Patent ID: 8138483

Claim:
A method of measuring a phase of a phase shift mask, the method comprising: dividing initial extreme ultraviolet (EUV) light into secondary EUV light portions; irradiating the secondary EUV light portions onto the phase shift mask as incident EUV light portions; and measuring the phase of the phase shift mask from reflected incident EUV light portions reflected from the phase shift mask wherein irradiating the secondary EUV light portions onto the phase shift mask as incident EUV light portions comprises reflecting the secondary EUV light portions back towards the phase shift mask, and directing the reflected secondary EUV light portions towards the phase shift mask as the incident EUV light portions, and wherein reflecting the secondary EUV light portions back towards the phase shift mask comprises controlling a reflection angle for a first mirror, such that the reflected secondary EUV light portions are directed to a location above a path of the secondary EUV light portions.