Ink-Jet Printing, Self-Assembled Polyelectrolytes, and Electroless Plating: Low Cost Fabrication of Circuits on a Flexible Substrate at Room Temperature

Ink-Jet Printing, Self-Assembled Polyelectrolytes, and

Electroless Plating: Low Cost Fabrication of Circuits

on a Flexible Substrate at Room Temperature

Kevin Cheng,*1 Ming-Huan Yang,1 Wanda W. W. Chiu,1 Chieh-Yi Huang,1 Jane Chang,*1 Tai-Fa Ying,1 Yang Yang2

1Opto-Electronics & Systems Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, RO ChinaFax: 886-3-5917446; E-mail: [email protected]; [email protected]

2Department of Materials Science and Engineering, University of California-Los Angeles, Los Angeles, California, USA

Received: October 10, 2004; Revised: December 5, 2004; Accepted: December 6, 2004; DOI: 10.1002/marc.200400462

Keywords: adhesion; electroless plating; ink-jet printing; polyelectrolytes; via hole

Introduction

In recently years, the ability of plastics to function as semi-

conductors, diodes, and transistors in plastic integrated cir-

cuits has aroused attention in industry and academia. Of

particular interest have been their basic mechanical proper-

ties, such as strength, flexibility, and lightweightedness. In

particular, flexible electronics are attracting significant

attention because of the potential advantage of being able to

form three-dimensional (3D) circuits by using a multilayer

technique. Unfortunately, the method of forming metal

wires on such flexible electronics has not yet been well

explored, since the traditional lithographic process is not

compatible with organic compounds. Traditional printing

wire board (PWB) processes require the use of selective

masking and etching technology to create regions of metal-

lization on non-conducting substrates.[1] This kind of fab-

rication process has served the industry well and has

provided the desired image resolution with an acceptable

cost. However, all of these ‘‘analog’’ processes require the

manipulation of digital data for the initial production of a

screen or photo tool, and, therefore, are not cost-effective

for small patch production. Generally, the fabrication of

new masks requires lengthy substrate processing steps that

can lead to 1–2 d down time. It further increases the pro-

duction cost and time delay for the preparation of boards,

particularly in prototype and small patch of board

production.

Summary: The driving forces behind the development offlexible electronics are their flexibility, lightweightedness,and potential for low-cost manufacturing. However, becauseof physical limitations, traditional thermal processes causedeformations in the flexible substrate. As a result, the ad-hesion quality of the printedwires is deteriorated. This articlereviews recent developments in printing circuits on a flexiblesubstrate by combining self-assembled polyelectrolytes, ink-jet printing of a catalyst, and electroless plating of metals.The limitations and potential applications of this technologyare also discussed. Experiments implementing this technol-ogy demonstrated significant results. By a vibration-inducedassistance during an ink-jet printing catalyst process, linewidth and blurring can be controlled towithin�3%variation.Following the IPC 6013 standard for flexible electronics, theresults after thermal cycling (288 8C, 6 times) and a hot oiltest (260 8C, 3 times) indicated that the metallic circuit hadretained excellent adhesion properties and electric character-istics. We also report the first successful demonstration of ametal film in a via-hole inner wall on a flexible substrate. This

novel fabrication method is ideal for the realization of largearea, flexible electronics and future multilayer flexible sub-strate application, such as flexible display, chip on flexiblesubstrate, etc., particularly where traditional lithographicprocesses can not be applied.

Flexible high-density circuit on an FR-4 substrate (left) andpicture of via hole with copper inner wall (right).

Macromol. Rapid Commun. 2005, 26, 247–264 DOI: 10.1002/marc.200400462 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Feature Article 247

Kevin Cheng received his B.S. degree in Mechanical Engineering from the University of Tamkang inTaiwan in 1992, and a Ph.D in Aeronautics & Astronautics from National Cheng Kung University in1998. Subsequently after his PhD degree, he worked as an engineer of Center of Aeronautics &Astronautics of Industrial Technology Research Institute (ITRI) in Taiwan, where he was responsible forthe system design & integration of gas turbine. In 1999, he transferred to the Printing TechnologyDivision, Opto-Electronics and Systems Laboratories (OES), ITRI. In 2003 he received the First Awardpaper for National Instrument Days, and Second Award for 2004. Cheng has published more than 29conference or journal papers, and has filed or been granted 13 individual patent in USA, Taiwan, andChina. In 2004, his group was invited to give a presentation at NIP 20, the International Conference onDigital Printing Technologies. The same year, Cheng got the certificate of Discipline of Innovation fromStanford Research Institute. Cheng is a system integration section manager in the Printing TechnologyDivision since 2000. He has focused on the field of the system integration and industrial ink-jet printingprocesses development, such as printed circuit board (PCB), color filter, polymer light emitting device(PLED), micro-lenses, organic TFT by ink-jet printing, etc. In 2004, Cheng’s group delivered the firstmulti-task ink-jet platform in the world to PLED manufacturer, and in the end of this year, a substratesize of 550 mm� 650 mm standard platform with multiple ink-jet heads for TFTs would be funded. Hisresearch interest covers opto-mechanical system development, ink-jet printing control, and new processfor organic material development. E-mail: [email protected]

Ming-Huan Yang received his Master degree in Chemistry from National Cheng Kung University in2002. He is now a system integration engineer in the Printing Technology Division, Opto-Electronicsand Systems Laboratories of Industrial Technology Research Institute in Taiwan. His work has primarilyfocused on the industrial ink-jet printing processes development, especially in PCB fabrication by ink-jet printing. E-mail: [email protected]

Wanda W. W. Chiu received her Master degree in Mechanical Engineering from National ChengKung University in 1996. She is now a system integration engineer in the Printing Technology Division,Opto-Electronics and Systems Laboratories of Industrial Technology Research Institute in Taiwan.Her work has primarily focused on the industrial ink-jet printing processes development, especially incolor filter, PWB, MEMS device fabrication by ink-jet printing. E-mail: [email protected]

Chieh-Yi Huang received the B.S. degree in electrical engineering from National Taiwan University ofScience and Technology, Taipei, Taiwan in 1996. He is currently working at Opto-Electronics & SystemsLaboratories of the Industrial Technology Research Institute, Hsinchu, Taiwan, as a section manager,and studying M.S. degree in electrical and control engineering from Nation Chiao Tung University,HsinChu, Taiwan. His current research areas include image coding and printing system design.E-mail: [email protected]

Jane Chang is Manager of Printing Systems in Printing Technology Division, Opto-Electronics andSystems Laboratories of Industrial Technology Research Institute in Taiwan. She received her B.S.degree in Mathematics/Computer Science from the University of California, Los Angeles, in 1986 and aMaster degree in Business Administration from the Pepperdine University in 1992. Prior to joining ITRI,she has worked in Xerox, USA until 1995 as a software engineer and development manager. Her workhas focused on areas such as print control software, operating system, printer system integration andindustrial ink-jet printing processes development. E-mail: [email protected]

248 K. Cheng et al.

Macromol. Rapid Commun. 2005, 26, 247–264 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Interest has recently grown in the rapid prototyping

capabilities of direct write technology (DWT), which pro-

pel the development of a novel class of deposition technique

for the fabrication of micro-electronic components and

devices. DWTeliminates the need of expensive lithography

and high-vacuum processing, including plasma and che-

mical vapor deposition (PVD and CVD), plasma etching,

etc. The simplification of circuit pattern design to result in

almost instantaneous manufacture of a circuit board is

highly attractive. An advantage of using DWT in circuit

interconnect manufacturing is that the process is additive,

like the ink-jetmethod.Material is only deposited in desired

locations, thereby eliminating the amount of chemical and

material waste generated in the circuit-board-formation

process. In addition, the ability of using ink-jet technology

to produce an entire circuit pattern designed by computer

graphics software makes it a powerful prototyping tech-

nology. There has been growing interest towards ink-jet

technology used in the fabrication of low-cost polymer

electronics, polymer light-emitting diode displays etc.,

where the polymer structure and molar mass, the solvents,

and concentration dominate the ink-jet printability of the

polymer.[2] Studies conducted by Bharathan and Yang[3]

and Hebner et al.[4] have successfully demonstrated the

rapid, low-cost processing capabilities of ink-jet printing to

create electro-luminescent devices.

