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Semiconductor technology is advancing below 50 nm critical dimensions bringing unprecedented challenges to process
engineering, control and metrology. Traditionally, interconnect metrology is put behind high-priority gate metrology;
however, considering metrology, process and yield control challenges this decision is not always justified. Optical
scatterometry is working its way to interconnect manufacturing process control, but scanning electron microscopy
(SEM) remains the number one critical dimension (CD) metrology for interconnect process engineering and optical
proximity correction (OPC) modeling. Recently, several publications have described secondary electron (SE) trapping
within narrow high-aspect ratio interconnect structures. In these papers, pre-dosing of the sample helped to extract SE
from the bottom of the hole and measure its diameter. Based on current understanding of the phenomenon, one should
expect that high-aspect ratio interconnect structures (holes and trenches) with critical dimensions below 100 nm may
show signs of SE trapping of various degree. As a result, there may be an uncontrolled effect on SE waveform and,
therefore, bias of CDSEM measurement. CD atomic force microscopy (AFM) was employed in this work as a reference
metrology for evaluation of uncertainty of trench and hole measurements by CDSEM. As the data indicates, CDSEM
bias shows a strong dependence on pitch of periodic interconnect structure starting from drawn CD of 50 nm. CDSEM
bias variation for the evaluated set of samples is about 19 nm. A typical OPC sample consists of both photoresist and
etched interlayer materials. As the AFM data for photoresist material indicates, the hole diameter changes quite
significantly with depth and the hole profile varies from one OPC structure to another. Abe et al. [1] have used a clever
way to correlate physical bottom diameter of holes with CDSEM measurements and demonstrated that for their process
and dimensions the SEM "top" diameter and physical bottom diameter correlate well. Unfortunately, this conclusion
can't be generalized and measurement uncertainty of CDSEM must be evaluated on an individual technology/process
basis. A more general approach to improve CDSEM accuracy is necessary which is based upon SI-traceable CDSEM
bias measurements, modeling and correction.
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