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Topics Covered
About Park Systems
General
Considerations
XE-series System Overview
Patterned Sapphire Substrates (PSS) Investigation
Conclusion
Park
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.
Nitride-based semiconductor materials (i.e., GaN, InGaN, AlN) have attracted
considerable attention in the past years for their applications to electronic
and optoelectronic devices, including blue/green light emitting diodes (LEDs),
laser diodes (LDs), and solid state transistors. Researchers have long known of
the properties of the group-III nitrides, including their wide bandgaps and
robust nature, but had not been able to find a suitable substrate on which to
grow thin films of these compounds. To be practical, the substrate needed a
crystalline structure that matched GaN (utilized as standard n/p-type contact)
and it had to be available in large sizes at a reasonably low cost. Recent
advancements in the epitaxial thin film growth techniques and substrate material
selections partially addressed this problem. However, the lattice parameters of
sapphire, the substrate of choice for most LED manufacturers, are not an exact
match to GaN (16% lattice mismatch), which results in a high density of
crystalline defects (i.e., dislocations, point defects) SiC is better suited as
a substrate material, but is still very costly and also generates stacking
mismatch boundaries in the device structure, which significantly shorten the
device lifetime. Large area Si substrates (4", 6" and beyond) are rapidly
becoming the substrate of choice for electronic device manufacturers, however
substrate mismatch effects are still present and a high degree of surface
cracking (due to the built-in strain) occurs when the critical film thickness is
surpassed. In addition, Si substrates are less resistant from a mechanical
standpoint when compared to both sapphire and SiC.
To further enhance the brightness of blue/green LEDs grown by metal organic
chemical vapor deposition (MOCVD), most of the established manufacturers in the
Asian region (Japan, Korea, Taiwan, and China) have adopted surface patterning
techniques for both (a) the sapphire to epitaxial film interface (sapphire
substrate patterning achieved by substrate preparation techniques) and (b)
epitaxial film to fabricated contact interface (p-side roughening achieved by
MOCVD epitaxial growth methods). These methods, if properly designed and
implemented, allow for a more efficient coefficient of light extraction from the
LED device to be achieved. For both patterning cases, proper information
concerning the surface morphology is vital to achieving target requirements.
Most of the traditional microscopy techniques (i.e., optical microscopy,
scanning electron microscopy, etc.) have been found to have major limitations in
terms of applicability to monitoring the above mentioned surface roughening
techniques. Included are spatial resolution limitations, sample preparation
techniques, reliability and reproducibility issues, amount of time spent to
collect data and so on. Atomic force microscopy (AFM) offers a simple, efficient, and
non-destructive alternative to investigating the various types of patterns
achievable for both sapphire substrate and top LED surface cases.
Traditional AFM tools have been designed with the core of the scanning
mechanism relying on an XYZ piezoelectric tube scanner. As such, crosstalk and
nonlinearity were inherently built in this design. The XE-Series
scan system, which has been developed by Park
Systems decoupled the XY and Z-scanners and was introduced as the next
generation AFM design. As a result, the True
Non-Contact mode was enabled.
For all products of the XE-Series, the Z-scanners controlling the vertical movement of
the probe tip, which is fundamental to acquiring the surface morphology
information, is completely decoupled from the XY-scanner, which moves the sample
in the XY directions. This fundamental architectural design change provides the
user with significant operational advantages enabling the True
Non-Contact mode. Physical separation of the XY and Z-scanners allows for
complete removal of the background curvature and effectively eliminates the
crosstalk and nonlinearity issues that are intrinsic to conventional
piezoelectric tube based AFM systems. This uniquely designed XE scan system not only
increases the data collection speed by at least 10× compared to conventional
piezoelectric tube scanners, but also minimizes the error due to the inherent
nonlinearity of the scanner itself.
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Figure 1. Cross-section view AFM's system. Perfectly decoupled
XY-Z scanner enables independent movement of Z scanner, which makes True
Non-Contact scanning with optimal feedback speed.
