VISUAL AIDS PROGRAM of the International Union for

VISUAL AIDS PROGRAM of the International Union for Vacuum Science, Technique and Applications Module 4 PARTIAL PRESSURE ANALYZERS AND ANALYSIS © 2004. IUVSTA PPA 0. 00

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 1 Introductory Material and Terminology © 2004. IUVSTA PPA 1. 00

Comparing the Need for a Total Pressure Gauge with that for a Partial Pressure Analyzer • • • WHEN Total pressure gauges are appropriate Partial pressure gauges are appropriate WHY Cost effectiveness Process demands © 2004. IUVSTA PPA 1. 01

Relationship of a PPA to an Ion Gauge and a Mass Spectrometer © 2004. IUVSTA PPA 1. 02

TERMINOLOGY - Review • Element • Isotope • Atom • Isotopic Abundance • Molecule • Compound © 2004. IUVSTA PPA 1. 03

TERMINOLOGY - General • PPA, Vacuum Analyzer, RGA • Mass Number & AMU • Mass to Charge Ratio, m/q • Gas Pressure & Gas Density • Units for Pressure © 2004. IUVSTA PPA 1. 04

TERMINOLOGY - General • Dynamic Range • Minimum Detectable Partial Pressure • Sensitivity • Resolution & Resolving Power © 2004. IUVSTA PPA 1. 05

DETERMINATION OF RESOLVING POWER/RESOLUTION Example © 2004. IUVSTA PPA 1. 06

EXAMPLES OF ALTERNATIVE DETERMINATIONS OF RESOLUTION ∆M Y Y XY XY M X Noise level m/e M M+1 XY (YM + YM+1 ) Y YM+1 YM M M+1 M m/e © 2004. IUVSTA M+1 m/e PPA 1. 07

TERMINOLOGY - Ion Source • Fragmentation/Cracking • Impact Fragmentation • Thermal Fragmentation • Ionization Efficiency/Ionization Probability • Ion Charge – Multi-Charged Ions • EID – Electron Induced Desorption © 2004. IUVSTA PPA 1. 08

TERMINOLGY - Ion Source & Analyzer • Charge Exchange • Space Charge • IID - Ion Induced Desorption • Off Axis Ions © 2004. IUVSTA PPA 1. 09

TERMINOLOGY - Analyzer • Mass Range • Scan Rate • Scan Parameter • Transmission • Ghost Peaks © 2004. IUVSTA PPA 1. 10

TERMINOLOGY - Detectors • Faraday Detector • Multiplier Detector • Ion Counting © 2004. IUVSTA PPA 1. 11

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 2 Influence of the Components of a PPA on Data © 2004. IUVSTA PPA 2. 00

BLOCK DIAGRAM OF A PPA SYSTEM © 2004. IUVSTA PPA 2. 01

OVERVIEW Potential Impact of the Ion Source on Data • Ion Source Surface Chemistry/Physics • Species Fragmentation by Heat • Species Fragmentation by Energetic Particles • Generation of Non-Representative Artifacts • Greatest Source of Information and Misinformation © 2004. IUVSTA PPA 2. 02

OVERVIEW Potential impact of the Analyzer Section on Data • Analyzer Bypass • Analyzer element collision effects • Tuning • Wall Collisions © 2004. IUVSTA PPA 2. 03

OVERVIEW Potential impact of the Ion Detector on Data • Ion Collision at the Detector • Ion Neutralization • Photon Effects • Secondary Electron Effects © 2004. IUVSTA PPA 2. 04

OVERVIEW Data Collection and Display • Conversion of Analog to Digital Information • Computer Data Storage • Possible Non-linearities • Noise © 2004. IUVSTA PPA 2. 05

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 3 Ion Sources © 2004. IUVSTA PPA 3. 00

