Nobuharu Noji

Feasibility Study of Multiple-Beam Scanning Electron Microscopy for Defect Inspection

Multiple beams are formed around an optical axis and scanned on a specimen. Secondary electrons (SEs) emitted from the scanned points are accelerated by an electrostatic objective lens and deflected by an E cross B (E×B) beam separator to a secondary-electron detector. By scanning over an 80 nm line & space pattern with four beams, Scanning Electron Microscopy (SEM) images are obtained. From these SEM images it is seen that there is no problem with cross-talk from the neighboring beams in multiple-beam SEM where there are plural detectors. The emittance and brightness of the electron gun with a LaB6 spherical cathode are measured to be 163 mrad µm and 1.5×105 A/cm2sr, respectively, for an emission current of 450 µA and a beam energy of 4.5 keV. A simulation is performed for a combination lens, with a lens, the magnetic gap of which is formed at the specimen side, and an electrostatic lens with an electrode to which a positive high voltage is applied. Based on these electron gun characteristics and simulated aberration characteristics, 8-columns by 8-rows multiple beams with a 160 nA total beam current and a 50 nm resolution are expected. A defect inspection with equivalent pixel frequencies of 4 GHz is expected. This pixel frequency is 10 times larger than that of a commercial available system, and then the multiple-beam SEM is a most promising system for a next-generation inspection.

Electron beam inspection system for semiconductor wafer based on projection electron microscopy: II

Optical inspection systems and/or electron beam inspection systems are quite useful tools for the yield management in the semiconductor process. However, they have some issues of difficulties for the application to the yield management after 100nm-technology node generation. Optical inspection systems have a resolution limit by diffraction phenomena. On the other hand, electron beam inspection systems based on scanning electron microscopy (EBI-SEM) have the limit of inspection speed. Both limits are serious matter for the application to yield management after 100nm-technology node generation. We have developed the electron beam inspection system based on projection electron microscopy (EBI-PEM), having both performances of inspection speed of optical types and spatial resolution of EBI-SEM. The system has been improved on the signal electron collection efficiency and transmittance of the electron optical system. We also have developed high rate and sensitive signal detection system. Then we considered that the inspection speed of several times faster than the conventional EBI-SEM is feasible at the spatial resolution less than 100nm.

Inspection performances of the electron beam inspection system based on projection electron microscopy

The production prototype of an electron beam inspection system based on projection electron microscopy (EBI-PEM) has been developed. Inspection performances of the EBI-PEM were evaluated using the programmed defect standard wafer delivered by SELETE. We confirmed the EBI-PEM had the same inspection speed, 9 cm2/h, as the conventional electron beam inspection system based on scanning electron microscopy (EBI-SEM) under the following conditions: pixel size of 50 nm and defect capture rate of more than 80%. Furthermore, the EBI-PEM achieved an inspection speed of 36 cm2/h. This inspection speed is four times higher than that of the EBI-SEM. The EBI-PEM would be an effective tool for inspection subsequent to the 90 nm technology node generation.

Mask inspection method using the electron-beam inspection system based on projection electron microscopy

We developed the electron beam inspection system based on projection electron microscopy (EBI-PEM), and then applied this system to inspection of mask defects. Usually, inspection of mask defects (such as monitoring of growing defects) is carried out with resist pattern on Si wafer by using an optical inspection tool. In recent years, the shrinking of the design rule for LSI devices has fueled demand for mask inspection for small defects, which are hard to detect with the resolution of an optical inspection tool. Therefore, a high-resolution electron beam inspection tool is desired. However, conventional electron beam inspection systems based on scanning electron microscopy (EBI-SEM) require very long inspection time (10-100 times longer than in the case of optical inspection tool) and inspection costs are very high. In addition, it is difficult to inspect resist pattern by using an electron beam inspection tool, because of the charge- up problem. In order to solve the problem, we examined a new mask inspection method using an electron beam inspection system based on EBI-PEM. Although, EBI-PEM have an advantage in terms of inspection speed, it is more difficult to inspect resist pattern by EBI-PEM than by EBI-SEM, because EBI-PEM is very sensitive to charge-up of a sample surface. Therefore, we tried a method in which inspection is performed after transferring a pattern to SiO2 thin film formed on Si wafer. By optimizing the thickness of SiO2 thin film and the electron beam condition of EBI-PEM, we were able to minimize the influence of charge-up and obtained a higher contrast image. Using this method, EBI-PEM achieved inspection sensitivity of 35nm in the case of programmed defect wafer. We confirmed the probability of realizing high-speed and high-resolution mask inspection by using EBI-PEM.