Helium Leak Testing Services — to 1 × 10⁻⁹ mbar·l/s

A vacuum chamber is only as good as its worst weld. Helium mass-spectrometer testing is the only method that finds a leak small enough to matter at UHV, and it is the number that a semiconductor customer will ask for before accepting the part.

Why helium, and why a mass spectrometer

Helium is inert, small, and present in the atmosphere at only 5 ppm, so there is almost no background to confuse the reading. A mass spectrometer tuned to mass 4 detects it directly. Pressure-decay and bubble testing cannot resolve the 10⁻⁹ range at all — they will pass a chamber that then fails to reach UHV in the customer plant.

Test methods we use

For a sealed or closed chamber we use the helium spray (sniffer) method with the chamber under vacuum and the operator passing a calibrated helium jet over every weld, flange and feedthrough. For small parts and subassemblies we use the helium bombing method — pressurising with helium in a chamber, then testing the part in vacuum. For critical chambers we run an integral helium test with the chamber evacuated and helium applied externally, which is the most sensitive configuration.

What the leak rate number means

A leak rate is a gas flow, normally quoted in mbar·l/s. The practical bands are: above 1 × 10⁻⁵ mbar·l/s is a gross leak you can find by spraying; 1 × 10⁻⁵ to 1 × 10⁻⁸ is a fine leak needing a mass spectrometer; below 1 × 10⁻⁹ is the UHV range where the test itself has to be done carefully or background helium will mask the signal.

What you receive

A per-chamber test record giving the method used, the detection limit, the measured leak rate at each test point, the date and the operator, together with the helium concentration and dwell time. This is the document a customer audit asks for, and it is issued whether or not a leak was found.

Reference table

Leak detection methods and what they can actually resolve

Method	Minimum detectable leak rate	Best for	Cannot do
Bubble / immersion test	~1 × 10⁻⁴ mbar·l/s	Gross leaks, field checks	Fine leaks; leaves residue
Pressure decay	~1 × 10⁻³ mbar·l/s	Sealed volumes, production	Anything at vacuum-grade tightness
Helium sniffer / spray	1 × 10⁻⁷ mbar·l/s	Locating a leak on a large weldment	Quantifying UHV performance
Helium bombing	1 × 10⁻⁹ mbar·l/s	Small sealed parts, subassemblies	Large chambers
Integral helium test in vacuum	1 × 10⁻¹⁰ to 1 × 10⁻¹¹ mbar·l/s	UHV and semiconductor chambers	Locating the leak (only confirms it)

What we control in-house

304 / 316L stainless steel and 6061 / 5083 aluminium fabrication
Clean-environment TIG welding with full penetration and no micro-porosity
Helium mass-spectrometer leak testing to 1 × 10⁻⁹ mbar·l/s
In-house electropolishing and passivation — not subcontracted
Chamber bake-out to 150–400 °C for UHV outgassing control
Precision-machined ISO-KF, ISO-K and CF (ConFlat) flanges
Zeiss CMM verification to ±0.02 mm across large work envelopes
12 m gantry CNC machining — 12 m × 4 m × 2 m envelope
SEMI-referenced documentation and inspection protocols

Frequently asked questions

What leak rate is acceptable for a UHV chamber?

Most semiconductor equipment builders specify better than 1 × 10⁻⁹ mbar·l/s for a UHV chamber, and 1 × 10⁻¹⁰ for critical process chambers. We test to 1 × 10⁻⁹ as standard and can go lower on request.

How do you find a leak in a large weldment?

With the sniffer method: the chamber is evacuated, and a calibrated helium jet is swept along each weld and joint while the mass spectrometer watches mass 4. The signal rises as the jet passes the leak, which locates it to within a few centimetres.

What is the difference between helium leak testing and helium leak detection?

They describe the same physics. Leak detection is the general term for finding and quantifying a leak with a mass spectrometer; leak testing usually refers to the formal, documented procedure performed against a specification. We issue documentation for both.

Can a chamber pass helium testing and still fail to reach UHV?

Yes — that is exactly what outgassing does. A chamber can be leak-tight and still never pump down because water vapour desorbs from the internal surfaces. That is why electropolishing and bake-out are specified together with leak testing, not instead of it.

Do you test at the customer site?

Chamber testing is performed at our facility before shipment. For large assemblies that cannot be shipped fully dressed, we test the base chamber and provide the procedure for the final site test so the customer can repeat it on installation.