Reiner Garreis

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.

Mask effects for high-NA EUV: impact of NA, chief-ray-angle, and reduction ratio

With higher NA (≫ 0.33) and increased chief-ray-angles, mask effects will significantly impact the overall scanner performance. We discuss these effects in detail, paying particular attention to the multilayer-absorber interaction, and show that there is a trade-off between image quality and reticle efficiency. We show that these mask effects for high NA can be solved by employing a reduction ratio <4X, and show several options for a high-NA optics. Carefully discussing the feasibility of these options is an important part of defining a high-NA EUV tool.

90 degree optical hybrid for coherent receivers

The power losses and noise of the present 90-deg hybrid optical coherent receiver are minimized through the use of a six-port configuration in which channel balancing is furnished by half-wave plates. The 90-deg shift is introduced between orthogonally polarized beam components by a quarter-wave plate, and the output ports are phase-shifted by 180 deg through the sole use of the half-wave plates and polarizing beam splitters.

Interactions of 3D mask effects and NA in EUV lithography

With high NA (>0.33), and the associated higher angles of incidence on the reflective EUV mask, mask induced effects will significantly impact the overall scanner-performance. We discuss the expected effects in detail, in particular paying attention to the interaction between reflective coating and absorber on the mask, and show that there is a trade-off between image quality and mask efficiency. We show that by adjusting the demagnification of the lithography system one can recover both image quality and mask efficiency.

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.

Optical lithography in the sub-50-nm regime

The use of immersion technology will extend the lifetime of 193nm and 157nm lithography by enabling numerical apertures (NA) much greater than 1.0. A definition of effective k1 is derived to assist in comparison of various technologies with differing optical characteristics. The ultimate limits of NA are explored by analysis of polarization effects at the reticle and imaging effects at the wafer. The effect of Hertzian or micro-polarization due to the size of the reticle structures is examined through rigorous simulation. For the regime of interest, 20nm to 50nm imaging, it is found that dense features on the reticle will polarize the light into the TE component upwards of 15%. Below this regime, the light becomes polarized in the TM direction. Additionally, oblique incidence on the reticle, resulting from large system NAs and 4x reduction, will cause PSM phase errors. The use of polarization in the illuminator for imaging will result in substantial gains in exposure latitude and MEF when the NA~1.3 with 45nm lines at 193nm. The end-of-line pullback for 2-dimensional patterns is reduced by the use of TE polarization in the illuminator. The overall polarization effects increase with decreasing k1. The lower limit of optical lithography can be extended by using source-mask optimization and double exposure to go below the classical resolution limit, i.e., k1<0.25.

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.

Catadioptric lens design: the breakthrough to hyper-NA optics

To enable optical lithography for sub 55 nm features, ArF immersion lithography requires numerical apertures to be significantly larger than 1 - thus leading to new challenges for optical design. Refractive lens designs are not capable to capture these extreme etendues. Catadioptric lens designs can overcome these fundamental issues by keeping the diameters of the optical materials acceptable. We have studied various catadioptric design approaches. The main criteria used to evaluate the potential of the different solutions include mechanical complexity, reticle compatibility, optical sensitivities, polarization capabilities, image field shape, as well as enabling extendibility to even higher NAs. Our assessment leads us to a new design type called catadioptric in-line design which shows superior performance for high NA systems with NA > 1.1.

Solutions for printing sub-100-nm contacts with ArF

This study assesses the various approaches to printing contacts in the sub 100nm regime using 193nm. Traditional techniques are analyzed along with the use of tri-tone contacts and pupil filtering. Approaches using attPSM masks looks promising down to pitches of 300nm. Below this, assist features may be needed to prevent residual artifacts due to sidelobes. For pitches > 400nm the use of tri-tone masks show a significant improvement in process latitude and ease of overlapping process windows. The pupil filter solution does not seem provide any significant improvement as compared to other solutions with the exception that it provides the lower MEF. Realization of this solution will increase machine complexity and will possibly impact throughput, especially if using transmission filters. However, pupil filtering can be an option for isolated contact layers that are printed with binary masks. We find that the process and enhancement techniques to print a dense contacts and isolated contacts to be vastly different. This may require a split into two exposures if an extensive pitch range is needed.

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

We derive an imaging budget from the performance of EUV optics with NA = 0.32, and demonstrate that the 22nm node requirements 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 influence of 3D mask effects and facetted illumination pupils. As test structures we consider dense to isolated lines, contact holes, and 2D elbows. We classify the contributions in a hierarchical order according to their weight in the 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 16nm and 11nm nodes. Future developments in optics manufacturing will keep the budgets controlled, thereby paving the way to enable printing of these upcoming nodes.