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Surface & Coatings Technolog

Low temperature deposition of metal films by metal chloride reduction

chemical vapor deposition

Yuzuru Ogura a,*, Chikako Kobayashi b, Yoshiyuki Ooba a, Naoki Yahata a, Hitoshi Sakamoto a

aAdvanced Technology Research Center, Mitsubishi Heavy Industries, Ltd., 1-8-1 Sachiura, Kanazawa-ku, Yokohama 236-8515, JapanbHITEC Co., Ltd., 1-8-1 Sachiura, Kanazawa-ku, Yokohama 236-8515, Japan

Available online 10 August 2005

Abstract

Tantalum nitride, titanium, and iridium films were deposited at lower than 300 -C temperature by metal chloride reduction chemical vapor

deposition (MCR-CVD) method using Cl2 plasma and respective metal targets. These results demonstrated features of MCR-CVD method:

(1) The N/Ta ratio in the tantalum nitride films can be controlled by gas flow ratio of N2 to Cl2, (2) Conformal coverage to bottom-up filling

via holes can be changed by substrate temperatures, (3) Deposition of iridium films needs protecting layers against damages by chlorine

radicals (Cl*). We discuss the deposition mechanism of MCR-CVD as a system including depositing and etching, which are competitive to

each other.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Chlorine; Plasma; CVD; Reduction

1. Introduction

The chemical vapor deposition (CVD) process is a

promising method for providing a conformal coverage on

various device structures with dimensions below a submi-

crometer. Metalorganic chemical vapor deposition

(MOCVD) is widely used for the deposition of metals

although impurities such as C, H, and O in deposited films

that originate from a metalorganic source deteriorate

electronic properties. An alternative metalorganic source is

chlorides, which exhibit reasonably high vapor pressures at

low temperatures. In particular, Cu CVD for the intercon-

nections of ULSI devices has been investigated by many

researchers. They used H2 or H2 plasma as a reducing agent,

and a high growth rate of Cu films was achieved [1–4].

However, a low deposition temperature with low impurities

is still required. CVD methods of refractory metals and

nitrides such as Ti, Mo, Ir, Ta and TaN, which are necessary

for the electrodes of devices, also have the same require-

0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2005.07.050

* Corresponding author. Tel.: +81 45 775 2437x9037; fax: +81 45 771

3879.

E-mail address: Yuzuru_Ogura@mhi.co.jp (Y. Ogura).

ment potentially [5]. Additionally since TaN film is

attractive as an advanced gate electrode because of

compatibility with high-k gate dielectric [6] and tunability

of the work function by [N]/[Ta] ration of TaN [7,8],

controllability of [N]/[Ta] ratio of TaN is also required.

We found that Cl* can be used as reducing agents for

the deposition of Cu [9]; simultaneous supply of Cu

chlorides and Cl* at a certain ratio leads to the deposition

of Cu, otherwise excess Cu chlorides deposit Cu chloride

films or excess Cl* etch Cu. If a Cu plate and a substrate

are placed at appropriate positions in Cl2 plasma, Cu

chlorides is produced by etching the Cu plate, and

simultaneously, Cu chlorides is reduced by Cl* at the

surface of the substrate to deposit a Cu film. It was

demonstrated that the deposition of Cu occurs at temper-

atures below 300 -C, and pure Cu, which contains less

than 15 ppm of Cl, was obtained [10]. We found that this

metal chloride reduction chemical vapor deposition (MCR-

CVD) method can be applied to other metals such as Ta,

and Ir, whose chlorides have reasonably high vapor

pressures at low temperatures and chemical instability

[11], but an issue of etching the surface of the substrate

during deposition was raised in the case of Ir.

y 200 (2006) 3347 – 3350

fcc-

TaN

(11

1)

fcc-

TaN

(20

0)

bcc-

Ta

(11

0)

Y. Ogura et al. / Surface & Coatings Technology 200 (2006) 3347–33503348

In this paper we report recent results of deposition of

metal and nitride films by MCR-CVD method; a controll-

ability of [N]/[Ta] ratio in TaN deposition, an alternative of

conformal deposition or bottom-up filling of Ti deposition,

and application of a protective layer before Ir deposition.

20 25 30 35 40 45 50

Inte

nsit

y (A

rb. U

nit.

)

2 (º)θθ

QN2/QCl2

0.061

0.040

0.020

0.012

0.000

Fig. 2. XRD spectra of TaN films grown with the N2/Cl2 gas flow rate from

0 to 0.061.

