PLASMA FLOW AND PLASMA EXPANSION AROUND 3D OBJECTS IN METAL PLASMA IMMERSION ION IMPLANTATION

Transcription

1 PLASMA FLOW AND PLASMA EXPANSION AROUND 3D OBJECTS IN METAL PLASMA IMMERSION ION IMPLANTATION Darina Manova & Stephan Mändl 1

2 Motivation 2

3 Motivation Visualisation of Water Flow from Dynamic Sand Dunes 3

4 Motivation Plasma Flow around Objects & Interaction with Expanding Plasma Sheath How to Measure? Analytic methods SIMS very sensitive method, absolute calibration is necessary RBS absolute calibration, low sensitivity Spectroscopic Ellipsometry precise method for simple systems: interference fringes from transparent layer, extinction from adsorbing layer Colour fast and global measurement, but very simplistic interpretation 4

5 Table of Contents Motivation Generation of Supersonic (Metal) Plasma Flow Experimental Set-up Sampling of plasma flow at surfaces Results Influence of background pressure Influence of high voltage Conclusions 5

6 MePIII: Vacuum Arc Vacuum Arc Self sustaining, high current, low voltage gas discharge Local heating (cathode spots) + thermal emission of electrons + ejection of metal atoms Plasma density near cathode ~ cm -3 Properties of arc plasma Small voltage drop between cathode and anode Ion flux parallel to electron flux from the cathode Fully ionised plasma, charge states Initial kinetic energy of ions 1 1 ev Supersonic flow 6

7 HD vs. MHD A. Anders, Surf. Coat. Technol., 136 (21) T. Arnold, Ph.D. thesis 7

8 Diffusionsprozesse Hydrodynamics Re ρul η transition from laminar to turbulent flow: Reynolds number Magnetohydrodynamics S τ τ R A 2 µ L / η = = L / v A µ val η time scale for diffusion vs. convection: Lundquist number for vacuum arc S 1 but resistive MHD dominant in plasma sheath regions 1/3 N << λ << c L H mean free path similar to system dimensions in our case transition between single particle and collective motion picture 8

9 MePIII&D and MHD Deposition vs. Implantation small & large sheath Addition of high voltage pulses change sampling volume of plasma flux Pulse length and pulse voltage as free parameters Modification of Child-Langmuir law with non-stationary initial plasma P.V. Akimov et al., Physics of Plasmas 8 (21)

10 Experimental Set-up 3 mm 6 mm Cathode Tail top Front side Back side Trigger Cathode Gas inlet 1 cm RF plasma source 39 cm 1 cm Filter (shield) Sample Pumping system Pulse generator Vacuum arc with simple shield as filter Cathode materials: Al and Ti Ar flow:, 15, 35 sccm Background pressure: 1-2,.9, 1.8 µbar High voltage Pulses (f = 3 khz): to 1 kv Pulse length: 2.5 to 5 µs Substrates: Si and SiO 2 /Si 1

11 MHD Flow Area Density (1 18 cm -2 ) 4 2 Al: 1-5 mbar kv 5 kv 1 kv Thickness (nm) Al_m Al_e Ti_m Ti_e Thickness (nm) µbar Al_kV Al_1kV Ti_kV Ti_1kV Distance from Edge (mm) 1-2 µbar.9 µbar 1.8 µbar Distance from center (mm) Front Side Minor pressure dependence as primary shield far away from substrate Thinner layer near edge caused by off-normal incidence and correspondingly higher sputter rate Mapping of sheath width for different materials complicated by different self-sputter yields and charge state distribution 11

12 Back Side top Schematic set-up of coupons on the back side of the sample holder outside inside bottom 12

13 MHD Flow Background pressure 1-2 µbar Al.9 µbar 1.8 µbar 1-2 µbar Ti.9 µbar 1.8 µbar 13

14 Back Side Color Index Position (mm) Al µbar 25% 5% 75% Thickness (nm) Color Index , 8 9,5 9, 8,5 8, 7,5 7, Ti (µbar) 25% 5% 75% Position (mm) Thickness (nm) Colour can be used as a substitute for layer thickness Reasonable agreement with layer thickness for Ti for thin films as long as film is still partially transparent Optical properties of Al (i.e. n + k) apparently dependent of thickness Gradient of film thickness directly comparable to mean free path (conversion factor still to be determined) 14

15 Total Current Ti 1-2 µbar µbar 1.8 µbar Al 1-2 µbar.9 µbar 1.8 µbar Total Current (ma) Pulse Length (µs) Voltage (kv)

16 Back Side I: small pulse length Ti, 1 kv, 1.8 µbar Ti, 1 kv,.1 µbar SiO 2 Thickness (nm) Position (mm) Ti Thickness (nm) Ti, 3,5 kv,.1 µbar Deposition concentrated at high pressures on tail region Only sputter removal of oxide from back side 16

17 Back Side: SE Exp Sim1 Sim2 Sim3 Psi Delta Winkel (Grad) Exp Psi Delta Sim ψ, (degree) nm Ti 1.6 nm. nm 5.1 nm Interface. nm. nm nm SiO nm nm Si substrate nm SiO 2 Si substrate Wellenlänge (nm) Wavelength (nm) Surfaces subjected to sputtering by ion bombardment need modelling with a graded interface (i.e. implanted layer) for reasonable results (1 kv, 1-2 µbar series) 17

18 Back Side II: large pulse length 1.5 kv, 5 µs 2.5 kv, 5 µs 3.5 kv, 5 µs 3.5 kv, 15 µs Peculiar behaviour at long pulse lengths and intermediate voltages can be traces to highly localized increase in ion flux Initial oxide of 2 nm is completely removed at 3.5 kv and 5 µs 5 kv, 5 µs Maximum depth could reach 5 nm 18

19 Discussion & Conclusions (Plasma) flow around obstacles is still a very modern problem Colour visualisations can be a very helpful tool Congruence of different analytic results from the same samples is not always achieved as each method may measure something different Metal plasmas originating from a cathodic arc can be guided by simple shields The fraction of lost flow depends on pressure and voltage (5 75%) MePIIID will lead to highly inhomogeneous deposition and implantation distributions; especially around low symmetry objects depending on pressure, voltage and pulse length Combinatorial materials science could be based on this approach to vary energy flux and deposition rate independently from each other Prediction of actual distributions requires much more work Sheath expansion in 3D geometry is a very complex function, in contrast to 1D geometries where a monotonous increase with time and voltage is observed. 19

20 Comparison to PIII Geometries 3 mm cm -2 6 mm 3 mm cm cm -2 9 mm 1 mm O O 3D samples B O O O Flat samples influenced by substrate holder Steel, 1.431, PIII, 1 kv, front side, back side before and after corrosion test 2

21 Summary & Conclusions Interplay of plasma flow, plasma sheath and geometry can create very strange and highly localized effects 21

22 Danksagung Johanna Lutz Susann Heinrich Sabine Schirmer Katharina Scholze David Haldan 22