Ultracompact optrode with integrated laser diode chips and SU-8 waveguides for optogenetic applications

This paper reports on the design, fabrication and characterization of an innovative silicon-based neural probe with optical functionality. This so-called optrode is intended as an ultracompact tool for optogenetic applications in neuroscientific research. Beside platinum microe-lectrodes for electrical recording applications, bare laser diode (LD) chips combined with waveguides (WGs) implemented in the negative photoresist SU-8 are integrated on the probe. The assembly of the bare LD chips applies flip-chip and wire bonding and benefits of a lateral alignment accuracy better ±5 μm required for the efficient coupling of light into the 15-μm-wide and 13-μm-high WGs. Undesired tissue illumination due to stray light is effectively blocked using a micromachined cover chip adhesively bonded to the probe base. Probe shafts with a length of up to 8 mm and a thickness of 50 μm carrying four electrodes and two WGs each have been realized. The maximum optical output power per WG was measured to be 29.7 mW/mm² for LDs with a center wavelength of 650 nm.