A low-stress assembly structure and method for a MEMS inertial device

Abstract

The application discloses a low-stress assembly structure and method of a MEMS inertial device, which comprises a substrate, a flexible buffer layer and a MEMS sensitive element; a high-precision assembly positioning groove is formed in the substrate; the flexible buffer layer is arranged in the high-precision assembly positioning groove, the flexible buffer layer is a modified polyimide material doped with nano-silicon dioxide particles, and the matching degree of the elastic modulus of the flexible buffer layer and the elastic modulus of the substrate is not less than 95%; an array type micro-pit structure is arranged on the surface of the flexible buffer layer and used for absorbing residual stress in assembly; the MEMS sensitive element is installed above the flexible buffer layer; a stress release groove is formed in the edge of the substrate of the MEMS sensitive element and used for relieving local stress concentration; and the MEMS sensitive element and the flexible buffer layer are fixed by low-temperature solidified epoxy adhesive. The application effectively reduces the stress generated in the assembly process and improves the device precision and stability.

Description

Technical Field

[0001] This invention relates to a low-stress assembly structure and method for MEMS inertial devices, belonging to the field of inertial device technology. Background Technology

[0002] Traditional MEMS inertial device assembly primarily relies on high-precision pick-and-place machines and epoxy adhesive bonding processes. However, as device dimensions shrink to the micrometer level, assembly stress has become one of the main factors affecting device accuracy. Industry data shows that stress generated during assembly can increase gyroscope zero-bias drift by more than 30% and accelerometer measurement error by more than 25%, severely restricting the performance improvement of high-end MEMS inertial devices. In high-end applications such as aerospace and defense, devices must withstand the effects of complex vibrations, shocks, and sudden temperature changes in extreme environments. Performance degradation caused by assembly stress directly affects system reliability. For example, during high-maneuverability flight of drones, sensor errors caused by assembly stress can lead to decreased navigation accuracy and even flight accidents.

[0003] Existing MEMS inertial device assembly and stress optimization technologies have many shortcomings, making it difficult to meet the assembly and use requirements of high-precision and high-reliability devices. Traditional epoxy adhesive bonding processes are prone to residual stress due to curing shrinkage, resulting in uneven stress distribution, poor adaptability to high and low temperature environments, and difficulty in process control. Newer assembly technologies such as solder welding, micromechanical assembly, and electrostatic bonding generally suffer from drawbacks such as assembly thermal damage risks, high manufacturing costs, demanding process conditions, and poor adaptability to mass production. Existing passive and active stress optimization methods can only correct measurement errors caused by stress in the later stages, failing to effectively address the physical deformation of devices caused by assembly stress, thus offering limited stress suppression effects. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a low-stress assembly structure and method for MEMS inertial devices, which effectively reduces the stress generated during the assembly process and improves the device's accuracy and stability.

[0005] To achieve the above objectives, the present invention employs a low-stress assembly structure for MEMS inertial devices, comprising a substrate, a flexible buffer layer, and a MEMS sensing element. A high-precision assembly positioning groove is provided on the substrate; The flexible buffer layer is disposed in the high-precision assembly positioning groove. The flexible buffer layer is a polyimide material doped with nano-silica particles. The elastic modulus of the flexible buffer layer matches the elastic modulus of the substrate by no less than 95%. The surface of the flexible buffer layer is provided with an array of micro-pit structures to absorb residual stress from assembly. The MEMS sensing element is mounted on top of the flexible buffer layer; a stress relief groove is formed on the substrate edge of the MEMS sensing element to alleviate local stress concentration; the MEMS sensing element and the flexible buffer layer are bonded and fixed together by low-temperature curing epoxy adhesive.

[0006] Furthermore, the positioning accuracy of the high-precision assembly positioning groove is ±0.5μm.

[0007] Furthermore, the particle size of the nano-silica particles is 20-40 nm; based on the mass of the polyimide material, the doping mass fraction of the nano-silica particles is 5-15%.

[0008] Furthermore, the array-type micro-pit structure has a pit diameter of 40-60 μm, a depth of 15-25 μm, and a spacing of 90-110 μm between adjacent pits.

[0009] Furthermore, the stress relief groove has a width of 8-12 μm and a depth of 3-8 μm.

[0010] Furthermore, the substrate is a silicon substrate or a glass substrate, and the substrate thickness is 500μm-700μm; the thickness of the flexible buffer layer is 95-105 μm.

