空气中和水下激光等离子体冲击波对硅表面形貌的影响

Silicon Surface Topography by Laser-Induced Plasma Shock Waves in the Air and under Water

利用Nd：YAG纳秒脉冲激光（波长为532nm）分别在空气中和水下对单晶硅进行单脉冲辐照，研究了在介质／硅片界面产生的激光等离子冲击波对硅表面形貌的影响。通过压电传感器对辐照过程中冲击波力学信号进行采集，利用扫描电子显微镜（SEM）对辐照后的硅片进行表征。结果表明：在相同能量强度下，水下辐照硅表面所产生的冲击波平均速度为空气中的1．5～2倍，力学强度约为空气中的10倍；水下硅表面的熔坑中心处出现了许多凸起的球状物以及下凹的孔洞，边缘处没有沉积物且具有波纹状结构，而空气中硅表面熔坑中心处较为光滑，边缘处具有一圈圈的沉积物。研究表面，在介质／硅片界面产生等离子冲击波所引起的热-力学效应是硅表面形貌形成的主要原因。与空气介质相比，在水下由于水的约束作用而引起更大的冲击波力学强度，以及由于水的存在而发生爆发式沸腾的热学现象共同导致了在水下和空气中硅表面形成了截然不同的形貌。

The morphologies of silicon surfaces are modified with the single Nd：YAG nanosecond laser pulse （wavelength 532 nm） in the air and water. The influence of laser induced plasma shock waves in the medium/silicon interface on silicon surface topography is studied. The shock wave mechanical signals are gathered by piezoelectric sensors and the morphology of silicon surfaces is observed by scanning electron microscopes （SEM）. It is found that at the same energy level, the average speed of the underwater shock wave generated by irradiating the silicon surface under water is 1.5-2 times higher than that in air, and the mechanical strength under water is about 10 times higher than that in air. By observing the silicon surface morphology, it is found that many raised bulbs and recessed holes appears at the center of the craters on silicon surface under water, with corrugated structure but no sediment at the edges. While the center of the craters on silicon surface in air are relatively smooth, with circles of sediment at the edges. Thermal-mechanical effect induced by plasma shock waves generated at the medium/silicon interface is the main reason of silicon surface topography formation. Under water, greater mechanical strength of the shock wave induced by restriction effect of water, and explosive boiling thermal phenomena result in completely different topography as compared with those in air.