|
As technology advances, the demand for tighter photomask final flatness specifications becomes greater. Studies have
shown that the process of mounting a pellicle induces the largest change in flatness in photomask fabrication. Photomask
pellicles play an important role in flatness due to the many components in the mounting process. For example, pellicle
frame flatness, pellicle adhesive, mounting force, mounting time, mounting orientation and mask backing shape during
mount all can play a role in changing the mask shape during pellicle mount. Many of these factors have been
investigated over the last several years [1][2][3][4]. Recent studies have demonstrated that the height of the pellicle
frame also has a significant impact on the final flatness with lower stand off frames resulting in reduced pellicle
influence on mask distortion [5]. This paper will examine the flatness influence factor as a function of mounting
direction and mask backing variations. For these experiments, the same pellicle frame was remounted for each set of
experiments to eliminate external pellicle frame flatness factors and to minimize the amount of data deviations. Four
different types of mask backing types were selected that differed in the contact area with the mask in particular pressure
points. The mask backing types consist of a border frame, 4 point pressure points, a full backing plate (quartz substrate),
and a pellicle frame. In addition to using the four different types of mask backings, the pellicles were also mounted both
in the vertical and horizontal directions in determining final photomask flatness. This work demonstrates that frame
flatness and shape play the largest roles and mounting force, backing plate and mounting orientation have less of an
effect.
|
