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INTERNATIONAL UNION FOR VACUUM SCIENCE,TECHNIQUE AND APPLICATIONS

UNION INTERNATIONALE POUR LA SCIENCE,

LA TECHNIQUE ET LES APPLICATIONS DU VIDE

INTERNATIONALE UNION FRVAKUUM-FORSCHUNG, -TECHNIK UND -ANWENDUNG

VISUAL AIDSFOR INSTRUCTIONS IN

VACUUM TECHNOLOGY

& APPLICATIONSSeries Editor

John L Robins

Module 4:

PARTIAL PRESSURE ANALYZERS AND

ANALYSISSecond Edition

Leonard Beavis

Presenters Notes

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INTERNATIONAL UNION FOR VACUUM

SCIENCE, TECHNIQUE AND

APPLICATIONS

VISUAL AIDS

FOR INSTRUCTIONS IN

VACUUM TECHNOLOGY

& APPLICATIONS

Module 4

PARTIAL PRESSURE ANALYZERS AND

ANALYSIS

Second Edition

Presenters Notes

by

Leonard Beavis

Series Editor

John L Robins

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Module 4: Partial Pressure Analyzers and Analysis IUVSTA Visual Aids Program

3

INTRODUCTION TO THE SECOND EDITION

This revision of the Partial Pressure Analyzers and Analysis module of the IUVSTA

Visual Aids Project replaces the 1stEdition which had been produced in 1972. Many

changes in partial pressure analysis have occurred in the 30 intervening years. It isintended that this revision accurately reflect these changes. As this collection ofslides is intended to be an instructional tool in the technology and not an historical

documentation of partial pressure analysis, all of the original material is beingreplaced with both new content and format.

It is assumed that the students have a familiarity with elementary chemistry and

physics and basic vacuum technology. It is further assumed that the instructor has

experience with partial pressure analyzers and analysis as well as an advanced level of

understanding of vacuum science. Accordingly, in most cases the material on the

slide will not be explained in detail, i.e. this material has not been prepared as a stand-

alone course for either students or instructors but is intended as material to enhancethe presentation of a course by a competent instructor.

This Instructors Guide is arranged in the following fashion: For each slide, from the

compendium of slides, there is accompanying text which includes a statement of the

purposeof the slide, some points of emphasisregarding the material on the slide andsometimes some advanced topicsthe students may wish to discuss (possible subjects

for an advanced course on partial pressure analysis).

Leonard BeavisAlbuquerque, USA.

June 2004

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4

LIST OF SLIDES

Section 1. INTRODUCTORY MATERIAL AND TERMINOLOGY

Slide 1.01 Comparative Need for Total Press Gauges and Partial Press Analyzers

Slide 1.02 Relationship of a PPA to an Ion Gauge and a Mass SpectrometerSlide 1.03 Terminology ReviewSlide 1.04 Terminology General 1

Slide 1.05 Terminology General 2Slide 1.06 Determination of Resolving Power/Resolution

Slide 1.07 Examples of Alternative Determinations of Resolution

Slide 1.08 Terminology Ion Source

Slide 1.09 Terminology Ion Source and Analyzer

Slide 1.10 Terminology Analyzer

Slide 1.11 Terminology Detectors

Section 2. INFLUENCE OF THE COMPONENTS OF A PPA ON DATA

Slide 2.01 Block Diagram of a PPA System

Slide 2.02 Overview: Potential Impact of the Ion Source on Data

Slide 2.03 Overview: Potential Impact of the Analyzer Section on DataSlide 2.04 Overview: Potential Impact of the Ion Detector on Data

Slide 2.05 Overview: Data Collection and Display

Section 3. ION SOURCES

Slide 3.01 Methods for Producing Ions

Slide 3.02 Electron Bombardment Ion Source

Slide 3.03 Ionization Efficiency Curves

Slide 3.04 An Open Ion SourceSlide 3.05 Nier Type Closed Ion Source

Slide 3.06 Table of Parent Ions Expected in a Leak Tight Vacuum System

Slide 3.07 Table of Parent Ions with Major Isotopes Included

Slide 3.08 Metastable Ions Generated in the Ion Source

Slide 3.09 Typical Hydrogen/Deuterium Spectrum

Slide 3.10 Triatomic Deuterium FormationSlide 3.11 Triatomic Hydrogen Ion Formation

Slide 3.12 Commonly Seen Multicharged Ions

Slide 3.13 Species Produced by the Filament

Slide 3.14 Fragmentation Species Produced in an Ion SourceSlide 3.15 Effect of Bombardment Voltage on Fragmentation

Slide 3.16 Thermal Fragmentation by Hot Filaments

Slide 3.17 Occasionally Seen Important Contaminants

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5

Section 4. SAMPLES OF SPECTRA; ION SOURCE EFFECTS

Slide 4.01 A Typical Spectrum of a Relatively Clean System

Slide 4.02 A Spectrum with Air and Organic Contamination

Slide 4.03 Scans during Pump Down at 70eV and 105eV Electron Energy

Section 5. ION DETECTORS

Slide 5.01 Faraday Cup DetectorSlide 5.02 Faraday Collector Features

Slide 5.03 Secondary Electron DetectorsSlide 5.04 Discrete Dynode Electron Multiplier

Slide 5.05 Continuous Dynode Electron Multiplier

Slide 5.06 Secondary Electron Multiplier for Extreme Low Pressure

Slide 5.07 Diagram of an Ion Counting System

Section 6. MASS ANALYZERS

Slide 6.01 Mass Analyzers Readily Available

Slide 6.02 Magnetic Sector AnalyzerSlide 6.03 Operation of the Magnetic Sector Spectrometer

Slide 6.04 Special Issues with the Magnetic Sector

Slide 6.05 Time of Flight Mass Analyzer

Slide 6.06 Operation of the Time of Flight Spectrometer

Slide 6.07 Quadrupole Mass Filter

Slide 6.08 Quadrupole Operation

Slide 6.09 Stability Diagram for the Quadrupole Mass Filter

Slide 6.10 Location in Time of Mass Peaks for Various Sweep ParametersSlide 6.11 Quadrupole Issues

Slide 6.12 Symptoms of Improper TuningSlide 6.13 Mislabelled Spectrum for Hydrogen Selenide

Section 7. DATA STORAGE AND DISPLAY

Slide 7.01 Data Display and Storage

Slide 7.02 Logarithmic Display of m/q = 1 - 72Slide 7.03 Linear Display of Data in Slide 7.02, m/q = 1 69Slide 7.04 Block Diagram of Data Acquisition with Feedback Control

Section 8. SOME APPLICATIONS

Slide 8.01 Air Leak Spectrum

Slide 8.02 Leaky N2Backfill Valve

Slide 8.03 Vacuum System Bakeout

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6

Section 9. QUANTITATIVE AND QUALITATIVE DATA

Slide 9.01 Qualitative vs. Quantitative Data

Slide 9.02 Diagram of an Orifice Flow Calibration System

Section 10. BIBLIOGRAPHY/REFERENCES

Slide 10.01 Bibliography I (Props of Elements, Isotopes & Compounds)Slide 10.02 Bibliography II (Vacuum)

Slide 10.03 Bibliography III (PPA Spectra Interpretation)Slide 10.04 Bibliography IV (Instrumentation)

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PRESENTERS GUIDE

Section 1. INTRODUCTORY MATERIAL AND TERMINOLOGY

Slide 1.01 Comparing the Need for a Total Pressure Gauge with that for aPartial Pressure Analyzer

Purpose: To introduce the idea that not every vacuum system requires a PPA but thatmuch of modern vacuum technology depends upon a qualitative and possibly

quantitative knowledge of the constituents of the gases in the vacuum.

