Formation and characterization of stochastic subwavelength structures on polymer surfaces [Elektronische Ressource] / von: Robert Leitel

Robert Leitel

Sujets

• Physik

Informations

• Publié par: friedrich-schiller-universitat_jena
• Publié le: 01 janvier 2009
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 8 Mo

Extrait

Formation and characterization of stochastic
subwavelength structures on polymer surfaces
Dissertation
zur Erlangung des akademischen Grades
doctor rerum naturalium (Dr. rer. nat.)
Optical Coatings Department
Fraunhofer-Institute for Applied Optics and Precision Engineering
Institute of Applied Physics
Friedrich Schiller University of Jena
vorgelegt dem Rat der Physikalisch-Astronomischen Fakultat
der Friedrich-Schiller-Universitat Jena
von: Dipl.-Phys. Robert Leitel
geboren am: 19.01.1980 in JenaGutachter
1. Prof. Dr. Andreas Tunnermann, Universitat Jena
2. Prof. Dr. Hans K. Pulker, Universitat Innsbruck
3. Prof. Dr. Herbert Welling, Universitat Hannover
Tag der letzten Rigorosumsprufung: 28.10.2008
Tag der o entlichen Verteidigung: 18.11.2008Contents
Nomenclature II
Abstract 1
Zusammenfassung 2
1 Introduction 3
2 Optics of rough surfaces and thin lms 6
2.1 Optical properties of structured media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Spectral properties of thin lms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Reverse engineering from optical spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4 Rigorous calculation of spectral properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3 Interaction of plasmas with polymers 15
3.1 Properties of plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Essential polymer properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3 Plasma treatment of polymer surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4 Manufacturing and instrumental analysis 33
4.1 Generation of subwavelength structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.2 Analysis of plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.3 Simulation of ion collision cascades in polymers . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.4 Surface topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.5 Optical spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5 Research on subwavelength structures 48
5.1 The erosion process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.2 Antire ection e ect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.3 Structure formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.4 Reverse engineering from optical spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.5 Origins of optical loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.6 Aspects of self-organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6 Conclusions 88
References 90
Acknowledgments 101
Curriculum vitae 103
INomenclature
AFM atomic force microscopy
AR antire ection
ATR attenuated total re ection
dc direct current
EMA e ective medium approximation
FDTD nite dierence time domain
FEM nite element method
FWHM full width at half maximum
IRRAS infrared re ectance absorption spectroscopy
LIPSS laser-induced periodic surface structures
MCM Monte Carlo method
MF merit function
MW microwave
PA polyamide
PC polycarbonate
PET polyethylenterephthalate
PMMA polymethylmethacrylate
PSD power spectral density
RCWA rigorous coupled-wave analysis
RF radio frequency
SEM scanning electron microscopy
SWS subwavelength structure
XPS x-ray photo-electron spectroscopy
A area
absorption coecient
degree of ionizationi
phase constant
c free-space velocity of light
C surface concentration of mobile species
II damping constant
Number of x atoms per monomer, C:::carbon, O:::oxygen(x)
d thickness
d;d penetration depthi p
D surface diusivityS
e elementary charge
E energy
E electric eld strength
electric constant0
complex dielectric function (of the material i)(i)
e ective dielectric functione
f frequency (of quartz crystal monitor)
f oscillator strengthj
F Coulomb force
h height
h Planck’s constant
half cone angle
angle of light incidence
i imaginary unit
i;j numerical index (subscript)
I Intensity(0)
J;J ion uxi
k Boltzmann constantB
k spatial frequency in x;y-directionx;y
extinction coe cient
l length (lateral)
wavelength
center wavelength0
Debye lengthD
(e ective) period
m (electron, ion) mass(e;i)
permeability
n refractive index (of material x), a:::ambient, s::: substrate(x)
