Soft X-ray multilayer mirrors by ion assisted sputter...

V. RIGATO 1Source: INFN-LNL-2009 SIF- Bologna September 21, 2010

Soft X-ray multilayer mirrors by ion assisted sputter deposition

Valentino Rigato

INFN Laboratori Nazionali di Legnaro

Bologna, September 21, 2010

V. RIGATO 2Source: INFN-LNL 2009 SIF Bologna , September, 21 2010

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 multilayersPreparation & results

X-Ray astronomy multilayers

Summary


The laboratory activity is supported by the experiments submitted and approved by INFN National Commissions and by external projects

The LNL Materials Laboratory

The LNL Materials Laboratory fields

ion–surface interaction processes(ion beam analysis, ion implantation, ion assisted film growth, irradiation )

thin films synthesis by PVD and hybrid CVD-PVD technologies

nanostructured, multilayer and advanced thin films materials

ion plasma diagnostics

electron and atomic force microscopy

electrical mechanical properties of coatings

structural properties of coatings

Publications&

Know how


The Materials LaboratoryAdvanced materials process development and synthesis

Reactive Plasma Sputtering Deposition

Plasma Diagnostics

Ion-solid interaction

Low Friction, High HardnessNanoscaled Materials and Multilayers

Soft X-ray multilayers

High Performance Plastics

Passivating layers

Scintillator materials

Research activity of the Laboratory 1) Advanced materials preparation2) Process development


Characterization of materials with advanced methods1) Composition, stoichiometry, microstructure, morphology, surface topography2) Electrical, optical and mechanical properties

The Materials LaboratoryCharacterization of Physical Properties

Composition / depth profile / structureIon Beam Analysis (RBS, NRA, ERD, PIXE)Micro-PIXE 2-D trace element analysisFT-IRNuclear cross section measurement

Other physical propertiesNano-Hardness, Elastic modulusAdhesion (Micro-Scratch)Residual StressAtomic Force Microscopy & SEM

Ca Si K Fe

Ca Si K Fe

2*2 mm2


Soft-X ray Multilayer MirrorsEUV and X-Ray High Reflectivity Multilayer mirrors are deposited on flat and curved substrates forapplications in X-ray astronomy, EUV lithography , “water-window” microscopy, Free Electron Laser Optics

70 75 80 85 90 95 1000,0

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0,6

B4C

Rifle

ttivit

à

Energia (eV)

SiO2/(B4C/Si/B4C/Mo)×40/SiBombardamento ionicoa bassa energia (~15eV)

SiO2/(B4C/Si/B4C/Mo)×40/SiBombardamento ionico modulatoper ogni strato 5/25/75eV

Mo

Si

B4C

~3 nm

struttura del periodo

EUV Bragg High Reflectivity

Mirror design for GI hard X-ray telescopes

Multilayer Technology

Low energy ion bombardment Modulated ion bombardment


NORMAL INCIDENCE SOFT X-RAY MIRRORS

MATERIALS

absorberspacer ddnm +=Λ )(

Λ=Γ absorberd

NUMBER OF LAYERS

INTERFACE ROUGHNESS

ATOMIC INTERMIXING


Soft X-Ray Multilayer Materials combinations

EUV-lithography (6 – 15 nm) (95 – 206 eV)

Astrophysics (17 – 30 nm) (41 – 73 eV)

Biology (water window 2.35 – 4.5 nm) (528 – 276 eV)


INTERFACE ATOMIC CONTROL

Sharp interfaces

Controlled intermixing

Ultra-Low interface roughness

LOW REFLECTIVITYHIGHEST REFLECTIVITY

HIGHEST REFLECTIVITY


HOW TO CONTROL INTERFACE QUALITYMultilayer are deposited by rf sputtering in Ar or Xe plasma

Ion bombardment energy and flux are controlled by plasma diagnosticsLangmuir probe with position controlPlasma 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 substratePlasma density: 1010 ions/cm3 is typical

Low pressure radio frequency sputtering is preferredAbout 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 ?


