''Formation and Deposition of Nanoclusters'' Rainer Hippler. University of Greifswald Hoboken 31 July 2008

Transcription

1 ''Formation and Deposition of Nanoclusters'' Rainer Hippler University of Greifswald Hoboken 31 July 28

2 Hansestadt Greifswald Home town of Caspar David Friedrich Rainer Hippler Hoboken 28 2 Marktplatz

3 University of Greifswald Founded 1456 Rainer Hippler Hoboken 28 3

4 Physics in Greifswald 27 Leibnitz-Institute for Low Temperature Plasma Physics Max-Planck-Institute for Plasma Physics Rainer Hippler Hoboken 28 4

5 Outline Plasma-assisted deposition Deposition of thin metal oxide films Nano-size clustern formation Cluster deposition Properties of deposited cluster Rainer Hippler Hoboken 28 5

6 Plasma-assisted deposition of, e.g., Thin metal oxide films Optically transparent Electrically conducting Large number of potential applications: Glas with low heat conductivity Solar cells Optical coatings Liquid crystal and plasma display panels optoelectronical elements Rainer Hippler Hoboken 28 6

7 Plasma-enhanced chemical vapor deposition: Magnetron discharge Langmuir probe Substrate Ellipsometry Magnetron Rainer Hippler Hoboken 28 7

8 Penning 194 Cylindrical magnetron discharge Utilises an external magnetic field Lower ignition pressure, and Increase of the discharge current at the same discharge voltage. C water-cooled cathode (Al, Cu,..) R1, R2 rings fixing the anode Deposition has been carried out onto mica plates M which could be changed in situ by magnetically moving the weight F. The whole tube was placed within a coil, providing a magnetic field H (arrow) parallel to the axis of C. Rainer Hippler Hoboken 28 8

9 Charged particles in a Magnetic Field Rainer Hippler Hoboken 28 9

10 Planar magnetron Electron trap region Positve Ions impinging on negatively biased cathode with kinetic energy E e U produce Sputtered target atoms and ions, sputtering yield Y.5 Secondary electrons, emission coefficient γ.5. Secondary electrons gain energy equal to cathode potential and become trapped in the trap region of perpendicular electric and magnetic fields where each energetic electron produces 1/γ 2 ions. Rainer Hippler Hoboken 28 1

11 Hall current Drift or Hall current can exceed the discharge current by factors of 3-7. Rainer Hippler Hoboken 28 11

12 Magnetron balanced and unbalanced mode Substrate Target N S N N S N N N Balanced mode: inner magnet up - outer magnet down inner Magnet outer Magnet Unbalanced mode: inner magnet down - outer magnet up Rainer Hippler Hoboken 28 12

13 Deposition of Functional layers: Glas with low heat conductivity Heat transport = heat conductivity + Radiation Glas/Argon/Blocker/Glas Solar,5 µm Room temperature 8 µm wave length Rainer Hippler Hoboken 28 13

14 Deposition of Functional layers: Glas with low heat conductivity Schutzschicht Blocker Keimschicht Schutzschicht Blocker Keimschicht Deckschicht Silber Grundschicht Deckschicht Silber Glassubstrat Rainer Hippler Hoboken 28 14

15 Plasma diagnostics Langmuir probe Energy-resolved mass spectroscopy (Plasma process monitor) Thermal probe to determine the energy influx Optical and tunable diode laser spectroscopy Rainer Hippler Hoboken 28 15

16 Plasma diagnostics: Langmuir probe Rainer Hippler Hoboken 28 16

17 Langmuir probe: electron density Rainer Hippler Hoboken 28 17

18 Mean electron energy Decrease caused by cooling due to sputtered metal atoms Rainer Hippler Hoboken 28 18

19 Plasma diagnostics: Energyresolved mass spectrometry Plasma process Monitor Mass spectrometer Energy filter Rainer Hippler Hoboken 28 19

20 Ion energy distributions Target: In/Sn (9:1) Ar: 2 sccm; O 2 : 2 sccm signal (cps) 1 5 Ar O O In Sn Positive ions signal rate (cps) 1 4 Negative ions InO O kinetic energy (ev) kinetic energy (ev) Rainer Hippler Hoboken 28 2

21 Ion energy distribution of Ti + ions vs. pressure High-energy tail of sputtered metal atoms Inreased cooling at larger pressures

22 Blue Tunable Diode Laser Absorption Spectroscopy Rainer Hippler Hoboken 28 22

23 Rainer Hippler Hoboken 28 23

24 Laser absorption profile (Al) Magnetron discharge Target: Al Rainer Hippler Hoboken 28 24

25 Aluminum Density density (1 17 m -3 ) μbar 5 μbar 7 μbar 95 μbar Eq power (W) Rainer Hippler Hoboken 28 25

26 Al density vs. Oxygen flow density (1 17 m -3 ) 2. Ar: 2 sccm oxygen flow (sccm) Target oxidation is responsible for the drastic drop (factor of 3) in Al atom density Change in sputtering yield is insufficient to explain the oyxgen dependence More likely, electron emission is drastically reduced To overcome target poisening, operate magnetron in ac or pulsed mode Rainer Hippler Hoboken 28 26

