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In this paper, we demonstrate an effective way of annealing in vacuum to enhance the -Factor of planar spiral inductors for RF applications. The impact on the enhancement of the Q has been quantitatively analyzed using an equivalent circuit model. For the on-top type spiral inductor on silicon substrates, the peak value of Q-Factor is increased by 32.4% and 45.9% after annealing at 250degC and 350degC, respectively. The annealed copper coil exhibits an observable reduction in cavity defects, leading to a significant -enhancement due to the improvement in conductivity. A decrease in substrate resistance for an annealed inductor is also observed, which has a negative influence on enhancement. However, the final increment of the Q-Factor by annealing proves that the contribution of the copper coil is dominant.