Effect of Ru crystal orientation on the adhesion characteristics

of Cu for ultra-large scale integration interconnects

Hoon Kim a,*, Toshihiko Koseki a, Takayuki Ohba b, Tomohiro Ohta b,Yasuhiko Kojima c, Hiroshi Sato c, Shigetoshi Hosaka c,

Yukihiro Shimogaki a

a Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo,

Bunkyo-Ku, Tokyo 113-8656, Japanb Division of University Corporate Relations, The University of Tokyo,

7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8654, Japanc Technology Development Center, Tokyo Electron AT Limited, 650 Mitsuzawa,

Hosaka-cho, Nirasaki-city, Yamanashi 407-0192, Japan

Available online 21 October 2005

Abstract

The adhesion of Cu on Ru substrates with different crystal orientations was evaluated. The crystal orientation of sputter deposited Ru could be

changed from (1 0 0) to (0 0 1) by annealing at 650 8C for 20 min. The adhesion of Cu was evaluated by the degree of Cu agglomeration on Ru. Cu

films on annealed Ru films with the (0 0 1) crystal orientation showed 28% lower RMS values and 50% lower Ru surface coverage than Cu as-

deposited on Ru having the (1 0 0) crystal orientation after annealing at 550 8C for 30 min, which suggest that Cu wettability on the Ru(0 0 1) was

better than that on the Ru(1 0 0) plane. The low lattice misfit of 4% between Cu(1 1 1) and Ru(0 0 1) may be the reason for this good adhesion

property.

# 2005 Elsevier B.V. All rights reserved.

PACS: 68.35.G; 85.40.L

Keywords: Cu wettability; Ru glue layer; Ru crystal orientation; Lattice misfit

www.elsevier.com/locate/apsusc

Applied Surface Science 252 (2006) 3938–3942

1. Introduction

After the 90 nm generation, Cu will completely replace Al as

the new ultra-large scale integration (ULSI) interconnect

material due to its favorable electrical conductivity and superior

resistance to electromigration. Chips with more conductive Cu

interconnects and low-k interlayer dielectrics will consume less

power and operate at significantly higher speeds due to

decreased resistance–capacitance coupling delay. However, to

prevent catastrophic contamination caused by Cu diffusion

through interlayer dielectrics into silicon, diffusion barriers,

such as tantalum (Ta) and tantalum nitride (TaN), should be

used to contain Cu interconnects [1]. In a conventional Cu-

damascene process, a continuous Cu-seeding layer (>7.5 nm)

* Corresponding author. Tel.: +81 3 5841 7131; fax: +81 3 5841 7131.

E-mail address: kimhoon1@dpe.mm.t.u-tokyo.ac.jp (H. Kim).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2005.09.042

is deposited over these diffusion barriers by physical vapor

deposition (PVD) to assure a good Cu electrofill [2].

As line widths shrink to less than 45 nm, there is concern that

PVD-based Cu-seed layers will not provide the continuous step

coverage which is required for void-free filling during Cu

electroplating. Thus, there was an attempt to electroplate Cu

directly onto diffusion barrier films [3]. However, thin barrier

layers like TaN (>200 mV cm) are too resistive to plate Cu

effectively, especially in the high-aspect-ratio damascene

features. Conformal deposition techniques, such as chemical

vapor deposition (CVD) and atomic layer deposition (ALD) of

Cu, are therefore of interest for the production of extendible

seed layers [4]. Unfortunately, integration of CVD and ALD

Cu-seed layers from fluorine-containing precursors has been

problematic for several reasons, such as nucleation on, and

adhesion to, Ta liners [5–8].

Once these issues became apparent, the search for alternate

seed layer materials was initiated, and several candidates were

H. Kim et al. / Applied Surface Science 252 (2006) 3938–3942 3939

Fig. 1. X-ray diffraction patterns from as-deposited Ru films and Ru films

annealed at 650 8C for 20 min.

identified; noble metals which could be deposited with CVD or

ALD processes were of particular interest [9–16]. Ru as all, or

part, of a barrier layer upon which copper could be electro-

deposited directly is a candidate for such an effort [15–16]. The

immiscibility of Ru in Cu is a particularly favorable attribute for a

seed layer, and the low resistivity of Ru (7.6 mV cm), which is

approximately half that of Ta (13.5 mV cm), is also an attractive

feature for this purpose [15]. Ru also showed good adhesion

properties to Cu deposited by sputtering, electroplating and even

CVD [9–18]. Thus, Ru should be a promising adhesion promoter

or seed layer for direct plating.

