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18 The PCB Magazine • June 2014 SUBSTRATES: PoLyESTER FILM FoR THE FLExIBLE ELECTRoNICS INDUSTRy continues of which was to achieve a defect reduced sur- face by a) Characterizing the effectiveness of substrate cleaning techniques b) Identifying the defects critical to device manufacture and tracking where the defects were introduced in the manufacture of the film and subsequent handling and transport of film into a device manufacturers facilities c) Identifying and applying strategies to reduce defects and improve device manufacturing yields Working with one of the project partners to identify defects with the potential to cause shorts and line outs, etc., an attempt was made to work back through film handling to identify where the defects were introduced. The image in Figure 2 was taken from a Plas- tic Logic device "mask,"—comprising an array of circuit lines 40 µm apart, 10 µm wide and 50 nm high. The mask plate was then electrically tested with an X Y grid array to locate the short regions, which were studied using the LAM. In this case, a particle of extraneous debris gave rise to a short. The defect regions were first quantified in terms of height significance; fur- ther analysis was then carried out to determine the composition of the defect. This particular study revealed that virtually all shorts were due to externally introduced detritus. Within this same "HighQSurf" program, a major audit was performed of the film as it was entering the manufacturer's clean rooms. In summary, this audit revealed that the debris on the surface was derived from human, packag- ing and post-process interactions and not the initial film manufacture. The primary conclu- sions suggest there is considerable scope for improving the surface film quality by address- ing hygiene factors. Attention to detail is one aspect of this, but given that film handling and transport is unavoidable, improved methods of surface cleaning become important. Trials involving program partner Teknek led to their development of their Nanocleen roller technol- ogy with the composition and configuration of rollers designed to remove sub-micron as well as larger particles. Improvements in static control and the provision of a protect film on the sur- face of the planarized films have led to further improvements in film surface quality and the adoption and exploitation of techniques such as described above have taken Plastic Logic to the stage where they now achieve the same yields on polyester film as they have achieved on glass. New Film Developments In addition to the above, new films are be- ing developed to make further improvements targeting surface cleanliness, but also to address additional film requirements. These include: 1. "On Demand" Clean Film The polyester film surface on manufacture is almost glass smooth, but as discussed above the surface becomes compromised during the subsequent handling and transport processes. One approach currently under development is to exploit coextrusion technology. As shown in Figure 3, a sacrificial film layer is coextruded onto the surface of the film substrate during the film manufacture. The polyester film surface be- Figure 3: schematic view of "on demand" clean film (a = peelable layer).