Enhancement periodic regularity of surface nano ripple structures on Si wafer using a square shaped flat-top beam femtosecond NIR laser

We report a new enhanced periodic regularity of surface nano ripple structures on a Si wafer using a square shaped flat-top beam femtosecond (190 fs) laser (λ = 1,030 nm). This periodic nanosized structure is called laser-induced periodic surface structure (LIPSS). Mechanical slit and tube lens were used to create a square shaped flat-top beam profile from the Gaussian beam profile of the femtosecond laser. Enhanced periodic regularity of the LIPSS was obtained by applying 100 pulses of flat-top beam irradiation slightly above the fluence of ablation threshold. The morphologies of the fabricated nanosized ripple structures on the Si wafer were analyzed using optical microscopy and atomic force microscopy (AFM). The periodic differences of the LIPSS obtained using the flat-top beam and Gaussian beam were approximately 60 and 400 nm, respectively, between the central and peripheral regions. It indicated that 6.7 times smaller periodic difference in LIPSS fabricated by flat-top beam. The periodic distribution widths of the LIPSS obtained using the flat-top beam and Gaussian beam were approximately 120 and 290 nm, respectively, in the fabricated LIPSS area. It indicated that 2.4 times narrower periodic distribution width in LIPSS fabricated by flat-top beam. Thus, the LIPSS produced using a flat-top beam has an enhanced periodic regularity than that of the LIPSS produced using a Gaussian beam. Moreover, the uniform periodic distribution of the LIPSS created using a flat-top beam induces a uniform electromagnetic field, which is called a hot spot. Owing to this characteristic of uniform periodic LIPSS, the efficiency of applications using hot spot is expected to improve. In addition, uniform LIPSSs with square and rectangular shaped types were demonstrated on Si wafers controlling open width of the mechanical slit and varying of the x-y axes mechanical stage.