ABSTRACT
Interference of light fields is the basis formany optical measurement
techniques, some of which use the large coherence length of lasers,
but some also benefit from the finite coherence length of wideband
sources. With the emergence of sophisticated methodologies that
improve the signal-to-noise ratio, such as common path interfer-
ometers, superquiet light sources, phase-shifting devices, and wide-
dynamic-range detectors, interferometry can provide axialmeasure-
ments with precision approaching 1 pm or even better. This fact is
being utilized in the large interferometers for detecting gravitational
waves such the laser interferometer gravitational wave observa-
tory (LIGO). In optical imaging, interference has been utilized
successfully in several well-known classical microscopy techniques
such as the Zernike phase contrast microscope and the Nomarski
differential interference contrast microscope. In the context of finite
coherence techniques optical coherencemicroscopy (OCM) has been
known for long time in the context of interference microscopy [1-
8], coherence radar [9, 10], and white light interferometry [11-16].
Today OCM is a general term for the optical microscopy techniques
that are based on relatively short coherence lengths of the light and
can be classified into two types, both acting as a 3D imaging tool. The
first is low-temporal-coherence (TC) microscopy and macroscopy,
also known as optical coherence tomography (OCT), which is being
used for medical diagnostics, particularly in ophthalmology and
dermatology [17-20]. The second is full-field OCT (FF-OCT), in
which imaging is done both in the reference and sample paths
using lenses or microscope objectives [21-25]. Several modes were
used such as heterodyne detection [26], wide field configuration
[27], phase shift technique [28], and a stroboscopic mode [29].
Applications vary from imaging of the eye [30] and other scattering
tissue [31-33]. FF-OCT uses low spatial coherence (SC) and TC
similar to the well-known coherence probe microscopy (CPM) that
has been in use for a long time in optical metrology [1-8]. “CPM”was
the term coined for this technique by the optical metrology division
of KLA-Tencor (a company based in Santa Clara, CA, USA) and it
has many advantages over conventional microscopy or conventional
interferometry in its ability to discriminate between different
transparent layers in a multilayered stack. A very-low-coherence
microscope (Linnik microscope) that uses high numerical aperture
(NA) objectives and broadband white light has been in use for many
years within the metrology tools [34] of KLA-Tencor [5, 35-37] used
for the inspection of the fabrication processes of microelectronic
devices, particularly for autofocus purposes, the measurement of
the critical dimension (CD), and the stepper misalignment errors
between lithographically overlaid layers. Theoretical modeling of
the interferogram from such microscopes has been established
recently [36] based on scalar imaging theory, taking into account
coherence effects following the treatment by Hopkins [38, 39].
