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...
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
Electrostatic Discharge
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
Electrostatic Discharge
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
Electrostatic Discharge
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
Electrostatic Discharge
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
Electrostatic Discharge
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
Electrostatic Discharge
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.
Electrostatic Discharge
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
More Protection DevicesMore Protection Devices
Circuit diagram and partial layout of two stage 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
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
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
- Gate poly and sidewall spacers
- Degradation of dielectric strength due to current
Antenna EffectAntenna Effect
- Electrostatic charge proportional to area of poly
- Small gates connected to large poly area can cause significant damage
Antenna EffectAntenna Effect
- Poly area acts as antenna
- Effect also seen during ion implantation of the source / drain regions
Antenna EffectAntenna Effect
- Measurement of effect
- Magnitude of effect proportional to
Exposed Conductor Area
Gate Oxide Area
Antenna EffectAntenna Effect
- Separate area ratios computed for multiple layers
- Significant damage
conductor area > several hundred gate area
Antenna EffectAntenna Effect
• 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
Antenna EffectAntenna Effect
• 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)
ContaminationContamination
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.”
ContaminationContamination
• 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
ContaminationContamination
• 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
ContaminationContamination
• 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
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.
Particle RemovalParticle Removal
It takes about a minute, but one must wait patiently for the particle removal process to complete
Particle RemovalParticle Removal
Air is filtered and supplied to the lab through a very elaborate duct system
Particle RemovalParticle Removal
The system return air is acquired though floor vents. The supply / return are both perpendicular to the room to minimize “swirling.”
Mobile Ion descriptionMobile Ion description
Dissolve in SiO2 at elevated temperatures
Loss of mobility at normal temperatures
Effect 1/2Effect 1/2
Induce parametric shift in MOS transistor at threshold voltage Long term failures slow drift of threshold voltage
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