27 Pulsed mode operation leading edge 15 μs ON working t = 15 μs pause t = 3.85 μs = 4. μs t periode 3.85 μs OFF i eff = T T 1 i() t dt I eff = 3 ma I peak 15 A cathode voltage: - 5 V up to - 62 V Rainer Hippler Hoboken 28 27

28 Discharge current 12 peak current [A] Ar, 5 Pa Ar, 15 Pa Ar+O 2, 5 Pa Ar+O 2, 15 Pa I eff = 24 ma I peak = 1 A ON OFF time [μs] real data (calculated by the help of Ohm law) from the oscilloscope ope measured between the output and input of pulsed source resistance plasma parameters are influenced by current behaviour Rainer Hippler Hoboken 28 28

29 Electron density peak current [A] Ar Ar+O 2 p = 5 Pa I = 1 A electron density [1 17 m -3 ] Electron density enhanced: up to 1 18 /m 3 i.e., factor of more than 1 compared to dc operation -2 ON OFF time [μs] Rainer Hippler Hoboken 28 29

30 Al density vs. Oxygen flow N Al-pulse [1 17 m -3 ] Im = 5 ma Im = 9 ma O 2 [sccm] density (1 17 m -3 ) Pulsed operation dc operation oxygen flow (sccm) Rainer Hippler Hoboken 28 3

31 Characterisation of thin films Photo electron spectroscopy (XPS) Probe transfer under vacuum X-ray diffractometry (GIXD) X-ray reflectometry (GIXR) Atomic force microscopy (AFM) Rainer Hippler Hoboken 28 31

32 XPS photo electron spectrum 8 6 Indium-Tin-Oxid film 5 nm thick intensity[a.u] 4 In [LM M] Sn-3d In-3d In3d 3/2 In3d 5/2 2 O[KLL] In2p 1/2 O-1s In2p 3/2 O1s Sn3d 3/2 Sn3d 5/2 In4d binding energy[ev] Rainer Hippler Hoboken 28 32

33 XPS: Sn-3d Sn3d, ito139.3 sccm O 2 V bias = V intensity (a.u) binding energy (ev) Rainer Hippler Hoboken 28 33

34 XPS: Sn-3d 3 28 Sn3d, ito138 Variation of O 2 flow 26. sccm O 2 V bias = V Intensity(a.u) sccm 2 34 Sn3d, ito sccm O 2, V bias = V B.E [ev] Intensity(a.u) sccm 35 Sn3d, ito sccm O V bias = V Intensity(a.u) sccm B.E [ev] B.E [ev] Rainer Hippler Hoboken 28 34

35 ITO film composition Metallic target Oxidised target 8 8 composition (%) 6 4 In O O In 6 4 composition (%) 2 Sn Sn oxygen flow (sccm) film thickness (nm) Rainer Hippler Hoboken 28 35

36 Thin film properties Metallic target Oxygen-poor (ca. 4 % ) High tin contents (ca. 15 %) Dependence on substrate bias 2 phases: metallic/in 2 O 3 and SnO Oxidised target Oxygen-rich (6 % or more) Low tin contents (6 %) In 2 O 3 und SnO 2 Rainer Hippler Hoboken 28 36

37 Deposition of TiO x thin films using pulsed dc planar magnetron system TiO 2 has good technological properties photocatalytic activity conversion of solar light into useful chemical energy self-cleaning surfaces, purification of air and water, photocatalytic self-sterilisation, TiO 2 amorphous structure anatase rutile brookite anatase photocatalytic properties temperature 3 ºC (thermal annealing) crystalline structures rutile high refractive index, higher hardness temperature 9 ºC (conversion of anatase) Rainer Hippler Hoboken 28 37

38 Structure of deposited TiO 2 thin films Grazing incidence XRD (Seiffert FPM HZG 4) dependence of structure on ratio Ar/O 2 gases and pressure of unheated samples TiO 2 was found in rutile, anatase, and amorphous structure, (α-titanium) 6 5 Anatase (11) Rutile (11) Rutile (11) α-ti (11) intensity [a.u] α-ti (2) p = 15 Pa Ar/O 2 2/2 p = 5 Pa Ar/O 2 2/2 p =.75 Pa Ar/O 2 2/2 1 α-ti (1) p =.75 Pa Ar/O 2 2/ Θ [ ] Rainer Hippler Hoboken 28 38

