Fill for Shallow Trench Isolation CMP
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Fill for Shallow Fill for Shallow Trench Isolation CMP Trench Isolation CMP Andrew B. Kahng Andrew B. Kahng 1,2 1,2 Puneet Sharma Puneet Sharma 1 Alex Zelikovsky Alex Zelikovsky 3 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
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Fill for Shallow Trench Isolation CMP. Andrew B. Kahng 1,2 Puneet Sharma 1 Alex Zelikovsky 3. 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. - PowerPoint PPT Presentation
Transcript of Fill for Shallow Trench Isolation CMP
Fill for Shallow Trench Fill for Shallow Trench Isolation CMP Isolation CMP
Andrew B. KahngAndrew B. Kahng1,21,2
Puneet SharmaPuneet Sharma11
Alex ZelikovskyAlex Zelikovsky33
1 ECE Department, University of California – San Diego2 CSE Department, University of California – San Diego
3 CS Department, Georgia State University
http://vlsicad.ucsd.edu
AcknowledgementsAcknowledgements We thank Prof. Duane Boning and Mr. Xiaolin Xie We thank Prof. Duane Boning and Mr. Xiaolin Xie
at MIT for discussions and help with abstractions at MIT for discussions and help with abstractions of physical CMP phenomena, as well as supplying of physical CMP phenomena, as well as supplying the STI-CMP simulator.the STI-CMP simulator.
OutlineOutline Introduction and BackgroundIntroduction and Background Problem formulationsProblem formulations Hexagon covering-based fill insertionHexagon covering-based fill insertion Experiments and resultsExperiments and results ConclusionsConclusions
CMP for STICMP for STI STI is the mainstream CMOS isolation technologySTI 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)
SiSi
OxideOxide NitrideNitride
Before CMPBefore CMP After Perfect CMPAfter Perfect CMP CMP goal: Complete removal of oxide over nitride, perfectly planar nitride and trench oxide surfaceCMP goal: Complete removal of oxide over nitride, perfectly planar nitride and trench oxide surface
Imperfect CMPImperfect 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 densityCMP quality depends on nitride and oxide density Control nitride and oxide density to enlarge planarization Control nitride and oxide density to enlarge planarization
window and to decrease step heightwindow and to decrease step height
Failure to clear oxideFailure to clear oxide Nitride erosionNitride erosion Oxide dishingOxide dishing
Key Failures Caused by Imperfect CMP
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
STI Fill InsertionSTI Fill Insertion
Top view of layout
Diffusion/Nitride
Area available for fill insertion
α α
Oxide
Nitride
Shrinkage = α (process dependent ~0.2µ)
Top View
OutlineOutline Introduction and BackgroundIntroduction and Background Problem formulationsProblem formulations Hexagon covering-based fill insertionHexagon covering-based fill insertion Experiments and resultsExperiments and results ConclusionsConclusions
Objectives for Fill InsertionObjectives 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 variationObjective 2: Maximize nitride density
Problem FormulationProblem Formulation Dummy fill formulation
Given: STI regions where fill can be inserted Shrinkage α
Constraint: No DRC violations (such as min. spacing, min .width,
min. area, etc.) Objectives:
1. minimize oxide density variation
2. maximize nitride density
Density Variation Minimization with LPDensity 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 design’s nitride features by α max. oxide density (|OxideMax|) per tile
To compute: insert max. fill, shrink nitride features by α Output: target oxide density (|OxideTarget|) per tile Dual-objective single-objective (nitride density) problem with oxide
density constrained to |OxideTarget |
Nitride Maximization Problem FormulationNitride Maximization Problem Formulation
Dummy fill formulation Given:
STI regions where fill can be inserted Shrinkage α
Constraint: No DRC violations (such as min. spacing, min .width,
min. area, etc.) Target oxide density (|OxideTarget|)
