Time-varying axial magnetic field is added to an inductively coupled plasma. Weak axial magnetic field of about 10-15 G can be obtained by a pair of Helmholtz coils attached to the chamber. This scheme has been applied to etch SiO2 and silylated photoresist, where processing pressures are generally below 7 Pa, and ions are major reaction species. In order to extend this concept to other etching conditions, this method is applied to photoresist ashing, where processing pressure is usually higher than 133 Pa, and downstream oxygen radicals are. important species. It is found that 60 Hz of the ac current gives the maximum ash rate and that ash rate is increased from 6.1 to 7.0 mum/min, and uniformity is improved from 8% to 4.5% over an 8 in. wafer. When aluminum baffle is placed between plasma source and reaction chambers, the ash uniformity improves further to 2.0% over a 300 mm wafer. The presence of axial magnetic field increases the plasma density and thus ash rate, and better uniformity comes from redistribution of ions and radicals during the off period in a periodic magnetic field.