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

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Evaporation 191

Table 4.7: (Continued)

Materials Form of

evaporant

Feeder

mechanism

Filament

temp.

◦

Substrate

temp.

◦

Comments on ﬁlms

Cr–SiO (60–100 mol

% Cr)

Sintered pellets,

∼0.7 mm size

Disk and wiper 2050 ± 60 200 Resistor ﬁlms, SiO content

equals that of source ±1–3%.

Rates: 20–30

−1

across 70

cm distance

Cr(15)–Si(85) Powdered alloy Vibrating chute 2000 200–500 Resistor ﬁlms

Si + TaSi

+Al

Mixed powders Worm drive 2500 200–400 Resistor ﬁlms with ±10%

control

Compounds

AlSb Powder, 100/150 mesh Vibrating trough 1400–1600 700 Imperfect epitaxial ﬁlms on

GaP Powder, 100/150 mesh Vibrating trough 1500 540 Epitaxial ﬁlms on Ge crystals

GaAs Powder, 100/200 mesh Vibrating trough 1450 300–670 Epitaxial ﬁlms on Ge crystals

above 600

◦

C. Rates:

2–30

−1

Powder, 100/200 mesh Worm drive 1400–1800 530 ± 10 Epitaxial ﬁlms on GaAs

crystals. Rates: 2–5

−1

across 21 cm distance

Powder, 100/150 mesh Vibrating trough 1300–1800 475–525 Epitaxial ﬁlms on Ge crystals

Powder, 40/60 mesh Micrometer

screw and piston

1325 525–575 Highly oriented ﬁlms on Ge

and GaAs crystals. Rates:

10–25

−1

across 10 cm

distance

GaSb Powder, 100/150 mesh Vibrating trough 1650 500 Epitaxial ﬁlms on Ge crystals

InP Powder, 100/150 mesh Vibrating trough 1400–1650 300 Epitaxial ﬁlms on Ge crystals

InAs Powder, 100/150 mesh Vibrating trough 1500 500 Epitaxial ﬁlms on Ge crystals

InSb Granules Vibrating trough 1600 450–460 Epitaxial ﬁlms on InSb

crystals. n-type, 10

–10

donors per cm

192 Chapter 4

Table 4.7: (Continued)

Materials Form of

evaporant

Feeder

mechanism

Filament

temp.

◦

Substrate

temp.

◦

Comments on ﬁlms

Powder, 100/150 mesh Vibrating trough 1650 300–400 Epitaxial ﬁlms on Ge crystals

S, Cu

Se Powders of

250–300 ␮m

Vibrating trough 1400 25 Semitransparent, conductive

ﬁlms of Cu

1,8

S and Cu

BaTiO

Sintered powder,

100/200 mesh

Vibrating chute 2300 500–700 Crystalline ﬁlms, dielectric

constants of 400–700. Rate:

−1

across 8 cm distance

Various perovskites Sintered powder,

100/200 mesh

Vibrating trough 2050–2300 500–700 Perovskite ﬁlms, epitaxial on

LiF crystal. Rates: 1–3

−1

across 8 cm distance

Evaporation 193

was deposited on WC–Co insets and high-speed steel samples. In order to improve the

adhesion of the nanocomposite ﬁlm, they applied −900 V bias voltages at the beginning of the

deposition. CrN is more malleable than TiN; however, it has better oxidation resistivity. By

adding another improved oxidation-resistant material such as CrN to TiAlN, the wear

resistance of the nanocomposite thin ﬁlm has been improved [126]. Chang et al. studied the

wear resistance and hardness with TiSiN alloys by cathodic arc plasma evaporation [127].

According to their results the wear mechanism is controlled by counter-materials; however, no

clear relation between bias voltage and wear resistance has been found.

4.8.4.1 Direct Evaporation

Table 4.8 gives the experimental conditions for the direct evaporation of refractory

compounds. Evaporation can occur with or without dissociation of the compound into

fragments. As seen from Table 4.8, the observed vapor species show that very few compounds

evaporate without dissociation. Examples are SiO, MgF

, CaF

, and other group IV

divalent oxides (SiO homologs like GeO and SnO).

In the more general case, when a compound is evaporated or sputtered, the material is

transformed to the vapor state not as compound molecules but as fragments thereof.

