stacked signal tsv for thermal dissipation in global ... · vlsi/cad related work • thermal via...
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Stacked Signal TSV for Thermal
Dissipation in Global Routing for
3D IC
Po-Yang Hsu, Hsien-Te Chen, TingTing Hwang
National Tsing Hua University, Taiwan
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VLSI/CAD
Outline
• Introduction
• Motivation
• Stacked-TSV Assignment & Relocation
• Experimental Results
• Conclusions
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VLSI/CAD
3D IC
• 3D IC reduces the interconnect delay and
integrates heterogeneous components
• High power density and lower thermal
conductivity produce critical thermal issue
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VLSI/CAD
Heat Dissipation in 3D IC
• The heat dissipation in vertical direction is the
critical issue in 3D IC
• Thermal conductivity of Si is much better than
oxide material
Thermally insulated materials
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VLSI/CAD
Related Work
• Thermal via insertion method – Zhang at el. “Temperature-aware routing in 3D ICs”, ASP-DAC’06
– Cong and Zhang “Thermal via placement in 3D ICs”, ISPD’05
– Goplen and Sapatnekar “Placement of thermal vias in 3D ICs using various
thermal objectives”, TCAD’06
– Wang et al. “Rethinking thermal via planning with timing-power-temperature
dependence for 3D ICs”, ASP-DAC’11
• Thermal aware routing and placement – Pathak and Lim “Performance and thermal-aware Steiner routing for 3-D stacked
ICs”, TCAD’09
– Cong et al. “Thermal-Aware Cell and Through-Silicon-Via Co-Placement for 3D
ICs”, DAC’11
– Chen et al. “A new design architecture for power network in 3D ICs”, DATE’11
Extra via overhead
Without fully utilization of stacked signal TSV 5/34
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VLSI/CAD
Outline
• Introduction
• Motivation
• Stacked-TSV Assignment & Relocation
• Experimental Results
• Conclusions
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VLSI/CAD
Non-stacked TSV vs. Stacked Signal TSV
Heat sink layer
Non-stacked signal TSVs Stacked signal TSVs
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VLSI/CAD
Temperature Comparison of Stacked
TSV and Non-stacked TSV
• Stacked TSV has better thermal dissipation
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VLSI/CAD
Temperature vs. Distance of Stacked TSV
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VLSI/CAD
Outline
• Introduction
• Motivation
• Stacked-TSV Assignment & Relocation
• Experimental Results
• Conclusions
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VLSI/CAD
Problem Definition
• Input:
– 3D cell placement result
– TSV budget
• Output:
– 3D cell and TSV placement result
• Objective:
– Wirelength
– Temperature
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VLSI/CAD
Overall Flow
TSV budget 3D cell placement result
Stacked-TSV assignment
Stacked-TSV relocation
3D cell and TSV placement result
1: Wirelength
2: Temperature
1: Temperature
2: Wirelength
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VLSI/CAD
• Assign TSV within bounding boxes for each
inter-tier net no wirelength overhead
• Place as many stacked signal TSVs as possible
reduce temperature
Stacked-TSV Assignment Stage
source terminal TSV
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VLSI/CAD
Stacked TSV Assignment Flow
Select an inter-tier net
Current tier = second bottommost tier of the net
Find candidate grids by Steiner tree
Place TSVs by gain function
Topmost tier? No
Yes
Rip-up and re-assignment TSV for critical region
All nets are
performed ?
