Manufacturing of an absorbing filter controlled by a broadband optical monitoring

Transcription

1 Manufacturing of an absorbing filter controlled by a broadband optical onitoring Bruno Badoil, Fabien Learchand*, Michel Caelinaud and Michel Lequie Laboratoire Institut Fresnel, Université Paul Cézanne, 3388 Marseille cedex (France) * Corresponding auor: fabien.learchand@fresnel.fr Abstract: his paper deals wi a broadband optical onitoring set up useful for e anufacturing of absorbing coatings. he onitoring strategy consists in siultaneous easureents of transittance and reflectance over a large spectral range. he resulting analysis allows en to deterine e real tie deposited ickness. A stage of design correction is possible after e deposition and analysis of each layer. his eod has potential for in etallic layers coatings. We en describe layer after layer e strategy for e control and anufacturing of a filter wi given colorietric properties. 8 Optical Society of Aerica OCIS codes: 3.6: Coatings, 3.395: Metallic, opaque, and absorbing coatings, 3.86: hin fils: Deposition and fabrication References and links. B.. Sullivan and J. A. Dobrowolski, Deposition error copensation for optical ultilayer coatings. II. Experiental results - sputtering syste, Appl. Opt. 3, (993).. A. V. ikhonravov, M. K. rubetskov, and. V. Aotchkina, Investigation of e effect of accuulation of ickness errors in optical coating production by broadband optical onitoring, Appl. Opt. 45, (6). 3. L. Li and Y. Yen, Wideband onitoring and easuring syste for optical coatings, Appl. Opt. 8, (989). 4. A. V. ikhonravov and M. K. rubetskov, On-line characterization and reoptiization of optical coatings, Proc. SPIE 55, (4). 5. D. Ristau,. Gross, and M. Lappschies, Optical broadband onitoring of conventional and ion process, in Digest of Optical Interference Coatings on CD-ROM (Optical Society of Aerica), paper ue (4). 6. M. Lappschies, B. Goertz, and D. Ristau, Application of optical broadband onitoring to quasi rugate filter by ion bea sputtering, in Digest of Optical Interference Coatings (Optical Society of Aerica), paper ue4 (4). 7. S. Wilbrandt, R. leitel, D. Gabler, O. Stenzel, and N. Kaiser, In situ broadband onitoring and characterization of optical coatings, in Digest of Optical Interference Coatings on CD-ROM (Optical Society of Aerica), paper ue6 (4). 8. B.. Sullivan and K. L. Byrt, "Metal/dielectric transission interference filters wi low reflectance.. Experiental results," Appl. Opt. 34, (995). 9. B. Badoil, F. Learchand, M. Caelinaud and M. Lequie, Interest of broadband optical onitoring for in fil filters anufacturing, Appl. Opt. 46, (7).. S.D. Browning, J.A. Dobrowolski, 7 OSA opical eeting on Optical Interference Coatings anufacturing proble, paper MB (7).. J. A. Dobrowolski, S. Browning, M. Jacobson, and M. Nadal, "7 opical Meeting on Optical Interference Coatings: Manufacturing Proble," Appl. Opt. 47, 3-45 (8).. O. Y. Borkovskaya, N. L. Ditruk, and O. V. Fursenko, Characterization of in etal fils wi overlayers by transparency and ultiangle including surface plason excitation reflectance ellipsoetry eod, Proc. SPIE 394, 5-54 (997). 3. A. V. ikhonravov, M. K. rubetskov, O. F. Prosovskiy, and M. A. Kokarev, Optical characterization of in etal fils, in Digest of Optical Interference Coatings on CD-ROM (Optical Society of Aerica), paper WDPDP (7). 4.. Csendes, B. Daroczy, and Z. Hantos, Nonlinear paraeter estiation by global optiization: coparison of local search eods in respiratory syste odelling, Syste Modelling and Optiization, (Springer- Verlag, Berlin 986) # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS 8

