A LED micro-display with 90×90 pixels on a 80 μm pitch

We report on the flip-chip packaging of 90×90 arrays of addressable microLEDs onto CMOS circuitry by a solder reflow process. The substrate emitting microLEDs were parabolically shaped and fabricated on free-standing GaN. The individual emitters were 15 μm in diameter with an array pitch of 80 μm where all bond pads were given a gold finish. The solder balls were dispensed onto the CMOS pads with an automated PacTech solder dispenser that dispensed 50 μm diameter solder spheres with a composition of Sn (96.5%)-Ag-(3%)-Cu (0.5%). After alignment of the LED array on top of the CMOS chip, the solder was reflowed at a temperature of 260 C in nitrogen ambient. The completed assemblies form micro-displays that can display computer generated patterns designed for the stimulation of neural cells. The displays were designed to be mounted on the camera port of a microscope to project the emitted light onto neural tissue.