Eugene Y. Ma

MECHANICS OF ROLLABLE AND FOLDABLE FILM-ON-FOIL ELECTRONICS

The mechanics of film-on-foil devices is presented in the context of thin-film transistors on steel and plastic foils. Provided the substrates are thin, such transistors function well after the foils are rolled to small radii of curvature. When a substrate with a lower elastic modulus is used, smaller radii of curvature can be achieved. Furthermore, when the transistors are placed in the neutral surface by sandwiching between a substrate and an encapsulation layer, even smaller radii of curvature can be attained. Transistor failure clearly shows when externally forced and thermally induced strains add to, or subtract from, each other.

AMORPHOUS SILICON TRANSISTORS ON ULTRATHIN STEEL FOIL SUBSTRATES

Thin-film transistors of hydrogenated amorphous silicon (a-Si:H) were fabricated on foils of stainless steel with thickness ranging down to 3 μm, which is less than three times the thickness of the deposited layers. Transistors made on foils from 3 to 200 μm thick exhibit comparable electrical performance. Two factors account for the feasibility of such thin device/substrate structures. One is that the built-in stress and the differential thermal contraction stress nearly cancel each other in steel/a-Si:H structures. The other is that on very thin foils the transistor structure offloads part of its strain to the steel foil.

Compliant substrates for thin-film transistor backplanes

The emergence of wearable electronics is leading away form glass substrates for the display backplane, to plastic and metal. At the same time the substrate thickness is reduced to make displays lighter. These two trends cooperate toward the development of compliant substrates, which are designed to off load mechanical stress from the active circuit onto the substrate. Compliant substrates made the circuit particularly rugged against rolling and bending. Design principles for compliant substrates include: (a) Moving the circuit p;lane as close as possible to the neutral plane of the structure, and (b) Using substrate and encapsulation materials with low stiffness. Design principle (a) is demonstrated on thin-film transistors made on thin steel foil. Such transistors function well after the foils are rolled to small radii of curvature. Principle (b) of compliant substrates is demonstrated with bending experiments of a-Si:H TFTs made on thin substrates of polyimide foil. TFTs on 25-micrometers thick polyimide foil may be bent to radii of curvature as low as 0.5 mm without failing. The reduction in bending radius, from R-2 mm on same- thickness steel foil, agrees with the theoretical prediction that changing from a stiff to a compliant substrate reduces the bending strain in the device plane by a factor of up to 5.

Tunable thin-film filters based on thermo-optic semiconductor films

Thermo-optic layers of thin film semiconductors are deposited by PEVCD to create thermally tunable bandpass filters for WDM optical networks. Amorphous semiconductor films, adapted from the solar cell and display industries, are the primary ingredient. Single-cavity tunable filters with FWHM=0.085 nm, >40 nm tuning range, and insertion losses 0.2-4 dB are demonstrated. Key enablers for this new family of index-tunable thin film devices are PECVD deposition, large internal temperature changes >400C, high conductivity polysilicon heater films, and extremely robust film adhesion. Possible applications include optical monitoring, add/drop multiplexing, dynamic gain equalization, and dispersion compensation.

Broadly tunable thin-film intereference coatings: active thin films for telecom applications

Thin film interference coatings (TFIC) are the most widely used optical technology for telecom filtering, but until recently no tunable versions have been known except for mechanically rotated filters. We describe a new approach to broadly tunable TFIC components based on the thermo-optic properties of semiconductor thin films with large thermo-optic coefficients 3.6X10[-4]/K. The technology is based on amorphous silicon thin films deposited by plasma-enhanced chemical vapor deposition (PECVD), a process adapted for telecom applications from its origins in the flat-panel display and solar cell industries. Unlike MEMS devices, tunable TFIC can be designed as sophisticated multi-cavity, multi-layer optical designs. Applications include flat-top passband filters for add-drop multiplexing, tunable dispersion compensators, tunable gain equalizers and variable optical attenuators. Extremely compact tunable devices may be integrated into modules such as optical channel monitors, tunable lasers, gain-equalized amplifiers, and tunable detectors.