Practical aspects of high vacuum leak detection

Practical received

aspects

10 February

Harald

1970; accepted

Westgaard,

of high

27 February

Veeco Instruments

vacuum

Ltd, Marshlands

Road.,

Introduction

The Mass Spectrometer 1~1s dcvcloped in 1910. Its major uses in today’s high vacuum technology are those of leak detection and residual gas analysis. By far most of the mass spectrometers in use in the high vacuum field today are used for leak detection. A mass spectrometer leak detector is a partial pressure gauge permanently tuned to one gas, usually helium, complete with its high vacuum system. A typical helium mass spectrometer leak detector is capable of measuring a partial pressure of helium in the order of 1O-1’ torr. Since the mass spectrometer measures the partial pressure of helium in the spectrometer head, the accuracy of the instrument in measuring leak rates depends on maintaining a constant pumping speed both of the instrument’s own high vacuum pumping system and of any external pumping system used during the test. This can only be obtained if the vacuum system is operating properly and kept clean. The most common causes of instability or helium background are: (a) Contamination in the vacuum system trapping and releasing helium. (b) Backstreaming and variation in pumping speed. (c) Ion scattering in the mass spectrometer head due to high operating pressure (at 2 ;,: IO-,’ torr mean free path of air is approximately 10 inches). (d) Leak in the leak detector itself (meter deflection due to residual helium in air at I \: lO-J torr may be IO-100 times the minimum detectable signal). The smallest leak detectable by normal operation of a typical leak detector is I ?.,10-l” atmcc/sec. This can be improved by approximately 2 decades by accumulation techniques at sacrifice of clean-up time. of large

systems

The throughput of a typical leak detector at its maximum operating pressure in the order of 3-5 :< 1O-3 torr litres per second, or 3-5 lusecs. An unbaked chamber of a few cubic meters will have a desorption load many times that value. This necessitates auxiliary pumping, taking only a sample of the gas load into the leak detector, thereby reducing the test sensitivity. The question of test sensitivity with the leak detector in the foreline of a diffusion pump versus pumping directly on the chamber is a much discussed topic among the world’s leak detection authorities. In practice the best guide is to connect the Vacuum/volume

20/number

4.

Pergamon

Press

detection

1970 Farlington,

Portsmouth,

Various methods of Mass Spectrometer leak Detection of high are discussed, with the emBhasis on the Dractical considerations of the test results.

Testing

leak

Ltd/Printed

Hampshire,

England

vacuum systems and components of the tests and interpretation

leak detector at whatever point one can take full advantage of getting maximum throughput to the instrument and get as large a proportion of the pumped gases as possible entering the leak detector. Vessels which present too large a gas load to obtain sufficient sensitivity by conventional vacuum testing, either because of their size. design or materials used, or because of gaseous devices mounted in the vessel, have to be tested by sniffing or pressure testing. In sniffing the leak detector responds to changes in the ambient concentration of helium in the immediate vicinity of the probe. Typical-no leak--concentration of helium in the air is 4 parts per million. At this concentration an amount of helium in the order of lO-8 atm cc/set is entering the leak detector (about 100 times the minimum detectable leak). By zeroing out this background, a change of concentration as small as 0.5 ppm can be detected, setting the lower limit of detectability. Quantitative calibration is extremely difficult to determine, since (a) There is no assurance how large a proportion of the helium leaking out is being pumped into the leak detector; (b) Once the helium concentration around the probe reaches 100 per cent there is no further increase in the leak rate signal; The sensitivity of sniffing can be increased by as much as IO6 by placing a hood on the outside of the section tested, with the sniffer probe inserted into the hood. Extremely sensitive testing of large welded vessels can be done by testing individual sections of the weld at a time. A vacuum box, designed specially to suit the welds to be tested, is placed over a section of weld at a time, the box is evacuated with a leak detector and an auxiliary pump while applying helium to the opposite side of the weld, either by “bagging” (inserting helium under a cover of sheet polythene) of unfinished vessels or by pressurizing the vessel in cases where the test is carried out on a completed vessel. The rate of flow of helium into the leak detector is related to the actual leak by’: g,=P, 1 +?w P3S3

