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 Application of polymer materials in thin-film

optical devices

Alexei Meshalkin

Institute of Applied Physics of Academy of Sciences of Moldova

Chisinau, Moldova

e-mail: alexei@asm.md

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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. 

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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

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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.

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Different deposition techniques of polymer layers:self-assemblyextrusion and co-extrusionmoldingspin-coating!sputteringchemical deposition.

Spin-coating is one of the technological and accessible method of obtaining polymeric thin films.

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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).

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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: 0-10000 RPM

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Thickness measurement of thin filmsMost common used and available methods:

Atomic-Force microscopy EllipsometrySpectral transmittance/reflectance methodInterferometry!

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.

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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.

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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

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D

dh

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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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Comparison of thickness measurement by AFM

research and interferometric OPTIC METER

d=15 nm±6nm d=14 nm±4nmThis 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.

 

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Used coating cycle

Rate/time schedule of spin-coating cycle 

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PEPC polymer films

Thickness as a function of PEPC solution 

Concentration

Thickness, μm

2.5% 0.17 ±0.01

5% 0.30 ±0.01

7.5% 0.46 ±0.01

10% 0.83 ±0.01

12.5% 0.94 ±0.01

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PETPC polymer films

Thickness as a function of PETPC solution 

Concentration

Thickness, μm

3% 0.76 ±0.01

5% 0.97 ±0.01

10% 1.74 ±0.01

15% 2.27 ±0.01

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Thickness variation depended on spin-speed (500-10000 rot/min, 20 sec).

Variation of spin-speed

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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

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Transmission spectra of polymer layers

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Comparison of thickness measurement by interferometric and

spectral methods.

The difference of obtained results of two methods averaged not more than 5%.

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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.

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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

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Light source (laser)

Output beaming

? IA

& E

«DOE»

wave-front

object

Applications of diffraction optical elements

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40%

40%

Relief of diffraction grating prepared by holographic recording and etching.

Phase diffraction grating

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up to 100%

Relief of diffraction grating prepared by holographic recording and etching.

Diffraction grating with one diffraction order. Phase plate acts as prism.

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Combination of diffraction grating and lens

Diffraction lens focusing in point line

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Laser beam

Diffraction element

Focused line

Diffraction element focuses light in thin line.

y

x

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Laser beam

perpendicular lines

y

x

Diffraction element

Diffraction element focuses light in perpendicular lines.

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Center: l 8мкм

Edge: l 0.6мкм

210-mm diffraction mirror

Diffraction mirror

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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.

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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).

Thin film

2. Holographicor e-beam recording

Etching solution

Substrate Substrate Substrate

1. Obtaining of thin polymer film

3. Selective etching to form

relief phase plate

resist

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The transmittance spectra of PEPC and T-PEPC:PEPC shows:

the films are transparent T=90% in visible region =450-900nm

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%.

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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

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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.

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Thank you for your attention!!!

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