In-situ polarimetry of illumination for 193-nm lithography

Ellipsometry is defined as a technique for determining the properties of a bulk material or a thin film, such as optical index and thickness, from the measurement of a polarization state of a reflected light or a change of polarization states between the incident and reflected lights. On the other hand, polarimetry is defined as a technique for determining a polarization state of a light. In other words, ellipsometry uses polarization as a probe and polarimetry measures polarization itself. We have constructed a theory of polarimetry of illumination used in lithography equipments, fabricated a polarimeter mask, and demonstrated it for a hyper-NA (numerical aperture) immersion lithography scanner. The polarimeter mask comprises polarizers and quarter-wave (λ/4) plates that are crammed into a narrow space with a height of 6.35 mm. The thin plate polarizers available at a wavelength of 193 nm are made of calcite, and the λ/4-plates insensitive to angle of incidence are made of four thin plates, two of which are crystalline quartz; the other two are sapphire. A light traveling through a window of the polarimeter mask reaches an image detector at the wafer level through projection optics. Stokes parameters of the illumination light can be measured without any influence from polarization characteristics of the projection optics between the mask and the image detector.