Imprinted Bragg grating sensors based on hybrid polymers

Integriert-optische Bragg-Gitter-Sensoren sind hervorragend geeignet für die Anwendung in der Analytik und der Prozesstechnik. Das Bragg-Gitter reflektiert einen spektralen Anteil des geführten Lichts. Die reflektierte Wellenlänge ist abhängig von Umgebungseinflüssen, wie beispielsweise der Temperatur, der Luftfeuchte oder dem Brechungsindex der unmittelbaren Umgebung, wodurch ein breites Anwendungsfeld für derartige Sensoren gegeben ist. Die vorliegende Arbeit befasst sich mit der Entwicklung einer innovativen Prozessoption zur günstigen Herstellung von planaren oberflächenstrukturierten Bragg-Gitter-Sensoren auf Polymerbasis. Als neues großtechnisches Herstellungsverfahren wurde ein Prägeprozess angewendet. Dabei wird eine Masterstruktur in eine Prägeform (aus PDMS) abgeformt, mit der die Strukturen wiederum in das eigentliche Prägematerial übertragen werden. In dieser Arbeit kam die „UV-gestützte Substratkonforme Imprintlithographie“ (UV-SCIL) zum Einsatz, womit die Strukturübertragung komplexer Mikro- und Nanostrukturen auf Substratgrößen bis 200mm Durchmesser möglich ist. Als Prägematerial für UV-SCIL wurden erstmals Hybridpolymere (Ormocer®e) evaluiert, welche im Vergleich zu anderen Polymeren thermisch und chemisch stabiler sind. Ein kompletter Prägeprozess konnte für ein kommerzielles Ormocer® (OrmoComp®)entwickelt werden, wobei ganzflächig Wellenleiterstrukturen mit Krümmungen und Verzweigungen abgeformt wurden, sowie Gitterstrukturen mit einer Periode von-300 nm. Verschiedene Problemstellungen entlang der UV-SCIL-Prozesskette konnten weiterentwickelt werden. Ein vereinfachter und verkürzter Prozess zur Abscheidung einer Antihaftschicht auf dem Silizium-Master wurde eingeführt. Dieser garantierte eine defektfreie Abformung von Nanostrukturen. Weiterhin wurde die Prägewerkzeugherstellung optimiert, durch eine neue Prozessführung bei einer Teilschicht. Anstelle von 24 Stunden bei 50°C benötigte der neue Herstellungsprozess nur 8 Stunden bei Raumtemperatur. Zusätzlich konnte dadurch die thermische Verspannung in den mehrlagigen Prägewerkzeugen deutlich verringert werden, was anhand von experimentellen Untersuchungen und dem Vergleich zu einem mechanischen Modell nachgewiesen werden konnte. Besonders Prozess-aufwändig ist die Entwicklung und Erzeugung der nanoskaligen Gitterstrukturen auf dem Silizium-Master, weshalb hier zeitsparende und flexible Methoden von Vorteil sind. In dieser Arbeit konnte die fokussierte Ionenstrahl-Technologie (FIB) zur Herstellung der Bragg-Gitter-Strukturen erfolgreich angewendet und analysiert werden. Durch die FIB-Nutzung werden die Gitterstrukturen direkt und lokal in den Silizium-Master geschrieben. Außerdem wurden funktionsfähige Bragg-Gitter direkt in OrmoComp® mit einer innovativen Prozessmethode hergestellt, basierend auf einer neuartigen Kombination von SCIL-Prägewerkzeugen und direktem Laserschreiben. Als zentraler Aspekt dieser Arbeit konnte erstmals die Funktionsfähigkeit von UV-SCIL-geprägten oberflächenstrukturierten Bragg-Gitter-Sensoren gezeigt werden. Die resultierenden Reflexionssignale der OrmoComp®-Bauteile wiesen eine Signal-Halbwertsbreite unter 0,44nm auf. Weiterhin wurden Schichtabscheidungen zur Sensitivitäts- bzw. Selektivitätssteigerung auf den Gitterstrukturen durchgeführt und analysiert. Für dünne Titandioxidschichten wurde eine sensitivitätssteigernde Wirkung gegenüber Veränderungen des Brechungsindexes der Umgebung nachgewiesen. Außerdem wurde der selektivitätssteigernde Einfluss von SiO2- und OrmoStamp®-Beschichtungen an den hergestellten Bauteilen nachgewiesen. Damit ist es möglich, den Einfluss einer einzelnen Messgröße (z.B. die Veränderung des Brechungsindexes der Umgebung) zu reduzieren und somit eine von dieser Größe unabhängige Analyse der Messumgebung zu erhalten. Die eigentliche Sensoranwendung konnte zum einen anhand von Temperaturmessungen demonstriert werden, mit einer sehr hohen Temperatursensitivität von über 200 pm/°C, im Vergleich zu vergrabenen OrmoComp®-basierten Bragg-Gitter-Sensoren mit 130 pm/°C [1]. Zum anderen wurde mit den Sensoren eine Verschiebung der Reflexionswellenlänge in Abhängigkeit vom Brechungsindex der Umgebung ermittelt und so deren Einsetzbarkeit für eine Brechungsindexmessung nachgewiesen. Neben der Messung an Luft und mit Öl-Gemischen konnte auch eine Sensoranwendung in wässriger Umgebung erfolgreich gezeigt werden. Die Brechungsindexbestimmung verschiedener Salz- und Zuckerlösungen führte zu Empfindlichkeiten von bis zu 30 pm/Gew.