Experimental investigations of laser-induced forward transfer

process of organic thin films

Benjamin Thomas a, Anne Patricia Alloncle a,*, Philippe Delaporte a, Marc Sentis a,Sebastien Sanaur b, Michael Barret b, Philippe Collot b

a Laboratoire Laser Plasma et Procedes Photoniques (LP3), UMR 6182 CNRS - Universite de la Mediterranee,

Cs 917 13288 Marseille Cedex 9, Franceb Centre de Microelectronique de Provence (CMP-GC), Ecole de Mines de St ETienne, Department of Packaging

and Flexible Substrates (PS2), 13541 Gardanne, France

Received 9 June 2007; received in revised form 14 September 2007; accepted 14 September 2007

Available online 21 September 2007

Abstract

This paper deals with transfer induced by laser of thin layers of a conducting polymer, the poly(3,4-ethylenedioxythiophene)-poly(styr-

enesulfonate), for applications in plastic electronics. This relatively simple technique of direct writing offers the ability to make surface micro-

patterning by localized deposits of material. The study of the various mechanisms (ablation, transfer and deposit) has been carried out according to

different conditions of irradiation: wavelength (from ultraviolet to infrared radiation), pulse duration (nanosecond and sub-nanosecond) and

fluence. The morphology of the transferred patterns has been analyzed by optical microscopy and scanning electronic microscopy. Our objective is

to understand the different mechanisms involved in the process in order to optimize it in terms of geometrical resolution while preserving the

properties of the transferred material.

# 2007 Elsevier B.V. All rights reserved.

www.elsevier.com/locate/apsusc

Available online at www.sciencedirect.com

Applied Surface Science 254 (2007) 1206–1210

Keywords: LIFT; Polymer; Ablation; Transfer; Laser

1. Introduction

Some applications in microelectronics are concerned by a

mass production requiring reduced manufacturing costs and a

high rate of manufacture. It is for example the production of

objects with simple electronic functionalities used in fields such

as radio frequency identification (RFID) tags, smart card or

micro-connecting systems.

The development of the direct printing technologies of the

components on flexible supports, associated with the use of

new conductive polymeric materials would allow avoiding the

use of complex and expensive techniques such as photolitho-

graphy.

The main process which is currently used is a printing

technique, similar to ink-jet techniques. It requires the

solubility of the organic material to deposit. New organic

compounds with very interesting electric characteristics are

* Corresponding author. Tel.: +33 491 829 381; fax: +33 491 829 289.

E-mail address: alloncle@lp3.univ-mrs.fr (A.P. Alloncle).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.09.042

regularly synthesized, and numbers of them do not have the

solubility properties which would enable them to be used by

this printing process. It is thus very interesting to study the

implementation of new techniques giving the possibility to

produce electronic components with non-soluble organic

materials on flexible supports.

The objective of the works presented here is to study the

potential of laser techniques in plastic microelectronics.

Particularly, the LIFT (laser-induced forward transfer) process

of a conducting polymer already used in ink-jet printing

technique: the PEDOT-PSS (poly(3,4-ethylenedioxythio-

phene)-poly(styrenesulfonate)).

2. Description of the experiments

The laser-induced forward transfer (LIFT) technique

consists in removing a small piece of a thin layer previously

deposited on a transparent substrate and transferring it on

another substrate using a pulsed laser (Fig. 1). This simple,

single step, direct printing technique offers the ability to make

surface micro-patterning or localized deposition of material.

Fig. 1. Principle of LIFT.

Fig. 3. Experimental setup.

Table 1

Different lasers used for LIFT experiments

Laser Wavelength

(nm)

Pulse duration and

energy

Nd:YAG (Spectra Physics) 1064 and 532 1 J, 8 ns, 10 Hz

LPX 210i Lambda Physics 248 0.5 J, 25 ns, 1–100 Hz

S pulse Amplitude Systemes 1025 200 mJ, 400 fs, 1 kHz

Nd:YAG (Leopard S10/20

Continuum)

1064 and 532 120 mJ, 50 ps, 10 Hz

B. Thomas et al. / Applied Surface Science 254 (2007) 1206–1210 1207

The irradiation occurs through the substrate, which has to be

transparent to the incident radiation. The radiated energy

absorbed by the layer is confined in volume defined by the size

of the irradiated surface and the absorption length of the

material. This energy is converted in thermal and mechanical

energy, leading to the ejection of the material. A receiver

substrate is placed in closed proximity.

