Investigation on tensional membrane force of the fluoropolymer dust covers in semiconductors

The photomask dust cover used in semiconductor manufacturing comprises a thin fluoropolymer membrane stretched tightly over an aluminum frame. The dust cover plays a vital role in protecting the photomask from dust pollution, thereby ensuring the purity of the exposed patterns and enhancing the wafer yield. However, the membrane is extremely thin and easily damaged during the manufacturing process. Therefore, this research proposes an analytical model for predicting the tensional force induced in the membrane under different distributed loads. The reflective fringe method, combined with a four-step phase-shifting technique, is used to measure the three-dimensional surface topography of the membrane under various distributed loads. A trial-and-error fitting method based on a simulation model of the dust cover is then employed to estimate the tensional force which yields the maximum vertical displacement closest to the experimental observations under different values of the distributed load. The analytical results confirm that a greater distributed load results in a higher membrane tensional force. However, the predicted tensional force reduces as the service life of the membrane increases due to the effects of plastic deformation. Overall, this study presents a simple approach of predicting the tensional force induced in the pellicle membrane under different loads. Thus, by taking the yield strength of the membrane as a threshold value, it serves as a useful tool for assessing the quality of the membrane and excluding its use from the production process if appropriate.