Nanoscale Inspection: A New Low for the Optical Microscope

Three-Dimensional nanoscale features studied by new technique

As technology incorporates more nanoscale features, quality control becomes more difficult. Verifying such small dimensions often requires special equipment which is expensive and relatively slow compared to traditional methods. A new twist on old technology promises to improve prospects.

With more companies manufacturing nanoscale devices, there is increasing demand for efficient inspection. The stalwart method of small scale inspection is the optical microscope, but with its simplicity comes limitations. Researchers at the National Institute of Standards and Technology (NIST) have developed a way to overcome the limitations while maintaining much of the simplicity.

The fundamental restriction on optical microscopy is resolution and that is based on the wavelength of light used. When objects reach dimensions smaller than the wavelength range of the visible light used, optical microscopy has to get more creative. This is especially true of three-dimensional components where all sides must be measured. As reported in a NIST press release, by interpreting scattered light, very small features can be measured accurately.


The data is compiled by sequentially collecting images while adjusting the focal point. The technique, called Through-Focus Scanning Optical Microscopy (TSOM), can then generate a 3-D representation of the feature. Unlike scanning electron microscopy, which provides a photographic-type image, TSOM represents the information as a heat map where colors represent dimensions. The operator is trained to recognize the colors and distributions as the features of interest and determine conformance to the specified dimensions.

TSOM is reportedly able to, “detect tiny differences in 3-D shapes, revealing variations of less than 1 nanometer in size among objects less than 50 nm across.” This is an impressive feat for an optical method which is typically limited to a resolution of 250 nm (for green light). The method was verified by first analyzing features with an atomic force microscope and then re-analyzing them with the TSOM method.

Creating new ways to use current, proven equipment is a wise way to deliver solutions. Although innovation is often about new technology, there is much to be said about using current technology better. In this case, it can maintain simplicity while increasing performance. A goal any engineer should have.

For more on the technique, see the video below:

Image courtesy of Ravikiran Attota of NIST