Hiroyuki Shindo

Measurement technology to quantify 2D pattern shape in sub-2x nm advanced lithography

We have succeeded in quantifying changes in 2D pattern shape, which are induced by exposure condition and Optical Proximity Correction (OPC), from CD-SEM image. In the current lithography technology, micro patterns which are close to resolution limit are printed on wafer by fully utilizing aggressive OPC technology. In such lithography technology, controlling the shape of printed patterns is extremely difficult. In order to control such difficult patterning process, a demand to precisely quantify the pattern shape of 2D patterns is significantly growing. SEM images captured by CD-SEM are used mainly for the measurement of one dimensional size such as line width and contact hole diameter. It has been said not easy to measure shape variation of 2D patterns such as corner and line end from SEM images. However, we have succeeded in quantifying pattern shape of 2D pattern by utilizing Advanced SEM contouring technology which is combined with CD-gap-free contouring technology [1] and Fine SEM Edge (FSE) technology [2]. By this, we could quantitatively measure shape variation which are induced by exposure condition variability and/ or OPC, which used to be considered difficult to quantify. For the verification of this new measurement technology, wafers on which printed 2D patterns that are exposed in different conditions and with varied SRAF changed in size and position are prepared. The 2D patterns are measured by CD-SEM and SEM images of the 2D patterns are taken. To the SEM images of the 2D patterns, this new measurement technology is applied to quantitatively analyze how the expose condition and SRAF variation affect the printed 2D pattern shape. In this paper, the results of above experiments are reported.

A non-uniform SEM contour sampling technique for OPC model calibration

OPC model calibration techniques that use SEM contours are a major reason for the modern day improved fitting efficiency in complex mask design compared to conventional CD-based calibration. However, contour-based calibration has a high computational cost and requires a lot of memory. To overcome this problem, in conventional contour-based calibration, the SEM contour is sampled uniformly at intervals of several nanometers. However, such sparse uniform sampling significantly increases deviations from real CD values, which are measured by CD-SEM. We also have to consider the shape errors of 2D patterns. In general, the calibration of 2D patterns requires higher frequency sampling of the SEM contour than 1D patterns do. To achieve accurate calibration results, and while considering the varied shapes of calibration patterns, it is necessary to set precise sampling intervals of the SEM contour. In response to these problems, we have developed a SEM contour sampling technique in which contours are sampled at a non-uniform rate with arbitrary mask shapes within the allowable sampling error. Experimental results showed that the sampling error rate was decreased to sub-nm when we reduced the number of contour points.

SEM-contour shape analysis method for advanced semiconductor devices

The new measuring method that we developed executes a contour shape analysis that is based on the pattern edge information from a SEM image. This analysis helps to create a highly precise quantification of every circuit pattern shape by comparing the contour extracted from the SEM image using a CD measurement algorithm and the ideal circuit pattern. The developed method, in the next phase, can generate four shape indices by using the analysis mass measurement data. When the shape index measured using the developed method is compared the CD, the difference of the shape index and the CD is negligibly small for the quantification of the circuit pattern shape. In addition, when the 2D patterns on a FEM wafer are measured using the developed method, the tendency for shape deformations is precisely caught by the four shape indices. This new method and the evaluation results will be presented in detail in this paper.

High-accuracy optical proximity correction modeling using advanced critical dimension scanning electron microscope–based contours in next-generation lithography

Optical proximity correction (OPC) modeling is traditionally based on critical dimension (CD) measurements. As design rules shrink and process windows become smaller, there is an unavoidable increase in the complexity of OPC resolution enhancement technique (RET) schemes required to enable design printability. The number of measurement points for OPC modeling has increased to several hundred points per layer, and metrology requirements are no longer limited to simple 1-D measurements. Contour-based OPC modeling has recently arisen as an alternative to the conventional CD-based method. In this work, the technology of contour alignment and averaging is extended to arbitrary 2-D structures. Furthermore, the quality of scanning electron microscope (SEM) contours is significantly improved in cases where the image has both horizontal and vertical edges (as is the case for most 2-D structures) by a new SEM image method, which we call fine SEM edge (FSE). OPC model calibration is done using SEM contours from 2-D structures. Then, the effectiveness of contour-based calibration is examined by doing model verification. The experimental results of the model quality with innovative SEM contours that was developed by Hitachi High-Technologies Corporation (Ibaraki-ken, Japan) are reported. This combination of advanced alignment and averaging, and FSE technologies, makes the best use of the advantage of contour-based OPC-modeling, and should be of use for next-generation lithography.

