Scanning White Light Interferometry (SWLI), now referred to as Coherence Scanning Interferometry (CSI), is established as a powerful tool for sub-nanometer surface metrology. The technique provides accurate and rapid three dimensional topographical analysis without contacting the surface under measurement. This paper will focus on recent developments of CSI using the Helical Complex Field (HCF) function that have extended its use for important thin film measurements. These developments now enable CSI to perform thin film thickness measurements, to measure the surface profile and the interfacial surface roughness of a buried interface and to derive optical constants (index of refraction n and extinction coefficient K).
A chromatic confocal microscope is a single point non-contact distance measurement sensor. For three decades
the vast majority of the chromatic confocal microscope use refractive-based lenses to code the measurement axis
chromatically. However, such an approach is limiting the range of applications. In this paper the performance of
refractive, diffractive and Hybrid aspheric diffractive are compared. Hybrid aspheric diffractive lenses combine
the low geometric aberration of a diffractive lens with the high optical power of an aspheric lens. Hybrid aspheric
diffractive lenses can reduce the number of elements in an imaging system significantly or create large hyper-
chromatic lenses for sensing applications. In addition, diffractive lenses can improve the resolution and the
dynamic range of a chromatic confocal microscope. However, to be suitable for commercial applications, the
diffractive optical power must be significant. Therefore, manufacturing such lenses is a challenge. We show in this
paper how a theoretical manufacturing model can demonstrate that the hybrid aspheric diffractive configuration
with the best performances is achieved by step diffractive surface. The high optical quality of step diffractive
surface is then demonstrated experimentally.
Publisher’s Note: This paper, originally published on 5/10/14, was replaced with a corrected/revised version on 5/19/14. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
This paper considers coherence scanning interferometry as a linear filtering operation that is characterised by a point
spread function in the space domain or equivalently a transfer function in the frequency domain. The applicability of the
theory is discussed and the effects of these functions on the measured interferograms, and their influence on the resulting
surface measurements, are described. The practical characterisation of coherence scanning interferometers using a
spherical reference artefact is then considered and a new method to compensate measurement errors, based on a modified
inverse filter, is demonstrated.
In the field of scanning white light interferometry (SWLI), it is well known that films of optical thickness in excess of the
coherence length may be measured by simply taking advantage of the fact that such films exhibit interference maxima
corresponding to each interface. In fact for the majority of such 'thick' films the determined thickness has a DC error arising
from the spectral phase-change on reflection at the two interfaces. For thinner films, the interference maxima coalesce and
it was for this regime that the HCF (helical complex field) was previously introduced to allow thin film extraction. This work
has now been significantly extended with a demonstrated capability to extract film interfaces with a lateral (XY) resolution
of 1.25μm and a (Z) surface rms noise of ~0.75A (angstrom). It is also capable of covering both the thin and thick film regimes (from
~50nm to several microns, both limits being material dependant).
Results are presented showing the performance of this approach, these include 'micro-scratches' that are apparent in the
thickness of the deposited layers as well as substrate/film and film/air interfaces. These are compared to the original 'surface'
as determined by SWLI and by AFM surface measurements. Additionally a brief comparison is made between film thickness determination using this approach, spectrophotometry, ellipsometry and stylus profilometry.
Scanning white light interferometry (SWLI) is now an established technique for the measurement of surface topography. It has
the capability of combining sub-nanometre interferometric resolution with a range limited only by the z-traverse, typically at
least 100&mgr;m. A very useful extension to its capability is the ability to measure thin films on a local scale. For films with
thicknesses in excess of ~2&mgr;m (depending on refractive index), the SWLI interaction with the film leads simply the formation
of two localised fringe bunches, each corresponding to a surface interface. It is evidently 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 ~2&mgr;m, 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. Following the derivation of the HCF function, examples of
extracted multilayer structures are presented. This is followed by a discussion of the limits of the approach.
Scanning white light interferometry (SWLI) is now an established technique for the measurement of surface topography. It has the capability of combining sub-nanometre interferometric resolution with a range limited only by the z-traverse, typically at least 100μm. A very useful extension to its capability is the ability to measure thin films on a local scale. For films with thicknesses in excess of ~2μm (depending on refractive index), the SWLI interaction with the film leads simply the formation of two localised fringe bunches, each corresponding to a surface interface. It is evidently 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. Following the derivation of the HCF function, examples of extracted multilayer structures are presented. This is followed by a discussion of the limits of the approach.
Through the consideration of a surface as a reflective phase grating, this paper establishes the various relationships between the statistical parameters of the surface topography and the far- field diffraction pattern. These relationships have been incorporated into Talyfine, a new measuring instrument designed primarily for the measurement of flats and cylinders. The performance of this instrument is compared against established stylus-based surface texture measuring instruments, Talystep and Form Talysurf.
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