The optimization of the source topology and mask design [1,2] is vital to future advanced ArF technology node development. In this study, we report the comparison of an iterative optimization method versus a newly developed simultaneous source-mask optimization approach. In the iterative method, the source is first optimized based on normalized image log slopes (NILS), taking into account the ASML scanner's diffractive optical element (DOE) manufacturability constraints. Assist features (AFs) are placed under the optimized source, and then optical proximity correction (OPC) is performed using the already placed AFs, in the last step the source is re-optimized using the OPC-ed layout with the AFs. The source is then optimized using the layout from the previous stage based on a set of user specified cost function. The new approach first co-optimizes a pixelated freeform source and a continuous transmission gray tone mask based on edge placement error (EPE) based cost function. ASML scanner specific constraints are applied to the optimized source, to match ASML's current and future illuminator capabilities. Next, AF "seeds" are identified from the optimized gray tone mask, which are subsequently co-optimized with the main features to meet the process window and mask error factor requirement. The results show that the new method offers significant process window improvement.
Maskless lithography imaging based on SLM tilt mirror architecture requires illumination of light on a non-planar reflective topography. While the actual mirror dimensions can be much larger than the wavelength of light, the spacing between mirrors and the tilt range of interest are on the order of the wavelength. Thus, rigorous electromagnetic solution is required to capture light scattering effects due to the non-planar topography. We combine high NA imaging simulation with rigorous simulation of light scattering from the mirrors to study its effects on 65nm maskless lithography imaging. We vary mirror size, mirror tilt arrangement, feature type and illumination settings and compare the rigorous light scattering imaging results with standard imaging simulations using Kirchoff approximation. While electromagnetic scattering effects are present in the form of lateral standing waves and edge streamers in reflected light near-field intensity, they have negligible effects on SLM imaging for mirror sizes more than 1µm 2 . The effects of mirror tilt arrangement on diffraction orders are used to study the through-focus behavior of alternating rows arrangement used in the SIGMA maskwriters as well as alternative arrangements. The good imaging properties of the alternating rows arrangement are confirmed and a multipass overlay scheme giving further image fidelity improvements is suggested.
Optical proximity correction (OPC) procedure for modifying designs requires an OPC setting effectively accounting for manufacturing and imaging constraints. Reticle-writing and imaging tool capabilities drive the choice for the minimum feature of an OPC model.Aggressiveness of an optical proximity correction is determined by a discretization setting for an OPC algorithm. Some OPC scheme parameters are there to restrict the minimum spacing and width to avoid circuit failures. The OPC minimum spacing parameter controls bridging lines. The OPC minimum width parameter limits the correction of trenches responsible for circuit breakdown. An aggressive choice of minimum spacing and width for an OPC setting can results in circuit failure: shortage or breakdown. The conservative approach results in poor circuit performance.The methodology was deployed at LSI Logic Corporation for empirical optimization of the OPC minimum spacing/width settings for a no failure imaging solution of OPCed masks. The proposed procedure is particularly beneficial for dark field metal interconnect masks. The approach was successfully validated for 130nm and 90nm backend technology metal layers.
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