CHAPTER-1
INTRODUCTION
1.1 Alumina
Aluminum Oxide is an amphoteric oxide with the chemical formula Al2O3,
commonly referred to as alumina or corundum, in its crystalline form. Alumina is a
white or nearly colorless crystalline substance that is used as a starting material for the
smelting of aluminum metal. It also serves as the raw material for a broad range of
advanced ceramic products and as an active agent in chemical processing.
Alumina is made from bauxite, a naturally occurring ore containing variable
amounts of water-containing aluminum oxides. Free alumina occurs in nature as the
mineral corundum and its gemstones (sapphire and ruby) can be produced synthetically
from alumina. Alumina is produced by melting bauxite in an electric furnace or it is
extracted from bauxite through the Bayer process. In Bayer process, bauxite is crushed,
mixed in a solution of sodium hydroxide, and is seeded with the crystals to precipitate
aluminum hydroxide. The hydroxide is heated in a kiln in order to drive off the water
and produce several grades of alumina.
1.2 Corundum, a-Al2O3
The alumina heated above 1200oC was named alpha alumina in 1916 by Rankin
and Merwin [Rankin and Merwin (1916)]. Natural crystalline aluminum oxide or
alumina is named as corundum and its monocrystals are colorless and transparent. If
they contain, microcrystals of magnetite (Fe3O4) or hematite (Fe2O3), they are called
emery. If the corundum monocrystals contain some elements in isomorphic substitution
for Al3+ in their structure, gemstones are formed, with specific names for their colors:
(i) sapphire-blue (Fe, Ti); (ii) ruby-red (Cr); (iii) topaz-yellow (Fe2+, Fe3+); (iv)
emerald-green (Fe2+).
Corundum crystallizes in hexagonal-rhombohedral system, space group D63d.
This structure type often referred as “corundum structure” in crystallography. Fig. 1.1
-1-
gives the packing of Al and O in basal plane. The structure of corundum can be viewed
as hexagonal closest packing of oxygen ions forming layers parallel to the (0001) plane
[Wyckoff (1964)]. The ionic radius of O2- ions is 1.35 Å and interstices between the
oxygen layers accommodate the smaller, 0.54 Å radius, Al3+ ions; each one is
octahedrally coordinated by six oxygen ions. Only two-thirds of the octahedral
interstices are occupied by Al3+ ions to maintain charge balance. Thus the lattice
consists roughly of alternating layers of oxygen and aluminum ions. Three different
arrangements of the aluminum layer are possible depending upon the position of vacant
sites. Calling the oxygen layers A, B and aluminum layers a,b,c, complete stacking
sequence is A-a-B-b-A-c-B-a-A-b-B-c-A. It is only reproduced after the sixth oxygen
layer or after the sequence a-b-c is repeated twice.
Fig. 1.1: Basal Plane of a-Al2O3 showing hexagonal close packed anion sublattice
(large open circles) and cations occupying two thirds of octahedral interstices
(small solid circles); small open circles are empty octahedral interstices.
The unit cell of a-Al2O3 defined in this way is called crystallographic or
structural unit cell, in contrast to morphological unit cell, where cation sequence is
repeated only once and height is half that of structural cell. The structure of a-Al2O3
results in coordination number of 6 and 4 for cation and anion respectively. Lattice
parameters in hexagonal axes, ao and co as 4.7589 and 12.991 Å respectively, have been
reported by several authors [Kronberg (1957); Swanson et al. (1960); Phillips et al.
(1980)].
-2-
Corundum can be synthesized by thermal and hydrothermal methods. Aluminum
oxide is formed by thermal dehydration of aluminum hydroxides. The extent of
conversion to corundum structure depends on the temperature and time of thermal
treatment. Total conversion occurs on heating above 1500oC for more than one hour. As
can be seen, a-alumina is thermodynamically stable and possesses very good
mechanical properties. These mechanical properties are preserved also at higher
temperatures [Gitzen (1970)]. Some of its properties are summarized in Table 1.1.
These properties combined with its chemical inertness have made it one of the
technologically most important ceramic material.
Table 1.1: Selected properties of (bulk) a-alumina (at room temperature).
Property
Value
Density (g/cm3)
3.96-3.99
Melting point (°C)
~2050
Bulk modulus (GPa)
239
Elastic modulus (GPa)
409-441
Hardness (GPa)
28
Thermal conductivity (W/(m·K))
46
Relative dielectric constant
10.5
Band gap (eV)
8.8
1.3 Transition Aluminas
Besides the thermodynamically stable a-phase, numerous metastable phases
exist. The number of reported metastable alumina phases, also known as transition
aluminas, exceeds ten, although some of the phases reported in literature appear similar
behavior of melting to solidification of the atomic structure [Levin and Brandon
(1998)]. Five crystalline aluminum hydroxides: Gibbsite, Bayerite, Nordstrandite,
Diaspore and Boehmite have crystals varying from micro to millimetric size and
aluminas are formed by dehydroxylation of these hydroxides on heating between 300oC
-3-
and 600oC. These fine grained powders change their structure at 1100oC, but remain as
powders even after increasing temperature from 1100oC upto near melting point of aalumina (2050oC). The powder particles start to coalesce, sinter and recrystallize with
a-alumina structure and are called tabular crystals of aluminas.
Stumpf et al. showed that between temperatures of dehydroxilation of aluminum
hydroxides and alpha alumina first crystallization, a number of well characterized and
reproducible intermediate crystalline alumina structures are formed; each one at a given
temperature range, with just one exception which is “amorphous” [Stumpf et al.
(1950)].
The structure of each alumina and its temperature range of existence are
determined by structure of starting hydroxide which are different for gibbsite, bayerite,
nordstrandite, boehmite and diaspore. These seven aluminas are called “Transition
Aluminas” and received Greek letters to identify them as: Gamma (g) alumina, delta (d)
alumina, theta (q) alumina, kappa (k) alumina, chi (c) alumina, eta (h) alumina and rho
(r) alumina. Fig. 1.2 shows the most recent review of thermal transformation sequence:
Aluminum hydroxide
transition aluminas
a-Al2O3
Fig. 1.2: Thermal transformation sequence of aluminum hydroxides [Wefers and
Misra (1987)].
