Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology

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Evaporation 201

produces a thin plasma sheath on top of the melt. The low-energy secondary electrons from the

plasma sheath are pulled upward into the reaction zone by an electrode placed above the pool

biased to a low positive DC or AC potential (20–100 V), thus creating a plasma-ﬁlled region

between the electrode and the electron beam gun. The low-energy electrons have a high

ionization cross-section, thus ionizing or activating the metal and gas atoms and increasing the

reaction probability on collision. Charge-exchange processes between positive ions and neutral

atoms take place in the plasma. In addition, as suggested by Yee [139], transient highly excited

compound species are formed. The formation of the compound is completed most probably on

the substrate from these energetic and excited transient species. The synthesis of TiC by

reaction of Ti metal vapor and C

gas atoms with a carbon to metal ratio approaching unity

was achieved with this process [131, 133]. Moreover, by varying the partial pressure of either

reactant, the carbon to metal ratio of carbides could be varied [133, 140] at will. The ARE

process has also been applied to the synthesis of all ﬁve different Ti–O oxides [141]. These

authors noted that in the ARE process (i.e. with a plasma) as compared to the RE process (i.e.

without a plasma), a higher oxide is formed for the same partial pressure of O

, thus

demonstrating a better utilization of the gas in the presence of a plasma. The same observation

was noted by Bunshah and Raghuram [137] as well as by Granier and Besson [142] for the

deposition of nitrides.

A variation of the ARE process uses a resistance-heated evaporation source. The basic ARE

process uses e-beam-heated sources, which are expensive and inconvenient for the evaporation

of low melting point, high vapor pressure materials. Nath and Bunshah [138] modiﬁed the

ARE process for resistance-heated sources, as shown in Figure 4.43. The metal vapors are

generated from the chamber; the reaction is enhanced by a plasma generated by injecting

low-energy electrons from a heated thoriated tungsten emitter towards a low-voltage anode

assembly. A transverse magnetic ﬁeld is applied to cause the electrons to go into a spiral path,

thus increasing the probability of electron/atom collision and subsequent ionization.

Modiﬁcations of the basic ARE process

The ARE process has substantial versatility since the substrate can be grounded, positively or

negatively biased, or it can be allowed to ﬂoat electrically. There are several modiﬁcations of

the basic ARE process, as illustrated in Figure 4.44.

1. Enhanced ARE process [143, 144]. This is the conventional ARE process using e-beam

heating with the addition of a thermionic electron emitter (e.g. a tungsten ﬁlament) for the

deposition of refractory compounds at lower deposition rates as compared to the basic

ARE process. The low-energy electrons from the emitter sustain the discharge, which

would otherwise be extinguished since the primary e-beam (used to melt the metal) is so

weak that it does not generate an adequate plasma sheath above the molten pool from

which low-energy electrons can be extracted by positively biased interspace electrode. The

substrate may be biased, grounded, or ﬂoating.

202 Chapter 4

Figure 4.44: Basic ‘ARE’ process and later variations.

2. Low-pressure plasma deposition process. Using e-beam evaporation sources, the electric

ﬁeld may be generated by biasing the substrate positively instead of using a positively

biased interspace electrode. In this case, it is called low-pressure plasma deposition

(LPPD) [145, 146]. However, this version has a disadvantage over the basic ARE process

since one does not have the freedom of choice to ground the substrate, let it ﬂoat, or bias it

negatively (the BARE process; see no. 4 below).

3. ARE using plasma electron-beam guns. The plasma e-beam gun, instead of the

thermionic e-beam gun, can be used to carry out the ARE process. The hot hollow cathode

gun has been used by Komiya et al. [147] to deposit TiC ﬁlms, whereas Zega et al. [148]

used a cold cathode discharge e-beam gun to deposit titanium nitride ﬁlms. The plasma

e-beam sources produce an abundant supply of low-energy electrons for the ARE-type

process.

