low-temperature plasma enhanced atomic layer...
TRANSCRIPT
M.S. DISSERTATION
Low-Temperature Plasma Enhanced Atomic Layer Deposition of Silicon
Nitride Thin Films for Encapsulation of Flexible OLEDs
by Ji Min Kim
February 2018
Department of Materials Science and Engineering College of Engineering
Seoul National University
Low-Temperature Plasma Enhanced Atomic
Layer Deposition of Silicon Nitride Thin Films for
Encapsulation of Flexible OLEDs
Advisor: Prof. Hyeong Joon Kim
by
Ji Min Kim
A thesis submitted to the Graduate Faculty of Seoul National
University in partial fulfillment of the requirements for the
Degree of Master of Science
Department of Materials Science and Engineering
February 2018
Approved
by
Chairman of Advisory Committee: Cheol Seong Hwang
Vice-chairman of Advisory Committee: Hyeong Joon Kim
Advisory Committee: Seong-Hyeon Hong
i
Abstract
In recent years, along with the development of organic light emitting
diodes (OLEDs) with excellent optical and mechanical properties, displays are
gradually evolving from a conventional form to a new form, and the
development for thinner, curved and flexible displays is rapidly increasing.
These types of display also require a high level of properties of encapsulation
layers which protect the OLEDs’ organic light emitting layer from moisture and
oxygen. Currently, silicon nitride (Si3N4) thin film encapsulation layers
deposited via chemical vapor deposition (CVD) have a disadvantage in that it
is too thick to impart enough mechanical properties required for the new
displays. Therefore, atomic layer deposition (ALD), which is currently
attracting attention as high-quality, ultra-thin, conformal and pinhole-free thin
film deposition, is able to be one of solution for replacing CVD method.
In this study, a silicon nitride thin film was deposited by plasma enhanced
ALD. By applying high-energy nitrogen plasma in process, ALD window
extended to the low temperature proper to OLEDs application. Thin film
analyses were performed to investigate the compositions, roughness, thickness
and density by Auger electron spectroscopy (AES), spectroscopic ellipsometry
(SE), X-ray reflectometry (XRR), atomic force microscopy (AFM), X-ray
photoelectron spectroscopy (XPS).
Atomic layer deposition of silicon nitride using bis(tertiary-butyl-amino)
silane (BTBAS) precursors and N2 plasma was deposited on the top of Si
substrates and polyethylene naphthalate (PEN) substrates at various deposition
ii
process temperature from 300 to 85°C, which is the maximum process
temperature of OLEDs. It was confirmed by measuring a refractive index of the
deposited thin films, which was ~ 2.0, that the deposited thin films had chemical
bond of the same energy as that of pure Si3N4. As the process temperature was
lowered, the refractive index dropped to ~ 1.6 and the carbon content in the film
increased to about 25 at%. The by-products formed during the plasma-enhanced
atomic layer deposition (PEALD) reaction were redeposited through activating
in the plasma and deteriorated the quality of the thin film. The film properties
were improved by adjusting the plasma exposure time and the gas flow rate,
and it was possible to deposit thin film having a refractive index of ~ 1.8 even
at the temperature as low as 85°C. In addition, to improve reliability of nitrides
which are easily oxidized in air, plasma post-treatment with N2 and Ar was
conducted and enhanced the stability of deposited thin films.
Silicon nitride was deposited on PEN and polyimide (PI) substrates by the
PEALD process, based on the optimized deposition conditions on silicon
substrates. As a result of XPS and scanning electron microscopy (SEM)
analyses, it was confirmed that a thin film chemically identical to the thin film
deposited on the silicon substrate was deposited, and that the grown per cycle
(GPC) value measured by SEM was also similar, indicating that the silicon
nitride ALD reaction also occurred stably on the polymer substrates.
In conclusion, through the control of process parameter including plasma
pressure and plasma gas flow rate, it is confirmed that silicon nitride thin films
were deposited at low temperature of about 85°C and were also deposited
reliably on the polymeric substrates.
iii
Keywords: Si3N4, low-temperature deposition, atomic layer deposition,
plasma enhanced atomic layer deposition, BTBAS
(bis(tertiary-butyl-amino) silane), redeposition, PEN, PI
Student Number: 2016-20776
Ji Min Kim
iv
Contents Chapter 1. Introduction .......................................................... 1
1.1 Overview ................................................................................... 1
Chapter 2. Literature Review ................................................. 3
2.1 Atomic Layer Deposition .......................................................... 3
2.1.1 General characteristics of ALD ................................................ 3
2.1.2 The Surface Chemistry of ALD ................................................ 6
2.1.3 Chemisorption Mechanisms ................................................... 16
2.1.4 ALD Process Window ............................................................ 18
2.1.5 Saturation of Surface .............................................................. 20
2.1.6 Effects of Temperature on Growth Rate in ALD .................... 22
2.2 Plasma-Enhanced Atomic Layer deposition ........................... 25
2.2.1 General Characteristics of PEALD ........................................ 25
2.2.2 Low temperature process ....................................................... 27
2.3 PEALD of Silicon Nitrides ..................................................... 29
2.4 ALD on Polymeric Substrates ................................................. 31
2.5 Thin Film Encapsulation for flexible OLEDs ......................... 35
Chapter 3. Deposition of Silicon Nitride Thin Films by Low-
Temperature Plasma-Enhanced Atomic Layer Deposition .. 37
3.1 Experimental Procedures......................................................... 37
3.2 Results and Discussions .......................................................... 41
3.2.1 PEALD of Silicon Nitride on Si substrate .............................. 41
3.2.2 Effects of plasma post-treatment ............................................ 50
3.2.3 PEALD of Silicon Nitride on Polymer ................................... 53
Chapter 4. Conclusions ........................................................ 60
REFERENCES ..................................................................... 62
v
List of Tables
Table 3.1 The detailed conditions for ALD. ......................................... 40
Table 3.2 Topographical AFM images and RMS values of PI, PEN, Si,
substrates and SiN films with 20 nm on the substrates. ....................... 56
Table 3.3 WVTR values of the silicon nitride thin films of various
thickness fabricated by ALD and CVD. ............................................... 59
vi
List of Figures Figure 2.1 Schematic illustration of each 4 steps in a ALD reaction
cycle.[16] ................................................................................................ 5
Figure 2.2 Graphical illustration of Eq. 2.8.[19] .................................... 9
Figure 2.3 Effect of the reactant partial pressure p on the coverage of
surface in an adsorption : (a) the equilibrium chemisorption coverage
Qeq in reversible adsorption with different equilibrium constant and (b)
the chemisorption coverage Q in irreversible adsorption.[16] ............. 14
Figure 2.4 The change in an adsorption amount over time t: (a)
irreversible adsorption (chemisorption) (b) reversible adsorption
(physisorption) and (c) irreversible and reversible adsorption.[3] ....... 14
Figure 2.5 Schematic explanation of five ALD cycles under an
assumption of irreversible adsorption: (a) Surface chemisorption
coverage Q over time t, (b) the amount of adsorbed reactants over time t,
and (c) the deposition rate of reactants over time t.[3] ......................... 15
Figure 2.6 Chemisorption mechanisms of ALD reaction : (a) ligand
exchange (b) dissociation (c) association.[16] ..................................... 17
Figure 2.7 ALD temperature window.[15] ........................................... 19
Figure 2.8 Factors identified to cause saturation of irreversible
chemisorption: (a) steric hindrance of the ligands and (b) the number of
reactive surface sites.[16] ..................................................................... 21
Figure 2.9 Change in the GPC with the ALD process temperature in ALD
window.[16] .......................................................................................... 24
Figure 2.10 Schematic representation of thermal ALD and plasma-
assisted ALD.[21] ................................................................................. 26
Figure 2.11 Growth rate of ALD as a function of growth temperature.[22]
.............................................................................................................. 28
Figure 2.12 TMA mass change measured by a QCM as a function of the
number of cycles : (a) for the 30 cycles and (b) for the first five cycles of
vii
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1
Chapter 1. Introduction
1.1 Overview
Organic light-emitting diodes (OLEDs) with many advantages such as
mechanical flexibility, realistic color and high contrast ratio have been
attracting great attention for its applications in flexible, stretchable and rollable
display.[1] However, they have a critical problem to overcome, which is that
the organic-based luminescent layer is easily degraded by oxygen and
moisture.[2] Therefore, the encapsulation technology which can realize a long
life time and a high reliability of device is a key technology in the OLED device.
