Absorption mappingfor characterization of glass surfaces

Mireille Commandre, Pierre Roche, Jean-Pierre Borgogno, and Gerard Albrand

The surface quality of bare substrates and preparation procedures take on an important role in opticalcoating performances. The most commonly used techniques of characterization generally give informa-tion about roughness and local defects. Aphotothermal deflection technique is used for mapping surfaceabsorption of fused-silica and glass substrates. We show that absorption mapping gives specificinformation on surface contamination of bare substrates. We present experimental results concerningsubstrates prepared by different cleaning and polishing techniques. We show that highly polishedsurfaces lead to the lowest values of residual surface absorption. Moreover the cleaning behavior ofsurfaces of multicomponent glasses and their optical performance in terms of absorption are proved to bedifferent from those of fused silica.Key words: Absorption, mapping, glass surface, local defects, photothermal deflection.

1. Introduction

In physics of thin films, the need for ultracleansurfaces and more generally the importance of well-defined substrate surfaces for achieving reproduciblethin-film properties has long been unanimously ac-knowledged.1 Morphology 1roughness, localized de-fects2 as well as chemical properties 1contamination2play an important role in the nucleation phenomenaand growth mechanisms of thin films.For example, a number of papers have focused on

nodular defects and their importance in optical coat-ings.2,3 All surface imperfections, such as dust, pol-ishing or cleaning residues, impurities, scratches, andsteps, can act as the starting point for thin-filmdefects. More generally, surface preparation tech-niques including polishing and cleaning a substrateprior to the deposition of a coating on its surface is animportant factor that influences the properties of thecoating. The substrate surface becomes a substrate–coating interface, and the nature of this interface hasa pronounced effect on the optical performance of a

The authors are with the Laboratoire d’Optique des Surfaces etdes Couches Minces, Ecole Nationale Superieure de Physique deMarseille, URA 1120 Centre National de la Recherche Scientifique,Domaine Universitaire de St. Jerome, F-13397 Marseille Cedex 20,France.Received 9 March 1994; revised manuscript received 11 July

1994.0003-6935@95@132372-08$06.00@0.

r 1995 Optical Society of America.

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finished coating, especially its resistance to laserdamage.4The most routinely used techniques to inspect

polished surfaces before coating are Nomarski micros-copy, scattering measurements,5 mechanical pro-filometry, and total internal reflection microscopy.6,7The sensitive photothermal deflection technique8

can give the mapping of low absorptance of baresubstrates9,10 to as low as 1 ppm 11 ppm 5 1 3 10262.We have shown that the absorption measured on baresubstrates is really nonuniform11: its average valueis approximately a few ppm at l 5 600 nm for fusedsilica, and local defects can be one hundred timesmore absorbing 1Fig. 12. Furthermore the spatiallyvarying absorption is localized on the substrate sur-face. Chemical analyses 1see, for example, Refs. 12–142 have shown contamination because of impuritiesand residues from the polishing compounds and clean-ing solvents 1metals, cerium or zirconium oxide, or-ganic solvents, water. . .2. Such absorbing residuesare likely to be responsible for the measured surfaceabsorption.After a short survey of the photothermal deflection

technique and a description of the experimentalsetup, we present a comparison of pictures of thesame surface obtained by photothermal mapping,scattering mapping, and Nomarski microscopy. Weshow that absorption mapping gives interesting andspecific information on surface and subsurface con-tamination of bare substrates 3fused silica, BK7,C20-36,15 D20-50 1Ref. 1524. Then we study to whatextent cleaning and polishing procedures of bare

substrates can modify their surface character withregard to optical absorption. The importance ofhighly polished surfaces is emphasized.

2. Photothermal Deflection Experimental Setup

When an electromagnetic wave is absorbed in asample, the part of energy converted to heat induceslocal variations in the refractive index of all theadjoining media. These refractive-index gradientsand the amount of absorbed light can thus be mea-sured from the deflection produced on a laser beam,also called a probe beam. We have used a collinearconfiguration in which the probe beam propagatesthrough the substrate and probes the radial thermalgradient in the substrate and in the air. These

Fig. 1. Absorption mapping of the same area on a fused-silicabare substrate: 1a2 top view, 1b2 oblique perspective.

radial gradients are created both by a Gaussianspatial profile of the pump laser beam and by heatdiffusion. This kind ofmeasurement requires calibra-tion of the photothermal deflection, using highlyabsorbing samples whose absorptance can be easilydetermined by a classical reflection transmissionmea-surement.11The layout of the experimental setup is shown in

