Fabrication and AC performance of flexible Indium-Gallium-Zinc-Oxide thin-film transistors

The internet of things or foldable phones call for a variety of flexible sensor conditioning and transceiver circuits. However, the realization of high-performance, large-area, and deformable analog circuits is limited by the materials and the processes compatible with mechanically flexible substrates. Among the different semiconductors, InGaZnO is one of the most promising materials to realize high-frequency flexible thin-film transistors (TFTs) and circuits. In this work, the effect of different geometries, including self-aligned, vertical, and double-gate structures on the AC behaviour of flexible IGZO TFTs is presented. All TFTs are based on Al2O3 insulating layers, InGaZnO semiconductor, and polyimide substrates. The presented TFTs exhibit state-of-the-art performance including a field-effect mobility up to 15 cm2 /Vs and a mechanical bendability down to radii of 3.5 mm. Due to different trade-offs required in the fabrication, flexible IGZO TFTs with the shortest channel length of 160 nm do not exhibit the highest measured frequency, whereas exceptional maximum oscillation and transit frequencies of 304 MHz and 135 MHz are demonstrated for 500 nm long self-aligned TFTs. Such optimized transistors can be used to realize entirely flexible analog circuits leading towards imperceptible electronic systems

Contact resistance and overlapping capacitance in flexible sub-micron long oxide thin-film transistors for above 100 MHz operation

In recent years new forms of electronic devices such as electronic papers, flexible displays, epidermal sensors, and smart textiles have become reality. Thin-film transistors (TFTs) are the basic blocks of the circuits used in such devices and need to operate above 100 MHz to efficiently treat signals in RF systems and address pixels in high resolution displays. Beyond the choice of the semiconductor, i.e., silicon, graphene, organics, or amorphous oxides, the junctionless nature of TFTs and its geometry imply some limitations which become evident and important in devices with scaled channel length. Furthermore, the mechanical instability of flexible substrates limits the feature size of flexible TFTs. Contact resistance and overlapping capacitance are two parasitic effects which limit the transit frequency of transistors. They are often considered independent, while a deeper analysis of TFTs geometry imposes to handle them together; in fact, they both depend on the overlapping length (LOV) between source/drain and the gate contacts. Here, we conduct a quantitative analysis based on a large number of flexible ultra-scaled IGZO TFTs. Devices with three different values of overlap length and channel length down to 0.5 μm are fabricated to experimentally investigate the scaling behavior of the transit frequency. Contact resistance and overlapping capacitance depend in opposite ways on LOV. These findings establish routes for the optimization of the dimension of source/drain contact pads and suggest design guidelines to achieve megahertz operation in flexible IGZO TFTs and circuits

Flexible In-Ga-Zn-O thin-film transistors with sub-300-nm channel lengths defined by two-photon direct laser writing

In this work, the low-temperature (≤ 150 °C) fabrication and characterization of flexible Indium-Gallium-ZincOxide (IGZO) top-gate thin-film transistors (TFTs) with channel lengths down to 280 nm is presented. Such extremely short channel lengths in flexible IGZO TFTs were realized with a novel manufacturing process combining two-photon direct laser writing (DLW) photolithography with Ti/Au/Ti source/drain e-beam evaporation and lift-off. The resulting flexible IGZO TFTs exhibit a saturation field-effect mobility of 1.1 cm2V -1 s -1 and a threshold voltage of 3 V. Thanks to the short channel lengths (280 nm) and the small gate to source/drain overlap (5.2 µm), the TFTs yield a transit frequency of 80 MHz (at 8.5 V gate-source voltage) extracted from the measured S-parameters. Furthermore, the devices are fully functional when wrapped around a cylindrical rod with 6 mm radius, corresponding to 0.4 % tensile strain in the TFT channel. These results demonstrate a new methodology to realize entirely flexible nano-structures, and prove its suitability for the fabrication of short-channel transistors on polymer substrates for future wearable communication electronics

Fabrication, modeling, and evaluation of a digital output tilt sensor with conductive microspheres

Recent advances in wearable computing ask for bendable and conformable electronic circuits and sensors, allowing an easy integration into everyday life objects. Here, we present a novel flexible tilt sensor on plastic using conductive microspheres as gravity sensitive pendulum. The sensor provides a digital output of the measurement signal without the need for any additional electronics (e.g., amplifiers) close to the sensing structure. The sensor is fabricated on a free-standing polyimide foil with SU-8 photoresist defining the cavity for the pendulum. The pendulum consists of freely movable conductive microspheres which, depending on the sense of gravity, connect different electric contacts patterned on the polyimide foil. We develop a model of the sensor and identify the amount of microspheres as one of the key parameters in the sensor design, which influences the performance of the sensor. The presented tilt sensor with eight contacts achieves an angular resolution of 22.5° with a hysteresis of 10° and less at a tilt of the sensor plane of 50°. Analysis of the microsphere movements reveals a response time of the sensor at ~ 50 ms