The encapsulation layer is now deposited directly on the device in the form of
a thin film which can reduces the encapsulation volume and imparts flexibility
to the device. Most of the encapsulation layers are composed of inorganic
materials including aluminum oxide (AlOx) deposited by sputtering[3], [4],
AlOx deposited by atomic layer deposition (ALD)[5], [6] and silicon nitride
(SiNx) or silicon oxide (SiOx) deposited by plasma enhanced chemical vapor
deposition (PECVD).[7], [8] As the thin film fabricated by ALD have the
advantages of high-quality, densely packed, conformal, and nearly pinhole-free,
it was reported that the encapsulation layer of various inorganic thin films
fabricated by ALD, including AlOx and titanium oxide (TiO2) exhibits more
excellent permeation characteristics than the sputtered or PECVD thin films.
[9]–[11] Recently, halogen-free PEALD process using bis(tertiary-butyl-
amino)silane (BTBAS) and N2 plasma was reported and high quality SiNx was
deposited in the temperature range of 300 ~ 500 C and at the plasma pressure
2
of 40 mTorr.[12] However, the quality of SiNx thin film obtained at low
temperature below 200 C was poor containing high carbon content and having
low density. Therefore, enhancement of the film quality at low temperature
strongly needs to be studied for OLEDs encapsulation application which
requires the low process temperature for long lifetime of them, though there are
few reports on low temperature ALD of SiNx.[13] In this dissertation, the
temperature dependence of PEALD SiNx from 85 to 300 C was investigated
and the quality of the films deposited at 85 C could be alleviated by controlling
two process parameters. Furthermore, the compatibility of the PEALD reaction
with polymeric substrates was confirmed and the permeation characteristic of
silicon nitride thin films was investigated by water vapor transmission rate
(WVTR) with a MOCON instrument.
The outline of this dissertation is same as follows. Chapter 2 covers the
basic concepts of ALD, general characteristics of its surface chemistry and
effects of temperature on growth rate in ALD and reviews the fundamental
literatures about PEALD of silicon nitride, the reaction mechanism of ALD on
polymeric substrates and thin film encapsulation technology for OLEDs.
Chapter 3 displays the temperature dependence of PEALD silicon nitride from
85 to 300 C. and covers the control of nitrogen plasma’s process parameters
successfully alleviating characteristics of thin films. Also, it covers PEALD of
silicon nitride on polymeric substrates including PEN and PI. Finally, chapter
4 covers the conclusion of the dissertation.
3
Chapter 2. Literature Review
2.1 Atomic Layer Deposition
2.1.1 General characteristics of ALD
Atomic layer deposition (ALD) is one of the bottom-up growth techniques
based on the sequential exposure of precursors and reactants based on self-
limiting surface reactions.[14], [15] To be specific, it is a deposition method
which enabled precise control of thickness in atomic-scale by a cycle which are
composed of alternating injection step of precursors and reactants. At the step
which pulses precursors on the surface of substrate, the precursors are
physically adsorbed at first and then chemical bonds between the surface and
precursors formed which make precursors chemically adsorbed on the surface.
Consecutively pulsed precursors are only physically adsorbed staking up on the
early adsorbed precursor molecules. These physically adsorbed precursors are
removed at a purge step which is followed by the precursors pulse step and the
surface is saturated with the chemisorbed precursors. Subsequently, reactants
are injected on the chemisorbed precursors forming chemical bonds between
surface groups and reactants. The remaining reactants are purged at a purge step.
After these four steps, the monolayer of thin films is deposited showing a self-
limited growth behavior. In this way, one cycle of ALD reaction is composed
of 4 steps precursor pulse, precursor purge, reactant pulse, and reactant purge
as shown in Fig. 2.1.[16] The growth rate, known as growth-per-cycle (GPC) is
the amount of added material to the surface during each reaction cycle in terms
4
of film thickness and the thickness of deposited material can be increased by
repeating reaction cycles.[17]
5
Figure 2.1 Schematic illustration of each 4 steps in a ALD reaction
cycle.[16]
6
2.1.2 The Surface Chemistry of ALD
As known in 2.1.1, a self-limiting reaction is the very distinguishing
feature in ALD reaction. This reaction mainly corresponds to the chemical
reaction between functional groups of a gaseous compound and that of solid
surface. In this section, several aspects of thermodynamics and kinetics in the
reaction will be illustrated. Generally, there are the two major surface reactions
that occur in ALD, physisoprtion and chemisorption. Two of the reactions are
able to be classified based on the strength of the interaction between the
adsorbing molecules (“adsorptive”) and the solid surface (“adsorbent”).
Physisorption is a phenomenon when precursors are physically adsorbed on the
surface by weak interactions, well-known as van der Waals forces. The value
of enthalpy of the adsorption, Had, is less than 200 kJ/mol, typically 20 kJ/mol.
