UC Berkeley

The photovoltaic semiconducting and transparent dielectric materials are of high interest in current industry. Femtosecond laser processing can be an effective technique to fabricate such materials since non-linear photochemical mechanisms predominantly occur. In this series of studies, femtosecond (fs) laser processing techniques that include laser drilling on Si wafer, laser scribing on CIGS thin film, laser ablation on Lithium Niobate (LN) crystal, and fabrication of 3D structures in fused silica were studied.

The fs laser drilling on Si wafer was performed to fabricate via holes for wrap-through PV devices. For reduction of the number of shots in fs laser drilling process, self-action of laser light in the air was initiated. To understand physical phenomena during laser drilling, scanning electron microscopy (SEM), emission, and shadowgraph images were studied. The result indicated the presence of two mechanisms that include fabrication by self-guided beam and wall-guided beam. Based on our study, we could fabricate ~16 micrometer circular-shaped via holes with ~200 laser pulses on 160-170 micrometer thick c- and mc-Si wafer.

For the fs laser scribing on ink jet printed CIGS thin film solar cell, the effect of various parameters that include pulse accumulation, wavelength, pulse energy, and overlapping were elucidated. In our processing regime, the effect of wavelength could be diminished due to compensation between beam size, pulse accumulation, energy fluence, and the absorption coefficient. On the other hand, for high PRF fs laser processing, pulse accumulation effect cannot be ignored, while it can be negligible in low PRF fs laser processing. The result indicated the presence of a critical energy fluence for initiating delamination of CIGS layer. To avoid delamination and fabricate fine isolation lines, the overlapping method can be applied. With this method, ~1 micrometer width isolation lines were fabricated.

The fs laser ablation on LN wafer was studied. The ablation thresholds and laser induced periodic surface structure (LIPSS) patterns according to the angle between laser polarization and crystal orientation were elucidated via SEM images. We could observe that both the ablation threshold and the LIPSS patterns were affected by the angle between laser polarization and crystal orientation. Additionally, we could fabricate ~45 nm dots on z-cut LN wafer surface along the x-axis.

Finally, 3D helical microchannel was fabricated in fused silica and utilized for protein mixing. The channel of ~50 micrometer diameter was fabricated by the femtosecond laser irradiation and chemical etching (FLICE) technique. For understanding of fluidic motions, numerical analysis was performed. The numerical analysis indicates the existence of an effective mixing condition due to compensation between residence time of the flow in the channel and the transverse flow patterns. Experiments supported the numerical analysis and we could achieve 90% mixing within ~400 micrometer distance from the confluence of two streams.