This paper presents the first known image reconstruction of complex probe tip shapes and the removal of those shapes from reentrant topologies that are encountered in critical dimension (CD) measurements. Algorithm improvements are described that enable reentrant image reconstruction. In addition, a solution is presented which eliminates image artifacts that result from noise when using Legendre transform or “slope-matching” reconstruction techniques. The new methods are compared to existing technology and demonstrated on reentrant features. Although demonstrated with two-dimensional profiles, the methods are readily extendable to three-dimensional morphologies. Tip wear effects on CD measurement are investigated and compared with the previous state-of-the-art method (“tip width subtraction”) and fully reconstructed images, clearly showing superior measurement stability for the image reconstruction method. Finally, CD measurements derived from reconstructed images are compared directly with Hitachi S4000 and S5000 cross-section SEM (X-SEM) data. The results show a close match between the SEM and CD AFM data.

Tip ͓in this article, tip refers to the apex region of an atomic force microscopy ͑AFM͒ probe. A probe consists of substrate, cantilever, and tip.͔ wear is a phenomenon that can reduce the accuracy and reliability of AFM. As both tip size and specimen approach nanometer scale, tip shape change due to wear becomes critical to topographical measurements such as critical dimensions and deep trenches. This article presents probe designs with specific wear-resistant features. Three categories of probe modification were selected to lessen wear, thereby improving lifetime and performance. These are probe material, surface coatings, and selective shape.

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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...

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