LEAKAGE CURRENT BEHAVIOR OF REACTIVE RF SPUTTERED HfO2 THIN FILMS
By
MICHAEL N. JONES
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2003
Copyright 2003
By
Michael N. Jones
This document is dedicated to the memory of my grandfather, Louis R. Jones. His support for me encouraged me to complete this work.
I am glad I was able to make my grandfather proud.
iv
ACKNOWLEDGEMENTS
First, I would like to thank my parents for encouraging me to attend the
University of Florida and to continue in pursuit of a higher education. I feel that I have
made the most of my college years, which can be attributed in part to my parental
support. Meanwhile, my sister, Kayla, has been a large part of my life and I am grateful
for that. I want to thank all of my friends for enriching my experience while at school,
and I thank Kathryn for her compassion and support.
I am very grateful to my research advisor, Dr. David Norton. Dr. Norton guided
and encouraged me throughout this project. I would like to express gratitude to Dr.
Amelia Dempere at the Major Analytical Instrumentation Center (MAIC) for providing
me with monetary support and the opportunity to work with a great staff.
I would like to thank all of my group members, especially Byoung-Seong Jeon
and Yongwook Kwon. Byoung-Seong taught me how to use the sputtering system and
worked with me to accommodate both of our schedules. Yongwook helped me make
electrical measurements and interpret data. I would also like to thank Joshua Howard and
Dr. Singh’s group for allowing me to use their CV/IV measurement system.
v
TABLE OF CONTENTS
Page ACKNOWLEDGMENTS ……………………………………………………………… iv
LIST OF TABLES ……………………………………………………………………... vii LIST OF FIGURES …………………………………………………………………….viii ABSTRACT ……………………………………………………………………………..xi CHAPTER 1 INTRODUCTION........................................................................................................... 1
1.1 Need for Alternate High-k Dielectric Material on Silicon....................................... 1 1.2 Deposition on ITO.................................................................................................... 2
2 BACKGROUND............................................................................................................. 4
2.1 Introduction to High-k Gate Dielectrics on Silicon ................................................. 4 2.2 Review of Candidate Materials ................................................................................ 8 2.3 Literature Review of HfO2 Thin Film.................................................................... 10
2.3.1 Structure .......................................................................................................... 10 2.3.2 Processing and Properties ............................................................................... 12 2.3.3 MOSFET Device Compatibility ..................................................................... 15
2.4 Sputtering ............................................................................................................... 15 2.4.1 DC Sputtering.................................................................................................. 15 2.4.2 RF Reactive Magnetron Sputtering................................................................. 18
vi
3 LEAKAGE CURRENT PROPERTIES OF RF SPUTTERED HfO2 ON INDIUM-TIN-OXIDE (ITO) ......................................................................................................... 19
3.1 Introduction ............................................................................................................ 19 3.2 Experimental Procedure ......................................................................................... 19
3.2.1 Substrate Preparation....................................................................................... 19 3.2.2 Film Growth .................................................................................................... 20 3.2.3 Measurements.................................................................................................. 20
3.3 Results and Discussion........................................................................................... 22 3.3.1 Dielectric Constant.......................................................................................... 22 3.3.2 Conduction Mechanism................................................................................... 23 3.3.3 Leakage Current .............................................................................................. 29 3.3.4 Microstructure ................................................................................................. 34
3.4 Conclusions ............................................................................................................ 40 4 LEAKAGE CURRENT PROPERTIES OF RF SPUTTERED HfO2 ON SILICON ... 42
4.1 Introduction ............................................................................................................ 42 4.2 Experimental Procedure ......................................................................................... 43
4.2.1 Substrate Preparation....................................................................................... 43 4.2.2 Film Growth .................................................................................................... 43 4.2.3 Measurements.................................................................................................. 44
4.3 Results and Discussion........................................................................................... 46 4.3.1 Microstructure ................................................................................................. 46 4.3.2 Dielectric Constant.......................................................................................... 53 4.3.3 Leakage Current Behavior .............................................................................. 55 4.3.4 Current Mechanism......................................................................................... 60
4.4 Conclusions ............................................................................................................ 62 5 SUMMARY .................................................................................................................. 64 REFERENCES.................................................................................................................. 66 BIOGRAPHICAL SKETCH ............................................................................................ 70
vii
LIST OF TABLES Table page
1 Band gaps and band offsets on silicon for candidate high-k dielectrics [3,4]............. 9 2 Film thicknesses of HfO2 samples on ITO determined by stylus profilometry ........ 21 3 Film thicknesses of HfO2 samples on p-type silicon determined by stylus
profilometry and ellipsometry. .................................................................................. 44
viii
LIST OF FIGURES Figure page 1 Band diagram of semiconductor/insulator interface at flat band ................................ 7 2 High-k gate dielectric candidates ................................................................................ 7 3 Schematic of a DC sputtering system ....................................................................... 17 4 Interactions with incident ions at the target surface.................................................. 17 5 Schematic cross-section of the metal-insulator-ITO structure used in this work. The
aluminum electrode was 150-200 nm thick; the thickness of the HfO2 film, as measured by profilometry, is shown in Table 2. ....................................................... 21
6 Schottky emission conduction mechanism. ФB is the schottky barrier height. Note
the magnitude of ФM, the energy required for thermionic emission from the metal.25 7 Leakage current of HfO2 on ITO measured at various temperatures. The sample was
sputtered with 5 mTorr O2 and 100°C substrate temperature. .................................. 27 8 Schottky emission plots of HfO2 on ITO measured at different temperatures. The
samples was sputtered with different deposition conditions: (a) 5 mTorr O2 and 100°C, and (b) 5 mTorr O2 and 25°C........................................................................ 28
9 Room temperature leakage current plots of HfO2 on ITO showing the effect of
substrate temperature. Each plot shows a different set of deposition oxygen pressures: (a) 1 mTorr O2, (b) 5 mTorr O2, (c) 10 mTorr O2. ................................... 30
10 Room temperature leakage current plots of HfO2 on ITO, showing the effect of
oxygen pressure during film deposition. Each graph displays a different substrate temperature: (a) 25°C, (b) 100°C, (c) 200°C. .......................................................... 32
11 Leakage current of HfO2 on ITO measured at room temperature and 2 V versus
growth conditions...................................................................................................... 33 12 XRD diffractogram of HfO2 on ITO grown with 10 mTorr O2 and varying substrate
temperature................................................................................................................ 34
ix
13 RMS roughness increase measured by AFM as oxygen pressure was varied during 200°C substrate temperature deposition.................................................................... 35
14 RMS roughness increase measured by AFM. The increase corresponds to increasing
substrate temperature when the films were grown with 10 mTorr O2. ..................... 36 15 AFM derivative image of HfO2 grown on ITO at 5 mTorr O2 and 200°C .............. 37 16 AFM image of HfO2 grown on ITO at 10 mTorr O2 and 100°C with highpass filter
applied to enhance grain edges.................................................................................. 38 17 AFM image of HfO2 grown on ITO at 10 mTorr O2 and 200°C with highpass filter
applied to enhance grain edges.................................................................................. 39 18 Drawing of MIS structure with interface layer ......................................................... 45 19 XRD diffractogram of HfO2 films grown on silicon at 200°C substrate temperature,
showing effect of oxygen pressure on crystallization. .............................................. 46 20 XRD diffractogram of HfO2 films grown on silicon at 25°C substrate temperature,
showing the effect of oxygen pressure on crystallization. ........................................ 47 21 XRD diffractogram of HfO2 films grown on silicon with 5 mTorr O2 pressure,
showing the effect of substrate temperature on crystallization. ................................ 47 22 RMS roughness as a function of substrate temperature for HfO2 on Si deposited at48 23 RMS roughness as a function of oxygen pressure for HfO2 on Si deposited at 200°C.
................................................................................................................................... 48 24 Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 10 mTorr O2
and 200°C.................................................................................................................. 49 25 Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 10 mTorr O2
and 100°C.................................................................................................................. 50 26 Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 10 mTorr O2
and 25°C.................................................................................................................... 51 27 Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 5 mTorr O2
and 200°C.................................................................................................................. 52 28 Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 1 mTorr O2
and 200°C.................................................................................................................. 53
x
29 Dielectric constant (calculated from accumulation capacitance) as a function of growth conditions for HfO2 grown on silicon........................................................... 54
30 Dielectric constant (calculated from accumulation capacitance) as a function of
growth conditions for HfO2 grown on silicon........................................................... 55 31 Room temperature leakage current measurement of HfO2 on silicon showing the
decrease in leakage current that corresponds with increasing substrate temperature during deposition. The graphs show the trend for films grown with (a) 5 mTorr and (b) 10 mTorr oxygen pressure. .................................................................................. 56
32 Room temperature leakage current measurement of HfO2 on silicon showing the
decrease in leakage current that corresponds with increasing oxygen pressure during deposition. The graphs show the trend for films grown at (a) 100°C and (b) 200°C substrate temperature................................................................................................. 57
33 Room temperature leakage current of HfO2 on silicon showing (a) effect of substrate
temperature when films are grown with 1 mTorr O2 and (b) effect of oxygen pressure when films are grown at 25°C .................................................................... 59
34 Schematic E vs. x band diagram of the Al/HfO2/SiO2/p-type Si structure in this
experiment................................................................................................................. 61
xi
Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
LEAKAGE CURRENT BEHAVIOR OF REACTIVE RF SPUTTERED HfO2 THIN FILMS
By
Michael N. Jones
December 2003
Chairman: David P. Norton Major Department: Materials Science and Engineering HfO2 is currently under consideration as a potential high-k material to replace
conventional SiO2. HfO2 thin films were deposited onto indium-tin-oxide (ITO) and p-
type silicon (100) substrates using reactive RF sputter deposition. Substrate temperature
and the amount of oxygen used during deposition were varied to determine their effect on
leakage current and microstructure. X-ray diffraction (XRD) and atomic force
microscopy (AFM) were both used for microstructural characterization. Al/HfO2/ITO
structures were used to measure leakage current and dielectric constant as a function of
deposition conditions. Temperature-dependent leakage current measurements allowed
for the extraction of the HfO2/ITO Schottky barrier height. In addition, metal-oxide-
semiconductor (MOS) structures were fabricated on silicon with aluminum electrodes.
