BackgroundOptical proximity correction (OPC) is an indispensable technology that has been propelling the advancement of computational lithography technology. To tightly control edge placement error (EPE) and maintain lithography process window, the demands on OPC computational resources and OPC turnaround time are growing rapidly with alarming acceleration. To tame the trend, machine learning technologies have been explored; however, an in-depth discussion on OPC solution learning limit is still lacking.AimWe aim to present an in-depth discussion on OPC solution learning limit and propose a general machine learning OPC framework that can be extended to curvilinear mask OPC technology.ApproachIn this study, we first investigate the machine learning OPC learning limit by examining noninverse lithography technology (non-ILT) OPC solution space characteristics inherited from edge segmentation and control point setting rules and then propose a general machine learning OPC framework that can take full advantage of deep convolution neural network (DCNN) learning capability while being able to preserve mask data high resolution.ResultsWith this machine learning OPC framework, we have achieved models with average absolute model error <1 nm for 14-nm metal layer. With single GPU, the average time for machine learning OPC models to produce results of 3840 nm × 3840 nm area is 8.74 ms for single channel input model and 12.65 ms for six channels input model.ConclusionsFor non-ILT OPC solution, there is an intrinsic learning limit inherited from edge segmentation rules. Machine learning OPC models should be content with learning low order OPC solutions. This intrinsic learning limit of non-ILT OPC solution may diminish for ILT OPC solution when the constraint on degrees of freedom of OPC solution is lifted. The machine learning OPC framework we proposed is general and extendable to curvilinear OPC technology.
Background: E-beam metrologies, both critical dimension scanning electron microscope metrology and defect scan metrology, have been playing a very critical role in gating patterning quality. SEM images can provide rich visual information for engineers to do qualitative and quantitative analyses. However, the low e-beam metrology tool throughput makes it impossible to obtain SEM images for larger area. Monte Carlo-based SEM image simulations or other SEM image simulations require postlithography or postetch pattern three-dimensional structures as prerequisite, and the simulation speed is not sufficiently fast for full chip implementation.
Aim: We aim to develop machine learning SEM models with sufficient accuracy and speed for full chip application in semiconductor manufacturing environment.
Approach: We have proposed a virtual SEM metrology solution based on U-Net neural network with physics-based feature maps as model input. With information in aerial image space encoded properly, SEM images of both postlithography and postetch can be predicted accurately enough for practical applications using our proposed virtual SEM metrology models. Equipped with GPUs, the machine learning-based SEM image models are fast enough to make it possible to realize full chip SEM generation from post-OPC data.
Results: Our machine learning SEM image models can predict SEM images with normalized cross correlation around 0.95 in reference to ground truth SEM images, each SEM image (512 × 512 in size) takes about 800 ms using single CPU, and the speed can be accelerated to about 10 ms with single GPU.
Conclusions: Using U-Net structure and physics-based feature maps as model inputs, machine learning-based SEM image models can be developed. The models are sufficiently accurate and fast to find their applications in semiconductor manufacturing, and they can be used as independent model for OPC data verification or generate SEM images as reference for SEM defect scan metrology.
Computational lithography has been playing a critical role in enabling the semiconductor industry. After source mask co-optimization (SMO), inverse lithography has become the ultimate frontier of computational lithography. Full chip implementation of rigorous inverse lithography remains impractical because of enormous computational hardware resource requirements and long computational time, the situation exacerbates for EUV computational lithography where mask 3D effect is more pronounced. One very promising technique to overcome the barrier is to take full advantage of the maturing machine learning techniques based on neural network architecture. Some success has been achieved using deep convolution neural network (DCNN) to obtain inverse lithography technology (ILT) solution with significantly less computational time. In DCNN, to extract features with sufficient resolution and nearly complete representation, the feature extract layers are very complicated and lack of physical meaning. More importantly, the training requires large number of well balanced samples, which makes the training more difficult and time consuming. To alleviate the difficulties relating to DCNN, we have proposed the physics based optimal feature vector design for machine learning based computational lithography. The innovative physics based feature vector design eliminates the need of feature extraction layers in neural network, only layers for mapping function construction are needed, which greatly reduces the NN training time and accelerates the NN model SRAF generation for full chip. In this paper, we will present our machine learning based inverse lithography results with adaptive and dynamical sampling scheme for neural network training.
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