Tilmann Heil

Polarization influence on imaging

We give a general introduction into polarized imaging and report on a Jones pupil approach for a complete evaluation of the resulting optical performance. The Jones pupil assigns a Jones matrix to each point of the exit pupil, describing the impact of both the global phase and the polarization on imaging. While we already can learn much about the optical system by taking a close look at the Jones pupil-and starting imaging simulations from it-a quantitative assessment is necessary for a complete evaluation of imaging. To do this, we generalize the concept of scalar Zernike aberrations to Jones-Zernike aberrations by expansion of the Jones pupil into vector polynomials. The resulting method is nonparaxial, i.e., the effect of the polarization-dependent contrast loss for high numerical apertures is included. The aberrations of the Jones matrix pupil are a suitable tool to identify the main drivers determining polarization performance. Furthermore, they enable us to compare the polarized and unpolarized performance of such a characterized lithographic system.

Impact of wavefront errors on low k1 processes at extremely high NA

This paper presents a comprehensive study of the impact of wavefront errors on low-k1-imaging performance using high numerical aperture NA lithographic systems. In particular, we introduce a linear model that correctly describes the aberration induced imaging effects. This model allows us to quantify the aberration requirements for future lithographic nodes. Moreover, we derive scaling laws characterizing the imaging performance in dependence on the key parameters exposure wavelength λ, NA, and k1. Our investigations demonstrate, first, that an accurate control of coma is and will be crucial, and, second, that spherical requirements will be very tight for k1<0.3 due to isolated contact printing. Finally, we summarize the results of this paper in a roadmap covering the aberration requirements in optical lithography down to the 45nm node. We conclude that the improvement of wavefront quality is necessary to enable imaging enhancement techniques, but is not sufficient to replace these techniques.

Performance of a 1.35NA ArF immersion lithography system for 40-nm applications

Water based immersion lithography is now widely recognized a key enabler for continued device shrinks beyond the limits of classical dry lithography. Since 2004, ASML has shipped multiple TWINSCAN immersion systems to IC manufacturers, which have facilitated immersion process integration and optimization. In early 2006, ASML commenced shipment of the first immersion systems for 45nm volume production, featuring an innovative in-line catadioptric lens with a numerical aperture (NA) of 1.2 and a high transmission polarized illumination system. A natural extension of this technology, the XT:1900Gi supports the continued drive for device shrinks that the semiconductor industry demands by offering 40nm half-pitch resolution. This tool features a projection lens based on the already proven in-line catadioptric lens concept but with an enhanced, industry leading NA of 1.35. In this paper, we will discuss the immersion technology challenges and solutions, and present performance data for this latest dual wafer stage TWINSCAN immersion system.

How to describe polarization influence on imaging (Invited Paper)

We give a general introduction into polarized imaging and report on a Jones-pupil approach for a complete evaluation of the resulting optical performance. The Jones pupil assigns a Jones matrix to each point of the exit pupil describing the impact of both the global phase and the polarization on imaging. While we can learn already a lot about the optical system by taking a close look at the Jones pupil - and starting imaging simulations from it - a quantitative assessment is necessary for a complete evaluation of imaging. To do this, we generalize the concept of scalar Zernike aberrations to Jones-Zernike aberrations by expansion of the Jones pupil into vector polynomials. The resulting method is non-paraxial, i.e. the effect of the polarization dependent contrast loss for high numerical apertures is included. The aberrations of the Jones-matrix pupil are a suitable tool to identify the main drivers determining the polarization performance. Furthermore, they enable us to compare the polarized and the unpolarized performance of the such characterized lithographic system.

Predictive modeling of advanced illumination pupils used as imaging enhancement for low-k1 applications

The specific properties of the illumination system are of increasing importance for the realization of low-k1 applications in modern lithography. In this paper, we present numerical investigations of optical imaging performance using real illuminator pupils in contrast to conventional simulations based on an idealized tophat pupil assumption. We study the impact of non-idealized radial and azimuthal intensity distributions as well as the consequence of local in-homogeneities in the pupil. Furthermore, we discuss the effect of scanning, and details of the numerical implementation. We quantify the imaging impact of the different illumination pupils by computing the through pitch, and through focus behavior of several low-k1 applications. We demonstrate that the tophat assumption often does not provide sufficiently accurate results. In particular, for annular and multi-pole settings, the real radial, and azimuthal intensity distribution have to be taken in to account. Accordingly, we introduce a simple heuristic model describing the real illumination pupil. Using this smooth pupil model, we demonstrate a significantly improved imaging performance prediction accuracy. Local pupil inhomogeneities have a minor impact. For coherent, and conventional settings, finally, we find that a modified tophat assumption gives already sufficiently accurate results, and can be applied for predictive simulations.

