An extensive test series was undertaken to validate image reconstruction algorithms used with critical dimension atomic force microscopy (CD AFM). Transmission electron microscopy (TEM) was used as the reference metrology system (RMS) with careful attention devoted to both calibration and fiducial marking of TEM sample extraction sites. Shape measurements for the CD probe tips used in the study were acquired both through the use of reentrant image reconstruction and independent (non-destructive) TEM micrographs of the probe tips. TEM images of the tips were acquired using a sample holder that provided the same projection of the tip as presented to the sample surface during AFM scanning. In order to provide meaningful validation of the CD AFM image reconstruction algorithm, widely varying sample morphologies and probe tip shapes were selected for the study. The results indicate a 1 -2 nm bias between the TEM and CD AFM that is within the uncertainty of the measurements given the Line Width Variation (LWV) of the samples and accuracy of the measurement systems. Moreover, each TEM sample consisted of a grid with multiple features (i.e., 21 to 22 features). High density CD AFM pre-screening of the sample allowed precise locating of the TEM extraction site by correlating multiple feature profile shapes. In this way, the LWV and height of the sample were used to match measurement location for the two independent metrology systems.Recent standards developed by VLSI and NIST, 1,2 in conjunction with on-going CD AFM development, 3 has enabled single nanometer uncertainties for critical dimension (CD) width measurements. 4 At present, this uncertainty is limited to features with uniform and near-vertical sidewalls. For this class of structure, subtracting the tip width from the acquired AFM feature lines provided an acceptable solution for semiconductor metrology during the "early days" of CD AFM. 5 However, semiconductor evolution results in diminishing feature size and uncertainty budgets, with concurrent increasing sample morphological complexity and tip-to-feature size ratio. Legacy 1-dimensional "tip width compensation" for correcting probe shape effects has been a progressively less-adequate solution for CD AFM metrology. The method retains residual tip shape artifacts in the AFM image, which in turn, lead to measurement bias. 6 To remove all vestiges of image "dilation" due to the tip shape (i.e., in regions contacted by the tip), the tip shape must be fully characterized and extracted from the image using an "erosion" process. 7 Removal of the entire tip shape contribution to image dilation is a powerful, general approach that can obviate the need to treat each tip shape contribution as a separate bias.A brief overview of reentrant capable image reconstruction can be seen in Figure 1. During the early 1990s, methodologies were developed for reconstructing conventional scanning probe images and tip shapes. 7-11 The methods were applicable for non-reentrant sample and tip morphologies (i.e., single-valued surfaces wh...
Improving device performance and yield is one of the primary goals of microelectronic research and development. In order to attain this goal, process engineers are focusing on the integration of new materials and the development of new device architectures. For production process control, the two main techniques to monitor device dimensions are CD-SEM and Scatterometry. Despite the excellent repeatability of these techniques, SEM and Scatterometry often suffer from unacceptably large measurement uncertainty, particularly when applied to newly developed device technologies. A consequence of these metrology limitations is a delay in the transition of new process technologies into production. Furthermore, these techniques have not been proven to be effective in measuring 3-dimensional characteristics such as Line Edge Roughness and Line Width Roughness in the Bottom-CD region.A potential alternative to SEM and Scatterometry in many applications is CD-AFM, a highly versatile metrology technique, which is capable of providing consistent, precise 3-dimensional measurements for a wide range of sample types and geometries.In this paper we present a recent CD-AFM scan algorithm enhancement that significantly improves Bottom-CD measurement bias and precision. In addition, we present a separate but complementary enhancement in the CD-AFM scan algorithm, which we have demonstrated to improve overall CD measurement resolution and precision, while increasing scan speed when using advanced CD-AFM Tips. Our results show that the use of these two techniques enhances Line-Edge Roughness and Line-Width Roughness resolution, precision and accuracy.
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