Valentin Satzinger

Application of two-photon 3D lithography for the fabrication of embedded ORMOCER ® waveguides

The idea of applying the two-photon 3D lithography (2P-3DL) to an industrial printed wiring board (PWB) fabrication process is quite pioneering. Taking advantage of the unique rapid prototyping properties of 2P-3DL--its particularly inherent true 3D capability and its high flexibility in processing- this lithographic method can be adapted and optimized concerning the direct laser-writing of integrated optical interconnects with tens of microns in diameter. This will push the method forward towards industrial fabrication of next generation PWBs with integrated optical layers, and put it on the leading edge of printed circuit board (PCB) technology. In this context, the concept of a direct laser-written embedded waveguide is based on the local increase of the refractive index of the exposed material, which is triggered by two-photon absorption (TPA) at the laser focus. The laser induced refractive index difference forms the core of the waveguide, whereas the unexposed surrounding material forms the cladding. Thus, only one optical material is required to form the waveguide using true 3D lithographic process compared to other devices, which significantly simplifies processes. The material is subject to stringent requirements concerning the PWB production process: beside its high refractive index change, a low optical loss of the fabricated optical interconnect is required. The integration of the waveguide into the volume of the material also requires thick films up to 500 microns on the PWB substrate, and the material has to withstand the complete PWB fabrication process, where the board is chemically treated and exposed to high temperatures as well as high pressure during the lamination processes of subsequent metal layers. For this application, an inorganic-organic hybrid polymer (ORMOCER) film is applied, casted onto a PWB substrate, and the two-photon 3D lithography system parameters and optics are tuned such that waveguides with a diameter of approx. 30 microns can be inscribed. The board is equipped with laser- and photodiodes, which are totally covered by the thick ORMOCER film. The integration of the waveguide in such a preconfigured board requires precise 3D registration of the sample prior to the waveguide writing in order to align the waveguide relative to the optoelectronic components. By means of the 3D registration, the waveguide alignment is an inherent part of the fabrication process. The 3D capabilities of the 2P-3DL permit not only the fabrication of single embedded waveguides with a simple geometry, but also more complex waveguide structure (e.g. bundles) with largely arbitrary waveguide configurations. In this paper, we present the development and realization of the two-photon 3D lithography for the fabrication of integrated optical interconnects on PWBs. The ultimate goal of this approach is the large-scale fabrication of leadingedge PWBs with an integrated optical layer for additional functionality. The functioning of the fabricated and embedded waveguides is demonstrated by measurements of the essential parameters of such an optoelectronic system (photocurrent, optical loss, throughput, etc).

Optoelectronic printed circuit board: 3D structures written by two-photon absorption

The integration of optical interconnects in printed circuit boards (PCB) is a rapidly growing field due to a continuously increasing demand for high data rates, along with a progressive miniaturization of devices and components. For high-speed data transfer, materials and integration concepts are searched for which enable high-speed short-range connections, accounting also for miniaturization, and costs. Many concepts are discussed so far for the integration of optics in PCB: the use of optical fibers, or the generation of waveguides by UV lithography, embossing, or direct laser writing. Most of the concepts require many different materials and process steps. In addition, they also need highly-sophisticated assembly steps in order to couple the optoelectronic elements to the optical waveguides. An innovative approach is presented which only makes use of only one individual inorganic-organic hybrid polymer material to fabricate optical waveguides by two-photon absorption (TPA) processes. Particularly, the waveguides can be directly integrated on pre-configured PCB by in situ positioning the optical waveguides with respect to the mounted optoelectronic components by the TPA process. Thus, no complex packaging or assembly is necessary, and the number of process steps is significantly reduced, where the process fits ideally into the PCB fabrication process. The material properties, the TPA processing of waveguides, and the integration concept will be discussed. Recent experiments employing vertical-cavity surface-emitting lasers demonstrated data rates exceeding 6 Gbit/s.

Structural and electrical properties of polymorphic pentacene thin films

Due to its outstanding carrier transport capabilities the aromatic hydrocarbon pentacene is still one of the most promising out of all organic semiconducting materials investigated so far. Pentacene appears in several polymorphic structures that significantly differ with respect to the d(001) spacing. It is shown, that precise control of the epitaxial growth process of thin films enables not only to adjust the formation of the polymorphic phases, but also to influence grain size and shape. The relative volume fraction of the pentacene polymorphs is determined by several parameters which are substrate material, deposition rate, film thickness and substrate temperature. A comparison of X-ray diffraction and Raman measurements reveals that the phase with the smaller layer-by-layer spacing grows on top of the other]. Moreover, there is a strict correlation between evaporation rate and maximum grain size. In addition to structural we also investigated the electrical properties of pentacene thin films focussing on polymorphism and its influence on the transport properties. Apart from the fact that the charge carrier mobility is strongly influenced by the grain size it turned out that the bulk phase is related to a lower intrinisic mobility than the thin film phase.