Biomimetic microelectronics for regenerative neuronal cuff implants

Smart biomimetics, a unique class of devices combining the mechanical adaptivity of soft actuators with the imperceptibility of microelectronics, is introduced. Due to their inherent ability to self‐assemble, biomimetic microelectronics can firmly yet gently attach to an inorganic or biological tissue enabling enclosure of, for example, nervous fibers, or guide the growth of neuronal cells during regeneration

Oxide thin-film transistors on fibers for smart textiles

Smart textiles promise to have a significant impact on future wearable devices. Among the different approaches to combine electronic functionality and fabrics, the fabrication of active fibers results in the most unobtrusive integration and optimal compatibility between electronics and textile manufacturing equipment. The fabrication of electronic devices, in particular transistors on heavily curved, temperature sensitive, and rough textiles fibers is not easily achievable using standard clean room technologies. Hence, we evaluated different fabrication techniques and multiple fibers made from polymers, cotton, metal and glass exhibiting diameters down to 125 µm. The benchmarked techniques include the direct fabrication of thin-film structures using a low temperature shadow mask process, and the transfer of thin-film transistors (TFTs) fabricated on a thin (≈1 µm) flexible polymer membrane. Both approaches enable the fabrication of working devices, in particular the transfer method results in fully functional transistor fibers, with an on-off current ratio >107 , a threshold voltage of ≈0.8 V, and a field effect mobility exceeding 7 cm2 V −1 s −1 . Finally, the most promising fabrication approach is used to integrate a commercial nylon fiber functionalized with InGaZnO TFTs into a woven textile

Design of bendable high-frequency circuits based on short-channel InGaZnO TFTs

A unique requirement of flexible electronic systems is the need to simultaneously optimize their electrical and mechanical performance. Amorphous InGaZnO thin-film transistors (TFTs) fabricated on free-standing large-area plastic substrates address this issue by providing a carrier mobility >10 cm 2 /Vs, and bendability down to radii as small as 25 μm. At the same time, limitations such as a constrained minimum lateral feature size, the lack of appropriate p-type materials, or the influence of strain have to be considered when designing circuits. Here, models describing the scaling and bending behavior of flexible InGaZnO TFTs, together with the design of strain insensitive circuits operating at megahertz frequencies are presented

Flexible InGaZnO TFTs with fmax above 300 MHz

n this letter, the AC performance and influence of bending on flexible IGZO thin-film transistors, exhibiting a maximum oscillation frequency (maximum power gain frequency) fmax beyond 300 MHz, are presented. Self-alignment was used to realize TFTs with channel length down to 0.5 μm. The layout of this TFTs was optimized for good AC performance. Besides the channel dimensions this includes ground-signal-ground contact pads. The AC performance of this short channel devices was evaluated by measuring their two port scattering parameters. These measurements were used to extract the unity gain power frequency from the maximum stable gain and the unilateral gain. The two complimentary definitions result in fmax values of (304 ± 12)MHz and (398 ± 53) MHz, respectively. Furthermore, the transistor performance is not significantly altered by mechanical strain. Here, fmax reduces by 3.6% when a TFT is bent to a tensile radius of 3.5 mm

Oxide thin-film electronics on carbon fiber reinforced polymer composite

In this letter, the direct fabrication of amorphous indium-gallium-zinc-oxide thin-film transistors (TFTs) and circuits on a commercial carbon fiber reinforced polymer (CFRP) substrate is demonstrated. The CFRP is encapsulated with a ≈10.6−μm -thick resin layer, although the surface roughness and temperature sensitivity of the substrate are not ideal for the fabrication of electronic devices, we present depletion mode TFTs exhibiting a field effect mobility of 18.3 cm2V−1s−1 , and a common source amplifier, providing a voltage gain of 8 dB and a −3 dB cutoff frequency of 11.5 kHz. The amplifier does not require any input bias voltage and can, hence, be directly used to condition signals originating from various transducers, e.g., piezoelectric strain sensors used to monitor the structural integrity of CFRP elements. This opens the way to the fabrication of smart mechanical CFRP parts with integrated structural integrity monitoring system