Laser Ablation for Polymer Waveguide Fabrication

An increase in interconnection density, a reduction in packaging sizes and the quest for lowcost product development strategy are some of the key challenges facing micro-optoelectronics design and manufacture. The influence of high-density, small-sized products has placed significant constraints on conventional electrical connections prompting various fabrication methods, e.g. photolithography, being introduced to meet these challenges and ameliorate the rapidly changing demand from consumers. While high-power solid state lasers are fundamental to large scale industrial production, excimer laser on the other hand has revolutionised the manufacturing industry with high precision, easy 3D structuring and less stringent production requirements. Micro-structuring using excimer laser, best known as laser ablation, is a non-contact micro- and nano-machining based on the projection of high-energy pulsed UV masked beam on to a material of interest such that pattern(s) on the mask is transferred to the substrate, often at a demagnified dimension with high resolution and precision. The use of mask with desired patterns and beam delivery system makes the fabrication in this case accurate, precise and easily controllable. The first part of this chapter introduces the fundamentals of laser technology and material processing. In the second part, optical interconnects as a solution to ‘bottlenecked’ conventional copper interconnections is introduced with emphasis on excimer laser ablation of polymer waveguides and integrated mirrors. Key research findings in the area of optical circuit boards using other techniques are also briefly covered

Laser ablation of polymer waveguide and embedded mirror for optically-enabled printed circuit boards (OEPCB)

Due to their inherent BW capacity, optical interconnect (OI) offers a means of replacement to BW limited copper as bottlenecks begin to appear within the various interconnect levels of electronics systems. Low-cost optically enabled printed circuit boards are a key milestone on many electronics roadmaps, e.g. iNEMI. Current OI solutions found in industry are based upon optical fibres and are capable of providing a suitable platform for inter-board applications especially on the backplane. However, to allow component assembly onto high BW interconnects, an integral requirement for intra-board applications, optically enabled printed circuit boards containing waveguides are essential. Major barriers to the deployment of optical printed circuit boards include the compatibility of the technique, the cost of acquiring OI and the optical power budget. The purpose of this PhD research programme is to explore suitable techniques to address these barriers, primarily by means of laser material processing using UV and IR source lasers namely 248 nm KrF Excimer, 355 nm UV Nd:YAG and 10.6 µm IR CO2. The use of these three main lasers, the trio of which dominates most PCB production assembly, provides underpinning drive for the deployment of this technology into the industry at a very low cost without the need for any additional system or system modification. It further provides trade-offs among the suitable candidates in terms of processing speed, cost and quality of waveguides that could be achieved. This thesis presents the context of the research and the underlying governing science, i.e. theoretical analysis, involving laser-matter interactions. Experimental investigation of thermal (or pyrolitic) and bond-breaking (or photolytic) nature of laser ablation was studied in relation to each of the chosen lasers with regression analysis used to explain the experimental results. Optimal parameters necessary for achieving minimum Heat Affected Zone (HAZ) and surface/wall roughness were explored, both of which are key to achieving low loss waveguides. While photochemical dominance – a function of wavelength and pulse duration – is desired in laser ablation of photopolymers, the author has been able to find out that photothermallyprocessed materials, for example at 10.6 µm, can also provide desirable waveguides. Although there are literature information detailing the effect of certain parameters such as fluence, pulse repetition rate, pulse duration and wavelength among others, in relation to the etch rate of different materials, the machining of new materials requires new data to be obtained. In fact various models are available to try to explain the laser-matter interaction in a mathematical way, but these cannot be taken universally as they are deficient to general applications. For this reason, experimental optimisation appears to be the logical way forward at this stage of the research and thus requiring material-system characterisation to be conducted for each case thereby forming an integral achievement of this research. In this work, laser ablation of a single-layer optical polymer (Truemode™) multimode waveguides were successfully demonstrated using the aforementioned chosen lasers, thus providing opportunities for rapid deployment of OI to the PCB manufacturing industry. Truemode™ was chosen as it provides a very low absorption loss value < 0.04 dB/cm at 850 nm datacom wavelength used for VSR interconnections – a key to optical power budget – and its compatibility with current PCB fabrication processes. A wet-Truemode™ formulation was used which required that optical polymer layer on an FR4 substrate be formed using spin coating and then UV-cured in a nitrogen oxygen-free chamber. Layer thickness, chiefly influenced by spinning speed and duration, was studied in order to meet the optical layer thickness requirement for multimode (typically > 9 µm) waveguides. Two alternative polymers, namely polysiloxane-based photopolymer (OE4140 and OE 4141) from Dow Corning and PMMA, were sparingly utilized at some point in the research, mainly during laser machining using UV Nd:YAG and CO2 lasers. While Excimer laser was widely considered for polymer waveguide due to its high quality potential, the successful fabrication at 10.6 µm IR and 355 nm UV wavelengths and at relatively low propagation loss at datacom wavelength of 850 nm (estimated to be < 1.5 dB/cm) were unprecedented. The author considered further reduction in the optical loss by looking at the effect of fluence, power, pulse repetition rate, speed and optical density on the achievable propagation but found no direct relationship between these parameters; it is therefore concluded that process optimisation is the best practice. In addition, a novel in-plane 45-degree coupling mirror fabrication using Excimer laser ablation was demonstrated for the first time, which was considered to be vital for communication between chips (or other suitable components) at board-level

