AFM Metrology for Polymer-Coated Wires
Introduction
Two new samples of polymer-coated wires were prepared and submitted for atomic force microscope (AFM) analysis. They are sample
#112 with smooth surface and sample #163 with rough surface. A Pacific Nanotechnology Nano-R2™ AFM system was used to scan
the samples in both air and liquid. Each sample was scanned on multiple localities. This work is to show the capability of the Nano-R2™
in topographic imaging and quantitative analysis. Examples results are shown and discussed below.
Experiment
All sample measurements were carried out using the Close-Contact Mode (also known as “Tapping”), which was found most suitable
for the present samples. Silicon close-contact probes with a spring constant of ~40 N/m and resonance frequency of ~300 KHz were
used in air scans. Softer silicon nitride probes with a spring constant of 0.32 N/m and resonance frequency of ~50 KHz were applied
to scan samples in liquid phase.
The Nano-R2™ system was calibrated in the X, Y, and Z axes before measurement. We utilized the standard samples to calibrate
Z-height 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. Scanning parameters and GPID of the feedback
system were adjusted to achieve optimal results.
Results and Discussion
Sample #112
We scanned many different localities of the sample, and found that the coating surface was smooth and uniform. Figure 1A is a typical
topographic image (30 × 30 μm) acquired by the Close-Contact Mode. Figure 1B and 1C show magnified views of the same locality.
The measured roughness of the structure (Figure 1B) is 5.91 nm in RMS. The sample morphology seems to depend on different spots.
Figure 3 shows the collected images.
Figure 1: #1 Hard Disk (A) Topography; (B) MFM image acquired by raising the
magnetic tip ~ 80 nm above
the surface.
Figure 2: Variable topography of the sample #112 in air. (A) and (B) are collected in two different localities,
which are also different from where the Figure 1 was collected.
To analyze the topography of the sample in a wet environment, these two sample wires were immersed in warm water during scanning.
The Environmental Cell (EC) and MicroCell options for the Nano-R™ AFM are designed to allow users to study samples in liquid and
gas phases. In the Close-Contact mode, because the damping of the cantilever induced by surrounding media would decrease the
mechanic Q-factor of vibrating probes, the soft triangular silicon nitride probe was used to obtain optimal results. Those commercially
available nitride probes have resonance frequency of ~50 KHz, tip radius of 60 nm, and Au back coating layer. The frequency sweep
was performed by putting probes very close to the samples (a few of hundreds microns) to avoid variable peaks during sweeping. Due
to damped Q and greater tip radius, images scanned in liquid are generally less crisp than in air. Silicon probes used in air scan have a
Q factor of ~600 and radius of < 10 nm.
Figure 3 shows liquid scan of the sample #112. No apparent
morphology change or deformation was observed in the present
samples. However, the peak-to-peak height and surface roughness
increase slightly in general. The roughness of the sample #112,
for example, increases from 5.91 nm to 11.33 nm (Figure 3). The
imaging scan for the sample #163 in liquid was similar to the sample
#112. The observed data changes in the acquired images are listed
in Table 1.
Figure 4 shows three topography images of the sample #163 in
air.
This sample is much less uniform than the sample #112. The
peak-to-peak height in Figure 4A (20 × 20 μm) is about 1.79 μm,
comparing to about 293 nm in Figure 1A (30 × 30 μm). This results
from polymer aggregation without uniformity. One can see some
where the surface is smooth, but others are rough. In some other
localities, the peak-to-peak height is more than 5-6 μm (not shown).
The peak-to-peak height is as large as 2.3 μm in Figure 4C. |
Figure 3: Liquid scan (5 × 5 μm) of the sample #112.
The wire was immersed into warm water during
scanning. There is no apparent morphology change
after soaking (see Figure 1B-C for dry images),
although the peak-to-peak height increases from
55 nm (Figure 1B) to 102 nm and the roughness
increases from 5.91 nm (Figure 1B) to 11.33 nm.
|
Figure 4: Topography of the sample #163 in air. (A) and (B) were collected in same locality. (C) was collected
from a different locality. This sample is much
rougher and less uniform than the sample #112 in terms of
peak-to-peak height and roughness.
Figure 5: Liquid scan (5 × 5 μm) of the sample #163. The wire was immersed into warm water during
scanning. (A) and (B) were from different localities.
and pushed the tip upward during scanning because of magnetic coupling between tip and the sample;
this results in the bit appearing higher than the surface. The pair 3 represents the bit-to-bit spacing of 362 nm. Another line analysis
shows a bit is about 1.45 μm long (not shown). The measurement method was similar to Figure 4.
In order to verify the degaussering process, a PNI reference hard disk (4GB) full of data was analyzed by MFM. Figure 5 shows a typical
MFM image. Similar to the #1 hard disk, the regular bit tracks were found uniformly on the disk surface.
Conclusion
| Table 1: Comparison of dry and wet samples |
| |
Peak-to-Peak Height (nm) |
Roughness (nm) |
Reference Figure |
| #112 (dry) |
54.89 |
5.91 |
Figure 1B |
| #112 (in water) |
101.79 |
11.33 |
Figure 3 |
| #163 (dry) |
241.04 |
28.17 |
Figure 4B |
| #163 (in water) |
372.99 |
44.45 |
Figure 5A |
This measurement shows example results of scanned topographies for two polymer-coated wires. The sample #112 is smoother
than the sample #163 in terms of peak-to-peak height and roughness (see Table). It is clear that an AFM can provide very detailed
quantitative information on wire surface. EC or MicroCell provides easy operation in liquid scan.
Pacific Nanotechnology’s Nano-R2™ AFM is shown fully capable for analysis and study of coating microstructures both in air and
liquid.
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