Silicon nanowire atomic force microscopy probes for high aspect ratio geometries
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(a) A nanowire AFM tip as grown (scale bar: 10 μm). The circular ring around the wire is caused by Au migration onto the bulk surface before the wire is fully catalyzed. This Au changes the morphology of the CVD growth. Inset (scale bar: 1 μm): detail of the nanowire tip as grown showing the faceted single crystal wire and nearly hemispherical Au catalyst tip. (b) An example of a nanowire AFM tip thinned via silicon oxidation (scale bar: 10 μm). The aspect ratio of the tip is 90:1. Inset right (scale bar: 1 μm): detail of the wire after thinning, the slight taper in the wire is carried over from the taper during wire synthesis. Inset top (scale bar: 100 μm): shows the full cantilever AFM probe, the cantilever is 5 μm thick, 125 μm long, and 30 μm wide.
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Single nanowire AFM tip imaging a 900 nm wide, 1.95 μm deep Cl etched trench during repeated thinning of the tip. The long axis of the cantilever was orthogonal to the long axis of the trenches for the scans. The angle on the negative X-axis is 13° and is caused by the mounting angle of the AFM probe. The depth traces plotted are the mean of column values in 2D images shown. The insets show the nanowire probes at the time of scanning (scale: 10 μm). The minimum wire diameters are (a) 464, (b) 160, and (c) 40 nm. The thinning was performed via oxidation in a dry oxygen atmosphere followed by removal of the oxide in a vapor HF chamber. The as grown diameter of the wire tip (a) when combined with the 13° tilt only just allows the wire to reach the bottom of the trench structure. Once the tip is thinned (b) the bottom of the trench is clearly imaged. Thinning the tip further (c) introduces artifacts in the image which cannot be eliminated by tuning the feedback settings of the Bruker Icon AFM used. These artifacts effectively reduce both lateral and vertical resolution.
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(a) The performance of the nanowire tip is compared to a conventional TESP tip, tilt-corrected FIB milled tip, and an SEM cross section of the same 1.95 μm deep, 900 nm wide trench. The symbols are plotted every 40th point for the nanowire and TESP AFM data and every 20th point for the SEM data and FIB milled AFM data. The nanowire tip significantly outperforms the TESP tip and agrees very well with the SEM data. The nanowire tip is comparable to the FIB milled AFM tip. The 90° image is scanned with the short axis of the cantilever orthogonal to the trench’s long axis and the 0° image is scanned with the long axis of the cantilever orthogonal to the trench’s long axis. The depth traces plotted are averaged by the same procedure as described for Fig. 2. The encroachment seen at the left and right side of the 0° scan from the SEM data indicate the tip diameter at the time of imaging. The nanowire diameter at the time of imaging in (a) was 244 nm; (c) the diameter was thinned to 84 nm. The commercial FIB milled tip had an 800 nm long chisel shaped tip terminated in a nominally 25 nm point attached to a nominally 5 μm long 200–500 nm wide shaft. This finer point allowed the FIB milled tip to probe channel on the negative X-axis but not the positive X-axis as the tip chisel shape is asymmetrical. On the positive X-axis the shaft diameter causes encroachment similar to the nanowire probe. The SEM data shown (a) is calculated from edge of the cross section shown in (b). (c) The probe as used during the scans shown in (a). It is possible to reach the bottom of a 2.05 μm deep, 300 nm wide trench by thinning the nanowire (d), although it does contain ringing artifacts that cannot be suppressed with the feedback settings on the Bruker Icon AFM used. The trenches being imaged in (d) are shown in (e). The probe at the time of imaging (d) is shown in (f). The scale bars for (b), (c), (e), and (f) are 2, 5, 1, and 5 μm, respectively.
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