Failure Mechanisms Team Members: Noah Boydston Kyle Brown Robert Colville Linsy Cook Wissam Khazem...

Failure MechanismsFailure Mechanisms

Team Members:

Noah Boydston

Kyle Brown

Robert Colville

Linsy Cook

Wissam Khazem

GeeHyun Park

Failure Mechanisms Failure Mechanisms

IC’s have many subtle flaws that dispose them towards failure

Engineers typically have two tools to minimize IC failure

- Operation under extremely stressful conditions to test them

- Rearranging or improving circuit layout for more robust circuit

Electrical OverstressElectrical Overstress

Very general type of IC failure of which there are 3 primary subtypes

-Electrostatic Discharge (ESD)

- Electromigration

- Antenna effect

Electrostatic Discharge Electrostatic Discharge

When two substances are separated an ES charge develops

Caused by the removal of electrons from surface atoms of materials

Factors• Magnitude of static

charge• Intimacy of contact • Rate of separation

Electrostatic Discharge

Failures – catastrophic

• Most hard damage to semiconductors occurs below human sensitivity around 4000V

• 2 primary hard failure types

• voltage punch through – CMOS and MOS with very thin oxide dielectric layers

• P-N junction degradation – bipolar circuits excessive power dissipation


Failures – noncatastrophic / intermittent upset

• Degradation

• Increased leakage current

• Lower breakdown voltages of P-N junctions

• Softening of the knee of V-I curve of a P-N junction

• Decreased dielectric constant

• Problems may not occur until later with additional stresses

• Intermittent upset – no hard damage, but results in data loss or noise


Failure Modes

• Thermal Secondary breakdown – high power, small junction causes junction melting

• Metallization Melt – ESD causes the metal to melt and bond wires to fuse, usually causes and open circuit

• Dielectric Breakdown – high potential difference across a dielectric region cause a punch through

• Bulk Breakdown – changes in junction parameters caused by excessive temperature at the junction


Susceptibility to ESD

• Human Body Test Model

• Simulates a charged

person or object that

comes into contact with

a device

• Uses a decaying exponential waveform

• Human Body Capacitance is 50 pF to 200 pF with a resistance of 1k to 5k

Human Body Test Model

Resistor

1.5k

0-1.5kV DC

Voltage SourceTestDevice

Current Limiting Resistor

Capacitor

100-150pF


Susceptibility to ESD

• Charged Device model

• Device is charged to between 1 to 1.5 kV

• One pin is discharged to a low impedance ground

Voltage Source

0-1.5kV DC

TestDevice

Impedance

Charged Device Model

Low

Current Limiting Resistor


Assembly Protection

• RFI / EMI Design zoning

• sensitive devices shielded by less

sensitive parts

• Faraday Shielding - conductive films or foils

• Increasing the number of ground

conductors in a PCB

• An ESD spark is more likely to hit a rough edge than a smooth one so ground is etched with a pattern

Input/OutputConnections

Zone 2ModeratelySensitive Devices

Barrier/Shielding

Barrier/Shielding

Zone 3Insensitive Devices

Zone 1Sensitive Devices

Barrier/Shielding


Assembly Protection

• Grounding

• Multipoint

• Fishbone type

• Single point

• Cabling – wiring to ESD sensitive devices

Twisted pair Decreasing

Shielded pair Effectiveness

Plain twisted pair

} Usually a hybrid of these is used.


ESD Protective Equipment• Wrist Straps connected to ground

• ESD protective work stations

• Protective packaging

• Protective bags

• Conductive foam

• ES detectors

• Conductive floors

• Special clothing

• Air Ionizers

Protective Circuits to Minimize Protective Circuits to Minimize ESD DamageESD Damage

Connect external leads to high series resistance, shunt paths, or voltage clamps

ESD protective circuits provide minimal protection often from only as much as 800 volts

Such measures do not totally eliminate ESD damage but reduce it drastically

More Protection DevicesMore Protection Devices

Use of Faraday Shielding to protect from ESD – elaborate but highly effective


Circuit diagram and layout of simple zener clamp


Circuit diagram and partial layout of two stage zener clamp


Buffered zener clamp

ElectromigrationElectromigration

• OverviewSlow wearout of metallization caused by excessive current densities

Becomes a problem when current densities closely approach or exceed 500,000 A/cm2