The MOS structures were used to measure current-voltage and capacitance-voltage
characteristics as a function of deposition conditions.
1
CHAPTER 1
INTRODUCTION
1.1 Need for Alternate High-k Dielectric Material on Silicon
The scaling of the metal-oxide-semiconductor field-effect transistor (MOSFET)
feature size to sub-100 nm dimensions requires a decrease in thickness of the current
thermal oxide SiO2 gate dielectric to less than 1 nm. At this thickness, direct tunneling
leakage current is significant for SiO2. A solution to the problem is to find materials with
high dielectric constants relative to SiO2. A material with a higher dielectric constant
(high-k material) can be made thick enough to avoid direct tunneling and still maintain
the capacitance of the thinner SiO2 film. An alternative gate dielectric material should
have a high dielectric constant, thermal and chemical stability in contact with silicon,
high quality interfaces, and low leakage current. Some of the materials that researchers
have considered are Y2O3, Al2O3, SrTiO3 (STO), BaxSr1-xTiO3 (BST), Ta2O5, and TiO2.
Materials like Y2O3, and Al2O3 do not provide a significant advantage over SiO2 because
their dielectric constants are not significantly higher. In contrast, ultrahigh-k materials
like STO and BST cause poor short channel effects due to the fringing field induced
barrier lowering effect. It has also been reported that some high-k dielectrics such as
Ta2O5, TiO2, and strontium titanate (STO) are unstable in direct contact with silicon and
have a high leakage current.
HfO2, however, with a dielectric constant on the order of 20-25, is a promising
alternative gate dielectric. HfO2 reportedly has good thermal stability on silicon and can
2
consume native oxide to form HfO2. It is the first high-k material to show compatibility
with the present complementary metal-oxide-semiconductor (CMOS) polysilicon gate
process. HfO2 has a high dielectric constant and relatively low leakage current.
In this study, HfO2 was investigated as an alternative gate dielectric. HfO2 was
deposited by reactive RF magnetron sputtering under varying oxygen partial pressures
and substrate temperatures onto silicon and indium-tin-oxide (ITO)-coated glass
substrates. Dielectric constant and leakage current properties were studied on ITO.
Metal-oxide-semiconductor structures were fabricated on silicon to measure capacitance-
voltage and current-voltage properties. The microstructures of these films were also
investigated as a function of deposition conditions.
1.2 Deposition on ITO Most of the research effort to date on HfO2 has focused on the dielectric and
leakage current properties of films deposited directly on silicon. In this case, the
measured properties of HfO2 on silicon reflect both the intrinsic properties of the HfO2
film as well as the effect of a SiO2 interface layer. While consistent with the gate
dielectric application, this structure makes it difficult to extract fundamental properties on
the HfO2 film itself.
To this end, we have investigated the dielectric and leakage current properties of
HfO2 films deposited by reactive RF sputtering onto ITO-coated glass substrates. The
objective of this study is to understand the electrical properties of the HfO2 without
influence from an interface layer. ITO is a conducting oxide, so the dielectric constant of
the HfO2 can be measured without accounting for the effect of interfacial layers. The
3
effects of substrate temperature and oxygen pressure on dielectric constant, leakage
current, and film microstructure are also investigated.
4
CHAPTER 2
BACKGROUND
2.1 Introduction to High-k Gate Dielectrics on Silicon Alternative, high dielectric constant materials are needed to replace SiO2 as the
gate oxide in metal-oxide-semiconductor (MOS) devices. The current device scaling
trend requires SiO2 film thickness thinner than 1 nm. SiO2 films thinner than 1 nm would
have direct tunneling leakage current greater than 1 A/cm2, which is unacceptable. A
gate dielectric forms a parallel-plate capacitor, where the capacitance is given by
dAC r ⋅⋅= εε 0 (2.1)
where A is the area of the capacitor, εo is the permittivity of free space, εr is the dielectric
constant, and d is the dielectric thickness, respectively. The use of an alternative high-k
dielectric would allow for a thicker gate dielectric layer to be used, reducing the
tunneling current while maintaining a given capacitance. The thickness of SiO2 film that
would give a capacitance-voltage curve equivalent to the new gate oxide is called the
equivalent oxide thickness (EOT). A new gate dielectric should be scalable to 1 nm
EOT. There are several additional conditions that an alternative gate dielectric must meet
to replace thermally grown SiO2 on silicon. They are summarized in the list below.
1. The dielectric constant of the new material should be significantly higher than the dielectric constant of SiO2 film (~3.9);
2. The new gate dielectric should be thermodynamically stable in contact with silicon;
5
3. The band gap of the material should be large, and the band offsets on silicon need to be greater than 1 eV to act as a tunneling barrier for both electrons and holes;
4. The interface trap defect density should be on par with current SiO2; 5. The defect density within the oxide layer should be minimal; 6. A stable amorphous phase material is preferred to avoid grain boundary leakage
current paths; 7. The new material should require minimal change from the existing oxide fabrication
process; 8. The dielectric should be thermodynamically stable in contact with poly-silicon; 9. There should be a low diffusion constant for dopant atoms of poly-silicon; [1]
The last three conditions refer to the compatibility of the new dielectric oxide
with the current MOSFET device processing. After the oxide film is grown on a
semiconducting substrate, the gate material is subsequently deposited on top of the gate
dielectric material. Heavily-doped polycrystalline silicon (polysilicon) is currently used
as the gate material. Therefore, to meet the requirements for current MOSFET
processing, the material must meet requirements dictated by the oxide/polysilicon
interface in addition to the oxide/silicon interface [1]. If these conditions are not met, the
use of a metal gate could be used to solve this compatibility problem [2].
The band offset requirement is very important to minimize leakage current. The
barrier at both the conduction and valence band needs to be high to act as a barrier for
electrons and holes, respectively. A schematic band diagram of the
semiconductor/insulator interface is presented in Figure 1. One of the problems with the
band offset condition is that materials that have large band gaps tend to have low
dielectric constants. Figure 2 illustrates this trend. Therefore, it is important to choose a
material that has a relatively high dielectric constant and a band gap that is large enough
6
to give low leakage currents. Fortunately, the large band gap materials are usually
favored by the stability criterion [3, 4].
If the new material is not stable in contact with silicon, an interface layer (most
likely SiO2) will form. In some cases the interface layers are shown to improve the
density of interface traps, but they provide a serious obstacle for device scaling. Two
capacitors in series result in the overall capacitance given in equation 2.1.
21
111CCCtot
+= (2.1)
Since the interface layer will have a lower dielectric constant than the high-k dielectric, it
will limit the maximum achievable capacitance – or the minimum achievable EOT. The
interface thickness must be strictly controlled so that the dielectric layer can be scaled to
EOT less than 1 nm. In summary, it is desirable to select a material with reasonably high
dielectric constant, large band offsets, and stability in contact with silicon. In the next
section, several candidate high-k dielectrics are reviewed in respect to the criteria that has
just been discussed.
7
Figure 1: Band diagram of semiconductor/insulator interface at flat band
3
4
5
6
7
8
9
10
0 10 20 30 40 50 60 70
Dielectric Constant, k
Ban
dgap
(eV)
SiO2
HfO2
ZrO2ZrSiO4
HfSiO4
TiO2
La2O3
Ta2O5
Y2O3
Si3N4
Al2O3
Figure 2: High-k gate dielectric candidates [3,4,5]
Semiconductor Insulator
Band gap
Valence Band Offset
Valence Band
Conduction Band
Conduction Band Offset
8
2.2 Review of Candidate Materials Presently, many researchers are searching for alternative gate dielectric materials
to replace silicon dioxide. Since a higher dielectric constant means we can grow thicker
films to reduce leakage current, why not use dielectrics with the highest dielectric
constants? Many metal oxides and ferroelectric materials have been investigated as
candidate materials, but most of them are not stable in contact with silicon. Furthermore,
the dielectric constant of materials generally tends to increase as the band gap decreases,
making it difficult to select a material with a large band gap and dielectric constant [1].
For example, SrTiO3 thin films have a dielectric constant of approximately 300 at room
temperature but a band offset of nearly zero. Therefore, this material cannot insulate
against tunneling current [1].
Robertson calculates that the barrier heights for Ta2O5, SrTiO3, and PbTiO3 on
silicon are very small as shown in Table 1. Schottky emission for these films figures to
be quite leaky [3,4]. Accordingly, schottky emission has been shown to be the primary
mechanism involved in the current transport in Ta2O5 films [6,7]. Lee et al. report that
annealing at 500-800 °C for 30 minutes is required to lower the leakage current of Ta2O5
films, which roughens the film surface and increases EOT [8].
9
Table 1: Band gaps and band offsets on silicon for candidate high-k dielectrics [3,4]
Dielectric Material Band gap (eV)
Conduction Band
Offset (eV)
Valence Band
Offset (eV)
SiO2 9 3.5 4.4
Si3N4 5.3 2.4 1.8
SrTiO3 3.3 -0.1
PbTiO3 3.4 0.6
Ta2O5 4.4 0.3 3
TiO2 3.1 0
ZrO2 5.8 1.4 3.3
HfO2 6 1.5 3.4
Al2O3 8.8 2.8 4.9
Y2O3 6 2.3 3.6
La2O3 6 2.3
ZrSiO4 6 1.5 3.4
HfSiO4 6 1.5
On the other hand, some materials have reasonable leakage current properties but
do not offer a significant dielectric constant advantage over SiO2. Al2O3 (K~7.6) has
been shown to have good leakage current characteristics [9,10], but does not have a
significantly higher dielectric constant than SiO2. The same can be said for Si3N4 and
most silicates. A promising gate dielectric should offer high dielectric constants,
reasonable barrier heights, and stability on silicon.
In addition to the tradeoff between dielectric constant and band gap, researchers
have shown that materials with dielectric constant greater then 25 show degradation in
MOSFET turn-off/on characteristics [11]. This is caused by fringing fields from the gate
10
to source/drain regions which further induce fields from the source/drain to the channel.
Furthermore, gate control is weakened by this so-called fringing field induced barrier
lowering [2]. Materials like TiO2, Ta2O5, BST, and STO, have high dielectric constants,
but require the use of barrier layer to suppress barrier lowering [2,11], and to prevent
reaction and interdiffusion with the silicon substrate [12]. As discussed in the previous
section, the low dielectric constant interfacial layer will dominate the capacitance-voltage
characteristics and limit the achievable EOT.
