1. Impact of RET on Physical Design ISPD 2001 April 2, 2001 F.M. Schellenberg, Ph.D. Calibre RET Group Luigi Capodieci, Ph.D. ASML MaskTools

2. F.M. Schellenberg / ISPD 2001 / April 2, 2001 2 Agenda n Agenda n Resolution Enhancement Technology (RET) –Lithography Basics –RET: OAI, OPC, PSM, and all that n Insertion in Process Flows n Impact on Physical Design –OAI –OPC –PSM n Importance of a “Target” layer n Conclusions

3. F.M. Schellenberg / ISPD 2001 / April 2, 2001 3 Lithography Basics n All IC layers are formed by Lithography n The light interacts with a reticle (aka mask) and the lens to form a reduced image in photoresist Illumination Mask Lens Wafer 4 100 UV Laser

4. F.M. Schellenberg / ISPD 2001 / April 2, 2001 4 Lithography Basics High spatial frequencies (dense lines) scatter at larger angles. The lens acts as a low-pass filter for high spatial frequencies

5. F.M. Schellenberg / ISPD 2001 / April 2, 2001 5 Lithography Basics n Fine pitches diffract at higher angles n The lens acts recollects light onto the wafer n The lens acts like a low pass filter n “DC” light passes directly through the lens –No contrast –No image –Wasted light

6. F.M. Schellenberg / ISPD 2001 / April 2, 2001 6 n The light interacting with the mask is a wave n Any wave has certain fundamental properties –Wavelength ( ) –Direction –Amplitude –Phase n RET is wavefront engineering to enhance lithography by controlling these properties Lithography Basics Amplitude Direction Phase

7. F.M. Schellenberg / ISPD 2001 / April 2, 2001 7 Wavefront Engineering: Direction Amplitude Direction Phase

8. F.M. Schellenberg / ISPD 2001 / April 2, 2001 8 Wavefront Engineering: Direction n Regular Illumination: Uniform disc n Off-Axis Illumination: e.g. Annular lens Lens

9. F.M. Schellenberg / ISPD 2001 / April 2, 2001 9 Wavefront Engineering: Direction n Many off-axis designs –Annular –Quadrupole / Quasar –Dipole n See your local stepper/scanner supplier + or

10. F.M. Schellenberg / ISPD 2001 / April 2, 2001 10 Wavefront Engineering: Amplitude Amplitude Direction Phase

11. F.M. Schellenberg / ISPD 2001 / April 2, 2001 11 Wavefront Engineering: OPC n Optical and Process Correction (OPC) for Amplitude Control n Modifies layout to compensate for process distortions –Add light where needed –Subtract light where not wanted –Add non-electrical structures to layout to control diffraction of light

12. F.M. Schellenberg / ISPD 2001 / April 2, 2001 12 Wavefront Engineering: OPC Mask layout Wafer result

13. F.M. Schellenberg / ISPD 2001 / April 2, 2001 13 Wavefront Engineering: Phase Amplitude Direction Phase

14. F.M. Schellenberg / ISPD 2001 / April 2, 2001 14 Wavefront Engineering: PSM n Phase Shifting Masks (PSM) n Etch topography into mask –Creates interference fringes on the wafer –Interference fringes can be extremely small n Make mask material phase shifting –“Attenuated” PSM

15. F.M. Schellenberg / ISPD 2001 / April 2, 2001 15 Wavefront Engineering: PSM For conventional steppers:  248 nm, NA=0.63 L  98 nm Mask

16. F.M. Schellenberg / ISPD 2001 / April 2, 2001 16 Wavefront Engineering: PSM n Interference effects boost contrast n Phase Masks can make extremely small gates n Phase Masks can double resolution –2X finer pitches 0° 180° 90 nm SEM image courtesy of IMEC

17. F.M. Schellenberg / ISPD 2001 / April 2, 2001 17 So What? n RET not done in isolation n Selection of RET technique carries an impact on design rules / layout restrictions

