6ECER Dec 2008 Zalina 6Haji Samadi%28online Journal Webpublica
Study of Floating Fill Impact on Interconnect Capacitance Andrew B. Kahng Kambiz Samadi Puneet...
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Transcript of Study of Floating Fill Impact on Interconnect Capacitance Andrew B. Kahng Kambiz Samadi Puneet...
Study of Floating Fill Impact on Interconnect Capacitance
Andrew B. Kahng Kambiz Samadi Puneet Sharma
CSE and ECE DepartmentsUniversity of California, San Diego
Outline
• Introduction
• Foundations
• Study of Capacitance Impact of Fill– Proposed Guidelines
• Validation of Guidelines
• Conclusions
Introduction
• Why fill is needed?• Planarity after chemical-mechanical polishing (CMP)
depends on pattern• Metal fill reduces pattern density variation• Stringent planarity requirements fill mandatory now
• Impact on capacitance• Grounded fill
• Increases capacitance larger delay• Shields neighboring interconnects reduced xtalk
• Floating fill• Increases coupling capacitance significantly
more xtalk signal integrity & delay• Increases total capacitance larger delay
Motivation• Floating-fill extraction is complex
• Floating-fill capability recently added to full-chip extractors• In past large buffer distance design-rule used
• Reduces coupling impact• Density constraints cannot be met reduce buffer distance
inaccuracy in capacitance estimation
• We systematically analyze capacitance impact of fill config. parameters (e.g., fill size, fill location, interconnect width, etc.)
• Propose guidelines for floating fill insertion to reduce capacitance impact
• Grounded fill used despite disadvantages (e.g., higher delay impact, routing needed)
• Designers use floating fill extremely conservatively Better understanding of capacitance impact needed
Assumptions & Terminology• Same-layer analysis
– Fill affects coupling of all interconnects in proximity
– We study effect on coupling capacitance of same-layer interconnects simplifies analysis
• Terminology– Fill and coupling interconnects are on Layer M (layer of interest)– ia and ib are interconnects of interest with coupling Cab
– We study increase in coupling ΔCab due to fill insertion– Dimensions measured in tracks (=0.3µ)
– Usability not compromised because:1. Coupling with same-layer neighbor large
– Validation: multiple configs with different densities on different layers considered2. Fill insertion between two same-layer interconnects, increases coupling
significantly– Validation: fill inserted everywhere
Large fraction of coupling impact captured by same-layer analysis• Synopsys Raphael, 3D field solver, used in all experiments
Outline
• Introduction
• Foundations
• Study of Capacitance Impact of Fill– Proposed Guidelines
• Validation of Guidelines
• Conclusions
Foundation 1
• Experimental Setup• Two interconnects on Layer M separated by three
tracks• Fill inserted on Layer M between two interconnects• M+1/M-1 density is set to 33%• 20% , 33% , 100% metal density for Layer M+2/M-2
tried
For ΔCab analysis, Layers M-2 and M+2 may be assumed as groundplanes
Foundation 2
• Experimental Setup• Two interconnects on Layer M separated by three
tracks• M+1 & M-1 density is set to 33%• M+2 & M-2 assumed groundplanes• Fill features inserted on Layer M at different locations
ΔCab is affected by fill geometries in the region REab only.
Outline
• Introduction
• Foundations
• Study of Capacitance Impact of Fill– Proposed Guidelines
• Validation of Guidelines
• Conclusions
Fill Size• Fill length (along the interconnects)
• Linear increase in ΔCab with Y-intercept
Guideline: Increase fill length instead of width
• Fill width• Increases super-linearly
• Using parallel-plate capacitor analogy, 1/w relation expected• Settings:
• Interconnect separation = 3 tracks• Layers M-1/M+1 have 33% density• 2 track width, 1 track length
Interconnect Spacing• ΔCab decreases super-linearly with spacing• For larger spacings (>10 tracks), coupling with
M-1 and M+1 wires more significant• Settings:
• Fill size = 2 tracks x 2 tracks• Layers M-1/M+1 have 33% density
Guideline: Insert fill where wire spacing is large
Fill Location• Y-axis translation
• Cab unaffected until fill close to an interconnect ending
Guideline: Center fill horizontally between interconnects
• X-axis translation • ΔCab increases ~linearly• Capacitance between fill & closer
interconnect increases dramatically• Settings:
• Wire spacing = 8 tracks• Fill size = 2 tracks wide, 4 long• Layers M-1/M+1 have 33% density
Edge Effects• Study two cases: (1) two interconnects horizontally
aligned, and (2) not horizontally aligned• With Y-axis translation of fill, edge effects observed
• When fill no longer in Rab, ΔCab dramatically decreases
• Settings:• Layers M-1/M+1 have 33% density• Interconnect width = 2 tracks• Fill size = 4 tracks long, 2 wide
Guideline: Insert fill in low-impact region (= outside Rab)
Rab
Interconnect Width
• Change width of one interconnect•Interconnect-fill spacing and
interconnect spacing constant• ΔCab increases rapidly, but saturates at ~ 4
tracks
Guideline: Insert fill next to thinner interconnects
Multiple Columns• Vertically aligned fill geometries are said to be
in a fill column • Change number of fill columns in fill pattern
•Fill area is kept constant • ΔCab reduces with number of fill columns
•Cf. Tran. Electron Devices ’98 (MIT)•Cf. VMIC-2004 invited paper (UCSD / UCLA)
Guideline: Increase number of fill columns
Multiple Rows• Horizontally aligned fill geometries are said to be
in a fill row• Change number of fill rows in fill pattern
• Fill area is kept constant • ΔCab increases with number of fill rows• As spacing between two fill rows decreases, the
ΔCab decreases
Guideline: Decrease number of fill rows and inter-row spacing
Outline
• Introduction
• Background & Terminology
• Study of Capacitance Impact of Fill– Proposed Guidelines
• Validation of Guidelines
• Conclusions
Application of Guidelines
Regular Staggered With guidelines
Guidelines applied1. Edge effects2. Maximize columns3. Minimize rows4. Centralize fill
ΔΔC = 6
4%
C = 6
4%
ΔΔC = 6
2%
C = 6
2%
ΔΔC = 1
6%
C = 1
6%
• Apply guidelines on 3 interconnect configurations• Reasonable design rules assumed• Configuration 1
Guidelines on Configuration 2
ΔΔC = 4
1%
C = 4
1%
ΔΔC = 4
1%
C = 4
1%
ΔΔC = 3
0%
C = 3
0%
Guidelines applied1. Wire width2. Minimize rows
Guidelines on Configuration 3
ΔΔC = 2
7%
C = 2
7%
ΔΔC = 2
7%
C = 2
7%
ΔΔC = 1
1%
C = 1
1%
Guidelines applied1. High-impact region2. Edge effects3. Wire spacing4. Minimize rows5. Centralize fill
Conclusions• Coupling with same-layer neighboring wires
significant and same-layer fill insertion increases it dramatically
• Systematically analyzed the impact of floating fill configurations on coupling of same-layer interconnects
• Propose guidelines for floating fill insertion to reduce coupling increase
• Ongoing work:– 3D extensions: Impact on coupling of different-layer
interconnects
– Timing- and SI-driven fill insertion methodology