Ultra-thin polymer insulating layers by initiated chemical vapor deposition and their applications to highly flexible transistors and flash memories

Abstract

Beyond the success of flexible displays, the studies on flexible electronics are now more diversified and intensified to address forthcoming applications such as wearable electronics, body-attachable patches, and smart healthcare.[1] The applications require challenging form-factors of strechability or epidermal adhesiveness, and organic electronic devices have been received great attention attributed their soft nature and low-temperature processibility.[2] The favorable nature of organic electronics have successfully led to ultra-flexible electronic devices, and consequently stretchable devices by wrinkling or impercepible bio-batches based on the ultra-flexible platform.[3] The possibilities of the organic electronics, nevertheless, have not been sufficiently opened yet due to the lack of organic insulating layers with sufficient performance, processibility, and down-scalability. Previous impressive ultra-flexible organic thin-film transistors (TFTs) and circuits have used inorganic insulating layers, and thus it suffered from significantly limited durability to mechanical stratin, below 1%.[4] And, also the use of inorganic insulating layers require high fabrication temperature, or unfavorable fabrication processes that are applicable only to a specific gate electroce in specific device structure or not compatible with large area fabrication. These problems become more significant in studying flexible flash memories, because it requires two ultra-thin insulating layers through which carrier conduction is elaborately contolled.

We recently proposed ultra-thin polymer films fabricated by initiated chemical vapor deposition (iCVD) as insulating layers.[5] Various kinds of monomers are applicable to the iCVD process, thus polymer insulating layers with variety of chemical structures can be fabricated. Among the polymers, poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane) (pV3D3) films deposited by the iCVD process showed almost ideal insulating property based on tunneling conduction for the thickness of about 10 nm, and maintained the excellent insulating property for the mechanical strain of up to 4%. The iCVD process, in addition, produces the ultra-thin polymer insulating layers for process temperature of near room-temperature. These are significantly favorable properties for insulating layers to develop electronic devices with both high performance, low power consumption, and high flexibility. Using the ultra-thin pV3D3 layers as gate dielectrics, low-voltage TFTs were successfully fabricated using organic, metal oxide, and graphene channels. The devices showed switching voltage of below 3 V and sufficiently high mobility for each channels, 1.5, 20, 5000 cm2/Vs respectively. Organic TFTs with the iCVD polymer gate dielectric layer showed homogeneous characteristrics for tensile strain of up to 2.3%, and then showed redection of channel mobility due to degradation of a small molecular organic channel. This is sharp constrast to the flexible TFTs with inorganic dielectrics, in which channel property entirely disappeared by the breakdown of insulating layers for a strain of near 1%. For TFTs with iCVD polymer gate dielectrics to have higher durability to strain, therefore, durability of channels need to be improved rather than the iCVD processed polymer insulating layers.