An electrochemical magnetorheological polishing apparatus and polishing method for semiconductor wafers

Abstract

This invention provides an electrochemical magnetorheological polishing apparatus and method for semiconductor wafers. The polishing apparatus includes: a spindle, a workpiece stage, a polishing disc, a conductive through-hole foam pad, a sealed electrolytic cell, a temperature control module, and a dynamic clustered magnetic field generator. During polishing, the conductive through-hole foam pad acts as an electrolytic framework, improving current utilization and uniformity of global anodic oxidation, enabling precise online control of the electrochemical anodic oxidation rate. Magnetic particles, under the action of the dynamic magnetic field, synergistically work with the foam pad to enhance shear yield stress and abrasive holding force, improving the uniformity and stability of mechanical removal. Abrasive particle renewal and self-sharpening are achieved through the clustering effect of the dynamic magnetic field. Combined with online processing gap control, this allows for staged rough and fine polishing of the wafer. This invention significantly improves wafer planarization efficiency and surface quality, possessing promising application prospects and industrialization value in semiconductor power device manufacturing.

Description

Technical Field

[0001] This invention belongs to the field of ultra-precision processing technology of semiconductor materials, specifically relating to an electrochemical magnetorheological polishing device and polishing method for semiconductor wafers. Background Technology

[0002] With the widespread application of third-generation semiconductor materials (such as silicon carbide (SiC) and gallium nitride (GaN)) in high-voltage power electronics, radio frequency communications, new energy vehicles, and aerospace, higher requirements are placed on wafer surface flatness, surface roughness, and material removal rate. However, these semiconductor materials have characteristics such as high hardness, high brittleness, and high chemical inertness. Existing chemical mechanical polishing (CMP) methods suffer from low material removal efficiency, thick surface damage layer, high material consumption, and insufficient processing uniformity when processing hard and brittle wide-bandgap semiconductor materials, making it difficult to meet the needs of manufacturing high-performance power devices and high-frequency electronic devices.

[0003] In recent years, electrochemical mechanical polishing (ECMP), as a surface planarization technology combining anodic oxidation and mechanical removal, has advantages such as minimal processing damage and high controllability. Its core lies in the ability to form a controllable oxide layer on the wafer surface through electrochemical anodic oxidation, which is then removed using polishing pads or abrasive grains, thereby reducing stress and damage. For example, Chinese patent (CN117020926A) discloses an electrochemical mechanical polishing device for silicon carbide wafers, which achieves electrolytic circuit construction and controllable material removal without damaging the wafer's adsorption structure through a dual-electrode system and an adsorption-type spindle fixation method. Meanwhile, Chinese patent (CN120461315A) discloses an electrochemical mechanical polishing device and process for silicon carbide wafers. In the rough polishing stage, hard abrasive grains are used at a high potential to quickly remove surface scratches and deep damage; in the fine polishing stage, soft abrasive grains are used and the potential is gradually reduced to remove shallow damage, resulting in a smooth, defect-free silicon carbide wafer. Another Chinese patent (CN114654380A) discloses an electrochemical mechanical polishing (ECMP) method for silicon carbide wafers, which uses small-diameter diamond abrasives in a NaNO3 electrolyte to achieve high-efficiency and high-quality polishing of silicon carbide. However, existing ECMP electrolytic circuits suffer from insufficient stability, difficulty in precisely controlling the oxide layer formation rate and uniformity, and the need for periodic dressing and replacement of polishing pads to maintain surface properties. Furthermore, the process is highly sensitive to the hardness and particle size inhomogeneity of the abrasive grains, easily causing scratches and subsurface damage to the semiconductor wafer surface.

[0004] Magnetorheological polishing (MRP) is a semi-fixed abrasive flexible processing technology. It utilizes magnetic particles arranged in a magnetic chain under an applied magnetic field to hold non-magnetic abrasive particles and remove small amounts of material from SiC wafers. This can achieve polished surface quality with little or no subsurface damage. However, it still has shortcomings in removal efficiency, abrasive particle renewal, and self-sharpening performance, making it difficult to meet the requirements for efficient planarization of large-size wafers. Dynamic magnetorheological flexible polishing pad structures driven by clustered magnetic fields have attracted much attention. For example, Chinese patent (CN110281085B) discloses a clustered magnetorheological grinding and polishing device. Through a clustered magnetic field formed by multiple magnets, the abrasive particles in the magnetorheological fluid form a dynamic flexible grinding head, improving polishing efficiency and surface quality. Chinese patent (CN103192297B) discloses a chemical clustered magnetorheological composite processing method for single-crystal silicon carbide wafers. This method mixes abrasive particles and acid / base chemical reagents into a magnetorheological fluid as a polishing working fluid. Under the action of a clustered magnetic field, a flexible polishing pad is formed to polish the single-crystal SiC wafer, achieving an ultra-smooth, high-quality surface. Chinese patent (CN110900322B) discloses an electro-Fenton cluster magnetorheological composite grinding and polishing device and method. During processing, it utilizes ·OH free radicals to oxidize the wafer surface and forms a dynamic polishing zone through the rotation and revolution of clustered magnets, thereby improving polishing uniformity and efficiency. While these technologies can improve abrasive renewal capabilities, significant abrasive escape still occurs during processing, resulting in low processing efficiency and poor uniformity.

