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16 The PCB Magazine • June 2014 er, it can be argued that perfection is not actu- ally required. What is important instead is to understand what defects are critical to a device manufacture, and to target and eliminate those defects where possible in a cost effective man- ner. Film makers and users continue to research surface quality, and experience over the past decade has shown that external debris (dust, externally introduced debris, scratches, etc.) rather than internal contamination is the ma- jor issue. One accepted approach to overcome this has been to cover any defects or marks on the surface with a planarizing coating, and it has been demonstrated that the introduction of such coatings significantly reduces the defect count on the film's surface. White light interferometry has been used extensively to character the surface of the films, but the use of roughness average (Ra) and root mean square roughness (Rq) can be misleading as they tend to be measured over a small sam- pling area and do not capture the occasional peaks that occur on the sample size of a display. Recent efforts have led to the development of a large area metrology (LAM) tool—the first of its type. This device has the capability to de- fect map the surface of film fast and precisely, achieving the optimum lateral resolution ob- tainable from white light interferometry. The equipment was designed to help distinguish between both intrinsic (polymeric based) and extrinsic (external, air borne debris) defects. Typical output: • Sampling area = 35 x 35 cm • Map of intrinsic and extrinsic defects, precise locations registered for compositional analysis • Full X'Y'Z measurements of the surface defects, enabling significance testing Such output can then be used to identify defects critical to device manufacture, and help determine the impact on the effectiveness of removal strategies. An example of this is where the LAM tool was used in support of a UK gov- ernment funded program (HighQSurf), the aim SUBSTRATES: PoLyESTER FILM FoR THE FLExIBLE ELECTRoNICS INDUSTRy continues Figure 2: lam image of Plastic logic (Pl) device mask showing debris.