view of a single high-magnification lens in optical inspection is only tens of micrometers. If multiple test units or a large area on the wafer need to be covered, the images must be scanned and stitched together block by block using an xy-axis moving stage. The delay of mechanical scanning and positioning increases linearly with the expansion of the inspection area.
[0064] The capacitance detection method directly acquires the capacitance signal formed by the upper and lower electrodes. The object of detection is the capacitance value rather than the spatial shape, which does not rely on lens field of view coverage or sensor pixel-by-pixel sampling. Regardless of the size of the test unit or the distribution density of the test units on the wafer, capacitance data can be acquired within microseconds simply by contacting the electrode pads with the probe. In batch testing, parallel detection can also be achieved through multi-channel signal acquisition, and the switching time is much lower than that of switching the field of view of an optical lens.
[0065] The following describes the specific method of fixing the upper electrode plate.
[0066] Reference Figure 2 , Figure 3 , Figure 4 In one possible implementation, the fixing part is an anchor point 36; the anchor point 36 is located directly below the upper electrode plate; the upper end of the anchor point 36 is connected to the upper electrode plate, and the lower end is connected to the insulating layer.
[0067] The anchor point fixing method achieves a mechanical connection between the upper electrode plate and the insulation layer through vertical anchor points 36, ensuring the upper electrode plate remains suspended. The fixing part of the upper electrode plate is the anchor point 36. For example, the anchor point 36 refers to a columnar or block-shaped structure with a certain cross-sectional area, perpendicular to the surface of the insulation layer, such as a cylindrical or rectangular column. The horizontal cross-sectional area of the anchor point 36 is usually matched with the cross-sectional area of the upper electrode plate fixing part to ensure connection strength. The anchor point 36 being located directly below the upper electrode plate indicates that the anchor point 36 is located below a small fixed support area of the upper electrode plate, rather than below the entire upper electrode plate. The anchor point 36 only provides support at the fixing point; the rest of the upper electrode plate has no supporting structure and can be freely bent.
[0068] In some embodiments, the first lower electrode plate and the second lower electrode plate are distributed on different sides of the fixing portion. That is, the first lower electrode plate and the second lower electrode plate are distributed on different sides of the anchor point 36.
[0069] In some embodiments, the first lower electrode plate and the second lower electrode plate are distributed on the same side of the fixing portion. That is, the first lower electrode plate and the second lower electrode plate are distributed on the same side of the anchor point 36. The first lower electrode plate and the second lower electrode plate are distributed on the side of the anchor point 36 facing the free end of the upper electrode plate.
[0070] For example, one end of the upper electrode plate is a fixed end, and the other end is a free end. The fixed end of the upper electrode plate is fixed by anchor point 36, and the free end can be bent freely. Its bending deflection increases linearly from the fixed end to the free end, with the free end having the largest deflection.
[0071] Capacitor amplification effect with symmetric distribution: Both lower plates are arranged on the same side of anchor point 36, for example, on the side of the free end extension direction of the upper plate, with the first lower plate closer to anchor point 36 and the second lower plate farther away from anchor point 36. When the upper plate bends due to stress gradient, the maximum deformation of the free end will directly act on the second lower plate, causing the change in the area or distance between the second lower plate and the upper plate to be much greater than that of the first lower plate.
[0072] When the two lower plates are distributed on the same side of anchor point 36, only one common upper plate is needed to form two independent capacitors with the first and second lower plates on the same side.
[0073] When the two lower plates are distributed on opposite sides of the anchor point 36, there must be a capacitor detection unit on each side of the anchor point 36. The upper plate needs to extend across the anchor point 36 to both sides. The distance the upper plate extends from the same anchor point 36 to both sides must ensure that it can cover the two lower plates, and also ensure that the distances between the two lower plates and the anchor point 36 are not equal. This will result in the upper plate being longer than when it is distributed on the same side, and occupying a larger overall area.
[0074] The embodiments of the present invention adopt a same-side distribution method. Compared with the opposite-side distribution method, only one upper plate with a smaller length dimension is needed to form two capacitors, resulting in a more compact structural layout.
