Low-loss inductors built on PECVD intrinsic amorphous silicon for RF integrated circuits

The quality factor (Q) of inductors on silicon (Si) is limited by the series resistance of the metal at low frequency and by the substrate resistivity at high frequency. Oxide is generally used to isolate the useful signal of the inductor from the lossy substrate. However, stoichiometric silica (SiO2) is processed at a high temperature, which eliminates the possibility of post-CMOS integration. By contrast, plasma enhanced chemical vapour deposition (PECVD) amorphous Si can be deposited at a low temperature and is easily integrated with most Si-based processes. Intrinsic amorphous hydrogenated silicon (i-a-Si:H) also displays low conductivity. In this work, i-a-Si:H deposited at a low temperature (250'C) is used in a novel approach as the isolation material for planar inductors on Si for RF integrated circuits. An improvementof more than 50% in Q is measured when 1.5 µm i-a-Si:H film is deposited on the Si substrate prior to fabrication of the inductor. This result demonstrates the influence of i-a-Si:H film on the RF performance of an inductor. Intrinsic a-Si:H is shown to be a promising material for the isolation of RF devices on low-resistivity Si.