Pump-down vocabulary: from up to air to base pressure

After a weekend open for a target swap, the chamber door swings shut and the lead tech says, "Soft pump it. Don't touch the gate till the turbo's at speed, and if it stalls in the fives, it's wet." Every phrase stands for a procedure, and getting one wrong can cost a day or a pump. Here is what they mean, in the order a pump down actually happens.

Getting off atmosphere

A chamber that is up to air has been vented, ideally with dry nitrogen rather than room air, because nitrogen leaves far less water clinging to the walls. Some techs just say it is "at atmo."

Getting it back down starts with the roughing pump: the oil-sealed mech pump humming on the floor or, where oil is unwelcome, a scroll pump or another dry pump. To rough out is to pull the chamber down through rough vacuum to the point where the high vacuum pump can take over.

The first minute is the delicate part. Throw the roughing valve wide open on a big chamber and the air expands so fast it cools, its moisture condenses, and the viewport goes cloudy. That is fogging, and it leaves water and stirred-up particles on everything you just cleaned. Soft pumping avoids it: crack a small bypass valve, let the pressure fall gently for a few minutes, then open the main line.

A mech pump that has swallowed too many wet chambers grows milky oil, pale and creamy in the sight glass. Running with the gas ballast open helps it cough the water back out; an oil change fixes it for good.

The handoff

The most watched moment in the sequence is crossover, where the roughing valve closes and the high vacuum valve opens. Cross too early and the high vacuum pump gets a slug of gas it cannot handle. Rough too deep, below about a tenth of a millibar, and roughing pump oil can backstream into a clean chamber.

Above or below the chamber sits the stack: the high vacuum pump, its gate valve, any trap, and the foreline running to the backing pump. A turbo spins up against a closed gate, and nobody opens that gate until the controller says it is at speed. Switched off, it takes many minutes to spin down. Venting it right means a slow, controlled bleed once it has coasted to about half speed. Dumping air into it at full speed is how people crash a turbo: a rotor full of shredded blades and a very quiet afternoon in the lab.

Short, fat pipe beats long, skinny pipe: conductance limits how much gas reaches the pump, so the effective pumping speed at the chamber always trails the data sheet.

A lab with a cryopump on the stack hears different words. A cryo pumps by freezing gas onto cold panels, so it slowly fills up and needs a regen: warm it, purge what it caught, cool it back down. "The cryo's due for a regen" means the afternoon's runs are cancelled.

Reading the gauges

No single gauge covers the whole trip, so a chamber carries two or three. From atmosphere down, a convection gauge, or a plain Pirani gauge once the pressure drops below a few tens of millibar, reads pressure from how fast gas carries heat off a hot wire. Below about a thousandth of a millibar, the job passes to an ionization gauge: a hot filament Bayard-Alpert gauge or a cold cathode gauge with no filament to burn out. A gauge stuck at the end of its scale is pegged, and a Pirani pegged at the bottom is not the same thing as good vacuum.

Units depend on whose lab you are standing in. A US fab talks in Torr, a European lab in millibar, a furnace shop in microns, and the calibration certificate in pascals. Nobody reads the exponent out in full. "Low sixes" means a few times ten to the minus six; "stuck in the fives" means the pump-down has stalled a full order of magnitude short.

When it stalls

Every healthy chamber has a pump-down curve, and the people who run it know it by heart: so many minutes to crossover, so many hours to base pressure. When the curve flattens early, the first suspect is water. A chamber left open on a humid day is wet: its walls are slowly giving up adsorbed water and the pressure crawls. That slow release of gas from walls and materials is outgassing, and in a clean, tight system it sets the floor.

The cure for a wet chamber is time or heat. A bake-out wraps the chamber in heater tape and holds it hot under pumping for a day or more, dropping the base pressure by one to three orders of magnitude. The cure for a dirty chamber is cleaning, and the classic culprit is a fingerprint: one bare-handed touch on a vacuum-side surface outgasses for weeks. Gloves are not a suggestion.

Then there is the stall that looks exactly like a leak and is not one. A virtual leak is gas trapped in a blind tapped hole under a screw, or between two surfaces clamped face to face, bleeding out slowly from the inside. No amount of helium sprayed on the outside will find it, which is why vacuum screws are vented and why the phrase makes old hands wince.

To tell real from virtual, isolate the chamber and run a rate of rise. Outgassing or a virtual leak gives a curve that bends over and flattens. A real leak climbs in a straight line, and a chamber that shows one will not hold vacuum until somebody finds it.

Chasing the leak

When the straight line shows up, someone gets told to chase a leak. A gross leak announces itself early: the chamber never reaches crossover and the roughing pump gurgles. Smaller ones need a helium leak detector, a mass spectrometer tuned to one gas, hooked to the foreline.

The method is the helium spray test: a short puff of helium at one joint at a time, starting at the top because helium rises. Spray too much and you get hang-up, helium soaking the system and holding the signal high so the next joint cannot be told from the last. "Easy on the helium" is the first thing a newcomer hears. For a quick yes or no on one flange, there is bagging: wrap it in a plastic bag full of helium and see if anything gets through.

Most leaks live at the joints. A KF flange seals with a clamp and an O-ring, so one hair across the seal is enough. A CF flange seals metal to metal, a knife edge biting into a copper gasket, and those gaskets are single use. Reusing one to save a few dollars is how a Monday pump-down becomes a Tuesday leak hunt.

To see what is actually in there, ask the RGA. A tall peak at mass 18 says water. Peaks at 28 and 32 in roughly a four to one ratio say air, and the helium bottle comes out.

Five words to learn first

• Crossover: the handoff from roughing to high vacuum, and the moment everyone watches.
• At speed: the turbo is ready and the gate can open. Not before.
• Wet: water on the walls, the usual reason a pump-down drags.
• Virtual leak: trapped gas that behaves like a leak and ignores helium.
• Rate of rise: the test that tells a leak from outgassing.

With those five, "it crossed fine but it's stuck in the fives, probably wet, give it a bake" stops being noise and becomes a plan for the rest of the day.