3D IC technology

Pouya DormianiChristopher Lucas

What is a 3D IC?

“Stacked” 2D (Conventional) ICsCould be Heterogeneous…

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.

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’shave 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

Performance Characteristics

Timing Energy With shorter interconnects in 3D ICs, both switching

energy and cycle time are expected to be reduced

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

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

Energy performance graphs

Design tools for 3D-IC design

Demand for EDA toolsAs the technology matures, designers will

want to exploit this design area Current tool-chains

Mostly academic We will discuss a tool from MIT

3D Standard Cell tool Design

3D Cell PlacementPlacement by min-cut partitioning

3D Global Routing Inter-wafer vias

Circuit layout managementMAGIC

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

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

Total wire length vs. Vias (Cont)

Plane assignment not made until detailed placement stage

Yields smaller total wire length but greater number of vias

Intro to Global Routing

OverviewGlobal 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

Illustration of routing areas

x

z

y

x

z

y

Detailed routing of net when routing areas are known

Hierarchical Global Routing

Tool uses a hierarchical global routing algorithmBased 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

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)

Illustration of Bisection

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

3D Routing ResultsPercentage Of 2D Total wire LengthMinimizing for Wire Length:

2 Layers ~ 28%

5 Layers ~ 51 %

Minimizing for via count:

2 Layers ~ 7%

5 Layers ~ 17%

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

Concerns in 3D circuit

Thermal Issues in 3D-circuits EMI Reliability Issues

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.

Heat Flow in 2DHeat generated arises due to switchingIn 2D circuits we have only one layer of Si to consider.

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

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.

Heat Dissipation in

Wafer Bonding versus Epitaxial Growth Wafer Bonding(b)

2X Area for heat dissipation

Epitaxial Growth(a)

Heat Dissipation in Wafer Bonding versus Epitaxial Growth

Design 1 Equal Chip Area

Design 2 Equal metal wire pitch

High epitaxial temperature

Temperatures actually higher for Epitaxial second layers Since the temperature of the second active layer T2 willBe higher than T1 since T1 is closer to the substrate and T2 is stuck between insulators

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

EMI

Interconnect Inductance EffectsShorter wire lengths help reduce the

inductance Presence of second substrate close to global

wires might help lower inductance by providing shorter return paths

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.

Implications on Circuit Design and Architecture

Buffer Insertion Layout of Critical Paths Microprocessor Design Mixed Signal IC’s Physical design and Synthesis

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

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)

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

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