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E-beam Inspection

A slower method for finding smaller defects.
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Description

In an e-beam inspection system, electrons are generated within the tool, which then hit the surface of a die. The electrons scatter and bounce back to a detector, enabling it to find defects in chips. Compared with optical inspection, e-beam inspection has significantly higher sensitivities — somewhere in the 1nm range. But it is much slower than optical and could take hours, if not days, to inspect a full wafer.

Because of that, e-beam inspection is used only to examine a small part of a die for defects, usually when optical can’t find certain defects. Often this happens in R&D, where problematic defects need to be located and rooted out. Then, in the fab, chipmakers use optical inspection tools to monitor and find chip defects in production.

E-beam inspection is also used for voltage-contrast defect applications. For this, an external bias is applied to a device. Then the e-beam analyzes the variations in the image contrast of the structure to locate shorts, opens and voids in chips.

All e-beam inspection systems are complex with several moving parts, including an electron gun, column, detectors and a wafer stage. In operation, a wafer is placed in a system. Then, in the system, an electron gun generates electrons, which move down the column.

At that point, a stage moves a wafer to a given location. Then, the electrons hit a small part on a die. This in turn creates an image of the die. The image is compared to a database to determine whether it’s a defect.

A system generates two types of electrons, secondary or backscattered, to help identify defects in devices. Secondary electrons are low-energy electrons, which bounce back from a sample and provide surface information. Backscattered electrons penetrate deeper into the sample.

E-beam is too slow to inspect a full wafer, so it’s used to look at a small part of a die. The throughput is measured in mm² per hour. Optical tools are measured by wafers per hour.

Both single-beam and multi-beam inspection systems are offered by equipment makers.