Increasing capabilities in precision manufacturing and micro technology are accompanied by increasing demands of high
precision industrial metrology systems. Especially for measuring functional surfaces, areal optical principles are widely
used. If, in addition, nanometer height resolution is needed interferometers seem to be the most promising instruments.
First, this contribution focuses on the transfer characteristics of white-light interferometers with microscopic field of
view. In general, microscopic instruments suffer from their limited lateral resolution capabilities. Hence, the transfer
function of these instruments is typically assumed to show a linear low-pass characteristic. We studied the transfer
characteristics of white-light interferometers by theoretical simulations and experimental investigations. Our results show
that in most practical cases these instruments behave nonlinear, i.e. the output surface profile cannot be obtained from the
input profile by a simple linear filter operation.
Although they are well-established, there are some further limitations of white-light interferometers if they are used to
measure micro or even sub-microstructures. If edges, steeper slopes or abrupt slope changes are present on a measuring
object characteristic errors such as batwings occur. Furthermore, a high effort concerning the correction of chromatic
aberration is necessary in order to avoid dispersion effects. Otherwise, there will be systematic discrepancies between
profiles obtained from evaluation of the coherence peak and those resulting from the phase of the interference signals.
These may lead to 2π phase jumps if the fringe order is obtained from the position of the coherence peak. Finally,
measurement artifacts may also result if the measured micro-structure shows discontinuities of the surface slope.
This contribution analyses the different phenomena and discusses approaches to overcome existing limitations.
Due to its outstanding depth resolution capabilities vertical scanning low-coherence or white-light interferometry is one
of the most used optical techniques in the field of 3D micro-metrology. Unfortunately, step height structures often lead to
disturbing effects known as batwings in SWLI measurement that overlay the real profile heights of a rectangular
structure. As a consequence, the lateral resolution capabilities and the transfer characteristics of white-light interference
microscopes are difficult to characterize. In general, the lateral resolution of such instruments is assumed to agree with
the lateral resolution of a conventional light microscope for 2D imaging and the measurement process of an optical
profiler is assumed to be linear similar to a microscopic imaging process.
Our results show that there are significant discrepancies between the instrument transfer function of a white-light
interferometer and the optical transfer function of a conventional microscope. In this paper we analyze the transfer
characteristics of current white-light interferometers based on theoretical considerations, simulation studies, and
experimental investigations. It turns out that under certain conditions a correct measurement of a rectangular profile is
possible even if only the first order diffraction component is captured by an objective lens with a given numerical
In addition to the discussion of current instruments new approaches to overcome existing limits will be introduced: In
order to reduce the batwing effect we combine a Mirau white-light interferometer with a confocal illumination system.
Furthermore, it is shown that proper adaption of the evaluation wavelength of the low-coherent light can improve the
measurement accuracy significantly if rectangular profiles are obtained from the phase information inherent in WLI
Scanning white-light interferometry (SWLI) provides the capability of fast and high-precision three-dimensional
measurement of surface topography. Nevertheless, it is well-known that white-light interferometers more than imaging
microscopes suffer from chromatic aberrations caused by the influence of dispersion. Chromatic aberrations lead to
systematic measuring errors in SWLI, especially on micro-structures with curved or tilted surface areas. For example, the
plane glass plates used in a Mirau-interferometer are a potential source of dispersion. If this influence is not completely
corrected for, errors in height measurement occur. In addition, the magnitude of these errors strongly depends on whether
the coherence peak's position or the phase of an interference signal is evaluated. This study is intended to overcome
these difficulties by a dispersion optimized white-light interferometer. The design corresponds to a Mirau-interferometer,
but in order to reduce dispersion phenomena, a reflective Schwarzschild microscope objective is used.
For beam splitting a so-called pellicle is positioned in-between the objective and the measuring object. The dominant
effect, which limits the accuracy of the interferometer is supposed to depend on multiple reflections from the front and
the back side of the pellicle beam splitter. As a consequence, ghost signals were measured in addition to the typical
white-light interference signals. This indicates that multiple reflections influence the results and finally limit the accuracy
of the interferometer.