Ink-Jet Technology Background

Ink-jet technology has become increasingly important dur-

ing the past ten years, particularly because of its popularity

in low-cost desktop printers. Basaran depicted several

recently developed methods for producing drops by ink-jet

printing.[5] The trend in ink-jet printing technology is to

enhance printing quality and speed by: 1) reducing droplet

size; 2) increasing the number of channels per head; 3)

increasing ejection rates; and 4) reducing problems such as

crosstalk between channels and satellite droplets.[6,7] There

have beenmany attempts to experimentally characterize the

performance of ink-jet printing, e.g., using instruments such

as a phase doppler particle analyzer (PDPA) to determine

the effect of crosstalk between adjacent nozzles,[8] and

stroboscopic techniques to record and analyze images of the

droplets.[9] Some new specially designed ink-jet platforms

had been developed for industrial application, and have

been overviewed by de Gans and Schubert.[10]

Tai-Fa Ying is Director of Printing Systems in Printing Technology Division, Opto-Electronics andSystems Laboratories of Industrial Technology Research Institute in Taiwan. He received his Masterdegree at the Department of Earth Science and Institute of Geophysics from the National CentralUniversity in 1986. In 1999, he got the Outstanding Award of Young Engineer from the ChineseInstitute of Engineers. He has published 41 international papers, including 11 papers for IEEE, and 3papers for J. Appl. Phys. In 2000, he was appointed as general manager of TECO Electro Devices. Heis the chief secretary of Taiwan Association for Magnetic Technology, and also serves as an editor ofDVD communication. E-mail: [email protected].

Yang Yang, obtained his PhD degree in physics and applied physics at the University of MassachusettsLowell in 1992, under the supervision of Professor Jayant Kumar and later Professor Sukant K.Tripathy. Subsequently after his PhD degree, he joined the Chemistry Department of University ofCalifornia–Riverside as a postdoctoral researcher to work on photochemistry hole burning effect. Inlate 2002, he joined UNIAX Corporation in Santa Barbara as a device physicist to work on polymerLEDs and transistors. His invention of conducting polymer/ITO composite electrode enabled thelifetime and device efficiency of the PLEDs to reach a level of commercial applications. In January of1997, he joined the Department of Materials Science and Engineering of UCLA as an assistantprofessor. He became an associate professor and professor in 1998 and 2002 respectively. Yang’sresearch focuses on conjugated organics and polymer materials and devices, such as light-emittingdiodes, memory devices, transistors, and solar cells. His group has been regarded very creative groupsin the polymer/organic electronics with inventions covering from highly efficient polymer LEDs,organic/polymeric nonvolatile memory devices, high speed organic diodes, and transparent polymerdevices. Yang has published more than 90 refereed papers, given more than 50 invited presentationson his research work, and has filed or been granted 22 USA patents. Currently, Yang’s group hasseven postdocs/visiting scholars, and 15 students. He has received the Outstanding Overseas YoungChinese Scientist Award from the Natural Science Foundation of China (2004), the NSF Career Award(1998), and the 3M Young Investigator Funds (1998). Yang serves on boards of several companies andgovernment committees. He is a co-founder of ORFID Corporation, a start up company located in LosAngeles focusing on organic transistor for displays and organic RFID. Yang can be reached [email protected].

Ink-Jet Printing, Self-Assembled Polyelectrolytes, and Electroless Plating: Low Cost Fabrication of Circuits . . . 249


Hydraulic Crosstalk and Overfilling

When ink-jet chambers are arranged in a tight array to aim

for a high device spatial resolution, they need to share one

common liquid supply. As a result, the pressure generated

from the firing chamber can affect themenisci at the nozzles

of its neighboring chambers, posing ‘‘hydraulic crosstalk’’.

Hydraulic crosstalk makes the droplet volume difficult to

control and even causes unexpected droplet ejection when

combined with thermal crosstalk.[11] Overfilling is another

important and related problem. It occurs when liquid

quickly refills back to the chamber after droplet ejection and

bubble collapse. Several provisions have been reported to

address the problem of hydraulic crosstalk during droplet

ejection and overfill during liquid refilling, such as incre-

asing the chamber length or placing extra slots in as reser-

voirs,[12] adding a narrow passage,[13] and driving the

nozzle by an intercooling and preheating waveform.[14]

Satellite Droplets

Satellite-droplet formation is one of the most troublesome

issues. The typical droplet ejection sequence of bubble jets

shows that a long tail separates from the primary droplet

and breaks into small satellite droplets. It randomly occurs

alongside the device pattern during ink-jet printing, and

results in line edge blurring and deteriorates the device

performance. Generally, the drops expelled by applying a

lower voltage and longer pulse-width signal yield a faster

drop velocity, a longer separation length, and a heavier drop

weight. If the drop speed is fast enough, the misdirected

satellite drops do not significantly influence the printing

quality.[15] Based on this observation, Cheng et al.[14] dis-

closed a driving waveform control method that helped to

modulate the ink-discharging condition. The preheating

and inter-cooling-compounded driving waveform can im-

prove the printing quality to result in significant reduction in

satellite-drop formation and the improvement of dot

circularity, as shown in Figure 1. The droplet has good

uniformity and few satellite occurrences.

Line-Width Limitation

The line width strongly depends on the drop size of the ink,

and it is controlled by the orifice of nozzle and the jetting

energy. After the drop lands on the substrate, it will spread

by a factor which is generally about 1.5, depending on the

surface property of the substrate. For example, a 10 pL drop

volume will spread to form a dot 27 mm in diameter.

Recently, Murata[16] developed a super-fine ink-jet system,

where the key technology is different from conventional

technologies, such as piezo-drive or thermal-drive. How-

ever, the details of the ink-jet system are confidential since

the patent is not yet open.Murata demonstrated an example

of ultra-fine patterning of fluorescent dyes on a silicon

substrate. The dot pitchwas 3mmeach, as shown inFigure 2.

In addition, they also presented the results of a wiring

pattern of a silver nanopaste (Harima Chemical Inc.) on

glass, which achieved a line width and line space width of

10 mm each. It was reportedly possible to further reduce the

line width to about 3.6 mm with a line spacing of 1.4 mm.

Figure 1. Illustration of ink-jet satellite occurred as dischargingcatalyst on glass substrate.

Figure 2. (a) An example of a fine lattice pattern obtained usingsub-micrometer diameter dots. (dot pitch, 3 mm). (b) Fine lines(bright) and spaces (dark) of silver nanopasteTM (Harimachemical Inc.) (line width is about 10 mm with 20 mm pitches)(from ref.[16]).

250 K. Cheng et al.


Direct Ink-Jet Nanoparticle

Ink-jet printing offers the additional advantages of low

capitalization of equipment, very high materials efficiency

(less than 5% of material is wasted), elimination of photo-

lithography process, and non-contact processing.[17] Some

have reported ink-jet printing of metal organic decomposi-

tion inks with nanoparticle additions or organic metal

precursor. Near-bulk conductivity of printed and sprayed

metal films has been achieved for Ag and Ag nanocompo-

sites.[18,19] A major challenge in applying a direct ink-jet

nanoparticle process is the ink formulation. The inks must

contain the appropriate precursors and a carrier compound,

and may further contain various binders, dispersants, and

adhesion promoters, depending on the nature of the precur-

sor and the particular application. Ink composition is

critical because it defines the process in which the ink is

jetted, the adhesion to the substrate, the line resolution and

its profile, and the electronic properties of metal forma-

tion.[19] So far, this approach faces the challenge of poor

adhesion between the metal film and the substrate; and

typically it needs a high temperature (�300 8C) sinteringpost-process to form the metal thin film for a good electric

property. This high-temperature process limits its applica-

tion, particularly for flexible electronics.

