Recent Development Work in IC Bond Pad Structure and Circuit Under Pad. Stevan Hunter

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Recent Development Work in IC
Bond Pad Structure and Circuit
Under Pad
Stevan Hunter
Presented to IMAPS Student Chapter, UI, 29 SEP 2011
1
Who did the work ?
• Stevan Hunter
– Principal Reliability Engineer, ON Semiconductor, Pocatello, ID
– PhD Student, ISU; IMAPS member
– (Project started in late 2008, early work constituted MS project, May
2010)
• Steven Sheffield, Cesar Salas, Jason Schofield, Jose
Martinez, Kyle Wilkins, Marco Salas
– Undergraduate student interns, Brigham Young University Idaho
– 1 per trimester starting in Fall 2009
– (Work is contuing with Jonathan Clark, Fall 2011)
• Bryce Rasmussen, Troy Ruud, many others...
– ON Semiconductor
** Work supported by ON Semiconductor **
2
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Scope of Today’s Presentation
Summarizing 3 presentations / publications in 2011, relating to
0.18µm CMOS IC Technology, and other CMOS with Al metallizations
1. Hunter, et al, “Use of Harsh Wafer Probe to Evaluate Various Bond
Pad Structures”, IEEE SWTW, Jun 2011
2. Hunter, et al, “Use of Harsh Wire Bonding to Evaluate Various IC
Bond Pad Structures”, IMAPS EMPC2011, Sep 2011
3. Hunter, et al, “Physically Robust Interconnect Design in Bond Over
Active Circuitry for Cu Wire Bonding”, IMAPS / SEMI Wirebonding
Workshop, Jul 2011
(Two new papers to be presented at IMAPS 2011 in Oct)
• Martinez, et al, “IC Bond Pad Structural Study by Ripple Effect”,
IMAPS 2011, Oct 2011
• Hunter, et al, “Bond Over Active Circuitry Design for Reliability”,
IMAPS 2011, Oct 2011
3
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Outline
•
•
•
•
•
•
•
•
•
•
•
4
Traditional IC Bond Pads
Wafer Probe
Wirebonding
The Cratering Test
Mechanism of metals deformation and SiO2 cracking
Need for improved bond pad structures, BOAC / CUP
Findings from harsh probing experiments
Findings from harsh bonding experiments
Design methodology for robust BOAC pad structures
Pad Design Solution examples
Summary
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Integrated Circuit Bond Pads
Example packaged
IC’s (from internet)
IC die example (photo: USC)
5
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Wafer Probe (Electrical Test)
• Probe needles simultaneously touch all bond pads of the die in order to
perform electrical testing of the die.
• Each touch damages the pad Al surface, leaving a “probe mark”
6
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Simplistic View of Probing
(Cantilever probe tips)
Wafer comes into contact
with the Probe Tip
wafer: bond pad
wafer Z-up
7
Stevan Hunter
Recent Development Work in IC CUP
Wafer moves up into
Overdrive position for test,
scraping the pad Al and
causing a Probe Mark
29 Sep 2011
Probed Pad Reliability Concerns
• Electrical OverStress (EOS) or Electrostatic Discharge (ESD)
– receives much attention and is usually eliminated in manufacturing
• Probe mark interferes with bondability
– probe mark area too large
– probe mark “gouge” too deep (too little Al remaining)
• Top intermetal dielectric (top IMD) cracks beneath pad Al
– crack weakens films adhesion and intermetallic bond strength
– crack in IMD causes crack in TiN barrier
• allows Al, Au, Cu diffusion into the circuitry
– crack may propagate during use (latent defect)
• physical weakening of pad and bond structure
– crack may extend to another metal feature
• electrical leakage or short
– crack is visible to a customer who does die analysis