Evaporation 203

4. Reactive ion-plating processes. If the substrate is biased in the ARE process, it is called

biased activated reactive evaporation (BARE). This bias is usually negative to attract the

positive ions in the plasma. The BARE process has been reinvented and called reactive ion

plating by Kobayashi and Doi [149]. Ishida et al. deposited Ni–TiC ﬁlms by ion plating

with cation ratios varying between Ti:Ni 0.35 and 3 [150]. Reactive ion plating (RIP) is

very similar to the reactive evaporation process in that metal atoms and reactive gases react

to form a compound aided by the presence of a plasma. Since the partial pressures of the

gases in reactive ion plating are much higher (> 10

−2

torr) than in the ARE process

(> 10

−4

torr), the deposits can become porous or sooty. The plasma cannot be supported by

lower pressure in the simple diode ion plating process; therefore, Kobayashi and Doi [149]

introduced an auxiliary electrode biased to a positive low voltage (as originally conceived

for the ARE process) to initiate and sustain the plasma at low pressure (∼ 10

−3

torr). This

is no different than the ARE process with a negative bias on the substrate reported much

earlier by Bunshah [135], which was designated by him as the biased ARE (or BARE)

process.

5. Another variation of reactive ion plating using a triode conﬁguration [151] involves

injection of electrons into the reaction zone between the e-beam-heated evaporation source

and the negatively biased substrate from a heated tungsten ﬁlament transversely to the

metal vapor path. These low-energy electrons are pulled across the reaction zone by a

positively biased anode located opposite to the cathode. The arrangement is very similar to

that shown in Figure 4.33 except for the use of an e-beam-heated evaporation source, and

is also very similar to triode sputtering. This adds versatility as well as complexity to the

process through the addition of another process variable.

6. Murayama [152] uses an e-beam-heated source with a negatively biased substrate and RF

activation of the reactants by means of a coil electrode of aluminum wire in the reaction

zone to deposit oxide and nitride ﬁlms.

7. ARE process using an arc evaporation source. Evaporation of metals using a low-voltage

arc in the presence of a plasma and a negatively biased substrate is used by Snaper [103,

104] and Dorodnov [153] to deposit nitride and carbide ﬁlms, with N

and hydrocarbon

reactive gases, respectively.

8. Reactive arc ﬁltered evaporation. The system consists of conventional bell jar vacuum

system and uses a magnetic ﬁeld steering arc and a plasma duct formed from a

free-standing and isolated high-current solenoid [154]. The advantage of this system is that

it preserves the target material and achieves higher ionization rates with respect to the

ion-plating process.

Recent developments in the ARE process

New techniques based on ARE are being developed for synthesis of novel and unique

materials. The emphasis of such developments is generally on two aspects: (1) new approaches

204 Chapter 4

to produce the vapor species; and (2) new plasma excitation and conﬁnement techniques and

development of modiﬁed plasma excitation geometries.

Westerwaal et al. studied the nucleation of MgH

ﬁlms by the ARE system [155]. Their results

showed that they had 10% unreacted Mg remaining in the structure, but by heat treating the

ﬁlms and subsequent rehydrogenation procedures they were able to obtain 100% MgH

ﬁlms.

New approaches to produce the various species The basic process involves evaporation of

the constituent metal alloy or compound using e-beam or resistance/induction-heated sources.

However, it is difﬁcult to use this approach with certain materials such as boron and carbon.

Two possible solutions can be used to overcome these difﬁculties: (1) use a low melting point

compound of the respective element, and (2) use a pulsed laser beam where the pulse rate and

pulse width can be appropriately adjusted to control the rate of material generation and

fragmentation. Moreover, in many cases, the energy of the laser beam can also be used as a

source for plasma excitation.

Both of these approaches have been explored. A process developed by Bunshah et al. [156] for

the synthesis of cubic boron nitride involves boric acid as an reactant, which can be easily

evaporated from a resistance-heated tungsten boat. In addition to the ease of evaporation, this

process excludes the toxicity problems associated with ﬁne boron particles which can be

produced during e-beam evaporation of boron. A similar approach can be extended to

evaporation of carbon using a low melting point carbon compound such as adamantine. It is

likely that many new materials hitherto difﬁcult to synthesize may possibly be deposited using

this routine. Moreover, this novel approach may contribute to further development in reactive

MBE processes and other vapor deposition processes involving organometallic compound

reactants.

The use of pulsed laser beams in an ARE type of process has been demonstrated in recent

literature on high T

superconducting ﬁlms. Films with high T

(90 K) and high critical current

density (0.7 × l0

A·cm

−2

at 77 K) have been produced [157]. It is claimed that pulsing of the

laser beam avoids fractionation of the compound and hence good control of ﬁlm stoichiometry

is achieved. It is also suggested that the photon energy is sufﬁcient to activate the reactive

gas/metal species thereby increasing their reactivity, leading to an increased oxygen

concentration in the deposited ﬁlms.

