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Is resistive RAM the next NVM star?

Posted: 22 Apr 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Resistive RAM? DRAM? NAND? 16nm?

Silver and germanium sulfide are both unusual materials for a semiconductor foundry and Adesto's business model is fabless. So there are limited options as to who can make the devices. In this case, Adesto has partnered with Altis Semiconductor, a speciality foundry based in Corbeil-Essones, France. The partnership is fairly close as Altis took an equity stake in Adesto in 2011, which also gave them a licence to Adesto's CBRAM technology.

Altis's technology roadmap (circa 2013) shows the eCBRAM technology as being under development on a 130nm CMOS process, but it now appears to be in production with the Adesto CBRAM.

Altis Semiconductor

Altis Semiconductor roadmap

We should point out that Adesto published a paper in 2013 where they introduced a second generation CBRAM using an amorphous alloy containing a semiconducting element for its anodes, and an amorphous oxide as the switching layer. This 2nd generation cell is said to offer improved data retention after solder reflow operations as compared to the 1st generation CR+BRAM devices.

Panasonic is the second company that TechInsights examines that is offering resistive RAM product (ReRAM) with their MN101LR series of microcomputers. The microcontroller is fabricated at Panasonic's former Tonami fab using a 180nm CMOS process. The Tonami fab is now operated as a joint venture with TowerJazz.

Panasonic uses a binary transition metal oxide (tantalum oxide) as a variable resistance layer sandwiched between an upper electrode (iridium) and a lower electrode (tantalum based electrode). Panasonic's '319 patent further describes the tantalum oxide as having two sub-layers, where a bottom tantalum oxide layer is formed by the reactive sputtering process of a Ta target to form an oxygen deficient layer (TaO1.43). This deposited tantalum oxide then undergoes an oxidation process to increase the oxygen content of its upper surface to form TaO2.45, which is close to the stoichiometric Ta2O5.

ReRAM cell

Panasonic ReRAM cell

The resistance of the tantalum oxide can be switched from a high resistance to a low resistance state by the application of a negative bias voltage pulse to the top electrode, and switched back to a high resistance state by applying a positive voltage pulse to the top electrode.

Panasonic provides few hints as to how the ReRAM cell works though they do indicate in their '319 patent that the oxygen deficient layer (TaO1.43) is somewhat conductive and the nearly stoichiometric layer (TaO2.45) is resistive and that the resistive switching occurs in the TaO2.45 layer. In a 2012 paper, Panasonic makes reference to conductive filaments being formed in the TaO2.45 with oxygen vacancies playing a role.

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