Secondaryionmassspectrometry

Transcription

1 Secondaryionmassspectrometry (SIMS) 1 Incident Ion Techniques for Surface Composition Analysis

2 Mass spectrometric technique 1. Ionization -Electron ionization (EI) -Chemical ionization (CI) -Field ionization (FI) -Field desorption (FD) - Secondary ion mass spectrometry (SIMS) 2. Fragmentation M+. positive ions negative ions neutral or molecular fragments 3. Ion mass analysis M: fi M+. + e- molecule molecular ion Bulk technique Surface technique Ex. Mass CH +. 4 = MW of CH 4 mass e - 2 = = 16

3 Ionization in SIMS Sputtering process or Ion etching Also called Ion Scattering spectroscopy (ISS) 95% of particles generated from first two layers (~1 nm) (~hundreds evto few kev) Primary ion Collision cascade in near-surface region, resulting in primary and secondary recoils Primary recoils Sample damages throughout the ion track Primary ion is implanted in material Secondary recoils Particles are emitted (sputtered) from the surface. 3

4 At 0 fs PI Primary recoils At 100 fs Secondary ions Time snapshots from a molecular dynamics simulation At 200 fs Secondary ions Secondary recoils 4

5 Sputtering process or Ion etching Useful technique for Oxide layer formed on Ta metal 1) Surface cleaning 2) Depth profiling (for AES and XPS) 3) Spectroscopy through fragment analysis Neutrals (>99%) Secondary neutral mass spectrometry (SNMS) Secondary ions (<1%) Secondary ion mass spectrometry (SIMS) 5

6 Secondary ion mass spectrometry (SIMS) Secondary ions originate from top few atomic layers surface sensitive Positive SIMS Negative SIMS Information from SIMS -Quantitative chemical composition -Structural information Key characteristics of SIMS spectra -Sensitive to isotopic components 6

7 Positive SIMS spectrum from PTFE Structural information from SIMS Fragmentation of PTFE 12 = C + 31 = CF + 50 = CF = C 2 F = C 3 F = C 2 F = C 2 F = C 3 F 5 + 7

8 Extremely low sputtering rate Continuous sputtering negligible surface damage (called non-destructive ) SSIMS The sampling depth of static SIMS is 1-2 nm 8

9 Potentials of SSIMS 1. Polymers fingerprint is related to fragmentation 9

10 Mass spectrum is related to average MW and polydispersity 10

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12 2. Adsorbed molecules Fe exposed to CO gas Fe 2 C 12

13 Energy between 1 30 kev Sputter rate = nm/sec 13

14 Secondary ion yield (implies the SIMS ionization efficiency) = fraction of sputtered atoms that become ionized Elemental effect Low IE high positive ion yield Positive ion High EA high negative ion yield 14

15 Fragmentation of organic substances CH 2 = 14 15

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17 Mass analyzer Quadrupole Most popular mass analyzers 1. Quadrupole 2. Time of flight (TOF) Limitation -typically m/z ranges of 0 to 1000) 4 parallel rods have fixed DC and alternating RF potentials applied to them Ions are passed along the poles and move depending on the electric fields, so that only ions of a particular m/z will be in resonance and passed to detector. 17

18 Time-of of-flight SIMS: How does it work? 2 nd ions are extracted and accelerated through the field-free flight tube All 2 nd ions have same E k E = k 1 2 mv v = (2E k /m) 1/2 2 v a (1/m) 1/2 m/z ranges of to t = L v = m L 2E k t = time of flight

19 Comparison of mass analyzers for SIMS Type Resolution m/ m Mass range Relative sensitivity Quadrupole < TOF > TOF-SIMS with high resolution is useful for identify the fragments having really close m/z. m/z ~15 19

20 Mass interferences High resolution mode M/ M ~ 5000 Positive SIMS spectrum of Silicon bombarded with oxygen ions 20

21 Trace Metal Contamination in semiconductor Low level metal contaminants can have a serious impact on semiconductor device performance. Fig.3: In the example shown above the mass 56 peak has been analyzed with a mass resolution (m/δm) of greater than 15,000 revealing metal and organic contaminants that are present at the same nominal mass as 28Si2+. 21

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24 1.Surface spectroscopy (static SIMS): TOF-SIMS analysis modes 24

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29 Surface modification Positive ToF-SIMS spectra of Cleaned PTFE and Ar-PTFE x10 6 Cleaned PTFE Intensity C + CF + CF + 2 C 2 F 2 + C 3 F 3 + C 2 F 4 + C 3 F 4 + C 2 F 5 + C 3 F 5 + x10 6 Ar-PTFE Intensity C 3 H + 7 C 3 H + 3 C 5 H + 5 C 4 H + 9 C 6 H 5 + C 9 H m/z In addition to (C x F y ) + fragments, the Ar-PTFE spectrum shows an increase in peaks due to unsaturated hydrocarbon moities 29

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32 Metals Analysis of the diffusion process of various elements into a polycrystalline metal oxide Mg + :a matrix component of a polycrystalline metal oxide composed of O 2.8 Mg 0.2 Ga 0.8 S 0.2 La 0.2. Y + and Fe + : the element which the surface was covered with for diffusion experiments Matrix component Diffused elements First row: Surface images. Second row: Images at a depth of 595 nm. Third row: Cross section along the green line (given in the top left image). 32

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34 TOF-SIMS Instrument Sample mounting Ion-TofTOF-SIMS IV (Germany) 34

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36 Case study Negative TOFSIMSspectra PMMA Composition in polymer blend Different m/z= 14 PEMA 36

37 Negative TOFSIMSspectra of a copolymer of PMMA and PEMA PEMA PMMA 37

38 Case study Study of micropatterningprocess Fabrication of patterned copper on PTFE via mcp PDMS TMS ink Master PDMS Printed pattern PDMS PTFE optical Achieved 200 µm lines with machined master 4 Metallisation Working on µm lines with Si / SiO 2 master 38 Copper pattern

39 TOFSIMS Images of Cu Patterned PTFE optical (500 x 500 µm 2 ) C CF CF 3 63 Cu 65 Cu Si SiCH 3 SiC 3 H 9 Si 2 C 5 H 15 O Pd 39

40 End of Chapter 40