:: AZoNanotechnology Article
AFM and the Semiconductor
High Performance Z-Servo
Aspect Ratio CD Measurement
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imaging in liquid and air environments, all systems are fully compatible with a
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services and expertise is matched only by our commitment to our customers.
AFM and the Semiconductor Industry
Force Microscopy (AFM) is emerging as an essential tool in the semiconductor
industry. As aspect ratios become higher, features become smaller, and
requirements for planarity tighten, AFM has
begun to replace profilometry both in development and in production, for
topographic measurements such as trench and via depths, step height, and
micro-planarity measurements. The critical dimensions remain as a major
challenge to previous AFMs employed as a semiconductor metrology tool. These AFMs based
on tube scanners do not have the sufficient Z-servo speed to properly measure
the high aspect ratios of feature sizes. Park
Systems, the original pioneer of AFM
technology, has changed the concept of AFM by
introducing the XE-series AFM (See XE-series product datasheets). With its
revolutionary XE scan system, the Z-scanner has a ten times higher bandwidth
than previous AFMs in the market. Such extraordinary Z-servo performance can
afford high speed Z-scanner feedback and enable True
Non-Contact AFM. In this application note, the high performance of Z-servo
is demonstrated in the critical dimension measurement of high aspect ratio
trenches using XE's True Non-Contact AFM.
High Performance Z-Servo
The XE scan system is a core feature that gives the XE-series
AFM its competitive edge. Park
Systems's innovative scanner design separates the Z-scanner from the XY, as
shown in Figure 1 (a), enabling exceptional Z-servo performance and scan
accuracy, unmatched by other AFMs.
The Z-scanner, being separate from the XY-scanner, is designed to have a
higher resonant frequency than conventional piezoelectricelectric tube scanners.
For this reason, a stacked piezoelectric actuator is used for the Z-scanner,
which has a very fast response speed with at least 10 kHz resonant frequency,
resulting in a high push-pull force. This enables more than 10 times the scan
rates of a conventional piezoelectric tube type scanner, vastly increasing the
speed of the measurements as shown in see Figure 1 (b)). Since the Z-servo
response of the XE scan system is very fast, the probe can precisely follow the
steep curvature of a sample without crashing or sticking to the surface,
enabling True Non-Contact AFM (see True Non-Contact XE Mode notes). The
majority of sample images by the XE-series
AFM is done in True Non-Contact AFM. Therefore, the high performance of
Z-servo not only boosts the speed of a profiling scan (consequently the
throughput), but also protects the tip, resulting in XE's ability to acquire
clear images for an extended period of time.
Figure 1. (a) XE Scan System separates the Z-scanner from
the x-y, enabling exceptional Z-servo performance and scan accuracy. (b)
Comparison of Z-scanner designs between XE and conventional piezoelectric tube
based AFM. The XE's Z-scanner is a flexure guided high force Z-scanner with
Z-servo bandwidth of more than 10 kHz.
Figure 2. (a) Image of a shallow trench sample by True
Non-Contact mode. (b) SEM image of a high aspect ratio tip before imaging, and
(c) SEM image of the same high aspect ratio tip after imaging the same area ten
High Aspect Ratio CD Measurement
The high performance of the Z-servo in XE-series
AFM is exemplified in Critical Dimension (CD) Metrology, as shown in Figure
3. The image was taken in True Non-Contact mode using a normal 5:1 high aspect
ratio tip. This high aspect ratio sample has a very narrow and deep trench,
where the width and the depth are about 160 nm and 550 nm accordingly. Not only
are these kinds of images impossible to obtain using a conventional AFM, but
they can also rival the performance of a CD SEM without having to cut the
sample. It is the high performance of the Z-servo that allows the tip to go down
and reach the bottom of the trench and climb up the trench wall instantly
without distorting the sample image. Sufficient bottom travel allows the
collection of enough data points to effectively represent the bottom of the
Figure 3. True Non-contact-AFM image of 0.16 µm wide,
0.55 µm deep trenches shown in 1:1 aspect ratio 3D rendering (1.5 × 1.5 µm).
What is more powerful in this image is the accurate detection and imaging of
the sidewall and trench edges. In XE's True
Non-Contact mode, the probe senses not only at the end of the probe, but
also in every direction of the probe. As the probe approaches a side wall, it
senses the wall at the side of the probe. The resonance frequency will change
and the z-scanner will instantly pull up until it reaches an equilibrium. This
provides the semiconductor industry customers with an ultimate solution for CD
metrology, including the side wall angle measurements. On the contrary, tapping
mode's interaction is limited to the bottom of the tip and the top surface of a
sample, and the feedback measures the amplitude change of the tip bouncing off
the sample. Therefore, when it comes to the side of a wall, the tapping mode
feedback does not comprehend the lateral force causing the tip to break.
A troublesome CD metrology tool in the market is the so-called 'lateral
roughness AFM' with a flared tip as shown in Figure 4. One must question
the effectiveness of such an AFM in the intended application of CD metrology.
The flared tip used is very large; (~ 200 nm), and as aspect ratios become
higher and features become smaller (~ 60 nm), one must wonder how these
questionable flared tips would fit into high aspect ratio trenches to perform
the lateral roughness in the first place. It is, with out a doubt, going against
the trend and the requirements of the industry.
Figure 4. (a) SEM image of a flared tip and (b) its
dimension. The width of a flared tip is more than 200 nm.
With its revolutionary XE scan system, the XE's Z-scanner has ten times the
bandwidth than that of other AFMs in the
market. The unprecedented high performance of XE's Z-servo allows for True
Non-Contact AFM that can accurately trace high aspect ratio features, and
obtain accurate detection and imaging of the sidewall and trench edges. This
provides the customers in the semiconductor industry with the ultimate solution
in the CD metrology including side wall angle measurement.
Source: Industrial Application Note: Critical Dimensions - Application
Note by Park Systems
For more information on this source please visit Park