Research Progress in Polishing Technologies for Ceramic Substrates

In recent years, high-speed development has been observed in fields such as electric vehicles, electric locomotives, semiconductor lighting, aerospace, and satellite communication. The electronic components in these applications operate with high currents, at high temperatures, and high frequencies. To ensure the stability of these components and circuits, higher demands are placed on chip carriers. Ceramic substrates, boasting excellent thermal properties, microwave performance, mechanical properties, and high reliability, find widespread use in these areas.

Broadly speaking, non-metallic and inorganic solid materials can be termed ceramics. They are compounds of metallic and non-metallic elements, primarily bonded by ionic bonds, covalent bonds, or a combination of both. They can be crystalline, non-crystalline, or polycrystalline. Consequently, ceramic materials are characterized by low ductility and toughness, but also feature high melting points, excellent insulation properties, and stable chemical performance due to their strong atomic bonds and limited free electrons. Ceramic substrates, fabricated from these materials, inherit characteristics such as high-temperature resistance, excellent insulation, and chemical stability. Furthermore, ceramic substrates exhibit high hermeticity, effectively blocking moisture, oxygen, and dust, thereby providing a stable operating environment for electronic components and circuits and ensuring their service life. Some ceramic substrates are also radiation-resistant, showing broad application prospects in aerospace and the nuclear industry. Ceramic materials currently used for substrates primarily include Beryllium Oxide (BeO), Alumina (Al₂O₃), Aluminium Nitride (AlN), Silicon Nitride (Si₃N₄), and Silicon Carbide (SiC).

Common Ceramic Substrate Polishing Technologies [5]

Ceramic substrates are characterized by high hardness, significant brittleness, a propensity for crack generation, and challenging surface processing. Therefore, stringent requirements are imposed on the surface machining of ceramic substrates. Typically, grinding and polishing are employed to remove surface contaminants, improve flatness, reduce surface roughness, and enhance dimensional accuracy and surface quality to meet thin-profile requirements. Given the varying properties and structures of different ceramic materials, selecting the appropriate polishing technology is crucial for achieving optimal results.

To improve flatness and obtain ceramic substrates with high surface precision and low surface roughness, the initial step involves a grinding process to remove surface defects, the altered surface layer, and scratches from the substrate. Subsequently, polishing techniques are used to further eliminate surface or sub-surface damage caused during grinding, resulting in a flatter surface with even lower roughness. Common polishing technologies for ceramic substrates include Chemical Mechanical Polishing (CMP), Abrasive Flow Polishing, Ultrasonic Vibration-Assisted Abrasive Flow Polishing, Electrophoretic Polishing, Electrolytic Polishing, and Magnetorheological Polishing.

2.1 Chemical Mechanical Polishing (CMP)

Chemical Mechanical Polishing (CMP) is renowned as the currently sole technology capable of achieving global planarization of wafer surfaces in integrated circuit (IC) manufacturing. The effectiveness of CMP directly impacts the final quality and yield of chips.

CMP utilizes the synergistic coupling of chemical corrosion from chemical agents in the polishing slurry and mechanical abrasion to remove surface defects at an atomic level, achieving global planarization. For instance, when polishing Al₂O₃ substrates using a silica sol-based slurry, a chemical reaction occurs as shown in Equation (1), and the resulting products are removed during the mechanical polishing process. This implies that the CMP process is more suitable for ceramic materials where interfacial reactions can occur, producing softer by-products.

2.1.1 Polishing Slurry

The chemical mechanical polishing slurry is a critical factor influencing polishing quality and efficiency. CMP slurry components generally include abrasive particles, oxidizing agents, and other additives. Additives typically comprise complexing agents, chelating agents, corrosion inhibitors, surfactants, and pH regulators. The specific composition is selected based on the physical and chemical properties of the material being polished and the desired polishing performance requirements.

