Soft X-ray multilayer mirrors by ion assisted sputter deposition

Transcription

1 Soft X-ray multilayer mirrors by ion assisted sputter deposition Valentino Rigato INFN Laboratori Nazionali di Legnaro Bologna, September 21, 2010 Source: INFN-LNL-2009 V. RIGATO 1 SIF- Bologna September 21, 2010

2 OUTLINE Laboratory Facilities at INFN-Laboratori Nazionali di Legnaro (LNL) Multilayers with nanometer periodicity for ultra-violet and X-ray mirror technology EUV soft X-ray multilayers Preparation & results X-Ray astronomy multilayers Summary V. RIGATO 2

3 The laboratory activity is supported by the experiments submitted and approved by INFN National Commissions and by external projects The LNL Materials Laboratory fields ion surface interaction processes (ion beam analysis, ion implantation, ion assisted film growth, irradiation ) The LNL Materials Laboratory thin films synthesis by PVD and hybrid CVD-PVD technologies nanostructured, multilayer and advanced thin films materials ion plasma diagnostics Publications & Know how electron and atomic force microscopy electrical mechanical properties of coatings structural properties of coatings V. RIGATO 3

4 Research activity of the Laboratory 1) Advanced materials preparation 2) Process development The Materials Laboratory Advanced materials process development and synthesis Reactive Plasma Sputtering Deposition Plasma Diagnostics Ion-solid interaction Low Friction, High Hardness Nanoscaled Materials and Multilayers Soft X-ray multilayers High Performance Plastics Passivating layers Scintillator materials V. RIGATO 4

5 Characterization of materials with advanced methods 1) Composition, stoichiometry, microstructure, morphology, surface topography 2) Electrical, optical and mechanical properties The Materials Laboratory Characterization of Physical Properties 2*2 mm 2 Ca Si K Fe Ca Si K Fe Composition / depth profile / structure Ion Beam Analysis (RBS, NRA, ERD, PIXE) Micro-PIXE 2-D trace element analysis FT-IR Nuclear cross section measurement Other physical properties Nano-Hardness, Elastic modulus Adhesion (Micro-Scratch) Residual Stress Atomic Force Microscopy & SEM V. RIGATO 5

6 Soft-X ray Multilayer Mirrors EUV and X-Ray High Reflectivity Multilayer mirrors are deposited on flat and curved substrates for applications in X-ray astronomy, EUV lithography, water-window microscopy, Free Electron Laser Optics EUV Bragg High Reflectivity Multilayer Technology Riflettività SiO 2 /(B 4 C/Si/B 4 C/Mo) 40/Si Bombardamento ionico a Low bassa energy energia ion (~15eV) bombardment 0,6 0,5 0,4 0,3 0,2 SiO 2 /(B 4 C/Si/B 4 C/Mo) 40/Si Bombardamento ionico modulato Modulated ion bombardment per ogni strato 5/25/75eV struttura del periodo B 4 C Mo B 4 C ~3 nm Si Mirror design for GI hard X-ray telescopes 0,1 0, Energia (ev) V. RIGATO 6

7 NORMAL INCIDENCE SOFT X-RAY MIRRORS MATERIALS Λ ( nm ) = d spacer + d absorber Γ = dabsorber Λ NUMBER OF LAYERS INTERFACE ROUGHNESS ATOMIC INTERMIXING V. RIGATO 7

8 Soft X-Ray Multilayer Materials combinations EUV-lithography (6 15 nm) ( ev) Astrophysics (17 30 nm) (41 73 ev) Biology (water window nm) ( ev) V. RIGATO 8

9 INTERFACE ATOMIC CONTROL Sharp interfaces HIGHEST REFLECTIVITY Controlled intermixing Ultra-Low interface roughness LOW REFLECTIVITY HIGHEST REFLECTIVITY V. RIGATO 9

10 HOW TO CONTROL INTERFACE QUALITY Multilayer are deposited by rf sputtering in Ar or Xe plasma Ion bombardment energy and flux are controlled by plasma diagnostics Langmuir probe with position control Plasma potential + substrate DC BIAS (Ion Energy) Plasma density, electron temperature (Ion Flux) Plasma is confined magnetically near substrate to increase the density of plasma facing the substrate Variable magnetic field (0-100 Gauss) implies variable plasma density hence flux to substrate Plasma density: ions/cm 3 is typical Low pressure radio frequency sputtering is preferred About 10-3 mbar pressure The ion/atom ratio is calculated by using the microbalance and by using Nucler Techniques to determine the impingement rate WHAT ION ENERGY IS BETTER? WHAT ION/ATOM IS BETTER? V. RIGATO 10

