Transient light emission microscopy for detecting the non-uniform junction temperature in flip-chip light emitting diodes

The non-uniform junction temperature may exist in the high-power light-emitting diode (LED) of flip-chip architecture. Detecting the local hot area on the chip is of great importance to the LED thermal management. Most of the common LEDs exhibit a lower forward voltage when its junction temperature is elevated. The local forward voltage deviation will lead to non-uniform current distribution and consequently affect the light distribution. In this paper, a hotspot detection method is proposed utilizing this intrinsic thermal-electrical property. The transient light emission distribution on the chip is measured by a system consists of a common microscope and an optical sensor. The spatial solution is high enough for practical application. The experiments are conducted on different LED chips. Various junction temperature distributions are generated by different patterns of the chip bonding pads. The measurement results suggest that the local hot area has a higher light intensity. The proposed method can be used to detect the location of the local hot area in the LED chip.