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Optical Nanolithography
Published in Bruce W. Smith, Kazuaki Suzuki, Microlithography, 2020
As previously seen, at high NA values (above 0.5) for high-resolution lithography, diffraction effects for TE and TM are different. When the vectorial nature of light is considered, a biasing between horizontally oriented and vertically oriented features results. Although propagation into a resist material will reduce this biasing effect [60], it cannot be neglected. Improvements on the reduction Dyson in Figure 2.93 have included elimination of the linear polarization effect by incorporating a second waveplate near the wafer plane. The resulting circular polarization removes the H-V biasing possible with linear polarization and also rejects light reflected from the wafer and lens surface, reducing lens flare. Improvements have also increased the NA of the Dyson design up to 0.7, using approaches that include larger-NA beam-splitter cubes, shorter image conjugates, increased mirror asphericity, and source bandwidths below 1 nm. This spectral requirement, along with increasingly small field widths to reduce aberration, requires that these designs be used only with excimer laser sources. Designs have been developed for both 248 and 193 nm wavelengths. Examples of these designs are shown in Figures 2.94 [61] and 2.95 [69]. Catadioptric 193 nm immersion lithography lens designs for numerical apertures above 1.0 are shown in Figure 2.96 [70].
Illuminators for Microlithography
Published in Fred M. Dickey, Scott C. Holswade, David L. Shealy, Laser Beam Shaping Applications, 2018
All lithographic projection lenses are designed to be telecentric on the image side, in order to maintain the same magnification through the DOF (Figure 1.2). A telecentric lens in the most basic terms is defined as one that has the chief ray normal at the image plane. This is the ray that emits from the edge of the field and passes through the center of the aperture stop. In more precise terms, it is the illuminator’s angular radiance distribution at the wafer that defines the telecentricity of imaging at the wafer. Typically, the projection lens has insignificant variations over the imaging field, so it is the illuminator that determines the degree of telecentricity correction. This is discussed in greater detail in Section II.C and Section III.E. The illuminator not only maintains the magnification of the image through the DOF, but can also increase the depth by tailoring the pupil profile for the specific objects being imaged. This is discussed in more detail in Section II.D. The DOF along with the exposure latitude defines the total process window. In the absence of other factors, the process window is usually illustrated as an exposure defocus (ED) window. Factors such as reticle flatness, wafer flatness, thermal drift, aberration correction of the projection lens, flare, and other characteristics can reduce the DOF. A larger ED window generally means a more robust lithography process and more wafers per hour.
Vision and Image Sensors
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
Stanley S. Ipson, Chima Okereke
The frame-transfer and ILT approaches both have performance advantages and disadvantages, and the hybrid frame-interline-transfer (FIT) approach [11] combines some of the advantages of both. This architecture includes a light-shielded field storage area between an interline imaging area and the horizontal output shift register. With this arrangement, the charge associated with the whole image is first transferred horizontally into the interline storage area, which facilitates exposure control. The charge is then transferred at maximum speed (as in the FT sensor) into the field storage area, which minimizes the occurrence of vertical streaking. For example, the NEC microPD 3541 array clocks the vertical registers at 100 times the normal rate for an ILT sensor. This gives a 20 dB improvement in streaking threshold and an overall threshold of 80 dB, making streaking effects less than other optical effects such as lens flare. On the other hand, the FIT approach does not improve the fill factor, and aliasing artifacts associated with ILT sensors and the noise levels are somewhat higher, and the CTE reduced, because of the higher clock frequencies. Manufacturers of FIT sensors and cameras include JVC, Panasonic, and Sony.
Sources of Error in HDRI for Luminance Measurement: A Review of the Literature
Published in LEUKOS, 2021
Sarah Safranek, Robert G. Davis
Like vignetting, lens flare stems from the optical structure of the camera lens. Any unwanted or stray light in the optical system of the camera lens, resulting from the internal diffraction of direct light rays, is considered to be lens flare. Lens flare is problematic in HDRI luminance measurements because it overestimates the luminance values of the pixels around a source by including information that is not present in the scene. At larger aperture settings (lower f-stop values), lens flare is exhibited in HDR images as the blurring of pixels surrounding a light source, and as the aperture setting is increased, the blurring gradually changes to more dramatic star-like artifacts as seen in Fig. 4 (left). The point spread function (PSF) is commonly used in optics to characterize lens flare, expressing an idealized scattering of light around a point source (Reinhard et al. 2005). Factors determining the PSF include aperture size, exposure time, and eccentricity (Inanici 2006) as well as less-manageable factors like oil and dust built up within the lens (Reinhard et al. 2005).