Proximity effect correction optimization considering fogging and loading effects compensation

A new advanced e-beam reticle writing system HL-950M has been developed to meet requirements for the production of 130 nm node reticles as well as the development of 100 nm node reticles. In order to improve the critical dimension (CD) accuracy and pattern positioning accuracy, several new technologies have been introduced. Fine address size is realized by a newly developed control electronics that enables the system to handle address unit of 2.5 nm, providing four times higher resolution than that of the previous systems. Reconstruction of sub-sub-field (SSF) pattern data has been developed so that the same pattern is exposed twice with reconstructed SSF pattern data sets with different SSF boundaries, realizing better stitching and positioning accuracy. High accuracy proximity effect correction has been developed with a new second order proximity effect calculation scheme, particularly promising better CD linearity. As main results of the system evaluation, the global CD accuracy of 9 nm and the global pattern positioning accuracy of 15 nm have been obtained. The overall performance of the HL-950M system has satisfied the specifications required for the 130 nm node reticle production and 100nm node reticle development.

We introduce well-developed optical proximity correction (OPC) techniques to the metasurface-based flat optics manufacturing process. Flat optics, formed by subwavelength scale nanostructure pillar (nanopillar) array, so called metasurface, has become promising substitutes for conventional bulky optical components. For its manufacturing, photolithography is preferable rather than the electron beam lithography (EBL) technique because of its time and cost effectiveness for mass manufacturing. However, the required feature size and pitch of the metasurface for the visible light is approaching the process limit of the ArF immersion lithography. It results in critical dimension (CD) errors due to optical proximity effect and could result in efficiency degradation of the flat optics. In the semiconductor manufacturing industry, OPC based on process modelling and numerical computation has been developed for the last few decades to control the CD on the wafer. Here, a machine learning (ML) model is constructed to avoid the time consumption of the conventional OPC method without losing the accuracy. Various pitches of flat optics metalens, from 465 nm to 160 nm, has been studied for the implementation of the ML OPC. The root mean square (RMS) CD errors < 1 nm and the CD accuracies < 6 nm can be achieved. The CD error percentages over the pillar diameters < 6 % is observed and the improvement of CD error and CD accuracy compared to rule based OPC in small pitches of metalens is demonstrated.

We experimentally evaluated the overall performance of our proximity effect correction method in electron projection lithography. A Si stencil mask for corrected data was fabricated and then exposed using an electron-beam stepper. Shape modification resulted in high linewidth controllability and uniformity of critical dimension (CD) even when the pattern density changed. The deviation in linewidth was 6.6 nm in linearity, and 4.7 nm in the pattern density dependence of a 100 nm line. As well as shape modification, we reduced the pattern density of large patterns by replacing the interior area with mesh patterns in order to produce isolated space patterns. The meshes were well fabricated on the mask. Isolated spaces were successfully formed without resolving the mesh patterns on a resist.

In cell projection lithography, critical dimension (CD) control is one of the important issues for device fabrication as well as resolution. Because plural patterns are exposed in one shot under the same dose, the proximity effect correction is more difficult than in the conventional variable-shaped beam (VSB) lithography. We have analyzed the CD deviation in order to obtain high CD accuracy of less than 0.02 µ m (range) which is sufficient for manufacturing 1 G dynamic randam access memory (DRAM). We have found that the Coulomb interaction effect plays an important role in CD deviation. We have proposed a new exposure intensity distribution (EID) function which contains a factor introduced for the first time to compensate the proximity effect and the Coulomb interaction effect simultaneously. The results indicate that the new EID function is very effective to compensate the Coulomb interaction effect and improve the CD deviation from 12% (0.03 µ m) to 6% (0.015 µ m) for 0.25 µ m lines-and-spaces (L/S) patterns.

