Plasma-assisted techniques for growing hard nanostructured coatings: an overview

18Plasma-assisted techniques for

growing hard nanostructured

coatings: an overviewR.S. Pessoa*,†, M.A. Fraga†,‡, L.V. Santos*,†, N.K.A.M. Galvao†,

H.S. Maciel*,†, and M. Massi**�IP&D, University of Paraiba Valley (UniVap), Sao Jose dos Campos/SP, Brazil;†Technological Institute of Aeronautics (ITA-DCTA), Sao Jose dos Campos/SP, Brazil;‡Faculty of Technology of Sao Paulo, Sao Paulo/SP, Brazil; ��Federal University ofSao Paulo-ICT, Sao Jose dos Campos/SP, Brazil

18.1 Introduction

Nanostructured materials are a new class of materials that are distinguished from

conventional materials by their extremely fine grain size from 100 nm down to a just

few nm. This is due to the small grain and the significant contribution of the grain

boundary regions causing the “nanoscale effect,” in which the properties are opti-

mized or exhibit completely new and/or different behavior compared to conventional

materials (Baer et al., 2003; Musil, 2012). Several papers refer to the properties of

nanostructured materials as “unique properties,” for example, superior hardness,

toughness, chemical stability, low friction and wear-resistance, and these unique

properties allow then to be applied in various technological sectors (Lukaszkowicz,

2011; Baer et al., 2003; Lu et al., 2006; Pogrebnjak and Beresnev, 2012). Table 18.1

presents some nanostructured materials and respective technological applications.

The thin (nanometer-sized) coating of a bulk material is a crescent category in the

field of nanostructured materials. The highly sophisticated surface-related properties

of nanocoatings, such as super-plasticity, and optical, magnetic, electronic and cata-

lytic properties, make them attractive for industrial applications in high-speed

machining, tooling, biomedical, automotive, optical applications and magnetic stor-

age devices (Musil, 2012; Baer et al., 2003).

There are many types of design models for nanostructured coatings, such as

nanocomposite coatings (Pogrebnjak and Beresnev, 2012), nanoscale multilayer

coatings (Hovsepian and Munz, 2003), superlattice coatings (Hovsepian and Munz,

2003; Munz et al., 2001), and nanocrystalline coatings (Malshe et al., 2002;

Hernandez-Torres et al., 2012). Moreover, there are different combinations for

nanocoating formation, namely metal�metal, metal�ceramic, ceramic�ceramic,

and solid�lubricant. Thus, by engineering these composite materials at the nanome-

ter scale, it is possible to enhance the materials properties such as hardness, making

it possible to obtain hard (with hardness above 20 GPa), super-hard (hardness above

Anti-Abrasive Nanocoatings.

© 2015 Elsevier Ltd. All rights reserved.

40 GPa), or even ultra-hard (80 GPa) materials, that rival diamond performance

(Zhang et al., 2003). As is known, diamond demonstrates both high thermal conduc-

tivity and electrically insulating properties and much attention has been put into

finding practical applications for this material. However, diamond has several lim-

itations for mass industrial applications, including its high cost and oxidation at

temperatures above 800�C (John et al., 2002). Therefore, recent research into super-

and ultra-hard materials has focused on compounds that would be thermally and

chemically more stable than pure diamond. Some examples of super-hard materials

are cubic boron nitride (c-BN), carbon nitrides, silicon carbide (SiC), and ternary

compounds such as boron-carbon-nitride (B-N-C). Sapphire and zirconia are exam-

ples of ultra-hard materials.

Table 18.1 Nanostructured materials and respective technologicalapplications in different fields of science and industry

Nanostructured

materials

Some technological applications in

different fields

References

Al-N based

coatings

Optical protective coatings Li et al. (2005)

Optoelectronic (field emission devices, flat

panel displays), heterostructure diodes

for UV light emitters and homojunction

light emitting diodes (LED)

Kasu and Kobayashi

(2000), Miskys

et al. (2003)

Protective layers against corrosion Miao et al. (1997)

Electromechanical/microelectromechanical

systems (MEMS) (piezoelectric sensing

layers)

Perooly et al. (2005)

B-N based

coatings

Field passivation and capping layers for

thermal annealing

Bathe et al. (2001)

Protective layers for metallic filaments Bathe et al. (2001),

Rudolph (1993)

Electrical insulation Rudolph (1993)

Si-C based

coatings

Optical coatings Fraga et al. (2012)

Optoelectronic (LED and heterojunction

diodes)

Kortright and Windt

(1988)

Piezoresistive sensing layers Fraga et al. (2011)

Si-N based

coatings

Microelectronics (sacrificial layers) Williams et al. (2003)

Electronic passivation of silicon surfaces

and antireflection coating on silicon

solar cells

Nagel et al. (1999)

Ti-C based

coatings

Industrial (high-speed machining, tooling,

and wear)

Steinmann and

Hintermann (2009)

Hard disk drives Sangkla et al. (2011),

Vitu et al. (2008)

Biocompatible implants Vitu et al. (2008)

Carbon based

coatings

Biomedical Takeno et al. (2010)

Tribological Bull (1995)

456 Anti-Abrasive Nanocoatings

All these designs and manipulations of materials with unique properties that are

often unachievable in bulk materials are now possible due to recent advances in tech-

niques for deposition and processing of thin films. Additionally, the design of hard

nanostructured coatings needs many factors to be taken into consideration, for exam-

ple, the interface volume, grain size, single layer thickness, surface and interfacial

energy, texture, epitaxial stress and strain, all of which depend significantly on mate-

rials selection, deposition methods, and processing parameters (Baer et al., 2003;

Malshe et al., 2002).

