A method for strengthening MEMS structure based on grain size gradient control

Abstract

The application discloses a kind of based on grain size gradient control MEMS structure strengthening method, belong to micro-electro-mechanical system technical field.The method includes: preparation does not contain sulfur brightener, nitrogen grain refiner and organic surfactant's basic electroplating solution;Small grain layer, middle layer large grain layer and bottom layer small grain layer are sequentially deposited on the surface of substrate, the ratio R of nucleation rate and growth rate is maintained above small grain mode threshold or below large grain mode threshold by linearly adjusting electroplating solution temperature, pH value and current density during deposition, form three-layer structure along the thickness direction Grain size presents small, big, small gradient distribution.It also collects cathode polarization curve on-line by closed-loop regulation system and deduces R value, dynamically adjusts electroplating parameter.The application realizes the collaborative optimization of MEMS structure strength and toughness, improves the anti-fracture and plastic deformation resistance of device, and does not need to add exogenous grain adjusting agent, process controllability and repeatability are good.

Description

Technical Field

[0001] This application belongs to the field of microelectromechanical systems (MEMS) technology, specifically relating to a method for strengthening MEMS structures based on grain size gradient control. Background Technology

[0002] Microelectromechanical systems (MEMS) technology, with its advantages of miniaturization, integration, and low power consumption, is widely used in consumer electronics, automotive electronics, aerospace, and biomedical fields. The microstructures constituting MEMS devices are typically fabricated using electroplating processes, and the microstructure characteristics of the plating layer have a decisive influence on the mechanical properties of the structure. According to the basic strengthening principles of metallic materials, smaller grain sizes result in higher yield strength but a corresponding decrease in plastic deformation capacity; larger grain sizes result in better toughness but a decrease in surface hardness and wear resistance.

[0003] Current electroplating fabrication methods for MEMS structures typically employ constant electroplating parameters to create a homogeneous coating with uniformly distributed grain size. This single, uniform microstructure struggles to simultaneously meet the dual demands of high strength and high toughness: pursuing high strength requires grain refinement, which risks brittle fracture; conversely, prioritizing high toughness necessitates grain coarsening, which reduces load-bearing capacity and wear resistance. This inherent conflict between strength and toughness has become a key bottleneck restricting further improvements in the performance of MEMS structures.

[0004] To overcome these shortcomings, the industry has attempted to optimize electroplating solution formulations, introduce grain size regulators, or improve post-processing techniques. However, limited by the traditional process's ability to uniformly control grain size, it has been impossible to achieve differentiated configurations and gradient distributions of grain size across different thickness regions of the same structure. Therefore, how to construct a reasonable grain size gradient along the thickness direction in MEMS structures through improvements in electroplating processes, thereby synergistically enhancing strength and toughness, is a pressing technical problem that needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a MEMS structure strengthening method based on grain size gradient control, which can effectively solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for strengthening MEMS structures based on grain size gradient control includes the following specific steps: Prepare a basic electroplating solution, wherein no sulfur-containing brighteners, nitrogen-containing grain refiners, or organic surfactants are added to the basic electroplating solution; A surface microcrystalline layer is deposited on the substrate surface. During the deposition process, the electroplating bath temperature is controlled between 15℃ and 25℃, the pH value is controlled between 3.5 and 4.5, and a current density is applied to adjust the ratio of nucleation rate to growth rate. Maintain at the preset small grain mode threshold above; A medium-sized large-grained layer is deposited on top of the surface small-grained layer. During the deposition process, the electroplating bath temperature, pH value, and current density are linearly adjusted from the parameter values ​​used when depositing the surface small-grained layer to 35°C to 45°C, 5.0 to 6.0, and 2 ASD to 3 ASD, respectively, to adjust the ratio of nucleation rate to growth rate. From above the small grain mode threshold Gradually reduce to the preset large grain mode threshold the following; A bottom layer of small grains is deposited on top of the intermediate large grain layer. During the deposition process, the electroplating bath temperature is restored to 15°C to 25°C, the pH value is restored to 3.5 to 4.5, and the current density is restored to achieve the desired nucleation rate to growth rate ratio. It rebounded and remained at the small grain mode threshold. This results in a three-layer structure with a small-large-small gradient distribution of grain size along the thickness direction.

[0007] Furthermore, the thickness of the surface small-grain layer accounts for 15% to 25% of the total thickness of the three-layer structure, the thickness of the middle large-grain layer accounts for 50% to 60% of the total thickness of the three-layer structure, the thickness of the bottom small-grain layer accounts for 20% to 30% of the total thickness of the three-layer structure, and the deviation between the average grain size of the bottom small-grain layer and the average grain size of the surface small-grain layer is within ±15%.

[0008] Furthermore, the average grain size of the middle large-grain layer is 2 to 4 times the average grain size of the surface small-grain layer.

[0009] Furthermore, during the deposition of the intermediate large-grain layer, the adjustment slopes of temperature, pH value, and current density are coordinated to ensure the ratio of nucleation rate to growth rate. The continuous linear variation avoids discontinuous breaks in grain size.

[0010] Furthermore, throughout the deposition process, the cathode polarization curve is acquired online using a linear scanning voltammeter. A closed-loop control system then uses a correlation model to analyze the characteristics of the cathode polarization curve and the current electroplating solution temperature and pH value. By deducing the ratio of nucleation rate to growth rate at the current moment. .

[0011] Furthermore, the closed-loop control system will deduce the ratio of nucleation rate to growth rate at the current moment. Ratio to the target of the current depositional stage The intervals are compared, and the electroplating solution temperature or current density is dynamically adjusted according to preset priorities and adjustment amounts based on the direction and magnitude of the deviation, so as to adjust the ratio of nucleation rate to growth rate. Maintain within the target range, with a closed-loop adjustment cycle of once every 5 to 30 seconds.

