Soft Ionisation Techniques, Terry Whitmore, Hiden Analytical

Transcription

1 Techniques Terry Whitmore Hiden Analytical Ltd. Introduction The RGA EI source EI at 70eV Threshold Ionisation Electron Attachment Ionisation in Residual Gas Analysis Summary The RGA EI Source EI at 70eV Ionisation Cross Sections I ions EI at 70eV e U = electron energy eV e U A e A 2e However: A e A 2 3e (multiple ionisation) AB e AB 2e (direct ionisation) A B 2e or A B 2e (dissociative ionisation) RGA-7, 20 September 2006, Daresbury 1

2 Faraday : torr 6e-06 5e-06 4e-06 3e-06 2e-06 1e-06 HAL IV RC RGA 201 #10940 N 2 m/e 28,14 Cracking Pattern! H O 2 m/e 32 2 O m/e 18,17,16 CO 2 m/e 44,28,16 Ar m/e Cycle number 1 mass : amu QMS signal (counts / sec) CH 4 E e - = 70 ev mass / charge (amu / e) CH 4 e ---> CH 4 2e CH 3 H 2e, CH 2 2H 2e CH 3H 2e, C 4H 2e H CH 3 2e,... 1% 13 CH 4 (isotopes a further complication) Interferences from the Gas Matrix Argon process gas, 10mTorr, 70eV electron energy The EI spectra from species of simple molecular structure become even more complex as the dynamic range is extended. N 2 produces peaks at 7, 7.5, 14, 15, 28, 29, 30 and 42 for example. H 2 O / 36 Ar 40 Ar 40 Ar At PPM levels tailing from larger peaks may affect the neighbouring peak as well (abundance sensitivity). 36 Ar In processing applications there will also be interferences from the gas matrix at these low levels. Interferences from the Gas Matrix Argon process gas, 10mTorr, 35eV electron energy : Principle 40 Ar H 2 O/ 36 Ar 40 Ar 36 Ar 1. Threshold Ionisation Different processes have different thresholds Operate around these thresholds Reduce fragmentation Simplify spectral interpretation Introduce selectivity RGA-7, 20 September 2006, Daresbury 2

3 1: Threshold Ionisation Ionisation energies of simple inorganic and organic molecules N 2 O 2 Ar CO CO 2 NH 3 NO CH 4 Air 70eV Solvent/ Air Mix Nitrogen Oxygen Argon monoxide dioxide Ammonia Nitric oxide Methane 0.0E00 0.0E C 2 H 2 Ethyne C 2 H 4 Ethene C 2 H 4 O 2 Acetic acid C 2 H 6 Ethane C 2 H 5 OH Ethanol C 3 H 6 O Propanal C 3 H 6 O Acetone C 3 H 8 O IPA 15eV E00 0.0E00 C 2 H 6 O C 6 H 6 C 6 H 14 C 6 H 12 CH 3 OH CH 2 O 2 CH 2 O CCl 4 DME 9.72 Benzene 9.24 Hexane (n) Formaldehyde Cyclohexane 9.86 Methanol Formic acid Tet eV m/e=40 NOT Ar! acetone IPA *10 0.0E00 *10 0.0E00 Standard 70eV TIMS Electronegativity sensitivity () selectivity (identification) versatility (overview) () time consumption quantification Also electron attachment to molecular species Electron impact ionisation cross section Electron attachment ionisation cross section Positive ion formation Negative ion formation RGA-7, 20 September 2006, Daresbury 3

4 bulk plasma power neutrals radicals reactive gas methane ions CF 3- ions from CF 4 Plasma Off (blue trace) e CF 4 CF - 4 (ground state) sheath abstraction addition growth cross linking zone annealing chemistry synergisms bulk electrons physics implantation sputtering displacement Intensity : c/s Plasma On (red trace) additional signal due to : e CF 3 (created in plasma) CF electron energy : V CF 3 - F C 4 F 8 Etching Plasma EAMS at 2.0eV Standard 70eV () Standard (70eV) MS, TIMS and EAMS are complementary sensitivity selectivity (identification) TIMS EAMS Ionisation at 70eV versatility (overview) time consumption quantification () () Teii et al so why not utilise all of them! in Residual Gas Analysis EAMS: Negative Ion Spectrum Ea=12eV, P tot <1E-9mbar in Residual Gas Analysis Exclusively ESD Peaks (Positive Ion Spectrum 70eV Electrons) O - virtually identical spectrum at 2.9eV ESD not gas phase spectrum? F H - HeH-? OH - F - CN - (?) O 2-35 Cl - 37 Cl - H O OH 35 Cl 37 Cl RGA-7, 20 September 2006, Daresbury 4

5 Summary - TIMS and EAMS are soft ionisation techniques that complement 70eV ionisation. - TIMS and EAMS introduce selectivity. - TIMS and EAMS are powerful tools in complex gas analysis and plasma diagnostics. - EAMS has potential for UHV/XHV RGA measurement. Thank you. RGA-7, 20 September 2006, Daresbury 5