1. Detailed Placement for Improved Depth of Focus and CD Control
Puneet Gupta1
Andrew B. Kahng1,2
Chul-Hong Park2
1 Blaze DFM, Inc.
2 ECE Department, University of California, San Diego

2. Outline OPC and SRAF: An Introduction The AFCorr Methodology
AFCorr Placement Perturbation
Experiments and Results
Summary

3. OPC (Optical Proximity Correction)
Before OPC
After OPC
C.-H. Park et al., SPIE 2000
Gate CD control is extremely difficult to achieve
Min feature size outpaces introduction of new hardware solutions
OPC = one of available reticle enhancement techniques (RET) to improve pattern resolution
Proactive distortion of photomask shape  compensate CD inaccuracies

4. SRAF (Sub-Resolution AF)
Process Margin (180nm)
Layout (or Mask ) Design
SB=0
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0.22
0.0
0.3
0.4
0.5
0.6
SB2
SB1
SB0
DOF
CD
Active
SB=1
SB=2
Wafer structure (SEM)
#SB = 0
#SB=1
#SB=2
CD (nm)
160
177
182
SRAF = Scattering Bar (SB)
SRAFs enhance process window (focus, exposure dose)
Extremely narrow lines  do not print on water
More SBs helps to enhance DOF margin and to meet the target CD

5. SRAFs and Bossung Plots
Bias OPC
SRAF OPC
Bossung plot
Measurement to evaluate lithographic manufacturability
Maximize the common process window
Horizontal axis: Depth of Focus (DOF); Vertical axis: CD
SRAF OPC
Improves process margin of isolated pattern
Larger overlap of process window between dense and isolated lines

6. Outline OPC and SRAF: An Introduction The AFCorr Methodology
AFCorr Placement Perturbation
Experiments and Results
Summary

7. Forbidden Pitches
#SB=1
#SB=2
#SB=3
#SB=4
Allowable
Forbidden
Forbidden pitch lowers printability, DOF margin and exposure margin
Typically based on tolerance of +/- 10% of CD
 Must avoid forbidden pitches in layout

8. Layout Composability for SRAFs
Better than
x+dx
 x 
Small set of allowed feature spacings
Two components of SRAF-aware methodology
Assist-correct libraries
Library cell layout should avoid all forbidden pitches
Intelligent library design
Assist-correct placement  THIS WORK
Intelligent whitespace adjustment in the placer

9. Outline OPC and SRAF: An Introduction The AFCorr Methodology
AFCorr Placement Perturbation
Experiments and Results
Summary

10. AFCorr: SRAF-Correct Placement
Before AFCorr
After AFCorr
Forbidden pitch
Cell boundary
By adjusting whitespace, additional SRAFs can be inserted between cells
Resist image improves after assist-aware placement adjustment
Problem: Perturb given placement minimally to achieve as much SRAF insertion as possible

11. Minimum Perturbation Approach
Objective:
Reduce forbidden pitch violation
Reduce weighted CD degradation with defocus
Minimum perturbation: preserve timing
Constraint:
Placement site width must be respected
How:
One standard cell row at a time
Solve each cell row by dynamic programming

12. Feasible Placement Perturbations
SaLP
gate
field
Sa-1RP xa
xa-1
Wa-1
Minimize  | di |
s.t. da-1 + da + Sa-1RP + SaLP + (xa – xa-1 – wa-1)  AF
wi and xi = width and location of Ci
i = perturbation of location of cell Ci
AF = set of allowed spacings
RP, LP = boundary poly shapes with overlapping y-spans
- Overlap types: g-g, g-f, f-f
S = spacing from boundary poly to cell border

13. Vertical Forbidden Pitches
Cell under
consideration
Row i
Row i-1
Handled in a way similar to horizontal overlap
Usually field poly
Typically, #vertical forbidden pitches < #horiz. F.P.
Due to restricted design rules like single orientation poly

