The disclosure relates to determining information about a target structure formed on a substrate using a lithographic process. In one arrangement, a cantilever probe is provided having a cantilever arm and a probe element. The probe element extends from the cantilever arm towards the target structure. Ultrasonic waves are generated in the cantilever probe. The ultrasonic waves propagate through the probe element into the target structure and reflect back from the target structure into the probe element or into a further probe element extending from the cantilever arm. The reflected ultrasonic waves are detected and used to determine information about the target structure.