solving process problems at the molecular flux level

8/3/2019 Solving Process Problems at the Molecular Flux Level

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Solving Process Problems at the Molecular Flux Level

Working from pressure readings alone can mislead and misdirect a search forprocess problems, but reaching down a layer further into the molecularflux level can provide real solutions.

Every vacuum process hassome sort of specifiedparameters that must be

met if the process is to beeither initiated or carried outsuccessfully. Most

commonly, theseparameters state that acertain vacuum level beachieved or maintained,

and these levels are usuallydenoted in either totalpressure or in partialpressures of specific gases.


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Although parametricspecificity at this level isoften sufficient, there are all

too many cases whereprocess problems erupteven though the vacuumspecs appear to be met orexceeded. This result, then,often causes the vacuumpractitioner to question the

original specifications eventhough they had alreadyproved to be apparentlyworkable and practical.


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What's really missing in this

situation is the awarenessthat all technologies, andvacuum in particular, existin layers of detail and

complexity. In this case, wecan assumeoverdependence on thefundamentalvacuumrelationshipQ=SP.Getting to the root of the problem

Although the Q=SPrelationship is an extremelyimportant tool in

The process gas enters the chamber,but most of it goes directly into thepump, and this provides a wrongpressure measurement.


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understanding the behavior

of vacuum systems, it does,however, have a built-intrap that can interfere with adeeper understanding if we

let it. We have to assumean equilibrium condition anda uniformity of gas load,pressure, and identity ofgas species throughout asystem to make use of it.When process or design

problems require us toreach into a deeper level ofunderstanding, it becomesapparent that the


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assumptions applied to the

Q=SP relationships aren'tnecessarily true at thislevel.

For example, the idea of alarge number of gasmolecules bouncing aroundwithin a chamber in purelyrandom and mutuallyequidistant motion begins tobreak down. The Q=SP

relationship is a bit of anabstraction that is useful forcalculating andunderstanding overall


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performance, but it cannot

take into account the actualdetailed motion ofmolecules within thechamber. This level is

where many problems areto be found. Specificsources of gas, often in theform of molecular beams,and specific gas sinks needto beconsidered and

analyzed.A modern day example

If we look at acommonly encountered

The process gas enters the chamber,but most of it goes into the gauge,which renders an incorrect pressuremeasurement.


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problem such as a tiny

pinhole leak, we can easilyconstruct a mental pictureof an expanding beam ofmolecules entering the

chamber from the leak site.If we know the leak rate incommon terms, such astorr-L/sec, we can do aQ=SP calculation anddetermine the chamberpressure change produced

by the leak providing weknow the pumping speed. Ifwe know the change inchamber pressure, we can


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calculate the leak rate

instead. A beam of non-polar air gases such asnitrogen (N2 ) or oxygen (O2) will enter an empty

chamber, continuallyrebound from surfaces,reach an equilibriumconcentration, and bepumped away.

But, working at a lower

level, we can see that thebeam of chemically activegas could easily pass rightthrough a work volume or


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impact a work surface. This

would have the same effecton the process as a totalpressure many orders ofmagnitude higher. Process

problems could then beencountered even thoughthe pressure reading(s) wasstill within the approvedprocess parameters.hinking in molecules

Thinking in molecular flux

not only allows us toquantify the numbers ofmolecules acting andinteracting within the


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system but also begins to

allow us to build mentalpictures of the performanceof the system. Out of thisability to construct mental

pictures will, in time anduse, allow the vacuumpractitioner to grow thepriceless ability to "feel" asystem perform. Thispainfully acquired skill canhave many important

practical applications inboth troubleshooting adesign or process and in


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the original system design

or process development.Calculating units

In the Q=SP relationship,we make our calculations in

units that are convenient forthe abstract relationship butare difficult to think in. Gasloads (Q) in mass flowterms of torr-L/sec are veryhard to picture mentally, buta stated number of

molecules/sec is mucheasier to handle effectively.After all, pressure is reallythe number of molecules in


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a given volume to anyone

working in the molecularflow regime where thenormal concept of pressureis essentially meaningless

anyway. In manyprocesses, the number ofmolecules passing througha process volume orimpacting upon a processsurface is key tounderstanding that process.