14. Dynamic Programming Solution
COST (1,b) = | x1-b| // subrow up through cell 1, location b
COST (a,b) = l(a) |(xa -b)| +
MIN{Xa-SRCH ≤ i ≤ Xa+SRCH} [COST(xa-1,i) + αHCost(a,b,a-1,i) βVCost(a,b)]
// SRCH = maximum allowed perturbation of cell location
HCost = horizontal “forbidden-pitch cost” = sum over horiz adjacencies of
[slope(j) |HSpace –AFj| * overlap_weight]
s.t. AFj+1 > HSpace  AFj
VCost = vertical forbidden pitch cost
l = perturbation weight
α, β = weights for horizontal vs vertical forbidden pitches
Slope = CD / Pitch = CD degradation per unit space between AF values
AFi = closest assist-feasible spacing ≤ HSpace
Overlap_weight = overlap length weighted by relative importance of printability for gate-to-gate, gate-to-field, and field-to-field

15. Outline OPC and SRAF: An Introduction The AFCorr Methodology
AFCorr Placement Perturbation
Experiments and Results
Summary

16. Experimental Flow Benchmark design Forbidden pitch SB OPC SB Insertion
Model-based OPC
(Best DOF model)
Lithography model
generation
(Best & Worst DOF)
Benchmark design
Placement
Assist Corrected
GDS
Route
Typical GDS
Post-Placement
OPCed GDSs
Delay
GDSII size
OPC Run Time
# Forbidden pitch
# SB
# EPE
Experiments

17. Experimental Setup KLA-Tencor’s Prolith Mentor’s OPCpro, SBar SVRF
Model generation for OPCpro
Best focus/ worst (0.5 micron) defocus
Calculating forbidden pitches
Mentor’s OPCpro, SBar SVRF
OPC, SRAF insertion, ORC (Optical Rule Check)
Cadence SOC Encounter
Placement & Route
Synopsys Design Complier
Synthesis

18. Experimental Metrics SB Count Forbidden Pitch Count EPE Count
Total number of scattering bars or SRAFs inserted in the design
Higher number of SRAFs indicates less through-focus variation and is hence desirable
Forbidden Pitch Count
Number of border poly geometries estimated as having greater than 10% CD error through-focus
EPE Count
Number of edge fragments on border poly geometries having greater than 10% edge placement error at the worst defocus level

19. Results: Increased SB Count
SB count increases as utilization decreases due to increased whitespace
#SB increases after AFCorr placement

20. Results: Reduced F/P and EPE
Forbidden pitch count (border poly only)
81%~100% in 130nm, 93%~100% in 90nm
EPE Count (border poly only)
74%~95% in 130nm, 83%~96% in 90nm

21. Impact on Other Design Metrics
Utilization(%) 90 80 70
Flow:
Orig
AFCorr
130nm
#EPE
4890
4721
5975
562
4276 15
R/T (s)
7821
7902
7876
7934
7913
7973
GDS (MB)
48.9
48.8
48.2
48.4
Delay (ns)
4.2
4.6
4.5
4.7
90nm
7523
1262
4813
532
2131
107
R/T(s)
6211
6327
6322
6431
6482
6499
GDS(MB)
43.1
43.3
43.2
Delay(s)
2.7
2.6
2.4
2.5
Impact : Data size < 1%, OPC run time < 2%, Cycle time < 4%
Other impacts are negligible compared to large improvement in printability metrics

22. Outline OPC and SRAF: An Introduction Forbidden Pitch Extraction
The AFCorr Methodology
Experiments and Results
Summary

23. Summary
AFCorr is an effective approach to achieve assist feature compatibility in physical layout
Up to 100% reduction of forbidden pitch and EPE
Relatively negligible impacts on GDSII size, OPC runtime, and design clock cycle time
Compared to huge improvement in printability
Ongoing research
Developing “correct-by-construction" standard-cell layouts which are always AFCorrect in any placement

24. Thank You!

25. Notation W = cell width; RP, LP = Boundary poly geometries
S = Spacing from boundary poly to cell border
O = Parallel adjacencies between poly features (g-f, g-g, f-f)
Example: Sa-1RP2 + (xa-1 – xa – wa-1) + SaLP3 should be assist-correct