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Compensation for Effects of Pitch-Split Double Exposure in Nanoscale SRAM

Compensation for Effects of Pitch-Split Double Exposure in Nanoscale SRAM. Wen Li and Matthew Weiner. Pitch Split Double Exposure (PSDE). Standard lithography Print everything on one layer using a single mask PSDE Print neighboring polysilicon lines on two different masks

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Compensation for Effects of Pitch-Split Double Exposure in Nanoscale SRAM

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  1. Compensation for Effects of Pitch-Split Double Exposure in Nanoscale SRAM Wen Li and Matthew Weiner

  2. Pitch Split Double Exposure (PSDE) • Standard lithography • Print everything on one layer using a single mask • PSDE • Print neighboring polysilicon lines on two different masks • Allows closer pitch with current technology • Only way to continue scaling without increasing n of current immersion liquids

  3. Effects of PSDE on SRAM • Same basic effect seen in standard lithography • Corner rounding and poly to diff mismatch • Now poly to poly mismatch added • Neighboring lines shift in different direction and by different amount

  4. Width Variation • Simulated contours in Calibre Workbench • 3σ overlay error of 7nm (Nikon NSR-S609B) • Double to account for more than 3σ variation • Maximum width variation within 6%

  5. Model of PSDE • Model systematic variation from PSDE as a variation in width • PD and PG widths vary • PU width stays roughly constant • Concerned with both: • Worst case read: PD decreases, PG increases • “Worst case” write: PG increases

  6. Effects on Margins • Simulation results of effects on margins

  7. PSDE Compensation • Dynamic bitline compensation and read assist • Want WM adjustment between half cells • Find VBLL and VBLR to equalize half cells’ VINT • Trade increased WM for RM

  8. Dynamic Compensation Circuit Design • Worst case half cells as sensors to find avg VINT • Feedback finds BL voltage to equalize VINT of each half cell

  9. Results of PSDE Compensation • Up to 0.33σ increase in WM

  10. Results of PSDE Compensation • Up to 0.22σ increase in RM • Increases yield by 4x

  11. Conclusions • Small change in RM and WM from PSDE • Needs to be verified experimentally • Both margins vary by about the same amount • Depending on amount of width variation, can get significant WM improvements • Can trade WM improvement for RM using traditional assist circuits

  12. Acknowledgements • Prof. Bora Nikolic • Lauren Jones • Lynn Wang

  13. Questions?

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