ZrO2 and HfO2 promise to be useful as alternative gate dielectrics. Amorphous
ZrO2 has been grown, and has thin EOT, low leakage current, negligible frequency
dispersion, interface density on par with SiO2, small hysteresis, and excellent reliability
[12]. ZrO2 and HfO2 have a dielectric constant of about 25 and a large band gap. The
band gap of HfO2 is about 6 eV. ZrO2 and HfO2 are theoretically more stable against the
formation of SiO2 on silicon than other oxides [12]. Robertson predicts ZrO2 and HfO2 to
have large band offsets on silicon of about 1.5 eV [3,4]. Both materials have been shown
to have good properties. In fact, successful transistors with ZrO2 and HfO2 have been
fabricated with the present polysilicon gate material [13,14,15,16,17].
2.3 Literature Review of HfO2 Thin Film 2.3.1 Structure Most of the work on HfO2 has been focused on amorphous films to replace SiO2.
Several factors favor an amorphous dielectric. Amorphous materials do not contain grain
boundaries or dislocations that can trap charge and offer fast diffusion pathways for
leakage current. In addition, stresses in amorphous materials can be taken up by small
variations in the random network, where they may likewise be taken up by misfit
11
dislocations in a polycrystalline material. Amorphous structures also tend to minimize
electronically active defects, but may give rise to shallow traps. Epitaxial films can be
free of grain boundaries and defects, but they require more difficult and expensive
methods to process [18]. Therefore, amorphous films are usually preferred for gate
dielectrics. Therefore, the crystallization of HfO2 films is an important issue.
If an amorphous structure is desired, the problem of growing films that remain
amorphous during device processing is not trivial. In fact, this is important for HfO2.
Crystallization often occurs during growth and post-growth annealing [13,19,20,21,22].
The monoclinic phase is the stable HfO2 phase, while metastable phases like
orthorhombic, tetragonal, and cubic are occasionally observed.
Sputtering and chemical vapor deposition (CVD) give the films with the best
amorphous character. Additionally, thin HfO2 films seem to crystallize at higher
temperatures than thick HfO2 films. 20-30 nm reactive sputtered films show monoclinic
crystal structure even before they are annealed. Meanwhile, 20-30 nm films sputtered
from a HfO2 target were amorphous until 500°C annealing. The films were a mixture of
monoclinic and orthorhombic phases up to 700°C annealing where only the monoclinic
phase dominated [19]. However, thinner sputtered films of 18.5 nm [20] and 0.5 nm
thickness [23] remained amorphous up to 700°C annealing temperature. A hafnium
metal layer was sputtered for both films. The first proceeded with subsequent reactive
sputtering of the hafnium target on top of the hafnium seed layer, and the second ended
with the rapid thermal oxidation of the hafnium film. In another case, thin DC reactive
sputtered HfO2 remained amorphous until 650°C annealing [21]. The highest
12
crystallization temperature reported was for CVD HfO2 that reportedly remained
amorphous after gate activation annealing at 950°C for 60 s [16].
Currently, there is a lot of interest in atomic layer deposition (ALD) of HfO2
films. There have been some promising results using ALD as the deposition method,
including the processing of a MOSFET device [17]. However, the nanocrystallinity of
HfO2 films continues to be a concern [13].
2.3.2 Processing and Properties
Several deposition processes have been used to grow HfO2 films: atomic layer
deposition, pulsed laser deposition, jet vapor deposition, chemical vapor deposition,
sputtering of pure HfO2 targets, reactive sputtering of hafnium targets, and sputter
deposition of hafnium metal and subsequent oxidation, among others. Most of the
literature has been directed toward the sputtering techniques. Sputtering is a fairly simple
and trusted thin film deposition process, especially for the growth of amorphous films.
The process is discussed further in section 2.4.
Even though the dielectric constant of evaporated films is reported to be about 25
[24], most of the reviewed literature gave effective dielectric constant values between 12
and 21. The discrepancy between values of 12 and 21 is because of the interfacial layers
and is greatly influenced by deposition and annealing procedure. One of the biggest
factors in high-k gate dielectric growth is interface thickness and quality. It greatly
influences properties such as dielectric constant, minimum achievable EOT, leakage
current, and interface trap density. Fortunately, results of HfO2 on silicon show that high
quality interfaces can be formed without special silicon surface treatment, and with the
use of surface treatment an EOT of 7.1 angstroms has been achieved [25].
13
Gutowski et al. stated through theoretical and experimental evidence that the
HfO2/Si interface is even more stable with respect to silicides than the ZrO2/Si interface
[26]. However, in most cases reviewed, the growth of an interface layer, usually hafnium
silicate, is observed. Several factors - including deposition method, oxygen pressure,
deposition temperature, annealing time/temperature, and annealing ambient – determine
the thickness and quality of the interface. Even though some researchers have shown that
the hafnium silicate interface layer is beneficial to reduce the interface trap density
[27,28], interface thickness control is a key issue in gate dielectric processing because it
limits EOT scaling. Several researchers have deposited HfO2 by reactive sputtering
[15,20,21,29,30,31,32]. In each case, the thickness of the interface layer increases with
increasing oxygen pressure and post-deposition annealing temperature. Callegari et al.
annealed HfO2 films in O2 at 600°C, then measured the thickness of the interface with
transmission electron microscopy (TEM). After correcting for the interface thickness, the
dielectric constant of HfO2 was 21 [29].
For thin HfO2 films, annealing lowers the leakage current [19,32]. It is also used
to fully oxidize and densify films that are deposited with hafnium metal targets instead of
reactive sputtering [23,25]. Fully oxidized films have better leakage current, and denser
films have higher dielectric constants [19]. For this reason, reactive sputtered films and
those sputtered from HfO2 targets tend to have better electrical properties than the films
that are oxidized after hafnium metal deposition. However, if the annealing conditions
are optimized, it is possible to scale post-deposition oxidized HfO2 below 1 nm more
easily than reactive sputtered films [28].
14
However, while leakage current and dielectric properties have been shown to
improve with annealing of thin oxide films, the opposite may be true with thicker films.
In the case of Ta2O5, where the main conduction mechanisms were the poole-frenkel
effect and the schottky effect, 20 nm films were improved by oxygen annealing while 80
nm films were severely degraded by oxygen annealing [33]. This phenomenon may be
caused by the large effect an oxide/Si interfacial layer can have on leakage current,
particularly when the oxide film is relatively thin.
The major drawbacks to annealing thin films are that excessive annealing also
causes more growth of the interfacial layer and may lead to crystallization of the HfO2
film. The scaling will become more difficult due to the increase in interface thickness
[22,31,32,34]; the crystallization can lead to increased leakage current. There are a few
steps to curb this interfacial effect. Annealing in N2 instead of O2 shows a more drastic
increase in leakage current and a smaller effect on the interface [15,21,32]. For example,
two similar HfO2 films were annealed in N2 and O2. For the film annealed in N2, EOT is
approximately 1.9 nm, while annealing in O2 results in EOT of 2.8 nm [21].
Additionally, surface preparation of the silicon substrate by annealing in NH3 or N2
suppresses the growth of an interface layer [25,27]. Nitrogen passivates the surface of
the silicon substrate, allowing an EOT as low as 7.1 angstroms to be achieved [25].
All of the studies reviewed by this author reported very low leakage current
values after annealing in either O2 or N2. There has not been much study into the leakage
current mechanism of HfO2. In one study, the Fowler-Nordheim tunneling mechanism
was observed in sputtered films 20-30 nm thick after annealing [19]. In another study,
Zhu et al. used a Fowler-Nordheim experiment at low temperature to extract band offsets.
15
The HfO2/Si conduction band offset was calculated to be approximately 1.13 ± 0.13 eV.
The calculated Pt/HfO2 offset is 2.48 eV, and the Al/HfO2 offset is 1.28 eV. A trap level
1.5 eV below the HfO2 conduction band was calculated as well, which would contribute
to poole-frenkel conduction. In the Al/HfO2/Si structure, schottky emission dominated
over poole-frenkel since the Al/HfO2 offset is smaller than the trap level [35]. Outside of
this, there is little reference to the current transport mechanism of HfO2.
2.3.3 MOSFET Device Compatibility
In addition to low leakage current, high dielectric constant, good reliability, thin
EOT, negligible frequency dispersion, interface density on par with SiO2, and small
hysteresis (as mentioned at the end of section 2.2), HfO2 is also compatible with current
MOSFET processing, including the use of polysilicon gate material. Polysilicon gate
MOSFETs with HfO2 gate dielectrics have been fabricated [16,17,23,25], and the
performance of these devices is good. The HfO2 devices have been better than ZrO2
MOSFET devices [17]. Using reactive sputtered HfO2 with a polysilicon gate, thermal
stability up to 1000 °C was reported along with EOT of 1.20 nm and leakage current of 1
x 10-3 A/cm2 at 1 V [23]. By annealing the silicon wafer in NH3 and using a TaN gate,
the EOT was ~ 7.1 angstroms and the leakage current was 1 x 10-2 A/cm2 at -1.5 V [25].
2.4 Sputtering 2.4.1 DC sputtering Sputtering has become a widely accepted process for the deposition of thin films.
Mirrors were first coated by sputter deposition as early as 1877. Until the late 1960s,
evaporation was preferred over sputtering. Then, the need for alloys with stringent
stoichiometric limits, conformal coverage, and better adherence, for magnetic and
16
microelectronic materials, increased the demand for sputtering deposition. The addition
of RF (which allows the sputtering of insulators) and magnetron sputtering extended the
capability of sputtering. Today, the capabilities of sputtering and the availability of high-
purity targets and gases make sputtering a popular choice for the deposition of thin films
[36]. Some of the benefits of sputtering include:
1. High uniformity of thickness;
2. Good adhesion of film to substrate;
3. Reproducibility of films;
4. Ability to deposit and maintain the stoichiometry of the target material;
5. Relative simplicity of thickness control. [37]
The sputtering mechanism is described in the following paragraphs.
