3D IC technology stacks multiple silicon layers vertically using through-silicon vias to connect the layers. This reduces interconnect length and delay. Motivations for 3D ICs include alleviating increasing interconnect delay issues and increasing the number of "nearest neighbors" for each transistor. Fabrication approaches include wafer bonding and epitaxial growth. Performance benefits include reduced timing delay and energy due to shorter interconnects. Design tools are needed to enable 3D IC design.

1. 3D IC technology
Pouya Dormiani
Christopher Lucas

2. What is a 3D IC?
“Stacked” 2D (Conventional) ICsCould be Heterogeneous…

3. Motivation
 Interconnect structures increasingly consume more of the power
and delay budgets in modern design
 Plausible solution: increase the number of “nearest neighbors” seen
by each transistor by using 3D IC design
 Smaller wire cross-sections, smaller wire pitch and longer lines to
traverse larger chips increase RC delay.
 RC delay is increasingly becoming the dominant factor
 At 250 nm Cu was introduced alleviate the adverse effect of
increasing interconnect delay.
 130 nm technology node, substantial interconnect delays will result.

4. 3D Fabrication Technologies
 Many options available for realization of 3D circuits
 Choice of Fabrication depends on requirements of
Circuit System
Beam
Recrystallization
Processed Wafer
Bonding
Silicon Epitaxial
Growth
Solid Phase
Crystallization
Deposit polysillicon
and fabricate TFTs
-not practial for 3D circuits
due to high temp of melting
polysillicon
-Suffers from Low carrier
mobility
-However high perfomance
TFT’s
have been fabricated using
low temp processing which
can be used to implement 3D
circuits
Bond two fully
processed wafers
together.
-Similar Electrical Properties
on all devices
-Independent of temp. since
all chips are fabricated then
bonded
-Good for applications where
chips do independent
processing
-However Lack of
Precision(alignemnt) restricts
interchip communication to
global metal lines.
Epitaxially grow a
single cystal Si
-High temperatures cause
siginificant cause significant
degradation in quality of
devices on lower layers
-Process not yet
manufacturable
Low Temp
alternative to SE.
-Offers Flexibilty of creating
multiple layers
-Compatible with current
processing environments
-Useful for Stacked SRAM
and EEPROM cells

5. Performance Characteristics
 Timing
 Energy
 With shorter interconnects in 3D ICs, both switching
energy and cycle time are expected to be reduced

6. Timing
 In current technologies, timing is
interconnect driven.
 Reducing interconnect length in
designs can dramatically reduce
RC delays and increase chip
performance
 The graph below shows the
results of a reduction in wire
length due to 3D routing
 Discussed more in detail later in
the slides

7. Energy performance
 Wire length reduction has an impact on
the cycle time and the energy dissipation
 Energy dissipation decreases with the
number of layers used in the design
 Following graphs are based on the 3D tool
described later in the presentation

8. Energy performance graphs

9. Design tools for 3D-IC design
 Demand for EDA tools
As the technology matures, designers will
want to exploit this design area
 Current tool-chains
Mostly academic
 We will discuss a tool from MIT

10. 3D Standard Cell tool Design
 3D Cell Placement
Placement by min-cut partitioning
 3D Global Routing
Inter-wafer vias
 Circuit layout management
MAGIC

11. 3D Standard Cell Placement
 Natural to think of a 3D
integrated circuit as
being partitioned into
device layers or planes
 Min cut part-itioning
along the 3rd
dimension
is same as minimizing
vias

12. Total wire length vs. Vias
 Can trade off increased total wire length for fewer inter-plane
vias by varying the point at which the design is partitioned
into planes
 Plane assignment performed prior to detailed placement
 Yields smaller number of vias, but greater overall wire length

13. Total wire length vs. Vias (Cont)
 Plane assignment not made until detailed placement
stage
 Yields smaller total wire length but greater number of vias

14. Intro to Global Routing
 Overview
Global Routing involves generating a “loose”
route for each net.
 Assigns a list of routing regions to a net without
actually specifying the geometrical layout of the
wires.
Followed by detailed routing
 Finds the actual geometrical shape of the net
within the assigned routing regions.
Usually either sequential or hierarchical
algorithms

15. Illustration of routing areas
x
z
y
x
z
y
Detailed routing of net when
routing areas are known

16. Hierarchical Global Routing
 Tool uses a hierarchical global routing
algorithm
Based on Integer programming and Steiner
trees
Integer programming approach still too slow
for size of problem and complexity (NP-hard)
Hierarchical routing methods break down the
integer program into pieces small enough to
be solved exactly

17. 2D Global Routing
 A 2D Hierarchical global router works by recursively
bisecting the routing substrate.
 Wires within a Region are fully contained or terminate at a
pin on the region boundry.
 At each partitioning step the pins on the side of the
routing region is allocated to one of the two subregions.
 Wires Connect cells on both sides of the partition line.
 These are cut by the partition and for each a pin is inserted
into the side of the partition
 Once complete, the results can be fed to a detailed
router or switch box router (A switchbox is a rectangular
area bounded on all sides by blocks)

