KEYWORDS: Metrology, Atomic force microscopy, Line width roughness, Line edge roughness, Scanners, Critical dimension metrology, Actuators, Silicon, Profiling, 3D metrology
Critical dimension atomic force microscope (CD-AFM or 3D-AFM) is an important metrology technique for full three-dimensional measurements of linewidth CD and sidewall shape. Recent improvements in the 3D-AFM platform design, including high-precision/low-drift sample stages and high resolution optics, have been coupled with 'enhanced CD' (eCD) scan mode and novel AFM tip design. Especially, the eCD mode features a fast scanning actuator system (FA) and a bottom corner transitional rescan algorithm (TRS). The actuation system utilizes high gain feedback electronics and high bandwidth piezoelectric actuator to pull away a slender tip much faster from a small trench sidewall. The transitional rescan algorithm detects a rising sidewall before rescanning the transition for better bottom corner profiling. The paper presents evaluation data to show these enhancements resulted in improved measurement capability for small trenches required for shrinking device size, better sidewall profiling, more accurate bottom CD and LWR/LER measurement, faster scan speed, and less tip wear [1]. All the improvements ensure 3D-AFM continues to have the lowest measurement uncertainty among all other dimension metrology techniques.
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.
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.
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