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LEAK DETECTION

Introduction

Ideal vacuum chamber

• What would an IDEAL vacuum chamber look like?– Stable wall surfaces– NO leaks– only source of gas is the gas delivery system(process in

the chamber)

• In a REAL vacuum system, there can be other sources of gas besides the gas delivery system.

• What problems can these other gas sources cause?– significantly increase the pump down time.– contaminants in the system.

Leak in vacuum systems

• Loading the unwanted gasses to the vacuum system and then pressure increasing in the system is known as leak.

• Leak a big problems in vacuum techniques and leak detection is a big challenge .

• In practice there are two main aspect of leak detection:

(1) Testing the components and

(2) Testing completed systems

Types of leaks

• There are 2 kinds of leak in the vacuum system

(1): Real leak

When the gasses passes from atmosphere into the vacuum system

(2) Virtual leak

When the gasses or vapors load or deliver to the system by

(a) Desorption the gasses from chamber`s wall and components

(b) Trapped spaces

Gas Sources

• The three main ways that undesired gasses can get into a vacuum system and the pressures increases:– Leaks( Vertual leak and Real leak)– Outgassing– Backstreaming

Gas Sources• Leaks (Real)

a crack or hole in the vacuum system drawing molecules from the surrounding atmosphere into the work chamber

• dirty/worn/damaged/improperly-seated seals (90%+)

• cracked or porous welds• cracks or flaws in the

chamber walls• loose components such as

feed-throughts• the migration of gas through

the wall material or O-ring

Gas Sources• Leaks (Virtual)

– apparent leaks caused by release of trapped gas inside the system

• threaded fastener cavity• flange connections and contacts

– Vented screws are used to eliminate virtual leaks.

Gas Sources• Outgassing: the natural evolution of

species inside the chamber, at low pressure, contributing to the gas load– Sublimation of solid chamber surfaces– Desorption from the walls of physically

adsorbed molecules– Out-Diffusion of gas that has been

absorbed into the grain boundaries of the metal

– Vaporization of liquids or solids in the chamber with relatively high vapor pressure

Gas Sources• When is outgassing likely to occur?

– after chamber clean– after being open to atmosphere

• The polar nature of water molecules causes them to persistently adsorb to chamber surfaces in many layers due to hydrogen bonding. These layers can take a very long time to outgas.

• How can outgassing be reduced?– Wear gloves during cleaning

• fingerprints are disastrous to a vacuum system!• ONE fingerprint can take several hours to outgas in a

high vacuum system!– Thoroughly remove all liquids and residues before closing– use correct materials for seals, cleaning, etc.

• How does vapor pressure affect outgassing?

Gas Sources

• Backstreaming:– an ideal pump only removes molecules and does not

give any back– real pumps “repeat” some gasses back into the

system• oil from diffusion pumps and rotary vane pumps

– oil-based pumping systems are designed with anti-suckback valves, and cold traps to minimize backstreaming

– this is the primary reason for the gradual replacement of oil-based pumps with “dry” pumps

Gas Sources

Outgassing

Leaks

Virtual

Real

Backstreaming

DiffusionPermeation

Trapped Volumes

Vented Screw

Double O ring sealed shafts

Atmosphere(760 torr)

Vacuum

Real leak and virtual leak curves

.

tt t

P

Real leak Virtual leakLeak present in vacuum system

Methods of leak detection

• There is two methods for detection of the leak: searching gas is inside (I) or out side (O) the system

Searching gasleak

Chamber

Searching gas

P in>p out

Pin<p out

gauge

P in

P out

(I)(O)

dete

ctor

Soap bubbles

.

He or Air

Soap bubbles (I)

It is possible to obtain higher sensitivities using liquids of lower surface tension

leak

Discharge tube Test

.

pump

chamber

Discharge Tube

leak

Day spray Test

pump

Dye spray

leak

Spark Test

.

pump

spark

Probe

..

Thermal cathode

Plalladium plaet

Vacuum chamber

Thermal cathode

- +

_

+

Palladium barrier leak detector

H2

Halogen detector

• .