Points of emphasis: Total pressure gauges are used when the reactants and/or

products of the vacuum process are known or controlled independently of the process.

For example: freeze drying, or chemical vapor deposition (CVD). Total pressure

gauges are also used when the results of particle to particle interaction are practically

independent of the species involved. For example: in high energy particleaccelerators. As total pressure gauges give a single output and are much less

expensive than a partial pressure analyzer, they are the gauges of choice in the above

instances.

Partial pressure analyzers (PPA) are used in applications where the species to bedetected or controlled is unknown or known to be a small fraction of the total gas

present. Examples include leak detecting, multi-layer semiconductor processing, gas

surface studies (catalysis) and contamination control processes. The requirement for

qualitative and/or quantitative gas species information is paramount. In these

instances it is necessary to discern a particularly active gas species or one that is at alow level compared to the total gas present. Anything but a PPA is inappropriate.

Advanced topics:

Relationship between advance of vacuum technology and the advent of

partial pressure analysis.

History of mass spectrometry and partial pressure analysis.

Development of the mass spectrometer leak detector (the first PPA) and

improved leak detection.

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Slide 1.02 Relationship of a PPA to an Ion Gauge and a Mass Spectrometer

Purpose: To illustrate the similarities and differences between these three very

important instruments.

Points of emphasis: Point out the increase in complexity and cost in going from therather simple ionization gauge to the analytical mass spectrometer.

Also discuss the similarities of the PPA to the mass spectrometer. For example theyeach measure mass to charge ratio. They each have a similar trade-off of sensitivity

and resolution.

The PPA is generally an instrument that can determine one mass number from an

integer adjacent mass number, whereas an analytical mass spectrometer can discern

masses that may be one thousandth of a mass unit or less apart.

Advanced topics: What must be sacrificed in order to increase resolution of a PPA?

Inlet systems for a PPA.

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Slide 1.03 Terminology - Review

Purpose: To review some of the terminology that will be used throughout the

remainder of the illustrations

Points of emphasis: Each element has unique chemical properties and has associated with it an

atomic number. The atomic number is the number of protons or electrons

associated with that element. For example: hydrogen is atomic number 1 with

one proton and one bound electron. Neon is atomic number 10 with ten

protons and ten bound electrons.

Most elements have more than one naturally occurring isotope associated with

them. An elemental isotope has the identical number of protons and bound

electrons and differs from other isotopes of a particular element in the number

of neutrons in the nucleus. Hydrogen has three naturally occurring isotopes.

These are uniquely named protium, deuterium, and tritium and have zero, one

and two neutrons in addition to a single proton respectively in their nuclei.

Neon also has three naturally occurring isotopes. These are designated neon-

20, neon-21 and neon-22 and contain in addition to ten nuclear protons 10, 11

and 12 neutrons respectively.

An atom has associated with it a mass, which for the purposes of partial

pressure analysis is an integer value. This integer, atomic mass, is the sum of

the number protons and neutrons in the nucleus. Thus hydrogen with threeisotopes has masses one, two and three. An atom is the single smallest

chemical entity of any element. Atoms of an element behave chemically

nearly identically no matter what the mass of the particular isotope

Associated with every element is the naturally occurring isotopic abundance ofthe element. These abundances are tabulated in various handbooks. In the

case of hydrogen most of it is protium, but 15 parts per hundred thousand aredeuterium and about 1 part in 1017 is tritium. The accurately determined and

known isotopic abundances are a tool used in partial pressure analysis.

Chemical combinations of atoms make molecules. Most elements that are

insulating and many semiconducting elements appear in the vapor/gas phase

as multi-atomic molecules. Examples include nitrogen as N2 and arsenic asAs3and As4. These have masses associated with the number of atoms present.

Thus N2has masses of 28, 29 and 30. Arsenic vapor has masses 225 and 300

associated with As3 and As4respectively.

Molecules may be mono-elemental, as above or multi-elemental as in water ormethane. Multi-elemental molecules are called compounds. Associated with

a compound is also a mass. It is the sum of the masses of the elements (byisotope) that make the compound. For example most water has a mass of 18,

and most methane has a mass of 16.

Advanced topics:

Determination of isotopic abundances of elements.

Determination of relative abundances of molecules.

Determination of isotopic masses to 1ppm.

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Slide 1.04 Terminology General 1

Purpose: To become familiar with terminology encountered in partial pressure

analysis.

Points of emphasis: Although at one time PPA, vacuum analyzer and RGA had somewhat different

meanings, for the past several years these names have been used

interchangeably. Occasionally one will also hear mass spectrometer mixed in

as a synonym also. It is probably not prudent to do so as most, if not all, mass

spectrometers have vastly different capabilities than PPAs.

In partial pressure analysis the mass number is always expressed in atomic

mass units (AMUs) rather than in metric or international mass units. 1 AMU

= 1.66x10-27 kg. Additionally, and in contrast to analytical mass spectrometry,

masses of atoms, molecules and molecular fragments are always expressed in

integer AMUs.

PPAs, as well as mass spectrometers always measure mass to charge ratio

(m/q) and not mass independent of charge. This is true of all possible schemes

for mass separation (analysis). This fact has large implications when making

quantitative interpretation of mass spectral output from a PPA. Multicharged

ions appear at mass numbers in the spectra different from singly charged ions.

Doubly charged carbon dioxide ions appear at mass 22 for example.

Gas pressure indicates a force per unit area. As pressure drops below about

10-8

atmospheres it becomes impossible to measure pressure directly. Gas

density (the number of molecules per unit volume) is the parameter that is

measured by ion gauges and PPAs. Gas density, !, is related to gas pressure,

p, through the ideal gas law: P=!RT. The measurement of gas density by aPPA becomes an issue when reporting pressures quantitatively measured at

elevated temperatures.

Whilst the SI units for pressure are the Pascal (Pa) (defined as one Newton per

square meter) and the millibar (mbar), an earlier unit, the Torr (or torr), is still

often used, particularly in the USA. 1 Torr = 1.33 mbar = 133 Pa. It is aconvenient approximation, particularly where orders of magnitude only are

being considered, to treat the units mbar and Torr as equivalent.

Advanced topics:

Peaks at non-integer mass numbers.

Elimination of multi-charged ions.

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Slide 1.05 Terminology General 2

Purpose: To define these four terms and show the relationship of the first two and the

relationship of the third and fourth.

Points of emphasis: The ability to detect one ion species in the presence of all other ion species is

called the dynamic range of the instrument. For some PPAs this may be as

small as one part per thousand. Modern quadrupoles have a dynamic range of

one part in 106-107.

The minimum detectable partial pressure is an expression of the possibility of

detecting the presence of a few molecules/unit volume of a particular

molecule. For example most modern quadrupoles can detect 10-14

mbar of

nitrogen under ideal conditions. Obviously these conditions must include a

total pressure of 10-8

mbar or less if the dynamic range of the instrument is

106. The dynamic range and minimum detectable partial pressure should be

specified in some relational form to assure there is no misunderstanding about

the capabilities of the PPA.

