CMP Slurries: Composition, Mechanisms of Action, and Applications in Wafer Fabrication and Advanced Packaging

What is CMP slurry?

As semiconductor devices evolve toward advanced process nodes, higher integration, and 3D architectures, wafers undergo multiple processing steps—such as thin-film deposition, photolithography, etching, material filling, and metal interconnection. The successive layering of different materials creates topographical variations (height differences) on the wafer surface. If these microscopic irregularities are not effectively controlled, they can compromise subsequent steps, including photolithographic depth of focus, pattern transfer accuracy, thin-film deposition uniformity, and device structure formation.

Chemical Mechanical Planarization (CMP) is the critical process used to address this issue. It achieves controlled material removal from the wafer surface through the synergy of interfacial chemical reactions and mechanical action, thereby ensuring high levels of both local and global flatness.

CMP slurry (also known as CMP polishing fluid or wafer polishing fluid) is a core consumable in the CMP process. Its chemical composition, abrasive properties, and dispersion stability directly influence material removal rates, material selectivity, within-wafer uniformity, and surface roughness, as well as the incidence of defects such as scratches, corrosion, and particle residues.

It is important to note that the outcome of semiconductor CMP is not determined solely by the slurry. Factors such as the polishing pad and its conditioning status, equipment parameters, pressure, relative velocity, wafer film structure, pattern density, temperature, and fluid delivery conditions all collectively influence the final polishing results. Consequently, a high-performance CMP slurry must not only possess stable material properties but also be compatible with specific wafer structures and process windows.

Composition and Mechanism of Action of CMP Slurries

Typical CMP slurries generally consist of nanoscale abrasive particles, chemical functional additives, and a liquid medium, with water-based systems being the most common. Certain metal CMP processes or specialized applications may employ alternative material removal methods, such as abrasive-free solutions. CMP does not rely simply on the mechanical grinding of the wafer by high-hardness particles; rather, its fundamental mechanism involves using a chemical system to alter the surface state of the target material—through processes such as oxidation, dissolution, complexation, or passivation—and then gradually removing the resulting reaction or modified layer via the mechanical interaction between abrasive particles, the polishing pad, and the wafer. Given the significant differences in surface chemical properties among various materials, CMP slurries typically require tailored designs based on the target material, film structure, and device fabrication process.

• Abrasive particles

Abrasives are a key component of most CMP slurries; common materials include silicon dioxide (SiO₂), cerium oxide (CeO₂), and aluminum oxide (Al₂O₃).

Silicon dioxide offers excellent control over particle size and dispersion stability, along with well-established surface chemistry modification techniques, making it widely used in dielectric layer CMP and various other CMP systems.

Cerium oxide exhibits strong chemical interactions with silicon dioxide surfaces. By manipulating particle surface chemistry, additives, and pH levels, high removal selectivity for SiO₂ over Si₃N₄ can be achieved; consequently, it is frequently employed in processes such as Shallow Trench Isolation (STI).

Aluminum oxide possesses high mechanical hardness and is suitable for CMP systems requiring significant mechanical material removal. However, abrasive selection cannot be based on hardness alone; factors such as the surface chemistry of the target material, defect levels, particle stability, and the process window must also be comprehensively considered.

For advanced CMP slurries, average particle size is no longer sufficient to fully characterize abrasive performance. Practical applications require attention to particle size distribution (PSD), particle morphology, the content of large particles and agglomerates, Zeta potential, and dispersion stability. Even a small number of abnormally large particles or irreversible agglomerates can significantly increase the risk of micro-scratches and particle residues on the wafer surface. Therefore, high-end CMP materials often place special emphasis on the distribution of large particles (the "tail" of the distribution) and changes in particle state during storage, transportation, circulation, and slurry delivery.

• Chemical functional additives

In addition to abrasives, CMP slurries incorporate various functional components—such as oxidizing agents, complexing agents, corrosion inhibitors, pH adjusters, dispersants, and surfactants—depending on the specific material system. Oxidizing agents modulate the oxidation state of metal or semiconductor surfaces; complexing agents facilitate or control the dissolution and migration of metal ions; corrosion inhibitors limit uncontrolled corrosion on metal surfaces; pH adjusters establish the appropriate chemical environment for reactions; and dispersants and surfactants regulate particle stability, wettability, interfacial adsorption, frictional behavior, and wafer surface residues. CMP chemical systems vary significantly depending on the material; for instance, dielectric CMP typically prioritizes material removal rate, selectivity, planarization capability, and surface defects, whereas metal CMP (such as for copper) requires a balance among oxidation, complexation, passivation, corrosion inhibition, and mechanical removal. Consequently, CMP slurries are essentially semiconductor process materials that rely heavily on material surface chemistry and specific process conditions.

