Selective laser sintering of Ag nanoparticles ink for applications in flexible electronics

Highlights

• Sintering of Ag nanoparticle ink with continuous wave and pulsed lasers.

• Simulation of the temperature distribution as a function of the laser fluence and pulse width.

• Electrical properties comparable to bulk Ag achieved on low T_g flexible substrates.

• The characteristics of three different pulse width regimes are compared and discussed.

Abstract

In this study the potential of three different laser sources (continuous wave (cw) or pulsed nanosecond (ns) and picosecond (ps)) operating at 532 and 1064 nm is investigated, as efficient tools for selective laser sintering of Ag nanoparticle (NP) ink layers on flexible substrates. Theoretical simulations indicate that picosecond (ps) laser pulses restrict the heat affected zone to a few micrometers only around the irradiated regions of the ink layer. For longer duration pulses or continuous wave operation, the laser beam profile and average power must be taken into account in order to avoid undesirably high temperatures reaching the substrate. These predictions were confirmed experimentally at 1064 nm and with ps pulses, efficient Ag ink sintering was achieved with no evidence of substrate degradation. On the contrary longer pulses may sinter Ag NPs in a broad wavelength range but inflict substrate damage over a wide laser fluence regime. Continuous wave mode may inflict damage to the low T_g polymers when applying laser power higher than 20 W, but it is the most flexible tool for sintering micro-patterns of homogeneous metallic features. The characteristics and the underlying mechanisms of each configuration are discussed.

Introduction

Electronic circuit fabrication on polymer substrates has been attracting significant interest towards the direction of low-cost or large-area electronics over the past few years [1], [2]. In this concept, selective laser sintering of deposited metal powder or nanoparticle (NP) inks has been explored as an alternative to conventional metal micro-patterning techniques [3], [4]. Standard integrated circuit (IC) fabrication processes are subject to limitations due to their multistep nature, high temperature processing, toxic waste byproducts and their high associated cost. Furthermore, the increasing size of electronic devices for applications in e.g. organic light emitting diode (OLED) displays or cellphones, poses great difficulty in adapting standard micro-fabrication processes, including lithographic patterning, at the proper scale. Noble metals (silver and gold) NP inks are highly stable during sintering and are in common use today [5]. The development of a metal NP ink based process has the dual advantage of (i) an inexpensive solution metal deposition approach without the need for costly vacuum deposition and (ii) a low-temperature process, which allows to use heat-sensitive and inexpensive polymers as the substrate. This is due to the thermodynamic size effect responsible for the drastic reduction of the nanomaterial melting temperature. For example, whilst bulk silver melting point is 960 °C the melting temperature of the material drops to around 100 °C when the size of the nanoparticle (NP) shrinks under 2 nm [6]. NP sintering is usually implemented with a convection oven. Therefore, alternative low temperature methods are needed in order to overcome associated drawbacks, such as the long sintering time and high thermal load. This is particularly important when heat sensitive substrates (polymers and paper) are involved making low-temperature sintering essential to avoid mechanical or thermal degradation of the substrate. Photonic sintering by either a flash lamp or laser [7], [8] can reduce significantly the input heat load thus protecting heat sensitive substrates. Flashlamp sintering has the advantage of large area exposure but lacks spatial resolution. It can therefore detrimentally affect device layers not intended for sintering. Laser sintering, on the contrary can be used selectively as a local heat source to enhance process resolution and further minimize thermal damage. This is achieved by careful selection of the laser wavelength tuning it to the strong absorption peak around the visible plasmon resonance wavelength (400–700 nm for most metals) [9] for efficient energy absorption. This is particularly important when polymer substrates with low glass transition temperature are used, which cannot be processed in a furnace or on a hot plate. Several examples of laser sintered metal NP patterns demonstrating much greater uniformity and higher resolution (down to 1–2 μm) than ink jet printed and sintered by a heater (∼100 μm) can be found in literature [10].

However, not all laser types are appropriate for efficient sintering. Many processing parameters must be taken into account before achieving metallic patterns with electrical characteristics comparable to those of bulk Ag; the input pulse energy, scanning speed, laser spot size and the beam profile may have significant impact to the sintering outcome. More importantly, both the wavelength and laser pulse duration, which relate to the optical and thermal penetration depth have to be investigated thoroughly, as heat diffusion to the substrate is critical. Low T_g flexible substrates such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN) or polydimethylsiloxane (PDMS) should be treated delicately and with caution in order to avoid permanent damage. In this paper, the sintering potential of three different laser sources, operating at cw, pulsed nanosecond and picosecond mode, is explored (synopsis in Table 1). An experimental study on the Ag NP sintering characteristics with different optical configuration parameters, relying on theoretical simulations, is presented. The results derived from each configuration are compared. Efficient sintering by ultra-short pulses leading to high electrical performance and minimum substrate thermal damage is proposed.