Objectives: maximize nitride density
OutlineOutline Introduction and BackgroundIntroduction and Background Problem formulationsProblem formulations Hexagon covering-based fill insertionHexagon covering-based fill insertion Experiments and resultsExperiments and results ConclusionsConclusions
Case Analysis Based Solution 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: Case 2: |OxideTarget | = |OxideMin| Case 3: |OxideMin| < |OxideTarget | < |OxideMax|
Case 1 Insert max. nitride fill Fill nitride everywhere where it can be addedFill nitride everywhere where it can be added Min. OD-OD (diffusion-diffusion) spacing ≈ 0.15µMin. OD-OD (diffusion-diffusion) spacing ≈ 0.15µ Min. OD width ≈ 0.15µMin. OD width ≈ 0.15µ Other OD DRCs: min. area, max. width, max. areaOther OD DRCs: min. area, max. width, max. area
Layout OD-OD Spacing Min. OD Width
Feature Nitride STI Well Diffusion expanded by min. spacing
Max. nitride fillWidth too small
} More common due to nature of LP
Case 2: Case 2: |OxideTarget | = |OxideMin| Need to insert fill that does not increase oxide density Naïve approach: insert fill rectangles of shorter side < α Better approach: perform max. nitride fill then dig square
holes of min. allowable side β Gives higher nitride:oxide density ratio
No oxide density in rounded square around a hole Cover nitride with rounded squares no oxide density
β
ααNitride
Hole
No oxide in this region
Top View
Covering with rounded squares difficult approximate rounded squares with inscribed hexagons
Cover rectilinear max. nitride with min. number of hexagons
Covering Bulk Fill with HexagonsCovering Bulk Fill with HexagonsHU-Lines
V-Lines
HL-Lines
V-LinesHU-Lines
HL-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
Case 3: Case 3: |OxideMin| < |OxideTarget | < |OxideMax| Holes give high nitride:oxide 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) < |OxideTarget|
increase oxide density by filling some holes If oxide density > |OxideTarget| decrease oxide density by partially
using Case 2 solution
Outloss = Oxide Area
Nitride Area
OutlineOutline Introduction and BackgroundIntroduction and Background Problem formulationsProblem formulations Hexagon covering-based fill insertionHexagon covering-based fill insertion Experiments and resultsExperiments and results ConclusionsConclusions
Experimental SetupExperimental 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
Layout After Fill InsertionLayout After Fill Insertion
Tiling-based fillTiling-based fill Fill with proposed approachFill with proposed approach
Inserted fill
Inserted fill
DesignfeaturesDesign
features
+ Higher nitride density+ Smaller variation in STI well size less variation in STI stress
Density Enhancement ResultsDensity Enhancement Results
0%
10%
20%
30%
40%
50%
60%
70%
Max. OxideDensity Var.
Min. NitrideDensity
Av. NitrideDensity
0%
10%
20%
30%
40%
50%
60%
70%
Max. Oxide DensityVar.
Min. Nitride Density Av. Nitride Density
Testcase: Mixed Testcase: OpenRisc8
Unfilled Tiled 0.5µ/0.5µ Tiled 1.0µ/0.5µ Tiled 1.0µ/1.0µ Proposed
Oxide Density Nitride Density
Tiled 0.5µ/0.5µ Tiled 0.5µ/0.5µProposed Proposed
Post-CMP Topography AssessmentPost-CMP Topography Assessment
133133
144144
146146
129129
143143
142142
Final Max. Step Final Max. Step
Height (nm)Height (nm)
50.450.4ProposedProposed
44.744.7Tiled 0.5µ/0.5µTiled 0.5µ/0.5µ
42.742.7UnfilledUnfilledOpenRisc8OpenRisc8
53.653.6ProposedProposed
46.546.5Tiled 0.5µ/0.5µTiled 0.5µ/0.5µ
45.3 45.3 UnfilledUnfilledMixedMixed
Planarization Planarization
Window (s)Window (s)Fill ApproachFill ApproachTestcaseTestcase
Step Height
Tiled 0.5µ/0.5µ Proposed
OutlineOutline Introduction and BackgroundIntroduction and Background Problem formulationsProblem formulations Hexagon covering-based fill insertionHexagon covering-based fill insertion Experiments and resultsExperiments and results ConclusionsConclusions
ConclusionsConclusions 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%
Thank YouThank You Questions?Questions?