[0012] Furthermore, when the ratio of nucleation rate to growth rate is detected... When the temperature is below the lower limit of the target range and the slope of the polarization curve decreases, the closed-loop adjustment system preferentially reduces the electroplating solution temperature in steps of 0.5℃ to 1℃. If the ratio of nucleation rate to growth rate is adjusted... If it still does not recover, increase the current density in steps of 5% to 10% of the current current density; When the ratio of nucleation rate to growth rate is detected If the current exceeds the upper limit of the target range and the slope of the polarization curve increases, then the electroplating solution temperature should be increased or the current density should be decreased.

[0013] Furthermore, the entire deposition process employs a constant current mode, and the current ripple coefficient is controlled within ±2%.

[0014] Furthermore, before depositing the underlying small grain layer, the parameters are restored in a stepwise sequence: first, the current density is increased to the target range, and after the cathodic polarization curve response stabilizes, the electroplating solution temperature is reduced to 15°C to 25°C, and finally the pH value is adjusted to 3.5 to 4.5, with the time interval between two adjacent adjustments being 30 seconds to 2 minutes.

[0015] Furthermore, the small grain mode threshold and the large grain mode threshold The determination method is as follows: For the electroplating solution system used, a series of electroplating pre-experiments were carried out by changing the temperature, pH value and current density. The cathodic polarization curves under each condition were recorded using linear sweep voltammetry to extract nucleation kinetic parameters, and the corresponding average grain size was measured, thereby fitting the ratio of nucleation rate to growth rate. The curve showing the relationship between grain size and the small grain mode threshold is selected on the curve. and large grain mode threshold .

[0016] In summary, this application includes at least one of the following beneficial technical effects: 1. This invention constructs a small-grain-size gradient structure. The small grain layers at the top and bottom provide high grain boundary density, significantly improving yield strength and wear resistance, and effectively resisting fracture failure caused by external loads. The large grain layer in the middle provides excellent plastic deformation capacity, effectively alleviating deformation damage caused by stress concentration, and achieving synergistic optimization of strength and toughness. By systematically analyzing the differential influence mechanism of temperature, pH value, and current density on the competitive relationship between nucleation rate and growth rate, a quantitative correlation model of parameter ratio and grain size is established to achieve precise gradient design of grain size.

[0017] 2. By monitoring the electroplating solution parameters and cathodic polarization curve in real time, and combining the nucleation theory model to back-calculate the current ratio, the electroplating parameters are dynamically adjusted to ensure that the ratio at each stage is always within the target range, thereby improving process stability and product repeatability.

[0018] 3. This invention utilizes existing conventional equipment such as constant temperature electroplating tanks, high-precision pH meters, and adjustable DC power supplies. Only a linear scanning voltammeter and an atomic force microscope are needed to achieve full-process monitoring. There is no need to introduce special chemical reagents or make large-scale equipment modifications, thus maintaining the simplicity and controllability of the process. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall MEMS structure strengthening method based on grain size gradient control. Figure 2 This is a schematic diagram illustrating the core principle of regulation based on the competitive relationship between nucleation rate and growth rate. Figure 3 This is a logic diagram showing the control of electroplating process parameters on the deposition process of small grain layer, large grain layer and secondary small grain layer; Figure 4 A schematic diagram illustrating the interaction and data flow between electroplating solution parameter monitoring, polarization curve acquisition, and dynamic parameter adjustment; Figure 5 This is a comparison chart showing the technical effects of grain size gradient structures and traditional homogeneous structures in terms of strength and toughness. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the following description is provided in conjunction with the appendix. Figure 1 To be continued Figure 5 The present invention will be further described in detail below with reference to specific embodiments.

[0021] This invention provides a MEMS structure strengthening method based on grain size gradient control. Its core lies in precisely controlling three key parameters during the electroplating process: temperature, pH, and current density. This dynamically balances the competition between nucleation rate and grain growth rate during electrodeposition, thereby achieving a grain size structure with small, large, and small gradient distributions on the MEMS structure surface. Ultimately, this results in a synergistic improvement in the strength and toughness of the MEMS structure. This method relies entirely on the precise control of electroplating parameters, requiring no external grain size regulators in the electroplating solution and no complex equipment modifications. Only a linear scanning voltammeter and an atomic force microscope are needed to monitor and verify the entire process on top of existing conventional electroplating equipment, maintaining the simplicity and controllability of the process.

[0022] According to the quantitative description of the Hall-Page relation, there is a clear mathematical relationship between the yield strength of a material and its grain size: in, Indicates resistance to deformation within the crystal. This represents the grain boundary strengthening coefficient, both of which are determined by the inherent properties of the coating material itself. Let be the average grain size. This relationship indicates that the smaller the grain size, the higher the yield strength of the material. This is because finer grains imply a higher grain boundary density, which effectively hinders dislocation movement and increases the external load required for material deformation. Based on this theory, when high strength performance is required in the surface layer of a MEMS structure, the formation of fine grains should be promoted; while when excellent toughness is required in the middle layer of the structure, grains should be allowed to grow sufficiently to obtain greater plastic deformation space.