Sputter deposition uses a momentum transfer process to deposit thin films of
target material onto substrates. A schematic of a typical DC sputtering system is shown
in Figure 3. In DC sputtering, a sputtering gas (usually argon) is introduced into a
vacuum chamber where an applied voltage ionizes the argon gas. The newly formed Ar+
ions accelerate toward the negatively charged cathode, which is at the target material.
When an ion approaches the target, there are five major interactions that can occur. They
are listed below and illustrated in Figure 4.
1. Reflection: The ion may be reflected; probably neutralized in the process.
2. Secondary Electron Emission: The impact of the ion may cause the target material to eject an electron.
3. Ion implantation: The ion may become implanted in the target material.
4. Rearrangements: The impact may rearrange the structure of the target material. This could result in point defects or even lattice defects.
17
5. Sputtering: The impact leads to a series of collisions between atoms, possibly leading to the ejection of target atoms [38].
Figure 3: Schematic of a DC sputtering system
Figure 4: Interactions with incident ions at the target surface [38]
Power Supply
Anode
Substrate
Target / Cathode
Gas Inlet
Vacuum Chamber
18
A plasma, or glow discharge, consisting mostly of ions, electrons, and neutral
atoms, is maintained between the target and substrate. During the glow discharge, it is
observed that a current flows and a film condenses on the substrate [36]. The major
drawback to DC sputtering is that it cannot be used for insulating targets.
2.4.2 RF Reactive Magnetron Sputtering
The thin film deposition process used in this thesis, RF reactive magnetron
sputtering, is a hybrid of several other types of sputtering. What follows is a
description of these sputtering types.
RF sputtering. DC sputtering cannot be used for insulating films because the
surface on the target becomes positively charged. The use of a RF power source with an
impedance matching network solves this problem and allows the sputtering of insulating
materials. The voltage is applied at a frequency of 13.56 MHz; this allows for charging
and discharging of the insulating target [38].
Magnetron sputtering. Magnetron sputtering utilizes magnetic fields to increase
deposition rates and allow for low operating pressures and temperatures. For example,
the application of a planar magnetron will cause electrons in the glow discharge to
follow a helical path, increasing the rate of collisions and ionization. Therefore,
magnetron sputtering grows high quality films at lower operating pressures [37].
Reactive sputtering. Reactive sputtering can be used to deposit films of such
materials like oxides or nitrides through the use of pure metal targets. In addition to the
sputtering gas (argon), a reactive species–oxygen or nitrogen, for example–is introduced
into the growth chamber. The sputtered target atoms react with the gas to form the new
material.
19
CHAPTER 3 LEAKAGE CURRENT PROPERTIES OF RF SPUTTERED HfO2
ON INDIUM-TIN-OXIDE (ITO)
3.1 Introduction Much of the research effort to date on HfO2 films as a gate dielectric has focused
on the dielectric and leakage current properties of films deposited directly on silicon.
While relevant for silicon MOSFET technology, the measured properties of HfO2 on
silicon reflect both the intrinsic properties of the HfO2 film as well as the effect of a SiO2
interface layer. This structure makes it difficult to extract fundamental properties on the
HfO2 film itself.
To this end, we have investigated the dielectric and leakage current properties of
HfO2 films deposited by RF reactive sputtering onto indium-tin-oxide(ITO)-coated glass
substrates. The objective of this study is to understand the electrical properties of the
HfO2 without influence from an interface layer. ITO is a conducting oxide, so the
dielectric constant of the HfO2 can be measured without accounting for the effect of
interfacial layers. The effects of substrate temperature and oxygen pressure on
microstructure, leakage current, and dielectric constant are investigated.
3.2 Experimental Procedure 3.2.1 Substrate preparation Reactive RF sputter deposition was used to deposit HfO2 thin films onto ITO-
coated glass substrates. Substrates were cut to a size of approximately 1 cm x 1 cm.
Then, they were ultrasonically cleaned in trichloroethylene, acetone, methanol, and
20
deionized water for five minutes each. They were blown dry with nitrogen gas. All
substrates were then mounted onto a platen with silver paint and loaded into the
sputtering system. The chamber was pumped down to a base pressure between 5 x 10-8
and 5 x 10-7 Torr.
3.2.2 Film growth
HfO2 films were grown by reactive RF magnetron sputtering from a 2 inch
diameter 99.99% pure hafnium target. The RF power was maintained at 100 W. Oxygen
partial pressure was varied from 1 mTorr to 10 mTorr, while the total pressure (Ar + O2)
was kept at 30 mTorr. Substrate temperature was varied from room temperature to
200°C. The deposition conditions as used in this study are shown in Table 2. Film
thickness was maintained at 60-80 nm for all samples.
After loading the substrates into the deposition chamber, the temperature was
ramped up to the appropriate setting. Meanwhile, the hafnium target was pre-sputtered
with the shutter closed to remove any oxidation from the surface. The shutter was
opened, and oxygen was slowly flowed into the system.
3.2.3 Measurements
Before deposition, a portion of each substrate surface was masked in silver paint.
When the paint is removed by sonication in acetone, it leaves behind a bare ITO surface.
The thickness of the step was then measured by stylus profilometry. Table 2 shows the
results of the profilometry measurements.
For electrical property measurements, circular aluminum electrodes with area π
*10-4 cm2 were sputtered as the top electrode using a shadow mask with a 200 micron
diameter pattern. The thickness of these electrodes was approximately 150-200 nm. The
21
ITO left bare by the silver paint mask served as the bottom electrode. The current-
voltage properties and dielectric constant of HfO2 was measured on ITO between the
aluminum and ITO electrode. Current-voltage curves were measured at various
temperatures to study the conduction mechanism, and capacitance was measured in order
to calculate the dielectric constant. Figure 5 is a schematic cross-section if the metal-
insulator-ITO structure used to make electrical measurements.
Table 2: Film thicknesses of HfO2 samples on ITO determined by stylus profilometry
Substrate Temperature
25°C 100°C 200°C
1 mTorr 67.5 nm 70.0 nm 60.0 nm
5 mTorr 63.0 nm 59.0 nm 76.5 nm
Oxygen Pressure
10 mTorr 65.0 nm 73.5 nm 64.0 nm
Figure 5: Schematic cross-section of the metal-insulator-ITO structure used in this work. The aluminum electrode was 150-200 nm thick; the thickness of the HfO2 film, as measured by profilometry, is shown in Table 2.
ITO
HfO2
Al
Glass
Thickness measured by profilometry
22
In addition to electrical measurements, x-ray diffraction (XRD) and atomic force
microscopy (AFM) were utilized to examine the film microstructure. Films without
deposited electrodes were scanned with the Philips APD 3720 diffractometer, and then
examined by tapping mode AFM on the Digital Instruments Dimension 3100. The
roughness, crystallinity, and grain structure were observed.
3.3 Results and Discussion
3.3.1 Dielectric Constant
For a parallel-plate capacitor, the dielectric constant can be calculated by
AdCk
⋅⋅=
0ε (3.1)
where C is the capacitance; d is the dielectric thickness; ε0 is the permittivity of free
space; and A is the area of the capacitor. For Al-HfO2-ITO structures, the capacitance
was measured at 1 MHz.
Note that there is little previous work on the dielectric constant of HfO2 thin films
on substrates other than silicon. In the case of HfO2 on silicon, the effective dielectric
constant is reduced due to the HfxSiyOz layer that forms between the oxide and the silicon
substrate. The interfacial layer acts as a capacitor in series with the dielectric film.
Accordingly, it is difficult to extract the dielectric constant of the HfO2 films in this way.
Even if the Si/HfO2 interface thickness is measured by cross-section transmission
electron microscopy (TEM), for example, the exact dielectric constant of the interface
remains unknown.
In the present structure, no significant interface is formed between the oxide and
ITO substrate. We can directly calculate the dielectric constant from capacitance and
film thickness measurements. Dielectric constant values were calculated for all the
23
samples listed in Table 2. The dielectric constant of HfO2 at 1 MHz was between 20 and
24 for all but one sample. The variation was not systematic with deposition conditions.
The sample grown at 100°C and 5 mTorr O2 had the only outlying data point, with a
dielectric constant of approximately 29. Most of the samples had dielectric constant
approximately 20-21. This value is in agreement with what was determined on silicon by
Callegari et al. after correcting for interface thickness [29].
3.3.2 Conduction Mechanism
A variety of conduction phenomena occurs when insulating films are sandwiched
between two electrodes. The identification of the dominant (rate-limiting) mechanism is
important in understanding the current-voltage characteristics of the structure being
studied. Two broad categories describe these mechanisms: barrier-limited and bulk-
limited. Barrier-limited mechanisms operate in the vicinity of the interface between
insulator and the contacts. The transport of charge into the insulator limits the
conduction. Schottky emission and tunneling are the most prominent examples of these
types of mechanisms. In the bulk-limited case, the current is limited by the transport of
carriers through the insulator. In other words, sufficient numbers of carriers are injected
into the insulator, but they experience difficulty in reaching the other electrode due to
bulk transport limitations. Examples are intrinsic and Poole-Frenkel conduction
mechanisms [36].
Schottky emission. The schottky mechanism resembles the thermionic emission
of electrons from a heated metal to a vacuum. Thermionic emission is described by the
following equation:
⋅Φ−
⋅⋅=Tk
TCJ MRD exp2 (3.2)
24
ФM is the work function of the metal and CRD is the Richardson constant. For
thermionic emission, electrons need to acquire ФM, typically 4-5 eV. Schottky barriers
heights (ФB), however, are smaller (in the range of 1 eV) because the electrons only need
to acquire enough energy to access the empty conduction band of an insulator. The value
of the schottky barrier height depends on the particular interface between the metal and
insulator/semiconductor. Figure 6 compares the thermionic emission of an electron from
a metal to vacuum with the schottky emission at the interface of a metal and insulator
[36].
The current density governed by schottky emission is given by the Richardson-
Dushman equation,
⋅⋅⋅⋅⋅
⋅⋅⋅=
2/1
0
32
41exp
d
EqTk
TAJεεπ
(3.3)
where
⋅Φ−
⋅=Tk
CA BRD exp (3.4)
The variables T and E are temperature and electric field, respectively. Constants k, q, εo,
and εd represent Boltzman’s constant, electron charge, permittivity of free space, and the
dynamic dielectric constant, respectively [33].
