Patent ID: 6346986
Filing Date: 2002-02-12
Classification: G01B,G03F

Abstract:
A method for monitoring a dimension of a pellicle, the method comprising:projecting a first light signal against a surface of the pellicle; detecting the first light signal reflected from the surface of the pellicle as a second light signal, the second light signal being a representation of the first light signal; and determining if the dimension of the pellicle is within an allowable value based on a characteristic of the detected second light signal, wherein projecting the first light signal comprises using a chopper to interrupt the first light signal, the second light signal comprising pulses of the interrupted first light signal reflected from the surface of the pellicle, the method further comprising: transforming the detected second light signal into an electrical signal having peaks corresponding to the pulses; determining a change in time periods between the peaks of the electrical signal; and correlating the change in time periods with a change in the dimension of the pellicle.