Measurement and modeling of coupling effects of CMOS on-chip coplanar inductors

The coupling effects between two adjacent coplanar spiral inductors are characterized for two cases, one where no guard structure is used and one where simple guard-rings are used. In addition, the effect of guard-rings is evaluated at different distances (190 μm to 1090 μm) between inductors. Measuring low levels of crosstalk is difficult, and in this context the effect of the test fixture itself is evaluated. Return current paths are found to have significant influence on low frequency results. In addition, based on laser cutting of test fixtures, a surrounding ground-ring is found to increase the crosstalk level. With the effect of ground-ring coupling removed, the use of simple guard-rings is shown to improve isolation by approximately 10-15 dB for closely spaced adjacent inductors. At larger distances, the gain from having a guard-ring reduces and eventually reduces to zero at a distance of 1000 μm. For closely spaced devices, a doubling of distance is found to provide an additional 20 dB attenuation of crosstalk. An extended model, including mutual inductive coupling and direct capacitive coupling, is shown to provide an accurate fit.