Especially, there is no chemical interaction between adsorbent and adsorptive
and physisorption could occur in the wide range of layers, multilayers.[18] On
the other hand, chemisorption originates from the chemical reactions between
adsorptive and adsorbent by forming chemical bonds. Therefore, Had is larger
than 200 kJ/mol (~2 eV) and occur in one layer, a monolayer. The reaction
between two reactants, AXx and BYy, generates compound AB and gaseous
byproducts, XY and xy. This can be represented as following chemical
equation.[19]
AXx(g) + BYy(g) AB(s) + XY(g) + xy(g) Eq. 2.1
7
This chemical reaction can be divided into the two consecutive reactions
(Eq. 2.2 and 2.3) which is the half reaction of one ALD cycle, the pulse step of
precursor and reactant.
BYy (g) + X*(AB)m(s) y*B(AB)m(s)+XY(g) Eq. 2.2
AXx (g) + y*B(AB)m(s) X*(AB)m+1(s) + xy(g) Eq. 2.3
Adsorption kinetics of ALD reaction correspond to the Langmuir
adsorption model as the self-limiting atomic layer growth of precursor and
reactant governs the ALD reaction. Langmuir adsorption model supposes the
condition that the additional adsorption of gaseous precursors is not occur on
the reaction sites that already-adsorbed by adatoms and the formation of next
layer occurs only following the full coverage of current layer. A model equation
expressed by fractional coverage, , showing the progress of layer formation
is as follows.[19]
Eq. 2.5
where and indicates the adsorption and desorption rates and
is the coverage of adsorptive A on the other adsorbent, B layer. Therefore,
superscript AB is applicable to the case when AXx adsorbs on the B layer.
is the pulse time of gaseous precursor, AXx, is its partial pressure and
and indicate the adsorption and desorption coefficients.
8
following means that already-adsorbed atoms are also able to be
desorbed. In Eq. 2.5, “1- ” express the assumption that adsorption does not
occur on the already-adsorbed sites. In the other case, e.g., when BYy adsorbs
on the A layer, superscript would be BA.
The differential equation, Eq. 2.5 can be solved as follows.
Eq. 2.6
Eq. 2.7
also can be solved with the similar method to and the growth
rate (GR) with the consideration of is as follows
Eq. 2.8
Fig. 2.2 provides the explicit relation between the two variables and GR
in the three-dimensional graph where and are x- and y-axis, and GR
is z-axis.
The surface chemistry plays a significant role in ALD reaction. Precursors
should have high enough vapor pressure to be delivered efficiently and have
the proper growth rate of ALD reaction. Also, they have good thermal stability
for wide temperature range of process.
9
Figure 2.2 Graphical illustration of Eq. 2.8.[19]
10
The coverage of ALD reaction from the Langmuir adsorption model also
can be obtained by the method investigated by Puurunen et. al.[16] To simply
identify the effect of the reactant partial pressure which plays a crucial role in
the adsorption process on the coverage, three assumptions which are frequently
used in many literatures concerning ALD chemistry are made; the possible
maximum amount of adsorbed species in a cycle is limited to one monolayer,
every adsorption reaction site is fairly distributed on the substrate surface, and
any possible interaction between neighboring adsorbed species is totally
neglected. Through the simplest example of molecular adsorption where
gaseous compound A adsorbs to a surface site S (Eq. 2.9), the characteristics of
adsorption kinetics in ALD can be explained.
A(g) + S(surface) A S (surface) Eq. 2.9
The coverage of adsorbed species is referred to the chemisorption
coverage and denoted as Q. The instantaneous rate of chemisoption coverage
change over time, dQ/dt, can be represented as the adsorption rate minus the
desorption rate. The adsorption rate can be expressed by the equation which is
adsorption rate constant ka, multiplied by the partial pressure p of the gaseous
precursor A, and multiplied by the fraction (1 Q) which means the unoccupied
remaining surface sites. The desorption rate can be expressed by the equation
which is the desorption rate constant kd multiplied by the fraction Q which
means the occupied surface sites.
11
= Eq. 2.10
In the equilibrium state, the chemisorption coverage is constant at any time
(Eq. 2.10 = 0), and the equilibrium chemisorption coverage Qeq can be
expressed as follows.
Qeq = Eq. 2.11
where p, K, ka and kd indicate the reactant partial pressure, the equilibrium
constant of the adsorption, adsorption rate and desorption rate respectively. In
a reversible adsorption ( 0, ), Qeq increases with p, as illustrated in
Fig. 2.3(a). However, in order to obtain a sufficient growth rate for a practical
process, the adsorption must be nearly irreversible. In the case of irreversible
reactions, the desorption rate, kd approaches to zero and then, the equilibrium
constant, K approaches to infinity. In this assumption, Eq. 2.11 can be solved
as follows.
Eq. 2.12
Therefore, the chemisorption coverage Q does not increase with p, but
always has a constant value at any pressure (Fig. 2.3(b)).
The chemisorption coverage Q can be expressed by a function of time by
integrating Eq. 2.10 over time and assuming constant pressure, adsorption rate
and desorption rate.
12
Eq. 2.13
Figure 2.4 illustrates three types of adsorption in graphs showing changes
in adsorption amount over time when a type of molecular precursor is pulsed
and then purged. The adsorption amount increases and becomes saturated over
time in the initial part and this is consistent with that illustrated in Eq 2.13.
However, in the pulse step, the three types of adsorption show different patterns
each other after the dotted line indicating the beginning of the purge step. Figure
2.5(a) indicates the irreversible chemisorption, Fig. 2.5(b) indicates the
reversible chemisorption and Fig. 2.5(c) indicates irreversible and reversible
chemisorption. After molecular species are adsorbed and the adsorbed amount
commonly saturated at a constant value on the surface (saturation occurs faster
if the p and ka are higher), in the case of (a), the adsorption amount does not
change since the reversible reaction that the adsorbed species is vaporized again
into gaseous molecular species does not occur. On the other hand, in the case
of (b), when the gaseous molecular species is removed from the surface by the
purge step, the adsorbed species is vaporized again to the gaseous species
through the reversible reaction, and the adsorbed amount returns to 0 over time.
The ALD reaction is similar to the case of (c) in which the irreversible and
reversible reaction are combined, and only the molecules adsorbed by the
reversible reaction are vaporized again into the gas phase. This is because, in
the case of the ALD reaction, physisorption which has a small adsorption
enthalpy and close to a reversible adsorption and chemisorption which has a
13
relatively large adsorption enthalpy and is similar to an irreversible adsorption
coexist.
In the ALD reaction, as a reactant A and B are injected alternatively, the
changes in surface coverage and adsorbed amount over time show different
patterns each other. Figure 2.5 shows the time-dependent changes in the
coverage, adsorbed amount, and deposition rate of A and B over five cycles of
the ALD reaction, assuming an irreversible reaction. In Fig. 2.5(b), the adsorbed
amount increases and saturated to a constant value, which is consistent with Fig.