2. Experimental details

The scheme of the MCR-CVD system employed in our

experiments is shown in Fig. 1. A metal source was

arranged opposite to a substrate in a reaction chamber.

Cl2–He gas mixture was introduced into the reaction

chamber and exhausted through a variable conductance

valve to keep the pressure in the reaction chamber constant.

To generate plasma, radio frequency (13.56 MHz) electrical

power was supplied via an antenna to Cl2–He gas mixture.

The chlorides produced by the chlorination of the metal

source by Cl* or Cl2 were transported to the substrate and

reduced at the surface of the substrate by Cl*. Prior to the

deposition, plasma was generated in the reaction chamber

until the metal source reaches the appropriate temperature to

start chlorination. The surface morphology and the crystal

structure of the films were observed by field-emission

scanning electron microscopy (FE-SEM) and X-ray diffrac-

tion (XRD) respectively. The concentrations of impurities in

the films were analyzed by X-ray photoelectron spectro-

scopy (XPS), and the resistivity of the film was measured by

the four-point probe method.

4

5 100

Gr)

3. Results and discussion

3.1. Deposition of tantalum nitride

Fig. 2 shows the XRD spectra of the obtained TaN films

grown at the range of the N2/Cl2 gas flow ratio from 0 to

0.061. At 0.040 and 0.061, diffraction peaks indexed as (111)

and (200) of fcc-TaN were detected clearly. Decreasing the

N2/Cl2 gas flow ratio to 0.020, these peaks were broaden and

a peak around 2h =37.5- appeared, which seemed to be

contributed from (110) of expanded bcc-Ta(N) by interstitial

penetration of N. With further decrease of the N2/Cl2 gas

Wafer

Metal (Nitride) film

Cl2 (+N2)

RF powerAntenna

Cl*

MxClyReduction

Etching

Plasma

Metal plate

Fig. 1. Schematic diagram of MCR-CVD process.

flow ratio, two peaks of fcc-TaN disappeared, and the peak

around 2h =37.5- was sharpened and shifted to that of bcc-

Ta (110). Therefore, it can be concluded that the N2/Cl2 gas

flow ratio can control the [N]/[Ta] ratio in the TaN films.

The variation of the resistivity and the growth rate of the

TaN films were shown in Fig. 3 as a function of the N2/Cl2gas flow ratio. The growth rate is suppressed by addition of

only 0.3 vol.% N2 (the N2/Cl2 gas flow ratio of 0.012). Ta

chlorides are produced on the surface of the Ta target by the

next reaction, because optical emission peak of TaCl was

observed during Ta deposition [12].

TaðsÞ þ Cl4ðgÞYTaClðgÞ ð1Þ

or

TaðsÞ þ 1=2Cl2ðgÞYTaClðgÞ: ð2Þ

Added N2 must be adsorbed on the surface of the target

chemically, since the temperature of the target reached

-5

-4

-3

-2

-1

0

1

2

3

0.00 0.02 0.04 0.06

QN2/QCl2

0

20

40

60

80 owth rate (nm

/min.)R

esis

tivi

ty (

m Ω

cm

Fig. 3. Variation of the resistivity and the growth rate of TaN films as a

function of the N2/Cl2 gas flow ratio.

Etchedarea

TiTiTi

(a) (b) (c)

Fig. 4. Cross-sectional FE-SEM images for Ti film on hole structure in the distance from the RF window to the substrate of 120 mm, with hole depth of 1.0 Amand the diameter of (a) f1.0 Am, (b) f0.5 Am, and (c) f0.4 Am.

Y. Ogura et al. / Surface & Coatings Technology 200 (2006) 3347–3350 3349

above 500 -C during deposition, therefore it is considered

that decrease of Ta chlorides flux to the substrate by the

decrease of reaction sites on the surface of the target cause

to suppress the growth rate.

TaCl(g) is transported to the substrate and adsorbed on

the surface of the substrate;

TaClðgÞYTaClðadÞ ð3Þ

where (ad) indicates an adsorbed state. The following

reaction is considered to be dominant on the surface of

the substrate,

TaClðadÞ þ ClðgÞYTaðsÞ þ Cl2ðgÞ ð4Þ

because the growth rate is almost independent on the N2/Cl2gas flow ratio under the presence of N2. Deposited Ta seems

to be nitridated by nitrogen radicals immediately.

TaðsÞ þ yNðgÞYTaNyðsÞ: ð5Þ

These results suggest that MCR-CVD method can be

applied to deposit metallic compounds films, if added gas

elements do not suppress formation of metal chlorides due

to reactions with a metal target.

