Thermal Limitations and Solutions for Microelectronic

Systems Driven by the Economics of Moore’s Law

Paul J. Boudreaux

Consultant & Senior ScientistUniversity Research FoundationLaboratory for Physical Sciences

College Park, MD 20740boudreau@eng.umd.edu Tel:301-935-6547

April 4, 2005 UMBC Presentation

Think the shuttle has a thermal management problem?

Consider the next generation of high performance chips:

>160 Watts/cm2

40W/cm2

68W/cm2

93W/cm2

What does the future portend for high performance microelectronic systems for thermal & power distribution?

State-of-the-Art Thermosyphon in Commercial Work Stations

• 100 Watt - 6 pipe finned tower

• 30 cubic inches• 30 fins per stack• 100-150 ft3/min air flow

required• Cools only 1 chip (1cm2 )

Modern Impediments to Chip Design

• Thermal Limitations < 90 oC jc

• Current Density: JAl < 4 x 105Amps/cm2

• Material Breakdown Potential, Emax= Vmax/dmin

Emax (typical) = 1 Volt/10 nanometers = 100 x 106 V/m

• Power Density < 250 Watts/cm2

• Random Soft Errors in Logic & Analog Circuits– Signals are a countable number of electrons ~ < 105 electrons

– Thermal noise fluctuations

– Quantum noise: particle decay, cosmic rays, radiation induced SEU

Consider a Mixed Analog/Digital/MEMs System’s In-Plane Thermal Gradients

• dT/dt , T and CTE are major problems• Analog voltages are temperature sensitive• In-situ stress/strain causes failure thru T

Potential Solutions:

Heat spreaders with high K values attached to the chip can help alleviate lateral T problems. Placing the system or components into a forced isothermal environment also reduces T, dT/dt and CTE related problems. Severing the thermomechanical heat path reduces or eliminates shock and vibration from entering the system while reducing weight.

Shannon-von Neumann-Landauer (SNL) Switching

Energy Per Bit*

Ebit>ESNL= kBT ln2 = 0.017eV @300K

Using the Uncertainty Relations for x & t, one calculates the max integration density of binary switches ~ 5 x 1013 devices/cm2, Power Dissipation per unit Area

~ 4 x 106 W/cm2 !

* ”Limits to Binary Logic Switch Scaling – A Gedanken Model”, Zhirnov, et al, Proc. of IEEE, Vol. 91, No. 11, Nov 2003, pp1934-1938.

We are at a Thermal Management Barrier (TMB) long before we get to the limits from Moore’s law

or the semiconductor laws of physics!

Moore’s Law

Spray Cooling: One Possible Answer

A Phase Change Methodology to Heat Removal that is Isothermal While Also

Severing the Thermomechanical Heat Path

Phase Change: Spray Cooling System Can Meet These Requirements

But It Requires An Active Cooling System

Heat can be acquired at high power densityHeat can be acquired at high power densitylevels, enabling circuitry to be compacted tolevels, enabling circuitry to be compacted tovery small board areas, while heat is rejectedvery small board areas, while heat is rejectedremotely, where providing larger condenserremotely, where providing larger condenser(radiator) areas and airflow is not a problem.(radiator) areas and airflow is not a problem.

Electronic Circuit BoardElectronic Circuit Board

Spray Cooling Cap

HeatHeatAcquisitionAcquisition

Delivery and ReturnDelivery and ReturnTubes (Can be Part ofTubes (Can be Part ofChassis Frame withChassis Frame withQuick-Disconnects)Quick-Disconnects)

Cooled MCMCooled MCMor IC Packageor IC Package

Liquid OutLiquid Out

Vapor ReturnVapor Return

FanFan

PumpPump FilterFilter

Compact Pump-Condenser UnitCompact Pump-Condenser UnitTypically Mounted at Rear of Enclosure or Typically Mounted at Rear of Enclosure or