Physical vapor deposition (PVD), chemical vapor deposition (CVD), ion implan-

tation, and laser beam treatments are the most widely applied procedures for modify-

ing the chemical composition and/or atomic structure of solid surfaces on a

nanometer scale. In this chapter an overview is presented of the hard nanocoatings and

the main plasma-assisted techniques for their synthesis. Among these techniques,

plasma enhanced chemical vapor deposition (PECVD), magnetron sputtering, and

atomic layer deposition (ALD) are highlighted. Characteristics of some hard coatings

such as DLC, SiC, c-BN, and aluminum oxide (Al2O3) as well as heterostructures

obtained by these techniques are presented.

18.2 Hard nanocoatings: from history to designsand properties

The concepts that seeded nanotechnology were first discussed in 1959 by renowned

physicist Richard Feynman in his talk “There’s Plenty of Room at the Bottom,” in

which he described the possibility of synthesis via direct manipulation of atoms

(Drexler, 1992). Only in 1974 was the term “nanotechnology” defined by Tokyo

Science University Professor Norio Taniguchi: “Nanotechnology mainly consists of

the processing of, separation, consolidation, and deformation of materials by one

atom or by one molecule” (Sandhu, 2006). The ideas contained in this definition

were further explored and developed by K. Eric Drexler in the late 1970s and

1980s, culminating in the release of his book Engines of Creation: The Coming Era

of Nanotechnology in 1986, which is considered the first book on the topic of nano-

technology (Drexler, 1986). Hence, emergence of nanotechnology as a field in the

1980s occurred through convergence of Drexler’s theoretical and public work,

which developed and popularized a conceptual framework for nanotechnology, and

high-visibility experimental advances that drew additional wide-scale attention to

the prospects of atomic control of matter.

At the same time, the German physicist Herbert Gleiter contributed significantly

and at various levels, to the development of nanostructured materials (Nordmann,

2009). Several papers cite H. Gleiter as the first to introduce the field of nano-

structured materials with his proposal to synthesize a new solid state structure � an

interfacial material � which differs from the crystalline and amorphous counter-

parts of the same chemical composition (Nordmann, 2009; Gleiter, 2000; Meyers

et al., 2006; Langdon, 2010). Gleiter’s proposal to achieve such a new structure by

457Plasma-assisted techniques for growing hard nanostructured coatings: an overview

introducing very large volume fractions of grain boundary or interface atoms by

crystal size reduction was based on the understanding of grain boundary structure.

Thus, by controlling the type and/or volume fraction of grain boundaries of conven-

tional engineering materials, unique material performance characteristics in bulk

materials as well as films and coatings can be achieved (Gleiter, 2000). After this

work, the interest in nanostructured materials greatly increased alongside the devel-

opment of techniques for engineering of these materials.

As mentioned in the Section 18.1, there are different design models for nanos-

tructured coatings where the main classes are: nanoscale multilayer coatings

and nanocomposite coatings (Musil, 2000; Lukaszkowicz, 2011; Pogrebnjak et al.,

2013). The multilayer coatings exhibit composition and phase modulation in one

direction of space (the growth direction), whereas the nanocomposite coatings are

modulated in all three directions of space. Due to the size effect, these new classes

of materials exhibit improved mechanical, electronic, magnetic and optical proper-

ties, and, of all the exotic properties, hardness is one of the most important for

applications in manufacturing industry (Pogrebnjak and Beresnev, 2012). For exam-

ple, nanocomposite coatings can present hardness two times greater than that of its

harder component (Pogrebnjak and Beresnev, 2012; Musil et al., 2001). From

knowledge of nanostructured materials emerged the following hardness classifica-

tion: materials with hardness greater than 20 GPa and less than 40 GPa are called

hard materials, super-hard materials and the ultra-hard materials refer to materials

with hardness $40 GPa and $80 GPa respectively (Zhang et al., 2002).

The hardness of nanostructured coatings as well as conventional bulk materials

is related to the material grain size (Lukaszkowicz, 2011; Zhang et al., 2003). The

relationship of the hardness and the grain size is illustrated in Figure 18.1; from this

Amorphous

Microcrystalline phase

Nanocrystalline

Har

dnes

s

100 101 102 104 106

Grain size, d (nm)

dc

Figure 18.1 Behavior of the hardness of materials as a function of grain size.

Source: Adapted from Lukaszkowicz, 2011; Pogrebnjak and Beresnev, 2012; Zhang et al., 2003.


it can be seen that there is a critical value of the grain size dc� 10 nm at which a

maximum value of hardness, Hmax, of the coating is achieved.