2.4(a). This pattern is accumulated and repeated five times which corresponds
to the number of ALD cycle. On the other hand, in the first cycle of Fig. 2.5(b),
the coverage of reactant A increases and be saturated to 1 but when the reactant
B is injected, the coverage of A decreases to 0 and the coverage of B is saturated
to 1. This is because, when the reactant B adsorbs to the surface, it forms a bond
with the fuctional groups of adsorbent A and substitutes the adsorbent A for the
adsorbent B on the surface. This process is reversed when reactant A is injected
again. Figure 2.5(c) shows the deposition rate over time. Since the unoccupied
reactive sites are abundant at the initial stage of the reactant injection, the
deposition rate is the highest and the rate gradually decreases as sites are
occupied by other reactants.
14
Figure 2.3 Effect of the reactant partial pressure p on the coverage of
surface in an adsorption : (a) the equilibrium chemisorption coverage Qeq in
reversible adsorption with different equilibrium constant and (b) the
chemisorption coverage Q in irreversible adsorption.[16]
Figure 2.4 The change in an adsorption amount over time t: (a) irreversible
adsorption (chemisorption) (b) reversible adsorption (physisorption) and (c)
irreversible and reversible adsorption.[3]
15
Figure 2.5 Schematic explanation of five ALD cycles under an assumption
of irreversible adsorption: (a) Surface chemisorption coverage Q over time t,
(b) the amount of adsorbed reactants over time t, and (c) the deposition rate of
reactants over time t.[3]
16
2.1.3 Chemisorption Mechanisms
There are three main types of chemisorption mechanisms in ALD
reaction as shown in Fig. 2.6. Figure 2.6(a) illustrates ligand exchange. ligand
exchange reaction is that a ligand of reactant is exchanged into a surface
reactive site and adsorbed on the surface as an absorbent. The ligand of
precursor forms gaseous byproduct with a surface functional group. The other
ligand of absorbent could be vaporized as a byproducts reacting with the other
surface functional group. Figure 2.6(b) shows dissociation. In this reaction, a
ligand dissociated from a precursor molecule and the precursor could be
adsorbed on the reactive site as it loses its one functional group. In Fig. 2.6(c),
a precursor does not lose its own ligands and forms a coordinative bond with a
reactive site. This is called association.
17
Figure 2.6 Chemisorption mechanisms of ALD reaction : (a) ligand
exchange (b) dissociation (c) association.[16]
18
2.1.4 ALD Process Window
The ideal behavior of ALD including self-limiting growth occurs only in
a proper temperature range or so-called “ALD window”. Figure 2.7 shows the
change in growth per cycles over growth temperatures in the ALD reaction.
ALD window specifies the temperature region where deposition rate does not
change and has a fairly constant value as the medium temperature range in Fig.
2.7. On the other hand, in the temperature ranges outside this ALD window, the
ALD reaction does not ideally occur and an increase or decrease in GPC occur
due to several reasons. There are 4 cases explaining this non-ideal behavior,
which is condensation (L1), decomposition (H1), incomplete reaction (L2) and
desorption (H2). At lower temperature, as their names indicate, the gaseous
reactant could physically condense on the surface or surface reactions may not
get enough thermal energy to the self-limiting growth. At higher temperature,
thermally unstable reactant could decompose by itself enabling additional
reactant adsorption. This self-dissociation of reactants causes an increase in
GPC forming multilayer deposition, which is similar to CVD. The adsorbed
species could also desorb from the surface by the reversible reaction and this
desorption would bring about the decrease in GPC at higher temperatures.
19
Figure 2.7 ALD temperature window.[15]
ALD
20
2.1.5 Saturation of Surface
Saturation of surface reactive sites is an important phenomenon for the
self-limiting growth of ALD reaction as mentioned in the previous chapters. It
is known that there are two dominating factors causing a saturation of surface
with adsorbed species in ALD reactions, as shown in Fig. 2.8: (a) steric
hindrance of the ligands and (b) the number of reactive sites.[16] Steric
hindrance of the ligands hinders the adsorption of other precursors by hiding
neighboring reactive sites with its ligands from other precursors. Adsorbent has
two or three ligand functional groups and these ligands repel the ligands of the
other adsorbents by van der Waals force, and be tilted to conceal the reactive
site. Therefore, all of the reactive sites could not be occupied by precursors
especially if their ligands are large enough, as in the case of metal organic
precursors. The number of reactive sites is also another important factor for the
saturation of surface. As shown in Fig. 2.8(b), the functional groups of surface
could react with each other and the reactivity of such sites is lowered. Then,
gaseous precursors could not be adsorbed on that surface sites and the surface
could not be fully saturated. This causes the decrease in GPC.
21
Figure 2.8 Factors identified to cause saturation of irreversible
chemisorption: (a) steric hindrance of the ligands and (b) the number of reactive
surface sites.[16]
22
2.1.6 Effects of Temperature on Growth Rate in ALD
As illustrated in 2.1.4, there is a temperature range that ALD could occur
stably. However, the growth rate per cycle (GPC) normally has a slight
temperature dependency even at the temperature range of ALD window. This
can be explained by the change in the main reaction mechanism and the number
of reactive surface site on the substrate over deposition temperature.
Figure 2.9 graphically illustrates the four main patterns of change in GPC
over temperature in ALD. Figure 2.9(a) shows the decrease in the GPC as
temperature increases. This is mainly because the number of reactive surface
sites decreases as deposition temperature increases. As mentioned in 2.1.5, the
number of reactive surface sites affects the number of adsorbed precursors and
the surface coverage. The GPC could have a constant value regardless of the
temperature (Fig. 2.9(b)). For example, even though the reactive surface sites
decrease with temperature, steric hindrance of ligand offsets the effect of the
number of reactive sites. As illustrated in Fig. 2.9(c), the GPC value increases
with temperature. This is mainly caused by increased thermal energy and the
relatively lowered activation barrier. Thus, the reaction which could not occurs
at lower temperature can actively occurs at higher temperature. Finally, the
GPC could increase at first and decrease again as temperature increases in Fig.
2.9(d). This can be easily understood by combination of Fig. 2.9(b) and Fig.
2.9(c). In an initial range of temperature, the GPC increases as adsorption
reaction rate increases due to thermal energy. As temperature further goes up,
23
the number of reactive surface sites becomes smaller and this factor dominates
the GPC value covering the effect of increase in thermal energy.