For sub-micron width leads, this translates to currents of only a few milliamps

Causes of ElectromigrationCauses of Electromigration

Impacting of electrons causes gradual shifting of Aluminum atoms from their normal lattice sites (see picture)

Aluminum atoms move away from grain boundaries causing voids to form between grains

Reduced area of wire increases resistance and worsens problem

Voiding in AluminumVoiding in Aluminum

Voiding in aluminum

due to electromigration

Preventing Damage From ElectromigrationPreventing Damage From Electromigration

Refractory barrier metals such as W, Ti, and Mo, can prevent catastrophic migration failure

When overlying Al shifts away, refractory metals remain

Can be deposited by vacuum metal deposition techniques or by sputtering

Sputtering is usually used because it is cheaper and more efficient

Refractories are especially useful in contacts and vias where Al metallization thins

New IBM Chip using only W New IBM Chip using only W and Cu, no Aland Cu, no Al

Cross section of same IBM ChipCross section of same IBM Chip

Preventing Damage from Preventing Damage from Electromigration - AlloysElectromigration - Alloys Al metallization is now alloyed with 0.5% to 4.0%

Cu

Due to low solubility in Al, Cu accumulates at the grain boundaries and helps prevent voiding

Such an alloy has 5-10 times the current carrying capacity of Al alone

Other Migration Prevention Other Migration Prevention MeasuresMeasures Rearranging leads to prevent crossing of oxide steps

Heating die to smooth corners of oxide steps before Al metallization is deposited – improves Al coverage of the steps

Use wider leads than normal when crossing oxide steps – leads should widen somewhat before they reach the steps

Compressively stressed overcoats inhibit void formation by confining Al under pressure

Other issues of importance in Other issues of importance in ElectromigrationElectromigration Displaced Al can short adjacent leads, so using

refractory metal is not a perfect solution

Displaced Al can also seep into damaged dielectrics causing a short

Antenna EffectAntenna Effect

• Dry Etching- Intense E-field

- Accumulation of electrostatic charges


- Gate poly and sidewall spacers

- Degradation of dielectric strength due to current


- Electrostatic charge proportional to area of poly

- Small gates connected to large poly area can cause significant damage


- Poly area acts as antenna

- Effect also seen during ion implantation of the source / drain regions


- Measurement of effect

- Magnitude of effect proportional to

Exposed Conductor Area

Gate Oxide Area


- Separate area ratios computed for multiple layers

- Significant damage

conductor area > several hundred gate area


• Prevention - Etching of poly and sidewall spacers

- Insert of metal jumper

- Escape route for charges

- Reduces area of the poly connected to gate oxide

- Removed after etching or implantation


• Prevention - Etching of metal layers

- Layers connected to diffusions provide leak path

- Jumpers inserted for layers not yet connected to diffusions

ContaminationContamination

• Vulnerability- Proper manufacturing techniques to minimize contamination

- Two major types of contamination - Dry Corrosion - Mobile Ion Contamination (Gee)


As stated by the textbook….

“The aluminum metal system will corrode if exposed to ionic contaminants in the presence of moisture. Only trace amounts of water are necessary to initiate this so-called dry corrosion

All modern integrated circuits are covered with a protective overcoat that acts as a secondary moisture barrier.”


• Dry CorrosionWater alone cannot corrode aluminum, but many ionic substances dissolve in water to form relatively corrosive solutions.

- Effects

-Water alone cannot corrode aluminum

- Phosphosilicate glasses - moisture phosphoric acid corrosion

- Halogen Ions - Chloride - Bromide


• Dry Corrosion

- Preventative Measures

- Design

- Minimize the number and size of all PO openings

- The production die should not include any unnecessary openings

- Metal should overlap bondpad openings on all sides

- Openings should be made as small as possible

- No circuitry should appear within the opening


• Prototype Design / Manufacturing

- Clean Room

- Cleanrooms are 10,000 times cleaner than hospital operating rooms

- Equipment

- Air / fluid filtration

- Clothing

- Can be a 43 step process

Particle RemovalParticle Removal

The clothing worn covers most of the body.


Air flows across the body to remove any foreign particles prior to entering cleanroom.