When the grain size is decreased to about 10 nm, the mobility of dislocations is

restricted. This occurs because the refined grain and the boundary regions suppress

the dislocation, causing the onset of incoherent deformations and increasing the

hardness of the material accord with the “Hall-Petch” relationship (Musil, 2012;

Zhang et al., 2003; Schiotz, 2001; Holleck, 1999):

HðdÞ5H0 1KUd21=2 (18.1)

where, H is the material hardness, K is strengthening coefficient and d is the grain

size. This effect is especially prominent for grain size down to tens of nanometers

(Lukaszkowicz, 2011; Zhang et al., 2003). In grain size of approximately 10 nm

there is a critical value where the maximal hardness is achieved. For grain size less

than dc, due to the increase in grain boundary a new deformation mechanism (i.e. the

Hall-Petch relation is inverted), called grain boundary sliding, replaces the disloca-

tion activity that dominates deformation process in conventional materials, thus

decreasing the hardness (Pogrebnjak and Beresnev, 2012; Zhang et al., 2003).

Softening caused by grain boundary sliding is mainly attributed to the large amount

of defects in the grain boundaries, which allows fast diffusion of atoms and vacan-

cies with the applying of stress. A further increase in the strength and hardness with

decreasing crystallite size can be achieved only if grain boundary sliding is blocked

by appropriate coating design and materials selection.

18.2.1 Classifications of hard nanocoatings

There are many design ideas for nanocomposite coatings, and embedding nano-

crystalline phases in the amorphous phase matrix is quite often applied which can

be fulfilled easily by PVD and CVD methods (Zhang et al., 2002). According to

Musil there are at least four types of nanostructures (Figure 18.2) that result in the

enhanced hardness of nanocomposite coatings: (1) bilayers with nanosize period λ;(2) the columnar nanostructure; (3) nanograins surrounded by a very thin tissue

phase, w (B1 to 2 monolayers); and (4) the mixture of nanograins with different

crystallographic orientations and/or different phases; here λ5 h11 h2, h1 and h2are the thickness of first and second layer of the bilayer, respectively (Musil,

2012).

Individual nanostructures are formed under different conditions using either a

sequential deposition of individual layers in the nanosize bilayers or in transition

regions where the coating structure changes from crystalline through nanocrystal-

line to amorphous. There are three transition regions: (1) the transition from the

crystalline to the amorphous material; (2) the transition between two crystalline

phases of different materials; and (3) the transition between two crystallographic

orientations of grains of the same material (Pogrebnjak and Beresnev, 2012).


Additionally, in 1999, Holleck et al. proposed a classification of hard coatings in

three groups according to their bonds (see Figure 18.3): (1) covalent hard materials;

(2) metallic hard materials; and (3) ionic hard materials (Zhang et al., 2003; Holleck,

1999; Zhang et al., 2005; Stuber et al., 2011).

(a) Nanosized bilayer

Layer 1

Layer 2

Layer 1

Layer 2

(b) Nanocolumn

(c) Nanograins surroundedby a tissue phase

(d) Nanograins with differentcrystallographic orientation

λ

Nanograin

w

Nanograin 1 Nanograin 2Tissue phase3D

sys

tem

s2D

sys

tem

sFigure 18.2 Schematic illustration of four nanostructures of the nanocomposite coating with

enhanced hardness: (a) nanosize bilayers, (b) columnar nanostructure, (c) nanograins

surrounded by a tissue phase, and (d) mixture of nanograins with different crystallographic

orientation (adapted from Musil, 2012; Pogrebnjak and Beresnev, 2012). Where w is the

tissue phase thickness.

Covalent bond

Hardness

Ionic bond

Stability, inertness

Metallic bond

Adhesion, toughness

Metallic hard materialsCovalent hard materials

Ionic hard materials

Titanium based

TiCTiBCN

TiN

TiSiCTiA1CN

Ti2N

VCWCCrN

Others

Diamond likeC

Others

SiCCN BN

A12O3ZrO2

A1N

Figure 18.3. Classification of some hard coatings with relation to their bond nature.

Source: Adapted from Zhang et al., 2003; Holleck, 1999; Zhang et al., 2005.


The specific properties of each hard material group are related to the bond type

between the elements. For example, covalent coatings such as diamond and boron

nitride are the hardest materials and have a high temperature strength, whereas the

ionic (ceramic) coatings exhibit the best chemical inertness and higher brittleness.

Metallic coatings have a good adhesion and toughness.

From Figure 18.3, S. Zhang et al. (2003) conclude that the best to design a nano-

composite coating with both high hardness and high toughness is probably to use

ternary, quaternary or even more complex systems (for example, multilayer struc-

ture Zhang et al., 2005), with a high-strength amorphous phase as matrix (such as

a-SiNx, a-BN, a-C) and hard transition metal-nitride nanocrystals (such as TiN, BN)

as the nanocrystalline phase to increase grain boundary complexity and strength.

These nanocrystalline phases should be refractory and immiscible with each other,

and could result in compositional modulation, segregation, and high thermal stabil-

ity of the nanostructure. The aim is to maximize interfaces and form well-defined

spinodal structure at interfaces.

18.2.2 Hardness mechanisms of crystallinenanostructured materials

There are few intrinsic super-hard materials highlighting diamond, c-BN and

some binary and ternary B-C-N compounds. Studies show that the high intrinsic

hardness of these materials is related to the following characteristics: a short inter-

atomic distance, a high binding energy, and a high degree of covalent bonding

(Gao et al., 2003).

