Imaging performance improvements by EUV mask stack optimization

Abstract

EUVL requires the use of reflective optics including a reflective mask. The mask contains a reflecting multilayer, tuned for 13.5 nm light, and an absorber which defines the dark areas. The EUV mask itself is a complex optical element with many more parameters than just the mask CD uniformity of the patterned features that impact the final wafer CDU. One of these parameters is absorber height. It has been shown that the oblique incidence of light in combination with the small wavelength compared to the mask topography causes a so-called shadowing effect manifesting itself particularly in an HV wafer CD offset. It was also shown that this effect can be essentially decreased by reducing absorber height and, in addition, it can be corrected by means of OPC. However, reduction of absorber height has a side effect that is an increased reflectivity of a mask black border resulting in field-to-field stray light due to parasitic reflections. One of the solutions to this problem is optical process correction (OPC) at field edges. In this paper we will show experimental data obtained on ASML EUV Alpha tool illustrating the black border effect and will demonstrate that this effect can be accurately predicted by Brion Tachyon EUV model allowing for a significant cross field CD uniformity improvement with mask layout correction technique. Also we show by means of rigorous 3D simulations that it is possible to improve the imaging performance significantly by performing global optimization of mask absorber height and mask bias in order to increase exposure latitude, decrease CD sensitivity to mask making variations such as CD mask error and absorber stack height variations. By sacrificing some exposure latitude throughput of exposure tool can be increased essentially and HV mask biasing can be reduced. For four masks with different absorber thicknesses from 44 nm to 87 nm it is proven experimentally by means of the EUV Alpha tool exposures of 27 nm L/S that the absorber thickness can be tuned to maximize exposure latitude. It was also proven that dose to size grows with absorber height and optimal feature bias depends on mask absorber height.