Sensitivity for PPAs is normally expressed in amp/torr (or amp/mbar) for a

particular gas, e.g. nitrogen, rather than in torr-1 (or mbar-1) as is done for

ionization gauges. To assure there is no confusion about such a specification

the amp/torr figure should include the electron bombarding current and the

electron accelerating potential. For example 1ma at 70eV electron energy.Resolution is expressed as the inverse of the peak width of a mass spectral peak in

AMU-1

. Resolving power is the ratio of the mass of the spectral peak being measured

to its width. Clearly resolution and resolving power are related. As mass peaks do

not have vertical sides to them the point on the side of the peak where width is to bemeasured must be specified. Slides 1.06 and 1.07 give examples of resolving power

and resolution measurements. Sensitivity and resolution are inversely related in all

PPAs. An increase in one of these factors comes at the expense of a corresponding

decrease of the other. Sensitivity " resolution-1

. For this reason specifications

involving resolution or sensitivity must be related i.e. specifying resolution without a

corresponding sensitivity and vise versa is meaningless.

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Slide 1.06 Determination of Resolving Power/Resolution

Purpose: To give an example of making a resolving power estimate.

Points of emphasis:

Resolving power is the mass number of a peak divided by its peak width inAMU.

In the example, the peak width for m/q=86 is found by taking the distance D

between peaks 84 and 86, in this case estimated to be 23 units, and dividing it

into the m/q difference of 2 AMU, and multiplying this quotient by the peakwidth of m/q=86 at the 10% amplitude point, which is estimated to be 11 units

(i.e. peak width = 11x(2/23) AMU).

Thus the resolving power is equal to 86/(22/23) #90.

In this example the 10% peak amplitude was taken as the place to measure the

peak width. This follows from the AVS recommended practice on PPA

calibration. Obviously the resolving power would be greater the higher up thepeak at which its width is measured.

Advanced topics

Is resolving power constant across the mass range?

How does resolving power vary with instrument type?

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Slide 1.07 Examples of Alternative Determinations of Resolution

Purpose: To demonstrate various concepts for measuring resolution.

Points of emphasis:

In the first example the peak width $m is measured at the 50% level. Clearlythe resolution will be greater the further up the peak that the width is measured

In the second example resolution is defined by the ratio Y/xY divided by themass number difference between the interfering peaks. As the peaks narrow

the resolution increases as expected.

In the third example resolution is defined by the ratio of the sum of the two

peak heights divided by the height of the valley between them divided by the

mass difference between the measured peaks. It is likely that as the peaks

narrow the valley height will decrease thus increasing the resolution.

In the fourth example the resolution is treated as in the third, above, except

that rather than dividing the valley into the sum of the neighboring peaks, thevalley height is divided into either of the equal amplitude peak heights.

Advanced topics:

Is resolution constant across the mass range?

Is it instrument type dependent?

Are there expressions that relate the various definitions of resolution

displayed above?

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Slide 1.08 Terminology Ion Source

Purpose: To discuss a number of terms relevant to ion source operation. Ion sources

are discussed in section 3, slides 3.01 to 3.17

Points of emphasis: Fragmentation and cracking are used interchangeably. These terms refer to a

molecule or molecular ion coming apart, i.e. fragmented, because of an

introduction of excess energy from one of a variety of sources. The term

cracking comes from the petrochemical industry; a major user of mass

spectrometers.

Impact fragmentation is caused by the collision of an energetic electron with a

molecule or molecular ion.

Thermal fragmentation occurs when a molecule or molecular ion collides with

a high temperature surface. The high temperature causes the particle tofragment.

Ionization efficiency is the number of electron/ion pairs produced per electron

colliding with an atom or molecule. Ionization probability is the probability

that an electron will ionize an atom or molecule. In each of these cases the

efficiency or probability is zero until the bombarding electrons energy

exceeds the ionization potential. At energies above the ionization potential theprobability and efficiency are greater than zero and a function of electron

energy.

When an ion is formed if it loses an electron the ion will take on one positive

charge. This is a singly charged ion. If the ion that is formed loses more than

one electron during the ionization process it becomes a multi-charged ion.

In an ion source, electrons collide with various electrode surfaces in thesource. It is possible if atoms/molecules are not tightly bound to the surface

that the electron can cause the atom/molecule to desorb into the vacuum. This

is called electron induced desorption EID.

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Slide 1.09 Terminology Ion Source and Analyzer

Purpose: To define terms that apply to the ion source and the analyzer sections of a

PPA.

Points of emphasis: Charge exchange occurs when an ion collides with a neutral molecule, and the

molecule loses an electron to the ion (possibly neutralizing the ion). The

molecule becomes an ion. Charge exchange becomes an issue only when the

gas density is high enough so that particle-particle collisions are likely, i.e. the

mean free path becomes smaller than the dimensions of the ion source or the

analyzer section.

Space charge occurs when the ion density becomes large enough to cause the

ions to be deflected, or spread. This generally causes the loss of ions.

Ion induced desorption (IID) is similar to EID. In this case the particle

causing desorption is an ion rather than an electron.

Depending upon the particular type of PPA, ions that are formed in the ion

source may be injected into the analyzer at an angle and from a position such

that they traverse the analyzer as if they were ions at the m/q in tune, thus

giving a false signal to the detector. These non-tuned ions are called off axis

ions.

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Slide 1.10 Terminology Analyzer

Purpose: To define terms that apply to the analyzer section of a PPA . Mass

analyzers are discussed in section 6, slides 6.01 to 6.13.

Points of emphasis The Mass Range of a PPA is defined by the operating parameter of the

analyzer section. It is typically from AMU=1 to 100, but may be 1 to 300 or

greater in some instruments. The Mass Range of magnetic sector instruments

may consist of multiple spans e.g. 1- 50 and 12 300. This typically is

accomplished by an adjustment to the magnet.

Scan Rate is expressed either as AMU/second or full range scans e.g. AMU 1-

100 per unit of time. In a quadrupole, mass peaks can be scanned at a

maximum rate of a few hundred AMU per second. A time of flight analyzer

can scan at rates 105 AMU /second depending upon the mass range

interrogated or thousands of scans /second.

Scan Parameter is the operating parameter of the instrument (analyzer) that is

adjusted, typically as a function of time, to move from one mass peak to

another. In a magnetic sector instrument, the scan parameter is typically the

ion injection energy into the magnet field region in less expensive instrument,

but may also be the magnet field intensity in some sector instruments. In the

time of flight instrument the scan parameter is a combination of injection

pulse length and ion energy. Quadrupoles scan the D.C. voltage applied to thequadrupole rods. The scan parameter relation to mass spectra presentation

will become more apparent in the Mass Analyzer section of this module, slides

6.01 to 6.13.

Transmission is usually expressed as amperes/pressure unit. It is anexpression of the efficiency with which the instrument (analyzer) can extract

from the ion source, transmit through the analyzer and detect ions of a specificspecies e.g. nitrogen.

Ghost Peaks are those apparent mass peaks that are not representative of any

actual species in the vacuum. For example: a peak at mass 19.5 is sometimes

seen in a magnetic sector due to ion induced desorption/charge change that

occurs from non-resonant ions striking the analyzer tube wall.