-4-
It may be concluded from Fig. 1.2 that all transition aluminas may be produced
from gibbsite and all of them are synthetic. a-Al2O3, in spite of its Greek letter, is not a
transition alumina but is the last crystalline material formed by the effect of increasing
temperatures on the transition aluminas. These transition aluminas are not considered as
true polymorphs of a-Al2O3. The sequence of transformation as shown above, is not
reversible that is neither a-Al2O3 nor any of high temperature alumina can be converted
to one of the transition alumina that occur at lower temperature, therefore according to
Wefers and Misra they may be classified as thermodynamically unstable although
reproducible, states of structural reordering of a-Al2O3 [Wefers and Misra (1987)].
Some authors prefer to call them “Metastable Alumina Polymorphs”. Table 1.2 (a) and
(b) shows X-ray powder diffraction data for all of the transition aluminas and a-Al2O3.
-5-
Table 1.2 (a): X-ray powder diffraction data for the transition aluminas (q, b, h-Al2O3).
d
5.73
5.46
4.53
3.55
2.86
2.83
2.82
2.73
2.73
2.57
2.45
2.31
2.26
2.26
2.25
2.03
2.02
1.95
1.91
1.91
1.91
1.87
1.82
1.81
1.80
1.78
1.78
1.73
1.68
1.65
1.65
1.62
1.57
1.54
1.51
1.49
1.47
1.46
1.46
1.43
1.43
1.41
1.41
1.41
1.41
1.41
1.39
1.39
1.39
q-phase (86-1410)
I
0.1
16.8
13.2
1.5
79.4
45.6
29.5
100
100
17
78.3
52.7
6.8
6.8
42.5
12.7
50.7
7.8
20.6
14.4
14.4
0.2
0.6
3.7
8.2
10.7
10.7
5.4
0.4
0.1
0.1
13.8
6.1
28.7
1.5
20.5
6.8
30.8
30.8
1.3
5.9
2.5
2.5
1.6
17.8
17.8
57.3
57.3
30.4
2q
15.47
16.23
19.61
25.07
31.23
31.63
31.72
32.81
32.81
34.93
36.71
38.91
39.82
39.82
40.05
44.67
44.83
46.49
47.62
47.70
47.70
48.76
50.12
50.31
50.71
51.45
51.45
52.78
54.56
55.64
55.64
56.76
58.74
59.95
61.44
62.32
63.06
63.98
63.98
65.14
65.42
66.06
66.07
66.27
66.53
66.53
67.47
67.47
67.62
b-phase (26-0031)
d
I
4.23
7
3.28
25
2.55
35
2.40
12
2.36
30
2.26
6
2.12
100
1.88
60
1.68
8
1.64
30
1.55
14
1.51
7
1.44
75
1.39
70
1.27
11
-6-
2q
20.98
27.17
35.22
37.46
38.10
39.95
42.65
48.49
54.48
55.92
59.47
61.48
64.58
67.53
74.40
h-phase (77-0396)
d
I
4.56
9.6
2.80
10.7
2.38
47.8
2.28
13.2
1.98
100
1.81
0.4
1.61
2.1
1.52
7
1.40
94.7
1.34
0.3
2q
19.43
31.99
37.71
39.45
45.87
50.26
57.02
60.83
66.89
70.40
Table 1.2 (b): X-ray powder diffraction data for the transition aluminas (k, g, d-Al2O3)
and a-Al2O3. 2q values are for l=1.54056 Ǻ.
a-phase (10-0173)
d
I
2q
3.48
75
25.58
2.55
90
35.14
2.38
40
37.78
2.09
100
43.36
1.74
45
52.55
1.60
80
57.52
1.55
4
59.77
1.51
6
61.16
1.51
8
61.34
1.40
30
66.55
1.37
50
68.20
1.28
4
74.27
1.24
16
76.88
1.23
8
77.23
1.19
8
80.69
k-phase (52-0803)
d
I
2q
6.09
12
14.53
4.47
11
19.86
4.18
3
21.24
3.79
3
23.48
3.15
4
28.30
3.05
29
29.23
3.04
33
29.33
2.97
3
30.04
2.80
75
31.90
2.58
100
34.81
2.43
14
37.03
2.42
13
37.16
2.34
15
38.55
2.32
23
38.75
2.32
23
38.75
2.17
12
41.69
2.13
27
42.48
2.12
68
42.68
2.08
5
43.51
2.06
13
43.92
1.91
3
47.60
1.89
6
48.26
1.88
14
48.45
1.87
25
48.62
1.87
17
48.73
1.83
8
49.75
1.82
5
49.99
1.82
5
50.02
1.76
4
52.04
1.75
5
52.31
1.71
3
53.52
1.64
8
55.89
1.64
18
55.99
1.64
24
56.15
1.64
19
56.24
1.49
5
62.19
1.49
7
62.29
1.49
5
62.44
1.45
15
64.09
1.45
15
64.11
1.44
30
64.81
1.44
62
64.96
1.39
31
67.12
1.39
31
67.15
1.39
18
67.29
1.39
12
67.56
1.35
4
69.78
1.34
14
70.06
d
4.56
2.79
2.38
2.28
1.98
1.52
1.40
1.14
1.03
0.99
0.88
-7-
g-phase (10-0425)
I
2q
40
19.47
20
31.96
80
37.64
50
39.52
100
45.90
30
60.95
100
67.09
20
85.10
10
97.29
10
102.43
10
121.40
d-phase (46-1131)
d
I
2q
5.47
3
16.21
5.06
2
17.51
4.55
15
19.51
4.07
0
21.82
3.61
0
24.66
3.23
2
27.62
3.05
6
29.28
2.88
29
31.04
2.81
32
31.82
2.79
3071
32.12
2.73
22
32.83
2.59
37
34.63
2.49
37
36.03
2.44
56
36.79
2.41
43
37.33
2.39
38
37.67
2.37
34
37.91
2.32
37
38.89
2.28
50
39.53
2.16
3
41.81
2.01
45
45.01
2.01
60
45.19
1.99
70
45.68
1.97
30
46.10
1.95
36
46.64
1.93
0
47.09
1.91
16
47.52
1.82
2
50.18
1.81
4
50.53
1.79
4
51.14
1.77
4
51.60
1.77
4
51.60
1.63
3
56.50
1.61
4
57.21
1.54
19
60.15
1.52
16
61.07
1.52
16
61.07
1.46
17
63.94
1.45
16
64.46
1.40
56
66.84
1.40
90
67.04
1.40
90
67.04
1.39
100
67.30
1.29
4
73.27
1.28
4
73.81
1.26
5
75.51
1.26
5
75.51
1.23
2
77.87
1.22
0
78.59
1.4 Alumina Coatings
Thin film applications in optics and electronics have made extraordinary rapid
progress in recent years. Consequently, the development of deposition techniques for
the preparation of thin films with reproducible and well defined properties plays an
important role in technological applications.