This technique has been widely investigated during the past

years mainly in nanosecond and sub-nanosecond regime

(deposition of metals, metal oxides, semiconductors even

biomaterial on various substrates and with various types of

laser) [1–5]. Some methods have been especially studied for the

transfer of organic material. Pique et al. [1,2,7] developed a

method adapted for materials sensitive to temperature elevation

(MAPLE DW, matrix assisted pulsed laser evaporation direct

write). Thin film, made with powders of the material to be

transferred mixed with a photosensitive polymer, is elaborated

on a donor substrate. Under the effect of the laser irradiation,

the polymer is vaporized and ejected from the substrate. Under

the effect of mechanical stress, particles are transferred on the

substrate receiver. Another method named LITI [6] (laser-

induced thermal imaging) has been developed especially for

plastic microelectronics application (fabrication of organic

light-emitting diode). A pre-coated substrate is irradiated by a

CW laser during few milliseconds. This thermal process

requires using materials whose properties are not modified in

the range of temperatures reached.

In our case, thin films of PEDOT have been spin-coated on

adequate substrates (BK7: Schott borosilicate crown glass and

quartz suprasil) transparent to the different wavelength. The

coatings have been characterized by scanning electronic

microscopy (SEM) and profilometry. The thickness is quite

Fig. 2. SEM visualization of the spin-coated PEDOT films: (a) ch

homogenous, varying from 280 to 300 nm depending on the

sample and a very low roughness has been measured (15–

20 nm). SEM visualizations of the layers can be seen in Fig. 2.

The LIFT experiments have been carried out using four

different lasers allowing different conditions of pulse duration

and wavelength (Table 1). We have observed different

mechanisms during the ablation and transfer phases depending

on the absorption of the PEDOT layer at a chosen wavelength.

The influence of the pulse duration has also been studied.

A receptor substrate is placed in contact or at 25 or 50 mm

closed to the donor. The influence of the distance is one of the

different parameters studied. The receptor is a sample of BK7

or silicon, or a thin flexible substrate used in plastic micro-

electronics, composed of several materials with nanometric

thickness.

The setup (Fig. 3) is almost the same for the different lasers. A

mechanical shutter placed, on the beam axis allows to select one

pulse. The energy per pulse is controlled using calibrated

aracterization of the layer thickness and (b) layer roughness.

Fig. 4. Ablation (a) and transfer (d) of PEDOT irradiated by a visible radiation (532 nm) with an 8 ns pulse duration. Fluence 1 J/cm2.

B. Thomas et al. / Applied Surface Science 254 (2007) 1206–12101208

attenuating plates and/or using a polarization device. A mask is

imaged on the thin layer using a converging length. The sizes of

the spots vary from 100 mm � 100 mm to 250 mm � 250 mm.

The precise positioning of the sample is obtained by micrometric

translation (x, y, and z) devices and controlled by imaging the spot

on a CCD camera. A shutter is necessary to select one shot.

The experiments have been performed under atmospheric

conditions.

3. Results and discussion

3.1. Influence of the absorption properties of the PEDOT

LIFT of a film of PEDOT has been first studied with a visible

radiation (l = 532 nm) in nanosecond regime. Microscopic

visualizations of the ablated and transferred film on a glass

Fig. 5. Ablation (A-a) and transfer (A-d) of PEDOT irradiated by an infrared radi

Fluence 1 J/cm2, substrates in contact. Ablation (B-a) and transfer (B-d) of PEDOT

multilayer flexible substrate. Fluence 0.6 J/cm2, distance between substrates: 25 m

substrate are presented in Fig. 4. The film is not uniformly

ablated even with fluences higher than 1 J/cm2. The transferred

spot is not homogeneous but formed with ejected fragments of

PEDOT. The 300 nm film of PEDOT is partially transparent to

this incident radiation and it is estimated that only 25% of the

incident energy is trapped in the layer. It appears that the

absorption takes place in multiple hot points inside the layer

leading to the fragmentation of the film. It will not be possible

to use theses conditions to get acceptable deposits.

On the other hand for 1064 nm, the polymer is much more

absorbing. The total ablation of irradiated surface is carried out

at a fluence threshold of 0.4 J/cm2. This fluence is not high

enough to carry out a homogeneous deposit. It is necessary to

irradiate the film of PEDOT with a fluence ranging from 0.6 to

1 J/cm2 to obtain a complete transfer of polymer as the optical

microscope visualizations shown in Fig. 5. Under these

ation (1064 nm) with an 8 ns pulse duration on a multilayer flexible substrate.

irradiated by an infrared radiation (1064 nm) with an 8 ns pulse duration on a

m.

Fig. 6. SEM visualizations of PEDOT transferred on silicon under 248 nm condition of irradiation; donor and target in contact. (a) Visualization of the entire spot and

the closest one, (b) visualization of the edge of the spot and (c) roughness.

B. Thomas et al. / Applied Surface Science 254 (2007) 1206–1210 1209

conditions a spot of identical size to the ablated zone is

reconstituted on a flexible multilayer substrate with an edge

accuracy from 10 to 15 mm. The analysis of the edge of the

ablated spot shows a direct rupture due to the effect of strong

pressure stresses. The radiation is absorbed by the polymer but

is kept confined in a small volume between the substrate and

the layer of the polymer non-directly affected by the

irradiation. Thus high pressure and high temperature are

generated and caused the rupture and the ejection of the layer.

