Design Of NBTI Effect Aging Monitoring Circuit For SRAM Array

With the rapid development of integrated circuits,the size of the manufacturing process is getting smaller to meet the design requirements of highly integrated circuits,which further reduces the threshold voltage on devices and results in challenging the reliability of integrated circuits.As the last stage of chip life cycle,circuit aging will lead to the failure of circuit function and increase circuit delay,which has a fatal impact on high-speed circuits such as processor and memory.Therefore,the aging monitoring and early warning on memory are important to the current circuit design.The most significant aging effect on circuits is the NBTI effect.This paper analyzes the influence of NBTI effect on the threshold voltage of the device under the stress by studying the mechanism of NBTI effect.According to the active state of the SRAM unit with six transistors,the aging mode of SRAM unit with NBTI effect is divided into symmetrical aging and asymmetric aging.Since the SRAM unit is subjected to the same stress for a long time in the asymmetric aging mode,the read stability,write stability and write speed of SRAM units are seriously affected,especially the write speed of the two states on on storage node is obviously different.Based on the asymmetry of write time,this paper designs a differential ring oscillation circuit with enabled control as the aging monitoring circuit of SRAM unit.Since the SRAM unit is considered as the load of the ring oscillation structure,the duty cycle of the oscillation signal can reflect the asymmetry of write time,which can be used to judge the aging degree on SRAM units.In order to further realize the NBTI aging monitoring on SRAM array,this paper connects the aging monitoring circuit to each bit line of the storage array,and determines the aging degree of the storage unit on the whole SRAM array through the control of word line,test signal and column selection signal.This aging monitoring circuit can complete the aging monitoring of SRAM array under different process angle,temperature and power supply voltage,but the monitoring effect is different.For obtaining the aging situation in the SRAM array efficiently,it is necessary to observe the duty cycle of monitoring signals intuitively.Therefore,this paper designs a digital duty cycle conversion circuit to convert the duty cycle of monitoring signals into the duty cycle that can be output by the system through the principle of level counting and shift control.This can visually observe the duty ratio of the monitored signal and effectively locate the aging SRAM cells in the storage array as well.