Why Atomic Force
AFM Products Utilizing Piezoelectric Tube
Scanners Cannot Meet the Industry Requirement
XE-series AFM Can Meet the Industry Requirements
Pole-Tip-Recession (PTR) 1: True Non-Contact
AFM vs. Tapping AFM
Pole-Tip-Recession (PTR) 2: Two Height
Peaks in ABS Area
Systems is the Atomic Force Microscope (AFM) technology leader, providing
products that address the requirements of all research and industrial nanoscale
applications. With a unique scanner design that allows for the True Non-Contact
imaging in liquid and air environments, all systems are fully compatible with a
lengthy list of innovative and powerful options. All systems are designed with
ease-of-use, accuracy and durability in mind, and provide your customers with
the ultimate resources for meetiong all present and future needs.
Boasting the longest history in the AFM
industry, Park Systems' comprehensive portfolio of products, software,
services and expertise is matched only by our commitment to our customers.
Why Atomic Force Microscope?
As the design rule becomes smaller, traditional metrology tools, such as
stylus and optical profilers, and CD-SEM are unable to meet the precision
metrology requirements from magnetic recording industry.
Virtually all the feature sizes in manufacturing steps are in nanoscale,
especially that of defect review and pole-tip-recession. Atomic Force
Microscope (AFM) routinely performs sub-nanometer measurements in ambient
atmosphere, and consequently has gained attention as a new candidate for
nanoscale metrology tool. However, old AFM systems
based on a piezoelectric tube scanner cannot meet the metrology requirements in
the industrial applications due to the intrinsic background curvature and
crosstalk between the XY & Z axes.
AFM Products Utilizing Piezoelectric Tube Scanners Cannot Meet
the Industry Requirement
The piezoelectric tube scanners are universally abandoned in modern AFM
technology, but unfortunately the magnetic recording industry has been somewhat
slow in catching up with the proven and latest advances. The limitation of the
piezoelectric tube scanner based AFMs are
best demonstrated in Figure 1. Old piezoelectric tube scanner based AFMs are
marginally acceptable in relative dimensioning of two objects when placed side
by side; however, the problem arises when one needs to know the absolute
dimensioning of an object as shown in Figure 1 (a). It gets even worse when two
objects are separated as shown in Figure 1 (b); even the relative dimensioning
of the two separated objects cannot be performed by the tube based AFMs. The
bottom line is that piezoelectric piezoelectric tube based AFMs are
completely useless and obsolete in absolute dimensioning of an object, the
critical requirement of today's magnetic recording industry.
Figure 1.Old piezoelectric tube based AFMs are (a)
marginally acceptable in relative dimensioning of two objects right next to each
other, but completely useless in absolute dimensioning of an object and (b) even
worse in the relative dimensioning of the two separated objects.
The problem originated from intrinsic limitations with the piezoelectric tube
scanner as shown in Figure 2. The piezoelectric tube is made of ferroelectric
ceramics that deform by several nm if a voltage is applied to the electrode on
the surface. As the piezoelectric tube itself is not orthogonal in three
dimensions, and each axis movement is non-linear, the XY-Z crosstalk with
non-linearity inevitably results in a significant background curvature as shown
in Figure 3.
Figure 2. The Piezoelectric tube scanner used in
conventional AFMs is not an orthogonal 3-D actuator. Each axis movement is
non-linear, the XY-Z crosstalk with non-linearity inevitably results in a
significant background curvature. Moreover, it has a low resonance frequency
(< 1 kHz) and low force.
Figure 3. (a) The background curvature of a piezoelectric
tube based AFM in raw data and (b) the line-profile cross section of the
background curvature. The XY-scanner of old piezoelectric tube based AFMs have
more than 80 nm out-of-plane deviation over only 15 µm scan.
