Laser Metrology

James Clerk Maxwell (1831–1879) has been one of the first to suggest using the wavelength as a natural gauge for length. In 1950, the standard meter in the new International System of Units (SI) is referred to the wavelength corresponding to the krypton-86 (605,780 nm).  The use of a precise wavelength as a reference for the measurement of  lengths is well established as the ideal measurement method.

In the 21st century, DHM® by Lyncée Tec refer to wavelengths for height measurements. By using ultrastable interferometric filters for selecting a precise wavelength band of a relatively broad spectrum laser source, DHM® operating wavelengths are precisely controlled and perfectly stable. The height values do not depend on any calibration,  precise positioning, or repeatability of interferometric piezo-controller, neither on any motorized displacement, or confocal detection system. DHM® provides thus precise measurements with very high repeatability.

How can the calibration of a DHM® be certified ?

Lyncée Tec provides optionally a certificate issued by the Swiss National Metrology Center METAS for the transmission spectrum of the interferometric filter used to control the central wavelength and spectrum width. The operating wavelengths of the DHM® and thus heights measurements are fully controlled since no other critical distance or displacement are involved in the height measurement.

Is the DHM® height measurement affected by the Numerical Aperture (NA) of the microscopic objective ?

DHM illuminates the sample with a parallel beam, whereas most of alternative systems use high NA illumination cone.

Illumination of the sample with a high NA cone implies that incident rays, carrying the length of the ruler (the wavelength), are not all parallel to the optical axis. As a consequence an empirical correction factor has to be applied to height measurement [Mykura 1963Creath 1989, Schultz 1991]. Such correction factor is not needed with DHM®.

Measurement of standardized samples, as well as direct comparison of measurement performed with other metrological tools have demonstrated the high accuracy and repeatability of DHM® [Colomb 2010].

8.9 nm certified step

How does the DHM® wavelength change with temperature?

The wavelength filters used in the DHM® have a minimal dependence on temperature. For instance, changes on the transmission wavelength due to a change of temperature of 10°C are negligible for most applications.