POWER GOLD FOR 175°C Tj-max
James J. Wang and Bob Baird
Motorola Inc.
Tempe, Arizona USA
James.J.Wang@motorola.com
ABSTRACT
Automotive is requesting engine control IC to operate in
145°C ambient. Power Gold technology can allow ICs to
operate hotter. Since more power dissipates from molded
package and across FR4 board with ICs at 175°C Tj-max
instead of at present 125°C or 150°C, Power Au technology
enhances power product. Continuous, reliable 175°C Tjmax operation is achieved with Power Au. Top silicon
metallization and wire bond pads are gold. Gold process
integrates with wafer flow having aluminum interconnect
layers underneath the top Power Au layer. Thick Power Au
carries current, minimizes electromigration through thinner
aluminum bus and provides an oxide free pad for Au wire
bonding. Both Au ball bonding speed and ball shear
strength improve with gold bond pads over conventional
aluminum bond pads. Gold to gold wire joints are
impervious to halides and do not corrode at high
temperature.
Rough gold surface adheres to mold
compounds and survives MSL1 preconditioning and
autoclave without delamination over die surface. Power Au
allows SMARTMOS product to survive 4000 hours of high
temperature bake at 190°C and 1500 cycles of air to air
temperature cycling from –65°C to 175°C. Although only
few selected packages are tested, results suggest Power Au
is compatible to standard molded packages. Package
Theta-jc remains same; nevertheless, power capability
increases when silicon junctions operate at 175°C. Circuit
boards also dissipate more power. Power Au cost for 8”
wafers is comparable to one layer of aluminum plus ILD.
Introduction of Power Au onto ICs improves power plus
reliability of packaging.
Key words: Power gold, power IC, Tjmax, 175C, Au-Au
bond.
BACKGROUND
Gold wire bonding to aluminum bond pads is common IC
practice. Au-Al metallurgy is reliable and is suitable for
applications up to 150°C IC junction temperature.
At
higher silicon junction temperature of 175°C, Au-Al
intermetallics formation limits IC operation to about 1000
hours [1]. To operate reliably at 175°C Tj-max longer than
1000 hours, monometallic aluminum wedge bonding to
aluminum bond pads is practiced on discrete power
semiconductors and low I/O count power ICs. However,
because 1 mil to 2 mils diameter aluminum wedge bonding
is rare, wedge bond option is not available for mainstream
molded packages. Switching from gold ball bonding over to
aluminum wedge bonding on existing 100 I/Os packaging
line is costly, requires pad layout design change and then the
exposed thin aluminum wires and aluminum bond pads are
still susceptible to corrosion.
Better package reliability is achieved with Power Au as top
metal. Au ball bonding to Power Au like Al-Al is also
monometallic that is not susceptible to intermetallics
formation. Furthermore, Au-Au bonds never corrode and
remain ductile and strong even within mold compounds that
contain moisture and trace corrosive halides. Electroplating
thick gold on silicon wafers is available in mass production
with excess capacity. Power Au process can be integrated
onto ICs fabrication flow as one additional interconnect
layer or as replacement to top aluminum or copper layer.
Power Gold top layer covers over exposed aluminum or
copper pads and present a superior bond pad surface to Au
wire bonding. Power Au allows some ICs currently limited
to 150°C Tj-max to be suitable for 175°C applications
inside the same over molded package.
Gold top wafer metallurgy had been practiced in the past.
With exception of GaAs and TAB, gold had been replaced
by aluminum interconnects and then by advanced copper
interconnects. Lower material cost plus ultra-fine line
capabilities of both aluminum and copper were reasons for
the displacement of gold as interconnect. However, to enter
high temperature IC applications, to achieve superior
reliability or to dissipate greater power, the resurrection of
gold as the top metal is both practical and effective. This
protective gold top is coined Power Au for the ability of
gold to increase power capabilities of ICs, packages and
systems.
INTRODUCTION
Power Au improves not just package and system but also
enhances IC devices. Improvements as current carrying
capacity for electromigration ruggedness, low Ron output
devices and metal debiasing are not detailed in this paper.
Neither are high temperature device issues as junction
leakage, HCI, transient energy capability, device
temperature coefficient, and hot testing of ICs addressed.