[0011] Furthermore, the curing temperature of the low-temperature curing epoxy adhesive is not higher than 80°C, the curing time is not more than 30 minutes, and the curing shrinkage rate is less than 0.1%.

[0012] A second aspect of the present invention also provides a low-stress assembly method for MEMS inertial devices, applied to the aforementioned low-stress assembly structure for MEMS inertial devices, comprising the following steps: S1. Surface pretreatment: Clean the surface of the substrate and MEMS sensitive element to remove surface impurities and oil, and improve the adhesion. S2. Buffer Layer Preparation: The pre-treated substrate is fixed on a rotating platform. A polyimide solution with suitable viscosity and doped with nano-silica particles is uniformly coated onto the high-precision assembly positioning groove of the substrate using a spin-coating process to form a flexible buffer layer with uniform thickness. Subsequently, heavy ions are accelerated using an accelerator to bombard the surface of the flexible buffer layer, forming a regular nuclear track damage structure on the surface of the flexible buffer layer. Then, the nuclear track damage area is directionally etched using an etchant, finally forming a uniformly distributed array of micro-pit structures on the surface of the flexible buffer layer. At the same time, stress relief grooves are pre-formed at the edge of the MEMS sensitive element substrate. S3. Positioning and Assembly: A vision-guided high-precision chip mounter is used to align the MEMS sensitive element with the high-precision assembly positioning groove on the substrate and attach it to the surface of the flexible buffer layer. S4. Low-temperature curing: The assembled structure is placed in a low-temperature curing oven, and the epoxy adhesive is cured and bonded using a low-temperature curing process. S5. Stress detection and calibration: The surface stress of the assembled device is detected using a micro Raman spectrometer. If the surface stress is greater than 10 MPa, the thickness of the flexible buffer layer and the curing process parameters are adjusted, and the device is reassembled and calibrated.

[0013] Furthermore, in step S2, the bombardment uses high atomic number ions (such as Xe⁺, Au⁺) with a mass number ≥ 100, an ion energy of 10keV-100keV, and a beam current of 1μA / cm. 2 -10 μA / cm 2 The bombardment angle is 0°-45° and the bombardment duration is 30s-60s; the etchant used is a sodium hydroxide solution.

[0014] Mechanism of the invention: Polyimide doped with nano-silica particles is used as a flexible buffer layer. The elastic modulus of the buffer layer is controlled by nanoparticle doping modification, achieving an elastic modulus matching degree of over 95% between the flexible buffer layer and the silicon-based substrate. This high modulus matching material system can significantly alleviate the stress abrupt change problem at the interface of heterogeneous materials, enabling uniform transmission, dispersion, and absorption of assembly stress at the interface. This avoids the stress accumulation and local overload deformation problems caused by modulus mismatch in traditional single rigid or flexible materials, thus constructing a uniform and stable stress buffer substrate at the material level.

[0015] Furthermore, this invention integrates an array-type micro-dimple structure, a substrate stress relief groove, and a high-precision assembly positioning groove to form a three-level hierarchical collaborative stress control system, achieving multi-dimensional stress management and assembly accuracy optimization. Specifically, the array-type micro-dimple structure on the surface of the flexible buffer layer can adaptively absorb instantaneous residual stress during the assembly process through microscopic flexible deformation, reducing stress residue during the assembly stage; the stress relief groove at the edge of the sensitive element substrate can effectively cut off the stress concentration transmission path, continuously releasing dynamically changing localized concentrated stress under conditions such as temperature alternation and vibration shock, suppressing micro-deformation of the sensitive element; the high-precision assembly positioning groove on the substrate can achieve micron-level limited assembly of the device, effectively avoiding additional stress caused by manual alignment deviations and assembly misalignments, significantly improving the assembly consistency and batch stability of batch devices. These three structures complement each other and work in a hierarchical manner, reducing the disturbance impact of assembly stress and complex external environmental stress on the MEMS sensitive structure from the dimensions of instantaneous assembly stress absorption, dynamic environmental stress dissipation, and batch assembly control, significantly improving the device's measurement accuracy, temperature stability, and vibration and shock resistance, meeting the application requirements of high-precision inertial measurement.

[0016] Compared with the prior art, the beneficial effects of the present invention are: (1) The present invention uses a composite stress control structure of a high modulus-matched flexible buffer layer combined with an array of micro-dimples and stress relief grooves to achieve uniform distribution and efficient absorption of assembly stress, thereby reducing the surface stress of the assembled MEMS sensitive element by more than 70%, effectively reducing the occurrence of micro-deformation and structural damage of the sensitive element, significantly weakening the negative impact of residual stress on the device's accuracy, stability and service life, and improving the problems of residual stress concentration and large residual stress in traditional assembly processes.

