Fabrication of arrays of (100) Cu under-bump-

metallization for 3D IC packaging Wei-Lan Chiu1, Chia-Ling Lu1, Han-Wen Lin1, Chien-Min Liu1, Yi-Sa Huang1, Tien-Lin Lu1, Tao-Chi

Liu1, Hsiang-Yao Hsiao1, Chih Chen1, Jui-Chao Kuo2 and King-Ning Tu3 1Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 30010, Republic of China.

2Department of Materials Science & Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China. 3Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA, USA.

Abstract—Due to the thousands of microbumps on a chip for3D ICs, the precise control of the microstructure of all the

material is required. The nearly -oriented nanotwinned and

fine-grained Cu was electroplated on a Si wafer surface and

annealed at 400–500 °C for 1 h, many extremely large -

oriented Cu crystals with grain sizes ranging from 200 to 400 μm

were obtained. The -oriented Cu grains were transformed

into super-large -oriented grains after the annealing. In

addition, we patterned the -oriented Cu films into pad

arrays of 25 to 100 μm in diameter and annealed the nanotwinned

Cu pads with same conditions. An array of -oriented single

crystals Cu pads can be obtained. Otherwise, single-crystal nano-

wire growth displays a process by one-dimensional anisotropic

growth, in which the growth along the axial direction is much

faster than in the radial direction. This study reported here a

bulk-type two-dimensional crystal growth of an array of

numerous -oriented single crystals of Cu on Si. The growth

process in 3D IC has the potential for microbump applications

packaging technology.

Keywords—-oriented Cu; nanotwinned copper; abnormal

grain growth.

I. INTRODUCTION To approache the limit of Moore’s law, the microelectronic

industry is turning to extend the limit using threedimensional

integrated circuits (3D ICs) in packaging, which focus on the

merging of chip technology and packaging technology. Due to

the requirement of higher I/O density in electronic devices, the

diameter of flip chip bumps continuously shrinks. In a 100um

diameter bump, the volume of solder is much larger than the

volume of under bump metallization (UBM). Once the

diameter of solder shrinks from 100 to 10μm, the volume of

solder consequently decreases by 1000 times. In this condition,

the volume of UBM in microbumps became larger than the

volume of solder. During reflowing and electric current

flowing, the solder would transform full intermetallic

compounds.

It has been reported that sputtering control the orientation of

grain and electroplating by pulse or dc could control grain size

and structure. Inculding of fine-grained or nanotwins could

increase the mechanism of Cu. Nanotwins could increase the

strain hardening capability and be able to combine the high

tensile strength and ductility in mechanism. Because of twins

with nanoscale lamellar spacings and coherent twin boundaries

(TBs), the nano-twinned structure allows room for the

dislocation multiplication and storage to continue to large

plastic strains. Nanotwins had fabricated by electroplated by

pulse in Cu film; they also showed prefer oriented (111) of

nanotwinned Cu (nt-Cu) have higher hardness and strength

than regular Cu.[1-2] But nanotwins are unstable in thermal

treatment, they are easy to be grain growth and transfer into

another orientation.

Controlling grain starts from crystal growth, which is one of

the most important topics in multi-disciplinary sciences

because the crystal growth controls the properties of

semiconductors, metals and ceramics in thermal processing.[3-

10] Several researchers have reported that giant grains up to

several hundred micrometers can be grown in Al, Ag and Cu

thin films.[11-15] However, it is unclear about the grain

crystallization in nanotwinned Cu. This study is a new type of

abnormal grain growth that results from the extremely

anisotropic two dimensional crystal growth of Cu via nearly

unidirectionally oriented nano-twins; the growth in the vertical

direction is much slower than that in the lateral directions.

II. EXPERIMENTAL

This experimental that included columnar nanotwins in Cu

film and different diameter about Cu pads. First, the nt-Cu

samples are divided into the pad and film. TiW of 200 nm

thickness and Cu of 200 nm thickness were sequentially

sputtered onto Si substrate. Ti was used as an adhesive layer,

and Cu was as a seed layer. The fabrication process of nt-Cu

pads induced one photolithographic procedures. The size was

the diameter of the Cu microbump, which varied from 25 to

100 μm were deposited by electroplating on the patterned

substrate. The nt-Cu films were 8 μm in thickness. The stirring

rate to grow the oriented nt-Cu was 600 r.p.m., and the current

density was 50mAcm− 2. The duty cycles were Ton=0.02 s and

Toff=1.5 s. The deposition rate was 1.2 nm s − 1 under this

electroplating condition. .The electroplating solution of Cu is

composed of CuSO4∙5H2O (196 g/l), HCl (1 ml/l), additive (3 ml/l), and deionized water. For the preparation of the

equivalent Cu layer without the nanotwins, we used the same

electroplating solution at 200mA cm− 2.