Polymer optical waveguide fabrication using laser ablation

Due to their inherent bandwidth capacity, optical interconnects are replacing copper as bottlenecks begin to appear within the various interconnect levels of electronics systems. Current optical interconnect solutions found in industry are based upon optical fibres and are capable of providing a suitable platform for inter-board applications. However, to allow high speed interconnects between components and within systems, optically enabled printed circuit boards containing waveguides are essential. One way in which this can be accomplished is through the integration of polymer optical waveguides into traditional printed circuit boards (PCBs). There are a number of routes to accomplish this including photolithography and laser direct imaging, however, this paper explores laser ablation using UV and IR sources namely: 248 nm Excimer, 355 nm UV Nd:YAG and 10.6 μm CO2, to form waveguide structures in optical polymer materials. The paper presents the process route and initial results of trials conducted to fabricate waveguides and indicate the variation in the structures formed by the different lasers. The demonstration of the use of these three lasers for optical waveguide fabrication may provide a route to the rapid deployment of this technology into the PCB industry through the use of existing infrastructure

Integrated optical and electronic interconnect printed circuit board manufacturing

Introduction: At high bit rates copper tracks in printed circuit boards (PCBs) suffer severe loss and pulse distortion due to radiation of electromagnetic waves, dispersion and bandwidth limitations. The loss can be overcome to some extent by transmitting higher power pulses and by changing the dielectric constant and loss tangent of the PCB substrate material. However, high power pulses consume power and can cause electro-migration which reduces the board lifetime, although the copper tracks can be surrounded by another metal to prevent this at the expense of further processing steps. The use of special board materials can be costly and some materials containing high dielectric constant crystallites can cause poor adhesion. The pulse distortion, dispersion and bandwidth limitations can be overcome to some extent by the use of pulse pre-emphasis and adaptive equalisation at further cost. Electromagnetic waves are radiated efficiently at high bit rates removing power from the track so causing loss, but more importantly they are also received efficiently by other nearby and distant copper tracks on the same PCB, or on adjacent PCBs, or PCBs and other electrical conductors outside of the system enclosure. This EMI crosstalk causes increased noise and so degrades the signal to noise ratio and the bit error rate of the copper track interconnections. Therefore, the main forces driving the development of alternative interconnect technologies are the EMI crosstalk, which becomes increasingly more serious as bit rates increase for longer and denser interconnects, and secondly the cost of overcoming the other problems that occur in copper interconnects at high bit rates. Optical fibres have replaced copper cables for long distance, backbone and submarine applications where they offer wide bandwidths for low loss, produce and receive no electromagnetic interference, and are relatively low cost. Optical interconnects are beginning to penetrate the markets at shorter distances, such as in local area networks, and as their cost is reduced, will be used within the system enclosure. The use of optics is expected to occur first where the problems for copper are most significant which is for high bit rate, dense interconnections in large area backplanes within non-conducting enclosures. Optical fibres are not the most convenient for interconnections within a system as they can only bend through a large radius of about 10 cm, otherwise light escapes from the fibre core into the cladding resulting in loss and signal corruption. Fibre connectors form a major part of the cost of the optical interconnect and a system with many fibres has many costly connectors. The fibres must be individually routed and errors in routing are time consuming to debug and correct. The fibres can be laid flat on the PCB plane and even bonded together within an epoxy layer, but this is not suited to low cost mass production. An alternative technology suitable for low cost mass production is that of multimode polymer buried channel optical waveguide interconnections within layers in the multilayer