Note that the XY- scanner of old piezoelectricx tube based AFMs have more
than 80 nm out-of-plane deviation over only 15 µm scan. These conventional AFMs
attempt to flatten the background curvature using the second or third order
fitting, and due to this every scan data of their AFMs are
the result of the "line-by-line" flattening, which is imbedded in their
operating software. As a marketing and sales policy, some AFM company
engineers are never allowed to demonstrate the raw data of their AFMs (Park
Systems can help customers with turning off the imbedded software
correction). It is unfortunate that even some of experienced engineers and
scientists are deliberately misled to believe that the processed and flattened
data of these old piezoelectric tube based AFMs are
the raw data.
The high order fitting and flattening of the background curvatures in
piezoelectric tube based ATMs become fatal issues in magnetic recording industry
as the feature sizes of manufacturing process become smaller to nanoscale and
the absolute AFM dimensioning of such feature is required.
Park Systems XE-series AFM Can Meet the Industry
Introduced by the original innovators and pioneers of AFM
technology after 4 years of intensive product development, the XE-series
AFM from Park Systems represents breakthroughs in every aspect of AFM
technology (For details, see "The XE-series: the Technology Behind the True
Non-Contact AFM" under XE Mode menu).
Physical separation of the XY-scanner from the Z-scanner completely removes
background curvature from the fundamental level, and effectively eliminates the
cross-talk and non-linearity problems that are intrinsic to old piezoelectric
tube based AFM systems (Figure 4 (a)). This uniquely designed AFM systems
Systems not only increases the data collecting speed by at least 10 times
compared to a conventional piezoelectric tube type scanner, but also improves
the error due to the inherent non-linearity of the scanner itself.
Figure 4. (a) In XE-series AFM, the Z-scanner is
separated from the XY-scanner; the XY-scanner actuates only the sample and the
Z-scanner actuates only the probe. (b) 2D guided XY flexure scanner used in
XE-series. 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.
In the XE-series AFM, the XY-scanner is a Body Guided Flexure scanner
as shown in Figure 4 (b), which is used to scan a sample in the XY direction
only. The flexure hinge structure of the XY-scanner guarantees highly orthogonal
2D movement with minimum out of plane motion. Due to the Parallel Kinematics
design, the XY-scanner of the XE-series
AFM also has low inertia and axis-independent performance.
Figure 5 (a) shows unprocessed raw AFM images
of a bare silicon wafer taken with a XE-series
AFM as shown in Figure 5 (a), and with a piezoelectric tube based AFM as
shown in Figure 5 (b). Since the bare silicon wafer is atomically flat, the
curvatures in the images are due to the artifacts induced by the scanner. Figure
5 (c) compares the line-profile cross sections of background curvatures in
Figure 5 (a) and (b). The 2D flexure stage of the XE-series
AFM has only 1-2 nm of out-of-plane motion for the scan range of 50 µm
(0.01% background curvature), compared to 80 nm by the tube scanner over the
same scan range (0.53% background curvature).
Figure 5. Unprocessed raw AFM images of a bare silicon
wafer taken with a (a) Park Systems's XE-series AFM, and with a (b)conventional
tube based AFM. (c) The line-profile cross sections of these background
In addition, the XE-series is the first and only AFM in the
market that realizes True Non-Contact mode in every specification, not just in
principle but in practice. True Non-Contact mode achieves an unprecedented tip-sample
distance, combined with superb tip and sample preservation. The advantages of true
Non-Contact mode enable the ultimate resolution of AFM and
measurement accuracy which are without peer in the AFM
industry. (For details, see "What is the Ultimate Resolution of AFM?" in True
Non-Contact AFM under XE Mode menu).
Defect Review Analysis
The immediate advantage of the zero background curvature in the XE-series
AFM is demonstrated in the defect review analysis of a hard disk surface.
Figure 6 shows images of a hard disk surface acquired by a Park Systems's
XE-series AFM as shpwm in Figure 6 (a) and a piezoelectric tube based AFM as
shown in Figure 6 (b). Both images came from a comparison study in real fab
condition by the same hard disk industry client. The images are obtained after
deglitching those lines due to door opening & closing in the fab facility as
shown in Figure 7.