• “obvious” reliability issue to customer
8
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
1 TD
2 Mils Overdrive
2 TDs
3 TDs
2 TDs
3 TDs
4 TDs
5 TDs
6 TDs
5 TDs
6 TDs
Pad #2 SLM Pads
1 TD
4 Mils Overdrive
Pad #2 SLM Pads
9
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
4 TDs
Probe Mark Analysis by AFM & Cratering Test
pad 8
pad 9
89
10
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Probe Mark Analysis by FIB Cross Section
FIB through the probe mark center reveals gross cracking,
with breakage of the top IMD and deformation of TM(-1)
11
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Bond Pads and Wirebonding
It is typical for 25µ Au wire or 20µm Cu wire to be
“bonded” to each pad, and the wire connects to a lead on
the package
Illustrations of ball bonds on bond pad structures (from ASE, Sep2011)
12
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Wirebonding
A ball is formed at the end of a Au or Cu wire, then the ball is squished onto
the Al pad surface and welded by ultrasonic energy
Upper photos: www.kns.com
13
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Wirebonding / Pad Reliability Concerns
• Bond ball poor adhesion
• Bond stability against corrosion and diffusion
– Au-Al IMC issues
– Resistance increase over time
– Loss of adhesion over time
•
•
•
•
•
•
14
IMD cracks (critical if BOAC / CUP pad)
Bending and deformation of BOAC / CUP metal circuitry
ESD protection (circuitry under pad in BOAC / CUP)
Bond wire adhesion to package lead
Bond wire integrity against corrosion or oxidation
Bond wire stiffness to combat bending during plastic
molding
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Bonded Pad Issues and Analysis Methods
1. Bond Pull Strength test (BPS)
–
–
–
–
Hook the wire loop and pull up until breakage
Record the breaking force and failure mode (accepted spec limits)
Ball may come off the pad at a low force due to poor adhesion
Ball may pull out a chunk of the pad as it pulls off (pad divot or crater)
2. Ball Shear test (BS)
–
–
–
–
Shear (bulldoze) the ball off the pad
Record the breaking force and failure mode (accepted spec limits)
Ball may come off the pad at a low force due to poor adhesion
Ball may pull out a chunk of the pad as it pulls off (pad divot or crater)
– Note that when the films adhesion is strong and the ball is stiff, BS test will
break something in the structure – interpretation of results may be difficult
3. Cratering Test
– Etch away the ball and pad Al, then optically inspect for damage
• Cracks, lifting barrier film, divots and craters, and ripple effect
– SEM inspection, FIB or cross section polish SEM
15
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Traditional bond pad: 4-level metal example
A 4-level metal pad structure within the
“pad window” is illustrated in concept:
Al metallization: TiN / Al(0.5%Cu) / TiN,
W vias, SiO2 dielectric
• sheets of metallization at all levels
• via arrays connecting the metals
• SiO2 dielectric surrounding
(Periphery of pad structure,
passivation, Si devices, etc. are not
shown)
16
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Al (MT)
Top Vias
MT(-1)
Vias
MT(-2)
Vias
MT(-3)
Pad Cracking: Process Flow
Wafer probe
Wirebond
Packaging
Operation
(electrical test)
1.
2.
3.
Compressive
stress from
probe force
Lateral stress
from scrub
Multiple
touchdowns
1.
2.
Compressive
stress from
C/V +
downforce
Lateral stress
from
ultrasonic
Cracks initiate, then propagate
(crack)
17
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
1.
2.
Thermal
cycling
Stresses from
packaging
1.
2.