[0005] Therefore, developing a technology that achieves high efficiency in semiconductor wafer planarization, uniform surface removal, and good oxide layer thickness distribution is an urgent technical problem to be solved. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of low processing efficiency, uneven surface removal, uneven oxidation, insufficient current utilization, and poor abrasive renewal and self-sharpening capabilities in existing semiconductor wafer planarization methods. This invention proposes an electrochemical magnetorheological polishing method and apparatus for semiconductor wafers. Through the synergistic design of conductive through-hole foam pads, electrochemical anodizing, an online temperature control system, and a dynamic magnetic field cluster mechanism, the organic coupling of electrochemical anodizing and magnetorheological mechanical removal is achieved, thereby significantly improving the efficiency, uniformity, and surface accuracy of semiconductor wafer planarization, meeting the urgent demand for ultra-precision processing in the manufacture of wide-bandgap semiconductor devices.

[0007] To overcome the aforementioned bottlenecks, this invention proposes an electrochemical magnetorheological polishing device and method for semiconductor wafers. Through the synergistic design of electrochemical anodizing and dynamic magnetic field cluster magnetorheological processes, it achieves efficient and high-quality semiconductor wafer processing. This technology not only improves current utilization and anodizing uniformity but also enhances the holding force and renewal capability of abrasive particles through the magnetorheological effect, thereby achieving efficient synergy between electrochemical oxidation and mechanical removal. Compared with traditional methods, this invention offers higher processing efficiency, superior surface quality, and stronger process controllability, demonstrating significant application necessity and industrialization feasibility.

[0008] Specifically, this invention provides an electrochemical magnetorheological polishing apparatus for semiconductor wafers. The polishing apparatus includes: a spindle, a workpiece stage, a polishing disc, a conductive through-hole foam pad, a sealed electrolytic cell, a temperature control module, and a dynamic clustered magnetic field generator. The conductive through-hole foam pad is fixed to the surface of a conductive copper sheet and serves as a conductive framework substrate for the electrolytic slurry. The sealed electrolytic cell is formed by epoxy resin and glass glue fixing an insulating ring acrylic, used to isolate the processing area and provide an insulating environment. The temperature control module and the dynamic clustered magnetic field generator use a cam mechanism to realize the overall oscillation motion of the clustered permanent magnets, used to generate a dynamic magnetic field, causing the magnetic particles to undergo magnetorheological effects during processing and form a semi-solidified flexible polishing pad, which makes surface contact with the wafer surface. The semiconductor wafer is fixed on the workpiece stage connected to the spindle, and the conductive through-hole foam pad is fixed on the conductive copper sheet at the bottom of the polishing disc. The conductive through-hole foam pad, the conductive copper sheet, and the polishing disc constitute a sealed space.

[0009] The polishing device can achieve the synergistic effect of electrochemical oxidation and magnetorheological mechanical removal, thereby improving the efficiency and surface quality of wafer planarization.

[0010] Furthermore, the spindle is connected to the power anode via a carbon brush; a conductive copper sheet is provided at the bottom of the polishing disc, and the conductive copper sheet is fixed by pins and connected to the power cathode via a carbon brush.

[0011] Furthermore, the temperature control module is an external digital display temperature controller composed of a heating element, a cooling element, and a wireless module. The heating element and the cooling element are embedded in the bottom of the polishing disc to heat and cool the polishing slurry. Together with the monitoring module, they form an online temperature control system to achieve real-time measurement and control of the polishing slurry temperature. The polishing slurry is a conductive polishing slurry containing conductive salts or weak alkaline solutions and abrasives to simultaneously achieve electrochemical oxidation and mechanical removal.

[0012] Furthermore, the dynamic cluster magnetic field generator employs a cam mechanism to achieve the overall yaw motion of the clustered permanent magnets. The magnetic pole rotation speed and magnetic field strength of the generator are adjustable to adapt to the requirements of abrasive grain distribution and self-sharpening capability at different processing stages. The processing gap set during polishing is matched with the compression amount of the through-hole foam pad to enhance shear yield stress and improve abrasive grain holding force.

[0013] Furthermore, the conductive through-cell foam pad is prepared through chemical doping, metal coating, or conductive medium filling to improve its conductivity and mechanical stability. The conductive medium is selected from conductive particles, one-dimensional carbon nanotubes, and two-dimensional graphene. Conductive particles, one-dimensional carbon nanotubes, and two-dimensional graphene are loaded onto the polishing pad as conductive media. The mechanical properties of the conductive through-cell foam pad are controlled by adjusting the pore size distribution, compressive modulus, and porosity to optimize the uniformity of current distribution and mechanical removal stability. In the preparation of the through-cell foam pad, the pore size distribution is adjusted by using the foaming agent ratio and curing temperature to make the pores uniform and enhance connectivity; the compressive modulus is controlled by changing the crosslinking density of the polyurethane matrix and the dosage of conductive fillers (such as CNTs, graphene, and carbon black) to achieve a gradient adjustment from flexible to semi-rigid; and the porosity is precisely controlled by hot pressing and through-cell template reconstruction processes.