Figure 6. Images of a hard disk surface taken by a (a)
Park Systems's XE-series AFM and by a (b) conventional piezoelectric tube based
Note the striking difference between the two in Figure 6. The image taken by
the piezoelectric tube based AFM is
processed by a line-by-line curvature flattening, and one can clearly notice the
distortion in the image as a result of the curvature flattening. The two edges
look more flat, and the center has more variation. As pointed out earlier, a
tube scanner has quite a bit of background curvature, approximately 80 nm over
15 µm scan. Since the current image by the piezoelectric tube based AFM is
about 10 µm in scan size, it would have a significant background curvature.
Without the curvature flattening, the this image would be so curved to the
point that one cannot even see theline features on a hard disk. Even with the
curvature flattening, the nanoscale features on the hard disk would undoubtedly
get distorted due to the high order fitting from the flattening process. Making
matters worse, the door deglitching adds to the distortion as it is impossible
to fit correctly both the glitches and the background curvature. The RMS
roughness from such data is bound to be wrong.
The image acquired using a Park Systems
XE-series AFM correctly reflects the texture of the original sample. As the
background curvature remains absolute minimum, the removal of door glitches is
the only correction to be made, and it can be easily implemented without using
high order fitting and changing the original raw image data as shown in Figure
Figure 7. The deglitching of a hard disk surface image
taken by Park Systems XE-series AFM. The glitches are generated when a door is
closed or opened in the actual fab facility of a hard disk industry client. Note
that the door glitches are removed without affecting the texture of the original
Pole-Tip-Recession (PTR) 1 : True Non-Contact AFM vs. Tapping
The density of information on a hard disc drive depends on the distance
between the read-and-write head of a hard-disk drive. Usually the head is
floating on an air cushion that builds up between the rotating hard-disk plate
and the read-and-write head that "flies" at a distance to the hard disk of
several nanometers. The active parts for read-and-write action are two ferritic
poles that guide and focus the magnetic field close to the disk as shown in
Figure 8. The pole-tip recession (PTR) is the difference in height between the
pole tip material and the air-bearing surface (ABS). As one way of increasing
the storage density in rigid disk drives, many efforts were taken to minimize
the head-media gap, and consequently resulted in signal loss; the pole tip had
to be recessed as little as possible, only to the point of preventing damage by
contact with the disk. For this reason, the nano-metrology of PTR is a critical
task6 with significant economic impact.
Figure 8. The pole-tip-recession (PTR) of a hard disk
drive is the difference in height between the pole tip material and the
air-bearing surface (ABS). The image is a cross section of a typical thin film
head showing the pole tips, undercoat/overcoat, gap, and the air bearing surface
Since the reference plane (ABS) is far from the pole tip, the small height of
PTR is easily buried in the background curvature of the piezoelectric tube
scanner, which is more than ten times larger than the PTR height. It gets even
worse when the images taken with piezo tube based AFMs are
line-by-line flattened with high order polynomial fitting. The background
curvature poses a significant bottleneck in the PTR metrology. As the pole tip
region has a typical height of a few nanometers and a width of 50 mm, the
resulting background curvature these AFMs would
be a few hundred nanometers over the scan range of 50 mm.
Figure 9 shows a flattened image taken by a piezoelectric tube based AFM using
tapping mode. First, note the residual background curvature even after
line-by-line flattening. Since there are air-bearing surfaces at the sides, the
average is flat, but the inside, recessed region, looks curved upward. That's
the major problem with image processing, since you do not know if the curvature
is coming from the original surface, from a scanner artifact, or from the sly
There is an even more pressing issue of tapping force changing the PTR
morphology in conventional piezoelectric tube based AFMs. Note
that the pole-tip (the half moon shaped area with two vertical stripes on its
left) in the PTR image by the piezoelectric tube based AFM is
located lower than its surroundings, contrary to the design as shown in Figure
8. Here, we have found that the tapping mode AFM can
cause serious artifacts due to the tapping force because the pole-tip is made of
softer materials and the tapping force depresses the pole tip more than its
surroundings and generate incorrect PTR values.