3.2. Conformal deposition and bottom-up filling

The optical emission spectrum in the plasma during the

deposition suggests TiCl(g) is the dominant species con-

tributing to the Ti deposition. It is considered that the Ti

(a) (b

Fig. 5. Cross-sectional FE-SEM images for Ti film on hole structure in the distance

The coverage for the diameters of (a) f1.0 Am, (b) f0.5 Am, and (c) f0.3 Am is

deposition process consists of the following three reactions:

etching the Ti target to form TiCl(g), adsorption of TiCl(g)

on the substrate surface, and reduction of adsorbed TiCl(ad)

by Cl*.

TiðsÞ þ ClðgÞYTiClðgÞ ð6Þ

TiClðgÞYTiClðadÞ ð7Þ

TiClðadÞ þ ClðgÞYTiðsÞ þ Cl2ðgÞ: ð8Þ

If the concentration of TiCl and Cl* ([TiCl] and [Cl*])

on the substrate surface are almost equal and the substrate

temperature is appropriate, TiCl would be reduced into Ti.

However, if [Cl*] is larger than [TiCl] or the substrate

temperature is higher, deposited Ti would be etched again.

Conversely, if [TiCl] is larger than [Cl*] or the substrate

temperature is lower, TiCl would condense on the

substrate.

In the case of deposition on walls of holes, [TiCl] and

[Cl*] must decrease with increasing depth because of

adsorption of TiCl and recombination of Cl* on the walls.

In a result, the gradient of [TiCl] and [Cl*] will be

formed from the opening to the bottom respectively,

depending on diameter of a hole, pressure, wall temper-

ature and so on.

In order to investigate deposition behaviour on walls of

holes, we performed Ti deposition on fine hole structures

covered with 50 nm thick TaN. Fig. 4 shows cross-sectional

) (c)

from the RF window to the substrate of 90 mm, with hole depth of 1.0 Am.

80%, 100%, and 90%, respectively.

Ir

SiO2

Si

SiO2

Si

Ir

Ta

(a) (b)

Fig. 6. Cross-sectional FE-SEM image of (a) damaged Si substrate under SiO2 layer by deposition of Ir without a protective film, and (b) Ir film deposited after

deposition of a Ta protective film.

Y. Ogura et al. / Surface & Coatings Technology 200 (2006) 3347–33503350

FE-SEM images of Ti film deposited on hole structures, all

with the same depth of 1.0 Am but different diameters of

f1.0, f0.5, and f0.3 Am. The tendency of bottom-up filling

becomes stronger with the smaller diameter holes, while the

etched area around the opening is narrower with the smaller

diameter holes in the distance from a RF entrance window

to a substrate of 90 mm. But almost conformally coated Ti

films were observed as shown in Fig. 5 in the case of the

distance of 120 mm; coverage of the Ti film is 80% for the

diameters of f1.0 Am, 100% for f0.5 Am, and 90% for f0.3

Am. The result suggests that the substrate temperature,

which depends on the distance from the RF entrance

window to the substrate, can switch the deposition

behaviour from bottom-up filling to conformal deposition.

3.3. Protective layer against substrate damage

Many etch pits on a Si substrate under a SiO2 layer,

which exhibited no recession, were observed after deposi-

tion of Ir films with MCR-CVD method as shown in Fig.

6(a). It is considered that penetrated Cl* through the SiO2

layer reacts with the Si substrate and formed Si chlorides are

diffused out. A possible cause of the etch pits formation is

likely the characteristic interactions of Ir and Si, because no

etch pits formation was observed in deposition of other

metal films such as Ta, Ti, Mo and Cu, even in cases that

occurred a significant recession of SiO2 layer by attack of

excess Cl*.

In order to avoid formation of etch pits on the substrate, a

Ta film deposited by MCR-CVD prior to Ir deposition was

used as a protective layer against penetration of Cl.

Consequently, a 50-nm-thick Ir film was successfully

deposited without damage of the substrate, as shown in

Fig. 6(b).

4. Conclusion

We demonstrated that films of tantalum nitride, titanium,

and iridium were deposited at lower temperature than 300

-C by MCR-CVD method using Cl2 plasma. It is found that

N2/Cl2 gas flow rate can control the [N]/[Ta] ratio in TaN

films as evident by XRD analysis. In the case of Ti

deposition it is shown that the distance between the RF

entrance window and the substrate influence the deposition

behaviour, which change from bottom-up filling to con-

formal deposition. The formation of etch pits on the

substrate during Ir deposition is likely the characteristic of

Ir. This damage was avoided by a Ta protective film

deposited by MCR-CVD prior to Ir deposition.

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