Transcript L11b_4345_Sp02

Failure Mechanisms
Team Members:
Noah Boydston
Kyle Brown
Robert Colville
Linsy Cook
Wissam Khazem
GeeHyun Park
Failure Mechanisms
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
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 Overstress
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Very general type of IC failure of which there are 3
primary subtypes
-Electrostatic Discharge (ESD)
- Electromigration
- Antenna effect
Electrostatic Discharge
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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
Resistor
Susceptibility to ESD
Current
1.5k
• Human Body Test Model
Lim iting
Resistor
• Simulates a charged
Test
Capacitor
Voltage Source
Device
100-150pF
person or object that
0-1.5kV DC
comes into contact with
Human Body Test Model
a device
• Uses a decaying exponential waveform
• Human Body Capacitance is 50 pF to 200 pF with a
resistance of 1k to 5k
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
Current
Lim iting
Resistor
Voltage Source Test
Device
0-1.5kV DC
Low
Im pedance
Charged Device Model
Electrostatic Discharge
Barrier/Shielding
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
Barrier/Shielding
Barrier/Shielding
Zone 1
Sensitive
Devices
Zone 2
Moderately
Sensitive Devices
Zone 3
Insensitive
Devices
Input/Output
Connections
Electrostatic Discharge
Assembly Protection
• Grounding
• Multipoint
Usually a hybrid
• Fishbone type
of these is used.
• Single point
• Cabling – wiring to ESD sensitive devices
Twisted pair
Decreasing
Shielded pair
Effectiveness
Plain twisted pair
}
Electrostatic Discharge
ESD Protective Equipment
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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
ESD Damage
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Connect external leads to
high series resistance, shunt
paths, or voltage clamps
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ESD protective circuits
provide minimal protection
often from only as much as
800 volts
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Such measures do not totally
eliminate ESD damage but
reduce it drastically
More Protection Devices
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Use of Faraday
Shielding to protect
from ESD – elaborate
but highly effective
More Protection Devices
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Circuit diagram and layout of simple zener clamp
More Protection Devices
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Circuit diagram and partial layout of two stage
zener clamp
More Protection Devices
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Buffered zener
clamp
Electromigration
• Overview
Slow 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 Electromigration
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Impacting of electrons
causes gradual shifting of
Aluminum atoms from their
normal lattice sites (see
picture)
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Aluminum atoms move away
from grain boundaries
causing voids to form
between grains
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Reduced area of wire
increases resistance and
worsens problem
Voiding in Aluminum
Voiding in aluminum
due to electromigration
Preventing Damage From Electromigration
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Refractory barrier metals such as W, Ti, and Mo, can prevent
catastrophic migration failure
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When overlying Al shifts away, refractory metals remain
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Can be deposited by vacuum metal deposition techniques or by
sputtering
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Sputtering is usually used because it is cheaper and more
efficient
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Refractories are especially useful in contacts and vias where Al
metallization thins
New IBM Chip using only W and
Cu, no Al
Cross section of same IBM Chip
Preventing Damage from
Electromigration - Alloys
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Al metallization is now alloyed with 0.5% to 4.0%
Cu
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Due to low solubility in Al, Cu accumulates at the
grain boundaries and helps prevent voiding
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Such an alloy has 5-10 times the current carrying
capacity of Al alone
Other Migration Prevention
Measures
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Rearranging leads to prevent crossing of oxide steps
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Heating die to smooth corners of oxide steps before
Al metallization is deposited – improves Al coverage
of the steps
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Use wider leads than normal when crossing oxide
steps – leads should widen somewhat before they
reach the steps
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Compressively stressed overcoats inhibit void
formation by confining Al under pressure
Other issues of importance in
Electromigration
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Displaced Al can short adjacent leads, so using
refractory metal is not a perfect solution
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Displaced Al can also seep into damaged
dielectrics causing a short
Antenna Effect
• Dry Etching
- Intense E-field
- Accumulation of electrostatic charges
Antenna Effect
- Gate poly and sidewall spacers
- Degradation of dielectric strength due to current
Antenna Effect
- Electrostatic charge proportional to area of poly
- Small gates connected to large poly area can cause significant
damage
Antenna Effect
- Poly area acts as antenna
- Effect also seen during ion implantation of the source / drain regions
Antenna Effect
- Measurement of effect
- Magnitude of effect proportional to
Exposed Conductor Area
Gate Oxide Area
Antenna Effect
- Separate area ratios computed for multiple layers
- Significant damage
conductor area > several hundred
gate area
Antenna 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 Effect
• Prevention
- Etching of metal layers
- Layers connected to diffusions provide leak path
- Jumpers inserted for layers not yet connected to diffusions
Contamination
• Vulnerability
- Proper manufacturing techniques to minimize contamination
- Two major types of contamination
- Dry Corrosion
- Mobile Ion Contamination (Gee)
Contamination
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.”