[0075] Figure 5 This is a schematic diagram of the cross-sectional structure of another test unit provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of another cross-sectional structure of the upper electrode plate bending upwards, provided in an embodiment of the present invention; Figure 7 This is a schematic diagram of another cross-sectional structure of the upper electrode plate bending downwards, provided in an embodiment of the present invention. (Refer to...) Figure 5 , Figure 6 , Figure 7 In one possible implementation, the fixing part includes two support beams 37 symmetrically arranged on both sides of the upper electrode plate, and anchor points corresponding to each support beam 37; the support beams 37 extend in a horizontal direction away from the upper electrode plate, with their proximal ends integrally formed with the upper electrode plate, and anchor points located directly below their distal ends; the upper ends of the anchor points are fixedly connected to the support beams 37, and the lower ends are fixedly connected to the insulating layer.
[0076] Two support beams 37 are symmetrically distributed on both sides of the upper electrode plate in the width direction, rather than the length direction. That is, from a vertical perspective, there is one support beam 37 on the left and one on the right side of the upper electrode plate, forming a layout where the upper electrode plate is centered and the support beams 37 are symmetrically held in place. The support beams 37 extend horizontally away from the upper electrode plate and are perpendicular to the length direction of the upper electrode plate. The portion of each support beam 37 near the upper electrode plate is integrally formed with the upper electrode plate, and an anchor point is located directly below the portion away from the upper electrode plate. The lower end of the anchor point is fixedly connected to the insulation layer, forming a mechanical transmission path between the insulation layer, the anchor point, the support beam, and the upper electrode plate.
[0077] In some embodiments, the first lower electrode plate and the second lower electrode plate are distributed on different sides of the fixing part. That is, the first lower electrode plate and the second lower electrode plate are distributed on different sides of the support beam along the line connecting the two ends of the upper electrode plate.
[0078] In some embodiments, the first lower electrode plate and the second lower electrode plate are distributed on the same side of the fixing portion. That is, the first lower electrode plate and the second lower electrode plate are distributed on the same side of the support beam. The first lower electrode plate and the second lower electrode plate are distributed on the side of the support beam facing the free end of the upper electrode plate.
[0079] In some embodiments, the side support method of the support beam results in no fixed structure directly below the upper electrode plate. Thus, the support beam can be positioned above the first lower electrode plate along the line connecting the two ends of the upper electrode plate, which is between the same-side method and the different-side method.
[0080] It should be noted that when fixed with a single anchor point, the stress of the upper electrode plate is concentrated in a small area of the fixing part. The connection surface between the anchor point and the upper electrode plate is prone to breakage of the fixing part under long-term use or with a large stress gradient. The double-sided support beam design disperses the stress of the fixing end of the upper electrode plate to two anchor points through two support beams. The stress borne by each support beam is only 1 / 2 of that of a single anchor point. Moreover, the extended structure of the support beam increases the mechanical transmission area, significantly reduces stress concentration, avoids the breakage of the upper electrode plate, and improves the service life and reliability of the test unit.
[0081] When fixed at a single anchor point, the upper electrode plate may twist around the anchor point. The double-sided symmetrical support beams provide two-point symmetrical constraints for the upper electrode plate, limiting its torsional freedom and ensuring that the upper electrode plate only bends in a direction perpendicular to the surface of the lower electrode plate. The deformation direction is unique, avoiding capacitance detection errors caused by torsion, such as torsion causing the facing areas of the upper electrode plate and the two lower electrode plates to be asymmetrical.
[0082] The support beam extends along both sides of the width of the upper electrode plate, without occupying the space below the length of the upper electrode plate. This means it does not affect the layout of the lower electrode plate below the upper electrode plate, and can reduce the length of the upper electrode plate.
[0083] The double-sided symmetrical fixing method of the upper electrode plate support beam combined with the anchor point in this embodiment of the invention disperses stress through the double-sided support beam, solving the stress concentration problem that may exist in the single anchor point fixing, while maintaining the suspension deformation capability of the upper electrode plate.
[0084] In one possible implementation, the first lower electrode plate and the second lower electrode plate are distributed on the same side of the fixing part.
[0085] In one possible implementation, the first lower electrode plate and the second lower electrode plate are distributed on different sides of the fixing part.
[0086] In one possible implementation, the test unit further includes a cover layer; the cover layer and the substrate form a sealed cavity; the cover layer is disposed above the upper electrode plate and has a gap between them.
[0087] For example, the capping layer is fixedly connected to the insulating layer or substrate through a support structure and is located above the upper electrode plate; a preset gap is maintained between the capping layer and the upper electrode plate, and the two are not in direct contact; the capping layer is used to isolate external dust and moisture or prevent the upper electrode plate from being impacted by external forces, and to protect the suspended state and structural integrity of the upper electrode plate.