Use
conditions
Thermal
cycling
“Cratering Test” (1)
• Cratering Test
(removal of bond ball and pad Al, then visual
inspect)
– Etch to dissolve or undercut the bond ball, and etch
away the pad Al, but purposely leave some of the
TiN barrier film if possible
– visually observe top SiO2 cracking
– visually observe other damage:
•
•
•
•
18
Stevan Hunter
“lifting barrier”
other loss of adhesion
divots in top IMD
craters
Recent Development Work in IC CUP
29 Sep 2011
Wafer Probe Cratering Test
• Cratering test removes the pad Al and the probe mark
• Usually etch only the Al to leave TiN barrier
– easier to see the cracks (highlighted in red below)
• Can also overetch the TiN to reveal etch damage in the
underlying metal layer
Pad 1
Pad 2
Pad 3
Probe
Mark
Normal
Etched
Over
Etched
19
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad 4
Pad 5
Pad 6
“Cratering Test” (2)
• Enables comparison between standard and harsh
wirebond, and various pad structures
Top Vias
No Top Vias
Au ball
bond
Harsh Au
ball bond
20
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
“Cratering Test” (3)
• optical “ripple effect”
– deformation in underlying metal interconnect (verify by FIB or XSEM)
Au ball bond
Harsh Au ball bond
FIB cross section of “traditional” bonded pad test structure
Al
Top SiO2
SiO2
Al
21
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
“Cratering Test” (4)
• Not all issues can be seen in cratering test
– cannot detect weakened locations
– may not see cracks in SiO2 if the TiN barrier is not broken
– cannot detect partially cracked locations on the bottom of
the SiO2
22
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Damage relating to top vias
• top vias participate in SiO2 cracking, giving traditional pads
with top vias the worst record for cracking
• lifting TiN, SiO2 divots, and craters are much more likely
with top vias
cracks propagate from via
an example of “lifting
to via
barrier”, relating to top vias
23
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Al Deformation & SiO2 Cracking
Basic issue with traditional pad designs:
• Large mechanical stress into brittle SiO2 film over
ductile Al film
• Underlying Al deforms into local “hills” and “valleys”, which
bends the SiO2 which cracks when its tensile fracture
strength is exceeded
– Cracks may initiate in the top of the SiO2 above deformed Al hill
– Cracks may initiate in the bottom of the SiO2 in deformed Al valley
•
•
•
•
24
Significant stress may be applied in wafer probing
Significant stress may be applied in wirebond, especially Cu
Additional stress may be applied in packaging
Cracks and weak regions in the SiO2 may go undetected,
but remain a reliability concern
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Stack Design Leads to Pad Cracking
Pad Window (probe and bond stresses here)
Al
SiO2
Al
SiO2
Al
SiO2
Al
SiO2
2.) SiO2 bends
and cracks
1.) Al film deforms
into hills and
valleys
Si
(TiN barrier layers not
shown, for simplification)
25
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Stack Design Leads to Pad Cracking (2)
Pad Window (probe and bond stresses here)
Al
SiO2
Al
SiO2
Al
SiO2
Al
SiO2
2.) SiO2 bends
and cracks
1.) Al film deforms
into hills and
valleys
Si
(TiN barrier layers not
shown, for simplification)
26
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Examples of Cratering Test Results
Pad crack from ball
bonding
Latent damage from
wafer probe “revealed”
from bonding
Pad “ripple” from
ball bonding
27
Stevan Hunter
Recent Development Work in IC CUP
Barrier lifting, and
divot in top IMD
FIB cross section of divot.
Sub-layer Al “ripple” caused the crack
29 Sep 2011
Needs for Al – SiO2 Bond Pad Structures
Need a methodology to accomplish the following together:
• Bond-over-active-circuitry (BOAC), 2 – 7 levels of metal
• Maximum pad design flexibility for small die size,
• Interconnect circuitry in all levels below the pad metal, (& ESD protection)
• “pad anywhere” is desirable
• Up to 6 wafer probe touchdowns
• Cu wirebond to replace Au wirebond
• Higher reliability
– No bending or deformation in sub-layers
– No cracks
– No other bonding issues
• Often cannot use thick top metal (MT)
• Decreased cost
28
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Engineering Tradeoffs / Issues
• Metal bending and deformation in interconnects is not
acceptable in BOAC / CUP
• Pad cracks are not acceptable in BOAC / CUP
• Traditional pad structure cracks easily from wafer probing
• Traditional pad structure cracks easily from wire bonding
• Thin top metal increases stress to reach underlying films
• Cu wirebond increases stress on the pad Al
• High number of probe touchdowns increases pad cracks
• Cannot add new layers or process steps
• Cannot introduce new materials or layer alterations
• No existing methods to solve these simultaneously
• …Need methodology for more robust pads
29
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Available Methods for Al – SiO2 Pads (1)
1. reduce the mechanical stress to the bond pad Al
– minimize touchdowns and force at wafer probe
– minimize stress at wirebond
but, Cu wirebond still requires more mechanical stress than Au
wirebond
2. modify the top of the pad to reduce the stress reaching
the underlying pad structure
– thicker pad Al
– alter the barrier film
– add films to the pad top
but, the above add cost and complexity, and adverse
engineering tradeoffs; deformation and cracking are
reduced but are not necessarily prevented in Cu wirebond
30
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Available Methods for Al – SiO2 Pads (2)
3. modify pad structure features for a specific purpose
– change the pad metal topology to improve Au wirebond adhesion
– modify the top via pattern in or around the pad to prevent cracks or
to contain the cracks within the pad window location
– eliminate top vias in the pad window to reduce cracking
– use vias in combination with specific metal patterns to “strengthen”
– remove metal features to reduce stress to devices beneath
– place dummy metal features for damping of bonding stress
– use specific connected bus structures suitable for BOAC
– use certain metal patterns to reduce pad capacitance
– use metal features to dampen the bonding stress
But, no method or combination of these methods meets the needs for
freeform design of BOAC with Al-based interconnects in all metal levels
beneath the pad window, especially not for achieving pad structures
robust enough for the increased mechanical stress of Cu wire bonding.
