Light emitting diodes (LED) have been rapidly developed in the past decades. In recent years, LEDs of the flip-chip structure attracted great attention from the LED manufacturers for its simpler manufacturing processes, better performance, and higher reliability. According to the flip-chip structure, the active layer is directly attached to the chip carrier. The bonding layer acts as a thermal interface. Therefore, this bonding layer plays a critical role in the LED performance. But in reality, due to the packaging design or manufacturing defect, it is impractical to guarantee a bonding layer of the full chip area and the partial bonding which only occupies part of the chip area will consequently influence the uniformity of the thermal state across the LED chip and generate the...[ Read more ]

Light emitting diodes (LED) have been rapidly developed in the past decades. In recent years, LEDs of the flip-chip structure attracted great attention from the LED manufacturers for its simpler manufacturing processes, better performance, and higher reliability. According to the flip-chip structure, the active layer is directly attached to the chip carrier. The bonding layer acts as a thermal interface. Therefore, this bonding layer plays a critical role in the LED performance. But in reality, due to the packaging design or manufacturing defect, it is impractical to guarantee a bonding layer of the full chip area and the partial bonding which only occupies part of the chip area will consequently influence the uniformity of the thermal state across the LED chip and generate the non-uniform junction temperature. Since the junction temperature is one of the dominating factors of the performance of an LED device, it is necessary to investigate such phenomenon.

The non-uniform junction temperature issues in the flip-chip LED have been reported in several papers. One of the key issues concerns about the measurement of the non-uniform junction temperature distribution. The measurement is usually accomplished by the thermography technique. However, there is no literature quantitatively reporting the characterization about the distribution of temperature. Furthermore, although the published literature has mentioned the current crowding effect from the non-uniform junction temperature, seldom of them quantified this current crowding effect.

In order to address all these questions and enhance the understanding of the non-uniform junction temperature in flip-chip LED, this study will focus on developing the characterization methods for the non-uniform junction temperature and developing models for the chip behavior relates to the non-uniform junction.

Firstly, the fabrication of the test vehicles in this study will be given in detail. Different commercial flip-chip LEDs are used and the silicon chip carrier is employed. Special bonding pads are designed so that non-uniform junction temperature can be created.

Since the radiation from the active layer has to pass through the transparent substrate of the LED, which is usually a single crystal sapphire, the thermal radiation or the electroluminescence radiation from the active layer is inevitably affected by the sapphire substrate. The sapphire substrate internally reflects the radiation and decreases the contrast of the radiation distribution, therefore, it is necessary to model the optical behavior of the substrate. Based on the proposed model, a correction method is developed to eliminate the influence from the substrate and recover the initial intensity of the radiation. Some necessary parameters are obtained by experimental tests according to the optical model. Meanwhile, these optical characterization results can further validate the optical model.

One of the most important characterizations for the LED is the junction temperature measurement. The forward voltage method and infra-red thermography method will be discussed in depth in this study. Besides, the measurement of the current distribution and the thermal power distribution are given. An important concept, the equivalent quantity, is introduced to assist the electrical characterization. The thermal power distribution results will next be used in the finite element simulation.

To validate the characterization methods, the comparison of the temperature profile from experiments will be conducted with numerical simulations. By the well-prepared characterization methods, the correct junction temperature distribution could be obtained and a good agreement could be observed between experimental results and simulation results.

The current crowding effect induced by the non-uniform junction temperature is investigated. It is confirmed that the non-uniform junction temperature can increase the current crowding effect because of the intrinsic properties of the LED chip. The current crowding effect can be further utilized to detect the non-uniform junction temperature by means of measuring the light distribution. A system for the transient light emission microscopic imaging is developed to implement the idea.