High-frequency microinductors with amorphous magnetic ground planes

For the first time, inductors were fabricated on silicon using standard silicon processing tools. The 2.75 turn device exhibits a cutoff frequency of 3.3 GHz. The study incorporated ground planes of amorphous CoTaZr below and above the inductor. CoTaZr has a resistivity of ∼ 100 μΩ · cm, a 4π M_s of 15 kG, a permeability of ∼ 870 up to 1.4 GHz, and an H_c of 0.2 Oe (unpatterned film). These properties were monitored during the process and showed no signs of degradation, even after being exposed to process temperatures of 400°C. Octagonal devices with slotted ground planes had the best frequency response with a cutoff frequency of 3.3 GHz for inductors with 0.4-μm-thick ground planes. Maximum inductive enhancement (over air-core inductors) of 50%-60% has been measured for devices with 1 μm of CoTaZr underneath the inductor. Similarly, two layers of 0.4-μm CoTaZr exhibit increased inductance of 30%-40% with one ground plane and up to 150% with two ground planes.