OPTICAL REVIEW Vol. 16, No. 2 (2009) 188–191 Wide-View-Angle =4 Plates for Diagnosing 193-nm Lithography Tools Yohko FURUTONO and Hiroshi NOMURA Semiconductor Company, Toshiba Corporation, 8 Shinsugita-cho, Isogo-ku, Yokohama 235-8522, Japan (Received July 18, 2008; Accepted November 13, 2008) We developed new =4 plates, which are insensitive to angle of incidence at 193 nm. Thanks to the development of the wide-view-angle =4 plates, a polarimeter reticle is able to measure the polarization of illumination of immersion lithography tools. The wide-view-angle =4 plates are composed of four crystal plates; two are made of crystalline quartz categorized as positive uniaxial crystals, and two are made of sapphire categorized as negative uniaxial crystals. Since the birefringent crystals differ in regard to which is the higher of the two refractive indices for the ordinary ray and for the extraordinary ray, their combination mitigates the retardation change at oblique incidence. We measure the retardation change of the new =4 plates by the Senarmont method. Results show that the change of retardation is mitigated within an angular range of incidence of Æ20. # 2009 The Optical Society of Japan Keywords: polarization, lithography, quarter-wave-plate, retardation, birefringent 1. Introduction Lithography is a key technology for the mass production of LSIs. For the progress of miniaturization of LSIs, it is necessary to control several properties of lithography tools. 0-λ plate ( n > n ) Polarization of illumination should also be one of these e o λ /4 plate properties in the near future.1) For the two-beam interference ( ne < no ) through an objective with an NA above 1, the p-component of polarization of an incident light in resist does not wholly Fig. 1. The wide-view-angle =4 plate composed of four plates. interfere with that of the other light, so that the contrast of A pair of crystalline quartz plates is a =4 plate, and the other pair fine images degrades. Techniques for measuring polarization of sapphire plates is a 0 À plate. states of illumination should be constructed by a system on the reticle stage, such as a special reticle with polarizers and =4 plates. It has been difficult to realize such a system in visible and deep UV regions. The phase of a light wave because of space restrictions. shifts depending on a light path in the material, and the light In 2006, H. Nomura proposed a polarimeter reticle, which path changes according to angle of incidence; consequently, is composed of two new optical devices.2,3) One is a thin the retardation also changes according to angle of incidence. plate polarizer. Conventional polarizing prisms such as To avoid this change of retardation, conventional =4 plates Nicol prism, Glan–Thompson prism, and Rochon prism require a collimating optics in front of them to penetrate all require a long space to split an input light into two orthogo- the rays to the face of the =4 plates at the normal incidence. nal linear polarizations. The thin plate polarizers made of It is hard to put the collimating optics on a reticle with a calcite (CaCO3), which has large birefringence at a wave- thickness of 6.35 mm. The wide-view-angle =4 plate makes length of 193 nm, can greatly economize on space. The other the polarimeter mask simple. device is the =4 plate with a wide range of view angles. This paper describes details of the wide-view-angle =4 3. Theory of Wide-View-Angle =4 Plates plates and measurement results of the retardation change The new =4 plates are composed of four crystal plates; with angle of incidence. two are made of crystalline quartz and two are made of sapphire. The two crystalline quartz plates compose a =4 2. The =4 Plates plate and the two sapphire plates compose a 0 À plate as Prior to discussion on the wide-view-angle =4 plates, illustrated in Fig. 1. Index ellipsoids of crystalline quartz and conventional =4 plates are briefly described. Widespread sapphire are shown in Figs. 2(a) and 2(b), respectively. =4 plates at visible to deep UV regions are composed of Crystalline quartz is categorized as a positive uniaxial crystal two or more plates made of uniaxial birefringent materials, with refractive indices of no > ne, and sapphire is categorized and one linear component of polarization, which is parallel as a negative uniaxial crystal with refractive indices of no < to the slow axis (s-axis), retards the phase by 90 compared ne, where no and ne are refractive indices for the ordinary ray with the other orthogonal component, which is parallel to the (o-ray) and for the extraordinary ray (e-ray), respectively. fast axis (f-axis), through the plates. On the surface of a