In this work, ellipsometric measurements were used to optimise the technology of machine working the polished parts made of MgF₂ optical ceramics. The ellipsometry is a high-performance contactless method to control quality of optical surfaces, which is used here due to a sharp response of light polarisation conditions to the properties and parameters of surface and subsurface layers in investigated reflective systems. It is shown that the highly productive technology of diamond polishing provides achievement of ellipsometric parameters at a level of conventional methods of polishing

Maslov, V.P.

Sarsembaeva, A.Z.

Sizov, F.F.

Semiconductor Physics Quantum Electronics & Optoelectronics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

199' 2004, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of UkraineSemiconductor Physics, Quantum Electronics & Optoelectronics. 2004. V. 7, N 2. P. 199-201.PACS: 81.05.JeEllipsometric control of quality of polishedMgF2 optical ceramicsV.P. Maslov, A.Z. Sarsembaeva, F.F. SizovV. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine41, prospect Nauky, 03028 Kyiv, UkrainePhone/fax: +380 (44) 265 0555, E-mail: maslov@isp.kiev.ua, sizov@isp.kiev.uaAbstract. In this work, ellipsometric measurements were used to optimise the technology ofmachine working the polished parts made of MgF2 optical ceramics. The ellipsometry is ahigh-performance contactless method to control quality of optical surfaces, which is usedhere due to a sharp response of light polarisation conditions to the properties and parametersof surface and subsurface layers in investigated reflective systems. It is shown that the highlyproductive technology of diamond polishing provides achievement of ellipsometric param-eters at a level of conventional methods of polishing.Keywords: ellipsometric control, diamond polishing, optical ceramics.Paper received 16.12.03; accepted for publication 17.06.04.1. IntroductionOptical ceramics based on MgF2 are polycrystallinematerials transparent in IR spectral range. Optical ce-ramics in comparison with single crystals are opticallyand mechanically isotropic, there are no planes of cleav-ages and cracks, and they are heat-resistant. Parts madeof optical ceramics are well grounded and polished withconventional processing techniques. The optical ceram-ics are base materials for manufacturing optical parts ofdevices, which operate in IR wavelengths range in condi-tions of large pressure gradients and temperatures (inputwindows, domes). One of the urgent questions to processsuch parts is the necessity of rising the productivity ofmanufacturing high-quality surface in these optical parts.With this purpose, the technology of effective pre-cise diamond polishing that  provides the increase ofproductivity by 2 to 5 times was designed. The ellipso-metric method was used as a method to control qualityof polishing [1].2. Method of ellipsometric investigationsWhen electromagnetic wave is reflected from an arbi-trary reflecting system (Fig. 1) [2,3], a phase differenceappears between components of an electric vector thatare perpendicular and parallel to the plane of incidence,which, in general, results in the elliptical polarisation ofthis wave. Reflection coefficient Rp and Rs of a systemand phase difference ∆ are related by the basic equationof ellipsometry:∆==ispetgRRψρ (1)Angles ψ and ∆ are called ellipsometric parameters ofthe system.The account of multiple reflections inside a layer atthe first and second optical boundaries allows to expressreflection coefficients of all the system, which enter intothe basic ellipsometric equation (1) through the Fresnelreflection coefficients at each boundary r1, r2 and thedepth of the layer. In this case, Eq. (1) gets the followingform:δδδδψississipippierrerrererretg−−−−∆++⋅++=2121121 11, (2)where ( ) 2/122221 sin4 ϕλpiδ nnd −=  phase difference in alayer.As a result of transforming Eq. (2), the angle depend-encies for the phase difference ∆ and angle ψ could beobtained:200SQO, 7(2), 2004V.P. Maslov et al.: Ellipsometric control of quality of polished MgF2 optical ceramics( )( )( )( )( )( )( )( ) ,1sin1411sin14222222122212221_222222122212221_−−−−+=−−−−=∆−∆ntgnntgnnnndtgtgntgnntgnnnndtgϕλϕϕpiψψϕλϕϕpi(3)where _ψ  and _∆   ellipsometric angles for substrate.At the Brewster angle (tgϕBr = n2/n1), from the secondequation of the system (3) the formula for the minimumvalue of ellipticity follows:( )( )( ) ( )2/1222221212221111+−−−= nnnnnndtgλpiψ . (4)Relation of Fresnel reflection coefficient for p- and s-components of the electric vector, tgρ and phase differ-ence between them can be counted by usage of metal-optics methods. The photoelectric method by Beattie andConn is the most suitable among them. The modificationof this method [4,5] allowing to apply it to transparentdielectrics was used in this work.The values directly measured are intensities of radia-tion reflected from a sample I0, I45, I90 measured at threeazimuths of an analyser ψα (equal accordingly 0°, 45°,90°) concerning incidence plane and fixed azimuth ofpolariser β = 45°. Ellipsometric parameters are calcu-lated using the formulae [6]:.22cos,90090045900IIIIIIItgtg−−=∆= βψ(5)As the measurements of ellipsometric parameters arecarried out within the limits of the Brewster angle, wherecos∆ passes through zero point, the error of phase differ-ence is minimum, and the following condition