KEYWORDS: Calibration, Line edge roughness, Scatterometry, Point spread functions, Process modeling, Scanning electron microscopy, Metrology, Scattering, Semiconducting wafers, Cadmium
Scatterometry has been proven to be effective in critical dimension (CD) and sidewall angle (SWA) measurements with
good precision and accuracy. In order to study the effectiveness of scatterometry measurement of line edge roughness
(LER), calibration samples with known LER have to be fabricated precisely. The relationship between ITRS LER
specifications and the feature dimension design of the LER calibration samples is discussed. Electron-beam-direct-write
lithography (EBDWL) has been widely used in nanoscale fabrication and is a natural selection for fabricating the
designed calibration samples. With the increasingly demanding requirement of lithography resolution in ITRS, the
corresponding LER feature of calibration samples becomes more and more challenging to fabricate, even for EBDWL.
Proximity effects in EBDWL due to electron scattering can cause significant distortion of fabricated patterns from
designed layouts. Model-based proximity effect correction (MBPEC) is an enhancement method for EBDWL to
precisely define fine resist features. The effectiveness of MBPEC depends on the availability of accurate electron-beam
proximity effect models, which are usually described by point spread functions (PSFs). In this work, a PSF in a double-
Gaussian function form at a 50 kV accelerating voltage, an effective beam size, and a development threshold energy
level of the resist are calibrated with EBDWL exposure tests. Preliminary MBPEC results indicate its effectiveness in
calibration sample fabrication.
Extreme ultraviolet (EUV) lithography is a promising candidate for high-volume manufacturing at the 22-nm half-pitch node and beyond. EUV projection lithography systems need to rely on reflective optical elements and masks with oblique illumination for image formation. It leads to undesired effects such as pattern shift and horizontal-to-vertical critical dimension bias, which are generally reported as shadowing. Rule-based approaches proposed to compensate for shadowing include changing mask topography, introducing mask defocus, and biasing patterns differently at different slit positions. However, the electromagnetic interaction between the incident light and the mask topography with complicated geometric patterns, such as optical diffraction, not only causes shadowing but also induces proximity effects. This phenomenon cannot be easily taken into account by rule-based corrections and thus imposes a challenge on a partially model-based correction flow, the so-called combination of rule- and model-based corrections. A fully model-based correction flow, which integrates an in-house optical proximity correction algorithm with rigorous three-dimensional mask simulation, is proposed to simultaneously compensate for shadowing and proximity effects. Simulation results for practical circuit layouts indicate that the fully model-based correction flow significantly outperforms the partially model-based one in terms of correction accuracy, while the total run time is slightly increased.
The modified transmission line theory is used to calculate equivalent refractive indices of the extreme ultraviolet (EUV) mask multilayer (ML) over wavelengths from 13.35 to 13.65 nm for finite-difference time-domain (FDTD) simulation. Generally speaking, a fine mesh requiring huge memory and computation time are necessary to get accurate results in an FDTD simulation. However, it is hard to get accurate results for ML simulation due to the thin thickness of each layer. By means of an equivalent refractive index, the ML can be treated as one layer with the bulk effective material. Using FDTD simulations, we study the reflectivities of 40 Mo/Si ML and bulk material cases. The ML structure and bulk material with periodic excessive surface roughness as well as patterned with periodic contact holes are also studied by using two- and three-dimensional FDTD simulations. The simulation cases for a single wavelength and for a full-bandwidth EUV light source with a 6 ML system are studied. The results from each simulation show that the root mean square error between ML simulations and the bulk material simulations are confined within 3.3%, and all cases indicate that the FDTD computation time of bulk material is about half as compared with a 40-ML simulation.
The Finite-Difference Time-Domain (FDTD) method is used to study the scattering effects of extreme ultraviolet (EUV)
mask. It requires significant amounts of memory and computation time as the fine grid size is needed for simulation.
Theoretically, the accuracy can be increased as the mesh size is decreased in FDTD simulation. However, it is not easy
to get the accurate simulation results for the multilayer (ML) structures by FDTD method. The transmission line theory
is used to calculate the equivalent refractive index for EUV mask ML to simulate the ML as one layer of bulk artificial
material. The reflectivities for EUV light with the normal incidence and small-angle oblique incidence in the bulk
artificial material and EUV mask ML are simulated by FDTD method. The Fresnel's equation is used to evaluate the
numerical errors for these FDTD simulations, and the results show good agreement between them. Using the equivalent
refractive index material for EUV multilayer mask can reduce the computation time and have the accuracy with tolerable
numerical errors. The ML structure with periodic surface roughness is also studied by this method, and it shows that only
half of computation time is needed to substitute ML to a bulk equivalent refractive index material in FDTD simulations.
This proposed method can accelerate the simulations of EUV mask designs.
Extreme ultraviolet (EUV) lithography is one of the promising candidates for device manufacturing with features smaller
than 22 nm. Unlike traditional optical projection systems, EUV light needs to rely on reflective optics and masks with an
oblique incidence for image formation in photoresist. The consequence of using a reflective projection system can result
in horizontal-vertical (H-V) bias and pattern shift, which are generally referred as shadowing. Approaches proposed to
compensate for shadowing effect include changing mask topography, modifying mask focus, and biasing features along
the azimuth angle, which are all rule-based. However, the complicated electromagnetic interaction between closely
placed circuit patterns can not only induce additional optical proximity effect but also change the shadowing effect.
These detailed phenomena cannot be completely taken into account by the rule-based approaches. A fully model-based
approach, which integrates an in-house model-based optical proximity correction (OPC) algorithm with rigorous three-dimensional
(3D) EUV mask simulation, is proposed to simultaneously compensate for shadowing and optical proximity
effects with better pattern transfer fidelity and process windows. Preliminary results indicate that this fully model-based
approach outperforms rule-based ones, in terms of geometric printability under process variations.
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