The model accuracy for Optical Proximity Correction (OPC) and Inverse Lithography Technology (ILT) applications is a critical factor for patterning success in advanced technology nodes. One difficult challenge has been the accurate and fast simulation of Negative Tone Development (NTD) photoresist processes. It has been widely observed that CD measurements, top-down SEM contour images and X-section or AFM resist side wall profile measurements cannot be adequately predicted by conventional lithography process models, e.g., . Therefore, conventional OPC models were often unable to meet the demanding accuracy requirements of advanced logic or memory manufacturing. A key to achieving the demanding model accuracy requirements in NTD photoresist processes is to consider the photoresist shrinkage effects both in the Post-Exposure-Bake (PEB) step and photoresist development process step. Starting from continuum elastic mechanics, e.g., , we have developed a fast and accurate full 3D compact shrinkage model and validated its accuracy and usefulness vs. experimental results for several advanced processes and vs. rigorous simulation using a full physical lattice model. The compact model captures the significant photoresist shrinkage and deformation behaviors such as surface topography, resist sidewall angle (SWA) and layout pattern dependency , with much faster runtime capable of use in OPC and ILT mask data optimization. The speed and flexibility of the model are such that it can also be applied to help increase accuracy of simulation for some complex physical behaviors seen in other photoresist processes such as EUV and positive tone develop (PTD) photoresist.
The optical proximity correction (OPC) model and post-OPC verification that takes the developed photoresist (PR) 3D profile into account is needed in the advanced 2Xnm node. The etch process hotspots caused by poor resist profile may not be fully identified during the lithography inspection but will only be observed after the subsequent etch process. A complete mask correction that targets to final etch CD requires not only a lithography R3D profile model but also a etch process compact model. The drawback of existing etch model is to treat the etch CD bias as a function of visibility and pattern density which do not contain the information of resist profile. One important factor to affect the etch CD is the PR lateral erosion during the etch process due to non-vertical PR side wall angle (SWA) and anisotropy of etch plasma source. A simple example is in transferring patterns from PR layer to thin hard mask (HM) layer, which is frequently used in the double pattern (DPT) process. The PR lateral erosion contributes an extra HM etch CD bias which is deviated from PR CD defined by lithography process. This CD bias is found to have a nontrivial dependency on the PR profile and cannot be described by the pattern density or visibility. In this report, we study the etch CD variation to resist SWA under various etch conditions. Physical effects during etch process such as plasma ion reflection and source anisotropy, which modify the local etch rate, are taken into considerations in simulation. The virtual data are generated by Synopsys TCAD tool Sentaurus Topography 3D using Monte Carlo engine. A simple geometry compact model is applied first to explain the behavior of virtual data, however, it works to some extent but lacks accuracy when plasma ion reflection comes into play. A modified version is proposed, for the first time, by including the effects of plasma ion reflection and source anisotropy. The new compact model fits the nonlinear etch CD bias very well for a wide range of resist SWAs from 65 to 90 degrees, which covers the resist profile diversities in most real situations. This result offers a potential application for both resist profile aware and etch process aware mask correction model in the mask synthesis flow.
The Optical Proximity Correction (OPC) model, a key to process yield in the mask synthesis flow, is getting more and more complicated and challenging at advanced technology nodes (1X nm). To achieve accurate critical dimension (CD) prediction and model robustness on varied designed patterns, a rigorously tuned compact model (RTCM)  that takes the photoresist chemical effects into considerations is strongly desired. A lithography process consists of three main stages: Exposure, Post-Exposure Bake (PEB), and Photoresist Development. Each stage is characterized by its fundamental physics or chemistry that governs the process of illumination induced photo-acid generation, thermally activated chemical reaction-diffusion, and developer dependent photoresist dissolution, respectively. The final resist profile is determined by the process details of all these stages directly or indirectly. For an ideal resist that the development contrast approaches infinity, resist development is aptly represented by a threshold model applied to the PEB latent image (acid or inhibitor concentration). So the quality of OPC modeling is largely determined by the fidelity of PEB latent image [2,3]. However, for some types of resist and developer used in Negative Tone Development (NTD), the development contrast shows a long tail without a sharp transition. For such low-contrast resist, the developed resist profile is no longer described well by the equilevel surface of PEB latent image. Going beyond the threshold approximation, we start from the fundamental equations of resist development physics and analyze the time evolution of development front that determines the resist profile. In this paper, a new compact model is derived to catch the main physics in resist Development, which is also simple and computationally efficient to suit for OPC applications. Comparison with S-LITHO rigorous solutions and real-wafer experiments with 1D and 2D test patterns have showed that the new compact model, with fewer free parameters, provides better CD prediction than the existing empirical lumped parameter models for low-contrast resists. The new physical compact model offers a more accurate and extendable solution for OPC modeling at the 10nm node and onward.
