Proceedings Article | 5 April 2007
KEYWORDS: Atomic force microscopy, Metrology, System on a chip, Transmission electron microscopy, Silicon, Critical dimension metrology, Carbon nanotubes, Scanning probe microscopy, Zoom lenses, 3D modeling
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
throughput.
