Formation of micropeak array on a silicon wafer

Marking on a silicon wafer with a small dot matrix has been performed using a diode-pumped second-harmonic generation (SHG) laser of yttrium aluminum garnet (YAG), liquid-crystal-display (LCD) mask, and projection optics. The image was obtained after laser irradiation through a patterned mask on the LCD. The each dot is a square with sides of 3.6 mu m, the pitch of each dot is 4.5 mu m and the height (not the depth) of each dot is approximately 0.5 mu m. The height of the dot strongly depends on the laser energy density. The dot topography is unique, with a central peak and peripheral depression. We have named this topography "micropeak." Regarding the shape of the micropeak, significant differences are observed from that of the textured Sombrero bump with a central peak. These differences are believed to result from the material of the substrate, i.e., silicon. We present a hypothesis of the micropeak formation mechanism: The density of silicon in the liquid state is greater than that in the solid state, which is a physical property. In the congealing process of molten silicon, a unique liquid-gas interface is formed, which first moves downward, and then upward, resulting in the micropeak.