The investigation results of holographic diffraction gratings recording processes by radiation of helium-neon laser have been represented. Inorganic As₄₀S₂₀Se₄₀ photoresist treated by the newly developed selective etching solution was chosen as a registering media. Angular and spectral dependencies of grating diffractive efficiency absolute values were measured. A comparison of fabricated gratings characteristics with corresponding parameters of gratings recorded on As₄₀Se₆₀ photoresist was carried out. Numericall simulation of groove profiles inherent to holographic gratings made in various experimental conditions was performed. Shown is that photoresists based on As₄₀S₂₀Se₄₀ are suitable for production of high quality holographic optical elements

Dan'ko, V.A.

Indutnyy, I.Z.

Litvin, O.S.

Min'ko, V.I.

Romanenko, P.F.

Shepeliavyi, P.E.

Semiconductor Physics Quantum Electronics & Optoelectronics

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

Semiconductor Physics, Quantum Electronics & Optoelectronics. 2004. V. 7, N 1. P. 88-92.' 2004, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine88PACS: 42.40.Eq, 42.70.LnRecording of high efficiency diffraction gratingsby He-Ne laserV.I. Minko, P.E. Shepeliavyi, V.A. Danko, P.F. Romanenko, O.S. Litvin, I.Z. IndutnyyV. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 41, prospect Nauky, 03028 Kyiv, UkraineAbstract. The investigation results of holographic diffraction gratings recording processes byradiation of  helium-neon laser have been represented. Inorganic As40S20Se40 photoresisttreated by the newly developed selective etching solution was chosen as a registering media.Angular and spectral dependencies of grating diffractive efficiency absolute values were meas-ured. A comparison of fabricated gratings characteristics with corresponding parameters ofgratings recorded on As40Se60 photoresist was carried out. Numericall simulation of grooveprofiles inherent to holographic gratings made in various experimental conditions was per-formed.  Shown is that photoresists based on As40S20Se40 are suitable for production of highquality holographic optical elements.Keywords: inorganic photoresist, holographic difraction grating, diffraction efficiency, etch-ing solution.Paper received 14.02.04; accepted for publication 30.03.04.1. IntroductionChalcogenide vitreous semiconductors (ChVS) owing tomanifold properties have already found various applica-tions, in particular, as inorganic photoresist [13].Photostimulated changes of ChVS layer solubility andoptical characteristics serve as a base of their use formanufacturing  diffractive optical elements. Holographicdiffraction gratings are the simplest among them. Holo-graphic difractive gratings fabrication using inorganicphotoresists comprises two main stages: exposure (record-ing) and after-exposure processing  etching in selectivesolutions. The quality of produced gratings depends onoptimization of these two stages. The first process is de-termined by the time of  exposure, photoresist sensitivity,recording wavelength. The second  by the dissolutionrate difference of unexposed and exposed areas of theresist, exposure pattern, time of etching.Since the spectral light sensitivity distribution of ChVSlayers is determined by the absorbed light energy andcorrelates with the layer absorption spectra [4], then forrecording the diffractive optical elements, usually usedis radiation of a rather powerful argon laser. Its radia-tion wavelengths (488.0 and 514.5 nm) are in the rangeof strong absorbance of ChVS photoresists [5]. But, theauthor of work [6] estimating the photosensitivity rangeof As40S60xSex layers demonstrated possibility to recordgratings by He-Ne laser radiation. The process of pro-duction of reflective holographic diffraction gratings us-ing He-Ne laser radiation was studied in [7]. It is neces-sary to note that photosensitivity value at the given ac-tinic wavelength [3,6] depends on both photoresist prop-erties and characteristics of developer used in etching.In this paper, we report the investigation results ofholographic diffraction gratings recording processes us-ing radiation of the most reliable helium-neon laser.Rather frequently used in holography helium-neon laser(λ = 632.8 nm) is characterized by stability of generation,absence of water-cooling and operational convenience.Inorganic As40S20Se40 photoresist (vacuum thermallyevaporated films) processed by the newly developed se-lective etching solution was choosen as a registering