Research progress on diamond polishing technology

Diamond has excellent performance and plays an outstanding role in many fields such as mechanics and optics (including semiconductors). In 2023, the market size of its semiconductor, heat dissipation and other fields will increase by hundreds of millions of dollars, and the popularity will continue. However, surface quality affects application, and efficient polishing to obtain high-quality surface has become a research focus, among which chemical mechanical polishing (CMP) has attracted attention due to its advantages.

Early Chemical Mechanical Polishing

In the early days of chemical mechanical polishing (CMP), high-temperature molten salts (such as KNO₃) were used as oxidants. Since 1974, the process has been continuously improved. For example, NaNO₃/KNO₃ rods were used for pressure polishing, and the RMS roughness could reach 0.2nm. Later, mixed oxidants were used to improve efficiency. Different formulations have their own advantages and disadvantages in removal rate and surface quality. There are many commonly used oxidants, some of which require high temperatures, but high temperatures can easily cause problems.

Chemical Mechanical Polishing With H2O2 and Its Mixture

Diamond surface microscopic interference image

H₂O₂ solution can be used for room temperature chemical mechanical polishing to obtain an atomically smooth surface. Tokuda et al. soaked and flattened diamonds in a H₂SO₄/H₂O₂ mixture to reduce roughness; Kubota et al. polished with a rotating iron rod in a H₂O₂ solution, oxidized the surface by ·OH, and obtained an atomic-level surface of Ra=0.092 nm, but the uniformity was poor. Yuan et al. showed that Fe²⁺ enhanced the polishing effect, and Fe₂(SO₄)₃+H₂O₂ reagent had the best polishing effect.

SEM photos of diamond surface before and after oxidation

Photocatalytic Assisted Chemical Mechanical Polishing

High speed horizontal spindle UV polishing machine

Diamond has a band gap energy of 5.45 eV. When it is irradiated with ultraviolet light at a wavelength less than 225 nm, it is excited to produce hole-electron pairs, which react with oxygen and water molecules to form bonds, produce O atoms and OH to oxidize the surface. Based on this theory, a photocatalytic assisted chemical mechanical polishing method was proposed. Anan et al. used ultraviolet light to polish single crystal diamond, and the polishing effect was significant, accompanied by the production of CO and CO2. Kubota et al. used sapphire polishing discs instead, and the effect was better. This method can improve the surface quality of diamond, but the equipment is highly complex and needs further research and optimization.

Conclusion and Prospect

The current diamond market is growing by hundreds of millions of dollars annually and its applications are expanding, and its surface quality affects the application effect. Chemical mechanical polishing has significant advantages, and the combination of H ₂ O ₂ method can achieve sub nanometer roughness and produce ultra smooth and low damage surfaces. In the future, achieving large-area, non subsurface damage polishing is a key prerequisite for expanding its applications.

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Research progress on diamond polishing technology

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