Application of CMP Slurry in Wafer Manufacturing

CMP technology has been widely applied in the manufacturing of logic chips, memory devices, and multi-layer interconnects; its primary applications include dielectric CMP and metal CMP.

• Dielectric CMP

Dielectric CMP is primarily used in processes such as Interlayer Dielectric (ILD) planarization and Shallow Trench Isolation (STI).

ILD CMP reduces height differences between various regions by removing excess dielectric material from the wafer surface, thereby providing a flatter surface for subsequent photolithography, thin-film deposition, and interconnect fabrication.

STI CMP requires precise control over the relative removal rates of silicon dioxide and silicon nitride. In a typical STI structure, Si₃N₄ often serves as the polish-stop layer; therefore, the design of abrasives and additives must achieve an appropriate SiO₂/Si₃N₄ removal selectivity to remove silicon dioxide while avoiding excessive damage to the underlying structures.

• Metal CMP

Typical applications of metal CMP include copper interconnect CMP, tungsten CMP, and CMP for associated barrier and liner layers.

Taking the copper damascene process as an example, after copper deposition and filling of trenches and vias, CMP is required to remove excess copper and barrier layer materials from the wafer surface while retaining the metal within the trenches and vias to form the desired interconnect structures.

Tungsten CMP is widely used in the fabrication of contact plugs, vias, and other tungsten-filled structures. Beyond controlling the removal rate of the target metal itself, it is necessary to balance the removal rates among tungsten, barrier layers, liner layers, and dielectric materials.

Compared to dielectric CMP, metal CMP involves more complex electrochemical processes. In addition to controlling the oxidation, complexation, dissolution, and passivation of the target metal surface, it is essential to mitigate static corrosion, dynamic corrosion, and galvanic corrosion that may occur between dissimilar conductive materials. Furthermore, copper CMP requires careful control of copper "dishing" and dielectric "erosion." Dishing typically manifests as the copper surface within metal lines or trenches receding below the level of the surrounding dielectric, whereas erosion generally appears as the excessive removal of both metal and dielectric in pattern-dense areas. Both phenomena are influenced by various factors, including pattern density, material selectivity, polishing pad deformation, local pressure, and the degree of over-polishing. Inadequate control can adversely affect wafer planarity, interconnect resistance, and the process window for subsequent manufacturing steps.

Application of CMP in Advanced Devices and Advanced Packaging

With the evolution of advanced device architectures such as FinFET, GAA (Gate-All-Around), and 3D NAND, the material combinations and structural complexities encountered in semiconductor CMP are increasing, placing higher demands on selectivity, nanoscale topography, and defect control. Simultaneously, CMP is playing an increasingly critical role in advanced packaging and 3D integration processes, including Through-Silicon Via (TSV) formation, wafer-level packaging, and hybrid bonding.

Taking Cu/dielectric hybrid bonding as an example, the bonding interface typically requires high local and global planarity. It also necessitates strict control over the relative height between the copper and the dielectric, as well as parameters such as dishing, erosion, surface roughness, particle defects, and surface residues. Notably, in certain hybrid bonding processes, a controlled, uniform nanoscale copper recess is an intentional part of the process design. During subsequent thermal treatment, the thermal expansion of the copper facilitates enhanced contact at the metal interface. Therefore, the objective of hybrid bonding CMP is not necessarily to achieve zero height difference between the copper and dielectric surfaces; rather, it is to maintain the copper recess, dielectric surface topography, and within-wafer uniformity within an optimal process window tailored to the specific bonding structure and workflow. As bonding pitches and copper pad sizes continue to shrink, nanoscale topographical features—such as dishing, copper recess, dielectric roll-off, local protrusions, pitting, and surface roughness—exert an increasingly significant impact on bonding interface contact, void formation, and long-term reliability. This underscores why advanced packaging applications demand CMP slurries capable of delivering low defectivity, high consistency, and precise control over surface topography.

Key Performance Indicators of High-Performance CMP Slurries

The evaluation of modern CMP slurries is no longer limited to the material removal rate; instead, it requires a comprehensive assessment of metrics such as material removal efficiency, uniformity, selectivity, planarization capability, surface quality, defect levels, and slurry stability.

• Material Removal Rate and Uniformity

Material Removal Rate (MRR) is one of the most basic process indicators of CMP. In the actual mass production process, it is not only required to have reasonable removal efficiency, but also to maintain stability between different batches, different times, and different wafers. Within-Wafer Non-Uniformity (WIWNU) and inter-wafer consistency are also important indicators for evaluating CMP mass production performance.