31
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Findings from Harsh Probing Experiments
• Optimize probing parameters for reduced force as required
– Multiple touchdowns still pose some risk for cracking
• Presence of a full sheet of Al metallization beneath the pad dominates
cracking behavior
• Top vias weaken the SiO2
• Thick pad Al reduces interconnect metal deformation and SiO2 cracking
• Prevent pad cracks at wafer probe by preventing bending of top SiO2;
i.e. prevent deformation in underlying Al film
– Lower metal pattern density of interconnect layers, especially MT(-1)
– In particular, ensure small metal width between spaces, slots, or holes in
MT(-1)
– (Cracks tend to occur where probe force is above MT(-1) areas)
– (Cracks tend to occur where probe scrub path transitions from MT(-1) space
to MT(-1) metal in the pattern)
32
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
MT(-1) experimental design examples
• After cratering test, cracks are visible in the TiN barrier
• MT(-1) patterns can be seen, after removal of pad Al and
TiN barrier film
Examples of metal test patterns in MT(-1)
full sheet,
no vias
under
full sheet,
no top vias,
but dense
vias under
[ref: Hunter, et al; IEEE SWTW JUN 2011]
33
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Harsh Probe Data Experimental Bond Pads
Fraction of cracked pads vs
metal pattern density in MT(-1)
34
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Harsh Probing: Pads cracked vs MT(-1) pattern
35
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Findings from Harsh Bonding Experiments (1)
Harsh Bonding results are very similar to harsh probing
results
• Pad structures crack due to high bonding stress, so we
optimize for low stress in manufacturing
– Cu bonding stress is “harsh” regardless
– Simulate the same high stress by un-optimized Au ball bond recipe
• Presence of a full sheet of Al metallization in pad sub-layers
dominates cracking behavior
• Top vias weaken the SiO2
• Thick pad Al reduces sub-layer interconnect metal
deformation and SiO2 cracking
36
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Al Thickness Reduces Sub-layer Deformation
Cratering test ripple: Au ball bond on traditional pad structure
1µm pad Al
7% pads cracked
1.5µm pad Al
4% pads cracked
3µm pad Al
0% pads cracked
37
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Findings from Harsh Bonding Experiments (2)
• Prevent pad cracks at bonding by preventing bending of top
SiO2; i.e. prevent deformation in underlying Al film
– Lower metal pattern density of interconnect layers, especially MT(-1)
– Presence of MT(-1) features and vias beneath may be beneficial,
unless high pattern density
– Reduce metal width between spaces, slots, or holes
• Other
– Bonding cracks don’t tend to interact with interconnect patterns
– Removal of metal in all layers beneath pad is unacceptable due to
potential harsh bonding strain on devices beneath and increased
cracking outside pad window
– Traditional reliability testing is not likely to reveal cracking issues
38
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Summary: Bond Pad Structure Cracking Results
Harsh Probing and Harsh Au Wirebonding
Traditional
Weakest
Slotted MT(-1)
Better
Missing MT(-1) Slotted MT(-2)
Best
39
Stevan Hunter
No top Vias
Slight improvement
Missing MT(-1)
More improvement
Waffle MT(-1)
Better
Missing MT(-1) and MT(-2)
Very good
Missing MT(-1) waffle MT(-2)
Best
Recent Development Work in IC CUP
29 Sep 2011
Missing MT(-1, -2,-3)
Strongest
“Harsh” Au Wire Ball Bonding Results
10206 pads analyzed in this experiment
Pad test structure
examples. These pads
have Top Metal
removed
Indicates Pad Designs with TM(-1) Missing.
