- ▸Czochralski (CZ) silicon crystal furnaces operate at 1400℃ molten silicon and 10⁻⁵ Pa high vacuum; any trace metallic impurities contaminate the silicon crystal, causing resistivity deviation
- ▸Carbon graphite seals are used at the quartz crucible shaft, seed crystal shaft, and furnace bottom flange — material purity requires total metallic impurities < 10 ppm (iron, copper, nickel each < 1 ppm)
- ▸High-purity isostatic graphite (e.g., IG-43) must be used, halogen-purified at 2800℃ to remove metallic impurities
- ▸Structural design uses metal bellows seal + water cooling jacket, controlling seal temperature below 200℃ to avoid high-temperature oxidation
- ▸Huahao Sealing achieved 200+ furnace cycles service life in a 12-inch silicon crystal furnace, with metallic impurity precipitation meeting SEMI standards
Czochralski (CZ) is the mainstream process for growing silicon crystals 300mm (12-inch) and below. CZ furnaces operate at 1400℃ high temperature and 10⁻⁵ Pa high vacuum; any component releasing metallic impurities will be carried by Ar flow to the molten silicon surface, contaminating the crystal. Carbon graphite seals as the furnace dynamic sealing key component directly determine silicon crystal resistivity uniformity.
1. Service Characteristics
CZ furnace chamber temperature 1400-1420℃ (molten silicon melting point 1414℃), vacuum 10⁻⁵ Pa, Ar protective gas maintaining slight positive pressure. The quartz crucible shaft and seed crystal shaft require rotational and pulling motion, requiring dynamic seals. If seals release iron, copper, nickel or other metallic impurities, they enter molten silicon forming recombination centers, reducing minority carrier lifetime and shifting resistivity from target.
2. Material Purity Requirements
2.1 High-Purity Isostatic Graphite
High-purity isostatic graphite (e.g., IG-43) must be used, halogen-purified at 2800℃ (Cl₂+F₂ mixed gas), reducing total metallic impurities below 10 ppm, with iron, copper, nickel each below 1 ppm. Conventional mechanical graphite (100-500 ppm metallic impurities) is strictly prohibited.
2.2 Purity Testing
Each batch requires GDMS (glow discharge mass spectrometry) testing of 30+ metallic elements, complying with SEMI PV17 standard. Huahao Sealing provides GDMS test reports for each batch.
3. Structural Design
3.1 Bellows Seal + Water Cooling
The quartz crucible shaft and seed crystal shaft use metal bellows seals (Inconel 625 material), with a water cooling jacket outside the bellows fed with 0.3-0.5 MPa cooling water, controlling seal temperature below 200℃. Without water cooling, seal temperature can reach 600-800℃, and graphite will oxidize to CO₂, destroying vacuum.
3.2 Furnace Bottom Flange Seal
The furnace bottom flange uses O-ring + fastener sealing; O-rings use fluororubber FKM (temperature rating 200℃, matching water-cooled flange temperature). O-rings are replaced after each furnace cycle to avoid thermal aging cumulative leakage.
4. Failure Modes
Main failures: graphite ring oxidation pulverization (40%, due to water cooling failure causing high-temperature oxidation), bellows fatigue cracking (25%), O-ring thermal aging (20%), metallic impurity exceeding standard contaminating crystal (15%).
5. Huahao Sealing Case Study
A semiconductor materials company's 12-inch silicon crystal furnace (crucible diameter 22 inches, charge weight 150 kg) originally used an imported brand graphite seal averaging 120 furnace cycles, once causing iron impurity to exceed standard and the entire crystal batch resistivity to deviate, losing about 500000 CNY. After switching to Huahao Sealing's IG-43 high-purity graphite + Inconel 625 bellows + water cooling jacket solution in 2022, service life reached 200+ furnace cycles, with GDMS-tested total metallic impurities stably below 5 ppm, and no further impurity contamination incidents.
Selection core: purity grade determines material grade (choose IG-43 + halogen purification), temperature control determines structural design (bellows + water cooling), seal reliability determines process stability.