The XE scan system is a core feature that gives the competitive edge to the
XE-series
AFMs. Park Systems's innovative scanner design separates the
Z-scanners from the XY-scanner, enabling exceptional Z-servo performance,
orthogonality, and scan accuracy. (See Figure 1.)
The Z-scanners, which controls the vertical movement of the AFM tip and
is fundamental to acquiring the surface morphology information, is completely
decoupled from the XY-scanner which moves a sample in X and Y horizontal
directions. From the structure, XE-series
AFMs remove background curvature from a fundamental standpoint, and
effectively eliminates the crosstalk and nonlinearity problems that are
intrinsic to conventional piezoelectric tube based AFM
systems.
The Z-scanners is designed to have a higher resonant frequency than
conventional piezoelectric tube scanners. For this reason, a stacked
piezoelectric actuator is used for the Z-scanners with a high push-pull force
when appropriately pre-loaded. Since the Z-servo response of the XE scan system
is very accurate, the probe can precisely follow the steep curvature of a sample
without crashing or sticking to the surface.
In the XE scan system, the XY-scanner is a Body Guided Flexure scanner, which
is used to scan a sample in the X and Y directions only. The flexure hinge
structure of the XY-scanner guarantees highly orthogonal 2D movement with
minimum out-of-plane motion. The 2D flexure stage of the XE scan system has only
1 nm ~2 nm of out-of-plane motion for the scan range of 50 µm, compared to the
inherent 80 nm by the piezoelectric tube scanner of conventional AFMs over the
same scan range.
The symmetrical flexure scanner design also makes possible to place much
larger samples on the sample stage than could normally be accommodated by a
piezoelectric tube scanner. Furthermore, the symmetry enables to keep the
scanner balanced even when a sample is loaded, so that the dynamics of the
XY-scanner is not distorted by the sample holder and/or the loaded sample. Since
the flexure scanner only moves in the XY direction, it can be scanned at much
higher rates (10 Hz ~50 Hz) than would be possible with a standard AFM.
The XE-series not only achieves a structural design innovation
that yields a trend setting AFM
performance, but it also brings state-of-the-art improvements to the
electronics. The XE Control Electronics incorporates advanced digital circuitry
with precision software and hardware components that empower high speed and high
capacity data processing, which are designed to enable the scanner, the core
unit of the AFM, to provide efficient, accurate and fast control, and to
facilitate the acquisition of stable images even beyond a scan speed of 10
Hz.
Besides the high speed measurement ability, XE's electronics controls the
movement of the AFM system precisely by the closed-loop scan system, which is
indispensable to map each additional property to the very point of enhanced
topographic details. Even though an AFM system
can acquire data with multiple modes, unless the system displays the exact
position of measurement, it needs software correction (or calibration) to map
the data on the exact position. Correction by software remapping usually works
well when the imaging area is comparatively small, but closed-loop scan is
applicable on any imaging area without distortion.
Shown in Figures 2 are AFM surface topography images (True
Non-Contact mode, amplitude channel) at the 20 µm scale of two different
types of patterned sapphire substrates (PSS) specimens acquired utilizing an XE-150
manufactured and commercialized by Park
Systems. Circular features (i.e., domes) are noticeable with diameters ~ 5
µm and about 1.7 µm in height independent on the scan axis direction. Although
the patterns were manufactured by the same method for both cases, the material
in Figure 2 (a) displays smooth, featureless dome surfaces with clean areas
between these domes, while the sample in Figure 2b shows a certain amount of
surface damage and/or residue for both top dome and areas in between. Figure 3
represents corresponding AFM surface topography views (Tapping Mode, amplitude
channel) at the 20 µm scale of the same PSS samples (as shown in Figure 2)
acquired utilizing a piezoelectric tube scanner instrument manufactured and
distributed by a different vendor.