METHODS FOR PRODUCING IONS © 2004. IUVSTA PPA 3. 01

ELECTRON BOMBARDMENT ION SOURCE © 2004. IUVSTA PPA 3. 02

IONIZATION EFFICIENCY CURVES © 2004. IUVSTA PPA 3. 03

AN OPEN ION SOURCE © 2004. IUVSTA PPA 3. 04

NIER TYPE CLOSED ION SOURCE © 2004. IUVSTA PPA 3. 05

TABLE OF PARENT IONS (PRINCIPAL ISOTOPES ONLY) EXPECTED IN A LEAK TIGHT VACUUM SYSTEM. Species Hydrogen Mass No. Formula 2 Expected peak size Source H 2 major metals major insulators & S. S. Water 18 H 2 O Carbon Monoxide 28 CO major-minor Carbon Dioxide 44 CO 2 minor insulators, copper & Cu alloys minor atmospheric contamination Hydro. Carbons 86 -142 C 6 H 14 - C 10 H 22 © 2004. IUVSTA metals PPA 3. 06

TABLE OF PARENT IONS WITH MAJOR ISOTOPES INCLUDED Species Mass No. Hydrogen 2 H 2 1 Water 18 20 H 216 O H 218 O 1 0. 002 Carbon Monoxide 28 29 30 12 C 16 O 1 0. 011 0. 002 44 45 46 12 C 16 O Carbon Dioxide Hexane 86 87 88 Formula 13 C 16 O 12 C 18 O 13 C 16 O 2 2 12 C 16 O 18 O 12 C 6 H 14 13 C 12 C H 5 14 13 C 12 C H 2 4 14 Peak normalized to parent 1 0. 011 0. 004 1 0. 066 0. 002 Decane 142 143 144 12 C 10 H 22 13 C 12 C H 9 22 13 C 12 C H 2 8 22 © 2004. IUVSTA 1 0. 108 0. 005 PPA 3. 07

METASTABLE IONS GENERATED IN THE ION SOURCE • H 3 + (m/q = 3) + • 0. 1 to 10% of H 2 (nonlinearly pressure dependent) see Slides 3. 09 -3. 11 • Depends upon operating conditions in the ion source • H 3 O + (m/q = 19) • Non linearly pressure dependent • Depends upon ion source operating conditions © 2004. IUVSTA PPA 3. 08

Typical Hydrogen/Deuterium Spectrum Deuterium and protium in a vacuum system. m/q = 1 - 6 © 2004. IUVSTA PPA 3. 09

TRIATOMIC DEUTERIUM FORMATION AS A FUNCTION OF ION SOURCE OPERATING PARAMETERS BOMBARDING ELECTRON ENERGY e. V © 2004. IUVSTA PPA 3. 10

TRIATOMIC HYDROGEN ION FORMATION AS A FUNCTION OF MOLECULAR HYDROGEN CONCENTRATION DEUTERIUM PRESSURE ( TORR ) © 2004. IUVSTA PROTIUM ( I 2 ) CURRENT AT MASS 2 PPA 3. 11

COMMONLY SEEN MULTICHARGED IONS m/q Argon Ar + + + Ar 4 + 40 20 13. 3 10 Carbon Dioxide C 02 + + 44 22 Carbon Monoxide C 0 + + 28 14 © 2004. IUVSTA PPA 3. 12

SPECIES PRODUCED BY THE FILAMENT Carbon Monoxide m/q = 28 especially by Tungsten Carbon Dioxide m/q = 44 especially by Tungsten Oxygen m/q = 32 by sputtering or evaporation from Thoriated Filaments © 2004. IUVSTA PPA 3. 13

FRAGMENTATION SPECIES PRODUCED IN AN ION SOURCE m/q (parent) m/q (fragments) Hydrogen 2 1 Water 18 17, 16, 1 Carbon Monoxide 28 16, 12 Carbon Dioxide 44 28, 16, 12 Methane 16 15, 14, 13, 12, 1 Heavier Organics 30 29, 27, 26, 15, 14, 13, 12, 1 44 43, ------ 36 plus those above 58 57, ------ plus those above © 2004. IUVSTA PPA 3. 14

Effect of Bombardment Voltage on Fragmentation 18 18 © 2004. IUVSTA PPA 3. 15

THERMAL FRAGMENTATION BY HOT FILAMENTS With La B 6 filament 2 1 With W(Th 02) filament Comparison spectra from a calibrated ML- 494 with lanthanum hexaboride filament and with thoriated tungsten filament. Total pressure about 5 x 10 - 8 mbar. © 2004. IUVSTA PPA 3. 16

