Hitoshi Sugahara

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.

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.

On-demand inspection recipe to detect defects of interest using Mahalanobis distance

An on-demand inspection recipe-setup method to detect defects of interest (DOI) was proposed. The method applies Maharanobis distance to recognize DOI-like defects without its own recipe. Moreover, actual application was evaluated and the method effectiveness was confirmed from viewpoints of on-demand processing time and DOI detection. The proposed method enables inspection tool managers to rapidly select an appropriate recipe which detects the most DOI from several initial recipes. The future research work will focus on several examinations for threshold decision based on reviewing all detected defects experimentally.

Focus measurement using SEM image analysis of circuit pattern

We have developed a new focus measurement method based on analyzing SEM images that can help to control a scanner. In advanced semiconductor fabrication, rigorous focus control of the scanner has been required because focus error causes a defect. Therefore, it is essential to ensure focus error are detected at wafer fabrication. In the past, the focus has been measured using test patterns made outside of the chip by optical metrology system. Thus, present focus metrology system can’t measure the focus of an arbitrary point in the chip. The new method enables a highly precise focus measurement of the arbitrary point of the chip based on a focus plane of a reference scanner. The method estimates the focus amount by analyzing side wall shapes of circuit patterns of SEM images. Side wall shapes are quantified using multisliced contours extracted from SEM-images high accuracy. By using this method, it is possible to measure the focus of the arbitrary circuit pattern area of the chip without a test pattern. We believe the method can contribute to control the scanner and to detect hot spots which appear by focus error. This new method and the evaluation results will be presented in detail in this paper.