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RESEARCH PAPERS: Micro/Nanoscale Heat Transfer

# Damage-Free Low Temperature Pulsed Laser Printing of Gold Nanoinks On Polymers

[+] Author and Article Information
Jaewon Chung1

Laser Thermal Laboratory, Department of Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94720-1740cgrigoro@me.berkeley.edu

Seunghwan Ko

Laser Thermal Laboratory, Department of Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94720-1740

Costas P. Grigoropoulos2

Laser Thermal Laboratory, Department of Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94720-1740

Nicole R. Bieri, Cedric Dockendorf, Dimos Poulikakos

Laboratory of Thermodynamics in Emerging Technologies, Department of Mechanical and Process Engineering, ETH Zurich, CH-8092 Zurich, Switzerland

1

Present address: Department of Mechanical Engineering, Korea University, Seoul, Korea.

2

Corresponding author.

J. Heat Transfer 127(7), 724-732 (Jan 12, 2005) (9 pages) doi:10.1115/1.1924627 History: Received June 09, 2004; Revised January 12, 2005

## Abstract

In this study, pulsed laser based curing of a printed nanoink (nanoparticle ink) combined with moderate and controlled substrate heating was investigated to create microconductors at low enough temperatures appropriate for polymeric substrates. The present work relies on (1) the melting temperature depression of nanoparticles smaller than a critical size, (2) DOD (drop on demand) jettability of nanoparticle ink, and (3) control of the heat affected zone induced by pulsed laser heating. In the experiments, gold nanoparticles of $3–7nm$ diameter dissolved in toluene solvent were used as ink. This nanoink was printed on a polymeric substrate that was heated to evaporate the solvent during or after printing. The overall morphology of the gold microline was determined by the printing process and controlled by changing the substrate temperature during jetting. In addition, the printed line width of about $140μm$ at the room temperature decreased to $70–80μm$ when the substrate is heated at $90°C$. By employing a microsecond pulsed laser, the nanoparticles were melted and coalesced at low temperature to form a conductive microline which had just 3–4 times higher resistivity than the bulk value without damaging the temperature sensitive polymeric substrate. This gold film also survived after Scotch tape test. These are remarkable results, considering the fact that the melting temperature of bulk gold is $1064°C$ and the polymeric substrate can be thermally damaged at temperatures as low as $500°C$.

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## Figures

Figure 1

Schematic of nanoink printing and curing system

Figure 2

Bipolar voltage waveform applied to piezoelectric actuator

Figure 3

Configuration of jet head and aluminum heating block. The nozzle to substrate distance was 1.2mm and in addition to Tj and Ts, the temperature at the aluminum heating block and the fluid temperature at the reservoir were measured to use as boundary conditions for numerical simulations. This configuration was used for numerical simulations.

Figure 4

(a) Variation of droplet velocity at 1mm from the nozzle in response to changing voltage amplitude and temperature of jetting head and substrate. Gap distance between the substrate and nozzle is 1.2mm. Ts and Tj represent the temperature of the substrate and the jetting head. When only Ts is changed, Tj is set to 37°C. However, Tj increases to 41°C when Ts⩾107°C. Note that the nozzle temperature (Tn) is approximately equal to Tj when Ts=Tj, and that Tn is higher than Tj when Ts>Tj due to heat conduction from the substrate. Images (b), (c), (d) are obtained by multiple 2μs exposures with a 98μs delay when Ts=107°C and Tj=41°C.

Figure 5

Variation of droplet velocity at 1mm from the nozzle changing dwell time, td [(B) in Fig. 2]. Ts changes and Tj is set to 37°C. Inset picture corresponds to the case for Ts=107°C and Vamp=13V and similar satellite droplet was observed at the dwell time of 30μs at different substrate temperatures and voltages.

Figure 6

Numerical result when Ts=147°C and Tj=37°C. (a) Velocity profile with stream lines. (b) temperature profile.

Figure 7

Estimated nozzle temperature (Tn) from experiment and numerical simulation due to substrate heating at Tj=37°C.

Figure 8

Morphology of printed and cured nanoink on polyimide film. The same voltage waveform in Fig. 2 was used with Vamp=14V. Before AFM (atomic force microscopy) scans, all lines were cured at 200°C to evaporate toluene solvent. (a) Shows the morphology of a single droplet deposition obtained at Ts=25°C. In (bd), the translation speed and jetting frequency were 2mm∕s and 30Hz, respectively. Therefore, the distance between the droplet centers is 67μm. Ts for (b), (c), and (d) was 25, 50, and 90°C, respectively.

Figure 9

(a) rms (root mean square) roughness and resistance per length of printed lines cured at different temperatures. (b) Cross-sectional profiles of printed lines cured at different temperatures.

Figure 10

Pulsed laser cured printed line at different scanning gaps, dgap. The same voltage waveform in Fig. 2 was used and Vamp=15V. Nanoink was printed on a polyimide film at room temperature and cured by a substrate heating at 200°C. During laser curing, the film was heated at 200°C using the vacuum chuck in Fig. 1. Laser power, beam waist (1∕e2), frequency and translation speed are 0.85W, 30μm, 100Hz, and 0.5mm∕s, respectively. (a) RL versus scanning gap of laser beam. Inset pictures are reflection images corresponding to cases circled with a dotted line. (b) AFM image before pulsed laser irradiation. (c) AFM image after pulsed laser irradiation at tpulse=10μs and dgap=40μm. (d) AFM image after pulsed laser irradiation at tpulse=20μs and dgap=10μm. rms roughness is 2.7nm.

Figure 11

Pulsed laser cured printed line at different laser pulse durations. Printed lines in Fig. 1 were used. During laser curing, the polyimide film was heated at 200°C using the vacuum chuck in Fig. 1. Laser power, beam waist (1∕e2), frequency, translation speed and scanning gap are 0.85W, 30μm, 100Hz, 0.5mm∕s, and 10μm, respectively. Inset pictures in (a) are reflection images corresponding to cases circled with dotted line. (b) and (c) are AFM images of the areas circled with a white line in (a).

Figure 12

Transient temperature profiles in depth with 40μs heating time

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