Figure 3 is a typical example of using nanoparticle ink for

ink-jet printing to form ametal film. Szczech et al.[20] chose

a piezo print head to discharge nanoparticle fluid suspen-

sion (NPFS) ink, which had basic properties of 1–

2 mN � s �m�2 viscosity, 27–29 mN �m�1 surface tension,

and 30 wt.-% Ag with 5 wt.-% Cu. The NPFS needed to

undergo thermal treatment to remove the remaining carrier

solvent and to allow the nanoparticles to sinter, typically

at 300 8C for 15 min. The reported line width was about

120 mm. It was found that the thickness variations were

attributable to deviations in droplet size resulting from

slight changes in excitation parameter settings. According

to the NPFS characteristics, improved electrical continuity

can be obtained if the processing temperature is increased to

650 8C, but unfortunately the organic substrates utilized in

this work cannot endure such high temperatures.

In a different approach, to form a patternedmetal wire by

ink-jet technology, Konishi et al.[21] printed a kind of amino

silane coupler as the seed material. Upon activation by an

acid catalyst, connections were subsequently made be-

tween the seed material and Cu or Ni ions by electroless

plating. Molesa et al.[22] demonstrated ink-jet printed Au-

nanoparticle (10 wt.-% hexanethiol-encapsulated 1.5 nm

gold nanoparticleswere dissolved in toluene) conductors on

plastic with a sheet resistance as low as 0.03 ohms per

square. Huang et al.[23] further demonstrated that reducing

the alkane chain length could significantly lower the pro-

cessing temperature requirements necessary to convert the

solution-deposited nanoparticles into low resistance, con-

tinuous films. There is a trade-off between stability and

annealing temperature because of the instability of thiols

with short alkane chains. The optimization of the process

reveals that 1.5 nm gold nanoclusters encapsulated with

hexanethiol have good stability and low resistance lines

with conductivities as high as 70% of bulk gold. Curtis

et al.[24] reported on the ink-jet printing of metal organic

decomposition (MOD) inks with and without nanoparticle

addition. Near-bulk conductivity of printed metal films has

been achieved for Ag and Ag nanocomposites. They show

good adhesion and ohmic contacts with a measured contact

resistance of 400 mO � cm�2. It is a function of the process

temperature and solvent. Fuller et al.[25] formed 3D

circuitry and a high-Q resonant inductive coil by ink-jet

printing gold and silver colloidal nanoparticle inks consist-

ing of 5–7 nm particles dispersed 10% by weight in

a-terpineol. After printing, the discharged filmwas sintered

at 300 8C, and its conductivity slowly increased over time.

Hong et al.[26] have demonstrated an a-Si/H thin film

transistor (TFTs) with an ink-jet printed copper source/

drain metallization. The maximum temperature of the

metallization is 200 8C, and the ink contained the precursormolecule of the metal-organic compound, copper hexano-

ate, Cu2(OH2)2(O2CR)4, where R¼ (C2H4)CH3.

In brief, the direct ink-jet printing of nanoparticles faces

the obstacles of insufficient adhesion to the substrate, and a

high-temperature process (200–300 8C) to sinter the metal

particles or to transfer precursor to the metal particles. For

most popular flexible substrates, like polyimide (Kapton)

and PET (polyester), the allowed temperature is limited to

275 and 150 8C, respectively. Therefore, the high-temper-

ature sintering process makes direct ink-jet printing of

nanoparticles difficult to apply to most of flexible subs-

trates. In addition, a high-temperature process will deform

the substrate, preventing precision alignment of other

necessary processes to form high-density circuits. To over-

come these problems, an alternative method is proposed in

this paper. First, the flexible substrate needs to be specially

treated to enhance its adhesion, and then the ink-jet ink can

be free of binder content and achieve high printing quality.

Figure 3. Partial cross section of a pad created with the silverNPFS, and observed with a scanning electronic microscope. Thepad was processed at 300 8C for 15 min. The average height ofthe pad was approximately 3 mm. The porous structure sug-gests that the sintering process is not complete. To furtherprocess the pad, a higher temperature would be required (fromref.[20]).



In addition, the ink has changed to a water-based catalyst

ink, to avoid the sintering process needed for nanoparticle-

containing ink. Under these conditions, the metal circuit

can then be fabricated by the transportation of catalyst and

metal ion in an electroless plating solution at room tem-

perature. Below, we introduce the processes of the surface-

treatment method, the self-assembled polyelectrolytes, and

the combination of ink-jet printing and electroless plating.

Self-Assembled Polyelectrolytes and Ink-Jet Printingand the Electroless-Plating Process

It is well known that a layer-by-layer ultrathin film can

be fabricated from oppositely charged polyelectroly-

tes. Since Decher and his co-workers[27,28] first put forward

this method, referred to as self-assembly in most refer-

ences,[29–34] it has been widely applied in recent years in a

variety of fields, including biofield,[35,36] charged parti-

cles,[37–39] thin metal films,[40–42] etc. But generally, the

factors that influence the film’s formation are still not fully

understood. Usually, the driving force of the film growth

was thought to rely on charge overcompensation of the

newly adsorbed polyions, i.e., the complement of electro-

static attraction of the cation–anion pairs formed in

successive adsorption steps.[28] But the electrostatic attrac-

tion was considered, especially in the recent past, not a

prerequisite since themultilayer film can be fabricated from

same-charge-carrying polymer.[43]

To characterize this type of film, UV-vis spectro-

scopy,[36,43] atomic forcemicroscopy,[31] andX-ray diffrac-

tion or neutron scattering[44,45] have been used. These

methods can provide details of the structure, component,

and morphology of the films, but characteristics such as the

wettability of the top layer, which influences the next

adsorption, cannot be revealed. Chen[46] first developed

dynamic contact angle (DCA), ya, measurement for study-

ing the surface wettability, roughness, heterogeneity, defor-

mation, and mobility. They found the ya of the film

fluctuates periodically with the layer’s alternative adsorp-

tion, as shown in Figure 4. When the polycation poly-

(diallyldimethylammonium) chloride (PDDA) is adsorbed,

the ya increases; while when the polyanion poly(styrene

sulfonate) (PSS) is absorbed, the ya decreases. However, yais different with different polycations. The hydrophilic

property of the polycationswill influence the next surface of

the PSS layer; e.g., from Figure 4, it can be observed that

various polycation layers exhibit quite different surface

behaviors in air but are only slightly different after hy-

dration. This is strong evidence that the layer surface is

reorganized going from a water to an air surrounding, or

vice versa. The hydration can make the ionic groups of the

polyelectrolytes stretch into the aqueous phase, resulting in

the film being more hydrophilic, as shown in Scheme 1a.

Nevertheless, when the layer is exposed in air, the soft and

hydrophobic moiety of the polycation should divert to the

top surface to meet the minimal surface free energy. The

hydrophobic moiety varies with different polyelectrolytes.

For PDDA, its hydrophobic ring structure is stiff and

difficult to rotate, so its layer stays hydrophilic both inwater

and in air. When PSS is adsorbed it cannot remain entirely

on the top layer. It penetrates into the under layers as well.

Moreover, the under layers also can move downwards or

upwards partly through reorganization; thus, the surface

nature is in fact the reflection of two or several layers.When

the surface is exposed to air, a reorganization of the charges

takes place and makes the surface more hydrophobic, as

shown in Scheme 1d.

Recently studies about the layer structure of polyelec-

trolytes and their control process, especially concerning

surface roughness, focus on the swelling of the salt solution

effect. Dubas and Schlenoff[47] found a quasi-linear swel-

ling response depending on the pair of polyelectrolytes

(PSS/poly(diallyldimethylammonium) (PDADM)) consti-

tuting the multilayer. The surface roughness of the multi-

layerwas observed to decrease significantly upon annealing

in salt solution. Fery et al.[48] prepared the polyelectrolyte

solutions of poly(allylamine) hydrochloride (PAH)/poly-

(acrylic acid) (PAA) with and without NaCl instead of pure

water, and compared the difference between the use of pure

water and salt solution in the washing process. Their

conclusions were similar to those of Dubas and Schlenoff; a

remarkable increase in surface roughness was found for the

multilayer deposited from salt-containing solutions and

washed with pure water after depositing each layer. The

mechanism of salt-induced roughness by the absorbing or

washing solution is helpful to change the lateral structure of

the PAH/PAA multilayer after their preparation.

