Removable Pixel Interconnect Development

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Removable Pixel Interconnect Development
H. F.-W. Sadrozinski, V. Fadeyev SCIPP, UC Santa
Cruz Bernard Phlips, Marc Christophersen Naval
Research Lab
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Motivation

• Progress in radiation testing of sensors is tied
  to ease of interconnect
• strips irradiate sensors separately from
  readout ASICs,
• wire bond pre- and post-rad
• Pixels up to now irradiate sensors and ASICs
  together
• Problem with testing pixel sensors at high
  irradiation doses ( 1016 neq/cm2)
• If bump-bond to present ASIC (FE-I3) prior to
  the irradiation, the ASIC would not survive the
  irradiation
• If bump-bond to the ASIC after the irradiation,
  the sensor will get high temperature exposure (
  typically gt 200oC for 1 min for solder reflow)
• Solve this by making structures that allow
  temporary interconnect for testing
• Based on silicone bumps with
  metallization on top.
• This is an alternative to methods using strip
  electronics readout proposed by Dortmund and
  Liverpool.
• If the method works. might be very useful
  for general production testing

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Thermal Characteristics of Bump Bonding Methods

• Thermal process tend to have typically 2 min
  profile, and tend to have fairly high
  temperature
• T/C Thermal compression gold film, balls gt
  200oC
• Snapcure fast curing of conducting epoxy with
  UV ?
• ACP, ACP film with gold balls inside 90oC
• (reports that it does not work for 50um pitch)
• In Studs 100oC
• Al, Au Studs with Ultra Sonic 180C

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Basic Structure
Flexible silicone bump with a metal contact (e.g.
Au) on top
Commercially available photo-patternable silicone
(WL-5150, Dow Corning Corporation) We have
obtained a silicone thickness from 10 to 70
mm The resolution of WL-5150 is moderately high
with 8 mm (features sizes)
The method is described in Ultra low stress and
low temperature patternable silicone materials
for applications within microelectronics by H.
Meynen et al, Microelectronic Engineering 76
(2004) 212218 .
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SEM Pictures
Examples of bumps manufactured at NRL all
shapes and sizes
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More SEM Pictures
Gold contact on bumps manufactured at NRL, no
metal line to the substrate.
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Contact schemes

• There are several ways to use
• the method
• Put bumps on ASIC only
• (requires alignment)
• b) Make a go-in-between structure with bumps on
  each side
• (requires x2 alignment, but
• more reusable)
• c) (An idea from Maurice Garcia-Sciveres)
• Put narrow bumps on one surface and wider
  ones on another to avoid the alignment during
  testing
• (more processing, but easier testing rad.
  hardness to be verified).

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Status
We are at initial stages of testing applicability
of this methodology. NRL is making a test sample
with pixel sensors pitches The
micro-fabrication of the devices will be done at
the Nanoscience Institute (NSI) of the Naval
Research Laboratory. The NSI is a
state-of-the-art nano- and micro-fabrication
facility. The Nanoscience Building has been
specially designed to minimize sources of noise
(vibration, acoustic, electromagnetic,
temperature and humidity fluctuations). The main
facility consists of a 5000 sq. ft. of Class 100
clean room and has all needed equipment for the
proposed micro-fabrication tasks. There was a
past history of cooperation with NRL in
GLAST/Fermi. We are looking forward to work with
them again.
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Post-Rad Tests

• If the bumps are put on the sensor, then there is
  an issue of radiation damage. Probably unlikely,
  but needs to be tested.
• How should we test the bumps after irradiation?
• Measurements relevant for interconnect
• Visual inspections
• Elasticity
• Sample conductivity
• Certainly, visual inspection is required. If that
  checks out, then
• NRL can make large pad (e.g. 1x1 cm) with metal
  on top, and has capability for compression
  measurements on such samples,
• then can do conductivity test.

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Issues
Understand high temperature annealing of
irradiated sensors might be manufacturer
dependent. Reduce temperature needed for making
contact (presently about 100oC) Radiation
resistance of silicone structure. Limitation on
bump sizes bumps on pixels ltlt pixel
size Testing contact resistance, electrical
breakdown, mechanical integrity
Yield AlignmentMechanical joint ASIC pixel
sensor