CD Measurement

The critical dimension (CD) is the smallest feature width on the wafer that determines whether a device works – classically the gate length of a transistor, but also line widths, spacer thicknesses or contact hole diameters. Because the CD feeds directly into electrical parameters such as switching speed, leakage current and threshold voltage, it has to be controlled to an accuracy in the low nanometre or even sub-nanometre range.

CD control takes place at several points along the process chain: directly after lithography (photoresist CD, to monitor exposure dose and focus), after etching (final CD, since the feature changes further during the etch step) and, on a sampling basis, after other critical steps. A distinction is made between the bias, the systematic deviation between the mask dimension and the actual feature, and the CD uniformity, the spread of the measured values across the wafer, from wafer to wafer and from lot to lot.

Several methods are available for the measurement itself, differing in resolution, speed and whether they are destructive. Two approaches dominate in production: direct imaging measurement using a scanning electron microscope (CD-SEM) and indirect, model-based optical measurement (OCD/scatterometry).

A CD-SEM is a scanning electron microscope optimised for throughput and reproducibility and designed specifically for the automated measurement of feature widths in semiconductor manufacturing. A finely focused electron beam scans the target feature line by line; at the edges of the feature the secondary electron yield changes abruptly (edge effect), because additional electrons can escape from the sidewalls there.

Secondary electron signal while scanning across an edge

The edge position is derived from the resulting intensity profile (linescan) using a threshold algorithm, for example as the point at which the signal reaches 50 % between baseline and peak. Since the shape and height of the secondary electron peak depend on the edge profile (vertical, sloped, rounded) and on the material, different evaluation algorithms return slightly different CD values for one and the same physical edge – the choice and consistency of the algorithm is therefore part of qualifying the measurement recipe.

To avoid charging effects and beam damage on sensitive materials, particularly photoresist, a low acceleration voltage and the lowest possible dose are used. Modern CD-SEM systems achieve a repeatability in the sub-nanometre range.