A combination of the processes of self-assembled mono-

layer (SAM) with polyelectrolyte multilayers (PEMs),

ink-jet printing, and finally, electroless plating to form a

large-area metallic circuit, is ideal for flexible electronics.

The PEMs will tangle with the flexible substrate, forming a

porous nanostructure, providing sites for following catalyst

Figure 4. The relationship of contact angle (ya; yr) vs. number oflayers (PDDA/PSS system). Number of layers: 0, bare mica; 1,PDDA; 2, PDDA/PSS; 3, PDDA/PSS/PDDA; 4, PDDA/PSS/PDDA/PSS; and so on (from ref.[46]).

252 K. Cheng et al.


to nucleate on the substrate. The ink-jet printing quality can

be improved when no binder is present in the ink. After ink-

jet printing the catalyst, the electroless plating step will

reduce the metal ions to the metal to form a metal circuit at

room temperature. No sintering process operation is

needed.

Monolayers can be deposited by self-assembly to build

multilayer coatings with alternating positive and negative

ionic layers absorbed on the substrate and form stable

coating layer. This is the so-called PEM process.[49] Weak

polyelectrolytes, such as poly(allylamine) hydrochloride

(PAH) and poly(acrylic acid) (PAA), are pH dependent with

their degree of ionization depending on the local electro-

static environment. Because of their ability to respond to

changes in the local environment, multilayers of weak poly-

electrolytes with many different architectures or properties

can be prepared by changing the pH of the assembly solu-

tion. Most of the researchers used a dipping process to form

polyelectrolyte layers on a substrate, for non-patterning and

the modification of substrate property needs.[50]

Alternatively, some researchers developed different ap-

proaches in the patterning step after polyelectrolyte

processing. Instead of ink-jet printing the catalyst, the

catalyst was patterned by micro-contact printing.[51–53] In

fact, some polyelectrolyte solutions have physical proper-

ties similar to that of purewater, like PAA and PAH, and are

suitable to dispense by the ink-jet method.[41] This

characteristic creates the possibility of modifying the local

surface on a substrate by image pattern definition and

avoiding the dip step to deform the substrate. In particular,

Shan et al.[42] has performed a pioneering work on the ink-

jet printing of catalyst and the formation of a metal circuit

by an electroless plating method. They formed metal cir-

cuits by direct printing of the Pt catalyst ink (0.5 mg �mL�1

concentration) onto the substrate, and then bathed the

substrate in electroless plating solution to form copper

circuits. After plating, the substrate was placed in a furnace

in flowing air, at 350 8C for 2 h, to sinter the copper linewith

100 mm width and 0.2–2 mm thickness. Guo et al.[41]

combined self-assembled PEMs, ink-jet printing, and

electroless metal plating technologies for the fabrication

of a Ni circuit. They found that PEMs significantly im-

proved the adhesion of Ni to the substrate, in addition to the

ink drop wettability behavior. The average thickness of the

Ni sheet was about 800 A. Similarly, Wang et al.[40]

concluded that the PAA/PAH PEM surface can be rendered

selectively or nonselectively toward catalyst binding by

choosing the appropriate pH conditions during PEM

fabrication with short activation times. They also demon-

strated an excellent selectivity of nickel plating for the

PAA-rich multilayer surface over the PAH-rich surface

when using Pd(NH3)4Cl2, and formed an excellent adhesion

metal film.

Experimental Part

Ink-Jet Platform

The ink-jet system consists of a specially designed thermal-bubble ink-jet head for patterning. The head has 300 nozzlesand the resolution is 600 dpi; each nozzle discharges ink dropsof a 35–85 picolitre (pL) volume. The printing system is basedon a three-axis X–Y–y table with a micro-step resolution of0.5 mm up to 4 inch � s�1 speed, a set of printing heads, and anarea charge coupled device (CCD) are fixed on the mechanicalsupport. In operation, the firing distance between the substrateand the print head is adjusted to 500 mm to achieve a betterprinting quality. A waveform driving procedure is adopted to

Scheme 1. The schematic structure of the surface layer ofpolycation and polycation/polyanion films. (a) A polycation filmwith a monolayer on mica after hydration with water, from which(film a) the contact angle was determined to be ya; (b) Film a afterexposure in air, from which (film b) the contact angle wasdetermined to be yr; (c) Film b after adsorption of a layer ofpolyanoin and hydration with water, from which (film c) thecontact angle was determined to be ya; (d) Film c after exposure inair, from which (film d) the contact angle was determined to be yr.Usually, ya represents the state of the surface in air while yr repre-sents the state of the surface after hydration, and Dy¼ ya� yr)represents the film’s mobility (from ref.[46]).



control the printing stability and quality, for further detailsrefer to Cheng et al.[14]

Measurement Tools

An optical-interferometry 3D surface profiler was used tomeasure the thin filmprofile (SNUPrecisionCo.,Korea). It hada vertical resolution of 0.1 nm, and lateral resolution of 0.5 mm.The scanning range can be adjusted from micro to nanometer,depending on the interferometric optics (2�–5�, Michelsoninterferometry, 10�–50�, Mirau interferometry). All theatomic force microscope (AFM) images (512 pixels wide)were taken using a Digital Instruments D3100 (Resolution: X,2 nm, Z, 0.05 nm, nonlinearity <1%) operated in the tappingmode in air. Scanning electronmicroscopy (SEM) imageswereacquired using an LEO 1530, and material analysis wasperformed using an energy dispersive X-ray (EDX) spectro-meter, of the EDAX Company.

Fabrication Processes

Before ink-jet printing of the catalystmaterial, i.e., Na2PdCl4, amodification of the surface property is required to increase thesurface adhesion to the catalyst. In this work, we use PEMs inour approach for the selective electroless plating of Cu. Thekey feature of PEMs based on PAH and PAA is the ability toalter themultilayer surface functionalitieswith a single layer ofpolyelectrolyte which can selectively bind with a Pd complex.A PAA-dominant surface binds a positively charged Pd com-plex, while a PAH-dominant surface resists binding. With anegatively charged Pd complex, the PAH-dominant surfacebinds the catalyst, while a PAA-dominant surface resistsbinding. Electroless plating was selectively promoted on onlythe PAA or PAH surface and inhibited on the other, with adifference of just one polyelectrolyte layer. PAH/PAA-multi-layer-coated substrates that contain regions of PAA or PAH inthe outermost layerwere used for direct plating only to the PAAor PAH surfaces. In this invention, the PEM process modifiedthe substrate surface, the catalyst material was ink-jet printed

onto this surface to obtain a patterned catalyst distribution, andfinally an electroless plating processwas used to form themetalwire over the pattern of the catalyst.

Chemical Preparation

The chemicals were prepared as described below. The chem-icals required were PAH (Mw ¼ 70 000, Aldrich Chem. Co.),sodium tetrachloropalladate (Na2PdCl4, Strem), PAA(Mw ¼ 90 000, 25% solution, Polysciences Inc), dimethyla-mine borane (DMAB, Acros Organics), sodium citrate, andlactic acid (which were obtained from Alfa Aesar). Allchemicals were used without further purification. Deionizedwater (>18MO cm,MillporeMilli-Q) was exclusively used inall aqueous solutions and rinsing procedures. The PAA andPAH solutions were maintained in a certain pH range, whichwas tuned by HCl and NaOH solutions, as suggested by Wanget al.[40] The catalyst solution enabled the Pd ions to beabsorbed with PAH due to interaction between the Pd ions andthe ammonium groups of PAH. Na2PdCl4 catalyst was dis-solved in deionizedwater as to obtain a solution of 10� 10�3

M

concentration. The copper electroless plating solution had twoformulas in this study, but therewas little apparent difference ingrowth rate, so ‘recipe b’ in Table 1 was preferred, because ofits easy preparation.