Weighting evaluation for improving OPC model quality by usingadvanced SEM-contours from wafer and mask

OPC modeling has been complex procedure in 28nm node, and it becomes difficult to obtain enough OPC modeling accuracy if calibration is done by using only CD value. Therefore it becomes essential to take pattern shape variation into consideration especially in 2D pattern calibration. Thus utilizing SEM-contour has become important technology. In SPIE advanced lithograpy 2010 [3], Contour-based OPC-modeling by using Advanced SEM-contour which is combined with Fine SEM Edge, alignment and averaging technologies was examined, and model quality was significantly improved. Also, in SPIE advanced lithography 2011, an advanced hybrid OPC modeling which uses 1D CD measurements by CD-SEM and 2D contours created by the advanced SEM-contouring technology and panoramic Mask SEM-contour showed high predictability for both 1D and 2D, even though the relationship between 1D and 2D calibration has trade-off. In this study, weighing function of Calibre ContourCal, a product of Mentor Graphics, was evaluated using the OPC data set same as that used in SPIE2011. The weighting can be set for 1D structure and 2D structure separately. In this paper, the quality of OPC model by applying different weighting is discussed.

SEM-contour shape analysis based on circuit structure for advanced systematic defect inspection

We have developed a practicable measurement technique that can help to achieve reliable inspections for systematic defects in advanced semiconductor devices. Systematic defects occurring in the design and mask processes are a dominant component of integrated circuit yield loss in nano-scaled technologies. Therefore, it is essential to ensure systematic defects are detected at an early stage of wafer fabrication. In the past, printed pattern shapes have been evaluated by human eyes or by taking manual critical dimension (CD) measurements. However, these operations are sometimes unstable and inaccurate. Last year, we proposed a new technique for taking measurements by using a SEM contour [1]. This technique enables a highly precise quantification of various complex 2D shaped patterns by comparing a contour extracted from a SEM image using a CD measurement algorithm and an ideal pattern. We improved this technique to enable the carrying out of inspections suitable for every pattern structure required for minimizing the process margin. This technique quantifies a pattern shape of a target-layer pattern using information on a multi-layered circuit structure. This enabled it to confirm the existence of a critical defect in a circuit connecting upper/lower-layers. This paper describes the improved technique and the evaluation results obtained in evaluating it in detail.

Contour-based metrology for complex 2D shaped patterns printed by multiple-patterning process

We developed a new measurement method enabling to quantitatively and accurately evaluate 2D pattern shapes, which becomes critical in patterning control of Metal layer patterns transferred by Litho-Etch-Litho-Etch (LELE) process. In LELE, a split patterning of a Metal-A (MA) layer and a Metal-B (MB) layer makes patterning control more challenging. Hence, it is essential to evaluate the shape of transferred patterns after final etching in order to verify that the patterning control of MA and MB layer patterns is performed within an allowable budget. For this, our Pattern Shape Quantification (PSQ) method [1][2][3], which enables to measure dimensional difference of the transferred pattern shape from their target-design, is an effective metrology. Patterns transferred through a LELE process contain the effects of two types of shape modifications. The first is the fidelity of the individual pattern shapes (e.g. pattern-end pull-back or push-out) whose determinative factors are adopted design (e.g. OPC and SRAF), process condition (of e.g. lithography and etching), etc. The second is the shift in position between MA and MB patterns induced by Pattern Placement Error (PPE) of MB with respect to MA. That means that the edge-placement errors (EPE) in the final pattern are not only due to the fidelity of the transferred pattern shape, but are also impacted by the PPE. Also, a space between MA and MB patterns will be affected by the PPE as well. A failure to maintain a required minimum space between patterns could lead to a leak-current between patterns (and hence directly affect device performance), so it is important that the PPE can be measured accurately. Therefore, we developed a method to measure local PPE in actual device patterns, from CD-SEM images, that also outputs a pattern-contour in which this PPE has been removed. Utilizing such a pattern-contour into the PSQ method enables to quantitatively determine the fidelity of transferred pattern shape solely induced by the 1st shape modification, while providing PPE data from the device patterns themselves. We believe that a high-quality patterning control (by e.g. optimization of process condition) of MA and MB can be performed only by using such a measurement result. This paper demonstrates and discusses the capability and effectiveness of our newly developed method.