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Slide 1.11 Terminology Detectors

Purpose: To define terms and describe briefly the types of ion detection used in

Partial Pressure Analyzers. Ion detectors are discussed in Section 5, slides 5.01 to

5.07.

Points of emphasis:

The Faraday detector is a detector that measures ions by the flow of electrons

to neutralize the ions as they collide with the metal surface of the ion

collector. The collector is usually at ground (earth) potential. Suppression of

secondary electron emission from ion bombardment or the photoelectron

effect is assumed.

The Multiplier detector is used to amplify ion currents in the vacuum system

through secondary electron emission from ion and electron collisions. It

depends upon high ion to electron and electron to electron conversion

efficiencies. These are undesirable attributes in a Faraday detector.

Multiplier detectors are used for detecting lower partial pressures than

Faraday detectors can detect.

Ion counting is a technique using a multiplier detector to detect very low

partial pressures. It is capable of measuring at pressure levels lower than

those using a multiplier detector in its analog mode. This detection capability

comes at the expense of more complex electronics.

======================= End Section 1 ====================

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Section 2. OVERVIEW OF INSTRUMENTATION

Slide 2.01 Block Diagram of a PPA System

Purpose: To define by diagram a complete partial pressure analysis system.

Points of emphasis:

In addition to the basic partial pressure analyzer instrument i.e. ion source,

analyzer and detector, the data collection, display, storage and analysis

equipment is an important part of any partial pressure analysis system.

The impact and interaction of each of these subsystems will be illustrated.

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Slide 2.02 Overview: Potential Impact of the Ion Source on Data

Purpose: To illustrate the importance of understanding the ion source as a generator

of data.

Points of emphasis: Much of the information in a PPA spectrum is not representative of the

contents of the vacuum system the PPA is interrogating, but is rather a

convolution of interactions of the vacuum species and the ion source materials

and potentials.

The superfluous information is often useful in interpreting the low-resolution

mass spectra of a PPA.

Nearly all of the information/misinformation in a PPA spectrum is generated

in the ion source. This fact must always be kept in mind when describing the

remainder of the vacuum based upon PPA data.

Atomic and radical ions are very reactive and capable of generating species

not present in the remainder of the vacuum system.

Neutral atoms and radicals are also very reactive and capable of generating the

same sorts of misinformation.

The sources of artifacts are alluded to on the slide.

Advanced topics:

How would misinformation be reduced?

Is there some useful misinformation?

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Slide 2.03 Overview: Potential Impact of the Analyzer Section on Data

Purpose: To give an overview of how the analyzer section influences PPA data.

Points of emphasis:

The analyzer section converts a heterogeneous mixture of different ions intoindividual atomic or molecular ionic constituents.

It is important that resonant (tuned to the chosen mass number) ions be

transmitted with maximum efficiency to the ion detector.

It is also important that non-resonant species not be transmitted to the detector.

They lead to detector signal noise. Analyzer bypass is such a source of noise.

Collisions with the walls or analyzer elements can lead to spurious signal

generation or analyzer detuning through insulating layer build-up.

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Slide 2.04 Overview: Potential Impact of the Ion Detector on Data

Purpose: To give an overview of the influence of the ion detector on data.

Points of emphasis:

Contrast the energies of ions collected on Faraday and electron multiplierdetectors.

Ion neutralization requires one or more electrons/ion to flow in the detector

circuitry.

Detectors in optical sight of the ion source may produce photoelectrons.

Describe the possible consequences.

Ions striking the ion collector surface can cause secondary electron emission.

Describe the possible consequences.

Advanced topics:

How can electron currents in the detector be modified?

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Slide 2.05 Overview: Data Collection and Display

Purpose: To describe some possible impacts of the data collection scheme on the

storage display and analysis of the data.

Points of emphasis: Data output from most PPAs is analog. Describe what is an adequate

conversion to digital information.

Describe how computers handle data.

Describe non-linearities in data management e.g. amplifier saturation,

logarithmic and linear data conversions.

Describe sources of noise in the data and its impact.

Advanced topics:

When is PPA data already digital?

How can non-linearities be minimized? How can noise be minimized?

======================= End Section 2 ====================

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Section 3. ION SOURCES

Slide 3.01 Methods for Producing Ions

Purpose: To illustrate some of the ways that energy input to a molecule can bereflected in changes to the molecule.

Points of emphasis:

The purpose of the ion source is to convert a neutral atom or molecule into an

ion that is an accurate reflection of the original neutral species, e.g. correct

mass, correct structure.

Not all energy inputs to a neutral atom or molecule accomplish this goal.

Advanced topics:

What are some ways of maximizing the ion sources purpose?

Can an ion source make a perfect conversion from neutral to ion?

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Slide 3.02 Electron Bombardment Ion Source

Purpose: To show what a typical ion source looks like and how ions are generated.

Points of emphasis:

Describe the purpose of each electrode and the corresponding potential appliedto the electrode.

Point out the flow of ions and electrons.

Describe what happens during electron collisions with neutrals or ions.

Describe the energetics of reactions in the ion source.

Advanced topics:

What reactions occur when neutrals or ions contact the filament?

What interactions occur when atoms or molecules contact the other

electrodes?

What interactions occur when ions contact the other electrodes?

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Slide 3.03 Ionization Efficiency Curves

Purpose: To show how the electron bombardment energy relates to the ability to

produce ions of various gasses.

Points of emphasis: Note that the peak in the ionization efficiency does not occur at the same

energy for all gasses.

Note that the peak efficiency of ionization is not the same for all gasses.

Describe what the impact of these two facts is.

Advanced topics:

Why do most PPAs operate with 70 eV electron energy?

What electron energy should be used for leak detecting with helium? With

neon?

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Slide 3.04 An Open Ion Source

Purpose: To show a schematic diagram of an open ion source used with most

commercially available PPAs.

Points of emphasis: Describe the advantages and disadvantages of this type of ion source.

Describe the materials usually used for each of the electrodes.

Describe the advantages and disadvantages of these selections.

Advanced topics:

How would you optimize the electrode materials? Why?

What are the optimum potentials on the electrodes? Why?

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Slide 3.05 Nier Type Closed Ion Source

Purpose: To show a diagram of a Nier type closed ion source.

Points of emphasis:

Describe where gas is injected into this type of ion source. Describe differential pumping and how it applies to this type of ion source.

Describe the advantages and disadvantages of the Nier type of ion source.

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Slide 3.06 Table of Parent Ions Expected in a Leak Tight Vacuum System

Purpose: To show a table of mass spectral peaks of an idealized hypothetical

spectrum of residuals in a non-leaking vacuum system.

Points of emphasis: This spectrum is not real. It is hypothetical only.

It assumes that only the major isotope of each element exists.

It also assumes no multiple ionization, no fragmentation, and no meta-stable

ions exist.

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Slide 3.07 Table of Parent Ions with Major Isotopes Included

Purpose: To show a table of a hypothetical spectrum similar to that in slide 3.06, but

with isotopic peaks added. The importance of isotopes in identifying elements

becomes apparent.

Points of emphasis:

This spectrum is not real.

Isotopic peaks to the 0.001 level of the major peak have been added.

It is through isotopic peaks that low-resolution mass spectra species are often

unequivocally identified.

