Gradient Reference Specimens for Advanced Scanned Probe
Microscopy (SPM)
Introduction
Motivation Recent
years have seen the development of a new generation of SPM techniques,
which intend to measure chemical, mechanical, and electro/optical
properties on the nanoscale. However, contrast in new SPM images
is difficult to quantify due to:
Unknown resolution and sensitivity
Topographic artifacts
Complex multi-source sample/probe interactions
Inconsistent Probe Quality Objective
Our research at the NIST Combinatorial Methods Center (NCMC)
aims to provide a suite of reference specimens for the quantification
of next-generation SPM data. Our specimens have a combinatorial
design and will:
Gauge the quality of custom-made SPM probes
Calibrate SPM image contrast through "traditional"
surface measurements
(e.g., spectroscopy, contact angle)
Provide information for understanding complex probe/sample interactions.
Experimental
Gradient
Micropatterned Specimens
Gradient combinatorial methods enable the fabrication of specimens
that vary in the properties that govern SPM image contrast in
a systematic, independent manner. Moreover, combinatorial samples
provide not one, but a multitude of calibration conditions.
The figure illustrates an example specimen design for quantifying
chemically sensitive SPM techniques such as friction-force SPM,
or Chemical Force Microscopy. The crux of this specimen is a
“gradient micropattern” (?-µp ): a series of
micron-scale lines that continuously change in their chemical
properties (e.g., surface energy) compared to a constant matrix.
Two “calibration fields” adjacent to the ?-µp
directly reflect the chemistry of the lines and the matrix.
Thus, traditional measurements (e.g., contact angle) along the
calibration fields (1) gauge local chemical differences in the?-µp
and thereby (2) calibrate contrast in SPM images collected along
the patterned strip.
Reference Specimen Fabrication
Fabrication of ?-µp specimens requires soft-lithography
of appropriate SAM molecules onto a planar substrate. A composite
stamp, which has both flat and corrugated areas, allows printing
of the m-patterned strip with the adjacent solid calibration
field. Next, a graded UV-ozonolysis (UVO), systematically modifies
the chemistry of the patterned SAM (and calibration field) along
one direction. For example, methyl-terminated alkyl chain SAMs
(hydrophobic) can be gradually converted into carboxylic acid
terminated (hydrophilic) chains. Subsequent "filling"
with a hydrophilic SAM completes the "matrix" of the
specimen.
Results
Gradient Reference Specimen Demonstration: SPM Friction
Force Image Contrast Calibration
Specimen fabricated via microcontact printing of a octyldimethylchlorosilane
SAM on a SiO2 matrix. The chemical gradient is achieved via
a graded UV-ozonolysis of the SAM.
Calibration of friction force SPM image contrast. The plot
abscissa gives the friction force contrast between the lines
and matrix for images collected along the ?-µp . The ordinate
expresses the corresponding g data collected along the calibration
fields. Thus, from a single specimen we create a comprehensive
calibration curve that relates SPM friction force to differences
in g. Also, the plot illuminates the smallest g difference sensed
by the probe (red arrow), which is useful for gauging the quality
of custom-made probes.
Future Direction
Target SPM Techniques for Reference Specimen Design:
CFM (Chemical Force Microscopy) - Chemical Imaging
AFAM (Atomic Force Acoustic Microscopy) - Mechanical/Adhesion
Imaging
Other Techniques – Optoelectronic (e.g. NSOM), Magnetic…
?-µp Specimens as Screening Tools for Film Nanomaterials
Publications
Duangrut Julthongpiput, Michael J. Fasolka and Eric J. Amis,
Microscopy Today , Press – August 2004.
Fasolka, M. J., Julthongpiput, D., and Briggman, K. A. ACS Polymeric
Materials: Science and Engineering Preprints, 90, 721, March,
2004, Anaheim, CA
Contributors:
Duangrut Julthongpiput
Michael J. Fasolka*
Polymers Division, MSEL
Donna Hurley (Materials Reliability Division, MSEL)
Tinh Nguyen (Materials and Construction Research Div., BFRL)
Sergei Magonov (Veeco/Digital Instruments)
