Ryuji Nishi

Phase change detection of attractive force gradient by using a quartz resonator in noncontact atomic force microscopy

Abstract Noncontact high-resolution imaging was succeeded in air by detecting a phase change of the quartz resonator, which made it possible to measure an attractive force gradient with high-quality factor even in air. The distance between a tip and a sample was controlled under the phase change of about −0.1° induced by attractive force gradient acting on the tip. A vertical and a lateral resolutions of 1 and 100 A, respectively, were obtained in noncontact imaging, respectively.

Non-contact atomic force microscopy study of atomic manipulation on an insulator surface by nanoindentation.

Experimental results on vertical manipulation on an insulator surface using non-contact atomic force microscopy are presented. Cleaved ionic KCl(100) single crystal is used as an insulator surface. With the nanoindentation method used, the vertical manipulation of a single atom in an ionic crystal surface is more difficult than in a semiconductor surface. Therefore, in many cases, more than one surface atom is manipulated while, in rare cases, single-atom manipulation is successfully performed. Lateral manipulation of a vacancy has occasionally succeeded on the KCl(100) surface. We have presumed that the lateral manipulation was induced by pulling.

Formation and reduction of streak artefacts in electron tomography

We have analysed the formation of streak artefacts in the reconstruction based on the filtered back projection algorithm in electron tomography (ET) and accordingly applied an adaptive interpolation technique to artefact reduction. In the adaptive interpolation to recover the missing information, the edge positions in a projection curve were tracked to reduce the interpolation error. A simulation was used to demonstrate the effectiveness of the artefact reduction. Furthermore, image reconstruction of integrated circuit specimens in the ET experiments with the ultra‐high voltage electron microscope show that the strong streak artefacts can be reduced effectively by our artefact reduction technique.

An automatic method of detecting and tracking fiducial markers for alignment in electron tomography

We presented an automatic method for detecting and tracking colloidal gold fiducial markers for alignment in electron tomography (ET). The second-order derivative of direction was used to detect a fiducial marker accurately. The detection was optimized to be selective to the size of fiducial markers. A preliminary tracking result from the normalized correlation coefficient was refined using the detector. A constraint model considering the relationship among the fiducial markers on different images was developed for removing outlier. The three-dimensional positions of the detected fiducial markers and the projection parameters of tilt images were calculated for post process. The accuracy of detection and tracking results was evaluated from the residues by the software IMOD. Application on transmission electron microscopic images also indicated that the presented method could provide a useful approach to automatic alignment in ET.

Multiple scattering effects of MeV electrons in very thick amorphous specimens.

Multiple scattering has an important influence on the analysis of microns-thick specimens with MeV electrons. In this paper, we report on effects of multiple scattering of MeV electrons on electron transmission and imaging of tilted and thick amorphous film specimens by experiment and theoretical analysis. Electron transmission for microns-thick epoxy-resin and SiO(2) specimens calculated by the multiple elastic-scattering theory is in good agreement with measurements in the ultrahigh voltage electron microscope (ultra-HVEM) at Osaka University. Electron transmission and electron energy are then presented in an approximate power law. The bright-field ultra-HVEM images of gold particles on the top or bottom surfaces of 5 and 15mum thick specimens further illustrate the effect of multiple scattering on image quality. The observed top-bottom effect for the very thick specimens appears to be mainly caused by multiple elastic scattering. With increase in the accelerating voltage from 1 to 2MV, image blurring, contrast, the signal-to-noise ratio, and the top-bottom effect are improved because of reduction in the influence of multiple scattering. However, the effect of specimen thickness on image blurring is shown to be stronger than that of accelerating voltage. At the 2MV accelerating voltage, the 100nm gold particle can be imaged with less blurring of approximately 4nm when located at the bottom surface of a 15mum thick epoxy-resin specimen.

Image quality of microns-thick specimens in the ultra-high voltage electron microscope.