[0014] Furthermore, the conductive copper sheet at the bottom of the polishing disc is provided with multi-point current distribution interfaces to improve the uniformity of current conduction and reduce local concentration gradient effects. The temperature control module includes a temperature sensor, a circulating cooling circuit, and a heating unit to stabilize the polishing slurry temperature and maintain the stability of the electrolytic environment.

[0015] Furthermore, the semiconductor wafer is a wide bandgap semiconductor material. Furthermore, the wide bandgap semiconductor material is selected from silicon carbide (SiC), gallium nitride (GaN), or gallium oxide (Ga2O3).

[0016] The present invention also provides a polishing method for a semiconductor wafer electrochemical magnetorheological polishing apparatus as described above, comprising the following steps:

[0017] S1. The semiconductor wafer is uniformly bonded and fixed to the spindle with conductive adhesive. The spindle is electrically connected to the power supply anode via carbon brushes to ensure reliable electrical contact and stable mechanical clamping.

[0018] S2. A conductive through-hole foam pad is fixed on the copper sheet at the bottom of the polishing pad; the insulating ring acrylic is fixed into a sealed electrolytic cell with epoxy resin and glass glue, and the pins of the copper sheet are electrically connected to the cathode of the power supply via carbon brushes to build a stable closed electrolytic circuit.

[0019] S3. The temperature of the polishing slurry is sensed in real time by the heating element and temperature sensor at the bottom of the polishing pad and transmitted to the digital display temperature controller via wireless network, so as to realize online monitoring and control of the polishing slurry temperature;

[0020] During the polishing process, conductive through-hole foam pads are used as the conductive framework substrate for the electrolytic slurry; this improves current utilization and achieves uniformity of global anodizing of the wafer, thereby enabling online precise control of the electrochemical anodizing rate.

[0021] S4. The chemical corrosion rate is controlled online by electrochemical parameters, electrolyte composition, pH and temperature parameters, thereby accurately obtaining a uniform and easily removable oxide film; the mechanical properties of the polishing pad are controlled online by setting the processing gap, and rough polishing and fine polishing are achieved under the action of a dynamic magnetic field.

[0022] Magnetic particles undergo magnetorheological effects, forming a semi-solidified flexible polishing pad that contacts the wafer surface through the solid-liquid coupling characteristics of the through-hole foam pad. Under the action of extrusion and shearing, the shear yield stress and abrasive holding force are enhanced. Under the action of dynamic magnetic field, the magnetic particles undergo magnetorheological effects and form a semi-solidified flexible polishing pad in the through-hole of the foam, achieving surface contact with the wafer surface. During the polishing process, the polishing pad enhances the shear yield stress and abrasive holding force under extrusion and shearing, improving the uniformity and stability of mechanical removal.

[0023] A dynamic magnetic field cluster magnetorheological device is used to drive the periodic aggregation and dispersion of magnetic particles, increasing the kinetic energy of the abrasive, promoting the self-sharpening and renewal of abrasive particles and maintaining the performance of the polishing pad, so as to achieve the synergistic effect of electrochemical oxidation and mechanical removal.

[0024] Under the influence of a magnetic field, magnetic particles aggregate, orient, and attract each other along magnetic field lines to form chain-like / cluster-like structures. These chains are "anchored" within the through-cells of the foam, causing the originally fluid slurry to undergo a reversible transition from fluid to solid locally, thus increasing the shear yield stress of the polishing pad. The through-cell foam provides a three-dimensional porous framework, allowing both magnetic / abrasive particles and slurry to flow through, while also providing mechanical support and shape constraint for the chains under pressure. This soft-hard coupling characteristic allows the polishing pad to conform to the microstructure of the wafer, balancing adaptability and removal capability. During processing, the semiconductor wafer undergoes an anodizing reaction to form a thin soft oxide layer. The magnetorheological foam polishing pad uses extrusion to embed abrasive particles into the oxide layer and applies shear force during relative motion. Once the shear force exceeds the yield strength of the oxide layer, the oxide layer is peeled off, improving processing efficiency and reducing substrate damage.

[0025] Furthermore, the electrochemical parameters in step S4 are current density and voltage, and the electrolyte includes neutral electrolyte and weakly alkaline electrolyte.

[0026] Furthermore, in step S4, the rough polishing stage employs a higher current density and a smaller processing gap compared to the fine polishing stage. The rough polishing stage uses a higher current density and a smaller processing gap to enhance the anodizing rate and the stiffness of the foam pad, achieving rapid removal and macroscopic planarization; the fine polishing stage reduces the current density and increases the gap to slow down the rate of oxide layer formation and removal, and, combined with the highly uniform shear force under the magnetorheological effect, obtains a low-damage, low-roughness surface.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] (1) By constructing a conductive skeleton through a conductive through-hole foam pad, the current utilization rate is significantly improved, the current distribution on the wafer surface is homogenized, and the thickness of the anodizing is highly uniform throughout the entire area.

