A VCO test structure for characterizing AC hot-carrier degradation

A test structure applying a voltage controlled oscillator (VCO) has been evaluated for investigating AC hot-carrier degradation. Degradation of output and inner waveforms and photon emission have been investigated by using an oscilloscope, an electron beam tester, a photon emission analyzer and a reliability simulator. Amplitude decrease of the inner waveforms due to channel resistance increase of the NMOSFETs in the VCO ring oscillator was observed and simulated as well as oscillation frequency decrease. It is made clear that a large stress was applied to the NMOSFET at the first CMOS inverter of the output buffer, which was caused by gate voltages given by a VCO inner waveform swinging around a large stressing point