In the present work, we have studied the etching characteristics of Si {100} and Si{110} in modified low concentration TMAH solution by adding different concentrations of NH2OH. The etch rate of silicon and thermal oxide, and etched surface morphology, which are important parameters to be known in the fabrication of MEMS structures using silicon wet bulk micromachining, have been studied in modified TMAH solution. In addition, the effect of aging time of the etchant solution on the etching characteristics is investigated. © Springer Nature Switzerland AG 2019.