24
Figure 2.9 Change in the GPC with the ALD process temperature in ALD
window.[16]
25
2.2 Plasma-Enhanced Atomic Layer deposition
2.2.1 General Characteristics of PEALD
As mentioned the previous chapters, one cycle of ALD consists of the
following four consecutive steps: a precursor pulse step, a purge step which
remove physically adsorbed precursors and byproducts of adsorption reaction,
a reactant pulse step, and a purge step. The main difference between thermal
ALD and plasma-enhanced ALD (PEALD) (also known as plasma-assisted
ALD) is the reactant pulse step. As shown in Fig. 2.10, PEALD exposes plasma
in the reactant pulse step.[21] By creating various plasma component including
radicals, ions and electrons, the reaction rate of ALD becomes higher, the film
quality gets better and ALD window can be extended to to lower temperature.
26
Figure 2.10 Schematic representation of thermal ALD and plasma-assisted
ALD.[21]
27
2.2.2 Low temperature process
There are many other advantages of PEALD including a wider choice of
substrate materials and precursors, better film qualities such as lower impurities
and higher density and electronic properties, a higher GPC value and much
more process parameters.[22] Among them, the most important advantage of
PEALD process in this dissertation is the extension of ALD window to lower
temperature region. Additional activation energy provided by plasma energy
facilitates the chemical reactions of precursors with less thermal energy, even
at room temperature. Also, unlike thermal ALD, the extension of process
window can be controlled by the optimization of plasma process parameter. As
shown in Fig. 2.11, thermal ALD has a narrower process window than PEALD
and especially, process window is widened to lower growth temperature limit.
Utilizing this property of PEALD, the process can be applied to the devices
have a sensitive thermal budgets such as organic based OLEDs, organic
semiconductors and fibers, which can greatly increase the availability of the
process.
28
Figure 2.11 Growth rate of ALD as a function of growth temperature.[22]
29
2.3 PEALD of Silicon Nitrides
As the feature size of integrated circuits shrinks, SiO2 and Si3N4, which
are the commonly used dielectric materials in the semiconductor market, has
been required to apply ALD method with perfect conformality. Unlike ALD
SiO2, which has a high GPC of and good film quality due to high reactivity of
oxygen plasma, Si3N4, has a high deposition temperature around 400°C and a
low GPC of 0.1-0.3 Å, so many experimental problems still exist.[23], [24] In
the chemisorption of precursors, Si3N4 has a much lower reactivity than SiO2,
requiring more than 100 times exposure to have the same GPC.[25] Ciaran et.
al. reported that unlike the surface OH groups, which is perpendicular to the
surface, the N-H groups has two H atoms on one N atom, which results in tilted
functional groups and prevents the chemisorption of the precursor. Recently,
halogen-free PEALD process using BTBAS (bis(tertiary-butyl-amino)silane)
and N2 plasma was reported and high-quality silicon nitride was deposited in
the temperature range of 300 ~ 500°C and at the plasma pressure of 40
mTorr.[12] Especially, by using N2 plasma, the surfaces are activated with
undercoordinated reaction sites without H atoms, which induces strong
adsorption of precursors. Rather, using NH3 plasma generates H-terminating
unreactive reaction sites and lowers the GPC value to 0.01 Å.[26] However, the
quality of silicon nitrides obtained at low temperature below 200°C was poor
containing high concentration of carbon and having low density. This is caused
by redeposition effect which is byproducts of the reaction is redeposited in the
films by being activated in N2 plasma and becomes even worse at lower
30
temperatures (e.g. 25 at% at 100°C).[27] Considering the growing the
importance of low temperature process having compatibility with polymeric
substrates for various applications including OLEDs, enhancement of the film
quality at low temperature therefore strongly needs to be studied, though there
are few reports on low temperature ALD of silicon nitride.[13]
31
2.4 ALD on Polymeric Substrates
Low-temperature ALD enables a deposition of thin films on thermally
sensitive materials such as polymer materials. ALD on polymeric substrates
expected to be applicable to the functionalization of polymer surface, to
fabricate unique hybrid materials including inorganic/organic composites, and
to fabricate encapsulation layer on polymeric substrates. ALD on polymers was
not well-researched until recently as polymer materials easily decompose at the
normal temperature range of ALD reaction. Also, a lot of polymer materials do
not have the enough surface reaction sites which were considered as a requisite
for ALD reaction.
Recently, as several studies on Al2O3 ALD has been reported and the high-
quality thin films were successfully obtained even at low temperature, a study
with Quartz crystal microbalance (QCM) explains the mechanism of ALD on
polymers.[28] This study observed the initial nucleation and growth of Al2O3
ALD on polymeric substrate with thicknesses of 2400 ~ 4000 Å. Various types
of substrates were studied, including polymethylmethacrylate (PMMA),
polypropylene (PP), polystyrene (PS), polyethylene (PET), and
polyvinylchloride (PVC). The most interesting observation investigated by the
QCM study was the noticeable mass gain and loss of Al(CH3)3 (TMA) by
diffusion into the surface of polymeric substrates during its pulse and purge step
at the initial ALD cycles.[14] QCM results of Al2O3 ALD for the initial ALD
cycles on PMMA, which the effects are especially pronounced, at 86°C are
shown in Fig. 2.12.[28] Figure 2.12 (a) suggests the QCM results for the first
32
30 cycles. Figure 2.12(b) magnifies the QCM results in the first 5 cycles. These
results indicate that the diffusion of TMA in and out of the PMMA substrates
was only observed during the very initial cycles, ~ 15 cycles. As the Al2O3 ALD
film grows on top of surface region of the polymer and begins to form a
continuous film, the Al2O3 ALD film hinders the TMA diffusion to the deep
part of polymer. This Al2O3 film acts as a barrier to prevent additional TMA
diffusion and the change in mass measured by QCM is not detectable after 15
ALD cycles and resulting in the linear growth of the film. These results suggest
a model with following mechanism for ALD on polymers: (1) precursor
molecules diffuse into the polymer chain and adsorbed on its surface; (2) ALD
reaction between two reactants occur and form a cluster material at the near
surface; (3) the clusters gradually grow and coalesce; (4) a continuous film
cover the surface and hinder an additional diffusion of precursors into the
polymer film; and (5) the ALD reaction ideally occur on the flat surface.[14]
Figure 2.13 schematically shows this ALD mechanism. The mechanism of
Al2O3 ALD reaction on polymer and has been successfully confirmed and
several studies reported experimental results about Al2O3 ALD on polymer.[29],
[30] However, the studies significantly concentrated on Al2O3 ALD and there
are few reports about the ALD reaction of other materials including Si3N4.