18. F.M. Schellenberg / ISPD 2001 / April 2, 2001 18 Insertion points of RET n OAI: –Inserted into Lithography Stepper n OPC –Typically inserted at Physical Verification n Verification modified to include process simulation n PSM –Modify P&R rules to allow finer pitches –Insert at P&R, Physical Verification, or Mask Data Preparation

19. F.M. Schellenberg / ISPD 2001 / April 2, 2001 19 Impact on Physical Design OAI OPC PSM

20. F.M. Schellenberg / ISPD 2001 / April 2, 2001 20 Impact on Physical Design: OAI n Off axis amplifies certain pitches at the expense of the others. n Concept of “Forbidden” pitches Half Pitch (nm) Depth of Focus (a.u.) Graph reference: Noguchi, M. et al. “Subhalf Micron Lithography System with Phase Shifting Effect”, in Optical/Laser Microlithography V, Proc. SPIE Vol. 1674 (1992), 92-104. Quadrupole Illumination

21. F.M. Schellenberg / ISPD 2001 / April 2, 2001 21 Impact on Physical Design: OAI Graph reference: Socha et al. “Forbidden Pitches for 130 nm lithography and below”, in Optical Microlithography XIII, Proc. SPIE Vol. 4000 (2000), 1140-1155. Acceptable Unacceptable Depth of Focus (  m) Pitch (nm) 130 nm lines, printed at different pitches Quasar illumination NA=0.7 Isolated Dense Quasar Illumination

22. F.M. Schellenberg / ISPD 2001 / April 2, 2001 22 Impact on Physical Design: OAI 45° lines vanish 110 nm lines Quasar illumination NA=0.7 Isolated Dense Quasar Illumination

23. F.M. Schellenberg / ISPD 2001 / April 2, 2001 23 Impact on Physical Design: OAI n Quasar / Quadrupole Illumination –Amplifies dense 0°, 90 ° lines –Destroys ±45° lines n Dipole Illumination –Horizontal Dipole prints only Vertical Lines –Vertical Dipole prints only Horizontal lines –Must decompose layout for 2 exposures n Vertical mask, horizontal mask

24. F.M. Schellenberg / ISPD 2001 / April 2, 2001 24 Impact on Physical Design OAI OPC PSM

25. F.M. Schellenberg / ISPD 2001 / April 2, 2001 25 Impact on Physical Design: OPC n OPC changes layout dramatically n OPC does not change design

26. F.M. Schellenberg / ISPD 2001 / April 2, 2001 26 Impact on Physical Design: OPC Designed Layout Final Layout Mask Wafer

27. F.M. Schellenberg / ISPD 2001 / April 2, 2001 27 Impact on Physical Design: OPC Original Designed Layout Layout with OPC Graphics courtesy of IBM

28. F.M. Schellenberg / ISPD 2001 / April 2, 2001 28 Impact on Physical Design: OPC SEM image courtesy of IBM

29. F.M. Schellenberg / ISPD 2001 / April 2, 2001 29 Impact on Physical Design: OPC Graphics & SEM image courtesy of IBM Simulation based check

30. F.M. Schellenberg / ISPD 2001 / April 2, 2001 30 Impact on Physical Design: OPC n OPC provides an automatic layout fix to achieve the target layer on the wafer n With simulation based checking, design rules can be more aggressive n Physical Verification becomes process-aware –Expands to add OPC –Verifies the results with process simulation

31. F.M. Schellenberg / ISPD 2001 / April 2, 2001 31 Impact on Physical Design OAI OPC PSM

32. F.M. Schellenberg / ISPD 2001 / April 2, 2001 32 Impact on Physical Design: PSM n PSM allows true resolution enhancement –Thin gates 90 nm wide in 180 nm process –Pitch doubling n Line size / pitch defined in –Libraries –Routing algorithms n Drives insertion to P&R 90 nm SEM image courtesy of IMEC

33. F.M. Schellenberg / ISPD 2001 / April 2, 2001 33 Impact on Physical Design: PSM n Maskmaking concerns for PSM –Phase etch effects: linewidth imbalance –No inspection technique –No repair technique –Desire to minimize final phase area n Drives phase assignment to the last possible moment

34. F.M. Schellenberg / ISPD 2001 / April 2, 2001 34 Impact on Physical Design: PSM n Best compromise: –Phase compliant Libraries, design rules n Phase assignment done at Verification –Allows PSM with OPC to be verified together –Verification includes mask manufacturing rules (e.g. imbalance).

