breaking the 3d ic power delivery walls using voltage...
TRANSCRIPT
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Breaking the 3D IC power
delivery walls using voltage
stacking
IEEE CAS DL program
Mircea Stan [email protected]
HPLP lab. http://hplp.ece.virginia.edu
ECE Dept., University of Virginia
May 2013
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Outline
• Why, What, How?
• 3D IC
• Power Walls
• Voltage Stacking
• Voltage Regulation for Stacking
• Voltage stacking in 3D IC
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Integrated circuits – 2D
Si wafers
Source: Wikipedia3
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What is 3D IC?
Multiple active layers - continue Moore’s law
Source: J.-Q. Lu et al., 2002 IITC, IEEE, 2002, pp. 78-804
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Why 3D IC? – Moore’s law!
5
Could be Heterogeneous… “Stacked” 2D (Conventional) ICs
Pouya Dormiani, Christopher Lucas ,“3D IC Technology”
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CMOS process cross section
Source: E. Levine – IC Fabrication and Yield Control
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Benefits of 3D IC
7
Yuan Xie, “ Cost/architecture/application Implication for 3D Stacking Technology
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3D IC “Power Walls”
• Physical stacking in 3rd
dimension exacerbates
the two-dimensional
power density explosion
• k-layered 3D IC : k-times
supply current, Lower
power pads
• TSV (Through silicon-
vias) : Adds resistance to
the PDN impedance
• TSV Area Over-head
Source :Synopsis ,Sematech Symposium 20098
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Power Density “Wall”
• Technology scaling => Increased Power Density
• Physical stacking in 3rd dimension exacerbates the two-
dimensional power density explosion
• Overhauling Heat Dissipation Capacity
Zhiyu Zeng et al, “ Tradeoff analysis and optimization of power delivery networks
with on chip voltage regulation”, DAC, 2010. 9
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Power Noise “Wall”
• EM effect, IR drop, Ldi/dt : with increasing current density
• Voltage Scaling : Noise margin
• Increased current demand => Lower PDN impedance needed
• IR drop triples from 45nm to 16nm
• 3.8% increase in IR drop -> 51% delay
overhead
• Slow scaling of PDN Impedance
Source: Runjhie Zhang et al, “Some Limits of Power Delivery in the Multicore Era” 10
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Thermal 3D IC bottleneck
• Power Pins at one end of the tiers, heat sink the
other end
• Current starved components placed near heat
sink, farthest from the power pins
• Current (3D)/Current(2D) = n, n number of layers
Let Rgrid = resistance of power grid
Vdrop-3D = n*Rgrid*Current(2D)
• Current starved layers getting lower voltage
headroom
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3D IC Power Delivery “Wall”
Heat Sink
Heat spreader
6th die
5th die
4th die
3rd die
2nd die
1st Die
Micro connect
TSV
Package
C4
Interposer
Interposer TSV
Power ~ O(Vol)
Vdd = ct.
I ~ O(Vol)
but,
C4s ~ O(Area)
TSVs ~ O(Area)
Unsustainable!
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In the past: 2D power delivery wall
Power ~ O(Area)
Vdd = ct.
I ~ O(Area)
but,
pads ~ O(Perim)
Unsustainable!
Source: http://gadgets.boingboing.net/2008/04/28/power-on-self-test-l.html13
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2D: Flip-chip to the rescue
Physical solution
Power ~ O(Area)
Vdd = ct.
I ~ O(Area)
C4s ~ O(Area)
2D Solved!
Not 3D though…
Source: Nextreme, Inc.
