As semiconductor devices evolve toward higher power, higher density, and miniaturization, substrate materials such as silicon, silicon carbide, and gallium nitride are approaching their performance limits. Diamond, with its extremely high hardness, ultra-high thermal conductivity, ultra-wide bandgap, high breakdown electric field, and broad spectral transparency from deep ultraviolet to far infrared, is regarded as the "ultimate semiconductor material." However, during processing, these excellent properties conversely become core obstacles to achieving precision machining of diamond surfaces. Traditional polishing methods struggle to balance high material removal rates with high surface quality, making this a key technological challenge restricting the widespread application of diamond in high-performance devices. Therefore, starting from the limitations of conventional diamond polishing techniques, this article will share several novel polishing technologies and their latest advances for atomic-scale surface finishing of diamond.
Conventional Polishing Technologies and Their Limitations
Traditional diamond polishing techniques mainly include mechanical polishing, thermochemical polishing, and laser polishing. Although these technologies have played important roles in the history of diamond processing, they all exhibit clear limitations when pursuing atomic-scale surface planarization.
(1) Mechanical Polishing: Mechanical polishing is the earliest method applied to diamond processing. Its principle involves using diamond abrasives or high-hardness abrasives (such as silicon carbide, alumina, etc.) on a polishing pad to mechanically abrade the diamond surface. Due to diamond's extremely high hardness, significant polishing loads are typically required to achieve material removal; however, such high loads tend to generate scratches, pits, and other surface and subsurface damages during processing.
(2) Thermochemical Polishing: Based on the mechanism of high-temperature interfacial diffusion, at elevated temperatures of 600–1800°C, carbon atoms on the diamond surface can diffuse and dissolve into transition metal polishing pads (e.g., iron, nickel), reducing processing difficulty. However, due to uneven heating of the metal substrate, the polishing process often suffers from uniformity issues, leaving the polished surface uneven.
(3) Laser Polishing: This technique uses a high-energy laser beam to directly irradiate the diamond surface, inducing laser graphitization (conversion of diamond phase to graphite phase), followed by mechanical removal of the graphitized layer. This method is highly efficient in the roughing stage, but the laser-induced heat-affected zone is relatively deep, easily leaving thermal damage layers on the surface and making it difficult to achieve global atomic-scale planarization.
Core Atomic-Scale Polishing Technologies for Diamond
To avoid strong-contact mechanical abrasion and minimize lattice damage, researchers have turned to novel atomic-scale polishing technologies centered on multi-energy-field synergy, such as chemical mechanical polishing (CMP), plasma-assisted polishing (PAP), and ion beam sputtering polishing (IBP).
01 Chemical Mechanical Polishing (CMP)
CMP is the most industrially promising technology for atomic-scale planarization. Its core mechanism involves the synergy of chemical oxidative modification and mild mechanical abrasion: oxidants in the polishing slurry convert sp³ bonds on the diamond surface into a loose, easily removable oxide layer, which is then gently scraped off by nano-abrasives under low stress, enabling layer-by-layer atomic-scale removal and fundamentally suppressing damage. However, conventional CMP still faces challenges with diamond polishing, such as low oxidation activity, slow reaction rates, and insufficient polishing efficiency, with material removal rates typically below 1 μm/hour. Currently, the industry is improving this through two main directions: external field assistance and optimization of the oxidant system in the polishing slurry, significantly enhancing polishing efficiency and surface quality.
(1) Oxidant Selection and Optimization: Oxidants are central to the chemical reaction in diamond CMP, directly determining the oxidation rate, surface modification quality, and final roughness. Based on the need to oxidize the inert diamond surface, the main optimized systems include:
High-valence salt oxidants: Potassium ferrate (K₂FeO₄), potassium periodate (KIO₄), potassium permanganate (KMnO₄), etc. These have high oxidation potentials and strong oxidation capabilities, accelerating the modification of the inert surface. For example, Yuan et al. demonstrated through comparative experiments that among such oxidants, the K₂FeO₄ system yielded the best polishing performance, efficiently transitioning from rough polishing to fine polishing and shortening overall processing time.
Hydrogen peroxide (H₂O₂) systems: Over the past decade, H₂O₂ and its mixtures have become a primary choice for diamond chemical polishing. As a strong oxidant at room temperature, H₂O₂ can directly react with the diamond surface to generate a hydroxylated oxide layer without high-temperature side reactions, serving as a foundational oxidant for atomic-scale polishing. However, the oxidation efficiency of H₂O₂ alone is limited by the generation rate of free radicals. Therefore, it is often combined with Fe²⁺ catalysis to establish a Fenton reaction, generating highly reactive •OH radicals, which multiplicatively enhance the oxidation rate of the diamond surface, achieving both high removal rates and atomic-scale surface quality, suitable for high-end semiconductor diamond substrate processing.
(2) External Field Assistance: Introducing high-energy fields can activate the diamond surface in situ, achieving more efficient removal. Currently, the main approaches are laser-induced and photocatalysis-assisted methods.
Laser-induced: While pure laser polishing enables rapid material removal, it tends to cause thermal damage and surface irregularities. However, if used as a rough polishing step to induce graphitization and quickly flatten the surface, followed by fine polishing with CMP, the roughness can be reduced to the nanometer or even atomic scale, while greatly improving the material removal rate and alleviating the low efficiency issue of traditional CMP.
Photocatalysis-assisted: Photocatalysts (e.g., TiO₂, ZnO, etc.) are added to the polishing slurry, and a specific wavelength of ultraviolet light (typically <387.5 nm) is applied during polishing. The valence band electrons of the photocatalyst are excited to the conduction band, leaving positively charged holes (h⁺) in the valence band. These holes oxidize water molecules (H₂O) or hydroxide ions (OH⁻) adsorbed on the photocatalyst surface, generating highly oxidative hydroxyl radicals (•OH). These radicals then react with carbon atoms on the diamond surface, achieving efficient removal of surface carbon atoms.
02 Plasma-Assisted Polishing (PAP)
Plasma-assisted polishing is a dry, contactless, chemical atomic-scale polishing method. A working gas such as O₂ is introduced and ionized to generate high-energy reactive species. These species react with carbon atoms on the diamond surface, producing volatile carbon oxides that desorb from the surface, achieving purely chemical atomic-scale etching. Subsequently, a slight mechanical action from a polishing pad enables efficient removal. The advantages of this method include stress-free, abrasive-free processing, high lattice integrity, precise control of etching depth, and mitigation of crystallographic anisotropy, making it currently the most promising technology for balancing efficiency and quality. However, the equipment cost is high, and achieving large-area uniform etching is challenging.
03 Ion Beam Sputtering Polishing (IBS)
Ion beam polishing is a high-energy physical sputtering-based, non-contact polishing method. Typically performed in a vacuum environment, an ion source generates high-energy ions (e.g., Ar⁺) that bombard the diamond surface at a certain angle. Through momentum transfer, surface atoms gain sufficient energy to overcome the surface binding energy and are ejected as sputtered atoms, achieving atomic-scale material removal and thus polishing.
Because it avoids contact pressure, friction, and associated subsurface damage, scratches, or deformation, this technology has already achieved roughness reduction of CVD diamond from 334 nm down to 0.5 nm using gas cluster ion beams (GCIB) generated from gases such as argon or sulfur fluoride, with future potential to reach the atomic level. However, the requirement for high vacuum, complex ion sources, and control systems makes the equipment expensive to purchase and maintain, limiting its widespread application in general industrial fields.