Hai-Bo Zhang

Note: Measuring effects of Ar-ion cleaning on the secondary electron yield of copper due to electron impact

In a measurement system of total secondary electron yield (SEY) with in situ ion cleaning, we investigate SEY characteristics of the Cu samples cleaned at different Ar-ion energies and cleaning time. Measured SEY data are compared with those before cleaning and simulated with the Monte Carlo method for an ideal surface of copper. We find that weakening the cleaning intensity, i.e., the ion energy or cleaning time, in some circumstances, can further reduce both the maximum SEY and the SEY at the high-energy end (>0.3 keV) of primary electrons, though the SEY is increased somewhat at the low-energy end. Accompanied by the analysis on the opposing contributions of contamination elimination and surface morphology to the SEY, this study thus provides a comprehensive insight into the effects of ion cleaning on the SEY in the investigation and suppression of secondary electron emission.

Tomography experiment of an integrated circuit specimen using 3 MeV electrons in the transmission electron microscope

The possibility of utilizing high-energy electron tomography to characterize the micron-scale three dimensional (3D) structures of integrated circuits has been demonstrated experimentally. First, electron transmission through a tilted SiO(2) film was measured with an ultrahigh-voltage electron microscope (ultra-HVEM) and analyzed from the point of view of elastic scattering of electrons, showing that linear attenuation of the logarithmic electron transmission still holds valid for effective specimen thicknesses up to 5 microm under 2 MV accelerating voltages. Electron tomography of a micron-order thick integrated circuit specimen including the Cu/via interconnect was then tried with 3 MeV electrons in the ultra-HVEM. Serial projection images of the specimen tilted at different angles over the range of +/-90 degrees were acquired, and 3D reconstruction was performed with the images by means of the IMOD software package. Consequently, the 3D structures of the Cu lines, via and void, were revealed by cross sections and surface rendering.

A 360°-tilt specimen holder for electron tomography in an ultrahigh-voltage electron microscope

A 360°-tilt specimen holder was designed for electron tomography in an ultrahigh-voltage electron microscope. The new holder is of simple noneucentric and single-axis tilting type, with a minimum tilt step of 0.01°. It is applicable to different types of specimens with clamping arrangement. Experiments show that images can be observed around the maximum tilt range of ±85°. The direction of the tilt axis is determined from the 0°- and 180°-tilt images. The possibility of correcting image alteration due to different specimen height is also discussed.

The surface potential of insulating thin films negatively charged by a low-energy focused electron beam

We report on the surface potential characteristics in the equilibrium state of the grounded insulating thin films of several 100 nm thickness negatively charged by a low-energy (<5 keV) focused electron beam, which have been simulated with a newly developed two-dimensional self-consistent model incorporating electron scattering, charge transport and charge trapping. The obtained space charge is positive and negative within and outside the region, respectively, where the electron and hole densities are greater than the trap density. Thus, the surface potential is relatively high around the center, then it decreases to a maximum negative value and finally tends to zero along the radial direction. The position of the maximum value is far beyond the range of e-beam irradiation as a consequence of electron scattering and charge transport. Moreover, a positive electric field can be generated near the surface in both radial and axial directions. The surface potential at center exhibits a maximum negative value in the condition of the approximately 2 keV energy non-penetrating e-beam in this work, which is supported by some existing experimental data in scanning electron microscopy. Furthermore, the surface potential decreases with the increase in beam current, trap density and film thickness, but with the decrease in electron mobility.

Utilizing the charging effect in scanning electron microscopy.

The charging effect of an insulating specimen from electron beam (e-beam) irradiation may be utilized to facilitate imaging in the scanning electron microscope (SEM). This has been confirmed by a great deal of experimental work during the last three decades. Particularly, recent investigations indicate that even located underneath insulating thin films that a low energy e-beam cannot penetrate, conductors not biased and overlay marks, are observable through a novel imaging pattern, charging contrast. Unlike conventional SEM contrasts, which usually reflect surface characteristics, the dynamic charging contrast can reveal information of underlying structures without any external exciting signal. The authors consider that this kind of charging contrast arises from the different redistribution rates of secondary electrons returning to the surface under the surface local field of the charged specimen. The charging contrast has the prospect of extending the SEM application and forming new testing methods matched with the fast development of integrated circuits.

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.

EFFICIENT COMPENSATION METHOD FOR REDUCING RIPPLE OF COCKCROFT-WALTON GENERATOR IN AN ULTRAHIGH-VOLTAGE ELECTRON MICROSCOPE

To get ultrahigh stability of the dc high‐voltage (HV) output, we propose a novel compensation method for reducing the periodic ripple included in the output of a Cockcroft–Walton (CW) generator circuit. A compensating signal is injected into the circuit via the ground end to compensate various ripple harmonics at the HV end. Considering the nonlinearity of a CW circuit, we first compose the signal with the linear superposition of harmonics minimizing individual ripple spectral components, and then reduce the deviation due to the nonlinearity through the fine adjustment on the synthesized signal. An experiment testing the method has been carried out in an ultrahigh‐voltage electron microscope, compensating the ripple components from the fundamental to the ninth higher harmonics. Ripple has been decreased by above one order of magnitude, resulting in a percent ripple of less than 1×10−6.