High-Vacuum Pumps — Beyond Roughing

Estimated time: 20–25 minutes

Orient

Roughing pumps remove the bulk gas and reach pressures in the 10^-2 to 10^-3 mbar range. Many applications need lower pressures — thin-film coating, semiconductor processing, scientific research. This is where high-vacuum pumps take over.

High-vacuum pumps work on fundamentally different principles than roughing pumps. At low pressure, gas molecules are in molecular flow — they don't behave like a fluid that can be scooped and compressed. High-vacuum pumps must capture or redirect individual molecules.

Core Content: The Turbomolecular Pump

The turbomolecular pump (turbo pump) is the most common high-vacuum pump in modern vacuum systems. A turbomolecular pump has many stages of angled blades spinning at extreme speed — each stage catches individual gas molecules and flings them toward the exhaust, like a series of high-speed fans working in relay.

How it works: A series of angled rotor blades spins at extremely high speed (typically 20,000–90,000 RPM). Gas molecules that enter the inlet collide with the fast-moving blades.

The angled blades transfer momentum to the molecules, directing them preferentially toward the exhaust. The molecules are "batted" downward through multiple rotor-stator blade stages until they reach the exhaust, where a backing pump (roughing pump) removes them.

Key principle: The turbo pump doesn't compress gas in volumes like a roughing pump. Instead, it uses molecular-level momentum transfer.

Each blade collision gives the molecule a push in the exhaust direction. After many stages, the cumulative effect produces a large pressure difference between inlet and exhaust.

Performance:

• Operating range: ~10^-3 to 10^-10 mbar (requires backing pump to handle the exhaust)
• Very high compression ratio for heavy molecules (nitrogen, oxygen) — excellent at keeping air out
• Lower compression ratio for light molecules (hydrogen, helium) — these are harder to pump
• Clean, oil-free pumping (no oil contacts the vacuum side)
• Fast start-up (reaches full speed in minutes)

Why it needs a backing pump: The turbo pump's exhaust side is at a relatively high pressure (typically 0.1–1 mbar). A roughing pump (rotary vane, scroll, or diaphragm) maintains this exhaust at low enough pressure for the turbo to function. If the backing pump fails, the turbo pump overloads and cannot maintain vacuum.

Connection to M05 (valve sequencing): The roughing pump must bring the chamber to the crossover pressure (~0.1 mbar) before the gate valve to the turbo pump is opened. Exposing a spinning turbo pump to atmospheric pressure can mechanically damage the rotor blades.

Other High-Vacuum Pump Types (Awareness)

For completeness, two other high-vacuum pump types are worth knowing about at an awareness level:

Diffusion pump: A diffusion pump uses heated oil vapour that rises in a jet and carries gas molecules downward with it — the oil condenses and recirculates while the captured gas is pumped away by the backing pump.

Once the standard high-vacuum pump, now largely replaced by turbo pumps in new installations. Still found in many existing systems. Concern: the hot oil can backstream into the chamber if not properly trapped.

Cryopump: Traps gas molecules by freezing them onto a cold surface (typically 10–20 K, cooled by a cryocooler). Very high pumping speed for water vapour.

Concern: the trapped gas is stored, not removed — when the cryopump warms up (regeneration), all the trapped gas is released. Must be carefully managed.

The Pumping Sequence

In a multi-pump system, pumps work in stages:

``` PUMPING SEQUENCE (Conceptual)

Phase 1: ROUGHING (atmosphere → ~0.1 mbar) Pump: Roughing pump (rotary vane, scroll, or diaphragm) Flow regime: Viscous → transition Duration: Minutes

Phase 2: CROSSOVER (~0.1 mbar) Action: Close roughing valve, open gate valve to high-vac pump Why: Roughing pump has done its job; high-vac pump takes over

Phase 3: HIGH-VACUUM PUMPING (~0.1 → target pressure) Pump: High-vacuum pump (turbo, diffusion, or cryo) Backed by: Roughing pump (maintaining low exhaust pressure) Flow regime: Molecular Duration: Minutes to hours (depends on gas load and target) ```

Why this matters for diagnostics: If the system isn't reaching target pressure, the pumping sequence tells you where to look. Is the roughing pump reaching crossover pressure?

Is the high-vac pump running at full speed? Is the backing pump maintaining adequate exhaust pressure? Each stage has its own potential failure mode.

Visual: Turbomolecular Pump Cross-Section

The description above explains momentum transfer in words, but the multi-stage blade geometry is much easier to grasp visually. The following cross-section shows how gas molecules interact with the angled rotor and stator blades from inlet to exhaust.

Observe how the blade angle alternates between rotor and stator stages — this is what prevents molecules from drifting back upward. Also note the exhaust connection: without a functioning backing pump maintaining low pressure at the bottom, the turbo pump cannot sustain the pressure gradient across its blade stack.

Visual: Turbomolecular Pump Spin-Up Demonstration

A turbomolecular pump accelerating to operating speed is a striking demonstration of the engineering involved. The following video shows the characteristic spin-up sound, the transition from audible whine to ultrasonic, and the controller readout confirming stable operating speed.

If you watch the controller readout during spin-up, you will see the current draw spike during acceleration and then drop once the rotor reaches rated speed. This behaviour is normal — a turbo pump that never reaches rated speed, or whose current remains high, would indicate a mechanical problem worth escalating.

What You Can Now Do

By the end of this section, you can:

• Explain why roughing pumps can't reach high vacuum (they rely on viscous flow)
• Describe how a turbomolecular pump works (molecular momentum transfer)
• Explain why turbo pumps need backing pumps
• Describe the conceptual pumping sequence (rough → crossover → high-vac)
• Identify the pump's role within the sequence (which pump does what)