Granted, the number ofmolecules/L/torr is a


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mindboggling number, but it

does give us something towork with. This can becalculated with the followingequation: (torr) x (3.54 x

1019 ) = molecules/L. Thiskind of calculation can beuseful when comparing thenumber of molecules in avolume and then adding ina particular beam-borne fluxof molecules from

something like a leak.

It is even more useful tothink (mental picture again)


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of the number of molecules

impacting a surface.Assuming that all theresidual molecules are N2 ,we can calculate the

number of collisions: (torr) x(3.95 x 1020 ) = number ofcollisions/cm2 /sec. This canbe compared to the numberof collisions/sec that wouldresult from an impactingbeam of molecules. The

same system can beapplied to the number ofmolecules passing througha process volume by merely


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calculating the surface area

of the volume.What's it good for?

Molecular flux concepts canbe applied in either a

qualitative or quantitativefashion. Qualitatively, wecan use the surface impactcalculations alreadydiscussed to determine theeffects.

For example, a freshlydeposited film of activemetal will react with theresidual gases as they


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impact on the surface, so it

becomes possible to makestoichiometric calculationsof the chemical changes onthe surface. That might well

be to determine how longthe fresh film can beexposed before aninsulating film results.

Before the quantitativecalculations become

important, though, we needto learn to think a system'spossible performancethrough using molecular flux


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thinking techniques to

determine whether therewould be any additionaleffects to the obviousresidual gas impacts.

We can then begin to thinkabout, and mentally picture,what might occur when aprocess gas is introducedinto a vacuum chamber.When a stream of gas is

allowed to flow from acontrolled leak valve intothe chamber, it will tend toform an expanding


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molecular beam that

traverses the chambervolume, and usually theprocess volume, inessentially a straight line.

The beam's diameter willincrease with distance andform a continuous cone withthe molecular populationgreatest at the cone'scenter in a condition calledcosine distribution. If a

pump is located directlyacross the chamber fromthe leak valve, the greatestportion of the gas will be


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directed into the pump's

throat with only the outlyingportion of the beam strikingthe chamber wall. If thegauge sensor head is

located 90o from the linebetween valve and pump, itwill only detect the gas thatdoesn't enter the pump as itscatters away from thechamber wall. This meansthat more gas is passing

through the chamber andthe process than would beexpected from the gauge'spressure reading. If the


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gauge and pump positions

are reversed, though, thegauge would detect aseemingly higher gas fluxthan would actually be the

case.

These two practicalexamples represent twocommon design mistakesthat lead to processproblems that aren't

immediately apparent whenthe process is monitored bypressure readings alone.


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Molecular flux thinking

allows us to take identifiedgas load sources andpicture, or calculate, theeffects of the resultant flux.

For example, a large O-ringon a door seal can easilyallow a diffusive flux fromthe O-ring's outgassing andpermeation of water vapor.This can result in the build-up of sorbed water vapor on

surfaces close to the O-ringthat can later be a problemfor a process if the surfacedesorption rate is too high.The outcome


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The number of examples

and possible uses for thistype of thinking and

analysis is endless, but

learning to use it on simpleand fairly standard

applications can help

master the technique andmake it a standard mental

reflex when considering a

design or trouble-shooting a

system. The Q=SP

relationship is a powerful

tool and should never be


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ignored, but any vacuum

practitioner should beconstantly ready to peel

back another layer in the

technology and thinkmolecular flux.

Phil Danielson

The Vacuum LabDowners Grove, Ill. 630-

434-1681

www.vacuumlab.com

solving process problems at the molecular flux level

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