Fig. 2. The pump beam laser can be a cw argon laser1in particular the 514.5-nm line2 or a cw dye laser1570–640-nm spectral range for Rhodamine 6G dye2.This s-polarized beam is directed so that the angle ofincidence on the sample surface will be approximately30° with respect to the normal. The beam is modu-lated by an acousto-optic modulator at a frequency of27 Hz. A power meter was used to measure thepump beampower. The incident power on the sampleis approximately 100 mW. The probe laser is an0.8-mW He–Ne laser that is normal to the samplesurface. The pump and probe beams are focusedonto the front surface of the sample through lenses offocal lengths 284 and 25 mm, respectively: the beamdiameters at this point are approximately 100 µm forthe pump beam and 40 µm for the probe beam1diameter at 1@e22. The relative position of the twobeams on the sample surface is adjusted in order tofind the maximum probe beam deflection. A quad-rant position sensor is utilized to measure the deflec-tion of the probe beam that is transmitted by thesample. The output signal of the sensor is directedto the differential input of a two-phase lock-in ana-lyzer. The sample is mounted upon an x–y transla-tion stage that allows one to move it in both directionsparallel to the sample surface. Accordingly we canmap the absorption variations at different points onthe sample surface. The spatial resolution, limitedmainly by the size of the illuminated area, is approxi-mately 100 µm; the increment step is 50 µm.This collinear photothermal deflection technique is

partially sensitive to the bulk absorption of thesubstrate. The order of magnitude of the contribu-tion of the bulk absorption can be estimated from thevalue of the thermal diffusion length of the substratematerial.11 Below we refer to surface absorptance as

Fig. 2. Experimental setup.

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the difference between themeasured absorptance andthe bulk estimated contribution.When a transparent substrate is illuminated at

oblique incidence with a source of high enough power,the spots related to multiple reflections at the frontand rear surfaces are often visible, at least the mostintense ones. In our setup a low f-number lenscollects the light scattered out of the specular direc-tions and an iris diaphragm is used for selecting thespot related to the reflection at the front surface.A silicon photodiode positioned behind the diaphragmwas used to measure part of the light scattered by thefront substrate surface. The ratio between this mea-sured value and the incident power is used in ourmappings for evaluation of scattering.

3. Specific Information Obtained byAbsorption Mapping

We have compared photothermal mapping, scatteringmapping, and Normarski microscope photography.For this purpose we have measured, on the samesamples and with the same spatial resolution, scat-tered light simultaneously with photothermal deflec-tion. Thus the scattering mappings that have beenobtained can first be comparedwithNomarski photog-raphy; these two techniques mainly test refractive-index and surface-profile variations. Hence it is notsurprising that these two images are alike even if thespatial resolution is not the same.On the other hand, photothermal mapping gives us

absorption or extinction coefficient variations. So itis normal to have no systematic correlation betweenabsorption mapping on the one hand and scatteringmapping and Nomarski photography on the otherhand. Indeed the behavior of surface defects de-pends on their nature. Some defects can induceabsorption 3Fig. 31a24 and scattering 3Fig. 31b24 simulta-neously. But in a general case 3Figs. 41a2 and 41b24 wefind scattering but nonabsorbing defects as well asabsorbing and nonscattering defects. So an emptyfield seen through a Nomarski microscope can beassociated with significant variations of absorption onphotothermal mapping. This is the case in Fig. 1.Thus photothermal mapping of absorption gives

specific information on the surface and subsurfacecontamination of bare substrates. Accurate absorp-tion measurements are probably a valuable tool forbetter preparation of surfaces prior to coating inorder to obtain low loss components as well as forstudying the influence of surface and subsurfacedefects on laser damage. Furthermore there can beno identification of the sites that could initiate laserdamage nor understanding of their role only byconsidering statistical data: one needs extensivecharacterization of the spatially varying surface de-fects with respect to their various properties 1optical,mechanical, and thermal2. Photothermal mappingof absorption is one of these means.

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4. Influence of Polishing and Cleaning Procedures onSurface Contamination

A. Some Cleaning Effects

If a sample is simply kept in a clean environment theabsorption mappings recorded on this sample at longintervals 1several days2 remain identical. However,any cleaning operation changes the absorption map-ping. Figure 5 shows absorption mappings of the

Fig. 3. Simultaneous mappings of 1a2 absorption and 1b2 totalintegrated scattering 1T.I.S.2 of the same area on a BK7 baresubstrate: absorption and scattering occur simultaneously in thecentral defect.

same area on a BK7 substrate 1T3 polished162 beforecleaning 1as received from the supplier2 3Fig. 51a24 andafter cleaning by a conventional procedure in anautomatic cleaning apparatus 3Fig. 51b24. One cancompletely modify spatial distribution of absorptionby cleaning when the average surface absorptancedecreases from 54 to 40 ppm. In Table 1 we givemean surface absorptance values before and aftercleaning for some BK7 and fused-silica substrates 1T3

Fig. 4. Simultaneous mappings of 1a2 absorption and 1b2 totalintegrated scattering 1T.I.S.2 of the same area on a BK7 baresubstrate: a defect can scatter but not absorb or can absorb andnot scatter.

