1. Adjusting the Abrasive System: From Hardness Matching to Functional Customization
The selection of abrasives, which are central to the mechanical removal process, must balance material hardness, polishing stage, and surface damage control. This goes far beyond simple hardness matching.
For silicon wafers (Mohs hardness ~7, moderate chemical activity), colloidal silica or alumina abrasives with a size of 0.5 to 2 micrometers are typically chosen for the rough polishing stage to balance removal rate and initial planarization. The fine polishing stage employs spherical silica abrasives, sized 20 to 50 nanometers, which can reduce surface roughness to below 0.1 nm. A key factor is controlling abrasive dispersion, often by adding agents like polyethylene glycol to prevent agglomeration and scratching.
For silicon carbide (Mohs hardness ~9.2, chemically inert, and anisotropic), rough polishing uses alumina or silicon carbide abrasives (0.5-1.5 μm) with thickeners like agar to prevent agglomeration. Fine polishing increasingly utilizes innovative abrasives, such as core-shell particles with a glucose core and silica shell (30-100 nm), or "smart" abrasives grafted with pH-sensitive polymer chains. These designs promote chemo-mechanical synergy for high-efficiency, low-damage polishing.
For gallium nitride (Mohs hardness ~9, but prone to corrosion with sensitive epitaxial layers), an extremely low concentration (0.5-2%) of small, spherical silica abrasives (30-80 nm) is preferred to minimize scratching. High-hardness abrasives like diamond are strictly avoided to prevent lattice damage.
2. Adjusting the Chemical Composition: From Simple Etching to Synergistic Catalysis
The chemical components must match the material's reactivity. The core challenge is addressed by designing synergistic systems involving oxidizers, catalysts, and complexing agents.
Regarding the oxidizer system, silicon polishing conventionally uses hydrogen peroxide as a mild oxidizer. Silicon carbide polishing is shifting from traditional, highly toxic nitric acid-hydrofluoric acid mixtures towards greener innovative approaches like visible-light-assisted Fenton reactions. Gallium nitride polishing often employs a composite oxidizer of hydrogen peroxide and potassium permanganate, coupled with manganese dioxide as a catalyst and acetate as a corrosion inhibitor to accelerate surface modification while suppressing over-etching.
Complexing and modulating agents are used to address specific issues like byproduct deposition and surface damage. For instance, sodium carbonate is added to SiC slurries to complex silicon ions, citric acid is used in GaN slurries to complex gallium ions, and EDTA is incorporated into silicon slurries to chelate metal impurities and enhance surface purity.
The use of eco-friendly components is a significant trend. This involves replacing highly toxic oxidizers with alternatives like hydrogen peroxide, using bio-based dispersants, and implementing recycling processes to recover and reuse catalysts and abrasives.
3. Adjusting Concentrations and pH: From Fixed Ranges to Dynamic Adaptation
Concentrations and pH values directly control the balance between chemical etching and mechanical removal, requiring dynamic adjustment based on the material and polishing stage.
For pH control, silicon polishing is typically performed under alkaline conditions (pH 10-11) to dissolve the silica layer. Silicon carbide polishing often adopts an "acidic first, alkaline later" strategy: low pH (2-4) for rough polishing to enhance oxidation, and higher pH (8.5-11.0) for fine polishing to improve surface quality. GaN epitaxial layer polishing requires a tightly controlled neutral-to-weakly alkaline environment (pH 8-9) to protect the sensitive surface.
Concentration optimization shows significant variation. The abrasive concentration (solid content) ranges from relatively high levels for silicon rough polishing to very low levels for GaN, reflecting the precise control needed over mechanical force. Oxidizer concentrations are also finely tuned according to the difficulty of oxidizing the material.
Dynamic adaptation characterizes advanced polishing processes. Examples include gradually decreasing the pH during silicon wafer polishing, utilizing pH-sensitive abrasives for self-adjusting SiC polishing, and simultaneously increasing the inhibitor concentration in acidic GaN polishing to counteract corrosive effects.
4. Addressing Specific Challenges with Targeted Adjustments
To tackle the anisotropy of silicon carbide (different removal rates on Si- and C-faces), specific surfactants can be added to modify the slurry's wettability on different crystal planes, making the removal rates more uniform for synchronous polishing.
For protecting sensitive gallium nitride epitaxial layers, specialized formulations with no strong oxidizers and minimal mechanical action are essential, employing mild solvents and surface modifiers to avoid damaging the ultra-thin layers.
To achieve global planarization of silicon wafers, inhibitors like benzotriazole are added to selectively protect the silicon substrate in recessed areas, enabling preferential removal of the surface oxide layer and protruding defects.
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