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

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Evaporation 181

Figure 4.36: Schematic of an RF sputtering system (after Davidse and Maissel) with

Used with permission of McGraw-Hill Book Co.)

Figure 4.37: Wheatstone-bridge circuit for resistance monitoring. (From Handbook of Thin Film

182 Chapter 4

weighed amount of source material to completion. Knowing the emission characteristics of

the source will allow the ﬁlm thickness to be calculated. Alternately, monitoring devices

discussed earlier can be calibrated to measure thickness directly.

4.7.4.2 Rate Control

Rate control is a more complex task and involves measuring the signal from a rate monitor and

using it in a feedback loop to control the power to the source and hence its temperature and

evaporation rate. Table 4.5 illustrates the pros and cons of various evaporation process control

methods.

4.8 Deposition of Various Materials

4.8.1 Deposition of Metals and Elemental Semiconductors

Evaporation of single elements can be carried out from a variety of evaporation sources subject

to the restrictions discussed above dealing with melting point, reactions with container,

deposition rate, etc. A typical arrangement is shown in Figure 4.1 for e-beam heating.

4.8.2 Deposition of Alloys

Alloys consist of two or more components, which have different vapor pressure and hence

different evaporation rates. As a result, the composition of the vapor phase, and therefore the

deposit, has a constantly varying composition. There are two solutions to this problem:

multiple sources and single rod-fed or wire-fed e-beam sources.

4.8.2.1 Multiple Sources

This is the more versatile system. The number of sources evaporating simultaneously is equal

to or less than the number of constituents in the alloy. The material evaporated from each

source can be a metal, an alloy, or a compound. Thus, it is possible to synthesize a dispersion

strengthened alloy, e.g. Ni–ThO

. On the other hand, the process is complex because the

evaporation rate from each source has to be monitored and controlled separately. The source to

substrate distance has to be sufﬁciently large (15 inches for 2 inch diameter sources) to have

complete blending of the vapor streams prior to deposition, which decreases the deposition

rate (Figure 4.38). Moreover, with gross differences in density of two vapors, it may be

difﬁcult to obtain a uniform composition across the width of the substrate owing to scattering

of the lighter vapor atoms. In another example, Nicholls et al. reported hot corrosion-

resistant Ni–Cr–Al coatings by magnetron sputtering or multiple target e-beam evaporation

[116].

Evaporation 183

Table 4.5: Evaporation process control

Control method

Electrical signal Prerequisites for

Available

Related to Thickness control

Rate control

Evaporation to

completion

No – Weighed evaporant change Not applicable

Control of source

temperature,

power, or current

Yes Evaporation rate Rates must be integrated Error-signal feedback

Ionization rate

monitor

Yes Evaporation rate Rates must be integrated Error-signal feedback

Particle impingement

devices

No – Direct observation on pivoted

models, manual stop

Direct observation on

torsion-wire models, manual

power adjustment

Microbalances Electromagnetic or

electrostatic

models: yes

Deposit thickness Strip-chart recorder, preset

microswitch for automatic

termination

Programmed weight increase or

second signal by electronic

differentiation

Crystal oscillator Yes Deposit thickness Counter, meter, or recorder;

preset microswitch for

automatic termination

Second signal by

differentiation; servoloop

Optical monitors With photocell: yes Deposit thickness Strip-chart recorder; fairly

complex circuitry for

automatic termination

Has not been implemented yet

Resistance monitors Yes Deposit thickness Strip-chart recorder and preset

microswitch, or digital

techniques

Second signal by electronic

differentiation; servoloop

Capacitance monitors No Deposit thickness

or rate

Planar capacitor substrate;

oscillator circuit with reference

capacity and comparator

Direct rate indication from

Riddle’s vapor capacitor

184 Chapter 4

Figure 4.38: Two-source evaporation arrangement yielding variable ﬁlm composition. (From

McGraw-Hill Book Co.)