3D Resist profile aware OPC has becoming increasingly important to address hot spots generated at etch processes
due to the mass occurrence of non-ideal resist profile in 28nm technology node and beyond. It is therefore critical to
build compact models capable of 3D simulation for OPC applications. A straightforward and simple approach is to
build individual 2D models at different image depths either based on actual wafer measurement data or virtual
simulation data from rigorous lithography simulators. Individual models at interested heights can be used by
downstream OPC/LRC tools to account for 3D resist profile effects. However, the relevant image depths need be
predetermined due to the discontinuous nature of the methodology itself. Furthermore, the physical commonality
among the individual 2D models may deviate from each other as well during the separate calibration processes. To
overcome the drawbacks, efforts are made in this paper to compute the whole bulk image using Hopkins equation in
one shot. The bulk image is then used to build 3D resist models. This approach also opens the feasibility of
including resist interface effects (for example, top or bottom out-diffusion), which are important to resist profile
formation, into a compact 3D resist model. The interface effects calculations are merged into the bulk image
Hopkins equation. Simulation experiments are conducted to demonstrate that resist profile heavily rely on interface
conditions. Our experimental results show that those interface effects can be accurately simulated with reference to
rigorous simulation results. In modeling reality, such a 3D resist model can be calibrated with data from discrete
image planes but can be used at arbitrary interpolated planes. One obvious advantage of this 3D resist model
approach is that the 3D model is more physically represented by a common set of resist parameters (in contrast to
the individual model approach) for 3D resist profile simulation. A full model calibration test is conducted on a
virtual lithography process. It is demonstrated that 3D resist profile of the process can be precisely captured by this
method. It is shown that the resist model can be carried to a different lithography process with same resist setup but
a different illumination source without model any accuracy degradation. In an additional test, the model is used to
demonstrate the capability of resist 3D profile correction by ILT.
A single compact resist model capable of predicting 3D resist profile is strongly demanded for the advanced technology
nodes to avoid the potential hotspots due to imperfect resist pattern shape and its lack of resistance in the subsequent
etch process. In this work, we propose a resist 3D (R3D) compact model that takes acidz-diffusion effect into account.
The chemical reaction between acid and base along z-direction is treated as second order effect that is absorbed into the
anisotropic diffusion length as a fitting parameter. Meanwhile, the resist model in the x-y wafer plane is still kept in
general by applying the compact solution of 2D reaction-diffusion equation. In order to have the 2D contour
predictability at arbitrary resist height, calibration from entire 3D data (CDs at several heights) areconducted
simultaneously witha single cost function so that the R3D compact model is described by a common set of resist free
parameters and threshold for all resist heights. With the low energy approximation, the acid z-diffusion effect is
equivalent to a z-diffused TCC that takes the form of linear combination of pure optical TCCs sampled at discrete
image-depth which can be pre-calculated. With this benefit, the R3D compact model offers a more physical approach but
adds no runtime concern on the OPC and verification applications. The predicted resist cross-section profiles from our
test patterns are compared those computed with rigorous lithography simulator SLITHO and show very good matching
results between them. The demonstration of the AF printability check from the predicted cross-section profile at AF
indicates the success of our R3D compact model.