Scatterometry

Notes

Faster and non-damaging compared with CD-SEM, but it needs a dedicated target and a good optical model.

Overview

Scatterometry, also called optical critical dimension (OCD) metrology, measures the shape of tiny repeating structures by shining broadband light on them and analysing how the reflected light changes with wavelength. The features are far too small to image optically, so their dimensions are inferred by fitting the measured signal to a physical model of the structure. It is fast and non-destructive, which makes it one of the main inline measurement methods in high-volume manufacturing.

How it works

Measurements are usually made on grating targets placed in the scribe lines between dies. The tool records a spectrum of reflected light, often combining spectroscopic reflectometry and ellipsometry. Software then solves the optics of a model of the structure, with parameters such as line width, height, sidewall angle and film thicknesses, and adjusts them until the calculated spectrum matches the measured one. Machine learning increasingly supports or speeds up this physical modelling.

The method is well suited to modern three-dimensional devices. Tall structures add internal reflections and therefore sensitivity, so high aspect ratios are not a barrier. For nanosheet transistors, scatterometry measures the thickness of the silicon and silicon germanium layers and their germanium content, and IBM researchers describe a scatterometry-based model as the only existing inline way to check inner spacer width and coverage. In 3D NAND, infrared versions measure channel hole dimensions and recesses deep inside the stack.

Its weakness is that it is only as good as its model. It does not measure film thickness directly and needs reference methods to anchor its results, and parameters can become correlated so that different shapes give similar spectra. It can also miss hidden features, such as a void at the bottom of a trench. X-ray methods such as CD-SAXS help for some cases but are much slower, so fabs often combine techniques.

Examples

• Measuring silicon and silicon germanium layer thickness and germanium content in a nanosheet stack before patterning.
• Checking the inner spacer recess in gate-all-around transistors, where too deep or too shallow a recess harms the device.
• Infrared OCD measuring channel hole dimensions and nitride recess in 3D NAND tiers.
• Infineon measuring the depth of trenches 42 µm deep at a 6.75 µm pitch with OCD.
• Combining scatterometry with top-down CD-SEM to measure fin width.

Related terms

• CD-SEM
• Ellipsometry
• Overlay metrology
• Film thickness

Where it appears on the Hub

Used on 9 pages of the Hub. In each group, the pages that mention it most come first.