Whether the uniaxial crystal is ‘‘positive’’ or ‘‘negative’’ holder, there is a scar that indicates s-axis or f-axis of the determines which ray retards the other at oblique incidence. =4 plate. Crystalline quartz ( -SiO2) and sellaite (MgF2) Therefore, combination between the positive and negative are widely used as materials of =2 and =4 plates employed uniaxial crystals may reduce the change of retardation with 188 OPTICAL REVIEW Vol. 16, No. 2 (2009) Y. FURUTONO and H. NOMURA 189 (a) Quartz (b) Sapphire Table 1. Azimuthal angles (deg) of each optical element. n z nz (a) (b) (c) (d) (e) n =4 plate Wollaston prism =4 plate Sample Wollaston prism y n no y no 45 0 þ 45 0 À45 45 45 þ 90 45 0 nx n nx þ À ne e 45 90 45 90 45 90° Fig. 2. Index ellipsoids of crystalline quartz and sapphire. Crys- talline quartz is categorized as a positive uniaxial material and 45° therefore, the refractive index for the ordinary ray is higher than that for the extraordinary ray. On the other hand, sapphire is categorized as a negative uniaxial material and therefore, the refractive index for 0° the extraordinary ray is higher than that for the ordinary ray. Optic axis (a) (b) (c) (d) (e) λ /4 plate Wollaston λ /4 plate sample Wollaston prism λ /4 plate prism (f) Fig. 4. Measurements were made in three cases of azimuthal FIX ° θ FIX FIX FIX 0 - detector Ray source angles, namely, 0, 45, and 90 , between the f-axis of the sample and ArF the incident plane. 193nm under N2 environment Fig. 3. The measurement configuration. The Senarmont method second Wollaston prism (e) for the measurement at three was adopted to measure the retardation change with angle of azimuthal angles of sample (d) are listed in Table 1. incidence. Figure 4 shows the azimuthal angles of sample (d). This measurement is done for these three cases. For example, the measurement of the retardation change angle of incidence. When the ray is incident to the crys- with angle of incidence when the incident plane lies at 45 talline quartz =4 plate at the normal, the retardation angle to the f-axis of the sample =4 plate (d) is explained. =4 of the o-ray to the e-ray is designed to be just 90. However, Plate (a), =4 plate (c), and Wollaston prism (e) were set at for an incidence other than the normal, the retardation azimuthal angles of 45, 90, and 0, respectively. The gradually exceeds 90 as the angle of incidence increases. intensities were measured by a power sensor (f), rotating the On the other hand, when the ray is incident to the normal Wollaston prism (b) in increments of 5. The retardation to the sapphire 0 À plate, the retardation must be nothing. error of the wide-view-angle =4 plate (d) from 90 is The retardation gradually arises as the angle of incidence connected by twice the angle of the first Wollaston prism increases, and the sign of the retardation is contrary to the (b) as =4 plate. Therefore, retardation change at the oblique ¼ 2; ð1Þ incidence can be mitigated by the 0 À plate. Interestingly, the retardation change depends on angles where is defined as the angle of Wollaston prism (b) to the between incidence plane and the s-axis of the =4 plate. sample (d) for the minimum intensity. Other cases such that When the incidence is in the plane that involves the f-axis or the incident planes are parallel and perpendicular to the the s-axis, the behavior of the retardation change is not f-axis of the sample (d) were also checked. complicated. When the incidence plane is parallel to the s- The measurement chamber was connected with argon axis, the o-ray retards the e-ray, that is, the retardation rises fluoride (ArF) excimer laser equipment. The wavelength of according to the angle of incidence. On the other hand, when the light is 193 nm. All the optical paths are in the nitrogen the incidence plane is parallel to the f-axis, the o-ray (N2) environment. The ArF laser emits a high-power light advances to e-ray, that is, the retardation declines. The linearly polarized in the horizontal direction. For fear that former is symmetric in curve to the latter. too high power of a light could cause errors from thermal expansion, the light was diminished by mirrors before being 4. Experiment led to the measurement chamber. Prior to the measurement, We adopted the Senarmont method in the measurement of we defined purity of polarization as retardation change with angle of incidence. Figure 3 shows I À I the measurement configuration. By exchanging =4 plate (c) I ¼ max min ; ð2Þ and sample =4 plate (d) for comparison with the original Imax þ Imin Senarmont configuration, we measured the retardation of and confirmed that the purity of the ArF laser is 0.998 at sample (d).