25
Figure 6: Schottky emission conduction mechanism. ФB is the schottky barrier height. Note the magnitude of ФM, the energy required for thermionic emission from the metal.
The pre-exponential term A is used to extract the schottky barrier height, ΦB.
Equation 3.4 can be written as
2/12/1
0
3
2 41)ln(ln EqTk
ATJ
d
⋅��
�
�
��
�
�
���
�
�
⋅⋅⋅⋅��
�
�
⋅+=�
�
�
�
εεπ (3.5)
Therefore the plot of ln(J/T2) versus E0.5 should give a line with y-intercept equal to
ln(A). CRD is the Richardson constant and is given by
3
2*4h
kmeCRD⋅⋅⋅⋅= π (3.6)
where m* is the effective mass [39]. For the free electron approximation CRD = 120
Acm-2K-2. The effective mass of an electron in HfO2 was measured to be 0.1m0 [35];
accordingly CRD = 12 Acm-2K-2.
Vacuum
Conduction Band
METAL INSULATOR
ФB ФM
e
26
Current-voltage measurements were made at temperatures ranging from 25°C to
250°C. The measurements were made for the substrate injection case (i.e. positive gate
voltage), where electrons were injected from the ITO and transported through the HfO2
film to the aluminum electrode. Since the injection of carriers into the insulator
conduction band concerns the interface between the first electrode and the insulator, this
allowed us to extract the height of the barrier at the ITO/HfO2 interface. The schottky
height of the aluminum/HfO2 has been studied before and was calculated to be ~1.28 eV
[35].
Figure 7 shows the temperature-dependent current-voltage curves for a film
deposited at 100°C in 5 mTorr oxygen. Figure 8 shows the corresponding schottky fits
for films grown at (a) 100°C and (b) 25°C. The ln(J/T2) versus E1/2 graphs for both
illustrate a good fit to the schottky model. However, only a self-consistent dynamic
dielectric constant can ensure that leakage current conduction mechanism in the film is
the schottky mechanism. This means that the dynamic dielectric constant, calculated
from the slope of the lines in the schottky plot, should be between the optical dielectric
constant and the static dielectric constant or very close to the optical dielectric constant
[8]. The square of the refractive index (n) gives the optical dielectric constant. For HfO2,
n is approximately 2 [40], so the optical dielectric constant is around 4. In the previous
section, we estimated the static dielectric constant to be 20~21 at 1 MHz. The dynamic
constant for the data in the graphs below is approximately 4-5. From these linear fits, we
extracted the barrier height at the HfO2/ITO interface. The range of data is
approximately 1.1 ± 0.2 eV.
27
Despite what appears to be a reasonable schottky fit, some of the samples
examined had dynamic dielectric constants less than 4, which are not consistent with the
schottky process. However, this phenomenon was observed in the films deposited at
higher substrate temperature and/or high oxygen pressure, thus yielding high leakage
current. For these films the conduction mechanism may be influenced by the schottky
emission process, another mechanism is also significant. For example, the contribution
from current through grain boundaries, if crystallization takes place, would cause the
measured dynamic dielectric constant to be small [33]. It is probable that films deposited
at the higher temperature and/or oxygen pressure have higher degrees of crystallinity that
would contribute to the overall leakage current and may cause deviation from the
schottky model. The data in following sections supports this explanation. It will be seen
that crystallinity is evident and increases with higher substrate temperature and oxygen
pressure.
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0 2 4 6Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
100 C150 C200 C250 C
Figure 7: Leakage current of HfO2 on ITO measured at various temperatures. The sample was sputtered with 5 mTorr O2 and 100°C substrate temperature.
28
-30-29-28-27-26-25-24-23-22-21-20
2000 4000 6000 8000
E0.5 (V/m)0.5
ln(J
/T2 )
[ln(
AK
-2cm
-2)]
100 C150 C200 C250 C
(a)
-34
-33
-32
-31
-30
-29
-28
-27
-26
5000 7000 9000 11000E0.5 (V/m)0.5
ln(J
/T2 )
[ln(A
cm-2
K-2
)]
25 C150 C200 C250 C
(b)
Figure 8: Schottky emission plots of HfO2 on ITO measured at different temperatures. The samples was sputtered with different deposition conditions: (a) 5 mTorr O2 and 100°C, and (b) 5 mTorr O2 and 25°C
29
3.3.3 Leakage Current
The effects of substrate temperature and oxygen partial pressure on the leakage
current behavior of HfO2 on ITO were also examined. Leakage currents as low as 8 x 10-
10 A/cm2 were obtained at 1 V for HfO2 films grown at 5 mTorr O2 and 25°C. Other
films displayed values on the order of 10-7 A/cm2. Figure 9 shows the effect of varying
substrate temperature (25°C, 100°C, and 200°C), while the oxygen pressure remains the
same. As a general trend, increasing substrate temperature caused an increase in leakage
current. In the 1 mTorr O2 case, it is difficult to differentiate whether the current increase
between the 100°C and 200°C films is due to the effect of substrate temperature or the
reduction of film thickness. However, those films grown with 5 mTorr and 10 mTorr O2
show a clear degradation in leakage current properties as the substrate temperature is
increased. In fact, this increase in leakage current between the films exists despite an
increase in film thickness. We believe that the increasing leakage current results from
increasing crystallinity as the films are grown under higher substrate temperature.
30
1E-101E-91E-81E-71E-61E-51E-41E-31E-21E-11E+0
0 2 4 6
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
25oC
100oC
200oC
1E-11
1E-101E-9
1E-8
1E-7
1E-61E-5
1E-4
1E-3
0 2 4
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
25oC
200oC
100oC
1E-91E-81E-71E-61E-51E-41E-31E-21E-11E+0
0 2 4 6
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
25oC
100oC
200oC
Figure 9: Room temperature leakage current plots of HfO2 on ITO showing the effect of substrate temperature. Each plot shows a different set of deposition oxygen pressures: (a) 1 mTorr O2, (b) 5 mTorr O2, (c) 10 mTorr O2.
(a) (b)
(c)
31
Figure 10 shows the effect of varying oxygen pressure (1 mTorr, 5 mTorr, 10
mTorr), while the substrate temperature remains the same. For films grown with room
temperature or 100°C substrates, leakage current is seen to decrease when the oxygen
pressure is increased from 1 mTorr to 5 mTorr. However, the leakage current then
increased when the oxygen pressure is further increased to 10 mTorr. One possible
explanation for the observed oxygen pressure dependence is that the films are not fully
oxidized at 1 mTorr O2. The increase of oxygen to 5 mTorr O2 oxidizes the films,
resulting in a leakage current decrease. However, when the oxygen pressure is increased
further, the leakage current increases due to the contribution of film crystallization. The
trend is more obvious for films grown at 200°C substrate temperature. The leakage
current is monotonically increased by additional oxygen pressure due to increasing
crystallization. Figure 11 shows the room temperature leakage current for each of the
nine samples studied measured at 2 V. It is easy to see the trend in leakage current as a
function of growth conditions.
32
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
0 2 4 6 8
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
1 mTorr O2
10 mTorr O2
5 mTorr O2
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
0 2 4 6 8
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
10 mTorr O2
5 mTorr O2
1 mTorr O2
1E-91E-81E-71E-61E-51E-41E-31E-21E-11E+0
0 2 4 6
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
10 mTorr O2 5 mTorr O2
1 mTorr O2
Figure 10: Room temperature leakage current plots of HfO2 on ITO, showing the effect of oxygen pressure during film deposition. Each graph displays a different substrate temperature: (a) 25°C, (b) 100°C, (c) 200°C.
(a)
(b)
(c)
33
15
10
25100
200
1.00E-09
1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
Leakage Current Density (A/cm2)
Oxygen Pressure (mTorr)
Substrate Temperature
(deg. C)
Figure 11: Leakage current of HfO2 on ITO measured at room temperature and 2 V versus growth conditions.
34
3.3.4 Microstructure
In addition to leakage current, we have also examined the microstructure of HfO2
films using X-ray diffraction (XRD). Figure 12 shows XRD results for HfO2 films
grown on ITO with 10 mTorr O2 pressure. The emergence of the broad hump at 2θ 28.3°
indicates the existence of crystallinity in the film deposited at 200°C. The peak is the
(-111) plane of the monoclinic HfO2 structure which is the most common HfO2 phase
[19,21,22,41]. The emergence of this peak supports the idea that enhanced crystallinity
at higher oxygen pressure and substrate temperature yields the observed increase in
leakage current.
20 30 40 50
Inte
nsity
(arb
.)
ITO (222)
ITO (400)ITO (441)
HfO2
M-(111)
2θ (deg.)
200°C
25°C
100°C
Figure 12: XRD diffractogram of HfO2 on ITO grown with 10 mTorr O2 and varying substrate temperature.
35
In order to gain further information about the film microstructure, tapping mode
atomic force microscopy (AFM) was used. The topography of the HfO2 films grown on
ITO strongly resembles the topography of the ITO surface on the several micron scale.
The polycrystalline grain structure of the ITO film is visible. The grains are about 0.5
micron in diameter and the rms roughness is about 3.416 nm. All of the samples
examined by AFM showed the ITO surface structure on the HfO2 film surface. However,
roughness differences in the films correlated with increasing oxygen pressure and
substrate temperature. Figure 13 and Figure 14 plot this trend. The increase in roughness
is consistent with an increase in crystallinity with high temperature and/or oxygen
pressure during deposition.
02468
1012
0 5 10
Oxygen Pressure (mTorr)
RM
S R
ough
ness
(nm
)
Figure 13: RMS roughness increase measured by AFM as oxygen pressure was varied during 200°C substrate temperature deposition.
36
02468
1012
0 100 200
Substrate Temperature (deg. C)
RM
S R
ough
ness
(nm
)
Figure 14: RMS roughness increase measured by AFM. The increase corresponds to increasing substrate temperature when the films were grown with 10 mTorr O2.
AFM images at a smaller scale show grain structure in the films.
Figure 15 shows an AFM image of the film grown at 5 mTorr O2 and 200°C. The figure
is actually an image of the derivative of the height. It represents what the surface might
look like to the eye or through a microscope. Figure 16 and Figure 17 have been
enhanced to show the grain boundaries better. This is done by the application of a
highpass filter. Each pixel is weighted according to the average of the surrounding
pixels. There is a noticeable grain size increase from Figure 16 to Figure 17 which
corresponds to the increase in substrate temperature from 100°C to 200°C. Since the
highest substrate temperature gives the maximum surface mobility, this is consistent with
island growth and coalescence [42].