PCB formed by the same, or slightly modified, processes already available within PCB manufacturing facilities. Copper tracks are still required in such substrates to transmit power through the backplane (or motherboard), Figure 1, in order to power mezzanine (or line, or drive, or daughter) boards and copper is still a practical and low cost option at low data rates. Hence, there is a need to develop a new type of multilayer hybrid PCB in which optical waveguide interconnects are used for the highest data rates, with copper tracks for lower data rates and for power lines and earth planes. These issues have been anticipated by system design companies such as Xyratex Technology, IBM Zurich and Siemens C-Labs, microprocessor designers such as Intel and materials development companies such as Dow Corning, NTT, Rohm and Haas and Exxelis, who have instituted research in their own laboratories and in associated universities into optical waveguide interconnect technology. Leading Universities and Research Institutions such as Cambridge (CAPE), University College London (UCL), Heriot Watt University, Loughborough University, National Physical Laboratory (NPL), IMEC - Ghent University, TFCG Microsystems, Belgium, Paderborn University, Germany, Helsinki University of Technology, Espoo, Finland and ETRI, South Korea are developing novel polymer materials, developing fabrication techniques, discovering design rules for waveguide layout and carrying out precision characterisation. Optical buried channel waveguides usually have a core with an approximately square or rectangular cross section made from a high refractive index (slow speed of light) material and a cladding surrounding the core of a lower refractive index (higher speed of light). They operate by total internal reflection (TIR) in a similar way to optical fibres. The cost of waveguide connectors is minimised by choosing to use multi-mode waveguides which typically have cores of 40 - 70micron width which can tolerate more misalignment than single mode waveguides. The optical buried channel waveguides are formed on a plane by a variety of fabrication techniques which can be implemented, after slight adaptation, in PCB manufacturers. Arrays of low-cost vertical cavity surface emitting lasers (VCSELs) emitting 850 nm wavelength and arrays of photodiodes operating at 1 0 Gb/s are readily available at low-cost for use in optical transmitters and receivers. At this wavelength, polymer is a convenient low-loss material for use as the core and cladding. Polymers can be chosen or designed which can be easily processed to form waveguides at low temperatures, have low cost, and can withstand subsequent high temperature reflow soldering processes. For optical printed circuit boards to be brought into widespread use, layout tools must be made readily available which design both the copper tracks and the optical waveguides [1]. In 2006 David R. Selviah of UCL, formed a large consortium of complementary universities and companies and led a successful bid to carry out a Flagship project entitled “Integrated Optical and Electronic Interconnect PCB Manufacturing (OPCB)” in the Innovative Electronics Manufacturing Research Centre (IeMRC). The consortium companies represented a complete supply and manufacturing chain and route to market for the polymer waveguide technology including companies manufacturing PCB layout tools, computer programs for modelling the behaviour of multimode waveguides, developing and supplying low loss polymer formulations, manufacturing multilayer PCBs, supplying printer fabrication equipment together with end user system companies who require optical printed circuit boards. The following sections describe the project’s objectives, the approaches being taken and some examples of what has been achieved so far in the project with an indication of future directions

Innovative optical and electronic interconnect printed circuit board manufacturing research

An overview of the £1.3 million EPSRC and company matched funded Innovative electronics Manufacturing Research Centre (IeMRC) Flagship project between 3 UK universities and 10 companies entitled "Integrated Optical and Electronic Interconnect PCB Manufacturing". The project aims to develop of optical waveguide design rules, layout software, fabrication methods compatible with commercial production, characterisation techniques and optical connector design to provide a supply chain for Polymer Multimode Optical Waveguide Printed Circuit Boards (OPCB) for 10 Gb/s board-to-board interconnections