We adopted SiO2 film by means of plasma enhanced atomic layer deposition (PEALD) as the first gate insulator of top gate IGZO TFT. TFT post-annealed at 300°C shows mobility, Vth, and S.S of 39.9 cm2/V.s, -1V, and 0.32 V/dec., respectively. We investigated the effect of H in the gate insulator during the annealing with the comparison of bottom gate coplanar TFT with ALD grown alumina film as the gate insulator.