In our previous paper [17], we compared the adhesion

properties of Cu on Ta and Ru by measuring the wetting angle

of Cu on these layers. The wetting angle of Cu on a Ru substrate

was 438, which was one-third of that on a Ta substrate (1238),which means Ru has better Cu adhesion than Ta. The improved

Cu adhesion of Ru compared to Ta was explained by the

concept of lattice misfit. The lattice misfit between Cu and Ru is

4%, however, that between Cu and Ta is 22%. Lower lattice

misfit results in a stable interface, however, this calculated

lattice misfit value is between Cu(1 1 1) and Ru(0 0 1).

In present work, therefore, the effect of Ru crystal

orientation on Cu wettability was investigated. In order to

control the crystal orientation of the Ru thin film, the effect of

Ru film thickness on the TaN substrate and post-annealing were

examined. The Ru film thickness did not show an apparent

effect on its crystal orientation, while annealing at 650 8C for

20 min changed the Ru(1 0 0) crystal orientation to the (0 0 1)

orientation. The Cu wettability on as-deposited Ru(1 0 0) and

annealed Ru(0 0 1) was, thus, evaluated by measuring the Cu/

Ru/TaN substrate surface after an anneal at 550 8C for 30 min

using XPS. The RMS roughness values of the sample surface

were also examined by AFM. Both measurements clearly

suggested that Ru(0 0 1) had better adhesion characteristics

than Ru(1 0 0), which coincides with our lattice misfit concept.

2. Experimental details

A Si wafer coated with a 100 nm thermal oxide was used as

the substrate. TaN was deposited on the SiO2/Si wafer using the

reactive sputtering method and direct current magnetron

sputtering. The deposited TaN film was Ta-rich, namely,

60 at.% Ta and 40 at.% N and had low crystallinity. This TaN

substrate (i.e. the TaN/SiO2/Si wafer) was cleaned in a solution

of 28% ammonia water:30% H2O2 (1:1) for 1 min at 100 8Cbefore Ru deposition, because this treatment will oxidize the

TaN surface and make its surface to amorphous phase. Such an

amorphous TaN top surface enables us to control the Ru crystal

orientation and to eliminate the effect of the TaN substrate. Ru

and Cu layers were deposited by RF magnetron sputtering with

a base pressure of less than 5 � 10�6 Pa, and employing multi-

target system which can deposit four different materials without

breaking the vacuum between depositions. The sputtering

targets were 2 in. diameter disks of Cu (99.99%) and Ru

(99.9%). The deposition was carried out at room temperature, at

a power density of 15.5 W/cm2, a working pressure of 1.33 Pa

and with an Ar gas flow rate of 20 sccm. The thickness of the Ru

films was targeted to be 8, 16 and 24 nm. Each Ru sample was

annealed in a rapid thermal annealing (RTA) furnace (with a

vacuum system) at 650 8C for 20 min in a vacuum (10�6 Pa)

environment to change the crystal orientation. The crystal

orientation of the as-deposited and annealed Ru films was

evaluated by using a u–2u X-ray diffractometer (XRD; MAC

Science, M18XHF) with Cu Ka radiation generated at 55 kV

and 250 mA. As-deposited and annealed Ru samples were

dipped in HF (50%) solution for 3 s to remove surface oxide.

Next 20 nm Cu films were deposited by sputtering onto the as-

deposited and annealed Ru substrates. Each samples was

annealed in a RTA furnace at 550 8C in a H2 (400 Pa)

environment to investigate the wettability of Cu [17,19].

The Cu wettability of each Ru substrate was evaluated by

measuring the degree of Cu film agglomeration. The surface

morphology of the Cu films after annealing were investigated

by using a field emission scanning electron microscope (FE-

SEM; JEOL JSM6340F). The agglomeration of Cu on Ru was

evaluated by measuring the RMS roughness using atomic force

microscopy (AFM; NPX 2100 Seiko Instrument Inc.) and the

surface coverage of Ru was measured by X-ray photoelectron

spectroscopy (XPS; ULVAC–F XPS Model 1600C) with Mg

Ka radiation generated at 300 W.

