Fill for Shallow Trench Isolation CMP

1
Fill for Shallow Trench Isolation CMP

• Andrew B. Kahng1,2
• Puneet Sharma1
• Alex Zelikovsky3

1 ECE Department, University of California San
Diego 2 CSE Department, University of California
San Diego 3 CS Department, Georgia State
University
http//vlsicad.ucsd.edu
2
Acknowledgements

• We thank Prof. Duane Boning and Mr. Xiaolin Xie
  at MIT for discussions and help with abstractions
  of physical CMP phenomena, as well as supplying
  the STI-CMP simulator.

3
Outline

• Introduction and Background
• Problem formulations
• Hexagon covering-based fill insertion
• Experiments and results
• Conclusions

4
CMP for STI

• STI is the mainstream CMOS isolation technology
• In STI, substrate trenches filled with oxide
  surround devices or group of devices that need to
  be isolated
• Relevant process steps
• Diffusion (OD) regions covered with nitride (acts
  as CMP-stop)
• Trenches created where nitride absent and filled
  with oxide
• CMP to remove excess oxide over nitride
  (overburden oxide)

Before CMP
After Perfect CMP

• CMP goal Complete removal of oxide over nitride,
  perfectly planar nitride and trench oxide surface

5
Imperfect CMP

• Planarization window Time window to stop CMP
• Stopping sooner leaves oxide over nitride
• Stopping later polishes silicon under nitride
• Larger planarization window desirable
• Step height Oxide thickness variation after CMP
• Quantifies oxide dishing
• Smaller step height desirable
• CMP quality depends on nitride and oxide density
• ? Control nitride and oxide density to enlarge
  planarization window and to decrease step height

6
STI Fill Insertion

• CMP is pattern dependent ? Fill insertion
  improves planarization window and step height
• Fill inserted in the form of nitride features
• Deposition bias Oxide over nitride deposited
  with slanted profile ? Oxide features are
  shrunk nitride features
• Size and shape fill to simultaneously control
  nitride and oxide density

7
Outline

• Introduction and Background
• Problem formulations
• Hexagon covering-based fill insertion
• Experiments and results
• Conclusions

8
Objectives for Fill Insertion

• Primary goals
• Enlarge planarization window
• Minimize step height i.e., post-CMP oxide height
  variation
• Minimize oxide density variation
• ? Oxide uniformly removed from all regions
• ? Enlarges planarization window as oxide clears
  simultaneously
• Maximize nitride density
• ? Enlarges planarization window as nitride
  polishes slowly

Objective 1 Minimize oxide density
variation Objective 2 Maximize nitride density
9
Problem Formulation

• Dummy fill formulation
• Given
• STI regions where fill can be inserted
• Shrinkage a
• Constraint
• No DRC violations (such as min. spacing, min
  .width, min. area, etc.)
• Objectives
• minimize oxide density variation
• maximize nitride density

10
Density Variation Minimization with LP

• Minimize oxide density variation
• Use previously proposed LP-based solution
• Layout area divided into n x n tiles
• Density computed over sliding windows ( w x w
  tiles)

• Inputs
• min. oxide density (OxideMin) per tile
• ? To compute shrink designs nitride features
  by a
• max. oxide density (OxideMax) per tile
• ? To compute insert max. fill, shrink nitride
  features by a
• Output target oxide density (OxideTarget) per
  tile
• Dual-objective ? single-objective (nitride
  density) problem with oxide density constrained
  to OxideTarget

11
Nitride Maximization Problem Formulation

• Dummy fill formulation
• Given
• STI regions where fill can be inserted
• Shrinkage a
• Constraint
• No DRC violations (such as min. spacing, min
  .width, min. area, etc.)
• Target oxide density (OxideTarget)
• Objectives
• maximize nitride density

12
Outline

• Introduction and Background
• Problem formulations
• Hexagon covering-based fill insertion
• Experiments and results
• Conclusions

