High-selectivity neural probe based on a Fabry–Perot optical filter and a CMOS silicon photodiodes array at visible wavelengths

Abstract. This paper presents a silicon neural probe with a high-selectivity optical readout function and light emitting diodes for neurons photostimulation and fluorophore excitation. A high-selectivity Fabry–Perot optical filter on the top of a CMOS silicon photodiodes array can read the emitted fluorescence, which indicates the neurons physiological state. The design, fabrication, and characterization of the optical filter are presented. The SiO2  /  TiO2 based optical filter thin films were deposited by RF sputtering. The performance of the optical filter deposited on the top of the silicon photodiodes array, implemented in the neural probe, was tested through in-vitro fluorescence measurements. The transmittance peak of the fabricated optical filter is 81.8% at 561 nm, with a full width at half maximum of 28 nm. The peak responsivity of the CMOS silicon photodiode with the optical filter deposited on its top is 273.6  mA  /  W at 578 nm. The in-vitro fluorescence measurements results show a CMOS photodiode current proportional to the fluorophore concentration with a good linearity (R2  =  0.9361). The results validate the use of the neural probe with the high-selectivity optical readout function to determine the presence of different fluorophore concentrations. The development of the device in a conventional CMOS process allows on-chip electronics readout.