US Patent 7388267 Selective stress engineering for SRAM stability improvement

An integrated circuit (IC) structure including a SRAM cell is provided in which the performance of the pass-gate transistors is degraded in order to increase the beta ratio of the transistors within the SRAM cell. In particular, the increased beta ratio is obtained in the present invention by intentionally improving only the performance of the pull-down transistors, while degrading the performance of the pass-gate transistors. This result is achieved in the present invention by implementing stress memorization technique on logic complementary metal oxide semiconductor (CMOS) nFETs and SRAM pull-down transistors to improve the nFET performance. The stress memorization technique is not performed at the pFET region to avoid performance degradation as well as at the SRAM pass-gate transistors to avoid the improvement. With performance improvement at the pull-down transistors and no performance improvement at the pass-gate transistors, the beta ratio of the SRAM transistors is improved.