Owing to many applications of the material, much research has been done on
alumina coatings. Despite this, many questions still remain to be answered concerning
the control and understanding of crystalline alumina thin film formation and properties.
One of the main reasons for these complexities is the existence of numerous different
crystallographic phases [Levin and Brandon (1998)].
Alumina is technologically an outstanding ceramic, which is used as a high
temperature, corrosion resistant refractory material. Due to its high hot hardness,
chemical durability and oxidation resistant properties, there has been an enormous
interest in the development of crystalline alumina films for their application as wear
resistant protective coatings on high speed machine tools for milling and cutting of cast
iron and low carbon steel. Alumina coatings are also of interest in microelectronic
industry as an insulating material due to their excellent electrical properties like large
band gap and high dielectric constant.
1.4.1 Properties
Some of the main properties of alumina on which numerous applications of alumina
coatings are based upon:
·
High compression strength
·
High hardness
·
Resistant to abrasion
·
Resistant to chemical attack by a wide range of chemicals even at elevated
temperatures
·
High thermal conductivity
·
Resistant to thermal shock
-8-
·
High degree of refractoriness
·
High dielectric strength
·
High electrical resistivity even at elevated temperatures
·
Transparent to microwave radio frequencies
·
Used as a dehydration catalyst
1.4.2 Applications of alumina coatings
a) Antireflective coatings: Pseudo-boehmite nanocrystals precipitated Al2O3 gel films
(flowerlike alumina) prepared on PMMA (poly(methyl methacrylate)) substrates
through the immersion of films in hot water, can be used as an antireflective coatings
because the reflectance of PMMA substrates coated with flowerlike Al2O3 films is less
than 0.8% in visible light region. Therefore these flowerlike alumina films present
excellent potential for practical applications to polymer optical components [Tadanaga
et al. (2008)].
b) Catalysis sensors: Laser irradiated sintered Al2O3 induces extremely fast
melting/quenching processes resulting in the evolution of g-alumina [Laude et al.
(1995)]. This g-Al2O3 is responsible for catalytic activity of alumina ceramic towards
electroless metal plating. Therefore electroless Ni plating of an excimer laser processed
Al2O3 thin films can be used as an alternative method for design of catalysts [Starbova
et al. (2005)].
c) Wave guide sensors: Optical waveguide sensors made by PAA (Porous Anodic
Alumina)/Al multilayers could detect a small change in the refractive index of a
solution and it provides high resolution than SPR (Surface Plasmon Resonance) sensors
[Yamaguchi et al. (2009)].
d) Buffer layer for superconductors: Superconductors can be fabricated successfully by
incorporating Al2O3 as a buffer layer. For example: PbTiO3 thin films could be
fabricated using Al2O3 as a buffer layer between Si substrate and Pt electrode. This
buffer layer effectively suppressed the interdiffusion of Pb and Si through Pt electrode.
-9-
The dielectric property of PbTiO3 film with Al2O3 buffer layer can be improved
compared with that of film without buffer layer [Kim et al. (1997)].
e) Protective coating on cutting tools: Crystalline alumina coatings of thickness in the
range of 5-10 µm are applied on cemented carbide substrates in conjuction with other
hard coating materials like TiN, TiCN and TiC to improve the service line of cutting
tools [Bunshah (2001); Kumar et al. (2007)].
f) Used as a template: Alumina can be used as templates for the synthesis of platinum
Y-junction nanostructures [Mahima et al. (2008)]. Recently much effort has been
directed towards the fabrication of shape selective nanostructures because of their size
and shape dependent properties. These nanostructures are generally synthesized in
many forms, including highly monodispersed spherical nanoparticles to several
anisotropic nanostructures as wires/rods, tubes etc. The template assisted route has been
one of the widely investigated routes to develop these nanostructures. Alumina, due to
its porous nature have been extensively used because of many desirable characteristics
including tunable pore dimensions and lengths, good mechanical and thermal stability
and well developed fabrication methods.
g) Applications in nanoscience: Alumina nanorods, nanowires and nanotubes have great
potential for nanodevice applications in nanoelectronics, photonics, data recording
media, gas sensing and gas storage. Alumina nanorods have excellent high temperature
stability and can be extensively used for catalytic supports in gas-gas separation and
petrochemical processing, especially during re-forming and isomerisation [Kuiry et al.
(2005)].
h) Applications in microelectronics: Alumina is used in microelectronics applications
because it combined economic, physical and electrical advantages. It offers
compatibility with thick film resistors and dielectrics and excellent adhesion with thick
film conductors.
i) Application in electronics: The material called solution-deposited beta alumina has
important applications in transistor technology and devices such as electronic books.
This form of sodium beta alumina has some very useful characteristics. The material is
produced in a liquid state, which means it can be deposited onto a surface in a precise
- 10 -
pattern for the formation of printed circuits. But when it is heated it forms a solid thin
transparent film. In addition, it allows us to operate at low voltages, meaning it requires
less power to induce useful current. Transparency and thinness of material make it ideal
for use in increasingly popular e-books, which rely on see-through screens and portable
power sources [Kuiry et al. (2005)].
j) Application in biology: Alumina is used as an orthopaedic biomaterial. Other
applications for alumina encompass porous coatings for femoral stems, knee prosthesis
and polycrystalline and single crystal forms in dental applications as tooth implants. It is
used as an implant material in knee and hip replacement.
1. 5 Deposition of Alumina Coatings
1.5.1 Chemical Vapor Deposition
Chemical vapor deposition is a process of chemically reacting a volatile
compound of a material to be deposited, with other gases, to produce a non volatile
solid that deposits atomistically on suitably placed substrates. CVD do not require
vacuum. Many variants of CVD processing have been developed including APCVD
(Atmospheric Pressure Chemical Vapor Deposition), LPCVD (Low Pressure Chemical
Vapor Deposition), PECVD (Plasma Enhanced Chemical Vapor Deposition), LECVD
(Laser Enhanced Chemical Vapor Deposition) and MOCVD (Metal Organic Chemical
Vapor Deposition). Various reaction types used in CVD environment include pyrolysis,
reduction, oxidation and disproportionation reactions etc.