3. Results and discussion

3.1. Effect of post-annealing and Ru film thickness on

crystal orientation of Ru films

XRD patterns obtained from Ru films produced by as-

deposited and annealed samples are shown in Fig. 1. A change

in the preferred orientation from (1 0 0) to (0 0 2) was observed

after annealing for 20 min at 650 8C. For a HCP structure, such

as Ru, the most thermodynamically stable face is (0 0 1).

Annealing of the as-deposited Ru film caused a change to the

closed-packed (0 0 1) plane. The crystal orientation of the Ru

film was examined and it had changed to (0 0 1) at a higher

deposition temperature [10,18] or when remote plasma was

added [14]. These results illustrated that additional energy

stimulated the change of the Ru crystal orientation to (0 0 1),

the most thermodynamically stable face. As the feature size

decreases, the thickness of the glue layer should also be

decreased and this can also change the crystal orientation

[20,21]. Therefore, we investigated the dependence of the

H. Kim et al. / Applied Surface Science 252 (2006) 3938–39423940

Fig. 2. X-ray diffraction patterns of as-deposited Ru films having different

thickness (8, 26, 24 nm).

Fig. 4. Plan view and 3D-SEM images of 20 nm thick Cu film on: (a) an as-

deposited Ru substrate and (b) a Ru substrate annealed at 650 8C for 20 min

after subsequently annealing at 550 8C for 30 min in a H2 (400 Pa) environment.

crystal orientation the thickness of the Ru film. XRD patterns of

the as-deposited Ru film are shown in Fig. 2. It seems that there

was a small change in the crystal orientation as the Ru film

thickness changes. XRD peaks were visible even for the 8 nm

thick Ru film. As-deposited Ru films on an oxidized TaN

substrate show mainly the (1 0 0) peak. Considering the

intensity of (1 0 0) in powder diffraction is only 40% of the

(1 0 1) intensity [22], the as-deposited Ru has strong (1 0 0)

orientation. Annealed samples showed mainly (0 0 1) crystal

orientation regardless of the thickness of the Ru film (Fig. 3).

3.2. Effect of Ru crystal orientation on Cu wettability

To evaluate the Cu wettability on different Ru film crystal

orientations, a 20 nm Cu film was sputtered onto as-deposited

and annealed 24 nm thick Ru substrates and the samples were

annealed at 550 8C for 30 min in a H2 (400 Pa) environment.

The agglomeration behavior of 20 nm Cu films was shown in

SEM images which were taken with a 48 angle to show a three-

dimensional (3D) view of the Cu islands together with a plan

view of the same samples (Fig. 4). Agglomeration of the Cu

film occurred on as-deposited Ru(1 0 0) (Fig. 4a) and annealed

Ru(0 0 1) (Fig. 4b) substrates. The Cu film on the as-deposited

Ru(1 0 0) substrate changed to a discontinuous copper layer.

The Cu film on the annealed Ru (0 0 1) substrate, however,

remained a continuous thin Cu film around Cu islands. The

degree of agglomeration represents the adhesion between Cu

and the substrate because if there is poor adhesion between Cu

and the substrate, Cu atoms can migrate easily during annealing

to minimize the surface energy [17]. The RMS roughness

values for the Ru film obtained from AFM and the surface

Fig. 3. X-ray diffraction patterns of 650 8C, 20 min annealed Ru films having

different thickness (8, 26, 24 nm).

coverage of the Ru substrates after annealing obtained from

XPS were used for quantitative analysis. Surface coverage

measured by XPS also shows the degree of agglomeration of

Cu. This is because the escape depth for XPS-photo-electrons is

about 2 nm [23], and thus, the Ru peak will not appear if the Cu

films remained a continuous layer. The observation of Ru peaks

in XPS means that the Cu film is not continuous and a high

surface area of Ru means that the agglomeration of Cu on the

Ru substrate is more progressed and the Cu adhesion is poor.

Fig. 5 shows AFM images of Cu films deposited on an as-

deposited Ru(1 0 0) substrate (Fig. 5a) and an annealed

Ru(0 0 2) substrate after 30 min of annealing at 550 8C in a

H2 (400 Pa) environment (Fig. 5b). Table 1 summarizes the

RMS roughness values and surface coverage of Cu on as-

deposited Ru and annealed Ru substrates. The RMS roughness

value of Cu on as-deposited Ru (26.2 nm) was found to be 39%

higher than that on annealed Ru (18.9 nm). The surface

coverage of Ru on the as-deposited substrate (14 at.%) is also

twice as high as that on the annealed Ru substrate (7 at.%).

