Two samples were prepared and submitted for AFM analysis. They are lithography-patterned Al Josephson junctions on silicon and an atomically flat rubrene crystal surface. A Nano-R2™ AFM system was used in this measurement and analysis. This work is to show the capability of Pacific Nanotechnology’s Nano-R2™ AFM to image and analyze nanosized structures. Example results are shown and discussed below.
All sample measurement was carried out in air using the Close-Contact (or “Tapping”) Mode of the Nano-R2™ AFM equipped with a
light lever scanner. The Close-Contact Mode was found the most suitable for these samples. Commercially available silicon probes with a
normalized radius of < 10 nm were used in all data acquisition.
The Nano-R2™ system was calibrated in the X, Y, and Z axes before measurement. We utilized the standard samples to calibrate Zheight
channel, and resulting system’ accuracy was ± 1 %. The resonant frequency of used probes was tested to comply with the
manufacturers’ datasheet. Data were processed using the NanoRule+™ software. Different scanning rates, resolutions and angles and
GPID of the feedback system were used to achieve optimal results. Grain analysis of Al film was done using the Grain Analysis function
of the NanoRule+™.
There are two Josephson junction arrays on the 7 × 7 mm2 silicon substrate. The junction array #1 is shown in Figure 1, array #2 in
Figure 4. Figure 1A is a 24 × 24 μm scan for an overview of the junction array. The arrow indicates the ~ 700 nm wide top layer of Al
deposits where grain analysis was done (see Figure 6). Figure 1B and C show magnified views of the junctions for details of dimension
analysis. Measurement results of Al wires are discussed below.
Figure 1: The Josephson Junction Array #1. (A) The 24 × 24 μm; (B) and (C) 5 × 5 μm Close-looks about the junction. The arrow indicates the ~ 700 nm wide top layer of Al deposits where the grain structure was analyzed (see Figure 6).
Figure 2: Dimesion Analysis for Al linewidth and height (junction array #1). The pair 1 (red) shows
that the top layer film of the horizontal wires is 25.33 nm higher than the bottom layer; pair 2 (blue)
represents the vertical Al wire as 25.48 nm high above the substrate; the pair 3 indicates the linewidth
of the second vertical wire is 0.118 μm.
The Line Analysis in the NanoRule+™ is capable of virtually any pixel-to-pixel dimension. Figure 2 shows an example dimension
measurement of the Al film for the junction in Figure 1B. We found that the horizontal Al wires have two layers (one layer on another).
The top layer looks brighter and rougher than the bottom layer. The pair 1 (red) in Figure 2 shows that the top layer film is 25.33 nm
higher than the bottom layer; pair 2 (blue) representing the vertical Al wire is 25.48 nm high above the substrate; the pair 3 indicates the
linewidth of the vertical wire is 0.118 μm. The measured results are shown in Figure 3A also. We measured linewidth for many different
lines (not shown) and found the results are comparable to the SEM measurement. Figure 3 shows clear 3D images for the two layers of
Al films. The highest feature in the structure (Figure 1B) is the intersection of two orthogonal Al lines, which is 91.753 nm high above
the substrate. In Figure 1B, the bottom layer of the horizontal lines is about 10 nm higher than the vertical lines, as indicated by pair 1-1
and pair 2-1 in Figure 2.
Figure 3: 3D views of the Josephson Junction Array #1.
Figure 4: The Josephson Junction Array #2. (A) The 18 ? 18 ?m; (B), (C) and (D) are close-looks for individual junctions. The Z-scale of (A), (B), (C) and (D)
are 270.83 nm, 248.11 nm, 117.12 nm and 172.42 nm respectively.
Grain Analysis
The regions of Al thin film were selected for grain analysis using the Grain Analysis function of the NanoRule+™. The first region is
the top of the ~50 μm wide wires connected to the outside pads. The topography and phase image are shown in Figure 5 (1 × 1 μm
scan). The second region is indicated by the arrow in Figure 1A. The detailed quantitative analysis for the second region was shown in
Figure 6 (600 × 600 nm scan).
Figure 5: (A) Topography of Grain Structures for the Al film (scanned on the one of ~50 μm wide wires
connected to the outside pads). (B) Phase image of the same structure.
Figure 6 is the grain analysis for the Al film indicated in Figure 1A. Figures 6A and 6B are 600 × 600 nm topography and corresponding
phase image. The Grain Analysis can count and analyze nanoscale grain structures, and it provides area, perimeter, volume, height,
Max_height, radius, length, width and aspect ratio for individual grains and the statistical average. Figure 6C shows the grain boundaries
between the total of 168 grains in Figure 6A. The average Max_height and radius were 7.29 nm with 1.45 nm st. and 26 nm with 9.4
nm st. (data not shown due to the report length). The Max_height ranged from 2.88 nm to 11.26 nm. Figure 6E shows the Max_height
distribution of these 168 individual grains, and the mode is about 7 nm.
AFM is very accurate in Z-height, however, tip convolution effect limits its lateral resolution. Phase image is extremely sensitive to resolve
grain boundaries for the present sample (Figure 6B). The particle in Figure 6B is much smaller than Figure 6A. Figure 6D shows the
grain boundaries between the total of 946 grains in Figure 6B (phase image), which means lateral resolution of the phase image is
better than the topography image for this sample. Most boundaries are not visible in Figure 6B. The average radius is about 10 nm
with 4.9 nm standard deviation. Figure 6F shows the measured Radius distribution of the 946 grains in the phase image (mode is ~
8.5 nm).
