纳米SRAM寄生双极晶体管效应的仿真研究

Simulation of Parasitic Bipolar Transistor Effect in Nanometric SRAM

以65nm双阱CMOS(Complementary Metal Oxide Semiconductor)工艺的SRAM(Static Random Access Memory)为研究对象,采用三维数值模拟方法,结合SRAM中晶体管布局和邻近SRAM的相对位置,对寄生双极晶体管效应致纳米SRAM内部节点电势多次翻转的产生机制进行了深入阐述,对寄生双极晶体管效应致纳米SRAM发生MCU(Multiple Cell Upset)的影响因素进行了详细研究.发现寄生双极晶体管效应致SRAM内部节点电势多次翻转源于N阱中两个PMOS漏极电势的竞争过程,竞争过程与寄生双极晶体管效应的强弱相关,需综合考虑PMOS源极与N阱接触的距离、PMOS漏极与N阱的电势差两个因素.在纳米双阱CMOS工艺的SRAM中,PNP寄生双极晶体管效应对MCU起着重要作用.减小阱接触与SRAM单元的距离,可减弱邻近SRAM的寄生双极晶体管效应并降低MCU的发生概率,即使阱接触距离很近,特殊角度的斜入射和高LET(Linear Energy Transfer)值离子入射仍存在触发邻近SRAM的寄生双极晶体管效应并导致MCU的可能.

Considering the transistors distribution in a SRAM and the relative positions oftwoadjacent SRAMs,the mechanism of multiple upsets in 65nm twin-well CMOS SRAM internal nodes and the impact factors of multiple cell upset induced by parasiticbipolar effect are investigated through 3D TCAD device simulation.It is found that multiple upsets in SRAM internal nodes result from the competition of p+-drains in the n-well.The competition depends on the parasitic bipolar effect which is related to the distance between p+-source and n-well contact,as wellas the electric potential difference between p+-drain and n-well.PNP parasiticbipolar transistors play an important role in nanometric twin-well CMOS SRAM.Although reducing the distance between SRAM and n-well contact can weaken parasitic bipolar effect,ions with special incident angles or high linear energy transfers can also trigger the parasitic bipolar effect in adjacent SRAM and induce multiple cell upset.