Abstract

Based on the optimal process parameters of NaCl-modified sapphire CMP obtained from preliminary experiments, this work further explores the intrinsic modification mechanism of chloride ions via thermodynamic simulation, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and multiple microscopic characterization methods. The results reveal that chloride ions can induce mild chemical corrosion on sapphire surfaces and promote the generation of soluble sodium tetrachloroaluminate (NaAlCl₄), which realizes high-efficiency material removal under low-intensity mechanical abrasion. However, NaCl doping inevitably induces slight crystallization and aggregation of nano-silica particles in the slurry, which poses potential scratch risks to the polished wafer surface and limits industrial application. To solve this defect, potassium chloride (KCl) was adopted to replace NaCl as the chloride ion donor for slurry optimization. The KCl-modified slurry retains the excellent polishing optimization effect of chloride ions and completely eliminates silica particle crystallization defects, achieving high-efficiency and defect-free CMP of a-plane and c-plane sapphire. This study systematically clarifies the material removal mechanism of chloride ion-modified sapphire CMP and optimizes the slurry formulation, providing solid theoretical basis and technical support for industrial ultra-precision polishing of sapphire substrates.
Keywords: sapphire CMP; modification mechanism; Gibbs free energy; NaAlCl₄; slurry optimization; KCl additive

1. Introduction

The dynamic balance between chemical corrosion and mechanical abrasion is the core prerequisite for high-quality sapphire CMP. Moderate chemical etching can form a soft modified layer on the sapphire surface, reducing the mechanical removal resistance of surface materials and improving polishing efficiency. In contrast, excessive chemical reaction will break the chemical-mechanical synergistic equilibrium, leading to residual reaction by-products and deteriorated surface quality. Previous experiments have verified that 0.2 wt% NaCl doping can significantly optimize the polishing performance of multi-crystal-plane sapphire, yet the micro-scale chemical reaction mechanism and material removal model remain unclear. Meanwhile, the slurry crystallization defect induced by NaCl restricts the practical industrial promotion of the modified polishing system.
In this study, HSC thermodynamic software was used to calculate the Gibbs free energy of each potential reaction in the polishing system to verify the spontaneous feasibility of chloride ion-sapphire interfacial reactions. SEM-EDS and XPS were employed to characterize the surface corrosion morphology and chemical reaction products, clarifying the atomic-level modification mechanism of chloride ions. Targeting the slurry crystallization defect caused by Na⁺, the alkali metal ion type was optimized, and the polishing performance and slurry stability of the KCl-modified system were comprehensively evaluated. This work realizes the dual optimization of polishing efficiency and slurry stability, laying a foundation for industrial application of modified sapphire CMP slurry.

2. Mechanism Research and Characterization

2.1 Thermodynamic Analysis of Chemical Reactions

The standard Gibbs free energy change (ΔGθT) of each possible reaction in the Al₂O₃-SiO₂-H₂O-NaCl system within 0–60 °C polishing temperature range was calculated by HSC chemistry software. The calculation formula is as follows:
 
Within the actual polishing temperature range of 0–60 °C, the standard Gibbs free energy changes (ΔGθT) of all core reactions, including sapphire hydration, aluminum silicate formation, and aluminum chloride complexation, were negative. This indicates that all interfacial chemical reactions can proceed spontaneously under CMP conditions, verifying the theoretical feasibility of chloride ion-mediated sapphire CMP modification. The thermodynamic relationship between reaction Gibbs free energy and temperature is displayed in Figure 1.
 
Figure 1. Relationship between Gibbs free energy change and temperature of main reactions in the polishing system

2.2 SEM Morphology Analysis of Sapphire Surface Corrosion

The surface corrosion morphology of sapphire wafers soaked in NaCl solution was observed by SEM, and the elemental composition of corroded and uncorroded regions was analyzed via EDS spectroscopy. As shown in Figure 2, uniform triangular corrosion pits appeared on the sapphire surface after NaCl treatment. EDS detection confirmed that the corroded pits contained Na, Cl, Al, and O elements, while only Al and O elements were detected in the intact uncorroded regions. This morphological and elemental evidence directly proves that NaCl can undergo effective chemical corrosion reactions with sapphire substrate surfaces.
 
Figure 2. SEM-EDS results of sapphire surface after NaCl solution corrosion: (a) corrosion pit morphology; (b) element distribution in corroded area; (c) element distribution in uncorroded area

2.3 XPS Analysis of Polishing Reaction Products

XPS was utilized to comparatively analyze the surface chemical states of sapphire wafers polished with pure water and 0.2 wt% NaCl-doped slurry. The Al 2p and O 1s XPS spectra of pure water-polished sapphire (Figure 3) confirm that sapphire undergoes spontaneous hydration reaction in aqueous environments, generating hydrated products of Al(OH)₃ and AlO(OH).
 

