Measurement and modeling of high-speed interconnect-limited digital ring oscillators: The effect of dielectric anisotropy

IC interconnect characterization is growing in importance as devices become faster and smaller. Accurate numerical extraction of 3D interconnect capacitance is essential for achieving design targets in the multi-GHz digital regime. An AlGaAs-GaAs heterojunction-bipolar-transistor test chip was fabricated. The chip used 3-level metal with anisotropic polyimide interlevel dielectrics. Full differential current-mode logic circuit technology was employed. The chip contained a variety of interconnect capacitor structures (parallel plate, finger, crossover) and interconnect-limited ring-oscillator circuits. Capacitance and oscillator frequency measurements were performed to compare with CAD-tool predictions. Good agreement with measurements was achieved with an independently obtained 25% uniaxial polyimide dielectric anisotropy. Measured and predicted results for the capacitor test structures generally agreed to within 2%. Predicted ring-oscillator periods were within 4% of measurement.