Laser-Induced Forward Transfer of Organic LED Building Blocks Studied by Time-Resolved Shadowgraphy

Abstract

The patterned deposition of thin films is essential for many technological applications. One promising material deposition technique is laser-induced forward transfer (LIFT), where a thin layer coated on a transparent substrate is ablated by a laser pulse passing through the substrate. The ablated material is collected on a nearby receiver substrate in a pattern defined by the laser. The technique can be applied to heat and light sensitive materials, provided that they are not directly irradiated by the laser pulse. For this application, a sacrificial layer is introduced between the substrate and the transfer layer so that the laser energy is converted into mechanical propulsion while protecting the sensitive layer from radiation. In this work, the application of a triazene polymer as a sacrificial layer for LIFT has been studied with the final goal of transferring organic light-emitting diode (OLED) pixels. Donor films made of a stack of triazene polymer, metal, and optionally an electroluminescent polymer were irradiated from the back side by a pulsed XeCl excimer laser (308 nm, 30 ns), and the ablation process was imaged by lateral time-resolved shadowgraphy. Back-side ablation of a triazene/metal film produced the ejection of a metal flyer, whose stability in flight can be improved when the flyer contained a layer of undecomposed triazene polymer. The stabilization was provided by the composite structure of the metal/polymer film. A receiver substrate was placed at distances of 0.5 and 1 mm to collect the pixel. However, the transfer across the gap revealed an effect that complicates the system. The shock wave created by ablation was reflected from the receiver substrate and may cause the destruction of the flyer. Transfer studies in a vacuum showed that the shock wave can be eliminated but that the flyer speed increased substantially, leading to the disintegration of the flyer upon impact with the receiver substrate. The ejection of a flyer from a triazene/metal/luminescent polymer film, selected as a building block of an OLED pixel, shows an increased stability of the flyer as well, thus confirming the strengthening effect of a polymer layer on the metal flyer. Using this system, the transfer in air of incomplete pixels showed promise, opening the way to patterned deposition without contact between the substrates. † Part of the "Barbara J. Garrison Festschrift".

Ejection Regimes in Picosecond Laser-Induced Forward Transfer of Metals

“…Since the shock waves do not seem to influence the ejection regime or velocity, and detailed visualization requires a different experimental setup, we refer to Refs. [51,52] for a detailed discussion of shock waves.…”

“…The ejection time scale is estimated to be only τ ∼ V=L ∼ 100 ns, assuming a velocity V ≈ 100 m=s and a length scale L ≈ 10 μm [18], resulting in challenging visualization conditions. So far, time-resolved visualization has been achieved for relatively thick liquid-film [19][20][21][22][23] and solid-phase [24][25][26] or pastetransfer [27,28] processes. Observations of LIFT processing of Au [29], Ni [30], Al [31], and Cr [32] do not provide sufficient spatial resolutions to track the process in detail.…”

Laser-induced forward transfer of pure metals

“…Time-resolved imaging is a suitable method to visualize the ejection process and to gain further insight into the ejection process during LIFT. However, due to the challenging visualization conditions, time-resolved visualization has only been achieved for relatively thick liquid-film [21][22][23][24][25] and solid phase [26][27][28] or paste-transfer [29,30] processes. Attempts to visualize LIFT-processing of Au [31], Ni [32], Al [33], and Cr [34] do not provide sufficient spatial resolution to track the process in detail.…”

“…In the following years several alternative LIFT based processes were developed. These alternative processes mainly focus on reducing the droplet size [61], but also on transferring more complex, often more sensitive materials like biological materials or complete devices like micro-electronics [26]. Over the years, the flexibility of the LIFT process led to a growing interest of the research community on this topic, as indicated by figure 2.1.…”

“…These shock waves were observed only occasionally in the images, but were faint. Since the shock waves do not seem to influence the ejection regime nor the velocity of the ejections, and detailed visualization requires a different experimental setup, the reader is referred to literature [26,102] for a detailed discussion of shock waves.…”