On the other hand, there is a group of hard materials that have what is called

extrinsic hardness. These crystalline materials have the hardness determined by

their microstructure, that is, it is possible obtain hard and super-hard nanocoatings

from these materials only by hindering or retarding the deformation generated by

dislocation movements (Arzt, 1998).

The four main mechanisms that can modify the hardness of crystalline materials

are illustrated in Figure 18.4.

It is known that high hardness is achieved for maximal concentration of introduced

distortions. Grain boundary is an example of a lattice distortion. This mechanism

has been observed especially in nanocrystalline ceramics (Szlufarska et al., 2005),

Hardnessmechanisms

Grain boundary Solid solution Precipitation Energetic ionbombardment

Figure 18.4. Different mechanisms that can influence the hardness of crystalline materials.


nanocrystalline metals (Van Swygenhoven, 2002), and multilayer nanostructures

(Mayrhofer et al., 2005).

Solid solution mechanisms are characterized by lattice distortion due to the

insertion of atoms of the solute (also called alloy element) either in interstitial or

substitutional lattice positions. These mechanisms are limited by the solubility limit

of the solute. It has been observed that PVD processes at low substrate temperature

increase the solubility limit (Holleck, 1988). Examples of hard coatings that can be

obtained from the solid solution mechanism are Ti-C-N, Ti-Al-N, and Cr-Al-N

(Zhang et al., 2007).

Precipitation or dispersion strengthening is the process characterized by the pre-

cipitation of a second phase from a supersaturated metastable phase. Generally, this

mechanism is accelerated and controlled by a post-deposition annealing (Arzt,

1998). The literature reports the precipitation mechanism for nanocrystalline thin

films such as Ti-Al-N (Mayrhofer et al., 2003), Ti-B-N (Mayrhofer et al., 2006),

and Zr-Al-N (Sanjines et al., 2006).

Energetic ion bombardment is another mechanism used to obtain hard coatings

during film growth. It has been employed to modify the morphology of the films.

The ion bombardment creates interstitials and vacancies that can lead to the forma-

tion of dimensional defects (Mattox, 1989). These defects are associated with the

existence of internal stress in the films, which result in hardness increase. It has

been reported for different PVD coatings (Mayrhofer et al., 2006).

18.3 Main plasma-based techniques for synthesisof hard nanocoatings

A variety of modern coating techniques have been employed to realize hard nanos-

tructured coatings. Among these technologies are plasma-assisted PVD, CVD, and

PECVD processes, which make it possible to grow materials with extremely high

indentation hardness (40�80 GPa) (Veprek and Veprek-Heijman, 2007; Veprek

et al., 2000; Zou et al., 2010).

Table 18.2 provides a listing of hard thin film materials and compares the micro-

hardness of PECVD and PVD deposited thin films (note that nanocrystalline films

have the highest hardness) (Martinu et al., 2009). Several types of films deposited

by PECVD have microhardness in the super-hard range while the number is limited

for PVD deposited films. Hardness is generally improved by increasing the energy

of deposited atoms using high substrate temperatures, substrate bias, plasma bom-

bardment (ion assist), or a combination of all four.

Moreover, as can be seen in Table 18.2 the most widely used hard and super-

hard coatings are binary and ternary alloys of titanium, boron, chromium, tungsten,

silicon, and allotropes of carbon.

The sections that follow describe plasma-assisted techniques for growing hard

nanostructured coatings as well as discussing materials synthesis and its resulting

properties.


18.3.1 Physical vapor deposition

The PVD process comprises physical and reactive sputtering techniques. Basically,

these techniques differ when a neutral gas (physical sputtering) is added together

with a reactive gas (reactive sputtering). In physical sputtering, ions impinging on

Table 18.2 Microhardness of PECVD and PVD films

Note: a � amorphous, c � crystalline, pc � polycrystalline, and nc � nanocrystalline.Source: Adapted from Zhang et al., 2005.


the plasma bombard the target and release atoms (or molecules) of the target mate-

rial. Argon ions at 500�1000 V are usually used. The sputtered atoms diffuse

through the plasma and arrive at the substrate, where they can be deposited

(Bogaerts et al., 2003). In reactive sputtering, a molecular gas (for example, N2

and/or O2) is used together or without a neutral gas. Besides the positive ions from

the plasma that sputter bombard the target, the dissociation products from the reac-

tive gas also react with the target. Hence, the film deposited at the substrate will be

a combination of sputtered target material and the reactive gas (Bogaerts et al.,

2003; Lieberman and Lichtenberg, 2007; Berg and Nyberg, 2005; Pessoa et al.).

The sputter deposition process is schematically presented in Figure 18.5.

The steps of sputtering process are as follows:

1. The neutral gas is ionized by an external power supply (direct current, radio-frequency,

etc.), producing a glow discharge or plasma.

2. The source (the cathode, also called the target) is bombarded in a high vacuum by plasma

ions due to the potential acceleration drop in the cathode sheath.

3. Atoms from the target are ejected by momentum transfer and diffuse through the vacuum

chamber.

4. Atoms are deposited on the substrate to be coated and form a thin film.

Because sputter yields are of the order of unity for almost all target materials, a

very wide variety of pure metals, alloys and insulators can be deposited. Physical

sputtering, especially of elemental targets, is a well understood process enabling sput-

tering systems for various applications to be relatively easily designed. Reasonable

deposition rates with excellent film uniformity, good surface smoothness and adhe-

sion can be achieved over large areas (Lieberman and Lichtenberg, 2007).

