1 Application of polymer materials in thin-film optical devices Alexei Meshalkin Institute of Applied Physics of Academy of Sciences of Moldova Chisinau, Moldova

2 Advantages of polymer materials Excellent optical properties (high optical transmission 92-95%, variable refractive index 1,4÷1,7) Smaller density = smaller weight Easy processing technology Semiconducting properties, photosensitivity Much cheaper price!!! Polymers are gradually replacing inorganic optical materials.

3 The main aims: 1 – obtaining of thin polymer films with desired thickness 2 – accurate measurement of film thickness after deposition 3 – accurate measurement of refractive index of thin films Thickness measurement of polymer films requires the application of high precision methods

4 Polymer materials Polyepoxypropylcarbazole (PEPC) Polyepitiopropylcarbazole (PETPC) Chemical structure of selected polymers. Polymer materials were selected since they are known to have excellent film forming properties and be photosensitive.

5 Different deposition techniques of polymer layers: self-assembly extrusion and co-extrusion molding spin-coating! sputtering chemical deposition. Spin-coating is one of the technological and accessible method of obtaining polymeric thin films.

6 Spin coating technique A schematic model describing the film formation during the spin-coating process. After the initial spin-off stage (i), where solvent is evaporated (ii), the thin film is formed (iii).

7 Spin-coater SGS Spincoat G3P-8 Programmable spin-coater have the capacity to store and execute up to 30 programs, with up to 20 steps each. Spin profiles adjustable in 1.0 rpm rotation increments, 0.1 second timing increments, and 1.0 second increments for dwell time, with precise repeatability from cycle to cycle. Speed: RPM

8 Thickness measurement of thin films Most common used and available methods: Atomic-Force microscopy Ellipsometry Spectral transmittance/reflectance method Interferometry! Determination of film thickness by optical interferometry is widely used. Method is rapid and relies on the interference of two beams of light, where the optical path difference of these beams is related to film thickness.

9 Microinterferometr MII-4 Optical scheme of MII-4 interference microscope. 1 – reference beam, 2 – object beam, O – objectives, D – diaphragms, M – mirrors, P - beam-splitting plate, C – compensating plate, S – sample.

10 Interference shift fringe Interference fringes seen through an interference microscope. The step between the two fringe patterns correspond to a geometrical phase shift which depends on the film thickness

11 Modernized microinterferometer MII-4 equipped with photocamera for interferograms recording and saving. The proposed method is non-contact and can be applied for thickness measurement in the range of 50 nm – 5 μm.

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13 Comparison of thickness measurement by AFM research and interferometric OPTIC METER d=15 nm±6nm d=14 nm±4nm This experiment demonstrated sufficient convergence of the results of the interferometric method and AFM method of film thickness measurement. It shows applicable of interferometric method for thin submicrometre and nanometer thickness measurements.

14 Used coating cycle Rate/time schedule of spin-coating cycle

15 PEPC polymer films Thickness as a function of PEPC solution Concent ration Thickness, μm 2.5%0.17 ±0.01 5%0.30 ± %0.46 ± %0.83 ± %0.94 ±0.01

16 PETPC polymer films Thickness as a function of PETPC solution Concent ration Thickness, μm 3%0.76 ±0.01 5%0.97 ± %1.74 ± %2.27 ±0.01

17 Thickness variation depended on spin-speed ( rot/min, 20 sec). Variation of spin-speed

18 Optical constants determination from transmission spectra From the transmission spectra both the thickness and the refractive index of obtained films were determined by method of fitting curves proposed by Swanepoel

19 Transmission spectra of polymer layers

20 Comparison of thickness measurement by interferometric and spectral methods. The difference of obtained results of two methods averaged not more than 5%.

21 Conclusions It was shown, that the thickness of thin polymer films could be analyzed with high resolution by the interferometric method. The broad range of thicknesses (from 100 nm to 3 μm) can be covered by using polymer solution with varied polymer concentration. The film thickness dependence on the concentration of solution is linear, but the spin speed doesn’t lead to essential thickness variation. Therefore this linear dependence can be used to predict the film thickness of spin-coated polymers if the solvent is known. The described method of thickness measurement by MII-4 interference microscope provided of developed soft allows controlling the films thickness with accuracy ±10nm. Proposed spectral transmission method can be applied for simultaneous determination of thickness and optical constants for thin polymer films of a wide variety of materials.

22 The main elements of classical optics. The basis of classical optics is based on lenses, prisms, mirrors Classical optics Diffraction optics prism beam splitter lens

23 Light source (laser) Output beaming ? I A & E «DOE» wave-front object Applications of diffraction optical elements

24 Relief of diffraction grating prepared by holographic recording and etching. Phase diffraction grating

25 up to 100% Relief of diffraction grating prepared by holographic recording and etching. Diffraction grating with one diffraction order. Phase plate acts as prism.

26 Combination of diffraction grating and lens Diffraction lens focusing in point line

27 Laser beam Diffraction element Focused line Diffraction element focuses light in thin line. y x

28 Laser beam perpendicular lines y x Diffraction element Diffraction element focuses light in perpendicular lines.

29 Center: l  8мкм Edge: l  0.6мкм 210-mm diffraction mirror Diffraction mirror

30 HOLOGRAPHIC PROPERTIES OF POLYMER FILMS Optical scheme of holographic set-up. 1 –Ar laser, 2 – mirror, 3 – investigated photosensitive film, 4 – collimator, 5 –Не-Ne laser, 6 – photodetector, 7 – measuring card, 8 – PC, 9 – beam splitter. Scheme of etching set-up: 1 –He-Ne laser, 2 – etching curve, 3 – sample, 4 – etching agent, 5 –photodetector, 6 – PC with measuring card.

31 Methods of fabricating of photo- and electronoresists as main components of DOE As it was indicated main element of diffraction optical elements is photo- and electronoresists.The technological scheme is indicated below( Laser beam (λ=488 nm Ar+ laser; λ=632 nm He-Ne Laser ).

32 The transmittance spectra of PEPC and T-PEPC:PEPC shows:  the films are transparent T=90% in visible region = nm  irradiation by UV and Ar + laser (λ=488 nm) resulted in appearing of strong absorption band at 650 nm PEPC ETPC UV irradiation All synthesized polymers were sensitized with iodoform CHI3. It was determined that to achieve the maximum photosensitivity the optimal concentration of CHI3 in the polymer was about 10 mass%.

33 DIFFRACTION OPTICAL STRUCTURES ON THE BASIS OF POLYMERS The last results were achieved due to special elaborated methods of exposition by laser and electron beam and by selection of special condition of etching

34 Sample of a protecting hologram with the image of a flying stork bearing a grape, recorded in polymer layer; #1, #2 and #3 – 1,0 µm perioв gratings, #4 and #5 – 2,0 period gratings.

35 Thank you for your attention!!! Questions Answers