Where

in Great

Q,=Rate

of flow of helium into the leak detector

Q,=Actual p,=Pressure s,=Pumping Britain

leak rate in the box speed in the auxiliary

pump 143

H Westgaard:

Practical

p,=Pressure

aspects

of high

at the analyser

s,=Pumping detector

speed

vacuum

leak detection

head of the leak detector

at the analyser

head

of the leak

improvement of the carrier

in response time and sensitivity of sniffing by use gas-sniffer probe is illustrated in Figures I and 2.

Testing scaled components In order

to get maximum

test sensitivity.

the quantity

PISI P3.73

must be reduced as much as possible. ie one must get the box pressure as low as possible with a minimum of auxiliary pumping. In practice a box pressure lower than IO I torr is usually required. Flexible boxes with differentially pumped double gasketing made from highly resilient materials are described by Garrod and Nankivell’. The response time-the time required to yield a signal of 63 per cent of masimum signal-is determined by the time constants of the following parts of the system: (a)

Electronics

(b)

Leak

(c)

Sniffer

detector

(generally vacuum

and connecting

less than

0.5 set).

system

(less than

hose (with

0.5 set).

IO feet of host

typically

IO-30 set). (d) The leak itself. Here the time constant depends on the leak size and configuration, from a fraction of a second on large leaks to hours (or even days) on very small leaks with long paths. The response time determines the probe speed (a typical probe speed is 2 ft per minute). The time constant and test sensitivity can both be improved considerably by employing a newly developed sniffer probe which uses CO, as a carrier gas. The CO2 is injected into the line near the sniffer inlet, increasing the pressure in the connecting line. This changes the flow from molecular or transitional to viscous and thereby drastically increases the conductance of the line from the probe to the leak detector. The carrier gas is selectively pumped by the liquid nitrogen trap in the leak detector at a rate of several thousand litres per second. The

Veeco ACCU-PROBE steel bellows tubing

sniffer with 30 feel Mcasurcd response

of stainless time is ap-

proximately 5 seconds signal is approximately

Most of the mass production leak testing of small hcrn~etically scaled devices (such as transistors) is done with a regular helium leak detector using a multi-port manifold with batches of devices of up to ten or more tested in each port. Using a IO port manifold with a batch size of IO. one can test 100 devices on in a l-3 minute period, or LIP to 6000 per hour (depending the reject rate). A variety of automatic or semi-automatic machines have been introduced for this type of leak testing, some \vith a number of successive valved pumpdown chambers through which the devices are passed on their way to the mass spcctrometet vacuum system and back to atmosphere. Some of these instruments use a “Go/No-Go” signal to actuate a system of shutes to deposit the devices in the proper bins for leak tight or leaking units. The main problems common to most of these instruments are : (i) Complesity, resulting in high equipment cost and inherent maintenance problems. (ii) Helium background seriously reducing the test sensitivity. Lubricant for moving parts and polymer gaskets can be a considerable source of instability and helium background by absorbing and re-emitting helium. (iii) Sensitivity calibration can only be done on a stationary system. The sensitivity in leak testing batches of parts going through at production test rates is very much lower and is usually extremely difficult or impossible to determine. A recently developed instrument, the MS-90 UFT Production Leak Test Station, shown in Figure 3, overcomes these problems. This simple instrument has a very small test chamber pumped by extremely fast roughing and high vacuum pumping systems in turn. Using a very small test volume, and not having to cope with a large amount of moving parts with associated lubricants in vacuum, the MS-90 UFT does not experience the

Clean-up lime 3 5 seconds.

to 375;

of full

H Wesfgaard:

Practical

I I I i I 0, ” TVPF

aspects

of high

vacuum

leak detection

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5 SECOND EXPOSURE TOLEAKAREA

2

145

H Westgaard:

Practical

aspects

of high

vacuum

leak detection helium background problems common to so many other instruments. The calibration of the MS-90 UFT is done by attaching a Ranged calibrated leak directly onto the test chamber and cycling the instrument at its regular testing speed. giving an actual test sensitivity under production test conditions. Rcgardlcss of v,hich of these instruments is used, the prcparation of the devices-the helium bombing--\\ ill be the same. Graphs showing leak rate signal versus the actual leak rate for various situations are sho\vn in Figures 4 and 5. Figure 4 sho\\s a difl‘crent curve for bomb times of I. 3. IO, 30 and 100 hours, for one specific device v~l~~~ne of IO z cc. The Iowcr (dotted) curves are included to shop the clrect of csccssivc waiting bctwecn bombing and testing. Figure 5 indicates detectability limits for large leaks in devices of diKerent volumes. These figures (4 and 5) sho\v the curves for helium bomb pressure of I atmosphere. Bomb pressures of 4-5 atmospheres are commonly used. for which the curvcs have to be corrected. The amount of this correction depends on the Row characteristics of the leak. This is covered in the section entitled “Calibration of Leak Tests”. One precaution must be taken in helium bomb testing: helium desorption from the surface of the devices can cause a helium signal resulting in rejection of devices which are actually well within the leak tlghtncss specilication. This is usually avoided by proper cleaning procedures before bombing; in some cases a flushing of the devices with dry air between bombing and leak testing may be necessary. The same techniques used in helium bomb testing are used successfully in some instances to test porosity of ceramics and other materials.

Figure 3. MS-90 UFT Schematic.

Leak testing and backfilling sealed devices

of aircraft

instruments

and other

Sealed instruments which will be used at high altitudes will in service be operated in vacuum, with the pressure inside the case tending to stretch the case and open up any leak present. The best assurance of good performance can be had by leak testing the device under the same conditions under which it will be operating in service. This is simply accomplished in the MS9VBC shown in Figure 6. In this instrument the device can be evacuated and backfilled with helium while the leak detector itself is evacuating the outside cf the device. If a leak is present the helium will be pumped through it and into the leak detector and give a leak signal. The actual location of this leak can be found either by regular vacuum testing with a helium probe or by sniffing. Assuming the device is leak-tight it can be re-evacuated and backfilled with whatever fill gas specified, adding a trace of helium and pinched off. The pinch-off seal is then tested either by attaching the instrument directly to the leak detector pumping port, pumping directly on the pinched off tubing, or the device is left in the belljar which is evacuated with the leak detector. SIGNAL HELIUM TIME

10.I>/--,I-L,L’ II Id Id Figure 4 146

3 16” 16”

BOMBING BETWEEN 30 10000

v ACTUAL

OF DEVICES

LEAK

OF dcc

BOMBING AND TESTING: SEC UPPER CVRVE SEC-.. LOWER CURVE

RATE VOLUME

Calibration

of leak tests

In any type of leak detection it is very important that the test irsel/must be calibrated, not just the instrument. Some of the major considerations in this calibration are: (a) nature of the flow (viscous, transitional or molecular) (b) the gas in question (c) pressure differential across the leak (d) temperature

H Westgaard:

LEAK

Practical

RATE

aspects

of high vacuum

leak detection

ACTUAL

RATE

SIGNAL. FOR BOMB

VARIOUS

LEAK

VOLUMES

TIME

WAITING

TIME

3 HRS 30

SEC

no

ACTUAL

LEAK

RATE

Figure 5 In general one can usually assume that most leaks smaller than 1 x IO-’ atmcc/sec will be molecular by nature, while those larger than IO-,’ atmcc/sec are likely to take on viscous flow characteristics.

For molecular

flow of gases through

a cylindrical

tube?