C4 - Controlled Collapse Chip Connection14
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Power Pin “Wall”
Parameter 2009 2012 2015
Average Current Density (A/cm2) 64 108 150
Power Pins (% of total pins) 66 66 66
Off chip Data rate (Gb/s) 8 14 30
ITRS Roadmap 2009
• C4 Count ~ constant : current/pad
• For n layer 3DIC , Power pin count
~ 1/nth of 2D IC
• Electro migration can cause
open/short circuit
=> Chip failure
C4: controlled collapse chip connection
Current Per Power Pin (2D), ITRS
Source : Pingqiang Zhou et al, “ Reliable Power Delivery for 3D ICs”Source: Nextreme, Inc 15
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Off-Chip Versus On-Chip Regulation
• Reduces Off-Chip I2R losses in PDN parasitic
• Fast On-Chip voltage scaling
Zhiyu Zeng et al, “ Tradeoff analysis and optimization of power delivery networks with on chip voltage regulation”, DAC, 2010.
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On-Chip-Regulation Efficiency “Wall”
• Off-Chip High Voltage => On-Chip Low Voltage :
Power loss
• Switching Regulator : High efficiency, Difficult to
integrate On-Chip
• LDO efficiency : constrained by Vout/Vin
• Switched Capacitor : Switch Conductance/
Switching Loss
• Existing regulation techniques not energy
efficient to generate low voltage and exploit
DVFS
Source : Yu Pu, “Misleading energy and performance claims in Sub-Near Threshold Digital System”, ICCAD 2010
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3D IC: Voltage Stacking to the rescue
Electrical solution
Kirchoff’s current
and voltage laws!
Power ~ O(Vol)
I = O(Area)
Also for N > 2
Vdd = O(N) =
O(Vol/Area)
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Voltage Stacking
• 3D IC power delivery walls arise due to unsustainable increase in current
• Solution for delivering increased power without increase in current is to increase voltage
• Essentially the same idea used in macroscopic power distribution grids
• Simply increasing voltage for high voltage on-chip require explicit on-chip DC/DC regulators
• Voltage stacking uses implicit power regulation based on Kirchoff’s voltage law (Ohm’s law)
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Pros and Cons
• Power Pin : Implicit Regulation; k cores stacked need same/less
number of power pins as single core
• Off-Chip I2R Power Loss : k2 times
• IR Drop : k times
• Efficiency : Depending on “Imbalance”
• 3D IC : Physical layering of 3D IC naturally maps to voltage stacked
solution reducing TSV count
• Inter-layer core activity mismatch : Internal voltage noise
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Stacking for other power walls
Power efficiency wall
Active mode: parallel
During sleep: stacked
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Fabricated chip die photo
A. Cabe, M. Stan, “Standby Power Reduction using Voltage Stacking” GLSVLSI 2011
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Power Savings during Sleep
• 1 order of magnitude savings!
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Implicit Regulation : Resistive Versus CMOS
Stacked Load
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-100 -80 -60 -40 -20 0 20 40 60 80 100
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
CMOS Load
Resistive Load
Mid
-Voltage D
roop (
V)
Load Difference between top and bottom stack (%)
Resistive load: V α ICMOS load : V α √I
For CMOS load, less dependency of Voltage droop on load current
Voltage droop α ILoad
difference between the stacked layers
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Stacked CMOS Load
Charge conservation Itop = Ibottom
αtop ,αbottom : Top and bottom core activity factors,
Fc: Core frequency CL: Capacitive load
Vmid the output voltage delivered.
V1= Vdd – Vmid = αbottom/ (αtop + αbottom)
V2 = Vmid = αtop/ (αtop + αbottom)
Itop = αtop CL(Vdd – Vmid)Fc Ibottom = αBottom CLVmid Fc
αtop = αBottom Vmid = 0.5 Vdd
Itop
Ibottom
Vmid
Vdd
Kaushik Mazumdar et al, “ Charge Recycling On Chip DC-DC Conversion for Near-Threshold Operation”, IEEE SubVt,, 2012
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Explicit Regulation needed ?