In thermal treatment research, the Cu films were subjected

to annealing at temperatures ranging from 400 to 500 °C for

extended periods in vacuum oven at 5 × 10− 7 torr. All the

electroplating and reflowing processes were under air

condition. Using scanning electron microscope (SEM)

imagined microstructural features and focused ion beam (FIB)

polished cross-sections. X-ray diffraction (XRD) was used to

analyze the preferred texture of the Cu films. Electron Probe

X-ray Micro-Analysis (EPMA) were used to study the

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individual grain orientation in the Cu film and pads.

III. RESULTS AND DISCUSSION

Preferred orientation of Cu could be fabricated by pulse

electroplating. The spacing of the nanotwins ranged from 10

nm to 100 nm. We fabricated highly oriented nt-Cu

using oriented- Cu seed layer, likes as Figure 1a and

1b.They show the plan-view of the inverse pole figure map

from EBSD. Fig. 1c and 1d show the Cu film has extremely

high texture and the average 6.4° misorientation in the

statistical misalignment angles of the Cu(111) grains. After

annealing at 450 °C, we obtained exceptionally large grains

about 200–400 μm diameters and about ~ 65 times larger than

the surrounding grains which was called an abnormal grain;

see in Fig. 2a. Fig. 2b shows large grain merged small grains

in an enlargement and we observed two peaks of {111} and

{200} diffraction for various times by X-ray diffraction

spectra in Fig. 2c. To convert the abnormal grain growth to

solid-state crystal growth, we patterned an array of nt-Cu

bumps with diameters ranging from 100 to 25 μm and

thicknesses of 10 μm using a lithographic technique. Figs. 3a

and b present cross-sectional EBSD and FIB images,

respectively. There is an anisotropic growth of a grain

annealed at 350 °C for 30 min. Figs. 3c and d show a

grain had observed by annealed at 400 °C for 10min. Fig. 3e

shows grains grown to 170 and 207 μm, respectively,

after annealing at 400 °C for 20 min. It is the most significant

anisotropic growth in this condition.

As-fabricated columnar nt-Cu is showed by Fig. 4a. We

annealed at 400 °C for 20min and polished the cross-section

by Figs. 4b and c. The grain on the left-hand side grew

closely to 290 μm. Figs. 4b shows the plan-view of the

inverse pole figure map from EBSD and indicated oriented-

transferred into grains and grain size is

distributed for the grains in d. The anisotropic grain growth

has a very slow growth in the vertical thickness of 5 μm,

which is at least 50 times slower than the lateral growth. In the

-oriented grains, no nanotwins are detected.

Furthermore, the anisotropic grain growth started from the

bottom interface rather than from the free surface. We

produced an array of pads with diameters of 75, 50 and 25 μm

as-fabrication, as shown in Figs. 5a-c. This figure

demonstrates the precise control of growth of an array of

single-crystal Cu microbumps. Figs. 5d-f were completely

transformed into a -oriented single-crystal of Cu, and

the other three were very near complete transformation. The

75-μm Cu pads in Fig. 5d occupy ~ 55% of the surface area.

and the 25-μm Cu pads in Figure 5f occupy only ~8% of the

surface area. There are some electron-charging in the edge of

Cu pads and more severely occur in the sample with 25-μm

Cu pads. However, the anisotropic growth is more clearly

observed in different diameter of Cu pads.

Fig. 1 (a) Plan-view of the inverse pole figure map from for the -oriented surface grains with color coding of the inverse pole figure of the as-deposited Cu film. (b) The inverse pole figure of the -oriented Cu film. (c) X-ray diffraction for the as-deposited Cu film displays orientations. (d) The statistical results

showing the numerical fraction of grains as a function of the misalignment angle from the [111] direction.

Fig. 2. (a) Plan-view inverse pole figure map of the large grain obtained from electron backscattered diffraction in two-dimensional anisotropic crystal

growth in nt-Cu films. (b) Enlarged inverse pole figure map for the dashed rectangle. (c) Evolution of the X-ray diffraction intensities for the samples annealed at 450 °C for various times.