[0023] The final average size of grains during electroplating is determined by the competition between the nucleation rate and the grain growth rate. Based on classical nucleation theory, the relative magnitudes of the nucleation rate and growth rate directly determine the final grain size distribution characteristics. When the nucleation rate is much higher than the growth rate, a large number of dense nuclei will form on the substrate surface per unit time. These nuclei are difficult to grow sufficiently during subsequent growth due to space competition, eventually forming a fine and uniformly distributed small grain structure. When the growth rate exceeds the nucleation rate, a few preferentially nucleated grains will obtain more growth kinetic energy and spatial advantage, gradually engulfing the surrounding small nuclei that have not yet grown sufficiently, resulting in a typical grain growth phenomenon, ultimately forming a larger grain structure. The competitive relationship between the two can be quantitatively characterized by the ratio of the nucleation rate to the growth rate: the higher the ratio, the smaller the grain size; the lower the ratio, the larger the grain size. This invention precisely achieves gradient control of grain size in different regions of MEMS electroplated thin films by precisely controlling key electroplating parameters and dynamically balancing the competitive relationship between the nucleation rate and the growth rate.

[0024] The three major electroplating parameters—temperature, pH, and current density—each have distinct mechanisms of influence on the competitive relationship between nucleation rate and growth rate. Temperature's influence is primarily manifested in its dual effect on both the diffusion rate of metal ions and the migration rate of atoms on the grain surface. Increased temperature significantly accelerates the diffusion rate of metal ions in the electroplating solution. While this slightly lowers the energy barrier for nucleation, leading to a minor increase in the nucleation rate, its main effect is a substantial increase in the migration rate of atoms on the grain surface, driving rapid grain growth. Under these combined effects, increased temperature leads to a multiple increase in growth rate, while the increase in nucleation rate is relatively limited, ultimately decreasing the ratio of nucleation rate to growth rate and promoting larger grain size. Therefore, a lower temperature environment is more conducive to maintaining a higher ratio of nucleation rate to growth rate, creating favorable conditions for the formation of smaller grains.

[0025] pH affects the deposition driving force by altering the complexation equilibrium of metal ions. When pH decreases (i.e., acidity increases), the concentration of free metal ions rises, and the cathodic polarization increases significantly, which greatly enhances the nucleation rate. Meanwhile, the growth rate only increases slightly due to decreased ion adsorption stability, ultimately leading to an increase in the nucleation rate to growth rate ratio, resulting in grain refinement. Conversely, when pH increases (i.e., acidity decreases), complexed ions must first dissociate before deposition, leading to decreased cathodic polarization and a significant decrease in the nucleation rate. The growth rate increases due to improved ion deposition stability, but the nucleation rate to growth rate ratio decreases, resulting in grain coarsening.

[0026] The current density parameter directly determines the electron transfer rate on the cathode surface, and its relationship with the overpotential follows the Tafel equation. When the current density increases, the overpotential rises significantly, which greatly reduces the nucleation barrier, enhances cathode polarization, and significantly increases the number of activatable nucleation sites per unit area, resulting in an exponential increase in the nucleation rate. However, the growth rate is physically limited by the ion diffusion rate, and its growth rate is much lower than that of the nucleation rate, ultimately leading to a significant increase in the ratio of nucleation rate to growth rate, resulting in fine grains. When the current density decreases, cathode polarization weakens, the number of nucleation sites decreases, the nucleation rate drops rapidly, while the growth rate gradually approaches the ion diffusion limit and tends to stabilize. The ratio of nucleation rate to growth rate decreases, and the grain size increases.

[0027] For example, in nickel electroplating using a sulfate system, a small grain layer can be achieved using a temperature of 20-25℃, a pH of 3.8-4.2, and a current density of 5-7 ASD; while a large grain layer can be achieved using a temperature of 35-40℃, a pH of 5.0-5.5, and a current density of 2-3 ASD, thus realizing a gradient change in grain size from approximately 50 nm to approximately 200 nm. Based on the above principles and this specific example, and combined with knowledge of conventional electroplating processes, those skilled in the art can determine the specific parameter combinations for achieving small, large, and small grain size gradients through simple experiments for different plating materials.

[0028] Furthermore, the method of this application is implemented according to the following steps: Step S1: Material preparation and parameter setting, which shall be carried out in accordance with the following steps: Step S101: Determine the plating material and prepare the basic electroplating solution. Select the plating material according to the application scenario and mechanical performance requirements of the MEMS device. For accelerometers subjected to high-frequency vibration loads, nickel-based alloys or cobalt-based alloys with excellent fatigue performance are preferred; for pressure sensors in corrosive environments, corrosion-resistant gold or palladium plating can be used; for radio frequency devices that need to balance conductivity and mechanical performance, copper plating can be used.

[0029] Once the plating material is determined, the basic electroplating solution is prepared according to its electrochemical characteristics. The concentration of each component is adjusted according to the standard process specifications to ensure that the conductivity, metal ion concentration, and complexing agent ratio of the electroplating solution are all within the normal working range.

[0030] No grain size regulators, including sulfur-containing brighteners, nitrogen-containing grain refiners, or organic surfactants, are added during the formulation process to avoid interference from exogenous substances with the natural competition between nucleation rate and growth rate, ensuring that subsequent grain size control is dominated by electroplating parameters.

[0031] Step S102: Calculate the upper limit of the surface grain size based on the target yield strength, and determine the maximum grain size that meets the strength requirement using the Hall-Page relation. The expression for the Hall-Page relation is: In the expression, The yield strength of the material. The resistance to deformation within the grain represents the frictional resistance to the movement of dislocations within the grain. This is the grain boundary strengthening coefficient, reflecting the contribution of grain boundaries to strength; This represents the average grain size. and It is determined by the coating material itself and can be determined through a material handbook or experiment.

[0032] Let the required yield strength of the target MEMS structure surface be... The target size of the surface micrograins It should meet the following requirements: The maximum permissible average grain size is derived by inversely using inequalities, serving as the upper limit for the design of grain size in small grain layers.

