Recommendations for Using Flanges in CVD Furnaces

The flange connection in a CVD furnace is a key component for ensuring vacuum sealing and atmosphere control. Improper use is a major cause of poor vacuum performance, gas leakage, and even safety accidents.

Below is a detailed set of recommendations for selecting, installing, maintaining, and safely using flanges in CVD furnaces.

I. Core Flange Types and Selection Recommendations

Quick-Clamp Flange (QF/KF)

Structure:
Consists of two flanges with sealing edges, a centering ring, and a clamp. The clamp applies force to press the flange edges into the soft metal (usually nitrile rubber or Viton O-ring) inside the centering ring to achieve sealing.

Ventajas:
Easy to assemble and disassemble; very suitable for research scenarios that require frequent replacement of reaction chambers (such as quartz tubes). Low cost.

Disadvantages:
Vacuum level is usually limited to below high vacuum (e.g., 10^-7 mbar). Rubber O-rings have temperature limitations (typically <150°C) and outgassing issues.

Selection Recommendation:
Most suitable for CVD tube furnace inlet and outlet connections and low-vacuum pipeline connections. It is the standard interface between quartz reaction tubes and gas/vacuum lines.

Bolted Flange

Structure:
A metal sealing ring (usually oxygen-free copper) is clamped between two flanges. Uniform tightening of a circle of bolts creates high pressure, causing plastic deformation of the metal ring to fill micro-gaps and achieve sealing.

Ventajas:
Suitable for ultra-high vacuum (UHV, 10^-10 Pa and above), high-temperature resistance, and zero organic outgassing.

Disadvantages:
Very difficult to disassemble; the metal sealing ring must be replaced after each use, making the cost high.

Selection Recommendation:
Best for all-metal sealing UHV CVD systems, or connections between the furnace body and UHV components such as turbomolecular pumps.

VCR Metal Face-Seal Flange

Structure:
Two conical metal gasket surfaces (usually nickel or stainless steel) are joined and locked with a nut to create plastic deformation that achieves sealing.

Ventajas:
Zero leakage, high-temperature resistance, no outgassing, and easier to install than bolted flanges.

Disadvantages:
Gaskets must be replaced after every disassembly, leading to higher costs.

Selection Recommendation:
Ideal for key components in gas delivery systems such as valves, MFCs (mass flow controllers), and vacuum gauges, ensuring high purity and leak-free performance.

II. Installation Guide (Using Quick-Clamp Flanges as an Example)

Incorrect installation is the primary cause of leakage.

Cleanliness Is Critical

Before installation, use isopropyl alcohol (IPA) or lint-free wipes without acetone to thoroughly clean the sealing edges, centering ring, and O-ring.
Even tiny dust, hair, or old grease can cause leakage.

Inspect the sealing edges for dents or scratches. Light damage may be polished using a fine oilstone; severe damage requires replacement.

Correct Placement of the O-Ring

Ensure the O-ring is intact, with no aging cracks or permanent deformation.

Place the O-ring precisely in the groove of the centering ring, ensuring it lies flat without twisting or partial detachment.

Use Vacuum Grease (In Moderation!)

Apply a thin and even layer of high-vacuum grease on the O-ring.

Purpose:
Lubrication, improved sealing, and preventing the O-ring from sticking to quartz or metal at high temperatures.

Warning:
Do not use too much! Excess grease can be drawn into the vacuum system, contaminating pipelines or clogging valves.

Alignment and Tightening

Gently bring the two flanges together, ensuring natural alignment. Do not force them into position—misalignment will create stress and cause leakage.

Place the clamp and hand-tighten the screw to position it at the center.

Follow the “diagonal tightening sequence”:
Like tightening car wheels, tighten screws diagonally, in multiple rounds, gradually and evenly. Ensure uniform tightness at the end.
Never tighten one screw completely at once.

III. Routine Maintenance and Inspection

• Regular leak testing:
  Use alcohol/acetone spray or a helium leak detector to check all flange connections, especially before important experiments or if vacuum suddenly worsens.
• O-ring replacement:
  O-rings are consumables. Over time they may age, lose elasticity, or crack.
  Replace them regularly (e.g., every six months or based on usage frequency), instead of waiting for failure.
• Clean sealing surfaces:
  Clean the sealing surfaces after each disassembly to prevent buildup of residue.
• Spare parts:
  Keep common O-ring sizes, centering rings, and vacuum grease in stock.

IV. Safety Warnings and Common Mistakes

High-Temperature Warning

Rubber O-rings in quick-flange systems cannot withstand high temperatures.
Ensure the flange is located in the cold zone of the furnace (often achieved with water-cooling sleeves).
If the flange overheats, the O-ring may melt and fail, causing leakage or even fire hazards.

Pressure Warning

Never disassemble a flange while the system is under positive pressure—especially when internal pressure is unknown!

The correct procedure:
Purge with inert gas → pump down → vent back to atmospheric pressure → disassemble.

This prevents sudden release of toxic, flammable, or corrosive gases.

Common Mistakes

• Rough handling during installation, damaging flange edges or cracking quartz tubes.
• Forgetting the O-ring entirely, leading to impossible sealing.
• Using the wrong size or material O-ring—must use vacuum-grade nitrile or Viton O-rings.
• Clamp screws tightened too much or too little.
  Over-tightening damages O-rings and sealing edges; under-tightening results in leaks.

V. Summary

The flange is the “gateway” of a CVD furnace. If the gateway is not sealed properly, all precise temperature and gas control becomes meaningless.

Key points to remember:

• Choose according to application: frequent disassembly → QF/KF; UHV → bolted flange; high-purity gas lines → VCR.
• Clean installation: clean sealing surfaces and maintain a dust-free environment.
• Even tightening: diagonal tightening sequence is essential.
• Regular maintenance: O-rings are consumables—replace and leak-check frequently.
• Safety first: always monitor temperature and pressure risks.