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Fig. 3 (a) Cross-sectional map of the inverse pole figure from EBSD for the extremely anisotropic growth of the Cu grains

annealed at 350 °C for 30 min. (b) The corresponding cross-sectional FIB image. (c) Cross-sectional map of the inverse pole figure

from EBSD for the sample annealed at 400 °C for 10 min. (d) The corresponding cross-sectional FIB image for the sample in c. (e)

Cross-sectional FIB images for the sample after annealing at 400 °C for 20 min.

Fig. 4. (a) Cross-sectional FIB image of the as-fabricated nt-Cu sample. (b) Cross-sectional inverse pole figure map of the large

crystal with a lateral crystal size of 290 μm that showed extremely anisotropic growth of the Cu crystals. (c) Cross-

sectional FIB image of the sample in b. (d) Plan-view of the inverse pole figure map from electron backscattered diffraction for the

sample annealed at 450 °C for 25min. (e) Grain size distribution for the grains in d.

Furthermore, we prepared randomly oriented nanotwins to

compare the -oriented nanotwins in achieving the grain

growth of crystals. Fig. 6a presents a FIB cross-

sectional view of the randomly oriented nanotwins in Cu, and

Fig. 6b presents the corresponding X-ray diffraction spectrum,

where show the {111}, {200} and {220} reflections. The

normal grain growth occurred after annealing at 450 °C for 30

min,. Figs. 6c and d present the FIB cross-sectional image and

the corresponding X-ray diffraction spectrum, respectively. In

addition, an inverse pole figure map of the top view of the

grains obtained from EBSD in Fig. 6e reveals no extremely

anisotropic grains. Fig. 6f presents the average grain size is 6.6

μm.by the grain size distribution.

By the results, we could define the necessary and sufficient

conditions to obtain extremely anisotropic crystal growth

below. The first condition is the presence of a seeding layer

with a low density of -oriented seeds. So we prepared a

seeding layer with a very high density of -oriented

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Fig. 5 Different diameter of arrays of nt-Cu are (a)

75-μm diameter, (b) 50-μm diameter and (c) 25-μm diameter.

After annealing at 450 °C for 60 min, the Cu crystals

transformed into single crystals; like as (d) 75-μm

diameter, (e) 50-μm diameter and (f) 25-μm diameter.

seeds, we could obtain a very high intensity of -

oriented nt-Cu; however, upon annealing, we could hardly

detect the growth of grains because there were no

-oriented seeds. The second condition is a high density

of -oriented nanotins, which provide the stored energy

to serve as the driving force in the growth of the anisotropic

and untwinned {100} single crystals. Randomly oriented

nanotwins will not do serve as this driving force. The third

condition is that the substrate should have a large difference in

thermal expansion coefficient from that of the metal film, such

that a biaxial strain in the in-plane directions of the film occurs

under high-temperature annealing. The abnormal grain growth

in Cu has been explained by strain relaxation. Strain will

clearly have a role in this study of extremely abnormal grain

growth. In addition to the biaxial thermal strain mentioned

above, it has been reported that in pulse-electroplating nt-Cu, in

situ strain measurement using the bending beam method

reveals that the nt-Cu film is under in-plane biaxial tensile

strain.[16] We will not attempt to explain the mechanism of

abnormal grain growth here because what we have is the

microstructural evolution of an elastically anisotropic inclusion

(a two-dimensional grain) within an elastically anisotropic

matrix (the oriented nt-Cu).

We have applied this anisotropic growth to produce an array

of numerous -oriented single crystals of Cu microbumps

with sizes from 100 to 25 μm on Si wafer surfaces. An

extremely abnormal grain growth of - oriented single

crystals of Cu in a matrix of -oriented and nt-Cu

columnar grains had formed; the lateral growth is

approximately two orders of magnitude faster than the vertical

growth.

Fig. 6 (a) Cross-sectional FIB image for the as-fabricated Cu

film at 200mA cm−2. (b) The corresponding XRD spectrum for

the Cu film in a. (c) Cross-sectional FIB image for the Cu film

after annealing at 450 °C for 30min and (d) the corresponding

XRD spectrum for the Cu film in c. (e) Plan-view of the

inverse pole figure map from EBSD for the Cu grains annealed

at 450 °C for 30min. (f) Grain size distribution for the grains in

e. The average grain size is about 6.6 μm, which is

approximately the thickness of the Cu film.

ACKNOWLEDGEMENTS

Financial support from the Ministry of Science and

Technology, Taiwan under the contracts 102-2221-E-009-040-

MY3 is acknowledged. We would also like to thank the Center

for Micro/Nano Science and Technology at National Cheng

Kung University for assistance with the analytical equipment.

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