-%. Die hervorragenden Eigenschaften des Sensorsystems hinsichtlich Temperatur- und Brechungsindexsensorik unterstreichen das Potenzial des vorgestellten Konzeptes für sensorische Anwendungen.Integrated optical Bragg grating sensors are applicable in analytics and process technology. The Bragg grating is reflecting a spectral part of the propagating light. The wavelength of that spectral part depends on surrounding conditions, such as temperature, humidity, or the surrounding refractive index, allowing for a broad field of application of such sensors. The objective of this work is the development of an innovative process option for the low-cost fabrication of planar surface relief Bragg gratings in polymers. An imprint process was applied as a novel fabrication technology. It is a contact-based lithography technique, where a stamp (made of PDMS) is molded from a master substrate and then replicated into the actual imprint material. In this work, UV enhanced substrate conformal imprint lithography (UV-SCIL) was used, enabling the patterning of complex micro- and nanostructures on full wafers with a diameter of up to 200 mm. Further on, hybrid polymers (Ormocer®s) were evaluated as a new imprint material for UV-SCIL, featuring an increased thermal and chemical stability compared to other polymers. A complete imprint process was developed and introduced, using a commercial Ormocer ®, named OrmoComp®. A full-wafer structure transfer of planar test structures was realized, containing waveguides with bendings, Y-branches as well as Bragg grating structures with a period down to 300 nm. Various tasks along the UV-SCIL process chain were addressed and optimized, e.g. a process for the deposition of an antisticking layer on the silicon master wafer with reduced process time and effort. This allows for a defect-free replication of nanostructures. Further on, it was possible to improve the stamp manufacturing process by adapting a process step. The new procedure is conducted at room temperature and takes 8 hours instead of 24 hours. Additionally, it resulted in a reduction of the thermal stress induced bending of multilayered stamps, which could be experimentally verified and compared to a mechanical model. One of the most complex parts within the fabrication of surface relief Bragg gratings is the development and structuring of the nanoscale gratings on the master substrate, which is why fast and easy adaptable methods are advantageous. In this work, a focused ion beam (FIB) was successfully used to fabricate Bragg grating structures inthe master substrate. The gratings are written directly and locally into the waveguide structures of a silicon master. Within another approach, functional surface relief Bragg gratings could be fabricated directly in OrmoComp® by applying an innovative process option based on the combination of SCIL stamps and direct laser writing. As a main issue of this work, the functionality of UV-SCIL imprinted surface relief Bragg gratings was demonstrated for the first time. The resulting reflection signals of the imprinted OrmoComp® devices exhibited a full width half maximum of 0.44 nm. Further on, layer depositions were performed on top of the Bragg gratings for increased sensitivities. A thin titanium dioxide layer was found to strongly increase the Bragg gratings sensitivity towards refractive index changes in the surrounding medium. In this context, it was also proven that SiO2 and OrmoStamp® coatings decrease the Bragg grating’s sensitivity towards specific measurands. Thus, it is possible to reduce the influence of a specific measurand (e.g. a change of the surrounding refractive index) on the measurement result. In consequence, this enables to capture the surrounding independently from this particular parameter. The actual sensor application was demonstrated on the one hand by performing temperature measurements, revealing a remarkable temperature sensitivity of more than 200 pm/°C. On the other hand, the wavelength shift of the Bragg reflection towards a changing surrounding refractive index was analyzed, demonstrating the possibility to apply the sensor for refractive index sensing. The functionality of such imprinted Bragg gratings in water was also shown. Different concentrations of salt and sugar solutions were used for refractive index determinations, resulting in a shifting Bragg wavelength of up to 30 pm/wt.-%. The sensor’s excellent sensitivity towards temperature and refractive index changes underlines the high potential of the presented concept for sensing applications