Thermal effects, characterized by the ejection of micro-

droplets are visible on the edge of the deposited spot.

Complementary analysis is necessary to control the quality of

the transferred PEDOT especially to measure its electrical

properties.

Damages are created in the multilayer flexible substrate at

1 J/cm2 (black parts visible on photo A-d, Fig. 5) in the zone of

Fig. 7. Ablation (A-a) and transfer (A-d) of PEDOT at l = 1064 nm, t = 50 ps and 0.

and 0.39 J/cm2.

fluctuations of energy. The damage threshold is very close to

the optimal conditions of transfer. If the target substrate has a

damage threshold higher, glass for example, and for a fluence

still increasing there is progressively disappearing of the

deposit. The transfer is initiated in the first part of the pulse, but

the energy remaining in the same pulse contributes to ablate the

transferred material.

When the receiver substrate is placed at a 25 mm distance

from the donor, there is construction of interference patterns

due to the reflection of the incident beam by the target. The

homogeneity of the ablated spot is destroyed and as a

consequence the homogeneity of the transferred PEDOT also.

But the spot is still well solved (Fig. 5, B-d). This kind of

pattern does not exist if the target substrate does not reflect the

incident radiation. Also using a sub-nanosecond pulse could be

an option to solve this problem. If the distance increases from

18 J/cm2; Ablation (B-a) and transfer (B-d) of PEDOT at l = 1064 nm, t = 50 ps

B. Thomas et al. / Applied Surface Science 254 (2007) 1206–12101210

25 to 50 mm or more it is impossible to keep a good shape of the

transferred PEDOT.

The third condition of absorption experimented is 248 nm.

This radiation is strongly absorbed by the polymer but we have

to keep in mind that UV radiation destroyed the electrical

properties of the PEDOT. It was still interesting to work with

this radiation to study if the ablation and transfer occurs in a

different ways. We can presume in a first approach that the

conductivity will be only modified in the first nanometers of the

layer directly photo-ablated. Complementary electrical mea-

surements will be done on the deposited PEDOT. The ejection

of the polymer as a solid ‘‘confetti’’ occurs at a low fluence

(0.12 J/cm2), but a uniform and homogenous deposit is obtain

in a very small range of fluence. SEM visualizations of the

transferred PEDOT on silicon in such conditions are reported in

Fig. 6. The edge of the deposit is well solved and shows a

rupture under mechanical stresses (Fig. 6b). There is no

droplets ejection due to thermal effects that can be seen outside

the transferred PEDOT. A clean channel is obtained between

two spots. In this case the size of the transfer spot is

150 mm � 220 mm and the size of the channel is 20 mm. A

channel length around 10 mm can be easily obtain with the

same conditions. A high roughness (Fig. 6c) is visible at the

surface of the transferred PEDOT.

3.2. Influence of the pulse duration

Some results obtained in nanosecond regime show the

possible interest to use shorter pulses with the objective of

reducing the heating effects or working in a larger range of

fluence. The experiments undertaken with a pico-second laser

and a femto-second laser show similar results.

At 1064 nm the beam is absorbed by the polymer. At a low

fluence the film of PEDOT is separated from the substrate and

starts to lose its cohesion. The absorption of the beam occurs in

the PEDOT, at the interface with the substrate. It remains

confined by the not irradiated PEDOT layer. A kind of bubble

containing the products of the interaction is formed and begins

to expand (Fig. 7-a and -d). In this case, the pressure stresses

induced during the pulse are not high enough to break

completely the film of polymer. At the opposite of the

nanosecond regime the absorption of the radiation is very

uniform on all the spot. The all ablation of the film is obtained

for 0.4 J/cm2. Only a part of the film is transferred and forms a

homogenous deposit with a very small thickness. The shape of

the transferred part is difficult to control and varies from one

shot to another. Another part of the film is ejected as micro-

fragments.

4. Conclusion

Experiments have been carried out on the LIFT of 300 nm

PEDOT coatings under different conditions of irradiation and

laser pulse duration. Interesting results have been obtained in

nanosecond regime especially when the PEDOT is very

absorbing to the incident radiation (248 nm). The film keeps its

cohesion during the transfer and a precise cutting of the edge is

obtained. Moreover, the transfer occurs at very low fluence

conditions, under the damage threshold of the multilayer

flexible substrate.

The results obtained in the shorter pulse regimes show a

difficulty to control the transfer. But if this transfer is partial the

first electrical measurements, show that the transferred film has

electrical conducting properties.

Another interesting way to trap the incident energy is to use

a transition-absorbing layer in order to avoid the thermal effects

and the direct irradiation of the polymer layer.

Acknowledgments

This work is performed with a financial support obtained

trough the MICROPOLY framework (CIMPACA).

Special acknowledgments to the GCOM2 and the CRMCN

laboratories (Marseille).

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