Seung-Hune Yang

The new test pattern selection method for OPC model calibration, based on the process of clustering in a hybrid space

Model-based Optical Proximity Correction (OPC) is widely used in advanced lithography processes. The OPC model contains an empirical part, which is calibrated by fitting the model with data from test patterns. Therefore, the success of the OPC model strongly relies on a test pattern sampling method. This paper presents a new automatic sampling method for OPC model calibration, using centroid-based clustering in a hybrid space: the direct sum of geometrical sensitivity space and image parameter space. This approach is applied to an example system in order to investigate the minimum size of a sampling set, so that the resulting calibrated model has the error comparable to that of the model built with a larger sampling set. The proposed sampling algorithm is verified for the case of a contact layer of the most recent logic device. Particularly, test patterns with both 1D and 2D geometries are automatically sampled from the layer and then measured at the wafer level. The subsequent model built using this set of test patterns provides high prediction accuracy.

Verification of directed self-assembly (DSA) guide patterns through machine learning

Verification of full-chip DSA guide patterns (GPs) through simulations is not practical due to long runtime. We develop a decision function (or functions), which receives n geometry parameters of a GP as inputs and predicts whether the GP faithfully produces desired contacts (good) or not (bad). We take a few sample GPs to construct the function; DSA simulations are performed for each GP to decide whether it is good or bad, and the decision is marked in n-dimensional space. The hyper-plane that separates good marks and bad marks in that space is determined through machine learning process, and corresponds to our decision function. We try a single global function that can be applied to any GP types, and a series of functions in which each function is customized for different GP type; they are then compared and assessed in 10nm technology.

Study of lens heating behavior and thick mask effects with a computational method

Advances on techniques that enable small technology nodes printing benefit the lithography with cost. For instance, lens heating draws peoples attention when the NTD process is applied together with the bright tone mask. And the study of it requires the investigation of many other variables. In this paper we examine individual impact of several closely related process variables to understand the lens heating behavior. Meanwhile, though it is known that the PTD process is less sensitive to the lens heating effect, we do observe mask topography induced best focus shifts among different patterns with small spaces. It is of interest to discover the extent to which the NTD is affected. Thus in this paper we also compared the two processes with respect to the mask topography effect by simulating the best focus shifts of a series of test patterens.

Computational Process Optimization of Array Edges

DRAM chip space is mainly determined by the size of the memory cell array patterns which consist of periodic memory cell features. Resolution Enhancement Techniques are used to optimize the periodic pattern process performance. This is often realized with aggressively coherent illumination sources supporting the periodic pattern pitch only and making an array edge correction very difficult. The edge can be the most critical pattern since it forms the transition from periodic patterns to non periodic periphery, so it combines the most critical pitch and highest susceptibility to defocus. Non functional dummy structures are very effective to support the outermost edge but are very expensive, so their reduction or avoidance directly increases chip space efficiency. This paper focuses on how to optimize the DRAM array edge automatically in contrast to manual optimization approaches that were used effectively but at high cost. We will show how to squeeze out the masks degrees of freedom to stay within tight pattern tolerances. In that way we minimize process variations and the need of costly non-functional dummy structures. To obtain the best possible results the optimization has to account for complex boundary conditions: correct resist effect prediction, mask manufacturability constraints, low dose, low MEEF, conservation of symmetries and SRAF printing, simultaneous optimization of main features and SRAFs. By incorporating these complex boundary conditions during optimization we aim to provide first time right layouts without the need for any post processing.

Analysis of mask CD error by dose modulation for fogging effect

Mask critical dimension (CD) errors are analyzed in case fogging effect is corrected by dose modulation method with comparison of measurement and simulation. In the test mask, an extreme condition from pattern density 0% to 100% is applied for making fogging effect. On the ground of the utmost pattern densities which is one of the factors of fogging effect, various mask CD errors are observed with optical measurement in spite of fogging correction. Each error factor is distinguished from whole mask error using electron beam simulator which is adopting Monte Carlo (MC) calculation for electron scattering modeling, proximity effect correction (PEC) and even fogging effect correction. From error analysis, 3 kinds of mask error are observed. The first CD error is from an inaccurate modeling of fogging effect, the second is from fogging correction program. The third is error from development loading effect. The two formers are comparatively less important than the latter because they can be soluble problems by careful selection of fogging model or improvement of computing systems. However, error from develop loading effect is hard to solve so that not only chemical but also fluid mechanical approach is needed.