The gas phase hydrolysis of volatile halides, principally chlorides, in a mixture
of hydrogen and carbon dioxide gases, is one of the most useful methods, and
thermodynamic assessment of the reaction indicates that following oxides are among
those that can be prepared in this way: BeO, MgO, B2O3, Al2O3, Ge2O3, In2O3, SiO2,
TiO2, ZnO2, HfO2 and Ta2O5 [Maxwell and Rabouin (1965)]. Out of these substances,
alumina films have been the subject of extensive investigation in recent years, because
bulk properties are superior to those of silicon dioxide films in some respects [Salama
(1970)]. The general chemical reaction to produce alumina by CVD is as follows:
2AlCl3(g) + 3H2(g) +3CO2(g)
Al2O3(s) + 3CO(g) + 6HCl(g)
- 11 -
....(1.1)
Since 1970s, alumina has frequently been used in CVD wear resistant coatings
on cemented carbide cutting tool inserts. Two alumina polymorphs are generally used in
this application: metastable k-Al2O3 and stable a-Al2O3 [Lux et al. (1986); Ruppi
(2005)]. CVD k-Al2O3 has a smaller grain size and lower pore density compared with
a-Al2O3 [Park et al. (1983); Halvarsson and Vuorinen (1995)] and this is considered to
be favourable in cutting applications. However at temperature above 1000oC, often
reached in modern metal cutting, a phase transformation from k-Al2O3 to a-Al2O3 may
occur, and as this transformations associated with a volume decrease of about 8%,
adhesion and cohesion of transformed a-Al2O3 coatings may be degraded [Vuorinen
and Karlsson (1992)]. One important factor influencing which phase that forms has
been shown to be the nucleation surface on which alumina coatings are deposited
[Halvarsson and Vuorinen (1995)]. It has been suggested that by carefully controlling
nucleation stage of growth and promoting a-phase nucleation, higher quality coatings
have been obtained [Ruppi (2005)]. Further g-alumina can also be formed during CVD
processes, even though it is seldom beneficial considering the application point of view.
It has been shown that formation of this phase can be promoted by introducing H2S as a
catalyst and/or dopant during depositions [Ruppi and Larsson (2001); Larsson and
Ruppi (2001)]. Main drawback of CVD process is that it requires high deposition
temperatures (around 1000oC) [Ruppi (2005); Halvarsson et al. (2006)] that limit the
choice of substrate material to those that withstand high temperature such as cemented
carbides. For alumina deposition at lower temperatures, several research groups studied
alumina growth using Plasma Enhanced/Assisted Chemical Vapor Deposition
(PECVD/PACVD) [Kyrylov et al. (2005); Laimer et al. (2005)]. Ruppi and Larsson
deposited alumina films using CVD at a temperature range of 800-1000oC and found
that when properly nucleated, k-Al2O3 could be CVD deposited to a considerable
thickness under a wide range of experimental variables. CVD k-Al2O3 appeared to be
relatively stable and did not transform to a-Al2O3 during the long heat treatment during
deposition [Ruppi and Larsson (2001)]. Fig. 1.3 shows the XRD patterns of k-phase of
alumina along with peaks originating from underlying coating and substrate.
- 12 -
Fig. 1.3: XRD pattern showing the major k-Al2O3 peaks and the peaks originating from
underlying Ti(C,N) coating and substrate (WC,g) [Ruppi and Larsson
(2001)].
Later Ruppi found that nucleation control is of crucial importance for getting aalumina coatings. As a result of optimised nucleation CVD a-alumina layers were
composed of small defect free grains without any porosity [Ruppi (2005)]. Ruppi found
that prior to deposition of Al2O3, some nucleation steps control the alumina phase. Fig.
1.4 below shows the SEM micrograph of CVD deposited alumina which is composed of
multilayered Al2O3 with four alternating layers of k-Al2O3 and a-Al2O3 respectively
[Ruppi (2005)].
Fig. 1.4: SEM micrograph of CVD deposited alumina on Ti [Ruppi (2005)].
- 13 -
1.5.2 Physical Vapor Deposition
Physical Vapor Deposition (PVD) is a general term used to describe a method to
deposit coatings by condensation of vaporized form of desired film material onto
various surfaces. This coating method involves purely physical processes such as high
temperature vacuum evaporation with subsequent condensation or plasma sputter
bombardment rather than involving a chemical reaction at surface to be coated as in
chemical vapor deposition. Variants of PVD include cathodic vapor deposition, e-beam
PVD, evaporative deposition, pulsed laser deposition and sputter deposition.
Alumina coatings have been deposited by several physical vapor deposition
techniques such as e-beam evaporation [Shamala et al. (2004)], pulsed reactive
sputtering [Kohara et al. (2004)], AC magnetron sputtering [Khanna et al. (2006)] and
pulsed laser deposition [Hirschauer et al. (1997)].
1.5.2.1 Electron beam evaporation
Among the variety of vacuum deposition techniques, e-beam evaporation is
known to result in good quality amorphous Al2O3 films [Yu et al. (1995)]. Electron
beam evaporator can act as a possible candidate for low cost manufacturing of alumina
films in contrast to more expensive methods like Atomic Layer Deposition (ALD),
CVD, sputtering etc. In addition, e-beam evaporation is a powerful technique to prepare
well crystallized oxide films with higher deposition rate and ease of control from wide
range of evaporation [Huang et al. (2006)].
In e-beam evaporation, the evaporant charge (alumina, in this case) is placed
inside a water cooled crucible or in depression of water cooled copper hearth. The
purity of the evaporant is assured because only a small amount of charge melts or
sublime so that the effective crucible is unmelted material next to cooled hearth.
Therefore no contamination of evaporant by Cu occurs. In common configuration of
gun source, electrons are thermionically emitted from heated cathode filament that is
shielded from direct line of sight of both evaporant charge and substrate. In this way
contamination from heated tungsten filament is eliminated. The cathode filament is
negatively biased w.r.t anode from 4 to 20 kilovolts which serves to accelerate
electrons. In addition, a transverse magnetic field is applied that serves to deflect the
- 14 -
electron beam in a 270o circular arc and focus it on hearth and evaporant charge at
ground potential. At high evaporation rates, vapor just above the hearth provides a high
pressure viscous cloud of very hot evaporant. The energy transfer between vapor atoms
and electrons occur in this region of viscous cloud. The region beyond this dense cloud
is at much lower pressure and so molecular flow occurs. Therefore instead of evaporant
particles being ejected from various points on flat surface, they appear to originate from
the perimeter of viscous cloud. Now the virtual source plane has moved away from melt
surface towards the substrates. Fig. 1.5 shows the schematic diagram of e-beam
evaporator.