Image quality of MeV transmission electrons is an important factor for both observation and electron tomography of microns-thick specimens with the high voltage electron microscope (HVEM) and the ultra-HVEM. In this work, we have investigated image quality of a tilted thick specimen by experiment and analysis. In a 3 MV ultra-HVEM, we obtained transmission electron images in amplitude contrast of 100 nm gold particles on the top surface of a tilted 5 microm thick amorphous epoxy-resin film. From line profiles of the images, we then measured and evaluated image blurring, contrast, and the signal-to-noise ratio (SNR) under different effective thicknesses of the tilted specimen and accelerating voltages of electrons. The variation of imaging blurring was consistent with the analysis based on multiple elastic scattering. When the effective thickness almost tripled, image blurring increased from approximately 3 to approximately 20 nm at the accelerating voltage of 3 MV. For the increase of accelerating voltage from 1 to 3 MV in the condition of the 14.6 microm effective thickness, due to the reduction of multiple scattering effects, image blurring decreased from approximately 54 to approximately 20 nm, and image contrast and SNR were both obviously enhanced by a factor of approximately 3 to preferable values. The specimen thickness was shown to influence image quality more than the accelerating voltage. Moreover, improvement on image quality of thick specimens due to increasing the accelerating voltage would become less when it was further increased from 2 to 3 MV in this work.

Determination of the linear attenuation range of electron transmission through film specimens

We have investigated the linear attenuation range of electron transmission through film specimens and its dependence on the electron energy, the acceptance half-angle of a detector or an objective aperture, and specimen properties, in the scanning transmission electron microscope (STEM) and the conventional transmission electron microscope (TEM). Electron transmission in the bright-field mode was calculated by the Monte Carlo simulation of electron scattering, and its range of the linear attenuation in film thickness was then determined by a linear least squares fit. The corresponding linear thickness range was shown to increase with the electron energy and the acceptance half-angle, although it decreased with the increase in the atomic number of specimen materials. Under the condition of a 300kV STEM or a 3MV ultra-high voltage electron microscope (ultra-HVEM), the linear attenuation range could extend to several microns for light specimen materials, and this was validated by experimental data in the ultra-HVEM. The presented results can be helpful for accurately measuring the specimen thickness or mass from electron transmission, and estimating the deviation of electron transmission from linearity when tilting a specimen in electron tomography.

Lorentzian-like image blur of gold nanoparticles on thick amorphous silicon films in ultra-high-voltage transmission electron microscopy

We quantitatively analyzed the contrast degradation and blur of 20-nm gold nanoparticles adsorbed on the top of amorphous silicon films of thicknesses of 0.54, 1.09, 1.63 and 2.2 μm in bright-field transmission electron microscope (TEM) images taken at accelerating voltages of 0.5, 1, 2 and 3 MeV. The thickness dependence of the transmission was well explained and consistent with our calculations. The blur function, derived by assuming that the TEM image of a thick specimen can be reproduced by convolving the TEM image of a very thin specimen with it, was found to be expressed by a two-dimensional Lorentzian function. Considering the two characteristics of the Lorentzian function, a sharp peak around the center and a long tail, we concluded that, for TEM observations of thick specimens, the image contrast is degraded predominantly by inelastic scattering and the image is blurred predominantly by multiple elastic scattering.

An autofocus method using quasi-Gaussian fitting of image sharpness in ultra-high-voltage electron microscopy

An accurate method using image sharpness to determine the best focusing is proposed for ultra-high-voltage electron microscopy. This method maximizes image sharpness for adjusting the focus. Five images with different defocus values are used to calculate the image sharpness. To obtain the best focus value that produces greatest image sharpness, fitting the quasi-Gaussian function to five image sharpness is a suitable alternative. This method, which maximizes image sharpness, gives better accuracy than the wobbler method for the ultra-high-voltage electron microscope. The focusing area can be selected without moving the field of view, because the focusing area can be selected at almost any area in the image.

Atomic structure of Ge clusters on Si(111)-(7 × 7) by non-contact AFM

We present non-contact (NC) AFM results of Ge clusters on a Si(111)-(7 × 7) reconstructed surface. The low temperature NC-AFM allows us to directly observe the atomic structure of the Ge clusters on the Si(111). The Ge clusters reside in the middle of a half unit cell in the (7 × 7) reconstruction surface and they are ~1.4 A higher than the Si adatoms. By direct NC-AFM observation, the features of the Ge clusters on the Si(111) became clear. First, the Ge atoms reside in spaces between Si adatoms and on the Si/Ge atoms on the Si(111). Second, the Si adatoms shift from their original position through Ge adsorption, and interact with the Ge atoms accompanied by surface relaxation and a change in their spatial heights. In addition, the interatomic distance between the Ge atoms inside the clusters is approximately 4.0 A, which is larger than that between the Ge atoms in the bulk (2.4 A). Our NC-AFM results of Ge clusters provide valuable information for the basic study of clusters on semiconductor surfaces and may be useful for the manipulation and assembly of clusters for the realization of diverse nanostructures at the atomic level.