Based on the above wettability results confirmed by RMS

roughness and XPS surface coverage measurements, the

annealed Ru substrate with a (0 0 1) crystal orientation had

better Cu wettability than the as-deposited Ru substrate with a

(1 0 0) crystal orientation. The lattice misfit values between

H. Kim et al. / Applied Surface Science 252 (2006) 3938–3942 3941

Fig. 5. AFM images of a 20 nm thick Cu film deposited on: (a) an as-deposited

Ru substrate and (b) a Ru substrate annealed at 650 8C for 20 min after

subsequently annealing at 550 8C for 30 min in a H2 (400 Pa) environment.

Table 1

RMS roughness and surface coverage of Cu on as-deposited and annealed Ru

substrates

Cu on as-deposited Ru Cu on annealed Ru

AFM RMS roughness

value (nm)

26.2 18.9

Surface coverage Cu: 86%, Ru: 14% Cu: 93%, Ru: 7%

Fig. 6. AFM images of a 20 nm thick Cu film on a: (a) 8 nm; (b) 16 nm; (c)

24 nm thick Ru substrate after annealing at 550 8C for 30 min in a H2 (400 Pa)

environment.

Cu(1 1 1) and Ru(0 0 1) or (1 0 0) are 4 and 48%, respectively.

It was thought that the low lattice misfit between Cu(1 1 1) and

Ru(0 0 1) causes good Cu adhesion that was suggested in our

previous work [17]. This result shows that the lattice misfit

concept is a reasonable method for estimating the interface

adhesion property between Cu and the glue layer.

3.3. Effect of Ru thickness on Cu wettability

The wettability of Cu on different thicknesses of Ru film was

evaluated on as-deposited Ru substrates, (with thicknesses of 8,

16 and 24 nm) because in situ deposition of Cu and Ru

eliminated the effect of interface impurities. Fig. 6 shows AFM

images of a 20 nm thick Cu layer after annealing for 30 min at

550 8C in a H2 (400 Pa) environment. The agglomeration of the

Cu film was measured on 8, 16 and 24 nm thick Ru substrates

(Fig. 6). The morphological differences between the Cu films

on different Ru substrate thicknesses after annealing were

minor. For quantitative analysis, the RMS roughness values of

the Ru films obtained from AFM and the surface coverage of

the Ru substrates after annealing obtained from XPS were used.

The AFM surface roughness values for the Cu films on 8, 16

and 24 nm Ru substrates were 28, 24 and 25 nm, respectively.

The XPS surface coverage of Ru after annealing samples of

20 nm thick Cu films deposited on 8, 16 and 24 nm thick Ru

substrates were 19, 20 and 17 at.%, respectively. The RMS

roughness values of Cu and Ru surface coverage showed little

H. Kim et al. / Applied Surface Science 252 (2006) 3938–39423942

difference between the different Ru substrate thicknesses. From

the XRD results (Fig. 2), the thickness of Ru cannot affect the

crystal orientation of the Ru films, and the Cu wettability also

had no close relationship with Ru thickness as confirmed by the

RMS roughness values of the Cu films and the XPS surface

coverage of the Ru substrate after annealing. Thus, the crystal

orientation of the Ru film, which can be effectively controlled

by post-deposition annealing treatment, is the main controlling

factor affecting the Cu wettability.

4. Conclusion

The effect of the Ru crystal orientation on Cu wettability has

been studied. To change the crystal orientation of the Ru films,

post-annealing and different thickness of Ru film were used. A

change in Ru film thickness from 8 to 24 nm did not affect the

crystal orientation. Post-annealing, however, dramatically

modified the crystal orientation of the Ru films from (1 0 0)

to (0 0 1). The Cu wettability on Ru substrates was evaluated by

the degree of Cu agglomeration, using AFM surface roughness

and XPS surface coverage of the Ru substrate. The Cu

wettability was controlled by the crystal orientation of the Ru

substrate. The Ru(0 0 1) crystal orientation enhanced the Cu

wettability because it has a low lattice misfit with the Cu(1 1 1)

plane. From this result, we can suggest that the Ru(0 0 1) plane,

which can be easily obtained by post-annealing, is the most

suitable crystal orientation for good adhesion between Cu and

Ru and thus for producing high electromigration resistance.

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