Characterization of ferroelectric BST MIM capacitors up to 65 GHz for a compact phase shifter at 60 GHz

The voltage- and frequency-dependent material properties of (Ba,Sr)TiO₃ (BST) thin film have been extracted up to 65 GHz. To this aim, a set of MIM capacitors has been characterized by means of a lumped elements equivalent circuit validated by fitting on de-embedded experimental data. At 60 GHz the tunability is 80% (0-25 V bias), and the loss tangent is better than 0.16 (value at 0 V), demonstrating that the BST is a promising candidate to operate at millimeter frequencies. The results have been exploited in the design of the first tunable phase shifter based on MIM BST capacitors working at 60 GHz. The measured phase shift and the FoM at 60 GHz are, respectively, 135° and 23°/dB, when passing from 0 V to 30 V, and 150° and 22°/dB, when passing from 0 V to 90 V. The size is very compact (1.5 × 0.8 mm², i.e., area/λ₀² <; 0.05). The insertion loss is between -1.3 and -5.9 dB, and the return loss is better than -12 dB. The achieved performances are superior to the state-of-the-art.