Heiko Feldmann

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.

Concept and feasibility of aerial imaging measurements on EUV masks

On the road to and beyond the 22nm half-pitch on chip patterning technology, 13.5nm EUVL is widely considered the best next technology generation following deep ultraviolet lithography. The availability of an actinic measurement system for the printability analysis of mask defects to ensure defect-free mask manufacturing and cost-effective high-volume EUV production is an infrastructural prerequisite for the EUVL roadmap and represents a significant step toward readiness for commercialization of EUV for high-volume-manufacturing . Carl Zeiss and SEMATECHs EUVL Mask Infrastructure (EMI) program started a concept study and feasibility plan for a tool that emulates the aerial image formed by a EUV lithography scanner supporting the 22 nm half-pitch node requirements with extendibility to the 16nm half-pitch node. The study is targeting a feasible concept for the AIMSTM EUV platform, bridging a significant gap for EUV mask metrology.

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.

Use of diffractive optical elements in lithographic projection lenses

Projection lenses for high resolution lithography have high NA and work at small wavelengths. In the wavelength regime of VUV (e.g. 193nm), there is a very limited number of optical glasses available, namely fused silica and calcium fluoride. The latter is very expensive and used only sparely, leading to limited possibilities for chromatic correction. In addition to catadioptric approaches, another way to deal with chromatic aberrations is the use of diffractive optical elements (DOEs). They have negative dispersion coupled with positive power and they do not contribute to the Petzval sum. Moreover, it is easy to integrate an aspherical functionality into the structure of the DOE. Usually a DOE is placed close to the aperture stop to correct axial color. The stop of a lithographic projection lens often is located at the largest diameter, causing some serious fabrication difficulties for the DOE. For this reason a class of lenses with intermediate image is of interest. Here, the accessible conjugate of the aperture stop enhances the possibilities to arrange the stop and the DOE. This allows a convenient tradeoff between fabrication challenges and aberration correcting properties. We present different lens designs that take advantage of the named properties of DOEs at high numerical aperture.

Catadioptric projection lenses for immersion lithography

Recently, the development of high NA lenses for immersion lithography turned from dioptric concepts to catadioptric design forms. The introduction of mirrors involves the new challenge to deal with the inevitable obscuration of either field or pupil. We review the strategies used in this regard for microlithography, while focussing on the two most favored ones, folded and inline concepts. Although the vignetting situation is more complicated for inline systems, we report progress in this field of optical design yielding similar system performance for inline and folded designs. Since inline optical systems are much easier to realize, these are the concept of choice.

Use of diffractive lenses in lithographic projection lenses

Projection lenses for high resolution lithography have high NA and work at small wavelengths. In the wavelength regime of VUV (e.g. 193nm), there is a very limited number of optical glasses available, namely fused silica and calcium fluoride. The latter is very expensive and used only sparely, leading to limited possibilities for chromatic correction. In addition to catadioptric approaches, another way to deal with chromatic aberrations is the use of diffractive optical elements (DOEs). They have negative dispersion coupled with positive power and they do not contribute to the Petzval sum. Moreover, it is easy to integrate an aspherical functionality into the structure of the DOE. Usually a DOE is placed close to the aperture stop to correct axial color. The stop of a lithographic projection lens often is located at the largest diameter, causing some serious fabrication difficulties for the DOE. For this reason a class of lenses with intermediate image is of interest. Here, the accessible conjugate of the aperture stop enhances the possibilities to arrange the stop and the DOE. This allows a convenient tradeoff between fabrication challenges and aberration correcting properties. We present different lens designs that take advantage of the named properties of DOEs at high numerical aperture.

Simulation and error budget for high precision interferometry

An accurate calculation of the error budget of individual interferometer components can only be based on a simulation of the total interferometric setup. We have shown that such a simulation can in fact make accurate predictions of the achievable measurement precision. Furthermore, a simulation allows calculating the maximal allowable drifts magnitudes. Using a simulation of the interferometric setup therefore allows for focusing on critical components and drifts on the one hand and for relaxing requirements for non-critical components and drifts on the other hand.