is realized:ψρ tgtg = (6)3. Polishing by a diamond toolThe development of technology of a diamond polishingthe parts made of MgF2 optical ceramics was performedusing results of ellipsometric researches of tg  and ∆ϕ == ϕM  ϕB, values of which change after grinding withincrease of polishing depth, approximating to those val-ues of these parameters, which characterize an undis-turbed layer. The requirements on deviation from theshape of optical parts were no more than 1 micron ondiameters of details up to 80 mm.The laser null-ellipsometer LEF-3M-1 (λ = 6328Å)was used to investigate quality of polished MgF2 opticalceramics.The analyses of the influence of the sample flatnessand number of measurements on the accuracy of deter-mining ellipsometric parameters were previously con-ducted. It was ascertained that deviation from flatness ofa sample within limits N = –0.55 has no essential in-fluence on the error of measurements at the light beamdiameter 12 mm. With the purpose to obtaine reliableresults, we carried out not less than 6 measurements oneach sample. Under these conditions, the relative errorwas no more than 10 % [7].The changes of the main angle and ellipticity with thepolishing depth are conditioned by both changes withdepth of physicochemical properties of the damaged layerformed by grinding, and by those changes, which areintroduced during polishing and by the form of the pol-ished layer surface [8, 9].The depth of a polished layer, at which the minimumvalue of tgρ and minimum value of ϕM are reached, isadopted for the depth of the damaged layer for investi-gated processing. It was established that after diamondprocessing the depth of the damaged layer was 1.21.5times less than that after processing by a free abrasive ofthe same grain sizes. On the base of these results, thetechnological limits on processing were established. Dataabout the influence of polisher and abrasive materials onroughness and ellipsometric parameters of polished sam-ples are summarized in Table 1.The features of diamond polishing when using organicbases have exhibited in minor increase of ellipsometricparameters, which was conditioned by the presence ofconsiderable tangential efforts, and at directional inten-sive polishing on cloth (V = 800 rotation/minute) ellip-sometric parameters have been even more increased, andon a surface a corrugated texture, connected with increaseof concentration of structural defects in a direction ofprocessing, have appeared.Using the above ellipsometric method, designed wasthe technology of diamond polishing on organic flowbundle, which has allowed 2 to 5-fold increase in produc-tivity of processing surface of optical parts.Fig. 1. Reflection of flat simple harmonic wave from homogene-ous layer (n1, n2  refractiion indexes, and d  thickness of thelayer).Ej jERRdps12psV.P. Maslov et al.: Ellipsometric control of quality of polished MgF2 optical ceramics201SQO, 7(2), 20044. ConclusionsThe offered ellipsometric method of control has allowedto improve the technological process of diamond grind-ing and polishing MgF2 optical ceramics. It was shownthat after diamond processing the depth of the damagedlayer was 1.21.5 times less than that after processingby free abrasives of the same grinding. The features ofthe diamond polishing on the organic flow bundle wereexhibited in minor increase of ellipsometric parameters,which is conditioned by the presence of considerable tan-gential efforts of the tool.References1. T.V. Vladimirova, N.Ya. Gorban, V.P Maslov, T.S. Melnik,V.A. Odarich. The research of the optical proper ties andstructure of sitall // Optiko-mekhanicheskaya promylennost,„9, p. 31-33 (1979), in Russian.2. Fundamentals of an ellipsometry. Edited by A.V. Rzhanov //Novosibirsk, Nauka, 1979.3. T.V. Andreeva, V.A. Tolmachev. Methodological aspectsof ellipsometric experiments on optical materials // Optiko-mekhanicheskaya promylennost, „10, p. 36-39 (1986), inRussian.4. V.A. Odarich. Measurements of small ellipsometric param-eters by a photoelectric method // Zavodskaya laboratoriya,„43, p. 1093 (1977), in Russian.5. Beattie G.R, Conn G.K., Phil. Mag., , 46, p. 222-225 (1955).6. A.I. Voniurihin, V.P. Gerchanovskaya. Optoelectronic polari-sation devices, Kiev, Tekhnika, 1984, p. 136-138, in Russian.7. The reference book of the technologist  optician, Ed. byS.M. Kuznezcova, M.A. Okatova, Leningrad, Magnitostroe-nie, Leningrad department, 1983., 414 pages in Russian.8. A.I. Belyaeva, A.A. Galuza, T.G. Grebennik, V. Pyuriyev.Optical constants of surface layer on gadolinium galliumgarnet: ellipsometric study // Semiconductor Physics, Quan-tum Electronics and Optoelectronics, 2(4), p. 61-65 (1999).9. M.J. Riedl, Optical design fundamentals for infrared systems,SPIE Optical Eng. Press, Bellingham, USA (1995), p.136.Table 1. Results of measurements of ellipsometric parametersPolisher An abrasive Roughness RZ, tgρ•103 ÔM,°micrometesDiamond Diamonds with 0.0150.02 15.5 54°32′polisher on the size 5/3organic base in organic baseand diamondswith the size 5/3Colophon-pitch Chromium oxide 0.0250.03 13.0 54°35′resinThe cloth Chromium oxide Corrugated frame 19.0 54°30′appears

Ellipsometric control of quality of polished MgF₂ optical ceramics

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Ellipsometric control of quality of polished MgF₂ optical ceramics

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