Solution Processed Silver Sulfide Filament Memories
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1113-F02-09
Solution Processed Silver Sulfide Filament Memories Shong Yin, Steven K. Volkman, and Vivek Subramanian Department of Electrical Engineering and Computer Sciences, University of California-Berkeley ABSTRACT Ionic resistive switches are emerging as a potential successor for flash in non-volatile memory applications. In ionic switches, metal cations migrate through a solid electrolyte forming filaments on an inert cathode that results in an abrupt increase in conductivity. This process is reversible, and the switches may be reverted to a low-conductive state. These switches have low transition voltages and fast read speed. The low switching energy potentially makes them more scalable than many other resistive memories. Silver Sulfide Resistive switches have been fabricated by sulfidizing evaporated silver films in Sulfur solutions. XPS is used to quantify stoichiometry of resultant films. SEM and AFM indicate that surface roughness increases with sulfidation time as the bulk silver film is consumed in forming the silver sulfide. XRD has confirmed presence of the acanthite phase of silver sulfide in the films. The entire process is performed at temperatures below 200C, so the devices are potentially stackable over conventional CMOS substrates in a BEOL process, and are applicable to printable electronics on plastic substrates. Initial characteristics measured on these cells are very promising, exhibiting low energy switching and good programming margins. Write/Erase voltages for cells were about 400mV and -200mV respectively. Ron/Roff ratios range from 10 to as high as 10,000 depending on process conditions. Impact of bath concentration, bath temperature and post-annealing on the silver sulfide film structure are studied. Ionic switching is demonstrated in the films. INTRODUCTION Ionic memories are typically fabricated from mixed electronic and ionic conducting chalcogenide thin films (1). Terabe et. al. have reported fabrication of 150 nm x 100 nm crossbar switches composed of a silver/silver sulfide/air gap/platinum stack (2). The silver sulfide was formed by sulfidation of an evaporated silver film in ambient sulfur vapour (3). Ionic memories are two-terminal electrochemical devices, different from their three-terminal flash memory counterparts which are field drive MOSFETs. The switching mechanism in ionic memories results from the reversible electrochemical formation of silver filaments through the solid electrolyte layer. Bulk silver sulfide is a poor electronic conductor. Initially the device is in a high resistance state. Under proper biasing of electrodes, silver filament formation occurs through the electrolyte that shorts the electrodes, and transitions to a low-resistance state. Dissolution of the filament under subsequent bias reverts the device to a high-resistance state. Here we study a method of sulfidizing evaporated silver films in sulfur solutions at various bath temperatures. Elemental sulfur is soluble in aromatic solvents such as toluene, anisole and technical grade α-terpineol. This
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