7.4 Plasma Surface Modification
What can happen after surface modification?
• change of surface roughness
• change of surface chemistry
What these changes are used for?
• change of surface free energy, i.e. wettability
• improved adhesion of further coatings
• immobilization of biomolecules
Plasma Treatment
In contrary to depositions the changes are limited to a very
thin surface layer (in the order of nm) but please note that
the term “surface” is a matter of definition!
combination of various processes (chemistry, ions, UV) results in:
 removal of material
 modification of original material (especially important for polymers)
 grafting of new functional groups
ageing of treated surfaces
important issue of any surface treatment
processes
Effect of UV Radiation on Polycarbonate
suggested mechanism of carbonate bond breakage
due to UV radiation:
Plasma generates also UV photons and this effect is often forgotten!
Plasma modifications of polymers in inert gas
 discharge in argon, helium:
chemical bonds, such as C-H, C-C, C=C, are broken
 generation of free radicals at or near the surface
 radicals react with each other either directly (if polymer chain is flexible enough) or
due to migration along polymer chain („chain-transfer“)
 cross-linking, branching, removal of low molecular weight material or its
conversion into high molecular weight one (no new functional groups)
CASING
(cross-linking by activated species of inert gas)
R. H. Hansen, H. Schonhorn, J. Polym. Sci. B 4 (1966) 203
H. Schonhorn, R. H. Hansen, J. Appl. Polym. Sci. 11 (1967) 1461
increase of surface hardness, improvement of adhesive
forces at the interface
Additionally, changes of surface roughness
Plasma treatment in reactive gases gases
 plasma containing oxygen (O2, H2O, CO2 …)
• etching of surface carbon radicals by atomic oxygen
• new functional groups, e.g. C-O, C=O, O-C=O, C-O-O, CO3, OH
hydrophilic surface, change of roughness
 plasma containing nitrogen (N2, NH3 …)
• new functional groups such amine (N-C), imine (N=C), nitrile (NC), amide (N-C=O)
• incorporation of oxygen and its functional groups
• grafting of amine groups –NH2
hydrophilization, biocompatibility, imobilization of biomolecules
 plasma containing fluorine (SF6, CF4, C2F6 …)
F and CFx radicals react with surface and two different processes compete:
• etching
• grafting and deposition
hydrophobization, change of roughness
Why Plasma Modification of Polycarbonate?
Properties
 excellent breakage resistance (15-20x than acrylate, 250x than glass)
 good transparency (3 mm thick – 90 %)
 low inflammability, good workability, lighter than glass
replace glass and metals in:
• automobile headlamps, stoplight lenses,
• corrective lenses,
• safety shields in windows, architectural glazing
can be applied to:
• plastics vessels, parts of machines
• in optical grades for compact discs (CDs, CDROMs
and DVDs), optical fibers
 low hardness (0.2 GPa)
 low scratch resistance
 degradation by ultraviolet light
modification of PC surface
properties is necessary
Polycarbonates are attractive business article, the
most important PCs are based on on bisphenol A
(Diflon®, Macrolon®, Lexan®)
deposition of functional films
(scratch resistant, reflective, ...)
surface treatment for improved
film adhesion
Plasma treatment of polycarbonate
in Ar or O2 discharges (CCP)
 f = 13.56 MHz
 inner diameter of reactor 490 mm
 r.f. driven bottom electrode (420 mm)
 Ar, O2: Q = 5.7 sccm , p = 1.5 Pa
 r.f. power P = 100 and 400 W
External plasma parameters:
r.f. electrode
grounded
showerhead el.
55 mm
Plasma treatment of
polycarbonate –
etching rate and surface
free energy
0 5 10 15 20 25
10
20
30
40
50
60
70
80
g
g
p
g
d
g[mJ/m
2
]
treatment time [min]
P = 100 W, Ubias = -115 V, Qar = 5.7 sccm,
p = 1.5 Pa
Plasma treatment of polycarbonate –
surface chemistry by XPS
gas power
[W]
C [at. %] O [at.
%]
Si [at.
%]
N [at.
