Enhancing on-chip performance of single-turn octagonal inductor at millimeter wave and Sub-THz frequencies through grounded guard ring optimization

Abstract

The performance dependence of a single-turn octagonal inductor on its guard ring spacing at millimeter-wave (mm-wave) and sub-THz frequencies is investigated in this paper. The typical range of inductance that is employed in RFIC circuit designs at these frequencies is in the order of a few tens to a few hundreds of picohenries (pH). This paper shows that for such inductors at mm-wave/sub-THz frequencies, the quality factor (Q) is affected by the spacing of the guard ring (Sg) around it. At certain frequencies the Q sharply drops. This behavior is not typically seen at lower GHz-range frequencies and hence the guard ring design has not been critically studied. The coupling between the inductor and the guard ring surrounding it is observed to have magnetic resonant behavior and hence results in sharp reduction in Q at certain frequencies. The observed phenomenon is correlated to the theoretical framework developed for understanding this phenomenon including model and necessary equations. The dependence of quality factor on the guard ring current is modeled. An empirical model is also proposed to identify the frequency at which the sharp reduction in Q occurs. The impact of Sg on the inductors is relatively less pronounced for large-sized inductors. The Inductance (L) and Self Resonant Frequency (fSR) are shown to remain unaffected with changes in guard ring spacing. Guidelines, along with an example, are provided for determining optimal guard ring spacing for inductors operating at millimeter-wave/sub-THz frequencies.