Subsequently, the fragments have to recombine, most probably on the substrate, to reconstitute

the compound. Therefore, the stoichiometry (anion:cation ratio) of the deposit depends on

several factors including the deposition rate and the ratios of the various molecular fragments,

the impingement of other gases present in the environment, the surface mobility of the

fragments (which in turn depends on their kinetic energy and substrate temperature), the mean

residence time of the fragments of the substrate, the reaction rate of the fragments on the

substrate to reconstitute the compound, and the impurities present on the substrate. For

example, it was found that direct evaporation of Al

resulted in a deposit which was

deﬁcient in oxygen, i.e. which had the composition Al

3–x

[128]. This O

deﬁciency could be

made up by introducing O

at a low partial pressure into the environment. In other cases, for

example the direct evaporation of TiB

and ZrB

, the deposit contains both the monoboride

and diboride phases [129].

4.8.4.2 Reactive Evaporation

The difﬁculties involved in direct evaporation processes due to fragmentation of the vaporized

compounds are overcome in reactive evaporation where a metal is evaporated in the presence

of the reactive gas; the compound is formed by reaction of the evaporated metal species with

the molecules of the reactive gas. Though this technique has been extensively used to deposit a

variety of oxide ﬁlms for optical applications, it is generally observed that the ﬁlms are

deﬁcient in oxygen. It is also observed in some cases, especially in the synthesis of carbide

ﬁlms, that the deposition rate becomes a limiting factor governing the growth of the ﬁlms. In

such cases, stoichiometric TiC ﬁlms could only be deposited at very low rates (∼ 1.5

A/s max.)

194 Chapter 4

Table 4.8: Direct evaporation of inorganic compounds

Compound

Vapor species observed

(in order of decreasing

frequency)

mp,

◦

C T,

◦

C, at which

p*=10

−7

Torr

Comments on actual evaporation

temperatures, support materials used,

and related experience

Oxides

Al, O, AlO, Al

O, O

(AlO)

2030 ∼1800 From W and Mo supports at 1850–2250

◦

With telefocus gun at 2200

◦

C, no

decomposition

From W support: Al

ﬁlms have small

oxygen deﬁcits. O

-dissociation pressure at

1780

◦

C: 1.5 × 10

−15

torr

450 ∼1700 From Pt and Mo supports at 940–1370

◦

BaO Ba, BaO, Ba

(BaO)

,Ba

1925 1540 From Al

crucible at 1200–1500

◦

C. From

Pt crucible with only slight decomposition,

(1540

◦

C) = 3.5 × 10

−15

torr

BeO Be, O, (BeO)

n = 1–6, Be

2530 2230 From W support at 2070–2230

◦

C. With

telefocus gun at 2400–2700

◦

C, no

decomposition

– 817 1840 From Pt support

CaO Ca, CaO, O, O

∼2600 ∼2050 Support materials: ZrO

, Mo, W. The latter

two form volatile oxides, molybdates, and

wolframates at 1900–2150

◦

CeO

CeO, CeO

1950 – From W support without decomposition

In, In

O, O

– – From Pt support with only little

decomposition. Vapor species observed at

1100–1450

◦

C. At 1000–1450

◦

CfromAl

crucible, more In

O than In

MgO Mg, MgO, O, C

2800 ∼1560 Mo or W supports at 1840–2000

◦

form

volatile oxides, molybdates, and wolframates.

With telefocus gun at 1925

◦

C, no

decomposition. From Al

at 1670

◦

(Continued)

Evaporation 195

Table 4.8: (Continued)

Compound Vapor species observed

(in order of decreasing

frequency)

mp,

◦

C T,

◦

C, at which

p*=10

−7

Torr

Comments on actual evaporation

temperatures, support materials used,

and related experience

MoO

(MoO

)

, (MoO

)

n = 4.5

795 610 From Mo oven at 500–700

◦

C, the trimer is

the main species. Above 1000

◦

C, there is

some decomposition into MoO

(s)+O

(g).

At 730

◦

C, the oxygen-decomposition

pressure is 1.1 × 10

−14

torr. From Pt at

530–730

◦

NiO Ni, O

, NiO, O 2090 1586 From Al

crucible at 1300–1440

◦

C. Heavy

decomposition with po

=4× 10

−1

torr at

1586

◦

– 656 ∼450 Lower oxides result if evaporated from W

supports. Pt heaters do not produce

decomposition

SiO SiO – 1025 Usually evaporated from Ta or Mo heaters at

residual gas pressure below 10

−6

torr and at

temperatures between 1150 and 1250

◦

Dissociation into Si and O

begins above

1250

◦

C and may lead to oxygen-deﬁcient

ﬁlms

SiO

SiO, O

1730 ∼1250 With telefocus gun at 1500–1600

◦

C, no

decomposition. Ta, Mo, W supports are

attacked by SiO

and contribute volatile

oxides. From Al

at 1630

◦

C, SiO: vapor

species is present

SnO

SnO, O

– – From SiO

crucible at 975–1250

◦

C. Films

directly evaporated from W support are

slightly oxygen deﬁcient

196 Chapter 4

Table 4.8: (Continued)