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Slide 3.08 Metastable Ions Generated in the Ion Source

Purpose: To draw attention to two major meta-stable ions, that often appear, in PPA

spectra.

Points of emphasis: These meta-stable ion species are generated in the ion source.

The amount of each of these species that is generated is dependent upon: the

concentration of the materials that go to make them up in a non-linear fashion;

the electron current; the ion source geometry; and the electron energy in theion source.

H3+ and H3O

+ are only present in the PPA and not in the vacuum system.

Dempster discovered the meta-stable tri-atomic hydrogen ion in the early 20th

century.

Advanced topics:

What happens to these ions if they collide with a surface?

What happens to these ions if they drift freely in a vacuum?

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Slide 3.09 Typical Hydrogen/Deuterium Spectrum

Purpose: To show a spectrum of tri-atomic hydrogen (deuterium).

Points of emphasis:

Deuterium was used to accurately discern the amount of tri-atomic hydrogenions produced.

Identify the potential makeup of each mass peak from AMU=1-6.

Masses 1, 5 and 6 have single constituents.

Masses 2, 3 and 4 consist of multiple constituents.

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Slide 3.10 Triatomic Deuterium Formation as a Function of Ion Source

Operating Parameters

Purpose: To show the dependence of tri-atomic hydrogen ion formation upon various

operating parameters in an ion source.

Points of emphasis:

Point out dependence of tri-atomic ion formation on the electron current.

Point out how tri-atomic hydrogen ion formation depends upon electron

bombardment energy.

Advanced topics:

Why does the tri-atomic ion formation depend upon electron current?

Why does tri-atomic ion formation depend upon electron energy?

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Slide 3.11 Triatomic Hydrogen Ion Formation as a Function of Molecular

Hydrogen Concentration

Purpose: To show the dependence of tri-atomic hydrogen ion formation on the di-

atomic hydrogen concentration.

Points of emphasis:

The left side plot is for deuterium, but would apply to hydrogen (protium) as

well.

The data was taken from two different quadrupoles with differing geometriesin their ion sources.

The concentration dependence of the tri-atomic ion on di-atomic hydrogen is

of the form H3+ "H2

n . Where n = 1.3 and 1.5 respectively for left and right

plots.

Advanced topics: What would be the expected maximum value for n?

What conditions would maximize n?

Why does n differ for the two ion sources, and from nmax?

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Slide 3.12 Commonly Seen Multicharged Ions

Purpose: To give examples of the common multi-charged species and where they will

appear in a spectrum.

Points of emphasis: Multi-charged species are present in the PPA and not in the remainder of the

vacuum system.

They are, however, usually due to parent molecules that are extant in the

vacuum system.

They are stable until they collide with a surface or a gas molecule.

Multi-charged ions of other atoms and molecules are possible, but usually in

lower relative concentrations.

Advanced topics:

How is the concentration of multi-charged ions distorted in a Faradaycollector?

How is the concentration of multi-charged ions distorted in an electron

multiplier detector?

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Slide 3.13 Species Produced by the Filament

Purpose: To give examples of species that are generated from an ion source filament.

Points of emphasis:

Carbon oxides produced by a filament outgassing and reaction may beindependent of gasses in the vacuum system.

A common source of oxygen for production of carbon oxides by the filament

is water vapor from the vacuum system.

Oxygen ions (O2+) are produced by ion sputtering or evaporation from thoria

impregnated filaments.

Any of the filament-produced species may escape to the vacuum system,

although escape is unlikely.

Advanced topics:

How can carbon oxides be produced on the filament independent of carbon oroxygen in the vacuum?

Why doesnt a cold filament produce carbon oxides?

Is the amount of filament-produced species representative of the concentration

of the elements of these species in the ion source? Why?

Why is escape of filament-produced species to the vacuum unlikely?

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Slide 3.14 Fragmentation Species Produced in an Ion Source

Purpose: To tabulate expected fragment ions from commonly seen parent molecules.

Points of emphasis:

The tabulated species are nearly always present in PPA spectra. Fragment ion species peak amplitudes depend upon the operating parameters

of the PPA ion source i.e. filament material, operating temperature, electron

energy etc.

Fragment ions and neutrals are well confined to the PPA. They are generally

not out in the vacuum system.

Advanced topics:

What are the consequences of fragment species in the ion source?

Are fragment ions useful in identifying parent molecules? How?

What can be done to control fragmentation? Why do fragment species remain primarily in the PPA?

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Slide 3.15 Effect of Bombardment Voltage on Fragmentation

Purpose: To show the impact of electron bombardment energy on fragmentation.

Points of emphasis:

The spectra were taken in a stable, i.e. pressure constant, vacuum system. The only change was the electron bombardment energy being changed from

50 to 70 eV.

The mass 17 fragment ion of water is 0.31 of the parent mass 18 peak when

the electron energy is 50 eV and 0.39 when the electron energy is 70 eV.

Note that all of the peaks have smaller amplitude with the lower electronbombardment energy.

Advanced topics:

Are the mass peak amplitudes expected to increase as electron energy is

increased? Why? What is the optimum electron energy? Explain?

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Slide 3.16 Thermal Fragmentation by Hot Filaments

Purpose: To show the impact of filament operating temperature on fragment ion

production.

Points of emphasis: The PPA had filaments of W (ThO2) and LaB6 symmetrically mounted with

respect to the ion source and analyzer.

Ion source voltages and emission currents were held constant. Only the

operating temperature and material of the filament were varying. Thehexaboride filament operated at approximately 1000 K. The thoria filament

operated at approximately 1650 K.

The PPA was a R.F. linear type instrument.

Note the spectrum complexity increases with temperature.

The thoria filament shows the characteristic oxygen peak at m/q= 32.

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Slide 3.17 Occasionally Seen Important Contaminants

Purpose: To tabulate contaminant species and fragments from primarily acid

treatment residue.

Points of emphasis: Hydrochloric acid appears at masses 36 and 38 in a 3:1 ratio. Chlorine at

masses 35 and 37 will be seen in the same ratio as fragments.

Nitric acid residue often appears as nitrogen (m/q=28) and nitric oxide

(m/q=30). The parent (m/q=63) may or may not appear. Other sources ofN2and NO are also possible.

Sulfuric acid and other sulfur residues usually appear at m/q=48 (SO+) and

m/q=64 (SO2+). The appearance of only one of the peaks indicates that the

species is probably not sulfur related.

======================= End Section 3 ====================

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Section 4. SAMPLES OF SPECTRA; ION SOURCE EFFECTS

Slide 4.01 A Typical Spectrum of a Relatively Clean System

Purpose: To show a PPA spectrum of a relatively clean vacuum system and identifythe peaks.

Points of emphasis:

Point out and identify the parent, isotopic, meta-stable, multi-charged and

fragment ion mass peaks on this spectrum.

Note the logarithmic amplitude scale and its implications.

The amplitudes of the mass peaks are in amperes not pressure units.

Advanced topics:

Is there a possibility that the identification of any of the peaks is wrong?

Which one(s)? Why?

What would one need to do to relate peak amplitudes as pressure rather than

ion currents?

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Slide 4.02 A Spectrum with Air and Organic Contamination of the Vacuum

System

Purpose: To show and explain a PPA spectrum of a vacuum system with an air leak

and organic contamination.

