A bottom lens for an immersion exposure tool includes a hydrophobic coating on the sidewalls of the bottom lens. A bottom portion of the bottom lens is not coated with the hydrophobic coating to maintain the optical performance of the bottom lens and to not distort a pattern that is to be transferred to a substrate. The hydrophobic coating may reduce the thermal instability of the bottom lens. This may reduce overlay variation during operation of the immersion exposure tool, which may increase manufacturing yield, decrease device failures, and/or decrease rework and repairs.