The formation mechanism of tungsten (W) atoms in tungsten etching has been investigated with the SF6/Ar helicon plasma. The etch rate of W films decreases with increasing flow rate of Ar in the SF6/Ar gas mixture while the density of tungsten atoms increases as measured by optical emission spectroscopy Tungsten films are rarely etched with pure Ar plasma. The density of tungsten atoms generated by a direct sputtering is much less than the one in SF6/Ar plasma. By comparing the etch rate with the relative intensity of W atom as a function of source power and bias power, it was found that tungsten atoms are mainly formed through the dissociation reaction of stable product, WFx (0 less than or equal to x less than or equal to 6), and further by electron impact. The loading effect in tungsten etchback can be suppressed by enhancing the redeposition of TiNyFz (y and z greater than or equal to 0) by-products by increasing the source power. The suppression mechanism of the loading effect is investigated by an in situ monitoring of optical emission of tungsten atoms generated during overetching in tungsten etchback.