Fahlman B.D. Materials Chemistry

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crucible that is heated by a W ﬁlament. Temperatures as high as 1,800



C may be

reached inside the chamber, which is enough for some metals or metal salts to

vaporize. Deposition rates for this method are 1–20 A

1

. The use of an electron

beam to assist in the precursor evaporation results in temperatures on the order of

3,000



C, being more suited for the deposition of refractory metals/alloys and metal

oxides such as alumina, titania, and zirconia. Since the temperature of the chamber

interior is much higher than the walls, the gas-phase ions/atoms/molecules condense

on the sidewalls as well as the substrate; this may lead to ﬁlm contamination as the

nonselective coating ﬂakes off the chamber walls.

For high-purity metal or carbon ﬁlms, an ultrahigh vacuum (UHV) environment

(typically <1  10

6

Torr) must be used during PVD. This is necessary in order to

prevent the gas-phase reaction of metal/carbon atoms with atmospheric gases (e.g.,

O, O

) that would prefere ntially form metal oxides, hydroxides, or nitrides

rather than the desired ﬁlm. If mixed phases such as nitrides or oxides are desired,

puriﬁed nitrogen or oxygen may be introduced into the chamber, respectively. For

carbide ﬁlms, targets o f b oth the metal and carbon are placed together within the

vacuum chamber. As one would expect, since such a high-purity metal and high

vacuum chamber must be used, PVD is relatively quite expensive. Another limita-

tion is the selectivity and confo rmality of the procedure. While PVD wor ks well to

deposit material on surfaces in line-of-sight of the source, nonconformal deposition

of complex or rough surfaces (e.g., ﬁbers) is a critical limitation. This issue becomes

more important for integrated circuits with feature sizes less than 100 nm, often with

high aspect ratios (height/width ratio of the surface feature, Figure 4.53).

By contrast, CVD is a process in which gaseous precursors are reactively trans-

formed into a thin ﬁlm, coating or other solid-state material on the surface of a

catalyst or substrate. It should be stressed that CVD is no longer limited to thin ﬁlm

growth; this method is now the preferred route to generating ﬁber-optic preforms,

[47]

Figure 4.53. Illustration of conformal thin-ﬁlm growth for (a) trench ﬁlling and (b) step coverage.

The line-of-sight limitation of PVD, relative to a conformal CVD technique, is shown in (b).

298 4 Semiconductors

as well as an increasingly diverse nature of nanostructural architectures, especially

carbon nanotubes (CNTs) that will be detailed in Chapter 6. The CVD procedure is

often denot ed as metal-organic CVD (MOCVD), which more accurately speciﬁ es

the use of an organometallic precursor, containing a central metal and ancillary

ligands.

The steps involved in the growth of thin ﬁlms by CVD are shown in Figure 4.54.

Once the gas-phase precursor molecules enter the deposition zone, (a), they are

physisorbed to the substrate surface through weak van der Waals interactions, (b).

The ancillary ligands are removed through thermolysis, leaving the desired residual

species on the surface, (c), and ligands being removed from the deposition chamber

via the carrier gas ﬂow, (d). Strong covalent interactions are then formed between

the surface and adsorbed species, resulting in chemisorption. The surface-bound

species may migrate along the surface and/or react with other surface species,

eventually nucleating on thermodynamically favored positions, (e) en route toward

thin-ﬁlm growth. Since ﬁlm growth occurs through surface migration of intermedi-

ate species, CVD is the method-of-choice for depositions onto irregular surfaces

where conformality is not possibl e using PVD techniques (Figure 4.55).

In addition to the surface governed reactions described above, there are also

gas-phase reactions that may take place between precursor/intermediate molecules.

Although these reactions may also be important in the growth mechanism (e.g., for

plasma-enhanced CVD), signiﬁcant gas-phase reactions will result in less desirable

impure, granular, non-adhering, and non-conformal ﬁlms. At relatively high tem-

peratures, the gas-phase precursors may preferentially react with one another rather

than adsorbing to the substrate surface (b

). The gas-phase nucleation causes

granules to form and fall to the substrate surface due to gravitational forces (c

Typically, these reactions may be minimized by lowering the deposition tempera-

ture, which will facilitate the surf ace-bound growth route. Though gas-phase growth

is generally not desired for thin-ﬁlm applications, this technique has also b een

applied for the synthesis of nanopa rticles.

[48]

Figure 4.54. The important steps involved in CVD using the thermolysis of precursor molecules (see text

for details).

