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Number 3169 Application Note Making Ring Oscillator Measurements Series with the Model S530 Parametric Test System's Frequency Measurement Option Introduction (a) Vdd In the world of CMOS wafer parametric testing, the ring oscillator is one of the more important test structures because its test data helps confirm that logic gates are meeting their speed design criteria. This application note discusses techniques for testing these devices using Keithley's Model S530 Parametric Vosc Test System. Background A growing number of semiconductor fabs are incorporating ring (b) oscillators into their overall process control monitoring test structures. Frequency measurements on ring oscillator structures are used to determine gate propagation delay, one of the critical parameters that determines how quickly a digital circuit can operate. Every logic gate has input capacitance, so no device can Figure 1. Schematic (a) and block diagram (b) representation of a CMOS ring switch instantaneously because the input capacitance limits the oscillator (without trigger or buffer stage). speed at which a gate can switch. However, this gate propagation delay is too short for most test equipment to measure directly, a D-type flip-flop by a factor of 256 (or as high as 1024 for so test systems measure oscillation frequency instead and processes 0.25