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

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Atomic Layer Deposition 371

Figure 8.7: High index optical material, modiﬁed TiO

[10]. (HR TEM picture courtesy

of E. Kauppinen and H. Jiang, HUT, Finland.)

production in Finland since the mid-1980s. Similarly, the ﬁrst single-wafer ALD systems were

for 100 mm wafers, and current state-of-the-art production systems are for 300 mm wafers (see

Figure 8.15). More recently, photovoltaic and display industries have been driving the

development of high-throughput, large-area, single-substrate ALD systems, in both cluster and

in-line formats (Figure 8.9). Roll-to-roll ALD systems are also under development [11],to

meet the demand for ﬂexible electronics and packaging applications. This development and

progress demonstrate the scalability of ALD processes and equipment, to meet new business

and market demands.

8.3.5 Ultrathin Films

Most ALD ﬁlms grow in about 1

A sequences (growth per cycle), enabling precise deposition

of highly conformal ﬁlms in the nanometer and subnanometer thickness range (Figure 8.10).

These features attracted the semiconductor industry’s interest in ALD technology, starting

during the late 1990s [12, 13]. After extensive research and development work, ALD was ﬁrst

adapted in the semiconductor memory industry in 2005 [14], and at the end of 2007 in the

microprocessor industry, by Intel, for gate oxides [15, 16].

Another industry that has widely adapted ALD technology and is utilizing ultrathin ﬁlms is

hard disk drive application, for the insulator in the thin ﬁlm magnetic head. ALD enabled the

372 Chapter 8

Figure 8.8: ALD systems, from research to industrial production (courtesy of Beneq Oy):

TFS 200, TFS 500 and P800.

Atomic Layer Deposition 373

Figure 8.9: Scalability of ALD, 1.2 × 1.2 m

glass substrate coated with in-line TFS 1200 ALD

system. (Courtesy of Pekka Soininen and Jarmo Skarp, Beneq Oy.)

Figure 8.10: HfO

gate oxide on a 200 mm wafer. (Courtesy of Beneq Oy.)

374 Chapter 8

industry to continue to scale down dimensions and performance to meet the packaging density

requirements [17]. The pinhole-free feature of ultrathin ALD insulators has been a key enabler

in this application.

ALD technology is also being used actively in nanotechnology research. For example,

high-speed single-wall carbon nanotube ﬁeld-effect transistor memory elements have been

developed recently, based on ultrathin HfO

gate insulator, made with ALD [18]. This type of

nanotechnology research, enabled with ALD, paves the way beyond silicon-based

microtechnology.

8.3.6 Artiﬁcial Materials

Probably the most powerful beneﬁt of ALD is the ability to combine two or more different

materials in nanoscale, making possible the creation of new ‘artiﬁcial’ materials, with unique

features. This makes ALD a uniquely powerful enabling technology for state-of-the-art

nanotechnology research and for innovation to functionalize surfaces. These new functional

materials, combined with precision, high repeatability and scalability of ALD, make ALD

technology one of key enablers of new nanotechnology products.

The TFEL display structure (Figure 8.6) was developed in the early 1980s and the ATO

insulators are ALD nanolaminates of Al

and TiO

[19]. The insulator was originally

tailored to match optical properties by combining the high and low index materials. During

that work it turned out that by ﬁne tuning the nanolaminate ratios, it was possible to optimize

the new material for electrical breakdown strength (Figure 8.11). This invention solved the

early reliability and electrical breakdown yield issues and enabled production of the TFEL

displays. ALD enabled the nanomaterial breakthrough and production scale-up.

Figure 8.11: Electrical breakdown ﬁeld strength of ATO insulator vs Al

/TiO

ratio [19].

Atomic Layer Deposition 375

Another example of nanoscale modiﬁcations of material properties is modiﬁed TiO

(Figure

8.7) [10]. An optical refractive index of 2.3–2.4 at 550 nm is achieved by using a rather high

processing temperature of over 250

◦

C. The high processing temperature, however, leads to

polycrystalline TiO

and the grain boundaries cause unnecessary optical absorption. Crystal

growth is limited by using nanometer-thin intermediate layers of amorphous Al

. The ALD

method enables reliable processing of this modiﬁed material, with subnanometer precision.

8.4 Atomic Layer Deposition Precursors, Processes, and Materials

New ALD processes and materials are being actively developed at several Universities. ALD

was demonstrated for the TFEL application for the ﬁrst time by ZnS deposition using

elemental zinc and sulfur in the mid-1970s, followed by the use of ZnCl

and H

S [20]. After

the initial period, several other inorganic precursor types have emerged. During the second

half of the 1980s metal organic precursor chemicals were adapted for ALD material research

[21, 22]. This opened up plenty of possibilities for new ALD chemistries and processes. A

large number of ALD processes have been demonstrated and published to date, and the list

keeps growing (Table 8.2).