11 ION ENERGY -Bulk vs surface processes Ar+ ion bombardment calculation (normal incidence) Energy window for surface processes induced by Ar bombardment Si: ev Mo: ev Sputtering and intermixing Si: E(Ar)>50 ev Mo: E(Ar)>100 ev INVESTIGATED ENERGY E(Ar) = 25 ev E(Ar) = 75 ev Modulated Energy E d Mo = 33eV E d Si = 13eV * Z. Q. Ma and Y. Kido, Thin Solid Films 359, 288 (2000). V. RIGATO 11

12 ION/ATOM RATIO Molecular Dynamics calculations on metallic multi-layers indicate that for smoothing the interface appropriate combination of ion energy and ion/atom flux is necessary: the ion/atom ratio should be > 1 FOR MOLYBDENUM AND SILICON Growth rate: nm/s Ion/atom ratio = 2 * * X. W. Zhou and H. N. G. Wadley, Journal of Applied Physics 87, 2273 (2000). V. RIGATO 12

13 igh Resolution TEM & Reflectivity 0,7 0,6 0,5 % (pol s) % (pol p) angle of normal incidence: 10 Reflectivity 0,4 0,3 0,2 0,1 0, Photon Energy (ev) Examples of Si(white)/Mo(dark) multilayer with reflectivity higher than 60% at near normal incidence (@about 13.5nm) V. RIGATO 13

14 INTERFACE NATURE Montecarlo Calculations Calculate Silicon deposition rate depends on under-layer and layer thickness Interface thickness and composition Si Ar+ Mo 20nm V. RIGATO 14

15 XRR PROBES INTERFACE NATURE Interface study is necessary to simulate XRR spectra Interface widths (d Si/Mo and d Mo/Si ) Depend on ion bombardment d Mo/Si thickest E Ar =25eV E Ar =75eV V. RIGATO 15

16 Thermally stable engineered Mo/Si EUV multi-layers ( Si/B 4 C/Mo/B 4 C/Si ) Interdiffusion between elements is controlled by deposition of blocking thin layer barriers Nuclear Reaction Analysis is used to qualify the process and materials Elastic scattering of α-particles (RBS, ERD) for H, Si, Mo analysis 11 B(p,α) 8 Be 11 B(d,α 0 ) 9 Be 12 C(d,p) 13 C B x C interface atomic-layer Mo 2.5nm B x C amorphous Si(-H) B x C V. RIGATO 16

17 B4C monolayer blocking interface B(4.7)C monolayer at interfaces Thickness and composition depends on under-layer Mo Si Mo Si Mo Si B x C B x C Mo Si B x C 25eV vs 75eV bombardment on B4C NRA B x C on Mo low energy (25eV) B x C on Si low energy (25eV) B x C on Si High energy (75eV) B x C on Mo High energy (75eV) Mo Mo Si Si V. RIGATO 17

18 ION ENERGY MODULATION Ar ion bomardment energy may be changed during the layer growth GROWTH SiO 2 /(B 4 C/Si/B 4 C/Mo) 40/Si Ar+ energy ~25eV SiO 2 /(B 4 C/Si/B 4 C/Mo) 40/Si Ar+ modulated energy for each layer: 5eV(~<1nm)/25eV(~1nm)/75eV(remaining) E(Ar)=75eV (reamining) E(Ar)=25eV (=1nm) Lowest energy (<=1nm) Reflectivity (5 off normal) 0,6 0,5 0,4 0,3 0,2 0,1 ~3 nm periodic structure B 4 C 0, Photon Energy (ev) Mo B 4 C Si V. RIGATO 18

19 VARIABLE PERIODICITY MULTILAYERS - X-Ray Astronomy Hard X-Ray Grazing Incidence reflectivity optimized by varying the period: 2 to 20nm High Density Material (Pt, W) Low Density Material (C, Si) Softer X-rays Hard X-rays Single Layer Reflectivity V. RIGATO 19

20 SUMMARY Multilayer coatings have been developed since many years at the INFN- LNL In EUV and X-Ray mirrors technology nano-meter periodicity multilayers with either constant or variable period have quite unique reflection capabilities Ion bombardment of growing layers is necessary to improve and control the quality of interfaces and obtain highest reflectivity Ar energy <100 ev, ion/atom ratio >1 Best results obtained by changing the ion energy during the first stages of film growth The method is scalable to large area and applicable to many materials V. RIGATO 20