[0017] (2) Under the protection of the low-stress assembly system, the stress distortion problem in the assembly process of the device can be effectively suppressed. The core performance indicators such as gyroscope zero-bias drift and accelerometer measurement error are improved. The overall device measurement accuracy is improved by more than 80%, effectively breaking through the accuracy limitations brought about by the traditional assembly process, and meeting the high-precision measurement needs of high-precision navigation, precise attitude perception and other high-end scenarios.

[0018] (3) The present invention adopts a low temperature curing process, which effectively avoids the thermal stress and structural thermal damage caused by high temperature curing. Combined with the stress adaptive adjustment capability of the flexible buffer structure, the device temperature stability is improved by more than 60%. Under complex environments such as sudden changes in high and low temperatures and extreme temperature ranges, the device performance drift and attenuation are significantly reduced, and it can maintain excellent working stability, which is suitable for the use requirements of aerospace, military equipment and other harsh temperature conditions.

[0019] (4) The low-stress assembly structure of the present invention can buffer and absorb external vibration and impact loads, alleviate the measurement error fluctuation caused by repeated stress reconstruction under vibration conditions, improve the device's vibration resistance by more than 50%, and reduce the measurement error fluctuation amplitude in high-frequency and wide-frequency complex vibration environments, effectively improving the working reliability and measurement accuracy of unmanned equipment, vehicle navigation and other dynamic conditions.

[0020] (5) This invention simplifies the traditional complex stress control and high-temperature processing flow, with low overall process control difficulty and strong feasibility. It is compatible with existing mature semiconductor micromachining, MEMS packaging and precision surface mount processes, and has good process compatibility. Compared with the prior art, the process complexity of this invention is reduced by more than 40%, which can effectively improve the consistency of batch processing and production yield of devices; at the same time, the production efficiency is increased by more than 30%, and the overall manufacturing cost is reduced by more than 20%. While ensuring the high precision and high reliability of MEMS inertial devices, it also takes into account the application requirements of high-end devices and the economics of industrial mass production, and has good industrialization and promotion value. In the prior art, there is no disclosed composite stress control scheme that combines material modulus matching, multi-level microstructure stress synergistic release and precise positioning consistency control. This invention breaks through the technical limitations of traditional single stress optimization methods. According to actual tests, it can effectively reduce the residual stress of device assembly, improve the device zero-bias drift, measurement error, temperature stability and vibration resistance, and properly solve the long-standing problem of environmental stress interfering with device performance in the industry. Attached Figure Description

[0021] Figure 1 This is a cross-sectional schematic diagram of the assembly structure of the present invention; Figure 2 This is a schematic diagram of the array-type micro-pit structure on the surface of the flexible buffer layer of the present invention. Figure 3 This is a schematic diagram of the structure of the substrate of the present invention; Figure 4 This is a flowchart of the assembly method of the present invention. Detailed Implementation

[0022] The following embodiments are further illustrations of the present invention and serve as explanations of the technical content of the present invention. However, the essence of the present invention is not limited to the embodiments described below. Those skilled in the art can and should know that any simple changes or substitutions based on the spirit of the present invention should fall within the protection scope claimed by the present invention. Example 1

[0023] Combination Figures 1 to 4 As shown, a certain type of MEMS gyroscope is assembled using the low-stress assembly structure and assembly method of the present invention. The sensitive element of the MEMS gyroscope has a size of 5mm×5mm×0.5mm.

[0024] The specific assembly steps are as follows: First, the substrate (a silicon substrate with a thickness of 600μm is used in this embodiment) and the gyroscope sensing element are pretreated. The surface of the substrate and the sensing element is cleaned for 10 minutes using a plasma cleaning device with a power of 200W to remove surface oil, particulate impurities and oxides, which effectively improves the bonding force and bonding uniformity of the subsequent bonding interface. Then, based on the size of the gyroscope sensing element used, a high-precision assembly positioning groove is formed on the substrate surface using a dry etching method; Subsequently, a flexible polyimide buffer layer is prepared in the high-precision assembly positioning groove on the substrate surface using a spin-coating process. The flexible polyimide buffer layer is doped with 10wt% nano-silica particles (particle size of 20-40nm, 30nm in this embodiment) to ensure a high matching degree (96%) between the elastic modulus of the flexible buffer layer and the elastic modulus of the substrate. At the same time, the spin-coating process parameters are controlled to keep the thickness of the flexible buffer layer stable at 100μm, ensuring good stress buffering and absorption effects. Next, an array of micro-pit structures is formed on the surface of the flexible buffer layer using a combination of accelerator-accelerated heavy ion bombardment and etching. The diameter of the micro-pits is set to 45μm, the depth to 20μm, and the spacing between adjacent pits to 95μm. A stress relief groove with a width of 11μm and a depth of 6μm is reserved at the edge of the sensitive element substrate. Subsequently, a vision-guided high-precision chip mounter is used to complete the alignment and assembly by relying on the high-precision assembly positioning groove on the substrate surface. In this embodiment, the assembly positioning accuracy can reach ±0.3μm, ensuring the accurate assembly position of the sensitive components and avoiding additional assembly stress caused by assembly misalignment.

