A Design-of-Experiments Approach
light on this situation to realize that
the shortest
dwell time applied, 0.2 µs, is close to the bandwidth limit of the blanker and scan electronics of the HIM at TNO. Tus, it is not assured that the beam is fully settled into position when
jumping
Figure 6: High-magnification image of
two lines deposited at 16-nm pitch.
A line width of 13-nm is obtained, and even at this small spacing a 6-nm gap is maintained between the lines. The superimposed line scan shows that the lines are asymmetric, possibly due to precursor gas depletion.
from one line to the next. Tere is also quite an uncharted parameter space between this lowest dwell time and the next one applied, which was 10 µs. Nonlinearity in response might not be captured in this case. It is thought by the authors that a denser sampling of this factor might be in order. Te other two items, which are related, have to do with the impact of pitch. Te formula for line width does not contain a factor for a nearest-neighbor distance, but Figure 3 clearly indicates that it does matter. Tus, the fit
could surely be improved if this were accounted for. Te data show a relative insensitivity to this above about 24-nm pitch, so it may show as a non-linear factor, which is of consequence only at the smaller pitch. Related to this is the smaller than expected gap shrinkage. Narrower lines are obtained with small step size and long dwell times—both are factors that deplete more thoroughly the incoming precursor gas feed. Tus, it should not be surprising that when patterning two lines very close to one another that there is gas depletion in the space between the two lines. In fact the best results are obtained under such a condition, as Figure 6 illustrates. From this we measure a line width of just 13 nm and a gap of only 6 nm. It can be observed from the line scan insert in Figure 6 that the deposited material is “pushed” toward the outside of the pairs—at least according to the image gray level. Such a condition will require a more complex model.
Conclusion Te DOE methodology provides the nanoarchitect
with a tool to work through the many factors involved in characterizing beam-induced deposition, and it could be extended to other fabrication tasks. Te helium ion microscope shows a strong ability in the present case to produce fine features very reproducibly. In fact the main limitation at this point lies with the analysis of the results: line
26
www.microscopy-today.com • 2011 May
profile measurements by AFM would be a logical next step to get the most accurate measurements. Even then, what if the aspect ratio of the gap becomes large? Tere are many other factors that confront us, and each will have to be considered as we explore other deposition properties, such as resistivity and compositional purity. A DOE approach assists in the systematic identification of significant factors, which in turn provides valuable hints for improving the instrumentation, recipes, and metrology involved. Making accurate measurements at these length scales will provide interesting challenges for some time to come.
Acknowledgments Te authors would like to acknowledge the many
discussions and great technical assistance provided by Mr. Lewis Stern of Carl Zeiss NTS, LLC, during this work.
References [1] I Utke et al., J Vac Sci Technol B 26(4) (2008) 1197. [2] J Morgan et al., Microscopy Today 14(4) (2006) 24–31. [3] L Scipioni et al., J Vac Sci Technol B 28(6) (2010) C6P18.
[4] PFA Alkemade et al., Microsc Anal 24(7) (2010) 5. [5] D Smith et al., Nanotechnology 21 (2010) 175302. [6] D Montgomery, Design and Analysis of Experiments, Wiley, New York, 1991.
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