Ruth Houbertz

Inorganic–organic hybrid materials for application in optical devices

Abstract Integrated passive and active optical devices are the key components in current and future data transfer technologies. In order to fulfill future requirements in miniaturization for diffractive, refractive and integrated optical devices, new materials with higher thermal stability and a better compatibility to processing techniques used in conventional semiconductor devices production are needed. Inorganic–organic hybrid polymers (ORMOCER®s) produced at fairly low costs with a high degree of reproducibility are now proven candidates. The materials can be functionalized such that their physical and chemical properties can be tailored towards, e.g. optical applications on wafer-scale such as waveguides, gratings or microoptical devices. The materials behave as a negative resist and can thus be patterned by UV exposure with good resolution. Besides, the materials are very well suited for thin and thick film packaging technology. We here particularly focus on materials for optical (telecom/microoptics) applications. The optical behavior is characterized and discussed with respect to the chemical functionalities. Additionally, some application examples of selected optical components are given, produced either by UV lithography or by replication technology.

New wafer-scale fabrication method for stacked optical waveguide interconnects and 3D micro-optic structures using photoresponsive (inorganic–organic hybrid) polymers

The rapid growing demands on the number of data transport channels in integrated optical circuits are hitting the border of the integration density, which is realizable with the currently used planar technologies due to the limited wafer size. An approach to overcome this problem by the stacking of optical single-mode waveguides is presented. This newly developed technology is based on successive UV photolithography using the material system of hybrid inorganic–organic polymers (ORMOCER®s). To ensure a perfect structure homogeneity over the whole stack, a detailed theoretical and experimental investigation of the UV patterning process concerning the non-linear response of the material to the exposure light is done. The applicability of the newly developed wafer-scale fabrication method is presented by the exemplary realization of a four layer stack. Following this method of an UV induced local curing, the potential of a controlled photopolymerization for the realization of complex 3D micro-optical structures is presented on the basis of first experimental results.

Impact of photoinitiators on the photopolymerization and the optical properties of inorganic-organic hybrid polymers

Sol-gel synthesis allows one to produce inorganic–organic hybrid polymer materials which can be functionalized in order to tailor their physical and chemical properties. Besides, the resulting material properties are significantly influenced by further technological processing of the materials in thin film technology, i.e., the photochemical and thermal curing of the materials. In order to investigate the relationship between technological processing and material properties, a model system containing methacrylic groups as organically polymerizable units is chosen. The degree of conversion of the C=C double bond of the methacrylic group in dependence of the ultraviolet (UV) initiator concentration upon processing is characterized using Fourier-transform infrared spectroscopy. The data are correlated to measurements of the refractive indices at selected wavelengths.

Materials and technologies for fabrication of three-dimensional microstructures with sub-100 nm feature sizes by two-photon polymerization

The fabrication of sub-100 nm feature sizes in large-scale three-dimensional (3D) geometries by two-photon polymerization requires a precise control of the polymeric reactions as well as of the intensity distribution of the ultrashort laser pulses. The authors, therefore, investigate the complex interplay of photoresist, processing parameters, and focusing optics. New types of inorganic– organic hybrid polymers are synthesized and characterized with respect to achievable structure sizes and their degree of crosslinking. For maintaining diffraction-limited focal conditions within the 3D processing region, a special hybrid optics is developed, where spatial and chromatic aberrations are compensated by a diffractive optical element. Feature sizes below 100 nm are demonstrated.

Towards an atomistic model for ORMOCER®-I: application of forcefield methods

ORMOCER®s are an outstanding class of hybrid materials due to their tuneable properties, e.g. hardness, resistivity and refractive index. These materials are well-characterized with regard to their macroscopic properties, but understanding the system at the atomistic level still remains challenging. Understanding the material formation process at this level becomes especially important when three-dimensional nanoscale patterns are generated employing processes as laser-based multi-photon polymerization. We have developed an atomistic model based on the COMPASS forcefield to simulate the reference system ORMOCER®-I. We chose representative compositions for the condensation reaction product as well as for the organically cross-linked polymerized product. In the first part of the study, the results of forcefield validation experiments and the development of the atomistic model for ORMOCER®s are presented. The second part contains the results from molecular dynamics simulations at room temperature and under periodic boundary conditions, performed in order to test the feasibility of our model. The densities of the simulated materials are in very good agreement with experimentally determined densities for the unpolymerized as well as for the polymerized state, respectively.

