Low temperature and low damage silicon oxidation technique is highly required in various ULSI processes. Especially for trench isolation of memory cell, the oxidation should be ion-assisted for directionality but with low damage. For this purpose, a method of negative ion assisted silicon oxidation was experimentally studied employing a microwave O2 plasma. A feasibility of high rate silicon oxidation with low damage at low temperature by negative ion was examined.In particular, effects of transformer coupled RF bias was deeply studied at various frequencies. The plasma produced in a 6 inch stainless-steel chamber was employed and the downstream plasma was mainly concerned because the negative ion was highly populated.Ion mass and energy analysis showed that the dominant negative ion was O- and its density was more than one order higher than O2-. The RF bias was applied with a transformer couple in combination with a DC voltage to irradiate the negative ion or positive one separately. The frequency of RF bias was varied from 40 kHz to 13.56 MHz. The oxidation depth showed a strong dependence on RF bias frequency, and its maximum was obtained at around 1 MHz, close to the negative ion plasma frequency.Thus it was concluded that the oxidation was due to the negative ions.The oxidation rate at the negative ion irradiation showed as high as 50 A/min at Vpp=65 V and it was five times higher than that at the positive ion irradiation of the same energy.This high rate of oxidation by the negative ion is due to its high chemical reactivity. An XPS analysis showed that in the film formed by the negative ion there was less suboxide compared with that by positive ion.