Alan E. Rosenbluth

Optimum Mask and Source Patterns to Print a Given Shape

New degrees of freedom can be optimized in mask shapes when the source is also adjustable, because required image symmetries can be provided by the source rather than the collected wavefront. The optimized mask will often consist of novel sets of shapes that are quite different in layout from the target IC patterns. This implies that the optimization algorithm should have good global convergence properties, since the target patterns may not be a suitable starting solution. We have developed an algorithm that can optimize mask and source without using a starting design. Examples are shown where the process window obtained is between 2 and 6 times larger than that achieved with standard RET methods. The optimized masks require phase shift, but no trim mask is used. Thus far we have only optimized 2D patterns over small fields (periodicities of approximately 1 micrometer or less). We also discuss mask optimization with fixed source, source optimization with fixed mask, and the re-targeting of designs in different mask regions to provide a common exposure level.

Optimum mask and source patterns to print a given shape

New degrees of freedom can be optimized in mask shapes when the source is also adjustable, because required image symmetries can be provided by the source rather than the collected wave front. The optimized mask will often consist of novel sets of shapes that are quite different in layout from the target integrated circuit patterns. This implies that the optimization algorithm should have good global convergence properties, since the target patterns may not be a suitable starting solution. We have developed an algorithm that can optimize mask and source without using a starting design. Examples are shown where the process window obtained is between two and six times larger than that achieved with standard reticle enhancement techniques (RET). The optimized masks require phase shift, but no trim mask is used. Thus far we can only optimize two-dimensional patterns over small fields (periodicities of ;1 mm or less), though patterns in two separate fields can be jointly optimized for maximum common window under a single source. We also discuss mask optimization with fixed source, source optimization with fixed mask, and the retargeting of designs in different mask regions to provide a common exposure level.

Determination of thickness errors and boundary roughness from the measured performance of a multilayer coating

The influence of thickness errors and boundary imperfections on the performance of a multilayer x-ray mirror is discussed, and it is shown that both can be obtained separately from a measured reflectivity curve at a short x-ray wavelength. Multilayers have greatly reduced scattering compared to single films, and for this reason rough boundaries and gradual transition layers between the film materials give practically identical performance. A simple Debye-Waller factor is not sufficient to describe the performance of a multi-layer with many layers at short wavelengths.

Contrast properties of reflective liquid crystal light valves in projection displays

Projectors that use reflective light valves must employ beam splitters or analogous components to separate bright-state light from dark-state light, since both states must propagate in the space above the light valve. Polarization ray tracing shows that such beam splitters will not usually achieve high rejection of dark-state light when the beam has the typical angular divergence of about ±10°. At such propagation angles, different rays in the beam will have appreciably different planes of incidence at tilted optical coatings in the system (because of the compound angles involved). If the light valve is mirrorlike in dark state, we show that to correct the depolarization resulting from compound incidence angles, it is necessary that the optics introduce no rotation in the illuminating polarization. To a reasonable approximation, such a rotation in polarization will double in the return pass through the optics. To the same approximation, induced ellipticity in the illuminating polarization will cancel in double pass, and pure rotation can be converted to pure ellipticity with a quarterwave retarder. An important qualification, however, is that a light valve can only be exactly mirrorlike in restricted cases [i.e., if linearly polarized input light remains exactly linearly polarized (though possibly rotated) at all wavelengths when it reaches the mirror backplane of the light valve, independent of small manufacturing errors]. We calculate contrast loss in the more realistic case of a reflective twisted nematic liquid crystal (TNLC) light valve interacting with tilted coatings in the projection optics over finite numerical aperture (NA), and discuss the impact on LC thickness tolerances and spectral bandwidth Δλ. We extend our results to apply to more general light valves and more general projection optics configurations. Dark-state background is found to scale as NA2 (or in some cases as ∼NA2Δλ2). Because of this interaction, the complete system almost always shows a lower contrast than the light valve alone.

Experimental result and simulation analysis for the use of pixelated illumination from source mask optimization for 22nm logic lithography process

We demonstrate experimentally for the first time the feasibility of applying SMO technology using pixelated illumination. Wafer images of SRAM contact holes were obtained to confirm the feasibility of using SMO for 22nm node lithography. There are still challenges in other areas of SMO integration such as mask build, mask inspection and repair, process modeling, full chip design issues and pixelated illumination, which is the emphasis in this paper. In this first attempt we successfully designed a manufacturable pixelated source and had it fabricated and installed in an exposure tool. The printing result is satisfactory, although there are still some deviations of the wafer image from simulation prediction. Further experiment and modeling of the impact of errors in source design and manufacturing will proceed in more detail. We believe that by tightening all kind of specification and optimizing all procedures will make pixelated illumination a viable technology for 22nm or beyond. Publishers Note: The author listing for this paper has been updated to include Carsten Russ. The PDF has been updated to reflect this change.

