Metrology of Wide-Viewing-Angle /4 Plate in lithography

Abstract

Waveplates are among the most commonly used devices for altering the polarization state of light, and have been widely applied in polarization analysis of high-numerical-aperture (high-NA) imaging systems such as polarizing microscopes and immersion lithographers. As the NA of an optical system is increased, the effects of oblique incidence on the phase retardation of light rays passing through a waveplate become increasingly significant. This paper describes the design and manufacture of a 632.8 nm wide-viewing-angle (WVA) λ/4 plate. The method of phase compensation is employed to measure the phase retardation characteristics of this waveplate. These experimental results show that the phase retardation by the WVA λ/4 plate is consistently in the range between 84°and 96° for angles of incidence between ±20°, which confirms the effectiveness of the combination of positive and negative crystal in eliminating the influence of oblique incidence on phase retardation. INTRODUCTION As the feature size of integrated circuits shrinks, there is an increasing demand for high resolution in high-numerical-aperture (high-NA) optical imaging.1,2 However, the quality and resolution of such high-NA imaging systems are susceptible to the effects of polarization. Accordingly, the effect of oblique incidence of a light ray on its phase retardation by a waveplate—one of the key components of a polarimeter—has become a topic of great interest. In the polarimetry of a high-NA lithographer, measurements of the state of polarization of light and the polarization aberration of projection optics at the mask level are affected by the precision of the wide-viewing-angle (WVA) λ/4 plate. Consequently, a suitable method of measurement that ensures accurately gauged retardation is a prerequisite for minimizing errors induced by a waveplate, and therefore significant for its utilization in high-precision measurement. There are a variety of techniques for measuring retardation by a waveplate, including polarization-interference,3–7 phase-modulation,8,9 optical heterodyning,10,11 phase-shifting,12,13 and phase compensation,14–17 among others. Phase compensation is one of the most commonly used, owing to the relatively low complexity of the light path and its ease of adjustment, as well as the high precision of the results. PRINCIPLES OF PHASE COMPENSATION The principle of phase compensation, also known as de Senarmont compensation, is illustrated in Figure 1. The system consists of a polarizer, the waveplate to be measured, a standard λ/4 plate, and a polarization analyzer. The retardation by the target waveplate can be determined based on the variation in light intensity as the polarization analyzer is rotated while the first three devices are held fixed.