[0029] (2) Combining electrochemical anodizing with magnetorheological polishing flexible pads to form an electro-magnetic-force multi-field coupling system to achieve the synergistic effect of chemical removal and mechanical removal.

[0030] (3) By utilizing the dynamic magnetic field clustering mechanism to endow abrasive particles with self-sharpening and renewal capabilities, and combining the solid-liquid coupling characteristics of foam pads, the passivation failure and escape problems of abrasive particles are effectively solved, thereby improving the stability of long-cycle processing.

[0031] (4) By adjusting the processing gap online, the mechanical properties such as the stiffness of the through-hole foam pad can be controlled, so as to achieve flexible switching between rough polishing and fine polishing, shorten the wafer processing cycle, and take into account both processing efficiency and final surface quality.

[0032] (5) The processing equipment is simple, the processing method is easy to implement, the electrolytic system is highly stable, and it is suitable for ultra-precision planarization processing of wide bandgap semiconductor wafers such as silicon carbide and gallium nitride, and has broad prospects for industrial application.

[0033] (6) This technology uses a through-hole foam pad to constrain the movement of abrasive, causing the abrasive to gather on the wafer surface, improving the dispersion of abrasive, thereby improving processing efficiency and surface quality. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the semiconductor wafer electrochemical magnetorheological polishing apparatus of the present invention;

[0035] Figure 2 This is a schematic diagram of the electrochemical magnetorheological polishing process for semiconductor wafers according to the present invention;

[0036] Figure 3 This is a schematic diagram of the magnetic pole oscillation mechanism in the semiconductor wafer electrochemical magnetorheological polishing apparatus of the present invention;

[0037] Figure 4 This is a schematic diagram of the magnetic pole arrangement in the semiconductor wafer electrochemical magnetorheological polishing apparatus of the present invention;

[0038] In the diagram: 1. Spindle; 2. Workpiece stage; 3. Semiconductor wafer; 4. Conductive through-hole foam pad; 5. Polishing disc; 6. Temperature sensor; 7. Clustered permanent magnets; 8. Permanent magnet cluster assembly; 9. Cam mechanism; 10. Driven gear assembly; 11. Driven shaft; 12. Support base; 13. Outer spacer; 14. Drive shaft; 15. Second motor; 16. Base; 17. Second pulley; 18. First pulley; 19. First motor; 20. Drive gear assembly; 21. Conductive copper sheet; 22. Conductive copper ring; 23. DC power supply; 24. Carbon brush; 25. Heating element. Detailed Implementation

[0039] To enable those skilled in the art to more clearly understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments of the present invention are merely illustrative and not intended to limit the scope of protection. Various modifications and improvements can be made by those skilled in the art without departing from the core spirit of the present invention, and all such modifications and improvements should fall within the scope of protection of the present invention.

[0040] Example 1

[0041] like Figure 1 As shown, this embodiment provides an electrochemical magnetorheological polishing device. The polishing device includes a spindle 1, a workpiece stage 2, a semiconductor wafer 3, a conductive through-hole foam pad 4, a polishing disc 5, a temperature sensor 6, a cluster of permanent magnets 7, a permanent magnet cluster assembly 8, a cam mechanism 9, a driven gear assembly 10, a driven shaft 11, a support base 12, an outer spacer 13, a drive shaft 14, a second motor 15, a base 16, a second pulley 17, a first pulley 18, a first motor 19, a drive gear assembly 20, a conductive copper sheet 21, a conductive copper ring 22, a DC power supply 23, a carbon brush 24, a heating element 25, and a control unit (not shown in the figure).

[0042] The spindle 1 and workpiece stage 2 are connected. The workpiece stage 2 can hold standard-sized semiconductor wafers 3, ranging from 2 to 8 inches. The spindle 1 is connected to the anode of the power supply via conductive adhesive. The semiconductor wafer 3 is electrically connected to the spindle 1 via conductive adhesive and connected to a DC power supply 23, thus participating in the reaction as the anode in the electrochemical process. The polishing disc 5 is insulated and has a conductive copper sheet 21 at its bottom. The conductive copper sheet 21 serves as both a current transmission medium and a substrate for the conductive through-hole foam pad 4. The conductive copper sheet 21 can be a single-piece structure or a multi-zone structure to adjust the current distribution in different areas. The polishing disc 5 is preferably made of insulating bakelite material, and the conductive copper sheet 21 is made of T2 copper with a thickness of 0.5 mm to 5 mm.

