Atsuko Takafuji

An energy analyzer for high-speed secondary electrons accelerated in inspection SEM imaging

An energy analyzer has been developed to evaluate sample charging and voltage-contrast (VC) in a retarding-type scanning electron microscope (SEM) for inspecting semiconductor devices for defects. Since secondary electrons (SEs) are accelerated by the strong retarding field, a high degree of accuracy is needed to evaluate the energy of SEs. The energy analyzer consists of several electrodes for forming suitable electrostatic lens fields. Evaluation results showed that energy resolution of 2 eV for SE incident energy of 9.5 keV was attained and that evaluations of charging voltages on patterned samples were performed quantitatively.

Advanced inspection technique for deep-submicron and high-aspect-ratio contact holes

We developed a technique using electron beams for inspecting contact holes immediately after dry etching and detecting incomplete contact failures. Wafers with deep-submicron contact holes that had high aspect ratios of 10 could be detected during practical inspection time by controlling the charging effect on the wafer surfaces. Measurements of the energy distribution in the secondary electrons exhausted from the bottom of the holes indicated that they were accelerated by the charge-up voltage on the wafer surfaces. Our analysis showed that high-density electron beams must be used to charge the surfaces when the aspect ratio is high. The minimum thickness of the residual SiO 2 that could be detected at the bottom of the contact holes was 2 nm using an aspect ratio of 8. Applying this mechanism to optimize the dry etching process in semiconductor manufacturing showed that we could achieve reliable process control.

Numerical analysis of electron orbits in a compact high-current circular microtron

The performance of a compact high-current circular microtron is investigated by means of computer simulation. The microtron investigated here is operated with the Russian second type of acceleration and has an electron gun, composed of a cathode and an anode, located outside the accelerating cavity. The simulation results enable us to see the possibility of stable acceleration, to estimate the capture coefficient, and to predict the initial conditions necessary for stable acceleration.

A compact high-current circular microtron

A compact high-current circular microtron has been developed. This microtron is operated with the Russian second type of acceleration to reduce its size and has an electron gun outside the accelerating cavity to increase the beam current. The uniform magnetic field is about 0.19 T, and the energy gain per orbit is about 0.9 MeV. The beam current at a microwave power of 2.2 MW is 200 mA at 4 MeV and 80 mA at 9.5 MeV.

Optimized secondary electron collection in in-lens-type objective lens

Secondary Electron (SE) trajectories were numerically simulated for an ultra-high resolution Hitachi S-900 SEM with an in-lens type objective lens in order to optimize the SE detector position. The trajectories of SEs emitted from the sample were computed as follows. The electrostatic field of SE detector was calculated by a 2D rectangular finite element method (FEM). On the other hand, the magnetic field of the objective lens was calculated by a 2D axially symmetrical FEM. SE trajectories were then 3D simulated by Runge-Kutta method. The SE detector angle relative to the electron optical axis was changed from 45 degree(s) to 60 degree(s) and the SE trajectories were calculated for a various SE positions on the sample at 1 kV to 30 kV accelerating voltage. As a result, the position of the SE detector was optimized so that it gave no variation of brightness even in a low magnification of 250.