In recent years, mask critical dimension (CD) linearity and uniformity has become increasingly important. The ITRS roadmap shows the mask CD control requirements exceeding those of the wafer side beyond the 45nm node. Measurements show that there are systematic, uncorrected proximity effects even when a state-of-the-art proximity effect correction (PEC) algorithm is used. The uncorrected proximity effect is predictable with a computational model. The model for e-beam lithography and etch process contains terms to model short-range pattern density effects and plasma shadowing effect in Cr-etch. The model is calibrated using CD measurements on a test mask. The model is valid for arbitrary 2-D patterns. We present a model-based mask process compensation (MPC) method which applies geometric changes to polygons as in OPC. We discuss the goodness of model fit to the calibration data; verification of the calibrated model by SEM images; and the improvement obtained by MPC. The mask writing error, i.e. final inspection CD minus incoming database CD, was reduced by a factor of 2 through the use of MPC.

HL-7000M electron beam lithography system has been developed as a state-of-the-art reticle writer for the generation of 90nm node production and 65nm node development. It is capable of handling relatively large volume data files such as full Optical Proximity Correction patterns and angled patterns for System on Chip. Aiming at technological requirements, a newly designed electron optics column generating a vector-scan variable shaped beam and a digital disposition system with a storage area network technology have been integrated into HL-7000M. Since the requirement on the critical dimension uniformity is extremely demanding on the ITRS roadmap, HL-7000M has also needed to improve its beam shaping performance. The ability relevant to shaping beam size has a great impact on its line width or critical dimension accuracy. To reduce an aberration caused within the shaping lens system, the dual quadrupole electrostatic shaping deflector has been utilized. By applying advanced technologies, HL-7000M with a result of critical dimension uniformity (2.5nm and 2.8nm in 3&#963;) has achieved meeting its target requirement of the 90nm generation for production. Additionally HL-7000M has proved its potential, allowing the industry to establish quickly the processes further beyond the requirements of the 65nm node for development.

For the manufacturing of semiconductor technologies following the ITRS roadmap, we will face nodes well below a 32-nm half pitch in the next 2 to 3 years. Despite being able to achieve the required resolution, which is now possible with electron beam direct-write variable-shaped beam equipment and resists, it becomes critical to precisely reproduce dense line space patterns onto a wafer. This exposed pattern must meet the targets from the layout in both dimensions (horizontally and vertically). For instance, the end of a line must be printed in its entire length to allow a contact to be placed later. Up to now, the control of printed patterns such as line ends was achieved by a proximity effect correction mostly based on a dose modulation. This investigation of line end shortening (LES) includes multiple novel approaches, and contains an additional geometrical correction to push the limits of the available data preparation algorithms and the measurement. The designed LES test patterns, which aim to characterize the status of LES in a quick and easy way, were exposed and measured at Fraunhofer Center Nanoelectronic Technologies using its state-of-the-art electron beam direct writer and CD-SEM. Simulation and exposure results with the novel LES correction algorithms applied to the test pattern and a large production-like pattern in the range of our targeted critical structure dimensions in dense line space features smaller than 40 nm will be shown.

Critical Dimension (CD) Uniformity is one of the necessary parameters to assure good performance and reliable functionality of any integrated circuit (IC), and towards the advanced technology node 28nm and beyond, corresponding CD Uniformity becomes more and more crucial. It is found that bad mask CD Uniformity is a significant error source at 28nm process. The CD Uniformity on mask, if not controlled well, will badly impact wafer CD performance, and it has been well-studied that CD Uniformity issue from gate line-width in transistors would affect the device performance directly. In this paper we present a novel solution for mask global CD uniformity error correction, which is called as global loading effect correction (GLEC) method and applied nesting in the mask exposure map during the electron beam exposure. There are factors such as global chip layout, writing sequence and chip pattern density distribution (Global Loading), that work on the whole mask CD Uniformity, especially Global Loading is the key factor related to mask global CD error. From our experimental results, different pattern density distribution on mask significantly influenced the final mask CD Uniformity: the mask with undulating pattern density distribution provides much worse CD Uniformity than that with uniform one. Therefore, a GLEC model based on pattern density has been created to compensate the global error during the electron beam exposure, which has been proved to be efficacious to improve mask global CD Uniformity performance. Furthermore, it ’s also revealed that pattern type is another important impact factor, and GLEC coefficient need be modified due to the specific pattern type (e.g. dense line-space only, iso-space only or an average of them) to improve the corresponding mask CD uniformity.

