History of Flexible Electronics

  • Flexible electronics has a long history. It is anything thin and flexible.
  • Forty years ago single-crystalline silicon solar cells were thinned to raise their     power/weight ratio for use in extra terrestrial satellites. Because these cells were thin, they were flexible and warped like corn flakes. Today, silicon-integrated circuits are thinned to become compliant so that the owner of a smart card does not break it when he sits on it.
  • Flexible can mean many qualities: bendable, conformally shaped, elastic,        lightweight, nonbreakable, roll-to-roll manufacturable, or large-area.The field has open boundaries that move with its development  and application.
  • To the industrial community today, flexible electronics means flexible displays and X-ray sensor arrays. To researchers flexible means conformally shaped displays and sensors, electronic textiles, and electronic skin.
  • The development of flexible electronics dates back to the 1960s. The first flexible  solar cell arrays were made by thinning single crystal silicon wafer cells to∼100 μm and then assembling themon a plastic substrate to provide flexibility [1, 2].
  • In the mid-1980s, the active-matrix liquid-crystal display (AMLCD) industry  started in Japan by adopting the large-area plasma enhanced chemical  vapour  deposition (PECVD) machines that had been developed for a-Si:H solar cell fabrication.In 1997, polycrystalline silicon (poly-Si) TFTs made on plastic substrates using laser-annealing were reported. Since then, research on flexible electronics has expanded rapidly, and many research groups and companies have demonstrated flexible displays on either steel or plastic foil substrates. For example,in 2005 Philips demonstrated a prototype rollable electrophoretic display and Samsung announced a 7__ flexible liquid crystal panel. In 2006, Universal Display Corporation and the Palo Alto Research Center presented a prototype flexible organic light-emitting diode (OLED) display with full-color and full-motion with a poly-Si TFT backplane made on steel foil.
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  • Materials for Flexible Electronics

    A generic large-area electronic structure is composed of  :

  • (1) a substrate, eg: Thin Glass, Plastic Film, Metal Foil etc

  • (2) backplane electronics,

  •  (3) a frontplane, eg: Liquid Crystal Displays , Electrophoretic Displays , Organic Light-Emitting Displays , Sensors etc

    (4) encapsulation.

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To make the structure flexible, all components must comply with bending to some degree without losing their function. Two basic approaches have been employed to make flexible electronics:

 (1) Transfer and bonding of completed circuits to a flexible substrate  : In the transfer-and-bond approach, the whole structure is fabricated by standard methods on a carrier substrate like a Si wafer or a glass plate. Then it is transferred to or fluidic self-assembled on  a flexible substrate. The transfer-and-bond approach has been extended to the bonding of ribbons of Si and GaAs devices to a stretched elastomer, which upon relaxation forms a “wavy” semiconductor that can be stretched and relaxed reversibly .The transfer approaches have the advantage of providing high-performance devices on flexible substrates.

Their drawbacks are small surface area coverage and high cost. Bonded circuits will likely be added to large-area electronic surfaces at low density for highspeed communication and computation, lasing, and similarly demanding functions.

(2) Fabrication of the circuits directly on the flexible substrate : In many applications, the majority of the surface will be covered with electronics fabricated directly on the substrate. There are many approaches to integrating disparate materials and oftentimes flexible substrates are not fully compatible with existing planar silicon microfabrication processes. Direct fabrication may require :

(1) relying on polycrystalline or amorphous semiconductors because these can be own on foreign substrates,

(2) developing new process techniques,

 (3) introducing new materials, and

(4) striking a compromise between device performance and low process temperatures tolerated by polymer foil substrates.

Nanocrystalline silicon and printable polymers for OLEDs [33] are also materials of intense research.

 

HP and ASU Develop a Cheap Flexible Electronic DisplayImage

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Degrees of Flexibility

 Flexibility can mean many different properties to manufacturers and users. As a mechanical characteristic, it is conveniently classified in the three categories

(1) bendable or rollable,

(2) permanently shaped, and

 (3) elastically stretchable.

 The tools for microfabrication have been developed for flat substrates.

Therefore, at present all manufacturing is done on a flat workpiece that is shaped only as late as possible in the process. This approach benefits from the tremendous technology base established by the planar integrated circuit and display industries.

Flexible Checkerboard surface Image

Fabrication Technology for Flexible Electronics

1)    Fabrication on Sheets by Batch Processing :

Electronic devices and circuits and display panels are made by batch processing.Flexible foil substrates, cut to sheets, will be the drop-in replacement for the rigid glass plates or silicon wafers. Rigid substrates are best suited to free standing and loose mounting.

2)    Fabrication on Web by Roll-to-Roll Processing:

Flexible electronics are naturally associated with roll-to-roll processing

Roll-to-roll fabrication of large-area electronics, including solar cells, is desirable for cost reduction.

The roll-to-roll photolithography and etching tools available today are not capable of 2-μm resolution and overlay registration, particularly when combined with the tensioning applied for winding and with process cycles at elevated temperature, both of which cause substrate deformation.

The goal of roll-to-roll fabrication of flexible electronics is stimulating innovations in equipment and process design  process recipes, and system integration.

Tools for roll-to-roll processing that are available today include web cleaner, PECVD, sputtering, plasma etcher, die punch, evaporator,laser

writer, inkjet printer, screen printer, and inspection devices.

3)    Additive Printing :

Additive printing is roll-to-roll process compatible, is a high-throughput process, uses device materials efficiently, may not require vacuum, and may provide a solution to overlay registration problem through digital compensation. Noble-metal conductors, organic conductors,semiconductors, and insulators can be printed.

LG to debut first-ever flexible e-paperImage

Hermetic packaging of organic electronic devices is one of the most important challenges that must be overcome before flexible electronic devices can become a commercial reality. In this regard, thin-film barrier technology, especially on plastic substrates and also directly on the device, is a key enabling technology. From this review, it is clear that there are now multiple advanced barrier technologies that, at least in the research lab, can provide the right order of magnitude hermeticity.However, system integration challenges such as substrate deformation during device processing, thermomechanical stability, and compatibility with the device require more work and further improvements. Challenges also remain to demonstrate similar barrier performance using commercially viable R2R fabrication processes.Nevertheless, given the increasing rate of progress and increasing effort over the last decade, it seems likely that flexible thin-film barrier technology will mature in time to enable flexible electronics applications.

 

 

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