3D optical waveguides produced by two photon photopolymerisation of a flexible silanol terminated polysiloxane containing acrylate functional groups

Optical waveguides are becoming increasingly important in the developing area of broadband communications. The field of electronics is advancing rapidly, leading to further demands for larger data storage, smaller components and a better design of integrated optical circuits. The integration of optical interconnects on printed circuit boards (PCBs) requires precise technologies to make this emerging field possible. A promising new microfabrication technique, two-photon photopolymerisation (2PP) can be used to produce three dimensional structures in the sub-micron region. Near-infrared lasers can be used to create 3D optical waveguides by initiating the photopolymerisation of high refractive index monomers in polymeric matrix materials. Terminal silanol groups are intermediates for room temperature vulcaniseable (RTV) silicones and can be cross linked with functional silanes to produce flexible, transparent polymeric materials with high thermal stabilities. A silanol terminated polysiloxane; cross linked with a methyl substituted acryloxy silane has been developed as a suitable material for the fabrication of optical waveguides by two-photon absorption (TPA). A higher refractive index is achieved upon polymerisation of the acrylate functional groups. The material has been shown to be suitable in the fabrication of 3D optical waveguides with a high refractive index contrast. The cured material is fully flexible and exhibits high thermal stability and optical transparency. The material was characterised by Fourier transform infrared spectroscopy (FT-IR), simultaneous thermal analysis coupled with mass spectrometry (STA-MS) and near-infrared spectroscopy (NIRS). Waveguides were observed by phase contrast microscopy, cut back measurements and were additionally directly integrated onto specially designed PCBs by correctly positioning waveguide bundles between optoelectronic components using TPA.

Two-photon-induced thiol-ene polymerization as a fabrication tool for flexible optical waveguides

In this contribution, we present two flexible thiol-ene-based hybrid materials based on epoxy and acetoxy polysiloxane matrix materials. The latter cross-linking mechanisms allow for orthogonal curing of the matrix in the presence of thiol-ene monomers enabling fast one-step access to two-photon-polymerization (2PP) curable substrates for waveguide fabrication. Another time-saving feature of our concept is the straightforward UV-flood-curing after 2PP, which is also a progress compared to previous works with elaborate postprocessing. Optimization of the ratio of thiol/ene moieties with respect to reactivity and analyses of the thermal stability of the materials, which is required for the industrial process, were carried out. Besides investigations regarding the refractive index of the materials, the proof of principle for successful waveguiding will be given. Flexible optical waveguides were successfully fabricated inside a low refractive polysiloxane matrix material.

PCB with fully integrated optical interconnects

The increasing demand for miniaturization and design flexibility of polymer optical waveguides integrated into electrical printed circuit boards (PCB) calls for new coupling and integration concepts. We report on a method that allows the coupling of optical waveguides to electro-optical components as well as the integration of an entire optical link into the PCB. The electro-optical devices such as lasers and photodiodes are assembled on the PCB and then embedded in an optically transparent material. A focused femtosecond laser beam stimulates a polymerization reaction based on a two-photon absorption effect in the optical material and locally increases the refractive index of the material. In this way waveguide cores can be realized and the embedded components can be connected optically. This approach does not only allow a precise alignment of the waveguide end faces to the components but also offers a truly 3-dimensional routing capability of the waveguides. Using this technology we were able to realize butt-coupling and mirror-coupling interface solutions in several demonstrators. We were also manufacturing demonstrator boards with fully integrated driver and preamplifier chips, which show very low power consumption of down to 10 mW for about 2.5 Gbit/s. Furthermore, demonstrators with interconnects at two different optical layers were realized.

