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Characterization of Nodular and Thermal Defects inHafnialSilica Multilayer Coatings Using Optical,

Photothermal, and Atomic Force Microscopy

C. J. Stolz, J. M. Yoshiyarna, A. Salleo,Z. L. Wu, J. Green, R. Krupka

This paper was prepared for submittal to the29th Annual Boulder Damage Symposium

Boulder, COOctober 6-8,1997

December 24,1997\

DISCLAIMER

This document was prepared as an account of work sponsored by an agency ofthe United States Government. Neither the United States Government nor theUniversity of California nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California. The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernment or the University of California, and shall not be used for advertisingor product endorsement purposes.

Characterization of nodular and thermal defects in hafniahilica multilayer coatingsusing optical, photothermal, and atomic force microscopy

C. J. Stolz, J. M. Yoshiyama, and A. SalleoUnivemity of California

Lawrence Livermore National LaboratoryP. O. BOX 808, L-487Liverrnore, CA 94550

Z. L. WU*, John Green, and R. Krupka**Department of Physics and Astronomy

Eastern Michigan UniversityYpsilanti, MI 48197

ABSTRACT

Multilayer coatings manufactured from metallic hafnium and silica sources by reactive electron beam deposition, are beingdeveloped for high fluence optics in a fusion laser with a wavelength of 1053 nm and a 3 ns pulse length. Damage thresholdstudies have revealed a correlation between laser damage and nodular defects, but interestingly laser damage is also present innodule-free regions. Photothermal studies of optical coatings reveal the existence of defects with strong optical absorption innodule-fke regions of the coating. A variety of microscopic techniques were employed to characterize the &fects for a betterunderstanding of the thermal properties of nodular defects and role of thermal defects in laser damage. Photothermalrnicroseopy, utilizing the surface thermal lensing technique, was used to map the thermal characteristics of 3 mm x 3 mmareas of the coatings. High resolution subapcrturc scans, with a 1 pm step size and a 3 ~m pump beam diameter, W=conducted on the defects to characterize their photothermal properties. Optical and atomic force microscopy was used tovisually identify defects and characterize their topography. The defects were then irradiated to determine the role of nodular andthermal defects in limiting the damage threshold of the multilayer.

Key words: laser-induced damage, hafnia-silica multilayer coating, laser damage morphology, coating defects, opticalmicroscopy, atomic force microscopy, photothermal microscopy

1. INTRODUCTION

Large aperture (up to 0.34 mz) high damage threshold coatings are required for the National Ignition Facility (NIP) includingpolarizers and high reflectors with 3 ns pulse length damage thresholds of 10.9 and 21.9 J/cmz respectively.l Electron-beamphysical vapor deposition processes are being optimized to improve the laser resistance of these types of coatings. Onemodification over deposition processes used in previous fusion lasers at LLNL, was the replacement of hafnia as a startingmaterial with hafnium to reduce the amount of source ejections. 2-3This modification has led to a significant reduction (-10x)in the nodulm defect density. Although consi&rable effort has been spent on studying nodular defects, unfortunately thedamage threshold of coatings can also be limited by a multitude of different defect types. If one had a tool that couldnondestructively identify the specific defects with the lowest laser damage threshold, one could then characterize the defectswith a variety of tools to determine the proper coating process modifications and eliminate the fluence limiting defects thusraising the damage threshold.

Optical microscopy is a useful tool for characterizing a coated surface, but typical defect heights and depths are difficult toresolve with standard Nomarski optical microscopy. An Atomic Force Microscope (AFM) has sufficient lateral resolution toeasily image most coating defects, but there is only a weak correlation between nodular defect diameter or height with damagethreshold.ti A phototherrnrd microscope characterizes the thermal properties of a laser imadiatcd defect.’-’o If the damagemechanism is indeed driven by thermally-induced stresses, then this instrument shows promise as a tool for nondestructiveidentification of the small percentage of fluence limiting coating defects for further characterization by other methods.l’

Current addresses:* University of California, Lawrence Livennore National Laboratory, P. O. Box 808, L-487, Liverrnore, CA 94550** STN ATLAS Elektronik GrnbH, Abtcilung ETZ 51, Sebaldsbruecker Heerstrasse 235, 28305 Bremen, Germany

2. EXPERIMENTAL PROCEDURE

For this study, 1m Brewster’s angle plate polarizers wcm deposited by reactive electron-tam deposition from hafnium d

silica sources, The polarizer consists of an optimized nonquartcr-wave design of alternating hafnia and silica layers, Thesilica overcoat optical thickness is approximately W at 1w. Previous work has shown a significant increase (2-4x) in thefunctional damage threshold of a polarizer coating by increasing the overcoat optical thickness by an additional ?J2.” During

these experiments it was observed that delamination, the most prevalent damage morphology, initiated at ncdular defects andalso wmmd in areas that were free of nodular defects, 13 Tbercforc this coating design offers an opportunity to study the

relationship of defects, other than nodular defects, in the laser damage process.