Formation of Self-Assembled Polyelectrolytes Multilayers

Self-assembled polyelectrolyte films have been used as buil-ding units for constructing multilayer structures and asmodifiers of surface properties. Such surface modification,for example, can be used to promote adhesion and wettability,prevent corrosion, modify the electrical and optical propertiesof the material, or create electroactive monolayers suitablefor various optical and electronic sensors and devices.Polyelectrolyte layers are prepared by selective absorption ofcompounds at solid fluid interfaces to construct organizedoriented compact monolayers having a thickness ranging fromabout 1 to 3 nm. The molecular self-assembly process takes

Table 1. The sequence of events for the process of self-assembled polyelectrolytes, the ink-jet catalyst, and electroless plating to form aCu metal film.

Step Conditions/comments

Substrate cleaning Exposed to UV/ozone for 10 minImmerse into PAH(aq) PAH(aq) (10� 10�3

M) for 10 minSubstrate flushing deionized waterImmerse into PAA(aq) PAA(aq) (10� 10�3

M) for 10 minRepeat (PAH/PAA) bi-layer structure Up to three bi-layers of PAH/PAALast immersion into PAH(aq) (10� 10�3

M) PAH(aq) (10� 10�3M) for 10 min

Ink-jet printing Catalyst Na2PdCl4 (10� 10�3M)

Flushing deionized waterDipping HCl (aq) (pH¼ 2.5–3) 30 sElectroless plating Recipe a:[54] Cu, pH¼ 8.72,4 g �L�1CuSO4; 20 g �L�1 EDTA (disodium salt);

50 mL �L�1 triethanolamine, 4 g �L�1, DMAB(dimethylamine borane);1.6micromoles sodium cyanide; 22 micrograms 1,10 phenanthroline Recipe b:[55]

Cu, pH¼ 9, 0.032 MCuSO4; 0.04 M 1,5,8,12 tetraazadodecane;0.3 M triethanolamine;0.067 M DMAB (dimethylamine borane); 30–300 ppm 2,20-dipyridyl

Flushing deionized water

254 K. Cheng et al.


place as a layer-to-layer process, which is based on thespontaneous absorption of either nonionic polymers, poly-anions, or polycations from diluted aqueous solutions ontosurfaces that carry a functional group or a charge opposite tothat of the depositing polymer. Selective absorption of thesepolyelectrolytes is alternated to form a bilayer assembly thatleads to the formation of multilayer assemblies. The mole-cules, which are typically used for constructing the first mono-layer, have a terminal polar group and a non-polar functionalgroup at either the other end of the molecule or somewherewithin it.

Ink-Jet Printing Step

Print heads in the ink-jet system described above were filledwith Na2PdCl4catalyst in deionized water. The concentrationof the ink was 5� 10�3

M. The host then sent the desired PWBimage data for the system to print the pattern with catalyst.When dried, the Pd was adhered onto the substrate surface andhad diffused into the monolayer of PAH to leave a pattern onthe substrate.

Electroless Copper Plating Step

The printed Pd was meant to be the catalyst for subsequentcopper deposition. The recipe of copper plating in Table 1 wasprepared by reference to Jagannathan.[54,55] All of the reagents,which were dissolved in deionized water at room temperature,were stirred for several minutes. To keep the solution stable, abubble generator with air was used in the plating bath. The pHof the bath was maintained at pH¼ 9, and the copper depo-sition was carried out at room temperature. The immersiontime and temperature are two key factors for wire thicknesscontrol. To get a uniform wire, stirring was performed tocontrol the plating solution concentration, while bubbling inthe bath was used to stabilize plating solution as for traditionalprocesses. After removal from the plating solution, sampleswere rinsed with deionized water to remove loose copper andplating solution. They were then set aside to dry.

Process Tuning

PAH/PAA-based multilayers were fabricated on a glass, poly-(ethylene terephthalate) (PET), or FR-4 (flame retardant type 4glass reinforce epoxy resin) substrate. An aqueous solution ofPAH (10� 10�3

M) was adjusted to pH 7.5� 0.1 with 1 M

NaOH, and an aqueous solution of PAA (10� 10�3M) was

adjusted to pH 3.5� 0.1 with 1 M HCl. Other pH conditionswere similarly obtained by adding the appropriate amount ofacid or base. The pHvalue determines the fraction of functionalgroups that will be ionized during the assembly process; it ispossible to control the density of the non-ionic, reactive func-tional groups.Generally, the pHvaluewill control the completeionization of PAA and PAH to obtain stoichiometric pairing. Inthis study, multilayers were formed by first immersing subs-trates into the PAH solution for 10min followed by three 2minimmersions into water as the rinsing step. The substrates thenwere immersed into the PAA solution for 10 min followed byan identical rinsing step. The absorption and rinsing steps were

repeated until the desired number of bi-layers was obtained.One bi-layer is defined operationally as a single absorption ofPAH followed by absorption of PAA. The PEM was finallydried in air and stored under ambient conditions. The HCldipping step in Table 1 is a treatment of the PAA/PAH bi-layersstructure, as given by Rubner[56] who found the step to controlthe transformation of a dense multilayer film to a microporousmultilayer film. This HCl dipping step dominates the interfacebetween the catalyst and the PAA/PAH bi-layer, and indeeddetermines the adhesion property of the metal after electrolessplating.

Results and Discussion

Circuit Line Quality and Control

In this study, a pattern of metal circuits are realized by ink-

jet printing of a catalyst onto a substrate pre-coated with a

self-assembled multilayer structure, followed by a typical

electroless plating technique to form the metal wire. This

approach built on our previous study.[57,58] It modified the

procedure of Guo et al.[41] to improve poor selectivity of

metal deposition because of uncontrollable residual PAAon

a glass substrate.We developed a novel approach using ink-

jet printing of a catalyst, instead of the PAH, as the pat-

terning step, to avoid dipping the substrate into catalyst

solution and inducing contamination.

The electroless plating controls the metal film formation

and its thickness. Key factors are the plating bath concen-

tration, bath temperature, pH stability, additives, and the

plating time.Many prior studies have explored these factors

in the plating process.[59–63] In this work, we kept the same

condition of the plating bath (recipe b in Table 1, 50 8C,pH¼ 9), and controlled themetal thickness through theplat-

ing time. Shiratori and Rubner[64] pointed out that surface

roughness could influence the thickness of an absorbed

monolayer, with thicker layers being formed onmultilayers

of high roughness. In this study, the substrate FR-4 has a

primitive roughness of about 2 mm before polyelectrolyte

layer processing, and the processes follow the suggestions

as given in Table 1.

The distribution of the ink-jet printed catalyst and its

drying mechanism are controlled by the flow patterns in-

duced by evaporation. This wetting behavior described by

Deegan[65] is that a pinned contact line induces an outward,

radial fluid flowwhen there is evaporation at the edge of the

drop. The contact is pinned and there is a flow replenishing

the liquid to the edge. Under this assumption, Pd will

largely accumulate at line edge along the printing line

direction as the catalyst drops dry and trigger a non-uniform

reduction reaction rate of copper along the line profile in

electroless plating. As shown in Figure 5, it was observed

that the edge of the copper line had a thicker edge forming

because the contact line of printed catalyst was pinned at the

line edge. This subsequently resulted in greater Pd accu-

mulation along the edge of the line, which caused a faster



reduction reaction in the following electroless plating. In

Figure 5(a–c), three layers of PAH/PAAwere adsorbed on

the FR-4 substrate using the same condition as Yang

et al.,[57] and the outermost layer was PAH. We found that

the ink-jet-printed catalyst with a dot overlap at different

lengths affected the circuit line quality significantly. Here

we adopted a thermal print head developed by our group

with a 35 pL drop size. After ink-jet printing the catalyst and

sequential electroless plating, the metal line was formed.

We found that if the overlap of the dot is insufficient, the

formed metal line had blurring in its edge, as indicated in

Figure 5(a). However, an excess overlap of dots, although

smoothing the blurring, causes a wider line width, as pre-

sented in Figure 5(c). There is a compromise between the

requirement of line width and line uniformity. For different

substrates, like the smoother polyimide (PI) substrate, the

only difference is more PAH/PAA layers are needed to

modulate the absorbance on the substrate. The same obser-

vation was found as shown in Figure 5(a–c), and only the

case of a dot overlap distance of 50 mm is shown in

Figure 5(d) for PI. Figure 6 measured the line width along

line direction for the FR-4 substrate. The best overlap

condition is of 33.3 mm, which showed a more stable line

quality than that of 62.5 and 50 mm, while the maximum

deviation is 8%.

