Printing sub-micron structures using Talbot mask-aligner lithography with a 193 nm CW laser light source

A continuous improvement of resolution in mask-aligner lithography is sought after to meet the requirements of an ever decreasing minimum feature size in back-end processes. For periodic structures, utilizing the Talbot effect for lithography has emerged as a viable path. Here, by combining the Talbot effect with a continuous wave laser source emitting at 193 nm, we demonstrate successfully the fabrication of periodic arrays in silicon substrates with sub-micron feature sizes. The excellent coherence and the superior brilliance of this light source, compared to more traditional mercury lamps and excimer lasers as light source, enables the efficient beam shaping and a reduced minimum feature size at a fixed gap of 20 μm. We present a comprehensive study of proximity printing with this system, including simulations and selected experimental results of prints in up to the fourth Talbot plane. This printing technology can be used to manufacture optical metasurfaces, bio-sensor arrays, membranes, or microchannel plates

Resolution enhancement in mask aligner photolithography

Photolithographie ist eine unentbehrliche Technologie in der heutigen Mikrofabrikation integrierter elektronischer Schaltungen und optischer Komponenten auf verschiedenen Größenskalen. Die zugrundeliegende Aufgabe ist die Replikation der gewünschten Struktur, die kodiert ist in einer Photomaske, auf einem photolackbedeckten Wafer. In vergangenen Jahrzehnten gab es eine beeindruckende Weiterentwicklung photolithographischer Anlagen, was Auflösungen weit unterhalb eines Mikrometers ermöglicht. Das einfachste photolithographische Instrument ist der Maskenjustierbelichter, bei dem die Photomaske und der Wafer entweder in Kontakt oder in unmittelbare Nähe gebracht werden (Proximity-Modus), ohne zusätzliche optische Komponenten dazwischen. Vor über 50~Jahren eingeführt bleibt der Maskenjustierbelichter aufgrund seines wirtschaftlichen Betriebs das Instrument der Wahl für die Herstellung unkritischer Schichten, mit einer Auflösung von einigen Mikrometern im bevorzugten Proximity-Modus. Maskenjustierbelichter werden beispielsweise für die Herstellung von Mikrolinsen, lichtemittierende Dioden und mikromechanischen Systemen verwendet. Die erreichbare laterale räumliche Auflösung ist letztlich begrenzt durch die Beugung des Lichts an den Strukturen der Photomaske, was zu Verfälschungen der Abbildung auf dem Photolack führt. In dieser Arbeit entwickeln, präsentieren und diskutieren wir mehrere Technologien zur Auflösungsverbesserung für Maskenjustierbelichter im Proximity-Modus. Dies umfasst Photolithographie mit einer neuartigen Lichtquelle, die im tiefen Ultraviolett-Bereich emittiert, eine rigoros optimierte Phasenschiebermaske für periodische Strukturen, optische Proximity-Korrektur (Nahbereichskorrektur) angewandt auf nichtorthogonale Geometrien, und die Anwendung optischer Metaoberflächen als Photomasken. Eine Reduzierung der Wellenlänge verringert die Auswirkungen der Lichtbrechung und verbessert daher direkt die Auflösung, benötigt aber auch die Entwicklung geeigneter Konzepte für die Strahlformung und Homogenisierung der Beleuchtung. Wir diskutieren die Integration einer neuartigen Lichtquelle, ein frequenzvervierfachter Dauerstrichlaser mit einer Emissionswellenlänge von 193$\,$nm, in einem Maskenjustierbelichter. Damit zeigen wir erfolgreiche Prints von Teststrukturen mit einer Auflösung von bis zu 1,75$\,$µm bei einem Proximity-Abstand von 20$\,$µm. Bei Verwendung des selbstabbildenden Talboteffekts wird sogar eine Auflösung weit unterhalb eines Mikrometers für periodische Strukturen erzielt. Außerdem diskutieren wir die rigorose Simulation und Optimierung der Lichtausbreitung in und hinter Phasenschiebermasken, die unter schrägem Einfall belichtet werden. Mit einem optimierten Photomaskendesign kann dabei die Periode bei Belichtung unter drei diskreten Winkeln verkleinert abgebildet werden. Dies erlaubt es, Strukturen deutlich kleiner als ein Mikrometer abzubilden, wobei die Strukturen auf der Photomaske deutlich größer und damit einfacher herzustellen sind. Zudem betrachten wir eine Simulations- und Optimierungsmethode für die optische Proximity-Korrektur nicht-orthogonaler Strukturen, was deren Formtreue verbessert. die Wirksamkeit beider Konzepte bestätigen wir erfolgreich in experimentellen Prints. Die Verwendung optischer Metaoberflächen erweitert die Fähigkeiten zur Wellenfrontformung von Photomasken gegenüber etablierten Intensitäts- oder Phasenschiebermasken. Wir diskutieren zwei Designs für optische Metaoberflächen, die beide den vollen $2\,\pi$-Phasenbereich abdecken. Ein Design beinhaltet dabei noch einen plasmonischen Absorber, was zusätzliche Möglichkeiten bietet, den Transmissionskoeffizient anzupassen. Desweiteren beschreiben wir einen Algorithmus zur Berechnung des Maskenlayouts für beliebige Strukturen. Eine kontinuierliche Weiterentwicklung von Maskenjustierbelichtern ist unerlässlich, um Schritt zu halten mit der fortschreitenden Miniaturisierung in allen Bereich der Optik, der Mechanik und der Elektronik. Unsere Forschungsergebnisse ermöglichen es, die Auflösung der optischen Lithographie im Proximity-Modus zu verbessern und sich damit den zukünftigen Herausforderungen der optischen Industrie stellen zu können