[0043] A conductive through-hole foam pad 4 is fixed to the surface of the conductive copper sheet 21, serving as a conductive framework matrix for the electrolytic slurry. The conductive through-hole foam pad 4 has a three-dimensional through-hole structure with high porosity, exhibiting excellent electrolyte permeability and conductivity. As a conductive framework matrix, it not only improves current utilization but also uniformly distributes current density across the entire range, achieving homogenization of anodizing. Simultaneously, the conductive through-hole foam pad 4 exhibits soft-hard coupling characteristics under mechanical loads, providing semi-consolidated flexible support for the magnetorheological polishing slurry, thus balancing chemical oxidation and mechanical removal. The conductive through-hole foam pad 4 is preferably made of polyurethane material, with a thickness of 3-5 mm and a pore size of 800-1000 μm.

[0044] The polishing disc 5 is made of highly insulating acrylic and is sealed and fixed to the outside with epoxy resin and glass glue, forming an independent, enclosed processing space. This structure effectively prevents electrolyte leakage and ensures the stability of the electrochemical environment during the polishing process. The acrylic has an outer diameter of 200 mm and a wall thickness of 5 mm.

[0045] The conductive copper sheet 21 is connected to the conductive copper ring 22 via pins, and is connected to the negative terminal of the DC power supply 23 via a carbon brush 24, thus constructing a complete closed electrolytic circuit. This circuit can be driven by a constant current power supply or a pulse power supply to achieve current mode switching (constant current / constant voltage / pulse) for different processing stages, so as to precisely control the electrochemical anodizing rate. A further preferred power supply is the Maxtor MN-C series, with an input voltage of 220V, an input frequency of 50Hz, continuously adjustable output voltage / current, and external dimensions of 80×220×155mm.

[0046] Temperature sensor 6 and heating / cooling element 25 are embedded in polishing disc 5 to monitor and regulate the polishing slurry temperature in real time, ensuring the stability of reaction rate and slurry performance during processing and avoiding uneven processing quality caused by temperature fluctuations. Further, a multi-functional digital display industrial intelligent temperature controller E100 is preferred, featuring remote temperature detection and flashing alarm functions.

[0047] The permanent magnet cluster component 8 consists of eccentrically rotating permanent magnet poles, and the distribution of the permanent magnet poles is as follows: Figure 4 As shown. The first motor 19 transmits power to the drive shaft 14, and then through the drive gear assembly 20 to the driven gear assembly 10. Finally, the cam mechanism 9 realizes the overall yaw motion of the permanent magnet cluster assembly 8, which can form a dynamic magnetic field cluster effect in the polishing area. The motion mechanism of the entire dynamic cluster magnetic field is as follows. Figure 3 As shown, magnetic particles undergo magnetorheological effects under the influence of a magnetic field, coalescing into a microscopic semi-solid polishing "micro-grinding head." This, together with the support structure formed by the conductive through-hole foam pad 4, enhances the holding force and shear yield stress of the abrasive particles, achieving stable and efficient mechanical removal.

[0048] The control unit integrates multiple module functions, including power mode switching, current / voltage measurement, temperature and gap closed-loop control, magnetic pole speed control, machining gap adjustment, and spindle / polishing disc motion control. This control system enables coordinated adjustment of multiple parameters, ensuring that the machining process balances removal rate and surface quality.

[0049] The polishing apparatus also includes a sealed electrolytic cell, which is formed by epoxy resin and glass glue to fix an insulating acrylic ring, used to isolate the processing area and provide an insulating environment.

[0050] Example 2

[0051] like Figure 2 As shown, this embodiment provides a rough polishing method using the electrochemical magnetorheological polishing apparatus described in Example 1, comprising the following steps:

[0052] S1. The 6-inch semiconductor wafer 3 is fixed to the bottom of the spindle 1 with conductive adhesive, and a reliable electrical connection between the semiconductor wafer 3 and the positive terminal of the DC power supply 23 is achieved through conductive adhesive to ensure the anodic effect of the wafer during the polishing process; in order to enhance mechanical stability and electrical contact reliability, the thickness of the conductive adhesive is controlled at 0.1mm to ensure that the semiconductor wafer 3 is subjected to uniform force and does not warp.

[0053] S2. A 3mm thick conductive perforated foam pad 4 is adhered to the bottom of the polishing pad 5 and electrically connected to the conductive copper sheet 21. The conductive perforated foam pad 4 has a pore size of 300μm and is filled with 10 wt% carbon nanotubes (CNTs) as conductive filler, which allows it to maintain a porous and elastic structure while also serving as a conductive network framework to enable the electrolyte to permeate and conduct electricity within the pores of the pad. The conductive copper sheet 21 is located at the bottom of the conductive perforated foam pad 4. The conductive copper sheet 21 is connected to the power cathode via pins and a carbon brush 24, thereby forming a stable closed electrolytic circuit with the wafer anode. The outer periphery of the polishing pad 5 is sealed with epoxy resin and glass glue to fix an insulating acrylic ring, forming a sealed electrolytic cell.

