The present disclosure describes an apparatus of leveling a substrate in a charged particle lithography system. In an example, the apparatus includes a cantilever-based sensor that includes an optical sensor and a cantilever structure. The optical sensor determines a distance between the optical sensor and a surface of the substrate based on light reflected from the cantilever structure. In an example, a first distance is between the cantilever structure and optical sensor, a second distance is a height of the cantilever structure, and a third distance is between the optical sensor and the surface of the substrate. The optical sensor determines the first distance based on the light reflected from the cantilever structure, such that the third distance is determined from the first distance and the second distance.