Fig. 5. Absorption mapping of the same area on a BK7 baresubstrate 1a2 before and 1b2 after cleaning using a conventionalprocedure through an automatic cleaning apparatus. Spatialdistribution of absorption is completely modified whereas theaverage surface absorptance decreases from 54 to 40 ppm.

Table 1. Mean Surface Absorptance Values Appm B before and afterCleaning for BK7 and Fused-Silica Substrates, All T3 Polished

Status BK7 Fused Silica

Before cleaning 60 54 60 58 5.7 6.1After cleaning 45 40 54 45 6.0 5.2Ratio after@before 75% 74% 90% 78% 105% 85%

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polished162. Generally the first cleaning reducesmeansurface absorptance.It is interesting to compare the mappings of the

same sample after different or similar successivecleaning operations: thesemappings are hardly evercorrelated. As an example in Fig. 6 we present thephotothermal mappings of a BK7 substrate 1commer-cial grade polished2 central area after three differentand successive cleaning procedures: soft manualcleaning, soft automatic cleaning 1without ultra-sound2, and strong automatic cleaning 1with ultra-sound2. Mean surface absorptances are 42, 14, and26 ppm. Successive cleaning operations do not neces-sarily improve the results. Of course repeated ma-nipulations increase the risk of contamination.These effects are valuable for all the tested materi-

als; they show that the contaminants that causesurface absorption are not tightly bound to the sub-strate surface; generally the cleaning processes usedseem to displace absorbing impurities without com-pletely removing them. We now see in a more de-tailed study that the behavior of a surface depends onsubstrate material and polishing quality.

B. Study of BK7 Surfaces

BK7 substrates drawn from three sets polished indifferent conditions have been cleaned successivelywith increasingly effective cleaning procedures: softmanual cleaning, soft then strong ultrasonic auto-matic cleaning. In Fig. 7 we give minimum, mean,and maximum values for the absorptance measuredon a 750 3 550 µm2 surface. The contribution of thesubstrate bulk absorption appears clearly and corre-sponds to an absorptance of approximately 40 ppm inour experimental conditions. From these results wecan conclude that

1a2 for commercial grade polished substrates, softautomatic cleaning leads to the best uniformity andgenerally to the lowest mean absorptance 1Fig. 7, sets1 and 22,

1b2 for commercial grade polished substrates, ultra-sonic cleaning leads to amean value and peak-to-peakvariations of absorptance, both of which are higherthan those obtained with soft automatic cleaning 1Fig.7, sets 1 and 22,

1c2 on the other hand, the three cleaning proce-dures of T3 polished16 substrates give similar resultsfor mean as well as maximum values.

We can tie in these conclusions to parallel observa-tions with a Nomarski microscope:

1a2 A commercial grade polished BK7 substratecleaned through soft manual or soft automatic proce-dures displays microcorrugations interspersed with afew shallow scratches; it is the current view of stan-dard optical surfaces.

1b2 If we run the same surface through a strongultrasonic cleaning process, we can observe numer-

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Fig. 6. Absorption mapping of the same area on a BK7 baresubstrate after three different successive cleaning procedures, 1a2,1b2, and 1c2: these mappings are hardly ever correlated.

ous new scratches. The old ones become deeper androughness increases. It is a well-known result17:standard polishing techniques create fine scratchesthat cannot be observed immediately after polishing.These latent scratches are attacked during glasscleaning, all the more so because the procedure isdrastic: they are enhanced and thus become visible.This seems to create active sites with strong opticalabsorption.

1c2 On the other hand the view of T3 polished16BK7 substrates is not modified by our different clean-ing procedures.

C. Study of Fused-Silica Surfaces

A similar study has shown a smaller difference be-tween various cleaning techniques for fused-silicasubstrates. It was impossible to distinguish be-tween soft and ultrasonic cleaning procedures with anautomatic cleaning apparatus. Therefore in Fig. 8we present only manual and ultrasonic cleaningprocedures. For fused silica, the contribution of thesubstrate bulk absorption cannot be distinguishedfrom the noise. We observe, as previously men-

Fig. 7. Minimum, mean, and maximum surface absorptance forthree BK7 substrates polished in different conditions and cleanedsuccessively with increasingly effective cleaning procedures: softmanual cleaning 1-----2, soft 1- - -2, then ultrasonic 1—2 automaticcleaning. For each sample, measurements are performed on thesame area: com. gr., commercial grade; the double asterisks referto Ref. 16.

tioned, great similarity between different procedures.The principal conclusion is that the levels of surfaceabsorptance of commercial grade polished substratesare much higher than those of T3 polished16 samples,whatever the cleaning procedure used.

