Winfried Kaiser

Optics for ASML's NXE:3300B platform

Shipping in 2013, the NXE:3300 is the second generation of ASML’s EUV exposure platform. We review the current status of EUV optics production for the NXE:3300 tools. Four customer systems of the StarlithTM3300 series have been delivered so far. These sets of optics are characterized by a numerical aperture of 0.33 as well as significantly lower flare and wave-front levels compared to the StarlithTM3100. Meanwhile imaging down to 14 nm node features was demonstrated with the StarlithTM3300 pilot optics. Starting with this generation we introduce a fully new illumination system which allows for setting changes without efficiency loss. In this paper we focus on mirror fabrication and at wavelength qualification results of the optical systems produced so far. We also give an outline of potential solutions for the next generation of EUVL optics using higher NA.

EUV lithography optics for sub-9nm resolution

EUV lithography for resolution below 9 nm requires the numerical aperture of the projection optics to be significantly larger than 0.45. A configuration of 4x magnification, full field size and 6’’ reticle is not feasible anymore. The increased chief ray angle and higher NA at reticle lead to non-acceptable shadowing effects, which can only be controlled by increasing the magnification, hence reducing the system productivity. We demonstrate that the best compromise in imaging, productivity and field split is a so-called anamorphic magnification and a half field of 26 x 16.5 mm². We discuss the optical solutions for anamorphic high-NA lithography.

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.

Actinic review of EUV masks

Management of mask defects is a major challenge for the introduction of EUV for HVM production. Once a defect has been detected, its printing impact needs to be predicted. Potentially the defect requires some repair, the success of which needs to be proven. This defect review has to be done with an actinic inspection system that matches the imaging conditions of an EUV scanner. During recent years, several concepts for such an aerial image metrology system (AIMS™) have been proposed. However, until now no commercial solution exists for EUV. Today, advances in EUV optics technology allow envisioning a solution that has been discarded before as unrealistic. We present this concept and its technical cornerstones.While the power requirement for the EUV source is less demanding than for HVM lithography tools, radiance, floor space, and stability are the main criteria for source selection. The requirement to emulate several generations of EUV scanners demands a large flexibility for the ilumination and imaging systems. New critical specifications to the EUV mirrors in the projection microscope can be satisfied using our expertise from lithographic mirrors. In summary, an EUV AIMS™ meeting production requirements seems to be feasible.

Real-time wavefront measurement with lambda/10 fringe spacing for the optical shop

We will report on a new interferometer developed at Carl Zeiss, which has real-time measuring capability with instant visualization of results, is nearly insensitive to vibrations, has a variable fringe spacing from one lambda to lambda/1O (lambda represents the wavelength of the light used in the interferometric test), and can give lambda/100 accuracy through a simple calibration procedure. It can be handled with the same ease and in just the same way as conventional interferometers.

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.

High-precision interferometric testing of spherical mirrors with long radius of curvature

Surface deviations of spherical mirrors from a best fitting, mathematically ideal sphere were measured to an absolute precision of 0.25 nm rms. Because of the long radius of curvature, a Hindle-type arrangement was used as interferometric setup, resulting in a test arm length of about 1.4 m. A special calibration procedure was implemented to eliminate systematic, setup-dependent errors. A very fast data acquisition technique was combined with real-time wavefront averaging to eliminate the effects of random errors, such as wavefront variations due to the turbulent atmosphere in the beam path. For the evaluation of one mirror surface, all in all 400,000 individual wavefront measurements at 400 x 400 points were combined, requiring an overall measurement time of only one to two days.

Advanced technology for extending optical lithography

This paper discusses the technological consequences of extending optical lithography down to 50 nm. A total systems approach is used to define the overall error budgets on CD and overlay. The feasibility of extremely high NA optics is studied. With extremely high-NA optics, Depth of Focus decreases and focus control becomes more critical. Using a Step and Scan system, the potential of system dynamics is evaluated. Mask issues are briefly reviewed. Experimental data on field extension with butting is included. As well as being technically feasible, the lithography step for the future technology nodes must be cost effective. It is concluded that optical extension should preferably be supported by reduced field, large magnification scanners. The transition to Next Generation Lithography will be pushed to at least the 50 nm node.

EUCLIDES: European EUVL Program

In August 1998 a new European research program named extreme ultraviolet concept lithography development system (EUCLIDES) was started. The program, headed by ASM Lithography (ASML), partnered by Carl Zeiss and Oxford Instruments, is evaluating extreme ultraviolet lithography (EUVL) as a viable lithographic solution for resolutions of 70 nm and smaller. In this article a summary of program objectives and status will be given, followed by an overview of recent highlights obtained by the various program partners and subcontractors. Results of the system architecture study will be presented. These include the (preliminary) results of the comparison study between laser plasma and synchrotron sources in terms of throughput and cost-of-ownership. A novel technique will be presented which mitigates resist outgassing without the need of a window between the projection optics and the wafer. Using this idea the EUV flux on the wafer can be doubled hence increasing the throughput. An EUVL test bench, containing an E...

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.