Ultraviolet spectrophotometer using photomultiplier tubes in photon counting mode for measurement of optical properties of lithography optics.

A scheme of dual-beam spectrophotometry is proposed for polarization-resolved spectral characterization of ultraviolet lithography optics at high ray angles of incidence. The spectrophotometry utilizes photomultiplier tubes (PMTs) working in photon counting mode as the photon detector. Influences of the dark counts of the PMT detectors and the pileup effect of the data acquisition system on the systematic error of the spectral measurement are analyzed. The voltages of the PMTs are optimized to achieve a large ratio of photon events to dark counts. Slit width of the monochromator is chosen to reduce the influence of the pileup effect. Based on optimized measuring parameters, high-precision angle-resolved transmittance and reflectance measurements on bare and coated test wafers are demonstrated at the typical lithography wavelengths. The work paves a solid route toward accurate spectral characterization of the optical components in high numerical aperture lithography optics.