[show abstract][hide abstract] ABSTRACT: In several optical systems, a specific Point Spread Function (PSF) needs to be generated. This can be achieved by shaping the complex field at the pupil. The Extended Nijboer-Zernike (ENZ) theory relates complex Zernike modes on the pupil directly to functions in the focal region. In this paper, we introduce a method to engineer a PSF using the ENZ theory. In particular, we present an optimization algorithm to design an extended depth of focus with high lateral resolution, while keeping the transmission of light high (over 60%). We also have demonstrated three outcomes of the algorithm using a Spatial Light Modulator (SLM).
[show abstract][hide abstract] ABSTRACT: We have performed an in vitro and in vivo study, based on laser speckle contrast analysis, to detect fluid pulsation in the presence of artifacts caused by the relative motion between the sample and the illumination source. We observe that the pulsation signal is clearly detectable for a range of motion amplitudes and oscillation frequencies; however, for higher amplitudes and oscillation frequencies of motion, the signal, due to pulsation, becomes increasingly difficult to detect.
[show abstract][hide abstract] ABSTRACT: In this paper we experimentally demonstrate the proof of concept for predictive control of thermally induced wavefront aberrations in optical systems. On the basis of the model of thermally induced wavefront aberrations and using only past wavefront measurements, the proposed adaptive optics controller is able to predict and to compensate the future aberrations. Furthermore, the proposed controller is able to correct wavefront aberrations even when some parameters of the prediction model are unknown. The proposed control strategy can be used in high power optical systems, such as optical lithography machines, where the predictive correction of thermally induced wavefront aberrations is a crucial issue.
[show abstract][hide abstract] ABSTRACT: Using the subspace identification technique, we identify a finite dimensional, dynamical model of a recently developed prototype of a thermally actuated deformable mirror (TADM). The main advantage of the identified model over the models described by partial differential equations is its low complexity and low dimensionality. Consequently, the identified model can be easily used for high-performance feedback or feed-forward control. The experimental results show good agreement between the dynamical response predicted by the model and the measured response of the TADM.
[show abstract][hide abstract] ABSTRACT: Non-interferometric phase retrieval from the intensity measurements in Coherent Fourier Scatterometry (CFS) is presented using a scanningfocused spot. Formulae to determine the state of polarization of the scattered light and to retrieve the phase difference between overlappingscattered orders are given. The scattered far field is rigorously computed and the functionality of the method is proved with experimentalresults.
Journal of the European Optical Society Rapid Publications 07/2013; 8(2013):13048. · 0.93 Impact Factor
[show abstract][hide abstract] ABSTRACT: In recent times, coherent Fourier scatterometry has been considered as an emerging optical
grating scatterometry technique for semiconductor metrology since it shows large sensitivity
owing to its scanning ability. However, further utilization of coherence is possible by making
additional measurements using the principle of temporal phase-shifting interferometry. In this
paper, through numerical simulation, we show how scanning and interferometry can be
coupled together to improve the sensitivity of coherent Fourier scatterometry, to extend its
range of applicability and to obtain sufﬁcient information to calculate the complex scattering
matrix for all angles of incidences inside the numerical aperture of a microscope objective.
Journal of optics 05/2013; 15(7):075707. · 1.99 Impact Factor
[show abstract][hide abstract] ABSTRACT: Through-focus phase retrieval methods aim to retrieve the phase of an optical field from its intensity distribution measured at different planes in the focal region. By using the concept of spatial correlation for propagating fields, for both the complex amplitude and the intensity of a field, we can infer which planes are suitable to retrieve the phase and which are not. Our analysis also reveals why all techniques based on measuring the intensity at two Fourier-conjugated planes usually lead to a good reconstruction of the phase. The findings presented in this work are important for aberration characterization of optical systems, adaptive optics and wavefront metrology.
[show abstract][hide abstract] ABSTRACT: We report an experimental and theoretical study on the alignment error
tolerance of a 2 μm-size solid immersion lens (SIL) illuminated by
different types of focused spots. Tightly confined focal spots are of
great interest for improving the performance of many optical systems, so
that a study on the alignment tolerance is of interest. In particular,
it was found that micro-SILs can be largely misaligned with respect to
the optical axis of an objective lens focusing light onto it and yet
allow for a reasonably good immersed spot. In fact, a displacement of
approximately 400 nm, i.e. one fifth of the lens diameter, is tolerable.