Contamination
• Dry Corrosion
Water 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
Contamination
• 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
Contamination
• 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 Removal
The clothing worn covers
most of the body.
Particle 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 Removal
It takes about a minute, but
one must wait patiently for
the particle removal process
to complete
Particle Removal
All must be serious about maintaining a
“clean” environment.
Particle Removal
Air is filtered and supplied to the
lab through a very elaborate duct
system
Contamination
Some stations have independent air filter systems
Particle Removal
The system return air is acquired
though floor vents. The supply /
return are both perpendicular to
the room to minimize “swirling.”
Contamination
Fluids are filtered
Contamination
Tools are kept clean when not in use
Contamination
Materials are protected when
not in use (1/2)
Contamination
Materials protected when not in
use (2/2)
Contamination
The cleanroom is kept in a very
orderly condition
Mobile Ion description
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Dissolve in SiO2 at elevated temperatures
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Loss of mobility at normal temperatures
Effect 2/2
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The positive gate repels mobile sodium to oxide-silicon interface
Effect 1/2
Induce parametric shift in MOS transistor at threshold voltage
 Long term failures slow drift of threshold voltage
Temperature Solution
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Bake at 200°C and redistribute mobile ions
Preventative Measures
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Purer chemicals and improve process technique
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Phosphor to gate oxide stabilize to improve alkali metal
contaminants
Dielectric Polarization
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Threshold shift by dielectric polarization more predictable than
mobile ions
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Use phosphorus-doped polysilicon gate rather than gate oxides
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Phosphorus-doped polysilicon immobilizes alkali metals
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Moisture from outside package brings in sodium. This can be
reduced by improving package material to slow ingress of
sodium ions
Protection Overcoate
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Silicon nitride impermeable to mobile ions
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Phosphorus-doped glasses
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It can serve as a final line of defense against impurities
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Minimum number of probe pads needed, and they should be
kept far away from sensitive analog circuit
Scribe Seals
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Narrow content strips surrounds active area of die and
continuous ring
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P-type diffusion
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Guaranteed minimum area of substance contacts
Surface Effect
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Surface region of high electric field intensity
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Surface electric field induces the formation of the parasitic
channels
Hot Carrier Injection
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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
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MOS can generate hot carriers when operated in saturation
region at high-drain-to-source voltages
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The pinched- off portion of the channel slowly grows wider, aAs
the drain-to-source voltage increases
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The electric field becomes large to generate hot carriers near
the drain end of the transistor
Effect On Performance
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Hot carriers produced at the drain end of the transistor collide with
the lattice atoms
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Few of the recoiling carriers travel upward into the overlying oxide
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Most of the carriers pass trough the oxide and return to the silicon
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Few get trapped at defect sites within the oxide, will represent a fix
oxide charge
Parametric Shift
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Caused by hot carriers
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Can be partially or completely reversed
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The parametric shift vanishes as the fixed oxide change
dissipates
Avalanche Junction
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It occurs near the surface in most diffused junction
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Some of the hot carriers produced travel into the overlying oxide
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The avalanche voltage slowly increases during operation (called
Zener walk-out)
Zener Walkout Mechanism
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Junction diode’s reverse break down is observed using a curve
tracer
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Emitter-base Zeners can exhibit up to 200mV of walk-out
Preventative Measures1/2
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Lightly Doped Drain (LDD) structure
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Redesign the circuit
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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 Measures2/2
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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
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Base-emitter Zener diode
Special Appearance by:
Carlos, Corey, Eric, and Fariba