Previous work on high reflector coatings illustrated that nodular defects that were taller than 0.7pm bad a high probability ofejection at fluences below the NE requirement.4 Coated samples were examined under an optical microscope for identification

of nodular defects. The defccta were then measured on an Atomic Force Microscope (AFM) to determine their height. llmcc.nodules that were taller than 0.7 pm were identified and characterized. A diamond tipped indentor was then used to scratch a

3 mm circumferential square around each ncdule to facilitate Incation of the defect for additional tests. The coating was then

cbaractcrized with the photothermal microscope with an Argon-ion laser km at normal incidence to the coated sample. Theentire 9 mm2 arm was mapped at low resolution and high resolution subaperture maps were generated of the bigher absorb]ngdefects. After phototbenmd mapping, the defects with the highest photothermal signal, that were previously not measured on

the AFM, were characterized. These defwts were then irmchated in single shot mode at increasing fluence until damageoccurred. The results were analyzed to determine if a correlation existed between the defects with the highest phototbermal

signal irradiated with an Argon-ion laser at normal incidence and defa with the lowest damage threshold inadiatcd at 1w atBrewster’s angle. Some ncdules that were fully chamctcrizcd were also cmss+ectioned by a Fwused Ion Beam (FIB) for

determination of the defwt seed composition and depth.” These cross sections could be used for future theoretical modeling ofthe defect temperature distribution for comparison with the measured phototbermal signal,] 1

2.1 COATING DEFECTS

The marking pcdure proved to be difficult as illustrated by the multiple scratches surrounding defect la in the opticalmicrograph (F]g. 2.1.1), Upon closer inspection by scanning electron microscopy (SEM), tbrcc defect ty~s were identified

after photothermal mapping, namely a nodule (defect la), delamination (defects lb and 1d), and surface contamination

(defect 1.),

Fig. 2.1 la) Coating defect la is observed as a scattering site Fig. 2,1. lb) H]gher magnification SEM image of site 1 atlerwhen viewed by optical microscopy. photothennal mapping reveals additional defects.

The second site, illustrated in Fig. 2.1.2, has a ncdule of 0.9 ~m height (defect 2a) that is easily detected in the opticalmicrogmph. The tbiid site illustrated in Fig. 2.1.3, consists of multiple defects including nodules (defects 3a, 3b, and 3c) and

an indention from the diamond tip used to mark the area and also to scribe the square perimeters (defect 3d),

Fig, 2.1.2 Coating defect 2a when viewed by opticalmicroscopy.

Fig. 2.1,3 Coating defwta 3ad when viewed by optical

microscopy.

2.2 PHOTOTHERMAL MICROSCOPY

A photothermal microscope was developed based on the principle of surface thermal lens (S’fl.), a schematic diagmm of whichisshownin F]g. 2.2.1.15 ~sdetection scheme uses amdulati laser hem, mfemdtom tie pump beam or beating beam,

to generate a lucafiz.ed temperature rise and hence a surface deformation of the sample. The detection of the temperature rise is

accomplished hy using a second laser beam, rcf- to as the probe beam, which is rcflcctcd from the sample surface andrecords the thermal deformation through optical diffraction effect. Since the surface thermal deformation distorts the wavefront

of the prohc k-mm in a way similar tu a thermal lens of finite size, as shown in Fig. 2.2.1, tbe detection scheme is thusreferred to as the surface thermal lens rcchnique. The surface thermal lens, as predicted by dhllaction calculation and vcdficd

by experimental observations, can cause either photothemml divergence (defocusing) / photothennal convergence (focusing) urcomplicated diffraction fringes, depending on the spccitic geometq used in the detection scheme. ‘5-17

chopperpump laser

eample—

motorized translation stageFig, 2.2.1 Depiction of the photothennal micruscupe using Fig. 2,2.2 Depiction of surface thermal lensing technique.

surface thermal lensing tcduique.