Device characterization and test issues need to be addressed
to create reliable products at 175°C. This paper presents
Power Au and its broad effectiveness to improve power plus
reliability of semiconductor ICs within conventional over
molded packages.
POWER AT HIGHER Tj-max
As more functionality is integrated into same die size,
power density rises with advanced ICs. Industry has
improved interconnects, devices, packages, and FR4 boards
to both minimize ohmic heat generation and to dissipate
generated heat quickly and thereby keeping IC chip
temperature below 150°C. To reduce device Ron, to cut
junction leakages and to decrease metal resistances all help
to minimize heat generation within ICs. Package with
exposed heat sink pad, thermal vias drilled through boards,
and cooling fins help to dissipate heat from IC into
surrounding ambient.
As one explores options and
associated costs, also consider increasing junction
temperature and allowing greater power dissipation through
natural heat conduction. ICs are limited to maximum
power, P, by thermal dissipation performance of package
and board, Θja, and temperature difference between Tjmax
to the surrounding ambient temperature, Tamb.
P = (Tjmax – Tamb) / Θja
(1)
Many low power applications, Tamb is near room
temperature. But for automotive applications, Tamb is at
95°C for behind the dashboard ICs and rising as control
modules are tightly crammed under the dashboards. For
lack of space behind dashboard, some electronic modules
are already forced under the hood where Tamb rises up to
125°C. New demand for engine control IC with direct
engine mount is specifying even hotter Tamb. If radiator
fluid were used as coolant, Tamb is 130°C. Engine lubricant
oil as IC coolant raises Tamb above 140°C. With higher
temperature ambient, 175°C Tj-max is better suited. Even
at modest Tamb, the benefit of hotter Tj-max can be
significant. Designers may consider hotter operating Tjmax to avoid using costly packaging with low Θjc or to
minimize boards requiring thermal vias or cooling metal
chassis. Benefit derived with 175°C Tj-max is universal
because increased power dissipation through wider delta T
is independent of both package type and board selection.
Power dissipation gain %
250.0
Power dissipation gain achieved with 175°C Tj-max over
150°C is calculated from equation 1 and plotted as figure 1.
As an example, with Tamb at 105°C, 56% more power is
dissipated from packaging plus board if Tj-max were
increased from 150°C to 175°C.
CORROSION RESISTANCE
Aluminum is relatively resistant to corrosion with its
naturally protective Al2O3 oxide. With care during shipping
finished wafers, dry storage, wet sawing process, using low
halides mold compound, the semiconductor industry has
nearly eliminated corrosion for applications up to 150°C.
Nevertheless, exposed, 0.6 microns thin, aluminum bond
pads are still susceptible to corrosion. Moisture, trace
contaminants and higher operating die temperature
accelerate the corrosion of unpassivated aluminum pads.
Corrosion is a complex interaction of factors plus test
conditions and many models have been created to fit data
obtained with mold encapsulated packages. Models to
estimate the time to failure (TF) of aluminum bond pads due
to corrosion contain a scale factor (B0), the effect of
moisture (RH), and applied test voltage function f(V). And,
all models contain the same temperature based Arrhenius
exponential factor [3]. Preference of corrosion model is not
important to estimate the acceleration factor (AF) due to an
increase in chip temperature from T1 =150°C to T2 = 175°C
(448°K).
Using exponential corrosion model as an
example:
TF = B0 exp[(-a)RH] f(V) exp[Ea/kT]
(2)
With Eyring assumption of independent variables built into
constants and into models as equation 2 and accepting
industry consensus for the activation energy (Ea) of
aluminum corrosion in the range from 0.7 to 0.8 eV then:
AF = exp[(Ea/k) (1/T1 – 1/T2)] = 3.2
(3)
Boltzmann’s constant k = 8.62E-5 eV/°K. Applying Ea =
0.75 eV, bond pad aluminum corrosion is approximately 3x
faster if Tj-max rises by 25°C to 175°C. Corrosion damage
currently below detection can become a problem with
slightly higher temperature application.
200.0
150.0
100.0
50.0
0.0
25
50
75
100
125
150
Temperature ambient - C
Figure 1. Power dissipation improvement at 175°C Tj-max
over 150°C versus Tamb.
Sb2O3 as flame retardant in mold compound dissolves with
moisture and its ions then catalyze the corrosion of
aluminum. Organic bromine compounds form voids similar
to Au-Al bonds exposed to halides together with moisture.
