Material characterization with TOF-SIMS

Transcription

1 Material characterization with TOF-SIMS Jukka Lausmaa Department of Chemistry and Materials Technology,, Borås, Sweden Part 1: General - secondary ion mass spectrometry - time-of-flight mass spectrometry - instrumentation - general spectral features Part 2: Applications - spectroscopy of complex materials - imaging examples

2 TOF-SIMS is a mass spectrometric technique Prerequisites for mass spectrometry: (i) free molecules in gas phase (ii) molecules in an charged state (measure m/z) This can be achieved by, for example: thermal desorption, field emission, laser ablation, electrospray, electron impact, chemical ionization, plasmas, matrix assisted laser desorption/ionization,

3 SIMS: secondary ion mass spectrometry Primary ions Secondary ions (~1-50 kev) Sputtering Solid

4 Sputtering process PI Primary ion collision cascade - primary recoils - secondary recoils Primary recoils cause sample damage thoughout ion track PI s implanted in material Secondary recoils particle emission from surface (sputtering) surface damage Sputtered particles - atoms and clusters - molecules and fragments - neutrals and secondary ions -electrons

5 Important numbers and consequences Sputter yield, Y s = removed particles / PI typically 10 kev limit for non-destructive analysis Secondary ion yield, Y SI = secondary ions / PI can vary from (depends on ionization energy and matrix effects) quantification difficult Large number of secondary recoils with low energy: emission depth 3 monolayers (high surface sensitivity) excited region diam nm (dependent on binding energy) low energy distribution (influences mass resolution) - elements (1-5 ev, Thompson) - molecules ( ev, M-B) PI Sputtered particles

6 Dynamic SIMS High PI intensity Beam damage Analysis at continuously increasing depth Main applications: Bulk analysis (trace elements) Depth profiles (e.g. surface films, dopant profiles) Elemental imaging (e.g., grain boundaries, trace elements in biological samples)

7 Static SIMS Number of particles removed from surface during measurement should be negligible: Typically atoms cm -2 in one monolayer (ML) Primary ion dose density (PIDD) ions cm -2 Examples: Analysis area PI current Time 100 x 100 µm 2 1 pa ~ s 10 x 10 µm pa ~1-10 s

8 Detection limits Typically atoms or molecules cm -2 in monolayer (1 ML) Assume sputter yield, Y SI = 1 and secondary ion yield Y SI = 10-4 Analysis area Atoms/ML Secondary ions formed 100 x 100 µm 2 ~10 10 ~ x 10 µm 2 ~10 8 ~10 4 static SIMS with high detection sensitivity requires mass analyzer with high transmission time-of-flight (TOF) analyzer

9 Ion sources Ga + liquid metal ion source: Au + x Bi + x liquid metal ion source: liquid metal ion source: Cs + liquid metal ion guns: Electron impact guns (poor focus): Ar + : O 2+ : SF 5+ : C 60+ : High focus and intensity Higher sputter yields, similar focus Present state-of-the-art enhances negative SI yields, poor focus general purposes, ion etching (depth profiling) enhances positive SI yields enhanced yield for large molecular ions shallow depth profiling (less ion mixing) enhanced yield for large molecular ions shallow depth profiling (less ion mixing) depth profiling of organics

10 Time of flight mass spectrometer Pulsed ion beam Lens Sample holder V1 (20 kv) Mirrors Extraction electrodes V2 Signal Photodiode Computer Ion detector Trigger For details, see for example: R.J. Cotter, Time-of-Flight Mass Spectrometry ACS Symp. series, Vol.549 (1994) Y Variable attenuator Flight path Counts 0 Pulsed laser Electrostatic reflector TOF spectrum Ion detector Urefl (>20 kv) Time Constant kinetic energy from acceleration field: 2 mv E = = z U k 2 Flight time: 2 2 L t = 2 v 2 m 2 U t z = 2 L m z = a t 2 + b a, b = constants (calibrated by two known masses)

11 Advantages of TOF-analyzer Unlimited mass range (in practice limited by ion formation and ion stability) High mass resolution; M/ M (FWHM) > (single mass resolution at amu) High accuracy (calibration dependent); - absolute mass error typically 10-3 amu for <100 amu - relative errors in 10 ppm range High transmission (parallell detection, no filtering) Drawback: Pulsed measurement (low duty cycle) low signal intensities (compared to other MS)