The measurements are compared with a rigorous finite element method
model for a micro-SIL, showing an excellent agreement.
[show abstract][hide abstract] ABSTRACT: We carry out performance characterisation of a commercial push and pull deformable mirror with 48 actuators (Adaptica Srl). We present a detailed description of the system as well as a statistical approach on the identification of the mirror influence function. A new efficient control algorithm to induce the desired wavefront shape is also developed and comparison with other control algorithms present in literature has been made to prove the efficiency of the new approach.
[show abstract][hide abstract] ABSTRACT: We report on the experimental and numerical demonstration of immersed submicron-size hollow focused spots, generated by structuring the polarization state of an incident light beam impinging on a micro-size solid immersion lens (μ-SIL) made of SiO2. Such structured focal spots are characterized by a doughnut-shaped intensity distribution, whose central dark region is of great interest for optical trapping of nano-size particles, super-resolution microscopy and lithography. In this work, we have used a high-resolution interference microscopy technique to measure the structured immersed focal spots, whose dimensions were found to be significantly reduced due to the immersion effect of the μ-SIL. In particular, a reduction of 37% of the dark central region was verified. The measurements were compared with a rigorous finite element method model for the μ-SIL, revealing excellent agreement between them.
New Journal of Physics 10/2012; 14(10):103024. · 4.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: Incoherent Fourier Scatterometry (IFS) is a successful tool for high accuracy nano-metrology. As this method uses only far field measurements, it is very convenient from the point of view of industrial applications. A recent development is Coherent Fourier Scatterometry (CFS) in which incoherent illumination is replaced by a coherent one. Through sensitivity analyses using rigorous electromagnetic simulations, we show that the use of coherence and multiple scanning makes Coherent Fourier Scatterometry (CFS) more sensitive than Incoherent Fourier Scatterometry (IFS). We also report that in Coherent Fourier Scatterometry it is possible to determine the position of the sample with respect to the optical axis of the system to a precision dependent only on the experimental noise.
Journal of the European Optical Society Rapid Publications 07/2012; Vol 7. · 0.93 Impact Factor
[show abstract][hide abstract] ABSTRACT: We study decoupled and coupled types of surface plasmons in the near UV
wavelength range (λ = 193, 365, 405nm) in circular metal film
diaphragms composed of concentric sub wavelength nanoslit grooves.
[show abstract][hide abstract] ABSTRACT: Under appropriate laser exposure, a thin film of InSb exhibits a sub-wavelength thermally modified area that can be used to focus light beyond the diffraction limit. This technique, called Super-Resolution Near-Field Structure, is a potential candidate for ultrahigh density optical data storage and many other high-resolution applications. We combined near field microscopy, confocal microscopy and time resolved pump-probe technique to directly measure the induced sub-diffraction limited spot in the near-field regime. The measured spot size was found to be dependent on the laser power and a decrease of 25% (100 nm) was observed. Experimental evidences that support a threshold-like simulation model to describe the effect are also provided. The experimental data are in excellent agreement with rigorous simulations obtained with a three dimensional Finite Element Method code.
[show abstract][hide abstract] ABSTRACT: We apply a phase retrieval algorithm to the intensity pattern of a Hartmann wavefront sensor to measure with enhanced accuracy the phase structure of a Hartmann hole array. It is shown that the rms wavefront error achieved by phase reconstruction is one order of magnitude smaller than the one obtained from a typical centroid algorithm. Experimental results are consistent with a phase measurement performed independently using a Shack-Hartmann wavefront sensor.
[show abstract][hide abstract] ABSTRACT: We present a rigorous numerical model to study the near-field
characteristics of the focused spot embedded in a Super Resolution Near
Field stack layer. The results indicate that a focused spot beyond the
diffraction limit can be achieved and its characteristics can be modeled
by proper choice of optical parameters.