[0088] It should be noted that optical testing requires removing the capping layer to expose the upper electrode, and this process can easily damage the movable structure. However, the dual-capacitor testing method in this embodiment measures the capacitance value and does not require exposing the upper electrode. Therefore, in this embodiment, stress gradients can be tested not only after the movable structure is fabricated and before the capping layer is closed, but also after the capping layer is closed.
[0089] The stress gradient testing method is described below. In this embodiment of the invention, the entity performing the stress gradient testing method can be an automated testing device.
[0090] Figure 8 This is a flowchart illustrating the implementation of a stress gradient testing method provided in an embodiment of the present invention. (Refer to...) Figure 8 This invention provides a stress gradient testing method for testing MEMS wafers with integrated stress gradient testing units as described above; the method includes:
[0091] Step 801: Obtain the capacitance values of the first capacitor and the second capacitor;
[0092] For example, the capacitance values of the first capacitor and the second capacitor are collected by a capacitance tester by contacting the external pads of the test unit with the probes of the probe station.
[0093] In one possible implementation, after obtaining the capacitance values of the first capacitor and the second capacitor, the method further includes: if the capacitance value of the first capacitor is equal to the capacitance value of the second capacitor, then the upper plate is not bent.
[0094] Under stress gradient-free conditions, when the upper plate is not bent, since the vertical projection area and spacing of the upper plate on the two lower plates are the same, the capacitance value of the first capacitor is equal to the capacitance value of the second capacitor, it can be determined that the upper plate is not bent.
[0095] Step 802: If the capacitance of the first capacitor is greater than the capacitance of the second capacitor, then the upper plate bends upward.
[0096] When the upper plate bends upward, the distance between the plates of the first capacitor is smaller than that between the plates of the second capacitor. Since the distance between the plates is inversely proportional to the capacitance value, the capacitance value of the first capacitor is greater than that of the second capacitor.
[0097] Step 803: If the capacitance of the first capacitor is less than the capacitance of the second capacitor, the upper plate bends downward.
[0098] When the upper plate bends downwards, the distance between the plates of the first capacitor is greater than the distance between the plates of the second capacitor, making the capacitance of the first capacitor less than the capacitance of the second capacitor.
[0099] Step 804: Based on the ratio of the capacitance values of the first capacitor and the second capacitor, obtain the degree of bending of the upper plate;
[0100] In the same bending direction, the farther away from the fixed part, the greater the degree of bending of the upper electrode plate, and the smaller the distance between the upper electrode plate and the lower part. For the same change in bending of the upper electrode plate, the farther away from the fixed part, the greater the change in the distance between the upper electrode plate and the lower part. Therefore, based on the ratio of the first capacitor to the second capacitor, the degree of bending of the upper electrode plate can be determined.
[0101] Step 805: Obtain the stress gradient of the MEMS wafer based on the bending direction and degree of bending.
[0102] The bending direction of the upper electrode is determined by the difference in stress distribution along its thickness. If the upper electrode bends in one direction, it indicates that the film surface on that side is under tensile stress, while the other side is under compressive stress. The degree of bending of the upper electrode is determined by the magnitude of the stress gradient: the greater the stress gradient, the greater the stress difference between the upper and lower surfaces of the film, the stronger the driving force for bending the upper electrode, and the more significant the bending; the slighter the bending, the smaller the stress difference between the upper and lower surfaces of the film, and the smaller the corresponding stress gradient. By combining the two dimensions of direction and magnitude, the stress gradient can be completely reconstructed. The bending direction determines the direction of the stress gradient; the degree of bending determines the magnitude of the stress gradient; combining the two yields the complete stress gradient result of the MEMS wafer thin film. It should be noted that here, the stress gradient detection result of the upper electrode characterizes the stress gradient of the same layer of material in the MEMS wafer.
[0103] In one possible implementation, after step 804, the range of the detection quality stress gradient can be determined based on the ratio of the capacitance values of the second capacitor and the first capacitor to determine whether the stress gradient of the detection quality meets the requirements. This eliminates the need for precise calculation of the stress gradient value. For example, if the ratio of the capacitance values of the second capacitor and the first capacitor is less than a preset value, it can be determined that the stress gradient of the detection quality meets the requirements.