Some examples are given in Table 4.6. It is possible to evaporate each component sequentially,

thus producing a multilayered deposit which is then homogenized by annealing after

deposition. This procedure makes it even more difﬁcult to achieve high deposition rates. A

multiple source arrangement for production of alloy deposits at high rates is not known.

4.8.2.2 Single Rod-Fed Electron Beam Source

The disadvantages of multiple sources for alloy deposition can be avoided by using a single

source [117, 118]. It can be a wire-fed or rod-fed source; the latter is shown in Figure 4.39.

There is a molten pool of limited depth above the solid rod. If the components of an alloy,

A1B10, have different equilibrium vapor pressures, then the steady-state composition of the

molten pool will differ from the feed rod, e.g. A1B10. Under steady-state conditions, the

composition of the vapor is the same as that of the solid being fed into the molten pool. One

has the choice of starting with a button of appropriate composition A1B10 on top of a rod

A1B1 to form the molten pool initially, or starting with a rod of alloy A1B1 and evaporating

until the molten pool reaches composition A1B10. Precautions to be observed are that the

temperature and volume of the molten pool have to be constant to obtain a constant vapor

composition. A theoretical model has been developed and conﬁrmed by experiment. Ni–20Cr,

Ti–6Al, Ag–5Cu, Ag–10Cu, Ag–20Cu, Ag–30Cu, Ni–xCr–yAl–xY alloy deposits have been

successfully prepared. To date, experimental results indicate that this method can be used with

vapor pressure differences of a factor of 5000 between the components. This method cannot be

used where one of the alloy constituents is a compound, e.g. Ni–ThO

Evaporation 185

Table 4.6: Two-source evaporation, experimental conditions, and types of ﬁlms obtained

Evaporated

constituents

Evaporation conditions and

method of control

Substrate

temp. (

◦

Films obtained

Alloy and multiphase

ﬁlms

Cu + Ni Sequential evaporation Low Stratiﬁed ﬁlms annealing at 200

◦

C yields

two-phase alloy ﬁlms

Cu, Ag, Au, Mg, Sn,

Fe, Co

Simultaneous evaporation from two sources

Isolation-rate monitor control, ±1%

−193 Binary alloy ﬁlms of metastable structures

Cu, Ag, Au, Al, Ni,

and others

Simultaneous evaporation from two sources.

Rates adjusted by varying source temperature

25–600 Binary alloy ﬁlms of varying composition

and structure

Ni + Fe Two wire-ring sources, evaporation rates

controlled by quartz-crystal oscillator

300 Permalloy ﬁlms. d



≈ 10

−1

Nb + Sn Two sources, rates monitored by particle

impingement-rate monitor. Impingement ratio

25–700 Superconducting Nb

Sn ﬁlms,



≈ 10

−1

. ˛

≈ 1

V, Nb + Si, Sn Two electron-gun sources, rates monitored by

measuring ionization current

– Superconducting ﬁlms of approx

composition Nb

Sn and V

ZnS + LiF Two sources, rates monitored by a

micro-balance. Variable impingment ratios

30–40 Mixed dielectric ﬁlms of different

composition. d



≈ 10–30

−1

Au, Cr + SiO, MgF

Two-source evaporation with ionization-rate

monitor content (±1–2%)

25–300 Au–SiO, Au–MgF

, Cr–SiO, and Cr–MgF

;

resistor ﬁlms of different compositions

Cr + SiO SiO source at 1100

◦

C, Cr source at 1500

◦

Impingement ratio varied with location on

substrate

400 High-resistivity Cr–SiO ﬁlms of variable

composition

Cd + S Two effusion overs, Cd at 400–450

◦

C,Sat

120–150

◦

C. Cd excess

400–650 Stoichiometric CdS crystals

Cd + Se Impingement ﬁlms controlled by source

temperature. N

≈ 2 × 10

≈ 10

–10

−1

200 Stoichiometric CdS ﬁlms

(Continued)

