Bondability, Reliability and Yield Benchmarks for High Volume Specialty Gold Fine Bonding Wire

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Bondability, Reliability and Yield Benchmarks for
High Volume Specialty Gold Fine Bonding Wire

• By Heiner Lichtenberber, Micheal Zasowski, Gery
  Lovitz Daniel Ha
• For Williams Advanced Materials Buffalo, NY and
  YCMC Taipei, Taiwan

Presentation by Prasad Weerakoon
Date September 19th , 2007
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Introduction

• The demand for smaller diameter wires is
  increasing
• There is a need for processes used to make this
  wires to be made simpler.
• This paper studies the effects of controlling the
  dopant chemistry, break strength and elongation
  on production of these fine wires.

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Background

• Low loop and long length characteristics require
  a short HAZ which is controlled by the additions
  of certain metals in the parts per million range.
  
• Be and Ca are used to obtain high strength and
  recrystallization temperature.
• Bonding wire is produced by a combination of
  continuous casting and step drawing processes.

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Parameters

• The effects of deliberate additions of Be, Ca,
  Pt, Cu, Al and Ag are discussed.
• A gold wire was used.
• The effects of some 20 elements were determined
  using a DOE software.
• Once the desired chemistry of the dopants were
  determined, the properties of the wire was
  compared to the hydrostatically extruded test
  samples.

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Correlation of HAZ to Recrystallization
Temperature
The recrystallization temp. is inversely
proportional to the HAZ as a higher temp means
that the wire has to get hotter to recrystallize
the grain structure. BL- Break load
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Experimental Procedures-Chemical

• To determined the effects of minor additives to
  high purity gold, binary gold alloys of Be, Al,
  Ca, Cu, Ag and Pt and multi-component of alloys
  were prepared.
• These were vacuum melted, annealed and
  hydrostatically extruded to 0.001 diameter.
• Individual dopant levels varied from 3 to 30
  parts per million.
• Analysis were done using a software (ICP-MS).

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The hydrostatic extrusion equipment used in the
experiment has processing pressures from 100,000
psi to 200,000 psi depending on the strength of
the ingot.
Extrusion speeds varied from 250fpm at large
diameters to over 1000fpm at final size.
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Experimental Procedures-chemical Contd.

• Break load and elongation were determined.
• Tests were done for wires at their extruded
  condition and after annealing to 2.5 elongation.

This picture shows the large grain structure in
the ball of the wire and how it gets finer
further away from the ball
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Experimental Procedures- Mechanical

• Continuous casting was used to produce 0.312
  wire.
• Continuous casting was chosen since it is
  superior to ingot casting because the integrity
  and the grain structure of the casting is uniform
  throughout the length.
• This method is done by melting the metal in a
  graphite crucible with a graphite die located at
  a bottom.

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Experimental Procedures- Mechanical Contd.

• After the final annealing to the desired
  properties, the wire was bonded onto ceramic
  substrates (ICs)using a machine.
• The results were tested both
• visually and electrically to
• determine the consistency of
• the wire interconnections.

The picture shows the ceramic being connected to
the wires.
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Results-Chemical

• Dopants showed some increase in strength in the
  hard as extruded condition
• Be and Ca had the greatest effect while Cu and Ag
  had the least.
• Ca and Be had the most significant strengthening
  effects after annealing even though most dopants
  strengthened the wire to some extent during
  extrusion and drawing.

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Break Strength Vs. PPM of dopant
We can see that Be has an immediate strengthening
effect while Ca requires a minimum level before
any strengthening can occur.
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Recrystallization temp Vs. PPM of dopants
This graph shows that calcium had the most
significant effect on the recrystallization temp
while silicon had a negative effect.
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Results-Mechanical

• Wires produced by continuous casting, wire
  drawing and hydrostatic extrusion produced
  identical mechanical and physical properties as
  those processed by hydrostatic extrusion alone.
• Wire drawing is best for high volume production
  as it can handle long wire lengths.
• Complete numerical values of the results are
  discussed in the handout

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Wire pull tests
Wire pull tests were carried out at 24 hour
intervals. Ball shear strength showed a slight
increase at 48hrs and held steady throughout the
test.
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X-ray images

• X-ray images were taken of the ICs to show the
  sweep of the wires. The worst case deviation was
  2.98 which was within the specifications and
  tolerances.

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Discussion

• Since the atomic size and lattice parameters of
  Ag, Cu, Al, and Pt are similar to that of Au,
  they had little effect on the properties of gold.
  
• Since Be is 30 smaller than Au, Be atoms can
  occupy interstitial sites in the gold lattice,
  thus hardening and strengthening the gold.
• Ca is 30 larger than gold and incorporates
  itself to the grain boundaries of gold. This
  increases the recrystallization temp.

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Summary

• By choosing the appropriate dopants in the
  correct level, a series of alloys that exhibit
  low loop and long length characteristics can be
  produced.
• Consistency of the bonding wire and other
  characteristics are due to the chemical
  formulation of the bonding wire and the
  manufacturing method.

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References

• (1)G. G. Harman, Wire Bonding in
  Microelectronics, McGraw-Hill 1997
• (2)H. Lichtenberger, et.al, Gold Bonding Wire
  The Development of Low Loop, Long Length
  Characteristics IPMI 1998