Copper Thin Film on Polyimide Substrates by Vacuum Web Sputtering System for Chip on Film

Jung Cho, Byung-Jae Kim, and Young-Seop Kim

Toraysaehan Inc., Kongduk-Dong, Mapo-Gu, Seoul 121-721, Korea

Jong-Yong Park and Won-Kook Choi

Thin film Technology Research Center, Korea Institute of Science and Technology,

Cheongryang P. O. Box 131, Seoul 130-650, Korea

ABSTRACT

Copper foil without adhesive on polyimide films have been widely adopted in information

technology such as production of flexible printed circuits, drive IC substrates for liquid crystal

display and optical pickup for hard disk suspensions. Especially, substrates market for LCD and

OLED are rapidly increasing with 35 ~ 40% every year and will be 2.5 billions $ in 2007. In

order to make thinner layer pattern and higher thermal stabilized substrates, copper foil should be

attached between copper and polyimide substrates without adhesives. For these applications, two

kinds of flexible copper clad laminates (F-CCL) have been developed. One is casting type two

layer F-CCL which polyimide varnish is coated on copper foil and cured on copper foil surfaces.

The other is oppositely making of copper foil on polyimide films by using both vacuum

sputtering and electroplating. In this paper, we deposit a thin tie layer (6.5~15 nm) and Cu seed

layer (100~200 nm) on polyimide film (Kapton-EN and Upilex-S) after very low energy ion

beam irradiation with various gases for the improvement of adhesion and thermal stability. After

electroplating 9㎛ thick Cu foils on polyimide, the thickness uniformity, adhesion strength and a

thermal stability of the FCCL are investigated by a x-ray thickness measurement, a 90o peel

strength tester and thermal curing treatment. As the results, the peel strength of the as-received

FCCL is higher than 0.8 kgf/cm and kept 0.65 kgf/cm even after thermal treatment 7 days at 150 oC and a 9 ㎛ thick FCCL shows a very good thickness uniformity with only ± 2.8% standard

deviation over 520 mm in width.

INTRODUCTION

Cu metallized polyimide (PI) has been widely and typically utilized in electronic packaging

applications such as in flexible printed circuits (FPC) and Tape Automated Bonding (TAB). As

electronic devices in information technology (IT) are highly demanded to be thin, light, free in

shape and multi functioned, the utilization of FPC in mobile phone, digital camera, digital

multimedia broadcasting, computers and their peripherals. In particular, the LCD display market

will be extended 6, 7 generation soon and the number of output channel of IC driver should be

U

W

IBS

C1

C2

C3

R

W

MD

increased. Inner lead bonding (ILB) pitch in TCP has reached pitch limit 45 mm and thus at the

level of 480 number of output channel pitch size should be as small as 30 mm and thus COF (chip

on flex) technology will be adequate. For the achieving the fine pitch, an adhesiveless flexible

printed circuit will be a necessary precondition and a sputter and electrodeposition method is

superior to other casting and laminating process because of small grain size and easy tailoring the

Cu thickness. A lot of techniques using plasma and ion beam have been reported to enhance the

adhesion between Cu and polyimide.[1-6] In this article a low energy ion beam with high fluence

was adopted as a new surface modification technique to improve the adhesion of PI to metal and

a new tie layer was tried to enhance the thermal stability. Finally, the peel strength before and

after thermal treatment will be compared and discussed.

EXPERIMENT

Vacuum web coating system

In this study of copper thin film sputtering on polyimide film, schematically shown in Figure 1,

each function is installed in a separate vacuum vessel containing the winding module and process

modules. Each process module can be equipped with a maximum of three sputter deposition

sources in separately pumped process compartments

In case of surface pre-treatment, PI films were modified by very low energy ion beam with 160-

180 eV generated by a stationary plasma thruster for improvement of adhesion to Cu by the

formation of hydrophilic groups. This source is an closed electron-Hall drift ion source and

showed the current up to 1 A. O2, N2O, and the mixture of Ar with those were used for plasma

source gases. As a tie-coated material Ni-Cr-Zn compound were used. The thickness was varied

from 5 to 20 nm to check the dependence of the peel strength on thickness. Also Ni-Cr was also

deposited in parallel to be compared. Cu seed layer with 150-200 nm thick was deposited on tie-

coated PI surfaces by pulsed DC sputtering and then Cu film with 9-mm thick was electroplated

over it.