33
Figure 2.12 TMA mass change measured by a QCM as a function of the
number of cycles : (a) for the 30 cycles and (b) for the first five cycles of (a).[28]
34
Figure 2.13 A model for Al2O3 ALD on a polymeric substrate: (a) a cross
section of the polymer chains at the surface, (b) Al2O3 nucleation clusters
formed from ALD reaction at the near surface, (c) coalescence of Al2O3 clusters
and hindrance of additional TMA diffusion and (d) formation of a continuous
Al2O3 film that linearly grows at the surface.[14]
35
2.5 Thin Film Encapsulation for flexible OLEDs
OLEDs with many advantages have been attracting great attention for its
applications in flexible, stretchable and rollable display.[1] However, they have
a critical problem to overcome, which is that the organic-based luminescent
layer is easily degraded by oxygen and moisture and deteriorated severely
forming black spots as shown in Fig. 2.14.[2] Therefore, the encapsulation
technology which can realize a long life time and high reliability of the devices
is a significant technology in the OLED devices. The encapsulation layer is now
deposited directly on the device in the form of a thin film which can reduces
the encapsulation volume and imparts flexibility to the device as shown in Fig.
2.15. Most of thin film barriers are composed of inorganic materials including
AlOx by sputtering[3], [4], AlOx by ALD[5], [6] and SiNx or SiOx by
PECVD.[7], [8] As the thin films deposited by ALD have the advantages of
high-quality, densely packed, conformal, and nearly pinhole-free, it was
reported that the ALD layer of various inorganic thin films including AlOx and
TiO2 exhibits more excellent permeation characteristic than the sputtered or
PECVD layer.[34–36]
36
Figure 2.14 Degradation of OLEDs with black spots.
Figure 2.15 Shematic digrams for OLED encaplation structures with (a)
glass lid and (b) thin films barriers.[2]
37
Chapter 3. Deposition of Silicon Nitride Thin
Films by Low-Temperature Plasma-Enhanced
Atomic Layer Deposition
3.1 Experimental Procedures
Before silicon nitride thin films deposition, native oxide on Si substrate
was removed by dipping p-type Si (100) substrates in a dilute HF (3.7 %)
solution for 45 seconds. The silicon nitride films were deposited on substrates
by PEALD using BTBAS as a Si precursor and N2 plasma as a reactant gas at
300°C, 200°C, 120°C and 85°C and at the various plasma pressure. The bubbler
canister containing the precursor was heated to 80°C to increase its vapor
pressure. The depositions were carried out using capacitively-coupled plasma
(CCP) ALD reactor generated at radio frequency (RF) of 13.56 MHZ and was
operated at 400 W. The ALD process repeated one cycle composed of 4 steps,
source dose, N2 purge, N2 plasma and N2 purge. The standard recipe showing
saturating behavior which is an important indicator of ALD reaction was 0.8 s
BTBAS dose, 10 s N2 purge, 15 s N2 plasma and 10 s N2 purge. Plasma gas
residence time is adjusted by two means, by controlling the throttle valve
position and the flow of N2 plasma gas. Plasma post-treatment process was
conducted in the same ALD reactor N2 and Ar was used as plasma gas and the
process condition was the pressure of 1.5 Torr and the power of 400 W. The
process was carried out at 85°C, 120°C and 300°C and was repeated 30 cycles
with the plasma exposure for 15 s and purged for 15 s. In the case of polymeric
38
substrates, another CCP PE-ALD reactor was utilized to prevent contamination
of the equipment. Operating conditions are almost similar with the other one.
PI and PEN was used as substrate each for appropriate process temperature and
cleaned by 5 min sonication in isopropanol and deionized water prior to ALD
deposition. The film thickness and optical properties of the layers were
measured by spectroscopic ellipsometry (SE), using a J.A. Woollam Co. ESM-
300 ellipsometer over a wavelength range of 300 ~ 1500 nm. The optical model
consisted of a silicon substrate and a silicon nitride layer modeled with a
Cauchy dispersion equation. The refractive index values are reported at a
wavelength of 632.8 nm. The film thickness deposited on polymeric substrate
was measured by secondary electron microscopy (SEM; Hitachi, S-4800). The
surface roughness of thin films was measured by aomic force microscopy
(AFM; JEOL, JSPM 5200). The chemical composition of the films was
investigated with X-ray photoelectron spectroscopy (XPS, ThermoVG SIGMA
PROBE) and Auger electron spectroscopy (AES, Perkin-Elmer PHI 600). The
physical density of the films was analyzed by x-ray reflectometry (XRR,
PANalytical X’Pert PRO MPD). The transmittance spectra in s sectral range
from 1100 to 200 nm were obtained at UV/Vis spectroscopy (Jacos V-770).
39
Figure 3.1 Schematic diagram of the PEALD system.
Figure 3.2 Schematic drawing of a direct plasma reactor.[21]
40
Table 3.1 The detailed conditions for ALD.
Deposition material Si3N4
Substrate Si
Precursor BTBAS
Source temperature 70°C
Reactant N2
Substrate temperature 85~300°C
Wall temperature 150°C
Carrier N2 flow 1000 sccm
Purge N2 flow 1000 sccm
41
3.2 Results and Discussions
3.2.1 PEALD of Silicon Nitride on Si substrate
Figure 3.3(a) and (b) shows that as the deposition temperature decreases
from 300°C to 85°C, refractive index and density decreased being far from the
theoretical values, 2.02 and 3.2 g/cm3, respectively, and growth per cycle (GPC)
increased.[31], [32] This means that the bulky film which is thicker and has a
lower density was deposited at lower temperature. Figure 3.4 shows AES depth
profiles at 300, 200, 120 and 85°C, respectively. Surface oxidation seems to
occur during the samples’ transportation from ALD reactor to AES analysis and
it leads the decrease in nitrogen and carbon contents at the samples’ surfaces.
The carbon content in the film relatively increased as the temperature dropped
and this is the main reason that make the film bulkier. In order to investigate
the chemical binding energy state of silicon nitride films, XPS analysis was
conducted. Figure 3.5 shows N 1s and C 1s XPS core level spectra for ALD
silicon nitride deposited at the different temperatures. XPS data of peaks and
binding energies were calibrated with the C-C bonds (284.5 eV) in the C 1s
binding state. At 300°C, the binding energy of the N 1s electrons was 397.4 eV
in Fig. 3.5(a) and this value is consistent with the stoichiometric Si3N4 thin films’
binding energy [33], [34]. However, at 120°C and 85°C, the N 1s peaks
certainly comprise more than one peak centered at 397.2, 398.5 and 400.0 eV,
which are correspond to N-Si bonding, N bonded with sp3-hybridized C and
bonded with sp2 -hybridized C, respectively. Correspondingly, these are two
carbon peaks at 285.9 and 287.7 eV for these two binding states in Fig. 3.5(b)
42
and the peaks are broader with a higher intensity at 85°C than the other
samples.[35], [36] This increase in the carbon content makes the degree and
rate of surface oxidation more severe in lower temperatures as shown in Fig.