4.2. Silicon-Based Applications 299

By deﬁnition, CVD is a non-equilibrium process. Although thermodynamics may

provide useful information about the overall energetics of the growth process,

kinetics must be used to provi de information regarding reaction pathways or the

transformation rates of the gaseous precursors. The kine tic description of CVD is

divided into two parts: mass transport and the rates of the speciﬁc chemical reactions

involving the precursor and intermediate species. Since the growth rate for CVD

is relatively slow, the differences in forward and reverse reaction rates for interfacial

events are much smaller than the absolute rates themselves.

[49]

Thus, CVD is

frequently treated as a pseudo-equilibrium system, considering only the vapor and

solid immediately adjacent to the interface, the boundary layer.

Figure 4.55. SEM images of an electroluminescent phosphor particle, ZnS (used in backlight displays

for cell phones, watches, etc.), before (a) and after (b) the deposition of an aluminum oxide thin ﬁlm.

This ﬁlm is a transparent coating that prevents the phosphor particle from undergoing humidity-

accelerated decay. A technique known as ﬂuidized-bed CVD was used, where a carrier gas both

delivered the precursors to a vertically aligned CVD chamber, and dispersed the powdery sample in

order to expose all surface regions to the precursor vapors.

300 4 Semiconductors

As illustrated in Figure 4.56, the deposition may be controlled by surface-reaction

kinetics (e.g., high gas velocity at low temperatures/pressures), or by diffusion/mass

transport (e.g., low gas velocity at elevated temperatures or pressures (such as

atmospheric)).

[50]

Whereas the deposition rate in the former system is dependent

on the concentration/reactivity of the prec ursor gases, the deposition rate in the latter

is dependent on the diffusion rates of reactants and byproducts. Though the substrate

is often placed horizontally in the CVD chamber, it is more desirable to tilt the

substrate in order to increase the deposition rate and ﬁlm-thickness homogeneity.

For horizontally-positioned substrates, the velocity of the precursor vapor does

not remain constant across the substrate surface, but will decrease downstream.

Accordingly, the thickness of the boundary layer will increase at downstream

substrate positions, giving rise to depressed thicknesses of the deposited ﬁlm

Figure 4.56. Schematic of the kinetically-controlled processes involved in MOCVD. When molecular

precursors are introduced into the reaction chamber, they must ﬁrst diffuse through the boundary layer,

(a). The precursors or reactive intermediates/radicals are then adsorbed onto the substrate surface,

(b), where surface chemical reactions take place, (c). The ancillary ligands and organic residues from

the precursor are then desorbed from the substrate surface, (d), and must diffuse through the BL,

(e), en route from the deposition chamber as gaseous by-products, (f). Also shown are the conditions

that give rise to i) diffusion/mass transport, and ii) surface-reaction controlled kinetics.

4.2. Silicon-Based Applications 301

along the ﬂow direction. By tilting the substrate, the gas velocity over the wafer

surface will increase, leading to a uniform boundary layer thickness. This will also

enhance the residence time of precursor vapors over the substrate surface, leading to

increased growth rates.

At the microscopic level, there are a few key stages of thin-ﬁlm growth. From

adsorbed monomers and surface migration, the ﬁrst stage is formation of subcritical

embryos of varying sizes. These particles will further nucleate while taking on more

precursor adsorption, forming a supercritical sized cluster. These clusters will

coalesce into growth islands, which will expose new regions of the substrate that

serve to adsorb additional precursor species . Isolated surface islands eventually

grow together, leaving holes and channels that are ﬁlled by adsorbing precursor

molecules to form a continuous thin ﬁlm.

Two general types of reactors are used in CVD proce sses. In a hot-walled reactor

(Figure 4.57), a tube furnace completely surrounds the deposition chamber contain-

ing the substrate. The desired deposition temperature depends on the ther mal

stabilities of the substrate to be coated, and the reactor (i.e., glass (T < 600



C) or

quartz (T < 1,100



C)). Hot-walled reactors are used extensively for laboratory

studies, and also in industry for the CVD of semiconductors and oxides. These

reactors are often preferred d ue to the simplicity of setup, while being able to

maintain a uniform temperature over a large number of substrates. However,

secondary coating of the reactor walls is unavoidable; frequent cleaning of these

reactors is theref ore necessary, as deposits may easily ﬂake off these surfaces and

contaminate the growing ﬁlm. To prevent such problems, cold-walled reactors may

also be used for a CVD process, wherein only the substrate is heated. This focuses

the surface and gas-phase reactions to a region immediately surrounding the sub-

strate surface. However, this reactor type generally results in narrower deposition

zones and slower growth rates than hot-walled analogues.

