A Monolithic Micromachined Thermocouple Probe With Electroplating Nickel for Micro-LED Inspection

This paper presents a pair of trapezoid microcantilever probes with a monolithic micromachined thermocouple for inspecting the electrical and thermal properties of Micro-LED. To meet testing requirements, one of the microcantilevers was designed as a single-layer Ni structure for electrical conduction, while the other one, which consists of n-type poly-Si, SiO 2 and Ni layers, was used for temperature sensing. Both microcantilevers were fabricated by using Si bulk micromachining, thin film deposition, and electroplating processes. Fabrication result shows the length of each probe is about 78~\mu \text{m} and 118~\mu \text{m} , and the thicknesses of the microcantilevers and probe tip are 7.2~\mu \text{m} and 5.5~\mu \text{m} . Experiment results depict that the thermocouple junction between Ni layer and poly-Si achieves a good ohmic contact, and the measured sheet resistances for poly-Si and electroplated Ni are 150~\Omega /sq and 0.01~\Omega /sq. This suggests that the induced thermoelectric voltage can be generated and detected in response to a temperature difference. Mechanical tests verified that the probes provide an available elastic deflection to guarantee that the probe tips can exactly contact the Micro-LED electrodes, with a low contact force. The thermoelectric characteristics of the device have been further confirmed and calibrated on the basis of the measurement results of a commercial mm-sized LED in cooperation with an infrared camera. For the subject of the Micro-LED chip with a square size of 75~\mu \text{m} , its electrical and thermal properties were successfully determined, as the measured chip temperature is 52 °C at the applied current of 100~\mu \text{A} . [2021-0053]