A new method for the measurement of acid diffusion parameters in negative-tone e-beam chemically amplified resists and its application for proximity effect correction is presented. It is based on the measurement of the diameter of lithographic features (pillars) obtained from single-pixel e-beam exposures. Generally, long post exposure bake (PEB) times or low post apply bake (PAB) temperatures lead to wider features compared with short PEB times and high PAB temperatures, due to enhanced acid diffusion. The acid diffusion coefficient was found to depend exponentially on cross-linking density. Additionally, proximity effect correction parameters (triple Gaussian parameters) are easily calculated. These parameters are t PEB dependent and in the limit (t PEB→0) are almost equal to the corresponding values for non-amplified resists. The method has been applied to SAL-601 (Shipley), and the calculated proximity parameters were used successfully to correct the proximity effect of complicated layouts with a 0.15 µ m critical dimension.

The simplified electron energy flux (SEEF)model by which the backscattered energy distribution in the multilayered structure is calculated has been applied to the analysis of critical dimension (CD) variations caused by the thickness variations in copper interconnect. The SEEF model defines the reflection, downward transmission, and upward transmission of electron energy fluxes in each layer. Parameters of the SEEF model are expressed as functions of the depth from the substrate surface and are modified by the approximation in regard to the thickness variation. Using this approach, the process window for the dose and thickness variation has been analyzed quantitatively and the necessity of improving the process window has been confirmed especially at 45nm node and beyond. Moreover, a variation-aware proximity effect correction method, in which CD variations caused by process variations are reduced and dose margins for various patterns with the same linewidth are equalized, is proposed. The correction method improves the process window for isolated line pattern and 1:1 line-and-space pattern when the sixth wiring layer is fabricated.

Fast and accurate proximity effect correction algorithm based on pattern edge shape adjustment for electron beam lithography

In this paper, a quantitative method to analyze the effective range of fogging effect from massive data is presented. According to the calculated effective range, we use two approaches to correct the pattern-dependent CD error that come from e-beam writing. One is the fogging effect correction(FEC), which uses a Gaussian distributed model to describe FE. Second, we implement dosage modulation based on the assumption that the error caused by FE is linearly proportional to the pattern density of a mask. In summary, we are able to successfully predict the map of CD error for various layouts, and correct the error caused by FE in mask-making.

Easy to adapt electron beam proximity effect correction parameter calibration based on visual inspection of a “Best Dose Sensor”

A theoretical model of photoresist topography at the high step is proposed for the process in implant layers. By optimizing the layout design of the exposure pattern, the influence of the distribution, density, width and distance of the local high step on the global photoresist topography and critical dimension (CD) at the exposure pattern is studied. And the feasibility of the theoretical model is verified. It is found that the uniform distribution of the local high step is helpful to improve the half-size effect of the global photoresist thickness and CD. The global photoresist topography and CD uniformity are related to the distance of the high step.

OPC (Optical Proximity Correction) has been employed for over decade to address local lithographic printing effects. With the intensive scaling down of the designs as well as the increasing complexity of layout routing, lithographic process is being pushed to its theoretical limit and it has led to continuously shrinking DoF (Depth of Focus). Complex OPC model components are hence included into optical lithography simulation to ensure tolerable CD (Critical Dimension) variation and sustainable DOF of concerned layouts. For example, very complicated segmentation needs to be applied in mask correction, which comes at the cost of long runtime and requires an effective approach to consolidate the adequacy of model components during the flow of correction parameter tuning. In this paper, an approach is demonstrated to improve the accuracy and efficiency of OPC parameter tuning for mask correction. The approach starts with analyzing the target points in post-OPC database to identify those intolerable variations, followed by a pattern similarity grouping for the above intolerable layouts. Then, a concern index is established based on the CD out-of-tolerance ratio, dissection and pattern type for prioritizing the problematic variations. Then the corrective parameters are accordingly optimized to reduce the variation on highly prioritized patterns. During the iteration flow of OPC parameter optimization, the combination of pattern grouping and concern index greatly reduces required optimization iterations for OPC recipe tuning and enhances OPC convergence.