Figure 4.57. Schematic of a horizontal hot-walled CVD reactor. Shown is a two-precursor system, where

the water sensitive precursor contacts water vapor directly over the heated substrate.

302 4 Semiconductors

Perhaps more than any other mater ials synthesis technique, semantics becomes a

challenge due to the plethora of acronyms that are used to describe a speciﬁc CVD

process. In particular, it is not sufﬁcient to simply cite “CVD” alone in an article

title; a more explicit acronym must be used that states the type of deposition

chamber and precursor decomposition methodology employed. If no preﬁx is

afﬁxed to CVD, it usually denotes simple thermolysis of a prec ursor within a

standard cold- or hot-wall reac tor. Howeve r, if more energetic sources of energy

are used to degrade the precursor such as laser, plasma, or micro wave plasma, the

acronyms laser-assisted CVD (LACVD), plasma-enhanced CVD (PECVD), and

microwave plasma CVD (MPCVD) are used, respectively. Rather than using high-

energy plasma sources, a relatively new CVD technique uses a heated ﬁlament to

degrade precursor gases and form reactive intermediates. This technique is referred

to as either catalyzed CVD (Cat-CVD) or hot-wire CVD (HWCVD, Figure 4.58).

[51]

The pressure of the reactor must als o be deﬁned, and acro nyms APCVD, LPCVD,

and SCF-CVD are used to denote ambient, in vacuo, and supercritical pressure

conditions within the deposition chamber, respectively. In general, the resultant ﬁlm

conformality of a CVD process follows the order LPCVD > APCVD, due to the

relative migration rates of intermediate species along the surface of the substrate.

However, it has recently been shown that conformal thin ﬁlms of metal oxides may

be deposited on nonplanar substrates such as carbon ﬁbers under APCVD, even at

low temperature.

[52]

While plasma-enhanced methods are very useful to lower the substrate tempera-

ture, the as-deposited ﬁlms are typically less conformal and often contain more

surface impurities than competing methods. In this method, reactive radicals, ions,

and atoms/molecules are formed in the gas phase that interact with the relatively

Figure 4.58. Schematic of a hot-wire CVD system. Reprinted from Povolny, H. S.; Deng, X. Thin Solid

4.2. Silicon-Based Applications 303

low-temperature substrate to generate a ﬁlm. Some of the more recent applications

for plasma CVD include growth of cubic boron nitride (c-BN) thin ﬁlms.

[53]

In recent years, a complementary process known as atomic layer deposition

(ALD) has been widely utilized.

[54]

In contrast to CVD, ALD features the sequential

exposure of a substrate to two or more precursors. The precursor vapors are pulsed

into the reactor one at a time, bein g separated by inert-gas purges or evacu ation

(Figure 4.59). This establishes user-controlled deposition cycles, each comprising

surface saturation by one precursor, followed by the self-limiting surface reaction of

a second precursor. Consequently, deposition is unaffected by varying vaporization

rates of solid precursors – especially problematic for CVD. Further, ALD is gener-

ally carried out at lower temperatures than CVD since co-reactants are highly

reactive toward each other (e.g., trimethylaluminum and water vapor). In contrast,

using the same co-reactants for CVD results in hard-to-control gas-phase reac-

tions, even at low temperatures.

[55]

Since ﬁlm growth occurs sub-monolayer at a

time, ALD allows for an unprecedented control over the resultant ﬁlm thickness,

conformality, homogeneity, and stoichiometry. This strategy will become most

useful for the deposition o f the thin high-k gate oxide layers (e.g., HfO

, ZrO

)

required for next-generation CMOS devices.

Ideally, the ﬁlm thickness (i.e., growth rate) will be uniform across the entire

substrate surface. The ﬂux of gas molecules that impinge on a substrate surface may

be expressed as Eq. 17:

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

2pmkT

ð17Þ

where: J is the rate at which gas-phase molecules hit a surface (in units m

2

1

); m

is the mass of a gas molecule; k is the Bolt zmann’s constant (1.38  10

23

1

);

T is the temperature in K. Hence, the growth mechanism of ALD is unaffected by

the presence of an inert carrier gas, as long as it exhibits a purity level of at least the

parts-per-billion (ppb) level.