me-dia. Moreover, a comparison of fabricated gratings char-acteristics with corresponding parameters of gratingsrecorded on As40Se60 photoresist was carried out.2. ExperimentThe samples for holographic diffractive grating record-ing were deposited on high-quality polished glasssubstrates using thermal vacuum evaporation at  the pres-sure 105 Torr. To provide the necessary adhesion to theV.I. Minko et al.: Recording of high efficiency diffraction gratings by He-Ne laser89SQO, 7(1), 2004qlass substrates (it is extremely important for etchingprocess) and to eliminate the interference effects con-nected with reflectance from the substrate back surface,a chromium layer was deposited on the glass beforephotoresist deposition. The layer thickness was controledduring deposition process by the quartz-crystal-oscilla-tor monitoring system (KIT-1) and reached 60 nm forchromium layer and 600 nm for photoresist (As40S20Se40or As40Se60) layer.Samples for optical measurements were deposited ontopolished silica substrates. The transmittance, air-inci-dent, and substrate-incident reflectances of the sampleswere measured at normal incidence using a spectropho-tometer (KSVU-23). The optical constants of the investi-gated films were calculated, taking into account interfer-ence effects, by using these measured values, the samplethickness and the optical constants of the silica substrate(for details, see [8]).In the course of grating recording, an interferencepattern with the spatial frequency of 1130 mm1 wasformed using radiation of the helium-neon laser LHN-222. Both photoresists were exposed simultaneously us-ing the same light field intensity distribution. The exposevalues varied from 0.34 to 2.0 J/cm2.   After exposure, the samples were chemically treatedin non-water alkaline organic solutins. The duration ofetching process was chosen equal to the time of completesolution of unexposed photoresist (negative etching). Thefabricated relief-phase gratings were covered with reflec-tive thin-film (~ 100 nm) Al layer.    Angular (at the wavelength 632.8 nm) η(ϕ) (ϕ is theangle between incidend beams and the normal to thesubstrate plane)  and spectral  η(λ) (λ  wavelength) de-pendencies of the diffraction efficiency η for the first or-der of diffraction were measured for s- and p-pola-rizations. The η (λ) measurements were carried out in400800 nm spectral range in the setup close to the Littrowscheme and the angle between incident and diffractedbeams was 4 deg. The grating diffraction efficiency wastaken as the ratio of intensity of light diffracted into thefirst order to the intensity of incident light (absolute valueof diffractive efficiency). The surface patterns of thegratings were examined with a Dimension 3000 scan-ning probe microscope (Digital Instruments) in the AFMtapping mode.3. Results and discussionsThe exposure dependencies of diffraction efficiency ofgratings recorded on As40Se60 (curve 1)  and As40S20Se40layers (curve 2) are presented in Fig.1 (η was measuredat λ = 632.8 nm, p-polarization). These dependencies arecurves with maxima, which correspond to the optimumexposure (1.34 J/cm2). The optimum exposure results inthe greatest value of diffraction efficiency of gratings. Adeviation from this exposure to any direction leads todecrease of diffraction efficiency. Hereafter, all the in-vestigated gratings were recorded at the optimum ex-posure.Fig. 2 shows groove profiles of the gratings obtainedusing these inorganic photoresists. The groove profilesof both gratings are close to sinusoid, as seen from Fig.2.The relief depth h was as large as 316 nm and 286 nm, themodulation depth (h/d, where d is grating period) reached0.36 and 0.32 for gratings on As40Se60 and As40S20Se40,respectively. Thus, in accordance with [9], we obtainedhigh-modulation gratings. It is necessary to note thatabsorbance of investigated by us photoresists at the recor-ding wavelength differs nearly four times (1.104 cm1 As40Se60,  2.7⋅103 cm1  As40S20Se40 ). At the same time,0.5 1.0 1.5 2.0 2.501020304050607080901  As40Se602  As40S20Se4012H, J/cm2h,%Fig. 1. Expose dependencies of the diffraction efficiency of thegratings recorded using inorganic photoresists.025025002.50 5.00anmµm025025002.50 5.00bnmµmFig. 