Cross-talk suppression in SOI substrates

Cross-talk between the noisy digital circuitry and the highly sensitive analog circuitry of integrated “system-on-a-chip” circuits is a serious problem. Silicon-on-insulator (SOI) substrates can offer a solution for co-integration of such components as the buried oxide (BOX) layer provides a degree of isolation between adjacent circuits. However, at frequencies above a few hundred MHz, the BOX layer becomes transparent to ac signals and cross-talk occurs through the silicon handle wafer. Improvements in cross-talk suppression can be achieved through the use of high resistivity silicon (HRS) handle wafers. However, it has been shown that oxide charge can induce a mobile charge at the interface of the BOX and HRS handle wafer. A polycrystalline silicon layer under the BOX traps the induced charge so that the resistance at the surface of the HRS is not compromised. Importantly, cross-talk suppression with this silicon based SOI technology is shown for the first time to be as effective as that obtained in silicon-on-sapphire (SOS) control samples.