Figure 1. Schematic cross section of a web sputtering system with 3 deposition sources.

Characterization

Wetting angle measurement was carried out for calculating the change of surface energy and the

surface chemical composition before/after the surface modification was investigated by x-ray

photoelectron spectroscopy. Wetting angle was automatically measured by sessil drop method

using deionized water and ethylenegrycol. The adhesion strength was checked by a 90o peel test

before/after a chemical and thermal treatment. As the checking solution of chemical endurance,

Cu electroplated PI was soaked in NaOH (10%) and HCl (10%) solution. Moreover, the

thermal resistance was tested according to the method of IPC-FC-240C, in which the sample was

annealed at 150oC for 168 hr. The flexural endurance was also examined by the method (IPC-

TM-650) using the patterned sample with 101.6 mm x 17.3 mm.

RESULTS AND DISCUSSION

Surface modification

Figure 2 shows the energy distribution of Ar ion beam generated by a stationary plasma thruster

which was measured by energy analyzer at the discharge voltage, Vd=230 V. The average energy

(E) is approximately 178 eV corresponding to 70% of Vd. This is in accordance with the result

assuming E is equal to 2/3~3/4Vd.[7,8]

Figure 2. Energy Distribution of Ar+ ion beam generated by SPT .

Figure 3 represents the change of wetting angles when PI surface is treated with different ion

species. When Ar+ ion only is irradiated on PI, the wetting angle was just reduced from 78o to 48o

with the increase of ion fluence. In case of N2+ ion irradiation, the wetting angle becomes smaller,

but not lower than 15o even at high ion fluence of 5x1017/cm2. On the other hand, the wetting

angle greatly decreases down to 8o at the O2+ ion fluence of 5x1015/cm2 and could be reduced

below 4o at the fluence higher than 1x1017/cm2. So reactive oxygen ion irradiation was believed

to be very effective in increasing the surface energy of PI by the formation of hydrophilic group

such as carbonyl group which agrees well with the previous reports. [9,10] In a similar way

N2O+ ion irradiation shows the same trend with that of O2

+ ion and can further reduce at lower

fluence of 5x1016/cm2 than O2+. This can be attributed to the combined effects of both reactive

ion irradiation and transfer of larger mass which plays a major role in creating the unstable

chemical bond by scission. This is supported by the rapid reduction of wetting angle with the

irradiation of Ar+ and O2+ mixture at very low fluence of 5x1015/cm2 (not shown here).

Table 1 illustrates the relative concentration of PI components obtained from x-ray photoelectron

core-level spectra for Kapton Tab-100E. The ratio of C:N:O in pristine PI is 78.7:7.2:14.1.

When Ar+ and N2+ ion are irradiated, C ratio increases and reversely O ratio decreases. O2

+ and

N2O+ ion irradiation largely increase the O ratio up to 22% and 29% at 5x1015/cm2 and

5x1016/cm2 fluence respectively. This result indicates that O–contained gas is very effective in

the accumulation of oxygen in PI surface which is basically necessary for the formation of

hydrophilic functional group. In particular, it takes only 1 sec to treat the PI with O2+ at the

fluence of 5x1015/cm2 which is very promising in view of production processing.

