**5. Femtosecond laser induced forward transfer**

Laser Induced Forward Transfer (LIFT) represents a challenging technique to the conventional etching microfabrication techniques. It becomes particularly interesting when a very small quantity of material has to be deposited on a substrate. Many kinds of materials such as metallic films, semiconductors, polymers, or even biological material can be transferred (Thomas et al, 2007; Sanz et al, 2010; Colina et al, 2005). The material to be transferred is initially deposited in thin films on a transparent substrate named donor substrate, or "ribbon" (transparent at laser radiation used for LIFT process). Usually, but not necessarily, a very thin metallic layer is deposited as buffer between the donor substrate and the film to be transferred. The donor sample, is placed at a short distance, parallel to another acceptor substrate (virtually any material). The donor film is backward irradiated with a pulsed laser, like in figure 14.

Fig. 14. The schematic of LIFT principle.

The laser is focused on the donor thin film at the interface with the donor substrate. Then, a small amount of buffer material is ablated and transformed in gaseous faze. This gas expands pushing forward the rest of the material which is projected to the acceptor substrate. If the parameters are correctly chosen, the ejected material is deposited on the acceptor's surface. The role of the buffer layer is only to protect the material to be transferred and is used especially in the case of organic materials susceptible to be affected by a direct exposure to the laser beam. Otherwise, in absence of a buffer layer, the material itself can be vaporized at the interface with the donor substrate, the pressure of the created gas transferring a small quantity of material from a substrate to another.

In LIFT experiments some parameters, like distance *d* between the donor film and acceptor substrate, or laser fluency, have to be investigated in order to find the optimal processing conditions for the deposition of a certain material. In our LIFT experiments, we demonstrated the transfer of a polymer material, an Ormocer photoresist, using our laser processing workstation. The polymer layer was directly deposited by spin coating on a glass substrate, without any buffer layer. The distance between donor and acceptor was fixed at 15 m. Series of 5x5 pixels were created by single pulses, shot by shot. The laser source was the Clark MXR CPA-2101 laser, with 200 fs pulse duration and 775 nm wavelength, externally triggered for single shot experiments. The sample was translated from a pixel to another by a computer controlled translation stage. The distance between pixels was 50 m. The laser was focused to the donor layer by a 75 mm focusing lens with about 25 m focus spot diameter. The energy per pulse was varied from 2.5 to 7.5 J.

Optical images of the structures, as transferred to the acceptor substrate at different pulse energies, are shown in figure 15. The quality of the obtained structures strongly depends on the pulse energy. At the highest pulse energy used, non uniform droplets results, sparse on the donor surface. Decreasing the pulse energy the transferred droplets remain well defined.

Fig. 15. LIFT generated microstructures at different laser energy. Scale bar: 100 m.

The smallest size of the droplets obtained in these experimental conditions was about 2 m. Smaller structures, such as nanodroplets, can be also transferred (Banks et al., 2006) and even an entire microstructure or a microdevice could be deposited by LIFT (Piqué et al., 2006). This technique can be efficiently used as a microprinting method.
