Proc. SPIE. 9231, 30th European Mask and Lithography Conference
KEYWORDS: Ellipsometry, Statistical analysis, Error analysis, Scanning electron microscopy, Spectroscopic ellipsometry, Photomasks, Line width roughness, Critical dimension metrology, Line edge roughness, Chemical elements
Critical dimension and line edge roughness on photomask arrays are determined with Mueller matrix spectroscopic ellipsometry. Arrays with large sinusoidal perturbations are measured for different azimuth angels and compared with simulations based on rigorous coupled wave analysis. Experiment and simulation show that line edge roughness leads to characteristic changes in the different Mueller matrix elements. The influence of line edge roughness is interpreted as an increase of isotropic character of the sample. The changes in the Mueller matrix elements are very similar when the arrays are statistically perturbed with rms roughness values in the nanometer range suggesting that the results on the sinusoidal test structures are also relevant for “real” mask errors. Critical dimension errors and line edge roughness have similar impact on the SE MM measurement. To distinguish between both deviations, a strategy based on the calculation of sensitivities and correlation coefficients for all Mueller matrix elements is shown. The Mueller matrix elements M<sub>13</sub>/M<sub>31</sub> and M<sub>34</sub>/M<sub>43</sub> are the most suitable elements due to their high sensitivities to critical dimension errors and line edge roughness and, at the same time, to a low correlation coefficient between both influences. From the simulated sensitivities, it is estimated that the measurement accuracy has to be in the order of 0.01 and 0.001 for the detection of 1 nm critical dimension error and 1 nm line edge roughness, respectively.
We report on Mueller Matrix spectroscopic ellipsometry (MM-SE) to examine undesired asymmetries in
structural parameters, i.e. line edge roughness (LER). The investigation was done on a photomask containing
line space arrays with intentionally modulated line edges. The Mueller Matrix (MM) elements were measured
within the complete azimuth angle range (0 - 360°) and a wavelength range from 300 nm to 980 nm. The results
are presented in polar coordinates with the azimuth angle and wavelength as the angular and radial coordinate,
respectively. It was found that LER significantly impacts the MM elements, which is indicated by the increase of
the isotropic character of the array. The experimental data are confirmed by Rigorous Coupled Wave Analysis
(RCWA) simulations on perturbed arrays. Based on RCWA the impact of LER amplitudes in the nm range is
determined. It was found that both deviation of critical dimension (CD) and LER amplitude impact the MM
elements. Based on the intensity ratios of the elements and their spectral distribution both errors create a
characteristic finger print, which allows to separate them. Finally, the required measurement precision for LER
in the nm range is estimated at 0.001. This precision is challenging but achievable with today’s metrology.
We investigated the potentials, applicability and advantages of spectroscopic ellipsometry (SE) for the
characterization of high-end photomasks. The SE measurements were done in the ultraviolet-near infrared (UVNIR)
wavelength range from 300 nm to 980 nm, at angle of incidences (AOI) between 10 and 70° and with a
microspot size of 45 x 10 μm<sup>2</sup> (AOI=70°). The measured Ψ and 𝛥 spectra were modeled using the rigorous coupled wave analysis (RCWA) to determine the structural parameters of a periodic array, i.e. the pitch and
critical dimension (CD). Two different types of industrial photomasks consisting of line/space structures were
evaluated, the reflecting extreme ultraviolet (EUV) and the transmitting opaque MoSi on glass (OMOG) mask.
The Ψ and 𝛥 spectra of both masks show characteristic differences, which were related to the Rayleigh
singularities and the missing transmission diffraction in the EUV mask. In the second part of the paper, a
simulation based sensitivity analysis of the Fourier coefficients α and β is presented, which is used to define the required measurement precision to detect a CD deviation of 1%. This study was done for both mask types to
investigate the influence of the stack transmission. It was found that sensitivities to CD variations are
comparable for OMOG and EUV masks. For both masks, the highest sensitivities appear close to the Rayleigh
singularities and significantly increase at very low AOI. To detect a 1% CD deviation for pitches below 150 nm
a measurement precision in the order of 0.01 is required. This measurement precision can be realized with
advanced optical hardware. It is concluded that UV-NIR ellipsometry is qualified to characterize photomasks
down to the 13 nm technology node in 2020.