[0054] S3. Real-time monitoring and control of the polishing slurry temperature is achieved through an external digital display temperature controller. The polishing slurry consists of: diamond abrasive with an average particle size of 1μm, serving as the main mechanical removal unit; KOH electrolyte with a mass fraction of 1wt%, ensuring the conductivity of the electrolyte and the uniformity of anodic oxidation; and carbonyl iron powder with a particle size of 3μm and a mass fraction of 10wt%, which forms chains within the foam pores under an external dynamic magnetic field, combining with the three-dimensional framework of the foam to form a solid-liquid semi-solid polishing pad (the magnetorheological stiffness can be adjusted by the magnetic field strength and particle concentration). Temperature control is achieved through an external digital display temperature controller connected to the electrolytic cell, ensuring that the slurry temperature is maintained within 30℃ to avoid uneven oxide film caused by thermal effects.

[0055] S4. During the polishing process, the power supply is set to constant current mode, and the current density is controlled at 10mA / cm². 2 To ensure uniform growth of the anodic oxide layer, the polishing disc speed is set to 35 r / min to maintain sufficient exchange between the electrolyte and the foam pad at a low rate. The workpiece speed is set to 300 r / min to enhance shearing action and improve removal uniformity. Since the rough polishing stage requires enhanced mechanical action, a smaller processing gap of 0.8 mm is selected in this embodiment to generate a squeezing / shearing effect with the set speed, thereby increasing the shear yield stress and improving the material removal rate. The magnetic pole speed is controlled at 35 r / min, combined with a magnetic field strength of 250 mT, to allow the carbonyl iron powder to form a chain structure, thus enhancing the mechanical properties of the through-hole foam pad. The slurry temperature is monitored throughout the polishing process, and the temperature stability error is controlled within ±1℃. An electrochemical workstation is used to monitor the current / voltage, polarization curve, and online ellipticity or impedance signals in real time, controlling the pulse / dissolution sequence to precisely adjust the oxide layer growth and chemical dissolution rate, thereby controlling the oxide film morphology, thickness, and formation rate online to achieve a uniform and removable thin oxide layer.

[0056] During polishing, the magnetic / abrasive particles move within the pores of the foam pad, generating friction and a geometric clamping effect against the pore walls. This inhibits radial particle dispersion, thereby increasing the effective abrasive concentration (local enrichment) at the polishing interface and preventing abrasive particles from being carried away from the polishing zone by the slurry circulation. Under an applied magnetic field, the magnetic particles form chains, which are "anchored" within the foam pores and work together with the foam skeleton to bear the load. The axial tension of the chains and the friction of the foam walls together increase the resistance of the abrasive particles to displacement, making it more difficult for individual or clustered abrasive particles to detach, achieving a significantly improved "holding force." When the polishing pad is subjected to the squeezing / shearing action of the workpiece disk, the chains within the pores become denser and are compressed by the foam, leading to an increase in local shear yield stress. Under high shear yield stress, the abrasive particles are more likely to act on the oxidized film in an embedded-shear mode (rather than escaping by rolling or sliding), thereby improving removal efficiency and reducing the probability of random scratches. In addition, the microflow generated by the foam pores promotes uniform distribution of abrasive particles and avoids agglomeration. Meanwhile, under the influence of the dynamic magnetic field, the chain will undergo periodic reorganization, causing the passivated or encapsulated abrasive grains to detach and be replaced by new sharp grains, thus achieving self-sharpening and continuous effective grinding.

[0057] When a SiC wafer is immersed in a KOH solution and a positive potential is applied as the anode, an anodic oxidation reaction occurs on the SiC surface, forming a soft surface. The reaction process of the oxide layer is as follows:

[0058] ;

[0059] ;

[0060] When the oxide layer reaches a certain thickness, its insulating properties become significant. Surface charge accumulates, leading to an increase in surface potential and causing the oxide layer to break down. Due to the low resistance at the breakdown point, current concentrates there, forming oxide protrusions. In a weakly alkaline KOH solution, an etching reaction occurs between the KOH solution and the oxide layer. The reaction process is as follows:

[0061] ;

[0062] To prevent oxide layer cracking and excessive surface protrusion without reducing the oxidation rate, a relatively uniform and smooth oxide layer is generated. Simultaneously, the magnetorheological effect endows the foam pad and abrasive grains with synergistic soft-hard coupling characteristics, improving shear yield stress and abrasive grain stability and holding force; mechanical shearing achieves efficient removal of the oxide layer from the substrate. The rough polishing stage not only significantly improves the material removal rate but also maintains the overall uniformity of the surface morphology, achieving efficient removal of SiC surface material.

[0063] Example 3

[0064] This embodiment provides a fine polishing method using the electrochemical magnetorheological polishing apparatus described in Example 1. The specific steps are the same as in Example 2. The difference between this embodiment and Example 2 is that in this embodiment, the processing gap is controlled within the range of 1.1 mm, and the current density is controlled within 1 mA / cm². 2 Compared to Example 2, the size is moderately increased, which can reduce the contact stress and local shear force per unit area, avoid deep scratches and stress concentration effects that may occur in the rough polishing stage, and make the wafer surface gradually become atomically smooth in the fine polishing stage.