Generally, the drop size deviation from the nozzle is

about 5–10%. When combined with other factors such as

uniformity of substrate properties, plating variation, opera-

tion deviation etc., the total blurring deviation of the line

formed on the substrate would be larger. It is critical for

high-frequency applications that the circuit has lowblurring

(<10%) and uniform line profile. To overcome this pro-

blem, we operated an actuator device beneath the substrate

to generate vibration along the normal direction of the

substrate while the catalyst ink (Na2PdCl4) drops were con-

tinuous discharged, andmaintained the vibration force until

these drops were dried. The vibration energy oscillates the

ink-drop surface while these drops are merging, and pro-

vides an opposite external force to the capillary force,

which prevents the solute from separating at the periphery.

In addition, this acoustic streaming behavior will simulta-

neously change the vapor pressure near the drop surface. It

resulted in a faster evaporation rate and reduced the prob-

ability of Pd particles migrating toward periphery bound-

ary, and led to an excellent Pd distribution and small line

width variation for the ink-jet catalyst. Figure 7 shows the

relation between the line width variation and the frequency

of the actuator. To enlarge the distinguishable range of line

width; here we adopt a thermal print head with an 85 pL

drop size to discharge on FR-4 substrate, so the measured

line width is wider than in Figure 5. After imposing

vibrational energy during ink-jet printing of the catalyst, the

linewidth reduced from 172 mmdown to near 140 mm in the

same process. The blurring behaviorwas also inhibited. The

variation of line width was improved from �10% (without

control) to �3% (with a frequency of 500–800 kHz), as

marked by the circle in Figure 7.

Based on the vibration improvement method, Figure 8

shows the high-density circuit main board for a personal

Figure 5. Line quality for FR-4 substrate and PI substrate. (a)Ink-jet catalyst with dot overlap distance of 62.5 mm on FR-4.(b) Ink-jet catalystwith dot overlap distance of 50mmonFR-4.(c) Ink-jet catalyst with dot overlap distance of 33.3 mm onFR-4. (d) Ink-jet catalyst with dot overlap distance of 50 mmon PI.

Figure 6. Measured line width variation along line printingdirection at different drop overlap.

256 K. Cheng et al.


computer. The major pattern is composed of a circle pad, a

square pad, and a narrow metal wire. The copper metal line

was formed by ink-jet printing a catalyst, and then elec-

troless plating on the FR4 substrate. The line width is less

than 100 mm at a thickness of 12 mm, and the closer line

pitch is about 500 mm. Overall fabrication time was only

about three hours, during which most of the time was spent

on the thickness forming required in plating. In the

experiment, it took 2 h to achieve a thickness of 12 mm.

Key factors controlling the process are the plating bath

concentration, bath temperature, pH stability, additives, and

plating time. In our work, we kept all the factors constant

except the plating time.

Layer-by-Layer Interface

The surface property dominates the film quality because

adhesion between the substrate, the electrolyte layers, the

sequential catalyst layer, and the metal layer achieved by

electroless plating all depend on each other. The roughness

is expected to influence the adhesion of the deposited metal

coating. Chong et al.[66] found that roughness increases

with increasing weight loss of sodium hydroxide for the

treated films. A maximum roughness was obtained in sam-

ples with a weight loss of approximately 15–20%, beyond

which the roughness of the samples decreased. The adhe-

sion of the electroless-plated metal film was dependent on

the contact area produced by chemical treatment. This

treatment produced smaller diameter pores of greater depth

and induced better adhesion.

To further analyze the detail of each layer, Figure 9(a–c)

show the atomic forcemicroscopy (AFM) images depicting

the surface topography of the three PAH/PAA double layers

deposited on the glass substrate. For comparison, an AFM

image of the glass is shown in Figure 9(a), and an AFM

image of the outermost PAH layer on glass is shown in

Figure 9(b). In Figure 9(a), the maximum roughness of the

glass is about 7 nm, indicated by the level bar. After three

layers of PAH/PAA and one outermost PAH layer coating, a

porous structure is formed with a large surface coverage as

shown in Figure 9(b). The 3 mm� 3 mm scans showed the

PAH film forming high-density small grains approximately

50–250 nm in size. It is suggested that the larger grain size

of nearly 250mmis either contaminated by particles or is the

primitive property of the substrate, the same as that ob-

served in Figure 9(a). Therefore, because of this higher

surface area and microporous structure, the self-assembled

polyelectrolyte layers are of great benefit to the adsorption

of the catalyst used for metal deposition. As presented in

Figure 9(c), the structure of the outermost PAH forms a

cone array arrangement. After printing of the catalyst, the

remaining Pd atoms permeated into the root of the cone.

When Cu replaced Pd by the reduction reaction in the

electroless plating, copper is deposited near the root. This

resulted in an excellent adhesion property between Cu and

the substrate. Figure 9(d) shows a fast-fourier transfer

(FFT)[67] topology image for Figure 9(c), to examine the

orientation of the outermost PAH layer. The FFT of an

image is a two-dimensional array of complex numbers. It

represents the frequency of occurrence of pixel-intensity

variations in the spatial domain. The low frequencies,

located at the corner of the image, correspond to smooth and

gradual intensity variations found in the overall pattern of

the source image. Thehigh frequencies, located at the center

of the image, correspond to abrupt and short-intensity

variations found at the edges of objects (like the PAH

clusters). The photo was decomposed into the green color

channel, and the FFTs operated at the green-grey level

image. The results showed that the outermost PAH doesn’t

Figure 7. The line width variation controlled with differentexcitation frequency.

Figure 8. A flexible high-density circuit on an FR-4 substrate.The operation conditions for electroless plating were 50 8C, 2 h.



have obvious specific orientation. Similar results were

found for the red color channel and the blue color channel.

Rabini et al.[68] explained that the nanoparticle self-assem-

bly behaviorwould fluctuate because of solvent evaporation

dynamics. Homogeneous and heterogeneous evaporation

createvariousmorphologiesof thefinal self-assembly struc-

ture. It was not clear if the evaporation of the PAH solution

changes the orientation of the PAH arrangement, and its

interaction with the polyelectrolyte layers. More discus-

sions are needed. For further observation of the ink-jet-

printed Pd distribution, this Pd film was immersed in an

electroless plating solution of recipe (b) in Table 1 for 1 s, to

enhance the contrast of Pd. Figure 9(e) shows the surface

roughness of the ink-jet-printed Pd catalyst absorbed on the

outermost PAH layer with three PAH/PAA double layers

beneath. The data shows a root-mean-square roughness of

4.059 nm, and a maximum roughness of 6.687 nm for a

scanning length of 22.461 nm.

The principle of self-assembled polyelectrolytes is a

layer-by-layer assembly of oppositely charged species.

Typically, alternate layers of positively and negatively char-

ged polymers are sequentially adsorbed on the substrate

from a very dilute solution to build up the interpenetrated

multilayer structures. They are composed of one polyca-

tion–polyanion polyelectrolyte complex layer per dipping

cycle. Because of the usage of weak polyelectrolytes, the

charge density in the polymer chains of the PAH and PAA in

our experiment is controlled by the pH of the solution. By

careful control of the pH of the dipping solution, molecular

interactions, including electrostatic attraction and hydrogen

bonding, can be incorporated into the polymer layers.[41,42]

To further observe the morphology during the layer-by-

layer process, Figure 10 depict the surface topography of a

series ofAFM images for each layer in the sequence of PET/

PAH/PAA/PAH/PAA. . . . ./PAH/Na2PdCl4 catalyst. Each

image has an area of 3 mm� 3 mm. The PET has undergone

plasma treatment to enhance its wetting capability and to

enable the first PAH layer to be easily absorbed by the PET

substrate. After that, up to five layers of the PAH/PAA bi-

layer structure and one outmost PAH layer were coated, of

which the surface topography is observed. We found that

the PAH/PAA bi-layers would form a cone structure on the

PET surface, as in Figure 9(c), and presented tiny white

spots as in Figure 10(b–f). With an increase in PAH/PAA

bi-layers, the roughness increases, and the surface mor-

phology changes from cone to cluster. Therefore, because

of the higher surface area and the formation of microporous

structures, these self-assembled polyelectrolyte layers are

of great benefit to the adsorption of the catalyst for metal

deposition.