Two-photon polymerization of hybrid polymers for applications in micro-optics

Miniaturization and higher integration of opto-electronic components require highly sophisticated optical designs. This creates the demand for freeform technologies like Two-Photon Polymerization (2PP) and new specially adapted materials like hybrid polymers (ORMOCERRs). Recent progress in the fabrication of microoptical structures using 2PP and specially designed hybrid polymers is presented. Among the structures are freeform and aberration-optimized microlenses and multilevel diffractive optical elements. These components are discussed with respect to fabrication process and their resulting optical performance. Furthermore, 2PP-initiated refractive index modification, offering high potential for energy-efficient fabrication of 3D optical interconnects, is discussed.

Towards High Precision Manufacturing of 3D Optical Components Using UV-Curable Hybrid Polymers

Hybrid polymers have been already widely applied in photonic applications to manufacture microlenses or 2D and 3D waveguides. Thus, they are promising candidates to manufacture optical systems down to the chip level. A brief review on hybrid polymers consisting of both inorganic and organic functional units and thus combine superior material properties in just one material class will be given in this report. The material properties, which can be adjusted to the application in wide ranges enable to fabricate micro-optical elements (e.g. microlenses) using replication techniques such as UV-assisted replication or nano-imprint lithography. Aside of their applicability in 2D, emphasis will be in particular on the evaluation of hybrid polymer materials for two-photon absorption lithography, which is employed to directly manufacture sophisticated 3D photonic structures impossible to be generated with conventional 2D techniques.

High-precision 3D printing as a versatile tool for integrated photonics (Conference Presentation)

Driven by IoT, Industry 4.0, and social media the amount of data to be transferred is tremendously increasing, pushing the need for energy-efficient device concepts for a vast variety of products such as photonic integrated circuits or sensors. Low energy data transfer can be achieved, for example, by replacing part of the electronic circuitry by optical data lines in chip-level packaging, or by introducing optical elements such as specially designed microlenses into semiconductor laser packaging. This also allows to drastically reduce footprints of systems, and – at the same time – to increase functionality. On the other hand, a significant demand is seen in providing lower cost and scalable manufacturing processes with technologies which provide highest flexibility. High Precision 3D Printing as novel emerging fabrication technology is a promising tool for optical packaging. It enables to reduce the necessary process steps for packaging to only three to five, independently of the packaging task. This is enabled by a novel and versatile packaging concept where the chips and the dies are already mounted prior to the fabrication of optically functional elements such as optical interconnects or microoptics to couple, for example chip-to-chip or dies to fiber, with passive alignment only. Flexible exposure strategies using High Precision 3D Printing provide both, scalability and high throughput with fabrication times from seconds for optical waveguides and single microlenses to only a few minutes for more complex lens systems. The impact of the fabrication strategy will be discussed with respect to the performance of the optical devices.

Novel hybrid polymers for fabrication of advanced micro-optics

The enduring search for innovative materials, as solutions for the fabrication of advanced micro-optics, remains a crucial part in achieving the commercial success of emerging production technologies. In this regard, innovative optical polymers pave the way for easy, cost-effective, and reliable fabrication of micro-optical devices. To enable a broad range of applications, however, it is necessary for these materials to satisfy numerous requirements. These requirements derive from both the initial processing itself (e.g., the need for broad and reliable process windows) and from the final application (e.g., the need for transparency at a certain wavelength or wavelength bands, as well as non-yellowing, specific refractive index, and high-temperature stability properties). On the basis of Fraunhofer-Gesellschaft’s ORMOCER R

Evaluation of hybrid polymers for high-precision manufacturing of 3D optical interconnects by two-photon absorption lithography

The fabrication of optical interconnects has been widely investigated for the generation of optical circuit boards. Twophoton absorption (TPA) lithography (or high-precision 3D printing) as an innovative production method for direct manufacture of individual 3D photonic structures gains more and more attention when optical polymers are employed. In this regard, we have evaluated novel ORMOCER-based hybrid polymers tailored for the manufacture of optical waveguides by means of high-precision 3D printing. In order to facilitate future industrial implementation, the processability was evaluated and the optical performance of embedded waveguides was assessed. The results illustrate that hybrid polymers are not only viable consumables for industrial manufacture of polymeric micro-optics using generic processes such as UV molding. They also are potential candidates to fabricate optical waveguide systems down to the chip level where TPA-based emerging manufacturing techniques are engaged. Hence, it is shown that hybrid polymers continue to meet the increasing expectations of dynamically growing markets of micro-optics and optical interconnects due to the flexibility of the employed polymer material concept.