Light fields behind microstructures: study of the Babinet-principle in the Fresnel regime

The Babinet’s principle is a general theorem that holds for electromagnetic waves and is often applied to diffraction of light. Diffraction pattern of a body and that of the complement body are compared and the sum of both radiation patterns must be the same as for an unobstructed beam. In most cases of practical applications light fields are measured by recording the intensity and is limited by a numerical aperture smaller than one. In our contribution we study amplitude and phase behind amplitude structures. We demonstrate using the example of Talbot images what happens in experiments that try to use the Babinet’s principle to understand diffraction patterns for small periodic structures. Of particular interest is the regime close to the structure, the so-called Fresnel diffraction regime. We present results of simulation and measurement to show the impact of a limited numerical aperture

On the Characteristics of Highly Confined Light Fields

Myun Sik Kim

The Investigation of highly confined light fields created by different mechanisms, such as refraction, diffraction, and scattering, are the main subject of this thesis. We distinguish a specific scattering phenomenon of dielectric microspheres from general definition of light scattering that includes refraction and diffraction. Light confinement has been studied by numerous researchers in theoretical ways. However, the experimental approaches for microstructures were always limited due to the lack of resolution and functionalities of the measurement system. To understand the characteristics of optical fields propagating in free space or emerging form microstructures, intensity is not always sufficient. It is therefore necessary to measure the phase of light. The main goal of this work was to set up an optical instrument that is able to measure the amplitude and phase of the light field in the real 3D object space. By combining an optical interferometer (Mach-Zehnder type) with an optical microscope, we have developed a multi-functional high-resolution interference microscope (HRIM). Phase shifting interferometry (PSI) and the five frame (Schwider-Hariharan) algorithm have been introduced in the HRIM in order to directly measure the phase. Auxiliary techniques have been applied for the first time to the experimental setup of the HRIM. For example, multiple-wavelengths light sources for the study of the spectral dependency, immersion of object space for an enhanced resolution and a phase telescope (Bertrand lens) to image and manipulating the Fourier plane (the back focal plane of the objective) have been demonstrated. In addition to a plane-wave illumination, the engineering of the illumination beams allows to broaden the range of experimental investigations for a given sample, such as wavefront management by focused Gaussian beams and focused Bessel beams, and polarization-induced beam shaping for particular shapes of localized illuminations to a small sample. Our main subject is the confinement of light. Three physical optical mechanisms are discussed in detail: refraction, diffraction and scattering. Discussion is done by examples. We have chosen effects that represent the different effects of physical optics at a microscale: highly focused Gaussian beam for refraction, the spot of Arago and Talbot effect for diffraction and photonic nanojets for scattering. Note that even though scattering in general includes refraction and diffraction, here we use the term "scattering" only for scattered hotspots, i.e., photonic nanojets, by dielectric microspheres. Our study contributes to the understanding of strong confinement in submicron or sub-wavelength dimensions by such effects and show for the first time experimental data on phase and amplitude fields. Basic physical principles, like the Gouy phase shift for focused light were demonstrated for the photonic nanojet, for a complex optical phenomenon where focusing is based on scattering rather than refraction. We present the capabilities of the HRIM system through a series of measurements of aforementioned problems for highly confined light fields. To meet the proper experimental conditions for each problem of interest, the HRIM employs several auxiliary techniques of conventional microscopy, e.g., multiple-wavelengths light sources, an immersion technique, and a manipulation of the Fourier plane (the back focal plane of the objective). Engineering of the illumination including polarization-induced beam shaping broadens the range of applications of the instrument and we illustrate this by examples. Furthermore, the HRIM can be operated in two distinguished modes for measurements of the axial phase evolution, "longitudinal-differential mode" or "propagation mode". The longitudinal differential mode directly visualizes peculiar phase evolutions along the optical axis used to prove the phase anomalies. The HRIM with auxiliary techniques therefore serves as a powerful tool for the investigation of highly confined light fields in small volumes.