Bromine, when liberated at higher temperatures, can be
responsible for 'dry-corrosion', contrary to corrosion in
presence of moisture. All types of corrosion problems are
resolved with a corrosion resistant top layer of Power Au.
Gold is most noble metal with standard oxidation potential
at –1.50V. Power Au cap over exposed aluminum bond
pads or smaller aluminum contacts prevents possibility of
aluminum corrosion. Wire bonding is then to the Power Au
pads. Corrosion from moisture plus trace contaminants
GOLD – ALUMINUM INTERMETALLICS
Au wire bonded to aluminum forms many Au-Al
intermetallics. This interdiffusion of Au atoms into Al bond
pads is well studied [1-2]. At higher temperature, diffusion
and growth rate of intermetallics also accelerate. If the
entire thickness of aluminum bond pad were converted into
Au4Al intermetallic, then the poor adhesion of Au4Al to
barrier metal between aluminum layers can result in wire
bond separation and electrically open failure.
Even as Au4Al intermetallic is growing, Kirkendall’s voids
coalesce into hairline crack at intermetallics interface.
These weakened interfaces are susceptible to stress failure
and again result in electrically open failure.
Extensive historical testing is represented by time to failure
of Au wire bond to aluminum as represented on figure 2.
When compounded by aluminum corrosion, the time to
failure is even shorter. Under good conditions, lifespan of
Au-Al wire bond is approximately 1000 hours at 175°C
high temperature bake. 1000 hours wire bond lifespan is too
short for reliable automotive applications.
10000000
Time (hours)
1000000
100000
Au/Power Au
Au/Al
10000
1000
was noted after high temperature bake. Au-Au wire bond
interface appears to improve slightly with thermal anneal.
4
Resistance (Ohms)
within mold compounds are eliminated without employing
hermetic packaging.
3.6
Al/Au Daisy
Chain
3.2
Au line
2.8
2.4
0
500
1000 1500 2000
Cycles (-65C to 175C)
Figure 3. Au wire bond to Power Au with contacts to Al
daisy chain test structure and to gold serpentine lines.
Slight package resistance reduction is detected during
preconditioning stressing followed by 1500 cycles of
air to air temperature cycling.
The metal between Power Au and Al is not a perfect barrier
however. Under higher temperature testing, barrier metal
does eventually break down. Above 250°C plus self heating
from 860mA current, gold atoms punch through the barrier
metal and then gold diffuse into aluminum. Rapid diffusion
of Au into Al forms visible ‘purple plague’ within
aluminum. Scanning electron microscopy show a void in
the Power Au line immediately above contact to aluminum.
Missing gold has diffused down into the aluminum.
Voiding in Power Au line leads eventually to electrically
open failure. Both selection and thickness of barrier metal
can be optimized for even higher temperature applications.
100
80
100 120 140 160 180 200
Temperature (C)
Figure 2. Lifespan to 0.1% wire bond failure
Under Power Au, a high temperature barrier metal is first
sputtered over aluminum. This barrier metal serves as seed
metal for electroplating the thick Power Gold. Same seed
metal remains in between gold and aluminum to act as
diffusion barrier and prevents Au-Al intermetallic growth.
Wire bonding to Power Au is more durable as compared to
Au wire bonded directly to Al as intermetallic formation
occurs naturally. After oven bake reliability testing, Au to
Power Au wire bond survives 4000 hours at a bake
temperature of 190°C (figure 2). In comparison, Au-Al
bond is projected to weaken after 440 hours.
With Au wire bond to Power Au, contact resistance reduces
with anneal during temperature cycling of packaged test
structures (figure 3). Similar contact resistance reduction
Au
Au missing at
contact
Figure 4. Power Au line with void above contact to
aluminum after extremely high temperature testing and
860mA current. Gold diffused into aluminum and left a
void.
Recrystallization of electroplated Au grain from
heating is apparent in comparison to picture 5.
WIREBOND QUALITY
Quality is many features combined into superior product.
By most measurements, Au wire bond to Al is already good
quality. Au wire bond to Power Au is even better than Au
wire bond to Al.
There is no passivation over Power Au. Therefore, no
residual passivation film to hinder wire bonding.