2 . Introduction he developent and iproveents of array detectors in e last twenty years have opened up new horizons for optical onitoring. he precise control of deposited ickness of in fil stacks is now possible using a real tie analysis of e transitted flux over an extended spectral range for dielectric [-7] and etal/dielectic filters [8]. ypically, an in situ set up wi at least a white light source and a Charged Coupled Device (CCD) or Photo Diode Array (PDA) detector enables acquisitions of e light passing rough e coated substrate. Several techniques enable an efficient onitoring of e coated ickness. One exaple consists in finding e real tie ickness by coparing e easured transittance and e calculated spectral response as a function of layer ickness. his procedure can be repeated layer after layer. One ain advantage of Broadband Optical Monitoring (BOM) is to be adapted to a large variety of designs, and specifically to non quarter wavelengs. he ai of is article is to deonstrate e efficiency of such a onitoring syste on stacks including bo etallic and dielectric aterials. A siultaneous easureent of bo transittance and reflectance is very useful for onitoring such coating. o illustrate, we present e anufacturing of a filter wi given color properties on bo sides of e substrate.. Experiental set up he dedicated vacuu coater uses e Reactive Low Voltage Ion Plating technology. his e- bea deposition process is assisted wi Ar ions plasa. he following describes e start of e deposition of a layer: he first stage includes a pre-heating of e crucible wi an electron gun for about to 4 inutes under closed shutter. Last inute O flow is on in e case of an oxide aterial. hen e Ar plasa is started, iediately followed by e opening of e crucible shutter. And finally, deposition starts. he onitoring of in etallic layers presents difficulties, especially if a dielectric oxide layer follows. Indeed, oxide layers require oxygen inside e chaber, and a partial oxidisation of e etal is possible. We will see in section 4 e corresponding odifications on e spectral response. An in situ BOM syste easuring bo transittance and reflectance is an excellent tool to observe in situ e odifications of e optical properties of a in etallic layer. In our onitoring set up described in Fig., we use a tungsten halogen source ended by a µ diaeter fibre. he source is iaged on e substrate wi a spot diaeter about 8 and a negligible divergence. he substrate is en iaged on two fibres of 5 µ diaeter, corresponding to e transittance and reflectance channels. he two PDA detectors enable e recording of e spectra fro 4 to n wi a signal to noise ratio of 3 (respectively ) in transittance (reflectance). A precise description of e set up perforances is given in [9]. Note at e substrate, located at e periphery of e substrate holder, is at about 6 c away fro e center of e single rotary drive syste. As a consequence, all e signal acquisitions need to be sequential, wi an integration tie of about 4 s (signal tie window) and a periodicity of.8s (rotary drive speed). By coparing easured and calculated spectra, e deposited ickness is sequentially evaluated. When e ickness estiation atches e eoretical design, e coating is stopped. At e end of e deposition of a layer, if e deposited ickness is not exactly e one expected, a procedure consisting in correcting e design of following layers can be applied. # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS 9

3 M L L PDA Spectroeter M M Substrate holder Vacuu deposition chaber M : irror L : lens bs : beasplitter L M bs L L R Photodiode Halogen light source (HL OceanOptics) Fig.. BOM set up iplanted on a Balzers Ion Plating deposition achine 3. Design of an absorbing filter wi given properties In order to illustrate e potentialities of our onitoring syste, we decided to design and anufacture a filter wi given colorietric properties, following e anufacturing proble of OIC 7 conference [-]. he precise characteristics (intensity and colorietry) are given in table. able. characteristics of a colorietric filter fro [9] CIE standard observer color coordinate targets reflected fro side (backside) reflected fro side (filter side) transitted x y Y he erit function for e anufacturing contest was given by (). MF x x y y = =.x.y = = Y Y.Y / () # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS

4 We chose to look at a single side coated design, naed filter side. he residual absorption of 4% and e difference of color between e two reflected sides ipose to look for designs including at least one absorbing layer. We selected hafniu oxide (HfO, naed H aterial) and silica oxide (SiO, naed L aterial) as dielectric aterials and hafniu (Hf, M aterial) as absorbing aterial. he accuracy of e estiated refractive index of oxide dielectric aterials is better an for etallic aterials. A ere Cauchy law () describes e estiation of oxide indices on e visible range fairly well. a a n λ = a + + () ( ) 4 λ Wi a =.739, a = n, a = n 4 for HfO and a =.4686, a = n, a = n 4 for SiO For ese aterials, k ( λ) is very close to (k < -4 in e visible range) and is erefore taken to be null. he index of a sei transparent etallic fil is estiated eier on ellipsoetric eods [], or spectro-photoetry [3]. he latter consists in fitting a eoretical odel on reflectance and transittance curves of a bi-layer (Hf - HfO ). As e refractive index of HfO is supposed to be known, one has to look for icknesses of e two layers and optical constant of Hf using e spectral curves. Following is approach, we have decided to use a Cauchy odel for n and for k. A reverse engineering procedure gives e following values for Hf: a = 3.79, a = n, a = n 4 for n, real part of Hf index a = 3.346, a = n, a = 6.45 n 4 for k, iaginary part of Hf index his characterization does not take into account possible inhoogeneities of e etallic layer. he optical constants are supposed to be hoogenous for bo etallic and dielectric layer. Consequently, if e etallic layer is partially oxidised, e contribution of e refractive index gradient is included into e hoogeneous odel. One possible issue, considering in etallic layers, is at e optical constants n and k are dependant on e deposited ickness. Our experiental knowledge on in Hf layers is related to e deposition of light absorbers for spatial applications. In is case, we have considered ickness varying fro 7 to n. he deterination of refractive indices in is range of icknesses does not lead to significant changes. However, e difference wi an opaque (ickness ore an n) layer is significant, where for an opaque Hf layer n and k are noticeably higher. Under ese indices conditions, we sought a design including H,M,L layers and corresponding to e lowest MF. We used a global optiization algori derived fro [4]. Briefly, it is a clustering eod at evaluates rando initial designs, and en seeks several local inia wi a Quasi Newton algori. he best result is called global iniu. For e initial design, e requireent is to provide e nuber of layers and e appropriate chain of aterials. We choose a sequence of 8 layers substrate-h-l-h-l-h-l-m-h-air. For e following, is design presents two advantages: a single etallic layer and a positioning of is layer near e air. As e accuracy of refractive index deterination is better for dielectric aterials, it is preferable to insert only one Hf layer. Note also at after a etallic layer, e onitoring of e following layers is rendered ore difficult and less precise. Wi such a design, only a single layer will be coated after e etallic one. he optiization progra finally provides e eoretical design given in table. he erit function is about 6.6, and e colorietric coordinates are given in table 3. λ # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS

5 able. Colorietric filter structure icknesses (n) aterials layer 79.6 H layer 59.6 L layer H layer L layer H layer L layer 7.7 M layer 8 45 H able 3. Colorietric characteristics of filter defined on table CIE standard observer color coordinate targets reflected fro side (backside) reflected fro side (filter side) transitted x y Y On e Fig., one can observe e expected spectral responses in transittance (red curve) and in reflectance (filter side blue curve, backside green curve)..8 filter side REFLECANCE.6 RANSMIANCE.4. backside REFLECANCE WAVELENGH (n) Fig.. heoretical spectral responses of e colorietric filter design described in able. # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS

6 4. Manufacturing of e filter Our onitoring strategy consists in evaluating e real tie deposited ickness fro reflectance and transittance data. he siultaneous easureent of transittance and reflectance is not andatory as it is e case for optical characterization of etallic fil, but it provides additional inforation on e absorption level of e filter, and facilitates e onitoring of e last layer. he design can be corrected taking into account new values of icknesses already deposited, allowing a copensation of ickness errors. he deposition of e 8-layers stack is perfored as followed: for e first 6 dielectric layers, residual errors on ickness are negligible (below.5 n in absolute value). Results for e first 6-layers are presented in Fig. 3. Note at we have represented transittance and reflectance for a sei infinite substrate. he influence of backside is en subtracted. LAYER LAYER R/ R/ WAVELENGH (n) WAVELENGH (n) LAYER 3 LAYER R/ R/ WAVELENGH (n) WAVELENGH (n) LAYER 5 LAYER R/ R/ WAVELENGH (n) WAVELENGH (n) Fig. 3. Experiental reflectance and transittance of e first 6 layers. heoretical responses are in in lines. he onitoring of e 7 layer is e ost probleatic. First of all, e deposition is very fast (about 3 seconds), and second of all, e etallic layer is altered by factors: e # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS 3