ION ENERGY -Bulk vs surface processesAr+ ion bombardment calculation (normal incidence)

Energy window for surface processes induced by Ar bombardmentSi: 20 - 50 eVMo: 40 - 100 eV

Sputtering and intermixingSi: E(Ar)>50 eVMo: E(Ar)>100 eV

* Z. Q. Ma and Y. Kido, Thin Solid Films 359, 288 (2000).

EdMo = 33eV

EdSi = 13eV

INVESTIGATED ENERGY

• E(Ar) = 25 eV• E(Ar) = 75 eV• Modulated Energy


*

ION/ATOM RATIOMolecular 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: 0.05 - 0.1 nm/s

Ion/atom ratio = 2

* X. W. Zhou and H. N. G. Wadley, Journal of Applied Physics 87, 2273 (2000).


High Resolution TEM & Reflectivity

Examples of Si(white)/Mo(dark) multilayer with reflectivityhigher than 60% at near normal incidence (@about 13.5nm)

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angle of normal incidence: 10°

Ref

lect

ivity

Photon Energy (eV)

% (pol s) % (pol p)


Montecarlo CalculationsCalculate Silicon deposition rate

depends on under-layer and layer thickness

Interface thickness and composition

Mo

Ar+Si

20nm

INTERFACE NATURE


Interface study is necessary to simulate XRR spectra

Interface widths (dSi/Mo and dMo/Si) Depend on ion bombardmentdMo/Si thickest

XRR PROBES INTERFACE NATURE

EAr=25eV

EAr=75eV


Thermally stable engineered Mo/Si EUV multi-layers (…Si/B4C/Mo/B4C/Si…)

Interdiffusion between elements is controlled by deposition of blocking thin layer barriers

Nuclear Reaction Analysis is used to qualify the process and materials

Mo ≈ 2.5nm

amorphous Si(-H)BxC

BxC

Elastic scattering of α-particles(RBS, ERD) for H, Si, Mo analysis11B(p,α)8Be 11B(d,α0)9Be12C(d,p)13C

BxC interface atomic-layer


B(4.7)C monolayer at interfaces

Thickness and composition depends on under-layer

BxC on Mo – low energy (25eV)BxC on Si – low energy (25eV)BxC on Si – High energy (75eV)BxC on Mo – High energy (75eV)

Mo

Si

Mo

SiBxC

Mo

Si

Mo

Si

Mo

Si

Mo

SiBxC

B4C monolayer blocking interface

NRA

25eV vs 75eV bombardment on B4CBxC


Ar ion bomardment energy may be changed during the layer growth

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B4C

Ref

lect

ivity

(5° o

ff no

rmal

)

Photon Energy (eV)

SiO2/(B4C/Si/B4C/Mo)×40/SiAr+ energy ~25eV

SiO2/(B4C/Si/B4C/Mo)×40/SiAr+ modulated energy for each layer: 5eV(~<1nm)/25eV(~1nm)/75eV(remaining)

Mo

Si

B4C

~3 nm

periodic structure

ION ENERGY MODULATION

Lowest energy (<=1nm)

E(Ar)=25eV (=1nm)

E(Ar)=75eV (reamining)G

RO

WTH


VARIABLE PERIODICITY MULTILAYERS - X-Ray Astronomy

Hard X-Ray Grazing Incidence reflectivity optimized by varying the period: ≈ 2 to 20nm

Single LayerReflectivity

Softer X-ra

ys

Hard X-rays

Low Density Material (C, Si)

High Density Material (Pt, W)


SUMMARY

Multilayer coatings have been developed since many years at the INFN-LNL

In EUV and X-Ray mirrors technology nano-meter periodicity multilayerswith either constant or variable period have quite unique reflectioncapabilities

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 >1Best 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

Soft X-ray multilayer mirrors by ion assisted sputter...

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