Qiaolin Zhang

Analysis of OPC optical model accuracy with detailed scanner information

Production optical proximity correction (OPC) tools employ compact optical models in order to accurately predict complicated optical lithography systems with good theoretical accuracy. Theoretical accuracy is not the same as usable prediction accuracy in a real lithographic imaging system. Real lithographic systems have deviations from ideal behavior in the process, illumination, projection and mechanical systems as well as in metrology. The deviations from the ideal are small but non-negligible. For this study we use realistic process variations and scanner values to perform a detailed study of useful OPC model accuracy vs. the variation from ideal behavior and vs. theoretical OPC accuracy. The study is performed for different 32nm lithographic processes. The results clearly show that incorporating realistic process, metrology and imaging tool signatures is significantly more important to predictive accuracy than small improvements in theoretical accuracy.

Polarization aberration modeling via Jones matrix in the context of OPC

The increasingly stringent demand for shrinkage of IC device dimensions has been pushing the development of new resolution enhancement technologies in micro-lithography. High NA and Ultra-High NA (NA>1.0) applications for low k1 imaging strongly demand the adoption of polarized illumination as a resolution enhancement technology since proper illumination polarization configuration can greatly improve the image contrast hence pattern printing fidelity. For polarized illumination to be fully effective, ideally all the components in the optical system should not alter the polarization state during propagation from illuminator to wafer surface. In current OPC modeling tools, it is typically assumed that the amplitude and polarization state of the electric field do not change as it passes through the projection lens pupil. However, in reality, the projection lens pupil of the scanner does change the amplitude and the polarization state to some extent, and ignorance of projection pupil induced polarization state and amplitude changes may cause CD errors which are un-tolerable at the 45nm device generation and beyond. We developed an OPC-deployable modeling approach to model polarization aberration imposed by the projection lens pupil via Jones matrix format. This polarization aberration modeling capability has been integrated into the Synopsys OPC modeling tool, ProGen, and its accuracy and efficiency have been validated by comparing with an industry standard lithography simulator SolidE. Our OPC simulations show that the impact of projection lens pupil polarization aberrations on optical proximity effect (OPE) could be as large as several nanometers, which is not negligible given the extremely stringent CD error budget at 45nm node and beyond. This modeling approach is applicable to arbitrary polarization aberrations imposed by any components in the lithography system that can be characterized in Jones matrix format. Based on an experimentally measured Jones matrix pupil which intrinsically provides a much better approximation to the physical scanner pupil, we propose a more physics-centric methodology to evaluate the optical model accuracy of OPC simulator.

Novel apodization and pellicle optical models for accurate optical proximity correction modeling at 45 and 32 nm

An accurate optical model is the foundation of an accurate optical proximity correction (OPC) model, which has always been the key for successful implementation of model-based OPC. As critical dimension (CD) control requirements become severe at the 45- and 32-nm device generations, OPC model accuracy and hence optical model accuracy requirements become more stringent. In previous generations, certain optical effects could be safely ignored. For example, the transmission attenuation particularly at high spatial frequencies caused by lens apodization effects and organic pellicle films was ignored or not accurately modeled in conventional OPC simulators. These effects are now playing a more important role in OPC modeling as technology scales down. Our simulations indicate these effects can cause CD modeling errors of 5 nm or larger, at the 45-nm technology node and beyond. Therefore, they must be accurately modeled in OPC modeling. In our OPC modeling methodology, we propose two novel low-pass-filter models to capture the frequency-dependent transmission attenuation due to lens apodization and to pellicle films. These parameterized novel low-pass-filter models ensure that lens apodization and pellicle-film-induced transmission attenuation can be appropriately account for through regression during the experimental OPC model calibration stage in the case where no measured transmission data are available, thus enabling physics-centric OPC model building with considerably higher accuracy. We can then avoid overfitting the OPC model, which could cause instability in the OPC correction stage. The validity and efficiency of the proposed novel models are also verified using an industry-standard lithography simulator as well as an experimental OPC model calibration at the 45-nm technology node.