[0033] Step S103: Plan the thickness and grain size distribution of each grain layer, and allocate the proportion of each layer in the total coating thickness according to the stress distribution characteristics of the overall structure and the functional positioning of each layer.

[0034] The surface micrograin layer is used to provide high-strength support, and its thickness is controlled between 15% and 25% of the total coating thickness.

[0035] The middle layer with large grains is responsible for improving toughness, giving the structure sufficient plastic deformation capacity while maintaining overall strength, so as to alleviate stress concentration; the thickness accounts for 50% to 60% of the total thickness, and the average grain size is designed to be 2 to 4 times that of the surface small grains.

[0036] The function of the bottom small grain layer is to form mechanical symmetry with the surface layer, enhance the adhesion between the coating and the substrate, and account for 20% to 30% of the total thickness. The average grain size of the grains is controlled within ±15% of the small grains on the surface layer to ensure the mechanical balance of the gradient structure.

[0037] Step S104: Establish a quantitative characterization model of the competitive relationship between nucleation rate and growth rate, and define the ratio. nucleation rate With growth rate The ratio, that is .

[0038] According to classical electrochemical nucleation theory, the nucleation rate The growth rate is exponentially related to the overpotential. Controlled by ion diffusion and surface migration, therefore The value can be expressed as a function of parameters such as temperature, pH, and current density. To establish... Quantitative mapping between nucleation values ​​and grain size can be achieved through systematic preliminary experiments on the plating solution system used. This involves implementing a series of electroplating processes by varying temperature, pH, and current density. The cathodic polarization curves under each condition are recorded using linear scanning voltammetry to extract nucleation kinetic parameters. Samples are then taken and the corresponding average grain size is measured using atomic force microscopy or scanning electron microscopy, thereby fitting the nucleation parameters. The variation pattern of the value with the parameter and The correspondence between the value and the grain size.

[0039] On the calibrated curve, two thresholds are selected: This represents the lower threshold for the small grain mode. This represents the upper threshold for the large grain mode. When When the deposition enters a small-grain mode, nucleation events dominate, resulting in fine grains; when At this stage, deposition enters the large grain mode, grain growth becomes dominant, and coarse grains are obtained.

[0040] Step S105: Define the characteristics of each electroplating stage Value control target, surface small grain deposition stage, The value needs to be maintained at The specific control range is determined by pre-experimental calibration based on the target small grain size.

[0041] During the middle-layer large-grain deposition stage, control Values ​​from higher The area gradually decreased to below Furthermore, the transition uses a linear, gradual method to avoid... Abrupt changes in grain size result in discontinuities in grain size, ensuring a continuous transition in grain size from the surface to the middle layers.

[0042] During the bottom-layer small grain deposition stage, The value is restored to a level similar to that of the surface deposition stage, so that the grain size of the bottom layer is comparable to that of the surface layer.

[0043] Step S1, starting from the device service requirements, transforms the mechanical performance indicators into grain size and thickness distribution parameters of a multilayer gradient structure, and further establishes... The value serves as the core electrochemical control benchmark. Subsequent electroplating processes gain clear guidance for parameter switching and real-time feedback control, ensuring the stable reproduction of small, large, and small grain gradients that optimize strength and toughness.

[0044] Step S2, the small grain layer deposition step, is carried out according to the following steps: Step S201: Set and stabilize temperature parameters to suppress grain growth. The deposition of the surface small grain layer adopts a temperature environment lower than that of conventional electroplating, with a setting range between 15°C and 25°C. This low-temperature environment is maintained by the cooling cycle of the constant-temperature electroplating bath, which is equipped with a temperature sensor and a feedback controller to control temperature fluctuations within ±0.5°C.

[0045] The core mechanism of temperature setting lies in the fact that, according to the Arrhenius equation, lowering the temperature slows down the diffusion rate of metal ions in the electroplating solution and the migration rate of atoms on the grain surface. For every 10°C decrease in temperature, the ion diffusion coefficient decreases by approximately 30% to 40%, and the atomic stacking rate during grain growth decreases accordingly. This creates thermodynamic conditions for maintaining a high nucleation rate to growth rate ratio, thus slowing down the grain growth trend from a kinetic perspective.

[0046] Step S202: Adjust the pH value to a suitable acidic range to enhance cathodic polarization. The pH value of the electroplating solution is set between 3.5 and 4.5 during the surface small grain deposition stage to create a suitable acidic environment. Within this pH range, a high concentration of free metal ions can be maintained in the electroplating solution. The increase in free metal ions directly increases the effective deposition ion concentration on the cathode surface.

[0047] Simultaneously, an acidic environment enhances cathodic polarization, creating a stronger electric field on the cathode surface, which is beneficial for the reduction and deposition of metal ions and the formation of new crystal nuclei. It is important to clarify that suitable acidity refers to the pH range that can improve the nucleation rate while ensuring coating quality. Too low a pH value will deteriorate the coating quality or cause pinhole defects, therefore this range must be avoided. This step, from a thermodynamic perspective, increases the driving force of cathodic polarization, enhancing the advantages of the nucleation process.

[0048] In step S203, a high current density is applied to exponentially increase the nucleation rate. The current density for surface grain deposition is set to 1.5 to 2.5 times the conventional electroplating current density, with the specific value determined experimentally based on the plating material and electroplating solution system. Taking sulfate-based nickel electroplating as an example, the conventional current density is 2 to 4 ASD, while the current density in this stage can be set to 5 to 8 ASD.

[0049] The core function of high current density is to significantly increase the overpotential. Based on the relationship between overpotential and nucleation barrier, the increase of overpotential can reduce the energy barrier for the formation of new crystal nuclei, causing the number of activatable nucleation sites per unit area on the cathode surface to increase exponentially. This results in the generation of a large number of densely distributed crystal nuclei per unit time, making the deposition process more inclined to form new crystal nuclei rather than allow existing crystal nuclei to grow.

