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|Title: ||Laser ablation of polymer waveguide and embedded mirror for optically-enabled printed circuit boards (OEPCB)|
|Authors: ||Zakariyah, Shefiu S.|
|Keywords: ||Laser ablation|
2D 45-degree mirror
UV Nd:YAG laser
|Issue Date: ||2010|
|Publisher: ||© Shefiu Zakariyah|
|Abstract: ||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.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Mechanical, Electrical and Manufacturing Engineering)|
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