2. Relief profiles for the gratings recorded using As40Se60 (a)and As40S20Se40 (b) inorganic photoresists.90SQO, 7(1), 2004V.I. Minko et al.: Recording of high efficiency diffraction gratings by He-Ne laserother conditions for recording the gratings were identi-cal. The relief depths of etched samples of gratings haveonly 1.1 ratio. It means that the newly developed forAs40S20Se40  photoresist etching solution is more selec-tive in comparison to the used for As40Se60 developer basedon the solution of ethylenediamin in dimetilsulphoxide.Besides, As40S20Se40 photoresist photosensitivity is ap-proximately the same as for As40Se60.Angular and spectral dependencies of diffractive grat-ing efficiency  η  are the most informative and importantfrom the practical viewpoint. These dependencies for p-and s-polarization (respectively, parallel and perpendicu-lar arrangement of gratings grooves and electric vectorof incident light wave) were measured for studied gratingsand are presented in Figs 3 and 4 ((a)  for both figuresrefers to p-polarization, (b)  s-polarization). By com-parison of the experimental    dependencies with theoreti-cally calculated ones for high-modulated gratings withsinusoidal form of the groove profile [9,10], we can seethat maximum values of the diffraction efficiency arecomparable to theoretical predictions. Curves for bothangular and spectral dependencies, in principal, are inaccordance with predicted peculiarities. But for experi-mental curves, maxima differ from the predicted ones. Itindicates that the form of grating profiles is not the idealsinusoid.From the analysis of the data presented in Figs 3 and4, one can see that   dependencies for grating character-istics recorded using different photoresists are very simi-lar. This is understandable taking into account that themodulation depth and form of grating grooves determineangular and spectral dependencies of diffractive effi-ciency [9,10].Thus, we obtained gratings with practically equalparameters on photoresists, optical characteristics ofwhich differ to a great extent. In order to understand thisfact, a simulation of the grating production process, us-ing method described in [1] and characteristic curves forthe As40Se60 and As40S20Se40 layers obtained in [3], wascarried out. This method enable numerically to computethe form of the groove profile for holographic gratingsdepending on their production experimental conditions.The light intensity distribution in the photoresist in thecourse of recording and influence of photoresist opticalconstants change during exposure on light distributionpattern were  taken into account. It is necessary to men-tion that etching conditions influence the profile shape[1]. In our numerical calculation, varied parameters werethe photoresist absorbance at the recorded wavelength,exposure values and time of etching the samples afterexposure. Different values of abovementioned param-01a21  As40Se602  As 40S20Se40h,%2040400 500 600 700 800l, nm 608010001b21  As 40Se602  As 40S20Se40h,%2040400 500 600 700 8006080100l, nm Fig. 3. Spectral dependencies of the diffraction efficiency of thegratings (a  p-polarization, b  s-polarization).807060504030202020406080100012aj, deg.h,%1  As  Se2  As  S  Se4040 402060453525155403020202040608010001b2j, deg.h,%1  As  Se2  As  S  Se4040 402060Fig. 4. Angular dependencies of the diffraction efficiency of thegratings (a  p-polarization, b  s-polarization).V.I. Minko et al.: Recording of high efficiency diffraction gratings by He-Ne laser91SQO, 7(1), 2004eters, including the experimental ones, were used in cal-culations and their influence on the profile shape wasestimated. The obtained results of numerical modelinghas shown that the interference pattern created by re-cording radiation was reproduced in the photoresist inthe region of small photoresist absorption (linear responseof the system). Therefore,  creation of the sinusoidal re-lief by strongly absorbed radiation requires narrow com-bination of optimal parameters affecting  conditions ofrecording. And by the contrast, if the recording is in theregion of weakly absorbed radiation, of etching timechanging, for example, this leads to changing the reliefdepth, but not its form. It is clearly seen from Figs 5 and6 where  depicted are the dependencies of groove profileshapes on the etching time (curves 1,2,3,4,5  etchingtime 2, 4, 6, 8 and 10 min, respectively). (Zero on thevertical axis corresponds to the free interface ofphotoresist, the initial thickness of photoresist was 600nm). For medium (Fig. 5), absorbance α  of which at therecording wavelength is 2.7•103 cm1 (As40S20Se40, λ=632.8 nm), the 5-fold increase in etching time  (curve 1and 5) does not lead to visible changing the relief form,but changes the relief depth. If  the photoresist