Motoyuki Hirooka

Three-dimensional evaluation of an independent multi-walled carbon nanotube probe by tomography with high-resolution transmission electron microscope

We evaluated an independent multi-walled carbon nanotube (MWNT) probe by using tomography with a high-resolution transmission electron microscope to verify the three-dimensional structure of the probe tip. The new method of probe evaluation revealed the following features: (i) cutting the end of the MWNT probe caused the wall structure to disintegrate and encapsulated the graphene sheets fragmented by the discharged pulse; (ii) the cap of the MWNT probe was an open cylinder covered by walls similar in shape to a rectangular slit; (iii) the grooves of the inner walls of the MWNT probe, which were created by the discharge cutting method, maintained a cylindrical shape that was different from the peeling-off mechanism.

Dimension controlled CNT probe of AFM metrology tool for 45-nm node and beyond

Atomic Force Microscope (AFM) is a powerful metrology tool for process monitoring of semiconductor manufacturing because of its non-destructive, high resolution, three-dimensional measurement ability. In order to utilize AFM for process monitoring, long-term measurement accuracy and repeatability are required even under the condition that probe is replaced. For the measurement of the semiconductors minute structure at the 45-nm node and beyond, AFM must be equipped with a special probe tip with smaller diameter, higher aspect ratio, sufficient stiffness and durability. Carbon nanotube (CNT) has come to be used as AFM probe tip because of its cylindrical shape with small diameter, extremely high stiffness and flexibility. It is said that measured profiles by an AFM is the convolutions of sample geometry and probe tip dimension. However, in the measurement of fine high-aspect-ratio LSI samples using CNT probe tip, horizontal measurement error caused by attractive force from the steep sidewall is quite serious. Fine and long CNT tip can be easily bent by these forces even with its high stiffness. The horizontal measurement error is caused by observable cantilever torsion and unobservable tip bending. It is extremely difficult to estimate the error caused by tip bending because the stiffness of CNT tips greatly varies only by the difference of a few nanometers in diameter. Consequently, in order to obtain actual sample geometry by deconvolution, it is essential to control the dimension of CNT tips. Tip-end shape also has to be controlled for precise profile measurement. We examined the method for the measurement of CNT probe tip-diameter with high accuracy and developed the screening technique to obtain probes with symmetric tip-ends. By using well-controlled CNT probe and our original AFM scanning method called as Advanced StepInTM mode, reproducible AFM profiles and deconvolution results were obtained. Advanced StepInTM mode with the dimension- and shape-controlled CNT probe can be the solution for process monitoring of semiconductor manufacturing at the 45-nm node and beyond.

Evaluation of the Stiffness of Carbon Nanotube Probe by Force Curve Measurements

Buckling of arc discharge made multi-walled carbon nanotubes with various lengths was studied by alternating the length of a multi-walled nanotube by intermittent cutting. Buckling stresses were determined by measuring force-distance curves employing an atomic force microscope and the values were compared with those expected from the Eulers theoretical model. As the length of a nanotube was shortened, its buckling mode changed from elastic compressive bending with Youngs modulus of 1.2TPa, to inelastic compressive fracture. The inelastic behavior observed for short nanotubes can be attributed to the buckling mechanism, in which ripple-like distortions develop along the nanotube sidewalls.