Abrasive particles act on the material surface during polishing through mechanisms like micro-cutting, micro-scratching, and rolling, effecting mechanical material removal. The size of the abrasive particles directly affects polishing efficiency and surface quality. Excessively large particles increase the risk of scratches and other surface damage, while excessively small particles, although potentially providing better surface smoothness, significantly reduce the Material Removal Rate (MRR). The hardness of the abrasive must also match the material being polished. Excessively hard abrasives can cause surface scratches or more defects, while excessively soft abrasives lead to low material removal rates.

pH regulators optimize the chemical reaction conditions by adjusting the acidity or alkalinity of the polishing slurry, ensuring reactions proceed as intended. Oxidizing agents react with the workpiece surface to form a softened layer that is easily removed mechanically, enabling efficient and uniform material removal. Inhibitors are used to control the extent of chemical erosion, making the entire polishing process more controllable and stable [1]. Furthermore, selecting appropriate surfactants can simultaneously achieve effects such as abrasive dispersion, surface wetting, cleaning, and corrosion inhibition, indicating significant application potential.

2.1.2 Polishing Pad

The polishing pad plays a vital role in determining CMP performance parameters like Material Removal Rate and surface quality. Due to frictional forces and normal pressure applied by the wafer, the polishing pad undergoes surface wear and bulk compression, leading to surface microstructure degradation, asperity shearing, and pore clogging. Surface wear of the polishing pad causes a decline in its CMP performance.

2.2 Plasma-Assisted Polishing (PAP)

Plasma-Assisted Polishing (PAP) is a process that oxidizes the surface material into a softer oxide layer using plasma, while still relying on abrasive friction for material removal, thus assisting chemical mechanical polishing. The basic principle involves using a radio frequency (RF) generator to create plasma from reactive gases (such as water vapor, O₂, etc.), generating radicals (like OH radicals, O radicals). These highly oxidizing radicals oxidize and modify the material surface, forming a soft oxide layer, which is then polished away using soft abrasives (e.g., CeO₂, Al₂O₃), resulting in an atomically smooth surface.

2.3 Electrophoretic Polishing

Electrophoretic polishing is one of the highly promising non-contact polishing methods. This technology utilizes the differential migration speeds of charged particles in an electric field to achieve separation. This method induces virtually no damage commonly associated with mechanical processing on the machined surface, making it most suitable for the ultra-precision machining of Functional Ceramics.

Figure 3: Schematic Diagram of Electrophoretic Polishing

2.4 Electrolytic Polishing

Electrolytic polishing is a surface finishing technology that achieves surface flattening through selective electrochemical reactions on the workpiece surface. It combines electric current and chemical reactions. In an electrolyte, the metal workpiece acts as the anode, and an insoluble metal serves as the cathode. Applying a voltage between the two electrodes causes selective dissolution of the micro-protrusions on the anode, thereby reducing surface roughness and producing a smooth surface.

The electrolytic polishing method offers advantages such as simple and inexpensive equipment, straightforward and rapid operation, high production efficiency, the ability to rectify recessed areas inaccessible to mechanical polishing, and increased corrosion resistance of the workpiece.

2.5 Ultrasonic Vibration-Assisted Abrasive Flow Polishing

Similar to electrical discharge, plasma, and lasers, ultrasonic vibration can release substantial energy within a very short time and is widely used for processing hard and brittle materials. Ultrasonic Vibration-Assisted Abrasive Flow Polishing combines ultrasonic vibration technology with abrasive flow polishing. It applies ultrasonic vibration to the abrasive flow via an ultrasonic vibration system, leveraging the kinetic energy of both for polishing, forming a new composite polishing method. The introduction of ultrasound facilitates the formation and collapse of micro-bubbles, and the collapse of these micro-bubbles near the workpiece surface aids material removal, resulting in a more precise polished surface. This characteristic makes this polishing technique particularly suitable for surface polishing of precision optical components.

2.6 Magnetorheological Polishing (MRP)

Magnetorheological Polishing technology utilizes magnetorheological fluid, which undergoes rheological changes in a gradient magnetic field, forming a flexible "sub-aperture polishing tool" with viscoplastic behavior. A high relative speed is maintained between this flexible tool and the workpiece, subjecting the workpiece surface to significant shear stress, thereby removing surface material.

Conclusion

As power devices, microwave devices, photoelectric devices, etc., continue to evolve towards miniaturization, integration, and multi-functionality, higher demands are placed on ceramic substrates to maintain optimal signal transmission and heat dissipation performance.

As the substrate material for integrated circuits and copper-clad laminates, the surface quality of ceramic substrates directly impacts the service life and operational reliability of the final devices. To meet the developmental requirements of device integration, miniaturization, and high reliability, future demands for the surface quality of ceramic substrates will become increasingly stringent. The surface treatment technologies applied to ceramic substrates will consequently face ever more rigorous challenges.