0n complex refractive index
N particle density (of x), e:::electron(x)
volume lling factor (of material x)(x)
p pressure
IIIp dipole momentum
P dielectric polarization
q electric charge
r position vector
r electric eld re ection coe cient
R re ectance
speci c gravity
S scatter loss
rms-roughness
t electric eld transmission coe cient
t time
T transmittance
T electron temperaturee
T temperature of glass transitiong
TS total backscatteringb
TS total forward scatteringf
depolarization factor
U bias voltageB
U counter voltageC
U plasma potentialP
spectral data
v critical (Bohm) velocityc
V volume
’ angle of incidence
etch rate
sputtering yieldS
x;y;z components of propagation
susceptibility
! angular frequency
! eigenfrequency of a resonance0
! plasma frequencyp
net rate of re-deposition
IVAbstract
When polymethylmethacrylate (PMMA) is exposed to argon/oxygen low-pressure plasma, a stochas-
tic aligned surface morphology forms under certain conditions. The plasma-treated surface shows
a notable transmittance increase in the visible spectral region. The principle of antire ective sub-
wavelength structures is also used in nature to increase the eciency of the light coupling into the
"moth’s eye". Moreover, the technology is an inexpensive alternative to the deposition of interference
coatings to increase the light transmission of polymeric optical elements. This work is driven by the
need to fabricate such broadband antire ective morphologies on arbitrary shaped surfaces of di erent
kinds of polymeric substrates.
Beside PMMA, three transparent thermoplasts (polyamide, polyethylenterephthalate, polycarbon-
ate) have been chosen to analyze the underlying physical processes in order to understand the
structure formation during plasma treatment. The plasma’s optical emission spectra, the mass loss
of the polymers, and the surface temperature of the substrate have been measured in situ to get an
idea of the erosion process. The plasma-polymer interaction was supported by Monte Carlo method
to investigate the energy transfer of the impinging ions on the polymer substrate. The optical e ect
of the structured surfaces has been optimized for the polymers by the use of spectrophotometry that
was also available during the etch process. The structure formation has been analyzed by atomic
force microscopy and scanning electron microscopy. The revealed e ective pin shape was correlated
with the results from spectral reverse engineering.
The structure formation has been established on the surface of the four tested polymers and shows
a self-organized nature, since the topography was not prescribed from outside before or during the
process. The revealed dierences in size, shape, and antire ective e ect are speci c for the particular
polymer. The structure formation results from a combination of physical sputter-erosion and chemical
etching. Aspect ratios of the pin structure of height / width> 1 can only be explained by anisotropic
etching due to impact of high energetic plasma-ions. The subwavelength structure has been modeled
as e ective medium, which allows for the subsequent calculation of the optical properties by means
of interference coatings with nonlinear refractive index gradients. The occurrence of scatter losses
can be kept low and might be negligible for many applications. Particular PET surfaces show a high,
industrially appropriate transmittance increase in the visible spectral region after plasma treatment.
1Zusammenfassung
Die Behandlung von Polymethylmethacrylat (PMMA) durch ein Niederdruckplasma fuhrt zur Aus-
bildung einer stochastischen Oberachenstrukturierung. Die laterale Ausdehnung der individuellen
Struktureinheiten ist deutlich kleiner als die Wellenlange des Lichtes des sichtbaren Spektralbere-
ichs, wodurch sie entspiegelnde Eigenschaften analog zu den bekannten Mottenaugen-Strukturen"
"
aufweisen. Da diese Topographie weder vor noch wahrend des Prozesses von auen vorgegeben wird,
liegt es nahe das Wachstum auf Selbstorganisation zuruckzufuhren. Diese Arbeit wird motiviert
durch den industriellen Wunsch nach einer generellen Methode zur breitbandigen Entspiegelung von
Oberachen organischer Glaser durch Subwellenlangen-Strukturen.
Dazu werden neben PMMA noch drei weitere Materialien (Polyamid, Polyethylenterephthalat,
Polycarbonat) in die Untersuchungen zum Verstandnis der Strukturentstehung einbezogen. Die
optischen Plasmaemissionen, die Atzrate der Polymere und