Etching Shapes the Topography of Silicon Wafers: Lattice-Strain Enhanced Chemical Reactivity of Silicon for Efficient Solar Cells

Multiwire sawing of silicon (Si) bricks is the state-of-the-art technology to produce multicrystalline Si solar wafers. The massive indentation of the abrasive Si carbide or diamond particles used leads to a heavily mechanically damaged layer on the wafer surface. Etching the surface layer using typical HF HNO3-H2SiF6 acid mixtures reveals an unevenly distributed etch attack with etch rates several times higher than known for bulk Si etching. The present study follows the hypothesis that lattice strain, introduced by the sawing process, leads to an increase of the etch rate and determines the topography of the etched wafer, the so-called texture. Scratches were introduced into single crystalline Si surfaces in model experiments, and the magnitude and local distribution of lattice strain were extracted from confocal Raman microscopy measurements. The essential parameter used to describe the local reactivity of Si is the local etch rate, which was derived by confocal microscopy from the local height before and after etching. It was found that the reactivity of Si increases linearly with the magnitude of lattice strain. An increase in tensile strain raises the reactivity of Si significantly higher than an increase of compressive strain. The second decisive parameter is the reactivity of the etch mixture that correlates with the total concentration of the acid mixtures. Diluted acid mixtures with a low reactivity attack only the highest strained Si, whereas more concentrated and, therefore, more reactive acid mixtures can attack even slightly strained Si. Side effects, such as the behavior of amorphous or nanocrystalline Si and the generation of highly reactive intermediary species while etching, are discussed. The presence of unevenly distributed lattice strain of different magnitude and the resulting unevenly distributed reactivity of Si explain the features of a heterogeneous etch attack observed and the resulting topography of the etched wafer surface.