12 High resolution mass spectrum (m/z = 30) Si 29 SiH 28 SiH 2 Silicon wafer, as rec. 8 kev Ar + CH 2 O CH 4 N 13 CCH

13 TOF-SIMS Time-of-flight secondary ion mass spectrometry Vacuum Pulsed ion gun U a c HV (+/-) mass filter 1/2 t (m/z) 8 ion detector extractor/ion optics secondary ions primary ions (Ga, Cs,Ar, In, O, SF ) 5 + A atomic ions + B ~10 nm molecular ions + + ABC + AB C Static SIMS: Pulsed primary ion beam, dose <10 cm (surface layer not removed) Dynamic SIMS: Continuous ion beam, analysis at increasing depths (depth profiling) TOF: Method for mass filtration (measurement of flight time) ABC ~1 nm

14 TOF-SIMS analysis modes 1. Surface spectroscopy (static SIMS): High surface sensitivity (information depth 1-3 molecular layers) All elements, incl. isotopes are detected High mass resolution gives specific chemical information Low detection limits (% of monolayer down to ppm-ppb) 2. Microscopy (imaging): Submicron lateral resolution Analysis of composites, particles, fibres and microfabricated materials Imaging of lateral distributions at surface or in cross sections 3. Depth profiling (dynamic SIMS): Controlled sputter removal combined with spectroscopy or imaging Depth distribution from surface and into material (depth resolution < 1nm) Measurement of film thicknesses and diffusion profiles (< 1 µm thick) combined: 3D imaging on submicron scale

15 Silver, positive ions Na K x5 x 5 Ag Ag 2 Ag 3 x [amu] x kev Ga cm -2 Ag 5 Ag 7 Ag 9 Ag 11 Ag 13 Ag 15 Ag 17

16 KŽOH KClŽ K Cl K Cl KŽ K Cl KCl KŽCl K Potassium chloride KCl KCL1C_P K x x 500 K m Cl (m-1) [amu] Intensity K 2 K 2 Cl K 4 Cl 3 K 5 Cl 4 K 3 Cl 2 K 6 Cl 5 K 7 Cl 6

17 Hydroxyapatite, Ca 5 (PO 4 ) 3 OH 5 Ca++ Na CaŽ CaPO CaPOŽ CaŽPO CaŽPO CaŽOŽ CaŽHOŽ CaOH CaPOŽ CaŽO Ca2 O Ca 4.0 x 20 CaOH Ca2 CaPO CaPO3 Ca2 PO 3 Ca2 PO 4 Ca2 HO 2 CaOH CaPO2 Ca

18 Polymer identification Polystyrene 3 Polyethylene 3 CH CHŽ CH CH C H C H CH H CH CH CŽH CH CH CŽH 5.0 C x H 2x±1 C 2 H x C 3 H x CH CH CH 4.0 C H C H CH CH C H C H CŽH CH C H CH 3.0 C 3 H 3 C 7 H CH C 1 H x C 4 H x C 5 H x Jukka Lausmaa, Nordic Polymer Days, Gothenburg, August 17, 2005

19 Teflon (PTFE) 5 CFŽ CFŽ CŽF CF CF CŽF CF CF CF C C CF CF 2 CF 3 C 3 F 2 C 3 F 3 C 2 F 4 C 3 F

20 Polystyrene oligomer distribution PS 2200 dissolved in chloroform and deposited as monolayer on silver foil (Irgafos Ag) + (antioxidant) x 5 m = 104 (styrene repeat unit) Jukka Lausmaa, Nordic Polymer Days, Gothenburg, August 17, 2005

21 Interpretation of oligomer distribution x 5 Analysis of m/z = 1726 peak: 1.Assume silver cationized: Oligomer mass: = How many monomers? 1619 / = monomers 3. Mass of endgroups: 0.56 * = 58 (H + C 4 H 9 ) Jukka Lausmaa, Nordic Polymer Days, Gothenburg, August 17, 2005

