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

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Evaporation 211

Figure 4.47: STM topographs of Ag ﬁlms deposited at 300 K with thicknesses of (a) 0.3 nm,

(b) 1.0 nm, (c) 2.7 nm, (d) 8.5 nm, (e) 15.9 nm, and (f) 30.0 nm. Image size = 160 × 160 nm. The

inset in (a) shows an STM image (60 × 60 nm) of the amorphous substrate, and in (b), a line scan

between the arrows in the topograph, which reaches down to the substrate. The circles in

(b) enclose islands with GBs.

thicknesses of 1{1/2} atomic layers. He further observed that chemisorption of even a half

monolayer of oxygen severely inhibited epitaxial growth.

Sherman et al. [185] studied the deposition of thick Mo ﬁlms onto a rolled Mo sheet substrate

as a function of deposition temperature. They observed polycrystalline deposits at all

212 Chapter 4

Figure 4.48: Cross-sectional SEM images of the CuInS

thin ﬁlms deposited at different ﬂux

angles: (a)  =40



; (b)  =80



without substrate rotation; and (c)  =80

with substrate

rotation ω = 0.033 rev/s.

temperatures except in the range of 973–1188 K, where the surface oxide MoO

is unstable

and evaporates rapidly, thereby leaving behind a ‘clean’ Mo surface on which epitaxial growth

can readily occur aided by the high surface mobility at the elevated deposition temperature.

Once a continuous ﬁlm has formed, the subsequent evolution to the ﬁnal structure of the thin

ﬁlm is poorly understood at present. It undoubtedly depends on the factors mentioned above,

which in turn inﬂuence the primary variables of nucleation rate, growth rate, and surface

mobility of the adatom. The problem has been tackled by Van der Drift [186] and is also the

subject of a paper by Thornton [187].

The microstructure and morphology of thick single-phase ﬁlms have been extensively studied

for a wide variety of metals, alloys, and refractory compounds. The structural model was ﬁrst

proposed by Movchan and Demchishin [123] (Figure 4.49), and was subsequently modiﬁed by

Figure 4.49: Structural zones in condensates [123].

Evaporation 213

Figure 4.50: Structural zones in condensates [187].

Thornton as shown in Figure 4.50. Movchan and Demchishin’s diagram was arrived at from

their studies on deposits of pure metals and did not include the transition zone of Thornton’s

model, zone T, which is not prominent in pure metals or single-phase alloy deposits, but

becomes quite pronounced in deposits of refractory compounds or complex alloys produced

by evaporation, and in all types of deposits produced in the presence of a partial pressure of

inert or reactive gas, as in sputtering or ion-plating processes.

The evolution of the structural morphology is as follows.

At low temperatures, the surface mobility of the adatoms is reduced and the structure grows as

tapered crystallites from a limited number of nuclei. It is not a full-density structure but

contains longitudinal porosity of the order of a few hundred angstroms’ width between the

tapered crystallites. It also contains a high dislocation density and has a high level of residual

stress. Such a structure has also been called ‘Botryoidal’ and corresponds to zone 1 in Figures

4.49 and 4.50.

As the substrate temperature increases, the surface mobility increases and the structural

morphology ﬁrst transforms to that of zone T, i.e. tightly packed ﬁbrous grains with weak

grain boundaries, and then to a full-density columnar morphology corresponding to zone 2

(Figure 4.50).

The size of the columnar grains increases as the condensation temperature increases. Finally,

at still higher temperatures, the structure shows an equiaxed grain morphology (zone 3). For

214 Chapter 4

pure metals and single phase alloys, T1 is the transition temperature between zone 1 and zone

2 and T2 is the transition temperature between zones 2 and 3. According to Movchan and

Demchishin’s original model [123], T

is 0.3 T

for metals, and 0.22–0.26 T

for oxides,

whereas T

is 0.45–0.4 (T

is the melting point in K).

Thornton’s modiﬁcation shows that the transition temperatures may vary signiﬁcantly from

those stated above and, in general, shift to higher temperatures as the gas pressure in the

synthesis process increases. It should be emphasized that:



The transition from one zone to the next is not abrupt but smooth. Hence the transition

temperatures should not be considered as absolute, but as guidelines.



All zones are not found in all deposits. For example, zone T is not prominent in pure

metals, but becomes more pronounced in complex alloys, compounds, or in deposits

produced at higher gas pressures. Zone 3 is not seen very often in materials with high

melting points.

The reader is referred to a more extensive description given by Greene in Chapter 12, which

includes a discussion of the effects of substrate surface roughness and pressures.

Most thick deposits exhibit a strong preferred orientation (ﬁber texture) at low deposition

temperatures and tend toward a more random orientation with increasing deposition

temperature. Figure 4.51 shows the evolution of a large-grained columnar morphology in a Be

deposit from a much larger number of ﬁne grains which were originally nucleated on the

substrate. As growth proceeds, only those grains with a preferred growth direction survive,

presumably owing to considerations of the minimization of surface energy.

Figure 4.51: Photomicrograph of a Be deposit showing the evolution of large columnar grains.

