Abstract

Sapphire, namely single-crystal α-Al₂O₃, is a high-performance functional substrate extensively utilized in solid-state lasers, semiconductor chips, and gallium nitride-based light-emitting diode (LED) devices, attributed to its exceptional mechanical robustness, optical transparency, chemical inertness, and superior dielectric properties. The surface flatness and ultra-precise planarization of sapphire substrates serve as decisive factors governing the optoelectronic performance and service reliability of semiconductor devices. Chemical mechanical polishing (CMP) is currently the only mature manufacturing technique capable of achieving atomic-level smooth surfaces for sapphire substrates. Nevertheless, the inherent high hardness and chemical stability of sapphire lead to low polishing efficiency and poor surface uniformity in conventional CMP processes. Moreover, significant differences in polishing characteristics exist among sapphire crystal planes, severely constraining the high-precision and high-efficiency mass fabrication of qualified sapphire substrates.
To address the low material removal rate (MRR) and suboptimal surface quality of a-plane and c-plane sapphire substrates during conventional silica-based CMP, sodium chloride (NaCl) was introduced as a functional additive to modify and optimize the polishing slurry system in this work. Comparative CMP experiments were conducted on commercial 2-inch a-plane (1120) and c-plane (0001) single-crystal sapphire wafers. The experimental results indicate that the slurry doped with 0.2 wt% NaCl delivers the optimal comprehensive polishing performance. Under this optimized condition, both a-plane and c-plane sapphire substrates achieve prominently enhanced MRR and reduced surface roughness (Sq), with scratch-free and defect-free planar surfaces. This study systematically validates the modification effect of chloride ions on sapphire CMP performance, providing a reliable technical strategy for high-efficiency planarization polishing of multi-crystal-plane sapphire substrates.
Keywords: sapphire substrate; chemical mechanical polishing; chloride ion additive; material removal rate; surface roughness; crystal plane difference

1. Introduction

As a premium single-crystal oxide material, sapphire has emerged as an indispensable substrate for third-generation semiconductor optoelectronic devices, benefiting from its outstanding thermal stability, mechanical strength, and electrical insulation performance. In particular, it acts as the dominant epitaxial substrate for GaN-based LED chips. The surface planarization quality of sapphire substrates directly determines the epitaxial growth uniformity of functional thin films and the overall performance of optoelectronic devices, making ultra-precision CMP a core manufacturing process for high-end sapphire substrates.
The material removal behavior of sapphire CMP is governed by the dynamic synergism of chemical corrosion and mechanical abrasion. Variations in atomic stacking structures and chemical bond energies across different crystal planes lead to distinct polishing difficulty. The c-plane sapphire possesses an O-Al-Al-O atomic configuration, where the low-energy Al-Al bond (186 KJ/mol) is easily fractured during polishing. In contrast, a-plane and r-plane sapphire feature stable Al-O-Al-O structures with high-strength Al-O bonds (512 KJ/mol), which significantly increases polishing resistance. At present, existing research predominantly focuses on the CMP optimization of c-plane sapphire, while systematic investigations on the polishing performance and modification mechanism of a-plane sapphire remain insufficient, restricting the establishment of universal modification theories for multi-crystal-plane sapphire CMP.
Previous studies have verified that ionic additives, halogen elements, and chelating agents can effectively improve the CMP efficiency of sapphire substrates. Several reports have also confirmed that chloride ions can enhance the MRR of dielectric materials in CMP processes, yet their modification effect and intrinsic mechanism on multi-crystal-plane sapphire substrates lack systematic verification. Against this background, this work adopts alkaline silica-based slurry as the research carrier, employs NaCl as a chloride ion donor, and systematically investigates the influence of NaCl doping concentration on the polishing performance of a-plane and c-plane sapphire substrates. The research findings provide solid experimental support for the development of low-cost, high-efficiency sapphire polishing slurry.

2. Experimental Section

2.1 Experimental Materials and Equipment

Commercial 2-inch single-crystal a-plane (1120) and c-plane (0001) sapphire wafers were selected as experimental samples for comparative polishing tests. All CMP experiments were performed on an X62 S82 × 305-D-S single-sided polishing machine (Suzhou Herriot) equipped with a Suba600 polishing pad. A self-formulated alkaline silica-based slurry was applied, utilizing 40 wt% nano-SiO₂ sol as the abrasive agent with an average particle size of 80–90 nm, 0.2 vol% surfactant as the dispersant, and KOH as the pH regulator to maintain the slurry pH at 10.5. NaCl with mass concentrations ranging from 0 wt% to 0.4 wt% was incorporated into the slurry as a functional modifier. Each group of experiments was repeated no fewer than three times to eliminate random errors and ensure the repeatability and credibility of experimental data.
 