Table 8.2: Examples of materials deposited by ALD [20]

Materials Examples

Oxides

Dielectric Al

,TiO

, ZrO

, HfO

,Ta

,Nb

,Sc

, MgO,

, SiO

, GeO

,La

, CeO

, PrO

,Nd

,Sm

, EuO

GdO

,Dy

,Ho

,Er

,Tm

,Yb

,Lu

, SrTiO

BaTiO

, PbTiO

, PbZrO

,Bi

O, Bi

O, SrTa

, SrBi

YScO

, LaAlO

, NdAlO

, GdScO

, LaScO

, LaLuO

,Er

Conductors/semiconductors In

,In

:Sn, In

:F, In

:Zr, SnO

, SnO

:Sb, ZnO,

ZnO:Al, ZnO:B, ZnO:Ga, RuO

, RhO

, IrO

,Ga

,WO

, NiO, FeO

, CrO

, CoO

, MnOx

Other ternaries LaCoO

, LaNiO

, LaMnO

,La

1−x

MnO

Nitrides

Semiconductors/dielectric BN, AlN, GaN, InN, SiN

,Ta

,Cu

N, Zr

,Hf

Metallic TiN, Ti-Si-N, Ti-Al-N, TaN, NbN, MoN, WN

,WN

II–VI compounds ZnS, ZnSe, ZnTe, CaS, SrS, BaS, CdS, CdTe, MnTe, HgTe

II–VI-based TFEL phosphors ZnS:M (M = Mn, Tb, Tm), CaS:M (M = Eu, Ce, Tb, Pb), SrS:M

(M = Ce, Tb, Pb)

III–IV compounds GaAs, AlAs, AlP, InP, GaP, InAs

Fluorides CaF

, SrF

, MgF

, LaF

, ZnF

Elements Ru, Pt, Ir, Pd, Rh, Ag, W, Cu, Co, Fe, Ni, Mo, Ta, Ti, Al, Si, Ge

Others La

, PbS, In

,Cu

S, CuGaS

S, WS

,TiS

, SiC, TiC

TaC

,WC

,Ca

(PO

)

, CaCo

,Ge

376 Chapter 8

8.4.1 ALD Precursors in General

General ALD precursor requirements differ from other chemical gas-phase methods since all

gas-phase reactions should be excluded and reactions take place only at the surface. Although

chemical vapor deposition (CVD) precursors can sometimes be used for ALD, nowadays

speciﬁc precursors have been synthesized for ALD because this deposition technique allows

the use of signiﬁcantly more reactive precursors than CVD. In the ALD method, in order to

avoid uncontrolled reactions, sufﬁcient thermal stability of the precursors is needed in the gas

phase as well as on the substrate surface within the deposition temperature range, which is

typically 150–500

◦

C. Since ALD relies on self-limiting reactions, a sufﬁcient amount of the

precursor is only required during one pulse to cover the adsorption sites on the surface and the

excess will be purged by the inert gas between the reactive precursors. Because ALD is a

gas-phase process, solid and liquid precursors must be volatile under the operating

temperature and pressure, and if heating is required to obtain sufﬁcient vapor pressure, thermal

stability of the precursor over a prolonged period is necessary. Stability is key issue, especially

when stable industrial processes are designed.

In summary, there are some general requirements for ALD precursors, which include:



sufﬁcient volatility at the deposition temperature



no self-decomposition allowed at the deposition temperature



precursors must adsorb or react with the surface sites



sufﬁcient reactivity towards the other precursor, e.g. H



no etching of the substrate or the growing ﬁlm



availability at a reasonable price



safe handling and preferably non-toxicity.

8.4.2 Non-Metal Precursors

Because of the sequential nature of ALD, metal and non-metal precursors are typically

separated from each other. The ability to select an oxidizing or reducing precursor in

conventional ALD processes makes it possible to control reactivity and reactions of the metal

precursor.

In a non-oxidizing regime, reducing the precursor is also necessary for depositing elemental

metal ﬁlms. Hydrogen is perhaps the most widely used reducing agent [23], but metallic zinc

vapor [24, 25], silanes [26, 27], and B

[28] have also been successfully applied. Molecular

hydrogen is quite inert towards typical metal precursors and therefore quite high deposition

temperatures are needed to maintain ALD reactions.

