High Quality Near-subwavelength Ripples on Si Induced by Femtosecond Pulse Train Output from Fabry-Perot Cavity (Invited)

Laser-Induced Periodic Surface Structures (LIPSS) can be widely used in different processing fields, involving various materials such as semiconductors, dielectrics, metals, polymers. They have great potential in micro-nano processing. LIPSS are divided into two categories:Near-Subwavelength Ripples (NSRs) with period. larger than lambda/2 and Deep-Subwavelength Ripples(DSRs) with period Lambda smaller than lambda/2, where. is the incident laser wavelength. LIPSS can improve the surface properties of materials and can be used for modulation of the surface wettability of materials;enhanced surface Raman scattering; surface coloring;birefringence and optical storage, etc. They can be widely applied in many fields such as data storage, industrial manufacturing, and biomedicine. The formation of LIPSS is a complex process. Within a few nanoseconds after the femtosecond laser irradiation on the material surface, a series of ultrafast processes such as carrier excitation, carrier heating, lattice heating, plasma ejection, and nanoparticle ejection occur, which greatly increases the difficulty of processing regular NSRs. Therefore, how to improve the quality of NSRs has been an important research direction for femtosecond laser processing. Pulse shaping can regulate the distribution of laser energy in the time domain, which can then regulate the ultrafast process of laser-matter interaction. In order to fabricate high-quality LIPSS, we built a Fabry-Perot(F-P) cavity femtosecond laser pulse train processing system to conduct femtosecond laser time-domain shaping. The central wavelength of the laser is 1 030 nm, and we output femtosecond laser pulse train with flexible sub-pulse intervals in the range of 1 similar to 300 ps to fabricate NSRs on the silicon surface. The laser scanning velocity is altered by moving the sample through a 3D electronically controlled translation stage. By adjusting the distance between beam splitter and zero-degree mirror, we changed the sub-pulse interval and investigated NSRs induced by pulse trains with different sub-pulse intervals. After the samples were processed, we tested the surface morphology of the samples using a Scanning Electron Microscope (SEM). The NSRs induced by a Gaussian laser were slightly curved and had rough edges. However, the NSRs induced by the shaped pulse with a sub-pulse interval of 100 ps were uniformly oriented with smooth and straight edges. The peak of the spectrum obtained by its Fourier transform was (0.992 +/- 0.008) mu m(-1), corresponding to the period Lambda =(1 008 +/- 8) nm. The laser fluence window of the NSRs induced by the shaped pulse with a sub-pulse interval of 100 ps was 0.08 J/cm(2), which was four times as large as the NSRs induced by the Gaussian laser pulse. As the sub-pulse interval increased, regular NSRs could also be induced, but the width of single ripples was not uniform and the ripples started to become curved with inconsistent orientation. The surface morphology and depth of the sample were measured using a white light interference confocal microscope. The depth of regular NSRs induced by Gaussian laser pulse was (22 +/- 3.2)nm, and the depth fluctuation reached 14.4%. In contrast, the depth of regular NSRs induced by the pulse train of 100 ps was (45.7 +/- 2.7)nm, and the depth fluctuation was only 5.9%. The depth of NSRs induced by shaping pulse train was about twice that of the original Gaussian laser pulse. Meanwhile, the depth fluctuation was greatly reduced and the ripples were more regular. We used the ImageJ plug-in OrientationJ to measure the Divergence of Structure Orientation Angle(DSOA) of NSRs induced by Gaussian laser pulse and F-P cavity shaping pulses, and then analyzed the local orientation. The DSOA of the NSRs induced by Gaussian laser pulse was 7.9 degrees, while the DSOA of the NSRs induced by pulse train of 100 ps was only 2.8 degrees. The straightness and regularity of NSRs induced by all pulse trains were better than those induced by Gaussian laser pulse in the experimental range, which indicated that the NSRs induced by pulse trains were straight and regular. To quantitatively calculate the line edge roughness of the NSRs, we performed edge detection on the NSRs induced by different conditions. We obtained the one-dimensional straight line of the sample-averaged boundary by leastsquares fitting and calculated the standard error of the line edge. The best NSRs induced by pulse train of 100 ps had an edge roughness of 3.9 nm, which could meet the standard of lithography process. In summary NSRs induced by the femtosecond laser pulse train is superior to the NSRs induced by Gaussian laser pulse. The flatness depth and edge roughness of NSRs are significantly improved. Femtosecond laser pulse train can improve the quality of NSRs. Laser pulse shaping is expected to overcome the thermal effects ejected particles and insufficient energy deposition during femtosecond laser processing and improve the processing accuracy.