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Laser-induced forward transfer from healing silver paste filmsEmre Turkoz, Miguel Morales, SeungYeon Kang, Antonio Perazzo, Howard A. Stone, Carlos Molpeceres, andCraig B. Arnold

Citation: Appl. Phys. Lett. 113, 221601 (2018); doi: 10.1063/1.5060717View online: https://doi.org/10.1063/1.5060717View Table of Contents: http://aip.scitation.org/toc/apl/113/22Published by the American Institute of Physics

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Laser-induced forward transfer from healing silver paste films

Emre Turkoz,1 Miguel Morales,2 SeungYeon Kang,1 Antonio Perazzo,1 Howard A. Stone,1

Carlos Molpeceres,2 and Craig B. Arnold1,a)1Department of Mechanical and Aerospace Engineering, Princeton University, Princeton,New Jersey 08544, USA2Centro Laser, Universidad Politecnica de Madrid, Alan Turing 1, 28038 Madrid, Spain

(Received 21 September 2018; accepted 6 November 2018; published online 26 November 2018)

Laser-induced forward transfer (LIFT) is a nozzle-less printing technique where a controlled

amount of material is transferred from a thin film to a receiver substrate with each laser pulse.

Conventionally, each laser pulse is directed to a different spot on the donor ink film as the donor

substrate is moved together with the receiver surface after each pulse. In this letter, we demonstrate

that it is possible to do the LIFT printing of industrial grade silver paste using multiple pulses

on the same spot on the donor film due to the healing of the silver paste film. We modify the rheol-

ogy of the silver paste by adding a lower viscosity solvent and show that the change in material

rheology allows for printing in different regimes. Published by AIP Publishing.https://doi.org/10.1063/1.5060717

Laser-induced forward transfer (LIFT) is a technique

where a controlled amount of material is transferred from a

donor thin film to a receiving substrate by means of a laser

pulse.1 This printing technique is an additive manufacturing

technique2 that has been used to print various materials such

as electrodes for energy storage devices,3 DNA microar-

rays,4 layers of electronic devices,5 proteins,6 polymer/car-

bon nanotubes,7 biological materials with click chemistry,8

and light-emitting organic molecules.9,10 Also, silver micro-

and nanoparticle pastes have been used in LIFT applications.

Previously, structures such as metallic contacts11 and sol-

dered joints12 were fabricated with LIFT using silver paste.

In conventional LIFT applications such as those listed

above, the donor surface is coated with an ink composed of

the material of interest and a solvent. This donor surface

moves with the receiver substrate during the printing pro-

cess, which causes each laser pulse to interact with a fresh

ink region whose size depends on the laser spot size and

pulse energy [Fig. 1(a)]. In contrast to these more complex

ink layers, it is well known that a thin liquid film “heals” or

spreads inward to fill holes in a film. This phenomenon has

recently been investigated in detail for Newtonian liquid

films13,14 driven to close by surface tension, and the hole-

closing dynamics have been shown to be self-similar in

some physical limits. The “healing time” thealing for closing ahole in a Newtonian liquid film, with Hf� R0, is14

thealing ’lc

R40H3f

; (1)

where l is the liquid film viscosity, c is the surface tensionof the liquid-air interface, R0 is the hole radius, and Hf is thefilm thickness. In this letter, we demonstrate a similar capil-

lary healing phenomenon for viscoelastic silver paste films,

and we take advantage of this phenomenon to deposit mate-

rial by shooting the laser pulse at the same spot on the donor

surface. We show that by tuning the rheology of the silver

paste, it is possible to improve the quality of the deposits

obtained with this method.