37
Figure 15: AFM derivative image of HfO2 grown on ITO at 5 mTorr O2 and 200°C
38
Figure 16: AFM image of HfO2 grown on ITO at 10 mTorr O2 and 100°C with highpass filter applied to enhance grain edges.
39
Figure 17: AFM image of HfO2 grown on ITO at 10 mTorr O2 and 200°C with highpass filter applied to enhance grain edges.
40
3.4 Conclusions In conclusion, the leakage current behavior for HfO2 films deposited by RF
reactive sputtering onto ITO substrates with an Ar + O2 gas flow was examined. Oxygen
pressure and substrate temperature were varied to optimize films for high dielectric
constant and low leakage current. The dielectric constant of HfO2 films was calculated to
be ~20-21 from 1 MHz capacitance measurements. There was no systematic variation of
the dielectric constant according to deposition conditions.
A low leakage current of 8 x 10-10 A/cm2 was obtained for a Al/HfO2/ITO
structure sputtered at 25°C substrate temperature and 5 mTorr O2 pressure when the film
thickness was ~ 63 nm. Current-voltage curves were measured at various temperatures to
study the leakage current mechanism under substrate electron injection. For most films
deposited below 200°C, the data fit the schottky emission mechanism well. The schottky
barrier height at the ITO/HfO2 interface was extracted to be ~ 1.1 ± 0.2 eV.
Leakage current was increased by raising the substrate temperature; the lowest
leakage currents were obtained with room temperature depositions. Increasing the
oxygen amount from 1 mTorr O2 to 10 mTorr O2 also increased the leakage current. The
increase in leakage current with increasing oxygen pressure and substrate temperature
during deposition was attributed to increasing crystallinity. This assertion of increasing
crystallinity is supported by XRD data as well as AFM imaging and roughness data
obtained from the AFM study. However, for films grown at temperature less than 200°C,
the leakage current decreased when the oxygen pressure increased from 1 mTorr to 5
mTorr. To explain this, we asserted that films grown with 1 mTorr O2 were not fully
oxidized. The increase in oxygen pressure from 1 mTorr to 5 mTorr resulted in further
41
oxidation of HfO2 film, reducing the leakage current. The leakage current increased
when the oxygen pressure was increased further because the increase in crystallinity then
became the dominating factor.
The crystallization of the films was supported by increasing rms roughness values
as substrate temperature and oxygen pressure were increased. XRD seemed to indicate
the existence of small grains as well. Likewise, AFM imaging shows the appearance of
the grains in several samples.
The crystallization of the films was also reflected by schottky emission fitting.
The films grown at high oxygen pressure and temperature did not have self-consistent
dynamic dielectric constants in the schottky model. Since the most crystalline films were
not consistent with schottky dominated current transport, we inferred that the crystallinity
of the films contributes significantly to the conduction transport mechanism of these
HfO2 films.
42
CHAPTER 4 LEAKAGE CURRENT PROPERTIES OF RF SPUTTERED
HfO2 ON SILICON
4.1 Introduction The scaling of the metal-oxide-semiconductor field-effect transistor (MOSFET)
feature size to sub-100nm dimensions requires a decrease in thickness of the current
thermal oxide SiO2 gate dielectric to less than 1 nm. At this thickness, direct tunneling
leakage current is significant for SiO2. A solution to the problem is to find materials with
high dielectric constants relative to SiO2. A material with a higher dielectric constant
(high-k material) can be made thick enough to avoid direct tunneling and still maintain
the capacitance of the thinner SiO2 film. HfO2, with a dielectric constant on the order of
20-25, is a promising alternative gate dielectric. HfO2 has good thermal stability on
silicon and can consume native oxide to form HfO2. It is the first high-k material to show
compatibility with the present complimentary metal-oxide-semiconductor (CMOS)
polysilicon gate process. HfO2 has a high dielectric constant and relatively low leakage
current.
In this study, the properties HfO2 thin films were investigated as an alternative
gate dielectric. HfO2 was deposited by reactive RF magnetron sputtering under varying
oxygen partial pressures and substrate temperatures onto silicon substrates. Metal-oxide-
semiconductor structures were fabricated on silicon to measure capacitance-voltage and
current-voltage properties.
43
4.2 Experimental Procedure 4.2.1 Substrate Preparation Reactive RF sputter deposition was used to deposit HfO2 thin films onto p-type Si
(100) substrates. Substrates were cleaved to a size of approximately 1 cm x 1 cm. Each
substrate was ultrasonically cleaned in trichloroethylene, acetone, methanol, and
deionized water for five minutes each, then blown dry with nitrogen gas. Next, the
silicon substrates were immersed in a 10:1 H2O:HF solution for 45 seconds, then rinsed
twice successively in deionized water to remove native oxide and to hydrogen-terminate
the substrate surface. The substrates were again blown dry with nitrogen. All substrates
were mounted onto a platen with silver paint and loaded into the sputtering system. A
portion of each substrate surface was covered in silver paint for thickness measurement
after growth. The chamber was pumped down to a base pressure between 5 x 10-8 and 5
x 10-7 Torr.
4.2.2 Film Growth
HfO2 films were grown by reactive RF magnetron sputtering from a 2 inch
diameter 99.99% pure hafnium target. The RF power was maintained at 100 W. Oxygen
partial pressure was varied from 1 mTorr to 10 mTorr, while the total pressure (Ar + O2)
was kept at 30 mTorr. Substrate temperature was varied from room temperature to
200°C. There was a total of nine combinations of deposition conditions, as shown in
Table 3. Film thickness was 60-85 nm for all samples.
After loading the substrates into the deposition chamber, the temperature was
ramped up to the appropriate setting. Meanwhile, the hafnium target was pre-sputtered
44
with the shutter closed to remove any oxide from the surface. The shutter was opened,
and oxygen was slowly flowed into the system.
Table 3: Film thicknesses of HfO2 samples on p-type silicon determined by stylus profilometry and ellipsometry.
Substrate Temperature
25°C 100°C 200°C
1 mTorr 62.5 nm 65.0 nm 63.0 nm
5 mTorr 63.0 nm 59.0 nm 83.0 nm
Oxygen Pressure
10 mTorr 62.0 nm 65.0 nm 71.5 nm
4.2.3 Measurements
The thickness of the films was measured by stylus profilometry and ellipsometry
using the Filmetrics F20 thin-film measurement system. After the thickness was
determined, samples were prepared for electrical property measurement. Aluminum
electrodes with area π *10-4 cm2 were sputtered as the top electrode using a shadow mask
with a 200 micron diameter circle pattern. The thickness of these electrodes was
approximately 150-200 nm. The backside of the silicon substrate was mounted onto a
glass slide with conductive silver paint, the bottom electrode. The current-voltage
properties and capacitance-voltage properties of HfO2 were measured on silicon. The
current was measured under substrate electron injection. The capacitance-voltage curve
was obtained at 1 MHz. The voltage was swept from depletion to accumulation and then
back to depletion at sweep rate of 1.6 V/s. Figure 18 is a schematic cross-section of the
metal-insulator-semiconductor structure used to make electrical measurements in this
work.
45
In addition to electrical measurements, x-ray diffraction (XRD) and atomic force
microscopy (AFM) were utilized to examine the film microstructure. Films without
deposited electrodes were scanned with the Philips APD 3720 diffractometer, and then
examined by tapping mode AFM on the Digital Instruments Dimension 3100. The
roughness, crystallinity, and grain structure were obtained.
Figure 18: Drawing of MIS structure with interface layer
Silicon
HfO2
Al
Ag
HfxSiyOz
46
4.3 Results and Discussion 4.3.1 Microstructure X-ray diffraction (XRD) was used to investigate the formation of polycrystalline
grains as a function of processing conditions. The XRD results shown in Figure 19
through Figure 21 show that crystallinity is evident in the HfO2 films. The monoclinic
peak appears at 2θ = 28.3 ° for most films. The intensity of this peak increases for higher
oxygen pressure and substrate temperature used during deposition. This is undesirable
for gate dielectric applications as grain boundaries yield paths for enhanced leakage
current.
10 20 30 40 50
Inte
nsity
(arb
.)
2θ (deg.)
SubstrateM-(-111)
10 mTorr O2
5 mTorr O2
1 mTorr O2
Figure 19: XRD diffractogram of HfO2 films grown on silicon at 200°C substrate temperature, showing effect of oxygen pressure on crystallization.
47
10 20 30 40 50
Inte
nsity
(arb
.)
2θ (deg.)
Substrate
10 mTorr O2
5 mTorr O2
1 mTorr O2
M-(-111)
Figure 20: XRD diffractogram of HfO2 films grown on silicon at 25°C substrate temperature, showing the effect of oxygen pressure on crystallization.
10 20 30 40 50
Inte
nsity
(arb
.)
2θ (deg.)
Substrate
M-(-111)
200°C
100°C
25°C
Figure 21: XRD diffractogram of HfO2 films grown on silicon with 5 mTorr O2 pressure, showing the effect of substrate temperature on crystallization.
48
AFM studies were also performed for several films grown on silicon. Figure 22
and Figure 23 show the trend of increases roughness as oxygen pressure and substrate
temperature is increased. This is indicative of increasing crystallization.
0
2
4
6
8
0 100 200
Substrate Temperature (deg. C)
RM
S R
ough
ness
(nm
)
Figure 22: RMS roughness as a function of substrate temperature for HfO2 on Si deposited at 10 mTorr O2.
0
2
4
6
8
0 5 10
Oxygen Pressure (mTorr)
RM
S R
ough
ness
(nm
)
Figure 23: RMS roughness as a function of oxygen pressure for HfO2 on Si deposited at 200°C.