Dose-modulation-induced mask CD error on simultaneous correction of fogging and loading effect

In order to analyze a simultaneous correction of fogging and loading effect, the e-beam lithographic simulation was performed with dose modulation method. The in-house e-beam simulator which adopts Monte-Carlo method for electron scattering is used for performing Proximity Effect Correction (PEC) and fogging correction during the e-beam lithographic processes. Various values of theta, representative parameter which describes the deposited energy by fogging, are used for simulation. Fogging effect is well known phenomenon which is the additional energy deposition into large exposed area by second electron scattering, and this fogging correction is successfully achieved by dose modulation method. However, etch loading cannot be compensated properly by modulating dose due to its unique property. From the simulation results, it is obviously necessary to correct etch loading effect and fogging effect simultaneously in order to cure global and local CD errors. The bigger loading effect is, the bigger local CD error induced by dose modulation method is to be generated. This global error is reducible but irremovable perfectly owing the discrepancy between the property of etch loading effect and dose modulation. However, the proper selection of eta, the ratio of the energy deposition into resist from the back scattering electrons verse the forward scattering electrons, can remarkably reduce the global and local CD errors. As a consequence, the method of the dose modulation is not the perfect way to correct the CD errors induced by etch loading or positional induced error. Nevertheless, the dose modulation method with variable eta can be an alternative way to control the designed CD because of its precision and rapidity.

Fogging Effect Consideration in Mask Process at 50 KeV E-Beam Systems

To achieve higher resolution and critical dimension (CD) accuracy in mask fabrication, 50KeV E-beam systems are used widely. However, as a high acceleration system is adapted, the degree of fogging effect caused by multi-scattering electrons becomes more serious. Although considerable efforts have been made, fogging effect cannot be removed perfectly, therefore several compensation techniques are applied instead. Fogging effect not only deteriorates CD uniformity but also makes mean to target (MTT) control difficult. Moreover, Fogging effect causes proximity effect correction (PEC) error according to PEC methods such as dose modulation type usually used in variable shaped beam (VSB) system and GHOST type commonly used in Gaussian beam system. In this paper, we investigated the fogging effect under the various exposure conditions at raster scan Gaussian beam system and VSB system experimentally and analytically.

The effect of mask and source complexity on source mask optimization

More complex source and mask shapes are required to maximize the process window in low κ1 era. In simulation, the improvement can be shown well with ideal source and mask shapes. However imperfection of the source and mask can cause critical dimension (CD) errors and results in smaller process margin than expected one. In this paper, it is shown that how process margins can be improved with different source and mask complexities. Also the effect of source and mask complexities on CD errors and process margin degradation is discussed. The error source of the electron beam mask pattern generator is investigated and used for mask CD uniformity estimation with different mask complexity.

OPC modeling using AFM CD measurement

Most important factors in OPC model building will be sampling data for model calibration. We will demonstrate that how CD-AFM data can be used in OPC modeling and will show possibility to get a more predictive model by using CD-AFM data.

Optical performance comparison between negative tone development and positive tone development

A negative tone development (NTD) process has been considered as apromising candidate for the smaller contact solution due to the remarkable image quality over a positive tone develop (PTD) process. However, it has not been investigated why NTD has higher optical performance than PTD yet. In this paper, image log slope (ILS) and mask error enhancement factor (MEEF) of binary and phase shift masks (PSM) are investigated with considering mask bias, target critical dimension (CD) and pattern pitch. It is found that the irradiance slope is steep and wafer CD variation from mask CD variation is small when the target CD is relatively smaller than pattern pitch. Mathematical model is derived to analyze image quality of binary mask and PSM.Three-dimensional mask effect is also considered with rigorous simulation.