Evaporation 215

Figure 4.52: SEM micrographs of TiO

ﬁlms annealed at various temperatures. Substrate

temperatures: (a) 150



C, (b) 200



C, (c) 250



Chen et al. studied the effects of deposition temperature on the microstructure of TiO

ﬁlms.

Films were deposited by ion assisted deposition in which e-beam evaporation was used to

evaporate Ti

and a Kaufman-type ion-beam source was used to modify the morphology of

the ﬁlms [188]. Figure 4.52 shows the microstructure of the as-deposited 150

◦

C ﬁlm which

has a columnar structure. The microstructure persisted up to the annealing temperature of

300

◦

C. The increase in deposition temperature affected the columnar growth of the ﬁlm. As

the deposition temperature increased columns became denser. The as-deposited samples show

different recrystallization temperatures. The crystallization temperature was determined from

SEM images, X-ray diffraction (XRD) data and refractive indices of the ﬁlms. For ﬁlms

deposited at 150, 200 and 250

◦

C, the crystallization temperature was found to be 300, 250 and

200

◦

C, respectively.

Elegant proof of the importance of surface mobility was also provided by Movchan and

Demchishin [123]. Plots of the log of the grain diameter versus the inverse of deposition

temperature in zones 2 and 3 yield straight lines from which activation energies can be

computed. It was found that the activation energy for zone 2 growth corresponded to that for

surface self-diffusion and for zone 3 growth to volume self-diffusion. The morphological

results reported by Movchan and Demchishin for nickel, titanium, tungsten, Al

, and ZrO

216 Chapter 4

Table 4.9: Transition temperatures between various structural zones

Material Melting

temp., T

(K)

(K) T

(K)/T

(K) T

(K)/T

(K) Ref.

Ti 1945 653 ± 10 0.3 923 0.5 [98]

Ti 1945 673 ± 20 0.31 Phase

transformation

overlaps

– [113]

Ni 1726 543 ± 10 0.3 723 ± 10 0.45–0.5 [98]

Ni 1726 – – 777 ± 20 0.45 [114]

W 3683 1133 ± 50 0.3 1723 ± 50 0.45–0.5 [98]

Mo 2883 923 ± 20 0.3–0.34 1200 0.44 [189]

Fe 1810 – – Phase

transformation

overlaps

– [190]

Be 1573 473 ± 20 0.29 1023 ± 50 0.63 [191]

Ni–2OCr 1673 500 ± 50 0.3 870 0.52 [192]

ZrO

2973 648 ± 10 0.22 1273 0.45–0.5 [98]

2323 623 ± 10 0.26 1173 0.5 [98]

TiC 3340 1070 ± 30 0.31 Not observed

up to 1723 K or

0.51 T

– [113]

NbC Obeys the Movchan–Demchishin model [193]

ZrO

Obeys the Movchan–Demchishin model [193]

From structures observed at a speciﬁc deposition temperature, Au–Cr

and V

appear to obey the Movchan–Demchishin

model.

have been conﬁrmed for several metals and compounds. The data are given in Table 4.9

[29, 30, 194, 195].

Bunshah and Juntz [196] studied the inﬂuence of condensation temperature on the deposition

of titanium. Their microstructures, shown in Figure 4.53, agree substantially with those of

Movchan and Demchishin for zones 1 and 2 and T

, the transition temperature between zones

1 and 2. However, they failed to observe zone 3 at the temperatures above 700

◦

C found by

Movchan and Demchishin [123]. The structure was columnar up to 833

◦

C, which is the α:β

phase transformation temperature for titanium. At deposition temperatures above 833

◦

C, the

deposit crystallizes as the β phase and on cooling to room temperature, should transform to the

α phase, resulting in the typical ‘transformed-beta’ microstructure shown in Figure 4.53

(900

◦

C deposit), which could be mistaken for an equiaxed microstructure. Hence, the claim of

such a transition in structure from zone 2 to 3 by Movchan and Demchishin for titanium

deposits is confusing.

Evaporation 217

Figure 4.53: Structure of titanium deposits at various substrate temperatures [200].

Semaltianos and Wilson [197] investigated the surface morphology of thermally evaporated

gold thin ﬁlms on mica, glass, silicon, and calcium ﬂuoride substrates, and at different

temperatures. According to Figure 4.54, the average particle size of gold ﬁlms is 15, 20, and

25 nm for mica, CaF

, and Si substrates. However, for glass substrates large accumulations of

gold are formed consisting of many smaller particles. The increase in particle size for Si and

CaF

can be due to ionic interaction between the substrate and the ﬁlm.

Kane and Bunshah [198] observed the change in morphology in deposited nickel sheet. At

425

◦

C deposition temperature, the deposit showed a zone 2 morphology, whereas at 554

◦

the deposit showed a zone 3 morphology.

Chambers and Bower [199] studied the deposition of magnesium, copper, gold, iridium,

tungsten, and stainless steel. Of the photomicrographs presented, gold and magnesium showed

zone 2 columnar morphology at the appropriate substrate temperatures.

218 Chapter 4

Figure 4.54: STM images of gold ﬁlms grown on (a) glass, (b) mica, (c) CaF

, and (d) Si with the

substrates held at room temperature.

