Device miniaturization and complication is now stimulating the strong needs for the three dimensional (3D) geometry measurement of the structure. Now in the single-nanometer nodes, the CDs need to be known at multiple pattern heights especially in after-etch inspection. This means SEM measurements is expected to provide 3D contours. Unfortunately, CD-SEM is basically top-down imaging tool, therefore the acquired micrograph is the 2D projection of the device structure. Meanwhile, optical CD (OCD) works well to deduce the 3D geometrical parameters. Due to the spot size limitation, however, it is hard to obtain the local variation of the structure, or hard to access in-die metrology data. While TEM or slice and view by FIB-SEM is a reliable option to see the local 3D structure, they are destructive and low in statistics due to the long turn-around time. Therefore the challenge is, how to obtain in-die 3D geometrical information in non-destructive way.
The purpose of the present work is to extend CD-SEM capability to extract 3D geometry. The most intuitive approach is the direct imaging, where the structure can be extracted directly from the acquired image. The direct imaging includes, beam tilt SEM and topography detector. While the beam tilt is an intuitive method, at larger tilt over 10 degrees the image blur is unavoidable and the precise metrology application is difficult unless the small tilt angle is enough to see the side wall structure like high-aspect-ratio (HAR) structure over several microns in depth. Topography detector is another well-known approach, and works well for the isolated target like particles or pattern defects. In the smallest spacing, however, characteristic shadow is not created, and the topography extraction is basically difficult. By these reasons, we especially have a strong interest in indirect methods described below.
Indirect approach includes model-based geometry matching and hybrid metrology with other reference measurements. In these approaches, it is required to find the characteristic SEM waveform or signal intensity (gray-level) which are sensitive to the target geometry. The sensitivity of the response can be estimated by the simulation models, or the comparison with other 3D measurements tools like OCD or AFM. The concept is the generalization of the model-based library (MBL), where the SEM waveform is compared with pre-calculated library of the waveforms. In our approach, calibration could be done by using simulation data or real 3D profiling data. Another generalization is that a SEM operation condition is also altered to enhance the sensitivity of all the geometrical parameter. For example, by sweeping the beam voltage, the multiple waveforms are obtained and the low voltage data has a sensitivity in the pattern top geometry, and the higher voltage data in the pattern bottom. Similar parameter sweep can be applied to energy-filter voltages or focus heights. It is expected that this sort of the generalization enhances the correctness of the geometry profiles.
In the conference presentation, we will compare these 3D measurement approaches and discuss how to realize the reliable in-die 3D measurement in the coming advanced technology nodes.