[0050] In step S204, electroplating is started in constant current mode and parameters are fine-tuned. The temperature of the electroplating solution is stabilized at the target value in S201. After the pH value is fine-tuned to the target range in S202 using dilute sulfuric acid or dilute sodium hydroxide solution, the current density is gradually increased to the target set value in S203 through an adjustable DC power supply.

[0051] The entire deposition process employs a constant current mode, with the current ripple coefficient controlled within ±2% to ensure deposition stability. Gradually increasing the current density avoids instantaneous fluctuations in the nucleation process caused by sudden current changes.

[0052] Step S205: Dynamic monitoring and thickness control of the deposition process. During deposition, a closed-loop control system is used to monitor electroplating parameters and cathode polarization curves in real time. The closed-loop control system dynamically adjusts the current density based on the monitoring data to compensate for minor fluctuations in temperature or pH, ensuring that the ratio of nucleation rate to growth rate remains stable within the target range.

[0053] The deposition thickness of the small grain layer is controlled by the electroplating time, and the deposition rate is calculated based on the coulombic efficiency of the electroplating solution and the set current density. Typical deposition rates range from 0.5 to 2 micrometers per minute. When the thickness reaches the surface layer thickness planned in step S103, this stage of deposition ends.

[0054] In step S2, a high-density, fine-grained surface layer is formed on the surface of the MEMS structure substrate. After the high-strength surface layer is prepared, the process flow immediately proceeds to step S3, where the competitive pattern of nucleation and growth is changed by continuously adjusting the electroplating parameters, smoothly transitioning from the small-grain mode of the surface layer to the coarse-grain growth mode of the middle layer.

[0055] Step S3, large grain layer deposition, is carried out according to the following sub-steps.

[0056] Step S301: Set and execute temperature gradient heating. The starting temperature for the deposition of the middle large grain layer follows the value at the end of step S2, which is in the low temperature range of 15°C to 25°C.

[0057] As deposition proceeds, the temperature of the isothermal electroplating bath is controlled to rise linearly. The rate of temperature rise is determined based on the preset transition time and temperature increase, with a typical rate between 0.5°C / min and 2°C / min. The endpoint temperature is set in a moderate range of 35°C to 45°C, at which temperature both the metal ion diffusion rate and the surface atom migration rate are conducive to grain growth.

[0058] The heating process is achieved by sending a linearly changing temperature setpoint to the heating module of the constant temperature electroplating tank through a closed-loop control system, so that the temperature of the tank solution rises steadily.

[0059] In step S302, a pH gradient increase is set and executed. The initial pH value follows the value at the end of step S2, which is in the acidic range of 3.5 to 4.5. As deposition proceeds, a dilute sodium hydroxide solution is quantitatively added to the electroplating solution using a dosing pump, so that the pH value gradually increases linearly to the weakly acidic to neutral range of 5.0 to 6.0.

[0060] The pH adjustment slope and temperature adjustment slope are determined in advance through calibration and are synchronously controlled by the closed-loop control system according to the time-pH curve to ensure that the increase in pH and the increase in temperature are coordinated in time and the effect on the nucleation rate to growth rate ratio R is applied synchronously.

[0061] As the pH value increases, the dissociation equilibrium of complexed ions shifts towards the complexing direction, the concentration of free metal ions decreases relatively, the cathodic polarization decreases, the driving force for the formation of new crystal nuclei gradually weakens, and the nucleation rate continues to decrease; at the same time, the stability of ion deposition is improved, which is conducive to the continuous growth of nucleated grains.

[0062] Step S303: Set and execute the current density gradient reduction. The initial current density value follows the value at the end of step S2, which is in the high current density range of 5 ASD to 8 ASD. The closed-loop regulation system controls the adjustable DC power supply to gradually reduce the output current density linearly to a moderate level of 2 ASD to 3 ASD.

[0063] The slope of the decrease in current density is also coordinated with the adjustment slopes of temperature and pH value, ensuring that the combined effect of the three types of parameters on the R value is continuous and smooth.

[0064] When the current density decreases, the overpotential decreases, the nucleation barrier increases, the number of activatable nucleation sites per unit area decreases, and the nucleation rate drops rapidly. At the same time, because the ion diffusion rate is relatively sufficient, the growth rate approaches its limit value, and the competitive advantage gradually increases.

[0065] Step S304 involves implementing electroplating deposition under gradient parameters and dynamically maintaining a large grain growth mode, continuously performing electroplating deposition during the linear change of parameters. In the initial transitional phase with a high R value, the deposition still exhibits small grain nucleation characteristics, but existing grains begin to grow competitively. As the R value decreases, the rate of new grain formation decays, and the growth of existing nucleated grains gradually dominates the deposition process. Upon entering a stable period with a low R value, the main characteristics are continuous grain growth and inter-grain merger and reorganization, forming a large grain structure significantly larger than the surface layer. Throughout the process, the closed-loop control system uses a linear scanning voltammeter to acquire the cathode polarization curve in real time and determines the trend of the R value based on the change in the slope of the polarization curve and the characteristics of the limiting current plateau.

[0066] If a decrease in the slope of the polarization curve or an extension of the plateau region is detected, indicating an expansion of the growth rate advantage, the R value is suppressed by slightly reducing the temperature or slightly increasing the current density. If an unexpected increase in the slope is detected, the opposite fine-tuning measures are taken to ensure that the deposition is maintained in the target large grain growth mode.