Sometimes, residual oxide passivation film or polyimide
residue is still being attributed to poor wire bonds to
aluminum. Included with the Power Au process is a sputter
cleaning of aluminum pads before the seed metal sputter
deposition. Wafer process sequence from sputter cleaning
to seed metal deposition is completely within the same high
vacuum chamber.
Any thin film is sputtered off and
thereby the clean aluminum surface forms a more intimate
and consistent contact to the seed metal. Contact resistance
between Power Au and aluminum is consistent even at
small contact area.
Power Au does not oxidize. Au-Au bonding does not need
to scrub through an oxide film to form metallic bond. As a
result of Power Au, bonding force and ultrasonic power of
Au ball bond can be reduced. Even at lower bond force and
US scrub power, Au wire to Power Au results in slightly
better ball shear strength than Au to Al (210gm ball shear
strength for 2 mil diameter Au wire versus same Au wire to
Al pads average at 180gm). Au ball bonding can be
optimized at lower bonding force and achieve good shear
strength at faster bonding speed for throughput.
Figure 5. Electroplated Power Au line
PROBING ADVANTAGES
Fine pitch wire bonding has not been tested. Because thick
Power Au pads protrude above passivation instead of being
recessed below passivation opening, bonding is easier with
slightly finer pitch potential.
Likewise, probing the
protruding gold pads is easier than probing recessed
aluminum pads. Over-travel of probe needles sometimes
cracks edge of passivation, such probe damage is eliminated
with probing on Power Au.
Probing on oxide-less gold pads is consistent at elevated and
cold temperatures. Repetitive probing of thick gold does
not degrade subsequent wire bond. Power Au is more than
6x thicker than aluminum. Multiple probe tip touch-downs
that wear through thinner aluminum pads do not degrade
Power Au. High frequency wafer probing should yield
favorable result; similar to probing of GaAs semiconductors
with Au as top metal.
MOLD COMPOUNDS
For automotive ICs to operate up to 175°C Tj-max, over
molded package must operate reliably under temperature
swings from –50°C to 175°C.
Commercial mold
compounds are many. Glass transition temperature of mold
compounds, Tg, vary from 100°C to above 175°C.
Different mold compounds and different mold cure are
implemented at each packaging facility. Changing mold
compound to reduce stress or to minimize trace corrosive
agent requires evaluations with potential production line
disruption. Power Au appears to be suitable and compatible
to even low Tg mold compounds. Since gold does not
oxidize nor corrode, mold compounds tested under MSL1
plus autoclave do not corrode nor degrade Power Au.
Glass transistion signifies the temperature at which cured
mold compound’s coefficient of thermal expansion
increases abruptly. Using mold compound with low glass
transition temperature, a higher shear stress accumulate on
Au wires and on exposed Power Au lines encased within
such molding. Mold compound induced stress potentially
leads to mold compound delamination, cracking and wire
tear. Power Au was stressed within a Sumitomo mold
compound having measured Tg of 109°C (ramped from 60°C to 175°C at 3°C per minute on T. A. Instruments’
Dynamic Mechanical Analyzer, DMA, and using 3-point
bending clamp attachment. Samples were tested at 1 Hz.).
With additional post cure bake at 175°C for 1 hour, the Tg
of this mold compound increases to maximum of 131°C
(DMA) or 132°C (measured using TMA method). Both the
as received packages and added post cured packages were
similarly preconditioned and then stress within air to air
temperature cycling oven (–65°C to 175°C). Excellent
adhesion to the silicon die surface with Power Au is
observed with both post cured and as received packages.
Mold compound adheres extremely well to Power Au. Even
with relatively low Tg mold compound under test, die
surface delamination was not seen under scanning acoustic
microscopy (figures 6 and 7).
surface. Only after prolonged stressing is second bond
failure then noted. Limited results suggest Power Au can be
applied with different mold compounds and within many
types of packaging to achieve package reliability suitable
for high temperature applications.
Figure 6. Scanning acoustic microscopy of die surface after
extensive reliability stressing.
Delamination of mold compound is apparent around the die
and at the leads. However, mold compound adhesion to
Power Au is in tact.