Figure 18.5 Schematics of the sputtering process.


Typically, the sputtering process can be accomplished using a planar configura-

tion of electrodes and a direct current (DC) power supply, where one electrode is

biased negatively (cathode) and suffers the sputtering process. However, the sput-

tering yield is directly dependent on the gas pressure (the best sputtering rates are

in the range of mTorr), which compromises the efficiency of planar geometry for

this application: it is ideal for pressures above 100 mTorr. To solve this problem,

a magnetron discharge where the plasma is magnetically enhanced by placing

magnets behind the cathode target was developed, that is, a crossed electric and

magnetic field configuration is created. Figure 18.6 shows a schematic drawing of a

conventional DC magnetron sputtering discharge. The trapping of the secondary

electrons results in higher probability of electron impact ionization and hence high-

er plasma density, increasing the sputtering flux and allowing operation at lower

pressures, below 10 mTorr. Furthermore, the discharge voltage can be lowered into

the range of 300�700 V. The main problem with the magnetron sputtering configu-

ration is that the sputtering is confined to a small area of the target cathode gov-

erned by the magnetic field. The discharge appears in the form of a high-density

annulus of width w and radius R, as seen in Figure 18.6. Sputtering occurs in the

corresponding track of the target. This area, known as race track, is created by the

uneven ion density.

According to the Hall-Petch equation, the strength properties of the material rise

along with the reduction of the grain size. In case of coatings deposited by the PVD

processes, the structures obtained, with grain size B10 nm result in the maximum

mechanical properties. Coatings of this structure present very high hardness

(.40 GPa), ductility, stability at high temperatures, and so on (Lukaszkowicz,

2011). According to Musil, the addition of one or more selected elements into a

base material, for example, the addition of Si in the nitride transition metal (TM), is

a very effective way to produce hard nanocomposite coatings with enhanced hard-

ness, here TM5Ti, Zr, Hf, V, Nb, Ta, and so on (Musil, 2012). Moreover, it is

highlighted that the energy delivered to the growing film also has a crucial effect

on its structure, elemental and phase composition, and physical properties

Figure 18.6 Schematic drawing of a conventional dc magnetron sputtering discharge.

Source: Adapted from Bogaerts et al., 2003.


(Musil, 2012). This energy is delivered from the plasma environment and is com-

posed of (1) heating processes; and (2) conversion of the kinetic energy of bom-

barding ions and fast neutrals incident on the surface of growing film.

Recently, amorphous coatings with hardness ranging from B20 � B40 GPa

were reactively sputtered using an unbalanced magnetron deposition technique

(Musil, 2012):

1. a-(Si3N4/MeNx) composite coatings with high ($50 vol.%) content of a-Si3N4 phase; here

Me5Ta, Zr, Ti, Al, Mo, W, and so on.

2. a-(Si�B�C�N) coatings with strong covalent bonds. Magnetron sputtered films were

deposited co-sputtered using high purity graphite and boron targets in 50/50 Ar1N2 mix-

tures with formation of hybridized B-C-N bonds (Kim et al., 2004).

Also, super-hard amorphous SiC coatings are deposited by our group using co-

sputtering of silicon and carbon targets. Here the SiC coatings were obtained with

hardness of up to 50 GPa only modifying the negative bias applied at the substrate

holder (Medeiros, 2012).

Relative to the deposition of boron nitride films in cubic form (c-BN), it is reported

that the lower adhesion and extreme compressive stresses of these films are the main

problems in PVD techniques, which limits film thickness of c-BN up to 1 μm.

However, many researchers have reported adherent and thick multilayered films

recently (Cesur et al., 2008). Boron carbon nitride (BxCyN), also known as “heterodia-

mond” is essentially c-BN that contains between 5 at% and 30 at% C and is generally

deposited by PVD and CVD processes (Yamamoto et al., 2000). Magnetron sputtered

films were deposited co-sputtered using high purity graphite and boron targets in

50/50 Ar1N2 mixtures with formation of hybridized B-C-N bonds. Table 18.3

summarizes hardness values for the B-C-N family (Ulrich et al., 2009).

18.3.2 Chemical vapor deposition

In the field of chemical-based deposition techniques of hard coating thin films liq-

uid phase epitaxy (LPE), CVD techniques and their derivatives, such as PECVD

and atomic layer deposition (ALD), can be mentioned, among others. A very desir-

able feature of CVD-based techniques is that they can be carried out with well con-

trolled atmospheres and at lower temperatures (lower than 1500�C) in comparison

with other related techniques.

Currently, the most popular low-pressure thin film deposition techniques are

those based on chemical deposition processes such as hot filament chemical vapor

Table 18.3 Summary of hardness values for the B-C-N family(Solozhenko et al., 2009)

Material Diamond c-BC2N c-BC5 c-BN B4C ReB2

Vickers Hardness (GPa) 115 76 71 62 38 22


deposition (HFCVD) and PECVD (Veprek et al., 2000; Zou et al., 2010; Bogaerts

et al., 2003; Lieberman and Lichtenberg, 2007).

