Integration of microwave tunable (Ba,Sr)TiO3 based thin films with high-resistivity silicon substrates

In this study, we report on Si integration of Ba_0.6Sr_0.4TiO₃ (BST) thin film based microwave tunable devices by use of TiO₂ films as microwave buffer layer between BST and Si substrates. TiO₂ buffer layer were grown by Atomic Layer Deposition (ALD) onto Si substrates followed by Pulsed Laser Deposition (PLD) of BST thin films onto the TiO₂ buffer layer. The interdigital capacitor (IDC) fabricated on BST films grown on TiO₂/high resistivity Si substrates showed much enhanced tunability value of 33.2% while retaining an appropriate Q factor, as compared to 21% value obtained with BST films grown on MgO single crystal substrates. The microwave phase shifters were fabricated on BST thin films to investigate the potential feasibility of integrating BST films as microwave tunable devices. The phase shifter fabricated on BST films grown on TiO₂/Si substrate showed better figure of merit (FOM) of 30.7°/dB, as compared to 12.1°/dB of BST/MgO structure. ALD grown TiO₂ buffer layers enable successful integration of BST based microwave tunable devices onto high resistivity Si wafers.