Benefits and trade-offs of global source optimization in optical lithography

Source optimization in optical lithography has been the subject of increased exploration in recent years [1-4], resulting in the development of multiple techniques including global optimization of process window [4]. The performance advantages of source optimization have been demonstrated through theory, simulation, and experiment. This paper will emphasize global optimization of sources over multiple patterns, e.g. co-optimization of critical SRAM cells and the critical pitches of random logic, and implement global source optimization into current resolution enhancement techniques (RETs). The effect on optimal source due to considering multiple patterns is investigated. We demonstrate that optimal source for limited patterns does work for a large clip of layout. Through theoretical analysis and simulations, we explain that only critical patterns and/or critical combinations of patterns determine the final optimal source; for example those patterns that contain constraints which are active in the solution. Furthermore, we illustrate, through theory and simulation, that pixelated sources have better performance than generic sources and that in general it is impossible for generic sources to construct a truly optimal solution. Sensitivity, tool matching, and lens heating issues for pixelated sources are also discussed in this paper. Finally, we use a RETs example with wafer data to demonstrate the benefits of global source optimization.

Projection display throughput: efficiency of optical transmission and light-source collection

The optical system for a projection display based on three miniature reflective spatial light modulators (SLMs) is described. The total projection display light throughput is a function not only of the optical system efficiency but also of the light-collection and light-coupling efficiency referred to here as the lamp-SLM coupling. The optical system efficiency is the transmission of the optical components in the projection display. These are examined in detail through measurements and estimates of the components in the system. The various optical components include UV-IR filtering, illumination optics, polarization optics, color separation and recombination optics, SLM efficiency, and projection optics. The lamp-SLM coupling, which is the amount of usable light that can be collected from a particular lamp coupled to the projection optical system, is determined by the light-source luminance, the efficiency of the light-collection optics, and the optical system etendue. For small SLMs, less than 50 mm diagonal, for example, the lamp-SLM coupling efficiency falls off rapidly with SLM size and optical system f-number. The dependence of this coupling efficiency on SLM size is determined from measurements of the light-collection efficiency as a function of aperture size, where the apertures are used to simulate SLMs of the same dimensions. A variety of arc lamps were investigated for use in the projection display based on IBM reflective SLM devices. The lamp-SLM coupling dependence on arc gap was determined. The measurements are used to compare various lamps and to estimate directly the throughput for the complete projection system. The SLMs used in the projection display are liquid crystal devices which utilize only one polarization of light while discarding the second. Converting the discarded polarization into useful light can in principle double the throughput of the projector. However, polarization conversion results in doubling of the size of the light source and thus produces less efficient lamp-SLM coupling, particularly for long-arc-gap lamps. Measurements and analysis of throughput enhancement by polarization conversion are presented, and the dependence on arc gap and optical system etendue is discussed.

Design and fabrication of a prototype projection data monitor with high information content

A prototype 28-in.-diagonal desktop data monitor capable of displaying 2048 × 2048- pixel images has been designed, built, and evaluated. The monitor uses optical projection technology. A reflective, crystalline silicon active-matrix light valve using liquid crystal electro-optics and a digital electronic interface architecture is described. This rear-projection monitor has four million resolvable pixels, uses three light valves to achieve color, has a low-gain surface diffuser screen, and functions as a fully interactive, color personal computer monitor with motion video capability. The monitor is 20 in. deep.

Alignment errors from resist coating topography

The registration difficulty associated with a resist‐coated alignment mark depends on the type of optics used to illuminate and view the wafer alignment aid. The interaction of the light with the mark geometry is studied by calculating the outgoing wave front that results when an arbitrary collection of correlated or uncorrelated incident wave fronts is reflected from the nonplanar target structure. This solution is combined with a model of the partially coherent imaging system to predict the alignment signal. Bright‐field and dark‐field systems typical of those used in optical lithography are studied. Dark‐field viewing optics are generally less sensitive to offsets caused by resist coating asymmetries. The statistical alignment precision is related to the shape of the alignment signal and the signal‐to‐noise ratio. It is shown that the increased contrast in dark‐field systems can provide an alignment precision comparable to that of bright‐field systems despite a substantially lower photon collection eff...

Determination Of Thickness Errors And Boundary Roughness From The Measured Performance Of A Multilayer Coating

The influence of thickness errors and boundary imperfections on the performance of a multilayer x-ray mirror is discussed and it is shown that both can be obtained separately from a measured reflectivity curve at a short x-ray wavelength. Multilayers have greatly reduced scattering compared to single films, and for this reason rough boundaries and gradual transition layers between the film materials give practically identical performance. A simple Debye-Waller factor is not sufficient to describe the performance of a multilayer with many layers at short wavelengths.