[0104] This invention, through the provision of a first and second lower electrode plate fixed integrally to the upper surface of an insulating layer, and a suspended upper electrode plate, forms a first capacitor and a second capacitor with the same area facing each other. The suspended design of the upper electrode plate allows it to bend freely under stress gradient, changing the electrode spacing between the upper and lower electrodes, and thus altering the capacitance value. Because the distances between the two lower electrodes and the fixing portion of the upper electrode plate differ, the degree of bending of the upper electrode plate above the two lower electrodes also differs; the farther from the fixing portion, the greater the degree of bending. Therefore, the difference in the two capacitance values is only related to the difference in bending of the upper electrode plate. Based on the relationship between the capacitance values of the two capacitors, the bending direction and degree of bending of the upper electrode plate can be determined, thereby obtaining the stress gradient of the structural layer film in the same layer as the upper electrode plate in the MEMS wafer. This invention employs a capacitance testing method compatible with existing MEMS wafer manufacturing processes. By simultaneously testing the two capacitance values to determine the bending direction and degree of bending, and thus obtaining the stress gradient, it avoids the use of optical equipment for testing, reducing manufacturing costs.
[0105] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims
1. A MEMS wafer with an integrated stress gradient testing unit, characterized in that, The test unit, from bottom to top, includes: a substrate and an insulating layer; The insulating layer is provided with a first lower electrode plate and a second lower electrode plate that are isolated from each other. A suspended upper electrode plate is disposed above the first and second lower electrode plates; the vertical projection area of the upper electrode plate on the two lower electrode plates is the same; the stress gradient is the stress gradient in the thickness direction of the upper electrode plate; the upper electrode plate and the structure of the stress gradient to be measured are grown and prepared synchronously. The fixing part of the upper electrode plate is fixedly connected to the insulating layer; The distance between the fixing part and the first lower electrode plate is less than the distance between the fixing part and the second lower electrode plate; The upper electrode plate and the first lower electrode plate together form a first capacitor, and together with the second lower electrode plate, they form a second capacitor.
2. The MEMS wafer with integrated stress gradient testing unit as described in claim 1, characterized in that, The first lower electrode plate and the second lower electrode plate have the same area.
3. The MEMS wafer with integrated stress gradient testing unit as described in claim 1, characterized in that, The fixing part is an anchor point; the anchor point is located directly below the upper electrode plate; The upper end of the anchor point is connected to the upper electrode plate, and the lower end is connected to the insulating layer.
4. The MEMS wafer with integrated stress gradient testing unit as described in claim 1, characterized in that, The fixing part includes two support beams symmetrically arranged on both sides of the upper electrode plate, and anchor points corresponding to each support beam; The support beam extends horizontally away from the upper electrode plate, with its near end integrally formed with the upper electrode plate and an anchor point located directly below its far end. The upper end of the anchor point is fixedly connected to the support beam, and the lower end is fixedly connected to the insulation layer.
5. The MEMS wafer with an integrated stress gradient testing unit as described in claim 1, characterized in that, The first lower electrode plate and the second lower electrode plate are distributed on the same side of the fixing part.
6. The MEMS wafer with an integrated stress gradient testing unit as described in claim 1, characterized in that, The first lower electrode plate and the second lower electrode plate are distributed on different sides of the fixing part.
7. The MEMS wafer with integrated stress gradient testing unit as described in claim 1, characterized in that, The test unit further includes a cover layer; the cover layer and the substrate form a sealed cavity; The cover layer is disposed above the upper electrode plate, and a gap is provided between the cover layer and the upper electrode plate.
8. The MEMS wafer with integrated stress gradient testing unit as described in claim 1, characterized in that, The upper electrode plate, the first lower electrode plate, and the second lower electrode plate are all made of polycrystalline silicon.
9. A stress gradient testing method, characterized in that, The method is applied to testing a MEMS wafer with an integrated stress gradient testing unit as described in any one of claims 1 to 8; the method includes: Obtain the capacitance values of the first capacitor and the second capacitor; If the capacitance of the first capacitor is greater than the capacitance of the second capacitor, the upper plate will bend upwards. If the capacitance of the first capacitor is less than the capacitance of the second capacitor, the upper plate will bend downwards. The degree of bending of the upper plate is obtained based on the ratio of the capacitance values of the first capacitor and the second capacitor. The stress gradient of the MEMS wafer is obtained based on the bending direction and degree of bending.
10. The stress gradient testing method as described in claim 9, characterized in that, After obtaining the capacitance values of the first and second capacitors, the process also includes: If the capacitance of the first capacitor is equal to the capacitance of the second capacitor, then the upper plate will not be bent.