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CD-AFM can report CD measurements to several nanometer differences when different probing techniques including
probe types, scan methods, or data analyses are employed on the same sample despite using standard calibration
techniques. This potentially weakens the assertion that this instrument is inherently accurate. It is particularly important
to resolve these discrepancies given the measurement challenges where multiple probing techniques need to be
employed to get complete CD information. Probe type refers to tip construction methods that can significantly affect
geometrical aspects of probe such as effective length, width, and edge height as well as material composition and
coating. Scan code refers to CD or DT mode of tool operation. Analysis includes probe geometry deconvolution and
measurement algorithms. These challenges in measurement accuracy are especially significant for the foot or bottom
CD metrology of 3D structures. This paper explores the impact of these different probing techniques on the
measurement accuracy. In one series of experiments, measurements for different probing techniques are compared when
the test and the referencing structures are composed of similar material and possess smooth vertical profiles. The
investigation is then extended to explore the accuracy of bottom CD measurement of non vertical profiles encountered in
actual process development. A hybrid method using CD and DT modes has been tested to measure the bottom CD of
challenging pitch structures. The limited space for the probe is particularly problematic for CD mode but the accuracy
of DT mode for CD measurement is a concern. Other challenges will also be discussed along with possible solutions.
CD-AFM has increased uncertainty when it comes to measuring within 15 nm of the bottom of a structure. In this
regime details of the shape of the probe and the method by which this shape is extracted from the raw data become
important. Measured CDs can vary by a few nanometers depending upon the algorithm employed for data analysis.
These algorithms apply approximate methods for probe shape deconvolution from the raw data. Given all these sources
of variation in CD determination it important to understand their impact on the accuracy of measurement in order to
properly estimate uncertainty and drive improvement. Overall this paper provides a practical guideline in pursuit of
accurate CD metrology and scope for improvements for upcoming technology nodes.
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