Design and material contributions to second-harmonic nonlinearities in RF silicon integrated passive devices

Abstract

Second harmonic distortion in silicon integrated passive devices is sufficiently low to be of no concern in most applications. However, recent changes in frequency allocation for 4G cellular applications have raised concerns for interference with sensitive GPS signals. Previously, we reported a number of measurements on low-pass filters around 750MHz. These measurements suggested that harmonic distortion was mainly attributable to lateral surface fields in the circuit’s devices interacting with the substrate depletion layer. If this is the case, the nonlinearities in some cases could be reduced by design changes. We have performed further experiments and measurements aimed at improving our understanding of these issues and the relative importance of design versus substrate material. We have evaluated low pass filters with design variations aimed at reducing the interaction of the filters with the substrate. The designs were fabricated on substrates of different nominal resistivity, and with and without surface treatments aimed at reducing the so-called “parasitic surface conduction” effect. In substrate wafers that do not have surface passivation, harmonic distortion is generally high. In this case, we find that minor design variations may significantly reduce the second harmonic power. Furthermore, we observe in these devices that the harmonic power increases under conditions of illumination, suggesting that surface conduction plays a role. Importantly, however, in surface-passivated wafers we find that the harmonic power is significantly reduced and that design variations make less difference. Furthermore, we observe insensitivity to illumination, indicating that the photoconductive effect in these passivated samples is negligible. We have compared samples fabricated on different substrate resistivities. We find a very strong dependence. Comparison of samples fabricated on silicon substrates with 3500Ωcm resistivity, versus 1000Ωcm, shows roughly an order of magnitude reduction in second harmonic power. For 4G cellular applications in the 700MHz frequency band, substrate resistivity appears to be a critical consideration. As in other studies, we find that 2 nd -harmonic generation is closely correlated with losses in passive components and circuits. We have examined inductor quality factor, as well as insertion losses in transmission lines and filters, and find that higher losses are generally associated with higher harmonic power levels.