ExternallyExternally

Condenser/Condenser/Heat ExchangerHeat Exchanger

HeatHeatRejectionRejection

Low powerLow powerdensity, low density, low TTheat rejectionheat rejection

Isothermal Spray Cooling Directly Onto the Chip Surface

Pressurized LiquidPressurized Liquidfrom Pump Entersfrom Pump Enters

Spray ManifoldSpray Manifold

Spray Nozzles CreateSpray Nozzles CreateFine Cooling MistFine Cooling Mist

Impinging on IC ChipsImpinging on IC Chips

Vapor and Excess Vapor and Excess Spray Drawn Out Spray Drawn Out Suction Tube for Suction Tube for

Return to CondenserReturn to Condenser

SuctionSuctionOutletOutlet

LiquidLiquidInletInlet

Printed Circuit Printed Circuit BoardBoard

MCM SubstrateMCM Substrate(Illustrated with Integral (Illustrated with Integral

BGA Package)BGA Package)

IC DieIC Die(Shown Flip-Chip(Shown Flip-Chip

Mounted with Underfill)Mounted with Underfill)

1 atm1 atm(Not Pressurized)(Not Pressurized)

TypicalTypical

20-30 psi20-30 psiTypicalTypical

400 Watt Ceramic Package/ MCM with Spray Cooled Lid

Top View w/Underside of Lid Side View w/Lid Attached

Fluid InVapor Out

400W MCM Spray Nozzle Array

Phase Change: Spray Cooling Attributes

• Used for point-of-source cooling or system wide cooling• Demonstrated > 150 W/cm2 heat extraction• Forces Isothermal surface conditions wherever phase change occurs• Severs thermomechanical heat path enabling vibration and shock isolation• Direct heat removal from the IC chip surface for minimum j

• Conformable to surfaces, small liquid volumes required • Phase change fluids enable the choice of operating conditions over a large

temperature range (77K-380K)• Active cooling requires a “dead man’s hand” system to remove trapped heat

within the system when power fails• Capable of reliable zero-g operation, gives additional cosmic ray protection

from SEU due to electrostatic charge buildup in spacecraft• Mechanical pump reliability can be 99.999999% with redundancy• Phase change fluids require chemical and mechanical filters for long term

reliability; fluids must be chemically stable, environmentally friendly• Will not meet $0.10/Watt commercial costs in present form

Consider the Materials Used for the Thermal Conductivity Part of the Problem

• Simple thermal conductivity model -

P=dq/dt = K A T/d

• State-of-the-Art CMOS

• Random logic

• Max clock rate

• 3D Interconnected layersMan-made polycrystalline diamond could passively meet this requirement for thermal conductivity, K > 13W/cm C.

Note: KDiamond= 20 W/cm C @ 25C, and also,

KDiamond= 100W/cm C @ 100 Kelvin

Assumptions:

Electro-Magnetic Arc Deposition of Synthetic Diamond

Courtesy of Norton Diamond Film

Polycrystalline Synthetic Diamond

Visible &Thermal Images of Ceramic and Diamond MCMs

Visible Light Image

False Color IR Thermal Image @ 10 Watts per Chip

Ceramic Substrate

Diamond Substrate

Still Air & Room

Temperature

Cray Research J90 Supercomputer

MCM Version of a Cray J90

SGI/Cray Research J90 Supercomputer

Rack Mounted Flight Test Version

Cray Hard Drive

Aircraft Interface

J90 Cabinet Module

Monitor

Power Supply

All Computers Are Architecturally Two Dimensional

The first two architectural dimensions are used for the circuit layout.

The third architectural dimension is used for heat transfer.

25KW 3D Diamond Cube Design

Stacked Diamond MCMs Form Cube Design

Diamond MCM with Cu-Polyimide Interconnects Layers @ 625 Watts/MCM

Moving the heat laterally through the diamond substrate to the edge fins can produce reliable operational conditions in this 3D prototype demonstration system* at 2.5KW. Four 3D MCM interconnected substrates make up this prototype system. Spray cooling (phase change) extracts the heat from the two edge fin areas.* “Thermal Analysis of Spray Cooled 3D Interconnected Diamond Substrate MCMs: Comparison With Experimental Measurements”, Boudreaux, et al, IEEE on-line Journal Transactactions on Device and Materials Reliability, December 2004, pp594-604.