Figure 2 (a)
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Figure 2 (b)
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Figure 2. Surface morphology at the 20 µm scale of two different
patterned sapphire substrate samples (a) smooth dome surfaces and clean areas
between domes and (b) both dome top surfaces and areas between domes show damage
and/or residue. Images were acquired with an XE-150 AFM
instrument in True Non-Contact mode at 0.2 Hz with 256 pixel resolution.
Several image discrepancies are easily noticeable between Figures 3 and 2.
As such, in Figures 3,
- The features appear to be cylindrical (to some extent elongated along the
slow axis scan).
- Features heights are ~ 1.5 µm along the fast scan axis (X) and in the 1.1 ~
1.4 µm range along the slow scan axis (Y).
- No significant differences are noted in areas between the features of these
two samples.
Figure 3 (a)
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Figure 3 (b)
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Figure 3. Surface morphology at the 20 µm scale of the same two
patterned sapphire substrate samples shown in Figure 2. The images show (a)
asymmetrical features (somewhat elongated along the slow axis scan) with clean
areas between the features and (b) asymmetrical shape features displaying damage
and/or residue in the areas in between. These images were acquired with a
conventional piezoelectric tube scanner instrument in tapping mode at 1.0 Hz and
512 × 256 pixel resolution.
The only relevant imaging information in Figures 3 appears to be the fact
that the sample in Figure 3 (b) (if compared to Figure 3 (a)) has some amount of
surface damage and/or residue between the observed features, somewhat similar to
the information contained in Figures 2, but incomplete. The images in Figure 2
also indicate damage and/or residue for the top dome areas.
Direct optical microscopy investigation of the PSS samples discussed above
was performed in order to evaluate the exact nature (i.e., shape and size) of
the features imaged by AFM techniques. It was found (not shown) that the sapphire
pattern features were round, about 4 µm ~5 µm in diameter and approximately 1.6
µm tall for both samples discussed above. Based on these findings, the data
displayed by Figure 2, which was acquired utilizing the XE-150 was
validated.
The question of imaging discrepancies between the two types of AFM
instrument still remains. To further investigate, several images of the sample
shown in Figure 2 (a) were taken utilizing the XE-150
under various scan conditions. Figure 4 shows three-dimensional views of (a) True
Non-Contact mode at 0.2 Hz, (b) True
Non-Contact mode at 1.0 Hz, and (c) intermittent contact mode at 1.0 Hz. No
major (< 20%) scan parameters adjustments were employed during the image
acquisition. As easily observable, all three images look similar with no
significant feature changes (size, height) generated by the employment of
different scan modes and minor (< 20%) scan parameters adjustments. Even when
the XE-150 is operated utilizing the exact main scan conditions as
for the piezoelectric tube instrument (i.e., intermittent contact mode, 1.0 Hz),
the image details as shown in Figure 4 (c) are very different from the
piezoelectric scanner results as shown in Figure 3 (a). Perhaps additional scan
parameters conditions need to be examined for the piezoelectric tube scanner
tool in order to further improve its sample surface tracking and data
acquisition/rendering capabilities. It is not obvious from a simple user's
perspective.
Patterned sapphire substrates have been proven to significantly enhance the
light extraction coefficients of blue/green LEDs, and their use has been
experiencing a steady growth across all LED manufacturers. Information
concerning the exact pattern shapes is vital to achieving proper light
extraction efficiency. The atomic force microscopy technique provides a simple,
easy to use, non-destructive method to analyzing sapphire substrates prior to
epitaxial film growth. In addition to sapphire surface quality from a quality
control standpoint, atomic force microscopes can provide exact
measurements/imaging of various patterns employed by LED manufactures to better
extract light from the devices. The XE-series
AFM instruments manufactured and distributed by Park
Systems have been proven superior to traditional piezoelectric tube based
counterparts for these applications. The newest architectural designs of the XE-series
tools, comprising of decoupled XY and Z-scanners and achievement of True
Non-Contact mode, enable these instruments to record the correct surface
morphology information with a wide scan parameter range and flexibility.
Source: Park Systems
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Systems.