Ladislav Kuna

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).

Improvement of light extraction from high-power flip-chip light-emitting diodes by femtosecond laser direct structuring of the sapphire backside surface

We report on the structuring of the backside surface of sapphire substrates in high-power flip-chip light-emitting diodes (LEDs) by femtosecond laser direct writing. Varying the laser powers has been found to affect the sizes of the inscribed patterns on a submicron scale which facilitates the control of the structure sizes with high precision. Accordingly, since on the one hand the light extraction efficiency reveals a strong dependence on pattern sizes, and on the other hand, femtosecond laser structuring provides a simple opportunity to inscribe diverse structures along the LED surfaces, LEDs with laterally controlled light extraction efficiencies can be fabricated.

White LEDs and modules in chip-on-board technology for general lighting

At present, light-emitting diode (LED) modules in various shapes are developed and designed for the general lighting, advertisement, emergency lighting, design and architectural markets. To compete with and to surpass the performance of traditional lighting systems, enhancement of Lumen output and the white light quality as well as the thermal management and the luminary integration are key factors for success. Regarding these issues, white LEDs based on the chip-on-board (COB) technology show pronounced advantages. State-of-the-art LEDs exploiting this technology are now ready to enter the general lighting segments. We introduce and discuss the specific properties of the Tridonic COB technology dedicated for general lighting. This technology, in combination with a comprehensive set of tools to improve and to enhance the Lumen output and the white light quality, including optical simulation, is the scaffolding for the application of white LEDs in emerging areas, for which an outlook will be given.

On the Adjustment of the Color Temperature of White Light-Emitting Diodes by Femtosecond Laser Patterning

We show that femtosecond (fs) laser microstructuring of light-emitting diode (LED) encapsulates has a high potential for the fine-tuning of the color temperatures of white LED packages. The number of scattering centers that are fabricated within the volume of the encapsulate allows the precise adjustment of the color temperature of the device to the desired value. This opens possibilities to achieve color reproducibility among individual LED packages with highest accuracy.

Surface Texturing of High-Power Flip-Chip LEDs by Femtosecond Laser Direct Structuring

We report on the femtosecond laser direct structuring of the sapphire flipside surfaces of high-power flip-chip LEDs. It is found that diameter and depth of the created submicrometer-sized holes can be manipulated by varying the laser power at a constant number of laser pulses. This method enables the control of the structure sizes with high precision. Our study shows that the light extraction from such LEDs increases strongly with increasing hole sizes.

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.

Predicting solutions toward improved high power white LED light sources: a combined theoretical and experimental study

To compete with and to surpass the performance of traditional lighting systems, white LED development is still facing the necessity of further improvements. An important topic that has to be addressed in this context is the spatial homogeneity of the white light emitted, an issue that is directly associated with the geometry and the composition of the color conversion elements (CCE) in phosphor converted LEDs. In order to avoid the need for experimental realization and inspection of a large number of different configurations and compositions, optical simulation provides a time- and cost saving alternative. In this contribution we discuss a simulation procedure which allows us to predict optimized solutions for the CCEs in white LED light sources. The simulation process involves the set-up of a model for the blue emitting LED chip and the implementation of a multitude of different geometries and compositions of individual CCEs on top of the chip. Since the light is scattered within the CCEs, the respective scattering model, which considers the phosphor particle size distribution and the phosphor weight fraction is of particular importance. In the final sequence of the modeling procedure color uniformity is checked by monitoring the irradiance distributions both for the blue LED light and the yellow converted light separately on a detector. From a comparison of the simulation results for a significant number of different layouts we can deduce the impact of the individual materials parameters and predict optimized CCEs which are finally compared with real device set-ups in order to verify the accuracy of the simulation procedure.

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.

Focus image feedback-controlled 3D laser microstructuring

The availability of reliable ultrafast laser systems and their unique properties for material processing are the basis for new lithographic methods in the sector of micro- and nanofabrication processes such as two-photon 3D-lithography. Beside its flexibility, one of the most powerful features of this technology is the true 3D structuring capability, which allows fabrication with higher efficiency and with higher resolution compared to a sequential layer-by-layer structuring and build-up technique. Up to now, the two-photon method was mainly used for writing 3D structures quasi anywhere inside a bulk volume. In combination with a sophisticated and versatile machine vision support, the two-photon 3D-lithography is now targeting for micro- and nano-optical applications and the integration of optical and photonic components into optical microsystems. We report on a disruptive improvement of this lithographic method by means of an optical detection system for optical components (e.g. laser diode chips / LEDs and photo diodes) that are already assembled on an optical micropackage. The detection system determines the position coordinates of features of the optical microsystem in all three dimensions with micrometer resolution, combining digital image processing and evaluation of back reflected laser light from the surface of the system. This information is subsequently processed for controlling the fabrication of directly laser written optical and photonic structures inside and around such an optical microsystem. The strong advantage of this approach lies in its adaptation of laser written structures to existing features and structures, which also permits to compensate for misalignments and imperfections of preconfigured packages.

Foil based optical elements for beam shaping and color homogenization of phosphor converted white LED sources

Typically, light emission from light-emitting diodes (LEDs) occurs under a broad range of angles. On the other hand, for a lot of applications a more directed light emission is desired. This can be realized with the use of additional optical elements, like lenses. Still, this may provide some complications in case of light sources consisting of a plurality of individual LEDs, e.g., a panel light, which is expected to illuminate a target area homogenously. Instead of a homogeneous illumination, the use of lenses is prone to give reason for an inhomogeneous light distribution in which the emission from the individual LEDs is easily distinguishable. Therefore, there is a strong request for alternative strategies of beam shaping of LED light in LED-luminaires targeting both on a directed as well as homogeneous illumination of an area. In this contribution we discuss an alternative approach in this regard: Firstly, a collimator is designed, which strongly directs the light emitted from a single LED light source. Subsequently, a foil with an optical structure, that can be fabricated in a cost-effective way by soft-lithography and which diffuses the collimated light again, is applied on the collimator. The optical structure and the respective amount of light diffusion are designed in a way that the desired radiation patterns both from a single as well as a plurality of LED sources can be realized. In addition, we show that the realization of a desired radiation profile is not the only advantage of such an approach. A key benefit of this concept is the possibility to reduce the angle dependent inhomogeneity