How to Achieve High Purity Silica for Semiconductor-Grade Applications

Semiconductor manufacturing relies strictly on ultra-high-purity silica (quartz) as core raw materials for wafer diffusion tubes, etching chambers, quartz crucibles and optical components. Unlike ordinary industrial quartz, semiconductor-grade silica demands a minimum SiO₂ purity of 99.999% (5N), with critical impurity elements including Al, Fe, K, Na, Li, Ti and B controlled at ppb levels. Even trace silicate gangue or lattice metal contaminants will cause circuit leakage, wafer defects and chip failure. Based on professional quartz processing technology from quartz-mill.com, this article systematically introduces the full-chain integrated purification workflow covering raw ore screening, physical beneficiation, thermal activation, deep chemical leaching, high-temperature refining and clean quality control to produce qualified semiconductor-grade high-purity silica.

1. Strict Selection of Low-Background Quartz Raw Ore

The foundation of ultra-high-purity silica lies in high-quality raw material selection, as later purification cannot fully eliminate deep lattice impurities inherent in low-grade ore.

• Preferred feedstock: High-transparency vein quartz or natural quartz crystal with low fluid inclusions, low feldspar intergrowth and low alkali metal substitution inside crystal lattices. Deposits with high aluminum, boron and titanium must be rejected directly.
• Raw ore pre-inspection: XRF and ICP-MS rapid detection to screen out ore with total impurity content exceeding 150 ppm; only low-impurity quartz blocks enter the production line.
• Contamination-free primary crushing: Adopt all-ceramic lined crushers instead of iron alloy equipment to avoid mechanical iron pollution generated during size reduction, breaking bulk ore into 0.1–1 mm uniform particles to fully expose mineral grain boundaries and wrapped impurity inclusions.

2. Physical Pre-Beneficiation: Remove Coarse Gangue Minerals

Physical separation eliminates most symbiotic gangue such as feldspar, mica, iron oxide and clay before chemical treatment, drastically reducing acid consumption and purification difficulty in downstream processes. This stage includes four core procedures matched with complete quartz milling and classification equipment:

2.1 Attrition Scrubbing & Desliming

High-speed mechanical scrubbing with deionized water strips surface clay coatings and thin iron oxide films attached to quartz grains. Hydrocyclone desliming removes fine slime particles rich in aluminum and potassium to prevent impurity accumulation in follow-up tanks.

2.2 High-Gradient Magnetic Separation

4–5T high-gradient magnetic separators capture paramagnetic impurities: hematite, magnetite, ilmenite and wear iron from crushing and grinding media. This step reduces total iron content by over 95% and avoids iron contamination entering the flotation circuit.

2.3 Selective Reverse Flotation (Core for Feldspar Removal)

As the key separation process described on quartz-mill.com, fluorine-free acid flotation separates quartz from feldspar and mica by utilizing subtle differences in mineral surface wettability:

1. Adjust slurry pH to 2–3 with dilute sulfuric acid to activate feldspar surface;
2. Add cationic collectors to make feldspar hydrophobic and attach to air bubbles for upward floating removal;
3. Add quartz depressants to keep silica grains hydrophilic and settled in the tank.
   After flotation, Al₂O₃ content drops from 0.4–0.8% in raw sand to below 100 ppm, fundamentally solving the long-standing separation bottleneck of quartz-feldspar symbiosis.

2.4 Precision Closed-Circuit Milling & Air Classification

Specialized ultra-fine quartz roller mills with inert ceramic grinding rollers minimize metal abrasion pollution. Enclosed high-precision air classifiers control particle size distribution within a narrow target range, returning oversized particles for regrinding and isolating fine impurity micro-powders. The fully sealed system blocks ambient dust pollution during fine grinding.

3. Thermal Activation: Calcination & Water Quenching to Open Lattice Microcracks

Physical separation only removes surface and grain-boundary impurities; thermal pretreatment creates micro-fractures inside quartz crystals to expose lattice-substituted metal ions for deep leaching:

1. Calcinate purified quartz sand at 900–1050°C for 1–2 hours in a corundum-lined kiln;
2. Rapid water quenching with ultra-pure deionized water to generate uniform thermal stress microcracks inside grains;
3. Ultrasonic cleaning post-quenching to wash away loose surface impurity debris.
   This thermal cycle greatly improves the mass transfer efficiency of subsequent acid leaching agents, cutting leaching time by nearly half while reducing acid dosage.