49
Figure 24 through Figure 28 show three-dimensional views of the HfO2 film
surfaces grown at different temperatures and oxygen pressures. At 10 mTorr O2 and
200°C, the grains are largest and completely cover the scan surface. The figures show
that the grain size decreases and the films get smoother with lowering temperature and
oxygen pressure. This is because surface diffusivity is the highest at high substrate
temperatures. Accordingly, the growth rate of the films decreases 10-fold when the
oxygen pressure changes from 1 mTorr O2 to 5 or 10 mTorr O2. This is because the
target is oxidized at the high pressures. The slow deposition rate combined with the high
substrate temperature allows islands to grow larger since the surface diffusivity is high.
The result of this is large grains at high temperature and oxygen pressure.
Figure 24: Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 10 mTorr O2 and 200°C.
50
Figure 25: Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 10 mTorr O2 and 100°C.
51
Figure 26: Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 10 mTorr O2 and 25°C.
52
Figure 27: Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 5 mTorr O2 and 200°C.
53
Figure 28: Three dimensional AFM view of HfO2 surface grown on p-Si(100) at 1 mTorr O2 and 200°C.
4.3.2 Dielectric Constant
The dielectric properties of the HfO2 films were determined using capacitance
measurement with an applied voltage sufficient for accumulation. Overall, the dielectric
constant slightly increased with increasing substrate temperature and oxygen pressure.
This probably reflects the increase in crystallinity. The behavior is complicated by the
combined effects of increasing crystallinity, interface growth and composition, and the
presence of mobile ions. The dielectric constant decrease in the 1 mTorr series going
from 100 to 200°C in Figure 29 may be attributed to the growth of the interface layer.
The decrease in dielectric constant in Figure 30 going from 1 mTorr to 5 mTorr O2 at
25°C could result from the oxidation of the film. However, the effect of the
54
crystallization dominates most of the data as the dielectric constant increases with
increasing crystallinity.
10
15
20
25
30
35
0 100 200
Die
lect
ric C
onst
ant
1 mTorr Oxygen
5 mTorr Oxygen
10 mTorr Oxygen
Substrate Temperature (degrees C)
Figure 29: Dielectric constant (calculated from accumulation capacitance) as a function of growth conditions for HfO2 grown on silicon
55
10
15
20
25
30
35
0 5 10
Oxygen Pressure (mTorr)
Die
lect
ric C
onst
ant
25 C100 C200 C
Figure 30: Dielectric constant (calculated from accumulation capacitance) as a function of growth conditions for HfO2 grown on silicon
4.3.3 Leakage Current Behavior
The leakage current properties are affected by the crystallinity and stoichiometry
of the HfO2 film, as well as the quality and thickness of the interfacial layer at the
Si/HfO2 boundary. The behavior of leakage current with increasing substrate temperature
and oxygen pressure is shown in Figure 31 and Figure 32. Note that the interface layer is
thickest when the films are grown at high oxygen pressure and substrate temperature.
When the interface thickness is increased, leakage current of these films decreases. Some
small deviations in the trend can be explained by changes in overall film thickness.
56
)
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0 1 2 3 4 5 6
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
25°C
200°C
100°C
Figure 31: Room temperature leakage current measurement of HfO2 on silicon showing the decrease in leakage current that corresponds with increasing substrate temperature during deposition. The graphs show the trend for films grown with (a) 5 mTorr and (b) 10 mTorr oxygen pressure.
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0 2 4 6 8 10
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2
25°C200°C
100°C
(a)
(b)
57
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0 1 2 3
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
1 mTorr O2
5 mTorr O2
10 mTorr O2
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0 1 2 3 4 5
Gate Voltage (V)
Cur
rent
Des
nity
(A/c
m2 )
1 mTorr O2
5 mTorr O2
10 mTorr O2
Figure 32: Room temperature leakage current measurement of HfO2 on silicon showing the decrease in leakage current that corresponds with increasing oxygen pressure during deposition. The graphs show the trend for films grown at (a) 100°C and (b) 200°C substrate temperature.
(a)
(b)
58
The leakage current for the films grown at 25°C more generally reflect the
properties of the HfO2 film as minimal interfacial Si-Hf-O is formed. When the films
were grown with 1 mTorr O2, leakage current increased when substrate temperature
increased. When the films were grown with 25°C substrate temperature, the leakage
current increased when the oxygen pressure was increased. These trends are shown in
Figure 33. The lowest leakage current at 2 V was measured for the film grown with 1
mTorr O2 at 25°C. This is probably because of the increase in crystallinity, which has a
larger impact on leakage current than the interface growth in this particular case.
59
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0 1 2 3
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
25°C
200°C
100°C
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0 1 2 3 4
Gate Voltage (V)
Cur
rent
Den
sity
(A/c
m2 )
1 mTorr O2
5 mTorr O2
10 mTorr O2
Figure 33: Room temperature leakage current of HfO2 on silicon showing (a) effect of substrate temperature when films are grown with 1 mTorr O2 and (b) effect of oxygen pressure when films are grown at 25°C
(a)
(b)
60
4.3.4 Current Mechanism
In order to begin discussion of possible current mechanism combinations involved
in the complicated current transport through our structure, one needs to review several
transport mechanisms.
In schottky emission, carriers need to obtain enough energy to be transported over
the schottky barrier into the next material. Current governed by schottky emission
follows the form of the equation below [33].
⋅⋅⋅⋅⋅
⋅⋅⋅=
2/1
0
32
41exp
d
EqTk
TAJεεπ
(4.1)
Tunneling occurs when charge transport occurs through an insulating layer by the
penetration of carriers horizontally (at constant energy) through a barrier, rather than
surmounting enough energy to be transported over the barrier (as in schottky emission).
Tunneling is a quantum mechanical effect and requires a sufficient electric field.
Tunneling is described by equation 4.3 [36].
Φ⋅⋅
⋅⋅⋅⋅⋅⋅
Φ⋅⋅⋅⋅= 2/3
2/122
)(3
)2(8exp8 B
BT q
Eqhm
hEqJ π
π (4.2)
Third, Poole-Frenkel emission, is a bulk transport mechanism. Charge is trapped
in the insulator by impurity levels. The charge can be transported to the insulator
conduction band by internal emission processes. When a field is applied, the potential
well associated with the trap is distorted, increases the chances for the trapped electron to
escape. Equation 4.4 below describes the poole-frenkel conduction mechanism [36].
⋅⋅⋅⋅
⋅⋅
⋅Φ⋅
−⋅⋅=2/1
0
31expexpd
tPF
EqTkTk
qEcJ
εεπ (4.3)
61
The three mechanisms listed above are commonly seen in oxides, including HfO2.
The leakage current behavior of HfO2 grown on silicon can be explained by combining
the contributions of several mechanisms.
Figure 34: Schematic E vs. x band diagram of the Al/HfO2/SiO2/p-type Si structure in this experiment
The leakage current properties of HfO2 in direct contact with ITO has been
reported in Chapter 3. We concluded that the current transport was governed by schottky
emission at the ITO/HfO2 interface under substrate carrier injection. On the contrary,
HfO2 film does not grow in direct contact with silicon. An interface layer is formed
between the HfO2 film and the silicon substrate, most likely SiO2 or some HfxSiyOz.
Now, as seen by the band diagram in Figure 34 above, the barrier height makes it less
likely for schottky emission to occur from silicon to the interface layer than it would be
for schottky emission to occur from silicon to HfO2 in the absence of an interface. Our
data shows that schottky emission is not the dominant mechanism for the films on silicon.
Si SiO2 HfO2 Al
EV
EV EV
EC
EC EC
EF
62
In fact, there is probably a complicated assortment of mechanisms that control the current
transport in the Al/HfO2/SiO2/Si structure.
We would like to suggest a conduction mechanism for our samples, when
electrons were injected from the substrate. The injected electrons tunnel through the
interfacial layer into bulk trap levels inside the HfO2. Therefore, the conduction in these
films combines the tunneling through the interface and the poole-frenkel type in the
HfO2. The rapid discontinuous leakage current increase observed at an applied voltage of
3-5 V (not shown) in some of the samples grown with 1 mTorr O2 may be caused by the
breakdown of the thin HfxSiyOz layer at the interface with silicon. This rapid increase is
not observed in the films grown on ITO (in chapter 3). The mechanism described above
was also believed to occur in Ta2O5 films on silicon [33].
4.4 Conclusions HfO2 films were reactive RF sputtered onto p-type Si (100) substrates with an Ar
+ O2 gas flow. Oxygen pressure and substrate temperature were varied to determine their
effect on the dielectric and microstructural properties of HfO2 on silicon. AFM and XRD
were used to characterize film microstructure. XRD peaks grew more intense with higher
substrate temperature and oxygen pressure. In addition, AFM shows that grains were
largest and most prevalent in films grown at high oxygen pressure and substrate
temperature.
The dielectric constant was calculated for each sample based on the measure
accumulation capacitance and film thickness. Dielectric constant generally increased
with increasing substrate temperature and oxygen pressure due to the crystallinity. The
exception is for films grown at low temperature where the dielectric constant decreases
63
when the oxygen amount is increased from 1 to 5 mTorr. Apparently, the films are not
fully oxidized under low oxygen pressure at low temperature. At low temperature, the
oxidation of the film and/or the growth of an interface layer had a larger effect on the
dielectric constant than the film crystallization. Low leakage currents in high
temperature and/or high oxygen pressure films is due to the effect of the HfO2/Si
interfacial layer. The current transport mechanism in these films was speculated to be a
combination of tunneling through the interface layer, and then poole-frenkel and grain
boundary transport through the film.
64
CHAPTER 5 SUMMARY
HfO2 thin films were deposited onto indium-tin-oxide (ITO) and p-type silicon
(100) substrates using reactive RF sputter deposition. Substrate temperature and the
amount of oxygen used during deposition were varied to determine their effect on
leakage current and microstructure. X-ray diffraction (XRD) and atomic force
microscopy (AFM) were both used for microstructural characterization. Crystallinity
was determined to be present in films on both substrates and increases with increasing
oxygen pressure and substrate temperature.
Al/HfO2/ITO structures were used to measure leakage current and dielectric
constant as a function of deposition conditions. Temperature-dependent leakage current
measurements allowed for the extraction of the HfO2/ITO schottky barrier height. The
schottky barrier height at the ITO/HfO2 interface was extracted to be ~ 1.1 ± 0.2 eV.
