This work contributes to the understanding of anisotropic etching of silicon, for microsystems technology, by studying Si/Al selectivity during anisotropic etching of silicon in tetra-methyl ammonium hydroxide (TMAH). By under-etch experiments using a wagon-wheel mask pattern, Si/Al selectivity is studied in relation to trends in etch anisotropy, etched surface morphology, and variations of under-etch behavior with mask-edge angle, TMAH at 5 wt % is used. with or without the additives : dissolved silicon and ammonium persulfate, High Si/Al selectivity is accompanied by obvious changes in the roughness, flatness, and etch rate of the {100} cavity bottoms, by large changes in anisotropy as seen in under-etch rate curves, by lower ratio of {100} to {100} etch rates, and by more regular [101]-oriented steps on non-{111} cavity sidewalls. The conditions are consistent with a low rate of attack of [101]-directed periodic bond chains in the Si lattice,