Vacuum chamber

filament

Platinum plate

+

_

+

_

Halogen gasses

Leak covering by tape

• Leak covering (O)

leak

tape

A suspected leak can be covered over with a tape .

If a leak present then a fall of pressure will be observed.

leakH

L

Using thermal conductivity gauge as a leak

detector (trace gas detection)

As the thermal conductivity varies for different gases, an apparent change in the pressure is seen when different gasses is directed over the leak

Useful gas: Helium, Oxygen, Argon, CO2

Thermal conductivity gages

Carbon dioxide

Using Ionisation gauge and Ion pump as a leak detector

• The ion gauge current and ion pump current dependent on the gas properties.

• Chang in the ion current can be seen by using different tape of gasses or liquids over the suspected leak.

• Useful safe gasses are: He, Co2, Ar

• Useful safe liquids are Aceton, isopropyle alchohol

Using Mass spectrometers as a leak detector

Mass spectrometers is a excellent leak detector.

He

leak

chamber

Mass spectrometer

Mass spectrometer film

He

He source

Helium Leak Methods Sniffing Test (O)

Helium Leak Methods Sniffing Test (I)

HeHe

SIMS 180TD+

ALCATELChamber

Leak rate

• Leak rate is measured by standard cubic centimeter/sec (std. cc/sec).

• Although the leak rate in vacuum system is very low, but it is a big challenge in the vacuum techniques.

Leak rate of 1 x 10-1 std cc/sec

Leak rate of 1 x 10-3 std cc/sec

Leak Rates over Time

LEAK RATES

10 -1 STD CC/SEC --- 1 CC/10 SEC10 -3 STD CC/SEC --- 3 CC/HOUR10 -5 STD CC/SEC --- 1 CC/DAY10 -6 STD CC/SEC --- 1 CC/2 WEEKS10 -7 STD CC/SEC --- 3 CC/YEAR10 -9 STD CC/SEC --- 1 CC/30 YEARS

Why Helium is used for leak detection

HELIUM

• Helium is very light and small

• Low concentration in air (0.0005%)

• Permits dynamic testing

• Permits non-destructive testing

• Helium is safe

Why Helium?• Low Background (5-20 PPM)Low Background (5-20 PPM)

• InertInert

• SafeSafe

• LightLight

• Low CostLow Cost

?

He N

O

FNe

Cl

Ar

Kr

Xe

H

Why A Helium Leak Detector?

SIMS 180TD+

• High SensitivityHigh Sensitivity

• Wide RangeWide Range

• ReliableReliable

• QuantitativeQuantitative

• Dynamic TestingDynamic Testing

Helium Leak Testing: Vacuum Mode

• The “hard” vacuum test procedure measures the package’s increasing leak rate arising from all sources within the sample.

• This method enables the barrier performance of seals to be quantified.

Helium Test Methods “Hard” Vacuum

Helium Pre-Filled Samples

SIMS180TD+

1E-2 mbar

This method is good for vacuum equipments leak testing

Helium Leak Testing: Sniffer Mode

• Individual packages containing helium can be checked for leaks using a pencil-sized vacuum collector (sniffer) attached to a helium leak detector.

• The rate of gas sampling (flow rate) is set at 1cc/second; enabling a (ambient pressure) leak rate to be measured in ppm, or in cc/second.

• The sniffer mode test procedure is “site-specific” in that the probe collects helium escaping within ~ 1/8” of the leak site.

Helium Leak Methods Sniffing Test

HeHe100%

HeSIMS 180TD+

ALCATEL

This method is good for package`s leaks

Answer this question before taking any

further actions for solving leak problems 1-Has the pressure in the system changes since yesterday?

2-can any section of the vacuum system be isolated?

3-Has the gauge calibration been checked?

4-Are there any vapors present?

5-Has a real leak been established even though not found?

6-Can you tolerate the leak that has been established?

7-Have you used some different type of cleaning material after the last exposure to the atmosphere?

8-Have you add to or modified the interior of the vacuum system?

9-Is there any evidence of an air leak from an RGA spectrum with peaks corresponding to masses 14, 28, 32?