[0025] After assembly, the epoxy adhesive is cured at a low temperature using a low-temperature curing process. The curing temperature is set to 80℃ and the curing time is 30 minutes. The low-temperature curing conditions can effectively reduce curing shrinkage stress and residual thermal stress, and the curing shrinkage rate of the epoxy adhesive is less than 0.1%, further ensuring the low-stress assembly effect.

[0026] After assembly, stress detection and performance testing of the gyroscope's sensitive element surface were performed using a micro Raman spectroscopy instrument. The test results are as follows: After assembly, the residual stress on the surface of the gyroscope's sensitive element is only 8MPa, which is significantly lower than the 35MPa surface stress of the traditional process. The device zero-bias drift was reduced from 0.5° / h in the traditional process to 0.1° / h, and the device measurement accuracy was improved by 80%. Within a wide operating temperature range of -40℃ to 85℃, the device exhibits a zero-bias drift change of less than 0.05° / h, demonstrating a 60% improvement in temperature stability compared to traditional processes. It can maintain stable performance even under wide temperature alternation environments. Example 2

[0027] A low-stress assembly structure and assembly method were used to assemble a certain type of MEMS accelerometer. The sensitive element of the MEMS accelerometer has a size of 4mm×4mm×0.3mm.

[0028] The specific assembly steps are as follows: First, plasma surface cleaning pretreatment is performed on the substrate and accelerometer sensing element to ensure the cleanliness of the bonding interface and improve the bonding reliability. Then, based on the size of the accelerometer sensing element used, a high-precision assembly positioning groove is formed on the substrate surface using a dry etching method; A flexible polyimide buffer layer is spin-coated into a high-precision assembly positioning groove on the substrate surface. This flexible polyimide buffer layer is doped with 10 wt% nano-silica particles. An array of micro-pit structures is then formed on the surface of the flexible buffer layer using a combination of accelerator-accelerated heavy ion bombardment and etching. The micro-pits have a diameter of 50 μm, a depth of 20 μm, and a spacing of 100 μm between adjacent pits. The deformation of the array of micro-pits further absorbs residual assembly stress, improving stress concentration. Simultaneously, a stress relief groove with a width of 10 μm and a depth of 5 μm is pre-reserved at the edge of the sensitive element substrate to further optimize stress relief. The bombardment uses high atomic number ions (≥100), such as Xe⁺ and Au⁺, with ion energies of 10 keV-100 keV and beam current of 1 μA / cm². 2 -10 μA / cm 2 The bombardment angle is 0°-45°, and the bombardment duration is 30s-60s; the etchant used is a sodium hydroxide solution. After the buffer layer is prepared, a low-temperature curing epoxy adhesive is used to bond and fix the sensitive element to the flexible buffer layer. In this embodiment, the curing process parameters are optimized, with the curing temperature set to 75°C and the curing time set to 25 minutes. While ensuring the bonding strength, the thermal stress and curing shrinkage stress are further reduced. After assembly, the surface stress of the device was detected and calibrated using a micro Raman spectrometer. The measured surface stress was only 7 MPa, meeting the requirements for low-stress assembly. If the detected stress exceeds 10 MPa, reassembly and calibration can be performed by adjusting the buffer layer thickness and curing parameters to ensure batch assembly consistency.

[0029] The MEMS accelerometer assembled in this embodiment was subjected to performance and mass production performance tests. The test results are as follows: the accelerometer measurement error was reduced from 0.5mg in the traditional process to 0.1mg, and the overall measurement accuracy was improved by 80%. In a wide-frequency vibration environment of 20-2000Hz and 10grms, the device vibration rectification error is less than 0.02mg, the vibration resistance is improved by 50% compared with the traditional process, and the measurement stability under complex vibration conditions is significantly improved. Meanwhile, this invention simplifies the complex control process of traditional assembly technology, greatly reduces the complexity of process processing, and increases the daily output of a single machine from 500 pieces in the traditional process to 650 pieces, improving production efficiency by more than 30% and effectively reducing the cost of mass production.