%]
untreated 84.3 15.7 0 0
Ar 100 76.4 20.3 0.4 2.2
Ar 400 76.0 19.9 1.3 2.8
O2 100 74.0 24.0 0.4 1.7
O2 400 72.6 24.7 1.6 1.2
position [eV] assigment
C1 285.0 C-C, C-H
C2 286.6 C-O
C3 288/289 C-C(=O)-C /
O-C(=O)-O
C4 290.9 C-C(=O)-O
C5 292.1 shake up
7.5 Plasma Enhanced Chemical Vapor Deposition
7.5.1 Introduction to PECVD
Chemical Vapor Deposition (CVD)
thermally driven chemical deposition from gas phase:
R1 R2 R3
R4
R5R6R7
gas phase solid
surface
1. transport of reactants to the deposition
space
2. diffusion of reactants to the substrate
surface
3. adsorption of reactants
4. phys.-chem. processes  film growth and
by-products
5. desorption of by-products
6. diffusion of by-products in gas flow
7. transport of by-products from deposition
space
Low Pressure CVD (LPCVD) is often used in microelectronics or in
applications requiring excellent control over impurities
Plasma Enhanced (or Assisted) CVD
(PECVD or PACVD)
CVD method in which discharge is ignited in the gas mixture:
 collisions of energetic electrons with heavy gas particles
 production of highy reactive species
 more competing processes take place, deposition can be
generally divided into thermal and plasma branches
R1 R2 R3
R4
R5R6R7
gas phase solid
surface
R2
* R3
*
R5
*R6
*
R4
*
PECVD x CVD
plasma reaction branch at PECVD is much more important
because:
 sticking coefficient is much higher for reactive radicals
and activated surface
 activation energies of chemical reactions are lower for
excited reactants
BA 

 eAeA *
'*
BA 
thermal
plasma
reaction branch:
PECVD – lower deposition temperature, novel reaction schemes leading
to new materials, replacement of toxic and dangerous reactants
but
high complexity of chemical reactions and processes, worse selectivity
and reaction control, possibility of damages by energetic ions, UV
radiation or electrostaticaly (charge accumulation)
24334 H12NSiNH4SiH3 
,H3SiNHNHSiH 234 
700-900oC
250-350oC
7.5.2 PECVD of materials with silicon
 dielectric films for microelectronics
silicon nitride: SiH4+NH3 or SiH4+N2
T=250-400 oC
silicon oxide:
(final protective
passivation for
integrated circuit)
(insulating film – el.
separation)
SiH4+N2O/NO/CO2/O2
T=200-400 oC
Si(OC2H5)4 + O2
tetraetoxysilane (TEOS)
PECVD of materials with silicon
low-k dielektrics:
(el. separation for ULSI)
organosilicons + O2/… + …
 semiconducting films for microelectronics
organosilicon glass
(OSG)
epitaxial silicon: SiH4+H2 T=800 oC
polycrystalline silicon: SiH4/SiH2Cl2+H2/Ar T=450-700 oC
(gate electrode, connections in
MOS i.c., solar energy pannels)
 more dielectric films for microelectronics
 SiOx and SiOxCyHz for many other applications
scratch resistant films for plastics, anticorrosion films for metals,
barrier films for packaging and pharmacy,
biocompatible films
mixtures with organosilicons (TEOS, HMDSO, HMDSZ)
(hexamethyldisiloxane)
SiCH3
CH3
CH3
O Si
CH3
CH3
CH3
source of Si-O-Si bonds
SiO2-like films
source of CH3 groups
SiOxCyHz plasma polymers
• concentration of HMDSO in the gas
feed, especially oxygen
• power
• bias voltage / ion energy
• pressure
• pulsing
PECVD of films using HMDSO
 Qhmdso = 4 sccm, Qo2 = 0 – 80 sccm
 pressure 1 - 40 Pa
 rf power 100 - 450 W
 dc self-bias from –20 and –335 V
PECVD from HMDSO/O2 in CCP and ICP (13.56 MHz)
SOURCE
O2/CH4
P L A S M A
DIFFUSION
External Antenna
ELLIPSOMETER
OPTICAL FIBER /
LANGMUIR PROBE
Matching
box
Power supply
13.56 MHz
Modulator/polarizer
Analyser
HMDSO
 pressure 0.4 Pa
 rf power 300 W
 substrate at ground
5-100 % HMDSO in O2
CCP:
helical antenna in ICP mode:
Variation of film composition
 0.4 Pa: SiO2 structure, almost no impurities
 2.5 Pa: SiO2 structure, OH groups and H2O
 40 Pa: organosilicon films
1000 2000 3000 4000
norm.absorbance
CCP 40 Pa
CCP 2.5 Pa
ICP 0.4 Pa
###
wavenumber [cm
-1
] 600 800 1000 1200 1400
CCP 2.5Pa, 0ms
CCP 2.5Pa, 15ms
CCP 40Pa, 0ms
CCP 40Pa, 15ms
norm.absorbance
wavenumber [cm
-1
]
Si-O-Si
Si-OHH2O
Si-OH
CH3 Si-(CH3)x
Si-(CH3)x
5 % HMDSO in O2
Domains of stresses
without treatment
QHMDSO = 4 sccm
two different coatings
choosen for treatment
testing:
 P = 100 W, QO2 = 45 sccm,
d = 0.5 mm
 P = 400 W, QO2 = 10 sccm,
d = 1.2 mm
0 5 10 15 20
100
200
300
400
500
high
compressive
stress
P[W]
QO2
/ QHMDSO
deposition conditions
suitable for preparation
of intermediate layers
high tensile
stress
spontaneous
delamination spontaneous
cracking
for CCP 40 Pa
Film microstructure
15 ms off
0.5 ms off
cw
7.5.3 Carbon materials
Diamond, graphite and much more
Besides well known materials such as crystalline diamond or graphite
carbon can form many other interesting nanomaterials such as
fullerenes, carbon nanotubes, graphene..