Compound Vapor species observed

(in order of decreasing

frequency)

mp,

◦

C T,

◦

C, at which

p*=10

−7

Torr

Comments on actual evaporation

temperatures, support materials used,

and related experience

SrO Sr, O

, SrO 2460 ∼1760 From Al

at 1830

◦

C. Evaporation from

Mo or W at 1700–2000

◦

C produces volatile

Mo and W oxides, molybdates, and

wolframates

TiO

TiO, Ti, TiO

1840 – TiO

source material decomposes into lower

oxides upon heating. po, at 2000

◦

Cis

−10

torr. Nearly stoichiometric ﬁlms by

pulsed electron-beam heating

(WO

)

,WO

1473 1140 From Pt oven at 1040–1300

◦

C. From Pt

support at 1220

◦

C. From W heater with only

slight decomposition; po

at 1120

◦

Cis

3 × 10

−10

torr

ZrO

ZrO, O

2700 – From Ta support at 1730

◦

C, volatile TaO.

From W support, oxygen-deﬁcient ﬁlms. ZrO

source material loses oxygen when heated by

electron beams

Sulﬁdes, Selenides, Tellurides

ZnS – 1830 (p ≈ 150 atm) 1000 From Mo support. Minute deviations from

stoichiometry if allowed to react with residual

gases. From Ta at 1050

◦

ZnSe – 1520 (p ≈ 2 atm) 820

CdS S, Cd, S, Si, Su 1750 (p ≈ 100 atm) 670 From Pt oven at 740

◦

C. Films tend to deviate

from stoichiometry. Suitable support

materials: graphite, Ta, Mo, W, SiO

–coated W: evaporation at 600–700

◦

CdSe Se

, Cd 1250 660 From Al

crucible

Evaporation 197

Table 4.8: (Continued)

Compound Vapor species observed

(in order of decreasing

frequency)

mp,

◦

C T,

◦

C, at which

p*=10

−7

Torr

Comments on actual evaporation

temperatures, support materials used,

and related experience

CdTe Te

, Cd 1100 570 From Ta boat at 750–850

◦

C: ﬁlm

stoichiometry depends on condensation

temperature

PbS PbS, Pb, S

, (PbS)

1112 675 From quartz crucible at 625–925

◦

C. From

Mo support. Purest ﬁlms from quartz furnace

at 700

◦

C; Fe or Mo boats reacts and form

volatile sulﬁdes

– 546 550 From Mo support

, (SbSe)

, (Sb

(SbSe)

611 From graphite at 725

◦

C. From Ta oven at

500–600

◦

C, fractionation and ﬁlms of

variable stoichiometry

Halides

NaCl NaCl, (NaCl)

(NaCl)

801 670 From Ta, Mo, or Cu ovens at 550–800

◦

KCl KCl, (KCl)

772 635 From Ni or Cu oven at 500–740

◦

AgCl AgCl, (AgCl)

455 690 At 710–770

◦

C. From Mo support,

p*=10

−2

torr at 790

◦

MgF

, (MgF

)

(MgF

)

1263 1130 From Pt oven at 950–1230

◦

C. From Mo

support. Very little dissociation into the

elements

CaF

, (CaF) 1418 ∼1300 From Ta oven at 980–1400

◦

C. From Mo

support

PbCl

– 678 ∼430 Direct evaporation possible

198 Chapter 4

[130]. This limitation of deposition rate in the case of the reactive evaporation process is due to

the reaction kinetics of the compound formation by this process. The presence of a ‘plasma’ in

the ARE process inﬂuences the reaction kinetics by providing activation energy to the reactive

species, thereby making it possible to synthesize compound ﬁlms at considerably higher rates

[131–133] and lower temperatures.

Hass et al. studied the single and multiple source effect on metal oxide coatings (CeO

, ZrO

) by e-beam directed vapor deposition (DVD) [134]. In this approach, in order to make

the deposition uniform, transonic helium carrier gas was used with the e-beam to transport

metal vapor to a substrate. Metal oxide coatings were then produced by adding oxygen to the

carrier gas. Their model is based on the vapor pressures of each individual evaporation source.

