Mayuka Osaki

Accurate measurement of very small line patterns in critical dimension scanning electron microscopy using model-based library matching technique

Our purpose is to reduce the critical dimension (CD) bias for very small patterns with line widths of <15 nm. The model-based library (MBL) method, which estimates the dimensions and shape of a target pattern by comparing a measured scanning electron microscopy image waveform with a library of simulated waveforms, was modified in two ways. The first modification was the introduction of line-width variation into the library to overcome problems caused by significant changes in waveform due to changes in both sidewall shape and line width. The second modification was the fixation of MBL tool parameters to overcome problems caused by the reduction in pattern shape information due to merging of right and left white bands. We verified the effectiveness of the modified MBL method by applying it to actual silicon patterns with line widths in the range 10-30 nm. The CD bias measured by MBL method for three heights (20, 50, and 80%) was consistent with the atomic force microscopy results. The CD biases at all heights were <0.5 nm, and the slopes of the CD biases with respect to the CD were <3%. C 2011 Society of

CD bias reduction in CD-SEM of very small line patterns: sidewall shape measurement using model-based library matching method

The purpose of this study is to reduce the critical-dimension (CD) bias (i.e., the difference between actual and measured CD values) for very small line patterns with line widths smaller than 15 nm. The model-based library (MBL) matching technique, which estimates the dimensions and shape of a target pattern by comparing a measured SEM image waveform with a library of simulated waveforms, was modified in two ways to enable it to accurately measure very small patterns. The first modification was the introduction of line-width variation into the library to overcome problems caused by significant changes in waveform due to changes in both sidewall shape and line width. This modification improved the measurement accuracy. The second modification was the fixation of MBL tool parameters that relate to signal-intensity conversion to overcome problems caused by the reduction in pattern shape information due to merging of right and left white bands. This modification reduced the solution space and improved the measurement stability. We confirmed the effectiveness of the modification by using simulated images. We then verified the effectiveness of the modified MBL matching by applying it to actual SEM images. Silicon line patterns with line widths in the range 10-30 nm were used in this experiment, and the CD bias was evaluated by one-to-one comparison with atomic force microscopy (AFM) measurements. The CD bias measured by MBL matching for three heights (20, 50, and 80%) was consistent with the AFM results. The CD biases at all heights were smaller than 0.5 nm and the slopes of the CD biases with respect to the CD were smaller than 3%.

Application of model-based library approach to photoresist pattern shape measurement in advanced lithography

The model-based library (MBL) matching technique was applied to measurements of photoresist patterns exposed with a leading-edge ArF immersion lithography tool. This technique estimates the dimensions and shape of a target pattern by comparing a measured SEM image profile to a library of simulated line scans. In this study, a double trapezoid model was introduced into MBL library, which was suitable for precise approximation of a photoresist profile. To evaluate variously-shaped patterns, focus-exposure matrix wafers were exposed under three-illuminations. The geometric parameters such as bottom critical dimension (CD), top and bottom sidewall angles were estimated by MBL matching. Lithography simulation results were employed as a reference data in this evaluation. As a result, the trends of the estimated sidewall angles are consistent with the litho-simulation results. MBL bottom CD and threshold method 50% CD are also in a very good agreement. MBL detected wide-SWA variation in a focus series which were determined as in a process window by CD values. The trend of SWA variation, which is potentiality to undergo CD shift at later-etch step, agreed with litho-simulation results. These results suggest that MBL approach can achieve the efficient measurements for process development and control in advanced lithography.

CD-bias reduction in CD-SEM line-width measurement for the 32-nm node and beyond using the model-based library method

The measurement accuracy of critical-dimension scanning electron microscopy (CD-SEM) at feature sizes of 10 nm and below is investigated and methods for improving accuracy and reducing CD bias (the difference between true and measured CD values) are proposed. Simulations indicate that CD bias varies with feature size (CD) when the electron scatter range exceeds the CD. As the change in the CD-SEM waveform with decreasing CD is non-uniform, the CD bias in the results is strongly dependent on the algorithm employed to process the CD-SEM data. Use of the threshold method with a threshold level equal to 50% (Th = 50%) is shown to be effective for suppressing the dependence of CD bias on CD. Through comparison of experimental CD-SEM measurements of silicon line patterns (7-40 nm) with atomic force microscopy (AFM) measurements, it is confirmed that the threshold method (Th = 50%) is a effective as predicted, affording a largely invariant CD bias. The model-based library (MBL) method, which is theoretically capable of eliminating CD bias, is demonstrated to reduce the CD bias to near-zero levels. These experiments demonstrate the feasibility of next-generation CD-SEM for the measurement of feature sizes of the order of 10 nm and smaller.