Fig. 1.5: Schematic of electron beam evaporator.
Different researchers worked with e-beam evaporation and still no results of
crystalline alumina coatings deposited at room temperature have been found in
literature. Huang et al. deposited Al2O3-SiO2 thin films by e-beam evaporation in
oxygen pressure and found that in as-deposited amorphous films, even after annealing
to 1200oC, only a reflection of SiO2 phase appears (Fig. 1.6) [Huang et al. (2006)].
- 15 -
Fig. 1.6: XRD patterns of Al2O3-SiO2 films annealed at (a) 1000oC (b) 1200oC (c)
1400oC [Huang et al. (2006)].
Earlier work done by Shamala et al. on e-beam evaporated films did not help to
get crystalline alumina films even upto annealing temperature of 600oC [Shamala et al.
(2004)]. TEM showed improvement in the crystal structure for the evaporated films
annealed in air at 800oC for 12 h [Mansour et al. (1994)]. It was observed from
literature that polycrystalline Al2O3 thin films have been grown on Si substrates by
ionized beam deposition using Al solid surface in O2 at 700oC [Whangbo et al. (2001)].
Zywitzki and Hoetsch also found large crystallites of g-Al2O3 in amorphous matrix after
annealing the as-deposited films at 800oC that transformed to q-Al2O3 at 1000oC. Only
after annealing to 1200oC, there is nearly a complete phase transformation to a-Al2O3.
They also found that plasma activation allows the deposition of layers of aluminum
oxide at high deposition rate and lower substrate temperature [Zywitzki and Hoetsch
(1996)].
Also electron beam evaporated alumina films deposited at 65oC by Shamala et
al. reported to exhibit good optical properties but showed poor dielectric characteristics.
The refractive index of evaporated Al2O3 film is reported to be 1.71 and 1.60 at 300 nm
- 16 -
and 500 nm. Fig. 1.7 shows the refractive index versus wavelength for electron beam
evaporated film deposited at 65oC [Shamala et al. (2004)].
Fig. 1.7: Refractive index vs wavelength for electron beam evaporated film deposited at
65oC [Shamala et al. (2004)].
1.5.2.2 RF magnetron Sputtering
Sputtering is a thin film deposition process which is driven by momentum
exchange between ions and atoms in the material due to collision. The incident ion set
off collision cascades in the target, when such cascades recoil and reach target surface
with an energy above the binding energy of surface, an atom can be ejected or sputter
out. If the target is thin, collision cascades can reach backside of the target and atoms
can escape surface binding energy. The average number of atoms ejected from target
per incident ion is called sputter yield which depends on ion incident angle, energy of
ion, masses of ion and target atoms and surface binding energy of atoms in target.
Primary particles for sputtering process can be supplied in number of ways, for e.g. by
plasma ion source, an accelerator or by radioactive material emitting a-particle.
Sputtering system (Fig. 1.8) consists of a pair of parallel metal electrodes, one of
which is cathode or target of metal to be deposited. It is connected to negative terminal
of DC power supply and several kilovolts are applied to it. Facing the cathode is a
substrate or anode, which may be grounded, biased positively or negatively, heated or
cooled. After evacuation of chamber, a working gas typically argon is introduced that
act as a medium in which electrical discharge is initiated and sustained. Gas pressure
- 17 -
usually ranges from a few to hundred millions. After a visible glow, discharge is
maintained between electrodes, it is observed that current flows and a metal from
cathode deposits on substrates. Microscopically, positive gas ions in discharge strikes
cathode and physically eject or sputter target atoms through momentum transfer to
them. These atoms enter and pass through discharge region to deposit on growing films.
In addition, other particles as well as radiations are emitted from target. The electric
field accelerate electrons and negatively charged ions towards anode structure where
they impinge on growing films.
Fig. 1.8: Schematic representation of the sputtering chamber geometry and main
components: 1-Water cooling; 2-Heating resistors; 3-Substrates; 4-Target;
5-Permanent magnets; 6-Shield; 7-Insulator; 8-RF cable; 9-Thermocouple;
10-Gas inlet; 11-Pumping system.
RF sputtering was invented as a means of depositing insulating thin films e.g.
SiO2, Al2O3 etc. In this case, potential on target is periodically reversed. At frequencies
below about 50 KHz, ions have enough mobility to establish a complete discharge at
each electrode on each half cycle. DC sputtering conditions essentially prevails at both
electrodes that alternately behave as cathode and anode. Above 50 KHz, two important
effects occur. Electrons oscillating in glow region acquire enough energy to cause
- 18 -
ionizing collisions, reducing the need for secondary electrons to sustain discharge.
Secondly RF voltages can be coupled through any kind of impedance so that electrodes
need not be conductors. This makes it possible to sputter any material irrespective of its
resistivity. Typical RF frequencies employed range from 5 to 30 MHz. 13.56 MHz is
widely used for plasma processing. As a pulsating RF signal is applied to the target, a
large initial current is drawn during the positive half cycle. Only a small ion current
flows during second half cycle. This would enable a net current averaged over a
complete cycle to be different from zero but this cannot happen because no charge can
be transferred through capacitor. Therefore operating point on the characteristics shifts
to a negative voltage-target bias and no net current flows. Since AC electricity is
involved, both electrodes should sputter which presents a potential problem because
resultant film may be contaminated. While sputtering from only one of the electrode,
sputter target must be an insulator and can be capacitively coupled to an RF generator.
This equivalent circuit of sputtering system can be thought of as two series capacitorsone at target and other at substrate with applied voltage divided between them. Since
capacitive reactance is inversely proportional to the capacitance or area, more voltage
will be dropped across the capacitor with smaller surface area, therefore for efficient
sputtering area of the target, electrode should be small compared with total area of other
electrode.
In magnetron sputtering, a magnetic field is used to trap secondary electrons
close to target. The electrons follow helical path around magnetic field lines undergoing
more ionising collisions with neutral gaseous atoms near the target. This enhances the
ionization of plasma near the target leading to high sputter rate. It also means that
plasma can be sustained at lower pressure. The sputtered atoms are neutrally charged
and therefore unaffected by magnetic trap.