22 Molecular spectra: Cholesterol on silver Ag (M+Ag) Cholesterol on Ag 1 mg/ml, 5 yl (2M+Ag) Intensity Ion Mass [amu]

23 Substance identification Ex: Deprotonated molecular ion, (M-H) -, of cholesterol (C 27 H 45 O) Absolute mass ( u) Isotope pattern due to 13 C Theoretical spectrum (isotope distribution) % Isotope Cluster % 3.503% 0.365% 0.029% 0.002% 0.000% C27H45O Mass Measured spectrum /u Jukka Lausmaa, Nordic Polymer Days, Gothenburg, August 17, 2005

24 Spectral features Elemental targets (e.g. metals): - monoatomic ions (Me + ), clusters (Me n+ ) Inorganics (e.g., metal oxides, salts, ceramics): - monoatomic ions (Me + ), sometimes clusters (Me n+ ) - oxidized species (Me m O n± ) Polymeric materials: - predominantly molecular fragments - characteristic fragmentation patterns - characteristic fragments Adsorbates and surface contaminants: - predominantly fragments, often also intact (M+H) + or (M-H) - - cationized molecular ions (M + Me) + - oligomer distributions (M + Me) n +

25 Surface analysis: Challenges and questions Which elements and compounds are present on the surface? Which impurities/contaminants are present? How are they distributed over the surface? How are they distributed from the surface and into the material? Major challenges: - Distinguish the surface from the rest of the material - Minute amounts of materials (typically 1-10 ng/cm 2 )

26 TOF-SIMS analysis modes 1. Surface spectroscopy (static SIMS): High surface sensitivity (information depth 1-3 molecular layers) All elements, incl. isotopes are detected High mass resolution gives specific chemical information Low detection limits (% of monolayer down to ppm-ppb) 2. Microscopy (imaging): Submicron lateral resolution Analysis of composites, particles, fibres and microfabricated materials Imaging of lateral distributions at surface or in cross sections 3. Depth profiling (dynamic SIMS): Controlled sputter removal combined with spectroscopy or imaging Depth distribution from surface and into material (depth resolution < 1nm) Measurement of film thicknesses and diffusion profiles (< 1 µm thick) combined: 3D imaging on submicron scale

27 Surface contamination: fingerprint on Al foil 10x 100x y y Al C 2 H 3 Na Al Mg C 3 H y C 4 H y x 10 x 10 10x PDMS (147) Phthalate C 3 H y fragment C4 H y C C O O + O H PDMS (207) PDMS (221) x 100 x x m=14 (fatty acids) PDMS (281) Al foil + fingerprint m=14 (CH 2 ) Al foil, as received [amu]

28 Identification of surface contamination Na 3.0 Ti surface handled with PVC gloves Ti TiO x 5 phthalate fragment (DEHP+Na) + (DEHP+H) [amu]

29 Examples of common surface contaminants Type Characteristic peaks Plasticizers (phthalates) 149, 391 (M+H) +, 413 (M+Na) + Fatty acids CH 3 -(CH 2 ) n -COO - 227, 255, 283, m = 28 Synthetic oils m = 140 (C 10 H 20 ) n Polydimethyl siloxane 43, 73, 147, 207, 281, m = 74 (PDMS)

30 Injection moulding of polystyrene Collaboration with Dept Polymeric Materials, Chalmers (Francesco Pisciotti, grad. student) -Injection moulding widely used processing method -High pressures and high temperatures (<T m ) involved Objective: Study chemical composition of surfaces and interfaces, especially migration of impurities and additives

31 CH CH CŽH CH C H CŒH CHŒ CH C H C H CŽH C H CHŽ CH CH C H Injection moulded polystyrene: spectrum from cross section (interior of material) Characteristic PS spectrum (unsaturated C x H y species dominate) No additives or impurities detected x [amu]

32 Injection moulded polystyrene: spectrum from surface CH CŽH CH CH C H C H C H C H Not a characteristic PS spectrum - more saturated C x H y species - traces of C x H y Oand C x H y N detected No specific additives or impurities detected x [amu]

33 Injection moulded polystyrene: extract deposited on Ag-foil Ag Ag 2 x 10 Ag 3 Paraffin wax? m = 14 (CH 2 ) x 200 (Irganox Ag) + x 200 PDMS m = [amu] Jukka Lausmaa, Nordic Polymer Days, Gothenburg, August 17, 2005

