WIREBONDING CHARACTERIZATION AND OPTIMIZATION ON THICK FILM SU-8 MEMS STRUCTURES AND ACTUATORS

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WIREBONDING CHARACTERIZATION AND OPTIMIZATION ON
THICK FILM SU-8MEMS STRUCTURES AND ACTUATORS
LIGA and Biophotonics Lab

• NTHU
• Institute of NanoEngineering and MicroSystem
• SpeakerWen Cheng Yang

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Outline
LIGA and Biophotonics Lab

• Introduction
• Fabrication
• Testing
• Thermal actuator operation
• Conclusion

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Outline
LIGA and Biophotonics Lab

• Introduction
• Fabrication
• Testing
• Thermal actuator operation
• Conclusion

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Introduction
LIGA and Biophotonics Lab

• Although polymers for MEMS applications are
  becoming more common as an alternative to silicon
  for low temperature and microfluidic
  applications, few techniques are available to
  electrically integrate them into existing
  packaging technologies
• The epoxy based photoresist SU-8 has quickly
  become one of the most common polymers used in
  the MEMS field because of its ability to easily
  produce high aspect ratio structures with near-UV
  lithography

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Introduction
LIGA and Biophotonics Lab
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Outline
LIGA and Biophotonics Lab

• Introduction
• Fabrication
• Testing
• Thermal actuator operation
• Conclusion

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Fabrication
LIGA and Biophotonics Lab
Step 1 RIE 15 sccm O2 and 5 sccm of CF4 at 80W
and 68mtorr Step 2 Metal deposition
All hardbaking steps are done on a ramping
hotplate while constraining SU-8 structures
in-plane to avoid out-of-plane stress.
Cross-section of an SU-8 bond pad coated in metal
and electrically isolated from substrate.
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Fabrication
LIGA and Biophotonics Lab
Maximum wirebond yield onto 1 µm thick SU-8
hardbaked at 200 ºC with no surface treatments
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Outline
LIGA and Biophotonics Lab

• Introduction
• Fabrication
• Testing
• Thermal actuator operation
• Conclusion

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Testing
LIGA and Biophotonics Lab
SU-8 can exhibit significant visco-elastic
effects at higher temperatures, with a lower
Youngs modulus and higher energy absorption
When bonding at temperatures close to the Tg
value, plastic deformation of the SU-8 under the
ball bond reduces bond strength and yield.
Effect of a 30 minute hardbake on wirebond yield
to SU-8 of different thicknesses. Metal layer is
50/300 nm Cr/Au and no surface treatments are
used.
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Testing
LIGA and Biophotonics Lab
Wirebonds from 100 µm thick gold coated SU-8 test
structures to DIP.
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Testing
LIGA and Biophotonics Lab
Bonding to 16µm thick SU-8 structures at 123 ºC
resulted in yields above 95 for all but the 20
minute treatment time. Despite the high yields,
there were significant differences in the
mechanical strength of the bonds.
Failure force of bonds on 16 µm thick SU-8 for
different activation times and bonder
temperatures.
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Testing
LIGA and Biophotonics Lab
An activation time of 10 minutes produces maximum
bond strength for all bonding temperatures and
only wire failures when bonded at 123 ºC
Average failure force with standard deviation and
ratio of failure modes of ball bonds on 16 µm
thick SU-8.
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Outline
LIGA and Biophotonics Lab

• Introduction
• Fabrication
• Testing
• Thermal actuator operation
• Conclusion

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Thermal actuator operation
LIGA and Biophotonics Lab
The hot-cold arm thermal actuators were 40 µm
thick, 1 mm long, and had been activated with
O2/CF4 for 20 minutes prior to being coated with
50/300 nm Cr/Au.
For these actuators, a maximum input current of
63 mA was used at 2V without any signs of failure
at the ball bonds.
Performance of a thermal actuator
(insert), powered through direct wirebonding.
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Outline
LIGA and Biophotonics Lab

• Introduction
• Fabrication
• Testing
• Thermal actuator operation
• Conclusion

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Conclusion
LIGA and Biophotonics Lab

• Maximizing metal adhesion to SU-8 is the most
  critical step for successfully wirebonding onto
  SU-8 with thin metal films and optimized
  conditions can result in yields near 100.
• SU-8 test structures up to 100 µm thick have been
  electrically connected,and thermal actuators have
  been successfully connected to standard DIPs for
  operation.

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LIGA and Biophotonics Lab

• Thanks for your attendance .