The silver paste we use in this study is the commercial

Dupont PV17F microparticle paste. We use three different

thinner (DuPont 9450) concentrations to modify the rheology

of the paste: 0 wt. %, 15 wt. %, and 30 wt. %. The paste with

the specific thinner concentration is coated on a glass slide

using a blade coater. A frequency-tripled Nd:YVO4 laser

(Coherent AVIA, 20 ns) is used to generate a pulse with a

FIG. 1. Transfer of the silver paste with laser-induced forward transfer. (a)

The silver paste is coated on the transparent glass substrate and separated

from an acceptor surface by a distance d. After the transfer, surface tensionleads to closing of the hole and the thin film becomes flat again. (b) Time-

resolved images of an example of the compact transfer of the silver paste

without any thinner. The lighter blue background is ambient air. (c) The

deposited silver paste viewed under confocal microscopy. The individual sil-

ver microparticles can be seen in the image. (d) The compact transfer of the

silver paste with 15 wt. % thinner concentration. Particles are still visible

while they are not as packed together as the original paste presented in (c).a)Electronic mail: cbarnold@princecton.edu

0003-6951/2018/113(22)/221601/5/$30.00 Published by AIP Publishing.113, 221601-1

APPLIED PHYSICS LETTERS 113, 221601 (2018)

https://doi.org/10.1063/1.5060717https://doi.org/10.1063/1.5060717mailto:cbarnold@princecton.eduhttp://crossmark.crossref.org/dialog/?doi=10.1063/1.5060717&domain=pdf&date_stamp=2018-11-26

355-nm wavelength. The laser spot size used in this study is

20 lm. The LIFT process is captured using a time-resolvedimaging setup, which is the same as those used in previous

studies.15,16 The images of the transfer process from a side

view are obtained with a microscope (InfiniTube with a

Mitutoyo 10�Objective) attached to a CCD camera (SPOTInsight IN1820). Illumination is provided by strobing a 25-ns

plasma-discharge flash lamp (HSPS Nanolite) after a specified

time delay relative to the second laser pulse. The timing of the

laser and the strobe light is controlled with a digital delay gen-

erator (SRS DG535). The rheology measurements are per-

formed with a stress-controlled Anton Paar MCR 301 Physica

rheometer equipped with a roughened parallel plate geometry

with a 1.3 mm gap size to minimize the wall slip effects. The

ink thickness is measured using confocal microscopy.

LIFT of high viscosity silver pastes can take place in

different deposition regimes,18–21 where the ideal deposition

regime is the compact transfer (also known as concrete-dot

transfer18). An example for such a deposition is presented in

Figs. 1(b) and 1(c). Figure 1(b) shows the time-resolved

images of the process for a silver paste without addition of

the thinner, while Fig. 1(c) shows the deposited silver paste

under the confocal microscope. We also show the compact

transfer of the silver paste with 15 wt. % thinner in Fig. 1(d),

where the particles are still visible but they are less packed

compared to the 0 wt. % thinner case shown in Fig. 1(c). In

this study, we investigate how these three different rheologies

compare to achieve compact transfer with multiple pulses.

For the rheological characterization, we measured the

shear rate _c of the spindle as a function of the applied shearstress rs, and the results are reported in Fig. 2(a). The silverpaste exhibits a complicated non-Newtonian viscoelastic

behavior, which can be represented with the Herschel-

Bulkley model.22 We pick the Herschel-Bulkley model to

represent the rheology of our silver paste solutions because

this model has been shown to represent effectively the

behavior of many non-linear viscoelastic materials such as

solder paste,23 TiO2 paste,24 and other viscoplastic materials

used in 3D printing.25,26 According to this model, the stress

and the shear rate are related as rsð _cÞ ¼ ry þ k _cn, where ryis the yield stress, k is the consistency index, and n is theflow behavior index, where n< 1 for shear-thinning materi-als. Conventionally, the yield stress of the material is esti-

mated to be the value at which the material starts to flow

freely. For the stress values larger than the yield stress, small

increases in the shear stress leads to larger increases in the

shear rate, so the slope of the rsð _cÞ becomes smaller. Weestimate from Fig. 2(a) that the yield stress values are

approximately 930 Pa, 10 Pa, and 2 Pa, respectively, for the

0 wt. %, 15 wt. %, and 30 wt. % thinner added to the silver

paste.

