As industry prepares to introduce extreme ultraviolet (EUV) technology for the coming sub-10-nm lithography, this paper presents metrology approaches that utilize the prevalent Critical Dimension Scanning Electron Microscope (CD-SEM). Two technical approaches will be discussed. One is comprehensive solutions for new EUV characterized features, such as low resist-shrinkage electron beam optics and high efficiency metrology/inspection for EUV process monitoring. The other, like conventional minimization processes, is down-to-ångström-order metrology methodologies required for stricter CD process control. This paper is the first to conceptualize specifications for a stringent and multi-index tool matching, namely “atomic matching,” which is considered as a crucially important feature of any in-line metrology tools in the EUV era.
In recent years, semiconductor makers have been developing 1Xnm HP (Half Pitch) node devices. It is a requirement for CD-SEM to improve measurement uncertainty of not only the basic short-term repeatability, but also of the long-term stability and tool-to-tool matching. A conventional method called “Common Site Method” has been used to evaluate the uncertainty of CD-SEM measurement. In this method, target patterns on an identical sample and identical measurement point, which is the common site, is measured repeatedly. Common Site Method has possible instability factors as follows; 1) Carry-over due to contamination and/or charging caused by repeated measurement on the same points 2) Dimensional variation caused by the use of non-identical patterns between the groups These issues cannot be solved by averaging the results from many measurement points. In this paper we introduce a new method: “Fresh Site Method” to check the long-term stability and tool-to-tool matching with high precision. In this method, a target on arbitrary point extracted randomly from a large area of dense pattern is measured on multiple dies. Precision of the measurement is improved by increasing the number of dies for the measurement because influence of the CD non-uniformity among the dies is reduced by the averaging effect of the measurement points. In addition, “Macro-Area Measurement” method can reduce both the measurement uncertainty, caused by sample variation, and measurement time. As a result, the precision of CD-SEM monitoring using Fresh Site Method with Macro-Area Measurement produce sufficient results for the evaluation of CD-SEM measurement uncertainty, realizing a reproducibility performance below 0.1nm.
An image processing technique for estimating the incidence angle of an electron beam (beam-tilt angle) of a critical dimension scanning electron microscope (CD-SEM) has been developed. The estimation and correction of the error of the beam-tilt angle are indispensable for high precision measurement of CD and/or three-dimensional profiles of semi-conductor device patterns. In this technique, a pyramidal-shaped crystal sample made by anisotropic etching is used for calibration. From the top-down and tilted views of the sample, x and y directional beam-tilt angles relative to the top-down view are estimated simultaneously, with the geometrical variations of the pyramid ridge lines detected by image processing. Exact positioning of the sample is not required because the inclination and rotation of the sample towards the wafer surface are estimated separately from the beam-tilt angles.
Evaluation of 40 sample images, including 4 directional tilt angles, indicated that deviations of the estimated x and y beam-tilt angles were 0.13 and 0.12 degree respectively (3 sigma). It will also be shown that the technique is robust against characteristic SEM image distortion and low S/N.
This technique has achieved high precision and quantitative estimation for the beam-tilt angles, and will provide a method for high precision measurement of CD and three-dimensional profile for semi-conductor process monitoring and control in the future.