A novel masking method for high aspect ratio penetrating microelectrode arrays

Abstract

Neural interfaces connect signal processing electronics to the nervous system via implanted microelectrode arrays such as the Utah electrode array (UEA). The penetrating electrodes of the UEA are encapsulated with parylene-C. In order to form active electrode sites, parylene-C must be removed from the electrode tips. Masking the electrodes to selectively de-insulate the tips has been accomplished by poking the electrode through aluminum foil. This mechanical poking process lacks precise control over the length of exposed tips. The non-uniformity in tip exposure can change the electrode impedance across the array, which causes variability in recording and stimulating characteristics. This paper focuses on the development of a fabrication technology to produce uniformly exposed tip lengths, over a range of 30–350 µm. A novel batch-oriented and wafer-scale method to de-insulate the electrode tips of the UEA is presented which uses a photoresist to mask the samples and an oxygen plasma to etch parylene-C. The tip exposure is controlled by varying the spin speed during photoresist coating of the electrode array. At 100 rpm, the non-uniformity of the batch-processed arrays was 14.74 ± 7.49% while in the case of a wafer-scale process it was 3.0 ± 0.2% for 10 × 10 electrodes; this indicates that the wafer-scale process leads to better uniformity in tip exposure. A set of photoresist-based process parameters were also tested to ensure that the electrical and mechanical properties of parylene-C were not compromised by the process.