SUBSTRATES: Background Substrates for large area flexible electronics pose a different set of challenges than currently faced by non-flexible substrates. For one, they tend to be less dimensionally stable, which makes it more difficult to print multilayer devices reel-to-reel (R2R). Dimensional stability affects the resolution and registration of the features that can be printed. Also, depending on the material used, they are less tolerant to temperatures and solvents and are less smooth. Independent of substrate material surface quality will be a critical attribute and will be defined by the number of defects of a given shape and size per unit area (the unit area being defined by the area being processed for device manufacture). Perfection is unlikely and also chasing perfection will significantly raise substrate costs, therefore understanding what shape/ size defect will be critical to device manufacture and which surface defects can be ignored will be critical. At this stage this is an area that is not clearly understood but likely will be in 5 years time. A key component of this will be the capability to measure the surface smoothness of film over display size areas quickly and economically- this is an emerging requirement. Large area metrology will therefore be an area of growing importance in the next 5-10 years. Most likely advances in device processing over the next 5 to 10 years will alter the focus of substrate research. As an example for polymer film based substrates, although lowering shrinkage at a given temperature and lowering shrinkage at elevated temperatures approaching the Tm will always be of interest, advances in alignment compensation technologies for substrate distortion will require that consistency of substrate (sheet to sheet or roll to roll) will become more important than absolute shrinkage. Similarly although developing and commercializing polymer films with higher thermal stability are of interest, this increased temperature performance will come at a higher price. Therefore, advances in low temperature processing will likely mean that it will be possible to manufacture devices economically on existing commercially available films rather than necessarily requiring new expensive film substrates. Before selecting a substrate for a given application, there are many important properties to consider. The thermal and mechanical stability, resistance to moisture, gas and vapor transmission, and solvents are important to consider. In addition, surface smoothness, surface energy, optical transparency, commercial availability and costs must be considered. As with most materials, costs increase with substrate material property demands. Depending on the requirements of the application, a different flexible substrate will be required and for the more demanding applications the substrate will almost certainly be a multilayer composite structure. End-users seeking the best suited material for their application must balance the material properties and costs1.Different requirements for substrate properties based on application are summarized in Table 1. Some of the properties are ranked as design specific indicating that the substrate requirements depend on design or architecture of the devices. For example, in display applications, it is important that the substrate is transparent for a bottom-emissive design and not important for a top-emissive design. Table 1 Substrate properties requirements depending on application. Ranking is based on the importance (1 – very important, 2 – medium and 3 – less important, APS – application and product specific and DS – design specific). Application/Substrate Properties Smoothness Barrier Properties Optical Transparency Dimensional Stability Thermal Stability Mechanical Strength/ Flexibility RFID tag Antenna 2 3 3 2 2 2 Circuitry 1 2 3 1 2 2 OLEDs 1 1 DS 1 1 APS Passive 2 3 DS 2 2 APS Inorganic Active 1 2 DS 1 1 APS Display Backplanes Organic Active 1 1 DS 1 2 APS Organic Photovoltaics 2 1 DS 2 1 2 Batteries 3 2 3 2 2 2 Situation Analysis – Status and Current Developments In general, there are six major classes of substrates for large area flexible electronics: 1) polymer film, 2) metal, 3) paper, 4) textiles, 5) glass and 6) ceramics. More detailed information for each material is provided in the sections that follow. 2.1 Polymer Films Polyesters a. Applications Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are well-known polymer films used for a wide range of applications ranging from magnetic media and photographic applications, where optical properties and excellent cleanliness are of paramount importance to electronics applications such as flexible circuitry and touch switches, where thermal stability is key. More demanding polyester film markets, which exploit the higher performance and benefits of PEN include magnetic media for high density data storage and electronic circuitry for hydrolysis-resistant automotive wiring2,3. More recently these polyester films have emerged as the leading plastic based substrates for printable electronics4,5,6,7. Polyester films are being used in RFID, organic and inorganic AM backplanes and cholesteric liquid crystal (LC) displays. Plans are currently underway to commercialize e-paper displays based on active matrix (AM) backplanes fabricated on polyester film. b. Properties PET and PEN films are prepared by a process whereby the amorphous cast is drawn in both the machine direction and transverse direction. The biaxially oriented film is then heat set to crystallize the film. It is the fact that the films are both crystalline and biaxially oriented that imparts the unique property set associated with polyester films. This includes, -excellent clarity -low coefficient of thermal expansion (CTE) to minimize the stress in composite structures involving organic and inorganic layers-excellent solvent resistance (to cope with the wide range of solvents used in large area electronics -low moisture pickup to minimize dimensional change arising from film swelling during device manufacture -mechanical strength The polyester films can also be put through a further process whereby the films are allowed to relax under low tension at elevated temperatures. This process yields a film having low shrinkage up to the temperatures at which the films are heat stabilized. By careful control of thermal stress,