Voltage-Tunable Parallel-Plate Capacitors Fabricated on Low-Loss MBE-Grown BST

This paper describes the fabrication and measurement of tunable, lumped-element parallel-plate capacitors (PPC). The devices utilize electric-field tunable Ba
_x
Sr
_(1-x)
TiO
₃
(BST) thin films grown by hybrid molecular beam epitaxy (MBE) on SrTiO
₃
(STO) substrates. A high-quality interface was achieved by hot-sputtering epitaxial Pt top electrodes after an in-situ anneal in O
₂
. The Pt top electrode and the BST mesa were etched by two separate ion milling steps while the Pt metallization was thickened with Ti/Au to facilitate probing. The fabricated devices consisted of one-port PPCs which were directly probed by ground-signal-ground (GSG) RF probes. The devices were biased using a bias tee on the measurement port while wideband RF scattering parameters were measured from 100 MHz to 10 GHz. The PPCs exhibit a high quality factor of 720 at 100 MHz which drops to 120 at 1 GHz due to the large device area and parasitic electrode resistance. This parasitic resistance was exacerbated by measuring with relatively wide-pitch probes. The high quality factor combined with 2.2:1 tunability yields an exceptionally high commutation quality factor CQF of 750,000 at 100 MHz, 2000 at 1 GHz, and 200 at 10 GHz. These results are the first RF results of MBE-grown PPCs and demonstrate the extremely high quality of BST grown by hybrid MBE.