Imaging enhancements by polarized illumination: theory and experimental verification

The polarization properties of light become more and more important as numerical apertures of the projection lens increase. With unpolarized light the contrast of the image is degraded because of poor interference of the TM component of the light. By applying only TE linear polarized illumination light, the contrast loss can be minimized. The challenge will be to control the polarization variation throughout the imaged field. Besides contrast also the light incoupling in the resist depends on polarization. The different polarization directions (TE and TM) induce virtual dose differences. Immersion lithography reduces this effect due to reduced incident angles at a given lens NA. In the upcoming era beyond 0.9 NA, imaging enhancements by polarized illumination are needed. There are several components in a lithographic scanner which potentially influence polarization properties. Apart from illuminator and projection lens the reticle blank and the patterned mask absorber including 3D effects may impact the final intensity distribution in the resist. Last but not least the ability to measure the polarization state is a prerequisite to actively control polarization within the exposure system. The ability to assess the unpolarized and polarized projection lens performance with the on-scanner interferometer (ILIASTM) allows us to do this. In order to verify the benefits and challenges of polarized illumination systems, we built a prototype illuminator and tested it on both a 0.85 NA ArF system as well as on a 0.93 NA ArF system. Next to the successful qualification of illuminator and projection lens we were able to verify the expected gain in imaging performance with polarized light. In this paper we present results of the experimental work and compare the data with our simulations.

The future of EUV lithography: enabling Moore's Law in the next decade

While EUV systems equipped with a 0.33 Numerical Aperture lenses are readying to start volume manufacturing, ASML and Zeiss are ramping up their development activities on a EUV exposure tool with Numerical Aperture greater than 0.5. The purpose of this scanner, targeting a resolution of 8nm, is to extend Moore’s law throughout the next decade. A novel, anamorphic lens design, has been developed to provide the required Numerical Aperture; this lens will be paired with new, faster stages and more accurate sensors enabling Moore’s law economical requirements, as well as the tight focus and overlay control needed for future process nodes. The tighter focus and overlay control budgets, as well as the anamorphic optics, will drive innovations in the imaging and OPC modelling, and possibly in the metrology concepts. Furthermore, advances in resist and mask technology will be required to image lithography features with less than 10nm resolution. This paper presents an overview of the key technology innovations and infrastructure requirements for the next generation EUV systems.

EUV Imaging - an aerial image study

This work discusses the imaging properties of EUVL systems on the basis of an aerial image study in resist. A process window analysis for the lithographic structures which are driving the ITRS roadmap is presented. Here we cover the 45 nm and 32 nm node. In a first step we focus on the contribution of wavefront aberrations and flare effects to the imaging performance. In a second step we investigate the process latitude for different generic pattern of the above mentioned nodes. It becomes clear that EUVL tools are a very good choice for the printing of contact holes. Dense and semi-dense lines can be easily printed too, using a conventional illumination setting. From our current perspective, isolated features on bright field reticles are the most challenging structures for EUV imaging due to the flare impact on contrast and process latitude. Related to flare we discuss our progress in mirror surface manufacturing to reduce the overall flare level.

Imaging budgets for extreme ultraviolet optics: ready for 22-nm node and beyond

We derive an imaging budget from the performance of extreme ultraviolet (EUV) optics with NA = 0.32, and demonstrate that the requirements for 22-nm applications are met. Based on aerial image simulations, we analyze the impact of all relevant contributors, ranging from conventional quantities like straylight or aberrations, to EUV-specific topics, namely the influence of 3-D mask effects and faceted illumination pupils. As test structures we consider dense to isolated lines, contact holes, and 2-D elbows. We classify the contributions in a hierarchical order according to their weight in the critical dimension uniformity (CDU) budget and identify the main drivers. The underlying physical mechanisms causing different contributions to be critical or less significant are clarified. Finally, we give an outlook for the 16- and 11-nm nodes. Future developments in optics manufacturing will keep the budgets controlled, thereby paving the way to enable printing of these upcoming nodes.

The future of EUV lithography: continuing Moore's Law into the next decade

While 0.33NA EUV systems are readying to start volume manufacturing, ASML and Zeiss are ramping up development activities on a 0.55NA EUV exposure tool, extending Moore’s law throughout the next decade. A novel, anamorphic lens design, has been developed to provide the NA; this lens will be paired with new, faster stages and more accurate sensors and the tight focus and overlay control needed for future process nodes. This paper presents an overview of the target specifications, key technology innovations and imaging simulations demonstrating the advantages as compared to 0.33NA and showing the capabilities of ASML’s next generation EUV systems.