SEM (Scanning Electron Microscope)

Notes

The workhorse of physical failure analysis; with an EDX detector fitted it also identifies the elements present in a contaminant or residue.

Overview

A SEM (scanning electron microscope) is an instrument that scans a finely focused beam of electrons across a sample and detects the electrons that come back, building an image with resolution down to the nanometre scale. In the chip industry SEMs are used to inspect and measure patterns on wafers, to review defects and to examine cross-sections and failed parts in failure analysis.

How it works

An electron gun produces a beam that magnetic lenses focus to a tiny spot, and scanning coils move it across the sample line by line in a vacuum. Where the beam hits, it knocks out low-energy secondary electrons from near the surface, which reveal shape and texture, and scatters back higher-energy backscattered electrons, whose number depends on the atomic number of the material and so shows composition. Detectors count these signals point by point to form the image. X-rays emitted at the same time can be analysed to identify the elements present.

Fabs use specialised SEMs. Critical dimension SEMs look down on wafers and measure line widths and spaces automatically at chosen sites, with low beam energy to avoid damaging resist. Defect review SEMs drive to defect locations found by optical inspection and take high-resolution images so defects can be classified. Electron beam inspection systems use the same principle to scan larger areas for defects that light cannot resolve.

In failure analysis and process development, a dual-beam system combines a SEM with a focused ion beam that cuts precise cross-sections, so engineers can image the layers inside a single transistor or contact. ISO 16700 gives guidelines for calibrating SEM image magnification with reference materials, which excludes dedicated critical dimension SEMs.

Examples

• A critical dimension SEM measuring gate line widths on every production lot.
• A defect review SEM imaging particles found by an optical inspection tool.
• A dual-beam system cutting a cross-section through a failing via so it can be imaged.
• Backscattered electron imaging showing a void in a solder bump.
• Energy dispersive X-ray analysis identifying a contaminant particle's elements.

Why it matters

SEM images are the evidence behind many process and yield decisions. Metrology engineers run critical dimension SEMs, defect review engineers classify what inspection finds, and failure analysis and materials scientists use SEMs to find root causes.

Related terms

• TEM
• FIB
• PFA
• Nano-probing

Where it appears on the Hub

Used on 58 pages of the Hub. In each group, the pages that mention it most come first.