Application of thermal camera to optimize assembly methods of laser diodes

Abstract

The majority of electronic devices exert considerable amounts of heat during operation, and its effective disposal gains particular importance as the excessive temperature of semiconductor devices has a clearly negative impact on their operation. First of all, properties of semiconductor materials frequently depend to a large extent on temperature, and so temperature changes are a cause of a lack of stability of parameters of semiconductor devices. Furthermore, degradation processes of semiconductor materials are frequently thermally activated and occur much quicker at increased temperature, consequently shortening the lifetime of the devices involved. An additional mechanism which accelerates the degradation of electronic devices is the mechanical stress that occurs for materials having different temperature expansion coefficients, which is of particular importance during switching them on and off. This problem tends to gain importance, because with progress in miniaturization in electronics, the density of generated power grows. For optimizing the method of heat removal the knowledge of temperature distribution during normal device operation is crucial. The subject of this study were the determination of temperature distribution in violet light emitting, nitride-based laser diodes. Laser semiconductor diodes belong to devices characterized by the highest densities of generated power. The reason for this is that condition that allows lasing is achievement of a high current density in the active area of a laser diode. To meet this requirement the active area is designed in the form of a narrow strip of a width ranging from single microns for lasers of a lower power, up to a few tens of microns for higher power lasers [1][2]. As regards lasers produced on the basis of nitrides, power generated in the laser structure is even higher than in the case of red and infrared lasers, because owing to specific properties of those semiconductor materials, laser operation requires higher current density (3-10 kA/cm) and voltage (over 4V), which leads to a very high density of electric power (1250 kW/cm[3][4]. To achieve effective continuous wave laser operation it is indispensable to obtain appropriately low thermal resistance of the entire device. Determination of the active laser area temperature can be achieved by using various optical methods: electroluminescence, Raman spectroscopy, thermoreflection, or electrical methods, such as testing the voltage of the p-n junction. However, the above mentioned methods do not supply information on temperature distribution outside the active area of the laser and in its housing, which is necessary to allow optimization the heat dissipation. This type of data may be directly obtained from microscopic thermography.