The vertical cross-section of each layer was examined by

scanning electron microscopy (SEM). We coated 7.5 bi-

layers (seven PAH/PAA and an outmost PAH layer) on

PET with plasma treatment. This PET/7.5 polyelectrolyte

layer (PAH/PAA/PAH/. . . ./PAH)/catalyst/Cu layer has the

structure as shown in Figure 11. Figure 12(a) shows

the surface of the Na2PdCl4 catalyst, it indicated that the

catalyst has cracked trencheswhile drying, and its grain size

was about several microns. As shown in Figure 12(e), each

dried catalyst piece has a smooth surface, with about the

Figure 9. Observed layer characteristics of outmost PAH and Pd. (a) AFM tappingmode images of thesurface of a glass substrate; (b) AFM tappingmode images of the surface of the outermost PAH on glass;(c) a project view of PAH on glass; (d) fast-Fourier transfer of a green-color-abstraction-channel for theimage of (b); (e) Height profiles for a horizontal scan of ink-jet printing of Pd catalyst adsorbed on theoutermost PAH surface of three PAH/PAA double layers on glass. (f) Themorphology of the copper lineobserved by optical micrograph. A thicker edge than line center and blurring were found.

258 K. Cheng et al.


roughness of several nanometers. Figure 12(b) was a

vertical cross-section profile of PET/7.5 polyelectrolyte

layer (PAH/PAA/PAH/. . . ./PAH)/catalyst, before immers-

ing the substrate into the plating bath. The black dot

represents where some of the catalyst has penetrated

through the PAH/PAA layers, and touched the PET layer.

Analysis found that the catalyst layer was about 330 nm

thick. For the multilayer deposition process, the first layer

thickness is strongly dependent on the substrate properties.

After the formation of several layers, the thickness of the

multilayer increases with the number of layers linearly.

Figure 12(c) examined the position of the red spot in

Figure 12(b) by EDX to analyze its composition, and found

that the elements were C, O, and a little Cl coming from the

polyelectrolyte. The appearance of Pt and Ga were because

of the need of the EDX operation. Deposition of Pt was

needed as the electrode and Ga was used for the FIB (focus

ion beam) to cut the substrate. At the location of red spot, no

Pd was found. It indicated that no catalyst has penetrated in

this area. After electroless plating, the copper replaced Pd

by the reduction reaction, and the copper film formed over

the polyelectrolyte layers. It is observed in Figure 12(d) that

copper would tangle with PAH/PAA, presenting an excel-

lent adhesion of the metal film. Wang et al.[40] summarized

that the adhesion was improved by the tangling effect of

metal with polyelectrolyte layers. In the process, the poly-

electrolyte layers absorbed water, and hence behaved like

ionically cross-linked hydrogels. However, it is noted that

Durstock and Rubner[69] further found that the electric

characteristics of these PAH/PAA films would change with

respect to temperature, moisture content, and deposition

conditions. Therefore, in some thin-film applications, the

interface between the metal and the PAA/PAH needs to be

carefully controlled to obtain lines with low resistance.

Quality Standard

Mechanical properties are particularly sensitive to the

micro-structural length scale of the thin metal film. This is

particularly evident in electric deformation. The metal thin

films often appear to be more compliant than their bulk

equivalents,[70,71] presumably for microstructural reasons

such as imperfectly formed grain boundaries. The increase

of the yield stress with decreasing grain size is customarily

expressed by an inverse power law, known as the Hall–

Petch relation.[72–74] Huang and Spaepen[75] found the

average Young’s modulus has a 20% reduction for a multi-

layer metal film and is most likely the result of micro-

cracking of the grain boundaries. They concluded no

Figure 10. AFM images were shown for each layer process. (a) PET Image. (b) PET had been plasmatreated, and coated with one PAH layer. (c) PET/PAH/PAA. (d) PET/(PAH/PAA)2. (e) PET/(PAH/PAA)3. (f) PET/(PAH/PAA)4. (g) PET/(PAH/PAA)5. (h) PET/(PAH/PAA)5/PAH.

Figure 11. Layer structure of polyelectrolyte layers on PET.



softening with decreasing grain size was observed even at

the lowest values of bi-layer repeat length.

IPC 6013[76] covers mechanical, electrical qualification,

and performance requirements of flexible printed wiring. In

this standard, the flexible printed wiring may be single-

sided, double-sided, multilayered, or rigid-flex multi-

layered. All of these constructions may or may not include

stiffeners, plated-through holes, and blind/buried via. In this

study, the ink-jet-printed circuit had beenverified following

the testing standard of IPC 6013, as shown in Table 2. To

verify the adhesion capability, the 3M tape (3M, No.600,

1/200 width� 200 length) peeled in a vertical direction

presented excellent adhesion between the circuit and the

flexible substrate. No trifles were observed to be left on the

tape. The dramatic improvement of the adhesive properties

was because of the modification of the PEMs to the sub-

strate. Claesson et al.[77] mentioned that the adhesion

between one polyelectrolyte-coated surface and one bare

surface was initially stronger than that between the two

polyelectrolyte-coated surfaces.However, because ofmate-

rial transfer between the two surfaces, the adhesion decre-

ased significantly with the number of times that the surfaces

were driven into contact. For the polyelectrolytes of the

lowest charge density the results suggest that the entangle-

ment effects contributed to the adhesive interaction.

An important requirement of the circuit quality is the

resistance to thermal cycles, especially during the soldering

procedure. In the standard procedure, the sample was first

baked at 120–150 8C for 6 h to drive themoisture away, and

then cooled to room temperature. The circuit underwent a

thermal stress test at 288� 5 8C six times and was

immersed in 260� 5 8C hot oil three times. The circuit

was able to endure both testing conditions. Details can be

found in Table 2.

Circuits Performance

The line-circuit performance on the substrate surface has

been measured in prior studies.[57,58] The stable thickness

can be controlled following a linear average growth rate

of about 16 mm per 150 min, i.e., near 0.1 mm �min�1

deposition rate. A typical example has a thickness of about

4 mm, and a resistance less than 10 O per 10 cm. The best

conditions result in a resistance of about 0.304 O � cm�1

at a metal linewidth of 250 mm and line thickness of 17 mm.

This is roughly eight times the bulk resistivity of copper

(rbulk, Cu¼ 1.67 mO � cm�1). For typical applications like

printed circuit board (PCB), the thickness requirement is

between 12 to 35 mm. This suggests that a finer metal line

width is feasible for this type of application at the same

resistance requirement.

Metal Forming of Via Inner Wall

For the trend in inexpensive, higher performance, and smal-

ler size computers, there has been considerable develop-

Figure 12. SEM images for each layer. (a) Dried catalystsurface. (b) Cross-section profile for PET/7.5 polyelectrolytelayer/catalyst. (c) EDX analysis for the position of the red spot in(b). (d) Cross-section profile for PET/7.5 polyelectrolyte layer/catalyst/Cu at 2.5k�magnification.

260 K. Cheng et al.


ment in packaging and interconnection of integrated

circuits with increasing complexity. Given the increase of

input and output channels on a chip, the amount of wiring

required to interconnect the chip increases accordingly.

Moreover, the size of electronic devices continues to shrink

by increasing the number of signal planes and the wiring

density, reducing the line widths, and introducing smaller

holes and vias.[78,79] Some build-up technologies have been

rapidly developing to achieve such high-density multilayer

printed circuit boards.[80–84] As the via-hole becomes

smaller, i.e., the aspect ratio increases, the metallization of

these vias becomes an increasingly difficult task, because

these inside walls are not readily accessible to process-

ing solutions.[85,86] Several approaches have been explor-

ed,[87–93] but the successful metallization of blind vias

smaller than 50 mm has not yet been reported.

