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Sasaki and Tani: Via Formation Process for Smooth Copper Wiring (1/6)

1. IntroductionRecently, the performance and the density of semicon-

ductor devices have been improved remarkably. For ensur-

ing the improved performance and density, a large number

of I/O terminals and a high wiring density are required for

the package substrate. The ITRS roadmap for micropro-

cessor packages estimates that the width of a copper line

will decrease to less than 10 μm in 2014.[1]

Since the adhesion strength between copper and epoxy

resin, which is used as an insulation layer, is very low, the

surface of the epoxy resin is roughened to approximately 5

μm by using an etching solution such as permanganate in

order to improve the adhesion strength. However, while

making a fine pattern that is narrower than 10 μm, the

rough surface may cause a short circuit. This is because

some copper residue between the wirings remains after

etching because of the diffusion of the reflection during

the exposure process, where the diffusion depth depends

on the roughness.

Recently, some technologies that achieved the chemical

adhesion strength between electroless copper and the

insulation layer, e.g., by introducing hydroxyl groups and a

coupling agent on the insulation layer,[2] by introducing

other special coupling agents for the epoxy resin,[3–4] or

by modifying the surface of the insulation layer by UV

irradiation,[5–6] have been reported.

On the other hand, a fine-pitch wiring technology has

been developed by introducing the chromium compound

adhesion promoter layer on the surface of the smooth

epoxy resin.[7] In this reference, a chromium compound

was introduced by transferring the adhesion layer from a

copper foil to the smooth epoxy resin.

However, when this technique was applied to the manu-

facturing of multi-layer printed circuit boards, the conven-

tional via formation process was found to damage the

adhesion layer and roughen the epoxy resin. In order to

make vias in the insulation layer for the build-up layer, the

printed circuit board is exposed to the CO2 laser. After the

via formation, some residue called smear is removed by

permanganate. However, the permanganate solution

removes not only the smear but also the adhesion layer,

leading to the roughness of the epoxy.

In this study, a via formation process that does not dam-

age the adhesion layer is investigated. First, the adhesion

layer is transferred onto the insulation layer, followed by

the DFR lamination to protect the adhesion layer from the

[Technical Paper]

Via Formation Process for Smooth Copper Wiring on Insulation

Layer with Adhesion LayerShinya Sasaki and Motoaki Tani

Device & Materials Laboratories, Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan

(Received July 29, 2013; accepted October 9, 2013)

Abstract

We have developed a fine-pitch wiring technology for a package substrate by introducing a chromium compound adhe-

sion promotion layer on the surface of smooth epoxy resin. In order to apply this technology to the manufacturing of

multi-layer circuit boards, new via formation process is investigated. The outline of this via formation process is as fol-

lows: First, a copper foil with an adhesion layer is laminated on the epoxy resin. Then, the copper foil is removed using

a liquid etchant to leave the adhesion layer. Next, the adhesion layer is covered with a dry film resist (DFR) layer and

then, CO2 laser is exposed over the DFR layer to form via holes. The smear is removed using a plasma treatment. Finally,

the DFR layer is removed. In this study, the effect of the plasma condition on the smear removal rate and that of the type

of DFR remover on the adhesion layer are investigated. As a result, we find that the smear is effectively removed by the

plasma treatment with an O2/CF4 mixture gas and that the amine-type DFR remover does not damage the adhesion layer.

This will be a new via formation process with a high adhesion strength.

Keywords: Smooth Copper, Adhesion Layer, Via, Plasma Desmear, Amine-type Remover

Copyright © The Japan Institute of Electronics Packaging

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Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013

following process. Then, the effect of the plasma condition

on the smear removal rate and that of the type of DFR

remover on the adhesion layer are investigated.

2. Experimental Procedure2.1 Via formation process

The outline of the via formation process is shown in Fig.

1. First, a copper foil whose one side was coated with the

chromium compound adhesion layer was laminated on the

epoxy resin. The chromium compound was coated by

chromate treatment. The copper foil was very smooth as

its roughness was 1.2 μm, as shown in Fig. 2.