Printing sub-micron structures using Talbot mask-aligner lithography with a 193 nm CW laser light source

A continuous improvement of resolution in mask-aligner lithography is sought after to meet the requirements of an ever decreasing minimum feature size in back-end processes. For periodic structures, utilizing the Talbot effect for lithography has emerged as a viable path. Here, by combining the Talbot effect with a continuous wave laser source emitting at 193 nm, we demonstrate successfully the fabrication of periodic arrays in silicon substrates with sub-micron feature sizes. The excellent coherence and the superior brilliance of this light source, compared to more traditional mercury lamps and excimer lasers as light source, enables the efficient beam shaping and a reduced minimum feature size at a fixed gap of 20 µm. We present a comprehensive study of proximity printing with this system, including simulations and selected experimental results of prints in up to the fourth Talbot plane. This printing technology can be used to manufacture optical metasurfaces, bio-sensor arrays, membranes, or microchannel plates

High-power modular LED-based illumination systems for mask-aligner lithography

Mask-aligner lithography is traditionally performed using mercury arc lamps with wavelengths ranging from 250 nm to 600 nm with intensity peaks at the i, g and h lines. Since mercury arc lamps present several disadvantages, it is of interest to replace them with high power light emitting diodes (LEDs), which recently appeared on the market at those wavelengths. In this contribution, we present a prototype of an LED-based mask-aligner illumination. An optical characterization is made and the prototype is tested in a mask-aligner. Very good performances are demonstrated. The measured uniformity in the mask plane is 2:59 ± 0:24 % which is within the uniformity of the standard lamp. Print tests show resolution of 1 micron in contact printing and of 3 microns in proximity printing with a proximity gap of 30 microns