New plasma excitation modes and geometries As discussed earlier, the attributes of the ARE

processes are due to the possibility of controlling the plasma parameters independently of the

deposition process.

However, improvement in excitation and conﬁnement of the plasma, as well as control and

optimization of plasma parameters in the ARE processes, are likely to enhance the process

Evaporation 205

capabilities. Recent developments include (1) the use of inductively coupled RF with parallel

plate RF geometries, and (2) the use of multiple ﬁlaments and anodes with magnetic

conﬁnement. These enhancements have led to substantial improvements in ﬁlm properties as

well as process control. Examples are high-rate deposition of a-Si–H ﬁlms [158], transparent

conducting ﬁlms on polymeric substrates [159], and TiS

and MoS

[160, 161] ﬁlms with

variable x values.

Two additional modes of ionization are being explored. Currently, an auxiliary RF excitation

source similar to that reported by Oeschner [162] is being developed for use in ARE. It is

believed that the high electron density and energy selectivity offered by this source is

likely to enhance advantages of the ARE processes for compound synthesis. Also, work is

underway to integrate electron cyclotron resonance (ECR) excitation at microwave frequencies

with the ARE process. ECR plasmas are characterized by a very high level of ionization and

excitation, and may greatly enhance the use of ARE for the deposition and synthesis of

ﬁlms.

Sakai et al. studied plasma-enhanced reactive evaporation of TiO

thin ﬁlms for photocatalytic

applications [163]. The system consists of a Ti e-beam evaporation source and an ECR oxygen

plasma source, and a sample holder as shown in Figure 4.45. In conventional plasma-enhanced

systems the sputtering rate is low because the target gets oxidized and reduces the deposition

rate. In Sakai’s system the modiﬁcation is such that the Ti atoms and O

gas ﬂow to the

Figure 4.45: Reactive evaporation system [163].

206 Chapter 4

substrate independently so that the target does not get poisoned [163]. According to their

results the crystal structure of the TiO

thin ﬁlm changes as they increase the deposition rate

from 18 nm/min to 145 nm/min. The crystalline phase shifts from anatase to rutile as the

deposition rate increases.

Mechanism of the ARE process

A reactive evaporation process can be simply written as a reaction between the reactants

giving rise to the products. Illustrating this for the deposition of TiC ﬁlms, one may write:

xTi(vapor) + C

(gas)→xTiC(solid) + yH(gas)

In a plasma-assisted deposition process, the reactants dissociate into fractions/radicals and

ionic species are produced. Therefore a multiplicity of reaction paths are possible and the

overall reaction becomes more complex. Deshpandey et al. [164] studied the synthesis of TiC

and TiN ﬁlms, evaporating Ti in a plasma of C

gases for the synthesis of TiC ﬁlms and N

or NH

with Ti for the synthesis of TiN ﬁlms. Several spectroscopies were used to carry out

diagnostics on the plasma in the source-substrate volume to determine the species present and

the potential reaction paths leading to ﬁlm formation. Neutral mass spectrometry (MS),

plasma mass spectrometry (PMS), and optical emission spectroscopy (OES) were used to

examine the nature and relative concentrations of neutral, excited, and ionized species present

in the process.

The main results of these investigations are as follows:



Polymerizing reactions producing higher molecular weight hydrocarbon species are

dominant in the case of methane. Polymerization increases with increasing ﬂow rate of

for a given e-beam current. The above reactions lead to the formation of relatively

soft ﬁlms containing TiC and graphitic phases.



Hard, single-phase TiC ﬁlms are formed at ﬂow rates of about 50 standard cm

min

−1

for beam currents in the range of 0.2–0.3 A. Polymerization reactions do not

take place when C

is used as a reactive gas. Species such as carbon, CH, and CH

formed in the plasma from the dissociation of C

react with titanium to form TiC.

The PMS and MS data indicate the following possible routes for formation of TiC:

(a) formation of TiC in the plasma volume through reactions such as:

Ti + C→TiC

Ti + CH→TiC + H

Ti + CH

→TiC + 2H

Ti + CH

→TiC + 3H

followed by condensation of TiC molecules on the substrate; or

Evaporation 207

(b) formation of Ti

or Ti

complexes in the plasma volume followed

by condensation on the substrate to form TiC according to:

→TiC + C

+ CH

→TiC + C

Present data are not sufﬁcient to determine which of these two schemes is

dominant in the formation of TiC. PMS and MS sampling of the arriving ﬂux on

the substrate as well as studies with a biased substrate are necessary to resolve

this issue.

