Moisture Effects on MOS Devices

We have exposed MOS gate and field oxide transistors to moisture treatments for up to three weeks. These treatmentss were performed with all pins grounded at 130 °C with 85% humidity. Three different kinds of effects on MOS radiation response and/or low frequency noise were observed with moisture treatment at elevated temperature: (1) irradiated field oxide structures showed a significant decrease in low frequency noise, to levels well below pre-irradiation values [1]. This is attributed to the passivation of Si dangling bonds at or near the Si/SiO2 interface [2],[3]. (2) pMOS transistors showed up to an order of magnitude increase in pre-irradiation 1/f noise and a significant increase in post-irradiation oxideand interface-trap charge densities [4]. The increase in charge trapping and noise before and after irradiation is illustrated in Fig. 1, and attributed to moisture diffusion into and defect creation within MOS gate oxides. (3) nMOS transistors show much less sensitivity to moisture exposure than pMOS transistors [4], before and after irradiation. This is illustrated in Fig. 2 for devices from the same process lot and wafer as the devices of Fig. 1. Similar results were observed for nMOS and pMOS transistors from more than five different process lots [4], all of which included full CMOS processing, and included P-glass passivation layers. We attribute the differences in nMOS and pMOS sensitivity to these moisture treatments to (1) the enhancement of water diffusion in SiO2 by B atoms in the field oxide regions of the pMOS transistors, and (2) the retardation of moisture diffusion by P atoms in the field oxides of the nMOS devices [5],[6]. Whether defect creation or passivation dominates the response of a particular structure exposed to water is determined by the densities of defects and/or defect precursors prior to moisture exposure, and the detailed moisture exposure conditions, as we will discuss further in the presentation. We will also discuss the implications of these results for MOS noise models, as well as their implications for MOS reliability and radiation response.