Predicting and simulating resist defects with the help of rigorous nanomechanical/fluid dynamics simulations can be quite tedious and slow, especially for the use cases expected in EUVL. This is mainly because of the complicated meshing and discretization methods required. The finite element method (FEM) solver would need to solve a large system of equations due to the large number of higher order mesh elements making the overall simulations extremely slow. A total simulation time of several hours was observed while simulating a single rough profile for predicting collapse. Predicting resist defects with the help of simulations therefore requires newer strategies due to the overall randomness caused due to the numerous optical and chemical effects. This is where machine learning could help speed up the process.
Pattern collapse has become a problem area in lithographic manufacturing due to the added complexity as we move towards extreme ultraviolet (EUV) focused mass production of semiconductor devices. Collapse is influenced mainly by the geometry and mechanical properties of the pattern. Photoresist patterns with higher aspect ratios (ARs) or lower feature spacing (dense features) are prone to collapse. This, combined with a lower stiffness value (Young’s modulus), adds to the undesirable deformation of patterns at the wafer level as we move towards advanced technology nodes. Such a deformation in the resist is seen after the rinsing stage and is caused mainly due to the non-uniform drying of the rinse liquid after chemical development. The main causes of this are the unbalanced Laplace pressure ΔP difference across the liquid-air interface and the surface tension force (STF) along the three-phase line. In addition to that, the sidewall surface roughness leads to localized regions of higher aspect ratios which makes collapse modeling in EUV lithography more challenging. The irregular variation in the aspect ratios increases the risk of collapse and also requires additional model considerations. A machine learning (ML) based approach is introduced to predict deformation characteristics for rough cross-sections (XZ plane) which is then utilized for the computation of the overall deformation of an entire lines and spaces (L/S) pattern. The 3D profiles are converted into a 2D representation using modified Fisher vectors (FVs) and labeled based on the estimated deformation of the given rough profile. A convolutional neural network (CNN) is then trained with the data generated and used to predict collapse probabilities for a given data set. For the prediction of collapse in pillar patterns, a slightly different ML-based approach is used based on the cross sections in the XY plane. A finite element method (FEM) model is implemented to calculate the deformations δ for a given pillar arrangement which then serve as labels for the training data. The cross-sections are stacked together along the height/thickness and a 3D convolutional neural network is trained and used for collapse prediction.
Background: Negative-tone development (NTD) photoresists are prone to shrinkage effects during lithographic processing. Along with deformation seen during the postexposure bake (PEB), there are additional effects during the development that cannot be fully explained by a conventional PEB shrinkage model alone.
Aim: Understand the impact of PEB shrinkage on the development rate. Develop a model that can help predict resist profiles after chemical development.
Approach: A PEB shrinkage model for NTD resists is introduced, which uses the thermal properties of the resist material to help simulate shrinkage. The deformed state of the resist is used as an input to the development rate equation to predict the final feature dimensions observed in experiments.
Results: The strain concentration within the resist bulk can have an influence on the stability of the resist during the development. The strain influences the development rate depending on the resist feature shape and contours.
Conclusions: The results from this study can help improve optical proximity correction (OPC) modeling performance and help better understand the deformation characteristics of NTD resist materials. The model also shows that the development shrinkage has an influence on the edge placement error.
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