This paper reports on new developments of advanced CD AFM probes after the prior introduction of "trident probes" in
SPIE Advanced Lithography 2007 . Trident probes, having sharpened extensions in the tip apex region, make
possible bottom CD measurements within a few nanometers of the feature bottom corner; an area where other CD probes
have difficulties due to tip shape limitations. Moreover, new metrology applications of trident probes have been
developed for novel devices such as FinFET and vertical read/write hard disk heads.
For ever smaller technology nodes, new probes evolved from the design of the trident probe. For example, the number
of sharpened tip flares was reduced from three (trident) to two (bi-pod) to prevent possible interference of the third leg in
the slow scan direction, as shown in Figure 3.
Maintaining tip lateral stiffness as the tip size shrinks to less than 30 nm is vital for successful scanning. Consequently,
a significant recent improvement is the change of probe shank cross-sectional geometry in order to maintain tip vertical
aspect ratio of 1:5 (and lateral stiffness > 1 N/m). Finally, modifications of probe substrate are proposed and evaluated
for current and new CD AFM systems.
Hydrophobic, self-assembled monolayer (SAM) coatings were applied on CD probes to reduced tip "pull-away"
distance1 during CD AFM scanning. Test results show that the pull away distance can be reduced more than 5 times on
average (in some cases, by a factor of 15). Consequently, use of hydrophobic SAM coatings on CD probes mitigates
pull-away distance thus allowing narrow trench CD measurements.
We discuss limitations of prior CD AFM probes and design considerations of new CD probes. The characterization of
first prototypes and evaluation of scan performance are presented in this work.
The present paper is a continuation of an investigation to validate CD AFM image reconstruction using Transmission
Electron Microscopy (TEM) as the Reference Metrology System (RMS). In the present work, the validation of CD
AFM with TEM is extended to include a 26 nm diameter carbon nanotube (CNT) tip for non-reentrant feature scans.
The use of DT (deep trench) mode and a CNT tip provides detailed bottom feature resolution and close mid-CD
agreement with both TEM and prior CD mode AFM scans (using a high resolution Trident tip). Averaging AFM scan
lines within the ~80 nm thickness region of the TEM sample is found to reduce systematic error with the RMS.
Similarly, errors in alignment between AFM scan lines and TEM sample are corrected by a moving average method.
Next, the NanoCD standard is used for complete 2D tip shape reconstruction (non-reentrant) utilizing its traceable
feature width and well-defined upper-corner radius. The shape of the NanoCD is morphologically removed from the
tip/standard image, thus providing the tip's shape with bounded dimensional uncertainty. Finally, an update of the
measurement uncertainty budget for the current generation CD AFM is also presented, thus extending the prior work by
An extensive test series was undertaken to validate image reconstruction algorithms used with critical dimension atomic
force microscopy (CD AFM). Transmission electron microscopy (TEM) was used as the reference metrology system
(RMS) with careful attention devoted to both calibration and fiducial marking of TEM sample extraction sites. Shape
measurements for the CD probe tips used in the study were acquired both through the use of reentrant image
reconstruction and independent (non-destructive) TEM micrographs of the probe tips. TEM images of the tips were
acquired using a sample holder that provided the same projection of the tip as presented to the sample surface during
AFM scanning. In order to provide meaningful validation of the CD AFM image reconstruction algorithm, widely
varying sample morphologies and probe tip shapes were selected for the study. The results indicate a 1 - 2 nm bias
between the TEM and CD AFM that is within the uncertainty of the measurements given the Line Width Variation
(LWV) of the samples and accuracy of the measurement systems. Moreover, each TEM sample consisted of a grid with
multiple features (i.e., 21 to 22 features). High density CD AFM pre-screening of the sample allowed precise locating
of the TEM extraction site by correlating multiple feature profile shapes. In this way, the LWV and height of the
sample were used to match measurement location for the two independent metrology systems.
Proc. SPIE. 6518, Metrology, Inspection, and Process Control for Microlithography XXI
KEYWORDS: Metrology, Silicon, 3D modeling, Atomic force microscopy, Transmission electron microscopy, Zoom lenses, Scanning probe microscopy, Critical dimension metrology, System on a chip, Carbon nanotubes
As semiconductor and data storage industries apply Critical Dimension Atomic Force Microscopy (CD-AFM) for their
metrology needs in research and production, (1) measurement accuracy/repeatability and (2) measurement throughput
are the major criteria for acceptance. However, these two requirements are usually contradictory for a metrology
instrument. For example, a scatterometer can take a snapshot of a wafer in seconds, but such indirect CD measurements
are biased by the availability of library models and uncertainty of computer simulations. Transmission Electron
Microscopy (TEM) provides an atomic-scale resolution that is traceable back to the lattice structure of atoms, yet the
cross-section data is highly localized and can take days or weeks to acquire.
