Experimental approach to EUV imaging enhancement by mask absorber height optimization

Abstract

EUV lithography performance is improved significantly by optimizing and fine-tuning of the EUV mask. The EUV mask is an active element of the scanner optical system influencing main lithographic figure of merits such as image contrast, critical dimension uniformity (CDU), focus and overlay. The mask stack consists of Mo/Si multilayer acting as a bright field and a patterned absorber stack. In this work we will concentrate on investigation of EUV absorber.Absorber topography that is pronounced compared to the imaging wavelength of 13.5 nm, will give rise to various mask 3d effects such as shadowing or dependence of CD on feature orientation, best focus shift of different resolution structures, etc. [1] [2] . Light interference in the absorber layer results in swinging behavior of various lithography metrics as function of the absorber height [5] . Optimization of the mask absorber allows mitigating mask 3d effects and improving imaging performance. In particular, reduction of the absorber height mitigates the shadowing effect and relaxes requirements on Optical Proximity Correction (OPC), but can result in smaller Process Window due to lower imaging contrast and larger best focus shifts.In this work we will show results of an experimental approach to absorber height optimization. A special mask with 27 different absorber heights in the range 40-70 nm is manufactured by Toppan Photomasks. EUV reflectivity spectra are measured for the different absorber heights and an experimental swing curve is constructed.For each absorber height various resolution features are present on the mask. Lines of 27 nm and 22 nm are imaged on the wafer using the ASML EUV scanner NXE:3300B with an NA of 0.33. The experimental CD swing curve is constructed as well as HV change as a function of absorber height. The impact of the absorber height on Exposure Latitude (EL) and Dose to Size (D2S) is investigated. EL improves with increasing absorber height in some cases, however there is no clear EL gain for a 70 nm absorber compared to for example 52 nm absorber. D2S does show a clear trend through absorber height. In particular, D2S can be reduced by absorber height reduction: e.g. for 52 nm absorber D2S is 5% or 1 mJ/cm 2 smaller compared to 70 nm.The experimental results are used for calibration and verification of rigorous mask 3d simulations. This knowledge is crucial for accurate OPC of production masks and allows for accurate litho simulations of EUV user cases as a basis for lithography roadmaps towards High Volume Manufacturing and High NA EUV.EUV absorber behavior is determined by phase shifts Reflectivity Swing Curve X 8% k 6% 4% 2% awrmiim mm 55 6.5 75 85 9.5 10.5 Ab orbe heieht,A /2n Light retardation in the matter 4 phase shift Substrate: LTEM EUV mask n -refraction index of absorber 46, 53, 60, 67.5, 75, 82 (for n = 0.95) 180° phase shift between bright field and dark field: A h 67.5 (for n=0.95) 1. Figure 1 Two types of phase shifts in EUV mask: (a) Interference between reflection at the top of the absorber and from the multil...

Ni-Al Alloys as Alternative EUV Mask Absorber

“…The behavior of NiAl and Ni 2 Al 3 are expected to be like Ni 3 Al, as all three samples exhibit the same native oxide. The surface Al oxide dissolves in NH 4 OH, but remains stable in DIW, though metallic Al will be further oxidized and might increase roughness.…”

“…These effects are experimentally observable, as feature orientation-dependent shadowing effects [3], best focus variation through pitch [4], feature size-dependent pattern shift through focus [5,6], and pattern asymmetry and contrast loss [7]. Selecting the correct mask absorber material and thickness helps in reducing M3D effects [2,8,9].…”

“…Under more oxidizing condition, Al will react depending on the acidity. Highly acidic and highly basic condition will dissolve Al into 3 Al are compared between a reference sample, after 110 h H*, and after 24 h DIW. The basic cleaning condition with NH 4 OH was disregarded, as Al was not stable under this condition.…”

“…XPS measurement of Ni 2 Al 3 is presented in Figure 8. XPS data for Ni 3 Al and NiAl are available in the Supplementary Materials section Figure S1. Ni2p 3/2 , and Ni2p 1/2 peaks are found at 852.7 eV, and 870.0 eV respectively.…”

Novel EUV mask absorber evaluation in support of next-generation EUV imaging

“…More precisely, the current Ta-based absorber is at its limit for imaging extendibility. Thinning down below 50nm Ta-based absorber thickness will reduce the amount of absorbed light, reduce the NILS and increase best focus variation through pitch [10,11]. Although the current Ta-based mask has proven benefits from mask technology point, the imaging performance towards next technology nodes can benefit from mask optimization.…”