[0065] Compared to the rough polishing process in Example 2, this example significantly improves the smoothness and processing uniformity of the wafer surface while effectively removing residual processing layers and surface defects. It is suitable for the stringent requirements of ultra-flat, low-damage surfaces in power semiconductor device manufacturing. This example not only verifies the feasibility and stability of the electrochemical magnetorheological polishing method in the fine polishing stage, but also provides technical reference and process foundation for the subsequent ultra-precision processing of larger wafers (such as 8-inch SiC wafers), demonstrating broad application prospects and industrialization value.

[0066] Example 4

[0067] Based on Examples 2-3, this embodiment further introduces an online monitoring and closed-loop control mechanism. By real-time detection and feedback adjustment of multiple factors such as electrochemical parameters, temperature, processing gap, and magnetic field strength, it realizes intelligent switching and stable control of rough polishing and fine polishing during the electrochemical magnetorheological polishing process of semiconductor wafers.

[0068] Regarding electrochemical parameter monitoring, this embodiment includes a real-time monitoring module between the power supply output and the electrolytic cell to collect dynamic changes in current, voltage, and polarization curves during the polishing process. This module can adjust the power supply operating mode based on the monitoring results, such as switching between constant current and constant voltage modes, or applying pulsed current signals. This creates a dynamic balance between oxidation formation and oxide layer stripping, ensuring the uniformity and controllability of the electrochemical reaction and preventing oxide layer accumulation and localized breakdown due to excessive current density.

[0069] In terms of temperature monitoring and control, this embodiment sets up a digital display temperature sensor and a circulating cooling loop outside the electrolytic cell to form a closed-loop temperature control system. This system can monitor the slurry temperature in real time and achieve temperature regulation through coolant circulation, ensuring that the slurry temperature is stably maintained within the 30°C range. This avoids the electrochemical reaction rate shift and magnetorheological effect attenuation caused by temperature rise, thereby ensuring the stability and repeatability of the process.

[0070] Regarding the control of the machining gap, this embodiment incorporates a precision displacement sensor and actuator between the spindle and the polishing disc to achieve online measurement and dynamic adjustment of the machining gap. By adjusting the machining gap in real time, the degree of compressive deformation of the conductive through-hole polyurethane foam pad can be altered, thereby regulating its mechanical properties (stiffness, compliance) and enabling online switching between the rough polishing and fine polishing stages. For example, the machining gap can be reduced in the rough polishing stage to improve contact stiffness and shear stress, while the gap can be appropriately increased in the fine polishing stage to reduce mechanical forces and improve surface smoothness, thus balancing material removal rate and surface quality.

[0071] In terms of magnetic field control, this embodiment changes the magnetic field strength and distribution by adjusting the magnetic pole rotation speed and magnetic pole spacing online, thereby regulating the stability and clustering characteristics of the magnetic particle chains in the magnetorheological foam pad. Through the periodic perturbation and renewal of the dynamic magnetic field, the abrasive particle distribution is optimized, enhancing the self-sharpening and renewal capabilities of the abrasive and avoiding the negative impact of abrasive particle agglomeration and wear failure on surface quality.

[0072] Example 5

[0073] Building upon Examples 1-4, this embodiment further establishes a detection and evaluation method for the electrochemical magnetorheological polishing method and apparatus for semiconductor wafers, enabling systematic characterization of processing performance and verification of process effects. This method ensures the measurability and controllability of the proposed electrochemical-magnetorheological synergistic polishing technology by quantitatively measuring material removal rate, surface roughness, morphological evolution, and oxide layer composition and thickness.

[0074] For the evaluation of material removal rate (MRR), a precision electronic balance (accuracy 0.1 mg) was used to accurately weigh the 6-inch SiC wafer before and after polishing. The material removal rate was obtained by calculating the ratio of the mass difference to the polishing time, and correlation analysis was performed with process parameters (such as current density, magnetic field strength, and processing gap) to verify the differences in material removal efficiency under different process conditions.

[0075] For surface roughness and morphology inspection, a white light interferometer (ContourGT-X3), laser confocal microscope, or atomic force microscope (AFM) were used to characterize the surface before and after polishing at multiple scales. Specifically, surface roughness parameters (Ra, Rq, Sa, etc.) were measured at six different locations along the same radial direction, and the arithmetic mean was taken to characterize the overall surface level. The standard deviation was used to reflect the uniformity and stability of surface roughness. By comparing the test results under different parameter conditions, the improvement effect of the polishing process on wafer flatness and surface uniformity can be intuitively reflected.

[0076] In terms of oxide layer characteristic analysis, an ellipsometry was used to measure the thickness of the oxide layer on the wafer surface. Simultaneously, X-ray photoelectron spectroscopy (XPS) was used for qualitative and quantitative analysis of the oxide layer composition, supplemented by transmission electron microscopy (TEM) for detailed characterization of the microstructure and interface features of the oxide layer. These methods can reveal the formation and removal mechanisms of the oxide layer during electrochemical anodizing, providing a theoretical basis for process optimization.