3.3 because the formation of N-C bonds which have longer bond lengths than
that of N-Si lead to the decrease in the density of film from 2.3 g/cm3 to 2.0
g/cm3 so oxygen and vapors easily diffuse into the film and transform Si-N into
Si-O which have a higher binding energy than Si-N.[37] In order to deposit
silicon nitride thin films reliably at the temperature as low as 85°C, the carbon
contents, which is a main cause of degradation, must be decreased. Therefore,
it is necessary to clarify the cause of this nonideal ALD behaviors outside of
the ALD window and the source of those carbon impurities. Firstly, there is a
possibility that tert-butyl groups from the silicon precursor are not perfectly
removed because of the incomplete reaction of precursor and N2 plasma.
However, generally, the incomplete reaction cannot create enough reaction sites
on adsorbed precursors and hinder the consecutive adsorption of precursors and
low GPC values supports this.[38] Therefore, it cannot verify why the higher
GPC value was obtained at lower temperature in this study as shown in Fig. 3.3.
Secondly, there is another possibility that the precursor could condense on the
surface because its vapor pressure is lowered as the deposition temperature is
decreased.[39] However, it is unsuitable for this case that saturation behavior
for precursor dosing is observed as Fig. 3.7 illustrates. Therefore, these two
well-known causes for nonideal ALD behavior are inappropriate for this study.
Instead, recently, Knoops et al. reported that the redeposited carbon impurities
which are generated from fragmented reaction byproducts in the plasma can
43
affect the quality of ALD-silicon nitride at 200°C and they prevent the
byproducts from redepositing in the film by decreasing gas residence time
which is a measure of how long species remain in the reactor before being
flushed out.[27] Redeposition phenomenon can be more evident in the PE-ALD
reaction using N2 plasma which requires relatively higher plasma power and
longer plasm exposure time than the other ones using O2 or NH3.[40], [41] The
schematic diagram for redeposition phenomenon is illustrated in Fig. 3.6. Also,
byproducts are more likely to physically adsorb on the surface at lower
temperature and activated in the plasma by the fragmentation and contained in
the film.
In this study, the effect of plasma exposure time and gas residence time on
the quality of silicon nitride films at low temperature were investigated.
Comparison between samples are mainly based on refractive index which can
be measured directly after the film deposition. Figure 3.8 shows the effects of
N2 plasma exposure time on refractive index of silicon nitride films. The longer
exposure time contributed to the higher refractive index being close to the
theoretical value, 2.02. Figure 3.9 shows N 1s and C 1s XPS core level spectra
for ALD silicon nitride with different plasma exposure times. Similar to the
results in Fig. 3.6, it was observed that as the plasma exposure time was longer,
it is found that the N-C peaks intensities decreased and the N-Si peak intensity
increased decreased in Fig. 3.9(a) and C-N peaks intensities decreased in Fig.
3. 9(b). This implies that the longer plasma exposure time leaded to form denser
films and could improve the quality of film by detaching the redeposited carbon
impurities. Figure 3.10 shows the effect of the plasma pressure and plasma gas
44
flow on the refractive index of the silicon nitride films. Gas residence time is
expressed by V/q where V is the effective volume of the chamber and q is the
volumetric flow rate through that volume.[27], [42] Simply, gas residence time
can be expressed as a value proportional to the ratio of chamber pressure to
flow rate measured by mass flow controller. Accordingly, plasma pressure and
gas flow rate can be combined into one variable, gas residence time. As Fig.
3.10(b) indicated, the lower gas residence time led to the higher refractive
indices of the films. Also, samples with the same gas residence time among the
samples deposited at different gas pressures and gas flow rates had very similar
refractive index. This is because low gas residence time prevents the byproducts
from participating in plasma reactions and containing in the films due to high
flow rate.
45
100
200
300
400
1.4
1.6
1.8
2.0
2.2
Ref
ract
ive
inde
x at
632
.8nm
Den
sity
Tem
pera
ture
(o C)
Refractive index at 632.8nm
1.8
2.0
2.2
2.4
2.6
Density (g/cm3)
100
200
300
400
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Growth per cycle (A)
Tem
pera
ture
(o C)
46
Figure 3.4 Chemical composition of the silicon nitride films measured by
AES deposited at (a) 300°C (b) 200°C (c) 120°C (d) 85°C.
Figure 3.5 N 1s and C 1s XPS peaks corresponding to the silicon nitride
films deposited at different temperatures.
47
Fig
ure
3.6
Sche
mat
ic re
depo
sitio
n m
echa
nism
in th
e PE
-ALD
reac
tion.
48
Figure 3.7 GPC values of PEALD silicon nitride films as a function of
precursor dose time.
Figure 3.8 Refractive indices of PEALD silicon nitride films as a function
of N2 plasma exposure time.
49
Figure 3.9 N 1s and C 1s XPS peaks corresponding to the silicon nitride
films deposited with different N2 plasma exposure times.
Figure 3.10 Refractive index as a function of the plasma pressure (a) and
gas residence time (b) measured by spectroscopic ellipsometry.
50
3.2.2 Effects of plasma post-treatment
In addition to deposited films qualities, there was another issue, which is,
after a deposition, the thin films ware easily oxidized over time and the
composition was changed a lot. Particularly, oxidation rapidly progresses as the
deposition temperature is lowered. To solve this problem, plasma post-
treatment was applied to deposited films. Fig. 3.11 shows AES results of the
samples processed various treatment cycles at 300°C. AES was measured 5
days after deposition to identify the amount of natural oxidation. As the post-
treatment cycle increased, the oxidized thickness became thinner. Also, the
sputtering time was increased which means a denser film is formed. Fig. 3.12
indicates the refractive index and the thickness of thin films measured by
spectroscopic ellipsometry. The refractive indices of the films post-treated at
85°C, 120°C and 300°C using the N2 plasma were 1.83, 1.92 and 1.92,
respectively and the thicknesses were 106.68 Å, 101 Å, 94.1 Å. The denser film
was formed after the post-treatment at a higher temperature and the
densification which stronger atomic bonds are created and atoms are rearranged
was more facilitated by thermal energy.[43] However, it is meaningful that the
effect of treatment at 120°C was similar to that at 300°C. The post-treatment
processes using N2 and Ar plasma were conducted to identify which type of
plasma gas is more effective in the densification of the films. The refractive
indices of SiNx films post-treated with N2 and Ar plasma at 300 C was 1.92 and
1.88 and the thickness was 94.1 Å and 100.8 Å, respectively. Comparing the
two types of gas, N2, which delivers both chemical and physical energy, was
51
more effective than Ar, which transmits only physical energy. The post-
treatment process not only prevented the surface oxidation of the films, but also
improved the physical properties. The effect was the highest at 300°C and N2
gas was better than Ar. Through this short time post-treatment, we could
confirm the improvement of physical properties.