OCCASIONALLY SEEN IMPORTANT CONTAMINANTS formula m/q Nitric Oxide N 0 30 Nitrogen N 2 28 Hydrochloric Acid HCl 36, 38 Sulfur Dioxide S 02 64, 48 38=1/3 of 36 amplitude These species are often associated with incomplete removal of Acid Residues © 2004. IUVSTA PPA 3. 17

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 4 Samples of Spectra; Ion Source Effects © 2004. IUVSTA PPA 4. 00

A TYPICAL SPECTRUM OF A RELATIVELY CLEAN SYSTEM © 2004. IUVSTA PPA 4. 01

A SPECTRUM WITH AIR AND ORGANIC CONTAMINATION OF THE VACUUM SYSTEM © 2004. IUVSTA PPA 4. 02

SCANS DURING PUMP DOWN AT 70 e. V and 105 e. V ELECTRON ENERGY At 105 e. V At 70 e. V © 2004. IUVSTA PPA 4. 03

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 5 Ion Detectors © 2004. IUVSTA PPA 5. 00

FARADAY CUP DETECTOR © 2004. IUVSTA PPA 5. 01

FARADAY COLLECTOR FEATURES • Secondary Electron Suppression • Multicharged Ions Distort Number of Ions • Practical Lower Current Limit 10 -14 - 10 -15 amp • Corresponds to Lower Pressure Limit of 10 -10 - 10 -11 mbar © 2004. IUVSTA PPA 5. 02

SECONDARY ELECTRON DETECTORS • 2 Types – Discrete Dynode Electron Multiplier (DDEM) and Continuous Dynode Electron Multiplier (CDEM) • High Ion to Electron Conversion Efficiency Required (Ion Velocity Dependent) • DDEM Requires Stable and High Electron – Electron Conversion Efficiency • CDEM are Single (Channeltron) or Multichannel (channel plate) • Output Limited to About 10 µ A • Overall Gain: 10 4 - 10 8 © 2004. IUVSTA PPA 5. 03

DISCRETE DYNODE ELECTRON MULTIPLIER © 2004. IUVSTA PPA 5. 04

CONTINUOUS DYNODE ELECTRON MULTIPLIER © 2004. IUVSTA PPA 5. 05

SECONDARY ELECTRON MULTIPLIER FOR EXTREME LOW PRESSURE • Ion Counting • Pressure Range from about 10 - 22 mbar to about 10 -15 mbar • Lower Limit set by noise for cooled multiplier to approximately 0. 01 count/second • Upper Limit set by counting electronics and multiplier dispersion to approximately 100 mhz to 1 ghz • Complicated Electronics © 2004. IUVSTA PPA 5. 06

DIAGRAM OF AN ION COUNTING SYSTEM • Pulse Shaper – converts SEM pulse to shape useful for the counter • Discriminator – eliminates small amplitude noise pulses from the counter • Pulse Counter – counts each appropriate (ion created) pulse © 2004. IUVSTA PPA 5. 07

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 6 Mass Analyzers © 2004. IUVSTA PPA 6. 00

MASS ANALYZERS READILY AVAILABLE 1954 – 1965 Magnetic • cycloidal • sector • omegatron Non – Magnetic • R. F. Linear • Farvitron • Time of Flight 1965 - Present • Magnetic Sector – in niche applications • Time of Flight – For fast chemistry • Quadrupole - Dominant © 2004. IUVSTA PPA 6. 01

MAGNETIC SECTOR ANALYZER © 2004. IUVSTA PPA 6. 02

OPERATION OF THE MAGNETIC SECTOR SPECTROMETER • Ion trajectories are governed by the following relationship: where r = radius of the ion path (meters) V = energy of the ion (electron volts) B = magnetic field intensity (weber/m 2 ) • Resolution is improved by narrowing the entrance and exit slits (s 1 and s 2) but sensitivity is decreased as the slits are narrowed. • Scanning the mass range is accomplished by either scanning the injection energy (V) or the magnetic field intensity (B) © 2004. IUVSTA PPA 6. 03

SPECIAL ISSUES WITH THE MAGNETIC SECTOR • A time linear scan of either the magnetic field or the injection energy gives an m/q scan which is non-linear with time • Ghost peaks are produced by non-resonant ions colliding with walls and charge exchange • In order to adjust resolution/sensitivity entrance and exit slits must be mechanically adjusted © 2004. IUVSTA PPA 6. 04