Figure 7. Larger Power Au bus features are visible on the
surface of die under scanning acoustic imaging. Adhesion
of mold compound to Power Au is excellent even after
package had been subjected to MSL3 plus 1560 air to air
temperature cycles and then followed by autoclave. The
roughened surface of thick, electroplated Power Gold as
seen on figure 5 is believed to contribute to superior mold
compound adhesion.
Adding Power Au technology, packaged die exhibits
excellent resilience against mold compound delamination.
Devices under electrical testing indicate no change to
functional device parameterics and slightly lower wire bond
contact resistance (figure 3). With good die to mold
compound adhesion, CTE miss-match stress after extreme
temperature cycling from –65°C to 175°C does not degrade
first bonds nor damage exposed Power Au lines on the die
COST
Gold is expensive. However, the amount of gold plated as
Power Au on the die surface is typically less than the
amount of gold consumed by gold wires. Depending upon
die size, die coverage by Power Au, diameter of Au wires
and number of wires, total amount of gold employed varies.
For smart power ICs, gold wires typically consume 3x more
gold than electroplated Power Au. Although gold is
expensive, its superior chemical, electrical and mechanical
qualities are preferred both as wire material and as
protective top surface metal over the silicon die. In
comparison, the cost of Power Au is less significant than the
cost of gold wires.
With electroplated Power Au wafer fabrication, cost of gold
is even less significant when compared to expensive wafer
processing tools, chemicals, clean room facility and other
fixed plus variable costs. Fortunately, electroplating gold
on surface of silicon wafers is common wafer fabrication
practice for low cost LCD driver IC. Gold is electroplated
down to 60 microns pitch for tape automated bonding
(TAB) and for chip on glass assembly. Because liquid
crystal displays are universally used, subcontractors with
gold plating production capacity are available. Many
subcontractors are capable and willing to electroplate gold
on other types of silicon wafers besides MOS LCD driver
ICs. Multiple wafer plating subcontractors allow Power Au
technology to be applied quickly and cost effectively.
CONCLUSIONS
Wire bond and package reliability are hurdles limiting ICs
to 150°C Tj-max. The application of Power Au as top
metallurgy for ICs eliminates wire bond intermetallics,
corrosion and package mold stress effect. Since gold does
not oxidize nor corrode, noble gold top metal is better suited
against accelerated chemical reactions at higher
temperatures. Power Au enables ICs to operate reliably at
175°C Tj-max.
Power Au technology utilizes existing gold plating wafer
fabrication facilities. The application of Power Au can be
cost effective to improve reliability and or to achieve greater
power dissipation. Power Au is compatible to many types
of over-molded packaging and does not change the
packaging process flow. Power Au is added on after
completed wafer fab flow to prevent corrosion, to improve
mold compound adhesion and to enhance overall package
reliability. Improvements to both package and IC are so
noticeable that Power Au technology is then retested at
more stringent 175°C Tj-max applications. Power Au is
suited for medium to power ICs and packages. And, if
application is cost effective, Power Au may also be applied
within low power packaging to increase any IC’s power
dissipation.
Packaging and assembly houses can consider adding Power
Au over copper interconnects to allow standard gold ball
bonding to advanced copper wafers. Power Au further
improves device Ron, current capability, and reliability of
packages. Improvement is even more significant if the IC is
suitable for 175°C Tj-max operation. Power Au technology
increases flexibility, quality, and capability simultaneously.
ACKNOWLEDGEMENTS
We like to thank Roland Kallstedt, Dianne White, Russell
Shumway and Steve Ostrov for their expertise and help with
the myriad of tests performed during development and to
realize unique tests required by Power Au and by high
temperature reliability testing.
REFERENCES
[1] Noolu, N., Klossner, M., Ely, K., Baeslack, W., Lippold,
J., “Elevated Temperature Failure Mechanisms in Au-Al
Ball Bonds” International Microelectronics and
Packaging Society, IMAPS September 2002 Proceeding.
[2] Tung, C.H., Sheng, G.T., Teo, P.S., Lo, M.C.,
“Microstructure Studies of Under Bump Metallization
Systems Using Transmission Electron Microscopy”
Conference Proceedings from 28th International
Symposium for Testing and Failure Analysis, November
2002, pp. 505
[3] Blish, R., Durrant, N., “Semiconductor Device
Reliability Failure Models”, International SEMATECH
technology transfer #00053955A-XFR, May 31, 2000.