4. Deep Chemical Purification: Hot-Press Mixed Acid Leaching

Thermally activated quartz enters enclosed pressure-resistant acid leaching reactors to dissolve residual surface metal impurities and shallow lattice contaminants, the core step to lift silica purity to 4N–5N grade:

• Leaching formula: Compound solution of high-purity HCl, H₂SO₄ and trace oxalic acid (fluorine-free eco formula for mainstream semiconductor production);
• Reaction conditions: Liquid-solid ratio 5:1, temperature 70–90°C, sealed pressure environment with continuous stirring and ultrasonic auxiliary oscillation for 4–8 hours;
• Multi-stage countercurrent rinsing: After leaching, use class 1 ultra-pure deionized water for 5–7 times cyclic rinsing until residual acid and metal ion conductivity meet ppb standards.
  After acid leaching, Fe content falls below 10 ppb, Ca/Mg/Na/K alkali metals are removed by over 99%, and SiO₂ purity reaches 99.995%.

5. High-Temperature Chlorination Roasting: Attain 5N+ Ultra-High Purity

For advanced semiconductor processes (etching equipment, EUV optical parts, 3nm+ chip manufacturing), chlorination roasting is the ultimate purification step to eliminate stubborn deep lattice impurities (Al, Ti, Li) that acid leaching cannot remove:

1. Load pre-leached quartz sand into a high-purity alumina tube furnace;
2. Heat to 1250–1300°C under protective chlorine/hydrogen chloride mixed atmosphere;
3. Lattice metal oxides react with chlorine gas to form low-boiling volatile metal chlorides, which are extracted and exhausted from the furnace;
4. Purified silica is cooled under inert nitrogen shielding to avoid secondary oxidation pollution.
   This process stably boosts SiO₂ purity above 99.999% (5N), fully meeting strict semiconductor-grade material thresholds.

6. Controlled Post-Treatment & Cleanroom Packaging

Contamination prevention does not end after purification; standardized post-processing locks in ultra-high purity before delivery:

1. Vacuum low-temperature drying: 200–300°C inert gas drying to reduce free water and hydroxyl content below 50 ppm;
2. Dust-free screening: All-ceramic vibrating screens inside Class 100 cleanrooms to remove abnormal particles;
3. Sealed vacuum packaging: Double-layer inert gas-filled aluminum foil bags and food-grade PP ton bags, avoiding contact with metal packaging materials;
4. Full-spectrum quality testing: Batch ICP-MS, FTIR and XRF detection to record full impurity element data for semiconductor factory traceability.

7. Core Equipment & Process Design Principles from Quartz-Mill.com

The entire purification chain relies on anti-contamination, corrosion-resistant quartz processing equipment to prevent secondary pollution:

• Grinding and classification: All contact parts adopt corundum, silicon carbide or high-purity quartz lining, no iron or stainless steel media;
• Flotation, leaching and rinsing tanks: Polypropylene (PP) or PTFE anti-corrosion lining;
• Thermal furnaces: Alumina refractory without heavy metal additives;
• Closed circulation system: Full wastewater recovery and acid regeneration to balance high purification standards with environmental compliance, suitable for large-scale continuous semiconductor-grade silica production.

Achieving semiconductor-grade high-purity silica cannot rely on a single purification unit operation, but requires a closed-loop integrated workflow of low-impurity ore selection → physical gangue removal via scrubbing, magnetic separation and flotation → thermal lattice activation → deep hot-press acid leaching → high-temperature chlorination refining → clean drying and packaging. Each stage targets different forms of impurities: flotation eliminates feldspar silicate gangue, acid leaching removes surface metal contaminants, and chlorination roasting erases deep lattice substitution ions. Supported by complete contamination-free quartz crushing, milling and classification equipment systems, this standardized process steadily produces 5N ultra-high-purity silica that satisfies wafer manufacturing, advanced etching and high-end semiconductor optical component demands.