Cleanroom Tech Talks episode 1: What Is Vacuum and Which Types of Vacuum Pumps are There?
Welcome to the first article in our Cleanroom Tech Talks series! I’m Chris McNally, and I’ll be guiding you through some of the fundamentals of vacuum technology as it applies to cleanrooms and vacuum chambers. In this episode, we’ll start at the very beginning: what vacuum actually is, how we measure it, and the different types of pumps and techniques used to create it.
What Is a Vacuum?
At its simplest, a vacuum is a space where all “matter”, like air molecules, has been removed. On Earth, we are constantly surrounded by molecules in our atmosphere. When we remove those molecules from a given volume, we create a vacuum.
But a perfect vacuum, a space with absolutely zero molecules, does not exist on Earth. The closest we can get is around 30,000 molecules per square centimetre. That may sound like a lot, but compared to atmospheric pressure, it’s almost nothing.
In our Semiconductor Chamber Solutions division, we aim to achieve controlled vacuum inside a chamber. Gas molecules inside the chamber bounce around and hit the chamber walls, exerting force. By measuring the force of those collisions, we measure pressure, which is the same as measuring vacuum.
How Do We Measure Vacuum Pressure?
Vacuum pressure can be measured with different units; for this article we will use Torr.
760 Torr = Earth’s atmospheric pressure at sea level.
10⁻⁷ to 10⁻⁸ Torr = Typical semiconductor chamber pressure.
To put that into perspective, the pressure inside a semiconductor chamber is similar to the Thermosphere, where the International Space Station orbits.
The Two Main Types of Vacuum Pumps
To create vacuum, we use vacuum pumps, which fall into two main categories:
Transfer pumps – These physically move molecules out of the chamber.
Kinetic pumps: Add kinetic energy to molecules to move them (e.g., turbomolecular pumps, diffusion pumps).
Positive displacement pumps: Grab, compress, and exhaust molecules.
Entrapment pumps – These trap and hold molecules inside the system.
Cryogenic pumps: Use extremely cold surfaces to freeze and capture molecules.
Sputter ion pumps: Use plasma and magnetic fields to trap molecules as material.
In-chamber trapping: Cold surfaces inside the chamber (e.g., Edwards CTI-Cryogenics refrigerators, Polycold cryochillers) that directly capture molecules.
These different pumps achieve different target pressures in a given vacuum process, which varies by customer, segment, and market.
Target Pressure: Viscous Flow vs. Molecular Flow
The behaviour of molecules inside a vacuum chamber depends on how many are left.
Viscous Flow
Imagine you are in Grand Central Station in New York City during rush hour, there are thousands of people walking around. You are at one end of the station, you close your eyes and walk as fast as you can across the building to touch the wall on the other side. Doing so, you are going to collide with people and bounce off people trying to get across.
That’s viscous flow: gas molecules collide more often with each other than with the chamber walls.
Molecular Flow
Now imagine the same walk at 3 a.m. The station is nearly empty, so you can cross without bumping into anyone. That’s molecular flow: gas molecules are more likely to travel across the chamber, hit a wall, and continue their journey randomly bouncing around the chamber’s volume.
Since no single pump can work efficiently in both regimes, we use a stacked pump system:
First a primary pump or a backing pump, also called a rough pump, brings the system from atmosphere down to about 10⁻³ Torr (viscous flow). Then a high vacuum pump takes over, pushing the chamber into molecular flow, achieving the base pressure required for the process at hand.
What are Kinetic Pumps?
Kinetic pumps are a type of transfer pump that apply kinetic energy to molecules to create directional flow. We’ll look closer at two types of kinetic pumps:
Turbomolecular pumps act like turbines spinning at incredible speeds, up to 670 miles per hour. As it spins, a portion of the molecules are deflected down into the pump. The molecules continue downward through more and more purposeful deflections until ultimately being exhausted out of the system.
Diffusion pumps use high-velocity oil jets instead. The molecules are dragged down to a pool by the oil before being exhausted.
What are Entrapment Pumps?
Unlike transfer pumps, entrapment pumps don’t move molecules out, they trap them within the system.
Cryogenic pumps use cold surfaces to trap molecules and create vacuum. These molecules are held by the pump until it’s full, at which point the frozen molecules are thawed and flushed out of the system.
Sputter ion pumps trap molecules using plasma and magnetic fields; a plasma is controlled in a magnetic field that knocks electrons off to create ions. These positive ions are then attracted to a cathode and create sputtering, basically changing into materials. These pumps trap molecules as materials to create vacuum.
In-chamber trapping is not really a vacuum pump but rather a technique. It uses a dedicated gas trap that is placed directly inside the vacuum chamber. Examples include Gifford-McMahon refrigerators, such as Edwards CTI-Cryogenics refrigerators, and Polycold cryochillers. These systems insert a cold surface into the chamber, capturing molecules more effectively than a pump connected outside the chamber, since they are not limited by conductance losses.
Edwards CTI-Cryogenics cold traps (based on Gifford-McMahon refrigerators) provide extremely cold surfaces that can trap water vapour and other gases, like xenon.
Polycold cryochillers use Meissner coils to circulate cold refrigerant, cooling the coils and trapping water molecules. While they do not reach the ultra-low temperatures of CTI-Cryogenics cold traps, they are highly effective in applications that require very fast water pumping speeds.
Key Takeaways
Vacuum is about removing molecules to reduce pressure inside a chamber. The required pressure, or vacuum level, depends on the process at hand.
Different pumps serve different purposes, no one pump can do it all. We need to stack our vacuum pumps: a primary pump takes the pressure down to viscous flow, dedicated high vacuum pumping solutions can then pump down to molecular flow regime.
This wraps up the first episode of Cleanroom Tech Talks. In the next one, I’ll dive deeper into different vacuum technologies and how each type of pump supports specific cleanroom applications.