BitLines (BL) Type Failure in SRAM

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BitLines (BL) Type Failure in SRAM

Bitlines (BL) Type Failure in SRAM refers to a specific type of fault where bitlines (BLs) in the memory array consistently fail in a patterned way. This failure is characterized by issues along particular bitlines, often presenting as repeated faults in certain bitline groups or sections, rather than isolated faults within individual memory cells. In BL Type Failures, the cause is typically not located within the affected bitline itself. Instead, it may be related to defects in the bitline decoder or related interconnects, which control the entire group of bitlines. Possible issues could include short circuits with adjacent lines (like Vdd or Vss), open connections, or other structural problems within the bitline decoder circuitry. As a result, BL Type Failures often require more complex analysis techniques, such as layout tracing and physical failure analysis, to locate and address the underlying defect.

For instance, one troubleshooting procedure for such BL Type Failure in SRAM involves:

Failure Pattern Analysis: For a 48M SRAM chip, a recurring failure as shown in Figure 0091a was observed in specific bitlines (BLs 3 and 4 in every 8 BL group) within a sub-block, suggesting primary and secondary failure causes.

BL failure

Figure 0091a. BL type failure pattern showing hard failures in BLs 3 and 4 within each 8-BL group in the 192BLx512WL sub-block. [1]

Layout Tracing: Tracing started from the failing BLs, leading to the identification of three possible causes: (1) signal line shorting to adjacent lines like Vdd or Vss, (2) open interconnect from the BL decoder, or (3) a malfunction in the BL decoder.

BL failure

Figure 0091b. Layout tracing results highlighting the suspected defective BL decoder at the bottom. [1]

Physical Failure Analysis: After narrowing down the suspected defects, an undersized contact within the BL decoder area was detected using SEM imaging as shown in Figure 0091c.

Figure 0091c. SEM backscattered electron image revealing an undersized contact. [1]

Focused Ion Beam (FIB) Cross-Sectioning: FIB analysis confirmed the undersized contact was under-etched, indicating a physical defect.

BL failure

Figure 0091d. FIB cross-sectional image revealing the undersized contact with evidence of under-etching. [1]

This methodology above highlights systematic pattern analysis, followed by targeted tracing and physical examination, to isolate and confirm the cause of BL type failures.

[1] Z. Song, SRAM failure analysis evolution driven by technology scaling, Proceedings of the 21th International Symposium on the Physical and Failure Analysis of Integrated Circuits (IPFA), DOI: 10.1109/IPFA.2014.6898207, 2014.

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BitLines (BL) Type Failure in SRAM