40
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Au(1%Pd) 1 mil Wire Bond
(Non-optimized Recipe, 2000 pads)
Crater Test & Ball Shear Results
for Different MT(-1) Pad Structures
Only traditional pads exhibited consistent cracking for AuPd ball bond.
41
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Cu 1 mil Wire Bond
(2000 pads)
Crater Test & Ball Shear Results
for Different TM(-1) Pad Structures
Only traditional pads exhibited consistent cracking for Cu ball bond.
42
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Au & Cu Wirebond Reliability Test Results
• 1mil Au wirebond, and 1mil Cu wirebond rel tested in parallel,
• 3 different robust pad BOAC designs, electrically test IL
• 1 assembly lot (300 plastic packaged parts) each
• 0.8um Pad Al thickness on
robust pad
• Au bonds: large IMC
growth, some voiding
2
• Cu bonds: Al remaining
under bond, very little IMC, no
weakening or resistance
increase – more reliable for
high temperature
applications, don’t need
thick pad Al
BPS
BS
43
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Design of Physically Robust Pads
>>> don’t let the SiO2 “bend” significantly during pad stress <<<
Do this by preventing Al deformation beneath the SiO2
• Prevent Al “hills” and “valleys”, prevent plastic deformation
–
–
–
Do this by lowering the metal pattern density in the metal sub-layers
beneath the pad window, predominantly in MT(-1)
Limit the maximum allowed metal width (or distance) between spaces,
slots or holes in the metal sub-layers beneath the pad window, with
the most restriction imposed in MT(-1)
Vias beneath MT(-1) and MT(-2) features are encouraged
Below: Example of how robust pad design guidelines may be specified
(actual values are technology-specific)
44
MT
Very thin
nominal
THICK
MT(-1) density
MT(-1) width
0-50% dense,
very narrow
0-75% dense
narrow
0-85% dense
not as narrow
MT(-2) density
MT(-2) width
25–75% dense
wide
15-90% dense
wider
15-95% dense
widest
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Examples of Robust Pad Design Solutions
1. 3LM BOAC, Au wirebond
2. 3LM Power Device, Cu wirebond
3. 4LM standard pad for design library, Au or Cu wirebond
45
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Design Challenge 1
3LM BOAC
• 3-level Metal, older CMOS technology,
• Very thin top metal
• Die area limit requires BOAC pads
– Freeform interconnect circuitry in M1 and M2
– Some top vias required
– Need ESD protection circuitry under pad
• Traditional bond pads historically had issues with cracks in
non-optimized probing and bonding processes, partly due to
the very thin top metal
46
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Design Solution 1
3LM BOAC
• M1 and below for ESD protection circuitry under pad
– 93% M1 pattern density, mostly due to the natural spaces
• M2 sparse with only about 5% pattern density
• Top vias connect M2 to pad
• Pad electrical node conduction through dense vias outside
pad window
• Successful product qualification, including extended
reliability testing followed by wire pull, ball shear, and
cratering tests
• No cracking found in harsh probing and harsh bonding
tests with Au wire
• Issues with Cu wirebonding due to such thin top metal
47
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Design Challenge 2
3LM Power Device, Cu wirebond
• 3-level metal power device
• Thick top metal
• Cu wire bond
• “Pad anywhere” style BOAC
• Product failed qualification:
• Certain pads were having electrical shorts
– Caused by cracks in the top SiO2
– Pads with large area M2 features in the pad window had the cracks
48
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Design Solution 2
3LM Power Device, Cu wirebond
• DOE with a number of variations in lower metal density
– Various arrays of holes in M2 and M1
– Or selected slotting in M2 and M1
– And various via density changes between M2 and M1
• Arrays of holes in M2 were most successful at
preventing cracks
–
–
–
–
Don’t need holes in M1, only M2
Via placement had no effect
Lowered M2 pattern density to 80% on large area features
Uniform lower density in the pattern is a benefit of arrayed holes
• Completed full reliability qualification without issues
