PORTLAND, Ore. — Memristors made from pure silicon could enable resistive random access memory (ReRAM) that are simpler and cheaper to manufacture than Hewlett-Packard Co.'s titanium-based formulation, according to researchers at Rice University. In collaboration with fabless chip design house PrivaTran Inc. the team demonstrated a proof-of-concept ReRAM that packs only 1-kbit, but which they claim can be scaled beyond the densities of flash.
"Our memristors are made out of silicon instead of titanium like HPs," said Tour. "In its patent application, HP listed many oxides, but not silicon-oxide, which we have now turned into a bit cell for resistive RAMs."
Silicon oxide is the most common insulator today, used in all CMOS chips. Since its initial characterization in the 1960s, engineers have intentionally deposited silicon oxide in thicknesses that prevent its breakdown. However, by carefully crafting the voltage pulses going through it, thin layers of silicon dioxide can be made to change their resistance from near infinite to near zero, according to Rice and PrivaTran (Austin, Texas). SanDisk Corp. in fact has used this phenomenon to create write-once memories, but now Rice and PrivaTran claim to have made the process reversible, thereby enabling pure silicon ReRAMs.
Last year, Rice professor James Tour's lab demonstrated how thin-films of carbon—graphene—could be made into a memristor-like bit cell that could double the typical memory densities of flash, But during characterization of that prototype, Tour noticed that the bit cell, which had been using silicon oxide as an insulator, seemed to work even without the graphene. Careful observations of the phenomena by Tour and fellow professors Douglas Natelson and Lin Zhong led to the discovery of a reversible "soft" breakdown of silicon oxide, allowing its use as a bit cell.
"We had noticed in our work with graphene that the silicon oxide was breaking down, but we did not understand the mechanism—now we do," said Tour. "By applying the right voltage pulses we can cause a reversible soft breakdown in silicon oxide where oxygen atoms move out leaving a silicon filament between the source and drain electrodes, allowing current to flow--turning it into a memistor that can be used as a bit cell. All you need besides the silicon oxide is a crossbar and a vertical diode to build 3D resistive RAMs."
Silicon oxide memory chip fabricated by Rice and a commercial partner as a proof-of-concept. Silicon nanowire forms when charge is pumped through the silicon oxide, creating a two-terminal resistive switch.
(Images courtesy Jun Yao/Rice University)
According to Tour, the silicon filaments are composed of sub-five-nanometer nanocrystals, which should allow the bit cells to scale beyond the densities of flash which loose functionality beyond 20 nanometer sizes. In their prototype, the crossbar electrodes were made from polysilicon, enabling the entire memory array to be cast in silicon. In characterizing the all-silicon memories, the researchers found that they could switch in under 100 nanoseconds and could withstood 10,000 read/erase/write cycles, similar to flash memories, but with the possibility of going to much higher densities than flash.
Funding for the research was provided by the Army Research Office, National Science Foundation, Air Force Office of Scientific Research and the Navy Space and Naval Warfare Systems Command Small Business Innovation Research (SBIR) and Small Business Technology Transfer programs. Jun Yao and Zhengzong Sun, doctoral candidates at Rice, performed much of the lab work.