Similar studies on the deposition of TiN ﬁlms revealed the following:



Evaporation of Ti in a N

plasma showed that the predominant species leading to hard

stoichiometric TiN ﬁlms is 2Ti

→ 2TiN. The ratio of Ti

in the plasma was

1.05, i.e. close to unity. When this ratio was increased to 1.5, soft ﬁlms with excess Ti

in the deposit were produced. Yee [139] also proposed the same reaction path based on

his optical emission spectrographic studies.



Evaporation of Ti in an NH

plasma showed similar results. Under conditions where

the Ti

ratio was high, the ﬁlms were soft and titanium rich. With a higher ﬂow

rate of NH

, the N

concentration in the plasma was higher and the ﬁlms were hard.

4.8.5 Materials Synthesized by Evaporation-Based Processes

A variety of metals, alloys, and compounds (oxides, nitrides, arbides, sulﬁdes) have been

deposited using evaporation and related processes. In particular, the plasma-assisted variant of

the evaporation process, such as ARE, has been successfully used for deposition of a variety of

compounds for tribological as well as optoelectronic applications. Recently, a modiﬁed

process based on the ARE technique has also proved to be successful in synthesizing c-BN

[156, 165]. A representative list of the compounds synthesized by the ARE process is given

below. In a very recent development, the ARE process has been able to deposit Al

ﬁlms at

very high deposition rates (8 to 12 ␮m/h); these rates are 10–30 times higher than those by

sputter deposition [166].

The compounds synthesized by the ARE process include:



Carbides: TiC, HfC, ZrC, VC, W

C, TaC



Carbonitride: Ti (C, N)



Nitrides: TiN, HfN, ZrN, CrAlTiN, TiSiN



Oxides: TiO

, ZrO

,Al

, SiO

208 Chapter 4



Sulﬁdes: TiS

, MoS



Superconductors: low T

:Nb

Ge, CuMo

; high T

: YBa

7–8



Photovoltaic materials: a-SiH, CuInS



Optoelectric materials: In(Sn)O

, ZnO



Novel materials: c-BN, Diamond, i-C, a-C.

4.8.6 Deposition of Nanocomposite Materials

Nanocomposite thin ﬁlms containing embedding metals in polymer matrices can be prepared

by various techniques. Nanocomposites provide the ﬂexibility of tuning electrical, magnetic,

and other physical properties of the deposited ﬁlm independently by controlling the chemistry

of the constituents. Biswas et al. deposited Ni nanoparticles with Teﬂon layers by vapor-phase

tandem evaporation [167]. This technique allows metals and polymers to be evaporated at the

same time. They controlled the Ni nanoparticle size while generating low to relatively high

cluster-volume ﬁlling in the polymer. In evaporation of metals Takele et al. tuned the electrical

and structural properties by the simultaneously evaporation of metal (Ag, Au) and polymer

[168]. One of the most crucial factors in this deposition is the sticking coefﬁcient of metal

nanoparticles on polymers. The electrical properties mainly depend on the metal ﬁlling factor,

particle size, size distribution, and metal particle/insulator interface properties.

Zaporojtchenko et al. described the effects of metal interaction with various polymer matrices

by evaporating Cu, Ag, and Au on fully cured polymer ﬁlms [169].

4.9 Microstructure of PVD Condensates

4.9.1 Microstructure Evolution

PVD condensates deposit as single crystal ﬁlms on certain crystal planes of single crystal

substrates, i.e. by epitaxial growth [170], or in the more general case, the deposits are

polycrystalline. In the case of ﬁlms deposited by evaporation techniques, the main variables

are: (1) the nature of the substrate; (2) the temperature of the substrate during deposition;

(3) the rate of deposition; (4) the deposit thickness; (5) the angle of incidence of the vapor

stream; and (6) the pressure and nature of the ambient gas phase. Contrary to what might be

intuitively expected, the deposit does not start out as a continuous ﬁlm one monolayer thick

and grow. Instead, three-dimensional (3D) nuclei are formed on favored sites on the substrates,

e.g. cleavage steps on a single crystal substrate; these nuclei grow laterally and in thickness

(the so-called island growth stage), ultimately impinging on each other to form a continuous

ﬁlm. The average thickness at which a continuous ﬁlm forms depends on the deposition

temperature and the deposition rate (both of which inﬂuence the surface mobility of the

Evaporation 209

adatom) and varies from 10

A for Ni condensed at 15 K to 1000

A for Au condensed at 600 K.