Points of emphasis:

Contamination in the system is primarily organic molecules.

Because of the nature of fragmentation and complexity of organic

compounds, the spectrum becomes much more complex.

Major peaks of nearly all organic compounds are fragments. In the case of

inorganic gasses the major peak is nearly always the parent peak.

Odd numbered mass peak fragments of organics are larger than the adjacent

even numbered mass peak fragments.

Peak amplitudes are in amperes.

Advanced topics:

Identify the species contributing to each mass peak.

Why are odd numbered mass peak fragments larger than adjacent even

numbered mass peak fragments?

What causes the series of prominent peaks separated by fourteen mass units?

Are there ambiguities in the identification of the mass peaks? Why?

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Slide 4.03 Scans during Pump Down at 70ev and 105ev Electron Energy

Purpose: To show the impact of increasing the electron energy from 70 eV (left

spectrum) to 105 eV (right spectrum) on the mass peak amplitudes.

Points of emphasis: The two spectra were taken in sequence during a pump-down of a vacuum

system.

Most of the spectral peaks are seen to decrease as the system is evacuated.

The helium sensitivity increases by a factor of greater than two with the

increase of electron energy in the ion source in this particular PPA. See mass

4.

When leak detecting with helium the electron energy should be temporarily

increased in order to give greater sensitivity.

Note the appearance of O++(m/q = 8) when the electron energy is increased.

Advanced Topics:

Why does the helium sensitivity increase with electron energy?

What are the disadvantages of operating the ion source with greater electron

bombardment energy?

======================= End Section 4 ====================

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Section 5. ION DETECTORS

Slide 5.01 Faraday Cup Detector

Purpose: To show the electrode configuration of a typical Faraday Cup ion collector.

Points of emphasis:

The collector shape is important as well as the presence of the associated

electrodes.

Secondary electron suppression is important.

Advanced topics:

Why is the Faraday collector shape important?

Why is secondary electron suppression important?

Slide 5.02 Faraday Collector Features

Purpose: To give the main features of a Faraday collector.

Points of emphasis:

For high vacuum and ultra-high vacuum (10-5 to 10-9 mbar total pressure)

partial pressure measurements, a Faraday collector is adequate.

A Faraday collector is the simplest to use and is usually drift free.

Multi-charged ions generate larger signals due to their degree of charge.

Therefore multi-charged ion peaks appear larger than they are in a PPAspectrum.

The range of a Faraday collector detector can be extended (2-3 orders) tolower currents (pressure/gas density) through the use of a vibrating reed

electrometer amplifier in the detector circuit.

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Slide 5.03 Secondary Electron Detectors

Purpose: To describe features of two secondary electron multiplier detectors.

Points of emphasis:

All secondary electron multiplier detectors (SEMDs) are sensitive to theirenvironment. This causes the multiplier gain to drift downward with exposure

to the vacuum.

SEMDs come in two types. The older but still used discrete dynode electron

multiplier (DDEM) and the newer continuous dynode electron multiplier(CDEM).

Multi-charged ions, because they have greater velocity at a given energy lead

to potentially larger signals.

Determine the overall gain of an electron multiplier.

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Slide 5.04 Discrete Dynode Electron Multiplier

Purpose: To show a diagram of a discrete dynode electron multiplier and describe

major features of its operation.

Points of emphasis: Dynodes need high ion to electron and electron to electron conversion

efficiencies.

Common dynode materials include silver-magnesium and copper- beryllium

alloys.

Stability of dynodes is an issue. Some recent improvements are claimed. See

the final section of the bibliography (Slide 10.04).

Advanced Topics:

Calculate the average gain per electron required for 10 dynode and 14 dynode

multipliers to give a total gain of the multiplier of 106.

Ion to electron conversion is Gaussian. Assume the conversion of ions to

electrons is 3 with a %of 1. How many ions/100 are not counted?

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Slide 5.05 Continuous Dynode Electron Multiplier

Purpose: To show and describe important features of the continuous dynode electron

multiplier detector.

Points of emphasis: Explain the bend or twist in the CDEM type of detector.

Describe the coating and its functionality.

Describe degradation and end of life of the CDEM and DDEM multiplier

detectors.

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Slide 5.06 Secondary Electron Multiplier for Extreme Low Pressure

Purpose: Describe how DDEM or CDEM detectors are used for detecting extremely

low pressures or gas densities.

Points of emphasis: The multipliers are used in a pulse counting mode. Each ion is counted. But

spurious electronic (noise) signals are not counted.

The useful pressure range for this type of ion detection is about 10-22to about

10-15

mbar. The lower limit is determined by dark current of the entiremultiplier. This can be as low as 0.01 counts /sec with a cooled detector. The

upper limit is determined by counting electronics of about 100 Mhz to 1 Ghz.

and/or the multiplier output dispersion of 1-10 ns.

Ion counting requires more complicated electronics.

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Slide 5.07 Diagram of an Ion Counting System

Purpose: To show a block diagram of the type of electronics used for ion pulse

counting.

Points of emphasis: Cooling the electron multiplier decreases the number of non-ion produced

pulses.

Most ion counting systems are used in conjunction with short pulse time of

flight spectrometers.

Small gain changes in the multiplier are generally not critical to the ion count.

Large gain changes in the multiplier are compensated for by adjusting the

discrimination level.

======================= End Section 5 ====================

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Section 6. MASS ANALYZERS

Slide 6.01 Mass Analyzers Readily Available

Purpose: To describe briefly the history of the mass analyzers used in partial pressureanalysis and the dominance of the quadrupole analyzer for the past several years.

Points of emphasis:

There were a large variety of PPAs available and in use during the early use of

partial pressure analysis.

Presently the quadrupole analyzer is used for nearly all partial pressure

analysis.

Niche applications may still use the Magnetic Sector or Time of Flight type of

analyzers.

Most analytical mass spectrometry is still done with magnetic sector type of

instruments.

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Slide 6.02 Magnetic Sector Analyzer

Purpose: To show a diagram of a simple magnetic sector mass analyzer.

Points of emphasis:

The particular analyzer shown in the diagram is a 60 sector. Other possibledeflection angles of resonant ions are possible e.g. 45, 90, and 180 sectors.

Note that masses lower than the resonant ion mass strike the inner wall of the

analyzer tube. While those of mass greater than the resonant ion mass strike

the outer wall of the analyzer tube.

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Slide 6.03 Operation of the Magnetic Sector Spectrometer

Purpose: To describe the operation of the magnetic sector analyzer.

Points of emphasis:

Most inexpensive magnetic sector instruments use a permanent magnet andscan the mass range by varying the energy of the ions injected into the

magnetic field.

Most magnetic sector PPAs have fixed slit widths, and thus have a fixed

sensitivity and resolution that is mass dependent.

Because of the small size of magnetic PPAs, the instruments have two mass

ranges. The upper mass range is obtained by placing a magnetic shunt on the

magnet.

The lower mass range is typically from mass 2 to mass 15. The upper range is

typically from mass 12 to 100.

Advanced Topics:

What are the advantages of ion energy mass scanning?

What are the advantages of magnetic field mass scanning?

What other parameter could be used to scan the mass range? Is such scanning

practical?