EPFL2011

High power ultraviolet LED illumination systems: coherence properties and applications in photolithography

Johana Bernasconi

Mask-aligner photolithography is a technology used to replicate patterns from a mask to a photosensitive substrate. It is widely used in the fabrication of MEMS and micro-optical components, and for other applications with dimensions in the micrometer range. Traditionally, the light sources used for mask-aligners are high-pressure mercury arc lamps, which emit in the ultraviolet range with peaks at 365nm, 405nm and 435nm, the so-called g-, h- and i- lines. These lamps suffer from several disadvantages, such as a low efficiency, bulkiness, a short lifetime, and the toxicity of mercury. Finding an alternative to mercury arc lamps would be highly beneficial. In addition, specific techniques in mask-aligner technologies like Talbot lithography, multiple exposures, mask-source optimization or optical proximity correction lithography require high power sources with an increased control over the angular distribution and the spatial coherence. This is not easily done with a mercury arc lamp illumination. A method to easily measure the angular distribution and the spatial coherence in the mask-plane would be of great interest. In recent years, high power ultraviolet LEDs at the same wavelengths have appeared on the market. LEDs possess a smaller Etendue, they can be electronically driven at high frequencies and have a superior lifetime. This makes them ideal candidates to overcome the limitations of the mercury arc lamp illumination systems. The work focuses on the development and study of a novel LED-based illumination system for mask aligner lithography. This illumination system consists of an array of 7x7 LEDs, with individual reflectors. They form a modular 250W source which can replace a 1kW mercury arc lamp. The light is collected by the reflectors and brought onto a fly¿s eye integrator with two subsequent lenses which shape and homogenize the light field. Different patterns can be created by the source, determining the angles and the spatial coherence in the mask plane. The first part of the work presents the design and a complete set of characterizations of the final prototype. The achieved irradiance uniformity in the mask-plane of a MA/BA8 Gen3 SUSS mask-aligner is within ±1.2-2%. Prints tests in proximity printing, with a gap of 30µm, demonstrate a resolution of 3.5µm. In the second part of the thesis, the development of a method to measure the spatial coherence, which is based on a double slit approach and backed up by simulation, is described. The link between the spatial coherence length and the angular extent is made and the measurements show a good agreement with the analytical expression. A compact system is described, which enables the measurement of the spatial coherence in the mask plane, where only a little space is available. An original method is put into practice in order to measure the directional spatial coherence in two dimensions in the mask plane of a mask-aligner. This is achieved by using a circular double slits system. Prints are also made which illustrate the importance of the angular distribution and the spatial coherence. This novel LED-based illumination system is implementable in current, commercial, mask-aligners. It enables a greater efficiency, more functionalities and a longer lifetime compared to standard mercury arc lamp illumination systems. In particular, the spatial coherence properties of the source can be precisely managed and monitored thanks to the directional spatial coherence measurement.

EPFL2018

Phase calibration of a basic bright-field microscope for 3D metrology of transparent samples at the nanoscale

Martinus Gijs, Daniel Migliozzi, Bingying Zhao

Optical retrieval of the structure of transparent objects at the nano-scale requires adapted methods capable of probing their interaction with suitable light. Scattering events, for instance, depend on the content and the arrangement of the medium encountered by the light along its propagation path. Physical models of scattering were used in the past to understand image formation and phase retrieval in transparent objects. Here, we considered one of them, which is based on the acquisition of defocused images obtained with partially coherent illumination, and explored its phase retrieval capability over a wide range of illumination angles. We used a basic transmission bright-field microscope with a bandpass filter and an adjustable illumination aperture. The bandpass filter was used to select a specific wavelength range to avoid light dispersion in the sample and maximize illumination capabilities. The adjustable illumination aperture was used to probe and assess the calibration over a wide range of illumination angles, which give access to different parts of the spatial frequency spectrum of the sample. We subsequently employed a computational algorithm to retrieve the local 3-dimensional phase-shift induced on the light field by the scattering through the sample. We imaged several types of samples to explore the calibration and the results in different experimental configurations. We employed: (1) commercial dielectric nanospheres to assess the phase calibration when measured along the optical axis, (2) custom-made nanosteps micropatterned in a glass substrate to assess the phase calibration when measured along the transversal axis. We first verified the model prediction in the spatial frequency domain for the scattering induced by our samples, and subsequently obtained a consistent and linear phase-calibration for illumination numerical apertures ranging from 0.1 to 0.5. This enabled us to calibrate a very simple optical system in a wide range of illumination angles to obtain quantitative 3D metrology of transparent samples at the nanoscale.

SPIE-INT SOC OPTICAL ENGINEERING2021