TIME OF FLIGHT MASS ANALYZER © 2004. IUVSTA PPA 6. 05

OPERATION OF THE TIME OF FLIGHT SPECTROMETER • Ion trajectories are governed by the following relationship : L = drift tube length (meters) t = drift time (seconds) V = energy of ion (electron volts) • Resolution is improved by using shorter pulses of ions, sensitivity decreases proportionally • Drift times are microseconds therefore, detectors are electron multipliers • Snapshot of species present during the gated pulse • Only instrument capable of sub-millisecond response (Time resolved reactions) © 2004. IUVSTA PPA 6. 06

QUADRUPOLE MASS FILTER © 2004. IUVSTA PPA 6. 07

QUADRUPOLE OPERATION • Ions follow paths that are solutions to the Mathieu Differential Equations • For a given combination of D. C. and R. F. potentials applied to the Quadrupole rods, only ions of one given m/q value are transmitted from the ion source to the detector (see 6. 09) • As the D. C. /R. F. ratio approaches 0. 168, the resolution increases and sensitivity decreases • A voltage linear with time scan gives a linear with time m/q display as output • Ion injection energy has minimal impact as long as an Ion sees about 10 r. f. cycles • Off axis ions are discriminated against © 2004. IUVSTA PPA 6. 08

STABILITY DIAGRAM FOR THE QUADRUPOLE MASS FILTER _ © 2004. IUVSTA PPA 6. 09

LOCATION IN TIME OF MASS PEAKS FOR VARIOUS SWEEP PARAMETERS 2 Dominance of Quadrupole because: • linear mass scan • Resolution/sensitivity trade off controlled electronically by D. C. / R. F. ratio © 2004. IUVSTA PPA 6. 10

QUADRUPOLE ISSUES • Heavy Ions collect on Y rods. Over time, this can lead to electric field distortion • Ions bypassing rod structure to detector add to noise level • Quadrupole head and Quadrupole drive circuitry need to be tuned together (see 6. 12) • Quadrupole if not carefully mass number tuned can be misleading (see 6. 13) • Quadrupole with total pressure readouts are not to be relied upon © 2004. IUVSTA PPA 6. 11

SYMPTOMS OF IMPROPER TUNING Note peak narrowing toward higher mass numbers © 2004. IUVSTA PPA 6. 12

MISLABELLED SPECTRUM FOR HYDROGEN SELENIDE © 2004. IUVSTA PPA 6. 13

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 7 Data Storage and Display © 2004. IUVSTA PPA 7. 00

DATA DISPLAY AND STORAGE Linear Amplitude Display • Displays major contributors only • Range about 100: 1 in amplitude Logarithmic Amplitude Display • Tends to distort importance of minor contributors • The full dynamic range of the analyzer can be displayed. This is typically 5 to 7 orders • Requires less computer space to store data. That is a logarithmic display requires one word vs. 2 words for a linear display over the full range © 2004. IUVSTA PPA 7. 01

LOGARITHMIC DISPLAY OF m/q 1 - 72 © 2004. IUVSTA PPA 7. 02

LINEAR DISPLAY OF DATA IN SLIDE 7. 02, m/q = 1 - 69 10 -7 amp full scale display -7 Note m/q’s 2, 17, 18, and 28 are saturated (off scale) Note that very small peaks are nearly lost in the noise (baseline) © 2004. IUVSTA PPA 7. 03

BLOCK DIAGRAM OF DATA ACQUISITION WITH FEEDBACK CONTROL (dotted line) 0/1 = Digital to Digital device © 2004. IUVSTA PPA 7. 04

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 8 Some Applications - Leaks - Vacuum Bake-out © 2004. IUVSTA PPA 8. 00

AIR LEAK SPECTRUM • Note 4 to 1 ratio of N 2 to 02 (m/q = 28, 32) • Significant Argon Peak m/q = 40 and C 0 2 m/q = 44 © 2004. IUVSTA PPA 8. 01

LEAKY N 2 BACKFILL VALVE • Vacuum system nitrogen backfill valve has a leak at the seat • Note the absence of oxygen at m/q = 32 © 2004. IUVSTA PPA 8. 02

