Patent ID: 7535551

Claim:
A projection lens unit, comprising: a projection lens comprising a first plurality of lenses adapted to project an exposure beam of a first wavelength onto a semiconductor substrate, wherein N is the number of lenses in the first plurality of lenses; a measurement light emitting unit adapted to provide measurement light of a second wavelength different from the first wavelength, such that the measurement light irradiates the semiconductor substrate after passing through a second plurality of the lenses in the projection lens, wherein M is the number of lenses in the second plurality of lenses, and N is greater than or equal to M; a reference light emitting unit adapted to provide reference light of a third wavelength different from the first wavelength, such that the reference light irradiates the semiconductor substrate without passing through the first plurality of lenses in the projection lens; a reflected reference light receiving unit receiving a portion of the reference light reflected from the semiconductor substrate and generating a first intensity value associated with the received portion of the reference light reflected from the semiconductor substrate; and a reflected measurement light receiving unit receiving a portion of the measurement light reflected from the semiconductor substrate and generating a second intensity value associated with the received portion of the measurement light reflected from the semiconductor substrate, wherein divergence of the first and second intensity values from being identical indicates variation in a desired focus and level condition.