Atomic Layer Deposition 377

Deposition of nitride thin ﬁlms by ALD requires both a nitrogen source and a reducing agent

in order to obtain clean surface reactions. In many cases one compound, e.g. NH

, serves as

both nitrogen source and reducing agent. Ammonia has been used for depositing, for example,

TiN, Ta

N, NbN, and WCN thin ﬁlms by ALD, although its reactivity at low

temperatures is limited. Other nitrogen-containing compounds, such as (CH

)NNH

BuNH

and CH

CHCH

NH, have also been studied.

For chalcogenide thin ﬁlms it is possible to use elemental S, Se, and Te as precursors provided

that the other source is a volatile and reactive metal. The ﬁrst ALD process to be developed

was ZnS deposition using elemental zinc and sulfur. For other precursor types, including

halides, ␤-diketonates, and organometallics, simple hydrides, such as H

S, H

Se, and H

Te,

have typically been used as a second precursor, although their toxicity must be carefully

addressed. Recently, novel Se and Te precursors have been utilized for ALD, enabling the

deposition of selenides and tellurides [29].

Typically in the case of ALD-processed oxide ﬁlms, precursors attached to the surface can be

oxidized with H

O, H

[30],N

O [31],O

,orO

, the choice dependent on the metal

precursor selected. Water has frequently been used as an oxygen source and indeed it readily

reacts with many metal halides, alkyls, and alkoxides. For metal ␤-diketonate-type

compounds, only ozone or oxygen plasma can be used owing to the higher thermal stability of

the precursor. The use of a strong oxidizer guarantees that only a small amount of carbon is

left in the ﬁlm, as well as ensuring better interface quality.

8.4.3 Metal Precursors

The most used volatile metal-containing ALD precursors can be classiﬁed into ﬁve different

main categories, namely halides, ␤-diketonate complexes, N-coordinated compounds (amides,

amidinates), alkoxides, and true organometallics, i.e. metal alkyls and cyclopentadienyl-type

compounds. Other compounds have occasionally been used as ALD precursors for thin ﬁlms,

for example metal nitrates, carboxylates, and isocyanates [32, 33].

Several metal halide precursors have been applied in ALD processes, usually with water as an

oxygen source. They have enough high deposition rates and the price for industrial use is

reasonable. However, for delicate applications halide contamination of the ﬁlm may cause

problems at low deposition temperatures. In addition, HX (X = F, Cl, Br, or I) evolution during

the deposition process may cause problems such as corrosion and etching of the ﬁlm.

␤-diketonate-type metal chelates are known for their volatility and therefore they were

originally synthesized for the separation of metals by fractional sublimation [34], but they can

be also be used for CVD [35, 36]. The good thermal stability and reasonable volatility of the

␤-diketonate-type metal chelates make them suitable for ALD if a strong oxidizer can be used.

Coordinatively unsaturated ␤-diketonate-type compounds may oligomerize or react with the

378 Chapter 8

environment and become less volatile [37, 38]. This causes instability, especially with larger

and basic central ions such as strontium and barium.

Organometallic precursors, i.e. compounds with metal to carbon bond, usually offer high

reactivity in ALD. They were ﬁrst used for GaAs depositions by using either (CH

)

[39, 40] or (CH

)

Ga [41] as a gallium source, with AsH

as a second reactant. However,

their true capabilities were realized when their usefulness for the deposition of oxide materials

was discovered. Al

is maybe the most frequently studied ALD-processed material since.

For example, (CH

)

Al has been used with H

O, H

,NO

O, O

− plasma [42, 43] or O

[44] as oxygen sources [45]. Other metal alkyls such as (CH

)

AlCl, [46] (CH

)

AlH [47] or

(CH

)

Al have also been used as aluminum sources. The (CH

)

Al/H

O process works in

a wide temperature range of 100–500

◦

C with a reasonably high deposition rate of up to

1.2

A/cycle. At very low deposition temperatures OH-type impurities remain in the ﬁlm.

Different oxygen sources including plasma have been studied either to minimize the impurity

content or to optimize the electrical properties.

As a second example of metal alkyl precursors, zinc alkyls, especially diethyl zinc, have also

been used for the deposition of ZnO thin ﬁlms. ZnO thin ﬁlms have been deposited using H

as a second reactant and doped with boron [48, 49], gallium [50], or aluminum [51, 52] with

, (CH

)

Ga or (CH

)

Al, respectively, to increase the conductivity of the ﬁlms.

Cyclopentadienyl compounds (i.e. metallocenes) have at least one direct metal-carbon bond to

the C

ligand. As many metallocene compounds are volatile and thermally stable and they

are also suitable for use as ALD precursors, where high reactivity can be controlled by

sequential pulsing of the precursors. Although there exists a large family of different ligands

based on the C

, only a few of the simplest alkylated cyclopentadiene compounds have been

used as ALD precursors.