The assessment of the impact of mask pattern shape variation on the OPC-modeling by using SEM-Contours from wafer and mask

As design rules shrink, Optical Proximity Correction (OPC) becomes complicated. As a result, measurement points have increased, and improving the OPC model quality has become more difficult. From the viewpoint of decreasing OPC calibration runtime and improving OPC model quality concurrently, Contour-based OPC-modeling is superior to CD-based OPC-modeling, because Contour-based OPC-modeling uses shape based rich information. Hence, Contour-based OPC-modeling is imperative in the next generation lithography, as reported in SPIE2010. In this study, Mask SEM-contours were input into OPC model calibration in order to verify the impact of mask pattern shape on the quality of the OPC model. Advanced SEM contouring technology was applied to both of Wafer CD-SEM and Mask CD-SEM in examining the effectiveness of OPC model calibration. The evaluation results of the model quality will be reported. The advantage of Contour based OPC modeling using Wafer SEM-Contour and Mask SEM-Contour in the next generation computational lithography will be discussed.

Defect window analysis by using SEM-contour based shape quantifying method for sub-20nm node production

The impact on yield loss due to systematic defect which remains after Optical Proximity Correction (OPC) modeling has increased, and achieving an acceptable yield has become more difficult in the leading technology beyond 20 nm node production. Furthermore Process-Window has become narrow because of the complexity of IC design and less process margin. In the past, the systematic defects have been inspected by human-eyes. However the judgment by human-eyes is sometime unstable and not accurate. Moreover an enormous amount of time and labor will have to be expended on the one-by-one judgment for several thousands of hot-spot defects. In order to overcome these difficulties and improve the yield and manufacturability, the automated system, which can quantify the shape difference with high accuracy and speed, is needed. Inspection points could be increased for getting higher yield, if the automated system achieves our goal. Defect Window Analysis (DWA) system by using high-precision-contour extraction from SEM image on real silicon and quantifying method which can calculate the difference between defect pattern and non-defect pattern automatically, which was developed by Hitachi High-Technologies, has been applied to the defect judgment instead of the judgment by human-eyes. The DWA result which describes process behavior might be feedback to design or OPC or mask. This new methodology and evaluation results will be presented in detail in this paper.

Advanced CD-SEM imaging methodology for EPE measurements

Accurate EPE (edge placement error) characterization is important for the process control of high-volume manufacturing at N5 BEOL and beyond. In a CD-SEM metrology, the accurate edge-to-edge measurements among multiple layers and/or SEM-Contour extraction are required for the accurate EPE characterization. One of the technical challenges in CD-SEM metrology is to control charging effects caused by EB-irradiation during SEM image acquisition. In this paper, the effects of new charge control methods (Special Scan and Faster Scan), which are implemented in the latest Hitachi CD-SEM (CG6300), were examined with EUV resist hole-patterns. It was confirmed that Special Scan showed a profound effect on the suppression of the charge-induced errors. We also demonstrated the effects of the Special Scan for CD measurements and Contour Extraction for the EPE characterization of block on SAQP (SAQP lines + EUV block) pattern at imec iN7platform. Consequently, Special Scan is expected to be the solution for the accurate EPE measurements by CD-SEM.