Plasma diagnostics and Si etching were carried out in 13.56 MHz rf CF4 plasma mixed with He. In the plasma diagnostics, density of ion flux reaching electrode, and both ion sheath thickness and de self-bias voltage at powered electrode were measured. A small amount of He admixture in the CF4 plasma scarcely affects the plasma characteristics, but those greatly change when a few percent of CF4 is added into a He plasma : the ion sheath thickness greatly decreases and the self-bias voltage increases. This remarkable change was considered to be due to the CF4/He discharge becoming electronegative. The He admixture can produce additional CFn+ ions by deexcitation processes with both excited He* atoms and He+ ions, which affect considerably the Si etching rate and etching anisotropy in plasma etching process.