In theory, ALD growth proceeds by one atomic layer per cycle; however, due to

steric hindrances and limited number of reactive surface sites, the growth rate per

cycle is a fraction of a monolayer (ML) thickness – typically less than 0.9 ML.

Substrate

B layer

Pulse

A layer

Purge Purge Purge

Figure 4.59. Illustration of atomic layer deposition (ALD).

304 4 Semiconductors

The sticking coefﬁcient,S

, is often used to describe the fraction of incident

molecules that adsorb upon the substrate surface. In general, S

is dependent upon

the degree of coverage, temperature, and crystal structure/reactivity of the substrate

surface. It should be noted that S

is generally lower for CVD/ALD relative to PVD

techniques. Consequently, the lower S

results in being able to coat complex

topographies (conformal growth), as well as selected areas of the substrate.

A number of optimization runs must be performed in order to achi eve self-limited

growth, known as the ALD window. If the thickness is greater at the inlet end, it may

be due to insufﬁcient purging between pulses, or too low a deposition temperature

that results in condensation of the precursor (i.e., physisorption is occurring rather

than chemisorption). If the thickness increases toward the outlet end, it is usually a

sign of too high a deposition temperature that is causing surf ace decomposition of

the precursor. If the purge/evacuati on cycles are too long, the precursor may be

desorbed from the substrate. This would result in ﬁlm thickness that decreases

along the inlet-outlet direction. Finally, if the growth rate is too slow, the system

is activation-e nergy limited; that is, the temperature must be increased in order to

facilitate a suitable level of precursor reactivity.

Oftentimes, the deposition mechanisms between ALD and CVD are quite

different, even for identical precursor combinations. In particular, it is well known

that thin-ﬁlm growth by CVD is heavily inﬂuenced by side reactions (Figure 4.60)–

not as problematic for ALD (Figure 4.61). A CVD process often generates reaction

products such as CO, RH, or HCl that may be preferentially adsorbed onto the

substrate surface. This will cause ﬁlm-growth termination unless the competitive

adsorbents are removed through purging with inert gas or introduction of a reducing

gas such as H

The chemical nature of the prec ursor represents the most critical component of

a CVD/ALD process. Generally speaking, the choice of a particular precursor is

governed by the relative stabilities of the precursor and substrate, as well as the

volatility, cost, and hazards of the precursor. The coordination sphere of ligands

surrounding the central metal is extremely important; the organic ligands in these

precursors may lead to contamination of the ﬁlms if they are not completely

removed through a combination of pyrolysis, reduc tion, or oxidation processes.

Some precursors pose a high risk when being used. For example, Ni(CO)

has

a very high toxicity, Al(Me)

is pyrophoric, B

is explosive, and chloride-

containing species are corrosive. In general, organometallic precursors pose lower

hazards than hydrides and halides, but are much more costly.

Although it was once essential that volatile precursors be used, this is no longer a

synthetic limitation. Within the last decade, the gas/liquid properties of supercritical

ﬂuids (e.g.,CO

) have been used to solvate certain precursors, facilitating their

use for CVD.

[56]

Two variations of this tech nique may be used; supercritical ﬂuid

transport (SFT), using the ﬂuid as an aerosol-like delivery vehicle, or in situ thin-

ﬁlm growth within a high pressure reactor, known as supercritical ﬂuid deposition

(SFD). In these methods the precursor must be soluble in CO

, which is analogous

in solvating ability to hexane with an enhanced ﬂuorophilic character.

4.2. Silicon-Based Applications 305

Figure 4.60. Illustration of chemical vapor deposition (CVD) of an aluminum ﬁlm from Al(O

Pr)

showing the inﬂuence of temperature (decomposition route) on the resultant ﬁlm purity.

306 4 Semiconductors

A CVD process may involve the use of either single or mixed precursors. The best

precursor is a molecule that has sufﬁcient volatility (or CO

solubility), and contains

labile ligands that will leave no organic residue behind during its surface-catalyzed

decomposition. In general, volatile liquids and oils are attractive CVD precursors due

to the relative ease of vapor transport through simple carrier-gas bubbling, vaporiza-

tion, or direct liquid injection (DLI) techniques (Figure 4.62). However, low-melting

powders with high volatilities and low decomposition temperatures are regarded as the

Figure 4.61. Stepwise scheme for the atomic layer deposition (ALD) of a HfO

thin ﬁlm from Hf

(NMe

)

O sequential pulsing.

4.2. Silicon-Based Applications 307