There is also “self patting” of the body to

knock loose any stubborn particles.


It takes about a minute, but one must wait patiently for the particle removal process to complete


All must be serious about maintaining a “clean” environment.


Air is filtered and supplied to the lab through a very elaborate duct system


Some stations have independent air filter systems


The system return air is acquired though floor vents. The supply / return are both perpendicular to the room to minimize “swirling.”


Fluids are filtered


Tools are kept clean when not in use


Materials are protected when not in use (1/2)


Materials protected when not in use (2/2)


The cleanroom is kept in a very

orderly condition

Mobile Ion descriptionMobile Ion description

Dissolve in SiO2 at elevated temperatures

Loss of mobility at normal temperatures

Effect 2/2Effect 2/2

The positive gate repels mobile sodium to oxide-silicon interface

Effect 1/2Effect 1/2

Induce parametric shift in MOS transistor at threshold voltage Long term failures slow drift of threshold voltage

Temperature SolutionTemperature Solution

Bake at 200°C and redistribute mobile ions

Preventative MeasuresPreventative Measures

Purer chemicals and improve process technique

Phosphor to gate oxide stabilize to improve alkali metal contaminants

Dielectric PolarizationDielectric Polarization

Threshold shift by dielectric polarization more predictable than mobile ions

Use phosphorus-doped polysilicon gate rather than gate oxides

Phosphorus-doped polysilicon immobilizes alkali metals

Moisture from outside package brings in sodium. This can be reduced by improving package material to slow ingress of sodium ions

Protection OvercoateProtection Overcoate

Silicon nitride impermeable to mobile ions

Phosphorus-doped glasses

It can serve as a final line of defense against impurities

Minimum number of probe pads needed, and they should be kept far away from sensitive analog circuit

Scribe SealsScribe Seals

Narrow content strips surrounds active area of die and continuous ring

P-type diffusion

Guaranteed minimum area of substance contacts

Surface EffectSurface Effect

Surface region of high electric field intensity

Surface electric field induces the formation of the parasitic channels

Hot Carrier InjectionHot Carrier Injection

Weak electric field causes an overall drift of carriers but does not materially affect their instantaneous velocity, while a strong electric field actually increases the instantaneous velocity of the carriers

Effect Effect

MOS can generate hot carriers when operated in saturation region at high-drain-to-source voltages

The pinched- off portion of the channel slowly grows wider, aAs the drain-to-source voltage increases

The electric field becomes large to generate hot carriers near the drain end of the transistor

Effect On PerformanceEffect On Performance

Hot carriers produced at the drain end of the transistor collide with the lattice atoms

Few of the recoiling carriers travel upward into the overlying oxide

Most of the carriers pass trough the oxide and return to the silicon

Few get trapped at defect sites within the oxide, will represent a fix oxide charge

Parametric ShiftParametric Shift

Caused by hot carriers

Can be partially or completely reversed

The parametric shift vanishes as the fixed oxide change dissipates

Avalanche JunctionAvalanche Junction

It occurs near the surface in most diffused junction

Some of the hot carriers produced travel into the overlying oxide

The avalanche voltage slowly increases during operation (called Zener walk-out)

Zener Walkout MechanismZener Walkout Mechanism

Junction diode’s reverse break down is observed using a curve tracer

Emitter-base Zeners can exhibit up to 200mV of walk-out

Preventative Measures1/2Preventative Measures1/2

Lightly Doped Drain (LDD) structure

Redesign the circuit

Transistor– Used as a switch– Fully on in witch they are in linear region– Fully off in witch they are in cutoff region– They can withstand voltages far beyond the onset of the hot

carrier

Preventative Measures2/2Preventative Measures2/2

Long channel devices– Vicinity of the drain Produce the hot carrier– The rest of the channels remains unaffected – Increasing the channel length by a few micro far a few extra

volts

Base-emitter Zener diode

Special Appearance by:Special Appearance by:

Carlos, Corey, Eric, and Fariba

Failure Mechanisms Team Members: Noah Boydston Kyle Brown Robert Colville Linsy Cook Wissam Khazem...

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Transcript of Failure Mechanisms Team Members: Noah Boydston Kyle Brown Robert Colville Linsy Cook Wissam Khazem...