Figure 3. Variations of wetting angle at various ion irradiation and fluences

Table 2 shows the change of surface morphology of the untreated and treated PI at 5x1015/cm2

and 1x1017/cm2 with various ion species. The bare PI shows the root-mean-square (srms) of the

surface roughness of 2.9 nm. Most of ion species make slightly the PI surface rough as much as

3.0-3.5 nm at the ion fluence of 5x1015/cm2 and somewhat rough as much as 4.7-5.7 nm at

1x1017/cm2. Such a small change in surface morphology doesn’t influence the change of

dispersion force term related to surface roughness in surface energy. Because the surface energy,

the sum of the polar force and the dispersion force, increases from 42.1 erg/cm2 to 65.2 erg/cm2

by Ar+ ion irradiation and 81 erg/cm2 by O2+ or N2O+ ion irradiation, it is revealed that the polar

force related to chemical bonding dominantly contributes the increment of surface energy.

Tie layer Characteristics

Up to now Ni-Cr has been widely used as a tie layer materials for adhesiveless FPC, but the

peel strength fell down to 60% of the initial value after thermal annealing. In this article, Ni-Cr-

Zn (3%) instead of Ni-Cr is tested to improve the thermal property with the variations of the the

adhesion, tie- layers are sputtered on PI with/without surface modification.

Table 1. Relative C, N, and O atomic concentration in PI surfaces treated at various ion

beam irradiations

Sample Elemental ratio (%)

(/cm2) C N O

Bare PI 78.77 7.15 14.17

Ar

5x1015

5x1016

5x1017

83.50

90.28

85.01

3.94

2.99

3.64

13.10

6.73

11.35

O2

5x1015

5x1016

5x1017

73.19

76.01

85.40

4.76

4.05

4.27

22.05

19.94

10.33

N2

5x1015

5x1016

5x1017

84.73

89.03

88.76

8.32

5.82

6.31

6.92

5.15

4.92

N2O

5x1015

5x1016

5x1017

86.13

63.92

70.52

4.63

7.13

10.49

9.23

28.95

18.99

When tie layers are deposited on bare PI, Ni-Cr tie layer with 8 nm thick shows 0.25 kgf/cm and

0.35 kgf/cm after thermal annealing.In order to check how the low energy ion irradiation effect

on the improvement of However, Ni-Cr-Zn (8 nm) shows much better mechanical property of a

high initial peel strength of 0.65 kgf/cm and 0.57 kgf/cm after thermal annealing.

Table 2. Changes of surface roughness of various ion irraidiated PI surfaces.

5x1015/cm2 1x1017/cm2 Species

srms (nm)

Bare PI 2.9

Ar+ 3.0 4.7

O2+ 3.5 5.2

N2+ 3.4 4.5

N2O+ 3.5 5.7

Figure 4 shows the changes of the peel strength of flexible copper clad laminate (FCCL) with Ni-

Cr-Zn tie-coated layer on treated PI before/after thermal annealing at 150oC for 168 hr . In both

cases, the initial peel strength shows quite high value 0f 0.75-0.95 kgf/cm for all the tie- layer

thickness. In case of O2+ ion irradiation, after thermal annealing the peel strength quickly

decreases to 0.45 kgf/cm below 5 nm, but that still keeps high value as much as 0.65 kgf/cm at

the tie-layer thickness thicker than 6.5 nm. When N2O+ ion is irradiated, the initial value is a little

higher than those of O2+ ion irradiation, but any samples do not show high values after the

thermal annealing irrespective of the tie- layer thickness. This may result from the occurrence of

larger surface damages induced by ion irradiation at higher fluence than that of O2+ ion.

Figure 4. Changes of the peel strength of Ni-Cr-Zn coated FCCL.

Figure 5 shows the film uniformity of Cu sputtering on polyimide films by several sputter

cathode types. The sputtering uniformity of conventional planar type, high erosion type (patent

pending) and cylindrical type were 200nm±3.2%, 200nm±2.5% and 200nm±1.8%, respectively.

In comparison with conventional type sputter cathode and high erosion rate type cathode,

cylindrical type cathode realized more flat coating surface uniformity than those with a target

width of 700mm. Also it realized a target utilization above 50% at cylindrical cathode at a web

speed of 1 m/min each cathodes[11,12].

