Super computing is reaching out to ExaFLOP processing speeds, creating fundamental challenges for the way that
computing systems are designed and built. One governing topic is the reduction of power used for operating the system,
and eliminating the excess heat generated from the system. Current thinking sees optical interconnects on most
interconnect levels to be a feasible solution to many of the challenges, although there are still limitations to the technical
solutions, in particular with regard to manufacturability.
This paper explores drivers for enabling optical interconnect technologies to advance into the module and chip level. The
introduction of optical links into High Performance Computing (HPC) could be an option to allow scaling the
manufacturing technology to large volume manufacturing. This will drive the need for manufacturability of optical
interconnects, giving rise to other challenges that add to the realization of this type of interconnection. This paper
describes a solution that allows the creation of optical components on module level, integrating optical chips, laser
diodes or PIN diodes as components much like the well known SMD components used for electrical components. The
paper shows the main challenges and potential solutions to this challenge and proposes a fundamental paradigm shift in
the manufacturing of 3-dimensional optical links for the level 1 interconnect (chip package).
The FP7 Research for SME project IMAGINE - a low cost, high performance monolithic passive mm-wave imager
front-end is described in this paper. The main innovation areas for the project are: i) the development of a 94 GHz
radiometer chipset and matching circuits suitable for monolithic integration. The chipset consists of a W-band low noise
amplifier, fabricated using the commercially available OMMIC D007IH GaAs mHEMT process, and a zero bias
resonant interband tunneling diode, fabricated using a patented epi-layer structure that is lattice matched to the same
D007IH process; ii) the development of a 94 GHz antenna adapted for low cost manufacturing methods with
performance suitable for real-time imaging; iii) the development of a low cost liquid crystal polymer PCB build-up
technology with performance suitable for the integration and assembly of a 94 GHz radiometer module; iv) the assembly
of technology demonstrator modules. The results achieved in these areas are presented.
Optical technologies have changed the way people live since centuries, and the pace of knowledge creation and
implementation has strongly increased in the recent past. Prominent examples of recent change include the speed with
which information can be exchanged, allowing delay-free intercontinental communication, and the advent of the
broadband internet. The conception of planar waveguide optics has already ignited fundamental and manufacturing
research decades back, and its proclaimed uses were manifold, including data communication, bio analytics, or
The advances in waveguide optics have also generated many approaches to integrate optical technology into packaging
technology using fabrication methods known from the semiconductor or the printed circuit board (PCB) industry. These
technologies allow planar integration of optical waveguides and support the miniaturization of integrated systems. With
the first experiments dating back to the 1970's, the performance of planar integrated optical systems has risen from
proof-of-principle to a point where it is becoming increasingly appealing for many applications to use planar integrated
optical technology. A review of the state-of-the-art in integration technologies is given and the prospectus for the use of
integrated PCB based optical links is assessed and favorable conditions for successful implementation are proposed.
The integration of optical interconnections in printed circuit boards (PCBs) is an emerging field that arouses rapidly
growing interest worldwide. At present the key issue is to identify a technical concept, which allows for the realization of
optical interconnections that are compatible to existing PCB manufacturing processes. Above all, the material in which
the optical interconnections are embedded has to withstand increased temperatures and lamination pressures as well as
various wet chemistry processes.
AT&S uses so-called two-photon absorption (TPA) laser structuring - a rather new and innovative technology - to realize
optical circuits in a special polymer layer. In this case a near infrared laser is applied working in the femto-second
regime. The high photon density that can be reached in the laser's focus results in a modification of the optical polymer,
which is usually photosensitive in the UV-spectrum of light only. In our particular case, the refractive index of the
optical polymer is increased. Choosing the right laser intensity and focus propagation speed one achieves a waveguide
well embedded within the polymer layer, which has not been affected by the laser. In contrast to one-photon absorption,
which only allows a two dimensional respectively lateral modification of a polymer, this technology allows a
modification within the volume resulting in 3D-microstructures inside the polymer layer. Apart from the possibility to
realize structures in three dimensions, this TPA-technique has additional advantages. First of all, it allows one step
fabrication, which reduces costs and production time compared to etching procedures or conventional UV lithography
processes. Moreover, this technique allows varying the waveguide's cross section geometry and diameter simply varying
size and form of the structuring laser focus.
Whereas the realization of optical waveguides is not challenging anymore the coupling of waveguides with
optoelectronic components is rather delicate. That is, the waveguide's ends have to be accurately positioned close to the
emitting surface of the signal source and the sensing area of the light detector, respectively. Using the TPA technology to
structure optical waveguides AT&S has successfully evaluated a powerful method to solve this interface problem for the
realization of integrated optical interconnections (IOIs) on PCBs.
The development of integrated optical interconnections (IOIs) represents a quantum leap for the functionality of printed
circuit boards (PCBs). This new technology will allow highly complex product features and hence, higher product added
value. PCBs with optical interconnections will be used where applications call either for very high data streams between
components, modules or functional units (e.g. backplanes or multiprocessor boards) or for a space-saving design for
interconnection paths (e.g. mobile applications).
We discuss the different approaches towards integrating optical waveguides into PCBs and analyze the prerequisites for
a transfer to a product. Application scenarios for different markets are presented and steps proposed for required action
to deliver solutions that can be driven into a market.
In a second section a new and innovative concept for the integration of an optical interconnection system in PCBs is
presented. This revolutionary concept is highly supporting the worldwide trend towards miniaturization of not only
electronic but also optoelectronic systems in PCBs. The alignment of the optoelectronic components to the waveguides
has been addressed by this concept. It is shown that the process will allow the tolerances incurred in the manufacturing
processes to be dealt with in a separate process step, allowing existing standard methods for the production of electronic
interconnection systems to be used.
We report on the cost effective fabrication of 45° micromirror couplers within single-mode polymer waveguides for achieving fully embedded board-level optoelectronic interconnections. Compatibility with existing board manufacturing technology is achieved by making use of polymers with high thermal stability. The sol-gel polymers behave as negative photo resist and waveguides are patterned by UV exposure. Micromirrors are fabricated using excimer laser ablation, a very flexible technology that is particularly well suited for structuring of polymers because of their excellent UV-absorption properties and highly non-thermal ablation behavior. A coupling structure based on total internal reflection (TIR) is enhanced by developing a process for embedding a metal coated 45° mirror in the optical layers. The mirrors are selectively metallized using a lift-off process. Filling up the angled via without the presence of air bubbles and providing a flat surface above the mirror is only possible by enhancing the cladding deposition process with ultrasound agitation. Surface roughness of both the mirrors and the upper cladding surface above the mirrors is investigated using a non-contact optical profiler. Initial loss measurements at 1.3 μm show a propagation loss of 0.62 dB/cm and an excess mirror loss of 1.55 dB. During most recent experiments mirror roughness has been reduced from 160 nm to 20 nm, which will seriously reduce the mirror loss.