Waveguide Bragg gratings in Ormocer hybrid polymers

We report on the fabrication of Bragg gratings within rib-type waveguides of previously UV-cured inorganic-organic Ormocer hybrid polymers by applying the interferometric phase mask technique in conjunction with deep-UV laser radiation. The fabrication process as well as the influence of the applied laser fluence and the length of the Bragg grating on the characteristics of the Bragg gratings transmission and reflection spectra are discussed and compared to numerical simulations and calculations. Depending on the applied laser fluence and the chosen grating length, waveguide Bragg gratings with strong reflectivities of up to 98% and narrow bandwidths of down to 120 pm have been achieved

Waveguide bragg gratings in Ormocer®s for temperature sensing

Embedded channel waveguide Bragg gratings are fabricated in the Ormocer® hybrid polymers OrmoComp®, OrmoCore, and OrmoClad by employing a single writing step technique based on phase mask technology and KrF excimer laser irradiation. All waveguide Bragg gratings exhibit well-defined reflection peaks within the telecom wavelengths range with peak heights of up to 35 dB and −3 dB-bandwidths of down to 95 pm. Furthermore, the dependency of the fabricated embedded channel waveguide Bragg gratings on changes of the temperature and relative humidity are investigated. Here, we found that the Bragg grating in OrmoComp® is significantly influenced by humidity variations, while the Bragg gratings in OrmoCore and OrmoClad exhibit linear and considerably high temperature sensitivities of up to −250 pm/ ∘ C and a linear dependency on the relative humidity in the range of −9 pm/%

One-step nanoimprinted Bragg grating sensor based on hybrid polymers

Bragg grating sensors are used for real-time analysis of gases or liquids. Complex processing methods are typically required for the sensor fabrication. A reduction of process steps and costs during their fabrication is essential in order to broaden the field of application. Within this work, we demonstrate an innovative process for a one-step fabrication of integrated Bragg grating sensors and their successful application for temperature and refractive index sensing. UV-enhanced substrate conformal imprint lithography (UV-SCIL) is used to replicate surface relief Bragg grating (SBG) sensors on a full wafer scale. Multiple chips of a planar waveguide system including couplers and junctions were etched into a silicon wafer. Nanostructured SBGs were locally added on the waveguides by focused ion beam processing. The sensor structures were replicated by UV-SCIL into a commercially available hybrid polymer (OrmoComp®). Reflection measurements with different Bragg gratings were performed and compared to simulations. The results reveal narrow-band Bragg reflections, coinciding with the simulations. Further, the temperature dependence of the SBGs was investigated. The imprinted grating structures featured a very high temperature sensitivity of −202 pm/°C. The sensor response to a varying refractive index of the surrounding medium was determined for index matching liquids and aqueous solutions

TiO2 surface functionalization of COC based planar waveguide Bragg gratings for refractive index sensing

We demonstrate the applicability of a planar waveguide Bragg grating in cyclo-olefin copolymer (COC) for refractive index sensing. The polymer planar waveguide Bragg grating fabricated using a single writing step technique is coated with a high-index layer of titanium dioxide (TiO2) leading to a distinct birefringence. This in turn results in the splitting of the Bragg reflection into two distinct Bragg wavelengths, which strongly differ regarding their refractive index sensitivities. Where one wavelength is only slightly affected by the ambient refractive index, the second Bragg peak shows a strong sensitivity. Furthermore, we investigate the temperature behaviour of the functionalized sensor and discuss it with respect to applications in refractive index sensing

Hybrid polymers processed by substrate conformal imprint lithography for the fabrication of planar Bragg gratings

Within this work, we present an approach to use UV-enhanced substrate conformal imprint lithography (UV-SCIL) as a soft imprint technique combined with excimer laser irradiation to manufacture Bragg gratings within planar waveguides on a full wafer scale. For the first time, different hybrid polymers (OrmoComp®, OrmoStamp, OrmoCore, OrmoClad and OrmoClear) could be successfully patterned using UV-SCIL. For OrmoComp® (showing results very similar to OrmoStamp and OrmoClad) a complete imprint process could be realized. OrmoCore formed an inhibition layer in the presence of oxygen during the imprint, as could be observed for the use of OrmoClear as well. Processing options were elaborated to reduce the inhibition effect significantly, whereby the latter is mainly due to the atmospheric oxygen containing PDMS layer of the UV-SCIL working stamp. Further on, the successful realization of a planar Bragg grating operating at the telecom wavelength is demonstrated by tuning the refractive index (RI) of OrmoComp® using a phase mask and an UV excimer laser. FTIR-measurements show that the change in refractive index can be clearly correlated with a change in the chemical composition of the hybrid polymer during laser exposure