SRAM Bit-line Electromigration Mechanism and its Prevention Scheme

Zhong Guan1,  Malgorzata Marek-Sadowska1,  Sani Nassif2
1Dept. of Electrical and Computer Engineering, UC Santa Barbara, 2IBM Austin Research Laboratory


In this paper, we demonstrate that signal lines in SRAM arrays are prone to electromigration (EM). Our analysis shows that the read operation can cause unidirectional current flow in bit-lines. Thus the length of bit-lines should be bounded not only by performance requirements, but also by the Blech length constraint to avoid EM. We propose a method of determining the bit-line width under layout constraints to maximize the number of cells attached to a bit-line, while ensuing the reliability of the bit-line and maintaining SRAM performance. We also study the effects of SRAM parameter variations on the EM-safe bit-line length. Simulation results show that the EM-safe bit-line length decreases as technology scales, temperature or frequency rise, and parameter variations increase.