Thermal CVD is the process of gas phase heating (by a hot filament or hot wall,

for example (Berg and Nyberg, 2005)) in order to cause the decomposition of the

gas, generating radical species that, by diffusion, can reach and be deposited on a

suitably placed substrate. It differs from PVD, which relies on material transfer

from a condensed-phase evaporant or sputter target sources. A reaction chamber is

used for this process, into which the reactant gases are introduced to decompose

and react with the substrate to form the film. Figure 18.7(a) illustrates a schematic

of the reactor and its main components. Basically, a typical thermal CVD system

(HFCVD) consists of the following parts:

1. sources and feed lines of gases;

2. mass flow controllers for metering the gas inlet;

Figure 18.7 Schematic diagram of (a) HFCVD and (b) PECVD systems.


3. a reaction chamber for decomposition of precursor gases;

4. a system for heating up (hot filament or hot wall) the gas phase and wafer on which the

film is to be deposited; and

5. temperature sensors.

As a modification to the CVD system, PECVD arose when plasma was used to

perform the decomposition of the reactive gas source. By chemical reactions in

the plasma (mainly electron impact ionization and dissociation), different kinds of

ions and radicals are formed which diffuse toward the substrate where chemical

surface reactions are promoted, leading to film growth. The major advantage com-

pared to simple CVD is that PECVD can operate at much lower temperatures.

Indeed, the electron temperature of 2�5 eV in PECVD is sufficient for dissocia-

tion, whereas in CVD the gas and surface reactions occur by thermal activation.

Hence, some coatings, which are difficult to form by CVD due to melting pro-

blems, can be deposited more easily with PECVD (Lieberman and Lichtenberg,

2007; Peng et al., 2011). Among the kinds of plasma sources that have been used

for this application radiofrequency (RF) discharges (Lieberman and Lichtenberg,

2007), pulsed discharges (Peng et al., 2011), and microwave discharges (Berg and

Nyberg, 2005) stand.

Basically, in PECVD the substrate is mounted on one of the electrodes in the

same reactor where the species are created, as illustrated in Figure 18.7(b). Here,

we focused the RF discharge because it is the configuration more used in research

and industry. The RF PECVD reactor essentially consists of two electrodes of dif-

ferent areas, where the substrate is placed on the smaller electrode, to which the

power is capacitively coupled. The RF power creates a plasma between the electro-

des. Due to the electrons having higher mobility than the ions, a sheath is created

next to the electrodes containing an excess of ions. Hence, the sheath has a positive

space charge, and the plasma creates a positive voltage with respect to the electro-

des. The electrodes therefore acquire a DC self-bias equal to their peak RF voltage

(self-bias electrode). The ratio of the DC self-bias voltages is inversely proportional

to the ratio of the squared electrode areas, that is, V1/V25 (A1/A2)2 (Lieberman

and Lichtenberg, 2007).

Therefore, the smaller electrode acquires a larger bias voltage and becomes neg-

ative with respect to the larger electrode. The negative sheath voltage accelerates

the positive ions towards the substrate which is mounted on this smaller electrode,

allowing the substrate to be bombarded by energetic ions facilitating reactions with

the substrate surface.

In order to maximize the ion-to-neutral ratio of the plasma, the plasma must be

operated at the lowest possible pressure. Nevertheless, the ions are only about 10%

of the film-forming flux even at pressures as low as 50 mTorr. Lower pressures

cannot be used, as the plasma will no longer strike. A second disadvantage of this

source is the energy spread in the ion energy distribution, prohibiting a controlled

deposition. This energy spread is due to inelastic collisions as the ions are acceler-

ated towards the substrate. The effect of this energy spread is to lower the mean ion

energy to about 0.4 of the sheath voltage. Another disadvantage of the RF PECVD

source is that it is not possible to have independent control over the ion energy and


the ion current, as they both vary with the RF power. On the other hand, PECVD

allows the deposition of uniform films over large areas, and PECVD systems can be

easily scaled up (Zhao et al., 2010).

Figure 18.8 illustrates the deposition mechanism of the CVD technique (Zou

et al., 2010). The mechanism occurs as a result of the following steps:

1. A predefined mix of reactant gases and diluents inert gases are introduced at a specified

flow rate into the reaction chamber.

2. A heat source is applied in order to dissociate the reactant gases.

3. The resulting radical species diffuse to the substrate.

4. The reactants get adsorbed on the surface of the substrate.

5. The reactants undergo chemical reactions with the substrate to form the film.

6. The gaseous by-products of the reactions are desorbed.

7. The gaseous by-products of the reactions are evacuated from the reaction chamber.

In general, SiC PECVD is performed in conventional PECVD reactors with

heated substrate holders (Neyts, 2006). Methane and silane are commonly used pre-

cursors (Sarro et al., 1998; Pelegrini et al., 2010). It is also possible to use a single

precursor such as 1,3-disilabutane DSB or methylsilane. The film properties

strongly depend on the process parameters including deposition temperature, pres-

sure, plasma power, and gas phase composition. In general, PECVD SiC films

exhibit amorphous form and compressive stress, but process conditions can be opti-

mized to tailor the stress levels to desired values. Typically, low plasma power and

high temperature conditions yield low stress films. Sometimes post-deposition

annealing at temperatures of order of 1000�C may still be required to crystallize the

films. Typical values of hydrogenated SiC films deposited by PECVD ranging from

10 to 30 GPa (El Khakania et al., 1994; Kim et al., 2008).