13
Case Analysis Based Solution

• Given OxideTarget , insert fill for max.
  nitride density
• Solution (for each tile) based on case analysis
• Case 1 OxideTarget OxideMax
• Case 2 OxideTarget OxideMin
• Case 3 OxideMin lt OxideTarget lt OxideMax
• Case 1 ? Insert max. nitride fill
• Fill nitride everywhere where it can be added
• Min. OD-OD (diffusion-diffusion) spacing 0.15µ
• Min. OD width 0.15µ
• Other OD DRCs min. area, max. width, max. area

More common due to nature of LP
14
Case 2 OxideTarget OxideMin

• Need to insert fill that does not increase oxide
  density
• Naïve approach insert fill rectangles of shorter
  side lt a
• Better approach perform max. nitride fill then
  dig square holes of min. allowable side ß
• ? Gives higher nitrideoxide density ratio
• No oxide density in rounded square around a hole
• Cover nitride with rounded squares ? no oxide
  density

• Covering with rounded squares difficult ?
  approximate rounded squares with inscribed
  hexagons
• Cover rectilinear max. nitride with min. number
  of hexagons

15
Covering Bulk Fill with Hexagons
HU-Lines
HU-Lines
V-Lines
HL-Lines
HL-Lines
V-Lines
For min. number of hexagons At least one V-Line
and one of HU- or HL- Lines of the honeycomb must
overlap with corresponding from
polygon Approach Select combinations of V- and
HL- or HU- Lines from polygon, overlap with
honeycomb and count hexagons. Select combination
with min. hexagons. Also flip polygon by 90º and
repeat. Complexity Polygon V-Lines x (Polygon
HL-Lines Polygon HU-Lines) x Polygon
area ? Cover max. nitride fill with hexagons,
create holes in hexagon centers
16
Case 3 OxideMin lt OxideTarget lt OxideMax

• Holes give high nitrideoxide density
• ? insert max. nitride fill and create holes to
  reduce oxide density
• OK for nitride fill to contribute to oxide
  density
• ? approximate rounded squares by circumscribed
  hexagons
• When max. nitride is covered with circumscribed
  hexagons, oxide density increases
• If oxide density (outloss x max. nitride area)
  lt OxideTarget ? increase oxide density by
  filling some holes
• If oxide density gt OxideTarget ? decrease oxide
  density by partially using Case 2 solution

17
Outline

• Introduction and Background
• Problem formulations
• Hexagon covering-based fill insertion
• Experiments and results
• Conclusions

18
Experimental Setup

• Two types of studies
• Density analysis
• Post-CMP topography assessment using CMP
  simulator
• Comparisons between
• Unfilled
• Tile-based fill (DRC-correct regular fill shape
  tiling)
• Proposed fill
• Our testcases 2 large designs created by
  assembling smaller ones
• Mixed RISC JPEG AES DES
• 2mm x 2mm, 756K cells
• OpenRisc8 8-core RISC SRAM
• 2.8mm x 3mm, 423K cells SRAM

19
Layout After Fill Insertion
Higher nitride density Smaller variation in
STI well size ? less variation in STI stress
20
Density Enhancement Results
Oxide Density
Nitride Density
Proposed
Proposed
Tiled 0.5µ/0.5µ
Tiled 0.5µ/0.5µ
21
Post-CMP Topography Assessment
Step Height
Tiled 0.5µ/0.5µ
Proposed
22
Outline

• Introduction and Background
• Problem formulations
• Hexagon covering-based fill insertion
• Experiments and results
• Conclusions

23
Conclusions

• Imperfect STI CMP causes functional and
  parametric yield loss
• Our fill insertion approach focuses on (1) oxide
  density variation minimization, and (2) nitride
  density maximization
• Large nitride fill features contribute to nitride
  and oxide densities, small ones to nitride only ?
  shape fill to control both densities
• Proposed max. nitride fill insertion with holes
  to control oxide density and achieve high nitride
  density
• Results indicate significant decrease in oxide
  density variation and increase in nitride density
  over tile-based fill
• CMP simulation shows superior CMP
  characteristics, planarization window increases
  by 17, and step height decreases by 9

24
Thank You

• Questions?