The Atomic Force Microscope (AFM) is a powerful instrument
for nano-meter scale science and technology. Since its invention
in 19861, the AFM has evolved by refining its capabilities
and ease-of-use. The most common configuration uses a micro-machined
cantilever with a sharp tip on its edge which scans over a sample
utilizing a piezoelectric tube scanner (Figure 1). The deflection
of the cantilever is measured by casting a laser beam onto the
cantilever’s backside and then detecting the reflected beam
with a position sensitive photo detector (PSPD) 2.
1. Piezoelectric tube scanner used in conventional
AFM. When a mirror symmetric voltage is applied
to the opposite electrodes, the tube bends side
ways. However, it is not an orthogonal 3D actuator.
such a configuration, an AFM has high vertical sensitivity
and is relatively easy to implement. In order to adjust
the incident laser beam to fall on the small cantilever
and to make the reflected beam hit the center of the PSPD,
an aligning mechanism with fine screws is used. This probing
assembly unit, including an aligning mechanism, a laser,
a PSPD, and cantilever, has a considerable mass and it is
difficult to scan the probing unit at a sufficiently high
speed while accurately measuring a sample. Therefore in
earlier AFMs, the probing unit was kept stationary, and
the sample was scanned along the XY and Z axes as shown
in Figure 2.
However, such an AFM has an intrinsic problem of the Z-servo
performance being dependent on the sample mass. Since the
sample has to be moved in the Z direction, the Z-servo control
parameters need to be adjusted every time the sample mass
changes. A more serious problem arises when it is necessary
to image large samples such as large silicon wafers, which
cannot be scanned fast enough in the Z direction for sufficient
vertical servo frequency response. In order to solve this
problem, it is necessary to scan the probe cantilever and
to ensure the laser beam follows the cantilever’s
A simple means of accomplishing this result is to miniaturize
the aligning mechanism and to scan the entire probing unit as
shown in Figure 3.3 However, such a miniaturized probing
unit still has a considerable mass which degrades the Z-servo
response. It is also inconvenient to align the laser beam with
tiny screws and a special tool is often required.
Another method is to attach lenses on the tube scanner such that
the laser beam follows the cantilever motion and the reflected
beam hits the same point on the PSPD while scanning, as shown
in Figure 4.4,5 However, in this method, the laser
beam does not perfectly follow the cantilever motion and the reflected
beam does not remain on the exact same point on the PSPD, causing
measurement errors and tracking force variations during XY scanning.
In most AFMs, a laser beam bounce system is used to
detect the cantilever deflection and a piezoelectric
tube scanner is used to scan the sample in XY and
z directions. Large samples cannot be scanned.
For large samples, the tube scanner may scan the entire
miniaturized probing unit. However, it is inconvenient
to align the laser beam, and the Z-servo response
is slow due to the increased mass that must be scanned.
Figure 4. Another method
is to insert lenses inside the tube scanner to make
the laser beam follow the cantilever motion. However,
this method is complicated and there are residual errors
in the laser beam path.
In addition, most AFMs have problems with scanning errors
and slow scanning speeds. As shown in Figure 1, the commonly
used piezoelectric tube scanner is not an orthogonal 3 dimensional
actuator that can be moved in any of the three dimensions
x, y, and z independently of one another. Since the XY motion
relies on the bending of the tube, there is non-linearity
and serious cross talk between the XY and Z axes.
Position sensors can be used to correct the intrinsic non-linearity
of the piezoelectric actuator,6 but the cross talk
from flexing the tube cannot be eliminated and it causes background
curvature and measurement errors. Using a tripod scanner does
not improve the non-linearity and cross talk problem much.
Furthermore, the tube scanner has a low resonance frequency
(typically below 500 kHz) and does not have enough force to
drive a conventional probing unit at high speed.