Andersson et al. deposited a-Al2O3 coatings using RF sputtering technique at a
substrate temperature of 280-560oC on chromia template layers. Fig. 1.9 shows GIXRD
diffractograms for two samples, one deposited at substrate temperature of 280oC for
both a-Al2O3 and Cr2O3 (sample I in Fig. 1.9) and other sample deposited at substrate
- 19 -
temperature of 280oC for Cr2O3 but 430oC for Al2O3 (sample II in Fig. 1.9) and he
found crystalline alumina at a substrate temperature of 430oC [Andersson et al. (2004)].
Fig. 1.9: GIXRD pattern of sample I (below) and sample II (above). The bars show the
reference peak positions for chromia and alpha alumina. ‘S’ denotes the
substrate peak [Andersson et al. (2004)].
1.5.2.3 Reactive sputtering
In reactive sputtering, thin films of compounds are deposited on substrates by
sputtering from metallic target in presence of a reactive gas, usually mixed with inert
working gas (usually argon). The most commonly reactively sputtered compounds are:
Oxides (Al2O3, In2O3, SiO2); Nitrides (AlN, TiN, TaN); Carbides (TiC, WC, SiC);
Sulfides (CdS, CuS, ZnS). Irrespective of the material considered, during reactive
sputtering, resulting film is either a solid sol alloy of target metal doped with reactive
element; a compound or a mixture of two. In a perfect world reactive gas would react
only with growing film to make a thin film of required compound. But in practise, the
reactions occur with sputter target surface as well. This process is termed as ‘target
poisoning’ and it complicates reactive sputtering and reduces thin film growth rate.
Hence a too low reactive gas flow will lead to formation of under stoichiometric films,
whereas a too high gas flow will result in target poisoning causing reduction in
deposition rate. Fig. 1.10 shows a typical deposition rate versus reactive gas flow graph.
- 20 -
Fig. 1.10: Hysteresis behaviour of deposition rate as variation of reactive gas flow.
Two key points in reactive sputtering are:
a)
Part of reactive gas is going down the pumps, rest is going into growing thin
films
b)
Compounds tend to sputter more slowly than metals
These results in positive feedback loop, where poisoning of sputter target
reduces thin film growth rate, so uses less reactive gas, and so leads to more target
poisoning. This positive feedback is seen as ‘target hysteresis’- sudden jumps in thin
film deposition rate, target voltage or oxygen pressure. The reactive sputtering process
has been modelled by Berg et al. [Berg et al. (1987); Berg and Nyberg (2005)]. This
model is based on set of balance equations, describing the flux of metal and reactive gas
between target, substrates and pumps [Berg et al. (1987)].
It was found by Khanna et al. that CrOx template layer improved the
crystallinity of alumina coatings grown on it and also facilitated the formation of
thermodynamically stable a-alumina phase [Khanna et al. (2006)]. Eklund et al. also
reported the same results that growth of a-Al2O3 at a substrate temperature of 450oC is
obtained using a [1014]-textured a-Cr2O3 template layer, while only limited a-alumina
nucleation is seen on [0001]-textured a-Cr2O3 template. Fig. 1.11 shows the crossection
- 21 -
SEM and TEM image of a-Al2O3 deposited on [1014] textured a-Cr2O3 template. It can
be seen both in SEM and TEM that columnar structure of a-Cr2O3 continues into Al2O3
layer [Eklund et al. (2008)].
Fig.1.11: (a) Cross-section SEM micrograph of a-Al2O3 deposited onto [1014]
textured a-Cr2O3 template. (b) Cross-section TEM micrograph of same
sample [Eklund et al. (2008)].
Recently Edlmayr et al. studied thermal stability of reactively sputtered Al2O3
coatings by bombarding with ions at a substrate temperature of 640oC and they found
that coatings grown on Si substrate is either X-ray amorphous for low ion bombardment
or g-Al2O3 for enhanced ion bombardment. They also studied the influence of annealing
temperature on the films. The GIXRD patterns of sample deposited at low ion
bombardment in as-deposited state and after different annealing treatments are shown in
Fig. 1.12. They found the deposition of a-Al2O3 at enhanced ion bombardment and at
high annealing temperature (Fig. 1.13) [Edlmayr et al. (2010)].
- 22 -
Fig. 1.12: XRD pattern of sample deposited at low ion bombardment in as-deposited
state and at high annealing temperatures [Edlmayr et al. (2010)].
Fig. 1.13: XRD pattern of sample deposited at enhanced ion bombardment in asdeposited state and at high annealing temperatures [Edlmayr et al. (2010)].
- 23 -
Schneider et al. deposited crystalline alumina films (q and k-Al2O3) at substrate
temperature of 370-430oC using ionized magnetron sputtering. Fig. 1.14 shows the
schematic of ionized magnetron sputtering unit. It was found that both energy and flux
of ions influence the structure and properties of coatings and crystallinity increased with
increase in ion flux to substrate (Fig. 1.15 and Fig. 1.16) [Schneider et al. (1997)].
Fig. 1.14: Schematic drawing of ionized magnetron sputtering unit [Schneider et al.
(1997)].
It can be seen in Fig. 1.15 and Fig. 1.16 that how crystallinity increases as the
ion current density rises from 3.6 mA cm-2 to 4.9 mA cm-2.
- 24 -
Fig. 1.15: Bragg-Brentano diffractogram of k-Al2O3 deposited at an ion current density
of 3.6 mA cm-2 [Schneider et al. (1997)].
Fig. 1.16: Bragg-Brentano diffractogram of k and q-Al2O3 deposited at an ion current
density of 4.9 mA cm-2 [Schneider et al. (1997)].
- 25 -
Recently Musil et al. studied the thermal stability of alumina coatings deposited
by reactive magnetron sputtering technique. It was found that nanocrystalline g-Al2O3
phase in the film is thermally stable up to 1000oC even after 5 h of annealing. Only after
annealing the film at 1100oC or more, a-Al2O3 phase dominates for the films with
sufficient thickness. Fig. 1.17 shows GIXRD pattern of 1200 nm thick alumina film
[Musil et al. (2010)].
