Feedthroughs — Connecting Inside to Outside

Estimated time: 15–20 minutes

Orient

A vacuum chamber is sealed from atmosphere. But you often need to get things into or out of the chamber without breaking vacuum: electrical power for heaters, signals from sensors, rotary motion for sample manipulation, cooling water for temperature control.

Feedthroughs are the components that bridge this gap — like passing a wire through the hull of a submarine, you need a special seal at the penetration point, or the vacuum is lost. They pass something (electricity, motion, fluid) through the chamber wall while maintaining the vacuum seal.

Core Content: Types of Feedthroughs

Electrical Feedthroughs

Purpose: Pass electrical conductors through the chamber wall for power delivery (heaters, deposition sources), signal measurement (thermocouples, pressure sensors inside the chamber), or control (motors, actuators).

Construction: A metal or ceramic insulator holds one or more conductor pins. The insulator is sealed to the flange (usually CF or KF) and the conductors pass through without electrical contact to the chamber wall.

Vacuum concern: The seal between the conductor, insulator, and flange is a potential leak path. Ceramic-to-metal seals (brazed) are very reliable.

Glass-to-metal seals are also common. Epoxy seals are used for rough vacuum but outgas under high vacuum.

Rotary/Linear Motion Feedthroughs

Purpose: Transfer mechanical motion through the chamber wall — rotating a sample stage, moving a shutter, translating a deposition source.

Construction: A shaft passes through the chamber wall via a dynamic seal. The seal must allow the shaft to move while preventing gas from entering.

Types:

• Magnetically coupled: A magnetically coupled feedthrough transfers rotary motion through the chamber wall without any physical penetration — an external magnet drives an internal magnet through solid metal, so there is zero leak path. The wall is sealed completely.
• Bellows-sealed: Bellows are flexible metal tubes with accordion-like folds — they allow mechanical movement while keeping the seal intact. A bellows-sealed feedthrough uses this flexibility to permit linear motion while maintaining a static seal at both ends.
• Ferrofluidic: A ferrofluidic seal uses a magnetic liquid that clings to a rotating shaft — allowing smooth rotation while blocking gas passage, like a liquid film that never breaks. Low leak rate, smooth rotation, but limited temperature range.
• Elastomer shaft seal: A rubber seal around the shaft. Simplest and cheapest, but highest leak rate. Only suitable for rough vacuum.

Vacuum concern: Dynamic seals (where parts move relative to each other) are inherently harder to seal than static seals (where nothing moves). Motion feedthroughs are often the weakest point in a high-vacuum system.

Fluid Feedthroughs

Purpose: Pass cooling water, process gas, or cryogenic fluids through the chamber wall.

Construction: Tubes welded or brazed into a flange. The tube-to-flange joint is the seal point.

Vacuum concern: Welded joints are generally very reliable. But if a cooling water tube develops a pinhole leak inside the chamber, the result is catastrophic — water flooding into a vacuum system creates massive contamination.

Why Feedthroughs Are Potential Leak Paths

Every feedthrough is a penetration through the vacuum boundary. Each penetration has at least one seal. Each seal can fail.

In complex systems with dozens of feedthroughs, the cumulative leak risk from all these penetrations can dominate the system's leak budget.

Diagnostic relevance: When investigating a leak on a complex system, feedthroughs should be among the first locations checked. A leak at a feedthrough often shows up as a constant rate of rise (real leak) localised to one area of the chamber.

Visualising Feedthrough Seal Points

The three feedthrough types each bridge the vacuum boundary in a different way. In the diagram below, seal points are highlighted in red. Count the seal points in each design and note which type has the fewest — this directly relates to leak risk.

The electrical feedthrough has the most seal points (three), which is one reason electrical penetrations are common suspects during leak investigations. The fluid feedthrough's welded joints are typically the most reliable, but a pinhole failure in a cooling-water tube inside the chamber would introduce catastrophic contamination — reliability of the seal does not eliminate the consequence of its failure.

What You Can Now Do

By the end of this section, you can:

• Describe what feedthroughs are and why they're necessary
• Identify electrical, motion, and fluid feedthrough types
• Explain why feedthroughs represent potential leak paths
• Consider feedthrough locations when diagnosing leaks on complex systems
• Explain why magnetically coupled feedthroughs are preferred for high-vacuum applications (zero penetration)