CF (ConFlat) flanges are the standard connection hardware for high-vacuum and ultra-high-vacuum systems in semiconductor manufacturing. The flange itself is a relatively simple turned part, but the knife-edge sealing geometry, surface finish requirements, and cleanliness standards make it a demanding machining job. This case study covers material selection, machining strategy, quality testing, and cost drivers for CF flanges produced from 316L stainless steel.
| Item | Spec |
|---|---|
| Application | Semiconductor vacuum chamber connection (CF flange) |
| Primary Material | 316L stainless steel |
| Gasket Variant Material | OFHC copper (C10100) for gasket mating surface |
| Standard | CF flange per ISO 3669 / CF standard |
| Sealing Surface Flatness | ≤ 0.025 mm |
| Surface Roughness | Ra ≤ 0.8 μm (post-electropolish) |
| Vacuum Leak Rate | ≤ 1×10−&sup9; Pa·m³/s |
| Annual Volume | 50 – 2,000 pcs |
| Phase | Duration |
|---|---|
| Prototype (first article) | 5–7 days |
| Production order | 3–4 weeks |
| Helium leak testing | Included in lead time |
| Electropolishing | Included in lead time |
Material choice for vacuum flanges is governed by two requirements: the material must not outgas significantly under vacuum (which would contaminate the chamber), and it must resist corrosion in the semiconductor processing environment. Several materials are used across the vacuum industry, but for CF flanges on semiconductor equipment, the options narrow considerably.
| Material | Vacuum Compatibility | Outgassing Rate | Corrosion Resistance | Machinability | Cost |
|---|---|---|---|---|---|
| 316L Stainless Steel | Excellent | Very low (<1×10−¹&sup0; Torr·L/s·cm²) | Excellent — resists halogen and acid exposure | Good — standard tooling, moderate speeds | Moderate — 1.5–2x mild steel |
| 304L Stainless Steel | Good | Low | Good — adequate for general vacuum, less resistant to halogens | Slightly better than 316L (lower work hardening) | Lower than 316L by ~10–15% |
| OFHC Copper (C10100) | Good | Low — requires bake-out | Good in inert environments; oxidizes in air | Good — soft, gummy chips | Moderate — comparable to 316L |
| 6061-T6 Aluminum | Poor for UHV — porous oxide layer | Moderate — oxide layer traps moisture | Adequate for rough vacuum only | Excellent — easy to machine | Low — 0.5x stainless steel |
While 316L is a common material, its role in semiconductor vacuum flanges involves specific requirements that make it the standard choice.
At high vacuum (below 10−&sup6; Torr), any gas molecules released from internal surfaces become a significant fraction of the residual gas load. 316L, when properly cleaned and baked, achieves outgassing rates below 1×10−¹&sup0; Torr·L/s·cm². This is low enough to allow the chamber to reach the 10−&sup9; Torr range required for processes like physical vapor deposition (PVD) and chemical vapor deposition (CVD). Aluminum and other metals with porous oxide layers cannot reach these levels without special surface treatments.
Standard 316 stainless steel contains up to 0.08% carbon. During welding, chromium carbides precipitate at grain boundaries in the heat-affected zone, depleting the surrounding matrix of chromium and reducing corrosion resistance. This is called sensitization. 316L limits carbon to 0.03% max, which effectively prevents sensitization in most welding scenarios. Since CF flanges are typically TIG-welded to the chamber body, this distinction matters.
The CF sealing mechanism relies on a sharp knife-edge (typically 20° included angle) machined into the flange face. This knife-edge bites into a soft OFHC copper gasket when the bolts are torqued, creating a metal-to-metal seal. 316L can be precision-turned to form this geometry without chipping or excessive tool wear, provided correct feeds, speeds, and tool geometry are used.
The OFHC copper gasket used in CF connections is softer than the 316L knife-edge. When the bolts are torqued, the knife-edge deforms the copper gasket plastically, filling microscopic surface irregularities and forming a hermetic seal. The hardness differential between 316L (~150 HV) and OFHC copper (~40–50 HV) is well-suited for this mechanism.
The CF flange machining sequence follows a logical order, with the most critical operation — the knife-edge precision turning — performed last before cleaning and electropolishing.
Every CF flange undergoes a series of tests before shipment. The helium leak test is the definitive acceptance criterion — if the flange leaks, it is scrapped or reworked.
| Test | Method | Acceptance Criteria | Frequency |
|---|---|---|---|
| Helium leak test | Mass spectrometer leak detector (MSLD), external spray method | ≤ 1×10−&sup9; Pa·m³/s | 100% of parts |
| Surface roughness | Contact profilometer on sealing face and knife-edge | Ra ≤ 0.8 μm (post-electropolish) | First article + 5 pcs/lot |
| Knife-edge angle | Optical comparator or vision system | 20° ± 1° | First article + 3 pcs/lot |
| Flatness (sealing surface) | Optical flat with monochromatic light source | ≤ 0.025 mm across full sealing surface | First article + 5 pcs/lot |
| Bolt hole position | CMM (coordinate measuring machine) | Position ±0.05 mm relative to center | First article + 2 pcs/lot |
| Visual inspection | Bare eye + 10x magnifier on sealing surface | No scratches, dents, contamination, or tool marks on sealing face | 100% of parts |
CF flange pricing is higher than a typical turned stainless steel part of similar size. The premium comes from tight tolerances, post-processing, and testing requirements.
| Cost Driver | % of Unit Cost | Notes |
|---|---|---|
| Raw material (316L) | 20–25% | 316L bar and forgings are moderately priced. Large-diameter blanks for CF200+ flanges carry a premium. Material utilization is 40–60% due to the flange's geometry. |
| CNC machining | 25–35% | Turning and drilling are straightforward. The knife-edge finishing pass requires slow feed rates and frequent tool changes. Setup time for bolt hole drilling adds cost at lower volumes. |
| Electropolishing | 10–15% | Subcontracted to a specialty surface finishing shop. Batch processing reduces per-part cost. Fixturing for large flanges adds handling time. |
| Helium leak testing | 10–15% | Mass spectrometer equipment is expensive ($30K–80K). Each test takes 10–30 minutes per part including setup. 100% testing is required for vacuum applications. |
| Clean packaging | 5–10% | Vacuum-sealed bags with desiccant, clean room handling, no-touch packaging. Some customers require Class 100 (ISO 5) clean bagging. |
| Documentation and certification | 5–10% | Material certificates (MTR), dimensional reports, leak test certificates, electropolish certificates. Semiconductor customers often require full traceability. |
| Phase | Duration | Deliverable |
|---|---|---|
| DFM review and quotation | 2–3 days | DFM notes on drawing, material sourcing plan, formal quote |
| Material procurement | 3–5 days (stock) / 4–6 weeks (mill order) | 316L bar or forging with MTR |
| First-article machining | 3–5 days | 5–10 FAI parts, in-process dimensional reports |
| Electropolishing (first article) | 2–3 days | Electropolished parts with surface roughness verification |
| Helium leak testing (first article) | 1–2 days | Leak test certificates, first-article inspection report |
| Customer FAI approval | 3–5 days | Customer sign-off on first article |
| Production machining + electropolish + leak test | 2–3 weeks | Production quantity with full documentation |
| Total (DFM to delivery, stock material) | 3–5 weeks | Shipment with certificates |
Send us your drawing — we'll return a DFM review and quote within 3 business days.
Get a Quote →