The Impact of Scanner Model Vectorization On Optical Proximity Correction

Low pass filtering taking place in the projection tools used by IC industry leads to a range of optical proximity effects resulting in undesired IC characteristics. To correct these predicable OPEs, EDA industry developed various, model-based correction methodologies. Of course, the success of this mission is strongly dependent on how complete the imaging models are. To represent the image formation and to capture the OPEs, the EDA community adopted various models based on simplified representations of the projection tools. Resulting optical proximity correction models are capable of correcting OPEs driven by the fundamental imaging conditions such as wavelength, illuminator layout, reticle technology, and lens numerical aperture, to name a few. It is well known in the photolithography community that OPEs are dependent on the scanner characteristics. Therefore, to reach the level of accuracy required by the leading edge IC designs, photolithography simulation has to include systematic scanner fingerprint data. These tool fingerprints capture excursions of the imaging tools from the ideal imaging setup conditions. They quantify the performance of key projection tool components such as illuminator and lens signatures. To address the imaging accuracy requirements, the scanner engineering and the EDA communities developed OPC models capable of correcting for imaging tools engineering attributes captured by the imaging tools fingerprints. Deployment of immersion imaging systems has presented the photolithography community with new opportunities and challenges. These advanced scanners, designed to image in deep sub-wavelength regime, incorporate features invoking the optical phenomena previously unexplored in commercial scanners. Most notably, the state of the art scanners incorporate illuminators with high degree of polarization control and projection lenses with hyper-NAs. The image formation in these advanced projectors exploits a wide range of vectorial interactions originating at the illuminator, on the pattern mask, in the projection lens and at the wafer. The presence of these, previously subdued phenomena requires that the imaging simulation methodologies be refined, increasing the complexity of the OPE models and optical proximity correction methodologies.

Efficient Post-OPC Lithography Hotspot Detection Using A Novel OPC Correction and Verification Flow

An accurate process model has always been the key for successful implementation of model-based Optical Proximity Correction (OPC). As CD control requirements become severe at the 45nm and 32nm device generations, process model accuracy requirements become more stringent. In previous generations, certain systematic process and tool fingerprints could be safely ignored. For example, lens apodization and mask pellicle film induced transmission loss, lens vectorial fingerprint(i.e. Jones pupil), illuminator polarization profile, and etc were ignored in conventional OPC modeling approaches. These effects are now playing a more important role in OPC modeling as technology scales down. Using conventional OPC model may lead to under-correction of the design layout during OPC, and this will result in large number of post-OPC layout hot spots which have patterning issues when the OPCed layout is exposed on the scanner. We designed an OPC correction and verification flow which can efficiently capture the post-OPC layout hot spots due to under-correction using traditional OPC model, and this flow further fixes these detected hot spots. Our simulations demonstrated that this proposed flow is able to achieve an OPC performance of 2.25nm CD error range and 0.26nm CD error standard deviation on poly gate layer for 45nm SRAM design. And this validated the efficiency of the proposed flow.

Scanner-characteristics-aware OPC modeling and correction

As scanner projection lens captures only a finite number of IC pattern diffraction orders. This low pass filtering leads to a range of optical proximity effects such as pitch-dependent CD variations, corner rounding and line-end pullback, resulting in imaged IC pattern excursions from the intended designs. These predictable OPEs are driven by the imaging conditions, such as wavelength, illuminator layout, reticle technology, and lens numerical aperture. To mitigate the pattern excursion due to OPEs, the photolithography community developed optical proximity correction methodologies, adopted and refined by the EDA industry. In the current implementations, OPC applied to IC designs can correct layouts to compensate for OPEs and to provide imaged patterns meeting the design requirements.

Modeling of focus blur in the context of optical proximity correction

The concept of focus blur encompasses the effect of laser bandwidth longitudinal chromatic aberration and scanner stage vertical vibration. The finite bandwidth of excimer laser source causes a corresponding finite distribution of focal planes in a range of 100nm or larger for the optical lithography system. Similarly, scanner vertical stage vibration puts the wafer in a finite distribution of focal planes. Both chromatic aberration and vertical stage vibration could introduce significant CD errors, hence can no longer be ignored in current lithography processes development and OPC development that require CD control within a few nanometers. We developed several methodologies to model the laser chromatic aberration and vertical stage vibration in OPC (Optical Proximity Correction) modeling tool. Extensive simulations were done to calculate chromatic aberration and vertical stage vibration focus blurs impact on lithography patterning for a variety of test structures. Chromatic aberration and vertical stage vibration focus blur effect was further included as an regression term in experimental OPC model calibration to capture its impact on litho patterning, and significant benefit to OPC model calibration was observed.