Rapid prototyping of micro-optics on organic light emitting diodes and organic photo cells by means of two-photon 3D lithography and nano-imprint lithography

Provided that suitable materials are available, novel structuring methods, such as two-photon-3D-lithography (2P3DL) and nano-imprint-lithography (NIL) are promising approaches for the fabrication of organic complex 2D and 3D structures. Optical materials based on photopolymerizable resins combined with novel efficient multi-photon photoinitiators can be used for a fast and simple fabrication of μ-optical components for MOEMS. The true 3D capabilities and the high spatial resolution of the 2P3DL permit the fabrication of nearly any optical designs from CAD. With supplementary feedback controlled positioning of the laser focus, a material can be processed at an explicit target position, e.g. on an organic LED or photo cell. The position of fabricated μ-optics relative to such devices is determined by 3D sample registration prior to the structuring process. Therefore, the alignment of laser written structures to existing sample features becomes a part of the fabrication process and no further assembly is required. We demonstrate the design and the fabrication of various μ-optical structures such as waveguides and μ-lenses for photonic μ-systems by means of 2P3DL. Furthermore, μ-lens masters prototyped by means of two-photon-3D-lithography and their replication via a PDMS stamp by means of NIL are presented. In addition, it can be shown that such μ-optical systems can be fabricated in situ on organic LEDs or organic photo cells enabling powerful building blocks for μ-optical systems.

Optical Waveguides Embedded in PCBs - A Real World Application of 3D Structures Written by TPA

The integration of optical interconnects in printed circuit boards (PCB) is a rapidly growing field worldwide due to a continuously increasing need for high-speed data transfer. There are many concepts discussed, among which are the integration of optical fibers or the generation of waveguides by UV lithography, embossing, or direct laser writing. The devices presented so far require many different materials and process steps, but particularly also highly-sophisticated assembly steps in order to couple the optoelectronic elements to the generated waveguides. In order to overcome these restrictions, an innovative approach is presented which allows the embedding of optoelectronic components and the generation of optical waveguides in only one optical material. This material is an inorganic-organic hybrid polymer, in which the waveguides are processed by two-photon absorption (TPA) processes, initiated by ultra-short laser pulses. In particular, due to this integration and the possibility of in situ positioning the optical waveguides with respect to the optoelectronic components by the TPA process, no complex packaging or assembly is necessary. Thus, the number of necessary processing steps is significantly reduced, which also contributes to the saving of resources such as energy or solvents. The material properties and the underlying processes will be discussed with respect to optical data transfer in PCBs.

Development and characterization of optoelectronic circuit boards produced by two-photon polymerization using a polysiloxane containing acrylate functional groups

Research into the integration of optical interconnects in printed circuit boards (PCBs) is rapidly gaining interest due to the increase in data transfer speeds now required along with the need for miniaturized devices with increased complexity and functionality. We present a method that involves embedding optoelectronic components in a polymeric material and fabricating optical waveguides in one step. A silanol-terminated polysiloxane cross-linked with an acryloxy functional silane is utilized as a matrix material into which the 3D optical waveguides are inscribed by two-photon-induced polymerization. A pulsed femtosecond laser is used to directly write optical waveguides into the material, forming an optical link between lasers and photodiodes that are directly mounted on a specially designed PCB. The boards produced were characterized by monitoring the transmitted photocurrent as well as temperature-dependent data transmission properties. Data rates exceeding 4 Gbit/s were achieved.

3D-structuring of optical waveguides with two photon polymerization

Two photon photopolymerization (2PP) is a new and modern method in solid freeform fabrication. 2PP allows the fabrication of sub-micron structures from a photopolymerizable resin. By the use of near-infrared (NIR) lasers it is possible to produce 3D structures with a spatial feature resolution as good as 200 nm. This technique can be used in polymer-based photonic and microelectromechanical systems (MEMS), for 3D optical data storage or for the inscription of optical waveguides based on a local refractive index change upon laser exposure. Since the 2PP only takes place inside the focus of the laser beam, complex 3D-structures can be inscribed into a suitable matrix material. In the presented work, 2PP is used to write optical waveguides into a prefabricated mechanically flexible polydimethylsiloxane matrix. The waveguides were structured by selectively irradiating a polymer network, which was swollen by a monomer mixture. The monomer was polymerized by two photon photopolymerization and the uncured monomer was removed by evaporation at elevated temperatures. This treatment led to a local change in refractive index in the order of Δn = 0.02, which was significantly above the industrial requirement of Δn = 0.003. The measured optical losses were around 2.3dB/cm. Since all unreacted monomers were removed by evaporation, the final waveguide was stable up to temperatures of more than 200°C. In a second approach highly porous sol-gel materials (based on tetramethoxysilane (TMOS) as precursor and the surfactant cetylpyridinium chloride monohydrate as structural template) were utilized as matrix materials. The precursor was organically modified with poly(ethylene glycol) spacers in order to increase the toughness and thus facilitate the fabrication of transparent porous monoliths and flexible films. The pores of the sol-gel-derived matrix were filled with acrylate-based monomers of high refractive index and after selective irradiation using 2PP waveguides (Δn = 0.015) could be written into the material.