Roll-to-roll printed resistive WORM memory on a flexible substrate

The fabrication process and the operation characteristics of a fully roll-to-roll printed resistive write-once-read-many memory on a flexible substrate are presented. The low-voltage (less than 10 V) write operation of the memories from a high resistivity ‘0’ state to a low resistivity ‘1’ state is based on the rapid electrical sintering of bits containing silver nanoparticles. The bit ink is formulated by mixing two commercially available silver nanoparticle inks in order to tune the initial square resistance of the bits and to create a self-organized network of percolating paths. The electrical performance of the memories, including read and write characteristics, is described and the long-term stability of the less stable ‘0’ state is studied in different environmental conditions. The memories can find use in low-cost mass printing applications.

(a) A printed questionnaire card with a R2R printed 12-bit WORM memory bank, flexible battery and a Si-based light-emitting-diode assembled together as a system. The graphical printing, screen printing of electrical wiring with a conductive silver ink and the card assembly were performed at Stora Enso Oyj. (b) R2R flexographic printing of WORM memory banks for the questionnaire card with VTT’s ‘ROKO’ printing line. (c) WORM memory bank pre-sintering and the readout device. The electrical contacts were realized using an array of spring-loaded probes.

We have demonstrated fully R2R gravure and R2R flexographic printed resistive WORM memory devices based on Ag NPs on a flexible substrate. The low-voltage write operation utilizes the RES process and the read operation can be performed both via capacitive (non-contact) or resistive (contact) measurement. The bit ink properties, including the initial ‘0’ state resistance, were tailored by mixing two different commercial Ag NP pastes in different weight ratios. The resulting printed and dried layers exhibited size segregation of the two ink component NPs, leading to a self-aligned network of low-resistance percolation paths. The observed high variation in the ‘0’ state resistance of the bits can be controlled by pre-sintering the bits to a constant level. The memories show excellent ‘0’ state retention of over 19 months when stored in the dark with a desiccant. However, both elevated temperature and moisture were found to rapidly decrease the ‘0’ state resistance of laminated devices. The WORM memories can find usage in various printed low-cost applications. A simple electrical questionnaire card was presented as an example.

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