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EETE MAY 2015 Paper memory ready to roll RBy Julien Happich esearchers at the Finish VTT Technical Research Centre have demonstrated they could print memory circuits directly on paper, using simple roll-to-roll printing techniques with a particular mix of commercially available metallic inks. The researchers envisage their cheap paper memories to be used in applications such as sensor data recording, product originality marking, playing cards, interactive packaging and product information cards. The write-once-read-many (WORM) memories can be fabricated directly on the product or packaging using flexographic or inkjet printing machines, common in the packaging industry. Each writable memory bit, measuring about 0.2x0.3mm each in the researchers’ experiments, consist of a mix of two commercially available silver nanoparticle inks, dried after a regular R2R printing process of the actual bits and their associated writing/ reading electrodes. Before writing, each bit is in the 1 state of high resistance. Writing a bit is performed by applying a low voltage (under 10V) across the bit, which in effect sinters adjacent silver nanoparticles Electronic Engineering Times Europe May 2015 17 Future products_93x277_UK.indd 1 02/04/15 17:24 and creates a path of least electrical resistance, hence turning the memory bit from a high resistivity ‘0’ state to a low resistivity ‘1’ state. This sintering step is non-reversible, which means the data content of the memory is only written once, but it supports an unlimited number of read operations. The VTT lab was able to print a roll length of 150m containing more than 10 000 printed WORM memory banks on a 125um thin heat stabilized PET substrate, but in prior research, it had demonstrated the sintering capability of the memory bits on paper. Each memory bank consisted of a linear array of bits automatically die-cut on the R2R line. Then each bit could be read sequentially by measuring the resistance across it since there is typically an order of magnitude between the 0 and 1 states (a recent paper discussed a bit resistance dropping from R‘0’ of 1:4 kOhm to R‘1’ under 100Ohm during a write time under 5ms).

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