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Slide 6.04 Special Issues with the Magnetic Sector

Purpose: To describe some of the issues and concerns with using a magnetic sector

analyzer.

Points of emphasis: Mechanical slit adjustment becomes troublesome in high and ultra-high

vacuum applications.

Stray fields from the magnet can have an impact on either or both the ion

source and the detector. This is a particular issue when B field massscanning is being used.

Off axis ion injection contributes to spurious (ghost) mass peaks.

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Slide 5.05 Time of Flight Mass Analyzer

Purpose: To show a schematic diagram of the Time of Flight (TOF) analyzer.

Points of emphasis:

The drift tube of length L is about 1 meter for a TOF partial pressureanalyzer

Clearly to get ion separation in 1 meter or less at any reasonable ion injection

energy, the ion pulse must be short, e.g. 1s or less.

The injection circuitry and detector electronics must operate in the sub-

microsecond region.

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Slide 6.06 Operation of the Time of Flight Mass Spectrometer

Purpose: To describe the operation of the Time of Flight analyzer, and some of the

advantages and disadvantages of this analyzer.

Points of emphasis: The TOF analyzer is the only PPA that measures all of the masses present

during the time of the injection pulse.

The TOF is particularly useful in measuring rapidly occurring reactions. For

example: intermediates and by products from explosions, materials ablatedfrom a surface by a laser pulse, progress and products of combustion, etc.

A compromise between the injection energy and the electronic injection and

detection electronics must always be considered. Low injection energy leadsto the possibility of space charge driving ions, particularly those of larger

mass, out of the detector. Whereas greater injection energy means the drifttimes are shortened and the response of the detector must be faster.

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Slide 6.07 Quadrupole Mass Filter

Purpose: To show a diagram of the quadrupole type of analyzer.

Points of emphasis:

Cross sections of the quadrupole rods that solve the Mathieu equations arehyperbolic. Nearly all commercially available quadrupole analyzers use

circular cross section rods.

The value of r0 is modified to compensate for the rod cross section shape

change.

Rod length of commercially available quadrupoles varies from a few

centimeters to about !meter.

Paul and his students built and successfully operated a quadrupole analyzer

with 10 meter long rods. The resolution of such an instrument was between

10-5and 10-6.

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Slide 6.08 Quadrupole Operation

Purpose: To describe the basic operation of the quadrupole analyzer.

Points of emphasis:

Ions traversing the quadrupole fields follow paths that are solutions to theMathieu Differential equations. These are the same equations that are used to

focus ions in high-energy accelerators such as an alternating gradient

synchrotron.

Stable R.F. and D.C. power supplies are required for successful quadrupoleoperation.

The radio frequency used with most instruments is of the order of 10 Mhz.

The spectra produced by quadrupoles that use voltage scan linear with time are

m/q linear with time scans.

The injection energy of ions has little influence on the spectra produced.

Injection energies of a few to a few tens of electron volts are common.

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Slide 6.09 Stability Diagram for the Quadrupole Mass Filter

Purpose: To show a diagram of a stable solution to the Mathieu differential equation,

and point out that resonant ions in a quadrupole traverse this region in going from the

ion source to the detector.

Points of emphasis:

Ions follow paths that oscillate in both the X and Y direction.

Resonant ions follow paths where the magnitude of the oscillations are

confined to the region within the rod structure, and thus pass from the ionsource to the detector.

Ions with m/q less than resonance have confined oscillations in Y direction,

but oscillations in the X direction increase in magnitude until the ions strikethe X rods and the ions are collected (neutralized).

Ions with m/q greater than resonance have confined oscillations in the X

direction, but oscillate in the Y direction with increasing amplitude until they

strike the Y rods and are lost.

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Slide 6.10 Location in Time of Mass Peaks for Various Sweep Parameters

Purpose: To show a scan parameter plot and describe how it has led to the

ascendancy of the quadrupole for partial pressure analysis.

Points of emphasis: K is the sweep or mass scan parameter e.g. magnetic field intensity or voltage.

The top line shows the mass scan as the constant voltage ratio D.C./R.F. is

increased linearly with time in a quadrupole analyzer.

The middle line shows the mass scan for a linear-with-time B field scan in a

magnetic sector analyzer. This scan spacing is also how ions arrive in time at

the detector in a Time of Flight analyzer.

The bottom line gives the mass scan for a linear-in-time ion injection energy

(voltage) scan in a magnetic sector analyzer.

The easier to read and interpret top line generated by a quadrupole in

conjunction with the ability to electrically adjust the resolution-sensitivitytrade-off have led to the dominance of the quadrupole analyzer for partial

pressure analysis.

Non-linear-with-time scans of the scan parameter are possible and have been

used to give linear-with-time mass spectra from magnetic sector analyzers.Such scans do not resolve the resolution-sensitivity adjustment issues in the

magnetic sector analyzer.

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Slide 6.11 Quadrupole Issues

Purpose: To show some important issues encountered when using commercially

available quadrupoles.

Points of emphasis: Although quadrupole analyzers are the dominant instruments used for partial

pressure analysis, they do have some shortcomings.

At pressures above about the mid 10-5 mbar range most quadrupoles suffer

from serious non-linear output. Small quadrupoles exist that extend thepressure range an order higher, but these typically have high background noise

so they cannot measure below about 10-8

mbar.

Every electronic drive package must be specifically tuned to a particular

quadrupole head. This is required because of the small differences in

capacitance from head to head and cable to cable. These small electronicdifferences lead to distorted radio frequency voltages that are not tolerated by

the analyzer.

The total pressure read-out assumes a specific mixture of gasses in the vacuum

for accuracy. Vacuum systems seldom have the usual residual mixture. The

total pressure measure is probably no better than plus or minus a half order of

magnitude in pressure.

Advanced topics:

How might a quadrupole be modified so that it would be not only a mass filter

but also an energy filter?

What contributes to detector noise from the quadrupole analyzer?

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Slide 6.12 Symptoms of Improper Tuning

Purpose: To show some of the characteristics of an improperly tuned quadrupole, and

describe how improper tuning comes about.

Points of emphasis: In this particular spectrum the higher mass peaks (m/q >44) are lost due to

improper matching of quadrupole head and the electronic drive unit.

The head and drive unit are from the same manufacturer.

The drive unit was tuned to the head, after this spectrum was taken, and

spectra showed typical higher mass organic fragments.

Advanced topics:

What are other symptoms of improper tuning?

Is it possible to lose the low mass number peaks rather than the high masses

due to improper tuning?

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Slide 6.13 Mislabelled Spectrum for Hydrogen Selenide

Purpose: To show a spectrum misidentified because of an improper mass scale and

describe how to this has occurred and how to correct the problem.

Points of emphasis: This particular spectrum is displayed in bar mode. Thus the peaks do not

display their characteristic shape.

The mass spectrum numbering is one number low.

Advanced topics:

How could the numbering of the mass spectrum be displaced?

What information is available to assure that the spectrum is displaced?

How much of each peak is due to each H2Se species?

What are the advantages and disadvantages of a bar mode display?

======================= End Section 6 ====================

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Section 7. DATA STORAGE AND DISPLAY

Slide 7.01 Data Display and Storage

Purpose: To describe two methods of displaying and storing the same spectrum.

Points of emphasis:

Linear amplitude displays of mass spectra are particularly useful when only

the major constituents of the vacuum are of interest.