[show abstract][hide abstract] ABSTRACT: A Hartmann Wavefront Sensor (HWS) is used as a tool to measure phase aberration at the EUV wavelength. Nevertheless, a conventional HWS measures only the wavefront slope in each sub-aperture and is not able to measure the phase structure inside it. This leads to an accuracy loss in the aberration reconstruction. In this work a phase retrieval algorithm is applied to the intensity pattern data in order to reconstruct the phase feature inside the sub-aperture and hence improve the accuracy. Experimental data confirms our simulations making this technique feasible regarding both the achieved accuracy and computational time. The phase information can be used to develop an adaptive optics system dedicated to a EUV stepper.
[show abstract][hide abstract] ABSTRACT: Incoherent Optical Scatterometry (IOS) is a well-established metrology technique in the semiconductor industry to retrieve periodic grating structures with high accuracy from the signature of the diffracted optical far field. With shrinking dimensions in the lithography industry, finding possible improvements in wafer metrology is highly desirable. The grating is defined in terms of a finite number of geometrical shape parameters (height, side-wall angles, midCD etc.). In our method the illumination is a scanning focused spot from a spatially coherent source (laser) within a single period of the grating. We present a framework to study the increment in sensitivity of Coherent Fourier Scatterometry (CFS) with respect to the IOS. Under suitable conditions, there is a more than fourfold enhancement in sensitivity for grating shape parameters using CFS. The dependence of scanning positions on the sensitivity analysis is also highlighted. We further report the experimental implementation of a Coherent Fourier Scatterometer. The simulated and experimental far fields are compared and analyzed for the real noise in the experimental configuration.
Metrology, Inspection, and Process Control for Microlithography XXV; 02/2012
[show abstract][hide abstract] ABSTRACT: We are developing an experimental setup to characterize dynamic
scattering in presence of static scattering. We attempt to retrieve the
flow parameters like fluid concentration and velocity in presence of
phantoms providing static scattering mimicking the characteristics of
skin. Our measurement relies on an optimally-designed optical setup
coupled to a high speed detector and the use of appropriate light
sources. The flow of particles causes a time varying effect on the
speckle pattern which can be measured quantitatively by the speckle
contrast term. The speckle contrast is defined as the ratio of standard
deviation and mean intensity of speckle variation. Depending on the
concentration and velocity of moving particles, the speckle pattern will
decorrelate and this results in a drop in the contrast which can also be
seen in the recorded images as blurring of the speckle pattern. In
literature, measured contrast is related to the velocity and the
concentration of the scatterers as it plays a major role in the speckle
correlation time (ιc). In our experimental setup we attempt to
recover the properties of the moving scatterers in presence of static
scatterers. In parallel we present experimental simulations of our
experiment comparing it with theoretical studies describing dynamic
speckle in presence of static scatterers.
[show abstract][hide abstract] ABSTRACT: Scatterometry is a well established technique currently utilized in research, as well as in industrial applications, to retrieve the properties of a given scatterer (the target) by looking at how the light coming from a certain source is diffracted in the far field. Currently the light source is often a discharge lamp that, after wavelength filtering, can be thought as a quasi-monochromatic, but spatially incoherent, source. In the present work, benefits of using a focused spot from a spatially coherent light source, as that emitted by a laser, are investigated on a theoretical viewpoint. The focused spot is scanned over the object of interest and, for each scan position, a far-field diffraction pattern is recorded. Our results show that spatially coherent light can sensibly increase the accuracy of the technique with respect to the target’s geometrical profile.
Applied Physics B 12/2011; 105(4):775. · 1.78 Impact Factor
[show abstract][hide abstract] ABSTRACT: The invention of the femto-second frequency comb laser has
revolutionized the field of high-resolution spectroscopy, by providing
very accurate reference frequencies in the optical domain, acting as a
'frequency ruler'. Similarly, a frequency comb can be viewed as a ruler
for distance measurement, which is based on the fact that the vacuum
distance between subsequent pulses is known with the accuracy of the
used time standard. We have recently demonstrated absolute distance
measurements using a FC laser applying a cross-correlation technique,
which was supported by a theoretical and a numerical study on the
formation of cross-correlation in dispersive media. In this contribution
a measurement scheme based on dispersive (spectral) interferometry is
presented. For the measurement of distances up to 50 meter,
sub-micrometer accuracy is achieved.