Advanced XE Scan System
Park Systems’ advanced Crosstalk
Elimination (XE) scan system (shown in Figure 5) effectively
addresses all of the above-mentioned problems. In this configuration,
we used a 2-dimentional flexure stage to scan the sample in
only the XY direction, and a stacked piezoelectric actuator
to scan the probe cantilever in the Z direction only. The
flexure stage used for the XY scanner is made of solid aluminum
as shown in Figure 6. It demonstrates high orthogonality and
an excellent out-of-plane motion profile. The flexure stage
can scan large samples (~1 kg) up to a few 100 Hz in the XY
direction. This scan speed is sufficient because the bandwidth
requirement for the XY axes is much lower than that for the
Z axis. The stacked piezoelectric actuator for the Z-scanner
has a high resonance frequency (~10 kHz) with a high pushpull
force when appropriately pre-loaded.
Figure 5. In XE systems,
the Z-scanner is separated from the XY- scanner; the XY-scanner
scans only the sample and the Z-scanner scans only the probe.
XY flexure scanner used in XE-system. This single module parallel-kinematics
stage has low inertia and minimal out-of-plane motion, providing
the best orthogonality, high responsiveness,
and axis-independent performance.
A crucial point of our design
is the arrangement of the laser, PSPD, and the laser beam
aligning mechanism. As mentioned above, we have to ensure
that the laser beam falls on the same point on the cantilever
and the reflected beam hits the same point on the PSPD regardless
of the Z-scanner motion, so that only the deflection of the
cantilever will be monitored on the PSPD. We can achieve this
goal by casting the laser beam vertically from above and attaching
the PSPD to the Z-scanner, while the laser and laser beam
aligning mechanism are fixed to the frame. As shown in Figure
7 (a), the laser is mounted on one side of the probing head.
The laser beam is reflected by a prism, which is mounted on
a glass plate. The angle of the glass plate can be adjusted
by the two screws on the two diagonal corners of the glass
plate holder. Since the laser beam is falling on the cantilever
from the vertical direction, the beam always hits the same
point on the cantilever regardless of the Z-scanner motion.
The reflected beam is bounced at the steering mirror and hits
the PSPD. The angle of the steering mirror can be slightly
adjusted by the two screws on its diagonal edges such that
the bounced beam hits the center of the PSPD. Since the PSPD
and cantilever move together and the steering mirror is vertically
mounted, parallel with the Z-scan direction, the bounced laser
beam always hits the same point on the PSPD regardless of
the Z-scanner motion.
Figure 7. (a)
Initial design of the beam bounce detection mechanism for
the XE scan system. To detect the cantilever deflection, the
PSPD and the cantilever are moved together by the Z-scanner,
while the laser, laser aligning mechanism, and steering mirror
are fixed to the head frame. (b) One variation
of the beam bounce detection mechanism. The PSPD was lowered
to make clearance for the optical microscope. However, small
errors are introduced in this design. (c)
Another variation of the beam bounce detection mechanism.
In order to eliminate the error, a second mirror that is parallel
to the steering mirror was inserted. This mirror exactly compensates
for the effect of the steering mirror.
In order to accommodate an on-axis optical microscope, it
is desirable to have suitable clearance above the cantilever.
For this purpose, we lowered the position of the PSPD and
mounted the steering mirror at a certain angle such that the
path of the bounced laser beam became horizontal, as shown
in Figure 7 (b). However, in this configuration, the bounced
laser beam spot on the PSPD changes as the Z-scanner moves.
When the Z-scanner moves a distance h, there is an error of
h(1-sin2θ) in the position of the laser beam spot on
the PSPD, where θ is the angle of the cantilever. This
error term is very small compared to the amount of the laser
beam spot displacement when the cantilever is deflected by
a feature of height h on the sample surface. However, this
phenomenon still causes height measurement errors and introduces
spurious variations in the tracking force. Since the Z-scanner
motion is a known quantity, we can compensate for such errors
A better method is to eliminate such errors by introducing
another mirror, whose angle is in parallel with the steering
mirror, and positioning the PSPD accordingly, as shown in
Figure 7 (c). In this configuration, the second mirror exactly
compensates for the effect of the first mirror, and therefore
the laser beam hits the same point on the PSPD regardless
of the Z-scanner motion.
Figure 8. (a)Optical microscope
view and (b)sectional diagram of the
XE AFM stage This design allows
direct on-axis optical view.
