CD-SEM (Critical Dimension Scanning Electron Microscope)

Notes

Hitachi High-Tech has long dominated the CD-SEM market, with Applied Materials and others also supplying tools.

Overview

A CD-SEM (critical dimension scanning electron microscope) is a scanning electron microscope built specifically to measure the widths, spaces and hole sizes of patterns on product wafers, automatically and without breaking the wafer. Fabs use it after lithography and after etch to check that the most important features have printed at the intended size. It looks down on the wafer from above, so it measures lateral dimensions well but tells less about depth and profile.

How it works

A finely focused, low-energy electron beam is scanned across a feature, and detectors collect the secondary electrons given off where the beam strikes, building a top-down image. Software locates the feature's edges in the signal and reports the distance between them as the critical dimension. The tool loads wafers, navigates to programmed sites and measures them without an operator, following a recipe.

Low beam energy limits charging and damage, but even so the beam can shrink sensitive photoresist, especially the thin resists used in EUV lithography. Newer tools lower the landing energy further and spread measurements over several points to keep shrinkage acceptable. Because the edge position in the image depends on how electrons scatter in the feature, NIST and others have developed model-based methods that compare measured images with simulated ones to give more accurate widths and some shape information.

The beam spot is around a nanometre across, which becomes a significant share of the measurement as features approach the 15 nm range. For deep three-dimensional structures, higher-voltage CD-SEMs that detect backscattered electrons can measure buried features, while optical scatterometry, which infers profiles from how light diffracts off arrays, complements the CD-SEM for depth and shape.

Examples

• Measuring the width of resist lines after exposure, then the etched lines after pattern transfer, to track etch bias.
• Low-damage, multi-point measurement of thin High-NA EUV resist patterns, a feature of Hitachi High-Tech's GT2000 launched in 2023.
• Tracking contact hole diameters across a wafer to spot an across-wafer dose or focus problem.
• Using a model-based library approach, developed at NIST, to measure line width and sidewall angle from a top-down image.

Why it matters

Critical dimension control decides transistor speed, leakage and how well layers connect, and CD-SEM data feeds the process control loops that adjust lithography and etch settings. Metrology engineers, lithography and etch process engineers, process control engineers and metrology applications engineers use it every day.

Related terms

• Critical dimension
• Scatterometry
• LER
• Metrology

Where it appears on the Hub

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