Optimization of a Michigan-type silicon microprobe for infrared neural stimulation

This paper presents a Michigan-type deep brain silicon optrode capable of delivering infrared light into the neural tissue. Silicon optrodes were fabricated by deep reactive ion etching (DRIE) and subsequent wet chemical polishing. The proposed method is feasible to reduce the sidewall roughness significantly and is able to turn the substrate material into an infrared waveguide of sufficient efficiency for infrared neural stimulation. The advantage of our approach is that the fabrication process is fully compatible with that of functional neural microelectrodes. Moreover, there is no need to add further waveguide layers on top of such devices aiming IR stimulation, which may induce less tissue trauma. Our design also facilitates the spatially controlled illumination of the tissue through integrated micromirrors and microlenses and the precise alignment of an optical fiber through an integrated guide slot. An average system efficiency of 22.1% with a Gaussian beam profile (NA = 0.13) was achieved by reducing the RMS roughness of the sidewall down to 8.7 nm using 1310 nm wavelength illumination and coupled core diameter of 9 μm. To our knowledge, this is the first investigation of the optical properties of a Michigan-type silicon microprobe aiming infrared neural stimulation at the substrate level.