• Material selectivity and planarization capability

The removal selectivity between different materials directly determines the CMP process window. For instance, STI CMP typically requires high SiO₂/Si₃N₄ selectivity to protect the silicon nitride stop layer; meanwhile, multi-material systems—comprising copper, barrier layers, liner layers, and dielectrics—require the design of appropriate removal ratios for each CMP step. It is important to emphasize that the goal of CMP is not merely to increase the amount of material removed, but to progressively reduce surface height variations during the removal process. Therefore, in addition to the material removal rate (MRR), attention must be paid to planarization efficiency, pattern dependency, dishing, erosion, and both local and global surface topography.

• Defects and Surface Quality

Low defectivity is a critical technical requirement for advanced CMP slurries. Issues such as micro-scratches, particle residues, organic residues, metal residues, corrosion defects, pitting, and surface roughness can all impact subsequent processes—such as lithography, thin-film deposition, and bonding—or the electrical performance of the devices. As semiconductor device dimensions and advanced packaging interconnect pitches continue to shrink, even a small number of unusually large particles or localized defects can compromise downstream processing and manufacturing yields.

• Raw Material Purity and Contamination Control

Since CMP slurries come into direct contact with wafers, strict control is required over trace metals, abnormal particles, and other exogenous contaminants introduced during raw material sourcing, formulation, filtration, storage, packaging, and delivery. However, a distinction must be made between "functional formulation components" and "contaminants." As CMP slurries inherently contain acids, bases, salts, oxidizers, complexing agents, and other ionic functional components, slurry purity cannot be judged simply by the criterion that "lower ionic content is better." The primary focus must be on controlling exogenous impurities—those outside the intended formulation design that could affect the wafer surface and subsequent processes—as well as contamination introduced during production, packaging, and use. Consequently, contamination control for high-performance CMP materials typically encompasses the entire supply chain, including raw material purification, production environments, formulation systems, precision filtration, packaging materials, storage, and delivery.

• Particle Size Stability and Storage Performance

During the storage, transport, and delivery of CMP slurries, it is essential to maintain a controlled particle size distribution and dispersion state, preventing irreversible agglomeration, the growth of large particles, and sedimentation that cannot be redispersed. For systems where reversible sedimentation is permissible, specific conditions for mixing, circulation, or redispersion must be established to ensure that, following such treatment, the slurry's particle size characteristics and key process performance metrics are restored to within specified limits. Consequently, formulation design, particle stability, filtration capability, clean production practices, analytical testing, and batch-to-batch consistency serve as the critical foundations for the stable, large-scale production of high-end CMP slurries.

CMP抛光液的发展趋势

As advanced logic, memory devices, 3D integration, and advanced packaging continue to evolve, CMP slurries are advancing toward finer particle size control, more complex interfacial chemistry, and reduced defect levels. Regarding abrasives, narrower and more stable particle size distributions, lower levels of large particles and agglomerates, and enhanced stability in particle surface modification and dispersion systems help minimize micro-scratches and particle-related defects. Future slurry evaluation will increasingly focus on the distribution of large-particle "tails," agglomeration kinetics, and changes in slurry properties during storage, recirculation, and delivery.

In terms of chemical systems, precise control over oxidation, complexation, passivation, dissolution, and interfacial adsorption allows for the further tuning of material removal selectivity and process windows. Particularly in metal CMP, balancing a reasonable material removal rate (MRR) with the mitigation of corrosion, galvanic corrosion, and surface defects remains a critical focus for formulation development. With the rise of advanced packaging technologies such as hybrid bonding, CMP requirements are becoming increasingly stringent regarding surface roughness, nanoscale copper recess, dishing and erosion, and particle defect levels. Concurrently, CMP technology development is shifting from the isolated optimization of slurry formulations toward a synergistic approach that integrates slurry formulations, wafer materials, pattern structures, polishing pads, equipment parameters, and post-CMP cleaning processes.

Conclusion

CMP slurries are critical process materials in semiconductor wafer fabrication and advanced packaging; their particle characteristics, chemical systems, and contamination control standards directly influence material removal behavior, surface topography, defect levels, and process stability.

As technologies such as advanced logic, memory devices, and hybrid bonding continue to evolve, the requirements for semiconductor CMP regarding nanoscale topography control, low defectivity, and batch-to-batch consistency will continue to rise.

For CMP materials, future competition will extend beyond the formulations themselves to encompass comprehensive capabilities in nanoparticle control, raw material purification, ultra-clean manufacturing, precision filtration, trace analysis, defect detection, and collaborative development tailored to specific wafer processes.