34 Injection moulded polystyrene: spectrum from Ag-foil rubbed against surface Ag Ag Ag 3 Synthetic oil (-C 10 H 20 -) n x X15 m= (C 10 H 20 ) (314u+Ag) (DOP+Ag) + Ag 9 [amu] Irgafos 168 AGPRE1P, , 18: [amu] Jukka Lausmaa, Nordic Polymer Days, Gothenburg, August 17, 2005

35 Separation/purification of biomolecule samples by liquid chromatography Mixture of biomolecules Separation column Purified sample Y Y Y Y Y Surface functionalized separation medium Y Y Y

36 Surface characterization of media for liquid chromatography Collaboration with Amersham Pharmacia Biotech, Uppsala (Bo-Lennart Andersson and Mikael Andersson) Background - Function of LC media dependent on morphology and surface chemistry - Surface modification and ligands chemical specificity - Difficult materials ; beads, roughness, non-conducting, complex chemistry Objective: - Can ToF-SIMS provide useful information about surface chemistry?

37 Sepharose beads 20 µm Surface analyst s nightmare: Insulating + rough and porous + complex chemistry

38 Raw materials HO O OH O D-Galactose Agarose OH O O O HO 3,6-anhydro- L-Galactose Cross-linker OH OH O * O H H O O Dextran (branching exists) O HO O H H O O O HO O H H O O Alpha-1,6-D-Glucose O HO O n *

39 Comparison of raw materials Intensity Intensity C 4 H 5 O C 3 H 5 O 2 C 4 H 5 O 2 Agarose Dextran Ion Mass [amu]

40 Diethyl aminoethyl ligands on cross-linked agarose (DEAE Sepharose Fast Flow) C 2 H 5 O CH 2 CH 2 N C 2 H 5 H

41 Identification of surface ligands Sepharose + DEAE Agarose C 2 H 5 CH 3 N C 2 H 6 N C 3 H 6 N C 5 H 12 N C 6 H 14 N [amu] O CH 2 C 2 H 5 CH 2 N C 2 H 5 H

42 TOF-SIMS analysis modes 1. Surface spectroscopy (static SIMS): High surface sensitivity (information depth 1-3 molecular layers) All elements, incl. isotopes are detected High mass resolution gives specific chemical information Low detection limits (% of monolayer down to ppm-ppb) 2. Microscopy (imaging): Submicron lateral resolution Analysis of composites, particles, fibres and microfabricated materials Imaging of lateral distributions at surface or in cross sections 3. Depth profiling (dynamic SIMS): Controlled sputter removal combined with spectroscopy or imaging Depth distribution from surface and into material (depth resolution < 1nm) Measurement of film thicknesses and diffusion profiles (< 1 µm thick) combined: 3D imaging on submicron scale

43 Chemical imaging 1. Focused ion beam (Au + ) is scanned over surface; spectra from 128 x 128 points (alt: sample stage scan) A B C 10 µm 10 mm (128 x 128 pxl) 3. Images are constructed from raw data file, showing where substances are located D Mass spectrum from total area shows which substances that are present D B C /u Results are stored in raw data file, containing > mass spectra, with retained spatial information. A B C D Overlay A A signal

44 Drying stain on Si wafer Na 2 Cl Na 2 OH Al

45 Impurities on semiconductor devices Si (wafer) 25 µm Ti, W (evaporated) Användningsområden: Processutveckling och kvalitetskontroll Skadeanalyser Na (contamination)

46 ToF-SIMS imaging: Grain boundary segregation in polycrystalline silicon Si Al K 20 µm Ti Al K Ti Sample from: Dr J. Walmsley, SINTEF Technology, Trondheim, Norway

47 Analysis of fatigue crack Ion images Video Ion sputtered area Jukka Lausmaa, SP

48 Ion imaging of microstamped CH 3 - and COOH-terminated thiols on Au (40 and 60 µm stripes) 100 µm Au C 2 H 3 C 2 H 3 O Sample preparation by Department of Polymer Technology, Chalmers

49 Paints Often complex formulations, containing different additives (antioxidants, UV absorbers, leveling agents, ) Car paints; multilayer systems Interesting questions: - How are additives distributed? - How do additives diffuse? - Degradation mechanisms? Need for analysis methods that combine detailed chemical information with imaging capability TOF-SIMS

