EUVL mask performance and optimization

Abstract

d Toppan Photomasks inc., 131 Old Settlers Boulevard, Round Rock, TX 78664 USA EUV lithography requires an exposure system with complex reflective optics and an equally complex EUV dedicated reflective mask. The required high level of reflectivity is obtained by using multilayers. The multilayer of the system optics and the mask are tuned to each other. The mask is equipped with an additional patterned absorber layer.The EUV mask is an optical element with many parameters that contribute to the final image and overlay quality on the wafer and the productivity of the system. Several of these parameters can be tuned for optimal overlay, imaging and productivity results. This should be done with care because of possible interaction between parameters.We will present an overview of the EUV mask contributors to the imaging, overlay and productivity performance for the 27 nm node and below, such as multilayer and absorber stack composition, reflectivity and reflectivity uniformity. These parameters will be reviewed in the context of real-life scanner parameters for the ASML NXE:3100 and NXE:3300 system configurations. The predictions will be compared to actual exposure results on NXE:3100 systems (NA=0.25) for various masks and extrapolated to the NXE:3300 (NA=0.33).In particular, we will present extensive multilayer and absorber actinic spectral reflectance measurements of a state-ofthe art EUV mask over a range of incidence angles corresponding to an NA of 0.33 at multiple positions within the image field. The ML measurements allow calibrating ML stack for imaging simulations. It allows also the estimation of mask-induced apodization effects having impact on overlay.In general, the reflectivity measurements will give detailed variations over the image field of mask parameters such as ML centroid wavelength and absorber reflectivity which contribute to CD uniformity. Such a relation will be established by means of rigorous full stack imaging simulations taking into account optical properties of the coming NXE:3300 system.Based on this investigation we will propose optimal EUV mask parameters for the 22 nm node EUV lithography and below, to provide guidance for mask manufacturers to support the introduction of EUV High Volume Manufacturing.

Experimental approach to EUV imaging enhancement by mask absorber height optimization

“…A ML stack calibration was already performed in our previous work for other masks [3], [4] and a reasonably good match is achieved for peak reflectivity, centroid wavelength and full width at half maximum of reflectivity spectrum by assuming Mo-Si inter-mixing in the multilayer. However so far we could not achieve good match of the low reflectance tails of the spectrum.…”

High-power EUV lithography: spectral purity and imaging performance

“…Wavelength-dependent Jones pupils are used and the wavelength together with the n and k values of the ML and absorber materials are varied in the simulation package. The n and k values used for all materials were taken from the online CXRO database 12 and Davydova et al 13 For each wavelength of the EUV spectrum, the stack parameters are adapted to the correct value and the scanner Jones pupils are updated. Then a wavelength-dependent diffraction pattern was calculated, which results in a wavelength-dependent subimage.…”