αtop > αBottom Vmid > 0.5 Vdd :
Self-Regulation forces lower voltage
headroom for high activity cores
Unregulated Voltage Stacking oppose
DVFS
Explicit Regulator: Sink/Source
“imbalance” and compensate for the
natural “feedback”
Kaushik Mazumdar et al, “ Charge Recycling On Chip DC-DC Conversion for Near-Threshold Operation”, IEEE SubVt, Boston, 2012
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-60 -40 -20 0 20 40 60
-5
0
5
10
15
20
25
30
35
Unregulated
Regulated
Mid
-Vo
lta
ge
Dro
op
(%
)
Load Difference between Stacked Layers (%)
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On-Chip Regulator
• Switched Capacitor (SC) :
Assuming Current offset: Vout
droops below Vdd
Phase 1 : flycap1 charges to Vdd
+ ∆V while flycap2 to Vdd
- ∆V
Phase 2 : flycap1 and flycap2 swap, redirecting charge to Vout
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Voltage Stacking for more than 2 Layers
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Efficiency of V-S Regulated Technique
Efficiency: depends on mismatch between the
stacked domains
Mismatch :
• Activity of the circuits,
• Evaluation node capacitance
• Voltage swing in the domains.
Efficiency = Power_logic
Power_system =
VIN Itop + VINT |Ireg |
VIN ( Itop + Ireg | + Iq )
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Higher efficiency in Voltage Stacking
Implicit vs. Explicit
Regulate the current difference, not the sum!
Lower imbalance leads to higher efficiency
Reg
i1 i2
i1+i2
|i1-i2|
i1
i2
Reg
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Efficiency Comparison
0
10
20
30
40
50
60
70
80
90
100
50mA 100mA 200mA 300mA 400mA
Eff
icie
ncy
(%
)
ILoad(mA)
SC Efficiency
LDO Efficiency
Voltage Stacking Efficiency
Voltage Stacking Efficiency dependent on mismatch : More than 90% Efficiency for
closely matched stacked load
Worst case V-S Efficiency ~ SC Efficiency31
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Positive Vs. Negative Imbalance
Positive imbalance: similar to conventional regulator (Sourcing ILoad )
Negative imbalance: regulator absorbs current (Sinking ILoad )
8.5mA
10mA-1.5mA
-3mA
0.64V
1.2V
7mA
Regulator
8.5mA
7mA1.5mA
3mA
0.56V
1.2V
10mA
Regulator
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Feedback Control Circuitry
• Vout
= nVin
– iout
Rout (fsw
,Di,G
i)
• Hysteretic feedback scheme with lower and upper bounds to modulate the
switching frequency
State of O/P Out1 Out2 Select O/P Clock
Vout
> Vref
+∆ Toggle Low 1 Clk_high
Vref-∆
<Vout
< Vref+∆
Low Low 0 Clk_low
Vout
<Vref-∆
Low Toggle 1 Clk_high
Output Clock: Pulsed between high/low
frequency depending on comparator
detected “Select” signal
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Feedback with Conventional Load
0
10
20
30
40
50
60
70
80
90
10 50 100 150 200
Eff
icie
ncy
(%
)
ILoad (mA)
Hysteretic Feedback
No Feedback
Jain, R : S200mA switched capacitor voltage regulator on 32nm CMOS and
regulation schemes to enable DVFS,(EPE 2011)
Comparator o/p acting as
frequency modulated clockEfficiency : Improves at low power
with feedback
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Open/Closed Loop for Stacked Load ?