Simple metallocenes have been used for the deposition of metal oxides, such as for MgO [53],

[54], and In

[55]. For example, compared to the traditional ␤-diketonate/ozone

process [56],(C

)

Mg/H

O gives almost a ten times higher ﬁlm deposition rate and a wider

temperature window for MgO ﬁlm growth. However, because the larger size of the heavier

alkaline earth metals, simple (C

)

Sr and (C

)

Ba compounds do not possess sufﬁcient

thermal stability for controlled ALD growth. Therefore deposition of strontium- and

barium-containing ﬁlms has been studied using bulkier ligands. For example, SrTiO

[57, 58]

and BaTiO

[58] thin ﬁlms have been deposited from (C

)Sr, (C

)Ba and

)Ba as alkaline earth metal precursors and Ti(OCH(CH

)

and H

O as titanium

and oxygen sources, respectively.

For integrated circuit (IC) applications ZrO

and HfO

thin ﬁlms can be processed by ALD. In

addition to the conventional halide [59–61], nitrate [62], and amide [63, 64] precursors,

organometallic cyclopentadiene compounds have been used to produce insulating HfO

thin

Atomic Layer Deposition 379

Table 8.3: Published ALD processes for HfO

thin ﬁlms [66]

Metal precursor/

oxygen source

Range of

growth

(

◦

Preferred or most

frequently used T

growth

(

◦

HfCl

O 160–940 300

HfCl

300 300

HfCl

/Hf(NO

)

150–190 150–190

HfCl

OorH

225–500 300

HfCl

400–755 570–755

Hf(NEtMe)

O 200–350 250

Hf(NEtMe)

100–400 250–300

Hf(NMe

)

O 50–500 < 350

Hf(NMe

)

160–420 200–320

Hf(NEt

)

O 50–500 < 450

Hf(O



Bu)

350–480 350–480

Hf(O



Bu)

250 250

Hf(O



Bu)

(mmp)

275–400 360

Hf(mmp)

O 275–425 360

Hf(ONEt

)

O 250–350 300

Hf(NO

)

O 160–190 180

HfMe

O 300–500 350–400

Hf(CH

)

O 350

a = mmp 51-methoxy-2-methyl-2-propanolate (OCMe

OMe).

ﬁlms [65] (Table 8.3). Controlled growth has been obtained by using cyclopentadienyl-type

)

HfCl

and (C

)

Hf(CH

)

as a hafnium source. Because of the similar ionic radius

and analogous chemistry between zirconium and hafnium, organometallic hafnium

compounds behave almost identically to the corresponding zirconium precursors [66].

The selection of the best possible precursor for a desired material and application is not

straightforward. The ﬁlm quality, including optical, electrical, and mechanical properties, is

often the most critical quality factor. The selected precursor and deposition temperature will

affect the deposition temperature and impurity content.

8.4.4 Novel ALD Processes

Constant precursor development has been carried out for different types of materials.

Typically, the ALD growth of metal oxides and nitrides proceeds through two surface-limited

half-reactions. Recently, novel processes have been reported where a different type of

self-limiting growth has been obtained; for example, rapid deposition of SiO

using TMA as a

catalyst [67].

380 Chapter 8

Scheme 8.1: Catalytic SiO

deposition by ALD [67].

The low reactivity or high toxicity of the non-metal precursors often limit the use of novel

processes. For example, for metal ﬂuorides and phosphates the selection of reactive ﬂuorine or

phosphorous compounds is quite limited. In principle, it is possible to use HF, PH

derivatives,

and alkyl selenides as precursors, but owing to their corrosive nature and toxicity they are not

widely used.

The deposition mechanism of Ca–P–O has been studied by using Ca(thd)

and

(CH

PO/H

O as precursors. The ﬁrst Ca(thd)

cycle produces ﬁrst a monolayer of

CaCO

on the surface. During the second stage, the (CH

PO/H

O cycle, the phosphate

groups replace the carbonate species without changing the deposition rate.

Pilvi et al. [68] reported the deposition of metal ﬂuorides (MgF

, CaF

, LaF

) by using metal

␤-diketonates as a metal precursors and solid TiF

or TaF

as a second precursor. This reaction

proceeds through ligand exchange reaction between metal ␤-diketonate and volatile metal

ﬂuoride. It is interesting to note that although there are oxygen coordinated ␤-diketonate

ligands present, no signiﬁcant amount of oxygen is left on the ﬁlms. However, some titanium

or tantalum is left on the ﬁlms as an impurity.