Progress on fabrication for Retinal Readout project

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Progress on fabrication for Retinal Readout
project

• W Cunningham1,a, K Mathieson1, FA McEwan1, A.
  Blue, V OShea1, KM Smith1, A Litke2,
• E. J. Chichilnisky3, M Rahman1
• 1Department of Physics Astronomy, University of
  Glasgow, Glasgow G12 8QQ, UK
• 2SCIPP, University of California Santa Cruz,
  Santa Cruz CA 95064, USA
• 3The Salk Institute, 10010 N. Torrey Pires Road,
  La Jolla, CA 92037, USA

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Objective of Retinal Readout Project
To develop large area microelectrode arrays for
the detection of neural signals from retinal
tissue with an aim to increasing knowledge of the
way information is encoded for transmission to
the brain. This knowledge may allow the
production of artificial retina with similar
functionality.
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Steps in the Retinal Readout Project

• Develop easily reproducible methods for etching
  wires in ITO, and an etch for SiN passivation
  layer which doesnt damage the ITO layer
  underneath
• completed
• Continue developing passivation layer
• initial solution available
• Platinise electrodes
• Carry out experiment
• data available
• Begin to scale detector

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Process for etching wires in ITO
Etch depth
Images of etched wires, pattern centre and mid
wire regions
Dektak measurements of etch depths
I-V plots of the etched sample
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Process for etching holes in SiN passivation layer
Etch depth
AFM scan of the depth of widows
Positioning of windows for end electrodes
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Problems with Si3N4
Cracking or pitting of the Si3N4 layer leads to
platinisation failure
Unwanted platinum lumps
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Initial solution to Si3N4 problems
A combination of oxygen plasma cleans and
bi-layers of Si3N4 have solved the problems of
pinholes and micropores causing shorts in the
devices.
micropores
Si3N4
ITO wire
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Fabricated devices
Devices produced using photolithography
Using processes outlined previously
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Platinisation
Platinised electrodes with close up on left
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Experimental data
Spikes from retinal tissue recorded in Salk
Institute using Glasgow constructed detector
Y-axis Voltage 1mV
X-axis time 50 ms
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Scaling detectors (1)reducing electrode size
Reducing size of electrodes to look at smaller
groups of cells 2 mm electrodes
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Scaling detectors (1)reducing electrode size
platinised 2 mm electrode
AFM scan of 2 mm electrode
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Scaling detectors (2)Increasing density of arrays

• Photolithography limited to optical resolution
  scale
• Unsuitable for densely packed wires
• Use of e-beam lithography can produce dense
  arrays
• Large area patterns long write time
• combine e-beam and photolith

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Scaling detectors (2)Increasing density of arrays
8 mm wire ends wires 1 mm, 1 mm spacing
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Future work and developments

• Continue developing the processes for producing
  large scale arrays.
• mask techniques etc