This design provides clearance above the cantilever
and allows for a direct on-axis optical microscope view as
shown in Figure 8. The optical path from the sample to the
camera is a straight line. This configuration provides much
higher quality optical views than was provided by conventional
large sample AFMs,3,4
where an oblique mirror had to be inserted between the cantilever
and the objective lens. Since the oblique mirror may have
some defects and as it does not fully cover the light path,
the quality of the optical microscope is degraded in these
systems. Also, in order to pan the view, the objective lens
has to be moved out of its optical axis, introducing significant
blurring. In the XE-system, the objective lens, tube lens,
and CCD camera are rigidly mounted on a single body and move
together for panning and focusing to preserve the highest
quality optical vision.
XE-series AFMs are not only the most technologically-advanced
AFMs on the market, but also the most user-friendly. The intuitive
user interface minimizes the time needed to start taking measurements
and reflects our dedication to user-oriented product development.
XE-series AFMs allow customers of all experience levels to focus
on their experiments, not instruments. From developing the first
direct on-axis optics in industrial systems to our simplified
tip exchange procedure, Park Systems has led trend-setting and
market leading innovation in user convenience. At Park Systems,
the ease of use is the ultimate support we can provide to our
Figure 9. Zero background curvature
by Park Systems XE-system (a) and typical background curvature
of a conventional AFM system with a tube scanner (b). (c)
shows the cross section of these background curvatures.
Figure 9 shows unprocessed AFM images of a bare silicon
wafer taken with the XE-system (a), and with a conventional AFM7
(b). Since the silicon wafer is atomically flat, most of the curvatures
in the image are scanner-induced artifacts. Figure 9 (c) shows
the cross section of the images in Figure 9 (a) and (b). Since
the tube scanner has intrinsic background curvatures, the maximum
out-of-plane motion is as much as 80 nm when the X-axis moves
15 μm. The XE scan system has less than 1nm of out-of-plane
motion for the same scan range. Another advantage of the XE scan
system is its Z-servo response. Figure 10 is an image of a porous
polymer sphere (Styrene Divinyl Benzene), whose diameter is about
5 µm, taken with the XE-system in Non-Contact mode. Since
the Z-servo response of the XE-system is very accurate, the probe
can precisely follow the steep curvature of the polymer sphere
as well as small porous surface structures without crashing or
sticking to the surface. Figure 11 shows another example that
demonstrates the high performance of the z-servo response with
a flat background.
NC-AFM image of a polymer sphere taken with an
XE-100 (6 μm scan size). 1:1 aspect ratio of un-processed
Figure 11. NC-AFM image
of STI patterns on a 6 inch photomask taken with an
XE- 150 (5 μm × 5μm scan, 70 nm z range). 3D rendering
of un-processed raw data. The AFM probe traced both
upper and lower terraces faithfully and almost imaged
the side walls.
XE-series SPM was developed to have the following advantages:
1) Scan accuracy: There is no cross talk between the XY
and Z axes, and one can achieve high scan accuracy. 2) Sample
size: Since the sample is scanned by a flexure scanner
only in the XY direction, large samples as well as small samples
can be scanned at sufficiently high speed. 3) Z-servo speed:
Since the Z-scanner has a high resonance frequency with high force,
the Z-servo frequency response is much greater than in conventional
AFM. 4) Convenience: Since the laser beam aligning
mechanism is fixed to the head, it is possible to produce alignment
fixtures of adequate size for convenient and precise adjustment
without requiring any tools. 5) Optical vision:
Since there is enough clearance above the cantilever, it is possible
to accommodate a direct on-axis optical microscope.
1 G. Binnig,
C. Quate, and Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986).
2 G. Meyer and N. M. Amer, Appl. Phys. Lett. 53, 2400 (1988).
3 Agilent (formerly Molecular Imaging). Model 5500 (formerly PicoPlus)
4 Veeco (formerly Digital Instruments), Model Dimension 3100
5 P. K. Hansma, B. Drake, D. Grigg, C. B. Prater, F. Yashar, G.
Gurley, and V. Elings, S. Feinstein, and R. Lal, J. Appl. Phys.
76, 796 (1994).
6 R. Barrett, Rev. Sci. Instrum. 62, 1393 (1991).
7 Veeco (formerly Digital Instruments), Model Multimode