Schematic of 3D Machine

Photo of Sub-nanosecond Cycle Time Machine

Consider High Performance CMOS Operating at 1 Volt Bias

• A 100 Watt chip requires 100 Amperes• Electromigration concerns limit current densities

to < 105 A/cm2

• Power distribution with distributed power converters is only 40 to 60% efficient, generating even more waste heat

How to Handle the Power Distribution?

• Why are power converters so inefficient?– High values of RDS-on (>20 miliOhms) for semiconductor

switches

– Transformer inefficiency – poor ferrite performance

– Switching power converters use low frequency <150 KHz

• Is there a better way?– Integrated converters on chip at 100-200MHz

– New semiconductor switch design

– Nanoparticle ferrites for laminar transformers on chip

New Silicon NMOS 200A@1.2V Switch Assembly

• Si NMOS via MOSIS• 0.4 X 0.4 cm2 chip• Copper BCB Fine line Package

Interconnect• 10 micron thick Cu• C4 Flip Chip Bonded• 20 mil Cu-Invar-Cu Source Contact

Plane• 17 meter Gate length

Chip Packaged Chip

Top view

Side view

400 Amperes @ 1.2 Volts CMOSR DS(on)=179 microOhms

- 2 picosecond “Turn on” time

- CMOS Gate length 17 Meters

- Made with MOSIS technology

- 93% efficient power converter

Working prototype of switching power converter

Conclusions & Observations

• Spray cooling has demonstrated the capability to handle power density > 150 W/cm2

• Polycrystalline diamond is an excellent CTE matched heat spreader for Silicon ICs

• Power converters “on-chip” are possible with efficiencies > 90%

• On-chip currents of 200 to 400 Amperes are reliable at 1 volt• Random errors are a significant problem:

– Redundancy (voting) is often not a viable solution with limited power– New “random error hardened” logic designs are required– Analog designs are now limited by these random level signals

A Case Study in Thermal Management and Power Distribution For a 3D

Interconnected High Performance Microelectronic System

Paul J. Boudreaux

Consultant & Senior ScientistUniversity Research FoundationLaboratory for Physical Sciences

College Park, MD 20740boudreau@eng.umd.edu Tel:301-935-6547

Laser Embedded Graphite Resistors in Diamond

“As Grown” Diamond cut with a laser showing dark “residue”

around circumference

Laser induced graphite resistors in a diamond

substrate with metal interconnect*

1K Ohm @ 500 Watts

* Boudreaux, et al, “Laser Induced Graphite Resistors in Synthetic Diamond”, International Journal of Microelectronics & Electronic Packaging, 1996, Volume 19 , pp 169-177.

Moving & Expelling Heat

• Low system overhead• Isothermal environment• j < 90 oC• Die attach thermal impedance

minimized• 3D interconnections allowed• Clock speeds > 4GHz• Bias voltage ~ 1 Volt

• High thermal conductivity materials

• CTE matching materials• High (>90%) efficiency power

supplies• Phase change heat removal

WHY? The “best” switching supplies are limited by the switching device’s RDS-on impedance, typically 10 – 20 miliOhms. What if a switching transistor were made with a gate length of 50 meters instead of 2 – 3 microns? Then RDS-on would be < 20 microOhms! At a clock speed of 200 MHz instead of 100 kHz, the L-C-R components are fully embedded and Integrated into die

déjà vuThis is not the first time the industry has

run into TMB• Vacuum tube – ENIAC’s MTBF =15min

Solution: Change Technology to transistors

• Bipolar semiconductors – TTL, ECL, LSI~100W/cm2

Solution: Change Technology to CMOS

• CMOS – ULSI > 100 W/cm2 Solution: ? Today we are better situated with new materials and cooling technology to deal with the problem.

NOTE: Even if CMOS is not used, the thermal problem remains, i.e. SNL data.

3D structures introduce a paradigm shift in thermal design because prior systems were architecturally two dimensional; the third dimension was the surface where the heat was extracted. In a true 3D environment, this third dimension is used by interconnects, power distribution and bonding. New thermal concepts will be required to extract heat from such 3D systems.