Figure 9. Image of the pole-tip region taken by a
piezoelectric tube scanner based AFM in tapping mode. The background curvature
is similar to the height change in the pole tip area.
In tapping mode, as stated in the original paper, the tip is made to strike
the surface on each oscillation with sufficient oscillation amplitude and energy
to overcome the stickiness of the surface. In other words, the tip depresses the
sample surface by striking on it at each oscillation cycle. If the sample is
homogeneous, the overall depression would not change the topography. However, if
the sample is composed of various materials, the tapping force will depress the
soft areas more than the harder areas. This phenomenon will not be obvious if
the thickness of different materials is small, or if the sample surface is not
smooth. However, in the case of PTR, this effect is prominent since the sample
surface is very flat, and each material has large thickness.
In order to verify this, we performed Force Modulation Microscopy (FMM). As
shown in Figure 10, the pole tip area appears much darker than its surrounding,
which means the pole tip is much softer than its surrounding. This is in
accordance with the material properties of the pole tip, made of permalloy, and
its surrounding, made of alumina.
Figure 10. Force Modulation Microscopy (FMM) image
showing local variations in surface stiffness.
Figure 11 shows an unprocessed raw data of the same sample taken with Park Systems's
XE-series AFM in True Non-Contact AFM mode. As shown in the line profile of
Figure 11, the recessed region is completely flat with raised Air Bearing
Surfaces (ABS) at both sides. Moreover, the pole tip area is located higher than
its alumina coated region, as was designed to be. This conclusively demonstrates
the superiority of the XE-series AFM performance especially in its zero background
curvature and True Non-Contact AFM.
Figure 11. PTR region imaged with Park Systems's
XE-series AFM in True Non-Contact AFM mode. The image shows no background
Pole-Tip-Recession (PTR) 2: Two Height Peaks in ABS Area
As shown in Figure 8, the ABS area is used as a reference plane for the
subsequent measurements of the PTR. Therefore, the accurate reference height of
the ABS area is critical in performing more rigorously and reliably
characterizing the PTR. In Figure 12, the histogram of the ABS area image taken
Systems's XE-series AFM clearly shows two peaks. The peaks are separated by
about 1.65 nm. This simply reflects what we see optically in the ABS area, i.e.,
dark and bright regions. Two regions have obviously different heights. Please
note that the distinction of the two height peaks in the ABS area is an
important measure of an AFM performance in itself. Conventional piezoelectric tube
based AFMs cannot distinguish the two height peaks since such
nano-metrological differentiation is impossible due to the intrinsic background
curvature and subsequent flattening.
Figure 12. The histograms of two ABS and one PTR areas
whose images are taken by Park Systems's XE-series AFM. In other words, the mean
value of heights in ABS area will change depending on the size and location of
selected areas. However, the median values of the two peaks will remain the same
regardless of the size and location of selected ABS areas.
The averaged value of heights in a given ABS area will change by the
distribution of dark and bright regions in the area. In other words, the mean
value of heights in ABS area will change depending on the size and location of
selected areas. However, the median values of the two peaks will remain the same
regardless of the size and location of selected ABS areas. If one can
distinguish two height peaks in a given ABS area, median values of the ABS area
represent more accurate, more reliable, and more consistent reference height for
It is apparent that old AFMs based on piezoelectric tube scanners are not suitable in
hard disk metrology due to their scanner's artifacts. We also noticed that the
tapping mode can not only cause a serious amount of damage to the PTR, but also
change the relative height measurements between regions of different materials.
Moreover, there is always the undeniable fact that tapping causes faster tip
damage compared to True Non-Contact. Whether in defect review or in PTR
metrology, the zero background curvature and True
Non-Contact mode of the XE-series AFM are found to be critical in the
accurate and reliable metrology of the hard disk industry.
Source: Nano-Metrology of Magnetic Recording Industry - Application
Note by Park Systems
For more information on this source please visit Park