The present paper is a continuation of an investigation to validate CD AFM image reconstruction using Transmission
Electron Microscopy (TEM) as the Reference Metrology System (RMS). In the present work, the validation of CD
AFM with TEM is extended to include a 26 nm diameter carbon nanotube (CNT) tip for non-reentrant feature scans.
The use of DT (deep trench) mode and a CNT tip provides detailed bottom feature resolution and close mid-CD
agreement with both TEM and prior CD mode AFM scans (using a high resolution Trident tip). Averaging AFM scan
lines within the ~80 nm thickness region of the TEM sample is found to reduce systematic error with the RMS.
Similarly, errors in alignment between AFM scan lines and TEM sample are corrected by a moving average method.
Next, the NanoCD standard is used for complete 2D tip shape reconstruction (non-reentrant) utilizing its traceable
feature width and well-defined upper-corner radius. The shape of the NanoCD is morphologically removed from the
tip/standard image, thus providing the tip's shape with bounded dimensional uncertainty. Finally, an update of the
measurement uncertainty budget for the current generation CD AFM is also presented, thus extending the prior work by
NIST.
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.
The pitch of a Hitachi Standard Micro Scale was measured using NanoCal and a LEO 1560 SEM. The pitch pedigree and certification were intentionally withheld from Nanometrology team members to enable independent measurement and certification of an unknown Hitachi Micro Scale standard during this work. NanoCal allows one to achieve pitch measurements with sub-nanometer accuracy and precision as well as to perform SEM magnification calibration with the precision and accuracy required for sub 90 nm SEM metrology.
KEYWORDS: Calibration, Scanning electron microscopy, Metrology, Critical dimension metrology, Precision calibration, System integration, Logic, Roads, Material characterization, Data analysis
Precision can no longer be disassociated from accuracy. In fact, the two parameters must go hand in hand in the current 100 nm and below metrology environment. Therefore stating that "precision is more important than accuracy" does not capture the criticality of measurements of logic and memory chips with critical dimensions (CD) of 100 nm and below. The ITRS roadmap continues to set attainable milestones, calling for CD measurements with 3 σ to fall within a 0.9 nm error budget for the 100 nm mode, translating to ±0.9%. This implies tool calibration precision of 0.1% for 3σ. The methods in the industry today avoid both precision and accuracy and therefore cannot achieve the ITRS requirements. The resultant low precision and no accuracy in the SEM measurements lead to 'matching' and 'offset tables' that waste time, lose efficiency and potentially produce lower yield. Nanometrology's integrated system, NanoCal, a software/hardware product offers the first opportunity to combine both precision and accuracy in SEM magnification calibration. Using NanoCal in this work, we report that out of fab SEMs can be calibrated to 0.1% precision and 1 nm accuracy.
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