Fordable Flash Memories with Low Operating Voltage and Long Retention Time using Polymer Dielectrics

Abstract

Flash memories are most successful and essential non-volatile memory devices to store data in modern electronic products from thumb-scale devices to smart-phones and tablets, and are also the most promising candidate for wearable or both-attachable devices in future. Significant amount of studies have been proposed flexible flash memories, but favorable devices satisfying all key characteristics including low programming/erasing voltages, long retention time and high flexibility simultaneously have not been reported yet. The memory operations of programming, erasing, and retention are dominantly governed by two dielectric layers of a tunneling dielectric layer (TDL) and a blocking dielectric layer (BDL), that sandwich a floating gate (FG) to store controlled amount of charges. The insulating property satisfying the role of TDL can be found in ultra-thin dielectric layers, in which the carrier conduction behavior classified to direct tunneling in low field and Fowler-Nordheim (F-N) tunneling in high field. The major bottleneck to develop flexible flash memories is a lack of dielectric layers having both excellent insulating property based on ideal tunneling conduction and good flexibility itself. Here, we propose flexible flash memories by using polymer dielectric layers produced by initiated chemical vapor deposition (iCVD) for TDL and BDL, and by rational design based on flash memories operation mechanisms. We employed two kinds of iCVD processed polymers of pV3D3 and pEGDMA, and they showed different dielectric constant of 2.2 and 3 respectively and similarly excellent tunneling-based insulating property. By using pV3D3 as TDL and pEGDMA as BDL, electric field inside TDL can be much increased comparing to that in BDL due to lower dielectric constant, and thus much higher carrier transport through TDL by F-N tunneling occurs while carrier transport through BDL remains significantly low by direct. This rational design causing asymmetric carrier transport in two dielectric layers is highly favorable to flash memory operation. And with an ultrathin ca. 15 nm thick- pV3D3 layer and a ca. 40 nm-thick pEGDMA, the organic flash memory with a C60 channel showed sufficient memory window of 5 V with programming and erasing voltage of 10 V, and low retention time over 10 years simultaneously. In terms of flexibility, the device maintained the excellent memory operation for the mechanical strain of up to 2.8 %, attributed to the high durability of iCVD polymer dielectric layers. And finally the organic flash memories fabricated on a 6 um-thick Mylar substrate showed homogeneous characteristics with significantly deformation such as folding with bending radius of 300 um of over 1000 times. This result is the first flexible flash memories showing commercially considerable memory performance and outstanding flexibility at the same time, and will be a promising candidate for highly deformable products such as wearable devices or electronic patches.