Then, the copper foil was removed by the copper chlo-

ride etchant in order to leave the adhesion layer on the

epoxy resin.[7] Next, the adhesion layer was covered with

a protective layer. Then, the CO2 laser was irradiated on

the protective layer to form via holes in the epoxy resin.

The smear, which was the residual resin and was formed

during the laser abrasion process, was removed by using a

plasma treatment on the remains of the protective layer.

Finally, the protective layer was removed from the adhe-

sion layer. After these processes, 0.3 μm thick electroless

copper seed layer was deposited on the adhesion layer.

2.2 Insulating layerAn insulation layer composed of epoxy resin and 35-wt%

silica filler (diameter: 0.3–1.0 μm) was used. The thickness

of the insulation layer was 40 μm. After the lamination of

the epoxy resin and the copper foil, the insulation layer

was cured at 180°C for 1 h.

2.3 Selection of protection filmA 25 μm thick DFR layer was selected to protect from

the damage caused by the following processes because of

laser processibility, removability, and thickness control, as

shown in Table 1. The Cu foil could be used as the protec-

tive layer; however, the via diameter was larger than that

without the protective layer because of the laser power.[8]

A UV laser could be used for obtaining a small via, but the

laser equipment would have to be replaced in a manufac-

turing factory. In this paper, 40 μm thick insulation layer,

our target was to make a small via hole of 50 μm by using

CO2 laser.

2.4 Peel strength testA 35 μm thick copper layer was electroplated on the

electroless copper for the peel strength test. In the test

specimens, a number of 1 cm wide cuts were made

through the Cu layer. The peel strengths were measured

using a peel strength tester equipped with a digital force

gauge. The number of measurements was more than three

for each specimen and took an average. Considering the

total thermal impact of the four-layer double-side build-up

printed circuit board fabrication process, we cured the

specimens four more times at 180°C for 1 h and the reflow

process was carried out five times at 260°C for 1 min.

3. Results and Discussion3.1 Peel strength

Figure 3 shows the peel strength of the Cu layer on the

adhesion layer and that on the conventional desmear

Table 1 Selection of protective film.

PropertiesDry Film

Resist (DFR)Epoxy Resin

(B-stage)Cu foil

Laser processibility

Good Good Poor

Thickness control

Good Poor Poor

Removability Good Poor Good

Fig. 1 Process to transfer the adhesion layer including the via formation process.

Fig. 2 SEM micrograph of Cu foil (Rz~1.2 μm).

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Sasaki and Tani: Via Formation Process for Smooth Copper Wiring (3/6)

rough surface. High peel strength of 1 kN/m was obtained

for the Cu layer on the adhesion layer. It was higher than

that of the conventional desmear rough surface. The con-

ventional desmear process had 3-step, alkaline solvent for

pre-treatment, permanganese for etching and acid solution

for neutralize and remove the manganese residues. Even

after the reflow process, which was followed by the curing

process, the obtained peel strength was as high as 0.8

kN/m, which was still higher than that obtained in the

case of the desmear surface. The 10-point average rough-

ness of Rz for the transfer method was approximately 0.7

μm, which was very small as compared to the desmear

surface roughness of 3 μm. As shown in Fig. 3, very high

peel strength with a smooth epoxy surface is obtained by

transferring the adhesion layer. The surface of peeling

copper shows the residue of the resin, so the fracture

mode is destroying of the resin.

Figure 4 shows the SEM micrograph of a 10 μm line and

space on epoxy resin by using the transfer method. No

delamination was observed.

3.2 Adhesion mechanismIn order to clarify the mechanism of adhesion strength

improvement, x-ray photoelectron spectroscopy (XPS) was

carried out on the surface of epoxy resin after the transfer

of the adhesion layer and etched the copper foil by the cop-

per chloride as an etchant (Fig. 5). The peak at 574 and

583 eV, which indicate Cr 2p, were observed. Further, the

energy peaks from the carbonyl group and the metal-oxide

were observed using XPS narrow scan spectra as shown in

Fig. 6. The chromium compound was removed by using a

mixture of sulfuric acid and hydrogen peroxide as an

etchant, the peel strength was dropped to 0.4 kN/m after

the reflow.[7] Therefore, we believe that the chromate

compound such as chromium oxide or hydroxide bridges

the electroless copper and the epoxy resin to improve the

peel strength.