This familiar model of island growth of a polycrystalline ﬁlm during the initial stages of

deposition illustrates the case where there is limited interaction between depositing atoms and

the substrate. This is not always the case.

Petrov et al. [171] reviewed ﬁlm growth processes which include nucleation, coalescence,

competitive grain growth, and recrystallization. They also discussed evolution as a function of

deposition variables including temperature, the presence of reactive species, and the use of

low-energy ion irradiation during growth [171]. Types of substrates, such as amorphous and

polycrystalline, used in PVD processes play an important role in the microstructural

evolution in thin ﬁlms synthesized by low-temperature PVD. The use of amorphous substrates

allows the isolation of the effects of individual deposition variables on texture development.

However, polycrystalline substrates bias texture through local pseudomorphic epitaxy, with the

same overall microstructure evolution toward the ﬁnal state driven by the deposition

conditions. On both types of substrates, ﬁlm growth proceeds via a 3D or Volmer–Weber

model [171].

The growth processes controlling microstructure evolution, nucleation, island growth,

impingement and coalescence of islands, grain coarsening, formation of polycrystalline

islands and channels, development of a continuous structure, and ﬁlm growth, are presented

schematically in Figure 4.46. In the formation of ﬁlm, grain coarsening, i.e. recrystallization

through grain boundary (GB) migration, can occur both during and after island coalescence.

The nucleation barrier is generally expected to be small, leading to randomly oriented islands,

for low-temperature deposition on amorphous substrates [173, 174]. In situ transmission

electron microscopy (TEM) investigations conﬁrm this for studies of Au/SiO

[175, 176] and

In/C [177, 178]. Nucleation kinetics depend on the adatom binding energy, crystal structure of

the substrate material, lattice defects, surface steps, and contamination.

Polop et al. studied the initial stages of polycrystalline Ag ﬁlm formation deposited by thermal

evaporation on the amorphous Si layer [179]. The deposition temperature was 300 K and the

deposition rate 8 × 10

−3

nm/s. After the deposition the ﬁlms were analyzed by in situ scanning

tunneling microscopy (STM). Figure 4.47 displays STM topographs of the evolution of Ag

ﬁlm morphology as a function of the ﬁlm thickness. Polop’s results show that the ﬁlm

morphology is distinguished by three regimes: (1) ﬁlm thicknesses smaller than 0.8 nm – the

nucleation and island formation (Figure 4.47a), (2) ﬁlm thicknesses between 0.8 and 10 nm –

island growth regime (Figure 4.47b–d) and (3) ﬁlm thicknesses higher than 10 nm –

continuous ﬁlm regime (Figure 4.47e–f).

Akkari et al. deposited CuInS

ﬁlms by thermal evaporation with a ﬂux angle  with rotation

of the substrate [180].InFigure 4.48, when the ﬂux angle  increases, the tilting of the

columns also increases. When the substrate is not rotated the ﬁlm structure consists of

210 Chapter 4

Figure 4.46: Schematic diagram illustrating fundamental growth processes controlling

microstructural evolution [171, 172].

nanocolumns that are inclined towards the evaporation source. When the substrate is rotated

and the angle is at 80

◦

, the structure takes straight wire forms [180].

Important differences have been observed. Namba and Mori [181] found that by converting a

signiﬁcant fraction (∼ 10%) of the vapor ﬂux of Ag to positive ions, epitaxial growth of a

single crystal Ag ﬁlm on a single crystal NaCl substrate biased to −3000 V was observed,

whereas with vacuum evaporation, the Ag ﬁlm was polycrystalline. No clear explanation is

possible except to note that the mobility of the deposited species is much greater when partially

ionized than for neutral vapor species. The effective surface temperature of the growing ﬁlm is

much higher owing to ion bombardment, thus permitting greater surface mobility and resulting

in epitaxial growth. Taylor [182] used low-energy electron diffraction (LEED) techniques to

study the epitaxial deposition of Cu onto a single crystal [183] face of tungsten under ultrahigh

vacuum conditions. This represents the case where there is appreciable bonding between

depositing atoms and the substrate. The deposit on a clean tungsten surface was a uniformly

thin [184] Cu ﬁlm, i.e. no island growth prior to the formation of a continuous ﬁlm even at