[0067] In step S305, the thickness of the large grain layer is controlled by time to complete the preparation of the intermediate layer. The target thickness of the large grain layer is determined according to the plan in step S103, accounting for 50% to 60% of the total coating thickness. The deposition rate is calculated by the coulombic efficiency of the electroplating solution and the real-time current density, and the deposition time is set and precisely controlled accordingly.

[0068] Taking a total coating thickness of 20 micrometers as an example, the thickness of the middle large-grain layer is approximately 10 to 12 micrometers. When the deposition time reaches the preset value, the deposition of the middle large-grain layer is completed. At this time, a continuous transition structure from fine grains on the surface to coarse grains in the middle layer has been formed inside the coating.

[0069] In step S3 above, the competition between nucleation rate and growth rate is controlled by synchronously and linearly adjusting temperature, pH value, and current density, so as to obtain a large-grain layer with thickness and grain size that meet the design while maintaining a continuous transition. After the preparation of the middle large-grain layer is completed, the process immediately proceeds to step S4, where the R value is increased by switching parameters again, and a fine-grained structure is reconstructed in the bottom layer, finally completing the full-thickness construction of the small-large-small gradient structure.

[0070] Step S4, secondary deposition of the small grain layer, is carried out according to the following sub-steps.

[0071] Step S401: Perform a step-by-step parameter switching to restore the small grain deposition conditions. Before depositing the bottom small grain layer, the electroplating parameters need to be switched from the large grain growth mode at the end of step S3 back to the small grain mode of step S2. The specific parameter restoration targets are: temperature restored to a low temperature range of 15°C to 25°C, pH value restored to an acidic range of 3.5 to 4.5, and current density restored to a high current density range of 5 ASD to 8 ASD.

[0072] The parameter switching adopts a step-by-step sequence, rather than a simultaneous jump of all three parameters. During operation, based on the parameters at the end of step S3, the closed-loop control system controls the adjustable DC power supply to first rapidly increase the current density to the target range of 5 ASD to 8 ASD; after waiting for the cathodic polarization curve response to stabilize, the closed-loop control system then controls the cooling cycle of the constant temperature electroplating bath to rapidly reduce the temperature to the low-temperature target range of 15℃ to 25℃; finally, dilute sulfuric acid is quantitatively added to the electroplating solution through a dosing pump to finely adjust the pH value to the target range of 3.5 to 4.5.

[0073] During parameter switching, the time interval between two adjacent adjustments is predetermined based on the response characteristics of the electroplating solution system, generally ranging from 30 seconds to 2 minutes. The purpose is to allow sufficient time for the cathodic polarization state to re-establish equilibrium after each parameter step, thereby avoiding the introduction of disturbances to the deposition structure by the transient process.

[0074] In step S402, after confirming that the ratio of nucleation rate to growth rate has returned to the target level, and before the formal electrostatic deposition begins after the parameter switching in step S401, the closed-loop control system uses a linear scanning voltammeter to collect the cathode polarization curve and extracts the slope of the polarization curve for comparison and judgment.

[0075] When the slope of the acquired cathode polarization curve recovers to a larger value similar to the initial value recorded in step S2, it indicates that the cathode polarization has increased to the required level, the nucleation process has once again become the dominant mechanism, and the ratio R of nucleation rate to growth rate has returned to the level determined in step S105. That concludes the above. At this point, the closed-loop control system's criteria are met, allowing entry into the formal bottom-layer small grain deposition stage.

[0076] In step S403, electroplating of the underlying small grain layer is performed and the deposition thickness is controlled. The deposition of the underlying small grain layer continues to use a constant current mode, and the current ripple coefficient is controlled within ±2%. During the deposition process, the closed-loop control system continuously monitors the temperature, pH value, current density, and cathode polarization curve. When parameters deviate from the target, automatic compensation and adjustment are performed to maintain the R value in the preset high range.

[0077] The target thickness of the bottom small grain layer is executed according to the plan in step S103, generally accounting for 20% to 30% of the total coating thickness, and the thickness deviation from the surface small grain layer is controlled within ±20%. The deposition rate is calculated from the coulombic efficiency of the electroplating solution and the set current density, and the final thickness is controlled by the electroplating time.

[0078] Step S404: Control the grain size deviation between the bottom layer and the surface layer and terminate electroplating. During the deposition of the bottom layer small grain layer, the closed-loop adjustment system maintains the R value according to the mapping relationship between the R value and the grain size calibrated in step S104, so that the deviation between the corresponding average grain size and the surface layer small grain layer is within ±15%.

[0079] When the deposition thickness reaches the target value set in step S403, the output current is cut off by the adjustable DC power supply, and the substrate is removed from the electroplating solution, thus completing the entire electroplating process.

[0080] Thus, the MEMS structure coating forms a complete gradient structure along the thickness direction, with fine grains on the surface, coarse grains in the middle, and fine grains at the bottom.

[0081] Step S4, through orderly switching of electroplating parameters and confirmation of the R value based on the slope of the cathode polarization curve, reverses the competition between nucleation and growth from growth dominance to nucleation dominance, thereby depositing a fine-grained bottom layer that is mechanically symmetrical with the surface layer. Once the three-layer gradient structure is fully fabricated, the core deposition stage of the entire process is complete. During service, the high-density grain boundaries of the surface and bottom layers provide the required strength and wear resistance, while the large grains in the middle layer alleviate stress concentration through plastic deformation, thus addressing the synergistic requirements of strength and toughness at the material microstructure level.

[0082] Finally, step S5 involves closed-loop control and real-time monitoring, implemented according to the following sub-steps. The method used in this step is integrated throughout the entire electroplating process from steps S2 to S4, providing real-time assurance for the stable preparation of gradient grain structures.