• (Other products, including 2LM and 3LM, have been
improved by the same method, both Au and Cu wire)
49
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Design Challenge 3
Reduced area pad for design library
• Well established CMOS technology
• Nominal top metal thickness
• At least 4-levels of metal
• Standard robust pad design for no MT(-1) in pad window,
and full ESD protection under the pad
• Provide options for designers:
– Ability for designer to add MT(-1) interconnects for BOAC
– Permit use of ESD protection devices outside the pad instead
50
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Pad Design Solution 3
4LM standard pad for design library
• No MT(-1) in pad window
– MT(-1) and dense top vias for pad connection are outside pad window only
• Option for MT(-1) interconnect
– Simulated by bus structures with arrays of holes (75% pattern density in bus)
– Vias connecting MT(-1) to MT(-2)
• 4um wide bus structures traverse the pad window in MT(-2) for the various
electrical nodes, about 50% pattern density
• ESD protection circuitry under pad, including the M1 circuitry
– Option to replace ESD circuitry under pad and use other qualified ESD
protection circuitry as desired
• 3 pad designs qualified together
– Harsh probing, followed by harsh bonding tests with multiple wire types
– Standard and extended reliability qualification testing, with extra wire
pull, ball shear, and cratering tests following the reliability stresses
• 1mil Au wirebonding
• 1mil Cu wirebonding
51
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Summary
• Pad design challenges include various engineering tradeoffs
• Prior design methods for Al – SiO2 pad structures don’t
provide the needed solutions
• Basic BOAC / CUP pad design guidelines were developed
based on extensive data:
–
–
–
–
Various pad test structures
Harsh probing
Harsh Au wirebonding, AuPd, and Cu wirebonding
Extended reliability testing, with physical pad tests following
• “Ripple” effect in the cratering test is valuable in assessing
pad robustness
• Pad Al thickness is a less important parameter on a robust
pad structure
• Examples of improved pad design in product solutions
52
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Future Work
• Harsh Probing experiments are continuing
– More test structures in more CMOS technologies
– Different style probe cards
• Harsh Bonding experiments are continuing
– More test structures in more CMOS technologies
• Reliability testing Au and Cu wirebonded parts with new pad
structures
• Implementing improved pads and Cu wirebonding on
products
• Finite element modeling (FEM) to investigate robust pad
physical principles
• Seeking to refine bond pad Design Rules for BOAC / CUP
53
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
W stud pressing on Al over SiO2
Basic 3D
model
Von Mises
Stress after
downforce
54
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
3LM Bond Pad with W probe touching it.
Films, top to bottom, are:
1um pad Al, 1um SiO2, 0.5um Al M2, 1um SiO2 IMD1, 0.5um
Al M1, 1um SiO2 ILD, 600um Si
55
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Equivalent strain for 1E-5 uN probe
force on 3LM bond pad.
High strain points at the edge of the probe, with strain
continuing down through the layers.
56
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Probed Bond Pad FIB Cross Section
57
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
2D Surfaces Sketch (YZ & ZX symmetry)
with 5um radius flat tip
58
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
5u radius flat probe tip
100GPa pressure (all hidden except top SiO2
and MT(-1) Al)
59
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Bonding “Ripple” in bond pad layers
60
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Al “splash” in Cu ball bonding
61
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
2D Surfaces Sketch (YZ & ZX symmetry)
with 60um radius “bond ball”
62
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
Hiding everything but the top SiO2 and
MT(-1)
63
Stevan Hunter
Recent Development Work in IC CUP
29 Sep 2011
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