50 Spectrum from surface of car paint Intensity Intensity Intensity Intensity y * = silicone (anti foaming agent) g * * Artificially aged, 5000 h * Unaged Artificially aged, 5000 h Unaged mass mass Intensity Intensity Intensity Intensity * * * Artificially aged, 5000 h Artificially aged, 5000 h M205 = BHT (additive) CH 3 M647 = Irgafos 168 (phosphite) M662 = Irgafos 168 (phosphate) * Unaged Unaged mass mass

51 Sample preparation for cross section analysis Direction of microtoming of microtome PMMA Plastic Paint Paint Paint Paint PMMA Plastic Adhesive Epoxi

52 Imaging of cross sections Plastic Layer 1 Layer 2 Layer 3 (topcoat) Adhesive

53 Oxidation of paint system Sample aged in 18 O-enriched air Analysis: Imaging TOF-SIMS of 18 O - ions Result: Oxidation localized to pigmented layer 200 x 200 µm Jukka Lausmaa, Nordic Polymer Days, Gothenburg, August 17, 2005 Ref: Physical Electronics

54 Chemical mapping of biological samples 3 nm ~10 µm Sinauer Associates Inc. Lipid bilayer Knowledge about molecular composition of cells and tissues important for research on: understanding diseases drug development diagnostic methods Need for improved analytical methods: which molecules? spatial distributions? relevant length scales; nm mm Lipid molecule Protein molecule Alberts et al.: Molecular Biology of the Cell

55 Cell imprinting Transfer sample molecules to silver surface with retained lateral distribution Imaging TOF-SIMS of chemical imprint Advantages: + less fragmentation (Ag cationization) improves identification + higher SI yield improves sensitivity

56 Positive TOF-SIMS spectrum of cell imprint / u / u Intensity Intensity ph Ag Ag 2 Cl 2 Ag 3 Ag-ch Ag-ch 2

57 Spatially resolved chemical analysis of cells 10 µm TOF-SIMS images from imprints on silver SEM image of human leukocyte on glass 20 µm CH 4 N + (proteins, DNA) m/z = 184 (phosphocholine) m/z = 493 (cholesterol + Ag molecular ion) P. Sjövall et al. Analytical Chemistry (2003)

58 Lipid distributions in mouse brain cholesterol palmitate (255 u) sulfatide ( u) 261 u 429 u cholesterol / palmitat / sulfatid P. Sjövall, J. Lausmaa and B. Johansson, Analytical Chemistry,76, , 2004

59 TOF-SIMS analysis modes 1. Surface spectroscopy (static SIMS): High surface sensitivity (information depth 1-3 molecular layers) All elements, incl. isotopes are detected High mass resolution gives specific chemical information Low detection limits (% of monolayer down to ppm-ppb) 2. Microscopy (imaging): Submicron lateral resolution Analysis of composites, particles, fibres and microfabricated materials Imaging of lateral distributions at surface or in cross sections 3. Depth profiling (dynamic SIMS): Controlled sputter removal combined with spectroscopy or imaging Depth distribution from surface and into material (depth resolution < 1nm) Measurement of film thicknesses and diffusion profiles (< 1 µm thick)

60 Depth profiling using ion etching Ion beam Ion beam MS Signal intensity Layer thicknesses ~1-100 nm Sputter time vs depth can be calibrated Depth resolution a few nm Sputter time

61 Depth profiling (dynamic SIMS): 28 nm thermal oxide on HF etched titanium Interface Substance Mass Color F CH O TiO Time (S)

62 Applications of depth profiling Oxide layers (thickness and composition) Thin films (optical, conducting, hard coatings, ) Diffusion profiles Dopant profiles

63 Summary TOF-SIMS a versatile analysis technique, which combines: - detailed chemical information via high-res. MS - high detection sensitivity - high surface sensitivity - imaging capability at submicron scale Important applications: - polymeric materials (additives, molecular weight distr., ) - thin film characterization - cross section analysis - grain boundary segregation - microfabricated materials (e.g., microelectronics, µcp, ) - biological samples (emerging) However: - Best used in combination with other characterization techniques