sp3 C - diamond sp2 C - graphite
C60 - Buckminsterfullerene
carbon
nanotube
graphene
PECVD of carbon based materials
 amorphous diamond like carbon (DLC) films
 crystalline diamond films
 polymer hydrogenated carbon
films (a-C:H)
CH4/C2H2/… + (Ar/H2), T < 300 oC
0.1 - 5% CH4/C2H2/… in H2
RF plasma p=0.01-4kPa, Tgas=1000-1500oC, P=0.5-3kW
MW plasma p=2-10kPa, Tgas=2000-2500oC, P=0.5-2kW
T=700-1000oC
!! ion bombardment
1.2% of CH4
6% of CH4
Classification of carbon films
Classification of amorphous
hydrogenated carbon films
Methods for synthesis of carbon nanotubes (CNTs)
 arc discharge deposition
 laser ablation of graphite
costly apparatus not amenable for scaleup;
small quantities of high-quality SWNTs in 70-
90% purity
large quantities of CNTs but the purity is usually
about 30%; not suitable for direct synthesis of
supported aligned CNTs
 chemical vapor deposition (CVD)
plasma enhanced CVD
(PECVD)
thermal CVD
(energy source)
floating catalyst: Fe(C5H5)2,
Fe(CO)5
supported
catalyst:
Fe, Ni, Co ...
reactants: hydrocarbons or CO mixed
with H2, NH3, N2 ...
Microwave Plasma Torch at Atmospheric Pressure
Studied for the synthesis of
• carbon nanotubes (CNTs) using CH4
• iron-based nanoparticles (Fe-NPs) using Fe(CO)5
frequency 2.45 GHz, power 100 – 400 W
dual gas flow:
• central channel – Ar (500 – 1500 sccm)
• side channel – CH4, H2, Ar + Fe(CO)5
Surface Bound Deposition of CNTs
• mw power: 400 W
• Ar: 1500 sccm
• H2: 285-430 sccm
• CH4: 25-42 sccm
- Fe catalytic layer, 5-15 nm
(- SiO2 film, 200 nm)
• Si substrate with
-10 0 10 20 30
100
200
300
400
500
600
700
800
900
temperature[
o
C]
time [s]
CH4
, H2
flowcontrollers
opened
discharge off
1500/430/42 sccm
Si
Si
SiOCH
Fe
Nanostructuring of catalytical film is not
necessary for torch deposition of CNTs.
It takes place at the very beginning (few s)
of deposition.
400 mm
5 15 25 35 45 55 65 75 85 95
0
20
40
60
80
100
120
140
counts
diameter [nm]
Type of CNTs Growth
Fe 5 nm, 700 oC
edge
400 mm
8 10 12 14 16 18 20 22 24 26 28 30
0
2
4
6
8
10
12
14
16
18
20
counts
diameter [nm]
central area
1500/430/42sccm of Ar/H2/CH4
L. Zajickova et al. Plasma Process. Polym. 2007, 4, S245–S249
CNTs without Barrier SiO2
1500/430/42 sccm
700 oC, 45 s,
15 nm Fe on Si
# 174
# 174
L. Zajickova et al. Pure Appl. Chem. 82(6) 1259–1272, 2010
Iron Oxide NPs
phase type of
magnetism
Ms (300K)
(A.m2/kg)
applications
a-Fe2O3
weakly
ferromagnetic or
antiferromagnetic
0.3 photo-
chemistry
g-Fe2O3
ferrimagnetic

60-80 MRI,
ferrofluids,
drug
delivery,
cancer
treatment,
data storage
Fe3O4
ferromagnetic 90-100
e-Fe2O3
ferrimagnetic

20
hysteresis
high density
data storage
single domain crystals (superparamagnetic)
- below approx. 14 nm