Kou and Hung[94] reported a metallization of laminates

with blind vias on the dielectric side using electroless cop-

per plating with subsequent copper electroplating. Results

indicated that ultrasonic vibration during the pretreatment

processes of metallization enabled complete electroless

copper deposition on the inside wall of the small blind vias

without any voids. Figure 13 shows their results for an via

with aspect ratio of 1:1, a depth of 75 mm, and a diameter of

75 mm. For the same period of plating time, the thickness of

a copper deposit formed by periodic-pulse-reversal plating

was much thinner than that formed by direct-current

plating. Interestingly, the surface obtained by periodic-

pulse-reversal plating was smoother than the one from

direct-current plating or the original substrate surface.

To extend the application, the question of how to form the

metal layer on thevia-hole innerwall by amicro-dispensing

method was an important consideration. For a high-density

multilayer circuit board, thevia-hole diameter will decrease

to 30–50 m in the future. Screening printing is not a good

choice for its low yield rate at this resolution. An emerging

processing method is needed urgently to fulfill market

expectations. In general, the electric characteristics of a via

hole requires the thickness of the inner wall be larger than

15 mm, no pin hole can be observed, and resistance to be less

than 50 O under 200 V. This is the first study into the

formation of via-hole connections by polyelectrolyte layers

and ink-jet printing of a catalyst. The steps of forming the

metal layer of via holes are the same as the steps of forming

the metal wire, except for the ink-jet catalyst patterning

method. In our experimental observation, the hole diameter

is a dominant factor for ink-jet printing. For small holes,

several drops filled the holes and catalyst was absorbed onto

the innerwall. However, formediumholes or larger,most of

catalyst flowed onto the back side of substrate and caused

deterioration. To avoid this problem, the substrate was

attached to the back side of a porous film, to enable a flow

tuning mechanism of Darcy flow[95] as the catalyst was

discharged. After ink-jet printing the catalyst, the porous

film and inner wall of the hole temporarily retained the

catalyst, which then diffused into the porous film. The

catalyst that was in contact with the innerwall was absorbed

and left remnants of Pd. In the experiment, the ratio of the

screen size of the porous film to the via-hole diameter was

less than 1:10, which was found beneficial for modulating

the diffusion rate. In Figure 14, the metal layer has suc-

cessfully formed in the inner wall of the via hole in which

the minimum diameter is 300 mm, and the front side and

back side of substrate are connected successfully. The

flatness around the holewas also good,meeting the standard

requirement.

As mentioned earlier, a critical compromise between the

ink drop amount and the via-hole dimension characteristic

is needed to prevent overflow or underflow. However, we

found that themetal film fabrication process for the via-hole

inner wall is not yet stable. For example, in Figure 14(b),

void defects were found to form in the wall. Although the

hole can still connect the front side and the back side of the

substrate, as specified by IPC 6013 Class-1 standard (void

Table 2. Environmental testing results following the IPC 6013 standard for the circuit pattern in Figure 1.

Testing Item Results Testing Conditions

Peeling test Pass 3M tapeThermal stress Pass 288� 5 8C, 6 timesHot oil test Pass 260� 5 8C, 3 timesSoldering test Pass Soldering tin on circuit within 10 secDielectric withstanding voltage Pass Class 3, 100 V dc, 30 sec.Peeling stress – Excellent adhesion which can not peel the metal line to be tested. Stress should be

>245 MPa for 50–100 mm thickness, and an elongation rate larger than 12%at room temperature.

Figure 13. Vertical cross section of 75 mm diameter vias on75 mm thick epoxy dielectric processed by the ultrasonic vibra-tion process with periodic pulse reversal plating (from ref.[94]).



defect number less than three), but for a high end product

(IPC 6013, Class-3, no void defect),[76] these defects will

break or cause current leakage.

A possible solution to reduce the defects is fine tuning of

the self-assembled polyelectrolyte process. Recently, Sui

and Schlenoff[96] reported that a pH-responsive polyelec-

trolyte multilayer, like PAA and PAH, tends to be unstable

with respect to morphology and properties. Their solutions

were of non-pH-sensitive polymers blended with pH-

sensitive polymers, to decrease the net density of ionizable

groups within the multilayer, and yielded more stable metal

film fabrication. The pH-responsiveness of the multilayer

might be a possible cause for via-hole metal forming.

Similarly, Shiratori and Rubner[97] found that the pH value

of the solution plays an important role in the layer-by-layer

processing of the weak polyelectrolytes PAA and PAH. By

systematically altering the charge density of a weak

polyelectrolyte over a narrow range of pH, a dramatically

different polymer adsorption behavior was observed. With

controlled pH, the thickness of an adsorbed polycation or

polyanion layer can vary from 5 to 80 A. Dubas and

Schlenoff [98] found that the dissociation of multilayer

polyelectrolyte complexes was attributable to competition

for polymer/polymer ion pairs by external salt ions. The

multilayers were decomposed by protonating the weak

acid, hence decreasing polymer/polymer interactions,

which led to incomplete loss of polymer, probably because

of additional hydrogen bonding from the protonated weak

acid. Burke and Barrett[99] have demonstrated that the

loading and releasing behavior of small hydrophilic molec-

ules in weak polyelectrolyte multilayer films of PAH and

A (acid form) is controlled by a complex interplay between

the physicochemical properties of the films and the small

molecule species. Therefore, both electrostatic interactions

and degree of swelling of the films governed by the acid–

base equilibrium are important parameters in controlling

the formation of a thin film in the inner wall of a via.

Currently, the effect of pH on swelling and distribution of

Pd during drying, and consequently on electric character-

istic, are still unknown. Our group is performing further

studies on the film morphology and line profile.

Another possible reason for stability problems could be

the flow circulation during electroless plating. In our ex-

perimental setup, the plating bath passed through the hole

by diffusion and surface convection. The velocity distribu-

tion for each individual hole was not uniform and uncon-

trollable. In the near future, we plan to design a laminar flow

circulation device to better control the flow direction,

implementing different flow velocity in both normal and

parallel directions of the substrate surface to achieve amore

uniform plating condition and reaction rate.

Conclusion and Future Directions

This article reviews recent developments in printing circuits

on flexible substrates by combining self-assembled poly-

electrolytes, ink-jet printing of a catalyst, and electroless

plating ofmetals. The limitations and potential applications

of this technology are also discussed. We demonstrated the

patterning of multilayer electric circuits on various flexible

substrates by combining these three technologies. This

study found that this novel modification procedure gave

excellent selection capability and adhesion on different

substrates. To our knowledge, this is the first time such

technology has been implemented successfully in electro-

nic circuits to conform to the IPC standard, and have been

shown to effectively form a metal film in the via hole inner

wall.

However, some issues still need to be addressed. The first

is the line resolution and mushroom profile resulting from

ink-jet printing of Pd and the anisotropy growth in

electroless plating. These complex interconnections, which

include adryingmechanismof the catalyst and the interfacial

properties between each layer, need further identification.

In addition to using polyelectrolyte layers composed of

pH-sensitive polymers, it is also possible to induce both

reversible and irreversible morphological changes after the

assembly process is completed. Further studies will focus on

Figure 14. Picture of via hole with copper inner wall formed bypolyelectrolyte layers on hole inner surface, ink-jet catalyst intohole, and electroless plating of Cu. (a) Smooth and perfect metalforming. (b) Some void defect in the inner wall.

262 K. Cheng et al.


those uncertain factors influencing thin filmmorphology and

its relation to electrical characteristics.

Acknowledgements: This study was financially support by theMOEA (Ministry of Economic Affairs, RO China) andmeasurement assistance by the Unimicron Company. They aregratefully acknowledged.

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Documents

Ink-Jet Printing, Self-Assembled Polyelectrolytes, and Electroless Plating: Low Cost Fabrication of Circuits on a Flexible Substrate at Room Temperature