Figure 3. XPS spectra of sapphire polished with pure water: (a) Al 2p spectrum; (b) O 1s spectrum
 
The Al 2p, Cl 2p, and Na 1s XPS spectra of sapphire polished with NaCl-modified slurry are presented in Figure 4. The characteristic diffraction peaks of sodium tetrachloroaluminate (NaAlCl₄) were clearly identified in the spectra, verifying that chloride ions react with sapphire hydration products to form soluble NaAlCl₄. The intrinsic modification mechanism is attributed to the nucleophilic substitution effect: chloride ions act as nucleophiles to attack high-valence Al³⁺ in sapphire lattices, weaken the electrostatic bonding force between Al³⁺ and O²⁻, and finally form soluble [AlCl₄]⁻ complex ions, which can be easily removed by mild mechanical abrasion during polishing.
 
 
Figure 4. Schematic diagram of chloride ion nucleophilic substitution reaction on sapphire surface
 
 
Figure 5. Material removal model of chloride ion-modified sapphire CMP

3. Slurry Defect Analysis and Optimization

3.1 Defect Analysis of NaCl-Doped Slurry

During the CMP experiments, trace crystalline particles were observed at the bottom of NaCl-modified slurry. SEM-EDS characterization confirmed that these particles were aggregated nano-SiO₂ colloidal crystals (Figure 6). The essential cause of this phenomenon is that Na⁺ possesses a strong compression effect on the electric double layer of silica sol micelles, which reduces the electrostatic repulsion between abrasive particles and induces colloidal aggregation and crystallization. The generated hard crystalline particles are prone to scratching the sapphire surface during polishing, bringing irreversible defects and deteriorating surface quality.
Figure 6. SEM morphology and component analysis of crystalline particles in NaCl-doped slurry

3.2 Polishing Performance of KCl-Modified Slurry

To eliminate the crystallization defect of silica sol, KCl was selected as an alternative chloride ion donor to replace NaCl for slurry optimization. Compared with Na⁺, K⁺ exhibits lower electronegativity and weaker compression effect on the electric double layer of silica micelles, which effectively suppresses colloidal aggregation and crystallization. Experimental results demonstrate that the slurry doped with 0.2 wt% KCl achieves comparable excellent polishing performance to NaCl-modified slurry , with prominent improvements in MRR and surface flatness for both a-plane and c-plane sapphire. Notably, no crystalline particles were generated in the KCl-modified slurry after polishing, completely eliminating the potential risk of surface scratch defects.
 
 
Figure 7. Surface morphology of sapphire polished with KCl-doped slurry
 
 
Figure 8. Effect of KCl mass concentration on MRR of a-plane and c-plane sapphire
 
Figure 9. Schematic diagram of the influence of Na⁺ and K⁺ on the double electron layer structure of silica sol micelles
 

4. Conclusions

1. Chloride ions are effective functional additives for optimizing multi-crystal-plane sapphire CMP. During polishing, chloride ions undergo spontaneous nucleophilic substitution reactions with sapphire hydration products to generate soluble NaAlCl₄. The dynamic equilibrium between chemical corrosion and mechanical abrasion is optimized accordingly, realizing simultaneous improvement in material removal efficiency and surface planarization quality.
2. NaCl doping has inherent limitations in slurry stability. The strong compression effect of Na⁺ on the electric double layer of nano-silica micelles induces colloidal aggregation and crystallization, which brings potential surface defect risks for polished sapphire substrates.
3. KCl is verified as an optimal alternative chloride ion additive for high-performance sapphire CMP. The KCl-modified slurry fully retains the chloride ion-induced polishing optimization effect, completely eliminates silica crystallization defects, and presents superior slurry stability and industrial application prospects.
This study systematically clarifies the intrinsic chemical modification mechanism of chloride ions for a-plane and c-plane sapphire CMP, and develops a stable, high-efficiency, low-defect silica-based polishing slurry via ion-type optimization. The research outcomes provide valuable theoretical guidance and technical support for the industrial ultra-precision processing and high-quality manufacturing of sapphire optoelectronic substrates.
Based on the optimized polishing mechanism and slurry system for sapphire substrates proposed in this study, JXT provides high-quality multi-orientation and full-size sapphire wafer substrates. Supported by a mature integrated cutting, grinding and polishing manufacturing system, our products are compatible with both NaCl and KCl chloride-ion-modified polishing systems. This compatibility effectively avoids typical processing defects such as slurry crystallization and surface scratching, enabling the production of high-yield and high-flatness sapphire substrates that fully satisfy research and industrial mass-production requirements in the optoelectronic and semiconductor fields.
 
 
Related products：
1-12 inch Sapphire Substrate Wafer
Plane, R-plane, and M-plane Sapphire substrate 
Patterned Sapphire Substrate（PSS）