Silicon Substrate Surface Conduction and Dielectric Passivation Fundamentals
Trap-rich polysilicon passivation pins interface charges on high-resistivity silicon, suppressing surface conduction and harmonic spur spikes across RF switches.
Fixed Oxide Charge
Meaning
Immobile electrical charge located in a silicon dioxide layer near the silicon interface arises from incomplete oxidation and structural defects. This fixed oxide charge remains stable under normal electric fields and temperature variations. Its presence primarily alters the threshold voltage of metal-oxide-semiconductor transistors.
Physical Location
Structural mismatch occurs within the transition region between the bulk silicon substrate and the grown dielectric. This fixed oxide charge is located within a few nanometers of the interface, where excess silicon ions remain unbonded. Oxygen deficiency during the final stages of dielectric growth contributes directly to this defect state.
Device Influence
The electrostatic field generated by these charges causes a permanent shift in the flatband voltage. When fixed oxide charge density is high, the transistor threshold voltage shifts toward more negative values, which can increase leakage currents in n-channel devices. Controlling this parameter is necessary to maintain consistent operating margins across production lots.
Fabrication Control
High-temperature annealing in a nitrogen or argon ambient reduces the density of these defects by completing the oxidation reaction. The density of the fixed oxide charge is sensitive to the crystal orientation of the wafer, with the (111) surface exhibiting higher levels than the (100) surface. Process engineers monitor this parameter through capacitance-voltage sweeps on test wafers.