IWORID 2004, University of Glasgow 30 m Spacing 519-Electrode Arrays for In-Vitro Retinal Studies...
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Transcript of IWORID 2004, University of Glasgow 30 m Spacing 519-Electrode Arrays for In-Vitro Retinal Studies...
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
3030m Spacing 519-m Spacing 519-Electrode Arrays for In-Electrode Arrays for In-
Vitro Retinal StudiesVitro Retinal Studies
Debbie GunningDebbie GunningK. MathiesonK. Mathieson11, C. Adams, C. Adams11, W. , W.
CunninghamCunningham11, A.M. Litke, A.M. Litke22, E.J. , E.J. ChichilniskyChichilnisky33, M. Rahman, M. Rahman11
University of GlasgowUniversity of Glasgow11, University of California , University of California Santa CruzSanta Cruz22, Salk Institute for Biological Sciences, Salk Institute for Biological Sciences33
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
Outline:Outline:• Arrays:
• Design requirements • Fabrication processes • Electrical characterisation
• Tests: • Possibility of higher density arrays• Equivalent circuit predicting properties of
future arrays
• Conclusions
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
Array Array RequirementsRequirements• Transparent electrode array– Indium Tin Oxide, transparent
semiconductor commonly used in laptop displays
• High density of microelectrodes to ensure good detection of retinal cells
• Large area coverage to allow recording of correllated signalling
• Scalable fabrication process for future studies
512-channel readout board SCIPP,University of California Santa Cruz
Microelectrode Array
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
Fabrication of Fabrication of ArraysArrays
• Spin Resist onto Indium Tin Oxide (ITO) /Glass substrates
• Selective UV exposure of Resist, where ITO is to be removed
• Develop sample cleaning off exposed (weakened) Resist
• Dry etch ITO using Methane and Hydrogen
• Insulate with 1m Silicon Nitride (Si3N4)• Etch holes in Si3N4 to allow electrical
contacts to be made to ITO electrodes and bond pads
• Platinise electrodes to increase their surface area thereby decreasing impedance
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
Array Array DesignsDesigns• 61 Electrodes:
– 60m spacing = 0.17m2 coverage, 30m spacing = 0.04m2
coverage
• 519 Electrodes:– 60m spacing = 1.7m2 coverage, 30m spacing = 0.4m2
coverage
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
Fabrication of 519-Electrode Fabrication of 519-Electrode ArraysArrays
• Electron-beam lithography:• Inner section (wires, electrodes)• Feature size range: 1m-10m• Using UVIII as a fast e-beam
resist
• Photolithography:• Outer section (wires, bond pads,
vias)• Feature size range: 10m-100m
• Reactive ion etching:• Gases such as SiCl4, CH4/H2, SF6
used to etch Ti, ITO and Si3N4 respectively throughout fabrication of array
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
Characterisation of Characterisation of ArrayArray
• Short tests:• Ideally < 5 % of channels shorted• ~10% shorts possible i.e. ~50 shorts on 519-
electrode array has been achieved
• Impedance Measurements:• Measure the electrodes response to a sinusoidal
voltage stimulus • Impedance (magnitude) measured at 1kHz since
retinal signals have a duration ~ 1msec• Confirms that the electrode impedance will not
overwhelm small retinal pulses (<1mV)
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
3030m Electrode m Electrode ArrayArray
• Arrays platinise well, showing good electrical connection between electrode and bond pad
• Difference in impedances between 60m and 30m spaced electrodes, but not significant for retinal recordings
100 1k 10k 100k0
200k
400k
600k
800k
1M
Imp
edan
ce ()
Frequency (Hz)
60m electrode spacing 30m electrode spacing
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
Tests for Limits of Tests for Limits of FabricationFabrication
• What are the limits of fabrication for these arrays?
• Impedance and crosstalk tests on varying length, width and separation of traces
• Width: 10m - 300nm• Length: 40mm - 5mm• Short tests 15m spaced 519-
electrode array• Each ITO trace is passivated
with Si3N4 and has a 5m diameter electrode
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
Results: Trace Width Results: Trace Width InvestigationsInvestigations
• Limitations of this fabrication process ~300nm ITO width
• Impedances vary between 75k and 150k at 1kHz
• Impedance depends mainly on electrode/electrolyte interface not on ITO wire width
100 1k 10k 100k0
100k
200k
300k
400k
500k
Imp
edan
ce ()
Frequency (Hz)
10m 2m 1m 0.9m 0.8m 0.7m 0.4m
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
Modelling the ElectrodeModelling the Electrode
• All parameters are fixed by theory and only Asurface is altered (platinisation)
RITO ~ Resistance of ITO wire
Rs ~ Spreading resistance, Z 1/Ageometric
Ci ~ Interfacial capacitance, Z 1/Asurface
Rt ~ Charge transfer resistance, Z 1/Asurface
Rw ~ Warburg resistance
Cw ~ Warburg capacitance
Model adapted from G.T.A. Kovacs (Stanford)
electrolyte
Ci ~ 2nF
Rs~50kRITO
Rt~1M
Rw
Cw
electrode/electrolyte interface
Rw
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
SimulationsSimulations
• Model predicts trends Model predicts trends in experimental datain experimental data
• Small offset at high Small offset at high frequenciesfrequencies– Due possibly to Due possibly to
variations in variations in electrolyte electrolyte concentrationconcentration
• Model accurate Model accurate enough over range of enough over range of interestinterest
1k 10k 100k0
100k
200k
300k
400k
500k
Imp
edan
ce ()
Frequency (Hz)
Line width 0.4 m measurement PSPICE model
10 m measurement PSPICE model
IWORID 2004, University of IWORID 2004, University of GlasgowGlasgow
ConclusionsConclusions• Possible to fabricate 519-electrode arrays with m spacing• Possible to fabricate ITO traces down to 300nm width using
current processes– Impedance measurements indicate that they should
record retinal signals in a manner similar to existing arrays
• Dominant contribution to impedance results from electrode/electrolyte interface
• High density arrays with large area coverage could offer a valuable insight into how the brain functions– The ability for microelectronics and computing to record
and analyse large data sets means there is now a need for these arrays