[0083] Step S501: Establish a multi-parameter real-time acquisition link. Closed-loop control relies on the synchronous monitoring of electroplating solution temperature, pH value, current density, and cathode polarization curve. The acquisition methods for each parameter are as follows.

[0084] Temperature monitoring uses a platinum resistance temperature sensor or a thermocouple sensor with a measurement accuracy of ±0.1℃ and a sampling frequency of 1 to 5 times per second to provide real-time feedback of the tank liquid temperature to the control system.

[0085] pH monitoring uses a high-precision pH electrode in conjunction with a digital pH meter, with a measurement accuracy of ±0.01 and a response time of less than 5 seconds. The pH signal is continuously collected in the electroplating solution flow loop and transmitted to the control system.

[0086] Current density monitoring is achieved by connecting a precision sampling resistor in series in the cathode circuit and configuring a voltage acquisition module to measure the voltage drop across the sampling resistor. The voltage drop is then converted into instantaneous current density based on the effective area of ​​the cathode, with a measurement accuracy of ±0.5%, and is recorded synchronously by the control system.

[0087] The collected data on temperature, pH value, and current density are aggregated by a data acquisition card and then input into the control system. The control system compares these data point-by-point with the preset target parameter curve for the current electroplating stage. If any parameter deviates beyond the preset allowable range, the control system triggers a parameter adjustment command, driving the corresponding execution module to perform compensation.

[0088] Step S502: Online acquisition of the cathode polarization curve and extraction of characteristic parameters. The online acquisition of the cathode polarization curve is performed using a linear scanning voltammeter. The linear scanning voltammeter applies a potential signal that varies linearly with time to the cathode, scanning from the open-circuit potential towards more negative potentials. The scanning rate is set between 10 mV / s and 50 mV / s depending on the plating solution system, and the scanning potential range covers the deposition potential range of the plating material. The voltammeter simultaneously records the potential-current response data, forming a complete cathode polarization curve.

[0089] The control system extracts three key characteristic parameters from the polarization curve in real time. The initial potential, the value at which the deposition current begins to rise significantly, reflects the thermodynamic driving force of the electrodeposition reaction. The slope of the polarization curve, calculated from the linear segment of the Tafel interval, reflects the degree of electrochemical polarization and charge transfer rate. The limiting current plateau height, the value at which the current tends to saturate when the potential shifts further negatively, reflects the degree to which ion diffusion limits the overall reaction rate. These three characteristic parameters constitute the input basis for subsequent R-value estimation and control decisions.

[0090] In step S503, the ratio of nucleation rate to growth rate is inferred from the characteristics of the polarization curve. The control system is embedded with a correlation model established based on the preliminary experimental data from step S104. This correlation model maps the slope of the polarization curve and the height of the limiting current plateau to the ratio R, in the form R = f(slope, plateau current, temperature, pH value).

[0091] When calculating the R value, the control system incorporates the currently acquired polarization curve slope, limiting current plateau height, and current temperature and pH value into the correlation model. The model outputs an estimated R value through a combination of table lookup and interpolation: first, using the polarization curve slope and plateau current as indices, it retrieves the nearest calibration point from the multidimensional data table constructed in the pre-experiment; then, it performs weighted interpolation correction based on the deviations between the actual temperature and pH value and the calibration conditions, ultimately obtaining the nucleation rate to growth rate ratio R at the current moment. This R value characterizes the instantaneous competition mode of the deposition process and serves as a direct criterion for parameter adjustment.

[0092] Step S504: Determine the parameter adjustment strategy based on the deviation between the R value and the target value. The control system compares the current R value obtained by back-calculation in step S503 with the target R value range for the current stage set in step S105, and performs the corresponding adjustment action according to the direction of the deviation.

[0093] When the R value is detected to be below the lower limit of the target range, and the slope of the polarization curve decreases or the limiting current plateau lengthens, it indicates that the growth rate advantage is expanding and the nucleation process is suppressed. At this point, the control system generates adjustment commands according to the following priority: first, the electroplating solution temperature setpoint is reduced in small increments, with each reduction ranging from 0.5°C to 1°C; if the R value does not recover after temperature adjustment, the current density setpoint is increased in small increments, with each increase ranging from 5% to 10% of the current density. Through these measures, the growth rate is slowed, the nucleation rate is increased, and the R value is pulled back to the target range.

[0094] When the R value is detected to be higher than the upper limit of the target range and the slope of the polarization curve increases abnormally, it indicates that the nucleation rate is increasing too rapidly, which may lead to excessively fine grains and deviate from the design of large grains in the middle layer. At this time, the control system adjusts in the opposite direction: first, it increases the temperature or decreases the current density in small steps, with an amplitude comparable to the aforementioned suppression adjustment, so as to moderately promote growth and suppress nucleation, so that the R value returns to the target range.

[0095] When the R value falls within the target range, the control system maintains the current parameters unchanged and continues monitoring for the next cycle.

[0096] Step S505 involves closed-loop iterative monitoring and recording throughout the entire process. Steps S501 to S504 constitute a complete closed-loop cycle of monitoring-estimation-decision-execution. This cycle is repeated at fixed intervals throughout the electroplating process in steps S2 to S4. The execution interval is set according to the total electroplating time and the rate of parameter change, and is generally once every 5 to 30 seconds.

[0097] The monitoring data, estimated R value, adjustment actions, and adjusted parameter values ​​for each cycle are all recorded and stored by the control system, forming a complete process data archive. If a parameter deviation exceeds the hardware adjustment capability within a certain monitoring cycle, or if the R value cannot be pulled back to the target range for several consecutive cycles, the control system will issue an alarm, prompting operator intervention for inspection.

