Optimization of laser-induced forward transfer process of metal thin films
Introduction
Laser-induced forward transfer (LIFT) was first performed by Bohandy et al. in 1986 [1] and has been investigated by many researchers due to its unique process and the ability to fabricate microstructures. LIFT process consists of the sequence of the three events. First, laser beam is irradiated on metal thin film through a transparent support substrate and thin film is removed (Removal). Second, thin film is transferred to an acceptor substrate placed parallel to a support substrate (Transfer). Finally, thin film is hit on an acceptor substrate, and deposits there (Deposit). LIFT process can be performed in air and under room temperature without generating poisonous gases. Using mask projection method, optional patterns can be transferred on an acceptor substrate in optional size.
Taking a general view of LIFT investigations, its process can be categorized into two ways in a form of transfer. The one is the ablation process. Thin film is ablated by laser irradiation and transferred using different kinds of lasers. Pulse duration of laser beam is fs–ns order. The merit of this process is that various thin films, such as metals (Al [2], [3], Au [4], Cu [1], [4], [5], Cr [4], [5], [6], [7], Sn [6], V [6], Ti [6], [7], Pd [8], W [9], Ni [10], Ge/Se [11]), metal oxide (In2O3 [4]), semiconductor (Ge [6]), and superconductors (YBaCuO, BiSrCaCuO [12]) are available. But particles, which are transferred around a deposited material, affect the resolution of this material. The other is the solid phase process. Kántor et al. performed this process using long pulsed (100–1000 μs) lasers comparing the results of LIFT using short pulsed lasers [13], [14], [15]. Thin film is removed by the force of thermal expansion at laser irradiated zone and transferred in solid phase. Micrometer sized patterns of tungsten with clear contour was obtained as particles did not generate in laser irradiation. But it is considered that thin films with low melting temperature are not available because solid phase transfer is impossible.
On the other hand, in-process monitoring and calculations have done in order to investigate the mechanism of LIFT process as well as fabrication of deposited material. Bullock and coworkers photographed images of the Al vapor plume using shadowgraph and interferometer system [16], [17] and revealed that the damage of a support substrate was occurred by laser irradiation. This damage limited the transmitting fluence and affected the edge velocity of the plume. Nakata and Okada photographed the images of Au particles using two-dimensional laser-induced fluorescence (2D-LIF) method [18]. It was revealed that the velocity of atoms exceeds 2 km/s and the velocity of emissive particles was about 100 m/s. Adrian et al. [19] and Baseman and coworkers [20], [21] calculated the temperature distribution in thin film during laser heating using finite difference and finite element method, respectively. They suggested that thin film was heated and vaporized by laser beam at the interface between thin film and a support substrate, and the pressure reached a value enough for thin film to be removed and propelled thin film from a support substrate.
In this paper, the optimization of the ablation process in LIFT by experiments, that is, to get deposited materials with clear contour for various thin films is reported. And we evaluated the resolution of deposited materials, which were obtained changing the size of laser spot, as this evaluation is needed for fabricating of microstructures.
Section snippets
Experiments
LIFT set-up is shown in Fig. 1. Ni thin film was deposited on fused silica substrates of 2.3 mm thickness using ion sputtering method. Film thickness t (before LIFT process) is 180, 230 and 500 nm. The film was irradiated by a single pulse of KrF excimer laser (wavelength: 248 nm, pulse width: 30 ns) with flat-top profile using mask projection method. The removed thin film was deposited on Si wafers as acceptor substrates. Laser fluence and distance L, which is the distance between thin film and an
The optimization of the ablation process in LIFT
Fig. 2a shows the optical microscope image of the deposited Ni. The white square in this figure shows the size of laser-irradiated zone. The size of deposited nickel is equal to that of laser-irradiated zone, and droplet spread outward. In order to evaluate the resolution of deposited material, we defined distance d1 as the distance between the edge of the laser irradiated zone and the outer edge of droplets. In our recent work [10], it is revealed that making distance L shorter, distance d1
Conclusions
The optimization of the ablation process in LIFT was achieved and the dependence in the laser spot size on the resolution of a deposited material was revealed. Photographing the behavior of the plume, it was indicated that particles in the plume were transferred around a deposited material and worsened the resolution of the material in the conventional ablation process in LIFT. In the ablation process, deposited materials with clear contour were obtained under the condition that thin film was
Acknowledgements
The authors thank Mr. Y. Tsukuda, Sanwa Kenma Co. Ltd., for his support of preparation of metal thin films.
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