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).