In this feature, nanotechweb.org teams up with the journal Measurement Science and Technology to highlight progress in the field and flag up some of the hot topics.

Can cellulose nanocrystals rival carbon nanotubes? Only metrology will tell...

Biomass surrounds us, from the smallest alga to the largest redwood tree. Even the largest of trees owes its strength to a newly appreciated class of nanomaterials known as cellulose nanocrystals (CNCs). Cellulose is the world's most abundant polymer and, as such, is renewable and biodegradable. CNCs occur as whisker-like microfibrils that are biosynthesized and deposited in plant material in a continuous fashion.

However, accurate measurements associated with chemical, dimensional and structural characteristics must be developed for the full commercial exploitation of this material. Metrology and imaging of these cellulose-based nanomaterials are critical components in the value-added manufacturing chain from the discovery of nanomaterials to the commercialization of nano-enabled products.

The immediate tasks at hand are to develop protocols and adapt currently available nanoscale metrology and instrumentation to biological nanodimensional materials to obtain artifact-free property measurements. Instrumentation for probing biological materials at the nanoscale often requires both evolutionary and revolutionary developments and advances in measurement schemes and apparatus.

Researchers from NIST, Purdue University and the USDA Forest Service are using SPM nanoindentation to measure the transverse elastic modulus of individual tunicate cellulose nanocrystals at multiple locations.

To check out the results in full, click here (full reference available below).

Progress in SPM metrology at NIST

The ability to generate real-space, atomic-resolution images of surfaces has opened the door to the tantalizing prospect of reaching the quantum limit in dimensional metrology – a single atom. Conversely, the possibility of using the atomic crystal lattice as a measurement artifact has also been recognized.

Early scanning systems relied on notoriously nonlinear and hysteretic piezoceramics for motion generation, but as designs progressed, schemes were put in place to linearize the response of piezoceramics. Soon, national measurement institutes (NMIs) around the world began to develop metrological scanning probe microscope (SPM) instruments to realize the metrology potential of this apparatus. Many commercial ventures also began to offer SPMs with displacement sensors of various types on the motion axes to bring more metrological rigor to the SPM imaging process.

For more than two decades, the US National Institute of Standards and Technology (NIST) has been at the forefront of SPM-based dimensional metrology. Recent success stories include the Molecular Measuring Machine (M3), the calibrated atomic force microscope (C-AFM) and the critical dimension AFM (CD-AFM).

To find out more about the design and operation of all three of these tools, click here (full reference available below).

Bright-field optical microscope grabs nanometrology data

With the commercialization of nanotechnology, fast and reliable measurements of nanoscale features are becoming increasingly important. Several tools, such as the atomic force microscope (AFM), scanning tunneling microscope (STM) and scanning electron microscope (SEM), are used routinely to provide measurements at this scale, but optics-based systems are also making an impact thanks to their relatively low cost of ownership with high measurement throughput.

It is often a misconception that optical microscopes are not well suited for dimensional measurements of features that are smaller than half the wavelength of illumination (200 nm-sized features in the visible region) due to diffraction. This limitation can be circumvented by (i) considering the image as a "signal" that represents the target, (ii) using a set of through-focus images instead of one "best focus" image and (iii) making use of highly developed optical models.

Out-of-focus images contain additional useful data regarding the target. This information may be obtained using an appropriate data acquisition and analysis method. Based on this, and on the observation of a distinct signature for different parametric variations, researchers from the US National Institute of Standards and Technology (NIST) have introduced a new method for nanoscale dimensional analysis with nanometre sensitivity for three-dimensional, nanosized targets using a conventional brightfield optical microscope. The method is referred to as "through-focus scanning optical microscopy" or TSOM (pronounced as "tee-som") for short.

For a full description of this prize-winning method, click here (full reference available below).

The rapid evolution of scanning helium ion microscopy: a new tool for nanotechnology

The scanning helium ion microscope (HIM) is a new tool for nanotechnology. Thanks to enhanced resolution and new contrast mechanisms, the metrology performed with this instrument can be better for some materials than for its electron beam instrument counterparts. The HIM also promises greater depth of field (DOF), new imaging modes and the potential for charge-free imaging at higher landing energies without the need for conductive coating.

Since receiving one of the first HIM set-ups, scientists from the US National Institute of Standards and Technology (NIST) have been working with engineers from the IBM Semiconductor Research and Development Center to enhance the performance of the apparatus and to understand the science and capabilities of this new instrumentation for dimensional metrology.

To view a series of images obtained using the NIST apparatus, click here (full reference available below).

References

Development of the metrology and imaging of cellulose nanocrystals
Michael T Postek, András Vladár, John Dagata, Natalia Farkas, Bin Ming, Ryan Wagner, Arvind Raman, Robert J Moon, Ronald Sabo, Theodore H Wegner and James Beecher
Meas. Sci. Technol. 22 024005

Scanning probe microscope dimensional metrology at NIST
John A Kramar, Ronald Dixson and Ndubuisi G Orji
Meas. Sci. Technol. 22 024001

Nanometrology using a through-focus scanning optical microscopy method
Ravikiran Attota and Richard Silver
Meas. Sci. Technol. 22 024002

Review of current progress in nanometrology with the helium ion microscope
Michael T Postek, András Vladár, Charles Archie and Bin Ming
Meas. Sci. Technol. 22 024004

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Full details on all of these topics can be found in the journal Measurement Science and Technology. The material featured in this story was published as part of a feature issue on nanometrology. For a full list of special issues and features, visit the journal homepage.