1. Introduction of precursor gases

2. Transport of reactivespecies to the substrate

6. Transportation of reaction products

5. Desorption of reaction products

4. Migration of chemical species on the surface and film formation

3. Adsorption of reactive speciesonto the substrate surface

7. Removal of reaction products

Substrate

Figure 18.8 Chemical vapor deposition mechanism.

Source: Adapted from Zou et al., 2010.


Other hard materials synthesized by CVD-based techniques are carbon-based

materials. The carbon-based family of thin film materials includes diamond-like

carbon (DLC), tetrahedral amorphous carbon (ta-C), amorphous carbon (a-C), and

hydrogenated amorphous carbon (a-C:H). DLC and ta-C are disputably two of the

most widely developed and applied thin film materials used to increase wear resis-

tance and lubricity over the past two decades, and volumes have been published on

their properties. DLC is also deposited in microcrystalline, nanocrystalline, and

multilayer microstructures. Nanocomposite DLC films are being developed to have

lower wear and friction and significantly higher load carrying capability (Erdemir

and Voedvodin, 2009). High-quality DLC films have been demonstrated for a

number of applications, including low friction and wear, optical and anti-reflection

coatings, and barrier coatings. The various forms of DCL can be combined to form

a variety of nanoscale structures that are amorphous, flexible, and yet purely sp3

bonded “diamond.” The hardest, strongest, and most lubricious mixture of this type

is known as tetrahedral amorphous carbon, or ta-C. For example, a coating of only

2 μm thickness of ta-C increases the resistance of common (i.e. type 304) stainless

steel against abrasive wear, changing its lifetime in such service from one week to

85 years (Erdemir and Voedvodin, 2009). It can be considered to be the pure form

of DLC, since it consists only of sp3 bonded carbon atoms.

The properties of DLC and ta-C films depend on the amount of sp3 and sp2

bonding present, which can ultimately be related to the deposition process (Pierson,

1993). Figure 18.9 shows the percentage of sp3 bonding for thin film carbon

Figure 18.9 Percent sp3 bonding for thin film carbon allotropes.

Source: Adapted from Sasvvides and Bell, 1992.


allotropes (Sasvvides and Bell, 1992). It shows the two extremes, diamond and

graphite, and every combination of bonding in between. Note also that bonding

configuration is related to hydrogen content, and eventually back to the deposition

process. Microstructures range from purely amorphous to nanocrystalline. Hardness

definitely increases with an increase of sp3 percentage. CVD techniques allow con-

trolling sp3 percentage only modifying process parameters such as gas flow, sub-

strate bias, discharge power, etc.

Finally, we present some of the main advantages and disadvantages of CVD-

based techniques:

18.3.2.1 Advantages

� excellent interface and surface morphology;� growth of complex heterostructures with many different layers;� growth on patterned substrates;� multiple wafer scale-up;� high purity of the layers.

18.3.2.2 Disadvantages

� large quantities of serious toxic or flammable gases (AsH3, SiH4, etc.);� almost no possibility of in situ monitoring;� not the best technique for some specific material (lack of adequate precursors or of not

suited growth conditions).

18.3.3 Atomic layer deposition

18.3.3.1 Deposition technique

ALD is a nanocoating process based on the self-limiting gas�solid reaction where

it is possible to produce thin films of high quality (Violet et al., 2009; Marin et al.,

2012; Liang et al., 2008). This process is similar to CVD but in the ALD method

the reactions are divided into two separated processes (Marin et al., 2012). Thus,

ALD is a variant of the CVD process (Marin et al., 2009).

In the ALD method it is possible to control the film thickness at the atomic level

(Marin et al., 2009; Hoivik et al., 2003) over the deposition process. This control is

possible because unlike CVD, where the reactants are presented simultaneously dur-

ing the process, in ALD the precursor gases pulses are separated by purge steps of

an inert gas to remove the unreacted precursor and gases by products (Miikkulainen

et al., 2013; Wikimedia Commons; Leskela and Ritala, 2002; Puurunen, 2005). In

this process, the film thickness increases in each deposition cycle (Leskela and

Ritala, 2002). The cycle of the ALD reaction is showed in Figure 18.10.

When the alternating is realized with the ALD steps replaced by a pulsed CVD

step, the process is known as partial ALD (Granneman et al., 2007). Figure 18.11

shows a schematic model of ALD and partial ALD processes.


Precursor chemistry plays a very important role in the ALD process (Kim, 2011;

Kim, 2003). There are many requirements for the precursor used in the ALD pro-

cess, such as (Miikkulainen et al., 2013):

� it must be gaseous or vaporizable at a temperature lower than the ALD reaction

temperature;� it must react, preferably fast and aggressively, in a saturating, irreversible manner with

sites on the surface of the growth substrate;� the by-products formed should be inert not interfere with the ALD grown;

Figure 18.10 Schematic diagram of the ALD reaction cycle (adapted from Miikkulainen

et al. and Wikimedia Commons (Miikkulainen et al., 2013; Wikimedia Commons)): (1)

adsorbed reagent A molecule; (2) non-adsorbed reagent A molecule; (3) surface without �H

or �OH ligant; (4) unreacted site; (5) adsorbed reagent B molecule; (6) reagent A

contamination. Depending on temperature of the ALD process the as-deposited film can

contain reagent A contaminant.