According to their result the phase morphology is similar between a single metal source of

zirconia and reactive deposition of yttria and zirconia source samples.

4.8.4.3 Activated Reactive Evaporation

The ARE process generally involves evaporation of a metal or an alloy in the presence of the

plasma of a reactive gas [131, 135]. For example, TiC and TiN coatings are deposited by this

process by evaporating Ti in the presence of C

and N

plasma, respectively. The two basic

variants of the ARE process are shown in Figures 4.42 and 4.43. For more information on the

ARE process, refer to a review by Bunshah and Deshpandey [132]. The role of the plasma in

Figure 4.42: Schematic of the activated reactive evaporation (ARE) process.

Evaporation 199

Figure 4.43: Activated reactive evaporation (ARE) Process [136] using a resistance-heated

evaporation source.

this process is two-fold:



to enhance the reactions that are necessary for deposition of compound ﬁlms



to modify the growth kinetics and hence the structure/morphology of the deposits.

In the following section we discuss the above two aspects.

Thermodynamic and kinetic considerations in plasma-assisted deposition processes

For the formation of a compound by any chemical reaction, the corresponding thermodynamic

and kinetic constraints must be satisﬁed which also apply to the deposition of refractory

compound ﬁlms by reactive evaporation. In order to understand the role of plasma in

enhancing the chemical reactions essential for the formation of a particular compound, one has

therefore to consider the kinetics of these reactions.

Let us consider the reactions involved in the synthesis of some oxides, carbides, and nitrides

by reactive evaporation. Given below are the reactions for forming Al

, TiC, and TiN.

2Al +

→Al

G

◦

=−250 kcal (mol O

)

at 298 K

2Ti + C

→2TiC + H

G

◦

=−7.65 kcal (mol O

)

−1

at 298 K

2Ti + N

→2TiN G

◦

=−73.5 kcal (mol N

)

−1

at 298 K

As can be seen from the above reactions, the thermodynamic criterion of free energy of

formation is satisﬁed for the respective compounds.

200 Chapter 4

The reaction kinetics in reactive evaporation process can be treated in exactly the same manner

as for reactions occurring in heterogeneous systems of condensed phases. The model for

heterogeneous metallurgical kinetics involves: (1) transport of reactant to the reaction

interface; (2) transport of reaction products away from the reaction interface; (3) the chemical

reaction at the chemical interface; (4) the nucleation of new phase; and (5) heat transfer to or

away from the reaction interface.

For reactive evaporation, this model may be depicted as follows (e.g. for TiC formation):

Reactants

Products

Ti (metal atoms) TiC (deposit)

(gas) H

(gas)

Reaction interface

On the basis of the above model, the rate-controlling steps in the reactive evaporation process

are: (1) adequate supply of reactants; (2) adequate collision frequency; (3) the rate of chemical

reactions at the interface; and (4) the rate of removal of the reaction products from the interface.

It is easy to satisfy (1), (2), and (4) above for a reactive evaporation process. However,

condition (3), i.e. the rate of reaction, becomes the rate governing step. The ‘plasma’ in the

ARE process inﬂuences this step, i.e. the rate of reaction, by providing the necessary activation

energy to the reactive species. The effect of plasma on rate of reaction can be clearly

demonstrated by considering the results of Abe et al. [130] and Bunshah and Raghuram [133]

on deposition of TiC coatings. Abe et al. found that titanium carbide with a carbon to titanium

ratio of 1 could be formed by a reaction between Ti and C

or C

molecules on a

substrate at 300–500

◦

C only if the deposition rate is 1–1.5

A/s. At higher deposition rates, no

TiC was formed. Clearly, the activation barrier could not be overcome at the higher deposition

rates. Bunshah and Raghuram [133, 137] have similarly reported that the deposition of TiC by

reactive evaporation at higher deposition rates (150–200

A/s) required a very high substrate

temperature, exceeding 1000

◦

C. However, in the presence of plasma, these authors reported

that it was possible to deposit TiC at a high rate at a relatively low substrate temperature. The

plasma imparts sufﬁcient energy to the reacting species to overcome the activation energy

barrier, and hence condition (3), i.e. the rate of reaction, no longer remains the rate-governing

step.

Basic variants of the ARE process

The two basic variants of the ARE process are ARE with an e-beam evaporation source [131]

and ARE processes with a resistance-heated source [138].

ARE processes with an electron-beam-heated evaporation source are illustrated in Figure

4.42. In this process, the metal is heated and melted by a high-acceleration-voltage e-beam that