It is also believed that the chromium compound does

not remain between the wiring because there is no chro-

mium compound after etching by using a mixture of sulfu-

ric acid and hydrogen peroxide.[7] Figure 7 shows the

adhesion mechanism between the epoxy resin and the

electroless copper. Chromium compounds make strong

chemical bonds with Pd and resin surfaces, leading to the

strong peel strength between Cu and the epoxy resin.

Fig. 6 XPS spectra of C1s and O1s of epoxy surface after copper etching.

Fig. 3 Peel strength of Cu on transferred adhesion layer.

Fig. 4 SEM micrograph of epoxy resin surface.

Fig. 5 XPS spectrum of epoxy surface after copper etching by copper chloride.

(a) XPS spectrum of C1s

(b) XPS spectrum of O1s

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Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013

3.3 Via formation and via structureAfter the etching of the copper foil, DFR was laminated

on the adhesion layer. DFR consists of the acrylic resin.

Then, CO2 laser was irradiated over the DFR layer through

the epoxy resin; the structure of the sample is shown in

Fig. 8. The laser power was 1.63 mJ, 5 shot for 40 μm thick

epoxy resin and 25 μm thick DFR.

Figure 9 shows the SEM micrographs of the via hole.

The top view reveals that the shape of the via with DFR

shown in Fig. 9(a) is similar to that of the conventional one

as shown in Fig. 9(c). The via diameter with DFR is

smaller than the diameter of the conventional via because

of the total thickness. On the other hand, the cross-sec-

tional view as shown in Fig. 9(b) reveals that the tapered

shape of the newly formed via is similar to that of a conven-

tional one.

3.4 Plasma desmear conditionAfter the laser shot, the residue of the mixture of the

resin and some filler, the so-called smear, remains on the

bottom of the via hole, as shown in Fig. 10(a). Figure 10(b)

shows the bottom of the via hole after a plasma treatment

using an O2/CF4 mixture gas. It is seen that the smear is

removed by 10 min of the plasma treatment when the

plasma power is 6 kW at 250 mTorr. However, some resi-

due is remained after a plasma treatment using O2 gas only

or after 5 min of the earlier plasma treatment, as shown in

Figs. 10(c) and (d), respectively. The surface of the DFR is

also etched by using the plasma of the O2/CF4 mixture

gas. Under this condition, the etching rate of the epoxy

resin and the DFR is 0.4 μm/min and 1.5 μm/min, respec-

tively, as shown in Fig. 11. The etching rate of epoxy resin

with filler is almost 0, so the CF4 gas accelerate the etching

rate to remove the epoxy resin with filler.

15 μm of 25 μm thick DFR is etched during the smear

removal, and 10 μm of the DFR remains; therefore, there

is no damage on the surface of the adhesion layer on the

epoxy resin.

3.5 Removal of DFRThe removability of some removers is listed in Table 2.

After the plasma treatment to remove the smear, it was

found that DFR was difficult to remove by conventional

sodium hydroxide (NaOH); it took 10 times longer time

than in the case in which the plasma treatment was not

carried out because the temperature of the DFR surface

Fig. 7 Adhesion mechanism for epoxy resin and electroless copper by transfer method.

Fig. 8 Sample structure of via formation process.

Fig. 9 SEM micrographs of via hole.

Fig. 11 Plasma etching rate by O2/CF4 gas mixture.

Fig. 10 SEM images of bottom of via hole.

(a) After laser shot (b) Plasma desmear (O2/CF4 = 95/5, 10 min)

(c) Plasma desmear (O2/CF4 = 100/0, 10 min)

(d) Plasma desmear (O2/CF4 = 95/5, 5 min)

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Sasaki and Tani: Via Formation Process for Smooth Copper Wiring (5/6)

reached a value higher than 90°C during the plasma treat-

ment, which led to the reaction the DFR with epoxy resin.

So the DFR was hard to remove by conventional sodium

hydroxide and dipping long time in sodium hydroxide led

to damage to epoxy surface. On the other hand, an amine-

type remover can remove DFR in 90 s because it can easily

permeate into DFR.

