1. Processing Background and Technical Challenges
Gallium nitride (GaN), as a flagship material of the third-generation semiconductors, holds an irreplaceable position in power devices and radio-frequency applications due to its wide bandgap, high electron mobility, and excellent thermal stability. However, the chemical mechanical planarization (CMP) of GaN thin films has long been confronted with two fundamental contradictions: insufficient material removal rate (MRR) and the difficulty of achieving atomic-level surface roughness. Conventional colloidal silica slurries demonstrate poor efficiency in GaN processing and fail to simultaneously satisfy the dual requirements of high removal rate and ultra-low roughness. Furthermore, the mechanistic influence of acidic pH regulators on slurry performance has remained poorly understood, constituting a critical bottleneck constraining the advancement of GaN CMP technology.
2. Technical Approach and Research Methodology
This study employs CeO₂ as the abrasive system and systematically introduces acetic acid as the pH regulator, benchmarked against a conventional nitric acid system, to thoroughly investigate the performance differences and underlying mechanisms in GaN thin film CMP.X-ray photoelectron spectroscopy (XPS) was applied to precisely characterize the chemical states of GaN surfaces before and after polishing. Concurrently, quantum chemical calculations were employed to simulate and reveal the adsorption behavior and chemical interaction nature between acetic acid molecules and the GaN surface at the molecular level. By correlating macroscopic polishing performance data with microscopic chemical mechanisms, a comprehensive model of the acetic acid enhancement mechanism was established.
3. Core Mechanism Analysis
The research demonstrates that acetic acid exhibits substantially stronger adsorption and chemical interaction with the GaN surface compared to nitric acid. The carboxyl functional group within the acetic acid molecule undergoes specific bonding with active sites on the GaN surface, effectively promoting the formation of a softened surface layer. This reduction in the shear force required for mechanical removal enables CeO₂ abrasive particles to achieve high-efficiency material removal under relatively mild contact conditions.In comparison to nitric acid, the weak acidity of acetic acid renders the chemical etching action of the slurry system more controllable, avoiding the non-uniform over-corrosion of the GaN surface that occurs in strongly acidic environments. Consequently, while maintaining a high removal rate, the damage to surface integrity is minimized. This synergistic mechanism of “chemical softening-assisted mechanical removal” constitutes the fundamental reason why the acetic acid system achieves high-efficiency, low-damage GaN polishing.
4. Quantified Processing Results
The acetic acid-regulated CeO₂-based slurry achieved remarkable processing outcomes in GaN thin film CMP. The material removal rate (MRR) reached 495.2 nm/h, representing a 70% improvement over the nitric acid-regulated slurry, fully demonstrating the outstanding advantage of the acetic acid system in enhancing processing efficiency.
Regarding surface quality, the root mean square roughness Sq of the polished GaN surface was reduced to as low as 0.07 nm, genuinely realizing an atomically smooth surface free of scratches and subsurface damage. This meets the most stringent surface quality requirements imposed by high-end GaN device manufacturing.
5. Engineering Application Value
The acetic acid enhancement mechanism revealed in this study provides clear theoretical guidance for the optimization of CMP slurry formulations for GaN and analogous wide-bandgap semiconductor thin films. The strategy of substituting conventional strong-acid pH regulators with acetic acid is not only straightforward and readily implementable in engineering practice, but also ensures damage-free surfaces while substantially improving processing efficiency. This holds direct engineering significance for improving the production yield of high-value-added GaN-based products, including power devices and radio-frequency components.