39 Structures dependencies 1 R R+A R+A A 1 rutile rutile + anatase anatase A A O 2 /Ar R R+A A A,1 R R T T R+A a a T α-ti pressure [Pa] Rainer Hippler Hoboken a a a T amorphous A a a T

40 Surface morphology (AFM): Nano-size particles (clusters) rutile anatase Dependence of surface structure on pressure and/or crystalline phase measured cluster radius rutile 15.4 nm, anatase 1.25 nm measured cluster surface rutile nm 2, anatase 33.8 nm 2 Rainer Hippler Hoboken 28 4

41 How to produce nano-size particles? Particle formation in plasmas. Mass spectra of positive ions: Ar/CH 4 vs. Ar/C 2 H 2 ) H.T. Do, G. Thieme, M. Fröhlich, H. Kersten, R. Hippler, Contrib. Plasma Phys. 45, (25)

42 Mass spectrum of neutrals in a Ar/CH 4 dielectric barrier discharge A. Majumdar, J.F. Behnke, R. Hippler, K. Matyash, R. Schneider, J. Phys. Chem. A 19, (25)

43 Particle nucleation and growth Formation of primary clusters e Nucleation and cluster growth e, i Typical evolution Nucleation (3-body reaction): A + A + X A 2 + X Particle growth Coagulation Cluster growth (atom adsorption): A n + A A n+1 Coagulation: A n + A m A n+m C. Hollenstein (Ecole Polytechnique Lausanne, Switzerland)

44 Nano-size particle formation Nano-size particle growth from sputtered atoms in a magnetron discharge Depostion of nano-size particles on surfaces

45 Nano-size Ag particles on Si(1) Size distribution of deposited Ag clusters with and without mass filter. I. Shyjumon, M. Gopinadhan, O. Ivanova, M. Quaas, H. Wulff, C.A. Helm, R. Hippler, EJP D (26)

46 Cluster size vs. wall temperature counts K K K K K K K height (nm) surface coverage (clusters /µm 2 ) counts height temperature (K) mean height (nm) I. Shyjumon, M. Gopinadhan, B. Smirnov, C.A. Helm, R. Hippler, TSF (26) Rainer Hippler Hoboken 28 46

47 Deposition time 8 min 2 Deposition for 8 min 15 Number Dimension of clusters (nm) Clusters continue to grow on surfaces at prolonged depostion times Mean size: 5 nm

48 Deposition time 15 min

49 Deposited films from 5 nm clusters Intensity (arb. unit) Average size ~ 34.5 nm 8 Rainer 2nm Hippler Hoboken Cluster size (nm) 6 SiKα AgLα 1 Number of particles1 Energy (kev)

50 Rapid thermal annealing (473 K 173 K) Rainer Hippler Hoboken 28 5

51 Rapid thermal annealing (873 K and 173 K) Mean size: 7 nm Two-sizes: 3 nm and 6 nm Number of particles Average size ~ 7 nm Cluster size (nm) Number of particles Average size ~ 36 nm Cluster size (nm) Rainer Hippler Hoboken 28 51

52 Fragmentation of nanosize clusters? 14 Fragmentation: Number of particles K 873 K 873 K K Cluster size (nm) Rainer Hippler Hoboken 28 52

53 Conclusion Nano-size cluster show many interesting feature Much more work yet to be done Rainer Hippler Hoboken 28 53

54 Thank you Rainer Hippler Hoboken 28 54

55 1

56 1

57

58 Cluster size dependence on time N/µm 2-28 to to 24 nm counts K K K K K K K height (nm) Rainer Hippler Hoboken 28 58

59 Chemical composition counts O 1s(TiO 2 ) O 1s(SiO 2 ) 3 min 15 min 1 min 3 min 2 min 1 min Ref Si binding energy (ev) counts / s Ti2p3/ ev TiO2 4 2P 3/2 Ti in TiO 2 2P 1/ binding energy (ev) Clusters on the surface are fully oxidised TiO 2 Rainer Hippler Hoboken 28 59

60 Size of deposited Ti cluster size counts K K K K K K K height (nm) surface coverage (clusters /µm 2 ) the temperature dependence of size is due to the cluster-wall interaction temperature (K) counts height mean height (nm) Rainer Hippler Hoboken 28 6

61 Cluster size dependence on time surface coverage (clusters/µm 2 ) deposition time (s) mean height (nm) Around 3 s, the second layer growth - starts before the first layer is completed N/µm 2-28 to to 24 nm Rainer Hippler Hoboken 28 61

62 Melting of deposited nano-size Ag clusters intensity (a.u) RT-before 373 K 573 K 723 K 773 K 883 K RT-after Θ / melting point (K) ΔT = 4σMT ΔH m bulk rρ measured calculated cluster size (nm) Rainer Hippler Hoboken 28 62