Figure 1. Schematic diagram of the sapphire wafer polishing template

2.2 Process Parameters and Test Methods

The fixed CMP process parameters adopted in this study are summarized in Table 1, with a unified polishing duration of 60 min for all test groups. An AUY120 ASSY high-precision electronic balance (0.1 mg accuracy) was utilized to measure the wafer mass before and after polishing for MRR calculation. The surface morphology and root-mean-square surface roughness (Sq) of polished wafers were characterized via an Agilent 5600LS atomic force microscope (AFM). A NICOMP 380ZLS laser nanoparticle size analyzer was employed to test the particle size distribution of silica slurry under different NaCl doping concentrations.
 
Table 1. Fixed process parameters of sapphire CMP experiments
MRR was calculated according to the classical mass loss method, as shown in Equation (1):
MRR=M×10⁴/pπr²t
where m refers to the mass loss of the sapphire wafer (g), t is the polishing duration (h),ρ represents the density of sapphire (3.98 g/cm³), and r denotes the wafer radius (2.54 cm). Preston’s equation (MRR = K·P·V) was adopted to analyze the coupling relationship between mechanical polishing parameters (downward pressure, rotational speed) and material removal efficiency, revealing the mechanical regulation mechanism of the CMP process.

3. Experimental Results and Analysis

3.1 Effect of NaCl Concentration on Sapphire MRR

The variation trends of MRR for a-plane and c-plane sapphire with NaCl doping concentration are illustrated in Figure 2. With the increase of NaCl concentration from 0 wt% to 0.2 wt%, the MRR of both crystal planes increased significantly. At the optimal doping concentration of 0.2 wt%, the MRR values of a-plane and c-plane sapphire reached 2.481 μm/h and 9.128 μm/h, respectively. Further increasing the NaCl concentration beyond 0.2 wt% led to a gradual decline in the MRR of both crystal planes, indicating that excessive chloride ions would weaken the polishing modification effect.
 
Figure 2. Effect of NaCl mass concentration on MRR of a-plane and c-plane sapphire

3.2 Effect of NaCl on Slurry Particle Size and Mechanical Action

To clarify the intrinsic reason for MRR improvement, the average particle size of silica slurry under different NaCl concentrations was tested to exclude the influence of mechanical abrasion variation. As presented in Figure 3, the change of NaCl doping concentration exerted negligible effect on the particle size of nano-silica abrasives, demonstrating that the mechanical grinding performance of the slurry remained stable throughout the experiment. It can be concluded that the enhanced polishing efficiency is predominantly attributed to the strengthened chemical interaction between chloride ions and sapphire substrates, rather than the variation of mechanical polishing performance.
 
Figure 3. Variation of slurry average particle size with NaCl concentration

3.3 Effect of NaCl on Sapphire Surface Quality

The surface roughness variations of a-plane and c-plane sapphire before and after 0.2 wt% NaCl-assisted CMP are listed in Table 2. After optimized polishing, the Sq value of a-plane sapphire decreased from 0.340 nm to 0.249 nm, while that of c-plane sapphire reduced from 0.320 nm to 0.243 nm. The polished wafer surfaces exhibited uniform and smooth micro-morphology without visible scratches, residual particles, or corrosion defects (Figure 4), achieving high-quality atomic-level planarization.
 
Table 2. Surface roughness (Sq) of a-plane and c-plane sapphire before and after optimal NaCl-assisted polishing
 
Figure 4. Surface morphology of sapphire wafers polished with 0.2 wt% NaCl-doped slurry

4. Preliminary Conclusions

NaCl additive can effectively optimize the CMP comprehensive performance of a-plane and c-plane sapphire substrates. The optimal doping concentration is determined as 0.2 wt%, which simultaneously improves the material removal efficiency and surface planarization quality of multi-crystal-plane sapphire. The modification mechanism is independent of the mechanical performance change of silica slurry, but originates from the enhanced chemical reaction between chloride ions and sapphire crystal surfaces. The crystal-plane-adaptive polishing optimization effect of chloride ions provides a novel and feasible approach for universal high-efficiency ultra-precision polishing of sapphire substrates with different orientations.
 
In combination with the ultra-precision polishing process requirements of a-plane and c-plane sapphire substrates in this study, JXT  stably supplies full-specification single-crystal sapphire substrate wafers ranging from 2 to 12 inches. The product series covers commonly used crystal orientations including C-plane (0001), A-plane, R-plane, and M-plane, and supports customized single-sided polishing (SSP), double-sided polishing (DSP), and patterned sapphire substrate (PSS) processing. Relying on mature integrated cutting, grinding and polishing production lines, our products can adapt to the chloride-ion-modified CMP polishing process described in this study to realize atomic-level ultra-smooth surface fabrication. Featuring low total thickness variation (TTV), low warpage, and low dislocation density, our sapphire substrates are well applicable to GaN epitaxy, Mini/Micro LED, ultraviolet optoelectronic devices, and scientific research experiments. We provide small-batch sample services and stable large-batch mass supply, fully meeting the demands of semiconductor research, development and industrial mass production.
 
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