INTERACTION OF INTENSE ULTRASHORT VUV PULSES WITH DIFFERENT SOLIDS

RESULTS FROM THE TESLA TEST FACILITY FEL PHASE I

Jacek Krzywinski

Institute of Physics, Polish Academy of SciencesInstitute of Physics, Polish Academy of Sciencesand and DeutschesDeutsches ElektronenElektronen--Synchrotron DESYSynchrotron DESY

Outline:

� Motivation� Experimental� Damage and surface modification � short introduction� Results� Conclusions

Why XUV/x-ray ablation should be investigated?

1. estimating and minimizing damages to surfaces of heavily loaded XUV/x-ray optical elements developed and used for guiding and focusing of short-wavelength laser beams

2. durability assessments of materials suggested for the first walls of ICF reactors and optical elements exposed to intense XUV/x-ray radiation in a laser-plasma interaction chamber

3. diffraction-limited nanostructuring of solid surfaces for fabrication of microelectronic and micromechanical elementsand devices

4. determination of radiation field characteristics: imaging of spatial energy distribution in a focused beam ablatively imprinted on the irradiated material and determination of pulse energy content

Absorption of visible and VUV radiation

Au

nm

Si

µm

SiO2

km

VIS

For a first time it was possible to excite surfaces of metals, semiconductors, insulators in the very similar way: linear absorption, similar absorption coefficient

and concentrate on the following question:How different materials respond to a similar excitation?

VUV 90 nm Au Si SiO2

9 nm 10 nm 8.5 nm

Layout of the experiment

Sample holder X,Y,Z,ϕ

FEL XUV laser beam

Focusingmirror

TOF ions / electrons Video microscope

XUV detector

Evaporation cell

CCD

Metals:Au, Al, Cu

SemiconductorsSi, a-C

InsulatorsAl2O3 SiO2 YAG

MgF2 BaF2Organic compounds

PMMA, PTFE

Photon Beam Parameters

Samples

Wavelength 86-98 nmPulse length 50 fsPulse energy 1-0 uJMin. Spot size 10 umMax. Intensity ~1014 W/cm2

absorption & electron excitation, nonthermal meltingfemtoseconds

photoelectronshot electrons

hν

electron difussion & thermalization,

thermal melting, evaporation

picoseconds

heat difiussion, ablation front propagation

nanoseconds

Heat diffiusion by phonons,Pressure wave propagationnanoseconds, 1000 nm

Hot electrons diffusion phononspicoseconds, 100 nm

absorption + thermalization of electrons, femtoseconds, 10nm

hot electrons

phonons

Example of theoreticalpredictions: Au

hν = 14 eVλ= 86 nm

Photon beam

Boltzman equation

Two temperature model

Hydronynamical code: XUV � Ablator, Lagrangian finite-difference method, Mie-Grüneisen equation of state (EOS) based on the code developed by T. Anderson:. PhD thesis, University of California at Berkeley, 1996 and modified by V. Letal, L. Juha, A. M. Bittner, J. Krzywinski, J. R. Sobierajski)

ε

E2

energy

time

Ef=5.5 Ev

10 fs

Au

( ) ( ) ( )][][ εεε nCnCdt

dnfeee −− +=

E3

E4

collision integrals

Thermalization of electron gas in metalse-e and e-ph scattering (R.Sobierajski)

Time constant τ depends on energy deposition

For energy deposition ~ 1 eV/ atom τ~ 5 fs

Boltzman equation

Electron distributioncan be described by

temperature for t > 5fs

fluency ~ 0.005 J/cm2

fluency ~ 3 J/cm2

fluency ~ 0.4 J/cm2fluency ~ 0.05 J/cm2

50 µm

50 µm

50 µm50 µm

200 400 600 800 10000

1000

2000

3000In

tens

ity (a

.u)

Raman shift (cm-1)

Amorphization Micro Raman spectra

It is interesting to notice, (that except for the Au-10 and the PMMA) the ΦII values oscillate around 0.03 J/cm2 This number is in the order of the fluency Fcoh at which the absorbed energy per atom is equal to the cohesion energyEcoh: Fcoh= Ecoh ·n /αwhere n is theconcentration and α is theabsorption coefficient.

For typical values: Ecoh~3 eV, n~5·1022, and α~106 cm-1 Fcoh~0.025 J/cm2

corresponds to the measured value.

Table 1: Surface modification thresholds

0.010.01PMMA

0.020.02Ce:YAG

0.030.03SiO2

0.040.005Si

0.030.01a-C-40

0.010.01Au-15

0.020.02Au

ΦII[J/cm2]

ΦI [J/cm2]Sample

I

ΦI is the fluency at which we notice the change of the refractive indexΦII is the fluency at which we notice thedeformation of the surface detected by AFM.