We also note that the application of large shear stresses

by the rheometer on the 0 wt. % silver paste leads to rippled

outer edges as shown in Fig. 2(b). This edge fracture17 phe-

nomenon is due to elastic instabilities at the boundary of the

rotating plates where the sample gets infiltrated by air that

propagates as a fracture into the material, causing the mate-

rial to be expelled outside the plates.27

We examine the effect of rheology of the silver paste on

the laser-induced forward transfer. A critical parameter

regarding the printing process is the transfer threshold

energy, Eth, which is the energy value above which the mate-rial transfer takes place from the donor surface towards the

acceptor surface. Figure 3 shows the variation of the transfer

threshold fluence, Fth, as a function of the ink thickness, Hf,for the three thinner concentrations. The inset figure of Fig. 3

shows the variation of the threshold fluence at lower ink

thicknesses that are relevant to the practical printing applica-

tions. According to these results, it requires less energy to

transfer 0 wt. % thinner paste compared to 15 wt. % and

30 wt. % concentrations if the ink thickness is less than

15 lm. This result is counter-intuitive because it has beenpreviously shown that inks with lower yield stress16 and vis-

cosity28 can be printed with lower laser energy values.

However, for the higher ink thickness values, the result is

intuitive as it takes more energy to print as the thinner con-

centration decreases. We believe that the trends at lower

thickness values shown in Fig. 3 are because the silver

microparticles themselves absorb the laser energy. If the ink

thickness is lower, for 15 wt. % and 30 wt. % thinner concen-

trations, there are fewer silver particles to absorb the laser

energy, and more energy is needed to reach the transfer

threshold. On the other hand, if the ink thickness is larger,

the laser pulse is entirely absorbed by the particles regardless

FIG. 2. Parallel-plate rheology measurements of the silver pastes with the

addition of thinner. (a) The imposed shear stress rs from the rheometer isplotted against the measured shear rate _c. The results can be representedwith the Herschel-Bulkley model, which relates rs to _c as rsð _cÞ ¼ ry þ k _cn,where ry is the yield stress, k is the consistency index, and n is the flowbehavior index. We evaluate ry values to be approximately 930 Pa, 10 Pa,and 2 Pa for the 0 wt. %, 15 wt. %, and 30 wt. %, respectively. (b) The paste

with 0 wt. % thinner concentration exhibits the edge fracture viscoelastic

flow instability.

221601-2 Turkoz et al. Appl. Phys. Lett. 113, 221601 (2018)

of the thinner concentration, and the effects of viscosity and

yield stress are more apparent. For the larger ink thicknesses

in Fig. 3, it requires less energy to transfer inks with lower

yields stresses, as expected.

To investigate the effects of multiple pulses on the same

spot, we pulse the laser at a particular frequency. We present

an example set of time-resolved images of this process in

Fig. 4. These images are obtained 10 ls after each pulsesent to a Hf ¼ 35 lm film of 15 wt. % thinner concentration.The laser pulse energy is E¼ 11 lJ � Eth. The laser pulse

frequency is f¼ 5 Hz. With the ink thickness Hf ¼ 35 lm,laser spot size R0 ¼ 20 lm, ignoring the heat transfer effectsdue to the laser pulse and taking the measured viscosity,

which is evaluated using the data presented in Fig. 2(a) with

l ¼ drs=d _c as lð _c ¼ 1Þ � 235 Pa:s, and the surface tensionc � 30 mN/m, the formula (1), used with R0 � Hf, yields thea-ling � 0.03 s, which is shorter than the selected frequency.For about the first 15 pulses, the pulses deposit a larger

amount of ink, during which the laser pulses clean up the

excessive silver paste around the affected area. Then, the

deposition stabilizes and turns into a jet consisting of silver

particles attached to the filaments made of the solvent and

thinner. We report that this jet formation is stable and can

continue for more than 10 000 pulses.

We also note that, due to the inherent jet formation

mechanism of LIFT shots, the paste temperature increases

locally and the effective viscosity of the paste should

decrease accordingly.29 This set of experimental conditions

is a representative set that yields continuous material trans-

fer, and other conditions can be found for a given ink thick-

ness and thinner concentration.16 This result is notable

because each pulse sustainably creates a jet while the paste

in the vicinity of the affected spot is spreading inwards to fill

the hole again. However, as we observe from the time-

resolved images in Fig. 4, the formed jets consist of a few

drops containing silver microparticles connected to a thread

made of the thinner. The deposition as a result of this jetting

regime can be classified as the cluster transfer18 where the

resulting deposition consists of non-continuous clusters of

paste and is not desirable for real-life printing applications.