However, the films grown at high oxygen pressure and substrate temperature were not
consistent with the schottky mechanism because of the grain boundary contribution. In
general, leakage current on ITO was increased by increasing the oxygen pressure and
substrate temperature because of enhanced crystallization. However, at low temperatures
the films may not have been fully oxidized at 1 mTorr oxygen pressure. Therefore, the
leakage current was improved by more oxygen (5 mTorr) and then subsequently
degraded by the addition of excessive oxygen (10 mTorr). The dielectric constant of
HfO2 films was calculated to be ~20-21 from 1 MHz capacitance measurements on ITO.
65
In addition, metal-oxide-semiconductor (MOS) structures were fabricated on
silicon with aluminum electrodes. The MOS structures were used to measure current-
voltage and capacitance-voltage characteristics as a function of deposition conditions.
The dielectric constant was calculated for each sample based on the measure
accumulation capacitance and film thickness. Dielectric constant generally increased
with increasing substrate temperature and oxygen pressure due to the crystallinity. The
exception is for films grown at low temperature where the dielectric constant decreases
when the oxygen amount is increased from 1 to 5 mTorr. Apparently, the films are not
fully oxidized under low oxygen pressure at low temperature as was seen from leakage
current measurements on ITO as well. The leakage current for the Al/HfO2/Si structures
decreased for high substrate temperature and/or oxygen pressure during depositon. This
effect is attributed to the growth of the Si/HfO2 interfacial layer. The current transport
mechanism in these structures on silicon was speculated to be a combination of tunneling
through the interface layer, and then poole-frenkel and grain boundary transport through
the film.
66
REFERENCES 1. K. Cho, Computational Materials Science 23, 43-7 (2002). 2. B. Cheng, M. Cao, R. Rao, A. Inani, P. V. Voorde, W. M. Greene, J. M. C. Stork, Z. Yu, P. M. Zeitzoff, and J. C. S. Woo, IEEE Transactions on Electron Devices, 46, 1537 (1999). 3. J. Robertson, J. Vac. Sci. Technol. B 18, 1785 (2000). 4. J. Robertson, J. of Non-Crystalline Solids 303, 94-100 (2002). 5. J. S. Suehle, E.M. Vogel, M. D. Edelstein, C. . Richter, N. V. Nguyen, I. Levin, D. L. Kaiser, H. Wu, and J. B. Bernstein, Amer. Vac. Soc. 2001 6th International Symposium on Plasma Process-Incuced Damage, Monterey, CA, USA , 90 (2001). 6. A. Paskaleva, E. Atanassova, T. Dimitrova, Vacuum 58, 470-7 (2000). 7. T. P. Ma, IEEE, 6th International Conference on Solid-State and Integrated- Circuit Technology, 2001. Proceedings., 1, 297 (2001). 8. J. S. Lee, S. J. Chang, J. F. Chen, S. C. Sun, C. H. Liu, U. H. Liaw, Materials Chemistry and Physics 77, 242-7 (2002). 9. W. H. Ha, M. H. Choo, S. Im, J. of Non-Crystalline Solids 303, 78-82 (2002). 10. M. D. Groner, J. W. Elam, F. H. Rabreguette, S. M. George, Thin Solid Films 413, 186-197 (2002). 11. G. C. Yeap, S. Krishnan, and Ming-Ren Lin, Electronics Letteres, 34, 1150 (1998). 12. W.-J. Qi, R. Nieh, B. H. Lee, L. Kang, Y. Jeon, K. Onishi, T. Ngai, S. Banerjee, and J. C. Lee, Int. Electr. Devices Meeting. IEDM Technical Digest., 145-8 (1999). 13. P. S. Lysaught, P. J. Chen, R. Bergmann, T. Messina, R. W. Murto, H. R. Huff, J of Non-Crystalline Solids 303, 54-63 (2002).
67
14. L. Kang, Y. Jeon, K. Onishi, B. H. Lee, W.-J. Qi, R. Nieh, S. Gopalan, and J. C. Lee, 2000 Symp. On VLSI Technology Digest of Technical Papers, 44-45 (2000). 15. L. Kang, K. Onishi, Y. Jeon, B. H. Lee, C. Kang, W.-J. Qi, R. Nieh, S. Gopalan, R. Choi, and J. C. Lee, Int. Electr. Devices Meeting. IEDM Technical Digest., 35- 8 (2000). 16. S. J. Lee, H. F. Luan, W. P. Bai, C. H. Lee, T. S. Jeon, Y. Senzaki, D. Roberts, and D. L. Kwong, Int. Electr. Devices Meeting. IEDM Technical Digest, 31-4 (2000). 17. Y. Kim, G. Gebara, M. Freiler, J. Barnett, D. Riley, J. Chen, K. Torres, J. Lim, B. Foran, F. Shaapur, A. Agarwal, P. Lysagth, G. A. Brown, C. Young, S. Borthakur, H.-J. Li, B. Nguyen, P. Zeitzoff, G. Bersuker, D. Derro, R. Bergmann, R. W. Murto, A. Hou, H. R. Huff, E. Shero, C. Pomarede, M. Givens, M. Mazanec, and C. Werkhoven, Int. Electr. Devices Meeting. IEDM Technical Digest, 455-8 (2001). 18. A. M. Stoneham, J. of Non-Crystalline Solids 303, 114-122 (2002). 19. C.T. Kuo and R. Kwor, Thin Solid Films 213, 257-64 (1992). 20. B.-H. Lee, L. Kang, W.-J. Qi, R. Nieh, Y. Jeon, K. Onishi and J. C. Lee, Int. Electr. Devices Meeting. IEDM Technical Digest, 133-6 (1999). 21. S.-W. Nam, J.-H. Yoo, S. Nam, H.-J. Choi, D. Lee, D.-H. Ko, J. H. Moon, J.-H. Ku, S. Choi, J. of Non-Crystalline Solids 303, 139-143 (2002). 22. S. Xing, N. Zhang, Z. Song, Q. Shen, C. Lin, Microelectronic Engineering 1 66, 451-6 (2002). 23. B. H. Lee, R. Choi, L. Kang, S. Gopalan, R. Nieh, K. Onishi, Y. Jeon, W.-J. Qi, C. Kang and J. C. Lee, Int. Electr. Devices Meeting. IEDM Technical Digest, 39- 42 (2000). 24. C. T. Hsu, Y. K. Su and M. Yokoyama, Jpn. J. Appl. Phys. 31, 2501-4 (1992). 25. R. Choi, C. S. Kang, B. H. Lee, K. Onishi, R. Nieh, S. Gopalan, E. Dharmarajan, and J. C. Lee, Symposium on VLSI Technology Digest of Technical Papers, 15-6 (2001). 26. M. Gutowski, J. E. Jaffe, C.-L. Liu, M. Stoker, R. I. Hegde, R. S. Rai, and P. J. Tobin, Applied Physics Letters 80, 11 (2002).
68
27. P. D. Kirsch, C. S. Kang, J. Lozano, J. C. Lee, J. G. Ekerdt, J. Applied Physics 91, 7 (2002). 28. B. H. Lee, L. Kang, R. Nieh, W.-J. Qi, and J. C. Lee, Applied Physics Letters 76, 14 (2000). 29. A. Callegari, E. Cartier, M. Gribelyuk, H. F. Okorn-Schmidt, and T. Zabel, J. Applied Physics 90, 12 (2001). 30. L. Kang, B. H. Lee, W.-J. Qi, Y. Jeon, R. Nieh, S. Gopalan, K. Onishi, and J. C. Lee, IEEE Electron Device Letters 21, 181 (2000). 31. K. L. Ng, N. Zhan, M.C. Poon, C. W. Kok, M. Chan, and H. Wong, Electron Devices Meeting, Hong Kong. Proceedings. 51-4 (2002). 32. N. Zhan, K. L. Ng, M. C. Poon, C. W. Kok, M. Chan, and H. Wong, Electron Devices Meeting, Hong Kong. Proceedings. 43-6 (2002). 33. E. Atanassova, N. Novkovski, A. Paskaleva, M. Pecovska-Gjorgjevich, Solid- State Electronics 46, 1887-98 (2002). 34. B. K. Park, J. Park, M. Cho, C. S. Hwang, Applied Physics Letters 80, 13 (2002). 35. W. J. Zhu, T.-P. Ma, T. Tamagawa, J. Kim, and Y. Di, IEEE Electron Device Letters 23, 97 (2002). 36. M. Ohring, The Materials Science of Thin Films, Academic Press, San Diego, CA (1992). 37. J. George, Preparation of Thin Films, Marcel Dekker, Inc., New York, NY (1992). 38. B. Chapman, Glow Discharge Processes, John Wiley & Sons, New York, NY (1980). 39. C. K. Maiti, S. Maikap, S. Chatterjee, S. K. Nandi, S. K. Samata, Solid-State Electronics 47, 1995-00 (2003). 40. M. Fadel, O. A. Azim M., O. A. Omer, R. R. Basily, Appl. Phys. A 66, 335-43 (1998). 41. H. Ikeda, S. Goto, K. Honda, M. Sakashita, A. Sakai, S. Aima, and Y. Yasuda, Jp. J. Appl. Phys. 41, 2476-9 (2002).
69
42. K. Wasa and S. Hayakawa, Handbook of Sputter Deposition Technology, Noyes Publications, Westwood, NJ (1992).
70
BIOGRAPHICAL SKETCH
The author was born in Erie, Pennsylvania, and raised in Orlando, Florida, since
the age of two. In high school, the author was an accomplished trumpet player,
performing in ensembles such as the Florida All-State band and the Florida Symphony
Youth Orchestra. The latter included a performance tour of Australia that culminated in
concerts at Sydney Town Hall and the famous Sydney Opera House.
After graduation from Lake Howell High School in Winter Park, Florida, he was
accepted to the University of Florida in Fall 1998. He received a Bachelor of Science
degree in materials science and engineering in June 2003. As an undergraduate, ceramics
was his subject of specialization. Through the 3/2 degree program, the author was able to
enroll in graduate courses while still in pursuit of his undergraduate degree. Thus, he
graduated with a Master of Science degree in materials science and engineering in
December 2003, only a few months after earning his Bachelor of Science. During the
transition from undergraduate to graduate studies, he found a home in Dr. Norton’s
electronic oxides group. The author will remain in Dr. Norton’s group to study copper
diffusion barriers for his Ph.D. dissertation.