7 introduction of oxygen inside e vacuu chaber before e deposition of layer 8 and e triggering of e Ar plasa as soon as e deposition of layer 8 begins. Our strategy consists in perforing e evaluation ickness of layer 7 once e layer 8 deposition starts. We us take into account e two factors cited above and influencing drastically e etal ickness evaluation. Figure 4 shows e spectral responses at e end of e deposition of layer 7, after e introduction of oxygen, and at e triggering of e Ar plasa. One can also see e fit between eoretical and experiental spectra when a ickness of.3n of a hoogenous etallic layer is considered..3n is our ickness estiation and is not very different fro e target ickness (.7n) R/ R/.. AFER LAYER 7 (Hf layer) WAVELENGH (n).6 AFER LAYER 7 + O entrance WAVELENGH (n) R/ R/. 3 AFER LAYER 7 + O entrance + plasa WAVELENGH (n). 4 AFER LAYER 7 + O entrance + plasa fit eory / experient WAVELENGH (n) Fig. 4. Reflectance and ransittance fro e end of layer 7 deposition to e beginning of layer 8 deposition. It is judicious to optiize e layer 8 by e erit function MF (Eq. ()). As our set up easures e reflectance only on e filter side, we use a truncated erit function wi e two easured spectra (see Eq. (3)) x x y y = = x = y = / Y Y MF (3).Y he deposition process is stopped when e experiental MF is at its iniu value. he final spectra are given in Fig. 5. he good agreeent between eoretical and experiental curves indicates at e odelling of e etallic layer as an hoogeneous layer is a good approxiation for such filter. We evaluate e total erit function defined by (), including backside reflectance to a value of MF = 7.3. # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS 4

8 .6.5 RANSMIANCE filter side REFLECANCE WAVELENGH (n) Fig. 5. Reflectance and transittance of e 8-layer-filter. Experiental spectra are in bold lines, eoretical values are in in lines. Considering e good fit between eoretical and experiental curves, we now try to reduce e erit function by adding new layers. Our global optiization progra gives us a solution wi ree additional layers given in table 4, where e new layers are in bold font. he new erit function would be MF = Such a eoretical iproveent perits to try e additional deposition. We use e sae onitoring process as previously described. he last layer is onitored by e iniu of MF (see Eq. (3)). he final experiental and eoretical spectral responses easured wi a coercial spectrophotoeter (Perkin Eler Labda 8) are shown in Fig. 6, e experiental erit function is MF = 5.7. able 4. Iproved colorietric filter design. Additional layers are in bold font icknesses (n) aterials layer 79.8 H layer 6 L layer H layer L layer 5 77 H layer 6.5 L layer 7.3 M layer H layer L layer 74.6 H layer 66.8 L # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS 5

9 .8.6 filter side REFLECANCE RANSMIANCE.4. backside REFLECANCE WAVELENGH (n) Fig. 6. Spectral responses of e colorietric filter after e 3 additional layers: transittance (red curve), reflectance (filter side-blue curve), backside reflectance (green curve). Experiental easureents are in bold lines, eoretical values are in in lines. Fro spectral easureents we deduce e colorietric paraeters given by table 5. Figure 7 is a picture of e final coponent illuinated by e sun. CIE standard observer color coordinate targets reflected fro side (backside) reflected fro side (filter side) able 5. Colorietric paraeters x esure x y esure y Y esure Y transitted x, y et Y : eoretical colorietric paraeters, esure x, paraeters calculated fro easureents esure y et esure Y : colorietric # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS 6

10 a b c d Fig. 7. Reflected colors on filter side (a) and backside (b et d), and transitted color (c et d). 5. Conclusion hin fil filters including in etallic layers are extreely difficult to onitor. Coplex phenoena occur probably due to a partial oxidisation of e etal when an oxide is deposited after. he real etallic deposited ickness is en difficult to estiate. he siultaneous easureent of reflectance and transittance over a large spectral range is an efficient tool to analyse e etal optical properties variations. Our approach consisted in odelling e etallic layer as an hoogeneous layer of optical constants deterined on a bi layer stack. Its ickness was en deterined after e following layer deposition started. By cobining analysis and design, it is also possible to odify e icknesses of next layers in order to iprove e erit function. We have successfully applied is strategy for anufacturing a colorietric filter wi given properties. In e future, we plan to iprove our results by proceeding in a real tie optiization of e optical constant (refractive index). he benefit should be substantial in e case of seitransparent etallic layers whose oxidisation phenoenon probably depends on several factors such as teperature and O pressure. # $5. USD Received Oct 7; revised 8 Jan 8; accepted 4 Apr 8; published 5 Jul 8 (C) 8 OSA 4 August 8 / Vol. 6, No. 6 / OPICS EXPRESS 7