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Ultrafast laser processing of thin film interconnections in microelectronic, display, and photovoltaic applications

Final Report Summary - LASER-CONNECT (Ultrafast laser processing of thin film interconnections in microelectronic, display, and photovoltaic applications)

The overall aim of the Laser Connect project was to connect fundamental studies of ultra-short laser material interactions with volume production of future large area electronic devices, such as touch sensors displays, high density electronics, and photovoltaics. The project led to extensive training and knowledge transfer in the area of laser processing of electronic materials. This included the development and dissemination of new fundamental concepts for laser ablation of thin films and thin flexible glass. The project has led to the sustained expansion and development of a UK hybrid systems manufacturer, a Hong Kong based laser process developer, and an Irish laser applications research group. The project has had a direct impact on the professional development and training of 15 early and experienced researchers; the research also indirectly contributed to a further 10 researchers locally and a further 60 -100 associated with the broader project endeavours.

The highlights of the project can be summarised in terms of the original objectives described in the project proposal:
1. The project developed new industrial laser processes for improved next generation transparent touch sensors for displays. Two separate processes based on ultrafast lasers were developed and transferred to industry (Latvia, China).
A mechanical stress model was developed to explain the ablation of thin, highly doped, semiconducting ITO layers from PET substrates; a thermo-elastic effect was identified as the key release mechanism for this application.
Two separate ablative processes were identified for selective patterning of thin, highly doped, semiconducting ITO layers from glass substrates when nanosecond pulsed lasers are used; - a thermally driven melt flow and vaporisation process was identified for below band gap illumination (1064nm, 532, and 355nm) and above band gap illumination (266nm), respectively.
ITO ablation by ultrashort laser pulses was attributed nanoparticle ejection, triggered by an electron blast force, followed by the fragmentation of the thin film.
2. The project considered laser scribing of thin flexible glass.
The project implemented a comprehensive review of short and ultrashort pulse laser scribing of thin glass. A new proprietary high throughput process, known as mechanically inspired laser scribing, was developed specifically for thin flexible glass. A new technique for observing and minimising the rear surface damage obtained in via-hole drilling and scribing of dielectric substrates was developed and applied throughout the project.
3. The use of nanosecond, picosecond, and femtosecond pulsed laser sources were investigated to optimise the fabrication of second and third generation photovoltaic materials.
A photomechanical process was demonstrated for thin molybdenum layer on glass. Edge isolation was investigated for CIGS layers; a rear side ablation solution was adopted. Laser processes were also investigated for organic photovoltaic processes based on BCPM materials.
4. The academic understanding of ultrafast –laser matter interactions was also advanced.
The project identified the role of laser wavelength in creating different electron temperatures enabling "grain dependent" ablation for ultrathin gold layers. Computational models describing electron diffusion, lattice heating and mechanical stress for thin film ablation were developed for ablation of ITO / Mo thin films and for mechanically inspired laser scribing of thin flexible glass.

5. There were several activities in intersectoral training. Industrial researchers received formal training in optical design and computational modelling; these were developed for structuring of thin films and glass. Real time nanosecond plasma emission, shadowgraphy and Schlieren imaging diagnostics were developed during multiple secondments. Academic fellows received training in industrial laser production. Industry secondees developed a DUV laser process workstation at the academic partner on which new proprietary process for selective patterning of transparent conductive oxides on colour filter modules was developed.

In excess of 70 video conferences were undertaken using dedicated systems at industry and academic sites. Over 10 face to face meetings were undertaken. Over 64 internal project presentations were made between consortium members. Two dedicated researchers were recruited to the project; one recruited researcher was retained on expiration of the project funding, the other relocated to lead a similar activity in a European laser laboratory. 14 individuals, ranging from early stage, experienced, and very experienced researchers undertook inter-sectoral secondments. In excess of 20 different material systems were investigated.
In terms of the original project goals, Laser Connect has contributed successfully to:
• The transfer of two ultrafast processes for patterning touch panels to industry.
• The development of an alternative laser scribing process for thin flexible glass.
• The development of new laser processing concepts for photovoltaics.
• The real world inter-sectoral work experience for 5 early stage researchers.
• 10 completed conference presentations, 6 (+ 2 accepted, +2 submitted ) peer review publications, two patent applications and three new joint funding applications of which two were successful.
• The completion of 4 PhD topics on Au ablation, selective patterning of ITO, selective patterning of molybdenum, and mechanically inspired laser scribing will all be completed by 2016.
• The dissemination of research and knowledge transfer at two public workshops in April 2012 and May 2014, attended by 40+ and 60+ attendees, respectively.

The main results of the project confirm the significant potential for short and ultrashort laser processes to selectively pattern very thin films for large area electronics. The project made most progress on selective patterning of transparent conductive layers. The project also significantly advanced laser scribing of thin flexible glass. Both of these principal topics when combined enabled all three project partners to propose and progress the concept of high throughput short-pulse laser structuring of emerging micro- and nano- scale materials for future roll to roll manufacturing based on thin flexible glass. The realisation of this concept is being sustained beyond the Laser Connect project.
• The academic partner is developing a value chain for a future roll to roll additive and subtractive manufacturing platform in partnership with the UK partner (2015 – 2017).
• The UK partner is participating in a European initiative to develop a pilot line for organic light emitting diodes (OLEDs) (2016 to 2018).
• Both UK and Irish partners are collaborating on the development of real time process diagnostics for future laser processes on the roll to roll platform, selective laser patterning of graphene, and the development of biomaterials perspective for large area electronics.
• The Irish and UK partner are collaborating to form a future biomaterials / printed electronics pilot manufacturing line.