18. Illustration of Bisection

19. Extending to 3D
 Routing in 3D consists of routing a set of aligned
congruent routing regions on adjacent wafers.
 Wires can enter from any of the sides of the routing region in
addition to its top and bottom
 3D router must consider routing on each of the layers in
addition to the placement of the inter-waver vias
 Basis idea is: You connect a inter-waver via to the port
you are trying to connect to, and route the wire to that via
on the 2D plane.
 All we need now is enough area in the 2D routing space to route
to the appropriate via

20. 3D Routing Results
Percentage Of 2D
Total wire Length
Minimizing for Wire Length:
2 Layers ~ 28%
5 Layers ~ 51 %
Minimizing for via count:
2 Layers ~ 7%
5 Layers ~ 17%

21. 3D-MAGIC
 MAGIC is an open source layout editor developed at UC
Berkeley
 3D-MAGIC is an extension to MAGIC by providing
support for Multi-layer IC design
 What’s different
 New Command :bond
 Bonds existing 2D ICs and places inter-layer Vias in the design
file
 Once Two layers are bonded they are treated as one entity

22. Concerns in 3D circuit
 Thermal Issues in 3D-circuits
 EMI
 Reliability Issues

23. Thermal Issues in 3D Circuits
 Thermal Effects dramatically impact interconnect and device reliability in 2D
circuits
 Due to reduction in chip size of a 3D implementation, 3D circuits exhibit a sharp
increase in power density
 Analysis of Thermal problems in 3D is necessary to evaluate thermal robustness of
different 3D technology and design options.

24. Heat Flow in 2D
Heat generated arises due to switching
In 2D circuits we have only one layer of Si to
consider.

25. Heat Flow in 3D
With multi-layer circuits , the upper
layers will also generate a significant
fraction of the heat.
Heat increases linearly with level increase

26. Heat Dissipation
 All active layers will be insulated from each other by layers of dielectrics
 With much lower thermal conductivity than Si
 Therefore heat dissipation in 3D circuits can accelerate many failure
mechanisms.

27. Heat Dissipation in
Wafer Bonding versus Epitaxial Growth
 Wafer Bonding(b)
 2X Area for heat dissipation
 Epitaxial Growth(a)

28. Heat Dissipation in
Wafer Bonding versus Epitaxial Growth
 Design 1
 Equal Chip Area
 Design 2
 Equal metal wire pitch

29. High epitaxial temperature
Temperatures actually higher for Epitaxial second layers
Since the temperature of the second active layer T2 will
Be higher than T1 since T1 is closer to the substrate
and T2 is stuck between insulators

30. EMI in 3D ICs
 Interconnect Coupling Capacitance and cross talk
 Coupling between the top layer metal of the first active layer and the device on
the second active layer devices is expected

31. EMI
 Interconnect Inductance Effects
Shorter wire lengths help reduce the
inductance
Presence of second substrate close to global
wires might help lower inductance by
providing shorter return paths

32. Reliability Issues?
 Electro thermal and Thermo-mechanical effects
between various active layers can influence electro-
migration and chip performance
 Die yield issues may arise due to mismatches
between die yields of different layers, which affect
net yield of 3D chips.

33. Implications on Circuit Design
and Architecture
 Buffer Insertion
 Layout of Critical Paths
 Microprocessor Design
 Mixed Signal IC’s
 Physical design and Synthesis

34.  Buffer Insertion
 Use of buffers in 3D circuits to break up long interconnects
 At top layers inverter sizes 450 times min inverter size for the relevant
technology
 These top layer buffers require large routing area and can reach up to
10,000 for high performance designs in 100nm technology
 With 3D technology repeaters can be placed on the second layer and
reduce area for the first layer.
Buffer Insertion

35. Layout of Critical Paths and
Microprocessor Design
 Once again interconnect delay dominates in 2D
design.
 Logic blocks on the critical path need to
communicate with each other but due to
placement and desig constraints are placed far
away from each other.
 With a second layer of Si these devices can be
placed on different layes of Si and thus closer to
each other using(VILICs)
 In Microprocessor design most critical paths
involve on chip caches on the critical path.
 Computational modules which access the cache
are distributed all over the chip while the cache
is in the corner.
 Cache can be placed on a second layer and
connected to these modules using (VILICs)

36. Mixed Signal ICs and Physical
Design
 Digital signals on chip can couple and interfere with
RF signals
 With multiple layers RF portions of the system can be
separated from their digital counterparts.
 Physical Design needs to consider the multiple layers
of Silicon available.
 Placement and routing algorithms need to be
modified

37. Conclusion
 3D IC design is a relief to interconnect
driven IC design.
 Still many manufacturing and
technological difficulties
 Needs strong EDA applications for
automated design