Retroreflector for Photonic Doppler Velocimetry

In order to meet the goals of the Department of Defense (DoD) for smaller and more accurate weapons, the Munitions Directorate of the Air Force Research Laboratory (AFRL/RW) has numerous projects investigating the miniaturization of weapons and munition fuze components. One of these efforts is to characterize the performance of small detonators. The velocity of the flyer, the key component needed to initiate a detonation sequence, can be measured using a photonic Doppler velocimeter (PDV). The purpose of this research was to develop a microelectromechanical system (MEMS) device that would act as an optimal retroreflective surface for the PDV. Two MEMS solutions were explored: one using the PolyMUMPs™ fabrication process and one in-house fabrication design using silicon on insulator (SOI) wafers. The in-house de- sign consisted of an array of corner reflectors created using an SOI wafer. Each corner reflector consisted of three separate mirror plates which were self-assembled by photoresist pad hinges. When heated to a critical temperature (typically 140-160 ◦C), the photoresist pads melted and the resulting surface tension caused each mirror to rotate into place. The resulting array of corner reflectors was then coated with a thin layer of gold to increase reflectivity. Despite the successful assembly a PolyMUMPs™ corner reflector, assembling an array of these reflectors was found to be unfeasible. Although the SOI corner reflector design was completed, these devices were not fabricated in time for testing during this research. However, the bidirectional reflectance distribution function (BRDF) and optical cross section (OCS) of commercially avail- able retroreflective tapes were measured. These results can be used as a baseline comparison for future testing of a fabricated SOI corner reflector array

Antenna-coupled Infrared Focal Plane Array

In this dissertation a new type of infrared focal plan array (IR FPA) was investigated, consisting of antenna-coupled microbolometer fabricated using electron-beam lithography. Four different antenna designs were experimentally demonstrated at 10-micron wavelength: dipole, bowtie, square-spiral, and log-periodic. The main differences between these antenna types were their bandwidth, collection area, angular reception pattern, and polarization. To provide pixel collection areas commensurate with typical IR FPA requirements, two configurations were investigated: a two-dimensional serpentine interconnection of individual IR antennas, and a Fresnel-zone-plate (FZP) coupled to a single-element antenna. Optimum spacing conditions for the two-dimensional interconnect were developed. Increased sensitivity was demonstrated using a FZP-coupled design. In general, it was found that the configuration of the antenna substrate material was critical for optimization of sensitivity. The best results were obtained using this membranes of silicon nitride to enhance the thermal isolation of the antenna-coupled bolometers. In addition, choice of the bolometer material was also important, with the best results obtained using vanadium oxide. Using optimum choices for all parameters, normalized sensitivity (D*) values in the range of mid 108[√Hz/W] were demonstrated for antenna-coupled IR sensors, and directions for further improvements were identified. Successful integration of antenna-coupled pixels with commercial readout integrated circuits was also demonstrated

DESIGN, FABRICATION AND TESTING OF HIERARCHICAL MICRO-OPTICAL STRUCTURES AND SYSTEMS

Micro-optical systems are becoming essential components in imaging, sensing, communications, computing, and other applications. Optically based designs are replacing electronic, chemical and mechanical systems for a variety of reasons, including low power consumption, reduced maintenance, and faster operation. However, as the number and variety of applications increases, micro-optical system designs are becoming smaller, more integrated, and more complicated. Micro and nano-optical systems found in nature, such as the imaging systems found in many insects and crustaceans, can have highly integrated optical structures that vary in size by orders of magnitude. These systems incorporate components such as compound lenses, anti-reflective lens surface structuring, spectral filters, and polarization selective elements. For animals, these hybrid optical systems capable of many optical functions in a compact package have been repeatedly selected during the evolutionary process. Understanding the advantages of these designs gives motivation for synthetic optical systems with comparable functionality. However, alternative fabrication methods that deviate from conventional processes are needed to create such systems. Further complicating the issue, the resulting device geometry may not be readily compatible with existing measurement techniques. This dissertation explores several nontraditional fabrication techniques for optical components with hierarchical geometries and measurement techniques to evaluate performance of such components. A micro-transfer molding process is found to produce high-fidelity micro-optical structures and is used to fabricate a spectral filter on a curved surface. By using a custom measurement setup we demonstrate that the spectral filter retains functionality despite the nontraditional geometry. A compound lens is fabricated using similar fabrication techniques and the imaging performance is analyzed. A spray coating technique for photoresist application to curved surfaces combined with interference lithography is also investigated. Using this technique, we generate polarizers on curved surfaces and measure their performance. This work furthers an understanding of how combining multiple optical components affects the performance of each component, the final integrated devices, and leads towards realization of biomimetically inspired imaging systems