[0077] Furthermore, this embodiment systematically verifies the superiority of the present invention by comparing the comprehensive evaluation results under different experimental conditions. For example, when the current density is reduced and the processing gap is appropriately increased, the MRR remains within a stable range while the surface roughness (Sa) can be reduced to the sub-nanometer level, demonstrating high-quality processing capability in the fine polishing stage; under the action of a dynamic magnetic field, the magnetic particle chain can enhance the uniformity and renewal capability of abrasive particle distribution, thereby ensuring a high removal rate while avoiding local damage and surface defects.

[0078] Those skilled in the art will recognize that the embodiments and comparative examples described herein are intended to illustrate the electrochemical magnetorheological polishing method and apparatus for semiconductor wafers, and to help readers understand the advantages of the present invention. Therefore, the scope of protection of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can perform various other preparation processes and polishing parameter operations based on the technical teachings disclosed in this invention without departing from the essence of the invention, and these are still within the scope of protection of the present invention.

Claims

1. A polishing method for a semiconductor wafer electrochemical magnetorheological polishing apparatus, characterized in that, Includes the following steps: S1. The semiconductor wafer is uniformly bonded and fixed to the spindle with conductive adhesive, and the spindle is electrically connected to the anode of the power supply via carbon brush; S2. A conductive through-hole foam pad is fixed on the copper sheet at the bottom of the polishing pad; the insulating ring acrylic is fixed into a sealed electrolytic cell with epoxy resin and glass glue, and the pins of the copper sheet are electrically connected to the cathode of the power supply via carbon brushes to build a stable closed electrolytic circuit. S3. The temperature of the polishing slurry is sensed in real time by the heating element, cooling element and temperature sensor at the bottom of the polishing pad and transmitted to the digital display temperature controller via wireless network, so as to realize online monitoring and control of the polishing slurry temperature; S4. The chemical corrosion rate can be controlled online by electrochemical parameters, electrolyte composition, pH and temperature parameters; The mechanical properties of the polishing pad can be adjusted by setting the processing gap online, and rough polishing and fine polishing can be achieved under the action of a dynamic magnetic field; The semiconductor wafer electrochemical magnetorheological polishing device includes: a spindle, a workpiece stage, a polishing disc, a conductive through-hole foam pad, a sealed electrolytic cell, a temperature control module, and a dynamic cluster magnetic field generator. The semiconductor wafer is fixed on the workpiece stage connected to the spindle, and the conductive through-hole foam pad is fixed on the conductive copper sheet at the bottom of the polishing disc. The conductive through-hole foam pad, the conductive copper sheet, and the polishing disc constitute a sealed space. The dynamic cluster magnetic field generating device uses a cam mechanism to realize the overall yaw motion of the cluster permanent magnets, and the magnetic pole speed and magnetic field strength are adjustable; the permanent magnet cluster component is an eccentrically rotating permanent magnet pole, which is transmitted to the drive shaft by the first motor, and then to the driven gear component through the drive gear assembly, and finally to the overall yaw motion of the permanent magnet cluster component through the cam mechanism; the processing gap set during the polishing process is matched with the compression amount of the through-hole foam pad.

2. The polishing method of the semiconductor wafer electrochemical magnetorheological polishing apparatus according to claim 1, characterized in that, The spindle is connected to the power anode via a carbon brush; a conductive copper sheet is provided at the bottom of the polishing disc, which is fixed by pins and connected to the power cathode via a carbon brush.

3. The polishing method of the semiconductor wafer electrochemical magnetorheological polishing apparatus according to claim 1, characterized in that, The temperature control module is an external digital display temperature controller composed of a heating element, a cooling element, and a wireless module, used to realize real-time measurement and control of the polishing slurry temperature; the polishing slurry is a conductive polishing slurry containing conductive salt or weak alkali solution and abrasive.

4. The polishing method of the semiconductor wafer electrochemical magnetorheological polishing apparatus according to claim 1, characterized in that, The conductive through-hole foam pad is prepared by chemical doping, metal coating or filling with conductive medium.

5. The polishing method of the semiconductor wafer electrochemical magnetorheological polishing apparatus according to claim 1, characterized in that, The conductive copper sheet at the bottom of the polishing disc is equipped with a multi-point current distribution interface; the temperature control module includes a temperature sensor, a circulating cooling circuit, and a heating unit.

6. The polishing method of the semiconductor wafer electrochemical magnetorheological polishing apparatus according to claim 1, characterized in that, The semiconductor wafer is made of a wide bandgap semiconductor material.

7. The polishing method of the semiconductor wafer electrochemical magnetorheological polishing apparatus according to claim 1, characterized in that, The electrochemical parameters in step S4 are current density and voltage, and the electrolyte includes neutral electrolyte and weakly alkaline electrolyte.

8. The polishing method of the semiconductor wafer electrochemical magnetorheological polishing apparatus according to claim 1, characterized in that, Includes the following steps: Step S4, the rough polishing stage, employs a higher current density and a smaller processing gap compared to the fine polishing stage.

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