We report a self-aligned coplanar amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) using an a-IGZO/SiO2 stack layer. From the channel-length dependence of the total resistance for the TFTs, the channel and parasitic resistances were found to be 8.4 kΩ/μm and 9.7 kΩ/sq, respectively. The fabricated a-IGZO TFT exhibits field-effect mobility of 23.3 cm2/V ·s, threshold voltage of 3.6 V, and gate voltage swing of 203 mV/dec. A 23-stage ring oscillator made of the self-aligned TFTs exhibits a propagation delay time of 17 ns/stage at a supply voltage of 22 V.

Self-Aligned Coplanar a-IGZO TFTs and Application to High-Speed Circuits

Recently, there has been considerable interest in fl exible displays, as they facilitate the fabrication of displays that are thin, lightweight, robust, conformable, and fl exible. [ 1 ] To enable a fl exible display, a fl exible substrate must be used as the fundamental starting component in place of a conventional glass substrate. In general, metal foils, ultra-thin glasses, and plastic fi lms are considered candidates for a fl exible substrate. [ 2 ] In particular, fl exible displays using plastic substrates based on organic polymers have been a major topic not only because these show outstanding fl exibility and optical clarity at the same time, but also because they offer the possibility of a reduction in cost, coupled with a roll-to-roll process and ink-jet printing technology. [ 3 ] Common examples of commercially available polymers are polyethylene terephthalate (PET), polyether sulfone (PES), polyethylene naphthalate (PEN), polycarbonate (PC), and polyimide (PI). [ 1 , 4 ] To replace conventionally used glass substrates, a plastic substrate must be equipped with the properties of glass, such as optical transparency, thermal/chemical stability, O 2 /H 2 O permeability, low birefringence (or retardation), dimensional stability, and a low coeffi cient of thermal expansion (CTE). [ 1 ] Among these properties, the low CTE of plastic substrates coupled with dimensional stability is arguably the most important requirement, as it is directly related to compatibility with all other necessary display layers to be integrated onto them. [ 1 ] Although there have been extensive studies of fl exible devices built on polymer substrates, such as electrophoretic displays and organic thin-fi lm transistors (OTFTs), little progress have been made even with high-temperature processed devices such as active-matrix liquid crystal displays (AMLCDs). [ 5 ] Major obstacles include the high CTEs of polymers insuffi cient for the display layer integrations. Moreover, polymers usually have low glass transition temperatures ( T g s) where abrupt CTE changes are accompanied. This greatly limits their practical application in terms of the process temperature. Even polymers with a high T g , e.g., PES, are known to have a CTE of ∼ 50 ppm K − 1 , much higher than the typically required level (less than 20 ppm K − 1 ). [ 1 ] Other highT g polymers such as polytetrafl uoroethylene (PTFE) and poly(ether ether ketone) (PEEK) also have signifi cant drawbacks for implementation into large-area plastic electronics, as they are unfavorable in terms of cost. PI has a low CTE of ∼ 20 ppm K − 1 and a high

Rollable Transparent Glass‐Fabric Reinforced Composite Substrate for Flexible Devices

The piezoelectric energy generation properties of transparent flexible devices (TFD) based on PbZr 0.52 Ti 0.48 O 3 (PZT) films, which were fabricated by laser lift-off (LLO) process, were studied for a piezoelectric energy harvester. Through the introduction of indium-tin-oxide (ITO) and polyethylene terephthalate (PET) substrates, TFDs were implemented, respectively. The TFDs based on PZT films generated an AC-type output signal and output power of 8.4 nW/cm 2 , at periodically bending and releasing motion. In addition, inverted output signals were observed when the manufactured TFDs were connected to the measuring equipment in reverse and were bended to the reverse direction, demonstrating that the generating signals originated from the piezoelectric effect of TFDs. The experimental results clearly showed that the TFDs based PZT film have potential for use in next generation of electronic devices applications such as flexible devices, transparent electronics, and energy harvester.

Preparation on transparent flexible piezoelectric energy harvester based on PZT films by laser lift-off process

In today's society, displays are indispensable for digital interaction and personal contact. Therefore, the development of displays is an urgent need for the next generation. Leading the commercialization trends is the development of industrial, high-resolution, portable, and multifunctional displays. Moreover, for small-size portable displays, the most critical factor affecting future applications is their flexibility. Among the various materials that can act as the channel layer, low-temperature polycrystalline silicon (LTPS) is a potential candidate for next-generation portable displays, which require high resolution and stable reliability, and which can achieve virtual reality applications. This article is a review of the development of the LTPS thin-film transistors (TFTs) on soft and flexible electronics, especially the effect of mechanical strain. This article starts by providing an overview of the difficulties in fabricating LTPS TFTs on flexible substrates. The physical mechanism corresponding to each degradation caused by mechanical stress is presented next. Finally, to support the development of the flexible technologies and realize their commercialization, methods to overcome these mechanical strain-induced degradations are reported in three different aspects by understanding the physical degradation model.