Advanced CD-SEM metrology to improve total process control performance for hyper-NA lithography

In this research, we improved litho process monitor performance with CD-SEM for hyper-NA lithography. First, by comparing litho and etch process windows, it was confirmed that litho process monitor performance is insufficient just by CD measurement because of litho-etch CD bias variation. Then we investigated the impact of the changing resist profile on litho-etch CD bias variation by cross-sectional observation. As a result, it was determined that resist loss and footing variation cause litho-etch CD bias variation. Then, we proposed a measurement method to detect the resist loss variation from top-down SEM image. Proposed resist loss measurement method had good linearity to detect resist loss variation. At the end, threshold of resist loss index for litho process monitor was determined as to detect litho-etch CD bias variation. Then we confirmed that with the proposed resist loss measurement method, the litho process monitor performance was improved by detection of litho-etch CD bias variation in the same throughput as CD measurement.

High Precision CD Matching Monitoring Technology using Profile Gradient Method for the 32 nm Technology Generation

Measurement uncertainty requirement 0.37 nm has been set for the Critical Dimension (CD) metrology tool in 32 nm technology generation, according to the ITRS[1]. The continual development in the fundamental performance of Critical Dimension Scanning Electron Microscope (CD-SEM) is essential, as in the past, and for this generation, a highly precise tool management technology that monitors and corrects the tool-to-tool CD matching will also be indispensable. The potential factor that strongly influences tool-to-tool matching is the slight difference in the electron beam resolution, and its determination by visual confirmation is not possible from the SEM images. Thus, a method for quantitative evaluation of the resolution variation was investigated and Profile Gradient (PG) method was developed. In its development, considerations were given to its sensitivity against CD variation and its data sampling efficiency to achieve a sufficient precision, speed and practicality for a monitoring function that would be applicable to mass semiconductor production line. The evaluation of image sharpness difference was confirmed using this method. Furthermore, regarding the CD matching management requirements, this method has high sensitivity against CD variation and is anticipated as a realistic monitoring method that is more practical than monitoring the actual CD variation in mass semiconductor production line.

Performance verification of resist loss measurement method using top-view CD-SEM images for hyper-NA lithography

In this study, the principle of the resist loss measurement method proposed in our previous paper[1] was verified. The technique proposes the detection of resist loss variation using the pattern top roughness (PTR) index determined by scanning electron microscope images. By measuring resist loss with atomic force microscope, we confirmed that the PTR showed a good correlation with the resist loss and was capable of detecting variations within an accuracy of 20 nm for the evaluated sample. Furthermore, the effect of PTR monitoring on line width control was evaluated by comparing the error in line width control after eliminating undesirable resist loss patterns to that of conventional line width monitoring. The error of line width control was defined as the deviation range in post-etch line widths from post-litho values. Using PTR monitoring, the error in line width control decreased from 10 nm to less than 3 nm, thus confirming the effectiveness of this method.

Further study on the verification of CD-SEM based monitoring for hyper NA lithography

In our previous paper*[1], next generation lithography offering improved resolution by use of Hyper-NA and Low-k1, changes in exposure tool focus were seen to influence pattern shape and it was verified that pattern profile variation occurs even when measured CD values are similar. This shows the necessity for process control to include pattern shape information, conventional methods using the CD value alone will be insufficient as process latitudes continue to shrink. In such a situation, to be able to precisely measure the physical dimensions of design features becomes more and more important. In this study, we have investigated improved precision of Process Window (PW) determination by using the MPPC function that allows the pattern profile shape to be quantified. We have also evaluated pattern shape variation by means of Litho-simulation. As a result, it was confirmed that resist loss is the main change in shape that occurs. Therefore, we have focused our attention on resist loss and optimized the MPPC parameters by SEM simulation*[2]. As a consequence, it was possible to precisely detect the resist loss. Using this technique, it was possible to show the possibility for highly precise 3D measurement determination, for use in exposure tool monitoring, by using the MPPC measurement technique.