Electroluminescence measurement of microscale light-emitting diode wafers using a three-dimensional flexible probe head

Microscale light-emitting diodes (LEDs) could be used as the backlights of next-generation displays. However, high-density, large-area displays have stringent requirements in terms of manufacturing yields and the lack of effective tools—capable of high-throughput electroluminescence detection, which can facilitate known-good-die transfer printing—limits mass production of microscale LEDs. Here we show that a three-dimensional flexible probe head (analogous to the rigid probe cards used to conduct wafer-level tests of chips in semiconducting testing) and a corresponding electroluminescence detection system can measure the electrical and optical properties of microscale LEDs without introducing surface defects. Elastic microposts in the probe head can deform adaptively to match the surface morphology of the microscale LEDs and can tolerate height differences between the LED pads. The probe head has 32 × 32 pairs of probes that can simultaneously measure 1,024 microscale LEDs using a passive-matrix driving approach in 0.5 s. The contact stress applied from the probe head to the microscale LEDs is 0.91 MPa, which is at least two orders of magnitude lower than the yield stress that will typically create surface defects. We show that the system can perform more than 1 million repeated contact measurements with negligible probe wear.