The etch rate and etch profile of Si was investigated when Ar was added to an SF6 plasma in the etch step of the Bosch process. A Si substrate was etched with the Bosch process using SF6 and SF6/Ar plasmas, respectively, in the etch step to analyze the effects of Ar addition on the etch characteristics of Si. When the Ar flow rate in the SF6 plasma was increased, the etch rate of the Si substrate increased, had a maximum at 20% of the Ar flow rate, and then decreased. This was because the addition of Ar to the SF6 plasma in the etch step of the Bosch process resulted in the bombardment of Ar ions on the Si substrate. This enhanced the chemical reactions (thus etch rates) between F radicals and Si as well as led to sputtering of Si particles. Consequently, the etch rate was higher more than 10% and the etch profile was more anisotropic when the Si substrate was etched with the Bosch process using a SF6/Ar (20% of Ar flow rate) plasma during the etch step. This work revealed a feasibility to improve the etch rate and anisotropic etch profile of Si performed with the Bosch process.