Stress analysis of super high-pressure mercury discharge lamp bulbs under thermal loads

Abstract

Super high-pressure mercury discharge lamps have been used as a light source in photolithography. They are applied to manufacturing semiconductors, liquid crystal displays (LCD), printed circuit boards (PCB) and so on. It is well known that their working internal pressure is more than 1.0MPa and their temperature at bulb outer surface is more than 500°C. Therefore, it is essential to be examined whether their safety designs for fracture of the bulbs are enough or not. Because the bulb shapes have been designed empirically, research has yet been carried out on the optimal design of the lamps for preventing the bulb fracture, by using scientific methods such as the theory of elasticity, finite element method and so on. In this study, the Finite Element Method (FEM) is applied to obtain the thermal stress of bulbs in operation, and the effects of the thermal stress on the bulb fracture is examined by comparing the obtained thermal stress with the critical stress of silica glass. As the result, we found that the obtained thermal stress of the bulb was much smaller than the stress, which was occurred under internal pressure at room temperature. In addition, it was observed that the thermal stress distributions in the lamps were varied by due to the restricted condition at the bulb ends. Thus, it can be concluded that designers for bulbs should pay a lot of attention how to fix the bulbs, because the stress in the bulbs increases by the restricted conditions and ruptures of the bulbs may occur due to the stress concentration.