Figure 18.11 Schematic model of: (a) ALD and (b) partial ALD process.

(Adapted from Granneman et al., 2007).


� its reactants should not dissolve in the film and they must not decompose thermally during

storage, vaporization or ALD growth;� for some applications, high purity is required; and� it should present safe and easy handling, preferably not toxic or corrosive, thus ensuring

the integrity of the reactor and the user. And like any industrial process, low cost will

make it attractive to industries.

For reaction between the precursor and substrate surface an activation energy is

required. The activation energy is provided by the heating of the substrate and/or

the system (reactor chamber). In this case the process is known as “thermal ALD”

(Kim, 2011; Kim, 2003). Sometimes, precursors (molecular elements precursors)

are not sufficiently reactive, and thus they require more energy than provided in the

thermal ALD process. In order to reach the required energy, alternative methods

that complement the ALD process are used, for example plasma enhanced ALD

(PEALD). The difference between these two processes is that in PEALD the plasma

is activated during the step of exposure of the precursor (Kim, 2011; Kim, 2003)

and the film growth occurs at a lower temperature than in thermal ALD (Kim,

2011). Moreover, the reactant molecules of the precursor become more reactive and

the films deposited by PEALD have better properties, such as improved hardness

(Kim, 2011; Profijt et al., 2011).

Finally, the advantages of use the ALD technique include (Violet et al., 2009;

Marin et al., 2012; Leskela and Ritala, 2002; Granneman et al., 2007; Kim, 2003):

� deposition of several types of thin films (oxides, nitrides, sulfides and metals);� possibility of depositing films on various substrates, such as wafers, steel, 3D substrates,

micro- and nanoparticles, and others;� accurate thickness;� control of film thickness;� deposition of multilayers of different structures;� low deposition temperature.

18.3.3.2 Applications of ALD hard coatings

Although the main application of the ALD process is the deposition of ultra-thin

films, the hard coatings application is more geared towards the field of microelec-

tronics, more specifically in MEMS. MEMS comprise a variety of functional

mechanical devices fabricated on length scales from fractions of a micrometer to

millimeters. These devices are finding application in a wide variety of technologies,

including motion sensing, optical switching and other photonic applications, RF

switches and filters, mechanical actuators, chemical sensors, and others. As the size

of these devices shrinks, surface-to-volume ratio increases, and at dimensions

typical of MEMS devices, inertial forces become insignificant compared to sur-

face forces acting on these mechanisms. Then interfacial phenomena such as adhe-

sion, friction and wear can dominate the performance and reliability of MEMS

devices. The most sophisticated MEMS devices now in development employ sliding

contacts in rotary- or linear-motion structures, where friction and wear are a major

concern (Sniegowski and de Boer, 2000).


To mitigate these problems, the surfaces of MEMS devices can be modified

with coatings to prevent adhesion, and reduce friction and wear. Some of the most

commonly used/studied materials are the Al2O3, ZnO, WS2, W, and W/A12O3

nanolaminates. WS2 is a superior solid phase lubricant, W is a hard metallic,

wear-resistant material, and W/Al2O3 laminate films are potential super-hard coat-

ings with properties determined by the nanometer length scales of the laminate

layers.

Al2O3 is an advantageous material to use over various forms of silicon nitride, for

micro- and nanoelectromechanical systems, due in part to the low deposition temper-

ature that allows for integration with Complementary metal�oxide�semiconductor)

CMOS processing. Also, Al2O3, unlike silicon nitride, has a high chemical resistance

to dry-chemistry Si etchants (Tripp et al., 2006; Bull, 2012). Tripp et al. observed

that the hardness of ALD AL2O3 (order of 12 GPa) is comparable with the films pro-

duced by other deposition techniques (Tripp et al., 2006).

18.4 Conclusion

Engineered nanostructured thin films and low-dimensional structures are present on

thin film technology. New advanced deposition processes, such as ALD, are now

being developed and used to deposit advanced thin film materials and structures not

possible with conventional techniques. As a result, the hardness of thin films has

been steadily increasing, due primarily to new materials, improved deposition pro-

cesses and use of nanostructures and nanocomposites.

Plasma-assisted processes have contributed significantly to this development,

especially due to the ease automation of these processes allowing fine control of

the properties of matter. An example is the ALD process that has been well suited

to deposition of friction and wear-reducing films on MEMS structures. Sequential

reactant exposure and self-limiting surface chemistry result in exquisite control of

deposition rate and film thickness, and extremely conformal coating of high-aspect

ratio structures. However, the challenge remains to examine the mechanical and tri-

bological properties of thin and ultra-thin ALD films, and their suitability for appli-

cation in functional MEMS devices. Processes for preparation of a wide range of

materials, which are potential hard and/or lubricious coatings, have been demon-

strated by ALD (Ritala et al., 2002). The ease of fabrication of alloys and nanolami-

nates of ALD materials adds extra dimensions to the possibilities for tailoring the

properties of materials for tribological applications.

Acknowledgments

This work was supported by FAPESP-MCT/CNPq-PRONEX (process no. 2011/50773-0) and

PNPD-CAPES (process no. 02765/09-8).


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