University of Groningen

Pattern formation by capillary instabilities in thin filmsMorariu, Mihai Dorin

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Pattern Formation by

Capillary Instabilities in Thin Films

Mihai D. Morariu

MSC Ph.D.-thesis series 2004-06ISSN 1570-1530

“Pattern Formation by Capillary Instabilities in Thin Films”

M. D. MorariuPh.D. ThesisUniversity of Groningen, the Netherlands9th July, 2004

ISBN: 90–367–2072–9

This research was financially supported by the:“Dutch Polymer Insitute” (DPI);“Materials Science Center” (MSC);“Stichting voor Fundamenteel Onderzoek der Materie” (FOM);

Rijksuniversiteit Groningen

Pattern Formation by

Capillary Instabilities in Thin Films

Proefschrift

ter verkrijging van het doctoraat in deWiskunde en Natuurwetenschappenaan de Rijksuniversiteit Groningen

op gezag van deRector Magnificus, dr. F. Zwartsin het openbaar te verdedigen op

maandag 5 July 2004om 13.15 uur

door

Mihai Dorin Morariu

geboren op 4 Augustus 1973te Falticeni (Roemenie)

Promotor: Prof. Dr. U. Steiner

Beoordelingscommissie: Prof. Dr. Stefan HerminghausProf. Dr. Thomas P. RussellProf. Dr. David Andelman

ISBN: 90–367–2072–9

Never let the future disturb you.You will meet it, if you have to,with the same weapons of reason

which today arm you against the present.

Marcus Aurelius Antonius, Meditations, 200 A.D..

To Aurora

Abstract: Polymers, or macromolecules, dominate today’s materials in-dustry and play key roles in many areas of science and technology. The behav-ior of polymers at surfaces and interfaces has proved to be an important factorin understanding wetting issues, because the morphological features of a poly-mer film on a solid surface depend on the nature of interactions between thepolymer and the solid. Ultrathin films are unstable and break spontaneouslyvia a spinodal mechanism or by heterogeneous hole nucleations. In contrast,macroscopic films are stable. With the help of electric fields and temperaturegradients, a destabilizing interfacial pressure can be generated at their surface.If the external pressure overcome the stabilizing effect of the surface tension,it drives the film towards dewetting.

The external, either electric or thermic field, can be applied by placing thepolymer film between two electrodes in a capacitor geometry, including alsoan air gap. The developing morphology can be controlled by laterally varyingthe applied fields with the help of a structured electrode. As a consequence,the instability is driven towards the regions with high field intensity.

However, the interaction of the external fields with the polymer film isundesired when using an electrically active polymeric material. This limitationmay be circumvented by making use of film instabilities that are intrinsic tothe system. In case of a film made of a polymer blend, its surface can becomespontaneously unstable due to surface tension gradients at film–air interface.If confined by a topographically structured surface, the undulatory mode ofthe film instability is harnessed to induce directionality and functionality tothe film.

Polymer blends are also of practical interest as a means of obtaining micro–structured materials. We present a method of obtaining porous materialsbased on demixing of a polymer blend followed by a subsequent washing ofone of the polymers. If the phase morphology has a lateral length scale be-low 100 nm, the resulting nanoporous films can be used as high performanceantireflection coatings.

In conclusion, nanoscale materials and processes have potential applica-tions in many areas such as biological detection, controlled drug delivery,optical devices, sensors, etc. The advancement of new methods to controlthe pattern formation processes gives us the possibility to develop functionalmaterials that improve the technical inventory of our modern life.

Contents

1 Introduction i

2 Theoretical Background 12.1 Fundamental Notions of Polymer Physics . . . . . . . . . . . . 12.2 Phase Separation of Polymer Blends. . . . . . . . . . . . . . . . 4

2.2.1 Flory–Huggins Theory. . . . . . . . . . . . . . . . . . . . 52.3 Stability of Thin Homogeneous Polymer Films . . . . . . . . . 9

2.3.1 Navier–Stokes Equation . . . . . . . . . . . . . . . . . . 102.3.2 Interfacial Pressure . . . . . . . . . . . . . . . . . . . . . 152.3.3 Linear Stability Analysis and Dispersion Relation . . . . 21

3 Materials and Experimental Techniques 273.1 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1.1 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . 273.1.2 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . 283.1.3 Substrate preparation . . . . . . . . . . . . . . . . . . . 303.1.4 Spin–Coating Method . . . . . . . . . . . . . . . . . . . 31

3.2 Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.2.1 Optical Microscopy . . . . . . . . . . . . . . . . . . . . . 333.2.2 Atomic Force Microscopy . . . . . . . . . . . . . . . . . 35

3.3 Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4 Antireflection Coatings 454.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.2 Principle of AR Coatings . . . . . . . . . . . . . . . . . . . . . 464.3 Experimental method . . . . . . . . . . . . . . . . . . . . . . . 49

vi Contents

4.3.1 Novel Routes for the Preparation of Mineralized ARCoatings. . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.3.2 AR Coatings on Plastic Substrates . . . . . . . . . . . . 554.3.3 Improving the Cleanability of Porous AR Coatings . . . 60

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5 Instabilities by Thermal Fluctuations 655.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655.2 Fluctuation Induced Forces . . . . . . . . . . . . . . . . . . . . 665.3 Comparison to Earlier Experiments . . . . . . . . . . . . . . . . 725.4 Predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.4.1 Marginally van der Waals stabilized polymer films . . . 775.4.2 Film instabilities on composite substrates . . . . . . . . 785.4.3 Temperature dependence of film instabilities . . . . . . 795.4.4 Retardation effects . . . . . . . . . . . . . . . . . . . . . 79

5.5 The Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 805.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

6 Lithographic Techniques 936.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 936.2 Classical EHD Lithography . . . . . . . . . . . . . . . . . . . . 946.3 Hierarchical Structure Formation in E–Fields . . . . . . . . . . 966.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

7 Film Instabilities by Surface Tension Gradients 1097.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1097.2 Experimental Section . . . . . . . . . . . . . . . . . . . . . . . . 1127.3 Surface Tension Driven Lithography . . . . . . . . . . . . . . . 118

7.3.1 The Experimental Situation. Results and Discussions. . 1197.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

8 Conclusions 127

9 Samenvatting 131