It is interesting to notice, (that except for the Au-10 and the PMMA) the ΦII values oscillate around 0.03 J/cm2 This number is in the order of the fluency Fcoh at which the absorbed energy per atom is equal to the cohesion energy Ecoh: Fcoh= Ecoh ·n /αwhere n is the concentration and α is the absorption coefficient. For typical values: Ecoh~3 eV, n~5·1022, and α~106 cm-1 Fcoh~0.025 corresponds to the measured value.

a-C film on Si substrate

fluency ~ 0.05 J/cm2

Stress related damage

Multi-shot modification of silicon and quartz surfaces

0 10 20 30 40 50 600

50

100

150

200

250

300

350

X[µm]

Z[nm

]

0 100 200 300 400 500-160-140-120-100

-80-60-40-20

02040

Z [n

m]

X [µm]

Si

SiO2 Profiles of the craters

SiO2 ablation rate

-0,005

0,000

0,005

0,010

0,015

0,020

0,025

0,030

0,000 0,100 0,200 0,300 0,400 0,500 0,600

Fluence [J/cm^2]

Dep

th [u

m]

Serie1Serie2spalled [um]Wielom. (spalled [um])

40 shots22 shots

theory

Spallation depth were calculated with a help of the XUV ABLATOR hydrodynamic code

Printing interference pattern into surfaces of different insulators

PMMA

YAG

fluency ~ 3 J/cm2fluency ~ 0.1 J/cm2

Si02

Very „clean” ablation. Edges of the craters are sharp, no cracks, debris, columnstructures. It was easy to drill a nice holes trough the samples (SiO2, BaF2)Morphology of the craters’ interior hardly depends on the applied fluency.

Ion emission for conducting and insulating materials

0 20 40 60 80 100 120

0

0.2

0.4

0.6

0.8

1

Ion energy/charge [eV/e]

Nor

mal

ized

sig

nal [

a.u] Si+

O+ - Si+- O+

0 20 40 60 80 100 120

0

0.2

0.4

0.6

0.8

1

Ion energy/charge [eV/e]

Nor

mal

ized

sig

nal [

a.u] Si+

O+ - Si+- O+

0 200 400 600 800 1000

0

0.2

0.4

0.6

0.8

1

Ion energy/charge [eV/e]

Nor

mal

ized

sig

nal [

a.u] +1

+2+3+4

Only single ions states and low energetic ions (~50 eV) were detected for insulators for all irradiation conditions.

High charge states and energetic ions (~a few keV) were typical for conductors and semiconductors. Ion energu ~charge field emissionn

Si SiO2

LIPSS = spontaneously-grown periodical structures on the laser-illuminated surfaces

( spatial period of the structures is usually related to the laserlaser wavelength)

LIPSS in PMMA267 nm < Λ < 413 nm

10µmx10µm

Can we find LIPSS with a period correlated to the irradiation wavelength?

Does the wavelength always control the LIPSS period?

1. Can we find LIPSS with a period correlated to the irradiation wavelength?

Answer for the first question is : yes!, the period is 80 nm

Does the wavelength only control the LIPSS period?Clearly, in most cases, not!

120mmx120mm

µmx10µm10

LIPSS-II of the second kind induced at the bottom of craters ablated in PMMA at increasing fluency.

The spatial period increases with fluency.

Craters ablated before, behind, and exactly at the tight focus position. LIPSS-II in PMMA

267 nm < Λ < 413 nm

10µmx10µm

LIPSS-II in PMMA600 nm < Λ < 700 nm

70mmx70mm

4µmx4µm

80µmx80µm

LIPSS-II LIPSS-II in PMMA500 nm < Λ < 1000 nm

increasingfluence

Conlussion 1

� Measured damage threshold for all investigated materials was estimated to be between 10 - 40 mJ/cm2

� This value corresponds well to the fluency at which the absorbed energy per atom is equal to the cohesion energy.

� The result is surprising in case of bulk metals. Oneexpects that hot electrons should take most of the deposited energy from away the absorption volume and thus increase the threshold value.

Conclussion 2

� A distinct difference in the behavior of conducting materials and insulators was observed. The difference is dramatic while the absorption coefficients are similar for all the materials at the TT1 FEL wavelength.– For insulators the ablation is very „clean”. Edges of the

craters are sharp, no cracks, debris, column structures. Morphology of the craters’ interior hardly depends on the applied fluency. Ablation rate agrees with hydrodynamic simulations.

– In contrast, the irradiated silicon surface becomes very rough when the intensity exceeds the damage threshold.

– There was also a clear difference between insulators and conducting material with respect to ejected ions spectra. High charge states and energetic ions (~a few keV) were typical for conductors and semiconductors. Only single ions states and low energetic ions (~50 eV) were detected for insulators for all irradiation conditions.

Conclusion 3� LIPSS-I have been found at the rim of a few craters

created in a-C and PMMA by 98-nm and 86-nm radiation, respectively.

� LIPSS of the second kind (LIPSS-II) has also been found. LIPSS-II spatial periods depend more on laser intensity than wavelength and are significantly longer than the laser wavelength. Very common phenomenon.

Time resolved ablation /transmissionmesurements for TTF FEL2

� The VUV R can be rotated (in the theta �two theta mode) and can be set infront of the VUV T. Thisfeature allows calibration of both devices with respect to each other.

CCD

TOF

Microscope

XUV (in)

Probe

XUV (out)

XUV Transmission

XUV Reflection

Sample