The deposition from the jets shown in Fig. 4 is not ideal.

However, it shows that there should be an optimized set of

parameters that yields dense enough jets to produce the com-

pact deposition of drops for multiple pulses at the same posi-

tion on a film. To study this effect, we perform a new

experiment. A stage was designed to allow for the acceptor

surface to move while the donor surface is fixed in position,

so the laser pulse is directed towards the same spot on the

donor surface. While the laser is pulsed with a frequency of

5 Hz, the acceptor stage is moved, so the amount of material

that is deposited after each pulse can be observed separately.

The ink thickness for these experiments is measured as

45 lm. The deposition patterns on the acceptor surfaceimaged using confocal microscopy are presented in Fig. 5. In

this figure, the numbers (i.e., n) on top of each image denotethe (nth) laser pulse resulting in the displayed deposition.Figure 5(a) shows the deposition pattern for E¼ 18 lJ > Eth.We note that larger energy laser pulses clean the ink from

the substrate during the first few pulses, and the remaining

pulses result in consecutively fewer deposits. Figure 5(b)

shows the deposition pattern for E¼ 13.5 lJ. The desiredcompact deposition takes place for the first three pulses with

this energy, while the pulses after that result in chaotic depo-

sitions similar to those observed in Fig. 5(a). Figure 5(c)

shows the deposition pattern for E¼ 9.5 lJ. The low energylaser pulses transfer some amount of ink reproducibly at least

for ten pulses. However, the deposition pattern is not the

desirable compact pattern for practical applications.

In this letter, we explore the capillary healing phenome-

non to perform LIFT, which potentially increases the

FIG. 4. Time-resolved images of the LIFT process with multiple pulses at

the same position on the film using a paste with 15 wt. % thinner concentra-

tion. The ink thickness is Hf ¼ 35 lm and the laser pulse energy is E¼ 11 lJ(F¼ 3.50 J/cm2). The pulse frequency is f¼ 5 Hz. The numbers at the bot-tom right of each image denotes the number of the laser pulse after which

the time-resolved image is obtained. All the images are taken at 10 ls afterthe laser pulse is shot. The lighter blue background in each image is the

ambient air. The laser spot size is 20 lm.

FIG. 3. Variation of transfer threshold fluence Fth as a function of the inkthickness Hf for the pastes with three different concentrations. The inset fig-ure shows the same variation for the smaller thickness values relevant to

LIFT. For smaller Hf, it requires less energy to transfer the 0 wt. % thinnerpaste compared to the 15 wt. % and 30 wt. %. For larger Hf values, the laserpulse is absorbed completely for all three concentrations, and it requires less

energy to transfer inks with increasing thinner concentrations. The maxi-

mum standard deviation of the laser pulse energy values is below 1.8%.

221601-3 Turkoz et al. Appl. Phys. Lett. 113, 221601 (2018)

efficiency of the LIFT process by allowing the same donor

film spot to be irradiated multiple times. We show [Fig. 5(b)]

that through fine-tuning the rheological properties of the sil-

ver paste, high-quality silver compact structures can be

obtained for more than three pulses at the same spot on the

donor surface. We note that the material transfer with fre-

quency f should take place as long as fthealing < 1, where thealingdepends on the rheology of the viscoelastic film to be

printed. Our results also indicate that with proper control of

the rheology, laser pulse energy, and laser repetition rate, it

is possible to control the effective thickness of the ink for the

LIFT experiments. A lower effective thickness can be

achieved by using high-energy pulses initially to remove the

excessive ink, and the following low-energy pulses can be

used to deposit materials. We expect our finding to enhance

applications of silver pastes for LIFT processes and lead to

novel printing approaches such as printing multiple pixels on

top of each other with higher precision.

We thank the NSF for support via Grant No. DMR-

1420541 through Princeton Materials Research Science and

Engineering Centers (MRSEC) (to CBA). M.M. and C.M.

acknowledge support from European Commission APPOLO

FP7-2013-NMP-ICT-FOF.609355 and Spanish MINECO

Projects SIMLASPV-MET (ENE2014-58454-R) and

CHENOC (ENE2016-78933-C4-4-R).

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