70
75
80
85
90
95
100
100 75 50 25 0 -25 -50 -75 -100
Eff
icie
ncy
(%
)
∆I/ILoad (%)
No feedback
Hysteretic Feedback
70
75
80
85
90
95
100
100 80 60 40 20 0 -20 -40 -60 -80 -100
Eff
icie
ncy
(%
)∆I/ILoad (%)
No Feedback
Hysteretic Feedback
• Comparison of open-loop/close-loop SC circuit for high power (left: 10mW-
400mW, 2V→1V) and low power (right: 0.5mW-10mW, 1.2V→0.6V) loads
• Higher efficiency for Open loop regulation (low power loads)
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Switch Capacitor Model
ROUT has 2 asymptotic limits : Slow Switching Limit (RSSL) and Fast Switching Limit (RFSL)
RSSL => Ideal Switches, Current Impulsive in nature, Impedance inversely proportional to
Switching Frequency
RFSL => Switches and capacitance resistance dominate, capacitance act as fixed voltage
source, Impedance independent of Switching frequency
ROUT
~ √(RFSL
2 +RSSL
2)
m:n No-Load
Conversion ratio
Source : Seeman, “ A Design Methodology for Switched-Capacitor DC-DC Converter”, PhD Dissertation , UC Berkeley , May 2009
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Switch Capacitor Power Loss
• SSL impedance Loss : Charge transfer related loss =>
ILoad2.RSSL
• FSL impedance Loss : Switch conductance loss =>
ILoad2.RFSL
• Switch Drive Loss : Parasitic loss in the switches =>
Vswing2.N.Wswitch.Cgate.Fsw
• Bottom Plate Loss
• ESR Loss in Capacitor
Source : Michael Seeman, “ A Design Methodology for Switched-Capacitor DC-DC Converter”, PhD Dissertation , UC Berkeley , May 2009
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Power Loss Optimization
Source : Hanh-Phuc et al, “Design Techniques for Fully Integrated Switched-Capacitor DC-DC Converters”
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20
30
40
50
0
2
4
6
0
2
4
6
8
10
12
0
10
20
30
40
PTotal L
os
s (
mW
) X
10-3
PSSL L
os
s (
mW
) X
10-3
PFSLL
os
s (
mW
) X
10-3
PSwitching L
os
s (
mW
) X
10-3
X-Axis => Switch Area Y-Axis => Switching Frequency
Power Loss Breakdown
X
Y
39
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1.1
2.2
3.2
4.3
5.4
6.5
45
50
55
60
65
70
75
80
85
60
140
220
300380
460
Eff
icie
ncy (
%)
Switching Frequency (MHz)Switch Area (mm 2
)x 10 -3
1.1
2.2
3.2
4.3
5.4
6.5
45
50
55
60
65
70
75
80
85
60140
220300
380460
Eff
icie
ncy (
%)
Switching Frequency (MHz)
Switch Area (mm 2
)x 10 -3
Efficiency Versus Design Knobs
ILoad
= 200mA ILoad
= 50mA
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Output Impedance with imbalance
i1 (Layer1 current ) = i2 (Layer 2 current)
Rssl = 1/(2*fsw*Cfly)
Rfsl = 4R
i1 > i2 or i1<i2
Rssl = 1/(2*fsw*Cfly) + [2(∆V/Vin)]2
Rfsl = 4R[1+4 (∆V/Vin)2]
Increase in Rout with increase in imbalance and
lowering of Vin
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Output Impedance
0.00 0.02 0.04 0.06 0.08 0.10
1.35
1.36
1.37
1.38
1.39
1.40
1.41
Vin = 2V
Vin = 1V
Rfsl
Vdelta
(V)
More the imbalance, more Vdelta and more the loss
0.00 0.02 0.04 0.06 0.08 0.10
1.25
1.26
1.27
1.28
1.29
1.30
Vin: 1V
Vin: 2V
Rssl
Delta V (V)
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70
75
80
85
90
95
100
100 75 50 25 0 -25 -50 -75 -100E
ffic
ien
cy
(%
)
∆I/ILoad (%)
MIM Cap
Ideal Cap
Impact of Capacitor Parasitic
MIM cap Model (Including top/bottom plate
capacitance, plate and contact resistance
MOS Cap -> Highest Density(12nF/mm2), Max Bottom Plate Parasitic (7-10%)
MIM Cap -> Lower Density (2nF/mm2), Less parasitic (2-3%)
43
Impact of Capacitor Parasitic
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Interleaving – Ripple Mitigation
No interleaving
v
V/2 + ∆V
V/2 - ∆V
V/2 + ∆V
Phase 1
V/2 - ∆V
V/2 + ∆V
V/2 - ∆V
Phase 2
v
C1
C2
C2
C1
Fly caps never come parallel,
No energy loss through charge sharing
2-way interleaving
v
V/2 - ∆V
V/2 + ∆V
V/2 - ∆V
Phase 1
V/2 - ∆V
V/2 + ∆V
