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Introduction
In roughly 20 years the atomic force microscopes moved from an esoteric invention to a routinely used tool for
nanotechnology research and development. The AFM is credited with enabling many of the nanotechnology
advancements in science and engineering. The AFM has applications in almost all science and engineering disciplines
ranging from metrological measurements to visualizing nanostructures.
In the first AFM designs, derived from the scanning tunneling microscope, a light lever force sensor is used to measure
the force between a probe at the end of a cantilever and a sample's surface. In these initial designs, the sample was
scanned in the X, Y, and Z directions. Typically a piezoelectric tube was used for scanning the sample. Although fairly
simple in design, this type of microscope proved the usefulness of the AFM for research and engineering. The tube
scanner could be made relatively small and thus had a very high resonant frequency that facilitated fast scanning.
The greatest drawback of the sample scanning AFM using a piezo tube was the coupling that occurred between the X,
Y and Z axis. Although the sample scanning AFM was very helpful for visualizing nanostructures, it was very difficult
to make metrological measurements with an AFM having a tube as the primary scanner. A second drawback of sample
scanning atomic force microscopes is that they are useful for only small samples.
Atomic force microscopes that scanned the probe while the sample remained stationary were first introduced almost 20
years ago. Probe scanning microscopes had the primary advantage that they could scan any sized samples. However,
the structure of the tip scanning AFM is typically much larger than the sample scanning AFM. Thus, the resolution
of the tip scanning AFM is often lower than the sample scanning AFM. Presently, most AFMs that are used for
metrological applications scan the probe.
Design
Dual Scanner Technology (DST), innovated by Pacific Nanotechnology,
gives researchers the full range of scanning options because the
DST instrument incorporates both probe and sample scanning
capabilities. Often a research project requires both capabilities in the
same microscope. Figure 1 illustrates the design utilized in the Pacific
Nanotechnology Nano-DSTT AFM. The sample is supported on a
piezoelectric scanner, the rapid scanner. Above the sample is a probe
scanning, light lever AFM.
Figure 2 illustrates the electronic control system for an AFM with Dual
Scanner Technology. A control CPU is used to drive two independent
XYZ boards. Each XYZ board has all of the electronic functions
required to run an AFM including a PID control loop, phase/amplitude
detection circuit, and XY high voltage circuits. The XYZ boards then
drive the metrology and rapid scanners. Of course, specialized
software is essential for operating an AFM having DST. The Control
CPU must have a multi-threaded operating system such as LINUX so
that each of the scanners can be driven independently.
A piezoelectric driven flexure metrological scanner is used for the metrological applications. Its XY scan range is typically 90 microns but
can be as large as 360 microns. A range of 8 microns is available with the Z piezoelectric element. Calibration sensors in the X, Y and
Z axes facilitate accurate metrological measurements and positioning of the AFM probe. The flexure scanner advantage over traditional
piezoelectric tube scanners is that they have a minimal amount of bow in the Z axis. Additionally the flexure scanner has a minimal
amount of coupling between the X, Y, and Z axis.
The rapid scanner has an XY range of 2 microns and a Z dynamic range of about 0.5 microns. Because the scanner has a limited
dynamic range, it has a very high resolution in the XY axis and also a very low vertical noise floor. Additionally, the rapid scanner has a
very high resonant frequency and can be scanned in the XY axis at rates as high 250 Hz. The Z feedback for the AFM can be derived
from either the metrology or the rapid scanner.
Performance
A benchmark sample for showing the performance of an AFM sample is cleaved Mica. Viewing
atomic corrugations of such a sample is only possible if the noise floor of the system is
substantially below 0.08 nm and the horizontal resolution is substantially below 0.015 nm.
Figure 3 illustrates an image of Mica acquired with the rapid scanner. The corrugations are
clearly visible. This image was taken with both the metrological and the rapid scanner in the
microscope. Although it is sometimes possible, it is very difficult to measure images of mica
atomic corrugations with an AFM having a scan range in the XY axis that is greater than 50
microns. This is because of the dynamic range of the scanner.
There are several specifications that are critical for a high performance metrological AFM scanner
including bow, linearity, and cross talk. There are samples available for demonstrating each
of these specifications. For example, VLSI standard test patterns are used for demonstrating
linearity and calibration in the XY and Z axis. Scanning a flat piece of silicon will demonstrate
the residual bow in a scanner.
One of the most challenging samples to measure with an AFM metrological
scanner is a micro-fabricated "triangle" sample. The sample is comprised of a
series of parallel triangular patterns. The "triangle" sample will clearly illustrate
the residual coupling between the XZ and YZ axis. Figure 4 is the line profile of
a "triangle" sample measured with the flexure Nano-DSTT scanner.
The DST architecture further facilitates dynamic experiments. Such experiments
take advantage of the relatively high speed scanning capabilities of the
tube scanner in combination with the wide scanning range of the metrology
scanner. Such measurements are made by scanning the rapid scanner at a
rate of one frame per second and then using the metrology scanner to pan
around the sample's surface. This technique is very helpful for scanning at high resolutions and finding nanostructures on a surface that are present in relatively low densities. Additionally, this method can be
helpful for studying dynamic processes that occur on a sample surface.
Summary
Dual scanning technology is essential for researchers and engineers with the most demanding AFM applications. A DST atomic force
microscope incorporates two scanners: a flexure scanner for precise metrology/positioning and a rapid scanner for the highest resolution
scanning. Such as system is flexible so that any imaginable research project can be completed successfully without compromise. |
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