The change in beam profile of the probe beam is detected by a single-cell detector with a pinhole in front of it. The resulting

signals are then sent to a lock-in amplifier, which selectively amplifies the ac part of the signal. The pump laser beam isfccused to a beam size of about 3-5 ym for high spatial resolution, and the probe beam size about 15 ~m for the bestsensitivity. In this way the technique bas the ability to cbamcterizc localked optical and thermal properties, which are of

importance to the study of thin films for high power Iaaer applications. By scanning, the technique produces an image of a 2-dimensiomd spatially resolved photothernml response of the sample, which contains information on both surface ami

subsurface properties.

One important advantage of the surface thermal lens technique is that it combines the sensitivity of the pbototbermaldeformation methcd and the simplicity of the Eaditional thermal lens detection scheme. It has been demonstrated to be a

powerful tool for photothenmd characterization of weakly absorptive thin film coatings and bare substrates.’’-”

Fig. 2.3.2 shows a sketch of the experimental setup of the phototbermal microscope. A modulated Argon-ion laser is used asthe pump beam, and a stable HeNe her as the probe beam. In experiment this system can be slightly modified so thatmicroscopic. scanning of the surface reflectivity can be obtained for the same arm for comparative studies. The spatial

resolution of the system is mainly limited by its pump beam size, and is assessed to be about 5 pm. The sensitivity of thesystem at about 1 W incident pump laser power is less than 1 ppm.

3.1 RESULTS - CHARACTERIZATION

Fig. 3.1 shows the result of a 3 mm x 3 mm area scanned by the pbototbennal microscope. The image Iabcled as amplitude

reflects the absolute value of the laser-induced surface deformation, which is proportional to the hdizcd absorbance value.The phase map, on the other hand, is strongly related to lcxalizsd thermal properties ax well as the depth of the absorption

defect sites, Furtbcr understanding of the details is related to the modulation tlequency used, which in this case is 3.9 kHz,and requires substantial medeling effort, which goes beyond the scope of this paper.

Fig. 3.1 la) Photothenmd amplitude map of site 1 Fig 3.1. lb) Phototbermal phase map of site 1

There is excellent correlation between the clef- observed by photothemnal mapping in Fig. 3.1,1 and the SEM image inFig. 2.1. la. More detailed examination of the defect$ reveal cracking of the multilayer surrounding ncdular defd la inFig. 3.1.2 and around the delaminate defect lb in Fig. 3.1.3. These fractures provide evidence. of thermal failure. induced by

the phototbermd microscope. Although nodules can be weakly bound and show some evidence of cracking, probably due tothe cooling cycle after coating, these types of fiacturcs generally are not observed on coatings that have not been lawinadiatcd, The prcaence of cracks propagating at right angles to one another suggest that the cracka were formed at dit%rent

times since fractures in brittle materials occur normal to the smess gradient. ‘Ilk is consistent with the raster scan tecbniq”eused in phototbemmd mapping the defects. The delamination defects in Figs. 3.1.3 and 3.1.4 are more typical of l--induceddamage commonly observed on this type of coating and may be due to an excessive fluence in the pump beam of thephototbermal microscope, Since the operating pammetera for minimal surface modltication due to the phototbennal

Fig. 3.1.2a) SEM image of ncdular defect la. Fig. 3. 1.2b) SEM image of FIB cross section of deftireveal a silica seed near the substrate intmface,

la

Fig. 3.1.3 SEM image of delamination defect lb. Fig. 3.1.4 SEM image phase of delamination defect ld.

255

1

Fig. 3.1 .5a) Pbotothennal amplitude map of defect la. Fig. 3.1 .5b) Photothermd phase map of defect la.

microscope were unknown, srr upper bound for the pump Iaaer fluence waa determined during mapping of site 1. It is also

worth mentioning that the delamination depth is equal to the overlayer and there is no observable presence of a nwhdar defectThe cause of tfrk morphology could he the result of a defect in the vicinity of the interface between the two different coating

materials, that had high thermal absorption.

High resolution pbotothermal maps of nodular defect la show a discontinuity in the phaae map suggesting a thermal barrier

between the defect and the adjacent multilayer coating, This observation is consistent with the cm.cks that surround the nodulethat would minimize the nodulm surface in contact with the nuchdar defat thus reducing the heat tmrrsfer by conduction.

Unfortunately, it is not known when the crack warmed around the nudular defect

Site 2 was slso mapped by photothenmd microscopy as illustrated in Figs. 3.1.6. Defect 2a is a nodulsr defect of 0,9 #m

height that had a photothernrsl signal that wsa indistinguishable from the mrruunding multilayer. Defwt 2b is a partial

nwhdar defect of 0.17 pm height with approximately 50% of the center area missing and had a contraat ratio

(peak/background) of4,5. Itisunhown ifthepdalejection of thentiulewas a~sultoftie phototiemal mapping or&stm w relief during the coating, cleaning or hsndling prucesses,

Fig.3.1.6a) Phototherrrmla rnplitudem apofsite2. Fig, 3.1.6b) Phototherrnal phase mapofsite 2.

