Advanced metrology plays an important role in the research, production and quality control of optical components. With
surface finish, form error and other parameter specifications becoming more stringent, precision measurements are
increasingly demanded by optics manufacturers and users. The modern metrologist now has both contact and noncontact
measurement solutions available and a combination of these techniques now provides a more detailed
understanding of optical components. Phase Grating Interferometry (PGI) with sub-nanometre vertical resolution and
sub-micron lateral resolution can provide detailed characterization of a wide range of components including shallow and
steep-sided optics. PGI is ideal for precision form measurement of a comprehensive range of lenses, moulds and other
spherical or aspheric products. Because of the complex nature of these components, especially precision aspheric and
asphero-diffractive optics, control of the form is vital to ensure they perform correctly. Recent hardware and software
developments now make it possible to gain a better understanding and control of the form and function of this optics.
Another change is the use of high speed 3D non-contact measurement of optics which is becoming more popular. Often
scanning interferometric techniques such as coherence correlation interferometry (CCI) can be used to study components
not suited to 2D contact analysis, including fragile surfaces and structured surfaces. Scanning interferometry can also be
used to measure film thickness and uniformity of any coating present. In this paper the use of both PGI and CCI to
measure optical lenses and coatings is discussed.
Interferometry is now an established technique for the measurement of surface topography. It has the capability of
combining sub-nanometre resolution, high measurement speed with high repeatability. A very useful extension to its
capability is the ability to measure thick and thin films on a local scale. For films with thicknesses in excess of 1-2 μm
(depending on refractive index), the SWLI interaction with the film leads to the formation of localised fringes, each
corresponding to a surface interface. It is relatively trivial to locate the positions of these envelope maxima and therefore
determine the film thickness, assuming the refractive index is known. For thin films (with thicknesses ~20 nm to ~2 μm,
again depending on the refractive index), the SWLI interaction leads to the formation of a single interference maxima. In
this context, it is appropriate to describe the thin film structure in terms of optical admittances; it is this regime that is
addressed through the introduction of a new function, the 'helical conjugate field' (HCF) function. This function may be
considered as providing a 'signature' of the multilayer measured so that through optimization, the thin film multilayer
may be determined on a local scale.
Interferometry is now an established technique for the measurement of surface topography.
It has the capability of combining sub-nanometre resolution. A very useful extension to its capability
is the ability to measure thick and thin films on a local scale. For films with thicknesses in excess of
1-2μm (depending on refractive index), the SWLI interaction with the film leads simply the formation
of two localised fringes, each corresponding to a surface interface. It is relatively trivial to locate the
positions of these two envelope maxima and therefore determine the film thickness, assuming the
refractive index is known. For thin films (with thicknesses ~20nm to ~2μm, again depending on the
index), the SWLI interaction leads to the formation of a single interference maxima. In this context, it
is appropriate to describe the thin film structure in terms of optical admittances; it is this regime that is
addressed through the introduction of a new function, the 'helical conjugate field' (HCF) function.
This function may be considered as providing a 'signature' of the multilayer measured so that through
optimization, the thin film multilayer may be determined on a local scale.
The study of nanometre surface metrology is becoming more and more commonplace in industrial and research environments. Because of this expansion there are more and more technologies available for looking at the surface and due to the differences in the techniques each has its own specialist applications. Stylus profilometry, white light interferometry and confocal microscopy are common techniques used to measure surface metrology to nanometre precision. Strengths and weaknesses of each of the techniques are discussed with examples.
Optical interferometry is being used more and more to solve metrology problems. It is important in giving an insight into the properties of the top surface whether it is a processed surface or a thin film coating This paper will discuss metrology issues and ways in which to solve them using autocorrelation. Autocorrelation now commonly used to extract spatial information from surface profiles. It is suitable for looking at processing issues and identifying any periodic errors. An example showing the use of autocorrelation to solve process issues in diamond turning is shown.
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