Fig. 1.17: GIXRD pattern of ~1200 nm thick alumina films as a function of annealing
temperature at constant annealing time of 5 h. Peaks denoted as ‘s’ and ’h’
are reflections from substrate and substrate holder [Musil et al. (2010)].
- 26 -
1.5.2.4 Pulsed Laser Deposition (PLD)
The widespread use of laser ablation for thin film deposition, also referred as
pulsed laser deposition, has experienced an enormous growth in 90’s. Films of materials
for which more standard techniques have shown limited success have successfully been
produced by PLD and some optical devices based on heterostructures produced by PLD
have been reported in literature [Wu et al. (1998); Koinuma et al. (2000)].
Pulsed laser deposition is a deposition technique limited to investigation of
small area films. It has a capability of producing stoichiometric multicomponent films
and ability to deposit ceramic films. In its configuration, a high power laser situated
outside the vacuum deposition chamber is focussed by means of external lenses on the
target surface which acts as evaporation source. Lasers that have widely used for PLD
are Nd:YAG and gas excimer types. In case of Nd:YAG, that can deliver upto 2J/pulse
at a pulse rate of ~ 30Hz, 1064 nm radiation is frequency doubled so that output of 355
and 266 nm are produced. These are sufficiently intense for PLD work. Included among
popular gas excimer lasers are ArF (193 nm), KrF (248 nm) and XeCl (308 nm) types.
These deliver output of 500 mJ/pulse at a pulse repetition rate of several hundred hertz.
Irrespective of laser used, absorbed beam energy is converted to thermal, chemical and
mechanical energy, causing electron excitation of target atoms and plasma formation.
Evaporants form a plume above the target having collection of energetic atoms,
molecules, ions, electrons and droplets. The plume is highly directional and its contents
are propelled to substrate where they condense and form a film. Gases like O2 and N2
are often introduced inside the deposition chamber to promote surface reactions. Fig.
1.18 shows the basic set up for the conventional PLD configuration. For deposition of
layered film structure, multiple sources must be vaporized by laser beam. This can be
achieved using a single laser and beam splitters, two or more lasers emitting
simultaneously but independently or a single laser sequentially focussed on different
targets mounting on rotating wheel.
The species involved in PLD are the ones having highest kinetic energy (~10100 eV) in comparison with standard PVD processes like evaporation and sputtering
(~1-10 eV). This feature is essential for achieving films with very high density and good
- 27 -
adherence to substrates, both features being required for high performance coatings
[Voevodin et al. (1997); Lackner et al. (2004)].
Fig. 1.18: Schematics of standard PLD experimental setup.
However PLD is a pulsed process since most deposition takes place over several
tens of microseconds after each nanosecond laser pulse. This leads to an
“instantaneous” high deposition rate per pulse. This high deposition rate or arrival flux
of species to substrates is responsible for favoring production of metastable phases or
materials [Krebs et al. (1996); Gonzalo et al. (2003)].
One of the major drawback of PLD is splashing of macroscopic particles or
particulate deposition leading to rough and thus low quality films [Chen (1994);
Willmott and Huber (2000)]. Two main cases for particle formation during laser
deposition are breakway of surface defects under thermal shock and splashing of liquid
material due to superheating of subsurface layers. The method to reduce splashing
consists of using a mechanical particle filter that consists of a velocity selector that acts
as a high velocity pass filter to remove slow moving particulates. The second method is
to use a target of high density and smooth surface and effective improvement is to
polish the target surface before each run. The third method is to use relatively lower
energy density or lower deposition rates. Another drawback of PLD is lack of
- 28 -
uniformity over larger area, due to narrow angular distribution of plume that comes out
from target surface. This problem can be solved by rastering laser onto substrate by
rotation and translation in large area scale up.
Many process parameters such as substrate temperature, target to substrate
distance, laser fluence etc. contribute in getting PLD-grown alumina. Although the
underlying physical mechanisms are not yet fully understood, combined effects of such
parameters along with background gas and other parameters are known to have strong
influence on the final properties and structure of coatings [Chrisey and Hubler (1994)].
Hirschauer et al. deposited highly stoichiometric a-alumina films at a substrate
temperature of 850oC and 3 J cm-2 pulse energy using a high density Al2O3 target. It
was found that further annealing at 1000oC in air for 26 h slightly improved out of plane
orientation [Hirschauer et al. (1997)] (Fig. 1.19). Cibert et al. reported amorphous
alumina films at room temperature whereas g-type nanocrystallized structures are
pointed out for 800oC (Fig. 1.20) [Cibert et al. (2008)].
Fig. 1.19: X-ray diffractograms of a-alumina films grown at 850oC (magnified by
10000) and the same film after annealing in air at 1000oC for 26 h
[Hirschauer et al. (1997)].
- 29 -
The investigations done by Gottmann and Kreutz concentrated on the influence
of oxygen pressure, target to substrate distance and laser fluence on the refractive index
of the films, that is correlated with film density. They found that the compaction of the
films is achieved by particles impinging with kinetic energy above 30 eV on growing
films [Gottmann and Kreutz (1999)].
Gonzalo et al. found that films grown at high substrate temperature in low
oxygen exhibited higher degree of crystallinity and laser fluence have little effect
[Gonzalo et al. (1998)].
Fig. 1.20: GIXRD pattern of g-alumina thin films deposited by PLD at a substrate
temperature of 800oC [Cibert et al. (2008)].
1.6 Challenges in Deposition of Crystalline Alumina Coatings
Crystalline alumina is a highly insulating, optically transparent, chemically very
stable and hard material with numerous applications in many areas, from metallurgy to
microelectronics. The combination of excellent oxidation resistance and high hardness
(~30 GPa) has made it an interesting material for wear resistant coating applications
[Schneider et al. (1997); Li et al. (2000); Jin et al. (2002)], as well as for preventing
interlayer diffusion in thermal barrier coatings [Muller et al. (1999)]. The a-phase of
alumina is thermodynamically stable, but there are also a large number of metastable
- 30 -
phases. These are involved in a phase transition sequence, covering temperature range
of 300-1100oC and irreversibly ending at a-phase [Gitzen (1970); Levin and Brandon
(1998)]. Since a functional coating cannot be allowed to transform, it is necessary to
have a phase that is stable at the operation temperature of coating. For wear resistant or
diffusion barrier alumina coatings, this limits the group of unstable phases to stable aphase and in some cases, metastable k-phase which has a high transformation
temperature (~1050oC) [Gitzen (1970)].
