SRAM版图布局方向对产品失效率的影响

Influence of the Layout Direction of the SRAM on Product Failure Rate

静态随机存储器(SRAM)是集成电路中重要的存储结构单元。由于其制备工艺复杂、关键尺寸较小、对设计规则的要求最为严格,因此SRAM的质量是影响芯片良率的关键因素。针对一款微控制单元(MCU)芯片的SRAM失效问题,进行逻辑地址分析确认失效位点,通过离子聚焦束(FIB)切片及扫描电子显微镜(SEM)分析造成失效的异常物理结构,结合平台同类产品的设计布局对比及生产过程中光刻工艺制程的特点,确认失效的具体原因。对可能造成失效的工艺步骤或参数设计实验验证方案,根据验证结果制定相应的改善措施,通过良率测试及SEM照片确认改善结果,优化工艺窗口。当SRAM中多晶硅线布局方向与测试单元中一致时,工艺窗口最大,良率稳定;因此在芯片设计规则中明确SRAM结构布局方向,对于保证产品的良率具有重要意义。

The static random access memory(SRAM)is an important storage unit in integrated circuits.Because of its complex preparation process,small critical dimension and strict design rules,the quality of SRAM is the key factor affecting the yield of chip.In order to solve the problem of SRAM failure of a micro control unit(MCU),logical address analysis was carried out to confirm the failure site.The abnormal physical structure caused by the failure was analyzed by focused ion beam(FIB)slice and scanning electron miroscopy(SEM).Combined with the design layout comparison of similar products of the platform and the characteristics of lithography process in the production process,the specific cause of the failure was confirmed.For the process steps or parameters that may cause failure,the experimental verification scheme was designed,and the corresponding improvement plan was made according to the verification results.The improvement results are confirmed by the yield test and SEM images,and the process window is optimized.When the layout direction of polysilicon lines in SRAM is consistent with that in the test cell,the process window is the largest and the yield is stable.Therefore,it is important to specify the layout direction of the SRAM structure in chip design rules to ensure the yield of products.