The precise and fast acquisition of three-dimensional geometrical data of micro-components is mainly performed by two
alternative measuring techniques, either vertical scanning white-light interferometry (SWLI) or confocal microscopy.
Both are capable of measuring micro-structured surfaces with a very high precision while recording image sequences
during a so-called depth scan. For most applications the axial resolution of these systems is sufficient. Though, in certain
applications the lateral resolution of a measurement system is far more critical. In addition, there are several approaches
to define the lateral resolution in 3D microscopy. In this paper, we discuss some physical approaches to improve lateral
resolution and transfer characteristics in SWLI.
As a contribution to the EC-funded project "NanoCMM" we developed a special kind of Linnik white-light
interferometer, which provides a lateral resolution well below one micrometer even for working distances of more than
5 mm. The resolution enhancement was achieved by wavelength reduction, i.e. LEDs emitting in the blue and near UV
range were used for illumination. We compare the results obtained from a silicon pitch standard based on different
illumination sources with a conventional Mirau interferometer providing the same magnification. Finally, in another
setup we show that the batwing effect can be successfully reduced by using a confocal aperture in the illumination path
of the interferometer. The combination of these different modifications clearly improves SWLI measurement results in
comparison with those performed by conventional white-light interferometers.
We established an interferometric sensor for optical precision measurement of distance changes. A fiber-coupled micro-optical
probe with an integrated reference surface is mounted on a bending beam. A piezoelectric actuator deflects the
beam. Besides focus scanning this deflection modulates the optical path length of the measuring arm of the
interferometer, while the reference path remains unchanged. If the distance between optical probe and measuring object
changes, characteristic phase shifts of the corresponding interference signals appear. This enables us to achieve an
interferometric resolution. The problem of λ/2 ambiguity is solved by using the signal envelope resulting from confocal
For geometry measurement of high precision machined mechanical or optical workpieces a resolution in the nanometer
range is generally required. This can be reached by interferometric principles. In addition, measurement at steep flanks
can be achieved by optical systems with high numerical apertures. Unfortunately, a high NA is always accompanied by a
small depth of focus leading to a very limited measuring range. A possible solution in this context is a so-called depth
We realized a pointwise measuring interferometric sensor and use a piezo driven bending beam for the depth scan. A
micro-optical fiber probe with an integrated reference surface is mounted at the top of this beam. By use of a
piezoelectric actuator driven close to the resonant frequency of several hundred Hertz the beam deflects with a few
micrometers of amplitude. By this oscillation the optical path length of the measuring rays of the interferometer is
modulated, while the reference path remains unchanged. This leads to an interference signal which shows characteristic
changes in phase as the average distance between optical probe and measuring object changes.
Particularly in optical industries and in micro systems technology white-light interferometry has become a standard tool
for highly accurate topography measurement. Our work is based on a modified commercial white-light interferometer
with a tube lens of a rather short focal length. This allows a compact design and a large field of view without influencing
the numerical aperture of the objective. Furthermore, a LED illumination is used, which is a precondition for our
approach. The short focal length of the tube lens requires a proper optical correction in order to avoid measuring errors
caused by aberrations. Nevertheless, spherical surfaces with relatively large local surface tilts or MEMS with sharp edges
often give rise to systematic measuring errors. These are caused by diffraction and dispersion effects, which finally lead
to deviations between height values obtained from the envelope's maximum of a white-light interference signal and
those values obtained from the signal's phase. For certain cases this may result in ghost steps in the measured
topography. In order to identify these steps we use a second phase evaluation at a different center wavelength. During
the depth scan images are taken for both center wavelengths. A special evaluation enables us to clearly identify the
appearing phase steps and to correct the results in a second step. The main application of this technique is the
measurement of curved or structured specular surfaces with high resolution, which until now is limited by the occurring
effects. In addition, it might be possible to use low-cost optics in combination with the dual-wavelength technique in
order to correct the measuring errors resulting from optical aberrations.