Applying crystalline aluminum oxide can add additional physical properties to a
system. In particular, transformation of metastable phases from aluminum hydroxides
require high temperature (>800oC) [Levin and Brandon (1998)]. Post deposition thermal
annealing is a conventional method by heating both the film and substrate at elevated
temperatures, in order to transform an amorphous structure to a crystalline phase.
During heating of amorphous films, crack formation often occur for cases with
differential sintering leading to strain incompatibilities [Jagota and Hui (1991); Bordia
and Jagota (1993)] and cracks develop from intrinsic defects. For crystallization of
amorphous alumina films, post deposition annealing is usually required in the
transformation sequence (amorphous
g-Al2O3
a-Al2O3) in which crystallization of
g-Al2O3 is often observed at 300-900oC [Dragoo and diamond (1967); Taschner et al.
(1999)] and above 650-1000oC, crystallization of a-Al2O3 is achieved [Thornton and
Chin (1977)]. Post deposition annealing is accomplished by a change in ordering
structure and a transition from amorphous to crystalline metastable phases occurs.
Several authors reported crystallization of amorphous alumina films at a temperature
above 600oC [Eklund et al. (2009); Edlmayr et al. (2010); Musil et al. (2010)].
Another alternative method besides post deposition annealing to get crystalline
alumina films is to use high substrate temperature. The exploitation of hard, dense
coatings remain limited by the high substrate temperatures needed to produce them
[Andersson et al. (2004); Natali et al. (2005); Ruppi (2005)]. Pulsed laser deposition is
one technique that permits one to deposit compact films at lower substrate temperature
[Chrisey and Hubler (1994); Ruppi (2005)]. Fig. 1.21 shows the technologies available
and substrate temperatures required for the formation of a and k-alumina. For the
- 31 -
formation of a-alumina by sol-gel method, substrate temperature, Ts is between 1000oC
and 1300oC [Dynys et al. (1984)] and industrial CVD process for a-alumina requires a
substrate temperature of ~1000oC [Prengel et al. (1994)]. a-alumina was reported by
pulsed dc magnetron sputtering at Ts > 760oC [Zywitzki and Hoetsch (1996)]. At these
rather high substrate temperatures, choice of substrate material is limited to rather
expensive sintered substrates or Ni based alloys. Typical cutting tool and bearing steels
would change their metallurgical properties due to carbide growth and phase
transformation at Ts > 500oC [AMS handbook (1990)]. Replacing sintered substrates
such as cemented carbide with steel substrates would be economically beneficial.
Fig. 1.21: Schematic of substrate temperature versus deposition technology.
The solid state crystallization induced by ion beam irradiation has attracted
much attention because it can proceed at lower temperatures than that required for
thermal annealing [Nakata (1991); Nakao et al. (1996)]. This technique is considered to
be applicable for the crystallization of amorphous compound materials, such as oxides.
For example there are many studies on epitaxial crystallization of amorphous silicon
layer induced by ion beam irradiation at low temperature [Nakata (1991)]. There are
- 32 -
only a few studies on the crystallization of amorphous alumina thin films by ion beam
irradiation [Yu et al. (1995); Barbour et al. (2000)] and effectiveness of crystallization
at low temperature and mechanical properties of crystallized alumina films are not
always clarified.
Another approach to get crystalline alumina films is to use an isostructural
template layer. Jin et al.used pre-deposition of a chromia (Cr2O3) layer to grow aalumina thin films on silicon substrates at a substrate temperature of 400oC [Jin et al.
(2002)]. Chromia crystallizes in same hexagonal structure (corundum) as a-alumina
with a relatively small lattice mismatch (a and c axes for chromia are larger by ~ 4.1%
and ~ 4.6% respectively). Combined with ease of formation and hard, stable properties
makes chromia highly suited as a template to promote a-alumina growth [Ashenford et
al. (1999); Jin et al. (2002)]. Andersson et al. (2004) reported the growth of hard, single
phase a-alumina thin films at temperatures of 280-560oC using a chromia template
layer, on Si substrates. Chromia is shown to nucleate directly on amorphous silicon
dioxide with equiaxed grains of average grain size of 11 ± 2 nm, while a-alumina grains
grow with local epitaxy on chromia grains, inheriting their orientation and crystal
structure. This method has triggered research on the possibilities of stabilizing a-phase
of alumina in thin films by forming solutions of chromia and alumina [Schneider
(1997); Ashenford et al. (1999); Ramm et al. (2007)]. Kohara et al. deposited Cr2O3
nucleation layer by oxidizing a CrN film pre-deposited on cemented carbide substrates
[Kohara et al. (2004)]. Using this method, a-alumina dominant coatings could be
formed at substrate temperatures down to 700oC. Chromia pre-deposition has also been
claimed to aid the formation of a-phase in coatings deposited using inverted magnetron
sputtering [Khanna et al. (2006); Aryasomayajula et al. (2007)] at substrate temperature
in the range of 350-400oC [Cloud et al. (2008)]. Eklund et al. investigated the effect of
texture of chromia nucleation layer on the phase formation for sputter deposited
alumina films. They observed growth of a-alumina onto chromia at substrate
temperature of 450oC [Eklund et al. (2008)].
In order to reduce the formation temperature of crystalline phases in general,
and a-alumina in particular, several authors have turned to different ionized magnetron
- 33 -
sputtering techniques. Schneider et al. used an RF coil to increase ionization of
deposition flux and deposited films containing metastable k and q-alumina at
temperatures down to 320oC and 180oC respectively, by adjusting ion flux to substrate
[Schneider et al. (1998)]. Khanna et al. reported the growth of g-alumina without
deliberate substrate heating using inverted dual cylindrical magnetron sputtering
[Khanna and Bhat (2006); Khanna et al. (2006)]. Zywitzki and Hoetzsch deposited
alumina with conventional pulsed DC sputtering and showed that a-alumina could be
formed at 760oC and g-phase at temperature down to 350oC, if suitable substrate bias
and cathode power was applied [Zywitzki and Hoetzsch (1996)]. Li et al. used an
approach similar to Zywitzki et al. but also employed a solenoid creating a magnetic
field in the vicinity of substrate, as well as higher substrate bias values (up to 400 V) [Li
et al. (2000)]. Wallin reported the synthesis of a-Al2O3 at reduced substrate
temperatures using reactive HiPIMS [Wallin (2008)].
- 34 -
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