D. Statistical Results

In Figs. 9 and 10, in histogram form, we give theminimum, mean, and maximum values of surfaceabsorptance of 52 fused-silica 1Fig. 92 and 31 BK7substrates 1Fig. 102, all T3 polished16 and cleaned byusing the same automatic ultrasonic procedure.The maximum values correspond to local defects.In our experimental conditions 1pump beam diam-eter, 100 µm; increment step, 50 µm, sample’s mea-sured surface, 750 3 550 µm22 these highly absorbingareas generally take up nomore than one pixel, rarelytwo or three; furthermore there are generally nomore than one or two of these defects. We canobserve that 10% of fused-silica substrates present nodefect site of absorptance higher than 10 ppm.Severe pits of absorptance higher than some 100 ppmare generally visible in the Nomarski photography.

Fig. 8. Minimum, mean, and maximum surface absorptance forthree fused-silica substrates polished in different conditions andsuccessively cleaned with increasingly effective cleaning proce-dures: soft manual cleaning 1-----2, then ultrasonic 1—2 automaticcleaning. For each sample, measurements are performed on thesame area: com. gr., commercial grade; the double asterisks referto Ref. 16.

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Fig. 9. Histograms of minimum, mean, and maximum values ofsurface absorptance for 52 fused-silica substrates, all T3 polishedand cleaned using the same automatic procedure. Generally thedefects of absorptance higher than some 100 ppm are likewisevisible through a Nomarski microscope.

Fig. 10. Histograms of minimum, mean, and maximum values ofsurface absorptance for 31 BK7 substrates, all T3 polished andcleaned using the same automatic procedure.

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The mean values of surface absorptance for fused-silica substrates range from 3 to 20 ppm 1Fig. 92.Thosemeasured on BK7 substrates vary from 10 to 90ppm 1Fig. 102. The average value is 7.9 ppm for thewhole set of fused-silica substrates and 52 ppm forBK7 substrates. The behavior of these two kinds ofsurface in terms of optical absorption is different evenif their preparation procedures are similar. Further-more we have already seen that polishing procedureshave a significant influence on the surface absorptionof bare substrates: two sets of the same fused-silicasubstrates lead to 7.9-ppm average absorptance forT3 polished16 samples and to 25 ppm for commercialgrade polished samples.The statistical study on bare substrates is summa-

rized in Fig. 11. For each set of substrates 152 fusedsilica, 37 BK7, 12 C20-3615 and 5 D20-50152, mini-mum, mean, and maximum values of absorptance aregiven. This way of representing experimental datashows that

1a2 The three types of multicomponent glass havesimilar mean absorptance 1,62 ppm2, that is, approxi-mately eight times more than that of fused silica 1,8ppm2;

Fig. 11. Minimum, mean, and maximum surface absorptance forfour bare materials: fused silica, BK7, C20-36, and D20-50. Theindicated values are averaged for the following sets: 52 fusedsilica, 31 BK7, 12 C20-36, and 5 D20-50 substrates. Each of these100 samples was measured on a 750 3 550 µm2 area 1165 points2.

1b2 The minimum absorptance of these glasses is10 to 30 times higher than that of fused silica, whichgenerally cannot be distinguished from noise. Thisabsorptance is the same for BK7 and C20-3615 1whichpresent the same chemical passivity in the presenceof acids2; it is two times higher for the more sensitiveto chemical attack D20-5015.

1c2 The maximum values of absorptance have thesame order of magnitude. This may be explained bythe similarity of polishing and cleaning processes.

We can deduce that, in spite of similar polishingand cleaning, these materials of different chemicalcomposition lead to different kinds of surface withrespect to optical performance. As previouslyshown17,18 multicomponent glass adds significant com-plexity to polishing and cleaning behavior because ofpossible chemical changes. Therefore measure-ments of high-index glasses 1D20-50,15 nd 5 1.722 haveto be performed just after cleaning in order to avoidany degradation or chemical change of the nearsurface.

5. Conclusion

Optical surfaces are preferential places for contamina-tion that leads to strong degradation of optical proper-ties. Mapping absorption of optical surfaces enablesone to localize induced defects and to measure therelated absorption level. We have therefore charac-terized different polishing techniques and differentcleaning procedures on multicomponent glasses andfused-silica substrates, showing that absorption map-ping yields a specific view. For these materials thebest results 1lowest mean and maximum values ofabsorptance2 have been obtained with highly polishedsurfaces cleanedwith an automatic apparatus. How-ever measured surface absorptance is significantlydifferent for fused silica and glasses. For thesesubstrates it would be interesting to study the rela-tion between the surface contamination of bare sub-strates and the absorbing defects observed, on thesame area, in deposited thin films.

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11. M. Commandre, ‘‘Caracterisation de l’absorption dans lescomposants optiques en couches minces par deflexion photo-thermique,’’ These de Doctorat d’Etat 1Universite d’Aix-Marseille, France, 19922, Chap. 5, pp. 81–118.

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