[0030] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-stress assembly structure for MEMS inertial devices, characterized in that, Includes a substrate, a flexible buffer layer, and MEMS sensing elements; A high-precision assembly positioning groove is provided on the substrate; The flexible buffer layer is disposed in the high-precision assembly positioning groove. The flexible buffer layer is a polyimide material doped with nano-silica particles. The elastic modulus of the flexible buffer layer matches the elastic modulus of the substrate by no less than 95%. The surface of the flexible buffer layer is provided with an array of micro-pit structures to absorb residual stress from assembly. The MEMS sensing element is mounted on top of the flexible buffer layer; a stress relief groove is formed on the substrate edge of the MEMS sensing element to alleviate local stress concentration; the MEMS sensing element and the flexible buffer layer are bonded and fixed together by low-temperature curing epoxy adhesive.

2. The low-stress assembly structure for a MEMS inertial device according to claim 1, characterized in that, The positioning accuracy of the high-precision assembly positioning groove is ±0.5μm.

3. The low-stress assembly structure for a MEMS inertial device according to claim 1, characterized in that, The particle size of the nano-silica particles is 20-40 nm; the doping mass fraction of the nano-silica particles is 5-15% based on the mass of the polyimide material.

4. The low-stress assembly structure for a MEMS inertial device according to claim 1, characterized in that, The array-type micro-pit structure has pits with a diameter of 40-60 μm, a depth of 15-25 μm, and a spacing of 90-110 μm between adjacent pits.

5. The low-stress assembly structure for a MEMS inertial device according to claim 1, characterized in that, The stress relief groove has a width of 8-12 μm and a depth of 3-8 μm.

6. The low-stress assembly structure for a MEMS inertial device according to claim 1, characterized in that, The substrate is a silicon substrate or a glass substrate, with a thickness of 500μm-700μm; the thickness of the flexible buffer layer is 95-105 μm.

7. The low-stress assembly structure for a MEMS inertial device according to claim 1, characterized in that, The curing temperature of the low-temperature curing epoxy adhesive is not higher than 80℃, the curing time is not more than 30 minutes, and the curing shrinkage rate is less than 0.1%.

8. A low-stress assembly method for MEMS inertial devices, characterized in that, The application of the low-stress assembly structure for MEMS inertial devices according to any one of claims 1-7 includes the following steps: S1. Surface pretreatment: Clean the surface of the substrate and MEMS sensitive element to remove surface impurities and oil, and improve the adhesion. S2. Buffer Layer Preparation: The pre-treated substrate is fixed on a rotating platform. A polyimide solution with suitable viscosity and doped with nano-silica particles is uniformly coated onto the high-precision assembly positioning groove of the substrate using a spin-coating process to form a flexible buffer layer with uniform thickness. Subsequently, heavy ions are accelerated and bombarded on the surface of the flexible buffer layer to form a regular nuclear track damage structure. Then, the nuclear track damage area is directionally etched using an etchant to finally form a uniformly distributed array of micro-pit structures on the surface of the flexible buffer layer. At the same time, stress relief grooves are pre-formed at the edge of the MEMS sensitive element substrate. S3. Positioning and Assembly: A vision-guided high-precision chip mounter is used to align the MEMS sensitive element with the high-precision assembly positioning groove on the substrate and attach it to the surface of the flexible buffer layer. S4. Low-temperature curing: The assembled structure is placed in a low-temperature curing oven, and the epoxy adhesive is cured and bonded using a low-temperature curing process. S5. Stress detection and calibration: The surface stress of the assembled device is detected using a micro Raman spectrometer. If the surface stress is greater than 10 MPa, the thickness of the flexible buffer layer and the curing process parameters are adjusted, and the device is reassembled and calibrated.

9. A low-stress assembly method for MEMS inertial devices according to claim 8, characterized in that, In step S2, high atomic number ions with a mass number ≥ 100 are used for bombardment, with ion energies of 10 keV-100 keV and beam current of 1 μA / cm. 2 -10 μA / cm 2 The bombardment angle is 0°-45° and the bombardment duration is 30s-60s; the etchant used is a sodium hydroxide solution.

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