VACUUM SYSTEM BAKEOUT • Deuterium was introduced to this system. Peaks occur at masses 5 and 6 from Triatomic ions • Hydrocarbons give many of the notable peaks from m/q = 12 up © 2004. IUVSTA PPA 8. 03

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 9 Qualitative and Quantitative Data © 2004. IUVSTA PPA 9. 00

QUALITATIVE vs. QUANTITATIVE DATA • Calibration of PPA requires certification by a Standards organization of calibrated leak (Q) and use of orifice flow technique. • = Pc convenient if Pp << Pc C is calculated from its dimensions or by correcting measurements to another calibrated gas • Assure that isotope ratio is correct e. g. for methane mass 17 it should be 1. 1% of m/q = 16, for oxygen mass 34 should be 0. 4% of m/q = 32. Values larger than the expected ratio probably indicate gases are present which may also have fragments at the peak being calibrated e. g. water has fragments at m/q = 17 and 16, whilst mass 16 is the normally calibrated peak for methane. © 2004. IUVSTA PPA 9. 01

DIAGRAM OF AN ORIFICE FLOW CALIBRATION SYSTEM © 2004. IUVSTA PPA 9. 02

PARTIAL PRESSURE ANALYZERS AND ANALYSIS Section 10 Bibliography/References © 2004. IUVSTA PPA 10. 00

BIBLIOGRAPHY – I Reference Material on Physical and Chemical Properties of Elements, Isotopes and Compounds • CRC Handbook of Chemistry and Physics, 66 th ed. Robert Weast editor 1985 CRC Press Boca Raton FL • American Institute of Physics Handbook, 2 nd ed. 1963 Mc. Graw-Hill Inc. New York, NY • Lange’s Handbook of Chemistry, 14 th ed. 1992 Mc. Graw-Hill Inc. New York, NY • The Merck Index, 12 th ed. 1996 Merck & Co. Inc. Rahway, NJ © 2004. IUVSTA PPA 10. 01

BIBLIOGRAPHY – II Reference Material on Vacuum • Foundations of Vacuum Science and Technology, James M. Lafferty editor 1998 John Wiley and Sons Inc. New York, NY • The Physical Basis of Ultrahigh Vacuum, P. A. Redhead, J. P. Hobson and E. V. Kornelson 1968 Chapman and Hall, London (AVS reprint 1993) • Total and Partial Pressure Measurements in Vacuum Systems, J. H. Leck 1989 Blackie and Son Ltd. London • Recommended Practice for the Calibration of Mass Spectrometers for Partial Pressure Analysis, James A Basford 1991 AVS Recommended Practice © 2004. IUVSTA PPA 10. 02

BIBLIOGRAPHY – III Reference Material on PPA Spectra Interpretation • Mass Spectral Data – American Petroleum Institute Research Project 44 • Index of Mass Spectral Data, ASTM-STP 356, ASTM Philadelphia Pa. • Interpretation of Mass Spectra, F. W. Mc. Lafferty, 2 nd ed. 1973 W. A. Benjamin Inc. Reading, Massachusetts • A Beginners Guide to Mass Spectral Interpretation, T. A. Lee, 1998 John Wiley & Son Ltd. Chichester, England • Partial Pressure Analyzers and Analysis – AVS Monograph Series, AVS, New York, NY © 2004. IUVSTA PPA 10. 03

BIBLIOGRAPHY – IV Reference Material on Instrumentation • A Method of Preparation of La. B 6 Cathodes by M. Nasini and G. Redaelli, Rev. Sci. Inst. vol. 42 #12 (1971). • Electron Multipliers for 2000 and Beyond by John Gray, American Laboratory July 2000. • Dynamic Mass Spectrometers by Erich W. Blauth, 1965 Elsevier New York, NY. • Time of Flight Mass Spectrometer with Improved Resolution by W. C. Wiley & I. H. Mc. Laren, Rev. Sci. Inst. vol. 26 1150 (1955). • The Electric Mass Filter as a Mass Spectrometer and Isotope Seperator by W. Paul, H. P. Reinhard and U. Von Zahn, Z. Physik vol. 152, 143 -182 (1958) © 2004. IUVSTA PPA 10. 04