With a linear amplitude display it is important to assure that the largest peak is

fully displayed i.e. it does not saturate the amplifier with too large a signal.

Logarithmic amplitude displays of mass spectra are useful when minor

constituents may play a significant role in the vacuum.

Logarithmic data take up half the computer space that the same range in

amplitude of linear data occupy.

The full dynamic range of an analyzer (typically 5 to 7 orders) can be

displayed on a single logarithmic mass spectrum.

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Slide 7.02 Logarithmic Display of m/q 1 - 72

Purpose: To display a spectrum using a multi-decade logarithmic amplifier.

Points of emphasis:

Each major division e.g. 60 to 70 is a decade on the logarithmic amplifier. The dynamic range of data on this plot is at least 6 decades. The amplifier

used to obtain this data was a 7-decade amplifier.

Assign a mass number to each mass peak. Be careful with this.

This spectrum and the next one (7.03) were taken on the same pressure-stable

vacuum system. They were taken within 3 minutes of each other. Theyshould be viewed together.

Advanced topics:

What are the constituents of each mass peak

There is at least one non-integer mass peak present on this spectrum. Can youidentify its location and composition?

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Slide 7.03 Linear Display of Data in Slide 7.02, m/q 1 - 69

Purpose: To show a linearly displayed spectrum of the same data as displayed in

7.02.

Points of emphasis: Two mass peaks appear on this spectrum that do not appear on 7.02. Which

are they? Why dont they appear on 7.02?

A number of peaks that appeared on 7.02 do not appear on this spectrum.

Which are they? Why dont they?

Point out where this spectrum would fall on the logarithmic spectrum (7.02).

Identify by mass number and composition the peaks that appear on this

spectrum.

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Slide 7.04 Block Diagram of Data Acquisition with Feedback Control

Purpose: To give an illustration of how PPA data may be used as feedback control

information in process control.

Points of emphasis: The dotted line is the connection between the partial pressure analyzer i.e.

sensor and the control functions in the process chamber.

Establishing the proper feedback and control algorithms is a non-trivial

undertaking for any process control system.

Making the dotted line operate properly is usually the most expensive and

difficult part of an automatically controlled process.

Such systems do exist to control oxidation in semiconductor processing.

In experimental and non-repetitive processes a human often acts as the dotted

line and the data analysis part of the diagram.

======================= End Section 7 ====================

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Section 8. SOME APPLICATIONS

Slide 8.01 Air Leak Spectrum

Purpose: To show a PPA spectrum taken from a vacuum system with a large air leak.

Points of emphasis:

Point out the peaks from the major constituents of air, i.e. nitrogen, oxygen,

water, argon and carbon dioxide.

Point out the fragments and multi-charged species associated with the air

peaks.

Advanced topics:

What mass numbers are due to contaminant peaks?

Identify and justify your identification of the contaminants.

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Slide 8.02 Leaky N2Backfill Valve

Purpose: To show a spectrum of a vacuum system with another type of leak.

Points of emphasis:

This spectrum clearly does not indicate the presence of an air leak as can benoted by the absence of many of the constituents of air present in the previous

spectrum.

Many high and ultra-high vacuum systems are backfilled with dry nitrogen or

argon in order to make the return to the vacuum condition easier.

After many operations of backfill valves they often develop small leaks that

will eventually need to be repaired as in this case.

Identify all of the mass peaks due to nitrogen in this spectrum. There are more

than two.

Identify the remaining peaks in the spectrum.

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Slide 8.03 Vacuum System Bakeout

Purpose: To show a PPA spectrum of a vacuum system during bake-out with some

peculiar species present.

Points of emphasis: Identify the mass peaks in the spectrum.

Note the hydrocarbon fragments.

Note the large peaks due to zinc at masses 64, 66 and 68.

Advanced topics:

What is the evidence that the peaks at m/q=64, 66 and 68 are due to zinc?

Do other possibilities exist? What are they?

Do multiple possibilities exist for the other mass peaks in the spectrum? What

are they?

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Section 9. QUANTITATIVE AND QUALITATIVE DATA

Slide 9.01 Qualitative vs. Quantitative Data

Purpose: To describe how a PPA is used to acquire quantitative partial pressure data.

Points of emphasis:

A source of high purity gas is required for calibrating the sensitivity of a PPA

to that gas.

It is possible to use certified high purity gas mixtures to calibrate for several

species.

Calibrations should be made before and after making measurements of

unknowns until reliable stable performance of the instrument can be assured.

Calibration procedures as developed and refereed by reputable, knowledgeable

organizations should be used to calibrate partial pressure analysis systems.

See as an example the AVS Recommended Practice listed in the bibliography

(Slide 10.02).

Advanced topics:

In calibrating for methane one notices that the m/q=17 peak amplitude is 2.5%

of the m/q=16 peak amplitude at a presumed high purity methane pressure of

1X10-6mbar. Does this have an impact on the calibration for methane?

If it does have an impact, what is its magnitude?

What would the expected impact be when calibrating at a presumed pressure

of 1X10-7mbar?

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Slide 9.02 Diagram of an Orifice Flow Calibration System

Purpose: To show a block diagram of a recommended calibration system for

acquiring quantitative partial pressure information.

Points of emphasis: The diagram is taken from the AVS Recommended Practice for PPA

calibration (Slide 10.02).

The orifice flow procedure is one of several methods for calibrating a PPA.

In all procedures some sort of standard certified by a standards organization is

required. The certified standard may be a gauge calibrated for pressure of a

particular gas or a gas flow certified standard, as is used in the orifice flow

calibration system.

Using the method shown in the diagram, in addition to having gas flow

standards for calibrating the gases of interest, the orifice must be carefully

fabricated and measured in order to have an accurate calibration.

The value of C is gas and temperature dependent.

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Section 10. BIBLIOGRAPHY/REFERENCES

Slide 10.01 Bibliography I (Props of Elements, Isotopes & Compounds)

Slide 10.02 Bibliography II (Vacuum)

Slide 10.03 Bibliography III (PPA Spectra Interpretation)

Slide 10.04 Bibliography IV (Instrumentation)

Purpose: These slides display a bibliography of information pertaining to all Slides

from 1.01 through 9.02.

Points of emphasis:

The first reference material covers general data on physical and chemical

properties of elements and compounds. Information contained in thesereferences should be useful in determining parent peak masses as well as

isotopic peak masses.

The second set of references concerns information of a general vacuum nature.Basic operating principles of some PPAs and vacuum gauges are given.

Collisions of electrons, ions and neutrals with gasses and surfaces are

described in some detail.

The third set of references is about recommended calibration procedures for

gauges and PPAs. Some additional information on operating conditions and

principles for PPAs and gauges is given, particularly in Lecks book.

The fourth set of references deals with interpreting mass spectra. Referencespectra are given. Caution should be used when using reference spectra.

Although the location of m/q peaks for various species is useful theamplitudes quoted will most certainly be quite different for various PPAs.

The last set of information gives references with more detailed information

about the various currently used PPAs as well as those from the past. Other

instrumentation detailed information is also included.

The intent is not that the bibliography included here be all inclusive, but rather

that it be a start for further investigation and incentive to gain greater

understanding of the power and usefulness of partial pressure analysis.

======================= End Section 10 ====================

================== End Module 4================

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