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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