[0098] Step S5, through multi-parameter synchronous acquisition, online analysis of cathode polarization curves, and closed-loop feedback control of R value, maintains the competitive relationship between nucleation rate and growth rate on the target trajectory set in step S1, supporting the accurate preparation of small-large-small grain gradient structures in steps S2 to S4. This is a key technical step in the method of this invention from principle design to reliable process reproduction.

[0099] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects.

[0100] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for strengthening MEMS structures based on grain size gradient control, characterized in that, Includes the following steps: Prepare a basic electroplating solution, wherein no sulfur-containing brighteners, nitrogen-containing grain refiners, or organic surfactants are added to the basic electroplating solution; A surface microcrystalline layer is deposited on the substrate surface. During the deposition process, the electroplating bath temperature is controlled between 15℃ and 25℃, the pH value is controlled between 3.5 and 4.5, and a current density is applied to adjust the ratio of nucleation rate to growth rate. Maintain at the preset small grain mode threshold above; A medium-sized large-grained layer is deposited on top of the surface small-grained layer. During the deposition process, the electroplating bath temperature, pH value, and current density are linearly adjusted from the parameter values ​​during the deposition of the surface small-grained layer to 35°C to 45°C, 5.0 to 6.0, and 2 ASD to 3 ASD, respectively, so that the ratio of nucleation rate to growth rate is... From above the small grain mode threshold Gradually reduce to the preset large grain mode threshold the following; A bottom layer of small grains is deposited on top of the intermediate large grain layer. During the deposition process, the electroplating bath temperature is restored to 15°C to 25°C, the pH value is restored to 3.5 to 4.5, and the current density is restored to achieve the desired nucleation rate to growth rate ratio. It rebounded and remained at the small grain mode threshold. This results in a three-layer structure with a gradient distribution of small, large, and small grain sizes along the thickness direction.

2. The MEMS structure strengthening method based on grain size gradient control according to claim 1, characterized in that, The thickness of the surface small-grain layer accounts for 15% to 25% of the total thickness of the three-layer structure, the thickness of the middle large-grain layer accounts for 50% to 60% of the total thickness of the three-layer structure, the thickness of the bottom small-grain layer accounts for 20% to 30% of the total thickness of the three-layer structure, and the deviation between the average grain size of the bottom small-grain layer and the average grain size of the surface small-grain layer is within ±15%.

3. The MEMS structure strengthening method based on grain size gradient control according to claim 2, characterized in that, The average grain size of the middle large-grain layer is 2 to 4 times the average grain size of the surface small-grain layer.

4. The MEMS structure strengthening method based on grain size gradient control according to claim 1, characterized in that, During the deposition of the intermediate large-grain layer, the adjustment slopes of temperature, pH, and current density are coordinated to ensure the ratio of nucleation rate to growth rate. The continuous linear variation avoids discontinuous breaks in grain size.

5. The MEMS structure strengthening method based on grain size gradient control according to claim 1, characterized in that, Throughout the deposition process, the cathode polarization curve was acquired online using a linear scanning voltammeter. A closed-loop control system then used a correlation model to adjust the system based on the characteristics of the cathode polarization curve and the current electroplating solution temperature and pH value. By deducing the ratio of nucleation rate to growth rate at the current moment. .

6. The MEMS structure strengthening method based on grain size gradient control according to claim 5, characterized in that, The closed-loop regulation system will deduce the ratio of nucleation rate to growth rate at the current moment. Ratio to the target of the current depositional stage The intervals are compared, and the electroplating solution temperature or current density is dynamically adjusted according to preset priorities and adjustment amounts based on the direction and magnitude of the deviation, so as to adjust the ratio of nucleation rate to growth rate. Maintain within the target range, with a closed-loop adjustment cycle of once every 5 to 30 seconds.

7. The MEMS structure strengthening method based on grain size gradient control according to claim 6, characterized in that, When the ratio of nucleation rate to growth rate is detected When the temperature is below the lower limit of the target range and the slope of the polarization curve decreases, the closed-loop adjustment system preferentially reduces the electroplating solution temperature in steps of 0.5℃ to 1℃. If the ratio of nucleation rate to growth rate is adjusted... If it still does not recover, increase the current density in steps of 5% to 10% of the current current density; When the ratio of nucleation rate to growth rate is detected If the current exceeds the upper limit of the target range and the slope of the polarization curve increases, then the electroplating solution temperature should be increased or the current density should be decreased.

8. The MEMS structure strengthening method based on grain size gradient control according to claim 1, characterized in that, The entire deposition process was carried out in constant current mode, and the current ripple coefficient was controlled within ±2%.

9. The MEMS structure strengthening method based on grain size gradient control according to claim 1, characterized in that, Before depositing the underlying small grain layer, the parameters are restored in a stepwise sequence: first, the current density is increased to the target range, and after the cathodic polarization curve response stabilizes, the electroplating solution temperature is reduced to 15°C to 25°C, and finally the pH value is adjusted to 3.5 to 4.5, with an interval of 30 seconds to 2 minutes between two adjacent adjustments.

10. The MEMS structure strengthening method based on grain size gradient control according to claim 1 or 5, characterized in that, The small grain mode threshold and the large grain mode threshold The determination method is as follows: For the electroplating solution system used, a series of electroplating pre-experiments were carried out by changing the temperature, pH value and current density. The cathodic polarization curves under each condition were recorded using linear sweep voltammetry to extract nucleation kinetic parameters, and the corresponding average grain size was measured, thereby fitting the ratio of nucleation rate to growth rate. The curve showing the relationship between grain size and the small grain mode threshold is selected on the curve. and large grain mode threshold .

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