Shadowgraphy investigation of laser-induced forward transfer: Front side and back side ablation of the triazene polymer sacrificial layer
Introduction
Laser-induced forward transfer (LIFT) is a powerful and versatile method for the direct-write deposition of various materials. A transparent substrate (donor) is coated with a film of a material and placed close to a receiver substrate onto which the material shall be transferred. A laser pulse is imaged from the back side onto the donor film and induces a removal of the material at the irradiated spot with subsequent redeposition onto the receiver substrate [1], [2], [3]. Whereas the LIFT technique is readily applied to deposit metal films [4], [5], [6], sensitive materials cannot withstand direct irradiation and ablation. To overcome this limitation a sacrificial layer has been introduced between the donor substrate and the film to be transferred. This sacrificial layer is made of an aryltriazene polymer, which absorbs the laser light and protects the sensitive top layer from photodegradation and overheating. Upon irradiation, the triazene is decomposed into gaseous fragments, generating enough pressure to eject the top layer [7], [8], [9], [10]. The modified LIFT technique has been succesfully applied to transfer organic light-emitting diode materials [11] as well as living cells [12], nanocrystal quantum dots [13] and metal [14].
Up to now, deposition was realised in close contact between the donor and receiver susbtrates but no satisfactory results were obtained if the substrates were separated by a gap. However, achieving transfer without having the substrates in contact would greatly facilitate alignment of the donor and receiver. For this reason, it is important to understand how the material is ablated and transferred when the substrates are separated by a gap. A suitable method for this study is lateral time-resolved shadowgraphy imaging [15], commonly used for imaging the ejection process in LIFT [16], [17], [18].
In this work, laser ablation and forward ejection of a single layer of an aryltriazene polymer was studied by time-resolved shadowgraphy. Front side and back side ablation were compared at different film thicknesses and fluences, and the occurence of a flyer was investigated.
Section snippets
Experimental
The triazene polymer (poly[oxy-1,4-phenylene(3-methyl-1-triazene-1,3-diyl)-1,6-hexanediyl(1-methyl-2-triazene-1,3-diyl)-1,4-phenylene]) was synthesized as described in [19] (compound TP-6a). Films were prepared on fused silica substrates by spin coating from polymer solutions in chlorobenzene and cyclohexanone (1:1, w/w).
The ablation experiments were performed with single pulses from a XeCl excimer laser (Compex, Lambda Physik, λ = 308 nm and τ = 30 ns). A square mask with an aperture of 2 mm was used
Front side ablation
Ablation from the front side has been studied first, because most reference data exist for this configuration and therefore allow comparison with literature. It is also the most simple configuration, since the volume of ablated material is directly accessible from the crater depth measurement. A sequence of pictures taken at 110 mJ/cm2 for a 500 nm thick triazene polymer film is shown in Fig. 1. The film surface is on the right, while the laser pulse comes from the left.
Two propagating features
Conclusion
Time-resolved shadowgraphy imaging shows that front side ablation of the triazene polymer occurs in a clean way and produces no visible fragments. For back side irradiation, a flyer of polymer is observed under certain conditions. The shape and properties of the flyer depend substantially on the optimum relation between the film thickness and the laser fluence, which can be expressed as the precentage of undecomposed layer thickness. If this fraction is too high, only partial ejection is
Acknowledgement
Financial support from the Swiss National Science Foundation is gratefully acknowledged.
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