52
Figure 3.11 Chemical composition of the silicon nitride thin films (a) as
prepared (b) 30 cycles (c) 60 cylyes of post-treatment measured 5 days after
deposition by AES.
Figure 3.12 Refractive index and density (b) GPC as a function of process
temperature measured with spectroscopic ellipsometry.
53
3.2.3 PEALD of Silicon Nitride on Polymer
Prior to ALD on polymer, reference physical characteristics of thin films
are measured by sample deposited on Si substrates. GPC was 0.94 Å/ cycle and
Refractive index was 1.751 and mass density was 2.1 g/ . Figure 3.13 shows
the optical transmittance spectra for the silicon nitride thin film with thickness
of 40 nm. The optical transmittance is over 80 % in the wavelength range
between 390 nm and 800 nm, which indicates that transmission losses due to
the deposited thin films are negligible. Figure 3.14 roughly shows the flexibility
of SiN/PEN system. Table 3.2 indicates the surface roughness of PI, PEN, Si
substrates and deposited thin films on those substrates measured by AFM. As
shown in topographical AFM images, the roughness of the growth surface was
decreased or well-preserved after deposition. This indicates that the ALD
silicon nitrides were grown in layer-by-layer mechanism leading to the smooth
surface of thin film. Figure 3.15 shows the thickness of thin films deposited on
PEN substrates. As SE measurement method could not be applied to polymeric
substrates, the optical method using SEM was conducted for measuring rough
thickness of thin films. The thickness of the deposited films was about 40 nm
and GPC was 1 Å/cycle, which have similar values with the samples deposited
on Si substrate. Figure 3.16 shows N 1s XPS core level spectra for ALD silicon
nitride deposited on silicon and PEN substrates. PEN_2 indicates the thin film
deposited with N2 plasma exposure with 15 seconds and the others are thin films
deposited with N2 plasma exposure with 25 seconds. In PEN_2 sample, the
broadening of N spectra was detected on PEN substrate as Fig. 3.9. Also, the
binding energy of the N 1s electrons was consistent with the thin films
54
deposited on silicon substrate. This indicates that chemically identical thin films
are deposited on PEN substrates. Due to the high energy nitrogen plasma (400
W), the nitrogen surface termination, which is the most important factor in ALD
reaction, occurred regardless of the substrate materials, leading to thin films of
the same quality that could be deposited on from single crystal materials, silicon
to amorphous materials, PEN. Finally, Table 3.3 shows WVTR values of SiNx
films grown via ALD and CVD on polymeric substrates measured by a
MOCON instrument. The sample grown via ALD shows a lower value (< 0.05
g/m2 day) than the one grown via CVD even at the thinner thickness.
55
300 400 500 600 700 8000
20
40
60
80
100
Tran
smitt
ance
(%)
Wavelength (nm)
Figure 3.13 Optical transmittance spectra for PEALD silicon nitride thin
films.
Figure 3.14 An actual picture of the SiN/PEN system.
56
Tabl
e 3.
2 To
pogr
aphi
cal A
FM im
ages
and
RM
S va
lues
of P
I, PE
N, S
i, su
bstra
tes a
nd S
iN fi
lms w
ith 2
0 nm
on
the
subs
trate
s.
Subs
trate
PI
PE
N
Si
Stru
ctur
e U
ncoa
ted
PI
SiN
/PI
Unc
oate
d PE
N
SiN
/PEN
Si
N/S
i
AFM
imag
es
RM
S (n
m)
4.84
0.93
1.52
1.35
0.18
3
57
Figure 3.15 The tilted top view of the silicon nitride thin film deposited on
PEN substrate.
58
402 400 398 396 3940
1000
2000
3000
4000
Inte
nsity
(A.U
.)
Binding Energy (eV)
Si PI PEN PEN_2
Figure 3.16 N 1s XPS peaks corresponding to the silicon nitride films
deposited on different substrates and with different condition.
59
Tabl
e 3.
3 W
VTR
val
ues o
f the
silic
on n
itrid
e th
in fi
lms o
f var
ious
thic
knes
s fab
ricat
ed b
y A
LD a
nd C
VD
.
Mat
eria
ls
AL
D S
iN
CV
D S
iN
Thic
kn
ess
(nm
) 2
5
30
8
0
18
0
WV
TR
(g/m
2∙da
y)
< 0
.05
0.3
62
0.2
92
0.1
27
60
Chapter 4. Conclusions
PE-ALD silicon nitride was successfully deposited using BTBAS and a
nitrogen plasma as a Si precursor and a reactant, respectively. The high energy
nitrogen plasma of 400 W generated under-coordinated surface reaction sites,
which induced the strong adsorption of precursor and made the formation of
high quality thin films regardless of the substrate materials. Thin films,
deposited at a temperature of 300°C, showed the excellent properties, such as
theoretical density, refractive index and chemical bond, similar to those of the
crystalized Si3N4. But as the process temperature lowered from 300°C to 85°C,
the content of carbon redeposited on the thin film during the plasma exposure
increased, which induced both to lower the density of the thin film and to
enhance the oxidation of the surface. This phenomenon became alleviated by
extending the plasma exposure time or increasing the flow rate of the plasma
gas to remove the redeposited carbon, and thus the deposition of silicon nitride
having a refractive index of 1.8 was possible even at 85°C. In addition, in order
to enhance the above-mentioned oxidation of thin films, in-situ plasma post-
treatment was applied to the deposited thin films in the same ALD reactor to
improve the reliability of the thin films. Despite such short process time, it has
been observed that treatment improves not only the reliability of the thin film
but also physical properties of the thin film. At the same ALD deposition
conditions, silicon nitride could be deposited on polymer substrates, PI and
PEN. Both the GPC and the chemical bond of thin films had the similar values
as those of the thin film deposited on the Si substrate, and the surface roughness
of the thin films was also preserved or improved. In spite of the characteristics
61
of the ALD reaction, which is highly affected by the surface status of the
substrate, the ALD reaction could be stably performed and thus the thin films
of excellent physical properties could be deposited on the polymeric substrates.
Owing to the strong plasma power of 400 W, it was expected that
undercoordinated reaction sites, which induced strong adsorption of the Si
precursor, were sufficiently generated on the polymer substrate, where the
native functional groups did not exist enough. Based on these results, the
compatibility of ALD silicon nitride with polymeric substrates including PI and
PEN was identified and this enables the application of ALD silicon nitride to
OLEDs, as well as organic-based processes, organic semiconductors, polymer
fibers and fabrics. Furthermore, the WVTR value of deposited thin films on
polymeric substrates measured by MOCON equipment showed low enough
value (< 0.05 g/m2 day) to be applicable to the encapsulation of OLEDs at the
very thin thickness of thin film compared to the one fabricated by MOCVD.
62
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