**2. Length metrology**

The basis of any dimensional measurement technique is found on the realization of the SI unit of length via frequency-stabilized lasers and displacement interferometry. The measurement technologies employed include laser-ranging devices, large-scale coordinate measuring machines (CMMs), optical- and ultraviolet-light microscopes, scanning electron microscopes (SEMs), atomic force microscopes (AFMs), and scanning tunneling microscopes (STMs).

Both direct and indirect high accuracy measurements of length, distance and displacement make use of wavelength or optical frequency sensing techniques. Direct measurement techniques include laser interferometer calibration of computer numerical controlled (CNC) machine tools and CMMs, and commercial laser-based instrumentation is widely used both nationally and internationally for this purpose, to measure displacements and distance from typically a hundred nanometers to tens of meters. Multiple wavelength instrumentation is used to extend accuracy within well-controlled environments, whilst modulated laser ranging techniques (electronic distance measurements) are now widely applied in surveying over distances up to a few kilometers with, in some cases, sub-millimeter precision. Such precision instrumentation comprises laser wavelength sources as measurement transducers of varying degrees of stability and accuracy.

Dimensional metrology covers measurement of dimensions and in principle also geometries based on distance measurements in a wide range of more specific measurements, targeted on from primary sources, i.e. lasers to geometrical measurement of complex profiles, which typically include:


One of the most sophisticated challenges for science and the high technologies engineering is the growing need to address real industrial problems rather than the ideal measurement situation and embed the traceable length metrology directly into technological processes by performing precise dynamic measurements in more demanding environments than those of

This chapter will present a synopsis and analysis of literature and existing scientific and technical solutions of precision length calibration. It covers analysis of laser interferometers, line detection systems, measurement signals and algorithms, as well as measurement capabilities of state-of-the art calibration systems worldwide. The contribution also addresses a thorny issue of achieving reliable measurements and meeting contradictory requirements of accuracy and productivity of line scale calibration in non-ideal environmental conditions, under the influence of many external influencing factors. The problems will be also upon the anvil of the development of an interferometer-controlled comparator that is operated in dynamic mode and enables to trace the calibration of line

The basis of any dimensional measurement technique is found on the realization of the SI unit of length via frequency-stabilized lasers and displacement interferometry. The measurement technologies employed include laser-ranging devices, large-scale coordinate measuring machines (CMMs), optical- and ultraviolet-light microscopes, scanning electron microscopes (SEMs), atomic force microscopes (AFMs), and scanning tunneling microscopes (STMs).

Both direct and indirect high accuracy measurements of length, distance and displacement make use of wavelength or optical frequency sensing techniques. Direct measurement techniques include laser interferometer calibration of computer numerical controlled (CNC) machine tools and CMMs, and commercial laser-based instrumentation is widely used both nationally and internationally for this purpose, to measure displacements and distance from typically a hundred nanometers to tens of meters. Multiple wavelength instrumentation is used to extend accuracy within well-controlled environments, whilst modulated laser ranging techniques (electronic distance measurements) are now widely applied in surveying over distances up to a few kilometers with, in some cases, sub-millimeter precision. Such precision instrumentation comprises laser wavelength sources as measurement transducers

Dimensional metrology covers measurement of dimensions and in principle also geometries based on distance measurements in a wide range of more specific measurements, targeted on from primary sources, i.e. lasers to geometrical measurement of complex profiles, which

measurement of laser wavelength/frequency, stability, drift and line width of radiation

measurement of size or geometric features, like pitch, of 1D artifacts, for example end

measurement of size and/or locations of features in 2D structures common in the semi-

sources that are used for interferometry and distance measurement;

conductor industry, such as in the complex patterns of integrated circuits measurement of size location and orientation of features in 3D patterns;

calibration laboratories.

**2. Length metrology** 

typically include:

scale of up to L ≤ 3.5 m long to the wavelength standard.

of varying degrees of stability and accuracy.

standards and linear scales or encoders;


Calibration of a variety of parameters associated with the source, such as absolute wavelength or frequency, linewidth, stability or drift, are thus of primary importance to high precision length traceability. In parallel, techniques in wavelength metrology are targeted on other applications. These include spectral bandwidth characterization by wavelength division multiplexing (WDM) for optical communications, high resolution spectral analysis using Fabry-Perot standards, and high accuracy measurement of spectroscopic phenomena, which has strong input to scientific spectroscopy. Precision length metrology also plays a key role in the realization of derived units of pressure and current, for example. The highest wavelength/frequency accuracy and stability available contributes to the leading-edge determination of fundamental physical constants (DTI/NMDS, 2002a).

There are a number of sensor technologies and instruments with nanometer, or better, accuracy for measuring length that repeat well if used carefully, including the scanning probe and electron microscopes and some optical devices. However, universal measurement standards have not yet been established and even apparently sophisticated users of atomic force microscopes can produce large variations in their measurements of the same artifacts. Without agreed standards, tools or machines cannot be calibrated at the nanometer scale (Bureau International des Poids et Mesures, 2003).

Line graduated geometric bodies, with graduation spacings representing known distances, are the bases for all direct measurements of specific distances. It follows that instruments having line graduated elements as integral members may be considered the only mechanical means capable of carrying out direct measurements without complementary equipment or processes (Farago, F.T. & Curtis, 1994).

The need for reduced uncertainty in the "primary standard" aspect of length, i.e., in its definition and realization, and in the "secondary standard" aspect, i.e., in its transfer and dissemination through dimensional metrology, is linked strongly to tightening tolerances in industrial manufacturing.
