1. The Processing Challenge: The Dilemma Between Efficiency and Quality
Gallium nitride (GaN) single crystal substrates, owing to their outstanding wide-bandgap semiconductor properties, are indispensable in laser display, radio frequency (RF) device, and power electronics applications. However, the surface processing of GaN single crystal substrates has long been hindered by a fundamental contradiction: conventional standalone CMP processes typically require more than 15 hours to complete the processing of a single substrate, imposing severe limitations on production throughput. On the other hand, aggressively pursuing higher material removal rates inevitably introduces surface and subsurface damage, including lattice defects and impurity residues, which ultimately compromise the electrical performance of finished devices. Pure CMP alone is incapable of simultaneously achieving atomic-level surface quality and acceptable processing efficiency, and this has remained one of the most persistent unresolved challenges in the industry.
2. Technical Approach: The Synergistic ICP Etching and CMP Combined Strategy
To address this challenge, researchers have proposed an ICP etching and chemical mechanical polishing (CMP) combined processing strategy, in which the two processes function in a complementary manner to deliver both high efficiency and superior surface quality.
Stage One: ICP Isotropic Etching (Bulk Material Removal)
Inductively coupled plasma (ICP) etching serves as the first step, responsible for the rapid removal of the majority of material from the GaN substrate. The isotropic nature of ICP etching enables an extremely high material removal rate, dramatically reducing the overall processing time. However, the dry etching process inevitably leaves behind surface impurities and shallow subsurface lattice damage caused by plasma bombardment, which must be addressed in the subsequent stage.
Stage Two: CMP Wet Oxidation Polishing (Surface Refinement and Damage Elimination)
Following ICP etching, CMP is introduced as the precision finishing step. Through the synergistic action of wet chemical oxidation reactions and mechanical abrasion, CMP precisely removes the surface contaminants and subsurface damage layer left by dry etching, progressively bringing the surface to atomic-level smoothness. Since the bulk of the material has already been removed by ICP, the remaining stock requiring CMP treatment is substantially reduced, thereby significantly shortening the required CMP processing time.
3. Process Validation: Damage-Free Confirmation via FIB-STEM
To rigorously validate the processing quality of the combined approach, the research team employed focused ion beam transmission electron microscopy (FIB-STEM) to perform atomic-scale cross-sectional analysis of the processed GaN substrates. FIB-STEM results confirmed that after the combined ICP etching and CMP process, no detectable lattice damage, dislocation extension, or impurity enrichment was observed in either the surface or subsurface regions, demonstrating that true damage-free processing had been achieved. This high-resolution characterization technique provided solid experimental evidence for the reliability and integrity of the combined process.
4. Quantified Results: A Dual Breakthrough in Efficiency and Quality
The implementation of the combined process delivered significant and quantifiable improvements across two key dimensions:
In terms of processing efficiency, the total processing time was reduced from over 15 hours under conventional standalone CMP to approximately 1.5 hours, representing a tenfold improvement in throughput and substantially enhancing the production capacity potential for GaN substrates.
In terms of surface quality, the final processed GaN substrate achieved a surface roughness of less than 0.2 nanometers, reaching atomic-level smoothness that fully satisfies the stringent surface quality requirements of advanced semiconductor device fabrication.
5. Application Prospects and Industry Significance
The ICP etching and CMP combined strategy opens a viable and practical technical pathway for the high-efficiency, damage-free processing of large-diameter GaN single crystal substrates. This approach holds direct value across several critical application domains: in laser display, high-quality GaN substrates underpin the development of high-brightness, long-lifetime laser light sources; in RF devices, damage-free substrates contribute to reduced device noise and improved high-frequency performance; and in power electronics, atomically smooth surfaces are a prerequisite for fabricating highly reliable GaN power devices.The successful demonstration of this process strategy also provides important methodological insights for the broader semiconductor industry in tackling other hard and brittle wide-bandgap materials such as SiC and AlN. The process combination philosophy of dry-method rapid bulk removal followed by wet-method precision surface restoration is poised to become a mainstream technical paradigm for next-generation hard and brittle substrate processing.