35. F.M. Schellenberg / ISPD 2001 / April 2, 2001 35 n All RET make major changes to the layout n The “design” remains unchanged n Main consequence of RET: –Divorce between Design and Layout Impact on Physical Design

36. F.M. Schellenberg / ISPD 2001 / April 2, 2001 36 Impact on Physical Design “Target” Layer

37. F.M. Schellenberg / ISPD 2001 / April 2, 2001 37 “Target” Layer n All RET packages have one common assumption: –The layout presented is the desired structure for the wafer n In practice, this is NOT true.

38. F.M. Schellenberg / ISPD 2001 / April 2, 2001 38 “Target” Layer n Design Rules have evolved to “make things work” n These include compensations for physical phenomena (pre OPC) n The phenomena ebb and flow with process; n The rule remains as long as things work n With OPC, things may no longer work –The rules need to be reexamined

39. F.M. Schellenberg / ISPD 2001 / April 2, 2001 39 n Example: Historical rule on line extension n OPC software assumes the layout is the target, and adds OPC to the old OPC extension “Target” Layer Truly desired on wafer Layout according to design rule OPC on the OPC

40. F.M. Schellenberg / ISPD 2001 / April 2, 2001 40 “Target” Layer: SRAM Example

41. F.M. Schellenberg / ISPD 2001 / April 2, 2001 41 “Target” Layer: Example of Embedded OPC LI Design Hand Applied OPC: A = 240nm B = 255nm C = 270nm Bit Cell Slide courtesy of LSI Logic

42. F.M. Schellenberg / ISPD 2001 / April 2, 2001 42 “Target” Layer: Poly Layer De-OPC Rule Deck OPC Free Cell Slide courtesy of LSI Logic

43. F.M. Schellenberg / ISPD 2001 / April 2, 2001 43 “Target” Layer Plan: De-OPC on Bit Cells Memory I/O Analog Mixed Signal Etc. Cell Libraries De-OPC Memory Analog I/O Mixed Signal Tape Out De-OPC OPC Free Cell Libraries Memory I/O Analog Mixed Signal Etc. Compiler “Target” Layout DRC/LVS Process-based OPC Slide courtesy of LSI Logic

44. F.M. Schellenberg / ISPD 2001 / April 2, 2001 44 “Target” Layer Conclusion n Design rules are riddled with historical OPC n DON’T DO THAT! n With systematic OPC now part of the flow, it will do the heavy lifting n Consciously clean up libraries and design rules to create the actual “target” layout n If this is not done, mysterious failures will continue

45. F.M. Schellenberg / ISPD 2001 / April 2, 2001 45 Conclusions n Selection of RET is mandatory for future progress down Moore’s Law –It’s not going away n RET style has an impact on layout –Design and layout become very different n Design to a “Target” layer –Produce a layout that shows what is really desired –Allow RET to do its job

46. F.M. Schellenberg / ISPD 2001 / April 2, 2001 46 Acknowledgements n Emile Sahouria, Olivier Toublan Mentor Graphics n Bob Socha, ASML n Lars Liebmann, IBM n George Bailey, LSI Logic n Kurt Ronse, IMEC

47. F.M. Schellenberg / ISPD 2001 / April 2, 2001 47 Thank you for your attention.

48. F.M. Schellenberg / ISPD 2001 / April 2, 2001 48 www.mentor.com