(2) Where Q =Flow rate C,=Constant LI =Tube radius L =Tube length T =Temperature M= Molecular weight of the gas p? = Upstream pressure pI = Downstream pressure For viscous flow the flow rate is:

Q=c,$P.(P2-P,)=c,~ 2,1L(P22 -PI21

(3)

Where C, = Constant

Pa = Figure 6. Schematic of MS9 VBC

Pl+

P2

2

‘1 =Viscosity

of the gas 147

h Wesfgaard: The temperature

Practical

aspects

of high vacuum

leak detection

dependence on the viscosity is expressed by:

The pressure dilfcrential \vas covered briefly above. Since vacuum leak testing of high pressure systems is getting more and more common, however, it deserves a more detailed study. For a high pressure system the ratio between the leak rate under working conditions to that during leak testing for viscous flow condition is:‘:

Where the constant C is a measure of the strength of the attractive forces between molecules (C,,, = 122: C,, = SO). From formula (2) and (3) can be seen that the viscous flow rate is proportional to the difference between the squares of pressures upstream and downstream, while the molecular flow rate is proportional to the differences between the two pressures. Therefore, a viscous leak to atmosphere from a vessel operated at 2 atmospheres has a leak rate (22-lZ)=4 times that of same leak leaking from atmosphere to high vacuum during a leak test, while a molecular leak would have the same leak rate (assuming it would still exhibit molecular flow characteristics). Increased test sensitivity can be achieved in the case of viscous leaks by pressurizing the vessel with air in order to save expensive helium. If a vessel at a pressure of I atm (air) is pressurized with helium to 2 atm then to 4 atm with air, the ratio between the leak flow rate from 4 atm and the rate if only the helium were added is 4’--1

15

2--l

3

r=-=5.

p:,= External

pressure during

vacuum

p.,== Internal

pressure during

vacuum leak test=negligible.

The same ratio for molecular

leak test = I atm

flow is:

Qs PI -PL -=-= Q,

PI-P,

P1-Pr

If a molecular flow leak should be calculated as though it has viscous flow, an error is introduced which in most cases is acceptdble as an additional safety factor. The following formula can therefore be used as a general guide for vacuum leak testing of high pressure system9.

.

The ratio between the helium concentrations conditions is

resulting

Where p, y Working pressure p,-Outside pressure

in an increase in test sensitivity

under the same

of 2.5:l.

Looking at the type of gas used, formulae 2 and 3 show that the viscous flow rate is inversely proportional to the viscosity of the gas, while the molecular flow rate is inversely proportional to the square root of the molecular weight. In the case of helium as a test gas and air as a working gas, which is usually the case in leak detection, 2.7 and ?s -

=O.Y

This shows that under molecular flow conditions helium flow rates are approximately 2.7 times those of air, while the viscous flow rate for helium is slightly lower than that of air. Formulae (2) and (4) show that the molecular flow rate is proportional to the square root of the temperature while the relationship under viscous conditions is somewhat more complicated. Since the viscosity of gases increases with increased temperature, the viscous flow rate through a fixed size leak decreases with increased temperature, while the molecular flow rate increases. (For a temperature rise from 25°C to 50°C the increase in molecular flow rate is approximately 4 per cent while the viscous flow rate decreases approximately 6 per cent. For a temperature rise to 400°C the corresponding figures are 57 per cent and 76 per cent. 148

\ F

\

3 2

1 ld’

1

B

\ \

I) lo= loo Figure 7

2 3 5 810’

2

PRESSURE

(ATM)

H Westgaard:

Practical

aspects

of high

vacuum

leak detection the leak remains unchanged during variations in temperature and pressure. This is not entirely correct. Although for small variations in temperature of pressure they can be disregarded they can introduce a large error in extreme cases. Since these changes can be extremely complicated to calculate, one should always work with ample safety factors in such cases.

Qt=,$+ Where Q,

Vacuum

test leakage

/’

Multiplying

factor

Q,

Permissible

leakage

PI

= Working

pressure

P:!

External

pressure

A graph

showing

factor: plotted In the above

rate for converting rate under

under the

working

Quantity

the units working

conditions

conditions I p,‘-p2”

called

reduction

against p, is shown in Figure 7. it is assumed that the physical configuration

of

149