v
C1/2
C2/2
C2/2
C1/2
Fly caps come parallel to each other sharing 2∆V of charge
between them, leading to energy Loss 44
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Power Loss with Interleaving
Energy Loss (interleaving 2 way) =
1/2*c*(1/2*vin+vdel)^2+1/2*c*(1/2*vin-
vdel)^21/2*c*(vin1/2*(c*(1/2*vin+vdel)+
c*(1/2*vin-vdel))/c)^2-
1/8*(c*(1/2*vin+vdel)+c*(1/2*vin-vdel))^2/c
As ∆V increases, Powerloss due to charge sharing increases
More interleaving, less ∆V and less the intrinsic loss, but more the extrinsic loss
(from additional buffers and control circuitry needed for interleaving)
0.00 0.02 0.04 0.06 0.08 0.10
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
EnergyLoss/125MHz
Po
we
rLo
ss (
W)
Vdelta
(V)
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Finding Optimum Interleaved Stages
1way 2 way 4 way 8 way 16 way
0.070
0.072
0.074
0.076
0.078
0.080
0.082
0.084
0.086
0.088
0.090
0.092
Power Loss
Ripple
Interleaving
Po
wer
Lo
ss (
W)
0
20
40
60
80
100
120
140P
-P R
ipp
le (m
V)
46Tradeoff between P-P Ripple (performance) and Power Loss (Efficiency
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Efficiency : Conventional Versus
Stacked Load
50
55
60
65
70
75
80
85
90
50 100 150 200
Eff
icie
ncy
(%
)
ILoad (mA)
70
75
80
85
90
95
100
0 25 50 75 100
Eff
icie
ncy
(%
)∆I/ILoad (%)
Efficiency with varying conventional load (left) and stacked load (right). In Figure
(b), X-axis indicates relative imbalance (%) between the domains.
Source : Mazumdar, K.; Stan et al, Breaking the 3-D IC Power Delivery Wall. Asilomar Conference on Signals, Systems and Computers
(a) (b)
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3D IC scaling: more stacked layers
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SC Clock
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3V-2V
2V-1V
1V-0V
Cross coupled rows
of oscillators lock in
Phase
Horizontal and
vertical row buffer-
inverter delay
equivalent
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Phase-Frequency Locked Clock
50Time (ns)
Voltage(V)
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3D IC Power Delivery- TSV Bottleneck
• Smaller footprint : Fewer Power bumps
• Big P/G TSVs to deliver power to all the
stacked layers, causing congestion
• TSVs contribute to IR drop, reducing supply rail
integrity
Source: Sung Kyu Lim, “ 3D IC Circuit Design with Through-Silicon-Via : Challenges
and Opportunities “, GTCAD Laboratory 51
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TSV Trade-Offs
Change in IR drop with increasing
number of 3D IC layers and TSV
density
P-P Ripple improves due to TSV/3D
layers capacitive effect
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Layer1
Layer2
Layer3
Layer4
Layer5
Layer6
Power Grid
Layer1
Layer2
Layer3
Layer4
Layer5
Layer6
Power Grid
RegulatorsLayer1
Layer2
Layer3
Layer4
Layer5
Layer6
Power Grid
Conventional 3D:
Max No of TSVs
2_Layered 3D
Least No of TSVs
Tradeoffs between TSV count and
regulators
Clustered Voltage Stacking
3_Layered 3D
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Clustered Voltage Stacking
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Summary : Voltage Stacking in 3D IC
• 3D IC power delivery wall: at constant voltage cubic
increase in power/current but only quadratic
area/pins
• Voltage stacking can help break wall: quadratic
current and linear voltage
• Implicit regulation + explicit for imbalance
• Clustered Voltage Stacking
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Acknowledgments
• Funding from SRC, NSF, AMD, Intel
• Collaborators: Kaushik Mazumdar, Runjie
Zhang, Kevin Skadron
• IEEE CAS
• Universities of Victoria, British Columbia,
Washington and Portland State
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