Wei-En Fu

Preliminary study on nanoparticle sizes under the APEC technology cooperative framework

Despite the fact that there exist several techniques capable of characterizing nanoparticle sizes, their measurement results from the same sample often deviate from each other by an amount that is considered significant on the nanometre scale. In the absence of international standards, or worldwide recognized protocols dealing with nanoparticle characterization, an APEC-led preliminary interlaboratory comparison on nanoparticle size characterization was carried out among ten laboratories from six member economies. Test samples of nanoparticles of 20 nm and 100 nm nominal sizes were distributed for measurements by dynamic light scattering (DLS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microcopy (AFM) and differential mobility analyser (DMA). The comparison result showed fair agreement among the measurements on the certified reference materials (CRM) of 20 nm and 100 nm nanoparticles. Greater deviation was observed on the 20 nm nanosilver colloid sample. This comparison was regarded as a preliminary study on the measurement consistency among various nanoscale measurement techniques.

Particle deformation induced by AFM tapping under different setpoint voltages

The measured height of polystyrene nanoparticles varies with setpoint voltage during atomic force microscopy (AFM) tapping-mode imaging. Nanoparticle height was strongly influenced by the magnitude of the deformation caused by the AFM tapping forces, which was determined by the setpoint voltage. This influence quantity was studied by controlling the operational AFM setpoint voltage. A test sample consisting of well-dispersed 60-nm polystyrene and gold nanoparticles co-adsorbed on poly-l-lysine-coated mica was studied in this research. Gold nanoparticles have not only better mechanical property than polystyrene nanoparticles, but also obvious facets in AFM phase image. By using this sample of mixed nanoparticles, it allows us to confirm that the deformation resulted from the effect of setpoint voltage, not noise. In tapping mode, the deformation of polystyrene nanoparticles increased with decreasing setpoint voltage. Similar behavior was observed with both open loop and closed loop AFM instruments.

Deformation of polystyrene nanoparticles under different AFM tapping loads

Deformation induced by contact force from the tip is the major measurement uncertainty using atomic force microscope (AFM) for the apex height of nanoparticles. Additionally, the contact force by the AFM tip is difficult and not reliable in traditional tapping and contact modes. In this work, the contact forces applied by the AFM were varied using a peak-force tapping method, which is unique technique to perform force-controlled scanning, to characterize the deformation of nanoparticles. The obtained measurement results were compared with a theoretical model developed for predicting the deformation between PS nanoparticles and tip/substrate. It was found that the deformation occurred at low force as 0.5 nN for polystyrene nanoparticles on mica substrate. The deformation was fully plastic. In addition, the deformation has a linear relationship with contact force, which is consistent with contact mechanics model.

Reflective small angle electron scattering to characterize nanostructures on opaque substrates

Features sizes in integrated circuits (ICs) are often at the scale of 10 nm and are ever shrinking. ICs appearing in todays computers and hand held devices are perhaps the most prominent examples. These smaller feature sizes demand equivalent advances in fast and accurate dimensional metrology for both development and manufacturing. Techniques in use and continuing to be developed include X-ray based techniques, optical scattering and of course the electron and scanning probe microscopy techniques. Each of these techniques have their advantages and limitations. Here the use of small angle electron beam scattering measurements in a reflection mode (RSAES) to characterize the dimensions and the shape of nanostructures on flat and opaque substrates is demonstrated using both experimental and theoretical evidence. In RSAES, focused electrons are scattered at angles smaller than 1° with the assistance of electron optics typically used in transmission electron microscopy. A proof-of-concept experiment is combined with rigorous electron reflection simulations to demonstrate the efficiency and accuracy of RSAES as a method of non-destructive measurement of shapes of features less than 10 nm in size on flat and opaque substrates.