How to Choose the Right Fused Quartz Grade for Semiconductor Applications

Fused quartz (also called fused silica) comes in several grades with very different purity, OH content, and cost characteristics. Selecting the wrong grade can mean unnecessary cost, contamination problems, or premature component failure. This guide explains the key differences and how to match the grade to your application.

The Three Grades

Grade 1 — Natural Fused Quartz

Natural fused quartz is produced by melting high-purity natural quartz crystal in an electric arc furnace.

Key properties:

SiO₂ purity: ≥ 99.99%
Total metallic impurities: < 20 ppm
OH content: < 30 ppm (dry grade)
UV transmission: 200 nm – 3.5 μm
Cost: Baseline

Best for: Standard thermal oxidation and diffusion furnace tubes, wafer boats, push rods, flanges, and baffles where process temperature is below 1150°C and trace metal contamination at the ppm level is acceptable.

Grade 2 — Synthetic Fused Silica

Synthetic fused silica is produced from ultra-pure silicon compounds (SiCl₄ or TEOS) via vapor-phase hydrolysis or oxidation — never coming into contact with natural mineral sources.

Key properties:

SiO₂ purity: ≥ 99.9999%
Total metallic impurities: < 50 ppb (1000× cleaner than Grade 1)
OH content: 800–1200 ppm (wet grade) or < 1 ppm (dry grade)
UV transmission: 150 nm – 3.5 μm (superior DUV transmission)
Cost: 3–5× Grade 1

Best for: CVD reactor tubes, LPCVD process tubes, and any component where trace metal contamination at the ppb level cannot be tolerated — typically 300 mm wafer processes and advanced gate oxide growth.

Grade 3 — Opaque (Milky) Fused Quartz

Opaque quartz is produced from lower-purity natural quartz with controlled porosity that scatters light and reflects infrared radiation.

Key properties:

SiO₂ purity: ≥ 99.9%
Appearance: Translucent milky white
IR reflectivity: > 90%
Max temperature: 1100°C
Cost: Lower than Grade 1

Best for: Thermal shields, outer furnace tubes (where transparency is not needed), boat holders, and any insulating component where IR reflectivity reduces heat loss.

The Decision Matrix

Your Requirement	Grade 1	Grade 2	Grade 3
Temperature < 1150°C	✓	✓	✓
Temperature 1150–1200°C	—	✓	—
Metal contamination < 1 ppm	✓	✓	—
Metal contamination < 1 ppb	—	✓	—
UV transparency (< 200 nm)	—	✓	—
IR reflectivity needed	—	—	✓
DI water / HF / acid exposure	✓	✓	✓
Optical application	—	✓	—
Cost-sensitive	✓	—	✓

Common Mistakes to Avoid

1. Using Grade 1 where Grade 2 is required

If your process involves gate oxide growth, ultra-thin film deposition, or any step where sodium or iron contamination at even the ppb level can affect device yield — Grade 1’s < 20 ppm metallic content is insufficient. The 3–5× price premium of Grade 2 is trivial compared to the yield loss from contaminated process components.

2. Specifying Grade 2 everywhere “to be safe”

Grade 2 is not always better — it simply has lower metallic content. For mechanical components (flanges, push rods, baffles) or thermal shields, the extra cost of Grade 2 provides no benefit. Use Grade 1 or Grade 3 as appropriate.

3. Ignoring OH content

High-OH (wet-grade) synthetic silica transmits well at near-IR wavelengths but poorly in the 2.7 μm water absorption band. If your application involves mid-IR spectroscopy or pyrometry near the 2.7 μm absorption, specify low-OH (dry-grade) synthetic silica or sapphire.

Summary

Grade 1 — workhorse of the semiconductor furnace. Use for everything that doesn’t require ppb-level purity.
Grade 2 — mandatory for advanced CVD processes, gate oxide tubes, and any 300 mm wafer line component.
Grade 3 — thermal management applications where transparency is not required.

When in doubt, our application engineers can review your process conditions and recommend the optimal grade. Contact us with your application details — we respond within one business day.

How to Choose the Right Fused Quartz Grade for Semiconductor Applications