Fig. 3.l.7a) Photothernral amplitude mapofsite3. Fig. 3.l,7b) Phototherrnal phsaemapofsite3.

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Flg.3.l.8 H]ghresolution mphmdemaps ofcoating defects 3a+illustiate tiat Memplimde oftiephotothemd signalfwcoating defects can &. as high ss 126x over backgound.

Site 3 was also mapped by photothennal microscopy as illustrated inFigs. 3.l.7. Defect 3ehadthe highest phototherrnal signal of 126xover backgrorrnd. Itisa shallow pit with a depth of 0.25 Lm whichis witbin 10% of the theoretical thickness of the outermost twocoating layers. Interestingly, it is the only defect from site 3 that isnot visible in theoptical rnicrographin Fig. 2,1.3. The pit carrbeviewed by optical microscopy under higher magnification (200x).Higher resolution images ofall the site 3 defects arc in Fig. 3.1.8.Defects 3a-c are nndrdes and have the lowest phototberrnal amplitudesimmkwhiledefects3d-e arepits that havetbe highest phototbennal

F]g. 3.2.1 Relationship ofdefectaand darnagesitesto irradiated area

~plitude signal.

3.1 RESULTS - LASER INTERACTION

After sites 2arrd3were fully characterized, theywereirmdiated with a1053 nmNdYAGlas.erwitb aGaussian 9,4 ns pulse length and al,3mmdiarneter bcamatl/e2. Thelaser wasopsmtedin single shotmode. Laaerfluences were determined by beam protiling and totalenergy measurements. The sccrxacy of the fluence measurement iswith:n 15%. Adescription of thein-situ AFM laser damage tester isdesccibcdelacwhcrc.s

Ifdamagewsa notobserved, tbcsample wasimadia.tcd ataslightIy bigherfluence until damagccccurred. Tbe test areasaeshown in Fig. 3.2.1. In both areas tbedamage morphology wa.stbe exp%ted delamination of the outer silica layer. Site 2damaged at14.5J/cm’ after onlyonepulse mdsite3 badafluence ramp from 8-24 J/cm’. It is unknown if site3 had anyconditioning effwt due to the higher damage tbreahold.

In botbcases, thedefectthat badtbe highest phototbennal signal badtbelowest damage tbrcshold despite tbediffmcncesinthe test angles andwavelengtbs of thephototbermal micruscopeandlascr damage tester. In site 2, the partially damagencdulehad thelower dmnagetbrcshold, Insite3, tbedefect with thelowest damage threshold was a very shallow pit. Tbcseresults suggest tbataphotothernmlm icroscopeisa useful tool for identifying the lowest damwetbrcshold defects, Muvidedtiattiepurnp &m fluenceis lowenough to-not cause mdcuating sufiacem;tification. -

.

o

Fig. 3.2.2a) Typical delamination damage morphology at defect 3e. Fig. 3.2.2b) Laser shot sequence at site 3.

4. SUMMARY

Pfmtotbernml mapping obtained both ampfitude and phase information fur Iucal defects which were associated with absorptionand thermal properties for each specific defect. Fucused ion beam milling reveals nudule cross section to enable futuremcdeling of thermal properties of ncdule fur comparison with photothennal measurements. These preliminary resultssuggest a cumulation exists between defects with high phototberrwd signal, up to 126x over background, and low damagethreshold despite differences in the measurement wavelength and test angle, Evidence of thermal failure indicates the pumplaser of the phototbenmd microsco~ moditied the surface. Regardless, the delamination in site 1 imply the existence of ahighly absorbing very shallow defects, or thermal defwt+ that damage quite easily and may be the origination sites ofdelamination damage that has no evidence of a pit or ncdule as a damage initiation site, A careful balance between the highfluence required for ubserving weafdy absorbing defects and the lower fluence myired for minimal surface modification mustbe determined. I.ascr-induced damage uccumedat different defti morphologies, therefore.coating prucess improvements furhigher damage threshold coatings should have a bruader fucus than just elimination of source ejections.

5. ACKNOWLEDGMENTS

This work was performed under the auspices uf the U.S. Department of Energy by Lawrence Llvenuore National Laboratoryunder cormact No. W-7405-Eng-48.

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