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VLSI/CAD
Find Candidate Grids by Steiner Tree
Construct 2D Steiner tree
Map adjacent tier sinks to 2D plane
Aerial view Grid view
source terminal
Tier 1
Tier 2
Tier 3
1 2
3 4
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VLSI/CAD
Gain Function for Candidate Grids
• Select the TSV location by a gain function
– Grid with larger gain value is selected to place signal TSV
PowDensityi,j,k Power density in grid (i,j,k)
TSVNumi,j,k Number of signal TSVs in grid(i,j,k)
SDi,j Stacking degree in grid (i,j)
SDB < SDA
B
A
kji
n
kkji
jikji
TSVNum
PowDensitySDGain
,,
1,,
,,,
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VLSI/CAD
Signal TSV Placement
• For the given TSV budget N, we select N
candidate grids which have largest gain values
2D Steiner tree
If TSV = 1
If TSV = 2
Tier 1
Tier 2
Tier 1
Tier 2
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VLSI/CAD
Rip-up and Re-assignment TSV for
Critical Region
• Re-assign TSVs in overflow grids to nearby grids
that have whitespace
• Re-assign TSVs in congested grids to nearby
low routing demands grids
High routing demand
Sufficient whitespace
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VLSI/CAD
Overall Flow
TSV budget 3D cell placement result
Stacked-TSV assignment
Stacked-TSV relocation
3D cell and TSV placement result
1: Wirelength
2: Temperature
1: Temperature
2: Wirelength
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VLSI/CAD
• Not all hotspot grids are near the candidate grids
in bounding boxes
• Relocate stacked signal TSV to the hotspot grid
outside the bounding boxes with minimum wiring
overhead
Stacked-TSV Relocation Stage
hotspot
source terminal TSV
Stacked signal TSV 20/34
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VLSI/CAD
Stacked TSV Relocation Flow
Current tier = the bottommost tier
Hotspot grids identification in current tier
Matching saver nets and hotspot grids
Determination of stacking grid
Topmost tier?
No
Yes
Re-computation of thermal distribution
Current tier ++
output
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VLSI/CAD
Explore Saver Nets
Critical hotspot identification Hotspot source terminal
kji
n
kkji
kji
TSVNum
PowDensityThermalCri
,,
1,,
,,
Saver net
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VLSI/CAD
Hotspot grids Saver nets
Select Pairs of Nets and Grids
(wirelength overhead) (heat dissipation ability) greeStackingDeGridDistngEdgeCost ),(
Grid Distance
hotspot
Stacking Degree
hotspot
Net 1 Net 2
(Net1 > Net2)
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VLSI/CAD
Place Stacked TSV by Gain Function
n
k
n
kkjikjijiji TSVPowerDSSTGain
1 1,,,,,,
t
bk
t
bkkjikji WLWSDens ,,,,
Hotspot grids
A
B
C
Saver nets
2
1
3
A
DSST = Distance to hotspot grids
Power = Power density
TSV = TSV count
WSDens = Whitespace
WL = Wirelength
Candidate target grid
Thermal Wirelength
DSST
Power
TSV
WSDens
WL
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VLSI/CAD
Outline
• Introduction
• Motivation
• Stacked-TSV Assignment & Relocation
• Experimental Results
• Conclusions
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VLSI/CAD
Experimental Environment
• Benchmarks are from reference work [5]
• 3D placement results produced by a partitioning-
driven placement for 3D IC [5]
• Use FLUTE [19] to estimate global routing wirelength
• The topmost layer of chip and ambient temperature
set to 25 ℃
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VLSI/CAD
Wafer thickness 50 um 6670 (um * K/W)
TSV size 25 um2
2500 (um * K/W)
Interposer thickness 100 um 3*106 (um * K/W)
Low-K thickness 2 um 106 (um * K/W)
SiO2 of TSV thickness 0.2 um 106 (um * K/W)
Power dissipation 1 ~ 107 W/cm2
Key Parameters
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VLSI/CAD
Experimental Results
• Baseline result [5]
• Thermal-TSV insertion method [1]
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VLSI/CAD
Routing Congestion Results
• 3D Elmore delay model [11] is used to estimate
maximum delay value
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VLSI/CAD
# Stacked TSVs vs. Temperature in ckt1
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VLSI/CAD
Analysis of Each Step
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VLSI/CAD
Outline
• Introduction
• Motivation
• Stacked-TSV Assignment & Relocation
• Experimental Results
• Conclusions
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VLSI/CAD
Conclusions
• An algorithm is proposed to construct stacked
signal-TSVs for heat dissipation
• The experimental results show that our
algorithm has 17% temperature reduction
with 4% wiring overhead
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Thank You