Figure 4.55 shows surface and cross-section photomicrographs of a Ni–20Cr sheet deposited

by Agarwal et al. [194].At950

◦

C, 760

◦

C, 650

◦

C, and 427

◦

C deposition temperatures, the

surface and cross-section showed an equiaxed zone 3 morphology.

Mah and Nordin [201] found that the Movchan–Demchishin model was obeyed by beryllium.

They observed structures corresponding to all three zones with transition temperatures as

predicted by the model.

Neirynck et al. [202] studied the inﬂuence of deposition rate and substrate temperature on the

microstructure, adhesion, texture, and condensation mechanism of aluminum and zirconium

coatings on steel substrates and wires in batch and continuous-coating methods.

Kennedy [203] showed a change in morphology from columnar to equiaxed in Fe and

Fe–10Ni alloy with higher deposition temperature. Deposits of Fe–1%Y, which is a two-phase

alloy, showed columnar morphology only, the structure becoming coarser at higher deposition

temperature. The second phase appears to nucleate new grains so that the grain size in

Fe–1%Y alloys is much ﬁner than that of iron.

The microstructure of copper–nickel alloys [31] produced by co-deposition from two sources

showed a single phase, as might be expected for this system, which shows a complete solid

solubility. On the other hand, sequential deposition of Cu and Ni from two sources shielded

from each other onto a rotating substrate produced a microlaminate structure in the deposit

Evaporation 219

Figure 4.55: Photomicrographs of typical Ni–20Cr deposits at various substrate temperatures

[194].

where the laminate size could be varied from 0.01 to 40 ␮m by adjusting the deposition

parameters [204]. Similar structures were also developed in the Fe–Cu [204] and Ti–B

systems [204].

ln alloy systems showing the presence of several phases, e.g. Ni–B and Cr–Si, the deposits

showed that the phases present corresponded to those expected from the diagram [31].

Smith et al. [27] studied the deposition of the two-phase (α + β) type Ti–6Al–4V alloy

deposited from a single rod-fed source. The microstructure was very similar to wrought

material with the same characteristic α + β morphology present on a ﬁner scale in the

deposited material.

220 Chapter 4

Dispersion-strengthened alloys produced by co-deposition from multiple sources have also

been produced. Paton et al. [31] produced Ni–TiC, Ni–NbC and Ni–ZrO

alloys. The particle

size increased from 100 to 1000

A by changing the deposition temperature from 350 to

1000

◦

C. The size of the dispersed carbide phase particles increased on annealing at

1000–1100

◦

C owing to their slight solubility in nickel. On the other hand, the size and

distribution of ZrO

dispersion remained constant even after exposure at 1300

◦

C for 5 h, as

shown in Figure 4.56.

Movchan et al. [205] produced Fe–NbC and Fe–Ni–NbC dispersion strengthened alloys by

co-evaporation. The microstructure exhibited columnar morphology, with the inclusion of a

ﬁne dispersion of NbC particles.

Raghuram and Bunshah [206] studied the microstructure of TiC deposits from 500 to 1450

◦

(Figure 4.57). They observed the transition from the tapered crystallite (zone 1) to columnar

structure at 973 K, or 700

◦

C (0.3 T

). The highest deposition temperature (1450

◦

C) used by

these investigators was not sufﬁcient to produce an equiaxed structure, although this

temperature corresponds to 0.51 T

Xu et al. investigated the growth texture of MgO ﬁlms deposited by e-beam deposition at

different inclination angles [207] using SEM. The results are shown in Figure 4.58. Their

study showed that the deposition rate increases with the increasing inclination angle. However,

the surface loses its smoothness and shows more voids at higher angles, especially after 50

◦

The energy of the depositing beam of atoms can be increased if some of them are ionized. It

has been shown [23] that a small fraction of the vaporized species from an e-beam-heated

source is ionized owing to collisions with electrons in the plasma sheath above the molten

pool. Bunshah and Juntz [200] biased the substrate to −5000 V during the deposition of

beryllium at 570

◦

C and found that the columnar grain size was markedly reﬁned by the ion

bombardment as compared to the grain size produced without biasing the substrate at the same

deposition temperature. It may be postulated that the ion bombardment causes a localized

increase in temperature at the surface where deposition is occurring, thus causing a higher

nucleation rate and a ﬁner grain size. Similar results have been reported for tantalum [208].

The use of a hollow cathode gun intensiﬁes the degree of ionization of the vapor species,

resulting in a marked increase in kinetic energy of the vaporized atoms [209]. The effects of

substrate bias are, therefore, easier to observe. Increasing the substrate bias results in a change

in morphology from columnar to ﬁne, equiaxed grains for silver deposited on beryllium and

stainless steel [210], and silver and copper deposited on stainless steel [211].

The presence of a gas at high pressures (5–20 ␮m) results in a net decrease in kinetic energy of

the vaporized atoms due to multiple collisions during the transverse from source to substrate.

This degrades the microstructure to lose columnar grains [211] and eventually to an

agglomerate of particles. (This is a way to produce ﬁne powders by evaporation and