186 Chapter 4

Table 4.6: (Continued)

Evaporated

constituents

Evaporation conditions and

method of control

Substrate

temp. (

◦

Films obtained

PbSn + PbTe Source temperature varied around 750

◦

C 300 Epitaxial ﬁlms of PbSe

1∼s

on NaCl

crystals

Bi + Te Bi source at 750

◦

C, Te source temperature

variable. N

= 10–40

400–500 Stoichiometric ﬁlms of BiTe

. n-type,

2 × 10

electrons cm

−3

Bi + Se Rate control by quartz-crystal oscillator. Se

source at 250

◦

C; Bi source temperature

variable

52 Vitreous, semiconducting ﬁlms of

non-stoichiometric composition

Al + Sb Source temperature adjusted by quartz-crystal

oscillator to yield N

ratios of 1.6–16

550 Stoichiometric AlSb ﬁlms d



≈ 10

−1

Ga + As Ga source at 940–970

◦

C, As source at 300

◦

= 10; Ga implingement ﬂux:

−2

−1

550 Stoichiometric GaAs ﬁlms. Epitaxial on

(100); NaCl, polycrystalline on quartz

Ga + Aa Ga source at 910

◦

C, As source at 295

◦

Deposition rate: <2

−1

375–450 Stoichiometric GaAs ﬁlms on GaAs. Ga,

and Al

single-crystal substrates. Filter

featured to single-crystalline

In + As Incident ﬂuxes: N

=5× 10

−2

−1

=5× 10

−2

−1

230–680 Stoichiometric InAs ﬁlms: n-type

In + Sb Incident ﬂuxes: N

=5× 10

−2

−1

=5× 10

–5 × 10

−2

−1

400–520 Stoichiometric InSb ﬁlms: n-type

Source temperature adjusted by microbalance

to yield N

=1.1

250 Stoichiometric InSb ﬁlms: ˛

of Sb ≈ 0.6

Evaporation 187

Figure 4.39: Schematic of direct evaporation of an alloy from a single rod-fed source.

Suzuki et al. deposited TiAl alloy by intense pulsed ion beam evaporation [119]. When pure Ti

and Al targets were used they were unable to produce intermetallic compound. However,

sintered Ti/Al target successfully produced the Ti

Al thin ﬁlm. The schematics of the

apparatus is shown in Figure 4.40.

Shevakin et al. [120] investigated the relationship between the composition of the evaporant

material and the condensates for alloy evaporation using e-beam evaporation techniques. They

used this method to determine thermodynamic activities of the components of binary alloys at

temperatures above the melting point of the alloy.

4.8.3 Deposition of Intermetallic Compounds

Intermetallic compounds which are generally deposited, such as GaAs, PbTe, InSb, and TiAl,

have as their constituents elements with low melting points and high vapor pressures. These

compound semiconductors need to have a carefully controlled stoichiometry, i.e. cation:anion

ratio. Therefore, they can best be prepared by ﬂash evaporation or sputtering.

188 Chapter 4

Figure 4.40: Intense pulsed ion beam evaporation system.

compounds are harder to deposit because of their stoichiometry and the fact that the

elements of 5A group are volatile. Lida deposited InSb ﬁlms by ﬂash evaporation to investigate

the effects of dislocation levels on electrical resistivity. He found that the texture depends on

the evaporation source temperature [121]. Flash evaporation techniques have advantages on

depositing A

compounds for texture studies [122]. First, any excess antimony above the

stoichiometric composition is not effectively re-evaporated from the substrate if the substrate

is kept at temperatures lower than 350

◦

C. Therefore, deposition methods, like MBE, cannot be

used effectively in that low-temperature deposition region. Second, while the sublimation of

antimony leads to a vapor composed mainly of Sb

molecules, the evaporation of InSb leads to

a vapor containing more antimony in the form of Sb and Sb

molecules. This facilitates the

crystallization process, especially at low temperatures, and shifts down the transition

temperature between the amorphous and crystalline state.

