Analysis and measures against heat-expansion for sub-micron LD assembly by passive alignment

Currently, the conventional active alignment method is being gradually replaced by the passive alignment method, in which LD is bonded on silicon substrate with wave-guide. Transition to passive alignment was triggered by the drive for further cost reduction of optical device modules and demand for automated assembly operations. When applying the passive alignment method at the sub-micron level, the following points need to be considered. During heating operation, chip, substrate and heating tools are affected by thermal expansion. Position sift by temperature drift on recognition unit and bonding head also occur during processing. Therefore, the optimal operation cannot be achieved merely by raising recognition accuracy and alignment accuracy at the alignment table. Those conditions were carefully analyzed, and based on the following data, we have developed a unique calibration method using three cameras. The first camera is used to recognize the position of the chip before bonding. The second camera is used to recognize the position of the substrate before bonding. The third camera beneath the bonding stage is used to measure the displacement after bonding. By using those three cameras, placement accuracy of chip can be monitored, and any necessary data can be fed back, so as to maintain sub-micron accuracy throughout the processing. As a countermeasure against thermal expansion of chip, substrate and heating tools, a multi-layered ceramic heater and an air-floating head free of friction have been adopted, and the effects have been verified. This paper describes the data and the experiment to prove the methods described above, where successful sub-micron assembly was achieved.