Trap Rich Layer Physics for RF Silicon Substrates
Trap-rich polysilicon layers beneath buried oxide pin interface carriers to eliminate parasitic conduction and lower sub-6 GHz harmonic generation.
Fixed Oxide Charge Density
Meaning
Quantitative measure of the immobile ionized charges located near the interface between silicon and silicon dioxide. Interface quality for fixed oxide charge density identifies the number of charges per unit area that remain trapped within the transition layer of a dielectric film. This value influences the threshold voltage of transistors and the overall stability of the semiconductor device.
It is measured after the oxidation process and remains constant regardless of the applied gate bias.
Fabrication Variable
Chemical purity and oxidation temperature directly influence the number of trapped charges in the oxide layer. Reducing fixed oxide charge density requires precise control over the cooling rate and the ambient gas composition during the final stages of wafer processing. High concentrations of these charges shift the electrical characteristics of the device away from the design targets.
Annealing in a nitrogen or hydrogen atmosphere helps neutralize broken bonds at the atomic interface.
Device Influence
Threshold voltage shifts occur when these immobile charges create a parasitic electric field. Excessive fixed oxide charge density leads to leakage currents in the off state of a transistor.
Metrology Standard
Capacitance-voltage measurements provide the primary data used to calculate this density value. During the qualification of a new oxide process, the fixed oxide charge density is extracted from the flat-band voltage shift observed in a test capacitor. Variations in this metric across a wafer indicate non-uniformity in the oxidation furnace or contamination in the processing environment.
Low density values are essential for maintaining the reliability of sensitive analog components in radio frequency modules.