absorb-ance is increased, the change of the etching time lead todistored relief form. In Fig. 6, results of simulation forphotoresist based on As40Se60 (α =1.24•105 cm-1) whenrecording with the argon laser wavelength (λ = 488 nm)takes place. As seen from the figure, the form of grooveprofile of modulated gratings is changed with  the etch-ing time growth.Taking into consideration dependencies obtained bynumerical calculations, we can explain our experimen-tal results. Holographic gratings recording was made byHe-Ne laser radiation (λ = 632.8 nm) on As40S20Se40 andAs40Se60 photoresists, absorbance of which reaches2.7•103 cm1 and 1•104 cm1, respectively, (region of weakabsorbance) and response of media was linear (or closeto linear). Both photoresists were exposed using the samelight pattern, time of etching was chosen as the time ofcomplete solution of unexposed photoresist layers. In suchconditions for both used compositions of photoresists, thegratings with similar η  values were obtained in accord-ance with the results of numerical calculations.4. ConclusionsThe performed investigations show that the photoresistsbased on As40S20Se40, in spite of the low absorbance value,are suitable for recording the periodical holographicstructures by using helium-neon laser radiation. Use ofthe newly developed high-selective etching solution ena-bled to produce high quality reflective diffraction gratingswith maximal η values close to the calculated ones. Thegroove profile of diffraction gratings with the maximal ηvalues  is close to a sinusoid with groove depth  about300 nm. Maximal diffraction efficiency of gratings is ~83 % in polarized light and ~ 75 % in non-polarized one.Numerical computation of grating relief formation ena-bled us to understand experimental results of grating fab-rication using these  different photoresist composition.AcknowledgementsThe authors wish to express their gratitude toDr A.V. Stronski for valuable discussion and critical read-ing the manuscript.0 500 1000 1500 20006005004003002001000 As40S20Se40, l = 632.8 nm54321d, arb. un.h, nmFig. 5. Dependencies of the relief profile on etching time.d, arb. un.h, nm0 400 800 1200 16004003002001000As40Se60, l = 488 nm4321Fig. 6. Dependencies of the relief profile on etching time.92SQO, 7(1), 2004V.I. Minko et al.: Recording of high efficiency diffraction gratings by He-Ne laserReferences1. I.Z. Indutnyi, A.V. Stronski, S.A. Kostioukevich et al., Holo-graphic optical element fabrication using chalcogenide lay-ers // Optical Engineering, 34(4), pp.1030-1039 (1995).2. M. Vlcek, P.J.S. Ewen, T. Wagner, High efficiency diffrac-tion gratings in As-S layers//J. of Non-Cryst. Solids, 227-230, pp. 743-747 (1998).3. P.E. Shepeliavyi, S.A. Kostioukevich, I.Z. Indutnyi, A.V.Stronski, Fabrication of periodical structures with the help ofchalcogenide inorganic resists // Proc. SPIE, 2291, pp.188-192 (1994).4.  I.Z. Indutnyi, S.A. Kostioukevich, V.I. Min’ko et al. Laserlithography utilizing As2S3 layers // Optoelektronika iPoluprovodnikovaya Tekhnika, 25, pp. 52-59 (1993), in Rus-sian.5. P.F. Romanenko, I.I. Robur, A.V. Stronski, I.Z. Indutnyi, Therecording of relief-phase holographic gratings utilizing As2Se3layers // Ukrainskii Fizicheskii Zhurnal, 36(7), pp. 2028-2031(1991), in Russian.6. A.V. Stronski, Photostimulated processes in chalcogenidevitreous semiconductors and their application for the fabri-cation of the holographic optical elements. Manuscript. The-sis for Dr. Sci. degree, the Institute of Semiconductor Phy-sics, NAS of Ukraine, Kyiv, 2001.7. P.F. Romanenko, I.I. Robur, A.V. Stronski, Investigation ofrecording processes of holographic diffractive gratings onAs2Se3 layers by helium-neon laser radiation // Optoelekt-ronika i Poluprovodnikovaya Tekhnika, 27, pp. 47-50 (1994),(in Russian).8. I.Z. Indutnyi, A.I. Stetsun, Determination of the optical con-stants of thin absorbing films on a slightly absorbing substratefrom photometric measurements // Proc. SPIE, 2113, pp. 55-59 (1993).9. M.C. Hutley, Diffraction gratings, Acad. Press, London, 1982.10. R.R. Gerke, I.V. Golubenko, T.G. Dubrovina, G.M. Savitsky,Study of reflection properties of hologram diffraction gratingswith symmetric profile of lines // Sov. Optika and Spekt-roskopiya, 58(6), pp. 1318-1322 (1985).

Recording of high efficiency diffraction gratings by He-Ne laser

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Recording of high efficiency diffraction gratings by He-Ne laser

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