Design of VT-Sensitive Ring Oscillators for Monitoring Gate-TDDB Environmental Stress

This paper discusses the design methodology of a voltage and temperature-sensitive ring oscillator (VT_RO), whose frequency has a similar dependence on voltages and temperatures as that of worn-out stress strength of a gate time-dependent dielectric breakdown (gate-TDDB). The VT_RO is used to monitor the accumulated environmental stress of gate-TDDB in reliability-oriented products, such as automotive microcontroller units. Therein, the VT_RO drives the on-chip stress counters during the product operations over the years. Three types of VT_ROs have been designed and compared, which have been fabricated by using 28 nm HKMG test chips; multi-V
_th
VT_RO, single-Vth VT_RO; and combinational use of both a voltage-sensitive ring oscillator and a temperature-sensitive ring oscillator. The measured frequencies of the three types of ring oscillators are in good agreement with the target model of power-law V dependency along with an exponential inverse T dependency. The single-Vth VT_RO seems most promising in terms of product applicability, design difficulty, and low power.