In the case of CD-AFM, since the scanning probe physically interacts with the structure of interest at a close proximity,
the determination of sample morphology comes from direct measurements. Therefore, the measurement uncertainty can
be attributed to: (1) AFM probe tip shapes and (2) system control and scan algorithms. For the former, past efforts have
been mainly focused on improving metrology accuracy and repeatability by reducing the dimensional uncertainty of a
tip shape. This approach includes characterizing the probe tip shape periodically. Inevitably, such tip shape calibration
procedure takes time (approximately 5 min) and burdens production throughput.
In this paper, we introduce several new methods for AFM probe tip shape characterization with different designs of tip
shape characterizers. The new tip shape characterizers were designed to address the limitation of current structures.
First, a single silicon overhang structure with wear-resistant coatings was used as the characterizer for both tip width
and tip shape profile. Tip-to-tip scan repeatability data (0.7 nm 3 Sigma) and measurement statistics suggest an
improvement over present state-of-the-art practice. Tip shape profiles of several high aspect ratio (20:1 to 25:1), low
lateral stiffness probes were successfully characterized with this method. Furthermore, the use of single characterizer
provides an opportunity to shorten tool calibration time, and consequently, increase measurement throughput.
In addition, a carbon nanotube characterizer prototype is proposed for CD-AFM. When scanning probe geometry
shrinks with semiconductor technology nodes, it has become a challenge to characterize a probe with a few tens of
nanometer of width with a micrometer-size characterizer. Using a comparable or smaller size of characterizer for a
small (20 to 50 nm) AFM probe not only reduces the dimensional uncertainty, but also expands the 2-D profiling
capability of current tip shape characterization.
We will discuss limitations of current tip shape profiling techniques, proof-of-concept experiments for new
characterizers, implementation of new tip shape characterization methods, and approaches to increasing measurement
Three significant critical dimension atomic force microscopy (CD AFM) advances are presented in this paper. First, scanning probe image reconstruction methodologies that were formerly limited to parabolic type tip shapes and single-valued surfaces (i.e., non-reentrant topography), are extended to multi-valued surfaces and reentrant tip geometries. This crucial step allows the elimination of image artifacts associated with CD AFM scanning of complex feature shapes using reentrant tips. Second, in situ AFM tip images are provided in an automated tool that enables full image reconstruction. Consequently, for the first time, the combination of in situ tip reconstruction with the inherent reference measurement qualities of the AFM and full morphology reconstruction allow CD AFM metrology essentially free of tip shape effects. CD AFM is now primarily driven by development of tip geometries that contact the entire specimen surface while retaining adequate tip lifetime. The background of CD AFM image dilation is described, and the limitations of "legacy" 1D image reconstruction ("tip width subtraction") are illustrated with idealized probe shapes. Initial validation of the automated software is provided by comparison with TEM micrographs. Tip characterizations are presented for a morphologically complex ~20 nm diameter carbon nanotube tip and reentrant silicon CD32 tips (tip width ~ 30nm). Finally, the capability for CD AFM to scan a reentrant sub-45 nm width trench is demonstrated. An EUV resist trench was scanned with a CD32 tip (tip width = 27.4 nm). Minimum CD ranged from 42 to 45 nm. Reentrant image reconstruction is shown for the scan cross-section.
The use of carbon nanotubes (CNT) as probes for atomic force microscopy (AFM) has been studied worldwide for more than a decade; however, the industries have not widely accepted CNT probes in their day-to-day operation. In this work, we present a series of studies on the metrology performance of CNT probes in semiconductor industry. A total of 54 CNT probes were studied for tip geometry, and 11 probes were tested on production wafers from a variety of IC manufacturers. Five out of the 11 probes were further evaluated for tip lifetime in semiconductor manufacturing environments. Statistical measurement data and tip shape characterization results provide insights on the applications of CNT probes in microlithography process control. The recent advancements in AFM scan algorithms that enable the control and use of CNT probes were also discussed in this paper. Sidewall measurement data using tilted CNT probes, and the AFM image of a CNT probe showing a comparable resolution to that of transmission electron microscopy (TEM) were presented for the first time. The combination of advanced AFM system and CNT probes has proven to perform challenging metrology in 65 nm node and beyond.