Ring Oscillator Based Test Structure for NBTI Analysis

We have developed a new NBTI test structure comprising differential pairs of ring oscillators with stages of various circuit types. For stages consisting of inverters driving p-FET passgates, the gates of which are set at an adjustable DC potential, this structure allows high resolution absolute measurement of the average Vt shift of a large number (~ 100) product representative p-FETs in response to very short as well as traditional long duration pure NBTI AC or DC voltage/temperature stresses.