3.6 Adhesion strength after removal of DFRFigure 12 shows the peel strength of the Cu layer after

the DFR removal when two types of removers are used.

When the amine-type remover is used as the DFR remover,

a high peel strength of 0.9 kN/m is obtained as prepared

and 0.7 kN/m is obtained after reflow, which is almost the

same as that for a conventional rough surface. On the

other hand, when sodium hydroxide is used as the DFR

remover, the peel strength of 0.7 kN/m is measured as

prepared; the peel strength decreases to 0.4 kN/m after

the reflow. 0.8 μm of the surface roughness falled to 0.6

μm after sodium hydroxide treatment, there may be some

damage to the surface of the epoxy resin due to sodium

hydroxide. On the other hand, the roughness was 0.75 μm

after amine-type remover treatment, it was almost same as

without via formation process. Therefore, we can conclude

that the amine-type remover is good for removability and

peel strength. The fracture mode was the destroying of the

resin for without via formation process, the interfacial

peeling for with via formation process.

3.7 Adhesion layer after removal of DFRFigure 13 shows the XPS spectrum of the surface of the

epoxy resin after the DFR removal by an amine-type

remover. The energy peaks from the chromium compound

adhesion layer are still observed, which indicates the exis-

tence of the adhesion layer even after the via formation

process. The remaining adhesion layer maintains the high

peel strength.

4. ConclusionA new via formation process that helped achieves a high

peel strength and a smooth surface was developed, and

the mechanism of the high peel strength was clarified. The

transfer of the adhesion layer led to a high peel strength of

1 kN/m with a smooth epoxy surface, which was as high

as that of the rough surface obtained by the conventional

desmear process. In order to protect the adhesion layer,

the DFR was laminated on the adhesion layer, and then,

CO2 laser was exposed over the DFR. After the laser shot,

the smear was found to be removed effectively by the

plasma treatment with an O2/CF4 mixture gas. Finally,

DFR was removed by an amine-type remover, while some

of the adhesion layer remained on the epoxy resin, leading

to a high peel strength of 0.9 kN/m.

References[1] “International Technology Roadmap for Semiconduc-

tor, 2012 update-2011 edition Assembly and Packag-

ing,” ITRS, 2012.

[2] M. Tani, K. Nakagawa, and M. Mizukoshi, “Multilayer

Wiring Technology with Grinding Planarization of

Dielectric Layer and Via Posts,” Transactions of the

Japan Institute of Electronics Packaging, Vol. 3, No. 1,

pp. 1–6, 2010.

Table 2 Removability of DFR by some removers.

Plasma Desmear

DFR remover

Condition Removability

Nothing NaOH2 wt%, 2 min

Good

10m in (Smear Removal)

NaOH2 wt%, 2 min

Poor

NaOH2wl%,

30 minGood

Amine type

10 vol%, 90 sec

GoodFig. 12 Effects of different types of DFR removers on the peel strength.

Fig. 13 XPS spectrum after removal of DFR.

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[3] S. Siau, A. Vervast, E. Schacht, S. Degrande, K. C.

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[5] K. Matsui, K. Ishikawa, K. Inoue, M. Watanabe, I.

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[6] K. Baba, Y. Nishimura, M. Watanabe, and H. Honma,

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[7] S. Sasaki and M. Tani, “Formation Technology of

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[8] T. Hirogaki, E. Aoyama, K. Ogawa, S. Matsutani, and

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Shinya Sasaki received his B.S. and Ph.D. degrees in Organic Chemistry from Tohoku University, Sendai, Japan, in 1994 and 1999, respectively. He joined Fujitsu Laboratories Ltd. in 1999. Since then, he has been engaged in the research and development of advanced materials for microelectronics.

Motoaki Tani received his B.S. and M.S. degrees in Engineering Science from Osaka University, Osaka, Japan, in 1985 and 1987, respectively. He joined Fujitsu Laboratories Ltd. in 1987. Since then, he has been engaged in the research and development of advanced materials for microelectronics. He

received his Ph. D. degree from Osaka University, Osaka, Japan, in 2013. He is currently the Director of Electronics Packaging Laboratory.