Flexible low-temperature polycrystalline silicon thin-film transistors

This letter investigates the negative-bias temperature instability (NBTI) degradation of p-channel low-temperature polycrystalline-silicon thin-film transistors (LTPS TFTs) under mechanical tensile stress. Experimental results reveal that the interface state density Nit and grain boundary trap density Ntrap of tensile-strained LTPS TFTs are more pronounced than those of unstrained LTPS TFTs. Extracted density of states and conduction activation energy Ea both show increases due to the strained Si-Si bonds, which implies that strained Si-Si bonds are able to react with dissociated H during NBTI stress. Therefore, NBTI degradation is more significant after tensile strain than in an unstrained condition.

/pdf/nbti-degradation-in-ltps-tfts-under-mechanical-tensile-5459pwbsm1.pdf

NBTI Degradation in LTPS TFTs Under Mechanical Tensile Strain

We have studied the mechanical stability of poly-Si thin-film transistor on 50 μm-thick flexible metal foil with a field-effect mobility of 81.2 cm 2 /V s, a threshold voltage of −2.4 V, and an on/off current ratio of 10 6 . We have measured the electrical properties under various compressive and tensile strains by changing the bending radius of the base metal foil. We have found that the TFT is stable until the bending radius of 50 mm which corresponds to the strain of ∼1.4%.

Mechanical stability of poly-Si TFT on metal foil

In this letter, we have studied the inverted staggered thin-film transistor (TFT) using a spin-on-glass (SOG) gate insulator and a low-temperature polycrystalline silicon (poly-Si) by Ni-mediated crystallization of amorphous silicon. The p-channel poly-Si TFT exhibited a field-effect mobility of 48.2 cm2/V ldr s, a threshold voltage of -4.2 V, a gate-voltage swing of 1.2 V/dec, and a minimum off-current of < 4 times 10-13A/ mum at Vds = -0.1 V. Therefore, the gate planarization technology by SOG can be applicable to low-cost large-area poly-Si active-matrix displays.

Inverted Staggered Poly-Si Thin-Film Transistor With Planarized SOG Gate Insulator

Methylsiloxane-based spin-on-glass (M-SOG) has been applied to a polycrystalline silicon thin-film transistor (TFT) on a 40 μm-thick flexible metal foil as a gate dielectric and planarization layer as well. Triple spin coatings and curing of M-SOG layers reduce the surface roughness of the metal foil from 800 to 56 A. The p-channel metal-induced crystallization of a-Si using a cap TFT using M-SOG on a metal foil exhibited a field-effect mobility of 51.1 cm 2 /V s, a threshold voltage of -4.3 V, and a minimum off-state current of <1.74 X 10 -12 A/μm at V ds = -0.1 V. The poly-Si TFT was found to be stable against a negative gate-bias stress. Therefore, the low-cost, low-temperature polycrystalline silicon TFT can be applied to make flexible displays.

http://esl.ecsdl.org/content/11/4/H77.full.pdf

Coplanar Poly-Si TFT on Flexible Metal Foil Using Spin-On Glass as Gate Insulator and Planarization

We have studied the fabrication of hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) on flexible metal foil with polyimide planarization. The metal foil was coated with polyimide (PI) two times for palanarization with the total thickness of 2.6 μm. The PI was chosen because of its superior planarization capability, easy spin-on process and relatively high temperature process. To coat a PI layer two step process was carried out; room temperature coating and annealing at 180 °C for 1 h and then 300 °C curing for 1 h. The RMS surface roughness was changed from 663 to 20.6 A by two times coatings. The a-Si:H TFT on the PI planarized metal foil exhibited the field-effect mobility of 1.47 cm 2 /V s and a threshold voltage of 1.8 V. The flexibility of the high-performance TFT was studied for AMOLED backplane application.

High-performance hydrogenated amorphous silicon TFT on flexible metal foil with polyimide planarization

We have studied the oxygen-plasma effect on spin-on-glass (SOG) on the growth of low-temperature polycrystalline silicon (LTPS) by metal-mediated crystallization. The SOG buffer was coated on glass, and then it was exposed to oxygen plasma before the deposition of amorphous silicon (a-Si) to turn the SOG surface to hydrophilic. The a-Si on SOG was crystallized by metal-induced crystallization through a cap. The crystalline structure depended on the surface treatment: disk-shaped grains were grown on the O 2 -plasma-treated SOG, but needlelike grains were on the SOG without O 2 -plasma treatment. The p-channel poly-Si thin film transistor using the LTPS on O 2 -plasma-treated surface exhibited a field-effect mobility of 91.1 cm 2 /V s, a threshold voltage of -8.5 V, and a gate voltage swing of 1.2 V/dec.

http://esl.ecsdl.org/content/10/12/J155.full.pdf

Jung Ho Bae

Papers

Mechanical stability of poly-Si TFT on metal foil

Inverted Staggered Poly-Si Thin-Film Transistor With Planarized SOG Gate Insulator

Coplanar Poly-Si TFT on Flexible Metal Foil Using Spin-On Glass as Gate Insulator and Planarization

High-performance hydrogenated amorphous silicon TFT on flexible metal foil with polyimide planarization

Effect of Oxygen-Plasma Treatment on Formation of LTPS on SOG by Metal-Mediated Crystallization