In ﬂash evaporation, powder or chips of the two components are sprinkled onto a superheated

sheet to produce complete evaporation of both components. Various possible arrangements are

shown in Figure 4.41. Table 4.7 gives examples of the use of this technique.

4.8.4 Deposition of Refractory Compounds

Refractory compounds are substances like oxides, carbides, nitrides, borides, and sulﬁdes

which characteristically have a very high melting point (with some exceptions). In some cases,

they form extensive defect structure, i.e. exist over a wide stoichiometric range. For example,

in TiC, the C:Ti ratio can vary from 0.5 to 1.0, demonstrating vacant carbon lattice sites. In

other compounds, the stoichiometric range is not so wide.

Evaporation 189

Figure 4.41: Flash-evaporation mechanisms: (A) belt feeder (Harris and Siegel); (B) worm-drive

feeder with mechanical vibrator (Himes, Stout, and Thun, Braun and Lood); (C) disk feeder

(Beam and Takahashi); (D) disk magazine f eeder (Marshall, Atlas, and Putner); (E) mechanically

vibrated trough and cylinder source (Richards); (F) electromagnetically vibrated powder dispenser

Used with permission of McGraw-Hill Book Co.)

Evaporation processes for the deposition of refractory compounds are further subdivided into

two types: (1) direction evaporation [123], where the evaporant is the refractory compound

itself; and (2) reactive evaporation [124] or ARE [125], where the evaporant is a metal or a

low-valence compound, e.g. where Ti is evaporated in the presence of N

to form TiN or

where Si or SiO is evaporated in the presence of O

to form SiO

Recently, Alberdi et al. [126] deposited Cr–TiAl–N ﬁlms by simultaneous arc evaporation of

Cr and Ti–Al targets under a mixture of Ar and N

gas atmosphere. The nanocomposite ﬁlm

190 Chapter 4

Table 4.7: Flash evaporation of materials

Materials Form of

evaporant

Feeder

mechanism

Filament

temp.

◦

Substrate

temp.

◦

Comments on ﬁlms

Metals and alloys

Aa(64)–Cd(36),

Cu(52)–Zn(48)

Powdered alloys,

80/100 mesh

Moving bell – – Au–Cd and ˇ brass ﬁlms have

combination of source

Ni + Fe Mixed powders Disk and wiper 1930 – Ni–Fe ﬁlms with ±1% control

of composition

Ni(86)–Fe(14) Alloy wire Spool and guide

tube

2000 300 Ni–Fe ﬁlm composition equal

to that of source ±0.2%

Ni; Fe; Cu constants;

chromel; alumel

Pellets Disk magazine 2000 200–250 Thin ﬁlm thermocouples.

Rates: 5–300

−1

across

12 cm distance

Ni(80)–Cr(20) Alloy wire Spool and guide

tube

1620 – Nichrome ﬁlm, Cr content

varies with ﬁlament

temperature

Ni + Cr(20, 55, 70%

Cr)

Mixed powders,

100/300 mesh

Vibrating chute 1800 300 Alloy ﬁlms within 1% of

source composition. Rates:

1–10

−1

Sn; nylon Wire; strands Two spools and

cutting knives

200 (nylon) ∼0 Alternate layers of metal and

insulator

Metal-dielectric mixtures

Cu + SiO (1: 5) Mixed powders Rotating tube – −269 Highly disordered ﬁlms of

high resistivity

Cr + 30 mol% SiO Mixed powders,

325/500 mesh

Worm drive ∼2000 200 250 Ohm-sq resistor ﬁlms

with +20 to 50% deviations

Cr + SiO (82 and 74

mol% Cr)

Mixed powders,

125/325 mesh

Worm drive 2000 400 Resistor ﬁlms; SiO content

less than source. Rates:

−1

across 23 cm distance

(Continued)