Figure 5. Thickness uniformity profile of FCCL of 8, 10, and 12 mm thickness.

CONCLUSION

Low energy reactive ion beam irradiation with high fluence is proved quite effective in increasing

the surface energy up to 81 erg/cm of PI and thus improving the peel strength. Moreover this is

very productive and promising in web sputter system because it takes only 1 sec to treat PI. The

peel strength of Ni-Cr-Zn coated Ni-Cr-Zn tie layer shows a high value of 0.65 kgf/cm after

thermal annealing and this is considered as much better thermal endurance than conventional Ni-

Cr tie layer material. The copper sputtering thickness uniformity shows 200nm±1.8% at the

cylindrical cathode type.

References

1. A.J. Pertsin and Y.M. Paschuni,, “An XPS study of the in-situ formation of the

polyimide/copper interface,” Appl. Surf. Sci. 47, 115 (1991).

2. F.G. Egitto, L.J. Matienzo, K.L. Blackwell, and A.R. Knoll, “ Oxygen plasma modification of

polyimide webs:effect of ion bombardmwnt on metal adhesion,” J. Adhesion Sci. Technol., 8(4),

411 (1994).

-30 -20 -10 0 10 20 300.10

0.15

0.20

0.25

0.30

Cu S

putt

erin

g Th

ickn

ess (

mm)

Distance (cm)

Conventional Planar Type High Erosion Planar Type Cylindrical Type

3. R. Flitsch and D-Y. Shih, “A study of modified polyimide surfaces as related adhesion,” J. Vac.

Sci. Technol., A8(3), 2376 (1990).

4. W.J. Lee, Y.S. Lee, S.K. Rha, Y.J. Lee, K.Y. Lim, Y.D. Chung, and C.N. Whang, “Adesion

and interface chemical reactions of Cu/polyimide and Cu/TiN by XPS,” Appl. Surf. Sci., 205,

128 (2003).

5. S. Iwamori, T. Miyashita, S. Fukuda, S. Nozaki, K. Sudoh, and N. Fukuda, “Effect of an

interfacial layer on adhesion strength deterioration between a copper thin film and polyimide

substrates, ” Vacuum, 51, 615 (1998).

6. S.Y. lee, N. Mettlach, N. Nguyen, Y.M. Sun, and J.M. White, “Copper oxide reduction through

vacuum annealing, ” Appl. Surf. Sci., 206, 102 (2003).

7. A.I. Bugrova and V.P. Kim, “Plasma accelerators and ion injectors”, p.107, Nauka

Publishers, Moscow, 1984.

8. A.I. Morozov, A.I. Bougrova, V.T. Niskine, A.V. Dessijatskov, and D. Valentian, U.S. Pat.

5581155, “ Plasma accelerator with closed electron drift”.

9. W.K. Choi, Seok-Keun Koh, and Hyung-Jin Jung, “Surface Chemical Reaction

between Polycarbonate (PC) and keV Energy Ar+ Ion in Oxygen Environment,” J.Vac.

Sci.Technol., A14(4), 2366 (1996).

10. S.K. Koh, Y.B. Son, G.S. Gam, K.S. Han, W.K. Choi, and H.-J. Jung, “Formation of new

surface layers on ceramics by ion assisted reaction,” J. Mater. Res.,

11. J. Fehn Jr., W. Kittler Jr., D. Lievens, R. Ludwig, G. Philips, and A. Taylor, “Roll-to-Roll

Sputter Web Coater – A Status Report,” 41st Annual Technical Conference Proceedings of the

Society of Vacuum Coaters, 463, 1998

12. R. Kukla, H. G. Lotz, R. Ludwig, and P. Sauer, “ New Modular Roll- to-Roll PVD Web

Coater for Clean Room Production”, 41st Annual Technical Conference Proceedings of the

Society of Vacuum Coaters, 463, 1998

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