F. McEwan

Fabrication of microelectrode arrays for neural measurements from retinal tissue

The production of high-density, large-area microelectrode arrays for neurophysiology studies requires a combination of electron beam lithography and photolithography, together with dry etch pattern transfer. The retina is one of the few parts of the animal nervous system to which the input (image) and output (ganglion) signals are accessible. To read out the signals from the ganglion cells, we have made microelectrode arrays in the transparent conductor indium tin oxide (ITO) by CH4/H2 reactive ion etching (RIE) using a novel masking technique that avoids the usual problem of resist cross-linking. An electron beam lithography process has also been developed exploiting multiple plasma steps. The ITO layer lies on a glass substrate to allow the retinal cells and the electrode positions to be photographed using a CCD camera. SF6 RIE of vias through a thick protective silicon nitride coating deposited by plasma onto the ITO/glass permits the platinization of the electrodes. Both CH4/H2 and SF6 RIE require low electrical damage to the ITO, which we verify.

Pixel detectors for use in retina neurophysiology studies

One area of major inter-disciplinary co-operation is between the particle physics and bio-medical communities. The type of large detector arrays and fast electronics developed in laboratories like CERN are becoming used for a wide range of medical and biological experiments. In the present work fabrication technology developed for producing semiconductor radiation detectors has been applied to produce arrays which have been used in neuro-physiological experiments on retinal tissue. We have exploited UVIII, a low molecular weight resist, that has permitted large area electron beam lithography. This allows the resolution to go below that of conventional photolithography and hence the production of densely packed ∼500 electrode arrays with feature sizes down to below . The neural signals from significant areas of the retina may thus be captured.