Focus Blur Model to Enhance Lithography Model for Optical Proximity Correction

The concept of focus blur encompasses the effect of laser bandwidth longitudinal chromatic aberration and scanner stage vertical vibration. The finite bandwidth of excimer laser source causes a corresponding finite distribution of focal planes in a range of 100nm or larger for the optical lithography system. Similarly, scanner vertical stage vibration puts the wafer in a finite distribution of focal planes. Both chromatic aberration and vertical stage vibration could introduce significant CD errors, hence can no longer be ignored in current lithography processes development and OPC development that require CD control within a few nanometers. We developed several methodologies to model the laser chromatic aberration and vertical stage vibration in OPC (Optical Proximity Correction) modeling tool. Extensive simulations were done to calculate chromatic aberration and vertical stage vibration focus blurs impact on lithography patterning for a variety of test structures. Chromatic aberration and vertical stage vibration focus blur effect was further included as an regression term in experimental OPC model calibration to capture its impact on litho patterning, and significant benefit to OPC model calibration was observed.

Rigorous vectorial modeling for polarized illumination and projection pupil in OPC

High NA and Ultra-High NA (NA>1.0) applications for low k1 imaging strongly demand the adoption of polarized illumination as a resolution enhancement technology since proper illumination polarization configuration can greatly improve the image contrast hence pattern printing fidelity and the effectiveness of optical proximity correction (OPC). However, current OPC/RET modeling software can only model the light source polarization of simple types, such as TE, TM, X, Y, or sector polarization with relatively simple configuration. Realistic polarized light used in scanners is more complex than the aforementioned simple ones. As a result, simulation accuracy and quality of the OPC result will be compromised by the simplification of the light source polarization modeling in the traditional approach. With ever shrinking CD error budget in the manufacturing of ICs at advanced technology nodes, more accurate and comprehensive illumination source modeling for lithography simulations and OPC/RET is needed. On the other hand, for polarized illumination to be fully effective, ideally all the components in the optical lithography system should not alter the polarization state of light during its propagation from illuminator to wafer surface. In current OPC modeling tools, it is typically assumed that the amplitude and polarization state of the light do not change as it passes through the projection lens pupil, i.e. the polarization aberration of projection lens pupil is ignored. However, in reality, the projection lens pupil of the scanner does change the amplitude and the polarization state to some extent, and ignorance of projection pupil induced polarization state and amplitude changes will cause CD errors un-tolerable at the 45nm device generation and beyond. We developed an OPC-deployable modeling approach to model arbitrarily polarized light source and arbitrarily polarized projection lens pupil. Based on polarization state vector descriptions of a general illumination source, this modeling approach unifies optical simulations of unpolarized, partially polarized, and completely polarized illuminations. The polarization aberration imposed by the projection lens pupil is modeled via Jones matrix format, and it is applicable to arbitrary polarization aberrations imposed by any components in the lithography system that can be characterized in Jones matrix format. Numerical experiments were performed to study CD impact from illumination polarization and projection lens pupil polarization aberrations, and up to several nanometers impact on optical proximity effect (OPE) was observed, which is not negligible given the extremely stringent CD error budget at 45nm node and beyond. Based on an experimentally measured Jones matrix pupil which intrinsically provides a much better approximation to the physical scanner projection pupil, we propose a more physics-centric methodology to evaluate the optical model accuracy of OPC simulator.

Modeling scanner signatures in the context of OPC

The requirement for OPC modeling accuracy becomes increasingly stringent as the semiconductor industry enters sub- 0.1um regime. Targeting at capturing the IC pattern printing characteristics through the lithography process, an OPC model is usually in the form of the first principle optical imaging component, refined by some phenomenological components such as resist and etch. The phenomenological components can be adjusted appropriately in order to fit the OPC model to the silicon measurement data. The optical imaging component is the backbone for the OPC model, and it is the key to a stable and physics-centric OPC model. Scanner systematic signatures such as illuminator pupil-fill, illuminator polarization, lens aberration, lens apodization, flare, etc., previously ignored without significant accuracy sacrifice at previous technology nodes, but are playing non-negligible roles at 45nm node and beyond. In order to ensure that the OPC modeling tool can accurately model these important scanner systematic signatures, the core engine (i.e. the optical imaging simulator) of OPC simulator must be able to model these signatures with sufficient accuracy. In this paper, we study the impact on optical proximity effect (OPE) of the aforementioned scanner systematic signatures on several 1D (simple line space, doublet line and doublet space) and 2D (dense line end pullback, isolated line end pullback and T-bar line end pullback) OPC test patterns. We demonstrate that the scanner systematic signatures have significant OPE impact on the level of several nanometers. The predicted OPEs and impact from our OPC simulator matches well with results from an industry standard lithography simulator, and this has laid the foundation of accurate and physics-centric OPC model with the systematic scanner signatures incorporated.