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Polymer Thin Film Transistor Technology

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					Polymer Thin Film
Transistor Technology

          Michael Ajibola
Content
    Introduction
•   History of TFT
•   Applications
    Organic semiconductors
    Polymer semiconductors
    Polymer thin-film transistors
•   Deposition
•   Patterning
    References
Introduction – History of TFT
 In 1960's Weimer stated making TFT on glass substrate, and his
 paper "The TFT - A new Thin-Film Transistor" drew worldwide
 attention.
 Brody's group created a half-watt audio amplifier on a strip of
 aluminium foil.
 Later, people realized that the ideal application of TFT was display.
 In 1980's, people started working on flat-panel display for military
 applications, with emphasis on electroluminescence rather than
 liquid crystal display.
 In 1990's, as LCD technology matures, TFT found applications as a
 switch transistor at each pixel in the LCD.
 It is the liquid crystal display that brought thin-film transistors back to
 life.
Introduction - Applications
    Organics have long been
    attractive for use in electronics
    because of their light weight,
    flexibility, and low cost
    compared with their Si
    counterparts.
    Applications include:
•   Flat panel display
•   copy and fax machine
•   Digital X-ray
•   Smart cards, finger print
    sensor, electronic IC tags.
Introduction - Applications




          Application in Liquid crystal display
Organic semiconductors
                  Pentacene is the best known organic semiconductor.
                  Other examples are tetracene perylene


    Advantages
•   Low cost
•   Large area compatible
•   Low processing temperature
•   Compatibility with inorganic semiconductors

    Disadvantages
•   Low carrier mobility
•   Stability
•   Patterning of films
•   Requires novel fabrication technology
Polymer semiconductors
Advantages of polymer semiconductors over other organic semiconductors
• can be deposited from solution
• can form amorphous or semicrystalline films
• has good mechanical properties for flexible substrates
Polymer thin-film transistors


                   They are fabricated using a
                   combination of deposition and
                   patterning methods.
Deposition
 For polymer semiconductors, two forms of deposition are available:
 deposition of a soluble precursor from a solution and subsequent conversion
 to the final film , and direct deposition from solution. The motivation
 Spin-coating and solution casting are two popular ways ofdirect solution
 deposition, which is often used for polymers such as regioregular P3HT or
 various soluble oligomers . Post-processing treatments, such as thermal
 annealing, improve molecular ordering and grain sizes of the thin film and
 frequently result in better device performance.
 A major concern for solution processing methods is the effect of the solvent
 on underlying organic features, requiring chemically compatible materials. For
 this reason, dry processing methods are being developed .
 It is important to consider the organic solution concentration and solubility,
 solvent evaporation rate, and substrate surface. Because of their effect on
 the quality of the resulting semiconductor film, these processing parameters
 should be carefully controlled.
 Dielectric films are fabricated in a similar manner to the semiconductor layer.
 An example of a solution-processed dielectric layer is poly-4-vinylphenol
 (PVP) , which is deposited by spin-coating and then cross-linked at 200°C.
Patterning
    Screen Printing
•   this involves squeezing a specially prepared ink through a screen mask
    onto a substrate surface to form the desired pattern. This method has
    limited feature size resolution (75 µm or larger).

    Ink-jet Printing
•   this is similar to the operation of a conventional ink-jet printer, but uses
    specially formulated inks . Resolution on the order of 25 µm can be
    achieved without surface modifications.

    An additive dry printing method for depositing conducting polymers has
    been developed using a thermal (laser) imaging technique for the ablative
    transfer of a patterned layer onto a flexible receiver layer with resolution
    down to 5 µm. This can be used to process successive layers without the
    use of a solvent that could degrade the underlying organic layers.
Patterning
    Soft lithography
•   this encompasses a wide variety of patterning techniques in which a master
    structure is fabricated in a material, such as Si, by conventional lithographic
    processes, and then used to make elastomeric replicas in a material such
    as poly(dimethylsiloxane) (PDMS).

•   using microcontact printing, these elastomeric stamps are used for
    molecular transfer to produce a contact-induced chemical modification of a
    surface . The chemical modification can produce hydrophobic and
    hydrophilic patterns allowing for selective solution phase deposition of the
    organic semiconductor.

    Soft lamination
•   source and drain electrodes are deposited on one substrate, then laminated
    onto another substrate that already contains the gate, dielectric, and
    semiconductor, thus completing the transistor .
Patterning
    Optical lithography
•   this is a well-developed conventional technique for the patterning of
    features and components for microelectronic and photonic devices .
    In this process, geometric shapes from a mask are transferred to a
    substrate (e.g. Si wafer), enabling patterning of the active materials
    and electrodes. Both metal and conducting polymer electrodes can
    be fabricated using standard photolithography followed by lift-off .

•   Although optical lithography can achieve 100 nm resolution, it is a
    relatively expensive process. This method is also less suitable for
    the patterning of organic semiconductors because the exposure of
    organic semiconductors to solvents and etchant tends to cause
    degradation in device performance.
Ink-jet printing, with wax used in defining
the device structure.
References
 D. Knipp, Organic electronics lecture slides, spring 2006.
 Colin Reese et all, “Organic thin film transistors”, Materials today,
 sept. 2004.
 Alberto Salleo, “Fabrication and characterization of high-
 performance polymer thin-film transistors”, Palo Alto research
 center.
 Kateri E. Paul et al, “Additive Jet Printing of Polymer Thin-film
 Transistors”, Palo Alto research center.

				
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Description: TFT (Thin Film Transistor) LCD, which is an active matrix type liquid crystal display (AM-LCD) in one. LCD flat panel displays, in particular TFT-LCD, is the only brightness, contrast, power, life, size and weight to catch up and fully integrated performance than CRT display device, and its excellent performance characteristics of a good large-scale production high degree of automation, low-cost raw materials, the development is vast, and will quickly become a mainstream product in the new century, global economic growth in the 21st century, a bright spot.